<title>Real-time generation of intersection of surfaces for welding by video moire</title>

Abstract

A common problem in fabrication and welding of complex strucwres is that there is no simple way to determine where to cut one part so that it will fit another part unless both parts designed and built on a CAD/CAM system. Particularly in prototype or retrofit work, cutting and fitting parts for welding is more of an art than a science. We have developed a unique video moire system that generates the intersection contour in near real time with the contour output superimposed on a video image of the part, allowing the cut line to be marked while following the contour on the video momtot Moire techniques have been used to determine the shape of a surface or the deviation of a manufacwred shape from a desired shape. In a traditional moire system, distorted gratings on an object are viewed through an undistorted grating. The moire contours which result represent equal depth contours over the entire viewed surface. By generating the moire patterns in video, it is possible to view the distorted gratings on one object through a set of gratings that has been diStOrted by a different object For reasons to be explained in this paper, these moire contours can be thought of as a family of intersections between the two objects. The problem is how to find the unique intersection contour which is located where the parts are to be joined. We have solved this by using a specially modified Michelson interferometer to generate the gratings. By rotating one mirror, the interference fringes can all be made to move away (or towards) one central stationary fringe. This causes a corresponding movement in the family of intersection (moire) contours away from or towards the unique stationary intersection contour which is the desired cut line. The parts are moved until the intersection contour is located at the correct position. The interferometer mirror is then rotated while capturing a series of images. These frames are averaged, generating a composite image which contains only the unique stationary intersection contoui This image is then projected back on the targets and the parts are marked along the intersection contour. If the parts are cut along the marked curve, they fit exactly. Except for the frame average, taking a only a few seconds, the operation is essentially real time and involves no computation. In an industrial version of the system, the illuminators and cameras would be moved around thejigged parts so that all sides of the parts can be illuminated to continue the cut lines all around the parts.Once the shape of the prototype parts have been determined, the intersection contours can be added to the CAD files containing the shapes so that production parts can be manufacwred with the correct cut allowing easy fabrication. We have successfully applied this system to generating the intersection of two surfaces meeting at an oblique angle as well as to joining other structures. This moire system may have additional applications in 3-D location of parts for robotic assembly.