Machine tools for small work pieces are characterized by an extensive disproportion between workspace and cross section. This is mainly caused by limitations in the miniaturization of drives and guidance elements, since their physical working principle necessitates certain minimum sizes. Due to their high specific workloads and relatively small spatial requirements, Thermal Shape-Memory-Alloys (SMA) possesses an outstanding potential to serve as miniaturized positioning devices in small machines. Antagonistically arranged SMA actuators are especially feasible to fulfill these requirements.
This paper describes an adaptive closed loop control concept for actuators based on spring loaded or antagonistic arrangements of electrically-heated SMA elements. Due to their nonlinear stress-strain behavior such actuators are characterized by strain dependent load conditions at the activated SMA element. Consequently the actuator dynamic depends on its position. Hence an adaptive closed loop control concept to ensure a constant actuator dynamic over the entire stroke has to be developed. The approach is based on the determination of the transient transfer dynamics of the SMA Element. Two possible strategies are investigated and evaluated. Numerical models of both SMA wire arrangements are used to develop the adaptive control theoretically. An SMA wire test bench is designed to investigate the proof of the adaptive approach experimentally. Measurements of a conventional PI control are further compared to the achieved results of the new concept.