The Basics of X-Ray Tomography for Precision Measurement
April 26, 2011
X-ray technology has existed for some time in the industry. We could detect voids in material, cracks in welds, but not dimensionally measure the parts. Recent developments in computer tomography (CT) now provide high precision measurements, even on internal features, using X-ray technology. This article will explain the basics of CT that make this possible. We start with an understanding of 2-D optical measurement and relate that to how X-ray is used to completely capture and measure 3-D geometries. After understanding the basic principals, we can explore the inherent challenges with x-ray and the technology developed to overcome them. The advent of this technology opens new possibilities for verification of advanced manufacturing methods.
2-D Optical MeasurementIn 2-D optical measurement, positions of the parts’ edges are accurately located in relation to a datum. Edges are detected using a lens to magnify and project the part image onto a CCD chip with a pixel array. The light intensity that strikes each pixel produces electronic signals called gray scale values. Intense light, where the part does not block the light, produces high values. No light produces low values. Software evaluates where the pixels values dramatically change and establishes the locations of the edges. Algorithms compute the dimensional and spatial relationships of geometric elements to extract the part dimensions.
This, of course, is a simple explanation. It does not include optical system, subpixeling calculations, filtering, importance of lighting schemes, variable magnification for higher accuracy, and other techniques. These concepts were developed to overcome inherent errors in optical measurement. Without them, optical measurement is neither accurate and traceable nor consistent. Likewise, complex technical approaches were developed to provide x-ray with accuracy. However, these 2-D basics provide a starting point to understand 3-D X-ray measurement.
3-D Computer Tomography: Machine ConstructionAccurate tomography starts with machine construction, unlike X-ray machines of the past. A granite base provides the foundation for precision slides and scales from coordinate measuring machine design. It includes a high precision rotary axis. It can include an additional axis for a multi-sensor approach for the highest accuracy. Source: Werth Inc.
Gathering the Data: Point Cloud ReconstructionThe workpiece is placed on a high precision rotary axis between the X-ray tube and detector. The part is X-rayed and an X-ray projection is stored. The part is then rotated slightly and another X-ray is taken, and so on until the part is rotated through a complete 360 degrees with, typically, 400 or 800 X-ray projections. Software reconstructs these images with a known rotation angle into a voxel volume.
A voxel (from volumetric pixel) is like a 3-D pixel. Instead of gray scale light values, voxels represent density values inversely corresponding to the X-ray energy the workpiece absorbs. Evaluating where the voxel densities radically change determines the point locations on the inner and outer skins of the part. The use of calibrated gray scale algorithms provides sub voxel resolution and accuracy for the measurement points. Connecting those points with triangles produces an STL view resembling a CAD model view but representing the actual part.