Enhancing the actuation frequency of shape memory alloy wire by vibration-enhanced cooling

Abstract

Shape memory alloy wire actuators are lightweight, compact, and have high actuation stress, but their actuation frequency is limited by the rate they can be thermally cycled. Heating time can be reduced with high current, but rapidly cooling the material without introducing disadvantages is a challenge. This study establishes the feasibility of a novel approach to more rapidly cool shape memory alloy wires based on inducing their first mode of vibration. To achieve this, a steel spring pin placed on the wire was driven by an electromagnet as the shape memory alloy cooled. Experiments were conducted with a shape memory alloy wire (250 mm length, 0.381 mm diameter, 70°C austenite finish temperature) that was thermally cycled against constant forces and against an extension spring. The heating and cooling times were measured for the vibrating and non-vibrating trials to determine the factor of actuation frequency increase, which ranged from 1.55 to 2.10. A model of cooling time was developed and compared with the experimental results, with percent errors ranging from 5.3% to 32.8%. The approach was demonstrated in an application: a shape memory alloy–driven rotary ratcheting mechanism, with which factors of actuation frequency increase greater than 1.80 were measured due to vibration-enhanced cooling.