Fast soft contact motion using force control with virtual viscosity field

Interactions between robots and their environment are important to perform a variety of tasks. During interactions, contact is primitive but essential because a robot should generally contact an object before performing a task. Before contact, the robot should approach an object and then establish contact with a target force. Thus, force control plays an important role during contact. However, an impulsive force may occur if a robot has a non-negligible velocity upon contact. Such impulsive force may damage the object or robot and thus should be prevented. We propose a force control law using a virtual viscosity field. The viscosity coefficient in the proposed force control law changes according to the relative position between the robot and object. As a result, the proposed control law simultaneously enables fast approaching and the reduction of impulsive forces upon contact without requiring a motion planning algorithm or controller switching. In addition, the proposed control law provides robustness to position errors compared with conventional position-based control. Experimental results validate the proposed method on a two degree-of-freedom manipulator.