An Orthogonal-Axis Spherical Parallel Mechanism With Large Workspace for Humanoid Hip Joints: Design, Optimization, and Experimental Validation

Conventional serial-chain configurations for humanoid robot lower limbs exhibit issues such as concentrated driving torques, inertial coupling, and error accumulation, leading to inadequate joint stiffness, limited control precision, and reduced dynamic stability. To overcome these drawbacks, this study proposes a novel Orthogonal-axis Spherical Parallel Mechanism, inspired by the ball-and-socket structure of the human hip joint. The kinematic model of the mechanism is established. A multi-objective optimization framework is then constructed, considering workspace coverage, dexterity, and maximum joint torque, and optimal parameters are obtained through weighted decision-making. Simulations and experiments verify that the optimized mechanism fully covers the common range of motion of the human hip joint, demonstrates improved dexterity within common motion ranges, and supports a single-leg payload of 40 kg. Integrating high stiffness, a large workspace, and superior load capability, this design offers a comprehensive solution for humanoid robotic legs.