Nonlinear Position Control of Antagonistic Shape Memory Alloy Actuators

This paper presents a control scheme for angular position control of a rotary load using shape memory alloy (SMA) actuators in an antagonistic configuration. Hysteresis and significant nonlinearities in the stress-strain-temperature characteristics hinder effective utilization of SMA actuator. By considering the nonlinear behavior and thermal characteristics of SMA, a force control scheme based on partial feedback linearization and sliding surfaces is developed in this paper to regulate the torque applied by a differential SMA actuator pair. A position controller is then used to generate a desired torque which serves as a reference input for the force tracking controller. The controller is based on sliding surfaces and does not depend on characteristics of SMA. Thus an approximate model of SMA characteristics is sufficient. Experimental studies indicate that the proposed controller performs well in terms of achieving small tracking errors.