Spherical Foot Placement Estimator for Humanoid Balance Control and Recovery

One of the main challenges of bipedal gait is to avoid falling due to unknown disturbances. Compensating for these disturbances in bipeds is often achieved by leaning or stepping. In this work, the Spherical Foot Placement Estimator (SFPE) is introduced, which uses the biped's current kinematics and dynamics to predict if a step is needed, and if so where to step, to restore balance in 3D. An example of a controller using the SFPE is shown, which augments an existing optimal controller with both leaning and stepping: SFPE-based feedback is used to generate a desired momentum for momentum-based leaning while the SFPE point is used as a control reference for stepping. The new estimator outperforms existing balance criteria by providing both recovery step location prediction and momentum objectives with smooth dynamics.