KOMISJA BUDOWY MASZYN PAN ODDZIAŁ W POZNANIU Vol. 27 nr 2 Archiwum Technologii Maszyn i Automatyzacji 2007 GRZEGORZ BUDZIK * THE SELECTION CRITERIA OF SCANNING METHOD IN PROCESS OF REPRODUCING AN AIRCRAFT ENGINE BLADE GEOMETRY This paper presents possibilities of reproducing a geometric model of an aircraft engine blade using particular non-contact scanning methods in process of Reverse Engineering (RE). RE methods enable to obtain geometric data of existing element or assembly, as well as, geometric verification of manufactured elements. RE methods which enable to reconstruct a three dimensional element image among the others are composed: laser scanning and Computed Tomography. A laser scanning enables to reconstruct an external geometry of examined element. The Computed Tomography method enables to obtain a precision reconstruction of external and internal geometric dimensions. Nowadays, this is the only non-destructive method which enables to reconstruct internal and external geometry of an element with accurate precision. As a result of measurements it is possible to obtain a digital area of a solid as triangulated STL file or a point cloud. Key words: Reverse Engineering, 3D-scannig, Computed Tomography 1. INTRODUCTION Non-contact methods used in a process of Reverse Engineering can be divided in four groups: non-contact scanning, scanning with laser beam, photogrammetry, Computed Tomography. A non-contact scanning is performed with portable or stationary spatial scanners. In case of stationary closed scanners the operation area of a scanner limits dimensions of examined element. Portable scanners allow to scan objects with theoretically unlimited dimensions. A major advantage of Computed Tomography is a possibility of obtaining external and internal geometry of examined object in non-destructive way [1, 2]. A processing of data acquisited from computer tomo- * Dr inż. Chair of Machine Design, Rzeszow University of Technology.
64 G. Budzik graph using a specialist software enables to receive files for standards typical for CAD/CAM/CAE systems [3, 4, 5]. 2. COMPUTED TOMOGRAPH SCANNING The object of examination was an aircraft engine blade manufactured within WSK PZL Rzeszów (Fig. 1). For measurements, a computer tomograph was used. The preparation process of data using computer tomograph is complicated because appropriate working parameters of scanning machine needs to be set up to obtain accuracy required for aircraft engines elements [6, 7]. Fig. 1. View of examined blade Rys. 1. Widok badanej łopatki A model digitalizing in computed tomography is received by transitional imaging of examined object. 3-dimensional virtual image rises not as a result of measurements, but as a mathematical calculation. The element is being scanned slice by slice with required precision. CT images of particular slices have a half-tone image form. The main problem of 3D reconstruction is a correct definition of a border between the examined object and the environment. The geometry of the object (especially inner surface) is not always known, therefore unambiguous fixation of the threshold value of a grey shade scale for particular elements is a difficult matter. Accurate fixation of the threshold values influences significantly on segmentation
The selection criteria of scanning method... 65 process (finding a border between the object and the environment). This is a setting point for binarising the half-tone image (Fig. 2) [8]. Contour pixels isolated from the image determine inner and outer surface borders of examined object. Fig. 2. The 2D contour extraction, the DICOM data processing Rys. 2. Kontur 2D i jego obróbka w programie DICOM Data processing is executed in a process of singular 2D images (sections) preselection. Setting up the image coordinates is necessary for data set establishment. A standard type of data saving in tomograph used for examination is DICOM format. The object geometry reconstruction is a result of converting a half-tone image collection, where after this, comes the correction of mistakes and artifacts, image filtration, object classification, and finally a segmentation [8]. The maximum deviation of dimension obtained with Siemens Sensation 10 CT scanner is not more than: ε max = ±0.8 mm. After the calibration, the maximum deviation has reached value: ε max = ±0.26 mm [2, 6]. 2D contour section formed as a segmentation result (Fig. 3) represents a threedimensional representation of examined object. A contour model may be processed in CAD system (e.g. Mechanical Desktop). As a result of data processing it
66 G. Budzik is possible to obtain a CAD model, as well as, possibility to save it in a file and export to a readable format for other CAD systems and Rapid Prototyping. Fig. 3. Imaging of the analyzed part: a), b) set of contours for the whole 3D Region, c) the 3D model rendered with Mechanical Desktop program Rys. 3. Widok analizowanego elementu: a), b) widok konturowy, c) wyrenderowany model w programie Mechanical Desktop 3. NON-CONTACT SCANNING USING ZSCANNER 700 SCANNER Non-contact scanning can be executed with hand scanner. Such type of device is a hand scanner type ZScanner 700 manufactured by Z-Corporation, which allows to convert geometric objects into three-dimensional digital form (digitalizing). Examination performed by a scanner and software which belongs to CAR Technology Sp. z o.o. company, located in Kraków. The scanner has following technical parameters: weight: 980 grams, dimensions: 160 260 210 mm, measurement: 18 000 measurements/sec scanner class II (eyes safe), precision: up to 0.05 mm, resolution in z axis: 0.1 mm, type of data export: STL, RAW. The scanner is compatible with PC or portable computer. It works in Plug and Play system by connecting to the FireWire port. A view of the scanner is presented in Figure 4.
The selection criteria of scanning method... 67 Fig. 4. ZScanner 700 Rys. 4. ZSanner 700 The scanner enables a scanning of non-transparent objects with mat surface. Before the scanning it is necessary to stick white control markers at 5 mm dimension. In case of small overall dimension objects, a scanning may be executed on a special mat with markers that help scanner to orientate in 3D space (Fig. 5). Fig. 5. Process of a blade scanning Rys. 5. Łopatka w trakcie skanowania
68 G. Budzik Scanning precision in ZScan programme can be changed by virtual working space adjustment (a space with white markers) to the scanning space (virtual cube) (Fig. 6). During the scanning a scanner application presents a digital representation of the object. Fig. 6. Dimension space (cube) with markers and scanned blade Rys. 6. Obszar pomiarowy (sześcian) z markerami i obrazem skanowanej łopatki ZScan application has a possibility to save a virtual model with *.csf extension (ZScan programme format), *.stl (a format of the most Rapid Prototyping systems), *.txt (a format of a point cloud). These formats are readable for most of the CAD systems, which allows a further file processing in order to obtain a correct CAD model. CATIA V5R15 system is equipped with many tools for processing files saved as a point cloud (Fig. 7). As a processing result a virtual model is extracted. The model can be saved in formats which are suitable for CAD systems (IGES, STEP, CATpart, etc.). The deviation of dimension obtained with ZScanner 700 depends on the shape of scanned object. The maximum deviation for object like blade and impeller has reached value: ε max = ±0.32 mm.
The selection criteria of scanning method... 69 Fig. 7. A point cloud of a blade in CATIA programme Rys. 7. Widok łopatki w postaci chmury punktów w programie CATIA 4. SUMMARY Non-contact scanning enables to create a virtual model of a real element, which is an aircraft engine blade. Digitalizing process in case of computed tomography method and non-contact scanning, proceeds in different ways, but it allows to obtain a similar effects. The essential advantage of CT scanning is a possibility of representation the outer, as well as inner shape of examined object. Because of this, it is possible to examine objects with closed profiles or particularly complicated shapes. It has a special meaning for aircraft engine blades which can have inner cooling canals. CT scanning and data processing is a complicated and time consuming process, which requires using a special software. Scanning with a hand scanner enables to obtain a simple and fast virtual model creation, as a form of its external surfaces. ZScan software enables the scanned object optimization and some noise removal. Both scanning methods require using a specialist software to create a CAD model of examined blade. Point cloud or STL files processing is possible directly in CAD systems (e.g. CATIA, Mechanical Desktop) or with help of a specialist software compatible with reversal engineering or Rapid Prototyping formats (MagicLite, GeoMagic). A scanning method ought to be selected depending on a shape of examined object, also, digitalizing process precision, scanning time, the service cost, and format of received data in aspect of its further processing, should be taken into consideration.
70 G. Budzik REFERENCES [1] Budzik G., Miechowicz S., Marciniec A., Reverse Engineering Using for Reproduce Geometry of Rotor of Turbocharger, Journal of KONES Powertrain and Transport, 2006, vol. 13, no. 4. [2] Cygnar M., Miechowicz S., Budzik G., The Influence of CT Scanning Parameters on The Inaccuracy of The Computer Tomography Imaging of The Turbine Blade For Reverse Engineering, in: International Conference on Machining and Measurement of Sculptured Surface, MMSS 2006, Institute of Advanced Manufacturing Technology, Kraków 2006. [3] Gawlik J., Karbowski, K., Inżynieria odwrotna (Reverse Engineering) w wytwarzaniu wyrobów o złożonym kształcie, in: Nowoczesne techniki inżynierskie w szybkim rozwoju wyrobów. Materiały konferencyjne, Poznań 2004. [4] Faridani A., Introduction to the Mathematics of Computed Tomography, MSRI Publications, 2003, 47. [5] Liang, S., Lin, A., Probe Radius Compensation for 3D Data Points in Reverse Engineering, Computer in Industry, 2002, 48. [6] Miechowicz S., Sobolak, M., The Efficiency of Utilization of Siemens Sensation 10 CT Scanner for Reverse Engineering, Prace Naukowe Instytutu Technologii Maszyn i Automatyzacji Politechniki Wrocławskiej, 2004, nr 85. [7] Motavalli S., Reviev of Reverse Engineering Approaches, International Journal of Machine Tools & Manufacture, 1998, vol. 35, no. 1 2. [8] Tai C., Huang, M., The processing of data points basing on design intent in Reverse Engineering, Internationele Journal of Machine Tools & Manufacture, 2000, 40. Praca wpłynęła do Redakcji 16.03.2007 Recenzent: dr hab. inż. Jarosław Plichta KRYTERIA DOBORU METODY SKANOWANIA W PROCESIE ODTWARZANIA GEOMETRII ŁOPATKI SILNIKA LOTNICZEGO Streszczenie W artykule przedstawiono możliwości odtworzenia modelu geometrycznego łopatki silnika lotniczego za pomocą wybranych bezdotykowych metod skanowania w procesie inżynierii odwrotnej (ang. Reverse Engineering RE). Metody RE pozwalają na uzyskiwanie danych geometrycznych istniejącego elementu lub zespołu, umożliwiają również weryfikację geometryczną wytwarzanych elementów. Do bezstykowych metod RE pozwalających na odtworzenie trójwymiarowego obrazu elementu należą m.in. skanowanie laserowe i tomografia komputerowa. Skanowanie laserowe pozwala odtworzyć zewnętrzną geometrię badanego elementu. Metoda tomografii komputerowej pozwala na dokładne odtworzenie zewnętrznych i wewnętrznych wymiarów geometrycznych. Jest to obecnie jedyna nieniszcząca metoda pozwalająca na odtworzenie wewnętrznej geometrii elementu z odpowiednią dokładnością. W wyniku pomiarów możliwe jest otrzymanie obszaru cyfrowego bryły w postaci pliku STL lub chmury punktów. Słowa kluczowe: inżynieria odwrotna, skanowanie 3D, tomografia komputerowa