Optimal Design of Polycentric Joint for Semi-Active Knee Supporter

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 have developed several types of the biomimetic knee joint (BKJ) mechanism that mimics the rotational centers of biological knees. A polycentric BKJ which is a polycentric joint structure that mimics the rollback motions of knee joints by combining two gears with different radii is used in this study. From a viewpoint of the complexity of the device, the passive devices have a significant advantage. However, a lack of the automatic adjustment function is a big disadvantage. To enhance the usability and applicational field, we propose the Semi-active Knee Joint (SKJ) in this study. The SKJ has a polycentric gear mechanism and linear motion springs. In addition, the spring constant is adjustable from a weak spring to a strong spring. The basic mechanism, design method, and design processes are reported in this paper. As a result of an optimal design of the gear system based on the measurement result of a knee joint motion for a subject, we adequately designed a pair of gears for a tailer made SKJ with small misalignment of less than 5 mm even in the deep flexion over 90°. In addition, non-linear function of the assistive torque was analyzed for a mechanical model. As a result, spring constant, k is decided to be 45 N/mm for the upper knee and 20 N/mm for the lower knee.