Novel sensor system measures thickness of wet paint coatings

Micro-Epsilon has developed an innovative
combination sensor system, which attaches
directly to a robot arm to allow automotive
OEMs to inspect the thickness of sprayed paint
coatings on vehicle instruments, controls and
airbag cladding during the sp

Novel sensor system measures thickness of wet paint coatings and tyres

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 whilst 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 required 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, who manufactures 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.

Erich Winkler, product manager optical laser sensors at Micro-Epsilon comments: “Recticel had been using a laser sensor from a US company that just wasn’t performing correctly. 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.”

“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,” he adds.

Micro-Epsilon’s combination sensor system has a measuring range of up to 15mm, although most applications require 2mm or less. The system has a resolution of 1mm and an accuracy of 5mm. The system includes a multi-function controller, adapter board, sensor cable and power supply.

Thickness Measurement of Tyres

Utilising the same measurement technology, Micro-Epsilon has developed a system for Poland’s leading tyre manufacturer Debica, which measures the thickness of automotive tyres as it is applied to the metal/fabric former. The system is non-contact and offers high speed measurements in- process in real time, enabling Debica to accurately control the process with very high yield and quality control.

Rubber-coated textile and metal fabrics form the basis of tyre manufacture. The rubber is applied to the fabric by a ‘calendar rolling’ process, which demands a uniform layer thickness for high quality tyre manufacture. The strength and dimensional conformance of the tyre directly depends on this coating process.

Measurement of the thickness of the rubber on both rolls (3 & 4 in the diagram) is usually not possible due to the way the calendar is constructed. Therefore, additional measurements are made at the outlet.

However, two Micro-Epsilon ‘optoNCDT’ laser-based optical sensors can be positioned above and below the coated fabric web. The laser sensor is combined with a Micro-Epsilon ‘EU15 (05) flat eddy current sensor, which has a hole in the centre section to allow the laser spot to pass through and onto the wet rubber surface. A mechanical traversing system enables the sensors to move over the width of the web. Due to the small measuring spot and the high resolution, it is possible to not only measure the thickness, but also the surface structure of the coated fabric.

The sensors are protected against high ambient temperatures (up to 80°C) by a protective housing, which is cooled by compressed air. Measurement range is 20mm, resolution is 1mm and bandwidth is 10kHz.

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