Synthesis of current mode building blocks for fuzzy logic control circuits

Abstract

Since the introduction of the first digital-based fuzzy logic controller chip, efficient hardware design of fuzzy logic control systems (FLCS) has drawn substantial attention. The interest in fuzzy logic hardware systems is motivated by the desperate need to provide fast fuzzy logic-based systems capable of operating in real time process control conditions. Fuzzy logic hardware implementation problems can be classified into two basic categories: the digital approach and the analog approach. The digital approach was originated by Togai and Watanabe who developed the first digital fuzzy logic-based inference engine chip. However, the digital approach to the fuzzy logic hardware implementation seems to be less efficient in contrast to analog, especially when the complexity and speed of a control problem are crucial. The transformation of a real-world fuzzy data into a binary format requires tremendous processing power that must be provided in the real time. Therefore, the analog solution offers a sufficient strategy to solve fuzzy logic control problems in dedicated hardware. Recently, successful implementation of current mode fuzzy logic controller developed in CMOS technology was reported. This approach starts with the theoretic framework for fuzzy set operations that leads to the coherent representation of the fuzzy inference operations, including fuzzification and defuzzification. The proposed approach, utilizing the control strategy proposed by Mamdani, concentrates on the algebraic correctness and elegance. It is algebraically effective but it lacks the current mode circuit implementation insight. This drawback motivated the presented research. This paper presents a graph-oriented approach to the synthesis of fuzzy logic building blocks. The framework for synthesis of current-mode fuzzy logic circuits is derived and the graph representations of basic building blocks are developed. These blocks comprise the bounded difference circuit, the absolute difference circuit, the minimum circuit, the maximum circuit, and the membership function circuit. Thereafter, the circuit implementations are analyzed and the circuit implementation issues related to the accuracy of the fuzzy operations are discussed. >