Uncalibrated Stereo Hand-Eye Coordination

Abstract

This paper describes a system that combines stereo vision with a 5-DOF robotic manipulator, to enable it to locate and reach for objects by sight. Our system uses an affine stereo algorithm, a simple but robust approximation to the geometry of stereo vision, to estimate positions and surface orientations. It can be calibrated very easily with just four reference points. These are defined by the robot itself, moving the gripper to four known positions (selfcalibration). The inevitable small errors are corrected by a feedback mechanism which implements image-based control of the gripper's position and orientation. Integral to this feedback mechanism is the use of affine active contour models which track the real-time motion of the gripper across the two images. Experiments show the system to be remarkably immune to unexpected translations and rotations of the cameras and changes of focal length — even after it has 'calibrated ' itself. A future goal is to use affine stereo to implement shape-based grasp planning, enabling the robot to pick up a wide range of unidentified objects left in its workspace. At present it can only pick up simple wooden blocks and tracks a single planar contour on its target object. 1