Modeling software for industrial computed tomography problems

Transcription

1 Modeling software for industrial computed tomography problems Yuri V. Obidin, Konstantin V. Petukhov, Vladimir Y. Sartakov Technological Design Institute of Scientific Instrument Engineering (TDI SIE) Siberian Branch of the Russian Academy of Sciences (SB RAS) 41, Russkaya str., Novosibirsk, , Russia Tel.: + 7 [383] Fax: +7 [383] obidin@tdisie.nsc.ru Abstract Description of software for model technical and algorithmic solutions to be used in X-ray based industrial tomography is presented. The software has adjustable models of units under test and elements of registration channel, and generates projection data taking into account quantum noises and nonmonochromatism of the radiation. The software includes a set of approximate and local reconstruction algorithms, traditional image processing procedures and means for automatic detection and measurement of local defects. The software has been successfully used under development of a high-performance tomograph for quality check of fuel tubes. The performance of the developed tomograph are 60 articles per hour, sensitivity for pores - 0,1 mm, inaccuracy of measurements of linear size of pores - 20%. Keywords: Computed tomography (CT), industrial tomography, modeling software, panoramic view, reconstruction, measurement 1. Introduction The achievements of X-ray tomography has been widely used for flaw detection in manufactured articles. It is true for the areas where one relied upon radiographic methods of inspection. Along with technology development, X ray tomography has been in demand at the stage of series manufacturing when detailed information about physical qualities of the product is required. The same is valid for the articles securing safe functioning of equipment [1-3]. Implementation of a tomograph in a processing line creates a number of specific demands to the equipment installed. One of them is high performance. It takes full automatization of the inspection procedures, including flaw detection, measurement of the defects and pass-fail decision. Another serious demand is keeping up to safety standards for X-ray radiation. These specific problems arising under of X-ray tomography inspection technique for industrial articles can be determined and solved by mathematical modeling methods. 2. The software s structure TOMOFULL modeling software was designed as a part of a project to develop a tomograph for quality check of weld joints in the fuel tubes. The software has been developed to model different variations of technical solutions for the registration system. It requires a PC with Windows-XP OS. The software is written in C++ with use of Visual Studio 6.0. The developers used the methods of object-oriented programming, development and analysis (OOP, OOD, OOA)

2 The structure of the software is shown in Fig.1. kernel.ini Algorithm Parameters autos.spt Program Setup Instructions User s Interface tom.exe Application Framework Views.dll Templates Core Set Object Manager Mechanics tomkern.dll Core Set s Objects Algorithms Auto regimes Scaner Cameras Recon.dll Reconstruction KRecPan KRecSec Processing... Matrixf 3DMatrix f Image MechCOM MechUSB CamVSCTT CamEMUL Fig. 1. The structure of the software (the dot line marks separated files, the solid one are objects and their list headers). The software consists of the user s interface (tom.exe), core set (tomkern.dll), setup files (kernel.ini и autos.spt), and the external plug-in libraries (views.dll, mechanism.dll, camvsctt.dll, camemul.dll). Instead of the drivers, serving the X-ray detector and scanner, the software uses the modeling library as a source of projecting data for its core set. The program modules of that library are able to create and process 3-D registration scene (Fig. 2), describing mutual location of the radiation sources, units under test and the receivers. Each of the elements of the scene can be displayed in a set of primitives (the set includes 3-D figures as a sphere and a cylinder). Moreover, for each of the surfaces being modeled an attenuation coefficient and the direction in which radiation attenuates (depending on what side of the surface the absorbent is located) could be set. Calculating absorption of X-rays by a model object for each pair of points, one of which is the radiation source and the other is the receiver one figures out the parameters of the beam connecting the points. Then the coordinates of the points where the beam meets the objects of the scene are calculated. The points are sequenced as they go from the source of radiation to the detector. Thus, one divides the beam into stretches, and each of the stretches has an absorption coefficient set for it. The program processes the Fig. 2. Computer model for 3-D registration scene

3 calculations necessary to have brightness value on the detector. Having processed all the cells of the photoreceiver one obtains 2-D projection of the unit under test or a number of projections in a given range of angles. The library allows adding noises in projection data, including quantum noises that depend on radiation intensity. Taking into account that the radiation of the X-ray tube has nonmonoenergetic character one considers few sources of radiation that are located in one point of the scene. Each of the sources bears one spectral component of a general quantum stream. Distribution of power among the spectral components is done automatically the operator sets up only general power and the number of spectral components. Modeling the tomograph one used expended library of reconstruction algorithms (recon.dll) and data processing. The library has contained more than 10 different algorithms and allows reconstruction of cross-sections or 2-D developed panoramas using inverse Fourier transformation, reverse projection and algebraic methods. 3. Tomograms: panoramic view The complex inner structure of manufactured items can t always be properly displayed as a set of cross-sections. Detailed information about the structure can be obtained through 3-D reconstruction. The example of such a reconstruction one can see in Fig. 3, displaying a 3-D image of the weld joint. The method requires a full set of projection and takes maximum time to execute calculations. Moreover, the method highly depends on the operator s visual perception and his /her ability to analyze the structure of a unit under test. Automated quality control of manufactured articles may require representations based on randomly oriented and even non-planar cross-sections. Minimization of the initial information and calculations, convenience of the final information are achieved through development of reconstruction algorithms that are optimal for a particular quality check. The efficient quality check in the fuel tubes has been made possible through application of a set of panoramas, each representing a loft of a cylinder cross-section, located inside the unit under test. The equation for reconstruction in cylindrical 2 coordinates, z is, z p cos z) d, where: [ cos z] p( u, z) q( cos Fig. 3. 3D reconstruction of the weld joint. 0 ~ ( ~ p u) du are the filtered projections; z is a reconstructed cylindrical layer with radius ; p( u,, z) are the unfiltered projections; q[ cos u] is the filter core. The cylindrical layer unwinding to the 2-D rectangular image (panorama) makes it to be convenient either for visual investigation or for automated processing. Figure 4 here below visualizes the results of such panoramic reconstruction of the weld joint with porosity defect in the fuel tube

4 Fig. 4. Panoramic view of the cylinder layer and signal diagram. When one carries out a quality check of manufactured articles he should reconstruct a number of the layers. For this case processing the parameters of the defects for every image reconstructed is inefficient. That is why the tools developed to model registration process and further image processing have been used to optimize the regimes, components and algorithms of the high-performance tomograph to be used for fuel tubes quality check. 4. Results Modeling the tomograph has resulted in development of high-performance, approximate algorithm of reconstruction and processing of projection data that are optimal for a fuel tubes quality check. The processing includes deduction of regular components (projections of articles having no defects) that is followed by reconstruction of function of local differences containing only weld joint defects if any. The small size of the defects (hollow pores) and their contrast has let us skip logarithmation and decrease the time, taken by digital processing. Moreover, the reconstruction is carried out with the use of limited projections, that has resulted in decrease of resolution of defect location and the panorama restored has have information about the layers near by. The modeling has allowed defining the optimal depth of the layers and their amount. All the processing is displayed as a single image which is a result of compilation of 5 panoramic cylindrical layers. The reconstruction of the multi-layer image has decreased the number of the calculations required 5 times in comparison with cross-section reconstruction. It has affected neither the quality of pore detection nor the precision of their measurements. The time saved on repeated references to the initial projection data have resulted in using more precise algorithms for defect detection and measurement. One has studied a number of algorithms for measurement of pores. The best results have been shown when sum brightness is determined within a circle of certain radius. For that, one using thresholding methods defines a bright point that may be regarded as a defect marker in the image. Then the center of mass in the image is calculated around the point. The center is considered to be the center of the defect. Around the center, within the circle with radius R, one calculates the sum brightness of the points. Radius R should correlate with the threshold frequency of the filter, used for the reconstruction. The threshold frequency has been determined experimentally. The sum brightness value determined is converted into the size of the defect through gauge function with interpolation of values among the calibration points. The total time taking by processing projection data and pass-fail decision has not exceeded 6 seconds. The criterion of optimality for the parameters selected has been ratio of brightness in the images of the model defects and the noise value. The complex of the solutions used has 1-336

5 allowed decreasing X-ray radiation the minimum and applying algorithms of highperformance approximate parallel-beam reconstruction. 5. Conclusion The software developed has offered solutions for a number of problems in designing an automated tomograph to be used for weld joint quality check in fuel tubes: - with taking processing limits into account one has determined the requirements for registration regime and the tomograph s components; -one has developed the algorithms for reconstruction, flaw detection and measurement of defects like pores, allowing high-performance automated quality check; - one has demonstrated that the most effective method to process quality check in manufactured cylindrical articles is using panoramic reconstructive tomography; - one has developed and certified a high-performance automated industrial tomograph to carry out all operations of quality check of weld joints in fuel tubes. The technical characteristics of the tomograph developed: Performance in automatic regime 60 articles per hour Scanning time 54 seconds The number of elements in panoramic tomogram Spatial resolution limit, lines/mm 3 Sensitivity for pores, mm 0,1 Inaccuracy of measurements of linear size of pores 20% 6. Acknowledgements Authors would like to thank Dr. Anatoly K. Potashnikov for valuable contribution and helpful discussions. References 1. E.I. Vainberg, V.V. Klyuev, V.P. Kurozaev. Promyshlennaya rentgenovskaya vychislitelnaya tomografiya. Pribory dlya nerazrushaushego kontrolya materialov i izdelij: Spravochnik. M.: Mashinostroenie. 1986, Kniga 1, ss (in Russian). 2. E.I. Vasilieva, L.I. Kosarev, H.R. Kuzelev. Radiacionnaya komputernaya tomografiya v atomnoj energetike. Pod red. A.S. Shtan. M.: Energoatomizdat. 1998, 128 s. (in Russian). 3. E.I. Vainberg. Opyt primeneniya rentgenovskoi komputernoi tomografii v dvigatelestroenii. Dvigatel. 2005, No. 3 (39) (in Russian)