Force control of a jumping musculoskeletal robot with pneumatic artificial muscles

This paper introduces a method of force control for a jumping biped robot to correctly control the ground reaction force during continuous jumping. Assuming dynamic motion, such as jumping and running, the attitude of the robot depends on the dynamics in response to the ground reaction force. Therefore, controlling the ground reaction force is necessary for stabilizing the robot's motion. However, control of the ground reaction force involves a fundamental problem: feedback control does not work properly against the impact force owing to limitations in the control bandwidth. This study introduces a method for stiffness ellipse control that utilizes three antagonist pairs of six pneumatic artificial muscles. When the tip of the robot makes contact with the ground, an external force is induced in the direction of lower stiffness. By utilizing this property of the stiffness ellipse, it is possible to control the ground reaction force by feedforward control, which is not dependent on the control bandwidth. Based on this idea, impact force control at landing and jumping force control at takeoff were proposed to correctly control the ground reaction force during the continuous jumping of the robot. The results of several experiments conducted convince us that the relationship between the ground reaction force and the stiffness ellipse is almost linear, and that the ground reaction force can be controlled with high reproducibility by adjusting the stiffness ellipse.