Modelling and mechanical design of a flexible tube-guided SMA actuator

Shape memory alloy (SMA) wires are excellent candidates for wearable actuators since they are thin, low weight and have a high actuation force. The main drawbacks are that the wire should be kept straight and needs to be relatively long to enable a large enough actuation stroke. Embedding the SMA wire in a flexible tube largely enhances its applicability since then the counter forces are transferred by the tube material and the tube can be rolled up or attached to flexible surfaces or clothing layers. The performance of such tube-guided SMA actuators is, however, more complicated since it not only depends on the SMA behaviour but also on the tube materials and the actuator construction. In this research, a simple end-state model for a tube-guided SMA actuator system is proposed. We measure and model both the SMA and tube material properties, including tube creep effects, and derive an approximate prediction for the actuator stroke. Validation experiments showed that the predicted stroke during the second heating and cooling experiments agreed well with the measurements and that the average deviation is 9.6%, even though the deviation is much larger (27.3%) for the maximum applied force.