Experimental Study of a Shape Memory Alloy Actuation System for a Novel Prosthetic Hand

Recently, the development of compact, light-weight and powerful actuation systems has been in the centre of investigation at many scientific institutions and research groups all over the world. These systems can be used in devices of almost every aspect of modern life and based on their inherent technology they come with certain benefits and costs. One of the most demanding applications field in terms of actuator selection and design is the field of upper-extremity prosthetics. Modern commercial advanced hand and arm prostheses are conventionally actuated by electric servomotors. Although these motors achieve reasonable kinematic performance, they have been proven insufficient in meeting amputees’ demands, mainly due to their noisy operation and limited energy density which leads to the use of bulky and heavy driving systems (Herr, 2003). Therefore, an alternative nonconventional actuation technology is requisite in order to overcome these limitations which make a substantial proportion of upper-limb amputees avoiding the use of their prostheses. One of the most promising actuation technologies is based on Shape Memory Alloys (SMA) and phenomena related to change of their atomic structure. SMA are metallic alloys that can exhibit an actuation mechanism resembling the biological muscle they contract producing actuation forces. These muscle-like actuators present high power to weight ratio enabling the development of compact, lightweight prosthetic devices without too much compromising power capabilities and eliminating the forced-tradeoff between dexterity and anthropomorphic size, weight and appearance (Bundhoo, 2009). Additional benefits include an inherent position feedback method (given a near linear relationship between ohmic resistance and contraction), silent, smooth and life-like operation, and the lack of requirement for force or motion transmission devices (Kyberd et al., 2001). During the last two decades SMA have been studied and reviewed as possible actuation technology in prosthetics by many researchers but efficiency and response time are claimed as the most limiting factors (Del Cura et al., 2003). So, in order to render this material appropriate for application in upper-limb prostheses, these impediments must be overcome. Towards this scope, an innovative SMA actuation system for a newly developed prosthetic hand is constructed and studied. The technology applied in this hand offers a series of improvements when compared to current commercial prosthetic devices. Its design 5