Friedrich-Alexander-Universität Erlangen-Nürnberg

Detecting the keyhole geometry during laser beam welding

Detecting the keyhole geometry during laser beam welding

Since the 1980s, Laser beam welding is an industrially applied technology that allows for the realization of strong and highly precise weld seams. Recent developments of beam sources for laser welding enable higher process speeds, deeper weld seams and smaller foci. To ensure a stable and reproducible process, also under these demanding conditions, the process has to be monitored and actively controlled. 

Robot guided laser welding at LPT

Though closed loop control systems exist for certain special cases, the state of the art is still governed by a general lack of “suitable” sensors (representing the crucial part of a closed loop control system). Here, “suitable” denotes the ability to monitor the keyhole behaviour during the weld process with sampling  rates of several Kilohertz.

Current research at the Chair of Photonic Technologies within the framework of SAOT focuses on the tracking of the vapour plume in real time. This plume is formed by hot vapours that are ejected by the keyhole and therefore show a direct correlation to the keyhole geometry. The online determination of the centre and the size of the vapour plume are supposed to yield information on the keyhole length, tilt and diameter, i.e. quantities that are directly related to the process result. Therefore a sensor has been developed that observes the vapour plume during the weld process based on four pairs of concentrically arranged photodiodes. 

High Speed Imaging of the vapour plume

Offline calibration of the sensor permits the calculation of the absolute position of the centre of the vapour plume based on the signals of the photodiodes. First results on the detection of full penetration during the weld have been published. Further experiments will focus on the online measurement of the penetration depth and feed speed.

Calibration of the Sensor using technical light sources
To detect the centre of the vapour plume, the sensor is attached to the focussing optics


C. Brock, R. Hohenstein, M. Schmidt: Optical 3D position sensor for the fast tracking of light sources. In: Physics Procedia, Volume 5, Part B, 2010, Pages 437-445

C. Brock, R. Hohenstein, M. Schmidt: Towards Fast Tracking of the Keyhole Geometry. In: Physics Procedia, Volume 12, Part A, 2011, Pages 697-703

C. Brock: Laserstrahltiefschweißen – Prozessanalyse und Sensorkonzepte. In: Workshop “Prozesskontrolle bei der Lasermaterialbearbeitung”, Nov. 2011 (oral presentation only)


SAOT provides an interdisciplinary research and education program of excellence within a broad international network of distinguished experts to promote innovation and leadership in the areas

Optical Metrology
Optical Material Processing
Optics in Medicine
Optics in Communication and Information Technology
Optical Materials and Systems
and Computational Optics.