Whole-Body Motion Generation for Wheeled Biped Robots Based on Hierarchical MPC

Achieving stable movement on uneven terrains for wheeled biped robots (WBR) is nontrivial due to their under-actuated and inherently unstable nature. To address this issue, this article proposes a whole-body motion control framework based on hierarchical model predictive control (HMPC). First, the wheeled linear inverted pendulum (WLIP) model is proposed to analyze the dynamic coupling mechanism of WBR. Based on this coupling, an optimal ground reaction force (GRF) location control policy is formulated, which serves as parameters for the single rigid body (SRB) dynamic model, enabling the spatial motion of the under-actuated SRB fully controllable. Finally, the inverse kinematics control is utilized to generate the whole-body motion of the robot. This method directly considers the effect of GRF on the robot system, and balances the performance with computational efficiency of MPC. Experiments on a real hydraulic WBR verify that the proposed method provides excellent performance and robustness for both indoor and outdoor motion.