Energy-Efficient Braking Torque Control of Robotic Transtibial Prosthesis

The ankle joint usually rotates passively during early and middle stance due to the locomotion of human body. Based on this passive dynamics of human locomotion, we develop an energy-efficient braking torque controller for a robotic transtibial prosthesis. The controller first generates motor current/torque from human locomotion instead of the battery energy, and then controls the current/torque with alternating current sensing and voltage-controlled-resistor. On the basis of the low-level motor torque controller, we then propose the impedance model that determines the desired joint torque according to the joint angle and joint velocity, the prosthesis model that converts the desired ankle joint torque to desired motor torque, and the friction model that estimates the friction torque and compensates the joint torque. Step response and frequency response show that the low-level controller can reach the desired torque value within 0.07 s, and has a $-$3 $\text{dB}$ bandwidth of 25.2 $\text{Hz}$. Experiments of level-ground treadmill walking with three amputees show that the prosthesis with the proposed controller can effectively mimic the normal ankle performance and bring the amputees a natural gait pattern with low power consumption.