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Multi-layered glass thickness measuring solution

25 March 2014

In the manufacture of multi-layer curved glass for aircraft, automotive and architectural applications, precise thickness measurement of the individual layers, and combined overall laminate, are required. If the curvature of the different layers is not precise, when the layers are sandwiched together there could be a risk of stress points developing within the glass, which in turn could lead to defects or delamination of the glass.

For quality inspection and process control of manufactured transparent multi-layer materials, Micro-Epsilon has developed a range of confocal chromatic displacement sensors and controllers, which together, provide multi-peak measurement capabilities which can help improve product quality and increase production yields.

Confocal chromatic sensors offer one-sided thickness measurement and distance measurement of transparent materials such as glass. The sensors focus polychromatic white light onto the target surface using a multi-lens optical system. The lenses are arranged in such a way that the white light is dispersed into a monochromatic light by controlled chromatic deviation. A certain deviation is assigned to each wavelength by a factory calibration. Only the wavelength that is exactly focussed on the target surface or material is used for the measurement.

This light reflected from the target surface is then passed via a confocal aperture to the receiver, which detects and processes the spectral changes. This enables displacements and distances to be measured to nanometre accuracy.

Both diffuse and spectral surfaces can be measured and with transparent materials such as glass, a one-sided thickness measurement can be accomplished along with the distance measurement. Because the emitter and receiver are arranged in one axis, shadowing is avoided. The white light technology also provides greater stability compared to laser-based technologies, particularly against glass or mirrored surfaces.

The confocalDT IFS 2405 series of confocal sensors provide large stand-off distances (up to 100mm) offering greater flexibility in the variety of applications for the sensor. In addition, the tilt angle of the sensor has been increased significantly (up to 34°), which provides better performance when measuring across changing surface features.

The sensors are offered along alongside the confocalDT 2451 controller which offers fast measuring rates, improved signal-to-noise ratios, and real-time surface compensation for difficult-to-measure surfaces, including mirrored surfaces and glass.

The controller’s integrated multi-peak software enables users to evaluate up to five layers. In order to accurately determine the thickness of each layer, the controller retrieves the refractive index of each material layer from a database. Each refractive index is corrected depending on the wavelength: three refractive values are used for thickness calibration (beginning, end and middle of the layer). 

The controller is said to be simple to set up and operate. Users can remotely configure measurement channels via web-based software. The controller utilises either a white LED as the light source, which enables measuring rates up to 10kHz, or an external Xenon light source that provides up to 70KHz measuring rate. Ethernet, EtherCAT, RS422 and analogue output interfaces are provided.


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