This paper presents a stability criterion for biped robots on rough terrain. On rough terrain, which is not vertical to gravity, a robot may not fall even though its center of mass is out of the area of the supported polygon defined by the conventional VHP (Virtual Horizontal Plane) method. This is because there exists a certain force opposing the torque that causes the robot to fall down. We propose a new measure for strictly judging the limit of falling considering all the forces acting on the contact points. Numerical simulations and experimental results are also shown, which verify the proposed method.
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