A calibration method for spatial error of industrial robot under variable load conditions based on double-ball rotary structure and three contact displacement sensors

Abstract

A new calibration device using a double-ball rotary structure and three contact displacement sensors is proposed to measure the spatial errors of six-axis serial robot. The error model is divided into kinematics part and static stiffness part, established by Modified DH (MDH) model and virtual joint method. The MDH parameter errors are compensated by modifying the angles of each axis with Newton Raphson compensation method and ridge estimation. The stiffness parameters are also identified by the least square method. The self-designed double-ball rotary device is calibrated by coordinate measuring machine (CMM), while the three-point measuring device is calibrated by gauge block. Then, the kinematic and static stiffness calibration and compensation experiments are carried out on EPSON C4 A901 robot in turn. The spatial error under none load and 1.0 kg, 1.5 kg, 2.0 kg gravity load are measured separately. The experiment results show that the motion accuracy of the robot is improved to the range from 0.0163 mm to 0.0236 mm under the gravity load conditions between weights of 1.0 kg and 2.0 kg. Finally, by comparing the measuring results from the laser displacement sensor and laser interferometer, the measurement accuracy of each axis of the self-designed device is equivalent to the laser displacement sensor and the laser interferometer in micron range.