Visual Compliance: Task-Directed Visual Servo Control

This paper introduces visual compliance, a new vision-based control scheme that lends itself to task-level specification of manipulation goals. Visual compliance is effected by a hybrid vision/position control structure. Specifically, the two degrees of freedom parallel to the image plane of a supervisory camera are controlled using visual feedback, and the remaining degree of freedom (perpendicular to the camera image plane) is controlled using position feedback provided by the robot joint encoders. With visual compliance, the motion of the end effector is constrained so that the tool center of the end effector maintains “contact” with a specified projection ray of the imaging system. This type of constrained motion can be exploited for grasping, parts mating, and assembly. We begin by deriving the projection equations for the vision system. We then derive equations used to position the manipulator prior to the execution of visual compliant motion. Following this, we derive the hybrid Jacobian matrix that is used to effect visual compliance. Experimental results are given for a number of scenarios, including grasping using visual compliance.