Metrology grade CT scanning is a relatively new technology in the dimensional inspection world. This discussion will concentrate on ZEISS Metrotom CT scanners, as these are the systems I am most familiar with. The best machines are pricey, but the good news is that ZEISS offers CT scanning services on a contract basis at several locations in the USA.
Think of a ZEISS Metrotom CT scanner as a CMM with an X-Ray probe. The machines are built to metrology standards, and the accuracy statements are similar to CMM accuracy statements. For instance the volumetric accuracy specification for the ZEISS Metrotom 800 scanner equals 4.0 + L/100 (microns). For anyone unfamiliar with an ISO based CMM accuracy statement, this means that the system is accurate to 0.004mm, plus an extra 0.001mm of potential error for every 100mm of distance. It is worth mentioning that the Zeiss specifications are conservative, and the machines are typically more accurate than the published error specification.
The beauty of a CT scanner, is that these systems capture part geometry on both the outer and inner surfaces of the part. This allows features to be measured that are not accessible to touch probes, lasers, or optical systems. The data can then be used for dimensional inspection with ZEISS Calypso software, or can be exported as an STL file for reverse engineering and visualization. Assemblies can also be inspected to check for fit or surface contact.
Shown here is a cut-away view of a bottle and cap shown as assembled. The 3D imaging allows the fit to be inspected to make sure that there are no leakage areas.
Transparent materials are not a problem for a CT scanner, since it is X-Ray based. This is very useful for many applications in plastic or glass, as other types of sensors like lasers and white light scanners cannot be used on transparent surfaces.
CT scanners capture large amounts of high quality data. This allows the results of a scan to be used for tasks like “Part to CAD” comparison using a chromatic graphical report, showing the deviation from nominal (CAD) in color. This is more intuitive and much easier to explain to inexperienced staff than rows of numbers.
Flaw detection and analysis checks can be made to quantify flaw sizes and quantities. Shown here are images of reports with both 3D and cross-section views of voids in plastic parts.
This is also very useful for checking porosity in metal castings, and to look for large cracks and other flaws in welds and assemblies.
If you ever had to do a layout inspection on a plastic connector, especially if you were required to measure all the little locking elements inside the connector, then you will understand how much more wonderful life can be with a CT scanner. Just stick the part in a piece of foam and let the scanner find all those hundreds of features that used to be very difficult to access.
Because a CT scanner is a non-contact inspection method with very low inertia forces created during the inspection process, the holding fixtures can literally be chunks of foam cut to hold a part in place on the table.
Some parts (like silicone knee implants for example) are very challenging to hold with a fixture, while still allowing access to the features of interest. Being able to simply stage the part in a chunk of foam greatly simplifies the inspection setup.
Here is a picture showing a variety of part types that have been successful applications for the ZEISS Metrotom scanner.
So how does a Zeiss Metrotom CT scanner work? There is an X-Ray generator that sends out a cone-shaped X-Ray beam. The part is positioned on a rotary table between the X-Ray beam generator and a flat panel detector with a pixel array (kind of like your digital camera). A series of individual 2D images called “radiographs” are taken. The part is rotated capturing (typically 800 -1500 images), then this series of radiographs are processed and reconstructed to create a 3D data set. In the 2D vision the systems work with pixels. In the CT scanning world the 3D data set is comprised of “voxels”, which can be thought of as 3D cubic pixels. Zeiss holds a patent on making measurements from the raw voxel data. The images can be interpolated to calculate the location of a feature edge, providing a resolution of 1/10th of a voxel.
There are no line of sight issues, as the X-ray beam penetrates completely through the material. The beauty of this is that the 3D characterization of the part surfaces is very complete. Anyone who had ever worked with various 3D scanning systems for inspection or reverse engineering has run into situations where it’s just not possible to completely scan the surface of the part (let alone the inner surfaces). So in the case of reverse engineering the gaps in the surface data have to be recreated and filled in with software to create a “closed” file (data set) with no gaps in the surface if you want to re-create the part with a rapid prototyping system. Often the data gaps are near intersections of features and complex surfaces that the laser or white light scanners miss due to line of sight issues. So “filling in” these gaps in the surface can create large errors, with “invented” surface areas that may not accurately represent then true surface.
CT scanning technology continues to advance, with an ever-increasing set of good applications being discovered and developed. Over time these systems will also become more affordable. Eventually this technology will replace other less effective measurement technologies.
The author Dan Smith is employed at ZEISS, working at their Brighton MI tech center. For more information on ZEISS CT scanners, and ZEISS metrology services he can be contacted at:
Cell: 248 704-9846