FlexWhegBot: Toward bio-inspired multi-Terrain adaptability with a flexible spine and morphing wheel-legs

Wheel-legged robots face challenges in achieving the stability and flexibility of biological systems on complex terrains. To address this, we developed the FlexWhegBot, a bio-inspired robot integrating a stiffness-Tunable flexible spine and six morphing wheel-legs. Key innovations include a compliant spine enabling sagittal stiffness modulation (0.069-0.279 N/mm) and lateral tendon-driven bending (±37°), wheel-legs reconfigurable via a scissor mechanism, and a control system for gait and spinal adjustments. Experiments-including stiffness characterization, obstacle-crossing trials, and multi-Terrain locomotion tests-demonstrated that the low-stiffness spine reduced body roll fluctuations by a1/46 degrees, head height oscillations by 24.4-26.7%, and improved obstacle traversal speed by 12.5% compared to high-stiffness mode. A pure spinal lateral bendingbased steering strategy, without altering the gait, achieved consistent turning radii (a1/40.7 m) across speeds, simplifying gait transition control. Locomotion testing on grass, pebble, and cluttered wooden-strips terrains validated the FlexWhegBot's multi-Terrain adaptability. This work advances terrain adaptability in wheellegged robots through bio-inspired compliant spines and morphing mechanisms, paving the way for more flexible, lifelike bionic robots.