Calibration of an Industrial Robot Using a Stereo Vision System

Industrial robots have very good repeatability but still lack good absolute accuracy. The main reason is difference between the ideal robot kinematic model integrated in the robot controller and actual robot parameters. A method for identifying certain parameters of the robot model has been proposed. A noncontact method using a stereovision system attached to the robot arm is utilized for providing measurements of calibration points in space. Points are represented as spheres which localized by the stereo vision system project a circle in two image capture planes independent of the viewing angle. Spatial coordinates of each sphere center are acquired in different robot configurations. From these readings errors of robot absolute positioning are measured. The standard Denavit-Hartenberg (DH) notation is used when the modified model parameters containing joint encoder offset values are directly input to the robot controller. Calibration experiments carried out on a KUKA KR 6 R900 industrial robot show improved accuracy results. The maximum positioning error around calibration points was decreased from 3.63 mm prior to calibration, to 1.29 mm after the calibration procedure.