| A variable structure system is a control system that may contain more than one structure (sub-system) during operation. This thesis reports on our investigation and findings on techniques for analysing designing and controlling variable structure systems. We start with the simplest variable structure system: the sliding mode control system. In this system, the controller contains only two structures and the only way of changing them is by switching. A major drawback of this system is chattering, which is caused by a fast switching of the controller structure. An algorithm has been proposed and developed to eliminate this chattering.; Then we move on to the fuzzy logic control system which is a more general variable structure system. In this system, each fuzzy rule can be regarded as one structure, and the way of changing them can be smooth. Hence, the overall structure of the system can be very complicated so that the system performance and stability are difficult to be designed and analysed. To tackle these problems, we have developed a fuzzy logic controller (FLC) design approach for Takagi-Sugeno fuzzy model systems. Also, a stability analysis method has been developed for more general fuzzy logic control systems, which may or may not involve Takagi-Sugeno fuzzy models. The system stability can be proved by employing a common Lyapunov function for all sub-systems described by the fuzzy rules. To probe further, this stability analysis method has been applied to help the development of FLCs which combine conventional controllers. Examples on combining controllers like sliding mode controllers, state-feedback controllers and PI controllers have been studied and tested.; These developed variable structure techniques have been applied to many non-linear plants including inverted pendulum systems, mass spring damper systems and dc-dc power converters for improved dynamic performance and robustness to parameter uncertainties. In particular, a large-signal modelling and simulation method for switching dc-dc power converters has been developed. (Abstract shortened by UMI.)... |
