UKM-SUM24-PG32+33_Layout 1 26/06/2024 12:52 Page 32
SENSORS 8 THINGS YOU NEED TO KNOW ABOUT
CONFOCAL DISPLACEMENT SENSORS When specifying a non-contact confocal displacement sensor, several factors need to be considered including measuring speed, resolution, type of material and surface, the operating environment and whether thickness measurements are required, says Glenn Wedgbrow, business development manager at Micro-Epsilon UK.
re you familiar with confocal measurement technology? Are you aware of the benefits this particular measuring principle can offer for both displacement and multi-layer thickness measurement of transparent surfaces? The confocal measurement principle is designed for high accuracy, non-contact displacement and position measurement against any surface: solid, transparent, polished mirrored surfaces, low reflective matt surfaces and even liquids. If a surface is transparent, a one-sided thickness measurement can be achieved with inbuilt correction for the material refractive index. Confocal sensors are often selected when laser triangulation or other optical sensors are not accurate enough or stable enough on the surface being measured.
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Almost all industry sectors can benefit from this measurement principle as it can be used in both R&D and in-process measurements. In particular, the semiconductor, micro lens, automotive parts, medical, glass and MEMS (micro-electromechanical systems) industries use this sensor technology in many ways.
HOW IT WORKS
The confocal chromatic measurement principle works by focusing 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. 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. When selecting a confocal sensor, a number of factors need to be considered.
32 1. MEASUREMENT SPEED
If you need to measure the displacement, thickness or surface topography of a target material at high speeds, confocal technology is the most suitable optical displacement measurement technology. The latest confocal sensors and controllers from Micro-Epsilon, for example, achieve measurement rates of up to 30kHz using an LED light source integrated into the controller. With high measuring rates, it is important to adapt the exposure to the respective surface. This means the controller should dynamically regulate the exposure of the CCD line. This exposure control compensates for colour and reflectivity changes of the measuring object in order to increase the measurement accuracy at high measuring rates. Various interface options are available with these controllers, including Ethernet, EtherCAT, Profinet, EtherNet/IP, RS422 and analogue output, allowing the measurement data to be shared in real time with production and quality control systems.
In order to generate stable measurement signals on curved and structured surfaces, confocal sensors should tolerate a large measuring angle. Confocal sensors from Micro- Epsilon can tolerate a large tilt angle of up to 48 degrees.
Summer 2024 UKManufacturing
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