Large-scale shape measurement by a combined method based on three instruments

Abstract

The accuracy of optical three-dimensional metrology scanning decreases as the size of the parts for inspection increase to several tens of meters because of accumulated errors. A combined method that includes coordinate and surface metrology is proposed. The method features the use of a laser tracker, photogrammetric system, and a hand-held scanner that identifies fiducials. Each instrument provides a distinct measurement density of point data and ranges which are complementary to each other. The laser tracker is used to position and orient the scanner within a large volume, thus, to ensure accurate measurement, the data captured in the respective sensors' coordinate system is aligned to a common global coordinate system. Close-range photogrammetry is used to improve the speed with which fiducials are measured. The measurements obtained, using the photogrammetric system and laser tracker, are combined to produce highly accurate measurements of part surfaces. The performance, of the combined measurement systems, is verified by measuring calibrated lengths. Results show that the proposed technique enables quick measurement and yields submillimeter accuracy in measuring the absolute distance up to the laser tracker range. Error accumulation is therefore avoided.