This paper proposes a variable structure robust PI controller whose structure is continuously changed by fuzzy logic such that the controller makes the system respond quickly. If the error (e) is large or if its rate (e) is large the integrator gain is very large and vice versa in order to obtain a robust PI controller which is insensitive to both the plant noise and the observation noise. It is then practically applied to the speed control of an induction servomotor. Experiments show that the proposed controller is superior to both a sliding mode controller and a PID controller. The paper includes a stability analysis of the overall system and a design procedure using Popov's stability criterion.
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