| 1. |
- Sandberg, Henrik
(author)
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Model Reduction for Linear Time-Varying Systems
- 2004
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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)
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Dryer Section Control in Paper Machines During Web Breaks
- 2004
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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. |
- Gäfvert, Magnus, et al.
(author)
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Proposal for a distributed computer control system in heavy-duty trucks
- 2000
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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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| 4. |
- Johnsson, Charlotta
(author)
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Whitepaper: S88 for Beginners
- 2004
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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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| 5. |
- Johnsson, Charlotta
(author)
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Whitepaper: S88 for Universities
- 2004
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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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| 6. |
- Bengtsson, Johan
(author)
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Closed-Loop Control of HCCI Engine Dynamics
- 2004
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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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| 7. |
- Cervin, Anton
(author)
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Integrated Control and Real-Time Scheduling
- 2003
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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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| 8. |
- Eborn, Jonas
(author)
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On Model Libraries for Thermo-hydraulic Applications
- 2001
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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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| 9. |
- Grundelius, Mattias
(author)
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Methods for Control of Liquid Slosh
- 2001
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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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| 10. |
- Gäfvert, Magnus
(author)
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Topics in Modeling, Control, and Implementation in Automotive Systems
- 2003
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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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