Dynamic stress modeling of a shape memory alloy spring based on phase diagram analysis

Abstract

Soft actuators have recently been received attention as actuators of wearable robots. In particular, spring-type shape memory alloy actuators have the advantage of not only having high work density but also being able to take large deformation. However, control is still difficult. In order to make control, model-based control needs to be proposed. Using that, force control which can be made available for rehabilitation training is possible. As a first step, a model for the relationship between temperature and force at a fixed length is proposed. It was intended to interpret the phenomenon of increasing force from the low temperature, the change in force at the wide temperature interval, and the decrease in force at the same time as cooling began that differed from the existing model. To this end, we propose a model that reflects the geometry of the springs, the ordering of the critical stresses, and the non-parallel phase diagram. As a result, we confirm that the more the proposed model is used, the more similar results to the experimental results. In order to use this as a model for the actuator, simplification and integration with the heat transfer model were achieved.