During early and middle stance of level-ground walking, the ankle joint usually rotates passively due to the locomotion of human body. Based on this passive dynamics of human locomotion, we develop an energy-efficient torque controller with hierarchical structure for a robotic prosthesis. The low-level controller generates motor current from the human locomotion and controls the motor current/torque according to the high-level command. The high-level controller consists of a forward estimator and a feedback compensator. The forward estimator estimates the motor torque according to the desired ankle torque based on the prosthesis model that represents the transmission gain between the motor torque and the ankle torque. The feedback compensator compensates for the model and low-level control errors based on the torque measurements. Step response and frequency response show that the low-level controller could reach the desired value within 0.3 seconds and had a bandwidth of 4.2 Hz. Experiments of constant and linear torque tracking achieve small RMS tracking errors, which confirm the effectiveness of the proposed high-level controller.
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