Feedback Control Strategies for Shape Memory Alloy Actuators

Shape memory alloy (SMA) has been considered as an actuator for applications that require large force and displacement. Two factors have hampered the usefulness of such actuators: a hysteretic input-output relation and bandwidth limitations. This paper considers the hysteresis phenomenon from a control point of view. Instead of directly compensating for the hysteresis, which requires an accurate model, we use a closed loop approach which considers the feedback control of the beam strain. A simple lumped temperature and SMA force/displacement model is used for stability analysis. We show that with the SMA wire fixed between rigid surfaces, a proportional force feedback would render the closed loop stable. However, when SMA is coupled to a flexible structure, the resulting system can exhibit instability. We then show that a proportional position feedback is stabilizing, but there would be steady-state error. An adaptation scheme can be further added to remove the steady-state error. The analysis is verified experimentally in a simple experimental setup consisting of a flexible aluminum beam and a Nitinol shape memory alloy wire that applies a bending force to the end of the beam.