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

Abstract

In industrial X-ray computed tomography (CT), the application of more complex scan paths in comparison to the typical circular trajectory (360° rotation of the measurement object) can extend the potential of CT. One way to enable such 3D scan trajectories is to use a 6-degrees-of-freedom object manipulator system. In our case, a hexapod is mounted on top of the rotary table of a commercial CT scanner. This allows for adaptive tilting of the measurement object during the scan. For high accuracy, the geometry calibration of such setups is typically done using the X-ray projections of a calibrated multi-sphere object. Contrary to this, here, we demonstrate a procedure that is based on only a single sphere and can therefore experimentally be implemented with low effort. Using the intrinsic geometry parameters of the CT device as prior information, the hexapod coordinate system with respect to the CT machine coordinate system is determined by means of a one-step optimization approach. The resulting parameters are used to calculate projection matrices that enable the volume reconstruction for 3D scan trajectories. The method is validated using simulated X-ray images and experimental investigations including dimensional measurements. For the used setup, geometric measurement results for 3D scan trajectories that are calibrated with the presented method show in sum increased errors compared to the circular scans. A limited pose accuracy of the manipulator system is discussed as a potential cause. The results nevertheless indicate that the presented method is generally feasible for dimensional CT measurements provided that the pose accuracy is sufficient. The calibration procedure can therefore be a low-cost and easier to implement alternative compared to trajectory calibration methods based on multi-sphere objects, but with a tendency towards lower measurement accuracy. The methodology can in principle be transferred to different setups with 6-DOF manipulator systems, e.g. C-arm CT devices with a robot arm.

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

“…On the other hand, the data of a former publication [9] dealing with a trajectory calibration procedure based on a single sphere (see Fig. (b) Single sphere pressed into an cylindrical sample holder used for trajectory calibration according to [9]. The lower images are exemplary radiographs from the series of projections used for the calibration procedures.…”

“…The results are listed in table 2 in form of the reprojection error. The reprojection error is the root mean square of the absolute pixel distances between the calculated and measured ellipse positions (see [9] for details). In the case of the multi-sphere object, the mean reprojection error over all projections is used.…”

“…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.…”

“…(a) Multi-sphere object used for trajectory calibration according to [8]. (b) Single sphere pressed into an cylindrical sample holder used for trajectory calibration according to[9]. The lower images are exemplary radiographs from the series of projections used for the calibration procedures.…”

A density-controllable 3D measurement system based on linear profile sensor and universal manipulator

“…It has found extensive applications in quality inspection, reverse engineering, and other related fields [1][2][3][4][5][6]. In recent years, the emergence of industrial robots has led to the development of mounted 3D vision measurement systems [7][8][9][10][11][12][13]. These systems have gained popularity in industrial manufacturing due to the robot's flexibility and convenience.…”