In recent years, humanoid robot has been expected in various fields because of the shape and architecture of humanoid robot and aging of society. It is important that robot does not harm human so that we coexist with robot. Therefore, a lot of researchers have studied about falling down. Zero-Moment Point (ZMP) is most widely used as a stability indicator of biped robot. ZMP, however, is not unsuitable for falling indicator because of behavior of ZMP. On the other hands, Foot-Rotation Indicator (FRI) is useful when robot falls down. However, FRI point does not include content that whether robot keeps a balance on boundary surface of falling. In addition, FRI point is complicated and it is hard to calculate. A permissible amount of falling risk evaluation, therefore, is presented in this paper. This permissible amount is calculated by plantar contact points of the robot, acceleration value of center of gravity (COG) and reaction force. During robot is walking or standing, the falling risk is evaluated by this permissible amount. This method is applied for a condition of robot before robot falls. Proposed method is evaluated by the simulation and experimental results.
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