CAT detects auto component faults

WE HAVE all heard of CAT (Computer Aided Tomography) scans which can be used for non-invasive determination of the location, shape and size of any abnormal structure in the brain. Basically , the technique involves taking X-ray images of thin successive slices of the brain, digitally scanning the images and employing a computer to convert this data into a virtual image of the brain's interior.

Using a similar technique, engineers at DaimlerChrysler have been detecting deep-seated faults in vehicle components in a non- destructive manner. The standard method of testing the soundness of components is by radioscopy . However, since x-ray images give only a two- dimensional representation, depth information is lacking; highly reliable fault analysis is often not possible by this means. Computer tomography enables faults to be precisely located and analyzed. DaimlerChrysler is using computer tomography, above all, in the investigation of test, sample and prototype components.

How does it work? The computer tomography (CT) process is preceded by the classical x-ray procedure. In a fan-shaped array of beams with a thickness of a half to one millimeter, x-rays penetrate the object ofinvestigation. During the measurement, the component rotates in 1440 individual steps; this corresponds to four measurements for each degree of rotation.

The intensity distribution in the X-ray image is recorded by a line detector. A binary CT image is initially produced, which shows the gray values supplied by the line camera as a function of the angle measured. The computer reconstructs the image to produce the definitive cross-section, which indicates the precise position of the fault.

DaimlerChrysler'scombined radioscopy/CT unit, is unique in the automotive sector. In this apparatus, weighing 30 tons, X-ray and CT images can be successively recorded without the system having to be reset. With two x-ray tubes operating at differing acceleration voltages, components with a wall thickness of up to 150 millimeters can be examined. Objects measuring up to 210 millimeters across can currently be investigated, but this value can be doubled if each half of the object is measured separately. Depending on the nature of the component, a measurement takes up to 45 minutes.

In the next generation of computer tomography, the series of two- dimensional images will be replaced by three-dimensional ones; engineers from DaimlerChrysler are already working on 3D tomography and are aiming to produce their first three- dimensional CT images by the end of this year.

What we see here is not a newly discovered solar system, but a computer tomography image.

N. N. Sachitanand

in Bangalore

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