Balancing Using Vertical Center-of-Mass Motion: A 2-D Analysis From Model to Robot

Balancing strategies for humanoid robots often include the center-of-pressure control ("ankle" strategies), change of body's angular momentum (e.g., "hip" strategies), and taking a step. In this letter, we propose using vertical center-of-mass motion as an additional input for balance control. First, we specify analytic, theoretical capture regions under unilateral contact and height constraints only. Second, we add a vertical acceleration constraint and come to a simple control law for implementation. Third, we implement the control law in our momentum-based whole-body control framework. We test push recovery while standing on the NASA's Valkyrie humanoid robot and compare with a constant height controller, and we show that recovery can be improved using vertical motion. Furthermore, we discuss the differences that can be observed after the application of a simple model on a robot.