Abstract
Errors in the angle encoders of a laser tracker may potentially produce large errors in long range coordinate measurements. To determine the azimuth angle encoder errors and verify their values stored in the tracker’s internal error map, several methodologies were evaluated, differing in complexity, measurement time and the need for specialised measuring equipment. These methodologies are: an artefact-based technique developed by NIST; a multi-target network technique developed by NPL; and the classical precision angular indexing table technique. It is shown that the three methodologies agree within their respective measurement uncertainties and that the NPL technique has the advantages of a short measurement time and no reliance on specialised measurement equipment or artefacts.
Highlights
Results obtained at the National Physical Laboratory (NPL) from measurements of azimuth angle encoder errors of a laser tracker are presented and compared with the manufacturer’s internally stored error values
The metrological performance of a laser tracker is influenced by many factors including: compensation for atmospheric effects, thermal expansion of the instrument and its mount, thermal distortion of the work piece or artefact being measured, the wavelength of the laser radiation, the internal alignment of the gimbal axes and the linearity and alignment of the internal angle encoders
Errors in the horizontal and vertical angle encoders of the tracker may potentially produce large errors in measurements made by the tracker
Summary
Results obtained at the National Physical Laboratory (NPL) from measurements of azimuth angle encoder errors of a laser tracker are presented and compared with the manufacturer’s internally stored error values. Errors in the horizontal and vertical angle encoders of the tracker may potentially produce large errors in measurements made by the tracker. These angular errors result from spacing errors in the gratings of the angle encoders and centring errors of each encoder relative to its rotary mechanical axis. To evaluate the horizontal angle encoder errors and verify the values stored in the tracker’s error map, several methodologies were used, differing in complexity, measurement time and the need for specialised measuring equipment. All the measurement data were acquired using Spatial Analyzer [3]
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