A Total Hydraulically Powered Prosthetic Arm System

Abstract

The multi-functional externally powered arm prostheses currently available in the UK, suitable for fitting to children suffering from congenital deficiencies of both upper limbs, are all powered by compressed carbon dioxide gas's2. Arm articulation is accomplished by using three-way control valves in association with differential area piston actuators, or two three-way valves mounted back-to-back with double acting actuators, in closed loop position controlled servo-mechanisms. Several advantages are associated with the use of hydraulic fluid as the powering medium for such devices3 These mainly accrue from the relative compressibility of the two media. The virtually incompressible nature of hydraulic fluid results in safe operation of systems at higher working pressures and consequently results in better power to weight ratios. Thus hydraulically powered position control systems, designed to conform to the prosthetic requirements of safety, compactness and lightness, are able to meet load, response and stability specifications whch exceed the capabilities of their pneumatic counterparts. The output stiffness of hydraulic servos is also much greater, which decreases their disturbance by external loads. The construction of a hydraulically powered arm prosthesis has not been possible to date due to lack of a suitable power source. However, recent successful development of a prototype miniature hydraulic power unit indicates that portable hydraulically powered systems are viable'? 7 1 Energy storage for this hydraulic supply is accomplished by electric batteries which offer great advantages in ease of recharging or replacement compared with compressed carbon dioxide storage cylinders. This should make such a system popular in the United States where, due to the logistic difficulties of supplying gas cylinders, most externally powered prostheses are actuated by electric motors, resulting in heavier arm systems with poor dynamic performance. Other areas of hfficulty in implementing a hydraulic arm prosthesis include the development of suitable control valve and actuator system^^^^^ and the adaptation of suitable terminal devices to make them compatible with the arm structure and hydraulic power. This article describes the current state of the hydraulic prosthesis under construction at University College London, under the four main headings of arm structure, portable power supply, valve and actuator system and terminal device. It should be stressed that this project is aimed at producing a prototype hydraulic arm and that the arm now nearing completion will not be used for clinical trials, but could form the basis of a system which, suitably modified to permit small batch production, could be used by bilateral dysmelics.