Drives & Controls Magazine February 2023

42 n SENSORS February 2023 www.drivesncontrols.com The long and short of measuring thickness T here are many reasons why you might want to measure thickness. All materials have a tolerance in production. Materials that are produced too thin or too thick could cause problems further down the line, or for a customer who might use the material in a system that then fails or doesn’t fit. Changes in thickness during production can indicate component wear – in extrusion dies or on rolling stands, for example. Monitoring can give early warning signs. The traditional method for checking thickness in production processes has often been to take a sample from the start of the production run and then again at the end. But what happened in the middle? If you find a material is out of tolerance, that’s a lot of scrap to consider. So you might choose to take more readings during the process. If this is done manually, it often requires the production line to stop. Most process variability comes in start-and- stop phases, so keeping a line running is usually key to achieving consistency. Ultimately, checking the thickness of a product as it is manufactured ensures that the customer receives the quality of product expected. Thickness measurement is important in production monitoring, quality assurance and machine control. These measurements can be carried out using contact or non-contact sensors, but non-contact methods offer clear advantages in terms of accuracy and speed. This article describes several non- contact measurement techniques. These methods include: n One-sided thickness measurements carried out using non-contact sensors. A single sensor measures the complete thickness or part of the thickness of a target such as glass. n Two-sided thickness measurements are performed with at least one sensor pair mounted on the same axis and measuring a target synchronously. The difference between the individual measurement results gives the target thickness. n 2D/3D laser profile measurement is used for a variety of applications, including the completeness of weld seams, the optimum dosage of adhesives, or checking gap dimensions. Laser scanners can confirm the presence and size of adhesive beads and sealants. n with the through-beam principle, the transmitter of a laser micrometer produces a parallel light curtain that is transmitted to a receiver via a lens. If there is an object in the light path, the beam is interrupted. The shadow generated by this object is recorded by the receiver and output as a geometric value. This can be used to measure parameters such as diameter, gap, height and position. Combining technologies – for example, eddy current and capacitive or through- beam – allows single-sided measurements to be made. The complementary technologies detect one or other of the material or base surface and when the values are combined produce the required thickness. The various measurement technologies available include: n laser triangulation sensors which project red or blue laser beams onto a surface. The light reflected from the spot is imaged by an optical receiver 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 change in distance. The sensor controller conditions the distance signal and outputs a measured value. Because laser triangulation sensors are almost material-independent, a wide variety of materials can be measured. Because these sensors are not affected by electric or magnetic surface properties, they can be used to measure almost any material, including foods, metal, plastics, wood, silicon and rubber. They can be used for one- or two-sided measurements. n in confocal chromatic sensors , polychromatic white light is focused onto a target surface by an optical system using multiple lenses arranged so that the white light is dispersed into monochromatic wavelengths by controlled chromatic aberration. A specific distance is assigned for each wavelength by factory calibration. Only the wavelength that is exactly focused on the target is used for the measurement. An optical arrangement images the reflected light onto a light-sensitive sensor which detects and evaluates the spectral colour. In the case of multi-peak If you want to measure thickness, the first step is to choose an appropriate measurement technology. The material, surface and thickness of the target play a crucial role in deciding which option is best, as Glenn Wedgbrow, business development manager at Micro-Epsilon UK, explains. One-sided thickness use a single non-contact sensors to measure the target thickness