Towards high mobility and adaptive mode transitions: Transformable wheel-biped humanoid locomotion strategy

Wheel-biped humanoid robots offer a promising solution that combines the bipedal locomotion and manipulation capabilities of humanoids with the mobility advantages of wheeled robots. However, achieving high mobility and adaptive wheel-foot transitions while maintaining essential bipedal functionality in a transformable wheel-biped configuration (TWBC) presents a significant challenge. To address this, this paper proposes a transformable wheel-humanoid framework (TWHF), which enhances traditional humanoid robots by incorporating a compact, decoupled wheeled subsystem. This design effectively balances high-speed wheeling, seamless mode transitions, and fundamental bipedal locomotion. A novel key phase decomposition (KPD) methodology is introduced to analyze and decouple transition motions, providing structured guidance for subsystem design, motion planning, and control. Transition reference motions are optimized using a particle swarm optimization-based motion optimization (PSOMO) approach, leveraging sagittal modeling to ensure dynamic stability and kinematic feasibility. Additionally, the proposed trunk-ankle collaborative control (TACC) strategy further enhances transition adaptability to terrain discrepancies. Extensive experiments conducted on the wheel-humanoid BHR8-2 validate the proposed TWHF, demonstrating stable hybrid locomotion across diverse terrains and achieving wheeling speeds exceeding 10 km/h.