When talking about Measurement Uncertainty, it's been established that every piece of measurement equipment has some form of error associated with its measurements. No measurement can ever be perfect. Measurement uncertainty expresses this error as a mathematical formula. Machines with smaller measurement uncertainties are considered more accurate. For the sake of explaining the accuracy of industrial ct scanning, we refer to Measurement Uncertainty as Accuracy which are interchangeable in the context of this article.
To define the accuracy of industrial ct scanning, the CT manufacturer will select a testing protocol that is representative of typical measurements performed and can assure users of its capabilities. The tests must be thorough enough to verify the CT Systems specifications but not overly burdensome to the user. The protocol should be efficient so that it minimizes the amount of time and money required to perform the testing. The testing protocols developed to fulfill these requirements are VDI/VDE 2630 Part 1.3 from the German Association of Engineers and B89.4.23 from the American Society of Mechanical Engineers (ASME). Both standards are similar and meet ISO 10360 guidelines that the metrology world is already familiar with when evaluating traditional CMMs. These standards also give the ability to evaluate and compare different CT systems.
Below are some of these values for 3 different Zeiss CT Scanners:
Testing for long-range effects. Length is defined as the distance between the least-squares-determined sphere centers obtained from surface points taken on a hemisphere that is only in contact with air. Error is calculated as the difference between the measured length and the calibrated length.
Testing for short-range effects. Sphere size is calculated as the diameter of a least-squares sphere from surface points taken on a hemisphere that is only in contact with air. Error is calculated as the difference between the measured size and the calibrated size.
Sphericity is calculated from a least-squares sphere from surface points taken on a hemisphere that is only in contact with air. Measured value is calculated as the difference between the largest and smallest radial distance from the calculated center.
The test artifact should be made from a single material class with near constant density. Each CT system will have a max penetration length associated with each of the 3 material classes.
The test artifact should consist of a minimum of 8 calibrated coplanar spheres that will allow for the testing of different lengths in different spatial directions. There should be an obstructing body present between the x-ray source and the spheres being measured to simulate complex part geometries and to test penetration length and beam hardening effects.
The test artifact should have at least 28 calibrated center-to-center lengths. Each of the spheres should also have calibrated diameters sizes along with their spherical form (sphericity).
Below is the Test Artifact we use to test the Zeiss CT scanners in our lab and that satisfies all the requirements in the Testing Standards.
Often referred to as a Styli Forest Artifact, or in our laboratory as the “Cupcake” because of its multi layered design and shape. It consists of 22 spheres (14 on level 1, 7 on level 2, and 1 on level 3) with 35 calibrated lengths. The obstructing body consists of a plastic cover encompassing all 22 spheres. Artifact was calibrated in a DAkkS (German Accreditation Body) accredited lab with a high accuracy Zeiss Prismo CMM (0.0006mm measurement error)
Accuracy verification on a CT Scanner is an ongoing process that ensures the machine's capability to provide accurate measurements, contributing to quality assurance in various industries. The testing protocol discussed in this article helps to realize these goals.
Looking for an industrial CT scanning partner that employs the protocols necessary for the most accurate scanning?
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