Development of a Humanoid Shoulder Based on 3-Motor 3 Degrees-of-Freedom Coupled Tendon-Driven Joint Module

In this study, a new humanoid shoulder is developed using a coupled tendon-driven mechanism that consists of three motors to drive three degrees of freedom (DoFs). In the coupled tendon-driven mechanism, multiple motors simultaneously drive multiple joints with each joint driven by at least two motors coupled with tendons. The torque of a motor is redistributed to multiple joints in a certain proportion to improve the load capacity of the joints. The 3M3D (3-motor 3-DoF) coupled tendon-driven joint module is systematically analyzed and divided into four categories according to the torque transmission characteristics between the motors and the joints: fully routed motor-joint form, 1-unrouted motor-joint form, 2-unrouted motor-joint form, and 3-unrouted motor-joint form. The four categories were analyzed and compared with respect to the characteristics of the motor torque redistribution with different tendon routing forms. The 2-unrouted motor-joint form was adopted for the development of the 3-DoF shoulder joint. The problem of the coexistence of coupling and non-coupling in one joint is solved by introducing a 2-DoF rolling joint. The relationship between the angle and torque between the motors and joints was verified experimentally. The experimental results revealed that the 3M3D coupled tendon-driven shoulder realized torque enhancement effectively by tendon coupling, and the angle accuracy was sufficient as a humanoid shoulder.