Linguistic fuzzy modelling of an asynchronous motor

Sliding mode control and fuzzy logic control have been combined together in order to optimize the performance given by conventional sliding mode control; especially against the effect causes by the chattering phenomenon. One approach taken by many researchers is to use fuzzy logic to smooth the chattering in sliding mode control. In this paper, a novel direction to design the fuzzy sliding controllers is proposed. In fact, the control design is based on the fuzzy model of an induction motor using the approach of Ben-Ghalia. The aim of the proposed modelling approach is to provide a fuzzy set based representation of the cascade sliding mode control of an induction motor fed by PWM voltage source inverter, which operates in a fixed reference frame. For this purpose, a new decoupled and reduced model is first proposed. Then, a set of simple surfaces and associated control laws are synthesised. A piecewise smooth control function with a threshold is adopted. However, the magnitude of this function depends closely on the upper bound of uncertainties, which include parameter variations and external disturbances. This bound is difficult to obtain prior to motor operation. To solve this problem, a fuzzy modelling approach is presented to improve the design and tuning of a fuzzy logic controller using variable structure control theory. The fuzzy controller, is designed in order to improve the control performances and to reduce the control energy and the chattering phenomenon. Simulation results reveal some very interesting features.