Weld defects should be recognised in good time when welding by laser beam using integrated process monitoring. Welds were made on lap joints with Nd:YAG and CO_2 lasers in which a capacitive distance measurement with a sensor system was employed for controlling the position of the focus. In laser beam welding, however, the oscillator frequency of the sensor is not constant but there is noise. The structure of the noise depends on the feed rate and focus position and, when weld seam defects occur, shows characteristic changes. In a new process, the frequency distribution of the capacitive output voltage is evaluated using the frequency discriminator of the sensor system. In this way, evidence on the process parameters and advice on weld defects is obtained.
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