Strain fields measurement of non-rigid solid by scanning laser coupled with camera

¹ DRIVE, Université de Bourgogne, route des plaines de l’Yonne, 89 000 Auxerre, France
² Laboratoire Electronique, Informatique et Image, route des plaines de l’Yonne, 89 000 Auxerre, France

^a Jean-François FONTAINE: jffont@iut-dijon.u-bourgogne.fr

Abstract

Today, the techniques of measurement without contact by scanning laser are increasingly used. These techniques allow to measure 3D objects without contact and the measurement rates can reach few thousand points by second that is very adapted to complex shapes. To measure displacements and deformations, it is necessary to follow a solid particle in its motion between two configurations of the body before and after displacement. The method of stereovision based on grayscale patterns, obtained by applying a spray, is generally used. The results are obtained by two correlations: spatial and temporal correlations. An alternative method consists to use a scanner laser coupled with a camera. The laser scanner measures the 3D form and the camera permits to identify patterns on both configurations. The measurement process is described in the paper. The principle consists to acquire a point cloud of the surface before and after the movement as well as an image of the region of interest. The points clouds are given in the 3D frame and the position of pattern can be given in 3D coordinates. To have a coherent measurement, the camera and the laser scanner must be calibrated in the same frame using a single target. The difficulty is that the target must have contrast elements that are useful for calibrating the camera, but the response of the laser sensor is biased at a change of contrast. So, filtering must be used. Different kinds of targets have been tested (cube, checker board,…). The patterns are correlated in the images pair by classical correlation function. The point cloud is meshed with triangles (STL format) and projected on the corresponding image. On each image, the belonging of a pattern to a triangle is then tested on the basis of barycentric coordinates and the coordinates of the 3D pattern are calculated on the base of the conservation of these coordinates. So the localization of the pattern in 3D frame is possible. The advantage of this method is to reduce the imprecision of matching the search pattern unlike stereovision where two pairing are carried out. An application of deformation of soft body illustrates this approach. A comparison of the result is carried out with the stereo-correlation technique.