Inline CT

Authors: Dr. Ingo Stuke, Ahrensburg and Dr. Oliver Brunke, both from GE Sensing & Inspection Technologies, Wunstorf

The term InlineCT comprises high-speed computed tomography methods for mass production processes.

Non-destructive inspection methods are used as early as possible in the value chain during industrial production processes to minimize costs and reworking. High-speed radioscopic 2-D inline inspection systems with automatic defect analysis software have become state of the art, in particular for the inspection of castings.

In the future, high-speed CT technology based on enhanced medical tomographs will enable complete 3-D inspections even at high throughputs and in a variety of industrial applications. This CT method is particularly suitable for complex components made of cast light alloys, such as cylinder heads, with internal structures and wall thicknesses being inspected in a non-destructive manner directly on, or near, the production line.Industrial process monitoring using volumetric data (see Voxel) and three-dimensional analysis offers several advantages over conventional radioscopic 2-D inspection. It allows the reject rate to be reduced by also analyzing the 3-D position, shape and size of the defects, taking into account subsequent process steps. This means that any identified anomalies can be analyzed with a view to the final, finished areas and surfaces. Defects in workpiece sections which will be removed in subsequent machining steps can be ignored. Moreover, to avoid rejections, it is possible to check whether identified porosities (e.g. gas porosity, microporosity, hydrogen porosity) will be open on the final surface prior to machining. At the same time, the scanned workpiece geometry can be checked for deviations from the nominal CAD data. This means that any deviations in shape or size can be identified at an early stage of the production process. In addition, inclusions of a higher or lower density (e.g. oxide inclusions, slag inclusions) or residual sand core deposits (see Sand inclusion) can also be identified, localized and classified by their density and position using an inline CT.

Whilst computer tomography has been used in medicine for decades, over the last few years industrial CT has developed into a widely used 3-D inspection method for scientific and industrial applications. While, in the latter case, the specimen rotates in the X-ray beam which, thanks to its flexible magnification principle, enables extremely high resolutions of just a few micrometres or even less, in medical tomography, the CT gantry rotates with its generator, X-ray tube and the detector opposite to it at high speeds around a patient lying on an imaging table. Although the fixed magnification limits spatial resolution to several hundred micrometers, the benefits of this technology really come to the fore where this resolution is completely sufficient for defect analysis, as is the case with large light metal castings. Whilst in the past, industrial inline CT applications have focused on automatic loading and unloading devices using robot arms, for example, and on surface finish detectors (with comparatively long acquisition times and only a small imageable area), the use of a gantry scanner means that the workpieces can be conveyed simply and continuously through the tomograph on a conveyor belt, without any handling, and scanned comparatively quickly using helical multi-line technology. In addition, the throughput principle also means that various types of components can be inspected in sequence.

High-performance scanner for optimized throughput of specimens

The industrial inline CT system currently under development is based on medical tomographs from GE Healthcare. These have been adapted by GE Sensing & Inspection Technologies with the appropriate conveying equipment and defect evaluation software modules for continuous operation in high-speed industrial inspection systems. A specially developed air-conditioned safety housing not only shields the surrounding area from X-ray radiation, but also protects the tomograph from the dust and heat generated in harsh production environments.
The typical throughput requirements for the foundry industry range from 10 seconds for small pistons or chassis components to up to 80-90 seconds for complex engine components, such as cylinder heads. A fully automatic inspection method, including the whole data acquisition and analysis process, is required to meet these cycle times. The GE inline CT scanner enables a typical scanning and inspection speed of 5 to 10 millimeters per second or more – a very high throughput rate compared to typical industrial CT systems. To guarantee the required image quality at short measuring times, the system is fitted with a high-performance X-ray tube and a flexible 16-line detector, which represents an efficient compromise between scattered radiation on the one hand and scanning time on the other – particularly when scanning large components.

Although the scanning time could be reduced considerably further using detectors with 64 or more lines, the proportion of scattered radiation rises as the number of scanning lines increases and results in a major reduction in image quality and, therefore, less precise measuring results. This may be tolerable for scanning biological bodies, but light-metal components, in particular, generate a considerably higher amount of scattered radiation.

The required X-radiation is generated by a specially cooled GE rotary anode X-ray tube which ensures the high speed and complete transmission of radiation through the component with a tube voltage of up to 140 kV and a tube rating of several kW. The tube operating parameters have been adapted to meet the requirements of 3-shift production in 24/7 operation. Despite their high rating, these tubes work with a relatively small focus and therefore produce excellent image acuity. The data acquisition procedure uses a helical scan to ensure that the components to be inspected can be fed through the system quickly and continuously. In this case, the tomograph rotates around the test specimen in a helical motion. At relatively slow conveyor speeds, the various turns of the helical scan will overlap, which enables extremely high image quality, but also results in a lower throughput rate. Therefore, by carefully selecting the helix gradient, it is possible to achieve a perfect compromise between scanning speed and result quality for the application at hand. A conveyor speed of up to >30 mm/s can be achieved for high-speed scans.

Execution example: integration of the CT scanner into the production line

Different types of loading are possible depending on the area of application. In the simplest and lowest-cost version, the workpieces are removed from the production line by hand and placed onto a conveyor belt passing through the tomograph which is hermetically encased with lead for radiation protection purposes, and then moved out of it after the scan. This method has the benefit that random specimens of up to 500mm in diameter and with a length of up to >1000mm from various production lines can be scanned in parallel and at high speed. In contrast to continuous operation, an even higher rating of up to 53kW is possible with this version.

The parts handling system supports a mixture of all types of components and is also designed for direct integration into a production line. As shown in Fig. 3, the components to be inspected are moved through the tomograph continuously on conveyor belts or pallet conveyors. Automatic gates ensure that no radiation can escape during loading and unloading, ensuring a safe and continuous inspection process.

Evaluation parallel to the scanning process

In addition to efficient specimen handling and data recording, the main factors which determine the cycle time of a high-speed inline CT system include a parallel, fully automatic 3-D reconstruction and analysis process. This involves, for example, an automatic beam hardening correction procedure. The evaluation processes programmed for the particular workpiece are carried out automatically on the reconstructed 3-D volume in parallel to the scanning process. For metrological applications, for example, the workpiece surface including all the undercuts is extracted and the 3-D measurements are then carried out using pre-programmed measuring routines by special programs such as Polyworks Inspector®. Automatic porosity analyses in castings can be performed by means of the new 3D-SABA software generation from GE both on the basis of 2-D section slices and also in 3-D volumes.

The following are values which can typically be achieved with an inline CT system:

  • Spatial resolution minimum voxel size < 200µm
  • 3-D metrology: Sigma as little as < 2µm, absolute deviation by as little as < 10µm (thus, tolerances of only 200µm are “reliably” measurable according to Bosch Booklet no. 10)
  • Detection of defects: cavities of < 1mm
    • Fig. 1: With inline CT by means of modified medical scanners, components can be continuously moved through the tomograph, scanned at high speeds and analyzed three-dimensionally. The safety enclosure of this GE tomograph was removed for taking the picture, photo: GE Sensing &amp; Inspection Technologies
    • Fig. 2:  With the high-speed automatic helical inline CT, the gantry rotates around the workpieces on the conveyor belt together with the X-ray tube and the multi-line detector opposite to it, photo: GE Sensing &amp; Inspection Technologies
    • Fig. 3: For industrial applications, the GE inline CT scanner is equipped with automatic conveying systems and encased with a radiation-shielded housing comprising gates, photo: GE Sensing &amp; Inspection Technologies
    • Fig. 4: Automatic 3-D defect analysis of an aluminum casting. In this example, the smallest detected defects have a defect volume of ~0.95mm³ (blue), photo: GE Sensing &amp; Inspection Technologies
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