ENHANCED VARIABLE STRUCTURE CONTROL FOR NONLINEAR SYSTEMS

Abstract

Variable structure control (VSC) with sliding mode is a special type of control technique that is capable of making a control system very robust with respect to system parameters variations and external disturbances. In addition, the technique provides an easy way to design the control law for plant, linear or nonlinear. The fact that VSC did not receive wide acceptance among engineering professionals prior 1970's is probably due to the lack of design procedure and the existence of appreciable chattering in VSC systems. Important contributions have been made in VSC systems having linear plants, However, for nonlinear multi - input systems are problems remaining to be explored. They include characterization of possible switching schemes, simplification of design procedures, control of the performance of the reaching mode, reduction of chattering. Chattering occurs when the control-input switch discontinuously across the boundary, and it is undesirable because it involves high control activity and may excite high frequency dynamics. To eliminate chattering, various methods are used such as: sliding-mode controller with boundary layer (SMCB), fuzzy sliding­ mode controller (FSMC), sliding-mode controller (SMC) with sliding sector have been proposed and others. In all these works the basic idea is just to smooth the control action across the sliding surface while preserving the traditional variable­ structure control law. In this thesis a method called chattering-free and fast-response sliding-mode controller with a smooth control law is proposed, the proposed controller takes the distance of the system state from the sliding surfuce into account, the proposed SMCS uses a unified continuously varying term. As a result, not only is chattering completely eliminated, but system response, robustness, adaptability and steady­ state performance are all improved as well by choosing the new term appropriately. Compared with other sliding-mode control strategies, the proposed SMCS is believed to be one of the most practical methods ever proposed.