  Falling cat problem is well-known where cats show their super aerial
reorientation capability and can land safely. For their robotic counterparts, a
similar falling quadruped robot problem, has not been fully addressed, although
achieving safe landing as the cats has been increasingly investigated. Unlike
imposing the burden on landing control, we approach to safe landing of falling
quadruped robots by effective flight phase control. Different from existing
work like swinging legs and attaching reaction wheels or simple tails, we
propose to deploy a 3-DoF morphable inertial tail on a medium-size quadruped
robot. In the flight phase, the tail with its maximum length can self-right the
body orientation in 3D effectively; before touch-down, the tail length can be
retracted to about 1/4 of its maximum for impressing the tail's side-effect on
landing. To enable aerial reorientation for safe landing in the quadruped
robots, we design a control architecture, which has been verified in a
high-fidelity physics simulation environment with different initial conditions.
Experimental results on a customized flight-phase test platform with comparable
inertial properties are provided and show the tail's effectiveness on 3D body
reorientation and its fast retractability before touch-down. An initial falling
quadruped robot experiment is shown, where the robot Unitree A1 with the 3-DoF
tail can land safely subject to non-negligible initial body angles.
