The MIC JournalModeling, Identification and Control: A Nordic Open Access Research Bulletin
http://www.mic-journal.no
<![CDATA[A Cryptographic Toolbox for Feedback Control Systems]]>Feedback control systems consist of components such as sensory systems, state estimators, controllers, and actuators. By transmitting signals between these components across insecure transmission channels, feedback control systems become vulnerable to cyber-physical attacks. For example, passive eavesdropping attacks may result in a leak of confidential system and control parameters. Active deception attacks may manipulate the behavior of the state estimators, controllers, and actuators through the injection of spoofed data. To prevent such attacks, we must ensure that the transmitted signals remain confidential across the transmission channels, and that spoofed data is not allowed to enter the feedback control system. We can achieve both these goals by using cryptographic tools. By encrypting the signals, we achieve confidential signal transmission. By applying message authentication codes (MACs), we assert the authenticity of the data before allowing it to enter the components of the feedback control system. In this paper, a toolbox containing implementations of state-of-the-art high-performance algorithms such as the Advanced Encryption Standard (AES), the AEGIS stream cipher, the Keyed-Hash Message Authentication Code (HMAC), and the stream ciphers from the eSTREAM portfolio, is introduced. It is shown how the algorithm implementations can be used to ensure secure signal transmission between the components of the feedback control system, and general guidelines that the users must adhere to for safe operation are provided.]]>4143133321052672<![CDATA[Co-simulation as a Fundamental Technology for Twin Ships]]>The concept of digital twins, characterized by the high fidelity with which they mimic their physical counterpart, provide potential benefits for the next generation of advanced ships. It allows analysis of data and monitoring of marine systems to avoid problems before they occur, and plan for the future by using simulations. However, issues related to integration of heterogeneous systems and hardware, memory, and CPU utilization makes implementing such a digital twin in a monolithic or centralized manner undesirable. Co-simulation addresses this problem, allowing different sub-systems to be modelled independently, but simulated together. This paper presents the ongoing work towards realizing a digital twin of the Gunnerus research vessel by applying co-simulation and related standards. The paper does not present a complete, ready-to-use digital twin. Rather it presents the preliminary results, procedure, and enabling technologies used towards realizing one. In order to accommodate this goal, a novel co-simulation solution, developed in cooperation by members of the Norwegian maritime industry, is presented. Furthermore, a maneuvering case-study is carried out, utilizing pre-recorded sensor data obtained from the Gunnerus. Through a comparative study with the real maneuver in terms of speed, course, and power consumption, the proposed approach is verified in simulation.]]>4142973112423808<![CDATA[The OpenModelica Integrated Environment for Modeling, Simulation, and Model-Based Development]]>OpenModelica is a unique large-scale integrated open-source Modelica- and FMI-based modeling, simulation, optimization, model-based analysis and development environment. Moreover, the OpenModelica environment provides a number of facilities such as debugging; optimization; visualization and 3D animation; web-based model editing and simulation; scripting from Modelica, Python, Julia, and Matlab; efficient simulation and co-simulation of FMI-based models; compilation for embedded systems; Modelica-UML integration; requirement verification; and generation of parallel code for multi-core architectures. The environment is based on the equation-based object-oriented Modelica language and currently uses the MetaModelica extended version of Modelica for its model compiler implementation. This overview paper gives an up-to-date description of the capabilities of the system, short overviews of used open source symbolic and numeric algorithms with pointers to published literature, tool integration aspects, some lessons learned, and the main vision behind its development.]]>4142412955678080<![CDATA[An analysis of Model Predictive Control with Integral Action applied to Digital Displacement Cylinders]]>This article aims to analyze Model Predictive Control (MPC) for the control of multi-chamber cylinders. MPC with and without integral action has been introduced. Three different algorithms have been used to solve the optimization problem in the MPC. The different algorithms have been compared with an industrial solver. The influence of changing mass, choosing a different middle line pressure, system delays, signal noise, velocity estimation, and changing pressure levels has been investigated. It is concluded that for the small prediction horizon used in the paper a simple algorithm such as A* can produce results as good as the previously used Differential Evolution algorithm in less than half the time. It is further concluded that unknown software delays and unknown changes in mass have the largest effect on system performance.]]>4132232391530880<![CDATA[Extrinsic calibration for motion estimation using unit quaternions and particle filtering]]>This paper presents a method for calibration of the extrinsic parameters of a sensor system that combines a camera with an inertial measurement unit (IMU) to estimate the pendulum motion of a crane payload. The camera measures the position and orientation of a fiducial marker on the payload, while the IMU is fixed to the payload and measures angular velocity and specific force. The placements of the marker and the IMU are initially unknown, and the extrinsic calibration parameters are their position and orientation with respect to the reference frame of the payload. The calibration is done with simultaneous state and parameter estimation, where a particle filter is used for state estimation, and a Riemannian gradient descent method is used for parameter estimation. The orientation is described with unit quaternions, and gradients are developed in a Riemannian formulation based on the Lie group of unit quaternions. This leads to efficient derivations of gradient expressions involving orientations and provides added geometric insight to the problem. The efficiency of the method is demonstrated in simulations and experiments for a simplified crane payload problem.]]>4132072212002944<![CDATA[A Self-Contained Cylinder Drive with Indirectly Controlled Hydraulic Lock]]>This paper presents a self-contained pump-controlled hydraulic linear drive including an innovative load holding sub-circuit. For safety critical applications such as crane manipulators, locking valves or load holding valves are enforced by legislation, but the load holding functionality may also be used actively to decrease the energy consumption for applications where the load is kept stationary for longer periods of time. The system proposed in this paper is based on a simple hydraulic architecture using two variable-speed electric motors each connected to a fixed-displacement pump. This architecture is well-known in academic literature, but in this paper a novel load holding sub-circuit has been included. To control this load holding functionality, the low chamber pressure needs to be controlled accurately, while still being able to control the motion of the cylinder piston as well. Due to strong cross-couplings between cylinder piston motion and chamber pressures this task is non-trivial. The control for opening the locking valves is indirect in the sense that it is controlled via the chamber pressures, which are actively controlled. The fundamental control strategy presented in this paper is based on transforming the highly coupled physical states to virtual states, significantly reducing cross-couplings.]]>4131852054699136<![CDATA[Identification and Optimal Control for Surge and Swab Pressure Reduction While Performing Offshore Drilling Operations]]>In this paper, an unscented Kalman filter coupled with a nonlinear model-predictive controller for a hydraulic wellbore model with multi-variable control and tracking is presented. In a wellbore, high drill string velocities in operational sequences such as tripping might result in surge and swab pressures in the annular section of the wellbore. To overcome these challenges, a controller incorporating safety and actuator limits should be used. A second-order model is used to predict axial drill string velocity downhole. With a nonlinear model-predictive controller (NMPC) specifying the block position trajectory, choke flow reference, desired back-pressure pump flowrate and stand-pipe pressure, we can automatically supervise and control the pressure in the wellbore. To compensate for unmeasured states, an estimator is designed to predict the frictional pressure forces in the wellbore and filter noisy measurements. A stochastic approach for the hydraulic model is taken, including variance of the average fluctuations for the flow and pressure states. Comparing three NMPC configurations, the result of using an integration of the tracking error in the prediction model gave best offset-free tracking of the bottom-hole pressure. The controller compensates for the unknown fluctuations, and is shown to be robust towards model mismatch. Including the mechanical system in the NMPC prediction model, we can effectively constrain the predicted axial drill string velocity to reduce the pressure oscillations and achieve tracking of bottom hole pressure and choke differential pressure. The outcome is shown through extensive simulations to be an effective control strategy, reducing the pressure spikes while tripping.]]>4131651842533376<![CDATA[Review on Modeling and Control of Flexible Link Manipulators]]>This paper presents a review of dynamic modeling techniques and various control schemes to control flexible link manipulators (FLMs) that were studied in recent literature. The advantages and complexities associated with the FLMs are discussed briefly. A survey of the reported studies is carried out based on the method used for modeling link flexibility and obtaining equations of motion of the FLMs. The control techniques are reviewed by classifying them into two main categories: model-based and model-free control schemes. The merits and limitations of different modeling and control methods are highlighted.]]>413141163562176<![CDATA[Impact of fiber versus steel ropes on the lifetime of crane winches]]>Offshore crane winches are equipment installed on sea vessels and designed for accurate lifting and lowering of payloads at subsea levels in all conditions. In recent times, attempts have been made to replace conventional heavy steel ropes used in crane winches with high-strength synthetic fibre ropes for their various advantages, particularly for having comparable strength with steel at a much lower weight. Such an advantage makes it possible, in principle, to downsize a crane winch for the same lifting capacity leading to considerable savings or alternatively, to increase the service life of an existing crane winch and make the most out of its lifting capacity. However, it is still not known to what extent can replacing steel ropes in crane winches with fibre ropes increase their service life and in this paper, an attempt is made to answer this question.]]>413129139791552<![CDATA[Model-based optimal recovery of methane production in an anaerobic digestion reactor]]>An unhealthy condition of a biogas reactor is an abnormal situation characterized by a large reduction of the concentration of the microorganisms in the reactor, causing a reduction of the biogas production. This condition may occur due to too high feeding rate to the reactor. If an unhealthy condition continues for several days, the reactor may get into a washout situation where the methane production may drop considerably. By monitoring the alkalinity ratio or the methane production rate, an unhealthy condition can be recognized. Threshold of the alkalinity ratio or the methane production volume level can be determined by users for the start of applying the optimal recovery procedure. The goal of this study is an investigation on optimal recovery of methane production after determining an unhealthy condition. Our approach is model-based optimization, applied to a simulated reactor. The AM2 model is assumed.]]>412121128930816<![CDATA[Practice for determining friction in hydraulic winch systems]]>This paper presents a method for estimating friction in hydraulic active heave compensated (AHC) offshore winches. The method is a two-step approach where the first step is to model the friction loss in the hydraulic motors based on data from the sub-supplier. The second step requires real-life testing, where the remaining friction losses in the winch system is identified and modeled. In this context, a practice is characterized by obtaining a friction loss estimation with the highest possible accuracy over the widest possible range of operating conditions with a limited amount of experimental work. The method benefits from the use of parametric models, sub-supplier data, and real-life measurements on a 150 t AHC crane from National Oilwell Varco Norway (NOVN). The work is an important part of developing a simulation model that can be used actively in virtual testing and verification of crane operations at NOVN. A friction loss model developed from the proposed method was implemented in a NOVN simulation model. Computed and measured hydraulic pressures showed deviations of less than 10% from measured results for a 150 t crane operating in AHC.]]>4121091203978240<![CDATA[A Guide to Student-active Online Learning in Engineering]]>Online learning in higher education is becoming increasingly common as the possibilities of the available digital infrastructure expand. A recent emergent driver for online learning is the closing of universities to limit the spread of the coronavirus (COVID-19). Many educators are now faced with the need to make their teaching digital, though they have little or no experience with online teaching methods. In such a situation, learning outcomes may come second to what can be readily implemented by available digital resources. In this paper, a design for student-active online learning in engineering is proposed as a guide to help take account of learning objectives first, and the digital tools and resources necessary to achieve those objectives second. % in when choosing online learning strategies. In addition, the paper emphasises the social dimension of online learning, and recommends that explicit actions should be taken to increase positive social relations between students in an online course to be able to succeed with student-active learning methods. In the paper, a clear path is followed from objectives to learning activities, and then to assessments and evaluations, and appropriate digital tools and resources are suggested to support activities and evaluations in an online course. Online courses in engineering are targeted in particular, and challenges that arise from common activities such as problem solving and practical work in an online engineering course are addressed. The proposed guide emphasises usability to ensure that it can be used even by inexperienced digital educators, and an example on how the guide can be applied to design an online course in mobile robotics is given. The proposed guide aims to help shift online learning in engineering from traditionally teacher-active lectures to more student-active learning activities.]]>41291107920576<![CDATA[Adaptive Moving Horizon Estimator for Return Flow Rate Estimation using Fluid Levels of a Venturi Channel]]>Real-time estimation of the return drilling fluid during oil well drilling is investigated in this study. Online fluid level measurements from a Venturi channel which can be placed on the return flowline is used with a model-based estimator. A reduced order, 1-D, mathematical equation is used for the open flow in the Venturi channel for Newtonian or non-Newtonian fluid types. The volumetric fluid flow rate is estimated using a moving horizon estimator in real-time. The friction factor is also estimated together with the fluid flow rate. The effect of the variation of the channel slope on the flow rate estimation induced by the vibration of the channel during its operation is also studied. The method requires only two level measurements in the Venturi channel together with the channel geometry. The method is validated using a laboratory scale Venturi flow system. The proposed method shows promising potential to be used as a real-time return flow rate measurement in conventional drilling systems.]]>41279902343936<![CDATA[Dynamic model for simulating transient behaviour of rotary drum granulation loop]]>In this paper, a dynamic model for a rotary drum granulation loop with external product separator is developed. A population balance is used to capture dynamic particle size distribution in the 3-compartment rotary drum granulator model. Particle agglomeration along with particle growth due to layering are assumed as granulation mechanisms in the rotary drum. The model of the granulation loop includes models of the drum, screens and a crusher. Simulations using the developed model provide valuable data on dynamic fluctuations in the inlet and the outlet particle size distribution for the rotary drum. Simulation results showed that at smaller crusher gap spacings, the instabilities of the drum granulation loop occur, and damped oscillations are observed. Above the critical crusher gap spacing, sustained periodic oscillations are observed. The reason for oscillations is the off-spec particle flow that is recycled back to the granulator.]]>41265772537472<![CDATA[Steady State Counterbalance Valve Modeling with the Influence of Synthetic Ester Oils Using CFD]]>This study looks in details at the effects of synthetic esters being applied to a counterbalance valve from the perspective of a system engineer. There is limited literature on the subject of applied synthetic esters and as such limited unbiased sources for information. This creates reluctance against the use of these fluids in sectors and regions with no prior experience and knowledge of what to expect. This study expands the applied literature by investigating a commercially available valve using commercial oils, a basic synthetic ester, a fully saturated synthetic ester and a typical mineral oil type for benchmarking. The investigation is based on both computational fluid dynamics and experiments and is performed at 20, 40 and 60 deg. The product is a steady-state valve model including fluid dynamics and a parameter-dependent Coulomb friction. The CFD model reveals minimal oil type dependence in the resulting fluid dynamics model with flow forces and discharge coefficient being the same for mineral oil and esters. The experiments show that the esters primarily produce different levels of hysteresis with up to 40% less and 15% more hysteresis. The friction investigation showed that the relationship between hysteresis and pilot pressure was different for all oil types, and that the relationship between hysteresis and temperature was similar for all oil types. With full knowledge of mineral oil and the oil specific knowledge of the hysteresis and pilot pressure relationship at a single temperature, ester hysteresis was predicted with better than 88% accuracy across the three temperatures.]]>41251641617920<![CDATA[Compressor Surge Control Using Lyapunov Neural Networks]]>In this paper surge control in a compression system using a close-coupled valve (CCV) is proposed. The control design is based on Lyapunov control theory in combination with neural networks (NNs) and focuses on minimization of loss of energy in the compressor system. The approach allows for control design with guaranteed region of attraction when considering saturated controls. The CCV modifies the characteristics of the compressor and thus stabilizes the equilibrium beyond the original surge line at the expense of a pressure drop over the valve. Two control laws for the compression system are proposed. The first control law provides a global asymptotically stable equilibrium. The second control law focuses on minimization of the pressure drop over the valve, and local asymptotically stability is proven.]]>41241492844672<![CDATA[An Isogeometric Analysis Approach to Kinematics of Spatial Rigid Multibody Systems with Imperfect Joints]]>This paper proposes a novel generic methodology for kinematics of spatial rigid-multibody systems with and without lubricated joints. In this method CAD surface representations in the form of non-uniform rational B-splines (NURBS) are used to address the interface kinematics. This eliminates the time and effort needed to manually parameterize the interface geometry, by enabling a direct use of the engineering designs encapsulated in CAD systems. Furthermore, the use of NURBS for surface representation allows integration of tribodynamics into an isogeometric analysis (IGA) setting. The kinematic formulation is based on a new implicit matrix approach for implicitization of CAD surfaces in three-dimensional space. The construction of such implicit matrices and their properties are explained, and explicit expressions for the gap height distance, velocity and relative velocities in a general clearance joint are provided.]]>4112940647168<![CDATA[Variable Step Size P&O Algorithms for Coalescing Pump/Deoiling Hydrocyclone Produced Water Treatment System]]>This paper presents three variable step size P&O algorithms for optimizing the separation efficiency of a coalescing pump/deoiling hydrocyclone produced water treatment system. By continuously adjusting the pumping pressure, and subsequently the coalescing effect, the algorithms are used to minimize the oil concentration downstream the hydrocyclone. Due to the variable step size, the algorithms react rapidly to changes in the upstream produced water characteristics, at the same time as they reduce (or eliminate) steady-state oscillations. Based on both simulation and experimental testing, the study discusses advantages and disadvantages of the algorithms.]]>41113271184768<![CDATA[Automated drilling algorithms implementation on a laboratory drilling rig]]>Considering the state of art technology that exists today and the significant resources that are being invested into the technology of tomorrow, an idea of intelligent and fully automated machineries working on a drilling floor that is capable of consistently selecting best decisions or predictions based on realtime information available and providing drillers and operators with such recommendations, becomes closer to a reality every day. This work shows results of the research carried out on the topic of drilling automation. Its objectives are to design and test proof of concept technologies conducted on a laboratory-scale autonomous drilling rig developed at University of Stavanger, Norway. Main contribution of the study is on drilling speed (ROP) optimization with considering operational safety to personnel and environment (HSE) and drilling efficiency along with a digitized drilling program for directional drilling. The case studies are presented to show the different scenarios for drilling vertical wells and inclined wells.]]>4111113439616<![CDATA[Estimating uncertainty of model parameters obtained using numerical optimisation]]>Obtaining accurate models that can predict the behaviour of dynamic systems is important for a variety of applications. Often, models contain parameters that are difficult to calculate from system descriptions. Hence, parameter estimation methods are important tools for creating dynamic system models. Almost all dynamic system models contain uncertainty, either epistemic, due to simplifications in the model, or aleatoric, due to inherent randomness in physical effects such as measurement noise. Hence, obtaining an estimate for the uncertainty of the estimated parameters, typically in the form of confidence limits, is an important part of any statistically solid estimation procedure. Some uncertainty estimation methods can also be used to analyse the practical and structural identifiability of the parameters, as well as parameter inter-dependency and the presence of local minima in the objective function. In this paper, selected methods for estimation and analysis of parameters are reviewed. The methods are compared and demonstrated on the basis of both simulated and real world calibration data for two different case models. Recommendations are given for what applications each of the methods are suitable for. Further, differences in requirements for system excitation are discussed for each of the methods. Finally, a novel adaption of the Profile Likelihood method applied to a moving window is used to test the consistency of dynamic information in the calibration data for a particular model structure.]]>40421324319895296<![CDATA[Model-Free PI/PID Controller Tuning of Higher Order Nonlinear Dynamic Systems]]>This paper concerns model-free PI/PID controller tuning of possible (nonlinear) higher order systems. The method can be considered as a three-step procedure. The first step is to persistently excite the system in open loop and identify the dynamic model using a subspace identification method. The second step is to approximate the model to an integrator plus time delay or double integrator plus time delay dynamic model. The third step is to compute the PI/PID controller parameters. The proposed method/theory is verified on some motivated nonlinear higher order dynamic models.]]>404199211850944<![CDATA[Payload estimation using forcemyography sensors for control of upper-body exoskeleton in load carrying assistance]]>In robotic assistive devices, the determination of required assistance is vital for proper functioning of assistive control. This paper presents a novel solution to measure conveniently and accurately carried payload in order to estimate the required assistance level. The payload is estimated using upper arm forcemyography (FMG) through a sensor band made of force sensitive resistors. The sensor band is worn on the upper arm and is able to measure the change of normal force applied due to muscle contraction. The readings of the sensor band are processed using support vector machine (SVM) regression technique to estimate the payload. The developed method was tested on human subjects, carrying a payload. Experiments were further conducted on an upper-body exoskeleton to provide the required assistance. The results show that the developed method is able to estimate the load carrying status, which can be used in exoskeleton control to provide effectively physical assistance needed.]]>4041891983015680<![CDATA[Short-term ASV Collision Avoidance with Static and Moving Obstacles]]>This article considers colav for both static and moving obstacles using the bcmpc algorithm, which is designed for use by asv. The bcmpc algorithm originally only considered colav of moving obstacles, so in order to make the algorithm also be able to avoid static obstacles, we introduce an extra term in the objective function based on an occupancy grid. In addition, other improvements are made to the algorithm resulting in trajectories with less wobbling. The modified algorithm is verified through full-scale experiments in the Trondheimsfjord in Norway with both virtual static obstacles and a physical moving obstacle. A radar-based tracking system is used to detect and track the moving obstacle, which enables the algorithm to avoid obstacles without depending on vessel-to-vessel communication. The experiments show that the algorithm is able to simultaneously avoid both static and moving obstacles, while providing clear and readily observable maneuvers. The bcmpc algorithm is compliant with rules 8, 13 and 17 of the colregs, and favors maneuvers following rules 14 and 15.]]>4031771871341440<![CDATA[On-line parameter and state estimation of an air handling unit model: experimental results using the modulating function method]]>This paper considers the on-line implementation of the modulating function method, for parameter and state estimation, for the model of an air-handling unit, central element of HVAC systems. After recalling the few elements of the method, more attention is paid on issues related to its on-line implementation, issues for which we use two different techniques. Experimental results are obtained after implementation of the algorithms on a heat flow experiment, and they are compared with conventional techniques (conventional tools from Matlab for parameter estimation, and a simple Luenberger observer for state estimation) for their validation.]]>4031611763594240<![CDATA[Dynamic model adaptation to an anaerobic digestion reactor of a water resource recovery facility]]>This study deals with model adaptation of the AM2 model to an anaerobic digestion reactor of a water resource recovery facility, namely a 6000m3 reactor at VEAS WWRF, the largest of Norway. The model is based on the mass balance with six states including acidogens, methanoges, alkalinity, organic substrate, volatile fatty acid and inorganic carbon. The model adaptation is applied firstly to simulated reactor data for testing the algorithms, and then to experimental data. The experimental data are collected from laboratory analysis and online measurements from January to October 2017. The data of the first 100 days are used for model identification, and the remaining data for model validation. Identification analysis is based on the Fisher Information Matrix and the Hessian matrix. Also, a sensitivity analysis of the parameter estimates is accomplished.]]>4031431602169856<![CDATA[Nonlinear observer for hydropower system]]>Estimation of unmeasured states plays an essential role in the design of control systems as well as for monitoring of hydropower plants. The standard Kalman filter gives the optimum state estimates for linear systems. However, this optimality is not relevant for nonlinear models and a choice between stochastic and deterministic approaches is not so obvious in this case. Thus the application of a nonlinear observer in a hydropower system is of interest here as an alternative to the widely used extended Kalman filter. This paper provides a study and design of a reduced order nonlinear observer to estimate the states of a hydropower system. Implementation of the nonlinear observer is done in OpenModelica and added to our in-house hydropower Modelica library --- OpenHPL, where different models for hydropower systems are assembled. Simulations and analysis of the designed observer are done in Python using a Python API for operating OpenModelica simulations.]]>4031331421531904<![CDATA[Challenges in application of hybrid switched control to digital hydraulic motors]]>In this paper, the challenges regarding the application of a switched control approach to a digital displacement machine (DDM) are discussed. The system under consideration is initially presented. Subsequently, the reasons for considering the design and application of the specific, switched controller are discussed. Finally, the challenges are presented, which could function as future research.]]>402125132506880<![CDATA[An EKF for Lie Groups with Application to Crane Load Dynamics]]>An extended Kalman filter (EKF) for systems with configuration given by matrix Lie groups is presented. The error dynamics are given by the logarithm of the Lie group and are based on the kinematic differential equation of the logarithm, which is given in terms of the Jacobian of the Lie group. The probability distribution is also described in terms of the logarithm as a concentrated Gaussian distribution that is a tightly focused distribution around the identity of the Lie group. The filter is applied to estimation on SO(3) a case where a stereo camera setup tracks a crane wire with a payload. The wire, which is under tension and forms a line is monitored by two 2D-cameras, and a line detector is used to obtain a description of how the wire is projected onto each image plane. A model of a spherical pendulum is applied and the estimator is validated by applying it on simulated data, as well as experimental data.]]>402109124844800<![CDATA[Enabling Energy Savings in Offshore Mechatronic Systems by using Self-Contained Cylinders]]>This paper proposes a novel actuation system for an offshore drilling application. It consists of three self-contained electro-hydraulic cylinders that can share and store regenerated energy. The energy saving potential of the proposed solution is analyzed through a multibody system simulation. The self-contained system demonstrates superior energy efficiency compared to the benchmark system representing the state-of-the-art approach used today (i.e., valve-controlled cylinders by means of pressure-compensated directional control valves and counter-balance valves, supplied by a centralized hydraulic power unit). Due to the power on demand capability, the cancellation of the throttling losses, and the opportunity to recover energy in motoring quadrants, the self-contained system consumes 83.44% less energy without affecting the system's performance.]]>402891086230016<![CDATA[Feasibility of Deep Neural Network Surrogate Models in Fluid Dynamics]]>This paper studies reduced-order-models for the fluid flow problem of a digital valve, and whether it may efficiently be formulated by a deep Artificial Neural Network (ANN) to model e.g. the valve flow, flow-induced force, stiction phenomena and steep local pressure gradients that arise before plunger impact, which may otherwise require CFD to be accurately modeled. Several methodologies are investigated to evaluate both the required computation time and the accuracy. The accuracy is benchmarked against CFD solutions of flows and forces. As basis for comparison an analytical model is proposed where some fitting parameters are allowed, and the equation is tested outside its fitting range. A similar model is built as a deep ANN which is trained with data from the analytical model to investigate the amount of data required for an ANN and its fitting capabilities. The results show that in higher dimensions the required training data can be maintained low if data is structured by a Latin Hypercube, otherwise the amount becomes infeasible. This makes an ANN surrogate feasible when compared to a look-up table, and may be expanded to higher dimension where dynamical effects are included. However, the required data and computational cost for this is too extensive for the valve design considered as basis for the analysis. Instead, for this specific problem, the derived analytical model is sufficient to describe the valve dynamics and reduces the computation time significantly.]]>40271876553600<![CDATA[Quantitative genetics state-space modeling of phenotypic plasticity and evolution]]>Living organisms adapt to changes in environment by phenotypic plasticity and evolution by natural selection (or they migrate). At detailed genetic levels these phenomena are complicated, and quantitative genetics attempts to capture essential processes at a higher abstraction level. Phenotypic plasticity is then commonly modeled by reaction norms, which describe how individual traits in a population are expressed in response to changes in environmental variables. The mean reaction norms are evolvable, and here I present a general quantitative genetics state-space model for evolutionary reaction norm dynamics. Reaction norms make use of a reference environment, which is traditionally set to zero. This is problematic when the reference environment is the environment a population is adapted to, for the reason that this environment is a population property, which in itself may be evolvable. With reference to Ergon (2018), I describe models that take such evolvability into account. The resulting models are fundamentally different from most engineering system models, where given reference values are constant, and therefore without consequences can be set to zero. For simplicity I assume only temporal variations in environment, although there obviously are a lot of spatial variations in nature, and I assume that no mutations are involved. Fundamentals from quantitative evolutionary theory are given in appendices.]]>40151693277824<![CDATA[Master-slave H-Infinity robust controller design for synchronization of chaotic systems]]>This paper is devoted to robust master-slave controller design for generalized chaotic systems synchronization. The closed-loop system is asymptotically stable when the robust stability conditions hold and while the H-Infinity norm of the closed-loop transfer function with respect to defined output and input is strictly less than gamma>0. In this paper a modified L2 gain approach is used and an original design procedure is proposed to decrease the conservativeness of the former method. The effectiveness of the proposed method is shown in numerical examples.]]>4014150484352<![CDATA[Control System Commissioning of Fully Levitated Bearingless Machine]]>The bearingless permanent magnet synchronous motor (BPMSM) is a compact motor structure that combines the motoring and bearing functions based on well-designed integrated windings for generating both torque and magnetic suspension force. In order to achieve a successful high-performance control design for the BPMSM, an adequate model of the rotor dynamics is essential. This paper proposes simplified multiple-input and multiple-output (MIMO) control approaches, namely the pole placement and the linear-quadratic regulator (LQR), that allow to carry out identification experiments in full levitation. Additionally, the stability of the MIMO levitation controller is verified with the rotation tests. Compared with other recently published works, the novelty of this paper is to experimentally demonstrate that a stable fully levitated five-degrees-of-freedom (5-DOF) operation of a bearingless machine can be achieved by the proposed approach, and thereby, options for commissioning of such a system are obtained.]]>40127394913152<![CDATA[Methods for Experimentally Determining Stiffness of a Multi-Axis Machining Centre]]>This paper deals with global methods for experimentally determining the static stiffness of multi-axis machining centres. Different devices used for measuring deflection, in specific, are explored, where accuracy and usability are highlighted. The methods were tested on a 3-axis CNC milling machine, 2-axis trunnion table and a 6-DOF industrial robot.]]>40111254804608<![CDATA[Finding Clusters in Petri Nets. An approach based on GPenSIM]]>Graph theory provides some methods for finding clusters in networks. Clusters reflect the invisible grouping of the elements in a network. This paper presents a new method for finding clusters in networks. In this method, the user can adjust a parameter to change the number of clusters. This method is newly added to the simulator General-purpose Petri Net Simulator (GPenSIM) as a function for network analysis. With this GPenSIM function, in addition to the usual performance analysis of a discrete-event system via a Petri net model, supplementary information about the grouping of the elements can also be found. Finding clusters in discrete-event systems provides valuable information such as the ideal location of the elements in a manufacturing network. This paper also presents an application example on a flexible manufacturing system.]]>401110761856<![CDATA[A Novel Process-Reaction Curve Method for Tuning PID Controllers]]>A novel process-reaction curve method for tuning PID controllers for (possible) higher order processes/models is presented. The proposed method is similar to the Ziegler-Nichols process reaction curve method, viz. only the maximum slope and lag need to be identified from an open loop step response. The relative time delay error (relative delay margin), delta is the tuning parameter. The proposed method is verified through extensive numerical simulations and is found close to optimal in many of the motivated process examples. In order to handle the wide set of process models, two model reduction modes are presented.]]>3942732916478848<![CDATA[Modeling and Analysis of Fluid Flow through A Non-Prismatic Open Channel with Application to Drilling]]>This paper presents the development and validation of a simplified dynamic model of a Venturi channel. The existing dynamic models on open channels are based on the open channel flow principles, which are the shallow water equations. Although there are analytical solutions available for steady state analysis, the numerical solution of these partial differential equations is challenging for dynamic flow conditions. There are many complete and detailed models and numerical methods available for open channel flows, however, these are usually computationally heavy. Hence they are not suitable for online monitoring and control applications, where fast estimations are needed. The orthogonal collocation method could be used to reduce the order of the model and could lead to simple solutions. The orthogonal collocation method has been used in many chemical engineering applications. Further, this has been used in prismatic open channel flow problems for control purposes, but no literature is published about its use for non-prismatic channels as per the author's knowledge. The models for non-prismatic channels have more non-linearity which is interesting to study. Therefore, the possibility of using the collocation method for determining the dynamic flow rate of a non-prismatic open channel using the fluid level measurements is investigated in this paper. The reduced order model is validated by comparing the simulated test case results with a well-developed numerical scheme. Further, a Bayesian sensitivity analysis is discussed to see the effect of parameters on the output flow rate.]]>3942612725706752<![CDATA[Control of an Energy Efficient Hydraulic Cylinder Drive with Multiple Pressure Lines]]>This paper concerns a novel energy efficient hydraulic cylinder drive concept, consisting of three pressure lines used to supply a hydraulic cylinder(s). The proposed concept allows for reduced throttling losses compared to conventional solutions, while maintaining accurately cylinder control. One application where the concept shows great potential is as the actuation system for life tests of large mechanical structures, where structure deflections is made through cylinder force control. This work contributes to the development and control of this hydraulic concept, where the purpose is to obtain a high energy efficiency and an accurate cylinder force control. The final concept design is implemented in simulation models, where the performance of the developed control system is investigated to verify that the control performance is satisfactory. Furthermore, the energy efficiency is compared to a conventional hydraulic concept to verify the feasibility. Through simulation models, control performance similar to a conventional hydraulic concept has been obtained with the proposed control structure, while reducing the hydraulic energy consumption by up to 50%. Based on the findings in this paper it is assessed that the concept is ready for experimental validation.]]>3942452594012032<![CDATA[eXogenous Kalman Filter (XKF) for Visualization and Motion Prediction of Ships using Live Automatic Identification System (AIS) Data]]>This paper addresses the problem of ship motion estimation using live data from Automatic Identification Systems (AIS). A globally exponentially stable observer for visualization and motion prediction of ships has been designed. Instead of using the extended Kalman filter (EKF) to deal with the kinematic nonlinearities the eXogenous Kalman Filter (XKF) is applied and by this global stability properties are proven. The proposed observer was validated using live AIS data from the Trondheim harbor in Norway and it was demonstrated that the observer tracks ships in real time. It was also demonstrated that the observer can predict the future motion of ships. The motion prediction capabilities are very useful for decision-support systems since this can be used to improve situational awareness e.g. for manned and unmanned autonomous ships that operate in common waters.]]>39423324410133504<![CDATA[Transient conformal TEHL algorithms for multibody simulation]]>This article describes aspects of transient thermal elasto-hydrodynamical lubrication (TEHL) contact modelling for conformal contacts. This is to be utilized in a multibody simulation (MBS) framework for engineering purposes. The verification and proof of concept is done by implementation in the tool BEAST (Fritzson et al., 2014) and by comparision to published experiments and simulation results.]]>3932092325749760<![CDATA[The Norwegian Motion-Laboratory]]>This paper contains an overview of the equipment currently available in the Norwegian Motion Laboratory, a description of the IT networking infrastructure in the laboratory, a GitHub link to open source code developed, description of the PyQt-based graphical user interface, presentation of robot forward and inverse kinematics, presentation of equations of motion for the suspended load motion and a description of the full system kinematics. The paper ends with a list of research experiments and publications from the laboratory to date.]]>3931912085392384<![CDATA[Composite modelling in 3-D mechanics utilizing Transmission Line Modelling (TLM) and Functional Mock-up Interface (FMI)]]>Composite modelling and simulation is a solution to utilize investments in models and tools, use the right tool for the right task, increase the accuracy by means of more accurate modelled boundary conditions, switch between levels in model complexity for a specific sub-system, and facilitate co-operation in organizations. With the new Functional Mock-up Interface (FMI) standardization, efforts are increasing to make this happen. SKF BEAST is an advanced dynamic simulation tool for rolling bearings and other mechanical systems with contacts. The tool incorporates a framework for composite modelling and co-simulation, i.e., a Master Simulation Tool (MST). It uses Transmission Line Modelling (TLM) to ensure robust numerical behaviour of the complete composite system model and supports the Functional Mock-up Interface (FMI) for model import, including both model exchange and co-simulation. In this paper, the tools and the techniques for composite modelling are discussed in further detail and application examples are given.]]>3931791901338368<![CDATA[Model Predictive Control of Low-Speed Partial Stroke Operated Digital Displacement Pump Unit]]>To enhance the use of the Digital Displacement Machine (DDM) technology as the future solution for low speed fluid power pump and motor units, a Model Predictive Control (MPC) strategy is presented. For a low speed DDM, the conventional full stroke operation strategy is unsuitable, since the control update rate is proportional to the machine speed. This creates an incentive to utilize sequential partial stroke operation where a fraction of the full stroke is used, which thereby increases the control update rate and control signal resolution. By doing this, the energy loss is increased and may become undesirable large if the control objective is purely set-point tracking, why a trade-off is considered advantageous. Discretizing the full stroke based on a chosen update rate results in a Discrete Linear Time Invariant (DLTI) model of the system with discrete input levels. In this paper, the Differential Evolution Algorithm (DEA) is used to determine the optimal control input based on the trade-off between set-point tracking and energy cost in the prediction horizon. The paper presents a flow and a pressure control strategy for a fixed speed digital displacement pump unit and shows the trade-off influence on the optimal solution through simulation. Results show the applicability of the control strategy and indicate that a much higher energy efficiency may be obtained with only a minor decrease in tracking performance for pressure control.]]>393167177824320<![CDATA[Software Components of the Thorvald II Modular Robot]]>In this paper, we present the key software components of the Thorvald II mobile robotic platform. Thorvald~II is a modular system developed by the authors for creating robots of arbitrary shapes and sizes, primarily for the agricultural domain. Several robots have been built and are currently operating on farms and universities at various locations in Europe. Robots may take many different forms, and may be configured for differential drive, Ackermann steering, all-wheel drive, all-wheel steering with any number of wheels etc. The software therefore needs to be configuration agnostic. In this paper we present an architecture that allows for simple setup of never-seen-before robot configurations. The presented software is organized in a collection of ROS packages, made available to the reader. These packages allow a user to create her or his own robot configurations and simulate these robots in Gazebo using a provided plugin. Although the presented packages were created to be used with Thorvald robots, they may also be useful for people who are looking to develop their own robot and are interested in testing various robot configurations in simulation before settling on a specific design. To create a robot, the user lists modules with key parameters in one single configuration file and gives this as an input to the robot at startup. Example configuration files are provided within the packages. In this paper, we discuss key aspects of the ROS packages and provide directions on where to find updated information on how to install and use these.]]>3931571654421632<![CDATA[Tail Removal Block Validation: Implementation and Analysis]]>In this paper a solution for the removal of long tail blocktimes in a proof-of-work blockchain is proposed, implemented and analysed. Results from the mainnet of the Bismuth blockchain demonstrate that the variances in the key variables, difficulty level and blocktime, were approximately halved after the tail removal code was enabled. Low variances in difficulty and blocktimes are desirable for timely execution of transactions in the network as well as reduction of unwanted oscillations in the feedback control problem.]]>393151156507904<![CDATA[Downhole Temperature Modeling for Non-Newtonian Fluids in ERD Wells]]>Having precise information of fluids' temperatures is a critical process during planning of drilling operations, especially for extended reach drilling (ERD). The objective of this paper is to develop an accurate temperature model that can precisely calculate wellbore temperature distributions. An established semi-transient temperature model for vertical wellbores is extended and improved to include deviated wellbores and more realistic scenarios using non-Newtonian fluids. The temperature model is derived based on an energy balance between the formation and the wellbore. Heat transfer is considered steady-state in the wellbore and transient in the formation through the utilization of a formation cooling effect. In this paper, the energy balance is enhanced by implementing heat generation from the drill bit friction and contact friction force caused by drillpipe rotation. A non-linear geothermal gradient as a function of wellbore inclination, is also introduced to extend the model to deviated wellbores. Additionally, the model is improved by considering temperature dependent drilling fluid transport and thermal properties. Transport properties such as viscosity and density are obtained by lab measurements, which allows for investigation of the effect of non-Newtonian fluid behavior on the heat transfer. Furthermore, applying a non-Newtonian pressure loss model enables an opportunity to evaluate the impact of viscous forces on fluid properties and thus the overall heat transfer. Results from sensitivity analysis of both drilling fluid properties and other relevant parameters will be presented. The main application area of this model is related to optimization of drilling fluid, hydraulics, and wellbore design parameters, ultimately leading to safe and cost efficient operations.]]>392131149823296<![CDATA[Limiting Factors for the Ability to Achieve Accurate Pressure Control in Long Wells]]>Extended-Reach Drilling (ERD) with narrow pressure margin or uncertain geo-pressure is a challenge with respect to accurate pressure control. The back-pressure Managed Pressure Drilling (MPD) method has been widely used in drilling operations with the aim of controlling annulus pressure within safe bounds, and may also be applicable for ERD wells. However, the ability to control the pressure accurately is limited by several factors. Some of which are related to back-pressure MPD operations in general and some of which are more specific to ERD wells. In this paper, a study is presented on how pressure control is affected and sometimes limited by the actual data availability and quality, equipment, hydraulic models, control algorithms, and downhole conditions during an MPD operation in an ERD well. By using a transient well flow model, the theoretically obtainable MPD performance can be simulated. The benefit by utilizing real-time downhole pressure measurements transmitted by a wired drill pipe is demonstrated by simulations. It is shown quantitatively how variations in delay of measurement and bandwidth will influence the ability to control downhole pressure accurately in an ERD well. Benefit by this approach is a more accurate prediction of what is obtainable with MPD and how various factors may influence the ability to control downhole pressure.]]>3921151291296384<![CDATA[A model for torque losses in variable displacement axial piston motors]]>This paper includes a comparison of earlier presented models for torque losses in hydraulic motors and several proposed models that all rely on data typically available for a system engineer. The new models and the old ones are compared. The new models are all based on a model developed by Jeong 2007 with an expansion that include variable displacement. All of the new models yield very good accuracy down to approximately 50% of maximum displacement and down to approximately 15% of maximum speed. In these operational ranges the deviation in torque is less than 1%. The main purpose of the new models is to facilitate simulations of hydraulically actuated winches with a balance between accuracy and model complexity. This purpose is considered fulfilled with several of the proposed models.]]>3921071141809408<![CDATA[Challenges with Respect to Control of Digital Displacement Hydraulic Units]]>This paper investigates the many complications arising when controlling a digital displacement hydraulic machine with non-smooth dynamical behavior. The digital hydraulic machine has a modular construction with numerous independently controlled pressure chambers. For proper control of dynamical systems, a model representation of the systems fundamental dynamics is required for transient analysis and controller design. Since the input is binary (active or inactive) and it may only be updated discretely, the machine comprises both continuous and discrete dynamics and therefore belongs to the class of hybrid dynamical systems. The study shows that the dynamical system behavior and control complexity are greatly dependent on the configuration of the machine, the operation strategy, and in which application it is used. Although the system has non-smooth dynamics, the findings show that simple continuous and discrete approximations may be applicable for control development in certain situations, whereas more advanced hybrid control theory is necessary to cover a broader range of situations.]]>392911051893376<![CDATA[Energy Saving Potential in Knuckle Boom Cranes using a Novel Pump Controlled Cylinder Drive]]>This paper is considering the application of a novel pump controlled cylinder drive, the so-called Speed-variable Switched Differential Pump (SvSDP), for knuckle boom crane actuation. Especially the control system for the SvSDP drive is considered, and aiming on improving energy efficiency a refinement of the existing control structure is proposed. An energy efficient sizing algorithm for the SvSDP drive is developed, and fundamental differences between the achievable operating range for the SvSDP drive compared to a conventional valve-cylinder drive are discussed. A case study is conducted with knuckle boom crane actuation, and compared to a conventional valve actuation. Simulation results show that the motion tracking performance is on a similar level compared to the valve actuation approach, while the energy consumption is drastically decreased. For the given test trajectory the valve actuation system consumes 0.79 kWh of electrical energy, while the SvSDP drive consume 0.06 kWh, if ideal energy recovery and storage is assumed.]]>39273894843520<![CDATA[Lifetime Calculation of Irregularly Oscillating Bearings in Offshore Winches]]>The fatigue lifetime calculation of rolling bearings has been intensively researched in the past and has been well documented in standards. The majority of the conducted research in the field of bearing life expectancy only applies to bearings under continuously rotating conditions as can be found in many technical systems, such as vehicle transmissions. However, there are many other technical applications in which bearings perform oscillating motions, for which the fatigue life calculation is currently not standardized. For such oscillatory behaviour, there exist several modifications for the standardized lifetime calculation. The available modification approaches, however, assume purely sinusoidal oscillations. Thus, irregular non-sinusoidal oscillations require approximation with a simple sinusoidal oscillation to comply with the available approaches, which can lead to a loss of information. In this paper, a numerical calculation approach to modify the lifetime calculation for oscillating bearings underlying irregularly reversing speeds and varying loads is presented. The lifetime calculation results of this approach are compared to results of existing calculation approaches for fatigue life calculation of oscillating bearings. For this, the approaches are applied on simple sinusoidal speeds and loads. Subsequently, results of all calculation approaches are shown for irregularly varying speeds and loads, using the example of the drivetrain of an offshore crane winch with Active Heave Compensation~(AHC). The calculation results indicate, that available lifetime modification approaches for oscillating bearings seem to overestimate the lifetime for irregularly oscillating conditions.]]>3926172811008<![CDATA[Dynamic Interaction of a Heavy Crane and a Ship in Wave Motion]]>Previous work on the dynamics of vehicle-manipulator systems is extended to offshore ships with heavy cranes. The proposed method is based on a Newton-Euler formulation where the forces of constraint are eliminated using projection matrices based on the method of Kane's equations of motion. This leads to an efficient method for developing the equations of motion of a ship with a heavy crane so that the motion of the crane influences the motion of the ship and vice versa. The calculation of the projection matrices is made efficient and intuitive by observing that the columns of the projection matrices are the screw axes of the joint twists in Plucker coordinates. Wave excitation of the ship is modeled with force RAOs based on established wave spectra. This gives a model that is well suited for design and testing of crane control systems, and for studying the feasibility of demanding crane operations in different weather conditions.The resulting equations of motion have been validated in simulation experiments for a ship with a 3 DOF heavy crane with a payload, where the ship is excited by a JONSWAP wave spectrum using a simple controller based on feedback linearization. The simulations clearly demonstrated that the ship responded in a physically meaningful way to the motion of the crane.]]>3924560997376<![CDATA[A Semi-Heuristic Process-Reaction Curve PID Controller Tuning Method]]>A simple semi-heuristic method for designing PID controllers for time constant models are shown to be close to optimal. A Process-Reaction Curve method is proposed, composed by a method for approximating stable time constant models with an unstable DIPTD model, and relative time delay error delta-PID controller tuning. The Pareto-Optimal controller is used as reference.]]>3913743520192<![CDATA[Flow Characteristics and Sizing of Annular Seat Valves for Digital Displacement Machines]]>This paper investigates the steady-state flow characteristics and power losses of annular seat valves for digital displacement machines. Annular seat valves are promising candidates for active check-valves used in digital displacement fluid power machinery which excels in efficiency in a broad operating range. To achieve high machine efficiency, the valve flow losses and the required electrical power needed for valve switching should be low. The annular valve plunger geometry, of a valve prototype developed for digital displacement machines, is parametrized by three parameters: stroke length, seat radius and seat width. The steady-state flow characteristics are analyzed using static axi-symmetric computational fluid dynamics. The pressure drops and flow forces are mapped in the valve design space for several different flow rates. The simulated results are compared against measurements using a valve prototype. Using the simulated maps to estimate the flow power losses and a simple generic model to estimate the electric power losses, both during digital displacement operation, optimal designs of annular seat valves, with respect to valve power losses, are derived under several different operating conditions.]]>39123356874112<![CDATA[Parameter-dependent PWQ Lyapunov function stability criteria for uncertain piecewise linear systems]]>The calculation of piecewise quadratic (PWQ) Lyapunov functions is addressed in view of stability analysis of uncertain piecewise linear dynamics. As main contribution, the linear matrix inequality (LMI) approach proposed in (Johansson and Rantzer, 1998) for the stability analysis of PWL and PWA dynamics is extended to account for parametric uncertainty based on a improved relaxation technique. The results are applied for the analysis of a Phase Locked Loop (PLL) benchmark and the ability to guarantee a stability region in the parameter space well beyond the state of the art is demonstrated.]]>3911521314368<![CDATA[Iterative Learning Applied to Hydraulic Pressure Control]]>This paper addresses a performance limiting phenomenon that may occur in the pressure control of hydraulic actuators subjected to external velocity disturbances. It is demonstrated that under certain conditions a severe peaking of the control error may be observed that significantly degrades the performance of the system due to the presence of nonlinearities. The phenomenon is investigated numerically and experimentally using a system that requires pressure control of two hydraulic cylinders. It is demonstrated that the common solution of feed forwarding the velocity disturbance is not effective in reducing the peaking that occurs as a result of this phenomenon. To improve the system performance, a combination of feedback and iterative learning control (ILC) is proposed and evaluated. The operating conditions require that ILC be applied in combination with a feedback controller, however the experimental system inherently suffers from limit cycle oscillations under feedback due to the presence of valve hysteresis. For this reason the ILC is applied in combination with a feedback controller designed to eliminate limit cycle oscillations based on describing function analysis. Experimental results demonstrate the efficacy of the solution where the feedback controller successfully eliminates limit cycle oscillations and the ILC greatly reduces the peaking of the control error with reductions in the RMS and peak-to-peak amplitude of the error by factors of more than 30 and 19, respectively. Stability of the proposed solution is demonstrated analytically in the frequency domain and verified on the experimental system for long periods of continuous operation.]]>3911141520640<![CDATA[PD/PID controller tuning based on model approximations: Model reduction of some unstable and higher order nonlinear models]]>A model reduction technique based on optimization theory is presented, where a possible higher order system/model is approximated with an unstable DIPTD model by using only step response data. The DIPTD model is used to tune PD/PID controllers for the underlying possible higher order system. Numerous examples are used to illustrate the theory, i.e. both linear and nonlinear models. The Pareto Optimal controller is used as a reference controller.]]>3841851972035712<![CDATA[Dynamic Modeling of a 2-RPU+2-UPS Hybrid Manipulator for Machining Application]]>This paper presents a novel 5-DOF gantry hybrid machine tool, designed with a 2-RPU+2-UPS parallel mechanism for 3T2R motion. The 2-RPU+2-UPS parallel mechanism is connected to a long linear guide to realize 5-axis machining. A dynamic model is developed for this parallel-serial hybrid system. Screw theory is adopted to establish the kinematic equations of the system, upon which the dynamics model is developed by utilizing the principle of virtual work. A numerical example for processing slender structural parts is included to show the validity of the analytical dynamic model developed.]]>3841691846694912<![CDATA[Nonlinear Feedback Control and Stability Analysis of a Proof-of-Work Blockchain]]>In this paper a novel feedback controller and stability analysis of a blockchain implementation is developed by using a control engineering perspective. The controller output equals the difficulty adjustment in the mining process while the feedback variable is the average block time over a certain time period. The computational power (hash rate) of the miners is considered a disturbance in the model. The developed controller is tested against a simulation model with constant disturbance, step and ramp responses as well as with a high-frequency sinusoidal disturbance. Stability and a fast response is demonstrated in all these cases with a controller which adjusts it's output at every new block. Finally the performance of the controller is implemented and demonstrated on a testnet with a constant hash rate as well as on the mainnet of a public open source blockchain project.]]>3841571683302400<![CDATA[Accelerated Bearing Life-time Test Rig Development for Low Speed Data Acquisition]]>Condition monitoring plays an important role in rotating machinery to ensure reliability of the equipment, and to detect fault conditions at an early stage. Although health monitoring methodologies have been thoroughly developed for rotating machinery, low-speed conditions often pose a challenge due to the low signal-to-noise ratio. To this aim, sophisticated algorithms that reduce noise and highlight the bearing faults are necessary to accurately diagnose machines undergoing this condition. In the development phase, sensor data from a healthy and damaged bearing rotating at low-speed is required to verify the performance of such algorithms. A test rig for performing accelerated life-time testing of small rolling element bearings is designed to collect necessary sensor data. Heavy loads at high-speed conditions are applied to the test bearing to wear it out fast. Sensor data is collected in intervals during the test to capture the degeneration features. The main objective of this paper is to provide a detailed overview for the development and analysis of this test rig. A case study with experimental vibration data is also presented to illustrate the efficacy of the developed test rig.]]>3831431562281472<![CDATA[Model based flow measurement using venturi flumes for return flow during drilling]]>In an oil well drilling operation, a proper knowledge of the return fluid flowrate is necessary both for the stabilization of the bottom hole pressure of the well and also as a primary indication of a kick or loss. In practice, the drill fluid flowing through the return line is usually measured with Coriolis meters. However this method is both expensive and has some downsides. For instance there is a risk of blockage due to drill cuttings while measuring the discharge. The presence of gas and cuttings in the drilling fluid will also have a negative effect in the measurement i.e. for multi-phase fluid, the readings from Coriolis meters may not be accurate. A cheaper alternative would be to use an open channel for the measurement of the discharge from the return flowline. In this paper, a venturi rig is used as the open channel and modeled by the Saint Venant equations. Experimental verification of the simulation results show a promising behavior of the model based measurement of the return fluid flow.]]>3831351422669568<![CDATA[Comparison of Excitation Signals in Active Magnetic Bearing System Identification]]>Active magnetic bearings (AMBs) offer frictionless suspension, vibration insulation, programmable stiffness, and damping, among other advantages, in levitated rotor applications. However, AMBs are inherently unstable and require accurate system models for the high-performance model-based multi-input multi-output control of rotor position. Control electronics with high calculation capacity and accurate sensors of AMBs provide an opportunity to implement various identification schemes. A variety of artificial excitation signal-based identification methods can thus be achieved with no additional hardware. In this paper, a selection of excitation signals, namely the pseudorandom binary sequence (PRBS), chirp signal, multisine, and stepped sine are presented, applied, and compared with the AMB system identification. From the identification experiments, the rotor-bearing system, the inner current control loop, and values of position and current stiffness are identified. Unlike recently published works considering excitation-based identification of AMB rotor systems, it is demonstrated that identification of the rotor system dynamics can be carried out using various well-established excitation signals. Application and feasibility of these excitation signals in AMB rotor systems are analyzed based on experimental results.]]>3831231331390592<![CDATA[Modeling of a lay-flat plastic hose extrusion process]]>Many complex processes have a low degree of automation, and oftentimes important quality information is only available hours or even days after the production is completed. This article shows how multivariate design and response surface modeling were applied to a lay-flat plastic hose extrusion process in a full-scale experiment. Clear quantitative relationships were found, which to a large degree match existing qualitative process understanding. For instance, it was quantified how adhesion improves with increased extrusion screw speed and extrusion head temperature. The results can readily be used to inform the operators in real-time of important quality parameters of the hose currently under production. The clear results also indicate that increased process automation is achievable.]]>383111121564224<![CDATA[Tuning PD and PID Controllers for Double Integrating Plus Time Delay Systems]]>An existing method for tuning a PI controller for an integrating plus time delay plant are extended to be used for the design of a PD controller for a double integrating plus time delay plant. The PD controller is extended with integral action and an ideal PID controller is suggested in order to achieve optimality of the closed loop responses. Furthermore, some analytical results concerning the proposed PD and PID controller algorithm regarding the relative time-delay margin are worked out and presented. The algorithm and an existing method are successfully compared against each other on some examples, e.g. the planar movement control of a mariner vessel.]]>382951101668096<![CDATA[Relative Vessel Motion Tracking using Sensor Fusion, Aruco Markers, and MRU Sensors]]>This paper presents a novel approach for estimating the relative motion between two moving offshore vessels. The method is based on a sensor fusion algorithm including a vision system and two motion reference units (MRUs). The vision system makes use of the open-source computer vision library OpenCV and a cube with Aruco markers placed onto each of the cube sides. The Extended Quaternion Kalman Filter (EQKF) is used for bad pose rejection for the vision system. The presented sensor fusion algorithm is based on the Indirect Feedforward Kalman Filter for error estimation. The system is self-calibrating in the sense that the Aruco cube can be placed in an arbitrary location on the secondary vessel. Experimental 6-DOF results demonstrate the accuracy and efficiency of the proposed sensor fusion method compared with the internal joint sensors of two Stewart platforms and the industrial robot. The standard deviation error was found to be 31mm or better when the Arcuo cube was placed at three different locations.]]>38279933610624<![CDATA[Feasibility Study of Electromechanical Cylinder Drivetrain for Offshore Mechatronic Systems]]>Currently, there is an increasing focus on the environmental impact and energy consumption of the oil and gas industry. In offshore drilling equipment, electric motors tend to replace traditionally used hydraulic motors, especially in rotational motion control applications. However, force densities available from linear hydraulic actuators are still typically higher than those of electric actuators. Therefore, usually the remaining source of hydraulic power is thereby the hydraulic cylinder. This paper presents a feasibility study on the implementation of an electromechanical cylinder drivetrain on an offshore vertical pipe handling machine. The scope of this paper is to investigate the feasibility of a commercial off-the-shelf drivetrain. With a focus on the motion performance, numerical modeling and simulation are used when sizing and selecting the components of the considered electromechanical cylinder drivetrain. The simulation results are analyzed and discussed together with a literature study regarding advantages and disadvantages of the proposed solution considering the design criteria of offshore drilling equipment. It is concluded that the selected drivetrain can only satisfy the static motion requirements since the required transmitted power is higher than the recommended permissible power of the transmission screw. Consequently, based on the recommendation of the manufacturer, avoidance of overheating cannot be guaranteed for the drivetrain combinations considered for the case study presented in this paper. Hence, to avoid overheating, the average speed of the motion cycle must be decreased. Alternatively, external cooling or temperature monitoring and control system that prevents overheating could be implemented.]]>38259773799040<![CDATA[Sensitivity Analysis on the Reliability of an Offshore Winch Regarding Selected Gearbox Parameters]]>To match the high expectations and demands of customers for long-lasting machines, the development of reliable products is crucial. Furthermore, for reasons of competitiveness, it is necessary to know the future product lifetime as accurately as possible to avoid over-dimensioning. Additionally, a more detailed system understanding enables the designer to influence the life expectancy of the product without performing an extensive amount of expensive and time-consuming tests. In early development stages of new equipment only very basic information about the future system design, like the ratio or the system structure, is available. Nevertheless, a reliable lifetime prediction of the system components and subsequently of the system itself is necessary to evaluate possible design alternatives and to identify critical components beforehand. Lifetime predictions, however, require many parameters, which are often not known in these early stages. Therefore, this paper performs a sensitivity analysis on the drivetrain of an offshore winch with active heave compensation for two typical load cases. The influences of the parameters gear center distance and ambient temperature are investigated by varying the parameters within typical ranges and evaluating the quantitative effect on the lifetime.]]>3825158716800<![CDATA[Third Order Reconstruction of the KP Scheme for Model of River Tinnelva]]>The Saint-Venant equation/Shallow Water Equation is used to simulate flow of river, flow of liquid in an open channel, tsunami etc. The Kurganov-Petrova (KP) scheme which was developed based on the local speed of discontinuity propagation, can be used to solve hyperbolic type partial differential equations (PDEs), hence can be used to solve the Saint-Venant equation. The KP scheme is semi discrete: PDEs are discretized in the spatial domain, resulting in a set of Ordinary Differential Equations (ODEs). In this study, the common 2nd order KP scheme is extended into 3rd order scheme while following the Weighted Essentially Non-Oscillatory (WENO) and Central WENO (CWENO) reconstruction steps. Both the 2nd order and 3rd order schemes have been used in simulation in order to check the suitability of the KP schemes to solve hyperbolic type PDEs. The simulation results indicated that the 3rd order KP scheme shows some better stability compared to the 2nd order scheme. Computational time for the 3rd order KP scheme for variable step-length ode solvers in MATLAB is less compared to the computational time of the 2nd order KP scheme. In addition, it was confirmed that the order of the time integrators essentially should be lower compared to the order of the spatial discretization. However, for computation of abrupt step changes, the 2nd order KP scheme shows a more accurate solution.]]>38133501654784<![CDATA[UAV Path Planning using MILP with Experiments]]>In this paper, we look at the problem of tracking icebergs using multiple Unmanned Aerial Vehicles (UAVs). Our solutions use combinatorial optimization for UAV path planning by formulating a mixed integer linear programing (MILP) optimization problem. To demonstrate the approach, we present both a simulation and a practical experiment. The simulation demonstrates the possibilities of the MILP algorithm by constructing a case where three UAVs help a boat make a safe passage through an area with icebergs. Furthermore, we compare the performance of three against a single UAV. In the practical experiment, we take the first step towards full-scale experiments. We run the algorithm on a ground station and use it to set the path for a UAV tracking five simulated icebergs.]]>3812132962560<![CDATA[Energy Optimal Trajectories in Human Arm Motion Aiming for Assistive Robots]]>The energy expenditure in human arm has been of great interests for seeking optimal human arm trajectories. This paper presents a new way for calculating metabolic energy consumption of human arm motions. The purpose is to reveal the relationship between the energy consumption and the trajectory of arm motion, and further, the acceleration and arm orientation contributions. Human arm motion in horizontal plane is investigated by virtue of Qualisys motion capture system. The motion data is post-processed by a biomechanical model to obtain the metabolic expenditure. Results on the arm motion kinematics, dynamics and metabolic energy consumption, are included.]]>3811119522240<![CDATA[Unscented Multi-Point Smoother for Fusion of Delayed Displacement Measurements: Application to Agricultural Robots]]>Visual Odometry (VO) is increasingly a useful tool for robotic navigation in a variety of applications, including weed removal for agricultural robotics. The methods of evaluating VO are often computationally expensive and can cause the VO measurements to be significantly delayed with respect to a compass, wheel odometry, and GPS measurements. In this paper we present a Bayesian formulation of fusing delayed displacement measurements. We implement solutions to this problem based on the unscented Kalman filter (UKF), leading to what we term an unscented multi-point smoother. The proposed methods are tested in simulations of an agricultural robot. The simulations show improvements in the localization RMS error when including the VO measurements with a variety of latencies.]]>381191250304<![CDATA[Dynamic Relative Gain Array Estimation using Local Polynomial Approximation Approach]]>This article presents a procedure that utilizes the local polynomial approximation approach in the estimation of the Dynamic Relative Gain Array (DRGA) matrix and its uncertainty bounds for weakly nonlinear systems. This procedure offers enhanced frequency resolution and noise reduction when random excitation is used. It also allows separation of nonlinear distortions with shorter measuring time when multisine excitation is imposed. The procedure is illustrated using the well-known quadruple tank process as a case study in simulation and in real life. Besides, a comparison with the pairing results of the static RGA, nonlinear RGA and DRGA based on linearized quadruple tank model for different simulation cases is performed.]]>374247259924672<![CDATA[A Decoupled Approach for Flight Control]]>A decoupling method for flight control is presented that greatly simplifies the controller design. By approximating the higher order derivatives of the angle of attack and sideslip, it enables a rotation controller and a speed controller to be derived independently of each other, and thus gives access to a vast number of controller solutions derived for general classes of rotational and translational systems. For rotational control, a quaternion-based sliding surface controller is derived to align the wind frame in a desired direction, and using standard Lyapunov methods an airspeed controller is derived to ensure that an unmanned aerial vehicle moves with a positive airspeed. Simulations validate the potential of the proposed method, where the unmanned aerial vehicle is able to obtain leveled flight and move in a desired direction with a desired airspeed.]]>374237246701440<![CDATA[Industrial Evaluation of Integrated Performance Analysis and Equation Model Debugging for Equation-Based Models]]>The ease of use and the high abstraction level of equation-based object-oriented (EOO) languages such as Modelica has the drawback that performance problems and modeling errors are often hard to find. To address this problem, we have earlier developed advanced performance analysis and equation model debugging support in the OpenModelica tool. The aim of the work reported in this paper is to perform an independent investigation and evaluation of this equation model performance analysis and debugging methods and tool support on industrial models. The results turned out to be mainly positive. The integrated debugger and performance analyzer locates several kinds of errors such as division by zero, chattering, etc., and greatly facilitates finding the equations that take most of the execution time during simulation. It remains to further evaluate the performance profiler and debugger on even larger industrial models.]]>3742252361431552<![CDATA[On closed loop transient response system identification]]>Some methods for transient closed loop step response system identification presented in the literature are reviewed. Interestingly some errors in a method published in the early 80's where propagated into a recently published method. These methods are reviewed and some improved methods are suggested and presented. The methods are compared against each other on some closed loop system examples, e.g. a well pipeline-riser severe-slugging flow regime example, using Monte Carlo simulations for comparison of the methods.]]>374213223918528<![CDATA[Numerical and Experimental Study of a Novel Concept for Hydraulically Controlled Negative Loads]]>This paper presents a numerical and experimental investigation of a novel concept that eliminates oscillations in hydraulic systems containing a counterbalance valve in series with a pressure compensated flow supply. The concept utilizes a secondary circuit where a low-pass filtered value of the load pressure is generated and fed back to the compensator of the flow supply valve. The novel concept has been implemented on a single boom actuated by a cylinder. A nonlinear model of the system has been developed and an experimental verification shows good correspondence between the model and the real system. The model is used for a parameter study on the novel concept. From the study it is found that the system is stable for large directional valve openings and that for small openings a reduction of the oscillatory behaviour of the system can be obtained by either lowering the eigenfrequency of the mechanical-hydraulic system or by lowering the pilot area ratio of the counterbalance valve.]]>3741952112061312<![CDATA[Tube Model Predictive Control with an Auxiliary Sliding Mode Controller]]>This paper studies Tube Model Predictive Control (MPC) with a Sliding Mode Controller (SMC) as an auxiliary controller. It is shown how to calculate the tube widths under SMC control, and thus how much the constraints of the nominal MPC have to be tightened in order to achieve robust stability and constraint fulfillment. The analysis avoids the assumption of infinitely fast switching in the SMC controller.]]>3731811931788928<![CDATA[Structural observability analysis and EKF based parameter estimation of building heating models]]>Research for enhanced energy-efficient buildings has been given much recognition in the recent years owing to their high energy consumptions. Increasing energy needs can be precisely controlled by practicing advanced controllers for building Heating, Ventilation, and Air-Conditioning (HVAC) systems. Advanced controllers require a mathematical building heating model to operate, and these models need to be accurate and computationally efficient. One main concern associated with such models is the accurate estimation of the unknown model parameters. This paper presents the feasibility of implementing a simplified building heating model and the computation of physical parameters using an off-line approach. Structural observability analysis is conducted using graph-theoretic techniques to analyze the observability of the developed system model. Then Extended Kalman Filter (EKF) algorithm is utilized for parameter estimates using the real measurements of a single-zone building. The simulation-based results confirm that even with a simple model, the EKF follows the state variables accurately. The predicted parameters vary depending on the inputs and disturbances.]]>3731711801260544<![CDATA[A metamorphic controller for plant control system design]]>One of the major problems in the design of industrial control systems is the selection and parameterization of the control algorithm. In practice, the most common solution is the PI (proportional-integral) controller, which is simple to implement, but is not always the best control strategy. The use of more advanced controllers may result in a better efficiency of the control system. However, the implementation of advanced control algorithms is more time-consuming and requires specialized knowledge from control engineers. To overcome these problems and to support control engineers at the controller design stage, the paper describes a tool, i.e., a metamorphic controller with extended functionality, for selection and implementation of the most suitable control algorithm. In comparison to existing solutions, the main advantage of the metamorphic controller is its possibility of changing the control algorithm. In turn, the candidate algorithms can be tested through simulations and the total time needed to perform all simulations can be less than a few minutes, which is less than or comparable to the design time in the concurrent design approach. Moreover, the use of well-known tuning procedures, makes the system easy to understand and operate even by inexperienced control engineers. The application was implemented in the real industrial programmable logic controller (PLC) and tested with linear and nonlinear virtual plants. The obtained simulation results confirm that the change of the control algorithm allows the control objectives to be achieved at lower costs and in less time.]]>3731591691562624<![CDATA[Rate of Penetration Optimization using Moving Horizon Estimation]]>Increase of drilling safety and reduction of drilling operation costs, especially improvement of drilling efficiency, are two important considerations in the oil and gas industry. The rate of penetration (ROP, alternatively called as drilling speed) is a critical drilling parameter to evaluate and improve drilling safety and efficiency. ROP estimation has an important role in drilling optimization as well as interpretation of all stages of the well life cycle. In this paper, we use a moving horizon estimation (MHE) method to estimate ROP as well as other drilling parameters. In the MHE formulation the states are estimated by a forward simulation with a pre-estimating observer. Moreover, it considers the constraints of states/outputs in the MHE problem. It is shown that the estimation error is with input-to-state stability. Furthermore, the ROP optimization (to achieve minimum drilling cost/drilling energy) concerning with the efficient hole cleaning condition and downhole environmental stability is presented. The performance of the methodology is demonstrated by one case study.]]>373149158403456<![CDATA[Online Identification of a Two-Mass System in Frequency Domain using a Kalman Filter]]>Some of the most widely recognized online parameter estimation techniques used in different servomechanism are the extended Kalman filter (EKF) and recursive least squares (RLS) methods. Without loss of generality, these methods are based on a prior knowledge of the model structure of the system to be identified, and thus, they can be regarded as parametric identification methods. This paper proposes an on-line non-parametric frequency response identification routine that is based on a fixed-coefficient Kalman filter, which is configured to perform like a Fourier transform. The approach exploits the knowledge of the excitation signal by updating the Kalman filter gains with the known time-varying frequency of chirp signal. The experimental results demonstrate the effectiveness of the proposed online identification method to estimate a non-parametric model of the closed loop controlled servomechanism in a selected band of frequencies.]]>3721331471788928<![CDATA[Intraperitoneal Glucose Sensing is Sometimes Surprisingly Rapid]]>Rapid, accurate and robust glucose measurements are needed to make a safe artificial pancreas for the treatment of diabetes mellitus type 1 and 2. The present gold standard of continuous glucose sensing, subcutaneous (SC) glucose sensing, has been claimed to have slow response and poor robustness towards local tissue changes such as mechanical pressure, temperature changes, etc. The present study aimed at quantifying glucose dynamics from central circulation to intraperitoneal (IP) sensor sites, as an alternative to the SC location. Intraarterial (IA) and IP sensors were tested in three anaesthetized non-diabetic pigs during experiments with intravenous infusion of glucose boluses, enforcing rapid glucose level excursions in the range 70--360 mg/dL (approximately 3.8--20 mmol/L). Optical interferometric sensors were used for IA and IP measurements. A first-order dynamic model with time delay was fitted to the data after compensating for sensor dynamics. Additionally, off-the-shelf Medtronic Enlite sensors were used for illustration of SC glucose sensing. The time delay in glucose excursions from central circulation (IA) to IP sensor location was found to be in the range 0--26 s (median: 8.5 s, mean: 9.7 s, SD 9.5 s), and the time constant was found to be 0.5--10.2 min (median: 4.8 min, mean: 4.7 min, SD 2.9 min). IP glucose sensing sites have a substantially faster and more distinctive response than SC sites when sensor dynamics is ignored, and the peritoneal fluid reacts even faster to changes in intravascular glucose levels than reported in previous animal studies. This study may provide a benchmark for future, rapid IP glucose sensors.]]>3721211311914880<![CDATA[Rapid High-Frequency Measurements of Electrical Circuits by Using Frequency Mixer and Pseudo-Random Sequences]]>Frequency-response measurements at high frequencies have been shown to provide a valuable design tool in various fields of electronics. These measurements are often challenging when using most commercially available measurement tools due to their relatively low maximum sampling frequency and long measurement time. This effectively prevents frequency-response-based low-cost applications where fast and reliable measurements are required. This paper proposes the use of a combined frequency mixer applied with pseudo-random sequences. In this method, the applied pseudo-random excitation is upconverted to high frequencies by the mixer, and once injected into the device being tested, the system response is downconverted to lower frequencies. The method provides a low-cost solution that can be applied for rapid high-frequency measurements by using only modest data-acquisition tools. Experimental results based on a high-frequency resonator are presented and used to demonstrate the effectiveness of the proposed methods.]]>372113119475136<![CDATA[Modeling and Simulation of Multi-Room Buildings]]>Buildings are one of the largest energy consumers in the world which accounts for nearly 40% of the total global energy consumption. In the countries where cold climate conditions predominate, space heating is the key contributor to the increased energy consumption. Today there is a growing trend to use Building Energy Management Systems (BEMS) to control the energy consumption of buildings in an efficient manner. BEMS require a good heating model of the building to be integrated for better control purposes. This article refers to the development of different types of physics based buillding heating models, regarding single-zone, multi-floor and multi-room buildings. They address the propriety of each model in building heating control concerning the prediction accuracy and the prediction time. These models are verified for a residential building having three floors. According to the results, the multi-floor model is recognized to have the best qualifications obliged as a model for control.]]>37299111652288<![CDATA[Motion- and Communication-Planning of Unmanned Aerial Vehicles in Delay Tolerant Network using Mixed-Integer Linear Programming]]>Large amounts of data are typically generated in applications such as surveillance of power lines and railways, inspection of gas pipes, and security surveillance. In the latter application it is a necessity that the data is transmitted to the control centre ``on-the-fly'' for analysis. Also missions related to other applications would greatly benefit from near real-time analysis and operator interaction based on captured data. This is the motivation behind this paper on coarse offline motion- and communication-planning for cooperating Unmanned Aerial Vehicles (UAVs). A Mixed-Integer Linear Programming (MILP) problem is defined in order to solve the surveillance mission. To efficiently transmit the data back to the base station the vehicles are allowed to store data for later transmission and transmit via other vehicles, in addition to direct transmission. The paths obtained by solving the optimization problem are analyzed using a realistic radio propagation path loss simulator. If the radio propagation path loss exceeds the maximum design criterion the optimization problem is solved again with a stricter communication constraint, and the procedure is continued in an iterative manner until the criterion is met. The proposed algorithm is supported by simulations showing the resulting paths and communication topologies for different choices of delay tolerance. ]]>37277971627136<![CDATA[Inverse Kinematics for Industrial Robots using Conformal Geometric Algebra]]>This paper shows how the recently developed formulation of conformal geometric algebra can be used for analytic inverse kinematics of two six-link industrial manipulators with revolute joints. The paper demonstrates that the solution of the inverse kinematics in this framework relies on the intersection of geometric objects like lines, circles, planes and spheres, which provides the developer with valuable geometric intuition about the problem. It is believed that this will be very useful for new robot geometries and other mechanisms like cranes and topside drilling equipment. The paper extends previous results on inverse kinematics using conformal geometric algebra by providing consistent solutions for the joint angles for the different configurations depending on shoulder left or right, elbow up or down, and wrist flipped or not. Moreover, it is shown how to relate the solution to the Denavit-Hartenberg parameters of the robot. The solutions have been successfully implemented and tested extensively over the whole workspace of the manipulators.]]>37163751029120<![CDATA[Tracking a Swinging Target with a Robot Manipulator using Visual Sensing]]>In this paper we develop a method for loading parts onto a swinging target using an industrial robot. The orientation of the target is estimated by a particle filter using camera images as measurements. Robust and accurate tracking is achieved by using an accurate dynamic model of the target. The dynamical model is also used to compensate for the time delay between the acquisition of images and the motion response of the robot. The target dynamics is modeled as a spherical pendulum. To ensure robust visual tracking the position of the target mass center is estimated. The method is experimentally validated in a laboratory loading station with a swinging conveyor trolley as target, which is commonly used in industry.]]>37153621468416<![CDATA[Model-Free Predictive Anti-Slug Control of a Well-Pipeline-Riser]]>Simplified linearized discrete time dynamic state space models are developed for a 3-phase well-pipeline-riser and tested together with a high fidelity dynamic model built in K-Spice and LedaFlow. In addition the Meglio pipeline-riser model is used as an example process. These models are developed from a subspace algorithm, i.e. Deterministic and Stochastic system identification and Realization (DSR), and implemented in a Model Predictive Controller (MPC) for stabilizing the slugging regime. The MPC, LQR and PI control strategies are tested.]]>37141521369088<![CDATA[Automated Kick Control Procedure for an Influx in Managed Pressure Drilling Operations]]>Within drilling of oil and gas wells, the Managed Pressure Drilling (MPD) method with active control of wellbore pressure during drilling has partly evolved from conventional well control procedures. However, for MPD operations the instrumentation is typically more extensive compared to conventional drilling. Despite this, any influx of formation fluids (commonly known as a kick) during MPD operations is typically handled by conventional well control methods, at least if the kick is estimated to be larger than a threshold value. Conventional well control procedures rely on manual control of the blow out preventer, pumps, and choke valves and do not capitalize on the benefits from the instrumentation level associated with MPD. This paper investigates two alternative well control procedures specially adapted to backpressure MPD: the dynamic shut-in (DSI) procedure and the automatic kick control (AKC) procedure. Both methods capitalize on improvements in Pressure While Drilling (PWD) technology. A commercially available PWD tool buffers high-resolution pressure measurements, which can be used in an automated well control procedure. By using backpressure MPD, the choke valve opening is tuned automatically using a feedback-feedforward control method. The two procedures are evaluated using a high fidelity well flow model and cases from a North Sea drilling operation are simulated. The results show that using AKC procedure reduces the time needed to establish control of the well compared to DSI procedure. It also indicates that the AKC procedure reduces the total kick size compared to the DSI procedure, and thereby reduces the risk of lost circulation.]]>37131401218560<![CDATA[Modular System Modeling for Quantitative Reliability Evaluation of Technical Systems]]>In modern times, it is necessary to offer reliable products to match the statutory directives concerning product liability and the high expectations of customers for durable devices. Furthermore, to maintain a high competitiveness, engineers need to know as accurately as possible how long their product will last and how to influence the life expectancy without expensive and time-consuming testing. As the components of a system are responsible for the system reliability, this paper introduces and evaluates calculation methods for life expectancy of common machine elements in technical systems. Subsequently, a method for the quantitative evaluation of the reliability of technical systems is proposed and applied to a heavy-duty power shift transmission.]]>3711929900096<![CDATA[Hydraulic vs. Electric: A Review of Actuation Systems in Offshore Drilling Equipment]]>This article presents a survey on actuation systems encountered in offshore drilling applications. Specifically, it focuses on giving a comparison of hydraulic and electric drivetrains along with detailed explanations of their advantages and drawbacks. A significant number of industrial case studies is examined in addition to the collection of academic publications, in order to accurately describe the current market situation. Some key directions of research and development required to satisfy increasing demands on powertrains operating offshore are identified. The impact of the literature and application surveys is further strengthened by benchmarking two designs of a full-scale pipe handling machine. Apart from other benefits, the electrically actuated machine reduces the total power consumption by 70% compared to its hydraulically driven counterpart. It is concluded that electric actuation systems, among other advantages, in general offer higher efficiency and flexibility, however, in some specific applications (such as energy accumulation or translational motion control) hydraulic powertrains are favorable.]]>3711171058816<![CDATA[Model based control for run-of-river system. Part 2: Comparison of control structures]]>Optimal operation and control of a run-of-river hydro power plant depend on good knowledge of the elements of the plant in the form of models. Both the control architecture of the system, i.e. the choice of inputs and outputs, and to what degree a model is used, will affect the achievable control performance. Here, a model of a river reach based on the Saint Venant equations for open channel flow illustrates the dynamics of the run-of-river system. The hyperbolic partial differential equations are discretized using the Kurganov-Petrova central upwind scheme - see Part I for details. A comparison is given of achievable control performance using two alternative control signals: the inlet or the outlet volumetric flow rates to the system, in combination with a number of different control structures such as PI control, PI control with Smith predictor, and predictive control. The control objective is to keep the level just in front of the dam as high as possible, and with little variation in the level to avoid overflow over the dam. With a step change in the volumetric inflow to the river reach (disturbance) and using the volumetric outflow as the control signal, PI control gives quite good performance. Model predictive control (MPC) gives superior control in the sense of constraining the variation in the water level, at a cost of longer computational time and thus constraints on possible sample time. Details on controller tuning are given. With volumetric inflow to the river reach as control signal and outflow (production) as disturbance, this introduces a considerable time delay in the control signal. Because of nonlinearity in the system (varying time delay, etc.), it is difficult to achieve stable closed loop performance using a simple PI controller. However, by combining a PI controller with a Smith predictor based on a simple integrator + fixed time delay model, stable closed loop operation is possible with decent control performance. Still, an MPC gives superior performance over the PI controller + Smith predictor, both because the MPC uses a more accurate prediction model and because constraints in the operation are more directly included in the MPC structure. Most theoretical studies do not take into account the resulting time delay caused by the computationally demanding MPC algorithm. Simulation studies indicate that the inherent time delay in injecting the control signal does not seriously degrade the performance of the MPC controller.]]>364251263845824<![CDATA[Model based control for run-of-river system. Part 1: Model implementation and tuning]]>Optimal operation and control of a run-of-river hydro power plant depends on good knowledge of the elements of the plant in the form of models. River reaches are often considered shallow channels with free surfaces. A typical model for such reaches use the Saint Venant model, which is a 1D distributed model based on the mass and momentum balances. This combination of free surface and momentum balance makes the problem numerically challenging to solve. The finite volume method with staggered grid was compared with the Kurganov-Petrova central upwind scheme, and was used to illustrate the dynamics of the river upstream from the Grønvollfoss run-of-river power plant in Telemark, Norway, operated by Skagerak Energi AS. In an experiment on the Grønvollfoss run-of-river power plant, a step was injected in the upstream inlet flow at Årlifoss, and the resulting change in level in front of the dam at the Grønvollfoss plant was logged. The results from the theoretical Saint Venant model was then compared to the experimental results. Because of uncertainties in the geometry of the river reach (river bed slope, etc.), the slope and length of the varying slope parts were tuned manually to improve the fit. Then, friction factor, river width and height drop of the river was tuned by minimizing a least squares criterion. The results of the improved model (numerically, tuned to experiments), is a model that can be further used for control synthesis and analysis.]]>364237249888832<![CDATA[Experimental System Identification and Black Box Modeling of Hydraulic Directional Control Valve]]>Directional control valves play a large role in most hydraulic systems. When modeling the hydraulic systems, it is important that both the steady state and dynamic characteristics of the valves are modeled correctly to reproduce the dynamic characteristics of the entire system. In this paper, a proportional valve (Brevini HPV 41) is investigated to identify its dynamic and steady state characteristics. The steady state characteristics are identified by experimental flow curves. The dynamics are determined through frequency response analysis and identified using several transfer functions. The paper also presents a simulation model of the valve describing both steady state and dynamic characteristics. The simulation results are verified through several experiments.]]>364225235738304<![CDATA[Discrete Learning Control with Application to Hydraulic Actuators]]>In this paper the robustness of a class of learning control algorithms to state disturbances, output noise, and errors in initial conditions is studied. We present a simple learning algorithm and exhibit, via a concise proof, bounds on the asymptotic trajectory errors for the learned input and the corresponding state and output trajectories. Furthermore, these bounds are continuous functions of the bounds on the initial condition errors, state disturbance, and output noise, and the bounds are zero in the absence of these disturbances.]]>364215224840704<![CDATA[Control-Oriented Model of a Generating Set Comprising a Diesel Engine and a Synchronous Generator]]>A generating set (Genset) comprises a prime mover such as a Diesel Engine, and a synchronous generator. The most important controllers of such systems are the speed governor to regulate the engine or shaft speed and the automatic voltage regulator (AVR) to regulate the terminal voltage. The speed governor is a PID controller that uses the difference between the speed and its desired value as a feedback signal to change the fuel mass input by changing the fuel rack position. AVR is also a PID that uses the difference between the terminal voltage of the generator and its desired value, and changes it by manipulating the voltage of the field excitation circuit. Thus, the two controllers act separately. That is to say, if the speed varies from the desired value, the speed governor will react, while the AVR will not react as long as the voltage is stable, and vice versa. In this work, a control-oriented model is suggested for a Genset, and then a controller, that regulates the shaft speed and the terminal voltage, is designed by feedback linearisation. The proposed controller has two inputs: the fuel mass and the field circuit voltage. Simulations show that the proposed controller makes the two inputs act, simultaneously. Thus, any change of the speed e.g., forces the two input controls to react, in contrast to the ordinary PID controllers. Further, we discuss the robustness of the proposed controller to uncertainties and time delay.]]>3641992141204224<![CDATA[Parameter and State Estimation of Large-Scale Complex Systems Using Python Tools]]>This paper discusses the topics related to automating parameter, disturbance and state estimation analysis of large-scale complex nonlinear dynamic systems using free programming tools. For large-scale complex systems, before implementing any state estimator, the system should be analyzed for structural observability and the structural observability analysis can be automated using Modelica and Python. As a result of structural observability analysis, the system may be decomposed into subsystems where some of them may be observable --- with respect to parameter, disturbances, and states --- while some may not. The state estimation process is carried out for those observable subsystems and the optimum number of additional measurements are prescribed for unobservable subsystems to make them observable. In this paper, an industrial case study is considered: the copper production process at Glencore Nikkelverk, Kristiansand, Norway. The copper production process is a large-scale complex system. It is shown how to implement various state estimators, in Python, to estimate parameters and disturbances, in addition to states, based on available measurements. ]]>3631891985042176<![CDATA[Model-free optimal anti-slug control of a well-pipeline-riser in the K-Spice/LedaFlow simulator]]>Simplified models are developed for a 3-phase well-pipeline-riser and tested together with a high fidelity dynamic model built in K-Spice and LedaFlow. These models are developed from a subspace algorithm, i.e. Deterministic and Stochastic system identification and Realization (DSR), and implemented in a Linear Quadratic optimal Regulator (LQR) for stabilizing the slugging regime. We are comparing LQR with PI controller using different performance measures.]]>3631791881629184<![CDATA[Modeling and Design of a Spring-loaded, Cable-driven, Wearable Exoskeleton for the Upper Extremity]]>An approach to the design of wearable exoskeletons on the basis of simulation of the exoskeleton and a human body model is proposed in this paper. The new approach, addressing the problem of physical human-exoskeleton interactions, models and simulates the mechanics of both the exoskeleton and the human body, which allows designers to effectively analyze and evaluate an exoskeleton design for their function in concert with the human body. A simulation platform is developed by integrating a biomechanical model of the human body and the exoskeleton. With the proposed approach, an exoskeleton is designed for assisting patients with neuromuscular injuries. Results of the analysis and optimization are included.]]>3631671771509376<![CDATA[Online Identification of a Mechanical System in the Frequency Domain with Short-Time DFT]]>A proper system identification method is of great importance in the process of acquiring an analytical model that adequately represents the characteristics of the monitored system. While the use of different time-domain online identification techniques has been widely recognized as a powerful approach for system diagnostics, the frequency domain identification techniques have primarily been considered for offline commissioning purposes. This paper addresses issues in the online frequency domain identification of a flexible two-mass mechanical system with varying dynamics, and a particular attention is paid to detect the changes in the system dynamics. An online identification method is presented that is based on a recursive Kalman filter configured to perform like a discrete Fourier transform (DFT) at a selected set of frequencies. The experimental online identification results are compared with the corresponding values obtained from the offline-identified frequency responses. The results show an acceptable agreement and demonstrate the feasibility of the proposed identification method.]]>3631571651902592<![CDATA[On the Influence of Force Distribution and Boundary Condition on Helical Gear Stiffness]]>The gear stiffness has a direct influence on the dynamic response of transmission systems that include a gear box, the stiffness also controls the load distribution among the teeth in mesh. The stiffness of a gear tooth varies non-linearly as the contact line with the meshing gear tooth moves along the surface of the tooth and the resulting meshing stiffness also includes discontinuities. The stiffness estimation for helical gears can only be done using full 3D analysis contrary to spur gears where 2D often suffice. Besides the usual gear geometry defined by standards two factors are found to have a large influence on the stiffness. These two factors are the rim thickness included in the stiffness calculation and the contact zone size. In the contact zone the distribution of the load is also shown to be important. Simple possible simplifications in relation to the contact load distribution are presented. The gear stiffness is found using the elastic energy of the loaded tooth. In the finite element calculation the true gear tooth root profile is applied.]]>363143155894976<![CDATA[On Implementation of the Preisach Model: Identification and Inversion for Hysteresis Compensation]]>A challenge for precise positioning in nanopositioning using smart materials is hysteresis, limiting positioning accuracy. The Preisach model, based on the delayed relay operator for hysteresis modelling, is introduced. The model is identified from experimental data with an input function ensuring information for all input levels. This paper presents implementational issues with respect to hysteresis compensation using the Preisach model, showing the procedure to follow, avoiding pitfalls in both identification and inversion. Issues due to the discrete nature of the Preisach model are discussed, and a specific linear interpolation method is tested experimentally, showing effective avoidance of excitation of vibrational dynamics in the smart material. Experimental results of hysteresis compensation are presented, showing an approximate error of 5% between the reference and measured displacement. Consequences of an insufficient discretization level and a high frequency reference signal are illustrated, showing significant deterioration of the hysteresis compensation performance.]]>3631331421353728<![CDATA[Various multistage ensembles for prediction of heating energy consumption]]>Feedforward neural network models are created for prediction of daily heating energy consumption of a NTNU university campus Gloshaugen using actual measured data for training and testing. Improvement of prediction accuracy is proposed by using neural network ensemble. Previously trained feed-forward neural networks are first separated into clusters, using k-means algorithm, and then the best network of each cluster is chosen as member of an ensemble. Two conventional averaging methods for obtaining ensemble output are applied; simple and weighted. In order to achieve better prediction results, multistage ensemble is investigated. As second level, adaptive neuro-fuzzy inference system with various clustering and membership functions are used to aggregate the selected ensemble members. Feedforward neural network in second stage is also analyzed. It is shown that using ensemble of neural networks can predict heating energy consumption with better accuracy than the best trained single neural network, while the best results are achieved with multistage ensemble.]]>3621191326272000<![CDATA[Dynamic and kinematic observers for output coordination control of Euler-Lagrange systems: A comparison and applications]]>This paper compares a dynamic and a kinematic observer approach for output coordination control of mechanical systems formulated in the Euler-Lagrange framework. The observers are designed to estimate missing velocity and acceleration information based on position/attitude measurements to provide a full state vector to the coordination control algorithm. The kinematic observer approach utilizes a virtual system designed to mimic the kinematic behaviour of the leader in order to estimate unknown states of the state vector with a minimum of information available. The dynamic observer approach is based on utilizing the full dynamic model of the follower system when estimating the missing states. The two observers are compared in terms of estimation principles and practical performance, and applied to two practical examples; leader-follower robot manipulator synchronization control, and underway replenishment operations for surface ships.]]>362103118788480<![CDATA[Path Generation for High-Performance Motion of ROVs Based on a Reference Model]]>This paper deals with the generation of sufficiently smooth position, velocity, and acceleration references for guiding the motion of an ROV along purposefully defined curvature-continuous paths in automated missions. The references are meant to be employed in high-performance trajectory tracking and dynamic positioning applications. The path planning problem is not in the scope of this work. A reference model that synthesises references concerning a single Degree-of-Freedom (DoF) motion is initially described. Then, the use of the synthesised references as the parametrisation for other references concerning multiple DoF motion along curvature-continuous paths is exploited. Results from computer simulations and full-scale sea trials, both based on the NTNU's ROV Minerva, are presented and discussed. ]]>362811014205568<![CDATA[Multicopter Design Optimization and Validation]]>This paper presents a method for optimizing the design of a multicopter unmanned aerial vehicle (UAV, also called multirotor or drone). In practice a set of datasheets is available to the designer for the various components such as battery pack, motor and propellers. The designer can not normally design the parameters of the actuator system freely, but is constrained to pick components based on available datasheets. The mixed-integer programming approach is well suited to design optimization in such cases when only a discrete set of components is available. The paper also includes an experimental section where the simulated dynamic responses of optimized designs are compared against the experimental results. The paper demonstrates that mixed-integer programming is well suited to design optimization of multicopter UAVs and that the modeling assumptions match well with the experimental validation. ]]>36267791097728<![CDATA[Structural Observability Analysis of Large Scale Systems Using Modelica and Python]]>State observability of dynamic systems is a notion which determines how well the states can be inferred from input-output data. For small-scale systems, observability analysis can be done manually, while for large-scale systems an automated systematic approach is advantageous. Here we present an approach based on the concept of structural observability analysis, using graph theory. This approach can be automated and applied to large-scale, complex dynamic systems modeled using Modelica. Modelica models are imported into Python via the JModelica.org-CasADi interface, and the Python packages NetworkX (for graph-theoretic analysis) and PyGraphviz (for graph layout and visualization) are used to analyze the structural observability of the systems. The method is demonstrated with a Modelica model created for the Copper production plant at Glencore Nikkelverk, Kristiansand, Norway. The Copper plant model has 39 states, 11 disturbances and 5 uncertain parameters. The possibility of estimating disturbances and parameters in addition to estimating the states are also discussed from the graph-theory point of view. All the software tools used on the analysis are freely available.]]>36153651873920<![CDATA[Towards Qualifiable Code Generation from a Clocked Synchronous Subset of Modelica]]>So far no qualifiable automatic code generators (ACGs) are available for Modelica. Hence, digital control applications can be modeled and simulated in Modelica, but require tedious additional efforts (e.g., manual reprogramming) to produce qualifiable target system production code. In order to more fully leverage the potential of a model-based development (MBD) process in Modelica, a qualifiable automatic code generator is needed. Typical Modelica code generation is a fairly complex process which imposes a huge development burden to any efforts of tool qualification. This work aims at mapping a Modelica subset for digital control function development to a well-understood synchronous data-flow kernel language. This kernel language allows to resort to established compilation techniques for data-flow languages which are understood enough to be accepted by certification authorities. The mapping is established by providing a translational semantics from the Modelica subset to the synchronous data-flow kernel language. However, this translation turned out to be more intricate than initially expected and has given rise to several interesting issues that require suitable design decisions regarding the mapping and the language subset.]]>36123521114112<![CDATA[A Proactive Strategy for Safe Human-Robot Collaboration based on a Simplified Risk Analysis]]>In an increasing demand for human-robot collaboration systems, the need for safe robots is crucial. This paper presents a proactive strategy to enable an awareness of the current risk for the robot. The awareness is based upon a map of historically occupied space by the operator. The map is built based on a risk evaluation of each pose presented by the operator. The risk evaluation results in a risk field that can be used to evaluate the risk of a collaborative task. Based on this risk field, a control algorithm that constantly reduces the current risk within its task constraints was developed. Kinematic redundancy was exploited for simultaneous task performance within task constraints, and risk minimization. Sphere-based geometric models were used both for the human and robot. The strategy was tested in simulation, and implemented and experimentally tested on a NACHI MR20 7-axes industrial robot.]]>36111213923968<![CDATA[Offshore Wind Payload Transfer Using Flexible Mobile Crane]]>This article presents an offshore-simulated loading and unloading of a payload from a floating platform to a fixed structure. The experiments are performed in a dry-lab, where a Stewart platform is used to simulate the motion of the vessel. A hydraulically actuated vehicle loader crane is used to perform the tasks of payload transfer. The crane includes a hydraulic winch where the wire force is measured by a load cell. A mathematical model of the winch is derived and is experimentally verified. The control strategies include a heave compensation and a constant tension mode. A motion reference unit is used to generate the reference motion of the moving platform. Experimental results show the wire force while performing the load cases. This paper shows the advantage of using a reference motion as a feed forward control reference, instead of only relying on the constant tension.]]>361191289216<![CDATA[Verification and Examination Management of Complex Systems]]>As ship systems become more complex, with an increasing number of safety-critical functions, many interconnected subsystems, tight integration to other systems, and a large amount of potential failure modes, several industry parties have identified the need for improved methods for managing the verification and examination efforts of such complex systems. Such needs are even more prominent now that the marine and offshore industries are targeting more activities and operations in the Arctic environment. In this paper, a set of requirements and a method for verification and examination management are proposed for allocating examination efforts to selected subsystems. The method is based on a definition of a verification risk function for a given system topology and given requirements. The marginal verification risks for the subsystems may then be evaluated, so that examination efforts for the subsystem can be allocated. Two cases of requirements and systems are used to demonstrate the proposed method. The method establishes a systematic relationship between the verification loss, the logic system topology, verification method performance, examination stop criterion, the required examination effort, and a proposed sequence of examinations to reach the examination stop criterion. ]]>354333346728064<![CDATA[Experimental and Numerical Investigation of a Double-Acting Offshore Vessel Performance in Level Ice]]>In this paper a numerical model and experimental data are used to investigate the level ice performance of a double-acting intervention vessel. The icebreaking capability and maneuverability in level ice are analyzed by evaluating the behavior of the vessel when it is running both ahead and astern. The paper also presents the implementation of a random crack size model for more realistic icebreaking behavior, giving more consistent evaluation of the vessel's performance in various ice conditions. The numerical simulations are firstly conducted in model-scale for a direct comparison with the experimental results. The scaling of ship speed and ice resistance is then discussed by comparing the simulation results in both full-scale and model-scale. The effect on the vessel's performance of the different properties of scaled model ice and full-scale sea ice is also assessed. ]]>3543173327195648<![CDATA[A Unified Framework for Fault Detection and Diagnosis Using Particle Filter]]>In this paper, a particle filter (PF) based fault detection and diagnosis framework is proposed. A system with possible faults is modeled as a group of hidden Markov models representing the system in fault-free mode and different failure modes, and a first order Markov chain is modeling the system mode transitions. A modified particle filter algorithm is developed to estimate the system states and mode. By doing this, system faults are detected when estimating the system mode, and the size of the fault is diagnosed by estimating the system state. A new resampling method is also developed for running the modified PF efficiently. Two introductory examples and a case study are given in detail. The introduction examples demonstrate the manner to model a system with possible faults into hidden Markov model and Markov chain. The case study considers a numerical model with common measurement failure modes. It focuses on the verification of the proposed fault diagnosis and detection algorithm and shows the behavior of the particle filter. ]]>354303315856064<![CDATA[Image Techniques for Identifying Sea-Ice Parameters]]>The estimation of ice forces are critical to Dynamic Positioning (DP) operations in Arctic waters. Ice conditions are important for the analysis of ice-structure interaction in an ice field. To monitor sea-ice conditions, cameras are used as field observation sensors on mobile sensor platforms in Arctic. Various image processing techniques, such as Otsu thresholding, k-means clustering, distance transform, Gradient Vector Flow (GVF) Snake, mathematical morphology, are then applied to obtain ice concentration, ice types, and floe size distribution from sea-ice images to ensure safe operations of structures in ice covered regions. Those techniques yield acceptable results, and their effectiveness are demonstrated in case studies. ]]>3542933012628608<![CDATA[Autonomous Aerial Ice Observation for Ice Defense]]>One of the tasks in ice defense is to gather information about the surrounding ice environment using various sensor platforms. In this manuscript we identify two monitoring tasks known in literature, namely dynamic coverage and target tracking, and motivate how these tasks are relevant in ice defense using RPAS. An optimization-based path planning concept is outlined for solving these tasks. A path planner for the target tracking problem is elaborated in more detail and a hybrid experiment, which consists of both a real fixed-wing aircraft and simulated objects, is included to show the applicability of the proposed framework. ]]>3542792912985984<![CDATA[Using Autonomous Underwater Vehicles as Sensor Platforms for Ice-Monitoring]]>Due to the receding sea-ice extent in the Arctic, and the potentially large undiscovered petroleum resources present north of the Arctic circle, offshore activities in ice-infested waters are increasing. Due to the presence of drifting sea-ice and icebergs, ice management (IM) becomes an important part of the offshore operation, and an important part of an IM system is the ability to reliably monitor the ice conditions. An autonomous underwater vehicle (AUV) has a unique capability of high underwater spatial and temporal coverage, making it suitable for monitoring applications. Since the first Arctic AUV deployment in 1972, AUV technology has matured and has been used in complex under-ice operations. This paper motivates the use of AUVs as an ice-monitoring sensor platform. It discusses relevant sensor capabilities and challenges related to communication and navigation. This paper also presents experiences from a field campaign that took place in Ny-Aalesund at Svalbard in January 2014, where a REMUS 100 AUV was used for sea-floor mapping and collection of oceanographic parameters. Based on this, we discuss the experiences related to using AUVs for ice-monitoring. We conclude that AUVs are highly applicable for ice-monitoring, but further research is needed. ]]>3542632771477632<![CDATA[Modeling and Control for Dynamic Positioned Marine Vessels in Drifting Managed Sea Ice]]>This paper presents a development framework for dynamic positioning control systems for marine vessels in managed ice. Due to the complexity of the vessel-ice and ice-ice interactions a configurable high fidelity numerical model simulating the vessel, the ice floes, the water, and the boundaries is applied. The numerical model is validated using experimental data and coupled with a control application incorporating sensor models, control systems, actuator models, and other external dynamics to form a closed loop development platform. The ice drift reversal is simulated by moving the positioning reference frame in an elliptic trajectory, rather than moving each individual ice floe. A control plant model is argued, and a control system for managed ice is proposed based on conventional open water design methods. A case study shows that dynamic positioning in managed ice is feasible for some moderate ice conditions. ]]>3542492626029312<![CDATA[A Software Framework for Simulating Stationkeeping of a Vessel in Discontinuous Ice]]>This paper describes a numerical package for simulating stationkeeping operations of an offshore vessel in floating sea ice. The software has found broad usage in both academic and industrial projects related to design and operations of floating structures in the Arctic. Interactions with both intact and broken ice conditions can be simulated by the numerical tool, but the main emphasis is placed on modelling managed ice environments relevant for prospective petroleum industry operations in the Arctic. The paper gives a thorough description of the numerical tool from both theoretical and software implementation perspectives. Structural meshing, ice field generation, multibody modelling and ice breaking aspects of the model are presented and discussed. Finally, the main assumptions and limitations of the computational techniques are elucidated and further work directions are suggested. ]]>3542112486236160<![CDATA[The Arctic DP Research Project: Effective Stationkeeping in Ice]]>Stress on the environment from a potentially growing energy use is set to rise. Without doubt the energy resources in Arctic regions will be developed. An important goal will be to exploit the resources offered by for instance the Barents Sea as a new European energy province, and to do this in accordance with the principles of sustainable development that have successfully been used e.g. in the North Sea. The special edition of MIC on Arctic DP presents a set of articles that summarize to an extent the activities of the research project Arctic DP: Safe and green dynamic positioning operations of offshore vessels in an Arctic environment. This project was awarded in 2010 by the Research Council of Norway (RCN) as a competence-building project (KMB project) to NTNU and its partners Kongsberg Maritime, DNV GL, and Statoil. The objective was to target some of the challenges related to safe Arctic offshore operations by dynamic positioning. In this first article of the Arctic DP special edition we discuss the background for and establishment of the project, its planning and execution, and project closure. An overview is given for the scientific and engineering research performed in the project, with an account of what we have considered as Effective stationkeeping in ice by dynamic positioning. The corresponding research activities conducted under this main theme is summarized. ]]>35419121014271488<![CDATA[Recent Advances in Static Output-Feedback Controller Design with Applications to Vibration Control of Large Structures]]>In this paper, we present a novel two-step strategy for static output-feedback controller design. In the first step, an optimal state-feedback controller is obtained by means of a linear matrix inequality (LMI) formulation. In the second step, a transformation of the LMI variables is used to derive a suitable LMI formulation for the static output-feedback controller. This design strategy can be applied to a wide range of practical problems, including vibration control of large structures, control of offshore wind turbines, control of automotive suspensions, vehicle driving assistance and disturbance rejection. Moreover, it allows designing decentralized and semi-decentralized static output-feedback controllers by setting a suitable zero-nonzero structure on the LMI variables. To illustrate the application of the proposed methodology, two centralized static velocity-feedback H-Infinity controllers and two fully decentralized static velocity-feedback H-Infinity controllers are designed for the seismic protection of a five-story building.]]>353169190931840<![CDATA[Robust H-Infinity Filtering for Networked Control Systems with Markovian Jumps and Packet Dropouts]]>This paper deals with the H-Infinity filtering problem for uncertain networked control systems. In the study, network-induced delays, limited communication capacity due to signal quantization and packet dropout are all taken into consideration. The finite distributed delays with probability of occurrence in a random way is introduced in the network.The packet dropout is described by a Bernoulli process. The system is modeled as Markovian jumps system with partially known transition probabilities. A full-order filter is designed to estimate the system state. By linear inequality approach, a sufficient condition is derived for the resulting filtering error system to be mean square stable with a prescribed H-Infinity performance level. Finally, a numerical example is given to illustrate the effectiveness and efficiency of the proposed design method. ]]>353159168358400<![CDATA[Evaluation of Subjective and Objective Performance Metrics for Haptically Controlled Robotic Systems]]>This paper studies in detail how different evaluation methods perform when it comes to describing the performance of haptically controlled mobile manipulators. Particularly, we investigate how well subjective metrics perform compared to objective metrics. To find the best metrics to describe the performance of a control scheme is challenging when human operators are involved; how the user perceives the performance of the controller does not necessarily correspond to the directly measurable metrics normally used in controller evaluation. It is therefore important to study whether there is any correspondence between how the user perceives the performance of a controller, and how it performs in terms of directly measurable metrics such as the time used to perform a task, number of errors, accuracy, and so on. To perform these tests we choose a system that consists of a mobile manipulator that is controlled by an operator through a haptic device. This is a good system for studying different performance metrics as the performance can be determined by subjective metrics based on feedback from the users, and also as objective and directly measurable metrics. The system consists of a robotic arm which provides for interaction and manipulation, which is mounted on a mobile base which extends the workspace of the arm. The operator thus needs to perform both interaction and locomotion using a single haptic device. While the position of the on-board camera is determined by the base motion, the principal control objective is the motion of the manipulator arm. This calls for intelligent control allocation between the base and the manipulator arm in order to obtain intuitive control of both the camera and the arm. We implement three different approaches to the control allocation problem, i.e., whether the vehicle or manipulator arm actuation is applied to generate the desired motion. The performance of the different control schemes is evaluated, and our findings strongly suggest that objective metrics better describe the performance of the controller, even though there is a clear correlation between subjective and objective performance metrics. ]]>353147157732160<![CDATA[System Identification of a Non-Uniformly Sampled Multi-Rate System in Aluminium Electrolysis Cells]]>Standard system identification algorithms are usually designed to generate mathematical models with equidistant sampling instants, that are equal for both input variables and output variables. Unfortunately, real industrial data sets are often disrupted by missing samples, variations of sampling rates in the different variables (also known as multi-rate systems), and intermittent measurements. In industries with varying events based maintenance or manual operational measures, intermittent measurements are performed leading to uneven sampling rates. Such is the case with aluminium smelters, where in addition the materials fed into the cell create even more irregularity in sampling. Both measurements and feeding are mostly manually controlled. A simplified simulation of the metal level in an aluminium electrolysis cell is performed based on mass balance considerations. System identification methods based on Prediction Error Methods (PEM) such as Ordinary Least Squares (OLS), and the sub-space method combined Deterministic and Stochastic system identification and Realization (DSR), and its variants are applied to the model of a single electrolysis cell as found in the aluminium smelters. Aliasing phenomena due to large sampling intervals can be crucial in avoiding unsuitable models, but with knowledge about the system dynamics, it is easier to optimize the sampling performance, and hence achieve successful models. The results based on the simulation studies of molten aluminium height in the cells using the various algorithms give results which tally well with the synthetic data sets used. System identification on a smaller data set from a real plant is also implemented in this work. Finally, some concrete suggestions are made for using these models in the smelters. ]]>353127146968704<![CDATA[Gear fatigue damage for a 500 kW wind turbine exposed to increasing turbulence using a flexible multibody model]]>This paper investigates gear tooth fatigue damage in a 500 kW wind turbine using FLEX5 and own multibody code. FLEX5 provides the physical wind field, rotor and generator torque and the multibody code is used for obtaining gear tooth reaction forces in the planetary gearbox. Different turbulence levels are considered and the accumulated fatigue damage levels are compared. An example where the turbulence/fatigue sensitivity could be important, is in the middle of a big wind farm. Interior wind turbines in large wind farms will always operate in the wake of other wind turbines, causing increased turbulence and therefore increased fatigue damage levels. This article contributes to a better understanding of gear fatigue damage when turbulence is increased (e.g. in the center of large wind farms or at places where turbulence is pronounced). ]]>3521091252113536<![CDATA[Integrated Debugging of Modelica Models]]>The high abstraction level of equation-based object-oriented (EOO) languages such as Modelica has the drawback that programming and modeling errors are often hard to find. In this paper we present integrated static and dynamic debugging methods for Modelica models and a debugger prototype that addresses several of those problems. The goal is an integrated debugging framework that combines classical debugging techniques with special techniques for equation-based languages partly based on graph visualization and interaction. To our knowledge, this is the first Modelica debugger that supports both equation-based transformational and algorithmic code debugging in an integrated fashion. ]]>35293107626688<![CDATA[An Approach to Automated Model Composition Illustrated in the Context of Design Verification]]>Building complex systems form models that were developed separately without modifying existing code is a challenging task faced on a regular basis in multiple contexts, for instance, in design verification. To address this issue, this paper presents a new approach for automating the dynamic system model composition. The presented approach aims to maximise information reuse, by defining the minimum set of information that is necessary to the composition process, to maximise decoupling by removing the need for explicit interfaces and to present a methodology with a modular and structured approach to composition. Moreover the presented approach is illustrated in the context of system design verification against requirements using a Modelica environment, and an approach for expressing the information necessary for automating the composition is formalized.]]>35279911107968<![CDATA[Comparison and Implementation of a Rigid and a Flexible Multibody Planetary Gearbox Model]]>We propose algorithms for developing (1) a rigid (constrained) and (2) a flexible planetary gearbox model. The two methods are compared against each other and advantages/disadvantages of each method are discussed. The rigid model (1) has gear tooth reaction forces expressed by Lagrange multipliers. The flexible approach (2) is being compared with the gear tooth forces from the rigid approach, first without damping and second the influence of damping is examined. Variable stiffness as a function of base circle arc length is implemented in the flexible approach such that it handles the realistic switch between one and two gear teeth in mesh. The final results are from modelling the planetary gearbox in a 500 kW wind turbine which we also described in Jørgensen et.al (2013).]]>35259771067008<![CDATA[Modelling the heat dynamics of a residential building unit: Application to Norwegian buildings]]>The paper refers to the development of a continuous time mathematical heating model for a building unit based on the first principles. The model is described in terms of the state space variables, and a lumped parameter approach is used to represent the room air temperature and air density using mass and energy balances. The one-dimensional heat equation in cartesian coordinates and spherical coordinates is discretized in order to describe the thermic characteristics of the layers of the building framework and furniture respectively. The developed model is implemented in a MATLAB environment, and mainly a theoretical approach is used to validate it for a residential building unit. Model is also validated using experimental data for a limited period. Short term simulations are used to test the energy efficiency of the building unit with regard to factors such as the operation of heat sources, ventilation, occupancy patterns of people, weather conditions, features of the building structure and heat recovery. The results are consistent and are obtained considerably fast, implying that the model can be used further in modelling the heating dynamics of complex architectural designs and in control applications. ]]>3514357648192<![CDATA[Modeling and simulation of lab-scale anaerobic co-digestion of MEA waste]]>Anaerobic digestion model No.1 (ADM1) was applied and expanded in this study to model and simulate anaerobic digestion (AD) of an industrial carbon capture reclaimer MEA (monoethanolamine) waste (MEAw) together with easily degradable organics. The general structure of ADM1 was not changed except for introducing state variables of MEA and complex organics (CO) in the waste and biochemical reactions of MEA uptake and CO hydrolysis in the model ADM1_MEAw. Experimental batch test results were used for calibrating kinetics variables. The obtained kinetics were employed in the ADM1_MEAw to simulate semi-continuously fed experimental test for 486 days at room temperature (22 +/- 2oC). The validation results show that the ADM1_MEAw was able to predict the process performance with reasonable accuracy, including process pH, biogas generation and inorganic nitrogen concentrations, for a wide range of feed scenarios. Free ammonia inhibition, was observed to be the main inhibitory effects on acetoclastic methanogenesis, leading to volatile fatty acids (VFA) accumulation at high loads. Inhibition assumed to be caused by potentially toxic constituents of MEAw appears to be much less important than ammonia, suggesting that such constituents were broken down by AD.]]>3513141553984<![CDATA[Stiffness Analysis and Optimization of a Co-axial Spherical Parallel Manipulator]]>This paper investigates the stiffness characteristics of spherical parallel manipulators. By virtue of singular value decomposition, the 6x6 dimensionally inhomogeneous Cartesian stiffness matrix is transformed into two homogeneous ones, i.e., the rotational and translational stiffness matrices. The decomposed singular values and the corresponding vectors indicate the directions of high/weak stiffness and the stiffness isotropy for the manipulator at a given configuration. Two indices, one for rotation and the other for translation, are introduced to optimize the manipulator stiffness and to map the stiffness isocontours over the workspace to show an image of the overall stiffness. ]]>35121302678784<![CDATA[Bootstrapping a Compiler for an Equation-Based Object-Oriented Language]]>What does it mean to bootstrap a compiler, and why do it? This paper reports on the first bootstrapping of a full-scale EOO (Equation-based Object-Oriented) modeling language such as Modelica. Bootstrapping means that the compiler of a language can compile itself. However, the usual application area for the Modelica is modeling and simulation of complex physical systems. Fortunately it turns out that with some minor extensions, the Modelica language is well suited for the modeling of language semantics. We use the name MetaModelica for this slightly extended Modelica. This is a prerequisite for bootstrapping which requires that the language can be used to model and/or implement itself. The OpenModelica Compiler (OMC) has been written in this MetaModelica language. It originally supported only the standard Modelica language but has been gradually extended to also cover the MetaModelica language extensions. After substantial work, OMC is able to quickly compile itself and produces an executable with good performance. The benefits include a more extensible and maintainable compiler by introducing improved language constructs and a more powerful runtime that makes it easy to add functionality such as parser generators, debuggers, and profiling tools. Future work includes extracting and restructuring parts of OMC, making the compiler smaller and more modular and extensible. This will also make it easier to interface with OMC, making it possible to create more powerful and user-friendly OpenModelica-based tools. The compiler and its bootstrapping is a major effort -- it is currently about 330 000 lines of code, and the MetaModelica extensions are used routinely by approximately ten developers on a daily basis. ]]>351119457728<![CDATA[Simplified Human-Robot Interaction: Modeling and Evaluation]]>In this paper a novel concept of human-robot interaction (HRI) modeling is proposed. Including factors like trust in automation, situational awareness, expertise and expectations a new user experience framework is formed for industrial robots. Service Oriented Robot Operation, proposed in a previous paper, creates an abstract level in HRI and it is also included in the framework. This concept is evaluated with exhaustive tests. Results prove that significant improvement in task execution may be achieved and the new system is more usable for operators with less experience with robotics; personnel specific for small and medium enterprises (SMEs).]]>3441992111505280<![CDATA[Continuous-Curvature Path Generation Using Fermat's Spiral]]>This paper proposes a novel methodology, based on Fermat's spiral (FS), for constructing curvature-continuous parametric paths in a plane. FS has a zero curvature at its origin, a property that allows it to be connected with a straight line smoothly, that is, without the curvature discontinuity which occurs at the transition point between a line and a circular arc when constructing Dubins paths. Furthermore, contrary to the computationally expensive clothoids, FS is described by very simple parametric equations that are trivial to compute. On the downside, computing the length of an FS arc involves a Gaussian hypergeometric function. However, this function is absolutely convergent and it is also shown that it poses no restrictions to the domain within which the length can be calculated. In addition, we present an alternative parametrization of FS which eliminates the parametric speed singularity at the origin, hence making the spiral suitable for path-tracking applications. A detailed description of how to construct curvature-continuous paths with FS is given. ]]>344183198910336<![CDATA[Analysis of Offshore Knuckle Boom Crane - Part Two: Motion Control]]>In this paper design of electro-hydraulic motion control systems for offshore knuckle boom cranes is discussed. The influence of the control valve bandwidth along with the ramp time for the control signal are investigated both analytically with simplified system models and numerically with an experimentally verified crane model. The results of both types of investigations are related to general design rules for selection of control valves and ramp times and the relevance of these design rules is discussed. Generally, they are useful but may be too conservative for offshore knuckle boom cranes. However, as demonstrated in the paper, the only proper way to determine this is to evaluate the motion control system design by means of simulation. ]]>344175181414720<![CDATA[Analysis of Offshore Knuckle Boom Crane - Part One: Modeling and Parameter Identification]]>This paper presents an extensive model of a knuckle boom crane used for pipe handling on offshore drilling rigs. The mechanical system is modeled as a multi-body system and includes the structural flexibility and damping. The motion control system model includes the main components of the crane's electro-hydraulic actuation system. For this a novel black-box model for counterbalance valves is presented, which uses two different pressure ratios to compute the flow through the valve. Experimental data and parameter identification, based on both numerical optimization and manual tuning, are used to verify the crane model. The demonstrated modeling and parameter identification techniques target the system engineer and takes into account the limited access to component data normally encountered by engineers working with design of hydraulic systems. ]]>344157174782336<![CDATA[On-off and PI Control of Methane Gas Production of a Pilot Anaerobic Digestion Reactor]]>A proposed feedback control system for methane ow control of a real pilot anaerobic digestion reactor fed with dairy waste is designed and analyzed using the modified Hill model, which has previously been adapted to the reactor. Conditions for safe operation of the reactor are found using steady-state responses of dynamic simulations, taking into account the upper limit of the volatile fatty acids (VFA) concentration recommended in the literature. The controllers used are standard process controllers, namely the on-off controller and the PI controller. Several PI controller tuning methods are evaluated using simulations. Two methods are favoured, namely the Skogestad method, which is an open loop method, and the Relaxed Ziegler-Nichols closed loop method. The two methods give approximately the same PI settings. Still, the Skogestad method is ranged first as it requires less tuning time, and because it is easier to change the PI settings at known changes in the process dynamics. Skogestad's method is successfully applied to a PI control system for the real reactor. Using simulations, the critical operating point to be used for safe controller tuning is identified.]]>343139156690176<![CDATA[Using MPC for Managed Pressure Drilling]]>As production on the Norwegian shelf enters tail production, drilling wells with vanishing pressure windows become more attractive. This motivates use of automatic control systems for improved control of downhole pressure using Managed Pressure Drilling (MPD) techniques. PID SISO control solutions for MPD are by now relatively standard, and well understood. This article explores the potential benefits of using linear Model Predictive Control (MPC) for MPD. It is shown that in combination with wired drill pipe, the downhole pressure can be controlled at multiple locations in the open wellbore, by using both pumps and choke in applied backpressure MPD. Also, downhole pressure constraints (pore and fracture pressures) fit naturally in MPC. Illustrative simulations are presented from using a high fidelity well simulator called WeMod, and Statoil's MPC software SEPTIC. ]]>343131138340754<![CDATA[Model Predictive Control with Integral Action: A simple MPC algorithm]]>A simple Model Predictive Control (MPC) algorithm of velocity (incremental) form is presented.The proposed MPC controller is insensitive to slowly varying system and measurement trends and thereforehas integral action. The presented algorithm is illustrated by both simulations and practical experimentson a quadruple tank MIMO process. ]]>343119129828247<![CDATA[Temperature Control of a Pilot Anaerobic Digestion Reactor]]>Results of analysis and design and implementation of a temperature controlsystem for a practical pilot anaerobic digestion (AD) bioreactor fed with dairywaste are presented. A dynamic model of the reactor temperature is used asthe basis for theoretical results, including simulations. Controller functionsinclude on-off control, proportional plus integral (PI) control, andfeedforward control. Various PI controller tuning methods are compared. Theneed for adaptivity of PI settings is investigated. Results for asimulated full-scale reactor are given. ]]>34399117637874<![CDATA[Relaxed Ziegler-Nichols Closed Loop Tuning of PI Controllers]]>A modification of the PI setting of the Ziegler-Nichols closed loop tuningmethod is proposed. The modification is based on a combination of the Skogestad SIMCtuning formulas for ''integrator plus time-delay'' processes with the Ziegler-Nichols tuning formulas assuming that the process is modeled as an ''integrator plus time-delay'' process. The resulting PI settings provide improved stability margins compared with those obtained with the original Ziegler-Nichols PI settings. Compared with the well-known Tyreus-Luyben PI settings, the proposed PI settings give improved disturbance compensation. For processes with zero or a negligible time-delay, but with some lags in the form of time-constants, tuning based on ultimate gain and ultimate period may give poor results. Successful PI settings for such processes are proposed. ]]>3428397829828<![CDATA[On the Kinematics of Robotic-assisted Minimally Invasive Surgery]]>Minimally invasive surgery is characterized by the insertion of the surgical instruments into the human body through small insertion points called trocars, as opposed to open surgery which requires substantial cutting of skin and tissue to give the surgeon direct access to the operating area. To avoid damage to the skin and tissue, zero lateral velocity at the insertion point is crucial. Entering the human body through trocars in this way thus adds constraints to the robot kinematics and the end-effector velocities cannot be found from the joint velocities using the simple relation given by the standard Jacobian matrix. We therefore derive a new Jacobian matrix which gives the relation between the joint variables and the end-effector velocities and at the same time guarantees that the velocity constraints at the insertion point are always satisfied. We denote this new Jacobian the Remote Center of Motion Jacobian Matrix (RCM Jacobian). The main contribution of this paper is that we address the problem at a kinematic level and that we through the RCM Jacobian can guarantee that the insertion point constraints are satisfied which again allows for the controller to be implemented in the end-effector workspace. By eliminating the kinematic constraints from the control loop we can derive the control law in the end-effector space and we are therefore able to apply Cartesian control schemes such as compliant or hybrid control. ]]>3426982678382<![CDATA[Optimal control strategies with nonlinear optimization for an Electric Submersible Pump lifted oil field]]>In an Electric Submersible Pump (ESP) lifted oil field, the ESP of each oil well should be operated inside its operating window. The total power consumed by the ESPs in the oil field should be minimized. The speed of the ESPs and the production choke valve opening should be optimally chosen for maximizing the total oil produced from the oil field. At the same time, the capacity of the separator should not be exceeded. In this paper, nonlinear steady state optimization based on Sequential Quadratic Programming (SQP) is developed. Two optimal control structures are proposed in this paper. In the first case, the optimal pump speed is controlled by a PI controller by varying the electrical excitation signal to the motors. The optimal fluid flow rate through each oil well is controlled by another PI controller by varying the production choke valve opening. The paper shows that the production choke valve for each oil well has to be always 100% open to maintain the optimal fluid flow rate. In the second case, the production choke valves are considered to be always 100% open as hard constraints. The optimal fluid flow rate through each oil well is controlled by a PI controller by varying the pump speed. It is shown that when the optimal fluid flow rate is tracked by the controller, the speed of each of the pumps is equal to the optimal pump speed calculated by the optimizer. This basically means that we can achieve the optimization objective with the same optimal results as in the first case by using only a single PI controller. The limitations of these two optimal control structures for very low values and for very high values of the separator capacity are discussed. For the feasible range of separator capacities, the optimal locus of the fluid flow rate and the pump speed are shown in this paper. ]]>34255671471406<![CDATA[Adapting Dynamic Mathematical Models to a Pilot Anaerobic Digestion Reactor]]> A dynamic model has been adapted to a pilot anaerobic reactor fed diarymanure. Both steady-state data from online sensors and laboratory analysis anddynamic operational data from online sensors are used in the model adaptation.The model is based on material balances, and comprises four state variables,namely biodegradable volatile solids, volatile fatty acids, acid generatingmicrobes (acidogens), and methane generating microbes (methanogens). The modelcan predict the methane gas flow produced in the reactor. The model may beused for optimal reactor design and operation, state-estimation and control.Also, a dynamic model for the reactor temperature based on energy balance ofthe liquid in the reactor is adapted. This model may be used for optimizationand control when energy and economy are taken into account. ]]>3423554547948<![CDATA[CFD Wake Modelling with a BEM Wind Turbine Sub-Model]]> Modelling of wind farms using computational fluid dynamics (CFD) resolving the flow field around each wind turbine's blades on a moving computational grid is still too costly and time consuming in terms of computational capacity and effort. One strategy is to use sub-models for the wind turbines, and sub-grid models for turbulence production and dissipation to model the turbulent viscosity accurately enough to handle interaction of wakes in wind farms. A wind turbine sub-model, based on the Blade Momentum Theory, see Hansen (2008), has been implemented in an in-house CFD code, see Hallanger et al. (2002). The tangential and normal reaction forces from the wind turbine blades are distributed on the control volumes (CVs) at the wind turbine rotor location as sources in the conservation equations of momentum. The classical k-epsilon turbulence model of Launder and Spalding (1972) is implemented with sub-grid turbulence (SGT) model, see Sha and Launder (1979) and Sand and Salvesen (1994). Steady state CFD simulations were compared with flow and turbulence measurements in the wake of a model scale wind turbine, see Krogstad and Eriksen (2011). The simulated results compared best with experiments when stalling (boundary layer separation on the wind turbine blades) did not occur. The SGT model did improve turbulence level in the wake but seems to smear the wake flow structure. It should be noted that the simulations are carried out steady state not including flow oscillations caused by vortex shedding from tower and blades as they were in the experiments. Further improvement of the simulated velocity defect and turbulence level seems to rely on better parameter estimation to the SGT model, improvements to the SGT model, and possibly transient- instead of steady state simulations.]]>34119331302418<![CDATA[Control of Spacecraft Formation with Disturbance Rejection and Exponential Gains]]> We address the problem of state feedback translational motion control of a spacecraft formation through a modified sliding surface controller using variable gains and I^2 action for disturbance rejection. The exponential varying gains ensure faster convergence of the state trajectories during attitude maneuver while keeping the gains small (and the system less stiff) for station keeping. Integral action is introduced for rejection of disturbances with a constant nonzero mean such as aerodynamic drag. A direct consequence is a drop in energy consumption when affected by sensor noise and a decrease in size of the error states residual when operating close to the equilibrium point. A large number of simulation results are presented to show the control performance. ]]>3411118547437<![CDATA[On Active Current Selection for Lagrangian Profilers]]> Autonomous Lagrangian profilers are now widely used as measurement and monitoring platforms, notably in observation programs as Argo. In a typical mode of operation, the profilers drift passively at their parking depthbefore making a vertical profile to go back to the surface. This paperpresents simple and computationally-efficient control strategies to activelyselect and use ocean currents so that a profiler can autonomously reach adesired destination. After briefly presenting a typical profiler andpossible mechanical modifications for a coastal environment, we introducesimple mathematical models for the profiler and the currents it will use. Wethen present simple feedback controllers that, using the direction of thecurrents and taking into account the configuration of the environment(coastal or deep-sea), is able to steer the profiler to any desiredhorizontal location. To illustrate the approach, a few results are presentedusing both simulated currents and real current velocity profiles from the North Sea. ]]>3411101669272<![CDATA[The Good Gain method for simple experimental tuning of PI controllers]]> A novel experimental method -- here denoted the Good Gain method -- fortuning PI controllers is proposed. The method can be regarded as analternative to the famous Ziegler-Nichols' Ultimate Gain method. The approach takenresembles the Ziegler-Nichols' method as it is based on experiments with theclosed loop system with proportional control. However, the method does notrequire severe process upset during the tuning like sustained oscillations.Only well-damped responses are assumed. Furthermore, in the present study itis demonstrated that the approach typically gives better stability robustnesscomparing with the Ziegler-Nichols' method. The method is relatively simpleto use which is beneficial for the user. A theoretical rationale based on second order dynamics is given. ]]>333141151958464<![CDATA[Stochastic Stability Analysis for Markovian Jump Neutral Nonlinear Systems]]> In this paper, the stability problem is studied for a class of Markovian jump neutral nonlinear systems with time-varying delay. By Lyapunov-Krasovskii function approach, a novel mean-square exponential stability criterion is derived for the situations that the system's transition rates are completely accessible, partially accessible and non-accessible, respectively. Moreover, the developed stability criterion is extended to the systems with different bounded sector nonlinear constraints. Finally, some numerical examples are provided to illustrate the effectiveness of the proposed methods. ]]>333131139336963<![CDATA[3D Sensor-Based Obstacle Detection Comparing Octrees and Point clouds Using CUDA]]> This paper presents adaptable methods for achieving fast collision detection using the GPU and Nvidia CUDA together with Octrees. Earlier related work have focused on serial methods, while this paper presents a parallel solution which shows that there is a great increase in time if the number of operations is large. Two different models of the environment and the industrial robot are presented, the first is Octrees at different resolutions, the second is a point cloud representation. The relative merits of the two different world model representations are shown. In particular, the experimental results show the potential of adapting the resolution of the robot and environment models to the task at hand. ]]>3331231301222592<![CDATA[Multiobjective Optimum Design of a 3-RRR Spherical Parallel Manipulator with Kinematic and Dynamic Dexterities]]> This paper deals with the kinematic synthesis problem of a 3-RRR spherical parallel manipulator, based on the evaluation criteria of the kinematic, kinetostatic and dynamic performances of the manipulator. A multiobjective optimization problem is formulated to optimize the structural and geometric parameters of the spherical parallel manipulator. The proposed approach is illustrated with the optimum design of a special spherical parallel manipulator with unlimited rolling motion. The corresponding optimization problem aims to maximize the kinematic and dynamic dexterities over its regular shaped workspace. ]]>333111121648194<![CDATA[Numerical and Experimental Study of Friction Loss in Hydrostatic Motor]]>This paper presents a numerical and experimental study of the losses in a hydrostatic motor principle. The motor is designed so that the structural deflections and lubricating regimes between moving surfaces and, subsequently, the leakage and friction losses, can be controlled during operation. This is done by means of additional pressure volumes that influence the stator deflection. These pressures are referred to as compensation pressures and the main emphasis is on friction or torque loss modeling of the motor as a function of the compensation pressures and the high and low pressures related to the load torque. The torque loss modeling is identified as a Stribeck curve which depends on gap height. The asperity friction is decreasing exponentially with an increase in gap height. The parameters of the torque loss model are based on prototype measurements that include the structural deflections of the lubricating gap faces. ]]>33399109590386<![CDATA[Optimal passive-damping design using a decentralized velocity-feedback H-Infinity approach]]>In this work, a new strategy to design passive energy dissipation systems for vibration control of large structures is presented. The method is based on the equivalence between passive damping systems and fully decentralized static velocity-feedback controllers. This equivalence allows to take advantage of recent developments in static output-feedback control design to formulate the passive-damping design as a single optimization problem with Linear Matrix Inequality constraints. To illustrate the application of the proposed methodology, a passive damping system is designed for the seismic protection of a five-story building with excellent results. ]]>3338797448364<![CDATA[A Review on Approaches for Condition Based Maintenance in Applications with Induction Machines Located Offshore]]>This paper presents a review of different approaches for Condition Based Maintenance (CBM) of induction machines and drive trains in offshore applications. The paper contains an overview of common failure modes, monitoring techniques, approaches for diagnostics, and an overview of typical maintenance actions. Although many papers have been written in this area before, this paper puts an emphasis on recent developments and limits the scope to induction machines and drive trains applied in applications located offshore. ]]>3326986708641<![CDATA[An Explicit Formulation of Singularity-Free Dynamic Equations of Mechanical Systems in Lagrangian Form---Part Two: Multibody Systems]]>This paper presents the explicit dynamic equations of multibody mechanical systems. This is the second paper on this topic. In the first paper the dynamics of a single rigid body from the Boltzmann--Hamel equations were derived. In this paper these results are extended to also include multibody systems. We show that when quasi-velocities are used, the part of the dynamic equations that appear from the partial derivatives of the system kinematics are identical to the single rigid body case, but in addition we get terms that come from the partial derivatives of the inertia matrix, which are not present in the single rigid body case. We present for the first time the complete and correct derivation of multibody systems based on the Boltzmann--Hamel formulation of the dynamics in Lagrangian form where local position and velocity variables are used in the derivation to obtain the singularity-free dynamic equations. The final equations are written in global variables for both position and velocity. The main motivation of these papers is to allow practitioners not familiar with differential geometry to implement the dynamic equations of rigid bodies without the presence of singularities. Presenting the explicit dynamic equations also allows for more insight into the dynamic structure of the system. Another motivation is to correct some errors commonly found in the literature. Unfortunately, the formulation of the Boltzmann-Hamel equations used here are presented incorrectly. This has been corrected by the authors, but we present here, for the first time, the detailed mathematical details on how to arrive at the correct equations. We also show through examples that using the equations presented here, the dynamics of a single rigid body is reduced to the standard equations on a Lagrangian form, for example Euler's equations for rotational motion and Euler--Lagrange equations for free motion. ]]>3326168312847<![CDATA[An Explicit Formulation of Singularity-Free Dynamic Equations of Mechanical Systems in Lagrangian Form---Part one: Single Rigid Bodies]]>This paper presents the explicit dynamic equations of a mechanical system. The equations are presented so that they can easily be implemented in a simulation software or controller environment and are also well suited for system and controller analysis. The dynamics of a general mechanical system consisting of one or more rigid bodies can be derived from the Lagrangian. We can then use several well known properties of Lie groups to guarantee that these equations are well defined. This will, however, often lead to rather abstract formulation of the dynamic equations that cannot be implemented in a simulation software directly. In this paper we close this gap and show what the explicit dynamic equations look like. These equations can then be implemented directly in a simulation software and no background knowledge on Lie theory and differential geometry on the practitioner's side is required. This is the first of two papers on this topic. In this paper we derive the dynamics for single rigid bodies, while in the second part we study multibody systems. In addition to making the equations more accessible to practitioners, a motivation behind the papers is to correct a few errors commonly found in literature. For the first time, we show the detailed derivations and how to arrive at the correct set of equations. We also show through some simple examples that these correspond with the classical formulations found from Lagrange's equations. The dynamics is derived from the Boltzmann--Hamel equations of motion in terms of local position and velocity variables and the mapping to the corresponding quasi-velocities. Finally we present a new theorem which states that the Boltzmann--Hamel formulation of the dynamics is valid for all transformations with a Lie group topology. This has previously only been indicated through examples, but here we also present the formal proof. The main motivation of these papers is to allow practitioners not familiar with differential geometry to implement the dynamic equations of rigid bodies without the presence of singularities. Presenting the explicit dynamic equations also allows for more insight into the dynamic structure of the system. Another motivation is to correct some errors commonly found in the literature. Unfortunately, the formulation of the Boltzmann-Hamel equations used here are presented incorrectly. This has been corrected by the authors, but we present here, for the first time, the detailed mathematical details on how to arrive at the correct equations. We also show through examples that using the equations presented here, the dynamics of a single rigid body is reduced to the standard equations on a Lagrangian form, for example Euler's equations for rotational motion and Euler--Lagrange equations for free motion. ]]>3324560433361<![CDATA[Discrete LQ optimal control with integral action: A simple controller on incremental form for MIMO systems]]>A simple Linear Quadratic (LQ) optimal controller of velocity (incremental) form with approximately the same properties as a conventional PID controller of velocity form is presented, i.e. integral action. The proposed optimal controller is insensitive to slowly varying system and measurement trends and has the ability of stabilizing any linear dynamic system under weak assumptions such as the stabilizability of the system and the detectability of the system seen from the performance index. ]]>3323544671800<![CDATA[H-Infinity robust controller design for the synchronization of master-slave chaotic systems with disturbance input]]>This paper is concerned with the robust control problems for the synchronization of master-slave chaotic systems with disturbance input. By constructing a series of Lyapunov functions, novel H-Infinity robust synchronization controllers are designed, whose control regulation possess the characteristic of simpleness and explicitness. Finally, numerical simulations are provided to demonstrate the effectiveness of the proposed techniques. ]]>3312734464095<![CDATA[Optimization of lift gas allocation in a gas lifted oil field as non-linear optimization problem]]>Proper allocation and distribution of lift gas is necessary for maximizing total oil production from a field with gas lifted oil wells. When the supply of the lift gas is limited, the total available gas should be optimally distributed among the oil wells of the field such that the total production of oil from the field is maximized. This paper describes a non-linear optimization problem with constraints associated with the optimal distribution of the lift gas. A non-linear objective function is developed using a simple dynamic model of the oil field where the decision variables represent the lift gas flow rate set points of each oil well of the field. The lift gas optimization problem is solved using the fmincon solver found in MATLAB. As an alternative and for verification, hill climbing method is utilized for solving the optimization problem. Using both of these methods, it has been shown that after optimization, the total oil production is increased by about 4%. For multiple oil wells sharing lift gas from a common source, a cascade control strategy along with a nonlinear steady state optimizer behaves as a self-optimizing control structure when the total supply of lift gas is assumed to be the only input disturbance present in the process. Simulation results show that repeated optimization performed after the first time optimization under the presence of the input disturbance has no effect in the total oil production. ]]>3311325486948<![CDATA[Modeling and Parameter Identification of Deflections in Planetary Stage of Wind Turbine Gearbox]]>The main focus of this paper is the experimental and numerical investigation of a 750kW wind turbine gearbox. A detailed model of the gearbox with main shaft has been created using MSC.Adams. Special focus has been put on modeling the planet carrier (PLC) in the gearbox. For this purpose experimental data from a drive train test set up has been analyzed using parameter identification to quantify misalignments. Based on the measurements a combination of main shaft misalignment and planet carrier deflection has been identified. A purely numerical model has been developed and it shows good accordance with the experimental data. ]]>3311112218116<![CDATA[Modeling of Wind Turbine Gearbox Mounting]]>In this paper three bushing models are evaluated to find a best practice in modeling the mounting of wind turbine gearboxes. Parameter identification on measurements has been used to determine the bushing parameters for dynamic simulation of a gearbox including main shaft. The stiffness of the main components of the gearbox has been calculated. The torsional stiffness of the main shaft, gearbox and the mounting of the gearbox are of same order of magnitude, and eigenfrequency analysis clearly reveals that the stiffness of the gearbox mounting is of importance when modeling full wind turbine drivetrains. ]]>3241411491254181<![CDATA[Comparison of Nonlinearity Measures based on Time Series Analysis for Nonlinearity Detection]]>The main purpose of this paper is a study of the efficiency of different nonlinearity detection methods based on time-series data from a dynamic process as a part of system identification. A very useful concept in measuring the nonlinearity is the definition of a suitable index to measure any deviation from linearity. To analyze the properties of such an index, the observed time series is assumed to be the output of Volterra series driven by a Gaussian input. After reviewing these methods, some modifications and new indices are proposed, and a benchmark simulation study is made. Correlation analysis, harmonic analysis and higher order spectrum analysis are selected methods to be investigated in our simulations. Each method has been validated with its own advantages and disadvantages.]]>3241231401055951<![CDATA[Observer Based Sliding Mode Attitude Control: Theoretical and Experimental Results]]>In this paper we present the design of a sliding mode controller for attitude control of spacecraft actuated by three orthogonal reaction wheels. The equilibrium of the closed loop system is proved to be asymptotically stable in the sense of Lyapunov. Due to cases where spacecraft do not have angular velocity measurements, an estimator for the generalized velocity is derived and asymptotic stability is proven for the observer. The approach is tested on an experimental platform with a sphere shaped Autonomous Underwater Vehicle SATellite: AUVSAT, developed at the Norwegian University of Science and Technology.]]>3231131211734322<![CDATA[Multiple Property Cross Direction Control of Paper Machines]]> Cross direction (CD) control in sheet-forming process forms a challenging problem with high dimensions. Accounting the interactions between different properties and actuators, the dimensionality increases further and also computational issues arise. We present a multiple property controller feasible to be used especially with imaging measurements that provide high sampling frequency and therefore enable short control interval. The simulation results state the benefits of multiple property CD control over single property control and single property control using full feedforward compensation. The controller presented may also be tuned in automated manner and the results demonstrate the effect of tuning on input saturation.]]>323103112880533<![CDATA[Modeling of Human Arm Energy Expenditure for Predicting Energy Optimal Trajectories]]> Human arm motion can inspire the trajectory planning of anthropomorphic robotic arms to achieve energy-efficient movements. An approach for predicting metabolic cost in the planar human arm motion by means of the biomechanical simulation is proposed in this work. Two biomechanical models, including an analytical model and a musculoskeletal model, are developed to implement the proposed approach. The analytical model is developed by modifying a human muscle expenditure model, in which the muscles are grouped as torque providers for computation efficiency. In the musculoskeletal model, the predication of metabolic cost is conducted on the basis of individual muscles. With the proposed approach, metabolic costs for parameterized target-reaching arm motions are calculated and utilized to identify optimal arm trajectories.]]>32391101686061Editorial: Volume 32, No 2
http://www.mic-journal.no/PDF/Editorials/ED-2011-1.pdf
<![CDATA[Abstraction of Dynamical Systems by Timed Automata]]> To enable formal verification of a dynamical system, given by a set of differential equations, it is abstracted by a finite state model. This allows for application of methods for model checking. Consequently, it opens the possibility of carrying out the verification of reachability and timing requirements, which by classical control methods is impossible. We put forward a method for abstracting dynamical systems, where level sets of Lyapunov functions are used to generate the partitioning of the state space. We propose to partition the state space using an entire family of functions. The properties of these functions ensure that the discrete model captures the behaviors of a dynamical system by generating appropriate equivalence classes of the states. These equivalence classes make up the partition of the state space.]]>3227990494863<![CDATA[Semi-decentralized Strategies in Structural Vibration Control]]> In this work, the main ideas involved in the design of overlapping and multi-overlapping controllers via the Inclusion Principle are discussed and illustrated in the context of the Structural Vibration Control of tall buildings under seismic excitation. A detailed theoretical background on the Inclusion Principle and the design of overlapping controllers is provided. Overlapping and multi-overlapping LQR controllers are designed for a simplified five-story building model. Numerical simulations are conducted to asses the performance of the proposed semi-decentralized controllers with positive results.]]>3225777620489<![CDATA[Mean-Square Filtering for Polynomial System States Confused with Poisson Noises over Polynomial Observations]]> In this paper, the mean-square filtering problem for polynomial system states confused with white Poisson noises over polynomial observations is studied proceeding from the general expression for the stochastic Ito differentials of the mean-square estimate and the error variance. In contrast to the previously obtained results, the paper deals with the general case of nonlinear polynomial states and observations with white Poisson noises. As a result, the Ito differentials for the mean-square estimate and error variance corresponding to the stated filtering problem are first derived. The procedure for obtaining an approximate closed-form finite-dimensional system of the filtering equations for any polynomial state over observations with any polynomial drift is then established. In the example, the obtained closed-form filter is applied to solve the third order sensor filtering problem for a quadratic state, assuming a conditionally Poisson initial condition for the extended third order state vector. The simulation results show that the designed filter yields a reliable and rapidly converging estimate.]]>3224755342485<![CDATA[Analysis, Modeling and Simulation of Mechatronic Systems using the Bond Graph Method]]> The Bond Graph is the proper choice of physical system used for: (i) Modeling which can be applied to systems combining multidisciplinary energy domains, (ii) Analysis to provide a great value proposition for finding the algebraic loops within the system enabling the process of troubleshooting and eliminating the defects by using the proper component(s) to fix the causality conflict even without being acquainted in the proper system, and (iii) Simulation facilitated through derived state space equations from the Bond Graph model is solved using industrial simulation software, such as 20-Sim, www.20sim.com. The Bond Graph technique is a graphical language of modeling, in which component energy ports are connected by bonds that specify the transfer of energy between system components. Following a brief introduction of the Bond Graph methodology and techniques, two separate case studies are comprehensively addressed. The first case study is a systematic implementation of a fourth order electrical system and conversion to mechanical system while the second case study presents modeling of the Dielectric Electro Active Polymer (DEAP) actuator. Building the systematic Bond Graph of multifaceted system and ease of switching between different domains are aims of the first case study while the second study shows how a complex mechatronic system could be analyzed and built by the Bond Graph. The respective Bond Graphs in each case is evaluated in the light of mathematical equations and simulations. Excellent correlation has been achieved between the simulation results and proper system equations.]]>32135451041977<![CDATA[A Sensor Fusion Algorithm for Filtering Pyrometer Measurement Noise in the Czochralski Crystallization Process]]> The Czochralski (CZ) crystallization process is used to produce monocrystalline silicon for solar cell wafers and electronics. Tight temperature control of the molten silicon is most important for achieving high crystal quality. SINTEF Materials and Chemistry operates a CZ process. During one CZ batch, two pyrometers were used for temperature measurement. The silicon pyrometer measures the temperature of the molten silicon. This pyrometer is assumed to be accurate, but has much high-frequency measurement noise. The graphite pyrometer measures the temperature of a graphite material. This pyrometer has little measurement noise. There is quite a good correlation between the two pyrometer measurements. This paper presents a sensor fusion algorithm that merges the two pyrometer signals for producing a temperature estimate with little measurement noise, while having significantly less phase lag than traditional lowpass- filtering of the silicon pyrometer. The algorithm consists of two sub-algorithms: (i) A dynamic model is used to estimate the silicon temperature based on the graphite pyrometer, and (ii) a lowpass filter and a highpass filter designed as complementary filters. The complementary filters are used to lowpass-filter the silicon pyrometer, highpass-filter the dynamic model output, and merge these filtered signals. Hence, the lowpass filter attenuates noise from the silicon pyrometer, while the graphite pyrometer and the dynamic model estimate those frequency components of the silicon temperature that are lost when lowpass-filtering the silicon pyrometer. The algorithm works well within a limited temperature range. To handle a larger temperature range, more research must be done to understand the process' nonlinear dynamics, and build this into the dynamic model.]]>3211732569114<![CDATA[Implicit Identification of Contact Parameters in a Continuous Chain Model]]> Accurate contact modeling is of great importance in the field of dynamic chain simulations. In this paper emphasis is on contact dynamics for a time-domain simulation model of large chains guided in a closed loop track. The chain model is based on theory for unconstrained rigid multibody dynamics where contact within the chain and with the track is defined through continuous point contacts using the contact indentation and rate as means. This paper presents an implicit method to determine contact parameters of the chain model through the use of none gradient optimization methods. The set of model parameters are estimated by minimizing the residual between simulated and measured results. The parameter identification is tested on four different formulations of the Hunt-Crossly hysteresis damping factor with the aim of recognizing a superior model. ]]>321115655405<![CDATA[On Tuning PI Controllers for Integrating Plus Time Delay Systems]]>Some analytical results concerning PI controller tuning based on integrator plus time delay models are worked out and presented. A method for obtaining PI controller parameters, Kp=alpha/(k*tau), and, Ti=beta*tau, which ensures a given prescribed maximum time delay error, dtau_max, to time delay, tau, ratio parameter delta=d\tau_max/tau, is presented. The corner stone in this method, is a method product parameter, c=alpha*beta. Analytical relations between the PI controller parameters, Ti, and, Kp, and the time delay error parameter, delta, is presented, and we propose the setting, beta=c/a*(delta+1), and, alpha=a/(delta+1), which gives, Ti=c/a*(delta+1)*tau, and Kp=a/((delta+1)*k*tau), where the parameter, a, is constant in the method product parameter, c=alpha*beta. It also turns out that the integral time, Ti, is linear in, delta, and the proportional gain, Kp, inversely proportional to, delta+1. For the original Ziegler Nichols (ZN) method this parameter is approximately, c=2.38, and the presented method may e.g., be used to obtain new modified ZN parameters with increased robustness margins, also documented in the paper. ]]>314145164651142<![CDATA[Developing a Tool Point Control Scheme for a Hydraulic Crane Using Interactive Real-time Dynamic Simulation]]>This paper describes the implementation of an interactive real-time dynamic simulation model of a hydraulic crane. The user input to the model is given continuously via joystick and output is presented continuously in a 3D animation. Using this simulation model, a tool point control scheme is developed for the specific crane, considering the saturation phenomena of the system and practical implementation.]]>314133143610236 <![CDATA[Spacecraft Magnetic Control Using Dichotomous Coordinate Descent Algorithm with Box Constraints]]>In this paper we present magnetic control of a spacecraft using the Dichotomous Coordinate Descent (DCD) algorithm with box constraints. What is common for most work on magnetic spacecraft control is the technique for solving for the control variables of the magnetic torquers where a cross product is included which is well known to be singular. The DCD algorithm provides a new scheme which makes it possible to use a general control law and then adapt it to work for magnetic torquers including restrictions in available magnetic moment, instead of designing a specialized controller for the magnetic control problem. A non-linear passivity-based sliding surface controller is derived for a fully actuated spacecraft and is then implemented for magnetic control by utilizing the previous mentioned algorithm. Results from two simulations are provided, the first comparing the results from the DCD algorithm with older results, and the second showing how easily the derived sliding surface controller may be implemented, improving our results. ]]>314123131485225 Editorial: Volume 31, No 3
http://www.mic-journal.no/PDF/Editorials/ED-2010-2.pdf
<![CDATA[Model-Based Optimizing Control and Estimation Using Modelica Model]]>This paper reports on experiences from case studies in using Modelica/Dymola models interfaced to
control and optimization software, as process models in real time process control applications. Possible
applications of the integrated models are in state- and parameter estimation and nonlinear model predictive
control. It was found that this approach is clearly possible, providing many advantages over modeling
in low-level programming languages. However, some effort is required in making the Modelica models
accessible to NMPC software.

Particular consideration is given to implementation of gradient computation for real-time dynamic
optimization, where the dynamic models can be Modelica models. Analytical methods for gradient computation
based on sensitivity integration are compared to finite difference-based methods. A case study
reveals that analytical methods outperform finite difference-methods as the number of inputs and/or input
blocks increases.]]>313107121523576<![CDATA[A Tutorial on Incremental Stability Analysis using Contraction Theory]]>This paper introduces a methodology for differential nonlinear stability analysis using contraction theory (Lohmiller and Slotine, 1998). The methodology includes four distinct steps: the descriptions of two systems to be compared (the plant and the observer in the case of observer convergence analysis, the plant and the controller in the case of tracking controller analysis), the definition of an abstract system common to the two systems and denoted as the 'virtual system', and the convergence study of the virtual system using its virtual dynamics representation. The approach is illustrated on several simple examples. ]]>31393106693899<![CDATA[Comparing PI Tuning Methods in a Real Benchmark Temperature Control System]]>This paper demonstrates a number of PI controller tuning methods being used
to tune a temperature controller for a real air heater. Indices expressing
setpoint tracking and disturbance compensation and stability margin
(robustness) are calculated. From these indices and a personal impression
about how quick a method is to deliver the tuning result and how simple it
is to use, a winning method is identified. ]]>31379912387994 <![CDATA[Control of a Buoyancy-Based Pilot Underwater Lifting Body]]>This paper is about position control of a specific small-scale pilot underwater lifting body where the lifting force stems from buoyancy adjusted with an air pocket in the lifting body. A mathematical model is developed to get a basis for a simulator which is used for testing and for designing the control system, including tuning controller parameters. A number of different position controller solutions were tried both on a simulator and on the physical system. Successful control on both the simulator and the physical system was obtained with cascade control based on feedback from measured position and height of the air pocket in the lifting body. The primary and the secondary controllers of the cascade control system were tuned using Skogestad's model-based PID tuning rules. Feedforward from estimated load force was implemented in combination with the cascade control system, giving a substantial improvement of the position control system, both with varying position reference and varying disturbance (load mass). ]]>31267771149122<![CDATA[Oxygen Effects in Anaerobic Digestion - II]]>Standard models describing bio-gasification using anaerobic digestion do not include necessary processes to describe digester dynamics under the conditions of oxygen presence. Limited oxygenation in anaerobic digestion can sometimes be beneficial. The oxygen effects included anaerobic digestion model, ADM 1-Ox, was simulated against experimental data obtained from laboratory scale anaerobic digesters operated under different oxygenation conditions. ADM 1-Ox predictions are generally in good agreement with the trends of the experimental data.
ADM 1-Ox simulations suggest the existence of an optimum oxygenation level corresponding to a peak methane yield. The positive impact of oxygenation on methane yield is more pronounced at conditions characterized by low hydrolysis rate coefficients (slowly degradable feed) and low biomass concentrations. The optimum oxygenation point moves towards zero when the hydrolysis rate coefficient and the biomass concentration increase. Accordingly, the impact of oxygenation on methane yield can either be positive or negative depending on the digestion system characteristics. The developed ADM 1-Ox model can therefore be a valuable tool for recognizing suitable operating conditions for achieving the maximum benefits from partial aeration in anaerobic digestion. ]]>31255651402794<![CDATA[Piecewise quadratic Lyapunov functions for stability verification of approximate explicit MPC]]>Explicit MPC of constrained linear systems is known to result in a piecewise affine controller and therefore also piecewise affine closed loop dynamics. The complexity of such analytic formulations of the control law can grow exponentially with the prediction horizon. The suboptimal solutions offer a trade-off in terms of complexity and several approaches can be found in the literature for the construction of approximate MPC laws. In the present paper a piecewise quadratic (PWQ) Lyapunov function is used for the stability verification of an of approximate explicit Model Predictive Control (MPC). A novel relaxation method is proposed for the LMI criteria on the Lyapunov function design. This relaxation is applicable to the design of PWQ Lyapunov functions for discrete-time piecewise affine systems in general. ]]>3124554345113Editorial: Volume 31, No 1
http://www.mic-journal.no/PDF/Editorials/ED-2010-1.pdf
<![CDATA[Adaptive Backstepping Control of Nonlinear Hydraulic-Mechanical System Including Valve Dynamics]]>The main contribution of the paper is the development of an adaptive backstepping controller for a nonlinear hydraulic-mechanical system considering valve dynamics. The paper also compares the performance of two variants of an adaptive backstepping tracking controller with a simple PI controller. The results show that the backstepping controller considering valve dynamics achieves significantly better tracking performance than the PI controller, while handling uncertain parameters related to internal leakage, friction, the orifice equation and oil characteristics. ]]>3113544477141<![CDATA[Empirical Modeling of Heating Element Power for the Czochralski Crystallization Process]]> The Czochralski (CZ) crystallization process is used to produce monocrystalline silicon. Monocrystalline silicon is used in solar cell wafers and in computers and electronics. The CZ process is a batch process, where multicrystalline silicon is melted in a crucible and later solidifies on a monocrystalline seed crystal. The crucible is heated using a heating element where the power is manipulated using a triode for alternating current (TRIAC). As the electric resistance of the heating element increases by increased temperature, there are significant dynamics from the TRIAC input signal (control system output) to the actual (measured) heating element power. The present paper focuses on empirical modeling of these dynamics. The modeling is based on a dataset logged from a real-life CZ process. Initially the dataset is preprocessed by detrending and handling outliers. Next, linear ARX, ARMAX, and output error (OE) models are identfied. As the linear models do not fully explain the process' behavior, nonlinear system identification is applied. The Hammerstein-Wiener (HW) model structure is chosen. The final model identified is a Hammerstein model, i.e. a HW model with nonlinearity at the input, but not at the output. This model has only one more identified parameter than the linear OE model, but still improves the optimization criterion (mean squared ballistic simulation errors) by a factor of six. As there is no nonlinearity at the output, the dynamics from the prediction error to the model output are linear, which allows a noise model to be added. Comparison of a Hammerstein model with noise model and the linear ARMAX model, both optimized for mean squared one-step-ahead prediction errors, shows that this optimization criterion is 42% lower for the Hammerstein model. Minimizing the number of parameters to be identified has been an important consideration throughout the modeling work. ]]>3111934373803<![CDATA[Using Generalized Fibonacci Sequences for Solving the One-Dimensional LQR Problem and its Discrete-Time Riccati Equation]]> In this article we develop a method of solving general one-dimensional Linear Quadratic Regulator (LQR) problems in optimal control theory, using a generalized form of Fibonacci numbers. We find the solution R(k) of the corresponding discrete-time Riccati equation in terms of ratios of generalized Fibonacci numbers. An explicit Binet type formula for R(k) is also found, removing the need for recursively finding the solution at a given timestep. Moreover, we show that it is also possible to express the feedback gain, the penalty functional and the controller state in terms of these ratios. A generalized golden ratio appears in the corresponding infinite horizon problem. Finally, we show the use of the method in a few examples. ]]>311118439325<![CDATA[Oxygen Effects in Anaerobic Digestion]]> Interaction of free oxygen in bio-gasification is a sparsely studied area, apart from the common argument of oxygen being toxic and inhibitory for anaerobic micro-cultures. Some studies have, however, revealed increased solubilisation of organic matter in the presence of some free oxygen in anaerobic digestion. This article analyses these counterbalancing phenomena with a mathematical modelling approach using the widely accepted biochemical model ADM 1. Aerobic oxidation of soluble carbon and inhibition of obligatory anaerobic organisms are modelled using standard saturation type kinetics. Biomass dependent first order hydrolysis kinetics is used to relate the increased hydrolysis rate with oxygen induced increase in biomass growth. The amended model, ADM 1-Ox (oxygen), has 25 state variables and 22 biochemical processes, presented in matrix form. The computer aided simulation tool AQUASIM 2.1 is used to simulate the developed model. Simulation predictions are evaluated against experimental data obtained using a laboratory batch test array comprising miniature anaerobic bio-reactors of 100 ml total volume each, operated under different initial air headspaces giving rise to the different oxygen loading conditions. The reactors were initially fed with a glucose solution and incubated at 35 Celsius, for 563 hours. Under the oxygen load conditions of 22, 44 and 88 mg/L, the ADM1-Ox model simulations predicted the experimental methane potentials quite adequately. Both the experimental data and the simulations suggest a linear reduction of methane potential with respect to the increase in oxygen load within this range. ]]>304191201675515<![CDATA[A Bootstrap Subspace Identification Method: Comparing Methods for Closed Loop Subspace Identification by Monte Carlo Simulations]]> A novel promising bootstrap subspace system identification algorithm for both open and closed loop systems is presented. An outline of the SSARX algorithm by Jansson (2003) is given and a modified SSARX algorithm is presented. Some methods which are consistent for closed loop subspace system identification presented in the literature are discussed and compared to a recently published subspace algorithm which works for both open as well as for closed loop data, i.e., the DSR_e algorithm as well as the bootstrap method. Experimental comparisons are performed by Monte Carlo simulations. ]]>304203222412113<![CDATA[A Rudimentary History of Dynamics]]> This article presents a condensed overview of the development of dynamics from Ancient Greece through to the late 18th century. Its purpose is to bring an often neglected topic to the control community in as interesting a fashion as the author can achieve. ]]>304223235667211Editorial: Volume 30, No 3
http://www.mic-journal.no/PDF/Editorials/ED-2009-3.pdf
<![CDATA[Hooked on a New Technology: The Automation Pioneers in Post-War Norway]]> This paper presents the initial activities in servo engineering in Norway originating in the early 1950s based on contacts at the Massachusets Institute of Technology. The activities were initiated by a small group of servo enthusiasts who, through the Feedback Control Committee in the research council, managed to coordinate national activities and establish strong research groups in Trondheim, Bergen and Oslo. After the initial phase of establishing the research groups, there was a continuous strong focus on connections with industry and industrial applications. In the mid-1960s the committee was strengthened and became the Automation and Data Processing Committee. The initial group of automation pioneers have left a lasting impact on the academic and industrial fields of servo engineering and automation in Norway. ]]>303871001365114<![CDATA[Jens Glad Balchen: A Norwegian Pioneer in Engineering Cybernetics]]> This paper tells the story of Jens Glad Balchen (1926-2009), a Norwegian research scientist and engineer who is widely regarded as the father of Engineering Cybernetics in Norway. In 1954, he founded what would later become the Department of Automatic Control at the Norwegian Institute of Technology in Trondheim. This name was changed to the Department of Engineering Cybernetics in 1972 to reflect the broader efforts being made, not only within the purely technical disciplines, but also within biology, oceanography and medicine. Balchen established an advanced research community in cybernetics in postwar Norway, whose applications span everything from the process industry and positioning of ships to control of fish and lobster farming. He was a chief among the tribe of Norwegian cybernetics engineers and made a strong impact on his colleagues worldwide. He planted the seeds of a whole generation of Norwegian industrial companies through his efforts of seeking applications for every scientific breakthrough. His strength and his wisdom in combination with his remarkable stubbornness gave extraordinary results. ]]>3031011254943621<![CDATA[The Department of Engineering Cybernetics at NTNU: From 1994 Into the Future]]> A short overview of the developments at the Department of Engineering Cybernetics at NTNU over the last 15 years is given. The vision of the department is to stay among Europe's most well recognized universities in control engineering, both with respect to education and research. It is discussed how this is achieved, and will continue to be strengthened in the future. ]]>303127132109619<![CDATA[Modeling, Identification and Control at Telemark University College]]> Master studies in process automation started in 1989 at what soon became Telemark University College, and the 20 year anniversary marks the start of our own PhD degree in Process, Energy and Automation Engineering. The paper gives an overview of research activities related to control engineering at Department of Electrical Engineering, Information Technology and Cybernetics. ]]>3031331471317504<![CDATA[Feedback: Still the Simplest and Best Solution]]> Most engineers are (indirectly) trained to be "feedforward thinkers" and they immediately think of "model inversion" when it comes to doing control. Thus, they prefer to rely on models instead of data, although feedback solutions in most cases are much simpler and more robust. ]]>303149155535788<![CDATA[Past, Present and Future of Process Control at Xstrata Nikkelverk]]> MIC celebrates its 30th anniversary in 2009 and Xstrata Nikkelverk celebrates its 100th anniversary in 2010. Both anniversaries are certainly worth celebrating and with this article Xstrata Nikkelverk salutes MIC at this special occasion. ]]>303157165285248<![CDATA[Robot Control Overview: An Industrial Perspective]]> One key competence for robot manufacturers is robot control, defined as all the technologies needed to control the electromechanical system of an industrial robot. By means of modeling, identification, optimization, and model-based control it is possible to reduce robot cost, increase robot performance, and solve requirements from new automation concepts and new application processes. Model-based control, including kinematics error compensation, optimal servo reference- and feed-forward generation, and servo design, tuning, and scheduling, has meant a breakthrough for the use of robots in industry. Relying on this breakthrough, new automation concepts such as high performance multi robot collaboration and human robot collaboration can be introduced. Robot manufacturers can build robots with more compliant components and mechanical structures without loosing performance and robots can be used also in applications with very high performance requirements, e.g., in assembly, machining, and laser cutting. In the future it is expected that the importance of sensor control will increase, both with respect to sensors in the robot structure to increase the control performance of the robot itself and sensors outside the robot related to the applications and the automation systems. In this connection sensor fusion and learning functionalities will be needed together with the robot control for easy and intuitive installation, programming, and maintenance of industrial robots. ]]>3031671807773544<![CDATA[Trends in Research and Publication: Science 2.0 and Open Access]]> This paper considers current trends in academic research and publication, in particular as seen from the control community.
The introduction of Internet-based Web 2.0 applications for scientists and engineers is currently changing the way research is
being done. In the near future, participants in the research community will be able to share ideas, data and results on a whole
new level. They will also be able to manage the rapidly increasing amount of scientific information much more effectively than
today through collaborative efforts enabled by the new Internet tools. However, a necessary premise for such a development
is the availability of research material. Many research results are currently well protected behind expensive subscription
schemes that impede the free sharing of information. At the same time, an increasing amount of research is being published
through open access channels with unrestricted availability. Interestingly, recent studies show that such policies contribute to
an increased number of citations compared to the pay-based alternatives. In sum, the parallel development of new tools for
research collaboration and an increased access to research material may help spur a revolution in the way research is being
done.]]>303181190205128Editorial: Volume 30, No 2
http://www.mic-journal.no/PDF/Editorials/ED-2009-2.pdf
<![CDATA[]]>Kinematic and Elastostatic Design Optimisation of the 3-DOF Gantry-Tau Parallel Kinematic Manipulator One of the main advantages of the Gantry-Tau machine is a large accessible workspace/footprint ratio compared to many other parallel machines. The Gantry-Tau improves this ration further by allowing a change of assembly mode without internal link collisions or collisions between the links and end-effector. The reconfigurable Gantry-Tau kinematic design obtained by multi-objective optimisation according to this paper gives the following features: 3-D workspace/footprint ratio is more than 3.19, lowest Cartesian stiffness in the workspace is $5N/\mu m$ and no link collisions detected. The optimisation parameters are the support frame lengths, the actuator positions and the robot's arm lengths. The results comparison between the evolutionary complex search algorithm and gradient-based method used for the Gantry-Tau design in the past is also presented in this paper. The detailed statics model analysis of the Gantry-Tau based on a functionally dependency is presented in this paper for the first time. Both the statics model and complex search algorithm may be applied for other 3-DOF Hexapods without major changes. The existing lab prototype of the Gantry-Tau was assembled and completed at the University of Agder, Norway. ]]>30239562078085<![CDATA[]]>Constrained Control Design for Dynamic Positioning of Marine Vehicles with Control Allocation In this paper, we address the control design problem of positioning of over-actuated marine vehicles with control allocation. The proposed design is based on a combined position and velocity loops in a multi-variable anti-windup implementation together with a control allocation mapping. The vehicle modelling is considered with appropriate simplifications related to low-speed manoeuvring hydrodynamics and vehicle symmetry. The control design is considered together with a control allocation mapping. We derive analytical tuning rules based on requirements of closed-loop stability and performance. The anti- windup implementation of the controller is obtained by mapping the actuator-force constraint set into a constraint set for the generalized forces. This approach ensures that actuation capacity is not violated by constraining the generalized control forces; thus, the control allocation is simplified since it can be formulated as an unconstrained problem. The mapping can also be modified on-line based on actuator availability to provide actuator-failure accommodation. We provide a proof of the closed-loop stability and illustrate the performance using simulation scenarios for an open-frame underwater vehicle.]]>30257702137375<![CDATA[Closed and Open Loop Subspace System Identification of the Kalman Filter]]> Some methods for consistent closed loop subspace system identification presented in the literature are analyzed and compared to a recently published subspace algorithm for both open as well as for closed loop data, the DSR_e algorithm. Some new variants of this algorithm are presented and discussed. Simulation experiments are included in order to illustrate if the algorithms are variance efficient or not.]]>3027186357503Editorial: Volume 30, No 1
http://www.mic-journal.no/PDF/Editorials/ED-2009.pdf
<![CDATA[]]>A Matlab Toolbox for Parametric Identification of Radiation-Force Models of Ships and Offshore Structures This article describes a Matlab toolbox for parametric identification of fluid-memory models associated with the radiation forces ships and offshore structures. Radiation forces are a key component of force-to- motion models used in simulators, motion control designs, and also for initial performance evaluation of wave-energy converters. The software described provides tools for preparing non-parmatric data and for identification with automatic model-order detection. The identification problem is considered in the frequency domain. ]]>3011151034175<![CDATA[]]>Robust Explicit Moving Horizon Control and Estimation: A Batch Polymerization Case Study This paper focuses on the design and evaluation of a robust explicit moving horizon controller and a robust explicit moving horizon estimator for a batch polymerization process. It is of particular interest since there are currently no reported case studies or implementations of the explicit parametric controller/estimator for batch and polymerization processes. In this paper we aim at achieving tight offset-free tracking of a desired reactor temperature profile, making accurate states estimation despite of the possible perturbations, and demonstrating the practical applicability to a case with industrially relevant complexity. ]]>3011725418698<![CDATA[Robust H-Infinity Filter Design for Uncertain Linear Systems Over Network with Network-Induced Delays and Output Quantization]]> This paper investigates a convex optimization approach to the problem of robust H-Infinity filtering for uncertain linear systems connected over a common digital communication network. We consider the case where quantizers are static and the parameter uncertainties are norm bounded. Firstly, we propose a new model to investigate the effect of both the output quantization levels and the network conditions. Secondly, by introducing a descriptor technique, using Lyapunov-Krasovskii functional and a suitable change of variables, new required sufficient conditions are established in terms of delay-dependent linear matrix inequalities (LMIs) for the existence of the desired network-based quantized filters with simultaneous consideration of network induced delays and measurement quantization. The explicit expression of the filters is derived to satisfy both asymptotic stability and a prescribed level of disturbance attenuation for all admissible norm bounded uncertainties. ]]>3012737251396