QMT Features: October 2011
High speed inline CT
High-speed computed tomography for mass production processes. By Dr Ingo Stuke and Dr Oliver Brunke,  GE Sensing & Inspection Technologies.


Non-destructive inspection methods are used as early as possible in the added value chain during industrial production processes to minimise costs and reworking. Now  high-speed radioscopic 2-D inline inspection systems with automatic defect analysis software are state of the art for inspection of, for example, castings.  This article describes the next stage of evolution within X-ray technology, namely high-speed CT technology based on highly developed medical tomographs.

This technology makes it possible to carry out one hundred percent 3-D inspections for a very wide range of industrial applications, even at high throughput rates. High-speed CT is the only method currently available which allows internal structures of extremely complex components made of cast alloys, (such as wall thicknesses in cylinder heads), to be inspected on a non-destructive basis directly on, or near, the production line.

Industrial process monitoring using volumetric data and three-dimensional analysis offers several advantages compared with conventional radioscopic 2-D inspection. It allows the reject rate to be reduced by analysing the 3-D position, form and size of the defects, taking into account the subsequent processes the products must undergo. This means that any identified anomalies can be analysed with a view to the final, finished areas and surface finishes. Flaws in workpiece sections, which will be removed by subsequent stages of the process, can be ignored. Moreover, to avoid waste, it is possible to check whether identified porous areas will be open on the final surface before processing begins. At the same time, the scanned workpiece geometry can be checked for anomalies using the nominal CAD data. This means that any form and size deviations can be identified at an early stage of the production process. And finally, foreign substances, such as inclusions or residual sand casting core deposit, can be identified, localised and classified in terms of their density and position. In all these cases productivity can be increased, since the process parameters can be immediately adjusted to counter any identified flaws.

From medical to industrial process control
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. In the latter case, the sample rotates in the X-ray beam which, thanks to its flexible enlargement principle, allows extremely high resolutions of just a few micrometres, or better, to be achieved. However, in medical tomography, the CT gantry rotates with its generator, X-ray tube and corresponding detector at high speed around a patient lying on a imaging table which moves through the scanner.

Although the fixed magnification limits the local resolution to several hundred micrometres, the benefits of this technology really come to the fore when this resolution is completely adequate, as is the case for analysing flaws in medium size to large light metal castings. Whilst in the past, industrial inline CT applications have focussed 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 transported simply and continuously through the tomograph on a conveyor belt, without any handling, and scanned comparatively quickly using helix multi-line technology. In addition, the throughput principle also means that various types of components can be inspected in sequence.   

Improved sample throughput
The concept for the fast industrial inline CT system is based on medical tomography systems from GE Healthcare. These have been adapted by GE Sensing & Inspection Technologies with the appropriate transport facilities and automated 3D failure evaluation software modules for continuous operation in high-speed industrial inspection systems. A specially developed air-conditioned safety cabinet not only protects the surrounding area from the 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 extend from 10 seconds for small pistons or chassis components to as much as 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 inlineCT scanner enables a typical scanning and inspection speed of 5 to 10 millimetres per second or more - a very high throughput rate compared to typical industrial CT systems. To guarantee the required image quality with 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 imaging artefacts caused by scattered radiation on the one hand and scanning time on the other - particularly for scanning large components. Although the scanning time could be reduced considerably further by the use of detectors with 64 or more lines, the proportion of scattered radiation rises as the number of image lines increases and results in a major reduction in image quality and, therefore, less precise inspection results. Although this is tolerable for scanning biological bodies, light metal components, in particular, generate considerably higher scattered radiation.

The required X-ray radiation is generated by a specially cooled GE rotary anode X-ray tube which ensures the high speed and complete irradiation of 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 three-shift production in a 24/7 operation. Despite their high rating, these tubes function with a relatively small focus and therefore produce excellent image clarity. The data acquisition procedure uses a helix scan to ensure that the components for inspection can be fed through the system quickly and continuously. In this case, the tomograph rotates around the component. At relatively slow conveyor speeds, the various turns of the helix scan will overlap, which enables extremely high image quality to be achieved, but also results in a lower throughput speed. By carefully selecting the helix gradient, therefore, it is possible to achieve a perfect compromise between scanning speed and result quality for the application in hand. A conveyor speed of up to >30 mm/s can be achieved for high speed scans.  

Integration into the production line
Different types of loading are possible depending on area of application. In the simplest and lowest cost version, the workpieces are moved off the production line by hand and placed on to 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 samples of up to 500 mm in diameter and with a length of up to >1000 mm 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 53 kW is possible for 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 Figure 3 shows, the components for inspection are transported through the tomograph continuously on conveyor belts or pallet transport systems. Automatic gates ensure that no radiation can escape during the loading and unloading process, thus ensuring a safe and continuous inspection process.

Evaluation parallel to the scanning process
In addition to efficient sample handling and data recording, the main factors which decide the cycle time of a high speed inline CT system include a parallel, fully automatic 3-D reconstruction and analysis process. This includes, 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 parallel to the scanning process. For metrological applications, for example, the workpiece surface including all the undercuts are extracted; 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 carried out using the new 3-D ADR Software from GE both on the basis of 2-D section slices and also in 3-D volumes.

Industrial process monitoring
Advanced medical GE gantry computer tomography provides technology that has been established and developed over a period of three and a half decades. With GE’s new industrial CT concept, it is now going to be also available for high speed 100% in-process inspections within the typical cycle times used in production lines such as those used in foundries. Simply transporting the components through the tomograph and scanning them at speeds of up to several centimetres per second after which they can be automatically evaluated. This opens up a whole host of new applications for inline process controls, which have in the past been out of reach of conventional industrial CT.l

Dr Ing. Ingo Stuke is an Image Processing Engineer at GE Sensing & Inspection Technologies GmbH in Ahrensburg, Germany.
email:  Ingo.Stuke@ge.com
Dr Ing. Oliver Brunke is Product Manager for 3-D Metrology and Failure Analysis using Computer Tomography at GE Sensing & Inspection Technologies GmbH in Wunstorf, Germany.
email: Oliver.Brunke@ge.com
  
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