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Scanning system - micron accuracy with industrial components

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SmartDrive has developed the SatScan fine art scanning system that achieves micron accuracy using standard linear motion components from HepcoMotion.

A new, fully automated, fine art infrared scanning system is a prime example of what modern technology can achieve - and it also shows how a scientific tool with micron positioning accuracy can be created from standard industrial automation components. The system is the brainchild of motion control specialist, SmartDrive. Working with HepcoMotion, it has optimised the process of infrared scanning, thereby greatly extending the scope of the technology. Furthermore, the benefits of the system are not confined to infrared; the system control and image stitching software also enable distortion-free digital photographs of large works to be taken for further study and record. For the art conservation world this system is said to represent a significant imaging breakthrough.

The SmartDrive system is called SatScan and, as its name implies, the technology mimics that used in mapping systems with the optics 'flying' over or across the subject. SatScan works by moving digital and infrared camera heads incrementally around the artwork using encoder technology to achieve precision positioning.

Dennis Murphy, Managing Director of SmartDrive, explains: "We know the position of the carriage to within +/-1 micron. We are achieving this from automation components with industrial tolerances of 100–200 microns. In scientific terms we are also using relatively low-resolution, off-the-shelf cameras. But, by acquiring a small field of view – maybe just an inch or two square – and holding it perpendicular to the painting we can take images that are completely distortion-free."

Distortion-free imaging

The important aspect here is that each image has uniform scale throughout, so valid measurements can be taken anywhere on the complete reconstructed image. Also the scale makes individual tiles easier to match and automatically join in software. Users can select the digital quality by adjusting the camera lens; in the fully zoomed mode the resolution of the system can create images at over one thousand DPI over a large area. Using a single shot from the best digital camera available cannot match the SatScan's clarity and will always suffer from parallax issues and resultant distortion of the image. Murphy adds: "SatScan is a solid, proven digital technique that provides great application flexibility and stunning results."

SmartDrive reports that the first large-scale SatScan is installed at the Hamilton Kerr Institute near Cambridge, a centre of excellence for conservation services that is world-renowned for its work on tempura easel paintings. It provided the expert knowledge needed for the art version of the SatScan development and is now using the system on a daily basis. Infrared imaging is just one of a range of tools used by the Institute's conservators and is primarily used to see any under-drawings on the canvas or to examine previous re-touching. For example, it will show if an egg tempura painting has been retouched with oils. The information it provides guides the restoration process and also enriches the history of the work. It can also help with authentication by revealing details that may show consistency with a certain artist.

Another important area of work for the Hamilton Kerr Institute is research into the care of works of art. Paintings and the public thrive in very different environments so the Institute provides expert advice to the keepers of collections on how best to strike a compromise.

Historically infrared scanning at the Institute has been a time-consuming and costly exercise. Even the latest-generation equipment had significant pitfalls; it involved manually moving a Vidicon analogue TV camera into position, capturing a small infrared image and then uploading it using manual stitching software. Chris Titmus, an imaging consultant at Hamilton Kerr, says: "Imaging even a small painting was a day's work using this method. And even then the complete image was not clean, as stitching was by eye and the edges of most images were blurred."

A further problem with the old system was the danger of image burning onto the camera tube, necessitating regular replacement and adding to the process cost. Titmus adds: "Just having the camera switched on would cost in the region of £30 per hour."

Fully automated scanning

In contrast, the SatScan system is completely automatic. Titmus comments: "Once the work is in position we press a button, the image capture process begins and the software stitches it all together. In an hour or so we can achieve what used to require a whole day. Indeed, the system is also allowing us to take on work we would not have considered before. In the past it would only have been viable to look at selected areas of some paintings, whereas now we can examine the whole thing."

Another major benefit is the size of the painting or alter piece that can be scanned by the system. The Hamilton Kerr Institute recently used the SatScan to image a painted oak panel that was two inches thick and seven feet tall. It was simply a matter of moving it in its wheeled frame to a position in front of the system. The Institute specialises in subjects of this scale, so the reduced handling involved ensures the safety of both the artwork and the conservator.

The computing power that is now readily available is perhaps the single most important factor in the development of SatScan. This has enabled SmartDrive to use off-the-shelf components. The source of all the mechanical elements for the system was SmartDrive's longstanding partner, HepcoMotion. However, this linear motion specialist was not just responsible for product supply; having taken an outline brief from SmartDrive, HepcoMotion designed the system mechanics, then built and tested the system.

System design and manufacture

Creating sub-assemblies and, indeed, complete automation systems of this type is now a very important strand of HepcoMotion's business. It provides small manufacturers with a low-risk automation strategy, as HepcoMotion's systems are guaranteed fit-for-purpose. Murphy states: "We work with HepcoMotion a lot and have commissioned many successful joint projects; we know the range well but we needed to call their specialist knowledge on this project. While the system load is small, about 20kg for lights and the camera, we needed a large frame to accommodate large works of art. The HKI SatScan is designed to scan areas of 4.5m x 3.5m. This makes for a large installation and its metalwork – mostly aluminium extrusion – weighs over 400kg. The HepcoMotion technical team designed a strong, lightweight frame that was capable of handling the inertia of such a moving load yet maintains rigidity so that 'wobble' is minimised and the micron-positioning of the cameras that is so crucial is not compromised. Furthermore, we had to ensure everything was sufficiently robust, especially given the value of many of the subjects involved."

The HepcoMotion SBD (sealed belt-driven unit) was selected for carrying the camera heads in both the horizontal and vertical axes. This sealed belt drive is designed for high loads and demanding duty cycles and provides exceptionally clean linear motion. Each of the guideways is fitted with a scale and optical read head to provide positioning feedback. The high-strength aluminium SBD units are mounted onto the SatScan gantry that is made from the HepcoMotion MCS machine construction system. A small HepcoMotion GV3 guideway was chosen to provide fine adjustment of the camera carriage in the Z axis. Further HepcoMotion components included Zimm screw jacks that are responsible for adjusting the incline angle of the 4.5m x 3.5m frame to accord with the painting. The entire system was built at the HepcoMotion factory and the elements then transported to SmartDrive for cabling and the integration of the drives and controls. The working unit is located in a busy studio with typically three or four people working in close proximity; despite its scale the system emits no more than a pleasing hum as it automatically sets about its business of gathering high-resolution data.

SmartDrive called on all of its 21 years of motion control expertise to design the system capable of precise motion on such a large scale. This involved the careful consideration of load/motor inertia matching and the use of precision SmartDrive Taranis technology digital drives, the combination of which produces smooth, quiet and accurate motion. The company's stitching technology is, however, the vital component in this project. This Windows-compatible user interface and stitching technology took SmartDrive nearly three and half years to perfect. The patent-applied-for advanced algorithms ensure the stitching is accurate even when there is little or no data in the image to automatically work on. The user interface makes it easy to define and stitch areas with a few simple mouse clicks, quickly transforming digital data into exceptionally clear high-resolution infrared or visible spectra images that can be viewed or saved to disk.

New variants

SmartDrive is working on three versions of this automated system. In addition to the Art system developed for Hamilton Kerr for larger works, it has developed another for examining museum artefacts in standard 500mm square trays, typically archaeological collections, entomology or botanic specimens. Also developed is a microscope slide scanner version for imaging objects whose size is measured in microns rather than metres.

The Hamilton Kerr rig is also undergoing further development. The inclusion of a laser to provide dimensioning and profile data is currently in progress. Titmus adds: "We have already tested it out on an altarpiece from Westminster Abbey. During the course of restoration we needed to assess if the screen had grown or shrunk through changes in humidity. If the screen contracts too much, paint will flake off and, conversely, over-expansion will put pressure on the joints. The laser simply adds even more value by providing a micron-accurate 'Z' element to the high-resolution X-Y data SatScan already acquires. We can easily view this three-dimensional data in many CAD packages; the possibilities are endless."

Rupert Featherstone, Director of the Institute, states: "The SatScan system has proven itself to be a remarkable breakthrough for art restoration and preservation; the development has provided the Hamilton Kerr Institute with leading-edge technology that will ensure we continue to lead the field in conserving the nation's heritage."

 
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