Acceptance and reverification testing for industrial computed tomography – a simulative study on geometrical misalignments

Abstract

Acceptance and reverification testing for industrial X-ray computed tomography (CT) is described in different standards (E DIN EN ISO 10360-11:2021-04, 2021; VDI/VDE 2630 Blatt 1.3, 2011; ASME B89.4.23-2020, 2020). The characterisation and testing of CT system performance are often achieved with test artefacts containing spheres. This simulative study characterises the influence of different geometrical error sources – or geometrical misalignments – on these sphere measurements. The two measurands on which this study focuses are the sphere centre-to-centre distances and the sphere probing form errors. One difference between the current draft of the ISO 10360-11 standard (E DIN EN ISO 10360-11:2021-04, 2021) and the VDI/VDE standard 2630 part 1.3 (VDI/VDE 2630 Blatt 1.3, 2011) as well as the ASME standard B89.4.23 (ASME B89.4.23-2020, 2020) are the differences for the sphere centre-to-centre distances that need to be measured. The VDI/VDE standard and the ASME standard require measurements of these kinds of distances of up to 66 % of the possible maximum distance within the measurement volume, while the ISO draft asks for measurements of up to 85 % of the possible maximum distance. This requirement needs to be considered in connection with the maximum permissible error (MPE) specification for these sphere distance measurements. This MPE should be specified as a linear function of the nominal distance or a constant value or a combination thereof (compare definition 9.2 of ISO 10360-1:2000 + Cor.1:2002 (DIN EN ISO 10360-1:2003-07, 2003)), and thus, the linearity of the length-dependent maximum measurement error of the sphere distance measurements is of interest. This simulative study inspects to what extent this linearity can be observed for CT measurements under the influence of different geometric errors. Further, the question is whether measurement lengths above 66 % necessitate a change in the MPE specification. Thus, an automatic identification of cases that might affect the MPE specification is proposed, and these cases are inspected manually. A second aspect of this study is the impact of geometrical misalignments on the probing form errors of a measured sphere. The probing form error also needs to be specified. Thus, whether and how it is influenced by the misalignments is also of interest. Based on our simulations, we conclude that probing form errors and sphere centre-to-centre distances of up to 66 % of the maximum possible measurement length within the measurement volume are sufficient for acceptance testing concerning geometrical misalignments – each geometrical misalignment can be detected well with at least one of these two measurands.