Cooperative skating motion control for a quad-wheel-legged robot

This paper presents a novel cooperative skating motion control strategy for a quad-wheel-legged robot capable of dynamic leg configuration switching. The robot combines the advantages of legged and wheeled locomotion by using actively actuated legs with onboard wheels, enabling it to operate in multiple geometric configurations—namely, elbow-type, knee-type, inward- and outward-pointing postures. The proposed control framework integrates virtual model control for legged stability with Ackermann-based steering for wheeled maneuverability, enabling hybrid motion that supports continuous and dynamically stable transitions. A key contribution of this work is the development of a skating gait in which the forelegs maintain rolling contact to provide support and steering, while the hind legs alternate between push and swing phases to generate propulsion. To ensure robustness and smoothness, trajectory planning is performed using seventh-order Bézier curves, and ZMP-based stability criteria are enforced during each gait cycle. Furthermore, a configuration-switching controller allows on-line morphing between leg geometries, exploiting rolling contact to avoid foot lifting and preserve balance. Simulation results demonstrate the effectiveness of the proposed approach in executing hybrid skating motions, adapting to different terrain conditions, and maintaining posture stability throughout transitions. Comparative analysis with related works highlights improved versatility, support continuity, and locomotion robustness. This research advances the capabilities of wheel-legged robots in structured environments and lays the foundation for future development in real-world deployment scenarios. • A novel cooperative skating control framework is proposed for a quad-wheel-legged robot with on-line leg configuration switching. • The hybrid gait combines foreleg-supported rolling and hind-leg propulsion, ensuring stable and efficient locomotion. • Simulation results demonstrate smooth configuration transitions and robust skating up to 1.0 m/s with ZMP-based stability.