Improving the Speed of Shape Memory Alloy Actuators by Faster Electrical Heating

Abstract

This paper describes a method for improving the speed of actuators based on shape memory alloys (SMA) by increasing the rate at which an SMA element can safely be heated. The method consists of measuring the electrical resistance of an SMA element, calculating a maximum safe heating current as a function of measured resistance, and ensuring that the actual heating current does not exceed this maximum value. In effect, resistance is being used as a form of temperature measurement, and the maximum safe heating current is designed to prevent overheating. This method has been incorporated into a two-stage relay controller that controls the motion of a pantograph robot actuated by two antagonistic pairs of SMA wires. Experimental results show a substantial increase in the maximum velocity attainable by this robot, without any change in the cooling regime, purely as a result of faster heating.