Some issues about fuzzy logic control

Some issues about fuzzy logic control are discussed in this paper. Fuzzy logic deals with ambiguous events and situations. However, it does not mean that all events and situations are totally “fuzzy” and disorderly. The ideas about fuzzy logic should not be fuzzy. In this paper, several definitions which have not yet appeared in the literature are presented. A class of fuzzy numbers which the authors call “semi-closed, continuous, totally ordered, and well-defined fuzzy numbers” has very nice orderliness properties. Using this class of fuzzy numbers, it can be show that the fuzzy logic controller based on the filtered tracking error works like a variable structure controller. It also shows that the conventional PD or PI or PID controller is nothing but a special case of fuzzy logic control if the filtered tracking error is used as the feedback signals. Using this design technique, it is possible and easy to design a variable-gain PD- or PID-like controller. This design technique has been successfully used in control of a flexible-link robotic system