Design and Validation of Linkage-Switching-Based Wheel-Leg Composite Joint for Humanoid Robots

Lunar robots serve as the core equipment for surface exploration and operational tasks in lunar exploration projects, holding invaluable importance in deep space exploration and lunar scientific research. This study addresses the constraints of space and weight limitations for future lunar robots by proposing a humanoid robot composite joint based on a single joint-single drive, incorporating both wheeled and legged motion modes. To achieve independence between the two motion modes, this paper employs a planetary gear system combined with electromagnetic brakes to design a switchable transmission linkage. Furthermore, a dynamic model of the planetary gear system and a mathematical model of motor drive are established, leading to a motion control framework that includes torque feedforward. The results of co-simulation experiments indicate maximum tracking errors of 0.95% for the wheeled mode and 1.3% for the legged mode, validating the effectiveness of the control strategy. Prototype experimental results demonstrate that the proposed wheel-legged composite joint achieves stable switching between motion modes and independent motion output, confirming the feasibility of the design. In the future, the design of this composite joint can provide a reference for developing lunar robot movement systems.