Process control during laser machining

Controlled laser processing is required to improve and ensure the quality of welding and heat treatment. A sensor collects informations from the processing zone and passes them to a controller. The controller generates a signal adjusting the beam power according to the present situation. This feed-back structure using a pyrometer as sensor for the surface temperature was successfully employed for transformation hardening of workpieces with complex geometries. The complex phenomena in laser induced plasma require a modified sensor. the plasma signal detected by a photodiode contains implicit informations about the quality of the welding process. These informations have to be worked up before they can be passed to a controller. An experimental approach of process modelling, which requires a wide banded (0...300kHz) on-line mesurement of beam power for a recursive system identification algorithm, will be described.