Impact force control based on stiffness ellipse method using biped robot equipped with biarticular muscles

This paper introduces the control method for the impact force based on feed-forward control considering the stiffness ellipse of a biped robot. Assuming dynamic motion, such as jumping and running, the jumping direction depends on the dynamics in response to the ground reaction force. Therefore, it is necessary to control the ground reaction force in order to stabilize the system. However, control of the ground reaction force involves a fundamental problem: feedback control does not work properly against the impact force owing to the control bandwidth limitation. This study introduces a method for the stiffness ellipse control utilizing three antagonist pairs of six pneumatic artificial muscles. When the tip of the biped robot makes contact with the ground, an external force is induced in the direction that has lower stiffness. By utilizing this property of the stiffness ellipse, it is possible to control the ground reaction force by feed-forward control, which is not dependent on the control bandwidth. The results of several experiments convince us that the relationship between the ground reaction force and the stiffness ellipse is almost linear, and the ground reaction force can be controlled with high reproducibility by adjusting the stiffness ellipse. It is possible to stabilize the system during dynamic movements because this method can control the impact force.