Large-scale full-field metrology using projected fringes: some challenges and solutions

Abstract

The application of optical techniques to the measurement of shape and deformation of structures in the aerospace industry poses unique challenges resulting from the large length scales involved, which are typically in the 1-10 m range. For example, the relative immobility of large samples requires a network of sensors to be linked into a common global coordinate system; traceable calibration requires the development of new types of calibration artefact; and traditional interferometric techniques for displacement field mapping are frequently too sensitive to observe the physical effect of interest. We describe a system designed to address some of these problems based on the projected fringe technique combined with temporal phase unwrapping. Multiple cameras and projectors are linked into a common coordinate system using calibration concepts borrowed from the photogrammetry field. Traceable calibration is achieved through the use of reference spheres separated by a bar of known length. Traditional two-dimensional image processing techniques for recognizing circles (Hough transforms) have been extended to the automatic detection of spheres within the measured 3-D point clouds. Bundle adjustment software has been developed to refine the camera and projector calibration parameters as well as the rigid body translation and rotation coordinates defining the poses of the calibration artefact. An overview of all these aspects of the developed techniques is given in the paper. Typical results from a compression test on a large scale aluminium structure, performed on-site at Airbus UK using the developed system, are also presented.