Variable Interlock Mechanism Joining Shoulder Rotation and Elbow Flexion for Body-Powered Upper Limb Prostheses

People with upper limb amputations wearing body-powered prostheses perform elbow joint flexion and opening/closing the terminal device by pulling one control cable with the contralateral shoulder. When moving the terminal device close to the mouth during eating, the socket obstructs shoulder rotation motion. In this paper, we developed the variable interlock mechanism to realize shoulder rotation and elbow flexion simultaneously by pulling only one control cable for a body-powered prosthesis. We propose a mechanism consisting of a differential bevel gear and a continuous variable transmission (CVT) unit with a conical cone and roller. With the CVT unit, the user can change the interlocking relationship without multiple steps. The number of teeth of the bevel gear, the radius of conical cone, and roller were designed to realize the interlocking motion of the inner rotation of the shoulder joint at 80° when the elbow flexion angle is 145°. We measure the terminal device trajectory and each joint angle by fixing the prosthesis with the proposed mechanism in an aluminum frame and by pulling the control cable from the top using a motion capture system. For the two conditions of the designed CVT ratio (k_σ=1.00, 3.46), the measured CVT ratio becomes k_i=1.04, 1.54. We confirm the interlocking motion of our mechanism, and the error between designed and measured CVT ratio can be improved by reducing the backlash of the bevel gear and the slip between the conical cone and the roller at the CVT unit.