Thickness Measurement in Glass using Confocal Chromatic Sensors

16-09-2019

Confocal Chromatic Sensors provide non-contact automated measurements with precision and enhance the quality and efficiency of an object in a sustainable way. In a live production scenario, where operations and dynamic measurements are performed at a very high speed, confocal sensors are ideal to use. Due to their non-contact measuring technique, they are ideal for handling fast and sensitive measurement tasks that involve targets in challenging conditions.

The confocal sensor diffuses white light through a multi-lens optical system. This disperses the white light into different wavelength spectra, which are focused perpendicularly on the target object. The reflected light is then passed through a confocal aperture onto a spectrometer for processing the spectral changes further. Since these sensors are passive sensors and don’t give of heat, they can be used in a vacuum or in hostile environments such as potentially explosive atmospheres.

Bestech Australia has a range of confocal sensors to measure distance and single sided thickness measurement in glass. These sensors provide accurate measurement results even for shiny and reflecting surfaces. There are special types of sensors that can be used for measuring the diameter of a bore hole. We also offer special sensors to measure thickness of multi-layered objects such as glass or laminated glass.

Types of glasses that can be measured

Display glass sheets require uniformity in their thickness profile and to ensure that a homogenous thickness is maintained throughout the production, confocal sensors are used.  These sensors can be used to check the diameter and layer thickness of glass capillaries used in medicine. As a special feature, it can also measure and classify several layers of glass. Confocal sensors can also be used to measure the thickness of lenses, wall thickness of container glass, thickness of display and flat glass and bottom thickness measurement of container glass (even when the wall thickness vary).

How thickness is measured in multi-layer transparent material?

In measuring single or multi-layer transparent material, the operating principle is similar just with a couple of extra steps. The following figure shows an example of a multi-layer transparent material such as glass. To measure the thickness, the controller makes two separate calculations. First it measures the distances (D1, D2 and D3). Then it performs the calculations to find the thickness of Layer 1 and Layer 2. As a result, the thickness calculation for Layer 1 is D2 – D1 and the thickness calculation for Layer 2 is D3 – D2.

How wall thickness of a bottle is measured using a confocal sensor?

To measure thickness in a glass bottle, Confocal DT 2422 sensor is used. In order to measure thickness, the controller takes two measurements, Displacement 1 (D1) and Displacement 2 (D2), and both of these displacements are measured using one sensor. Then, to get the desired thickness, D2 – D1 is calculated. However, before this can be done, the sensor needs to be placed in the correct position for measurement, with regards to the bottle. The bottle or the “target” needs to be placed as indicated in the figure below, which means that the centre line of the bottle thickness needs to coincide with the centreline of the midrange of the sensor. Since this is difficult to achieve in practice, it is necessary to make sure that the target is near the midrange of the sensor.

The midrange of the sensor is calculated using the formula:

Mid-range = SMR + (0.5 x MR)

The sensor must be held in position securely with a special clamp/bracket. The idea behind this set-up is that the beam of light must hit the target at a right angle to avoid inaccurate measurements.

The minimum target thickness for the sensor IFS2403 (hybrid sensor) is approximately 15% of measuring range, whereas for IFS2405 (standard sensor), it is 5%. The maximum target thickness is calculated as:

Thickness_max = Sensor measuring range x refractive index of the target

Assuming that the refractive index for glass bottle is 1.514 and we are using the standard sensor with a 10 mm measuring range and an SMR of 50 mm (IFS2405-10), the following calculations are done for the standard IFS2405 sensor:

TargetminT =5/100×MR

Where,

  • TargetminT =Minimum thickness of the target.
  • MR Measuring range,

Putting in a number for our sensor we have, MR = 10 mm, therefore,

TargetminT =5/100×10 mm
TargetminT =0.5 mm

Now, we calculate the maximum target thickness using the following formula:

TargetmaxT=MR×RI

where,

  • TargetmaxT = Maximum thickness of target.
  • RI = Refractive index for the target, i.e. the glass bottle in our case.

TargetmaxT =10 mm ×1.514
TargetmaxT =15.14 mm

Therefore, the thickness of the bottle must lie between 0.5 mm and 15 mm (which is very large) for this given example. In fact, the next sensor down in the series, the IFS2405-3 sensor, could have also been used in this case with a much tighter range.

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