We present the design, control, and experimental evaluation of an energy regenerative powered transfemoral prosthesis. Our prosthesis prototype is comprised of a passive ankle and a powered knee joint. The knee joint is actuated by an ultracapacitor based regenerative drive mechanism. A novel varying impedance control approach controls the prosthesis in both the stance and swing phase of the gait cycle, while explicitly considering energy regeneration. This control method varies the impedance of the knee joint based on the amount of force exerted on the shank of the prosthesis. Furthermore, the controller promotes energy regeneration by precisely injecting a designated amount of negative damping into the system. Our control approach leads to a few tuning parameters that cover all of the gait phases for walking and all of the tested walking speeds and eliminates the need for tedious target impedance scheduling. Experimental evaluation is done with an amputee test subject walking at different speeds on a treadmill. The results validate the effectiveness of the control method. In addition, net energy regeneration is achieved while walking with near-natural gait across all speeds.
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