10 August 2010
Scanning the horizon
A few years ago, computed tomography (CT) scanning was cited as emerging technology with huge industrial potential. Machinery reviews its progress [video]
CT (or CAT) scanning stands for computed (axial) tomography and has its roots in the medical field. CT equipment is used to take a series of x-rays and uses a computer to put them together. Following its success in medical applications, a small number of progressive metrology companies took it upon themselves to see if CT scanning could have practical benefits for industrial applications. In this way, a CT machine could take pictures from different angles and give a series of cross sections or 'slices' through the part being scanned to build a very detailed picture of the inside of the component.
One of the metrology specialists taking a lead in this area is Carl Zeiss (01788 821770), which, through its Metrotom CT systems (see video at foot of page), can process components up to 300 mm diameter and up to 350 mm in length in a single scan (larger using multiple scans). Zeiss has two CT machines – the established Metrotom 1500 (introduced in 2007) and now the smaller Metrotom 800, launched in April this year. Indeed, Zeiss has its very own name for CT scanning: 'metrotomography' – the combination of metrology and tomography.
Image: The Metrotom 1500, the established machine from Zeiss, launched in 2007
Image: Inside Ziess' Metrotom
In practical terms, both Zeiss machines can process materials that include plastics (up to 200 mm accumulated wall thickness), aluminium (up to 100 mm) and steels (up to 15 mm). Metrotom has been designed using existing CMM sub-systems (guideways, drives, controller, rotary table, software) to ensure the accuracy of its measurement results: accuracy is quoted as 4.5 µm + L/100.
"In basic terms, the part is rotated 360° along its axis on a rotary table inside the machine where up to 720 images are taken," explains Des Hayward, UK sales manager at Carl Zeiss Ltd. "The high physical resolution of the x-ray detector of around 3 million pixels enables very high detail recognition to generate razor-sharp projection images. Typical cycle times are around 25-40 minutes. "In Europe, Metrotom systems are selling well, particularly in industries such as automotive and medical, largely for die cast aluminium components or injection moulded plastic parts," he continues.
"3D measurements and structural inspection of complex parts are driving all industrial CT applications, either in the form of internal geometric dimension collating or direct comparison to CAD models. In the UK, CT is still seen as an emerging technology that is in the early stages of acceptance. That said, we operate a bureau scanning service for which we are seeing reasonable levels of take-up."
This fits in with data from the US, where the Carl Zeiss Industrial Metrology Division reported that it surpassed 5,000 CT service scans earlier this year. The reason is simple: using CT scanning, parts with difficult-to-reach structures can be quickly and easily measured non-destructively. It is no longer necessary to cover the parts with synthetic resin and gradually destroy them to be able to measure in 2D, plane-by-plane.
According to Zeiss, using 'metrotomography' enables injection moulding or die casting toolmakers to reduce the number of corrections during development as the part deviations determined by the measuring software are used to optimise the tool form. In addition, non-destructive material and defect analysis – porosity analysis, material inspection and assembly checks – can also be performed, using a measured data set, without significant effort.
Another metrology specialist with a stake in this technology area is Wenzel (01452 728298), following its takeover of Volumetrik GmbH in 2008. According to Martin Hawkins, general manager of Wenzel UK, the company's CT workstation (called exaCT) has a major USP over its competitors. "The footprint of our system is very small," he states. "This is primarily because we use our own proprietary detector technology [where magnification takes place]. As a result, we have a far smaller footprint than equivalently specified machines and therefore a lower cost. Certain other machines need to move the part back and forth in front of an x-ray."
Image:Wenzel's claimed USP for its exact machine is that it has a very small footprint
The exaCT M is Wenzel's standard machine, which can handle parts up to 250 mm in diameter and up to 300 mm in length. Like Zeiss, there is also a smaller model, the exaCT S, which is for components up to 45 mm diameter and 75 mm long. It can process materials that include plastics, light alloys and carbon fibre – ie most materials of light density.
"Material density is probably the biggest issue in CT development at the moment," says Mr Hawkins. "To process a highly dense component such as a titanium turbine blade, for example, requires huge amounts of power. In turn, this requires bigger machines. However, at Wenzel we are already addressing this issue and will have a larger model available in 2011."
Wenzel's exaCT workstation includes a CT scanning machine with high quality image detectors, dual monitors, processor and software for projection acquisition, as well as reconstruction software for the evaluation of the generated 3D data.
"In the UK and Ireland, the biggest markets are aerospace and medical," adds Mr Hawkins. "In fact, we currently have a significant enquiry from a medical industry manufacturer based in Ireland." Wenzel UK also offers a bureau CT scanning service and Mr Hawkins says that take-up is quite high, particularly from manufacturers operating in Formula One motorsport.
"CT will come into its own as the technology develops and industry becomes more aware of its capabilities," he points out. "Most are surprised at the current functionality of our exaCT machines." Among the current measurement capabilities of Wenzel CT technology are dimensional control, wall thickness analysis, actual-to-nominal comparisons, tool and component optimisation, plus development, rapid prototyping and reverse engineering. Additionally, components can be assessed for material defects such as cracks, pores, pinholes, inclusions, inhomogeneities or variations in density. Flaws, delaminations (especially in composites) or material defects that are attributed to incorrect or deficient material composition or structure can also be detected reliably using exaCT. And there's more. Assemblies, for instance, can be inspected with exaCT to ascertain whether the position of separate components match, or whether parts have been wrongly assembled or seals are defective, for example. The examination of joints created by welding, soldering, riveting or gluing can also be realised.
Another relatively new entrant (2009) to the CT scanner market is the Y.CT Vario from Yxlon. Available in the UK via Aegleteq (01234 240729), the Y.CT Vario claims to set itself apart through a combination of high resolution and output performance, a high level of material penetration and short image acquisition times. The enabling factors behind this are the co-ordination of the new Yxlon Y.XST225-VF variofocus x-ray system with a polar manipulation concept and flat-panel detector technology.
Image: The Yxlon Y.XST225-VF variofocus x-ray system is a new kid on the block
The Y.CT Vario system can handle part heights up to 250 mm and diameters up to 180 mm, although the latter can be enlarged via measurement circuit extension to 320 mm.
In July this year, the company announced that Volkswagen AG had purchased its third CT system from Yxlon. Following two Y.CT Modular CT systems that are being successfully deployed overseas, yet another system is supporting the development and production of engine parts at the Volkswagen factory in Salzgitter, Germany.
Now then, in the same way that some CMMs feature touch, video and laser probes/sensors, would it not be possible to also integrate CT technology? Well, the answer is yes, because Werth Messtechnik has already done so. Represented in the UK by Mantech Sales (07799 068239), Werth says that its Tomoscope sees the world's first integration of CT into a multi-sensor CMM (See also).
Image: Werth says that its Tomoscope sees the world's first integration of CT into a multi-sensor CMM
Such technology opens up new measurement possibilities for manufacturers by allowing quick, complete and non-destructive inspection of most features with CT, combined with the exact measurements of external dimensions by traditional sensors/probes.
The system uses WinWerth measurement software, and the interactive graphic instructions are suited for use in a workshop environment, while the software consolidates all data-handling functions that are necessary for the measuring process.
Software, of course, not only provides the key to user friendly systems, but also to throughput speeds. Last year, Metris (now re-branded to Nikon Metrology 01332 811349), launched its Inspect-X 1.6 software that boosts throughput via increased CT reconstruction speed and improved system control. In combination with the latest hardware on new systems, the reconstruction of a 1,024 pixel cubed volume using 1,800 images is completed in only 38 seconds, whereas reconstruction previously lasted 120 seconds.
Image: Last year, Metris (now Nikon Metrology), launched its Inspect-X 1.6 software that boosts throughput via increased CT reconstruction speed and improved system control. Used here at Manchester University's Materials Science Centre
The usability of the system has been improved on various levels. For instance, the sample load position is now user programmable, so it can be reused for repeated operations, while the auto de-focus function allows for higher power operation, without risking target damage. In the UK, the company had success last year installing a 320 kV micro-CT system at the Henry Moseley Imaging Facility within Manchester University's Materials Science Centre. Henry Moseley is an open facility that provides researchers, ranging from aerospace through to palaeontology with access to world-class instruments for CT, from the millimetre to nanometre length scales. The imaging facility had already opted for a standard Metris XT H computer tomography system, equipped with a micro-focus source and high definition detector technology. A second system installed at Henry Moseley features a more powerful 320 kV micro-focus source to tackle denser or larger specimens, such as heavy engineering items. A premium 2,000 by 2,000 pixel Perkin Elmer flat panel detector digitises a field of view of up to 410 by 410 mm, generating accurate and pinpoint-sharp x-ray images and 3D CT visualisations.
The six-second scan
In the US, North Star Imaging (+1 763 463 5650) claims that its new eXpress CT system (http://chilp.it/86e4da) is the first and only one capable of 100 per cent 3D in-line inspection on production lines at production speeds. The system is capable of complete CT scans of a small component every six seconds.
The first eXpress-CT system currently being built for a customer performs complete CT scans of a small assembly device every six seconds. The system uses high speed robots for part handling and manipulation, and uses custom automatic defect recognition (ADR) software for automatic part quality verification and sorting.
The system uses fast, high resolution x-ray detectors, coupled with custom acquisition hardware and NSI software. North Star Imaging partners with NVIDIA (visual computing technology specialist) and uses graphics processing unit (GPU) technology for CT reconstructions that are 200 times faster than conventional CT hardware.
The latest developing innovation with this system is 4D CT scanning (4DCT). North Star Imaging is developing a CT scanning process capable of reconstructing a 3D model, complete with part movement.
View a video of North Star CT scanning here http://chilp.it/917be3
Carl Zeiss Metrotom CT Scanning video
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