ROBOT CALIBRATION BY MOBILE CAMERA SYSTEMS

A measurement technique for kinematic calibration of robot manipulator s, which uses a stereo hand-eye system with moving camera coordinates, is presented in this article. The calibration system consists of a pa ir of cameras rigidly mounted on the robot end-effector, a camera cali bration board, and a robot calibration fixture. The stereo cameras are precalibrated using the camera calibration board so that the 3D coord inates of any object point seen by the stereo cameras can be computed with respect to the camera coordinate frame [C] defined by the calibra tion board. Because [C] is fixed with respect to the tool frame [T] of the robot, it moves with the robot hand from one calibration measurem ent configuration to another. On each face of the robot calibration fi xture that defines the world coordinate frame [W], there are evenly sp aced dot patterns of uniform shape. Each pattern defines a coordinate frame [E(i)], whose pose is known in [W]. The dot pattern is designed in such a way that from a pair of images of the pattern, the pose of [ E(i)] can be estimated with respect to [C] in each robot calibration m easurement. By that means the pose of [C] becomes known in [W] at each robot measurement configuration. For a sufficient number of measureme nt configurations, the homogeneous transformation from [W] to [C] (or equivalently to [T]), and thus the link parameters of the robot, can b e identified using the least-squares techniques. Because the cameras p erform local measurements only, the field-of-view of the camera system can be as small as 50 x 50 mm2, resulting in an overall accuracy of t he measurement system as high as 0.05 mm. This is at least 20 times be tter than the accuracy provided by vision-based measurement systems wi th a fixed camera coordinate frame using common off-the-shelf cameras. (c) 1994 John Wiley & Sons, Inc.