We describe a method to estimate the scale errors in the horizontal angle encoder of a laser tracker in this paper. The method does not require expensive instrumentation such as a rotary stage or even a calibrated artifact. An uncalibrated but stable length is realized between two targets mounted on stands which are at tracker height. The tracker measures the distance between these two targets from different azimuthal positions. Each target is measured in both the front face and also in the back face. Low odd order harmonic scale errors can be determined from the difference in azimuth angle between front face and back face measurements to any one target. Low even order harmonic scale errors are determined from the residuals of the measured lengths at different azimuths from their average. These low order harmonics can then be used to correct the tracker s error map thereby increasing the tracker s angle measurement accuracy. We have demonstrated this for the second order harmonic in this paper as our measurements indicated their influence to be the dominant source of error among scale errors. It is important to compensate for even order harmonics as their influence cannot be removed by averaging front face and back face measurements; odd order harmonics can be averaged out by measuring in front-face and again in back-face. The length errors in a 7.75 m horizontal length placed 7 m away from a tracker were of the order of +-65 um before correcting the error map. They reduced to less than +-25 um after correcting the error map for second order scale errors.
Citation: Journal of Research of the National Institute of Standards and Technology
Pub Type: Journals
ASME B89.4.19, encoder scale errors, laser tracker, length tests, two-face tests