Transient characterisation and analysis of shape memory alloy wire bundles for the actuation of finger joints in prosthesis design

Abstract

Most current lower arm/hand prosthesis designs incorporate relatively bulky, heavy dc motors that produce substantial noise when performing actuation which renders
them uncomfortable for the end-user. The engineering challenge is to produce more effective powered upper limb prosthetic solutions. Identification, characterisation and testing of actuation methods with better force to weight ratios are essential pre-requisites for this. The main aim of this work is to carry out a comprehensive study to establish conclusively the feasibility of employing Shape Memory Alloys (SMA’s) in the actuation of prosthetic finger designs. A comprehensive review of existing literature has been undertaken in order to establish the maximum grip forces at each phalanx of the human hand under different loading conditions. An experiment was developed in conjunction with this review to estimate the time response of the hand during a gripping/releasing action. These results, in combination with physical dimensions, will be used to drive the design of a prosthetic limb.

A test rig has been developed which can facilitate complete transient and steady-state characterisation of a range of SMA wire diameters and bundle configurations. A
number of different configurations were tested, each configuration having a different combination of 150 μm and 300 μm diameter nitinol wires. A data acquisition system
was used to capture and retain data pertaining to the full characterisation of the bundles and in particular the strain and force capabilities of the various arrangements. A direct comparison is made between the actuator capabilities and the requirements of a working limb for basic gripping actions. This work will contribute to the development of an improved powered prosthetic solution.