Industrial computed tomography (CT) plays a key role in 3D coordinate metrology as an alternative to conventional coordinate measuring machines. CT measurement uncertainty depends on the setup parameters with which CT scans are performed. Currently, there is no established model that can describe the relationship between CT setup parameters and measurement uncertainty, and thus determine the optimal settings for a given measurement task. In practice, CT users choose setup parameters intuitively causing high variability in the measurement results. In this study, we enhance and validate an analytical method for optimizing imaging parameters. The proposed method is based on the assumption that minimizing the contribution of imaging parameters to measurement uncertainty corresponds to (1) maximizing image contrast and signal, (2) minimizing geometric blurring, and (3) minimizing image noise and CT artifacts. It also requires information about the workpiece nominal geometry and material composition. The proposed method calculates the optimal photon energy for precise surface determination, given the workpiece position and orientation. Tube voltage and prefilter are determined so that the resulting x-ray spectrum has an effective energy equal to the optimal energy and beam hardening artifacts are minimized. Tube current is chosen to maximize image signal, while avoiding blurring and excessive generated tube power. Exposure time is chosen as the shortest time that fully exploits the detector dynamics. The proposed method was validated by analyzing the standard deviations of CT measurements on a test phantom. An analysis of variance on the uncertainty components showed that the predicted parameters were globally optimal.

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Industrial X-ray computed tomography (CT) is a powerful technique for producing a three-dimensional model of an object and for performing dimensional measurements of an object’s inner and outer features. The diversity of the measuring objects causes a variety of CT setup parameters. The chosen CT setup parameters influence the uncertainty of dimensional measurements. A CT user determines the workpiece orientation and the distance between the workpiece and the X-ray source before a CT scan is commenced, and all other CT setup parameters are based on the workpiece placement inside the CT system. Hence, the workpiece placement is the trend-setting parameter of a CT scan. This paper presents a method for optimising a workpiece placement for dimensional measurements of multi-material workpieces. The method works with the stereolithography (STL) model of the workpiece and analytically tests the whole placement range to find the placement that minimises the attenuation power of a workpiece. The method is validated by analysing the standard deviation of CT measurements on a multi-material workpiece. An analysis of variance on the uncertainty contributions demonstrated that the predicted workpiece placement was optimal.

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