Friedrich-Alexander-Universität Erlangen-Nürnberg

Process Monitoring and Control of Focal Position in Laser Beam Cutting

Process Monitoring and Control of Focal Position in Laser Beam Cutting

The increasing use of high brightness lasers with output power in the kilowatt range for laser beam cutting leads to big advantages concerning yield and quality on the one hand, but on the other hand causes stability problems. Thus, the focal position, which can be influenced by many different factors during the process, cannot be regulated yet. Therefore, in cooperation with Maschinenfabrik Arnold GmbH und Co. KG and Precitec KG the Bayerisches Laserzentrum GmbH develops a process monitoring system for laser beam cutting, especially for the detection of instabilities due to focal position variations.

By a camera coaxially mounted inside the processing head an online monitoring of the cutting process is enabled. Specific values like the optical emission and kerf width can be derived from the image, different states of the process and changes of the cutting quality can be identified and detected, respectively. Afterwards, a correcting variable for a closed loop control of the axial focal position is generated.

Measuring the beam caustic of high brightness laser sources reveals that the focal position strongly depends on thermal effects within the optical system. Different states of the system influence the thermally induced focal shift (FS). The insight gained about the real focal position during the process enables feedback of the process variables to the focal position detected by the camera and thus, is the basis for a closed loop control.

Intensity distribution in focus position and measured focus shift (FS) at a) 100 W; b) 1000 W / clean cover glass; c) 1000 W / contaminated cover glass
Illustration of a possible detection of a focal shift by time-resolved coaxial camera-image analysis (Focal shift ≈ 1000 µm)


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