Application note: Spot welding monitoring using the MATRIX 1024 CORE-S infrared sensorThe spot welding process description and monitoring

Resistance spot welding (RSW) emerged in the 1950s, and is nowadays the predominant assembly technique in the automotive industry. The vehicle components (body in white, cradle, doors, ...) are made of thin metal sheets that are connected with spot-welded joints (or simply spot welds). To create a spot weld, two or more metal sheets are pressed together by electrodes, and an electric current is passed through. The resistance of the metal generates heat, and the sheets are welded together by means of local metal fusion: a spot weld has been created. No welding material is added in this process. Three regions are identified in a spot weld: a weld nugget with cylindrical shape, a heat-affected zone (HAZ) and the base material sheets.

Despite intensive work over the last 40 years, there has been little advancement in sensors systems for on line monitoring of the quality of spot welds. Most existing systems are based upon current, voltage and welding force. However, the temperatures reached during the majority of welding processes leads to the the infrared sensing as a powerful tool, and to the MWIR band in particular as the most useful spectral band for monitoring this type of processes. In addition, a high-speed monitoring is desirable to characterize the dynamics of the welding process. The MATRIX 1024 CORE-S module offered by New Infrared Technologies not only provides MWIR sensitivity, but also fast speed of response and high frame-rates due to the quantum nature of its infrared imaging detector, all to prices never seen before for a MWIR images thanks to the uncooled performance at room temperature.

Experimental set-up

The photo on the right shows the experimental set-up used during the capture of the data shown in this application note. In order to facilitate a comparative study between different welding parameters, all images were recorded using an integration time of 5 microseconds and frame rate of 80 Hz. The optics are a standard silicon lens with a focal length of 24 mm, F# 1.2 and FoV 7.6º x 7.6º. The samples under observation are anodized steel plates. The MATRIX 1024 CORE-S module can be seen on the right side.

The MATRIX 1024 CORE-S module

The module used for the data acquisition is a plug-and-play MATRIX 1024 CORE-S module, based on the MATRIX 1024 FPA, the only MWIR imaging FPA with uncooled performance from the market. These are some of its characteristics:

* FPA number of pixels: 1024 (32x32)* Detection band: MWIR (1-5 microns; peak WL @ 3.7 microns)* Integration time (us): Configurable, range 2-100 us* Frame rate (max): 100 frames per second* System communications interface: USB 2.0 full speed* Data format: Raw data, 14 bits* Module dimensions (mm): 57x40x40* Weight (grams): < 60 g* Electrical interface: USB (5 VDC, 0.2 A)

New Infrared Technologies - www.niteurope.com - Phone: +34916324363 - info@niteurope.com

Spot welding equipment

The MATRIX 1024 CORE-S module

The experimental set-up. The CORE-S module can be seen on the right side.

Experimental results

Low current (weak welding) Appropiate current (correct welding) Comments2D temperature map

MATRIX 1024 CORE-S provides 32x32 images of HAZ @ 80 fps. The images show a clear difference in IR signal according to the differences of temperatures reached at HAZ.

IR signal intensity provides in a straightforward way the weld nugget dimension.

3D temperature profileMATRIX 1024 CORE-S provides 3D temperature profiles with a resolution of 32x32 elements. The visualization helps to detect in a easy way local defects inside HAZ.This type of representation can be very useful for detection of defects and/or fatigue in the welding electrodes.

DynamicsMATRIX 1024 CORE-S provides IR signal vs time curves per each pixel. In the figure the red curve corresponds to central pixel of a good weld and blue curve corresponds the same pixel but in the case of a weak weld (low current).IR signal dynamics would allow to classify in a easy way the quality of a weld taking in account parameters such as increase / decrease slopes , duration, etc. An undesired effect when welding at high currents is splashing. This causes several effects in the production line such as defects and damages in the pieces and safety issues.Splashing can be detected and monitored in a easy way using fast frame rate IR imagers.

Conclusions

* The MATRIX 1024 CORE-S module by NIT is an outstanding tool for monitoring spot welding industrial processes in real time.

* It is the first low cost (few thousand euros) uncooled device in the market able to provide MWIR images @ high frame rates.

* A major breakthrough in weld inspection real time monitoring. For the first time there is available a low cost device which can provide images with all type of important information.

* NIT brings to the market the possibility to control events with infrared image detectors instead of infrared single point detectors, at the same price.

* NIT systems can work at different acquisition speeds.

New Infrared Technologies - www.niteurope.com - Phone: +34916324363 - info@niteurope.com

If you have any question or you would like to know more about our products and their applications, please visit www.niteurope.com or contact us at info@niteurope.com

New Infrared TechnologiesMakes uncooled IR imaging fast and affordable!