  We present a stepping stabilization control that addresses external push
disturbances on bipedal walking robots. The stepping control is synthesized
based on the step-to-step (S2S) dynamics of the robot that is controlled to
have an approximately constant center of mass (COM) height. We first learn a
linear S2S dynamics with bounded model discrepancy from the undisturbed walking
behaviors of the robot, where the walking step size is taken as the control
input to the S2S dynamics. External pushes are then considered as disturbances
to the learned S2S (L-S2S) dynamics. We then apply the system-level-synthesis
(SLS) approach on the disturbed L-S2S dynamics to robustly stabilize the robot
to the desired walking while satisfying the kinematic constraints of the robot.
We successfully realize the proposed approach on the walking of the bipedal
robot AMBER and Cassie subject to push disturbances, showing that the approach
is general, effective, and computationally-efficient for robust disturbance
rejection.
