Kinematic Design and Dimensional Synthesis of a Symmetric Five-bar Wheel-leg Mechanism for Quasi-linear Lifting and Self-recovery

Wheel-legged robots combine the efficiency of wheeled locomotion with the ter- rain adaptability of legged systems, but many existing mechanisms exhibit strong kinematic nonlinearity during body lifting, which complicates control and lim- its recovery capability. To address this issue, this paper proposes a symmetric five-bar wheel-leg mechanism that supports two motion modes within a uni- fied architecture: quasi-linear vertical lifting under synchronous actuation and body reorientation under differential actuation. A closed-loop kinematic model is established, and a reduced lifting model is derived under symmetry constraints. Based on this model, a boundary-constrained dimensional synthesis method with numerical optimization is developed to improve lifting linearity, enlarge the usable stroke, and avoid unfavorable kinematic regions. The optimized mechanism is implemented on a prototype robot with quasi-direct-drive actuation and slip-ring-based wiring. Simulation and experimental results show that the prototype preserves a quasi-linear angle–height relationship, with limited lateral drift, bounded joint torque, and small pitch variation during synchronous lifting. Workspace, Jacobian, and Monte Carlo sensitivity analyses further indicate that the selected design remains away from the prescribed singularity threshold under the tested perturbations. Self-recovery experiments under several tested conditions demonstrate that the differential mode provides repeatable posture-reconfiguration capability. These results support the feasibility of using the proposed mechanism for combined height adjustment and recovery-oriented reconfiguration in wheel-legged robots.