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

Shape Memory Alloys (SMA’s) are part of a new generation of lightweight, strong and relatively cheap actuators which have the potential to revolutionize the field of biomedical engineering. At present, DC motors are the most widely applied actuators employed in the design of prosthetic devices. These mechanical systems place restrictions on the number of degrees of freedom possible. SMA wires have the property of shortening (by up to 5%) [1] when heated and thus have the ability to apply forces. This is known as the Shape Memory Effect (SME) [2]. This phenomenon occurs when the wire is heated above a certain temperature, where its crystalline structure changes from a relatively soft martensitic state to a relatively hard austenitic state [3]. Heating can be achieved by applying a voltage drop across the wire and thus causing a current to flow, resulting in a resistive heating effect known as joule heating. One of the major issues with SMA behaviour is the substantial hysteresis which occurs during any cyclic heating/cooling stage. Work has been undertaken, with some success, by various groups [4, 5] attempting to compensate for the deviations caused by the slower cooling stage when used as dynamic actuators. Recently SMA’s have been applied in robotics actuation as they exhibit muscle-like properties [6-9]. SMA’s, owing to their small size and excellent force to weight ratios, provide an opportunity to facilitate additional degrees of freedom to prosthetic designs. This work focuses on characterizing SMA’s for use as actuators of individual phalanges in hand prosthesis. Characterisation of the human hand was carried out initially in order to establish the functionality requirements for comparative purposes. Knowledge of the range of motions of the fingers, as well as the forces required for everyday tasks, is viewed as critical if a prosthetic hand, whose performance matches as closely as is possible to that of a working limb, is to be developed. A typical 150 μm diameter SMA wire, while having a high force to weight ratio, still only produces a maximum force of 3.24 N [1]. Research indicated that forces substantially higher than this would be required for actuation of the fingers. As a result of this, bundling of the wires was investigated. This relatively new technique has been previously investigated by various groups. Moseley et al [10] demonstrated that large forces can be achieved without sacrificing actuator bandwidth. DeLaurentis et al [11] experimented with wire bundles of varying diameter with a view to establishing the optimal arrangement for attaining maximum forces. Characterisation of the transient and steady-state contraction and relaxation of Nitinol wires has been carried out by others [12]. Our work compares the transient and steady-state characteristics of SMA wire bundles with the force and displacement characteristics of the human hand. This comparison will provide the foundation for the development of a prosthetic hand with the required capabilities.