Model based position control of shape memory alloy actuators
Shape-memory-alloys (SMA) are easy to integrate into mechanical structures and capable of handling high specific workloads. Therefore, SMAs possess an outstanding potential to serve as positioning devices in various applications. However, due to the non-linear material behaviour it is often difficult to develop suitable controllers for SMA positioning applications. We present here the multi-domain modelling of an electrically heated SMA wire which includes changes of electrical parameters in conjunction to mechanical parameters. Due to the correlation between electrical resistance and mechanical stroke it was possible to implement a resistance-based position control without the necessity of an external positioning sensor. In order to design a linear position controller by common rules the highly complex and nonlinear model was simplified. Controller development yielded a PID algorithm that was implemented on a rapid prototyping system as part of an SMA wire test bench. However, a common problem of SMA materials is the shifting stroke-resistance-correlation which especially changes with increasing number of passed cycles. Accordingly, it is necessary to recalibrate the model after a specified operation time. Recalibration was performed by moving the actuator to defined positions where the resistance is measured. Subsequently, the stroke-resistance-correlation was determined by interpolation. The models accuracy was furthermore verified by various measurements with different wires and multiple loads. Based on these results, it was possible to design an actuator which utilizes a flexible socket instead of fixed mountings. This actuator is especially useful for applications with limited cross sections which, in a more general context, make it an ideal tool for applications in automotive, medical, and mechanical engineering.