Measuring all coating layers simultaneously

Thanks to a new technology, Volkswagen can now more easily measure all layers of a painted car body, in a non-contact and non-destructive manner. In this way, the Group gains more control over the painting process, taking an important step towards Painting 4.0 and greater sustainability.

Coating thickness measurement at a car body using terahers sensors
Not only car bodies, other components such as doors can be measured with the sensors mounted on robots. Picture: das-nano

Innovative terahertz technology is now being used for this purpose. “It was important for us to be able to measure all layers of the paint film in one automated measuring process,” explains Claudio Sole, who is responsible for the introduction of new technologies at VW. This is exactly what the new measuring system delivers. At Volkswagen, this means three to four layers of paint, depending on the production site.

The advantage of terahertz technology is that it can even measure up to seven layers simultaneously,  Business Unit Director Isreal Arnedo of das-Nano explains. The company is a supplier of the sensors that VW is now using. Volkswagen undertook its first experiments with the coating layer measurment technology several years ago, initially sending test sheets from its headquarters in Wolfsburg to potential technology providers to determine the suitability of the principle. Among other things, it was important that the sensors could measure the thickness of all sub-layers with an accuracy of one micrometre.

Proven Functionalty

After the first positive results, an initial small test cell was installed at Volkswagen Navarra in Spain and was able to measure, for example, doors and other smaller parts. The subsequent pilot system was then also installed in Pamplona and has since measured the layer thickness of more than 40,000 car bodies. “For a new technology to be rolled out in our country, 20,000 are needed,” explains Claudio Sole.

Coating thickness measurment
Not only car bodies, other components such as doors can be measured with the sensors mounted on robots.
Picture: das-nano

Bernd Engelmann, who coordinated the project and is involved in paint shop planning at Volkswagen, explains, “The measuring cell with the terahertz sensor is located directly behind the top coat dryer and measures 50 measuring points over the entire car body.” He adds, “The measuring head is on a robot that first measures 25 points on one side of the car body; this is then rotated and the robot moves to the remaining measuring points mirrored on the other side.” The actual measurement is quick, as Isreal Arnedo explains: “The positioning of the sensor and the subsequent measurement of the coating thicknesses takes less than 12 seconds. 10 minutes per car, 5 seconds per spot.”

Numerous hurdles overcome

There were several challenges to overcome in implementing the project, the team reports. “We started in Pamplona at the same time as the pandemic and could not be on site as usual,” says Claudio Sole. It helped that das-Nano, a Spanish supplier for the terahertz sensor, was found. The company’s headquarters are only a few kilometres away from Volkswagen Navarra, so there was no language barrier on site.

The vibrations that prevail in real production were also challenging. “Vibrations in the laboratory, however, are something completely different than in real operations. There are many different sources of vibration here due to numerous machines,” explains Bernd Engelmann. “We do not notice the vibrations as a person, but the car body does.” The problem had already been investigated in Wolfsburg during preliminary tests and therefore a technology was installed in the measuring head that compensates for vibrations and thus ensures accurate measurement.

Teraherz sensors save time and material

For Volkswagen, the project is a complete success. The measurement effort is now significantly reduced. “With previous methods, it was necessary to make strips, tape them off and manually re-measure the coating thickness. So now we save a lot of time and material,” explains Claudio Sole. It is also helpful that each colour only had to be calibrated once. “Before measuring the car body, the system independently checks whether it is working correctly using painted reference panels placed in the measuring cell,” explains Isreal Arnedo, adding: “It’s not a calibration, but a check of the system status.”

That’s not the end of it by any means. The measuring cell with the terahertz sensor will be equipped with further measuring devices to get an even better picture of the painting quality. Which sensors these will be is not yet revealed. The eventual goal is for the painting robot is to receive information directly from the measuring cell as to where it is applying too much or too little paint and to independently correct this.

Stable coating thickness measurement

“In principle, we are not far away from that,” says Claudio Sole. Compared to the previous system, Volkswagen is already getting much more information and data. And that’s despite the fact that the new terahertz sensors are only used to measure part of the car bodies. “The system runs very stably and the measurement delivers homogeneous results,” explains Claudio Sole. For a meaningful average value, it is therefore no longer necessary to measure all the car bodies.

The project was convincing. Terahertz technology is now state of the art for new paint shops and is now also being installed at the Bratislava plant in Slovakia and at the new Anhui plant in China. Terahertz technology is also expected to be used in Mexico.

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