A point and distance constraint based 6R robot calibration method through machine vision

•The tools are relatively cheap, and they are relatively easy to assemble, disassemble and carry.•The high sphericity of the ceramic reference sphere and resolution of the camera ensure high-accuracy measurements.•The calibration process can be executed on the robot station without disassembling or transporting the robot, and the space required for calibration is relatively small.•The measurement operation is relatively easy. This paper proposes a 6R robot closed-loop kinematic calibration method to improve absolute position accuracy with point and distance constraints though machine vision. In the calibration process, a camera attached to the mounting plate of the robot is used to capture a fixed reference sphere as a point constraint and to record robot joint angles and gauge block lengths that are used as a distance constraint. A first-order difference quotient is used to calculate the Jacobian matrix in the joint parameter identification process. The Staübli TX60 robot is successfully calibrated using the proposed method. After calibration, the average distance error of robot motion is decreased from 2.05 mm to 0.24 mm.