Test & Inspection: The Calibration and Accuracy of X-ray CT Systems
October 4, 2011
Modern X-ray computed tomography (CT) systems can produce extremely detailed images of internal and external features of complex components. Many of these features cannot be measured by traditional metrology methods, as they are hidden inside parts. This makes X-ray CT particularly valuable for high-value parts destined for aerospace and other mission-critical applications.
Inspection applications for X-ray CT are diverse, cutting across the automotive, aerospace, energy, medical, electronics and electronics packaging industries. The technique is applicable to a variety of materials, including metals, composites and (on suitable systems) single-crystal materials. Accompanying software tools enable analysis of part features against a computer-aided design (CAD) model, either via direct CT data-to-CAD comparisons or through geometric dimensioning and tolerance (GD&T) measurements. And with price points low enough to make it competitive with other techniques, X-ray micro CT is increasingly ready for mainstream metrology.
Current TechniquesLike other metrology technologies, X-ray CT systems must be traceably calibrated to give true metrological performance. Although technical committees are in the process of drawing up EU and ASME calibration standards, there currently is no internationally recognized standardized procedure for calibrating X-ray CT equipment. Instead, calibration must be derived and traced from another metrology method that does have such a standard, for example, a coordinate measuring machine (CMM).
There are currently two main methods of calibrating CT data at present:
Traceable calibration pieces
Combined CMM and CT systems.
It is recommended to use traceable calibration pieces, a lower-cost, simpler and more flexible calibration technique than combining CMM and CT systems in one.
Combined CalibrationUsers of the combined calibration method must first purchase a system consisting of both an X-ray CT system and a CMM-a relatively expensive proposition. Because the CMM is part of the system, its electronics must be protected from the X-rays. For this reason the CMM cannot be used when the X-ray CT system is in use. This reduces overall system flexibility and can complicate inspection routines.
The combined calibration method relies on use of the CMM to find a physical surface based on CMM settings before location of a surface based on the threshold of the X-ray CT scan. These two readings must then be calibrated to each other before the system can be used. The technique treats internal and external surfaces identically, and thus fails to account for beam hardening or scattered X-rays.
Combining use of X-ray CT and CMM not only is relatively expensive and inflexible; it also can lead to compromises in manipulator positioning because the CMM must be shielded from X-rays. Thus, combined systems are more complex, operators require additional training and can have longer scan times than systems using the preferred method. Combined systems also require CMM infrastructure-a granite bed and articulating arm-that further increase the cost, mass and complexity of the system.
The Preferred MethodIn comparison, the preferred calibration method is relatively low-cost and simple. It relies on having a traceable calibration piece consisting of a number of spheres, which are chosen to have scale and X-ray properties similar to those of the test piece.
Measuring the calibration samples with a calibrated tool such as a CMM allows operators to derive the center-to-center distances of the spheres. The X-ray threshold used to determine surface position in the X-ray CT volume data is irrelevant to the center-to-center distances. Thus operators can calculate voxel size in the CT data with accuracy of approximately 0.1%, more accurately than system calibration of approximately 1%. The only caveat here is required use of spheres-use of a linear threshold dimension would compromise accuracy.
Operators can then scale the CT data exactly so that it matches actual part dimensions and set the voxel size of the 3-D model to the value found from the traceable calibration. When combined with a true local surface determination, this will give a traceable calibrated 3-D CT model.
Multiple BenefitsIn the preferred method, operators need not own a CMM. An external CMM or similar metrology equipment can be used to traceably measure the calibration pieces. Measurement can be performed offline at any certified lab or equivalent at regular intervals. The X-ray system can be used to take the CT as required, and the CMM used for calibration is available to make other measurements of other parts at the same time.
Also, the method treats both the traceable calibration piece and the test object in the same way-that is, the surface is found from the CT scan. This approach provides confidence that the same surface has been located. Internal and external surfaces are found using local thresholds and gradients, minimizing or negating the effect of beam hardening and scattered X-rays.
There also is something to be said for having one apparatus designed to do a single job and do it well. In the preferred method, the X-ray CT system performs no functions that are not key to fast, accurate industrial computed tomography. Systems require no precision granite bed or large arm like a CMM would, so mass is lower. This results in faster and more accurate measurement.
Application ExamplesThe preferred method of X-ray CT calibration has been proved in real-world applications. For example, in turbine blade inspection, four slices are taken through the blade at different heights relative to the datum. The full-resolution scans of the turbine blade samples collected more than 8,000 projection images through 360 degrees. At least one major aerospace manufacturer has concluded that X-ray CT metrology is accurate through the results of numerous trials.
Using a minimum/maximum wall thickness measurement tool in the system’s image analysis software, operators can measure the wall thickness of the turbine blades at desired locations.
Blind trials have been carried out on diesel engine fuel injector nozzles to prove its case for the accuracy and utility of industrial X-ray CT. Accuracy of the shape and location of the tiny holes in automotive and diesel fuel injector nozzles is one of the keys to more efficient combustion and thus improved fuel efficiency, so hole size, location and form is controlled as tightly as possible.
An 8-minute X-ray CT scan of a diesel fuel injector nozzle can produce both 3-D data and 2-D slices through the component for GD& T measurement plus comparison to CAD data. Q
Quality OnlineFor more information on X-ray, visit www.qualitymag.com to read the following:
“2-D vs. 3-D: X-Ray Systems Go Under the Microscope”
“X-Ray Tomography: The Basics”
“X-ray Machine Goes Big”
Tech TipsModern X-ray computed tomography (CT) systems can produce extremely detailed images of internal and external features of complex components.
There are currently two main methods of calibrating CT data at present: traceable calibration pieces and combined CMM and CT systems.
The combined calibration method relies on use of the CMM to find a physical surface based on CMM settings before location of a surface based on the threshold of the X-ray CT scan.
The preferred calibration method relies on having a traceable calibration piece consisting of a number of spheres, which are chosen to have scale and X-ray properties similar to those of the test piece.
Attributes of Current X-Ray CT Calibration MethodsPreferred Method /Combined Method
*Low cost; no need to purchase a CMM / *High cost; must purchase CMM and X-ray CT
*Flexible-CMM and CT can be done on different objects at the same time/*CMM cannot be used (must be shielded) when X-ray machine is on
*Relies on CT consistency; objects calibrated from X-ray results/*Assumes that CMM and X-ray surface are the same
*Finds position of surface using gradients/*Assumes internal surface is the same as external surface
*Simple system; just does one job/*Relatively complex system
*Low mass-does not require the CMM mass/*Heavy-requires both CMM and CT systems