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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.
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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: εmax = ±0.8 mm. After the calibration, the maximum
deviation has reached value: εmax = ±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
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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
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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: εmax = ±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.
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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