OpTek Systems selects confocal sensors for precision measurements on highly reflective and diffuse surfaces in micromachining projects

Precision laser micromachining specialist OpTek Systems has installed non-contact confocal chromatic sensors from Micro-Epsilon on a number of its laser processing machines. These high precision confocal displacement sensors are capable of measuring the surface finish and groove depth of difficult materials, ranging from highly reflective, mirrored surfaces to dark, diffuse surfaces.

With an installed base covering Europe, USA and Asia, OpTek Systems is a global supplier of production-line laser systems for precision laser micromachining, laser processing and measurements, as well as providing subcontract laser processing services. The company has installed its machining equipment in most manufacturing environments, from clean rooms to construction sites.

As Mike Osborne, Technical Director at OpTek Systems' UK site in Abingdon, Oxford states: "The machines that we build here need to work to extremely high tolerances, typically to sub-micron accuracy. This means that any measurement and inspection systems that we install on these machines also need to operate at these levels of accuracy. On a recent project, we had some severe challenges to overcome in terms of the in-situ measurement capability of our machines, which would be required to measure the depth of features laser-etched in the surface of disc-shaped components to accuracies of 0.25 microns, as well as measure the surface finish [Ra] of these components to less than 0.1-micron accuracy."

"In addition, these same measurement systems would have to cope with changes in the texture and surface finish of the components, which might be high precision air bearings or seals. Typically, these components are machined from metals or hard ceramics, such as silicon carbide or tungsten carbide, and so the sensors need to measure on both dark, diffuse surfaces, as well as shiny reflective, mirrored surfaces. Finding a supplier that could provide us with one type of sensor that met these requirements was a challenge to say the very least," he adds.

According to Osborne, consideration was given to employing some of the contact-based, stylus measurement systems typically used for off-line measurements, but the practicalities of integrating (rather delicate) touch probe sensors within the automated environment of the machine were problematic. "Moreover, the ability of the non-contact probe to be able to measure a range of different sizes and shapes of parts and features with small tolerances both in depth and laterally without concern for mechanical crashes made the non-contact approach very attractive. Furthermore, the ability to avoid probe-tip cleaning and replacement provides a more reliable and stable long term solution."

At this point OpTek Systems decided to approach Micro-Epsilon for help. As David Jones, Sales and Applications Engineer at Micro-Epsilon UK comments: "After meeting with OpTek Systems to discuss their application in detail, we considered the various non-contact displacement measuring principles including laser triangulation sensors. We concluded that laser sensors would not provide OpTek Systems with the measurement accuracy it needed in this application, particularly when the surfaces were changing from dark, diffuse to highly reflective. We therefore recommended the confocal principle."

Micro-Epsilon recommended its confocalDT IFS 2405 non-contact confocal displacement sensor, which is able to measure on dark, diffuse to shiny, highly reflective surfaces, as well as providing the extremely high measurement precision required for the project.

The confocalDT IFS 2405 series of confocal sensors are designed for measurement tasks that require maximum precision - typically research and development tasks, laboratory and medical, semiconductor manufacturing, glass production and plastics processing. As well as distance measurements on highly reflective and dark, diffuse materials, the sensors can also be used for one-sided thickness measurement of clear film, boards or layers. The sensors also benefit from large stand-off distances (up to 100mm), providing users with greater flexibility in terms of the variety of applications in which the sensor can be used. In addition, the tilt angle of the sensor has been increased significantly (up to 34 degrees), which provides better performance when measuring across changing surface features.

The confocalDT IFS 2405 series comprises five sensors with measuring ranges from 0.3mm up to 30mm. Spot diameter is from 6µm up to 50µm. Maximum resolution is 0.01µm and maximum linearity is 0.3µm.

Confocal measuring principle
The confocal chromatic measuring 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 (aberration). A certain deviation (specific distance) 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. This light reflected from the target surface is passed through a confocal aperture onto a spectrometer, which detects and processes the spectral changes. Both diffuse and specular surfaces can be measured using the confocal chromatic principle.

Confocal measurement offers nanometre resolutions and operates almost independently of the target material. A very small, constant spot size through the measurement range of the sensor is achieved. Miniature radial and axial confocal versions are available for measuring the internal surfaces of drilled or bored holes, as well as the measurement of narrow apertures, small gaps and cavities.

As Mike Osborne explains: "Our machines, and indeed laser-machines in general, already have line-of-sight of the component that needs to be measured and so installing a confocal sensor close to this target was relatively straightforward. The confocal sensor can be positioned well away from the debris field produced by the laser machining process, and then brought in to measure the result almost in real time. The machines are now installed and fully operational at the customer's site and the confocal sensors are performing very well."

"In addition, being able to supply laser processing machines to our customers with in-situ measurement capabilities for difficult, changing surface textures, adds real value to our offering. The sensors are capable of measuring component and surface features that were previously beyond our capabilities, which in turn has opened up a lot of scope for providing new quality assurance features on our machines. We've also been very pleased with the technical support provided by Micro-Epsilon, particularly when it came to selecting the most appropriate measuring principle for the application. We look forward to working with them again on future projects," he concludes.

January 2015

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