Optical sensing boosts spot welding for auto makers

18 Mar 2014

Fraunhofer IPMS presenting prototype of an optical sensor system at the 2014 Smart Systems Integration expo.

Three research institutes and four industrial companies have collaborated for two years in European joint research project called SmartDress to develop a fully automated system that monitors, optimizes and processes the electrode tips for resistance spot welding in the automotive industry.

The Fraunhofer Institute for Photonic Microsystems (IPMS) in Dresden, Germany, provided the optical system to measure the wear that occurs on the welding electrodes rapidly and precisely.

Knowing this helps to optimize the quality of welded joints, increase the life cycle of electrode tips so making it possible to use spot welding economically even for sophisticated materials such as aluminum. Fraunhofer IPMS will present a prototype to a “wide specialist public audience” at the SmartSystemsIntegration 2014, in Vienna, Austria, from 26th-27th March 2014.

Nowadays, an average family car is held together by approx. 5,000 spot welds. For many years, the welding process used for this purpose - resistance spot welding - has been the undisputed leading technique for the long term joining of steel plates in the automotive industry, in bodywork and vehicle construction and in the sheet steel processing industries in general.

An essential quality characteristic of the process has not been systematically recorded, so far: the wear on the welding electrode. An increasing number of welding processes leads to the electrodes wearing out and consequently the cross-section of the contact areas increases in size due to thermal and mechanical influences.

This leads to the spot weld becoming ever larger and the melting area between the plates becoming irregular to the extent that they no longer bond. This is prevented by regularly dressing or reworking the electrode tips in order to maintain their original cross section. In case of galvanized steel, the wear is moderate and can be corrected on the basis of past experience.

Aluminum

However, the welding of aluminum components leads to such excessive wear that most auto body manufacturers consider aluminum spot welding to be unsuitable. For some assemblies, the electrodes are already worn out before a single part has been completed. Furthermore, until now, the intervals between the reworking of electrode tips are usually determined according to operator experience.

This reworking practice results in: avoidable production line downtimes during the maintenance intervals; unnecessary consumption of the copper electrodes and time and material intensive optimizing of the maintenance intervals during the set-up phase for new production lines.

At the core of the system is an optical sensor to record the condition of the welding electrode and thereby calculate the time and extent of cleaning or mechanical post processing required. Michael Leuckefeld, project manager at Fraunhofer IPMS, commented, “Our system monitors both welding electrodes simultaneously from various perspectives. We use diffuse monochrome LED lighting generated by opal glass for this purpose.

Light is sent to the electrode via a mirror arrangement and beam splitter and then back to the photo detector. Lighting and image recording thus use the same optical elements. The first prototype is based on a passive camera system controlled by an external computer via a USB interface. Later, the control is to be integrated into the sensor system. The first tests under live production conditions are scheduled to take place during 2014.

The SmartDress project was recently concluded with a series of successful tests. A prototype of the Fraunhofer IPMSs optical SmartSystem will be presented to the specialist public at its exhibition stand number A-02 at the SmartSystemsIntegration trade fair in Vienna from 26th to 27th March 2014.

About the Author

Matthew Peach is a contributing editor to optics.org.