Design, Control, and Validation of a Polycentric Hybrid Knee Prosthesis

This article presents a novel polycentric hybrid knee prosthesis (PHKP) by enabling a multibar linkage to work for high-torque activities in active mode and for low-torque activities in passive mode. The mode transition is implemented the servo-controlled telescopic slide rails. Distinguished from the single-axis hybrid knee prostheses, the PHKP knee provides geometric stability in stance and notably increases toe clearance in swing by using an optimally designed instantaneous center of rotation trajectory. In active mode, the PHKP knee provides high transmission ratio and continuous peak torque offering sufficient support for torque-demanding activities. Impedance control was employed to regulate joint torque as a response to knee motion in the active mode. It was found that due to the inherent geometric stability of the polycentric mechanism in stance, the damping coefficient of the controller can be tuned much smoother than the single-axis knees leading to more compliant and comfortable interactions among the wearer, the prosthesis and the ground. The stair ascent tests showed that the joint angle and torque profiles of the PHKP knee are similar to those of the able-bodied by demonstrating a step-over-step pattern with a peak angle of 73
$^{\circ }$
and a maximum torque of 0.92 N
$\cdot$
m/kg.