Beyond MPE

Qualification of industrial CT systems for dimensional measurements

By Dierck Matern / October 10, 2019
Thanks to most recent developments, computed tomography (CT) is now an affordable and efficient technology for non-destructive testing of workpieces made of a vast variety of materials. While digital radiography provides two-dimensional fluoroscopic images, computed tomography typically rotates the object between the X-ray tube and the flat-panel detector 360° and captures several hundred to well over a thousand X-ray images from various angles. Subsequently, reconstruction software is used to calculate a 3D volume from these images, facilitating a variety of analyses and, due to the high precision of today’s systems, measurements in internal structures. Metrology using CT is therefore an indirect, dimensional measuring procedure, because measurements, such as distances of components or diameters of drills inside a test object, are made virtually on its three-dimensional image and not on the object itself.

Same as with other tactile or optical measurement methods, qualifying CT systems is a prerequisite for comparability with other systems. The dominant form of measurement technology in this case for quite a few years has been the specification of MPE values (maximum permissible errors), i.e. the maximum permissible deviation of measurement results in stipulated benchmarks. For this purpose, the standard EN/ISO 10360 and its derived guideline VDI/VDE 2630 define procedures for comparison measurements of a computed tomography system to form a tactile coordinate-measuring machine (CMM). This means: Objects with known properties are scanned to check whether the measurements in the generated 3D volumes have the same values as the real objects measured with a CMM.

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