Laser welding process control with pco.dimax cs and Cavilux SMART
Filming the welding process is accompanied by an additional obstacle in addition to the highspeeds: during this process, a bright light covers the actual joint area. The bright process glare can be suppressed by a trick. As the glare is usually whitely, it can be concluded that its intensity is spread almost constantly across all (visible) wavelengths.
The light source which is mandatory for the high speed images only emits light in a small wavelength range (e.g. 640nm). Thus, its intensity is much greater than the process glare, even if the latter appears to be brighter to the human eye.
To suppress all other wavelengths, an optical bandpass filter is fitted to the camera in addition to the special light source. In the experiments, a laser with a nominal wavelength of 640 nm ± 10 nm is used together with a filter for 640 nm ± 5 nm. The laser is comparably easy to handle, because the light is visible and the emitted beam itself is neither coherent nor collimated. This avoids speckling and makes reflections harmless even despite the power of 400 W. The laser pulses are synchronized with the camera.
Since the pulse duration for typical repetition rates (>1000 fps) is not more than 1 µs it is recommended to use short exposure times (e.g. 1.5 µs for pco.dimax).
There are several technical challenges in this kind of application:
- Optic: a relatively high optical magnification is required. A typical field of view is about 20 x 20mm. At the same time a specific working distance is required to avoid the damage of the equipment.
- Reflections of the metal surface can saturate the image in certain areas accidentally.
- The field of illumination and reputation rate of the laser has to be coordinated to the field of view and frame rate of the camera.
As optics we used a 100mm Zeiss lenses with x2 Teleconverter. The best results we got with the setup shown below.