Omnidirectional capture steps for bipedal walking

Robust walking on two legs has proven to be one of the most difficult challenges of humanoid robotics. Bipedal walkers are inherently unstable systems that are difficult to control due to the complexity of their full-body dynamics. Aside from the challenge of generating a walking motion itself, closed-loop algorithms are required to maintain the balance of the robot using foot placements and other disturbance-rejection strategies. In this work, we propose a hierarchical, omnidirectional gait control framework that is able to counteract strong perturbations using a combination of step-timing, foot-placement, and zero-moment-point strategies. The perturbations can occur from any direction at any time during the step. The controller will not only maintain balance, but also follow a given reference locomotion velocity while absorbing the disturbance. The calculation of the timing, the footstep locations, and the zero moment point is based on the linear inverted pendulum model and can be computed efficiently in closed form.