The Design, Control, and Testing of an Integrated Electrohydrostatic Powered Ankle Prosthesis

This paper presents a prototype powered ankle prosthesis that can operate passively in most of the gait cycle and provide powered assistance for toe push-off and subsequent foot dorsiflexion. The use of an electrohydrostatic actuator (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 100 W brushless dc motor is used to drive a 0.45 cc/rev gear pump, controlling flow to an ankle cylinder through a bespoke manifold. The motor runs wet, pressurized to 6 MPa, avoiding the need for a pump shaft seal and a refeeding circuit for external leakage. A dynamic system model has been develop to help analyze the EHA performance. A motor control method is proposed based on heel strike recognition and a middle stance time delay. The prosthesis has been tested with a 70 kg transtibial amputee, and results are presented for walking on a treadmill at three different speeds (2.8, 3.8, and 4.8 km/h). The amputee has provided positive subjective feedback. We conclude that the hybrid passive-active approach has significant advantages for prosthesis design, and we outline future testing and development requirements.