A 3D Passive Laser Tracker for Accuracy Calibration of Robots

Laser trackers are usually used to measure spatial positional error of the moving targets on large-scaled objects, such as the airplane profile. It is a spherical coordinate system with a dual-axis tracking mirror actively controlled by the built-in servo system to direct the laser interferometer beam always toward the reflecting target. It is very expansive when used in testing the spatial positional accuracy of industrial robots, which are rapidly growing for factory automation in recent years. Therefore, simplified from the laser tracker structure, a 3D passive laser tracker (3DPLT) for measuring spatial trajectory of a moving target is proposed. It is composed of a 2D rotary stage and a linear telescopic mechanism with an end-ball, which can be magnetically sucked by the moving target. The rotational angles of the 2D rotary stage are measured by two respective encoders, and the radial displacement of the telescopic mechanism is detected by an equipped laser interferometer. The world coordinate of the target can be obtained by the coordinate transformation from the spherical measuring system of 3DPLT. Multibody system error modeling is used to analyze the measurement error of the instrument, and testing methods of the main error sources are also proposed. After accuracy calibration and error compensation, the measurement error of the 3DPLT is 49.2 mu m, which is good enough to calibrate the spatial trajectory error of the robot.