Modelling and correcting the effects of detector misalignments on dimensional measurements by computed tomography

The application and demands for inspection of advanced manufactured components by X-ray computed tomography (XCT), particularly in the field of dimensional metrology, are rapidly growing. However, specification standards to establish measurement assurance of XCT systems, e.g. performance verification protocols or calibration procedures, are still in the very early stages of development. In the absence of documented measurement standards, the comprehensive assessment of measurement uncertainty and subsequent establishment of related traceability are not ensured and are thus difficult to achieve. Evaluation of measurement uncertainty demands a thorough understanding of the major factors influencing the measurement. In the study presented here, the effects of geometrical errors in the XCT measurement instrument are investigated by way of simulation using Scorpius XLab®. Dimensional measurements are performed on a simulated cylindrical array of aluminum spheres in the presence of detector rotational errors of varying magnitudes. The simulated measurements are compared to the results from a simulation of the same measurement under ideal system geometry. This study is presented in the context of analysing the sensitivity of the geometrical influencing factors on the quality of the dimensional measurements made by XCT. Subsequently, the effects of detector rotations on the radiographic pixel space are modelled. A pixel distortion correction algorithm is introduced and applied to the simulated radiographs; the effects of applying the pixel correction algorithm on the dimensional measurements of the array of spheres are presented. Finally, the implications of the outputs from this study to the development of an effective test object for measurement of geometrical errors of an XCT test system are discussed.status: publishe