Fuzzy PWM-PID control of cocontracting antagonistic shape memory alloy muscle pairs in an artificial finger

Abstract

This paper presents biomimetic control of an anthropomorphic artificial finger actuated by three antagonistic shape memory alloy (SMA) muscle pairs that are each configured in a dual spring-biased configuration. This actuation system forms the basis for biomimetic tendon-driven flexion/extension of the metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints of the artificial finger, as well as the abduction/adduction of its MCP joint. This work focuses on the design and experimental verification of a new fuzzy pulse-width-modulated proportional-integral-derivative (i.e. fuzzy PWM-PID) controller that is capable of realizing cocontraction of the SMA muscle pairs, as well as online tuning of the PID gains to deal with system nonlinearities and parameter uncertainties. The main contribution of this paper is the proposed biomimetic cocontraction control strategy, which co-activates the antagonistic muscle pairs as a synergistic functional unit. It emulates a similar strategy in neural control, called “common drive,” employed by the central nervous system (CNS). In order to maintain a desired position of a joint, the corresponding agonistic muscle pairs are cocontracted by the CNS and stiffen the joint. The synthesis and parametric analysis of the proposed controller are carried out via numerical simulations using a dynamic model of the system. The performance advantage of the cocontracting fuzzy PWM-PID controller over the original PWM-PID controller is shown by both numerical and experimental results. A successful application of the new controller to fingertip trajectory tracking tasks using the MCP joint’s flexion/extension and abduction/adduction is also described.