Fraunhofer Institute for Industrial Mathematics ITWM
Fraunhofer Institute for Industrial Mathematics ITWM
Wickeldrahtrolle aus Kupferdraht
© istockphoto/wertorer
Wickeldrahtrolle aus Kupferdraht

Layers of Enameled Wire

Enameled wire is used for the construction of electric coils and transformers. We measure the layer thickness of the insulating varnish on fine and ultrafine wire without contact.

© istockphoto/wertorer

Measuring the Paint Thickness of Winding Wire

Coating Thickness Measurement for Enameled Wire – Precise Even at High Production Speeds

Enameled wire is a thin metal wire, usually used in coils – often between 0.01 and 1 mm in diameter – that is coated with an insulating varnish. This varnish layer performs an essential electrical insulation function and must therefore be applied precisely and evenly.

The challenge: High wire speeds are achieved in production, while at the same time the wire moves slightly perpendicular to the direction of movement, which rules out the use of classic point sensors such as chromatic or interferometric sensors.

Coatings often not only enhance aesthetics, but also have functional properties; these include protection against mechanical influences and harmful weather influences or the improvement of the haptics. Coating thickness is therefore of particular importance in industry.

OCT image of a thin coated wire with two opposing measuring heads. Assuming a circular wire, the wire diameter can also be determined.
OCT image of a thin coated wire with two opposing measuring heads. Assuming a circular wire, the wire diameter can also be determined.

Functional properties require a minimum thickness, unnecessary thick layers waste resources and thus increase production costs. There is now a solution for measuring very thin, semitransparent layers: optical coherence tomography (OCT). This method has been originally developed for the depth-resolved visualization of biological and medical materials. Thanks to intensive research, it has now also established itself outside medicine. High-resolution sample cross-sections, which are generated completely non-destructively in real time with visible or infrared light, make the OCT the ideal non-contact inspection technology for many applications.

Reflection Provides Information

The physical principle behind this method is the interferometric superposition of a light beam, which is reflected from different sample depths, with a reference beam. From the intensity and temporal shift of the reflected beam, the depth information of the sample can be determined mathematically and displayed as a cross-sectional image. The resolution is between 1 and 20 μm, depending on the spectral width. Optical coherence tomography is suitable for all materials that are at least partially transparent to visible or near-infrared light. These are many plastics, composite materials, glasses, ceramics or semiconductor materials.

 

With the help of optical coherence tomography, we are able to measure very thin, semitransparent layers.
© Fraunhofer ITWM
With the help of optical coherence tomography, we are able to measure very thin, semitransparent layers. The enamel thickness on copper wires varies between 15 µm and 3 µm in our measurements.

Inspection of Enamelled Wire

Enamelled wire, also known as winding wire, is a metal wire coated with an electrically insulating enamel layer during production. The thickness and weight of this lacquer insulation is very low compared to other insulating materials with the same effect. This wire is therefore preferred for the construction of electrical coils, transformers and machines. We successfully use OCT to measure the enamel thickness on copper wires. The thinnest wire had a diameter of 25 µm and a coating thickness of 3 µm.

OCT-Aufnahme eines dünnen Lackdrahts
© Fraunhofer ITWM
OCT image of a thin enameled wire: The lateral measurement range is larger than the wire to enable measurement despite lateral movement of the wire. The multiple reflections are used to increase measurement accuracy.

OCT System Measures Coating and Wire Thickness Simultaneously

An Optical Coherence Tomography (OCT) system provides a reliable solution to this problem. The technology makes it possible to

  • accurately determine the coating thickness on the wire
  • measure the wire thickness itself
  • and check the even distribution of the coating around the wire – all inline and in real time.

This enables complete quality control of the enamel wire during production – even under demanding conditions.