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SENSORS


Confocal sensor selection


❱ ❱ A range of confocal sensors are available to suit varying industrial requirements including uses in controlled environments and in restricted spaces


sensors, there is less familiarity surrounding confocal measurement technology and the benefits that this principle can offer for both displacement and multi-layer thickness measurement of transparent surfaces. The choice of which displacement sensor to use is based on a number of factors including measuring speed, resolution, type of material and surface, the operating environment and whether thickness measurements are also required.


A


WHAT IS CONFOCAL MEASUREMENT? 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 focused on the target surface or material is used for the measurement. Because of the variation, such factors as the required measurement performance, resolution, type of material or surface that needs to be measured, measuring speed, type of environment the sensor


2 /// DAQ, Sensors & Instrumentation Vol 1 No. 2


lthough many engineers involved in test and measurement processes are familiar with non- contact laser triangulation


Chris Jones of Micro- Epsilon provides guidance on the use of confocal displacement sensors as a measurement technology.


must operate in, how restricted or narrow the space the sensor must operate in and whether you need to measure the thickness of the target or material all need to be considered when making a sensor selection, according to Jones.


RESOLUTION AND SPOT SIZE Confocal sensors are often selected when laser triangulation or other optical sensors are not accurate or stable enough on the surface being measured. The confocal principle operates using a very small, constant spot size (typically 7-25 micron) through the measuring range, resulting in very stable, nanometre resolution measurements. On polished or highly reflective


Lenses


surfaces and even transparent materials, for example, confocal sensors offer greater stability than on dark, diffuse materials – the exact opposite of laser triangulation sensors.


Beam Target ❱ ❱ Confocal displacement sensors use


multiple optics to focus polychromatic white light on the target object


THERMAL STABILITY In terms of thermal stability, confocal sensors are more stable than laser triangulation or eddy current sensors. This is due to the design of the sensors, which typically comprises a cylindrical tube with a series of optical lenses. The sensor is considered “passive”, as the controller and electronics are housed separately and so can be located further away from the target object, which normally means they can be mounted in a more controlled temperature environment.


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