Computed Tomography Metrology in Industrial Research & Development

In non-destructive testing, computed tomography (CT) is increasingly used for dimensional measurement tasks such as the industrial inspection of work pieces. Besides the determination of the measurement uncertainty, the improvement of the image quality is subject of current research, in order to establish CT as a measuring device. 1. Cone-Beam CT and Metrology Dimensional measurement with CT has the advantage that in comparison with classical techniques measuring is not limited to the work piece’s surface any more, in fact it is possible to evaluate volumetric models of the work piece with high point densities of several million points and even contain information of commonly not accessible inner parts of the measurement object. CT typically features a great variety of different work pieces, materials and shapes from small plastic connectors used in electronic industry to large and heavy metal engine blocks. As dimensional metrology is a rather new field of application for computed tomography, many research projects are going on which are focused on establishing this technology in manufacturing metrology by determination of the task-specific measurement uncertainty and improvement of the image quality. In cone beam computed tomography the object to be inspected is rotated in the radiation field of an X-ray source. For a large number of beam directions (typically within 360°) the intensity distribution of the radiation after passing the work piece is determined by means of a flat panel detector and digitally stored as a projection. All the data necessary for a 3d reconstruction can be gathered much faster than in fan beam CT. After recording the projection data and 3D reconstruction the volume data can be evaluated. Figure 1 shows the set-up of a cone-beam CT system and the scheme of the CT recording process respectively. Figure 2 gives an overview about the different steps to measure geometrical measurands, characteristics or complete volumes (nominal-actual value comparison). Up to now there are no guidelines in order to ensure the traceability of CT measurements respectively to be able to indicate a task-specific measurement uncertainty. In 2004 the Technical Committee VDI/VDE-GMA-FA 3.33 has started it‘s work on the guideline VDI/VDE 2630 „Computed Tomography in Industrial Metrology“. So CT has not yet been generally recognized as a dimensional measuring device for geometry determination. Several research institutes in Germany are working in that field to establish industrial CT as a completed measuring device. For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=81 International Symposium on NDT in Aerospace December 3-5, 2008, Fürth, Germany