Tamara Reuter

The WIPANO research project series CTSimU deals with radiation simulation for determining the measurement uncertainty when measuring geometric features using X-ray computed tomography. Industrial X-ray computed tomography (CT) completely captures workpieces, including their inner and outer structures for metrology purposes. However, previous effort to determine task-specific measurement uncertainty is not always justifiable. Simulating the measuring process allows to estimate the measurement uncertainty numerically and to predict systematic measurement deviations. The CTSimU3 project aims to create a time-saving, resource-efficient, and reliable method that enables measurement uncertainty determination in CT through simulation using the GUM Supplement 1. This article presents the project idea as well as the first results of the requirement analysis.

Digital models using computer simulation for the acquisition of X-ray projections offer the possibility to virtually perform a X-ray computed tomography (CT) measurement. Virtual CT scans benefit research and development of X-ray computed tomography, serving various purposes while reducing physical scan time and therefore cost. The accuracy of a digital model determines its utility. Suitable test procedures of digital models can assure the sufficient accuracy of a digital model regarding a specific task. For the use of digital models in dimensional metrology an agreement of dimensional measurands compared to a physical counterpart is important. In this contribution, a physical and virtual CT measurements of a calibrated Multi Feature Check mini specimen are evaluated regarding surface transition metrics of the volume data and its metrological compatibility of dimensional measurands. For the evaluation of the gray value response at the surface transition, a methodology that uses local sampling patterns to generate a locally averaged edge response is presented. The digital model shows promising results to serve as a basis for further extension to determine the measurement uncertainty of dimensional CT measurements using the Monte-Carlo method in the future.

For the long-term goal of determining the measurement uncertainty of dimensional computed tomography (CT) measurements with simulation, it is necessary to prove the compatibility of the digital model with its real counterpart. A study of repeated CT measurements of a calibrated aluminum specimen was performed and afterwards simulated. A first indicator for a sufficiently good digital representation is the agreement of dimensional measurands, such as diameters and center-to-center distances. Results show in general good agreement between simulation and real measurement. A detailed discussion of the results as well as a proposed criterion for testing the simulation will be shown in the full paper.

Systematic and random measurement errors are the cause of uncertain conformance and non-conformance statements. A wrong conformance statement occurs if the workpiece is accepted and is a reject part (type II error or false-negative) and a wrong non-conformance statement if the workpiece is rejected and is an in-spec part (type I error or false-positive). In order to avoid type I and type II errors, measurement uncertainty must be taken into account in the conformance and non-conformance testing. In practice, some procedures are used to consider measurement errors or the uncertainty that deviate from the state-of-the-art in research and technology. As these methods have become established over many years, they are still widely used despite better theoretical knowledge. The standard ISO 14253-1:2017 specifies a procedure based on probability and measurement uncertainty that is in accordance to the internationally accepted “Guide to the expression of uncertainty in measurement” and its supplements but is often not used due to the complexity of the evaluation of measurement uncertainty. In this contribution we give an overview and comparison of the different existing methods and provide an suggestion for supplementing the standard ISO 14253-1:2017, as Monte Carlo simulations enable a direct probability-based conformance and non-conformance testing even for complex measurement processes.

This contribution deals with transferring measured focal spot intensity profiles to industrial X-ray computed tomography (CT) simulation software in order to gain a more realistic simulation for dimensional measurements and later a numerical determination....

The interest in using computer simulations of dimensional x-ray computed tomography (dXCT) for various metrological purposes—such as measurement planning, performance prediction, performance optimisation and, finally, measurement uncertainty estimation—is increasing along with the ever-growing demand for more reliable measurements with dXCT. However, before a piece of simulation software can be used for tasks related to coordinate metrology, it has to be ensured that it is able to simulate physical laws, characteristic effects and basic CT system functionalities correctly and with sufficient accuracy. In short, the software must be qualified for dimensional metrology tasks. As one part of such a qualification process, a method is presented here for determining conformity intervals of 2D tests (projection-based tests) based on 3D tests (testing based on dimensional evaluations in a reconstructed volume) for the assessment of dXCT simulation software. The method consists of varying relevant parameter values in order to verify their influence on 3D measurement results. The results of the 3D tests with varied parameter values are then transferred to the quantities tested in the 2D tests and used as the basis for determining conformity intervals. Two approaches are applied for determining whether or not a variation of a parameter value is significant: (a) statistical and (b) heuristic. Two examples are presented, each based on simulated images, which show the application of the two different approaches for determining conformity intervals for the results of the 2D tests.

The ability of industrial X-ray computed tomography (CT) to scan an object with several internal and external features at once causes increasing adoption in dimensional metrology. In order to evaluate the quality of a measurement value, the task-specific measurement uncertainty has to be determined. Currently, VDI/VDE 2630 part 2.1 gives a guideline to determine the uncertainty of CT measurements experimentally by conducting repeated measurements. This is costly and time-consuming. Thus, the aim is to determine the task-specific measurement uncertainty numerically by simulations (e. g. according to the guide to expression of uncertainty in measurement (GUM) Supplement 1). To achieve that, a digital twin is necessary. This contribution presents a simple first approach how a digital twin can be built. In order to evaluate this approach, a study comparing measurements and simulations of different real CT systems was carried out by determining the differences between the measurement results of the digital twin and of the measurement results of the real-world CT systems. The results have shown a moderate agreement between real and simulated data. To improve on this aspect, a standardized method to characterize CT systems and methods to implement CT parameters into the simulation with sufficient accuracy will be developed.