<p>In industrial control systems PID-control remains the prevalent strategy, also for processes that would benefit from model based control. The purpose of this thesis is to evaluate whether model based control can be readily implemented in an industrial control system. To this end a simulated surge tank with a simulated industrial control system is studied. For evaluation two scenarios with specified objectives are selected.</p><p>Following a review of LQR and versions of MPC, Predictive Functional Control (PFC) is considered the most suitable for implementation. PFC is a form of MPC developed with industrial applications in mind and therefore has several advantages for implementation in an industrial control system. Controllers for the surge tank-system are developed and implemented in the control system.</p><p>Basic analysis of stability, sensitivity and robustness suggests that PFC has some advantages that might be important in a non-simulated implementation. Compared with PID-controllers adjusted for control performance, PFC does not show any notable improvements in performance.</p><p>In conclusion, it is possible to implement model based control in an industrial control system and with PFC the implementation is considered easy.</p>

Vision is a flexible, non-contact sensor that can be used for position feedback in closed-loop control of dynamic systems. Current vision systems for industrial automation provide low sample rates and large sample delays relative to other types of position sensors. Poor sample rates and sample delays are a result of the vast volume of data that must be collected and processed by the vision system. A predictive visual tracker can help compensate for some of the deficiencies of current industrial vision systems. The objectives of the present research are to demonstrate that vision is a useful feedback sensor and prediction can be used to improve performance by compensating for the feedback delay of the vision system. An inverted pendulum was stabilized using a vision sensor as feedback to a state-feedback controller. The vision data was run through a d-step ahead predictor to compensate for the vision system delays. The system was simulated in Mat lab and an actual physical system was used to test the performance of the control system. The inverted pendulum provides a good test-bed for studying predictive control using vision feedback. The pendulum will fall without the constant adjustment of the cart position. The adjustment of the cart by the controller is delayed because of latency and quantization errors in vision feedback. The better the controller is able to compensate for delays and quantization errors, the greater its ability to stabilize the inverted pendulum. / Graduation date: 1996

A new decentralized nonlinear controller for Robot Manipulators is presented in this thesis. Based on concepts of Lyapunov stability theory and some control ideas proposed in [3]-[7], we obtain continuous nonlinear decentralized control laws which guarantee position and velocity tracking to within an arbitrarily small error. Assumptions based on physical constraints of manipulators are made to guarantee the existence of the controller and asymptotic stability of the closed loop system. Simulations show how well this rather simple control scheme works on two of the links of the Puma 560 Manipulator. The main contribution of this thesis is that it extends the results of a class of complex centralized control algorithms to the decentralized robust control of interconnected nonlinear subsystems like robot manipulators. / Graduation date: 1992

This thesis investigates the control of motion for a general class of vehicles that rotate and translate in three-space, and are propelled by a thrust vector which has fixed direction in body frame. The thesis addresses the problems of path following and position control. For path following, a feedback linearization controller is presented that makes the vehicle follow an arbitrary closed curve while simultaneously allowing the designer to specify the velocity profile of the vehicle on the path and its heading. For position control, a two-stage approach is presented that decouples position control from attitude control, allowing for a modular design and yielding almost global asymptotic stability of any desired hovering equilibrium. The effectiveness of the proposed method is verified both in simulation and experimentally by means of a hardware-in-the-loop setup emulating a co-axial helicopter.

An adaptive decentralized nonlinear controller for a robot manipulator is presented in this thesis. Based on the adaptive control schemes designed by Seraji [18], Dai [30], and Jimenez [31], we redesigned and further simplified the control algorithm and, as a consequence, we achieved better path tracking performance. The proposed adaptive controller is made of a PD feedback controller which has time varying gains, a feedforward compensator based on the idea of inverse dynamics, and an auxiliary signal. Due to its adaptive structure, the controller shows robustness against disturbances and unmodeled dynamics. In order to ensure asymptotic tracking we select a Lyapunov function such that the controller forces the negative definiteness of the time derivative of such a Lyapunov function. To do this, the tracking position and velocity error are penalized and used as a part of the adaptive control gain. The main advantages of this scheme are the comparably faster convergence of tracking error, relatively simpler structure, and smoother control activity. This controller only requires the position and angular speed measurement, it does not require any knowledge about the mathematical model of the robot manipulator. Simulation shows the capacity of this controller and its robustness against disturbances. / Graduation date: 1993

En diverses tècniques de control robust és habitual treballar amb plantes incertes, el comportament de les quals es pot descriure raonablement segons incertesa paramètrica mitjançant models lineals intervalars. La QFT (Quantitative Feedback Theory) és una d’aquestes tècniques de control robust, que treballa bàsicament amb especificacions donades en el domini freqüencial. L’aportació d’aquest treball es centra en diversos aspectes relacionats amb la problemàtica del control de sistemes amb incertesa paramètrica, amb l’estructura estàndard de control a llaç tancat amb realimentació negativa, i amb la possible existència de pre-filtre, si hi cal. L’ús de l’aritmètica intervalar facilita la representació d’incerteses i a més permet la implementació de tècniques d’anàlisi i de disseny que poden ser utilitzades per al control robust de sistemes. Per evitar la problemàtica associada a la representació de conjunts i els problemes que comporta en quant a multi-incidència i wrapping, el mètode adoptat en aquesta tesis per representar el conjunts utilitza una boira de punts i les operacions es realitzen de forma determinista (graella).Un primer aspecte del treball, en el cas que s’hagin comprovat la compatibilitat d’especificacions i l’existència de controlador, s’orienta en la recerca de l’espai de paràmetres del controlador d’estructura pre-fixada i baix ordre, aprofitant eines d’aritmètica intervalar tals com el concepte de projecció d’un conjunt i la seva implementació en algorismes concrets, basats en els principis de satisfacció de restriccions.Una qüestió a tenir en compte en treballar amb especificacions freqüencials és la dificultat de trobar una equivalència directa entre especificacions temporals i freqüencials. La tesi també fa incidència en el cas particular d’un tipus d’especificació temporal de seguiment de trajectòria o tracking que, sota determinades hipòtesis, alguns autors proposen una equivalència freqüencial. En el present treball es fa una aportació de simplificació del procés de disseny, remarcable per què una nova especificació (de sensitivitat) substituiria l’original (de seguiment). Aleshores, en cas d’existir una especificació addicional pròpiament de sensitivitat, ambdues quedarien integrades en una única especificació de sensitivitat, simplificant-se així el procés de disseny. I a més, el mètode proposat dóna de forma immediata el disseny del pre-filtre.També en la present tesi es treballa en una metodologia per automatitzar el disseny de controladors. Es proposa un algorisme de disseny automàtic (Automatic Loop Shaping, ALS) en el marc de la QFT, en el que s’aplica un criteri de minimització energètica de l’energia impulsional del controlador. Per validar les metodologies proposades es mostren un parell de casos d’estudi. El primer cas és el d’un interferòmetre real, utilitzat per al posicionament d’uns segments de mirall en un telescopi, on la problemàtica bàsica és l’existència de pertorbacions mecàniques. El segon cas és el d’un model simplificat de l’angle de capcineig d’un helicòpter de laboratori, amb incertesa estructurada i paramètrica. En ambdós casos els resultats simulats i experimentals han estat molt satisfactoris. / Robust control techniques often work with uncertain plants whose behavior can be described according to linear models using parametric uncertainty intervals. Quantitative feedback theory (QFT) is one such technique that works mainly with frequency domain specifications. This thesis focuses on aspects related to the problem of control systems with parametric uncertainty, using the standard closed-loop control structure, negative feedback and a pre-filter, if required.Interval arithmetic is helpful when representing uncertainty and allows the use of analysis and design techniques for robust system control. With a view to avoiding the problems associated with the representation of sets, multi-incidence and wrapping, in this thesis a cloud of points is used to represent the sets and the operations are performed in a deterministic way (grid).Once the compatibility of specifications and the existence of a controller have been verified, research is conducted into the controller parameter space with a pre-fixed and low-order structure using interval arithmetic tools such as the projection of sets and their implementation in specific algorithms based on constraint satisfaction principles. The difficulty of finding a direct equivalence between temporal and frequency specifications must be taken into account when one is working with frequency specifications. This thesis emphasizes on the temporal specification of tracking, for which under certain hypotheses some authors propose a frequency specification equivalent. A method is proposed to simplify the design process by replacing the original specification (tracking) with a new one (sensitivity). If there is an additional sensitivity specification, both are integrated in a single one, thus simplifying the design process. The proposed method also gives the pre-filter design. This thesis also deals with a methodology for automating controller design. An automatic design algorithm (automatic loop shaping, or ALS) is proposed in the QFT framework, in which a criterion of energy minimization on the controller’s impulse response is applied. Two case studies are presented to validate the proposed methodologies. First one is a real interferometer that is used for positioning mirror segments in a telescope. The basic problem here is the existence of mechanical disturbances. The second case study is a simplified model of a laboratory helicopter’s pitch angle with structured parametric uncertainty. In both cases the simulated and experimental results were highly satisfactory.

In industrial control systems PID-control remains the prevalent strategy, also for processes that would benefit from model based control. The purpose of this thesis is to evaluate whether model based control can be readily implemented in an industrial control system. To this end a simulated surge tank with a simulated industrial control system is studied. For evaluation two scenarios with specified objectives are selected. Following a review of LQR and versions of MPC, Predictive Functional Control (PFC) is considered the most suitable for implementation. PFC is a form of MPC developed with industrial applications in mind and therefore has several advantages for implementation in an industrial control system. Controllers for the surge tank-system are developed and implemented in the control system. Basic analysis of stability, sensitivity and robustness suggests that PFC has some advantages that might be important in a non-simulated implementation. Compared with PID-controllers adjusted for control performance, PFC does not show any notable improvements in performance. In conclusion, it is possible to implement model based control in an industrial control system and with PFC the implementation is considered easy.

This work focuses on the robustness issues related to distributed model predictive control (DMPC) strategies in the presence of model uncertainty. The robustness of DMPC with respect to model uncertainty has been identified by researchers as a key factor in the successful application of DMPC. A first task towards the formulation of robust DMPC strategy was to propose a new systematic methodology for the selection of a control structure in the context of DMPC. The methodology is based on the trade-off between performance and simplicity of structure (e.g., a centralized versus decentralized structure) and is formulated as a multi-objective mixed-integer nonlinear program (MINLP). The multi-objective function is composed of the contribution of two indices: 1) closed-loop performance index computed as an upper bound on the variability of the closed-loop system due to the effect on the output error of either set-point or disturbance input, and 2) a connectivity index used as a measure of the simplicity of the control structure. The parametric uncertainty in the models of the process is also considered in the methodology and it is described by a polytopic representation whereby the actual process’s states are assumed to evolve within a polytope whose vertices are defined by linear models that can be obtained from either linearizing a nonlinear model or from their identification in the neighborhood of different operating conditions. The system’s closed-loop performance and stability are formulated as Linear Matrix Inequalities (LMI) problems so that efficient interior-point methods can be exploited. To solve the MINLP a multi-start approach is adopted in which many starting points are generated in an attempt to obtain global optima. The efficiency of the proposed methodology is shown through its application to benchmark simulation examples. The simulation results are consistent with the conclusions obtained from the analysis. The proposed methodology can be applied at the design stage to select the best control configuration in the presence of model errors. A second goal accomplished in this research was the development of a novel online algorithm for robust DMPC that explicitly accounts for parametric uncertainty in the model. This algorithm requires the decomposition of the entire system’s model into N subsystems and the solution of N convex corresponding optimization problems in parallel. The objective of this parallel optimizations is to minimize an upper bound on a robust performance objective by using a time-varying state-feedback controller for each subsystem. Model uncertainty is explicitly considered through the use of polytopic description of the model. The algorithm employs an LMI approach, in which the solutions are convex and obtained in polynomial time. An observer is designed and embedded within each controller to perform state estimations and the stability of the observer integrated with the controller is tested online via LMI conditions. An iterative design method is also proposed for computing the observer gain. This algorithm has many practical advantages, the first of which is the fact that it can be implemented in real-time control applications and thus has the benefit of enabling the use of a decentralized structure while maintaining overall stability and improving the performance of the system. It has been shown that the proposed algorithm can achieve the theoretical performance of centralized control. Furthermore, the proposed algorithm can be formulated using a variety of objectives, such as Nash equilibrium, involving interacting processing units with local objective functions or fully decentralized control in the case of communication failure. Such cases are commonly encountered in the process industry. Simulations examples are considered to illustrate the application of the proposed method. Finally, a third goal was the formulation of a new algorithm to improve the online computational efficiency of DMPC algorithms. The closed-loop dual-mode paradigm was employed in order to perform most of the heavy computations offline using convex optimization to enlarge invariant sets thus rendering the iterative online solution more efficient. The solution requires the satisfaction of only relatively simple constraints and the solution of problems each involving a small number of decision variables. The algorithm requires solving N convex LMI problems in parallel when cooperative scheme is implemented. The option of using Nash scheme formulation is also available for this algorithm. A relaxation method was incorporated with the algorithm to satisfy initial feasibility by introducing slack variables that converge to zero quickly after a small number of early iterations. Simulation case studies have illustrated the applicability of this approach and have demonstrated that significant improvement can be achieved with respect to computation times. Extensions of the current work in the future should address issues of communication loss, delays and actuator failure and their impact on the robustness of DMPC algorithms. In addition, integration of the proposed DMPC algorithms with other layers in automation hierarchy can be an interesting topic for future work.

No description available.

Power system frequency is one of the key performance indices of system operation. Abnormal frequency deviations would incur negative impacts to power equipments and service quality. Thus, it is important to operate and regulate the system frequency within an acceptable range. North American Electric Reliability Corporation (NERC) has been using a Control Performance Standard (CPS) for frequency control performance assessment since 1997 which uses system frequency and inter--area power flow to evaluate the power system control performance. This thesis presents a design of CPS for isolated system and the results of Taiwan Power Company frequency control performance based on the proposed CPS1 formulation.