Design of soft artificial muscle based on high passive restoring force shape memory alloy spring

Abstract

Shape Memory Alloys (SMAs) possess a high potential for effective utilization in wearable robot systems requiring lightweight solutions due to their ability to produce significant output while being lightweight. However, they have drawbacks, such as slow actuation speed and difficulty in control due to their non-linearity, as they operate based on temperature changes. Previous research has reported that the non-linearity with respect to temperature decreases when SMAs are fabricated in a spring form, but these studies still presented challenges in improving actuation speed. In this dissertation research, we propose a new SMA spring actuator that maximizes one of the features of SMA springs, which can generate restorative force without energy input, possessing linear characteristics between force and displacement at room temperature. The proposed SMA spring in this study is manufactured by applying excessive torsional stress during spring fabrication, so that stress remains internally even after heat treatment. This internal stress transforms the atomic arrangement within the SMA spring from twinned martensite to detwinned martensite without deformation, thus, it does not exhibit non-linearity caused by detwinning when stress is applied at room temperature. Furthermore, in this study, an artificial muscle that applies a refrigerant circulation system is designed to enhance the actuation speed of the SMA actuator. The SMA spring bundle is encased in an elastic tube, designed to allow refrigerant to flow around it, and an artificial muscle, optimized in actuation speed and energy through thermal-mechanical modeling, is designed. The designed artificial muscle has a stiffness of approximately 1125N/m at room temperature and approximately 1732N/m at 70 degrees temperature, and has a bandwidth of 0.5 Hz for maximum contraction and relaxation motions. In this dissertation research, a wearable mechanism that can assist the movements of the upper limb’s elbow, wrist, and forearm by applying the developed high-output artificial muscle is presented. The manufactured wearable mechanism was able to compensate for the gravity of the elbow and enhance the stiffness of the wrist/forearm without energy input, utilizing the characteristic of generating large output while having linear restorative force.