Lateral Disturbance Rejection for the Nao Robot

Maintaining balance in the presence of disturbances is crucial for bipedal robots. In this paper, we focus on the lateral motion component. In order to attain disturbance rejection and to quickly recover balance, we combine three different control approaches. As a principal building block, we generate center of mass trajectories with a linear model predictive controller that takes scheduled footsteps into account. Strong disturbances generate unexpected angular momenta that can compromise stability. A second control layer extends the underlying preview controller with two recovery strategies that modify the planned CoM trajectories to dampen the rotational velocity of the robot and adapt the timing of the steps according to the expected orbital energy of CoM trajectories at support exchange. Experiments with a real Nao robot show that the system is able to recover from lateral disturbances as long as the robot does not tip over the current support leg.