Seam-tracking for high precision laser welding applications—Methods, restrictions and enhanced concepts

Abstract

Laser beam welding bears evident advantages regarding precision, quality, productivity, low heat input, and feasibility of automation. At the same time the process calls for high precision of the beam positioning on the workpiece, which therefore imposes high requirements of welding trajectory and feed rate accuracy; e.g., for butt welding the focal point of the laser beam with respect to the joint must be maintained within a typical accuracy better than 20–150 μm, depending on the focused beam radius. To meet these requirements, seam-tracking devices are used. A sensor measures the joint position and computes a correction vector to compensate the joint trajectory offset. The deviation is compensated either by a robot trajectory adjustment or by an additional tracking axis. This paper describes the basic concepts of seam tracking in detail and points out problems in the different control principles, which are evoked by the forerun of the sensor. State-of-the-art sensors and error compensating techniques are presented and analyzed. Further, a new approach for seam tracking is introduced. It uses a multisensor concept, which in addition to the seam position measures the relative displacement between the processing head and the workpiece. An integrated two-dimensional beam positioning system enables “self-guided” processing, which allows high-accuracy tracking of a joint independent of the motion system and disengages from time intensive sensor calibration and robot teaching necessity.

Method for vectorial robot movement determination enabling accuracy improvements

“…The substrate (4) is held by a pneumatic gripper (5) attached to the robot. Thereby, the substrate is guided relative to printing nozzle and camera, whereby the Aerosol flow can be interrupted with low latency by a shutter mechanism (7). For the image data processing a Desktop-PC I7 4820K (16 GB RAM; Mainboard with Intel X79 Chipset and direct connection of the PCI-E Lanes to the CPU) in combination with Halcon V10 have been used.…”

“…due to the said absolute accuracy of the robot or a displacement of the work-piece, this deviation is detected and the information is used to establish a closed-loop control of the robot [4], [5]. While these methods are based on a pre-programmed robot trajectory other characteristic visual servoing methods allow the guidance of a robot's TCP along an unknown three dimensionally winded profile [6] or a seam [7]. By this way of proceeding, a self-guided robot movement is achieved.…”

Robust vision-based joint tracking for laser welding of curved closed-square-butt joints

“…It considers surface scratches, uncertainty during tack welds, and welding of curved joints. Also, estimating the joint position where the laser beam hits the work piece eliminates the geometrical issue that occurs when the sensor measures a certain distance ahead of the laser beam spot (called sensor forerun in [7]). The solution is based on an implementation of the Hough transform [16], a Kalman-based filter [17] and a joint curve prediction model based on prior knowledge of the nominal welding path.…”

“…This requires a camera off-axis with careful positioning relative the welding tool. Regaard et al [7] present different concepts and principles regarding sensors for joint tracking and introduce a multi-sensor concept using a CMOS camera with a low power laser source for illumination, for tracking and also for measuring the displacement between the LBW tool and the work piece. A method using a CCD camera and a vision algorithm to track closed-square-butt joints are described in [8] with promising results shown for an arc welding application.…”

Remote laser welding with in-line adaptive 3D seam tracking

“…Even though a weld path is known in advance, there is often a deviation from the taught or preprogramed path due to tolerances of workpieces, clamping devices, and imprecise teaching [7][8][9]. The tolerance of laser beam positioning is of the order of the beam radius and its Rayleigh length [10].…”