A stepwise calibration method for the probe center of stereo target based on sensitive directions

The stereo target coordinate measurement systems are widely used due to their portability and flexibility. The probe of the stereo target contacts with the measured object. Accurate and efficient calibration of the probe center of the stereo target has become a key problem that needs to be solved urgently. The position invariance principle based on the optimized system measurement model is put forward. Then, the overall optimization calibration method using all probe positions in different target inclinations is proposed to improve calibration accuracy. By analysis and simulation, the target inclination changes parallel to the image plane are most sensitive to x- and y-directional calibration, whereas the target inclination changes relative to the image plane are most sensitive to z-directional calibration. The inclination range of the target is the main factor, and the number of inclination changes is a minor factor that affects the calibration accuracy. A stepwise calibration method based on two sensitive directions is proposed innovatively. The probe center calibration of x-, y-, and z-directions is obtained by only 7 larger inclinations each in the two sensitive directions. The calibration efficiency is improved greatly. The experiments prove that the stepwise calibration method improves the accuracy by three times compared with the one step calibration method. After the stepwise calibration of the probe center, the accuracy of the stereo target coordinate measurement system in the x-, y-, and z-directions is increased by 34.5%, 34.5%, and 54.2%, respectively.