K. T. O'Toole, M. M. McGrath, E. Coyle, Dublin Institute of Technology, Dublin, Ireland
It is widely acknowledged within the biomedical engineering community that Shape Memory Alloys (SMA’s) exhibit great potential for application in the actuation of upper limb prosthesis designs. These lightweight actuators, owing to excellent power-to-weight ratios and noiseless operation, are particularly suitable for prosthetic hand solutions. A four-fingered, twelve degree-of-freedom prosthetic hand has been developed by this group featuring SMA bundle actuators embedded within the palmar area. Joule heating of the SMA bundle actuators will facilitate the generation of sufficient torque at the fingers to allow a wide range of basic everyday tasks to be carried out. Transient characterisation of these actuator bundles has shown that the heating and cooling behaviours differ substantially. Natural convection will be insufficient to provide for adequate cooling during the elongation of the wire actuators in this embedded design.
An experimental test-bed has been developed so that the heat transfer characteristics of the appropriately sized SMA bundle actuators for use within the prosthetic hand design can be established. Various modes of heat sinking will be evaluated so that the most effective wire-cooling solution can be established. SMA bundles of varying size are used to achieve a generalised empirical mathematical model of heat transfer within SMA bundles under natural and forced cooling conditions. The optimum cooling solution will be implemented onto the mechanical hand framework in future work.
The wire bundles, whilst exposed to constant loading, will also be energised on a cyclical basis with a view to enhancing the dynamic performance (through forced cooling) of the actuators. Ultimately, these results coupled with phenomenological models of SMA behaviour, will be used in the development of an effective control strategy for this application in future work.
Summary: A novel four fingered hand prosthesis featuring Shape Memory Alloy (SMA) bundle actuators embedded within the palmar area of the hand has been developed. Analysis of the heat transfer characteristics of appropriately sized SMA bundles has been carried out to assess and optimise their dynamic performance during the heating and cooling cycle. Transient characterisation showed that natural convection is insufficient to provide for adequate cooling rates for a prosthetic application. Various modes of heat sinking have been evaluated to find the most effective wire-cooling solution for use within the hand prosthesis. A generalised empirical mathematical model of heat transfer in SMA bundles under natural and forced cooling conditions has been developed. The optimum cooling solution will be implemented onto the mechanical hand framework in future work.
