  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.
