AKL e.V.


Penetration Depth and Topography Measurement in Laser Materials Processing using Low Coherence Interferometry

Dr. Markus Kogel-Hollacher
Dr. Markus Kogel-Hollacher

Head of department R&D projects
Precitec Optronik GmbH
Neu-Isenburg, Germany

ZIP (english text and pictures)   ZIP (german text and pictures)

Members of the Project Team
  • Dr. Markus Kogel-Hollacher, Precitec Optronik GmbH, Neu-Isenburg, D (Team Representative)
  • Dr. Stephan Bichmann, Scheidt & Bachmann GmbH, Mönchengladbach, D
  • Dipl.-Phys. Niels König, Fraunhofer IPT, Aachen, D
  • M.Sc. Guilherme Mallmann, Fraunhofer IPT, Aachen, D
  • Dipl.-Ing. Thibault Bautze, Precitec GmbH & Co. KG, Gaggenau, D
  • Dipl.-Ing. (FH) Christian Fraas, Precitec Vision GmbH & Co. KG, Neftenbach, CH

Areas of Application
Nearly all industrial sectors using lasers for materials processing benefit from this new sensor technology, starting from deep penetration welding through material ablation up to additive manufacturing. Examples are online control of deep penetration welding depths in car body manufacturing or online topology measurement of structured moulds during the ablation process with short pulsed lasers in the tool making industry.

Technological Impact
  • New online process monitoring system on the basis of Low Coherence Interferometry technology
  • Improved quality assurance by detection and correction of defects during early production stages
  • Shorter development lead time
  • Reduced manufacturing costs for products

With respect to online quality assurance in laser processing there are still unsolved tasks like the online measurement of the penetration depth during laser welding. Up to now all attempts are based on the detection and evaluation of the electromagnetic radiation emitted from the keyhole or adjacent areas using camera devices or photo diodes as signal receivers. Even though measurements of real geometries, e.g. keyhole diameter or melt pool size, have been realized, the depth of the keyhole is still an estimation based on the detected values, physical relations and data processing.

A similar problem is the real-time measurement of parts structured by pulsed laser ablation. The generation of such surfaces is a complex task as the removal rates strongly depend on different machine, workpiece and environmental conditions, which influence process stability and product quality. Consequently there is a need to adapt a sensor system to the ablation process which is capable to measure online and coaxially to the processing beam the topology of the structured surface.

The innovation consists in the development of a new online sensor technology on the basis of Low Coherence Interferometry which is capable to exactly measure the depth of the keyhole during laser welding or the amount of ablated material during laser structuring or the level of material build up during additive manufacturing. The light of low coherence length with the aid of an interferometer is used for distance measurement of scattering materials like human tissue.

In 2006 the Precitec Optronik GmbH made the first trials with low coherence interferometry as a potential technology for distance measurement in remote laser welding. The work demonstrated that the measurement is not influenced by process emissions, i.e. electromagnetic radiation from the interaction zone. Aligning the measuring point of the sensor system into the keyhole makes an exact measurement of the depth of the capillary possible within the micrometer resolution which is requested by high precision laser welding processes. The sampling frequency of 70 kHz also ensures that the temporal density of measurement points is sufficiently high for typical welding speeds in industrial applications.

In 2009 the Precitec Optronik GmbH and the Fraunhofer Institute for Production Technology IPT started a collaborative project funded by the German Federal Ministry of Education and Research BMBF to enable this technology for high-precision topography measurement in laser structuring processes.

Some evolutionary steps later and based on the previous R&D results the standard for Low Coherence Interferometry was adapted to laser applications. In September 2013 a commercial system called IDM - In-Process Depth Meter - was presented to the public. All experience gained so far with the IDM system show an accuracy of the measured depth value of better than ± 10 microns.

Topography measurement of laser structured surfaces of tool steel,
(Picture Source: Fraunhofer IPT, Aachen)

Prototype of the laser micro machining system with integrated measuring device
(Picture Source: Fraunhofer IPT, Aachen)

In-Process Depth Meter IDM-System integrated in a laser welding head (left) and results of the IDM welding depth measurement compared to a cross section of the weld (right). Almost independent of the laser power the measured signal follows exactly the value of the capillary depth (St37 - structural steel).
(Picture Source: Precitec GmbH & Co. KG, Gaggenau)

HOME  |  Contact  |  Imprint  |  PRINT