A 3-DOF coupled tendon-driven humanoid waist

This study proposes a 3-degree-of-freedom (3-DOF) coupled tendon-driven humanoid waist featuring motion characteristics similar to those of the human waist. The tendon routing form of the waist was designed based on a 3-DOF coupled tendon-driven joint module considering the maximum balanced moment in the three directions. The ratios between the maximum joint torques for the pitch, roll, and yaw were designed to be 4:2:1 by the selection of a suitable tendon routing form and the adjustment of the pulley radius ratio. Two experiments were conducted to evaluate the maximum static torque and structural accuracy. The maximum static torques were 87.0, 53.0, and 22.2 Nm for the pitch, roll, and yaw, respectively. This was nearly consistent with the design considering the friction effect. An experiment for structural accuracy evaluation was conducted at different loads and different velocities to investigate the performance of the coupled tendon-driven mechanism. The experimental results indicate that, although the trajectory of the mechanism fluctuates greatly during the start and stop phases due to the large kinematic inertia of the mechanism and the stretching of the tendons, the trajectory also stabilizes when the velocity stabilizes.