Local and closed-loop calibration of an industrial serial robot using a new low-cost 3D measuring device

We propose an automated, closed-loop, and local calibration method for serial robots that uses a new, low-cost, 3D measuring device. The device consists of three Mitutoyo digital indicators, arranged in an orthogonal manner, and a mastering fixture based on kinematic coupling. The indicators communicate, via wireless connection, with a PC that controls the movements of the robot. To measure absolute Cartesian coordinates, the device is positioned incrementally over each of several 0.5-inch precision balls until all indicators are at zero, at which time the robot joint encoders are read. The balls are fixed with respect to the robot's base. The precise relative positions of the centers of these balls must be known in advance. In this study, the measuring device is mounted on the flange of an ABB IRB 120 robot. Only three precision balls are used, spaced 300 mm apart, and the distances between these balls are measured with a Renishaw telescoping ballbar. The absolute accuracy of the robot was enhanced by minimizing its position errors, using the least squares method. The feasibility of the calibration approach was demonstrated through a simulation study. Finally, an experimental validation showed that our calibration method caused the maximum position error of the robot, inside a sphere of 400 mm in diameter, to be reduced to 0.491 mm.