The fundamentals of thickness measurement
Published: 11 January, 2023
When selecting sensors for measuring thickness, the first step is to choose an appropriate method and measurement technology. The material, surface and thickness of the target play a crucial role in deciding which option is best for the application, says Glenn Wedgbrow*.
So why do we want to measure thickness? There are many reasons that thickness is important. All materials have a tolerance in production; materials produced too thin or too thick can cause problems further down the line or perhaps at your end customer’s site. Maybe they use this material and build it into a system that then fails or doesn’t fit?
Changes in thickness during production can indicate wear of components, for example, in extrusion dies or on rolling stands. Monitoring trends can point to early warning signs.
The traditional method of checking thickness in a production process is often to take a measurement sample from the start of the production run and then again at the end. But what happened in the middle?
Of course, if you find it’s out of tolerance, that’s a lot of scrap to consider. So, you might choose to take more readings during the process. If that check is being done manually then that often requires the production line to stop. Anyone will tell you that most process variability will come in start-and-stop phases so keeping a line running is generally key to getting things consistent.
Ultimately, checking the product thickness as it is produced ensures that the end customer receives the quality of product expected.
Thickness measurement is an important step in production monitoring, quality assurance and machine control. Although thickness measurements can be carried out using contact or non-contact sensors, non-contact measuring methods offer clear advantages in terms of accuracy and measuring speed. This article describes the use of non-contact measurement techniques.
Measurement methods
One-sided measurement: One-sided thickness measurements are carried out exclusively using non-contact sensors. In this case, only one sensor is used to measure the complete target thickness or part of the target thickness (e.g. glass thickness).
Two-sided measurement: Two-sided thickness measurements are performed with at least one sensor pair mounted in one axis to each other. This sensor pair measures synchronously onto the target. The difference between the individual measurement results gives the target thickness.
Profile Thickness measurement: 2D/3D laser profile measurement is used for a wide variety of applications, e.g. the completeness of weld seams, the optimum dosage of adhesives, or the correct gap dimensions. For the inspection of adhesive beads and applied sealants, laser profile scanners check the presence and size of adhesive beads.
ThruBeam measurement: With the ThruBeam principle, the transmitter of a laser micrometer produces a parallel light curtain that is transmitted via a lens arrangement into the receiving unit. The beam is interrupted if there is an object in the light path. The shadowing generated by this object is recorded by the receiving optical system and output as a geometric value. Parameters such as diameter, gap, height and position can be measured.
Technology combination: Combining technologies, for example Eddy current and capacitive or Eddy current and ThruBeam allow for single sided measurements to be made. The complementing technologies detect one or other of the material or base surface and when the values are combined produce the required thickness.
Measurement technologies
Laser triangulation sensors
Laser triangulation sensors project a red or blue laser beam onto the surface. The light reflected from the spot is imaged by an optical receiving system onto a position-sensitive element in the sensor. If the sensor or the measurement object are moved towards the laser beam, the laser sensor determines the correlating distance change. The sensor controller conditions the distance signal and outputs the measured values via interfaces.
As laser triangulation sensors are nearly material-independent, a large range of different material types can be measured. As laser triangulation sensors are not affected by electric or magnetic surface properties, almost all materials can be measured e.g. food, metal, plastics, wood, silicon, rubber, etc.
Measurement methods:
- One-sided measurement.
- Two-sided measurement.
Confocal chromatic sensors
With confocal chromatic measurements, polychromatic white light is focused onto the target surface by an optical system with multiple lenses. The lenses are arranged so that the white light is dispersed into monochromatic wavelengths by controlled chromatic aberration. To each wavelength, a specific distance is assigned by factory calibration. Only the wavelength that is exactly focused on the target is used for the measurement. An optical arrangement images the light reflected onto a light sensitive sensor element, on which the corresponding spectral colour is detected and evaluated. In the case of multi-peak measurements, several distance points are evaluated accordingly. Measurements can be made on practically all types of surfaces, including for mirrored and glass surfaces.
Measurement methods:
- One-sided thickness measurement of transparent materials.
- Two-sided measurement.
- Multi-layer measurement (e.g. coatings, solar cells, photo masks, smartphone screens)
Capacitive sensor technology
The principle of capacitive displacement measurement using the capaNCDT (capacitive Non-Contact Displacement Transducer) system is based on how an ideal plate-type capacitor operates. The two plate electrodes are represented by the sensor and the opposing measurement object. If a constant alternating current flows through the sensor capacitor, the amplitude of the alternating voltage on the sensor is proportional to the distance between the capacitor electrodes. The alternating current is demodulated and output as, for example, an analogue signal.
Capacitive sensors are designed to measure against any electrically conductive surface, even semi-conductors.
Measurement methods:
- One-sided thickness measurement of electrically non-conductive materials on metallic objects.
- Two-sided measurement.
2D 3D laser scanners
Laser scanners produce a wide laser line on the target to solve various measuring tasks in industry and automation: profile, width, height, depth, edge, bead, gap, angle, roundness and many more.
Measurement methods:
- One-sided measurement.
- Two-sided measurement.
- Profile measurement
- Edge Measurement
ThruBeam laser micrometers
Optical micrometers are primarily used as part of the manufacturing process and quality control of a production line, measuring continuous material, as well as single parts. A wide beam is projected between the transmitter and receiver to create a curtain of light. Objects breaking the beam create edges or shadowed areas that are used to determine the object size or position. Suitable for measuring diameters, gaps, segments and edges, these sensors can measure any material that breaks or reduces the light received.
Measurement methods:
- One-sided measurement.
- Two-sided measurement.
- Profile measurement
- Edge Measurement
*Glenn Wedgbrow is Business Development Manager at Micro-Epsilon UK.
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