Low-cost and automated calibration method for joint offset of industrial robot using single-point constraint

Purpose - The industrial robot has high repeatability but low accuracy. With the industrial robot being widely used in complicated tasks, e.g. arc welding, offline programming and surgery, accuracy of the robot is more and more important. Robot calibration is an efficient way to improve the accuracy. Previous methods such as using coordinate measurement machines, laser trackers or cameras are limited by the cost, complex operation or the resolution. The purpose of this paper is to propose an approach and calibration equipment to address these issues. Design/methodology/approach - The proposed method relies mainly upon a laser pointer attached on the end-effector and single position-sensitive devices (PSD) arbitrarily located on the workcell. The automated calibration procedure (about three minutes) involves aiming the laser lines loaded by the robot towards the center of the PSD surface from various robot positions and orientations. The localization is guaranteed by precise PSD feedback servoing control, which means physically the intersections of each pair of laser lines (virtual lines) are on the same point. Based on the untouched single-point constraint, the robot joint offset calibration is implemented. Using the authors' proposed approach, a portable, low-cost, battery-powered, wireless and automated calibration system was implemented. Error analysis was conducted on the system. Findings - The localization error of the developed calibration system is within 2 mu m. The errors in joint space are magnified in PSD plane, and consequently the resolution in the joint space is improved. The standard deviation of the identified parameters was small (10-2), indicating the stability of the calibration method. Both simulation and experimental results verify the feasibility of the proposed method and demonstrate the developed calibration system can identify joint offset with uncalibrated laser tool parameters. Originality/value - The paper shows how a portable calibration system for joint offset of industrial robots was developed and how the goal of fast, automated, low-cost, portable, and high precision calibration methods for joint offset was achieved.