Complex 3D scan trajectories for industrial cone-beam computed tomography using a hexapod

Abstract

Industrial x-ray computed tomography (CT) represents an established measurement technique in the field of nondestructive testing and dimensional metrology. However, the measurement accuracy is sometimes limited by measurement artifacts that cannot be avoided using a standard circular scan trajectory. This problem can be addressed with the aid of flexible 3D trajectories, but up to date, the application of these is mainly restricted to special CT devices using robot arms. In this paper, we present results using a hexapod as an additional positioning system in a commercial industrial CT scanner. In addition to the 360∘ rotation, task-specific tilting of the part during the scan is possible in this way. We used and adapted geometry calibration procedures based on a multi-sphere reference object to enable reconstruction with high accuracy. Using a demonstrator test fixture with high absorbing elements, we show that severe metal and truncation artifacts can be avoided for a region-of-interest scan. Furthermore, cone-beam artifacts, which are inherent to circular scan trajectories, can be reduced significantly. Using measurement objects that can be measured well with a circular trajectory, we found that applying a 3D trajectory leads to dimensional measurement deviations in the same range or even lower than those of a circular trajectory. This suggests that the pose repeatability of the hexapod is sufficient to perform complex scan paths without general loss of accuracy. The obtained results could be relevant for end users of conventional CT scanners, as upgrading existing devices is in principle possible. The presented investigations form the basis for the application of trajectory optimization algorithms.

CT scan trajectory calibration based on projected metal spheres: When and how should errors from elliptical distortion be corrected?

“…We tested the relevance of the ellipse correction for two different objects and types of scan trajectory calibration. On the one hand, trajectory calibration according to [8] was applied, using one of several multi-sphere objects that were manufactured at our institute for different magnifications (see Fig. 10(a)).…”

“…For 3D trajectory calibration as investigated in [8], the source-detector-distance d SD is known and can therefore be used for the ellipse correction. In the following, we show how the original derivation of the correction formula of Deng et al [11] can be adapted to use this additional information and make the correction more robust.…”

“…In [7], a method and a phantom that is suitable for a twin robotic CT system was presented. In our own work [8], we implemented multi-sphere based scan trajectory calibration for a CT device with a hexapod as additional object manipulator on top of the rotary table. In a recent contribution [9], we presented an alternative approach based on a single sphere.…”

IntelliScan: Improving the quality of x-ray computed tomography surface data through intelligent selection of projection angles

“…Most practitioners will agree that for the vast majority of cases, a circular scan trajectory leads to an acceptable quality of XCT data. However, as engineering components become more geometrically complex due to advances in manufacturing technology, perhaps more exotic XCT scan trajectories may offer some advantages [1][2][3][4]. Computer aided design (CAD) data is almost always available for modern engineering components;…”

Selection and evaluation of spherical acquisition trajectories for industrial computed tomography

“…Another With the broader availability of new, flexible configurations, the influence of the selected trajectory design becomes increasingly relevant and has attracted attention from industrial [37] and medical sides [38,39]. While some research has been performed on the effect of reducing the number of projections [40][41][42] or the adjustment of the acquisition trajectory with respect to metrological measurements [43], the influence of the trajectory choice onto the image quality in non-destructive evaluation has barely been investigated so far [37]. While for measurement tasks typically a high number of projections need to be acquired, this number can be reduced to a fraction if only the decision of a certain feature (which typically can be a defect, e.g.…”