Design of Polycentric Assistive Device for Knee Joint*

Abstract

Movements of knee joints are relative motions between tibia and femur, which include rolling and sliding. Conventional wearable knee assistive device utilized a hinge joint and generates nonnegligible mismatched motions under deep flexions. To assist the knee motion including the deep flexion, we developed a polycentric assistive device for knee joints (PAD-KJ) by using two gears with the same module and different radii. These radii were designed so that the center of the smaller gear moves on the trajectory of the knee joint with small error. According to the results of parametric design, the minimum error was about 3 mm to the model trajectory of knee joint. We also proposed a torque generator for this joint. The combination of a lever arm and a linear spring with the pair of gears successfully generated the assistive torque for standing motions. According to an evaluation test, the maximum torque of the single unit was about 1.2 Nm and the assistive torque in real usage will be 4.8 Nm.