QMT Features: March 2007
Wet paint measurement
Automotive OEMs can inspect the thickness of sprayed paint coatings on vehicle instruments, controls and airbag cladding during the spraying process itself - while the paint is still wet.

An innovative combination sensor system has been developed, which attaches directly to a robot arm, so that automotive OEMs can inspect the thickness of sprayed paint coatings on vehicle instruments, controls and airbag cladding  during the spraying process itself – while the paint is still wet. The sensor system is non-contact, easy to mount and offers high accuracy when used against shiny black PU surfaces.

To perform the thickness measurements, Micro-Epsilon’s ‘EU15 (05)’ eddy current sensor is used in combination with the company’s ‘optoNCDT’ laser-based optical triangulation sensor. The eddy current sensor measures the distance to the spray mould. The sensor has an opening in the centre, through which the optoNCDT laser sensor measures the distance to the sprayed component.

The eddy currents pass through the sprayed skin to the nickel-coated spray mould. The laser sensor supplies the reference distance to the skin surface. These processed signals are then subtracted from one another to provide an accurate measurement of the thickness of the applied paint skin.

In the automotive industry, paint coatings are normally sprayed in a heated mould, using robot-guided nozzles. Here, tight tolerances are required, particularly with safety-critical parts such as airbag cladding. For automotive OEMs, this means it is preferable to measure the thickness of the paint skin during the spraying process itself, rather than waiting for the paint to dry.

Up to now, automotive OEMs have been hampered by having to use single eddy current sensors to perform coating thickness measurements. An eddy current sensor would normally be positioned to touch the paint skin, which could cause damage to the surface of the coating. Also, companies have had to wait until the paint is dry before taking such measurements. Several automotive OEMs therefore approached Micro-Epsilon to try to help solve this problem.

The sensor system was developed initially for Belgian company Recticel, which makes automotive components such as vehicle dashboards, HVAC systems, seating, head and arm rests and under-bonnet systems. The company specialises in manufacturing dashboards to house airbag cladding systems. Ensuring the correct thickness of these skins is crucial to airbag performance and safety.

Prior to using Micro-Epsilon’s combination sensor system, Recticel had been inspecting the thickness of dashboards manually with a micrometer after production was complete. Scrap levels were unacceptably high, so in 2005, the company approached Micro-Epsilon for a solution.

 “After installing our system, the company’s reject and scrap levels have been significantly reduced and their production process is now faster and more efficient. The robot arm sprays a couple of layers then inspects the thickness of the coating with our sensor system whilst the coating is still wet,” says Erich Winkler, product manager optical laser sensors at Micro-Epsilon.

“To date, we’ve sold around 20 such systems to various European automotive companies. To my knowledge, no other sensor manufacturer is measuring the thickness of coatings in this way.”  l

email: chris.jones@micro-epsilon.com

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