Lateral capture steps for bipedal walking

Bipedal walkers are difficult to control, inherently unstable systems. Besides the complexity of the walking motion itself, the balance of the robot constantly has to be maintained with good foot placements and other disturbance-rejection strategies. In this work, we are presenting a new, closed-loop control approach that addresses both, the problem of complexity and the challenge of maintaining balance during walking. We decouple walking motion from balance and combine them in a hierarchical framework allowing a foot placement-based balance regulator to control the timing and footstep coordinates of central pattern- generated stepping motions. Furthermore, we decompose the balance controller into three simple, independent modules that compute suitable estimates of timing and sagittal and lateral coordinates for the next footstep to maintain a nominal center of mass trajectory. We implemented the timing and the lateral step size components using the equations of a parameterized version of the linear inverted pendulum model that we fit to data collected from a walking robot. The parameter optimization has a significant impact on the accuracy of our predictions. We demonstrate the efficiency of our approach by performing experiments on a real biped. Results show that the robot is able to reliably recover from any lateral push in only a few steps as long as it does not tip over the current support leg.