A Seam Measurement Method Based On Micro Depth Of Field Lens For Laser Welding

There are many advantages in laser welding, such as high energy density, highwelding speed, narrow heat-affected zones, small welding deformation, and easy to realizeautomatic control, etc. Therefore, laser welding has been widely applied to the mechanicalmanufacturing, aerospace industry, automotive industry, shipbuilding, powder metallurgy,biomedical, microelectronics industry, etc.The allowable seam gap width and misalignment in laser welding is very small. Thelaser focus must be positioned accurately along the seam trajectory, and the defocusdistance from the laser focus to the seam surface must be suitable. The welding laser beammust be adjusted to the local surface normal vector for3-D complex seam on the surface.Therefore, the seam gap width, seam position and seam surface normal vector must bemeasured before laser welding and tracked when laser welding, so as to guarantee thewelding quality.For the seam feature measurement of the tight butt joint without misalignment inlaser welding, current vision sensor cannot obtain the seam position and gap width stablyand reliably. A method for the measure of the laser beam welding seam based on the microdepth of field is proposed, in which a four times optical enlarger lenses, a CCD cameraand a LED external lighting device are used to constitute the measure system. The opticalaxis of the camera was placed at a certain angle. The vision detection device is mountedon a cross-slide. The method solves not only the problem of the small field in the micrurgybut also the problem in the observation of the subtle object.This paper presents the nonlinear model of the seam measurement system based onmicro depth of field lens and the system parameters calibration method, according to thelaser welding butt joint measurement system characteristics. A reasonable and efficientalgorithm of digital image processing is proposed for the weld image. The algorithm canextract the micro depth of field area, the central straight line of the high-definition belt, thecenter of the seam and the width of the seam in the weld image quickly and accurately.The3-D seam information can be obtained for the2-D seam information in the weldimage, according to the nonlinear model of the seam measurement system based on microdepth of field lens. The experiment shows that the seam measurement method based onmicro depth of field is fully satisfied the demand in laser welding.