Evaluation of the impact of faulty scanning trajectories in robot-based x-ray computed tomography

Abstract

X-ray computed tomography (CT) imaging for industrial applications is limited to certain physical conditions to be fulfilled. The size of the measuring object and the accumulated wall thickness are two fundamental conditions. An omission of these conditions by not capturing object attenuation information by the x-ray detector leads to missing data in the 3D reconstruction process and results as a consequence in image degradation and artifacts. Conventional industrial x-ray CT is based on cone-beam projections and circular or helical scanning trajectories using linear axis and a rotary (lift) table. For many inspection tasks on big-sized or unusually shaped objects the physical limits for obtaining a sufficient high image quality are reached very quickly when using conventional CT systems. Industrial six-axis robots offer much more flexibility with respect to the conditions mentioned earlier and can overcome the limitations of conventional scanners. In the present work we characterized an industrial six-axis robot in its working space following ISO 9283 in terms of pose accuracy and pose repeatability. These results are then used to simulate faulty scanning trajectories in terms of pose deviations where a single robot is used as an object manipulator to rotate virtual specimens on a circular trajectory resulting in different (faulty) reconstruction datasets. These datasets are evaluated visually and by using performance parameters and geometrical features in order to determine the reproduction fidelity (performance) of a one arm robot-based CT system depending on different pose errors. With the results obtained it was shown that a robot-based CT system of type B (in our classification scheme) using one robot as object manipulator should be able to reach a spatial resolution power in the range of the voxel size (in our case 200 µm) and smaller (neglecting effects from focal spot size, detector unsharpness from x-ray to light conversation and scatter radiation) if systematic pose errors are compensated using appropriate calibration methods.

Calibration of 3D scan trajectories for an industrial computed tomography setup with 6-DOF object manipulator system using a single sphere

“…The achievable accuracy for robot based setups however again depends on the positioning accuracy of the manipulator devices. While the pose repeatability of robot arms might be sufficient [2], comparatively high systematic deviations might make it more feasible to use calibration methods such as the multi-sphere based procedure in [8] that compensate for systematic errors.…”

“…In recent years, there is a trend towards extending scan trajectories to task-specific, non-planar shapes, especially for large measurement objects that are scanned with the help of robot arms as manipulator systems for the x-ray tube and detector [1][2][3][4]. Particularly with focus on such setups, there are several publications dealing with different methods to optimize 3D scan trajectories for industrial CT with regard to imaging quality and measurement time [5][6][7].…”

Generation of a 3D model of the inside volume of shoes for e-commerce applications using industrial x-ray computed tomography

“…We think that it is worth to investigate also the usage of 2D projection images (without 3D reconstruction) to obtain information on the inner shoe volume in order to reduce scanning time. Subsequently, a high-speed x-ray system integrated in the goods conveyor, as shown in figure 20, could be developed and realised for the field using, for instance, a multi-source (see [17,18]) or a robot-based system (see [19]) as soon the questions above are answered satisfactorily.…”

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

“…Custom-built or prototype devices all use robot arms to either position the source and the detector or the part. In [8], an overview of possible arrangements is given.…”

“…The possibility to scan components of large scale assemblies such as entire cars is the main benefit of using robot arms. Current research on robot arm CT systems addresses the characterization and improvement of the positioning accuracy [8,14]. As a precise knowledge of the geometry description for each single x-ray projection is necessary for a high quality volume reconstruction with good structural resolution (according to guideline VDI/VDE 2630 part 1.3 defined as the smallest structure that can still be measured dimensionally [15]), the positioning accuracy of the robot arms can be seen as a major challenge.…”