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.

A new approach to the design and control of shape memory alloy (SMA) actuators is presented. SMA wires are divided into many segments and their thermal states are controlled individually in a binary manner. The Peltier effect is used for heating and cooling individual segments of the SMA. Unlike the traditional way of controlling the wire length by driving a current to the entire SMA wire, the new method controls the binary state (hot or cold) state of each segment. The total displacement is then proportional to the number of the segments having the heated state, i.e. austenite phase. This architecture has three salient features, which would overcome fundamental difficulties of SMA. 1) Although the inherent property of SMA is highly nonlinear and uncertain with a prominent hysteresis, the binary state/phase control does not depend on the complexity of SMA state transition. 2) With use of the Peltier effect thermoelectric devices the response of SMA becomes more controllable, stable, and more accurate compared to the traditional air cooling and electric wire heating. 3) By operating at the heated state, SMA shows considerable load disturbance rejection compared to traditional methods. First, the basic principle and architecture of the segmented SMA actuator system are described. Initial implementation and feasibility tests are then presented, followed by discussion of the experimental results.