Design of a Serial-Parallel Hybrid Leg for a Humanoid Robot

This paper presents a 6 DOF leg mechanism for a humanoid robot. The proposed Hybrid Leg is designed to combine serial and parallel mechanisms and consists of a pair of twin 3 DOF serial chains in parallel. A 5-bar-linkage mechanism is implemented to the serial mechanism to generate 2 DOF motion regarding hip and knee pitch rotation. The hardware prototype is designed by matching the kinematic specification of a commercial robot's leg to compare the proposed mechanism with a conventional serial leg. We derive the analytical expressions of its forward and inverse kinematics. End-effector workspaces are shown with plots and inverse dynamics analysis of Hybrid Leg and serial leg with a given walking gait trajectory is presented. Hardware experiment is conducted with a prototype to verify the simulated workspace and trajectory tracking performance.