In this paper we study the instability of biped robots that is a combination of both sliding and tipping over. Specifically, when robot falling occurs, the ground reaction forces and moments on a foot may determine if sliding also happens. We deal with the situation that tipping over is impending, and treat the following three possible types of contact stress distribution on the foot: point contact, line contact, and area contact. In line and area contact regions we assume that normal stresses are Hertzian, then tangential stresses may be determined by utilizing theory of instantaneous center of zero velocity in planar kinematics. From these normal and tangential stresses we may determine force combinations that cause sliding.

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