MATERIAL TYPE The confocal principle is considered surface-independent, enabling measurements on any type of surface, both diffuse and specular – from dark, diffuse materials to highly reflective, shiny, mirrored or even transparent surfaces. With translucent or transparent
materials such as glass, a one-sided thickness measurement can be achieved using a single confocal sensor, along with the distance measurement. Also, because the emitter and receiver on a confocal sensor are arranged in one axis (i.e. the sensor measures vertically down and the beam is reflected back vertically up from the target), shadowing is avoided, which can be an issue if using laser triangulation sensors.
If you need to measure inside restricted spaces such as drilled holes, bored holes, cavities or recesses, miniature radial and axial confocal versions are now available. Some confocal sensor suppliers can offer miniature versions with diameters of just 4mm, allowing the sensor to be inserted into very tight or narrow gaps and cavities.
ENVIRONMENT As confocal sensors are considered passive, they are suitable for vacuum applications in semiconductor and microelectronics production. In clean rooms or vacuum environments, specific confocal sensors can be provided to suit either a low level vacuum or an ultra high vacuum with zero outgassing. As there are no electronic components inside the sensor, this means they do not emit any heat radiation during operation, which in turn prevents mechanical expansion of parts inside the sensor or the target being measured. The result is a much more stable, accurate sensor.
There are some limitations when using confocal technology. The operating environment for the beam path must be relatively clean and free of dust, dirt or fog, although not to clean room standards. In addition, measuring ranges are relatively small compared to other non-contact displacement technologies such as laser triangulation. The largest measurement
❱ ❱ The optoNCDT2403 controller can be used
remotely from the sensor and can handle very high measurement rates
Confocal sensors are connected to their controller (which houses all the conditioning electronics) via fibre optic cable. Cable runs can be long (up to 30m) with no degradation of the signal. And for vacuum environments, a vacuum feed-through connector is supplied.
MEMS TOPOGRAPHY Confocal chromatic sensors provide significant advantages when it comes to inspecting the shape, size and surface topography of MEMS (micro-electromechanical systems) structures during or post-production. These benefits include extremely high sensitivity and sub-micrometre resolution. The sensors can also be integrated to linear X-Y stages, machine tools or special purpose inspection systems with closed loop feedback control. Confocal sensors are capable of measuring the surface finish and groove depth of difficult materials, ranging from highly reflective, mirrored surfaces to dark, diffuse surfaces.
COONTROLLER OPTIONS Traditionally, most confocal controllers perform poorly when trying to compensate for difficult and changing
CONFOCAL SENSOR LIMITATIONS
range it typically up to 30mm, although sensors can still have relatively good stand off distances from the target being measured. The controller needs
to be separate from the sensor, which in some applications, can actually be a benefit rather than a hindrance. For example, where the sensor is measuring very hot targets or is mounted in a
hazardous environment, the controller can be installed at a safe distance away from where the measurements are taking place. In terms of operating temperatures, confocal sensors typically withstand ambient temperatures up to 50°C, although sensors can be protected for hotter environments with enclosures, air cooling and a protective window.
surface conditions, particularly in high speed surface scanning tasks. However, the latest confocal controllers from Micro-Epsilon use intelligent software algorithms based on the company’s experience in the design of optical laser sensors. These algorithms enable the controller to compensate in real time for surface reflectivity, enabling users to scan surfaces very rapidly at high resolution. The controllers also provide high speed triggering that allows them to be synchronised with encoders and other motion control devices. The result is a controller that provides more stable, higher accuracy measurements, down to nanometre resolution if required. The Micro-Epsilon range of confocal sensors can be used with the confocalDT measurement system. This controller can achieve measurement rates of 10kHz using an LED light source and 70kHz using a Xenon light source. A wide range of confocal sensors can be used with the controller and various interface options are available including Ethernet, EtherCAT, RS422 and analogue output. Measurement channels can be set up and configured via an intuitive web browser interface, which means there is no need to install any separate software. n
❱ ❱ Laser triangulation and other displacement sensing technology can be more applicable for larger scale measurement
DAQ, Sensors & Instrumentation Vol 1 No. 2 /// 3
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