Research On High Accuracy Angular Error Compensation Technology For Laser Tracker

The laser tracker is a large-scale three-dimensional coordinate measuring instrument,which works in mirror-tracking mode. The reflecting mirror in the tracker’s head, whichis followed by the rotating of horizontal shaft and pitch shaft in the two-dimensionalprecision tracking turntable, can steer the laser beam to the target. The angular data oftarget is obtained by circular grating encoder measurement systems which are mountedin horizontal shaft and pitch shaft of the laser tracker while the distance data is obtainedby Interferometer(IFM) and Absolute Distance Meter(ADM). Then the spacialthree-dimensional coordinate of target is calculated in the spherical coordinate system.The measurement accuracy of the laser tracker is directly affected by angular errors,which are the main error sources of instrument. In order to achieve high-precisionangular measurement, error compensation technique need to be used.In this paper, the error source and error model of laser tracker metal circular gratingencoder measurement system were firstly analysed. Then the principle of mounting tworeadheads diametrically opposed was studied. Thirdly, a variety of error compensationalgorithms and models were analysed. Then angular errors were calibrated discretely byautocollimator combined with multi-mirror polygon. A compensation model wasdeveloped with error harmonic compensation method based on nonlinear least squaremethod, whose components are built up from harmonic wave components whoseamplitudes are larger and phases remain essentially constant. Horizontal angular errorsand vertical angular errors of laser tracker were compensated with the methodmentioned above, which was also compared with other compensation methods, such aslinear compensation, cubic spline compensation and polynomial compensation.The uncertainty of angular error calibration was calculated. And angular errorscomponents and angular accuracy before and after compensating were also analysed.After compensation, horizontal angular measurement accuracy was reduced from1.60"to0.90", and vertical from4.89" to0.91", whose confidence probability is within arange of99.73%. The accuracies were improved by44%and81%respectively, andreached sub-arc second accuracy. The experimental results showed the effectiveness ofthe error compensation method proposed in this paper, which can improve theinstrument measurement accuracy greatly.