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1.
  • Sandberg, Henrik (author)
  • Model Reduction for Linear Time-Varying Systems
  • 2004
  • Doctoral thesis (other academic/artistic)abstract
    • The thesis treats model reduction for linear time-varying systems. Time-varying models appear in many fields, including power systems, chemical engineering, aeronautics, and computational science. They can also be used for approximation of time-invariant nonlinear models. Model reduction is a topic that deals with simplification of complex models. This is important since it facilitates analysis and synthesis of controllers. The thesis consists of two parts. The first part provides an introduction to the topics of time-varying systems and model reduction. Here, notation, standard results, examples, and some results from the second part of the thesis are presented. The second part of the thesis consists of four papers. In the first paper, we study the balanced truncation method for linear time-varying state-space models. We derive error bounds for the simplified models. These bounds are generalizations of well-known time-invariant results, derived with other methods. In the second paper, we apply balanced truncation to a high-order model of a diesel exhaust catalyst. Furthermore, we discuss practical issues of balanced truncation and approximative discretization. In the third paper, we look at frequency-domain analysis of linear time-periodic impulse-response models. By decomposing the models into Taylor and Fourier series, we can analyze convergence properties of different truncated representations. In the fourth paper, we use the frequency-domain representation developed in the third paper, the harmonic transfer function, to generalize Bode's sensitivity integral. This result quantifies limitations for feedback control of linear time-periodic systems.
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2.
  • Ekvall, Jenny (author)
  • Dryer Section Control in Paper Machines During Web Breaks
  • 2004
  • Licentiate thesis (other academic/artistic)abstract
    • Web breaks in the dryer section of a paper machine cause loss of production and quality problems. After a web break, the steam pressure in the cylinders must be reduced to avoid overheating. The goal of this project is to determine optimal steam pressure trajectories during web breaks, so that the production is restarted with the same drying properties of the cylinder as before the break. A detailed physical dynamic model of the drying cylinder has been developed. The model describes the relations between the steam valve position, the steam pressure, the cylinder temperature, and the paper temperature. The model is based on partial differential equations that describe heat conductivity for the cylinder and the paper web, and mass balances of water and dry material in the paper. The accuracy of the model has been verified through experiments made at the M-real paper mill in Husum, Sweden. Verifications are made both during normal operation and during web breaks. The dynamic model has been reduced in order to derive simple transfer functions between the steam pressure and the cylinder temperature, and between a logic signal that is active during web breaks and the cylinder temperature, respectively. The transfer functions obtained were used to find the optimal steam pressure trajectory during web breaks. A new feed-forward strategy for steam pressure control during web breaks is presented. The strategy has been tested on a paper machine with good results. The strategy is built on feed-forward compensation and has been well received at the mill.
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3.
  • Fält, Mattias (author)
  • Convergence Analysis and Improvements for Projection Algorithms and Splitting Methods
  • 2021
  • Doctoral thesis (other academic/artistic)abstract
    • Non-smooth convex optimization problems occur in all fields of engineering. A common approach to solving this class of problems is proximal algorithms, or splitting methods. These first-order optimization algorithms are often simple, well suited to solve large-scale problems and have a low computational cost per iteration. Essentially, they encode the solution to an optimization problem as a fixed point of some operator, and iterating this operator eventually results in convergence to an optimal point. However, as for other first order methods, the convergence rate is heavily dependent on the conditioning of the problem. Even though the per-iteration cost is usually low, the number of iterations can become prohibitively large for ill-conditioned problems, especially if a high accuracy solution is sought.In this thesis, a few methods for alleviating this slow convergence are studied, which can be divided into two main approaches. The first are heuristic methods that can be applied to a range of fixed-point algorithms. They are based on understanding typical behavior of these algorithms. While these methods are shown to converge, they come with no guarantees on improved convergence rates.The other approach studies the theoretical rates of a class of projection methods that are used to solve convex feasibility problems. These are problems where the goal is to find a point in the intersection of two, or possibly more, convex sets. A study of how the parameters in the algorithm affect the theoretical convergence rate is presented, as well as how they can be chosen to optimize this rate.
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4.
  • Gäfvert, Magnus, et al. (author)
  • Proposal for a distributed computer control system in heavy-duty trucks
  • 2000
  • Reports (other academic/artistic)abstract
    • In designing safety-critical real-time distributed control-systems a great potential lies in combining methods within the areas of dependable computer systems and control theory. In this report a brake system on a heavy-duty truck is used as a case study to discuss the choice of distribution level of the computer system and the control algorithms. The distrubution levels of these system aspects are othogonal in the sense that they may be chosen independently for a certain system configuration. A system configuration is obtained by mapping a specific control algorithm architecture on a computer/communications architecture. To explore different system architectures the combinations of three different distribution levels for the control algorithms and computer/communication systems are investigated with respect to dependability and required information exchange. The result is a proposal on the choice of final distribution level of the brake system.
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  • Johnsson, Charlotta (author)
  • Whitepaper: S88 for Beginners
  • 2004
  • Reports (other academic/artistic)abstract
    • Are you interested in or curious about Batch Control, ISA S88, and/or Plant Automation in general, then this whitepaper is for you. In the following three pages you will learn the basics about Batch Control and ISA S88 and you will understand how you can benefit from knowing them. For a deeper understanding I recommend you to read the other whitepapers in this series as well as reading the ISA S88 standard.
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9.
  • Johnsson, Charlotta (author)
  • Whitepaper: S88 for Universities
  • 2004
  • Reports (other academic/artistic)abstract
    • If you are working at a university teaching a course in automation, and/or process control or if you are a student in interested in those subjects 14 this paper is for you! You might already have heard about the S88 batch control standard 14 how it tries to define terminology and concepts that makes design and operation of process control plants easier. In this paper, we will try to briefly explain, in understandable words and easy phrases, why it is definitely worthwhile for you to read and understand this standard. So that you, being a teacher, can provide your students with something very interesting that also makes your students extra valuable on the market when graduating, or so that you, being a student, have the knowledge to chose a course that really inspires you and teaches you the stuff that industry looks for.
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10.
  • Jouini, Taouba (author)
  • Network Synchronization and Control Based on Inverse Optimality : A Study of Inverter-Based Power Generation
  • 2021
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis dwells upon the synthesis of system-theoretical tools to understand and control the behavior of nonlinear networked systems. This work is at the crossroads of three topics: synchronization in coupled high-order oscillators, inverse optimal control and the application of inverter-based power systems. The control and stability of power systems leverages the theoretical results obtained for synchronization in coupled high-order oscillators and inverse optimal control.First, we study the dynamics of coupled high-order nonlinear oscillators. These are characterized by their rotational invariance, meaning that their dynamics remain unchanged following a static shift of their angles. We provide sufficient conditions for local frequency synchronization based on both direct, indirect Lyapunov methods and center manifold theory. Second, we study inverse optimal control problems, embedded in networked settings. In this framework, we depart from a given stabilizing control law, with an associated control Lyapunov function and reverse engineer the cost functional to guarantee the optimality of the controller. In this way, inverse optimal control generates a whole family of optimal controllers corresponding to different cost functions. This provides analytically explicit and numerically feasible solutions in closed-form. This approach circumvents the complexity of solving partial differential equations descending from dynamic programming and Bellman's principle of optimality. We show this to be the case also in the presence of disturbances in the dynamics and the cost. In networks, the controller obtained from inverse optimal control has a topological structure (e.g., it is distributed) and thus feasible for implementation. The tuning is analogous to that of linear quadratic regulators.Third, motivated by the pressing changes witnessed by the electrical grid toward renewable energy generation, we consider power system stability and control as the main application of this thesis. In particular, we apply our theoretical findings to study a network of power electronic inverters. We first propose a controller we term the matching controller, a control strategy that, based on DC voltage measurements, endows the inverters with an oscillatory behavior at a common desired frequency. In closed-loop with the matching control, inverters can be considered as nonlinear oscillators. Our study of the dynamics of nonlinear oscillator network provides feasible physical conditions that ask for damping on DC- and AC-side of each converter, that are sufficient for system-wide frequency synchronization.Furthermore, we showcase the usefulness of inverse optimal control for inverter-based generation at two different settings to synthesize robust angle controllers with respect to common disturbances in the grid and provable stability guarantees. All the controllers proposed in this thesis, provide the electrical grid with important services, namely power support whenever needed, as well as power sharing among all inverters.
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14.
  • Alriksson, Peter (author)
  • State Estimation for Distributed and Hybrid Systems
  • 2008
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis deals with two aspects of recursive state estimation: distributed estimation and estimation for hybrid systems. In the first part, an approximate distributed Kalman filter is developed. Nodes update their state estimates by linearly combining local measurements and estimates from their neighbors. This scheme allows nodes to save energy, thus prolonging their lifetime, compared to centralized information processing. The algorithm is evaluated experimentally as part of an ultrasound based positioning system. The first part also contains an example of a sensor-actuator network, where a mobile robot navigates using both local sensors and information from a sensor network. This system was implemented using a component-based framework. The second part develops, a recursive joint maximum a posteriori state estimation scheme for Markov jump linear systems. The estimation problem is reformulated as dynamic programming and then approximated using so called relaxed dynamic programming. This allows the otherwise exponential complexity to be kept at manageable levels. Approximate dynamic programming is also used to develop a sensor scheduling algorithm for linear systems. The algorithm produces an offline schedule that when used together with a Kalman filter minimizes the estimation error covariance.
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15.
  • Bengtsson, Johan (author)
  • Closed-Loop Control of HCCI Engine Dynamics
  • 2004
  • Doctoral thesis (other academic/artistic)abstract
    • The topic of the thesis is control of Homogeneous Charge Compression Ignition (HCCI) engine dynamics. HCCI offers a potential to combine high efficiency with very low emissions. In order to fulfill the potential benefits, closed-loop control is needed. The thesis discusses sensors, feedback signals and actuators for closed-loop control of the HCCI combustion. Closed-loop control of the HCCI combustion using ion current is demonstrated. Models of the HCCI dynamics suitable for purposes of control design are presented. It is shown that low-order models are sufficient to describe the HCCI dynamics. Models of HCCI combustion have been determined both by system identification and by physical modeling. Different methods for characterizing and controlling the HCCI combustion are outlined and demonstrated. In cases where the combustion phasing in a six-cylinder heavy-duty engine was controlled, either by a Variable Valve Actuation system using the inlet valve or a dual-fuel system, results are presented. Combustion phasing is a limiting factor of the load control and emission control performance. A system where control of HCCI on a cycle-to-cycle basis is outlined and cylinder individual cycle-to-cycle control on a six-cylinder heavy duty engine is presented. Various control strategies are compared. Model-based control, such as LQG and Model Predictive Control MPC, and PID control are shown to give satisfactory controller performance. An MPC controller is proposed as a solution to the problem of load-torque control with simultaneous minimization of the fuel consumption and emissions, while satisfying the constraints on cylinder pressure.
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  • Cervin, Anton (author)
  • Integrated Control and Real-Time Scheduling
  • 2003
  • Doctoral thesis (other academic/artistic)abstract
    • The topic of the thesis is codesign of flexible real-time control systems. Integrating control theory and real-time scheduling theory, it is possible to achieve higher resource utilization and better control performance. The integration requires new tools for analysis, design, and implementation. The problem of scheduling the individual parts of a control algorithm is studied. It is shown how subtask scheduling can reduce the input-output latency in a set of control tasks. Deadline assignment under different scheduling policies is considered. A feedback scheduling architecture for control tasks is introduced. The scheduler uses feedback from execution-time measurements and feedforward from workload changes to adjust the sampling periods of a set of control tasks so that the combined performance of the controllers is optimized. The Control Server, a novel computational model for real-time control tasks, is presented. The model combines time-triggered I/O with dynamic, reservation-based task scheduling. The model provides short input-output latencies and minimal jitter for the controllers. It also allows control tasks to be treated as scalable real-time components with predictable performance. Two MATLAB-based toolboxes for analysis and simulation of real-time control systems have been developed. The Jitterbug toolbox evaluates a quadratic cost function for a linear control system with timing variations. The tool makes it possible to investigate the impact of delay, jitter, lost samples, etc., on control performance. The TrueTime toolbox facilitates detailed cosimulation of distributed real-time control systems. The scheduling and execution of control tasks is simulated in parallel with the network communication and the continuous process dynamics.
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18.
  • Cescon, Marzia (author)
  • Modeling and Prediction in Diabetes Physiology
  • 2013
  • Doctoral thesis (other academic/artistic)abstract
    • Diabetes is a group of metabolic diseases characterized by the inability of the organism to autonomously regulate the blood glucose levels. It requires continuing medical care to prevent acute complications and to reduce the risk of long-term complications. Inadequate glucose control is associated with damage, dysfunction and failure of various organs. The management of the disease is non trivial and demanding. With today’s standards of current diabetes care, good glucose regulation needs constant attention and decision-making by the individuals with diabetes. Empowering the patients with a decision support system would, therefore, improve their quality of life without additional burdens nor replacing human expertise. This thesis investigates the use of data-driven techniques to the purpose of glucose metabolism modeling and short-term blood-glucose predictions in Type I Diabetes Mellitus (T1DM). The goal was to use models and predictors in an advisory tool able to produce personalized short-term blood glucose predictions and on-the-spot decision making concerning the most adequate choice of insulin delivery, meal intake and exercise, to help diabetic subjects maintaining glycemia as close to normal as possible. The approaches taken to describe the glucose metabolism were discrete-time and continuous-time models on input-output form and statespace form, while the blood glucose short-term predictors, i.e., up to 120 minutes ahead, used ARX-, ARMAX- and subspace-based prediction.
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19.
  • de Maré, Lena (author)
  • Feeding Strategies Based on Probing Control for E. coli and V. cholerae Cultivations
  • 2006
  • Doctoral thesis (other academic/artistic)abstract
    • The recombinant DNA techniques have made it possible to produce many different proteins for a wide range of applications. The vector encoding for the recombinant protein is often inserted in the bacterium E. coli as it is a well studied and a well-known organism. To achieve a high productivity, it is important to reach a high cell density. This is obtained through fed-batch operation where the nutrient, usually glucose, is added continuously. The feeding strategy should be designed to avoid starvation and overfeeding. This is not an easy task as on-line measurements of key variables are normally not available. A probing feeding strategy using the measurements of the standard dissolved oxygen sensor is developed by M. Åkesson. The key idea is to superimpose pulses on the feed-rate and make use of the responses in the dissolved oxygen in a feedback algorithm. Also, when the maximum transfer capacity of the reactor is reached the feed-rate is decreased in order to keep the reactor working under aerobic conditions. The strategy has been successfully implemented in cultivations with different E. coli strains and on different scales. The probing feeding strategy is further developed in many aspects in this thesis. Firstly, it is implemented with good results also in V. cholerae cultivations, which shows how general the strategy is. Also, a model of a bio-reactor operating in fed-batch mode is presented and verified. The effect on the tuning rules of the probing controller is investigated. The probing feeding strategy is further improved for a more efficient product synthesis. This new fermentation technique manipulates the temperature when the maximum oxygen transfer capacity of the reactor is reached. The strategy consists of a mid-ranging controller structure and a modified probing controller. It is analysed and evaluated in experiments and simulations. Furthermore, some E. coli production strains need additions of amino acids or complex media besides the carbon nutrient to grow and produce the recombinant protein. The probing control concept is therefore extended in order to handle these situations. Feeding strategies for dual feeding of amino acids or complex media and glucose are developed.
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20.
  • Dressler, Isolde (author)
  • Modeling and Control of Stiff Robots for Flexible Manufacturing
  • 2012
  • Doctoral thesis (other academic/artistic)abstract
    • To survive on a global market, small and medium size enterprises (SMEs) require affordable and competitive industrial automation for high quality flexible manufacturing. This thesis contributes to the development of robot concepts that fit the needs of SMEs. A major part of the thesis deals with the modeling of the three degree-of-freedom (DOF) Gantry-Tau parallel kinematic robot, which has the potential to fulfill the requirements on accuracy, mechanical stiffness and conceptual flexibility of a robot for SMEs. Additionally, concepts that aid the SMEs to achieve the required accuracy and a more intuitive robot operation were developed and evaluated. The modeling of the Gantry-Tau robot includes both kinematic and dynamic modeling. Based on the nominal kinematic model, kinematic error models were developed, as well as kinematics for the F1-type Gantry-Tau, a Gantry-Tau architecture extended to 6~DOF. The modeling was evaluated in kinematic calibration experiments. A rigid body model was derived and identified, including friction in the actuators. As noticeable flexible behaviour was observed, the compliance dynamics were identified by black box modeling. Kinematic calibration was not only considered for evaluation of the kinematic models developed, but it was also studied how to automize the kinematic calibration procedure, so that it can be executed by non-expert SME staff after a possible geometric reconfiguration of the robot. In the search of affordable, accurate and reusable measurement devices for kinematic calibration in SMEs, the usage of camera vision for kinematic calibration was evaluated. To make the programming of a robot trajectory fast and intuitive, lead-through programming was recently introduced. A new concept for lead-through programming in contact situations is proposed in this thesis, where two force sensors are used. While the first sensor is used for guiding the robot, the second force sensor measures the tool force, which can prevent damage of the tool or workpiece and can help to keep a steady contact between tool and surface. The concept was demonstrated in two example applications. A possibility to improve the performance for a repeatedly executed motion is iterative learning control (ILC). An ILC algorithm is evaluated on the Gantry-Tau robot, which uses an estimate of the tool motion, based on measurements from an accelerometer mounted at the end-effector plate and in addition to measurements on the motor side. The performance of the tool motion was shown to be considerably improved compared to the case when only motor side measurements are used.
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21.
  • Eborn, Jonas (author)
  • On Model Libraries for Thermo-hydraulic Applications
  • 2001
  • Doctoral thesis (other academic/artistic)abstract
    • Mathematical modelling and simulation are important tools when dealing with engineering systems that today are becoming increasingly more complex. Tightly integrated production and process optimization are trends that give rise to heterogeneous systems, which are difficult to handle without expertise in several engineering disciplines. Model libraries provide an excellent way to package engineering knowledge of systems and units to be reused by non experts. Many commercial simulation packages provide good model libraries, but they are usually domain specific and closed. Heterogeneous, multi-domain systems requires open model libraries written in general purpose modelling languages. Modelica(TM) is such an open standard for an object-oriented modelling language for dynamic systems. The thesis describes principles for object-oriented equation-based model libraries. The main topic is modelling of thermo-hydraulic applications. Two different model libraries are presented, the Omola model library K2 for thermal power plants and the Modelica base library ThermoFlow for general thermo-hydraulic applications. The models are based on first principles. Lumped or 1D-discretized control volumes contain the thermodynamic balance equations. The base library is built for flexibility; the control volume can have different medium descriptions, single- or multi-component, and the momentum dynamics can be replaced by static descriptions. Some applications of the libraries are also described: a heat recovery steam generator, a drum-boiler model and a model of evaporating two-phase flow in a pipe. The thesis consists of four articles. Papers I and II describe the two model libraries. Paper III covers the two-phase flow application and gives a simplified physical analysis that shows under what conditions there will be pressure-drop oscillations in an evaporating pipe. Model libraries for industrial use must be validated against measured data. Paper IV describes how parameter estimation methods can be used for model structure validation. The thesis also has a short discussion on other model validation methods.
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22.
  • Eker, Johan (author)
  • Flexible Embedded Control Systems : Design and Implementation
  • 1999
  • Doctoral thesis (other academic/artistic)abstract
    • Embedded control system design is a complex and error prone task, and there is a great need for better development tools. Today most real-time control systems are static systems, designed in an ad-hoc fashion. A more interactive way of working would give shorter development times, reduced costs for testing, and safer, more robust systems. This thesis presents an interactive prototyping tool called Pålsjö and a controller description language called PAL. Pålsjö allows the user to interactively configure a system. Control systems are defined using block diagrams and blocks may be added and replaced on-line. The static nature of most embedded control systems makes them very sensitive to changes in the hardware or software configuration. There is rarely any interaction between the control loops and the underlying real-time kernel or operating system. From a control engineering perspective, the system executes in open loop. In this thesis, the possibility of using feedback in the scheduling of real-time tasks is explored. This is called feedback scheduling. An algorithm for selecting sampling rates based on the linear quadratic cost is presented. The dependence of the cost function on the sampling interval is investigated. The influence on the control performance from the interaction between the control tasks running concurrently in the real-time kernel is studied. A simulation toolbox for this is presented in the thesis. Designing flexible control systems is much simplified given the proper language support. Therefore the PAL language concept is extended into a new proposed language Friend, which is designed to support the development of flexible embedded control systems through the use of contracts.
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23.
  • Garpinger, Olof (author)
  • Analysis and Design of Software-Based Optimal PID Controllers
  • 2015
  • Doctoral thesis (other academic/artistic)abstract
    • A large process industry can have somewhere between five hundred and five thousand control loops, and PID controllers are used in 90–97% of the cases. It is well-known that only 20–30% of the controllers in the process industry are tuned satisfactorily, but with the methods available today it is considered too time-consuming to optimize each single controller. This thesis presents tools for analysis and design of optimal PID controllers, and suggests when and how to use them efficiently. High performing low-level controllers are also likely to be beneficial for higher-level advanced process control, thus promoting the economy of whole factories. Controller design is often a trade-off between conflicting criteria, such as load disturbance attenuation, robustness, and noise sensitivity. In this thesis, a MATLAB-based software tool is used to solve a constrained optimization problem, with respect to all three requirements. This gives tuning of both the PID parameters and a low-pass filter time constant. A large batch of benchmark models, representative for the process industry, has been used throughout the whole thesis for controller analysis. This includes comparisons between PID controllers derived using either optimization or tuning rules. Trade-off plots are also presented, which explicitly show the relationships between performance, robustness and the PID parameters. A new procedure for software-based optimal PID design is suggested, which leads to a set of PID, PI, and I controllers. The user can then select the best performing controller with an acceptable control signal activity. It is shown that the resulting controllers are optimal or near optimal with respect to the three above mentioned criteria. The same procedure can also be used to analyze the benefit of the derivative part by comparing optimal PI and PID controllers with the same level of noise sensitivity. The efficiency of the procedure is demonstrated on an industrial friction stir welding machine. For a more wide-spread use of the proposed procedure, it is shown that better modeling techniques are needed, and guidelines for such methods are also included.
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24.
  • Gattami, Ather (author)
  • Optimal Decisions with Limited Information
  • 2007
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis considers static and dynamic team decision problems in both stochastic and deterministic settings. The team problem is a cooperative game, where a number of players make up a team that tries to optimize a given cost induced by the uncertainty of nature. The uncertainty is modeled as either stochastic, which gives the stochastic team problem, or modelled as deterministic where the team tries to optimize the worst case scenario. Both the stochastic and deterministic static team problems are stated and solved in a linear quadratic setting. It is shown that linear decisions are optimal in both the stochastic and deterministic framework. The dynamic team problem is formulated using well known results from graph theory. The dynamic interconnection structure is described by a graph. It appears natural to use a graph theoretical formulation to examine how a decision by a member of the team affects the rest of the members. Conditions for tractability of the dynamic team problem are given in terms of the graph structure. Tractability of a new class of information constrained team problems is shown, which extends existing results. For the presented tractable classes, necessary and sufficient conditions for stabilizability are given. The state feedback $mathcal{H}_2$ and $mathcal{H}_{infty}$ dynamic team problems are solved using a novel approach. The new approach is based on the crucial idea of disturbance feedback, which is used to separate the controller effect from the measured output, to eliminate the controller's dual role. Finally, a generalized stochastic linear quadratic control problem is considered. A broad class of team problems can be modeled by imposing quadratic constraints of correlation type. Also, power constraints on the control signals are very common. This motivates the development of a generalized control theory for both the finite and infinite horizon case, where power constraints are imposed. It is shown that the solution can be found using finite dimensional convex optimization.
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25.
  • Grundelius, Mattias (author)
  • Methods for Control of Liquid Slosh
  • 2001
  • Doctoral thesis (other academic/artistic)abstract
    • Horizontal movement of liquid containers is a common operation in an industrial packaging machine. During the movement the acceleration of the container induces motion of the liquid within the container, this is referred to as liquid slosh or liquid vibration. If there is too much slosh the liquid might wet the sealing surfaces of the container and even contaminate the machine. There is no measurement of the slosh so the only way to control the slosh is through the acceleration reference that defines the movement. The work presented in this thesis is focused on development of systematic methods for calculation of acceleration references that move the container as fast as possible without too much slosh. The methods are based on a simple model of the slosh phenomenon which is derived from fluid dynamics and system identification. The acceleration reference is calculated both directly using optimal control techniques with various cost functions and constraints and iteratively using iterative learning control. To enable practical evaluation of the acceleration references and the use of iterative learning control an experimental setup has been used where it is possible to measure the surface elevation on one side of the container using an infrared laser displacement sensor. The experimental evaluations show that it is possible to achieve fast movements by solving a minimum energy optimal control problem and tuning of the model parameters. It is also shown that the iterative learning control methods are successful in finding good acceleration references in practice using only a simple model of the slosh phenomenon. A method that utilize tilting of the container is also derived, this enables faster movements with less slosh. The methods simultaneously calculates the horizontal and rotational acceleration references by solving a minimum energy optimal control problem. Experiments show that the method is successful if the maximum allowed surface elevation is not too large.
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26.
  • Gäfvert, Magnus (author)
  • Topics in Modeling, Control, and Implementation in Automotive Systems
  • 2003
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis treats different aspects of automotive control-systems and consists of four papers covering different areas in this large field. The first paper presents a 9-degrees-of-freedom dynamic model of a tractor-semitrailer vehicle, aimed at simulation and evaluation of active chassis-systems for stability enhancements. Special focus is put on inclusion of the dynamics of load transfer, which are of importance in active yaw-control and roll-over prevention. The second paper presents a new easy-to-use semi-empirical tire model for combined-slip conditions. The model is based on combining the standard rigid-carcass brush model with available empirical pure-slip models. A new method is presented for partitioning empirical pure-slip forces into components of sliding and adhesive forces. The third paper regards control of gasoline direct injection engines. Strategies based on feedback of the effective torque are shown to relax the requirements of accurate engine maps, in simulations. It is exemplified that extremum control may be used for realtime optimization of the engine operation with respect to fuel consumption. A new control structure that combines the extremum controller with disturbance rejection is used to render the probing signals invisible in the engine output. The fourth paper presents a new method to limit the effects of transient faults that occurs in hardware hosting implementations of feedback controllers. The idea is to introduce artificial signal limits that are based on closed-loop signal bounds, and combine them with an anti-windup scheme.
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27.
  • Hast, Martin (author)
  • Design of Low-Order Controllers using Optimization Techniques
  • 2015
  • Doctoral thesis (other academic/artistic)abstract
    • In many applications, especially in the process industry, low-level controllers are the workhorses of the automated production lines. The aim of this study has been to provide simple tuning procedures, either optimization-based methods or tuning rules, for design of low-order controllers. The first part of this thesis deals with PID tuning. Design methods or both SISO and MIMO PID controllers based on convex optimization are presented. The methods consist of solving a nonconvex optimization problem by deriving convex approximations of the original problem and solving these iteratively until convergence. The algorithms are fast because of the convex approximations. The controllers obtained minimize low-frequency sensitivity subject to constraints that ensure robustness to process variations and limitations of control signal effort. The second part of this thesis deals with tuning of feedforward controllers. Tuning rules that minimize the integrated-squared-error arising from measurable step disturbances are derived for a controller that can be interpreted as a filtered and possibly time-delayed PD controller. Using a controller structure that decouples the effects of the feedforward and feedback controllers, the controller is optimal both in open and closed loop settings. To improve the high-frequency noise behavior of the feedforward controller, it is proposed that the optimal controller is augmented with a second-order filter. Several aspects on the tuning of this filter are discussed. For systems with PID controllers, the response to step changes in the reference can be improved by introducing set-point weighting. This can be interpreted as feedforward from the reference signal to the control signal. It is shown how these weights can be found by solving a convex optimization problem. Proportional set-point weight that minimizes the integrated-absolute-error was obtained for a batch of over 130 different processes. From these weights, simple tuning rules were derived and the performance was evaluated on all processes in the batch using five different feedback controller tuning methods. The proposed tuning rules could improve the performance by up to 45% with a modest increase in actuation.
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28.
  • Haugwitz, Staffan (author)
  • Modeling, Control and Optimization of a Plate Reactor
  • 2007
  • Doctoral thesis (other academic/artistic)abstract
    • A new chemical reactor, the Alfa Laval Plate Reactor, is being developed by Alfa Laval, a Swedish world-leading heat exchanger company. The plate reactor combines the high-heat-transfer capabilities of plate heat exchangers with the efficient mixing and reaction control typical of microreactors. With this new concept, highly exothermic reactions can be produced using more concentrated reactants and more accurate temperature control. This will reduce the reaction time and the need for downstream separation, thus saving energy and reducing the impact on the environment. The focus of this thesis is to develop and apply control methods to take advantage of the full potential of the novel plate reactor concept. A nonlinear model of the reactor is derived based on first principles to conduct a system analysis and enable model-based control. The physical model allows a detailed investigation of the potential control inputs and how the process design and choice of inputs may affect the control design. Two control concepts are examined, decentralized control using multi-loop PID controllers and centralized control using Model Predictive Control. The concepts are evaluated and compared in terms of design methods, performance and practical aspects. A cooling system is designed and experimentally verified, where a mid-ranging control structure is implemented to increase the operating range of the hydraulic equipment. The start-up control problem is challenging due to process uncertainty, highly nonlinear dynamics and input and temperature constraints. The dynamics and the constraints are easily captured by the process model in the optimization problem. The open question is how to address the process uncertainty. Here, robustness to uncertainty is achieved by introducing state-space constraints in the optimization formulation, which decrease the sensitivity of the optimal solution. The start-up control problem has been approached from two sides, a time-driven continuous approach and an event-driven hybrid approach. Some of the results are specific for the plate reactor, but many parts may be generalized to other applications, for example the decentralized and centralized control design, the start-up/transition control design and use of mid-ranging control to handle two-input one-output systems.
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29.
  • Hedlund, Sven (author)
  • Computational Methods for Optimal Control of Hybrid Systems
  • 2003
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis aims to find algorithms for optimal control of hybrid systems and explore them in sufficient detail to be able to implement the ideas in computational tools. By hybrid systems is meant systems with interacting continuous and discrete dynamics. Code for computations has been developed in parallel to the theory. The optimal control methods studied in this thesis are global, i.e. the entire state space is considered simultaneously rather than searching for locally optimal trajectories. The optimal value function that maps each state of the state space onto the minimal cost for trajectories starting in that state is central for global methods. It is often difficult to compute the value function of an optimal control problem, even for a purely continuous system. This thesis shows that a lower bound of the value function of a hybrid optimal control problem can be found via convex optimization in a linear program. Moreover, a dual of this optimization problem, parameterized in the control law, has been formulated via general ideas from duality in transportation problems. It is shown that the lower bound of the value function is tight for continuous systems and that there is no gap between the dual optimization problems. Two computational tools are presented. One is built on theory for piecewise affine systems. Various analysis and synthesis problems for this kind of systems are via piecewise quadratic Lyapunov-like functions cast into linear matrix inequalities. The second tool can be used for value function computation, control law extraction, and simulation of hybrid systems. This tool parameterizes the value function in its values in a uniform grid of points in the state space, and the optimization problem is formulated as a linear program. The usage of this tool is illustrated in a case study.
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30.
  • Henningsson, Maria (author)
  • Data-Rich Multivariable Control of Heavy-Duty Engines
  • 2012
  • Doctoral thesis (other academic/artistic)abstract
    • The combustion engine is today the dominant technology for transportation of goods and people world-wide. Concerns for global warming, toxic exhaust emissions, as well as cost and availability of fuel have in recent years created incentives for technological evolution of combustion engines. More sophisticated engine instrumentation with additional degrees of freedom has been added to the engine design to reduce emissions and fuel consumption. But, as engines become more complex, the task of calibration and control becomes more challenging. This thesis investigates approaches to utilize rich sensor information for multivariable engine control. Different combustion modes, and different combinations of sensors and actuators have been studied and evaluated experimentally on a full-scale six-cylinder heavy-duty engine. The work is divided into four areas: virtual emissions sensing, dynamic emissions models, optimal engine control, and control of sensitive combustion modes. The theme of the thesis is to show how feedback control based on rich sensor information can be exploited to improve the engine operation and reduce the off-line calibration effort. The virtual sensing work presents a data-mining method for predicting exhaust emissions from cylinder pressure data. Principal component analysis was used to reduce the dimensionality of the high-resolution data, and a neural network model was trained to predict emissions on a cycle-to-cycle, cylinder-individual basis. The work on dynamic models investigates how system identification can be used to find multivariable dynamic models from a set of engine actuators to a set of variables related to high-level engine specifications, namely emissions, work output, combustion phasing, and peak pressure derivative. It was shown how fairly simple Wiener models can capture the main dynamics of the engine at a grid of operating points. One of the identified multivariable models was used for optimal control of the engine. In contrast to most previous work in the field, integration of fuel- and gas-path control into a single framework was pursued. A model predictive controller was designed based on a cost function expressed in terms of high-level engine control objectives, and feedback was based on measured emissions as well as cylinder pressure data. The final part of the thesis presents work on two sensitive combustion modes, HCCI and dual-fuel operation. Here, feedback control is necessary to achieve robust operation. For both types of combustion, it was shown how a combination of two actuators can be used to successfully control the combustion process.
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31.
  • Henningsson, Toivo (author)
  • Stochastic Event-Based Control and Estimation
  • 2012
  • Doctoral thesis (other academic/artistic)abstract
    • Digital controllers are traditionally implemented using periodic sampling, computation, and actuation events. As more control systems are implemented to share limited network and CPU bandwidth with other tasks, it is becoming increasingly attractive to use some form of event-based control instead, where precious events are used only when needed. Forms of event-based control have been used in practice for a very long time, but mostly in an ad-hoc way. Though optimal solutions to most event-based control problems are unknown, it should still be viable to compare performance between suggested approaches in a reasonable manner. This thesis investigates an event-based variation on the stochastic linear-quadratic (LQ) control problem, with a fixed cost per control event. The sporadic constraint of an enforced minimum inter-event time is introduced, yielding a mixed continuous-/discrete-time formulation. The quantitative trade-off between event rate and control performance is compared between periodic and sporadic control. Example problems for first-order plants are investigated, for a single control loop and for multiple loops closed over a shared medium. Path constraints are introduced to model and analyze higher-order event-based control systems. This component-based approach to stochastic hybrid systems allows to express continuous- and discrete-time dynamics, state and switching constraints, control laws, and stochastic disturbances in the same model. Sum-of-squares techniques are then used to find bounds on control objectives using convex semidefinite programming. The thesis also considers state estimation for discrete time linear stochastic systems from measurements with convex set uncertainty. The Bayesian observer is considered given log-concave process disturbances and measurement likelihoods. Strong log-concavity is introduced, and it is shown that the observer preserves log-concavity, and propagates strong log-concavity like inverse covariance in a Kalman filter. A recursive state estimator is developed for systems with both stochastic and set-bounded process and measurement noise terms. A time-varying linear filter gain is optimized using convex semidefinite programming and ellipsoidal over-approximation, given a relative weight on the two kinds of error.
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32.
  • Henriksson, Dan (author)
  • Resource-Constrained Embedded Control and Computing Systems
  • 2006
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis deals with methods for handling resource constraints in embedded control systems and real-time computing systems. By dynamic feedback-based resource scheduling it is possible to achieve adaptability andincreased performance for these systems. A feedback scheduling strategy is presented, which uses feedback from plant states to distribute computing resources optimally among a set of controller tasks. Linear-quadratic controllers are analyzed, and expressions relating the expected cost to the sampling period and the plant state are derived and used for on-line sample-rate adjustments. A flexible implementation of model predictive control (MPC) tasks is described. A termination criterion is derived that, unlike traditional MPC, takes the effects of computational delay into account in the optimization. A scheduling scheme is also described, where the MPC cost functions being minimized are used as dynamic task priorities for a set of MPC tasks. A method for optimizing the use of computational resources in a multi-camera-based positioning system is studied. The covariance of the estimation error is minimized, while meeting computation time constraints. A novel predictor for delay control in server systems is introduced. The predictor uses instantaneous measurements of queue length and arrival times and is continuously updated as new requests arrive according to a receding horizon principle. The predictor is evaluated in simulation and by experiments on an Apache web server. The MATLAB/Simulink-based simulator TrueTime is presented. TrueTime is a codesign tool that facilitates simulation of distributed real-time control systems. TrueTime also supports simulation of wireless communication and resource constraints associated with wireless sensor/actuator networks.
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33.
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34.
  • Ingimundarson, Ari (author)
  • Dead-Time Compensation and Performance Monitoring in Process Control
  • 2003
  • Doctoral thesis (other academic/artistic)abstract
    • The thesis contains two parts, dead-time compensation and performance monitoring. The first part on dead-time compensation is about robust tuning procedures for dead-time compensating controllers (DTC). Both stable and integrating processes are considered. Simple experiments are performed to obtain process models as well as bounds on the allowable bandwidth for stability. The DTCs used have few parameters with clear physical interpretation so that manual tuning is possible. In the second part on dead-time compensation the performance of PID controllers is compared to the performance of DTCs. The aim is to answer the question: ``When can a simple dead-time compensator be expected to perform better than a PID?''. The performance criterion used is the integrated absolute error (IAE). It is compared for PI and PID controllers and a simple dead-time compensator (DTC) when a step load disturbance is applied at the plant input. The topic of the second part of the thesis is performance monitoring of lambda-tuned feedback controllers. A lambda tuned loop is a loop with an one-degree-of freedom controller whose set point response is of first order plus dead time. Lambda is the time constant of the set-point response. For these loops a non intrusive performance monitoring methods is described which is thought to be a first indicator of bad performance after which existing diagnosis algorithms could be applied. For online implementation in distributed control systems a simple recursive algorithm to estimate the index is presented. All parameters of the monitoring method are set by using the lambda tuning. The method applies equally to stochastic or deterministic disturbances. Finally an algorithm to estimate a synthetic gradient of a quadratic cost function is presented. It is demonstrated that the gradient can provide valuable information for maintenance of controllers as the gradient gives information about the disturbances affecting the loop.
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35.
  • Johannesson, Erik (author)
  • Control and Communication with Signal-to-Noise Ratio Constraints
  • 2011
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis is about two problems in the intersection of communication and control theory. Their common feature is that they involve communication over an additive white noise channel with a signal-to-noise ratio (SNR) constraint. The first problem concerns the transmission of a real-valued signal from a partially observed Markov source. The distortion criterion is the mean squared error and the transmission is subject to a delay constraint, which introduces the need for real-time coding. The problem is first considered for scalar-valued signals when the channel has no feedback and then, in turn, generalized to each of the cases with non-white channel noise, vector-valued signals or channel feedback. It is shown that jointly optimal encoders and decoders within the linear time-invariant (LTI) class can be obtained by solving a convex optimization problem and performing a spectral factorization. The functional to minimize is the sum of the well-known cost in a corresponding Wiener filtering problem and a new term that is induced by the channel noise. The second problem, which can be viewed as a generalization of the first problem, concerns a networked control system where an LTI plant, subject to a stochastic disturbance, is to be controlled over the channel. The controller is based on output feedback and consists of an encoder/observer that measures the plant output and transmits over the channel, and a decoder/controller that receives the channel output and issues the control signal. The objective is to stabilize the plant, satisfy the SNR constraint and minimize the variance of the disturbance response. The problem is studied for channels without and with feedback. In both cases, it is shown that optimal controllers within the LTI class can be obtained by solving a convex optimization problem and performing a spectral factorization. Previously known conditions on the SNR for stabilizability follow directly from the constraints of these optimization problems.
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36.
  • Johansson, Karl Henrik (author)
  • Relay Feedback and Multivariable Control
  • 1997
  • Doctoral thesis (other academic/artistic)abstract
    • This doctoral thesis treats three issues in control engineering related to relay feedback and multivariable control systems. Linear systems with relay feedback is the first topic. Such systems are shown to exhibit several interesting behaviors. It is proved that there exist multiple fast relay switches if and only if the sign of the first non-vanishing Markov parameter of the linear system is positive. It is also shown that these fast switches can appear as part of a stable limit cycle. A linear system with pole excess one or two is demonstrated to be particularly interesting. Stability conditions for these cases are derived. It is also discussed how fast relay switches can be approximated by sliding modes. Performance limitations in linear multivariable control systems is the second topic. It is proved that if the top left submatrices of a stable transfer matrix have no right half-plane zeros and a certain high-frequency condition holds, then there exists a diagonal stabilizing feedback that makes a weighted sensitivity function arbitrarily small. Implications on control structure design and sequential loop-closure are given. A novel multivariable laboratory process is also presented. Its linearized dynamics have a transmission zero that can be located anywhere on the real axis by simply adjusting two valves. This process is well suited to illustrate many issues in multivariable control, for example, control design limitations due to right half-plane zeros. The third topic is a combination of relay feedback and multivariable control. Tuning of individual loops in an existing multivariable control system is discussed. It is shown that a specific relay feedback experiment can be used to obtain process information suitable for performance improvement in a loop, without any prior knowledge of the system dynamics. The influence of the loop retuning on the overall closed-loop performance is derived and interpreted in several ways.
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37.
  • Johansson, Mikael (author)
  • Piecewise Linear Control Systems
  • 1999
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis treats analysis and design of piecewise linear control systems. Piecewise linear systems capture many of the most common nonlinearities in engineering systems, and they can also be used for approximation of other nonlinear systems. Several aspects of linear systems with quadratic constraints are generalized to piecewise linear systems with piecewise quadratic constraints. It is shown how uncertainty models for linear systems can be extended to piecewise linear systems, and how these extensions give insight into the classical trade-offs between fidelity and complexity of a model. Stability of piecewise linear systems is investigated using piecewise quadratic Lyapunov functions. Piecewise quadratic Lyapunov functions are much more powerful than the commonly used quadratic Lyapunov functions. It is shown how piecewise quadratic Lyapunov functions can be computed via convex optimization in terms of linear matrix inequalities. The computations are based on a compact parameterization of continuous piecewise quadratic functions and conditional analysis using the S-procedure. A unifying framework for computation of a variety of Lyapunov functions via convex optimization is established based on this parameterization. Systems with attractive sliding modes and systems with bounded regions of attraction are also treated. Dissipativity analysis and optimal control problems with piecewise quadratic cost functions are solved via convex optimization. The basic results are extended to fuzzy systems, hybrid systems and smooth nonlinear systems. It is shown how Lyapunov functions with a discontinuous dependence on the discrete state can be computed via convex optimization. An automated procedure for increasing the flexibility of the Lyapunov function candidate is suggested based on linear programming duality. A Matlab toolbox that implements several of the results derived in the thesis is presented.
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38.
  • Johansson, Rolf (author)
  • Multivariable Adaptive Control
  • 1983
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis treats the problem of direct adaptive control of linear multivariable systems. The parametrization problem of adaptive control is discussed extensively. A pole-placement problem and a model-matching problem are formulated and interpreted in terms of model reference control. The problem is solved via a discussion on system invariants of multivariable systems as presented by Pernebo. The attention is then directed towards problems of identification and two different estimation schemes are formulated. Parameter convergence is guaranteed provided some conditions on /a priori/ information are satisfied. The requested prior knowledge is formulated in terms of the non-invertible system for the suggested prediction error identification algorithm. The second parameter adjustment law is shown to converge when a certain approximant of the left structure matrix, /i.e./, the system invariant is known. This result relaxes a result by Elliott /et al./ where the interactor is required to be known.The important question of stability of adaptive systems is also treated. The major result is a method for construction of Lyapunov functions for a class of single-input systems. Stability in the sense of Lyapunov and exponential convergence are shown.
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39.
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40.
  • Johnsson, Charlotta (author)
  • A Graphical Language for Batch Control
  • 1999
  • Doctoral thesis (other academic/artistic)abstract
    • In this thesis a graphical language for sequential control is presented and its application to batch control is examined and discussed. The name of the language is Grafchart. Its main feature is that it makes it possible to express complicated control problems in a compact and intuitive manner.Grafchart exists in two versions; a basic version and a high-level version. The basic version was available when the work presented in this thesis started. The basic version of Grafchart is mainly based on Grafcet, i.e., it has a clear and intuitive syntax. In addition to this it has support for parameterization and methods and message passing as well as extended facilities for exception handling. The high-level version of Grafchart takes the concepts of the basic version one step further. The concepts of object tokens and multi-dimensional charts are introduced. These features are inspired by high-level Petri nets and constructs from object-oriented programming languages.The main application area of Grafchart is batch control. In the thesis it is shown how Grafchart can be used for recipe structuring and recipe analysis. By using the features of Grafchart in various ways, recipes can be given different structures with different advantages and disadvantages. All structures comply with the international batch standard IEC 61512 (also refered to as ISA S88.01). To test and evaluate the different recipe structures, a real-time batch plant simulation platform was used. The batch recipe structures can be combined with resource allocation and a visualization of the allocation is thereby achieved. By transforming the recipes into an equivalent Petri net structure, formal analysis methods that exist for Petri nets can be used to look for possible deadlock situations.Several commercial batch control systems exist today. They all have a similar approach to recipe structuring, resource allocation and user presentation. The advantages of using Grafchart compared to some of these systems, regarding these features are presented in the thesis. A toolbox implementation of Grafchart exists. It is implemented in G2, an object-oriented programming environment. G2 is also an industrial environment, this makes the toolbox and the results of this thesis directly usable in industry.
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41.
  • Johnsson, Ola (author)
  • Perturbation-based Control of Industrial Fed-batch Bioprocesses
  • 2015
  • Doctoral thesis (other academic/artistic)abstract
    • The topic of this thesis is bioprocess control, more specifically control of industrial-scale microbial fed-batch bioprocesses. Its focus is therefore on methods which are easy to implement in an industrial setting, which gives certain limitations on sensors, actuators and control systems. The main part of the work in the thesis concerns control of the microbial substrate uptake rate by manipulation of the feed rate of liquid substrate to the bioprocess. This is an important parameter for improving process yields, as too low feed rates cause starvation of the microorganisms while too high rates lead to production of undesirable by-products. By-product formation decreases metabolic efficiency and the by-products have inhibiting effects on microbial growth and production. At high concentrations these can even halt growth completely, leading to process failure. Due to large batch-to-batch variations and the complexity of the pro- cesses, model-based control can be difficult to use in this type of system. The approach used in this thesis circumvents this problem by utilizing perturbations in the feed rate. It has previously been shown that the metabolic state with regard to substrate uptake rate can be determined by analysing the perturbation response in the dissolved oxygen level of a microbial process. In this thesis, the concept is developed through the use of perturbations at a predefined frequency. This provides a number of advantages and allows for estimation of the metabolic state through observing the perturbation frequency in the measured signal. The concept has been tested experimentally in industrial pilot and pro- duction scale. It has been demonstrated that a controller based on this concept can be used to compensate for batch-to-batch variations in feed demand and can rapidly compensate for changes in the demand. It has also been shown that the method can be used for monitoring and control in bioprocesses with a volume over 100 m3, using a low-complexity estimation algorithm suited for industrial use. The thesis also concerns mid-ranging control in non-stationary processes. A modified mid-ranging controller suited for such processes is proposed, which allows control signals to increase in unison during the course of a fed-batch process while maintaining the advantages of classical mid-ranging control. The concept can for instance be used for control of dissolved oxygen, an important process parameter in many bioprocesses. It has been successfully used for this purpose in pilot scale alongside the type of perturbationbased feed rate controller which is the main topic of this thesis, also showing how the latter can be used in conjunction with other control systems.
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42.
  • Kjaer, Martin Ansbjerg (author)
  • Disturbance Rejection and Control in Web Servers
  • 2009
  • Doctoral thesis (other academic/artistic)abstract
    • An important factor for a user of web sites on the Internet is the duration of time between the request of a web page until an answer has been returned. If this response time is too long, the user is likely to abandon the web site and search for other providers of the service. To avoid this loss of users, it is important for the web site operator to assure that users are treated sufficiently fast. On the other hand, it is also important to minimize the effort to optimize profit. As these objectives often are contradictory, an acceptable target response-time that can be formulated. The resources are allocated in a manner that ensures that long response times do not occur, while, at the same time, using as little resources as possible to not overprovision. The work presented in this doctoral thesis takes a control-theoretic perspective to solve this problem. The resources are considered as the control input, and the response time as the main output. Several disturbances affect the system, such as the arrival rate of requests to the web site. A testbed was designed to allow repeatable experiments with different controller implementations. A server was instrumented with sensors and actuators to handle requests from 12 client computers with capability for changing work loads. On the theoretical side, a model of a web server is presented in this thesis. It explicitly models a specific sensor implementation where buffering occurs in the computer prior to the sensor. As a result, the measurement of the arrival rate becomes state dependent under high load. This property turns out to have some undesirable effects on the controlled system. The model was capable of predicting the behavior of the testbed quite well. Based on the presented model, analysis shows that feed-forward controllers suggested in the literature can lead to instability under certain circumstances at high load. This has not been reported earlier, but is in this doctoral thesis demonstrated by both simulations and experiments. The analysis explains why and when the instability arises. In the attempt to predict future response-times this thesis also presents a feedback based prediction scheme. Comparisons between earlier predictions to the real response-times are used to correct a model based response time prediction. The prediction scheme is applied to a controller to compensate for disturbances before the effect propagates to the response time. The method improves the transient response in the case of sudden changes in the arrival rate of requests. This doctoral thesis also presents work on a control solution for reserving CPU capacity for a given process or a given group of processes on a computer system. The method uses only existing operating-system infrastructure, and achieves the desired CPU capacity in a soft real-time manner.
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43.
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44.
  • Larsson, Per-Ola (author)
  • Optimization of Low-Level Controllers and High-Level Polymer Grade Changes
  • 2011
  • Doctoral thesis (other academic/artistic)abstract
    • Two design problems at different levels in the control hierarchy are considered; optimization of robust low-level controllers with constrained control signal activity and optimization of economical high-level polyethylene grade changes. As for the first design problem, a constraint on control signal activity due to measurement noise is presented and used when optimizing and comparing PI/PID controllers with measurement filters of different orders. The results show increased performance when roll-off is present in the feedback loop and that similarities exist between PID and high-order Youla-parametrized controllers. Robustness margins separating the dead-time uncertainty from other process uncertainties are presented. Methods to compute the margins, posed as optimization problems based on Nyquist diagram interpretations, are given. PID and predictive PI (PPI) controllers with measurement filters are optimized and compared using the presented control signal activity constraint and robustness margins. The two controllers show similar performance on industrially representative processes, with a few exceptions where the PID controller outperforms the PPI controller. Concerning the second design problem mentioned above, a cost function for optimization of economical polyethylene grade changes is proposed. It considers inflow costs, on- and off-grade polymer production revenues and polymer quality variable intervals to define on-grade production as well as economical incentives for on-target production. Using the JModelica.org platform, several stationary operating points and dynamic grade changes are optimized with regards to economy. The optimizations are based on Modelica models of both a gas-phase polyethylene reactor and the polyethylene plant PE3 at Borealis AB. The results show that economically optimal grade changes can be divided into three phases with distinguishing features, and that the synchronization of control flows and the usage of recycle area off-gas flows are important. A Modelica library for the plant PE3 at Borealis AB, including three reactors and three distillation columns, is presented.
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45.
  • Lincoln, Bo (author)
  • Dynamic Programming and Time-Varying Delay Systems
  • 2003
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis is divided into two separate parts. The first part is about Dynamic Programming for non-trivial optimal control problems. The second part introduces some useful tools for analysis of stability and performance of systems with time-varying delays. The two papers presented in the first part attacks optimal control problems with finite but rapidly increasing search space. In the first paper we try it reduce the complexity of the optimization by exploiting the structure of a certain problem. The result, if found, is an optimal solution. The second paper introduces a new general approach of relaxing the optimality constraint. The main contribution of the paper is an extension of the Bellman equality to a double inequality. This inequality is a sufficient condition for a suboptimal solution to be within a certain distance to the optimal solution. The main approach of solving the inequality in the paper is value iteration, which is shown to work well in many different applications. In the second part of the thesis, two analysis methods for systems with time-varying delays are presented in two papers. The first paper presents a set of simple graphical stability (and performance) criteria when the delays are bounded but otherwise unknown. All that is needed to verify stability is a Bode diagram of the closed loop system. For more exact computations, the last paper presents a toolbox for Matlab called Jitterbug. It calculates quadratic costs and power spectral densities of interconnected continuous-time and discrete-time linear systems. The main contribution of the toolbox is to make well known theory easily applicable for analysis of real-time systems.
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46.
  • Lindholm, Anna (author)
  • Hierarchical Scheduling and Utility Disturbance Management in the Process Industry
  • 2013
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis deals with control of production at large-scale process industrial sites in the presence of disturbances. The main focus is on disturbances in the supply of utilities such as steam, cooling water and electricity. A general method for reducing the revenue loss due to disturbances in utilities is introduced, which may provide both proactive and reactive disturbance management strategies. Utility availability and area availability are introduced as performance indicators. These measures are used to obtain quick estimations of the revenue losses related to each utility. To obtain reactive strategies for utility disturbance management, a simple model of how utilities affect production in an area, and how utilities are shared between areas, is introduced. The modeling approach is utilized to formulate the production control problem at disturbances in utilities as an optimization problem. Measurement data are used to obtain empirical models of utility disturbances at an industrial site, which may be used as input to the optimization. Since production control closely relates to production scheduling, the integration of production scheduling based on orders and forecasts with production control at disturbances in utilities is studied in the final part of the thesis.
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47.
  • Madjidian, Daria (author)
  • Low-Rank Distributed Control with Application to Wind Energy
  • 2014
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis addresses three different topics in wind power plant operation. Most of the research is focused on controlling a wind farm that is required to meet a power set-point. In this mode of operation, the wind turbines are able to vary their power production as long as the total power demand is met. The research investigates how this freedom can be used in order to reduce the fatigue loads experienced by the wind turbines. The problem is studied in a linear-quadratic control setting where the objective is to minimize the total fatigue load experienced by the turbines, while satisfying a requirement on their joint power production. It is shown that, under certain assumptions, the design problem can be drastically simplified. In particular, the computational effort needed to obtain the solution is independent of the number of wind turbines and the only centralized operation required to implement the optimal control law is a single summation. The research also explains the mechanisms that make power allocation schemes useful for load reduction. Part of the research addresses wake effects in wind farms by developing a low-complexity model of the aerodynamic interaction between wind turbines. The model is used in a series of examples, where the wind turbines coordinate their power productions in order to maximize the power production of the wind farm. The examples indicate that the benefit of power coordination increases with the number of turbines in the wind farm. They also identify the underlying mechanisms behind this effect. The last topic of the thesis is to investigate the benefits of using preview of the incoming wind speed in order to reduce structural loads on the wind turbine tower. The main focus is to understand how measurement distortion influences the achievable load reduction as well as the required length of preview. Results from high-fidelity simulations based on real wind turbine measurements indicate that the use of preview can lead to a significant reduction of tower fatigue loads and that the length of preview needed to attain the reduction does not exceed a few seconds.
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48.
  • Malmborg, Jörgen (author)
  • Analysis and Design of Hybrid Control Systems
  • 1998
  • Doctoral thesis (other academic/artistic)abstract
    • Different aspects of hybrid control systems are treated: analysis, simulation, design and implementation. A systematic methodology using extended Lyapunov theory for design of hybrid systems is developed. The methodology is based on conventional control designs in separate regions together with a switching strategy. Dynamics are not well defined if the control design methods lead to fast mode switching. The dynamics depend on the salient features of the implementation of the mode switches. A theorem for the stability of second order switching together with the resulting dynamics is derived. The dynamics on an intersection of two sliding sets are defined for two relays working on different time scales. The current simulation packages have problems modeling and simulating hybrid systems. It is shown how fast mode switches can be found before or during simulation. The necessary analysis work is a very small overhead for a modern simulation tool. To get some experience from practical problems with hybrid control the switching strategy is implemented in two different software environments. In one of them a time-optimal controller is added to an existing PID controller on a commercial control system. Successful experiments with this hybrid controller shows the practical use of the method.
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49.
  • Mårtensson, Karl (author)
  • Gradient Methods for Large-Scale and Distributed Linear Quadratic Control
  • 2012
  • Doctoral thesis (other academic/artistic)abstract
    • This thesis considers methods for synthesis of linear quadratic controllers for large-scale, interconnected systems. Conventional methods that solve the linear quadratic control problem are only applicable to systems with moderate size, due to the rapid increase in both computational time and memory requirements as the system size increases. The methods presented in this thesis show a much slower increase in these requirements when faced with system matrices with a sparse structure. Hence, they are useful for control design for systems of large order, since they usually have sparse systems matrices. An equally important feature of the methods is that the controllers are restricted to have a distributed nature, meaning that they respect a potential interconnection structure of the system. The controllers considered in the thesis have the same structure as the centralized LQG solution, that is, they are consisting of a state predictor and feedback from the estimated states. Strategies for determining the feedback matrix and predictor matrix separately, are suggested. The strategies use gradient directions of the cost function to iteratively approach a locally optimal solution in either problem. A scheme to determine bounds on the degree of suboptimality of the partial solution in every iteration, is presented. It is also shown that these bounds can be combined to give a bound on the degree of suboptimality of the full output feedback controller. Another method that treats the synthesis of the feedback matrix and predictor matrix simultaneously is also presented. The functionality of the developed methods is illustrated by an application, where the methods are used to compute controllers for a large deformable mirror, found in a telescope to compensate for atmospheric disturbances. The model of the mirror is obtained by discretizing a partial differential equation. This gives a linear, sparse representation of the mirror with a very large state space, which is suitable for the methods presented in the thesis. The performance of the controllers is evaluated using performance measures from the adaptive optics community.
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50.
  • Möllerstedt, Erik (author)
  • Dynamic Analysis of Harmonics in Electrical Systems
  • 2000
  • Doctoral thesis (other academic/artistic)abstract
    • Frequency domain analysis and design of power systems is complicated in the presence of harmonics, switching dynamics, nonlinearities, unbalances, and for systems with mixed ac/dc dynamics. The reason is that linearization of the system does not lead to a time invariant system, but a system with periodically time varying dynamics, which implies that there is coupling between different frequencies. Often one has to rely on simplifying assumptions and simulation. The thesis uses linear time periodic (LTP) models to analyze power systems. The harmonic transfer function (HTF) for LTP systems is introduced. Using the HTF, the system can be treated as an infinitely dimensional linear time invariant system, which means that the system, under certain convergence conditions, can be analyzed using the well developed theory for LTI systems. The thesis contains four papers with power system applications. Paper I describes the modeling and analysis of networks including components with switching dynamics, such as diodes and thyristors. An algorithm for parameter estimation from experimental data is presented. Papers II and III treats modeling and analysis of single-phase railway systems. The modeling of the locomotives is performed in collaboration with industry. Paper IV treats analysis and control aspects of a converter for grid connection of a micro-turbine used for distributed power generation. This is a three-phase application done in collaboration with the industry.
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