Every year, tens of thousands of people in the United States alone require an above-knee leg amputation. These amputees require a prosthetic leg to regain mobility and to improve quality of life. Current prosthetic knees range from simple hinges to sophisticated computer controlled hydraulic dampers. Despite the advances in technology, above-knee amputees still show significant deficiencies in their walking patterns, walking speed, and energy consumption. Also, without a compact energy source and small, high power actuators, current prostheses prohibit ascending stairs step over step, and provide no assistance when standing up. Development in powered prosthetic knees has mostly focused on the control, with little emphasis on creating an efficient, low energy consumption, untethered device.

This thesis presents the first energetically autonomous hydraulically powered prosthetic knee. The primary innovation of the knee is a hybrid design with two separate hydraulic modes of operation: an active mode driven by a pump, and a passive mode controlled by a variable position valve. The electrohydraulic design combines the safety, comfort, and versatility of a state of the art microprocessor controlled hydraulic knee with three new benefits. First, by pumping fluid into the hydraulic cylinder, the necessary knee angle during the swing phase of walking to clear obstacles in the path of the foot is achieved. Second, adding power at the knee reduces hip torque required to swing the leg forward. Third, the power added at the knee helps lift the amputee's body weight, allowing the step over step ascent of ramps and stairs. These benefits should reduce back and hip pain, and should improve mobility to afford the amputee a healthier life.