ROBOT CALIBRATION BY MOBILE CAMERA SYSTEMS

A measurement technique for kinematic calibration of robot manipulators, which uses a stereo hand-eye system with moving camera coordinates, is presented in this article. The calibration system consists of a pair of cameras rigidly mounted on the robot end-effector, a camera calibration board, and a robot calibration fixture. The stereo cameras are precalibrated using the camera calibration board so that the 3D coordinates of any object point seen by the stereo cameras can be computed with respect to the camera coordinate frame [C] defined by the calibration board. Because [C] is fixed with respect to the tool frame [T] of the robot, it moves with the robot hand from one calibration measurement configuration to another. On each face of the robot calibration fixture that defines the world coordinate frame [W], there are evenly spaced 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 measurement. By that means the pose of [C] becomes known in [W] at each robot measurement configuration. For a sufficient number of measurement configurations, the homogeneous transformation from [W] to [C] (or equivalently to [T]), and thus the link parameters of the robot, can be identified using the least-squares techniques. Because the cameras perform 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 the measurement system as high as 0.05 mm. This is at least 20 times better than the accuracy provided by vision-based measurement systems with a fixed camera coordinate frame using common off-the-shelf cameras. (c) 1994 John Wiley & Sons, Inc.