Multi-Segment State Coordination for Reducing Latency Time of Shape Memory Alloy Actuator Systems

Abstract

This paper describes a new approach to the control of highly nonlinear shape memory alloy (SMA) actuator systems, in which SMA wires are divided into many segments and their thermal states are controlled individually as a group of finite state machines. Instead of driving a current to the entire SMA wire and controlling the wire length based on the analogue strain-temperature characteristics, the new method controls the discrete state (austenite or martensite) of individual segments and thereby controls the total displacement proportional to the number of the austenite segments. Although the inherent property of SMA is highly nonlinear and uncertain with a prominent hysteresis, this Hysteresis Loop Control is robust and stable, providing characteristics similar to a stepping motor. Furthermore, this method can apparently eliminate the latency time associated with phase transition of SMA actuators. Coordination of the multitude of segments having independent thermal states allows for fast response with zero latency time even for thick SMA wires. The new control method is implemented using the Peltier effect thermoelectric devices for selective segment-by-segment heating and cooling. Experiments demonstrate effectiveness of the proposed method, which exploits the inherent hysteresis and nonlinearity of SMA rather than compensating for the nonlinearity.