Automated High Speed Volume Computed Tomography for Inline Quality Control

Increasing complexity of innovative products as well as growing requirements on quality and reliability call for more detailed knowledge about internal structures of manufactured components rather by 100 % inspection than just by sampling test. A first-step solution, like radioscopic inline inspection machines, equipped with automated data evaluation software, have become state of the art in the production floor during the last years. However, these machines provide just ordinary two-dimensional information and deliver no volume data e.g. to evaluate exact position or shape of detected defects. One way to solve this problem is the application of X-ray computed tomography (CT) [1]. Compared to the performance of the first generation medical scanners (scanning times of many hours), today, modern Volume CT machines for industrial applications need about 5 minutes for a full object scan depending on the object size. Of course, this is still too long to introduce this powerful method into the inline production quality control. In order to gain acceptance, the scanning time including subsequent data evaluation must be decreased significantly and adapted to the manufacturing cycle times. This presentation demonstrates the new technical set up, reconstruction results and the methods for high-speed volume data evaluation of a new fully automated high-speed CT scanner with cycle times below one minute for an object size of less than 15 cm. This will directly create new opportunities in design and construction of more complex objects. Introduction: Increasing competition in the car industry particularly forces the suppliers to enhance their product quality and to optimize the production processes in order to improve or at least sustain the sales opportunities. Regarding the issue of quality control, Austria Alu-Guss (AAG), a wheel casting company, belonging to the BORBET group, is cooperating with the Fraunhofer-Gesellschaft in the field of automated radioscopic inline inspection. Since 1983, this company is producing light alloy wheels by millions for the European car industry each year. However, the changing philosophy to Just In Time (JIT) manufacturing in the car industry significantly increased the demand for quality and reliable supply for the cooperating companies. In 1996, after the acquisition of Austria Aluguss by Borbet, the programme AAG2000 was started, targeting for increasing the production capacities, linking processes and increasing automation in production processes. Automation mainly means higher process reliability and productivity. A maximum flexibility is guaranteed by modern automated production and complete quality management. One part in this quality chain is the inspection of the cast wheels by X-ray fault detection. During the casting process, defects like pores, blow holes or cracks can appear. Such defects are, in a certain range, permissible according to size, location and density with criteria prescribed by the car industry. As wheels are safety relevant parts, these instructions usually are very strict and today 100 % completely checked by radioscopic inline inspection of every cast wheel. State of the Art and Vision: The inspection, in former times done by manual/visual control of the scrolling radioscopic images of the x-rayed wheels, today is performed automatically. The applied ISAR system (Intelligent System for Automated Radioscopy) is inspecting the wheels without any human support immediately after the casting process [2,3]. The software is evaluating the x-ray images, detecting the defects and comparing them to a predefined quality instruction. Objects, classified as defective are rejected. The state-of-the-art solution, which is based on two-dimensional image processing, however, provides no volume information and thus, no exact defect location and size, regarding the third dimension in beam direction, is available in the case of radioscopic imaging. This is not a real problem for wheels manufactured today because all known and relevant defects can be found by this method. However, according to innovation processes in design, construction and production, future quality characteristics can appear that can no longer be checked by simple twodimensional radioscopy. The car industry continuously asks for weight reduction, engendering the development of new and innovative wheels. AAG has developed a so called “Nature Wheel” similar to the construction of human bones. In case of this new product, inline computed tomography could help to examine and measure the position of the integrated core (with lower weight than Al), which then could reduce one inspection step of the current production process. Innovations like this are the challenge for the future non-destructive inspection systems and reveal the limits of the current radioscopic methods. To overcome the limits of standard radioscopic methods, namely the lack of information about the shape of defects, about their exact location or moreover about the inner construction of complex object structures, the threedimensional computed tomography has to be applied, which has proven its potential for the near future to solve those problems. During the last five years, the 3D-CT has made significant progress fostered by the development of a new flat panel detector technology [4,5] on the one hand and by the continuously increasing power of computers on the other hand. Due to these developments, reconstruction times today are in the range of some minutes for object volumes with diameters up to 15 cm. This time, in this case compared to cycle times of standard inline 2D inspection systems of about 30 s for such objects, is not far away from industrial maturity. The anticipating aspiration is to develop 3D-CT systems, combined with fully automated volume image processing, achieving cycle times in the range of production time for such and even larger objects. The major tasks for this challenging goal are • advanced high speed reconstruction algorithms, working with a set of limited projections, • high speed volume data evaluation, • more sensitive, faster and larger x-ray detectors and • high power x-ray tubes. This presentation will demonstrate that improvements already for the first two items will refute the prevailing opinion that CT is still too slow for industrial application. Results: The only chance to achieve short scanning times, is to use 3D or cone beam CT which means to get the total set of reconstruction data by one single object scan. Axial Computed Tomography (ACT) can be realized in one-, twoand three-dimensional systems related to the kind of imaging geometry (parallel, fan or cone beam, fig.1). 2D-CT Systems: One object layer (fig.1 middle) is reconstructed by one fan beam scan similar to the medical scanners. To acquire the total volume, several scans of different layers have to be carried out and the slices are subsequently put together to one volume. A volume of 1024 3 voxels still needs a total scanning and reconstruction time of more than 10 hours! Nevertheless, this method has an important advantage due to scattered X-rays, which are superposed to the real X-ray absorption on the detector and thus reduce the quality of reconstruction. As the solid angle of line detectors in relation to the scattering object is much smaller as of array detectors, reconstruction results consequently should be better in the case of fan beam scanning. Figure 2 demonstrates this effect by comparing two slices, one reconstructed by 2D or fan beam geometry and the other by 3D or cone beam scanning. Although the fan beam setup was not ideal (the line detector was simulated by collimating a flat panel array detector), the difference is evident. Much better results are possible, if more suitable and adapted line detectors are applied. Especially in case of dimensional measuring, where exactly defined object boundaries are needed, reconstructions of cone beam or 3D systems still are unusable today.