ASME B89.4.19 standard for laser tracker verification – experiences and optimisations

Abstract

Laser trackers are becoming the tool of choice for large volume dimensional metrology applications such as the measurement of aerospace assemblies, power plant structures, civil engineering structures and terrestrial transportation vehicles. A laser tracker is a portable coordinate measuring system that tracks a moving target reflector and measures the position of the target in spherical coordinates (r, θ, φ). 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 workpiece 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 angular measuring scales. The most important of these potential error sources, which fundamentally limit the achievable uncertainty, are the internal mechanical and optical alignments and the quality and alignment of the angular scales. Several national and international standards exist or are in the process of being developed for performance verification of laser trackers. ASME B89.4.19-2006 is one of the established standards used to verify the performance of laser trackers. The main test relies on measuring a known reference length in a variety of configurations and ranges and comparison of the observed error (laser tracker measured length minus reference length) with the specified maximum permissible error (MPE) defined by the manufacturer. The establishment of an ASME B89.4.19 laser tracker verification facility at NPL is introduced. We highlight the importance of tracker verification and discuss the error sources, which contribute to the tracker measurement uncertainty. Some initial results obtained using this new facility are presented.