Validation of the network method for evaluating uncertainty and improvement of geometry error parameters of a laser tracker

Five National Metrology Institutes (NMIs) worked together to validate the assessment of laser tracker's (LT) uncertainty for large scale dimensional metrology in subsequent measurements using the network method. The LT uncertainty is assessed by measuring a set of a fixed network of targets from different LT positions. Afterward, we must perform a “bundled adjustment” of all the measurements to determine the transformations of the LT positions that minimize the residuals (differences) between a computed “virtual” group of targets, called composite points, and the redundant targets coming from the LT positions transformed to a unique LT position. Each residual is weighted with the LT's Maximum Permissible Error (MPE) specified by the LT manufacturer. The standard deviation of the weighted residuals becomes LT uncertainty. Afterward, with Monte-Carlo simulation, we propagate the LT uncertainty to the position of every target in the network and 3D distances between them. LT uncertainty is valid if 3D distance's uncertainties computed with composite points is greater than the error computed with these distances minus the same distances calibrated with the line of sight method (LOS) or if the absolute normalized error is less or equal to one. LOS method uses only the LT's interferometer for calibration, which dispense the use of a calibrated scale bar. Based on the network method, the NMIs developed two methods to improve LT geometry error parameters. The first improves the LT geometry error parameters by fitting the composite points to the LT geometry error model. The second evaluates the parameters placing the LT geometry error model in the minimization of the bundled adjustment's residual. LT geometry error parameters are valid if the parameter's uncertainties are negligent compared to its value. •A method to identify and improve geometry error compensation parameters of a laser tracker (LT).•Validation of a simple and practical method to report measurement uncertainty under real circumstances.•Avoid using a calibrated scale bar for future LT measurement comparisons between National Metrology Institutes (NMIs).•A starting point for the use of LT Maximum Permissible Errors (MPE) and uncertainty evaluation.•LT uncertainty comparison in similar environmental conditions with a similar setup of targets in a 3D point network. This work can be considered a method for future inter-comparison between NMIs.