A new control system for fast motion control of SMA actuator wires

This paper describes a new motion control system for actuators based on antagonistic-pair arrangements of electrically-heated SMA elements. Two different controllers have been tested, both of them incorporating a new rapid heating strategy that greatly increases the rate at which the elements can be heated without risk of overheating. Essentially, the current applied to the element is the lesser of two quantities: a desired heating current and a maximum safe current. The former is computed by a motion control law, and is a function of the position error. The latter is a function of the measured SMA resistance and predetermined threshold values. This strategy relies on the martensite and austenite phases having different resistivities, so that a range of resistance values can be identified for individual SMA elements which imply that these elements are strictly below their maximum operating temperature. Whenever the measured resistance lies within this range, a large heating current may be applied. If the resistance lies outside this range then the heating current must be cut back to a value that will not overheat the SMA. Experimental results are presented to confirm that this strategy can greatly increase the velocity of controlled motion without changing the cooling regime.