Biped Stabilization by Linear Feedback of the Variable-Height Inverted Pendulum Model

The variable-height inverted pendulum (VHIP) model enables a new balancing strategy, based on vertical motions of the center of mass, in addition to the well-known ankle strategy. We propose a biped stabilizer based on linear feedback of the VHIP that is simple to implement, coincides with the state-of-the-art for small perturbations and is able to recover from larger perturbations thanks to this new strategy. This solution is based on "best-effort" pole placement of the 4D divergent component of motion of the VHIP under input feasibility and state viability constraints. We complement it with a suitable whole-body admittance control law and test the stabilizer on the HRP-4 humanoid robot.