This paper presents a prototype powered ankle prosthesis which can operate passively in most of the gait cycle and provide assistance for toe push-off and subsequent foot dorsiflexion. The use of electrohydrostatic actuation (EHA) gives the ability to switch quickly and smoothly between passive and active modes. In this new powered ankle prosthesis, the motor-pump unit is integrated with the ankle joint and the battery and controller are held in a backpack. A 100W brushless DC motor is used driving a 0.45cc/rev gear pump. The motor runs in hydraulic fluid, pressurised to 60bar, avoiding the need for a pump shaft seal and a refeeding circuit for external leakage. A simulation model was developed to help analyse the performance characteristics of the EHA. The simulation results are compared with laboratory-based experiment results to validate the model. The compact prototype is suitable for prolonged testing with amputees, and an example set of results from amputee testing is presented.

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