Quadruped robots are likely to fall into the fault joint state in outdoor explorations. The unexpected joint lock may suddenly happen when quadruped robots are implementing normal gaits, and maintaining the primal movement patterns to finish targeted tasks could be disastrous. In this paper, a fault-tolerant motion planning and generation method for quadruped robots with joint lock is proposed. Fault-tolerant cases on three types of joints on legs are investigated, and equivalent geometric models are proposed to reconstruct kinematics. To make unnecessary deformation of gait patterns as small as possible, the body posture and standing height of quadruped robots are to be optimized based on the nonlinear equivalent geometry models with constraints. The proposed fault-tolerant method is applicable to constructing a quasi-static whole-body controller, and it does not require additional operations and constraints of the fault leg. To validate the consistency and stability of the proposed fault-tolerant method, the experiments are implemented on the three joint lock failure scenarios for quadruped robots.

Legged robots demand to keep the balance under required durations when performing standing and locomotion. It may be a challenge for legged robots to maintain balance during standing without actuation from ankle joints and involvement of support polygon plane to estimate the center of mass (CoM). In order to maintain the standing balance of legged robots under such a scenario, we propose a virtual suspension model control (VSMC) method which is concise and can get rid of updating model iterations on sophisticated dynamics. Furthermore, we optimize the range and propose the criteria for the virtual height of the CoM for quickly adjusting parameters and adapting to task characteristics. Simulation experiments on balance keeping are performed for single-leg robot vertical standing, quadruped robot diagonal standing and biped robot parallel standing cases, and the proposed method can achieve promising results within the approximative real setting condition that demonstrate the feasibility and strong anti-interference ability of the VSMC approach.

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