Variable compliance control based on soft-landing trajectory for hopping robot

In this paper, soft-landing of hopping robot is considered. In the landing phase, the robot is affected by reaction force from the ground. In order to achieve a desired COG (center of gravity) position at bottom, the soft-landing trajectory is introduced. This trajectory was planned to minimize impulse and maximum force from the ground. However the trajectory is not sufficient to apply to actual robots because the robots cannot adapt unknown (or inconstant) external force. Therefore, in this research, we propose novel variable compliance control with soft-landing trajectory. Compliance parameters are modified based to the soft-landing trajectory so that transition from force control to position control can be accomplished smoothly. While force control should be applied to absorb the impact from the ground, position control should be applied at bottom since the robot should track the desired trajectory for the next hop. With this method, transition from force control at touchdown to position control at bottom can be achieved appropriately. The robot can land with suppressing the impact force with accomplishing the desired COG position at bottom.