Tino Hausotte

As industrial computed tomography (CT) systems evolve toward robotic CT and freely selectable three-dimensional acquisition trajectories, spatial resolution must be evaluated not only globally but also with respect to position and direction within the three-dimensional measuring volume. The modulation transfer function (MTF) is a key metric for spatial resolution, yet the widely used ASTM E1695−20 method, based on edge response analysis of cylindrical objects, provides only averaged axial-plane values and lacks directional information. To overcome these limitations, this contribution proposes an extended MTF evaluation approach suitable for advanced 3D trajectories. The method analyzes individual gray value profiles along lines perpendicular to object surfaces and applies local averaging to improve robustness against voxel grid effects and noise. While a cylindrical object is used for comparability with ASTM E1695−20, a spherical object enables full 3D directional MTF assessment.

The WIPANO research project series CTSimU deals with radiation simulation for determining the measurement uncertainty when measuring geometric features using X-ray computed tomography. Industrial X-ray computed tomography (CT) completely captures workpieces, including their inner and outer structures for metrology purposes. However, previous effort to determine task-specific measurement uncertainty is not always justifiable. Simulating the measuring process allows to estimate the measurement uncertainty numerically and to predict systematic measurement deviations. The CTSimU3 project aims to create a time-saving, resource-efficient, and reliable method that enables measurement uncertainty determination in CT through simulation using the GUM Supplement 1. This article presents the project idea as well as the first results of the requirement analysis.

Digital models using computer simulation for the acquisition of X-ray projections offer the possibility to virtually perform a X-ray computed tomography (CT) measurement. Virtual CT scans benefit research and development of X-ray computed tomography, serving various purposes while reducing physical scan time and therefore cost. The accuracy of a digital model determines its utility. Suitable test procedures of digital models can assure the sufficient accuracy of a digital model regarding a specific task. For the use of digital models in dimensional metrology an agreement of dimensional measurands compared to a physical counterpart is important. In this contribution, a physical and virtual CT measurements of a calibrated Multi Feature Check mini specimen are evaluated regarding surface transition metrics of the volume data and its metrological compatibility of dimensional measurands. For the evaluation of the gray value response at the surface transition, a methodology that uses local sampling patterns to generate a locally averaged edge response is presented. The digital model shows promising results to serve as a basis for further extension to determine the measurement uncertainty of dimensional CT measurements using the Monte-Carlo method in the future.

The document JCGM 106:2012 outlines two probability-based approaches for evaluate the conformity or non-conformity of a measured value with a specification in Chapters 7 and 8. The ISO 14253-1:2017 is the applicable standard for conformity and non-conformity assessment using a probability-based approach with guard bands for measurement values from geometric measurements. The article presents a possible extension of the ISO 14253-1:2017 to enable the practical application and the use of various distribution functions and the results of a measurement uncertainty determination with Monte Carlo simulation. The article focuses in particular on the questions of which types of distribution functions should be assumed for the measurands depending on the number of measurement points and the value of the measurand, as well as what the effects are for two probability-based approaches from JCGM 106:2012 Chapters 7 and 8 when there is a large measurement uncertainty compared to the tolerance, and what needs to be taken into account. The influence of a measurement uncertainty which is too large in comparison to the tolerance is illustrated in the three relevant cases for both approaches, thus highlighting the need for an appropriately small measurement uncertainty for conformity and non-conformity assessment.

Selecting scan angles such that surface segments are aligned with straight X-ray paths (i.e., rays are tangential to the surface and therefore perpendicular to the local surface normal) is known to produce sharper transitions of those surface segments in the reconstructed volume. This enhances dimensional accuracy in sparse-view computed tomography (CT). However, existing approaches offer no direct means to exploit this criterion for automatic scan-angle optimization. We propose a method that uses a virtual representation of the CT setup, including an STL surface model of the inspected part, to automatically identify taskspecific scan angles. Using elementary vector calculus, the algorithm determines projection directions that generate tangential X-rays for targeted surface segments. To support different levels of geometric complexity, we introduce two variants of the angle-selection procedure. The methods were experimentally validated on two objects with distinct absorption and geometric characteristics. For a steel gauge block, employing the minimum number of task-specific projections required for surface-data completeness substantially outperformed a conventional high-projection scan. For a geometrically more complex test object, surface-related errors were still reduced within the region of interest. The proposed approach – particularly suited for flat surface structures and not accounting for image-degrading factors other than cone-beam artifacts – shows promise for high-throughput dimensional metrology of mono-material parts.

This article presents the development of a novel, active measuring head for coordinate measuring machines that is capable of measuring forces and torques in six degrees of freedom (6 DoF) during tactile sensing. It uses a so-called flotor as carrier of the stylus, which is kept in levitation by six electromagnetic voice coil actuators (VCA) in an hexapod-like arrangement. These VCAs function simultaneously as actuators and as force sensors, which are calibrated in situ according to the Kibble balance principle. Additionally, a position and orientation measurement system is integrated, consisting of three two-dimensional position sensors that can detect the position and orientation of the flotor in 6 DoF. The article describes the basic functionality as well as the most important technical functional elements and key design features. These features aim to optimise the metrological properties of probing force and position measurement in order to achieve the lowest possible measurement uncertainty. Furthermore, the structure of the peripheral electronics system for the controlling of the coordinate measuring head is described. So far, the conceptual and structural design has been implemented, two prototypes have been manufactured and put into operation. At the time of publication, initial metrological investigations are being carried out.

For the long-term goal of determining the measurement uncertainty of dimensional computed tomography (CT) measurements with simulation, it is necessary to prove the compatibility of the digital model with its real counterpart. A study of repeated CT measurements of a calibrated aluminum specimen was performed and afterwards simulated. A first indicator for a sufficiently good digital representation is the agreement of dimensional measurands, such as diameters and center-to-center distances. Results show in general good agreement between simulation and real measurement. A detailed discussion of the results as well as a proposed criterion for testing the simulation will be shown in the full paper.

Although powder-based additive manufacturing processes have great potential for the toolless production of complex and individual components with almost unlimited design freedom, they have only hesitantly been used for the flexible production of individual workpieces. One reason for this is the unacceptable quality deficits in terms of geometrical accuracy and component strength. In the case of selective laser melting (SLM), these are induced not only by component distortion during cooling, but also by fluctuations in the process parameters and influencing factors such as laser energy, layer thickness, particle size, temperature history, and relative position deviations between the laser beam and the powder bed caused during layer-by-layer production. Detection of warpage using retained samples and special test specimens is expensive and potentially inaccurate due to the correlation between part properties and position in the building chamber resulting from spatial temperature gradient dependencies [6]. As a workaround, a layer-wise incremental in-line measurement has to be carried out. The automatic correction of process fluctuation-related component errors through the regulation of process parameters that can be realized with this should allow a significant reduction in rejects, costs, and provision times.

Industrial x-ray computed tomography (CT) systems with high geometric flexibility are increasingly utilized for large-scale measurement objects or challenging measurement tasks. To maintain high accuracy when deviating from the established circular scan trajectory, trajectory calibration methods using multi-sphere reference objects with known marker positions are commonly employed. These multi-sphere objects can either be scanned together with the measurement object (online trajectory calibration) or in a separate scan (offline trajectory calibration). While offline calibration increases machine time, it generally results in higher scan quality. However, a sufficient pose repeatability is necessary to ensure comparable or even superior accuracy to online calibration. In this contribution, we present a straightforward procedure to compare both types of trajectory calibration in a way that the differences of the results can directly be traced back to the influence of the pose repeatability. The multi-sphere reference object is not only used for trajectory calibration, but simultaneously as a measurement object for repeated measurements. The methodology is tested on both a twin robotic CT system and a conventional CT system that is additionally equipped with a hexapod manipulator for adaptive object tilting. Results showed, independent from the type of trajectory calibration, systematic measurement errors in the order of 10⁻⁵–10⁻⁴ of measured sphere distances and sphericity values below 50 μ m . For sphere distances, random errors were increased by a factor of 5 due to the offline trajectory calibration, but were still low ( < 1 μ m ) in comparison to systematic errors and the spread of different measurement features. Overall, both investigated systems demonstrated sufficient positioning repeatability for offline trajectory calibration. The method is in general also applicable to any other types of manipulator systems used for CT devices. It provides a workflow for the decision which type of trajectory calibration is preferable for a given CT system.

The structural resolution describes the ability of a measuring device to detect small structures on the surface of a component or test specimen by means of a quantitative value. However, the structural resolution in the computer tomograph depends on the object and must therefore be determined separately for each measurement task. The previous approaches to structural resolution determination are only related to test specimens. In this paper, less discrete approaches based on a circular pattern are presented, which can be integrated into the measured component. A voxel-based methodology as well as two surface-based methodologies are described. The investigation results regarding the effect of the component position on the structural resolution are obtained on the basis of real CT measurements. A comparison is also completed with the well-known hourglass method. The results show that the resolution depends on the object being measured, with similar values being obtained for the same object using different methods.