The relation between the robot with the environment is very important issue for the biped robot's control because the collision with the environment occurs repeatedly during the walking motion. Generally, the position control is used for the biped robot's control. But the position control can not cope with the collisio force and the small environmental variations which the robot can not sens by the position sensors (visual sensor, ultrasonic sensors, etc.). In order to realize the stable contact with the environment, the robot must have a compliance by itself. So in this paper, we propose a compliance control strategy for a three dimensional biped robot with variable compliance gain. The goal of this control system is to realize the smooth state transition from the single support phase to the double support phase. The value of this variable compliance gain increases with the interaction force at the bottom of the foot. This variable compliance gain will maintain the performance of the control system in the free motion as well as contact conditions and plays an important role in adusting the stiffness of the biped robot system. The feasibility of the proposed method was confirmed by several numerical and experimental results.
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