This thesis presents an experimental study on how to track zero gaps between metal sheets to be joined by laser beam butt welding. Automated laser beam welding is gaining interest due to its ability to produce narrow and deep welds giving limited heat input and therefore less distortions compared to other processes, such as arc-welding. The automated laser beam welding process is however sensitive to how the high power laser is positioned with regards to the joint position. Deviations from the joint position may occur due to inaccuracies of the welding robot and fixturing, changes in joint geometry, process induced distortions, etc. Welding with an offset from the joint position can result inlack of sidewall fusion, a serious defect that is hard to detect. This work develops and evaluates three monitoring systems to be used during welding in order to be able to later control the laser beam spot position. (i) A monitoring systemis developed for three different photo diodes, one for the visual spectrum of the process emissions, one for the infrared spectrum, and one for the reflected highpower laser light. The correlation between the signals from the photodiodes and the welding position relative to the joint is analysed using a change detection algorithm. In this way an indication of a path deviation is given. (ii) A visual camera with matching illumination and optical filters is integrated into the laser beam welding tool in order to obtain images of the area in front of the melt pool. This gives a relatively clear view of the joint position even during intense spectral disturbances emitted from the process, and by applying animage processing algorithm and a model based filtering method the joint positionis estimated with an accuracy of 0.1 mm. (iii) By monitoring the spectral emissions from the laser induced plasma plume using a high speed and high resolution spectrometer, the plasma electron temperature can be estimated from the intensities of two selected spectral lines and this is correlated to the welding position and can be used for finding the joint position.

Laser beam welding is one important technology in automated production. It has several advantages, such as the ability to produce deep and narrow welds giving limited heat induced deformations. The laser beam welding process is however sensitive to how the high power laser is positioned with regards to the joint position. Therefore, to achieve a seam without defects, the joint position needs to be measured and controlled. The laser beam welding process is also sensitive to variations in joint gap width. Costly joint preparations are required to achieve the tight fit up tolerances needed to produce high quality welds. However, the demand on joint preparation can be somewhat relaxed by allowing the joint gap width to vary and controlling the process. One way of doing this is to control the filler wire feed rate based on joint gap width measurements.This thesis presents experimental studies on how to track closed-square-butt joints and also how to handle varying square-butt joints in laser beam welding.

Different optical sensor systems are evaluated for their performance to estimate the joint position and the joint gap width. The possibility of detecting beam offsets is studied by using sensors systems based on a photo diode and on a spectrometer. Estimations of the joint position, to be used for closed loop position control, is studied by using a camera and external LED illumination. Variations in joint gap width is evaluated using a spectrometer, a camera and a laser profile sensor. Experimental results show that both the photodiode system and the spectometer system is able to detect beam offsets and that the beam position can be estimated with sufficient accuracy when welding closed-square-butt joints. It is also shown that the joint gap width can be estimated by the selected sensor systems and that the estimates can be used for controlling the wire feed rate in order to obtain a constant weld geometry and avoid defects related to the gap width.