Thermal Compensation of Photogrammetric Dimensional Measurements in Non-standard Anisothermal Environments

Abstract

Manufacturers are currently facing large volume metrology challenges driven by thermal effects such as variation in refractive index and thermal expansion. Thermal expansion is one of the largest contributors to measurement uncertainty and it can often be difficult to realise the standard 20° C temperature required. The current process for dimensional measurement requires that the temperature is measured at the instrument, and the entire measurement volume is scaled linearly by the same factor. Unfortunately, this assumes that temperatures are uniform all over the measurand, which is seldom the case particularly at large volume scales.Useful for deformation measurement, photogrammetry is increasingly employed in industry, which in some cases can exhibit uncertainties comparable with the industry standard laser tracker. By measuring temperature more broadly and combining this data with finite element analysis, it is possible to compensate each of these points in 3D space along the X, Y and Z axes. Actively creating challenging metrology conditions with highly localized temperature gradients, and maximum temperatures in excess of 45° C has allowed this approach to be tested. Results show that in many cases it is possible to make localized predictions of displacement within the range of photogrammetric measurement uncertainty.