Design, construction, and calibration of a step gauge of nests for performance evaluation of Laser Trackers

Abstract

According to documentary standards for the performance evaluation of Laser Trackers (LT), long length reference artifacts are required. In this paper, we discuss the design, construction, and calibration of a long length artifact called step gauge of nests (SGN). The SGN has several nests in line to place the LT probe; the two extreme nests of the SGN are at a distance of 3 m approximately. The documentary standard establishes that the gauge's length must be known no matter the orientations it takes. However, for long gauges, factors like gravitational force, fixturing forces, change in the environmental conditions, among others, deforms the gauge, and its length changes when its orientation changes. To evaluate these factors, in the design stage, we use a finite element simulation of the SGN to predict such deformations (mainly length variations between the two extreme nests). The simulation takes into account the used material, its stiffness, straightness, distribution of the nest's weight, and geometry's change of the SGN to reduce the variations in its length. For the construction stage, we describe how the SGN was manufactured and how using high module carbon fiber, we reduce the influence of the temperature factor. The results of the finite element simulation show a length variation of around 20 ppm between the horizontal and vertical SGN positions. That variation was validated with the calibration results using two different methods. The first uses the line of sight (LOS) method, which involves the same LT under evaluation. The second uses an accurate CMM, using the overlap method for calibration. The traceability of the LOS method is accomplished with the wavelength calibration of the LT interferometer; meanwhile, the overlap method uses a CMM evaluated with a laser interferometer with calibrated wavelength.