QMT Features: December 2009
Multisensors step up
Evidence suggests that the uptake of multi-probe systems is growing. Multi-probe without compromise:is  the next step change in measurement technology.  By Andy Fulton, OGP UK.

Most engineers are aware of the progress made by machine tools in recent years. Individual milling machines, lathes and borers can today be replaced with a single multi-task machining centre. Relatively few, however, seem to be aware that measuring systems have now realised similar advances.

Today it is possible to measure component features using a combination of zoom optics and video analysis, automatically scan surface profiles with an inbuilt laser and deploy a touch probe to measure three dimensional features. All of the probes work within the same axis system, are programmed from a single software source, and are controlled automatically by the machine without the need for operator intervention.

While it has been possible for some time to add laser or touch probe technology to camera-based measuring systems, the inevitable compromise to the working distance of the optics has left many users dissatisfied. For this reason, conventional ‘bolt-on’ systems have never fulfilled the potential of true ‘multi-probe’ measurement that was promised when the first models arrived on the market.

Similarly, some builders of co-ordinate measuring machines have tried to add camera technology, although the portal motion of most CMMs means that the lighting changes with each machine movement, making reliable video measurement extremely challenging.

But now there is a new technology that is set to establish multi-probe measuring systems as the metrology devices of preference at manufacturing shops everywhere. For the first time in industry, the latest SmartScope CNC measuring machines from OGP feature a combination of probes that work to the maximum of their individual capability, without compromise.

This is no small feat and one that the manufacturing sector has been awaiting for some time. So how has it been achieved? Well, OGP’s prowess in the development of optical non-contact measuring systems is well documented. Add to that its close collaboration with Renishaw regarding the design of the compact SP25 touch trigger/scanning probe and the development of its own TeleStar TTL (through-the-lens) laser, and a vision of accumulated expertise starts to become apparent.

The future is clear
Importantly, TTL technology means there is no offset between the laser and the optics, providing a long working distance that allows users to measure recesses up to 200mm deep, without obstruction issues. In fact, TeleStar can still measure when its path is blocked by up to 60%.

For this reason, TeleStar is well suited to measuring small holes and channels. In one particular obstruction test, the TTL laser managed to collect data successfully from a 0.025mm gap set between two 25mm tall gauge blocks. It can perform this impressive feat because TeleStar is an interferometric system whereby accuracy is not a function of front lens diameter and working distance, unlike most triangulation lasers.

Flexible yet uncompromised multi-probe technology is now a reality, allowing machine shops to complete fast and repeatable measuring routines using a single system. As a result, set-up time and cycle time are reduced significantly, while accuracy is enhanced because all probes are calibrated to a single point of reference – the centreline of the optics. Additionally there is less capital expense, fewer calibration/service intervals and less labour requirement, while well thought out multi-sensor strategies reduce the need for operator training because all measurements are called from a single software interface. 

Other probes can also be integrated into the latest platforms, such as the Feather Probe, to which OGP has exclusive rights, which can measure small features that are inaccessible to video measurement or conventional touch probes, or too sensitive to withstand traditional probing forces.

A Rainbow Probe can also be integrated. This is an innovative electro-optical device that measures surfaces without contact by analysing changes in the optical spectrum as a function of part-to-probe spacing as it scans, making it suitable for measuring glass or translucent surfaces.

Seeing is believing
Adding to the flexibility of multi-probe measuring systems is the option to integrate dual rotary tables into the same frame of reference. This is particularly useful when measuring complex parts such as nozzle guide vane (NGV) components used in aero engine applications. Here, a touch probe can be used to establish the axis system before a camera probe is deployed to measure the side-facing seal slots. Once this is complete, a laser probe measures the profile of the aerofoil before the system reverts to the optical system to measure the cooling holes. Because of the common optical path, the switch between video and laser is instantaneous and at no point is there any need to re-establish part datums.

Measuring some of the intricate features of NGVs is not a problem for OGP video-based non-contact measuring machines as they already incorporate advanced lighting to provide high resolution optical clarity. For instance, when NGV cooling holes are spark eroded there is inevitably some break-out around the edge of the hole. While competitor machines will measure the break-out and record an oversize hole, OGP’s in-house manufactured TeleStar zoom optics are diffraction limited, colour corrected and fully telecentric for superior imaging, which allows enough light to get inside the hole (beyond the break-out) to measure the diameter correctly. As a result, any kind of mirco-component or micro-feature will benefit from this technology; after all, if you can’t see it, you can’t measure it!

While many would expect technology of this ilk to be particularly adept at measuring small features, few realise that the latest multi-probe metrology platforms can also handle large components such as aerospace blades and automotive cylinder heads. Previously the preserve of portal-type CMMs, few would have thought it possible to measure such big parts using camera-based systems, even as recently as 10 years ago.

Another sector that can benefit is medical, largely because measurement data is pulled straight from the machine into PDF format, so it cannot be changed. This provides complete integrity in accordance with CFR 21 part 11 (of the Code of Federal Regulations), which deals with the FDA’s guidelines on electronic records.
Evidence suggests that the uptake of multi-probe systems is growing. Around 40% of the systems sold by OGP now feature both camera and touch probes, while laser is also being increasingly integrated. Furthermore, it’s a trend set to gather pace, as more and more manufacturers begin to realise that today’s multi-probe measuring systems operate without compromise or restriction. The next step change in measurement technology has arrived.

email: andrewf@ogpuk.com

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