Quadruped robots are machines intended for challenging and harsh environments. Despite the progress in locomotion strategy, safely recovering from unexpected falls or planned drops is still an open problem. It is further made more difficult when high horizontal velocities are involved. In this work, we propose an optimization-based reactive Landing Controller that uses only proprioceptive measures for torque-controlled quadruped robots that free-fall on a flat horizontal ground, knowing neither the distance to the landing surface nor the flight time. Based on an estimate of the Center of Mass horizontal velocity, the method uses the Variable Height Springy Inverted Pendulum model for continuously recomputing the feet position while the robot is falling. In this way, the quadruped is ready to attain a successful landing in all directions, even in the presence of significant horizontal velocities. The method is demonstrated to dramatically enlarge the region of horizontal velocities that can be dealt with by a naive approach that keeps the feet still during the airborne stage. To the best of our knowledge, this is the first time that a quadruped robot can successfully recover from falls with horizontal velocities up to 3 m/s in simulation. Experiments prove that the used platform, Go1, can successfully attain a stable standing configuration from falls with various horizontal velocity and different angular perturbations.

翻译：四足机器人是为适应复杂恶劣环境而设计的机器。尽管步态策略已取得进展，但从意外摔倒或计划性坠落中安全恢复仍是一个未解决的问题。当涉及较大水平速度时，这一问题更加困难。本文提出一种基于优化的反应式落地控制器，该控制器仅利用本体感觉测量值，针对在平坦水平地面自由落体、且不知落地距离与飞行时间的力矩控制四足机器人设计。该方法基于质心水平速度估计，采用变高度弹簧倒立摆模型，在机器人坠落过程中持续重新计算足端位置。通过这种方式，四足机器人能在各个方向成功落地，即使存在显著水平速度时亦然。实验证明，该方法可大幅扩展采用空中阶段保持足端静止的朴素方法所能处理的水平速度范围。据我们所知，这是首次实现四足机器人在仿真中成功从水平速度高达3米/秒的坠落中恢复。实验表明，所用平台Go1能从不同水平速度及不同角度扰动的坠落中成功达到稳定站立构型。