Extrinsic calibration of a conoscopic holography system integrated in a CMM

In the present work, a procedure for extrinsic calibration of a 1D conoscopic holography sensor integrated in a CMM was developed, whereby it is possible to relate the coordinates of points captured by the sensor with the coordinate system of the machine. The procedure uses the information of the CMM error map to compensate the influence of these errors when associating the sensor measurements with the machine coordinate system. The calibration method was based on digitizing a test sphere and minimizing the error between the diameter of a least-squares fit of the digitized points and the value of its certified diameter. Results were compared for two test spheres of different diameter and material. To evaluate the efficacy of the calibration method, 3D distances between the spheres of a ball-plate were measured as well as several 3D geometrical features on a tailor-made test part. The tests carried out showed that the calibration method is independent of the type of test sphere used and that it is able to suitably compensate the positioning errors of the machine. Therefore, the calibration method proposed is applicable to integrate these type of sensors in CMMs and CNC machine tools with several controlled axes.