·Table of Contents
·Aeronautics and Aerospace
The PC-Based videodefectoscopic system DX 3 SCH for the inspection of honeycomb panels in aircraft turbines
A A Ketkovich, V N Filinov, M V Filinov, V Maklashevsky
The honeycomb seals (HS) are the heavy duty elements of modern gas turbine engines (GTE). Usually HS is produced from the corrugated metal ribbons as honeycombed structures, by subsequential fastening by means of soldering on an inner side of GTE stator. Soldering defects (local absence of welding alloy, violation of the thermal mode, etc) result in HS disbonding and separation, and this, in its turn, has the fatal influence on GTE operation. Regarding to this, the inspection of HS is an actual task.
At the present moment, acoustic and visual methids are the basic ones for HS inspection. The perspective photothermal method is not applied widely due to complication of control procedures. In some cases (inaccessibility HS for ultrasonic inspection because of rising tides and similar elements on the rotor, small size of honeycombs, etc) visual method (VM) is widely used. The VM is realized in two ways - honeycombs inspection in reflected light, and observation of the effect of light "infiltration" from honeycombs illuminated locally by optical lightguide, into adjacent cells, that takes place in case of absence of welding alloy.
By the inspection in reflected light, HS with defectous soldering form the characteristic image with an obvious point in center of each cell. Absence of welding alloy and other defects result in leaps in the structure of light dazzles from a cell (general brightness fades, the spot in center disappears, etc). However by the existing methods (visual inspection) the foregone modifications of observation conditions (geometry, brightness) result in operator fatigue, missing of defects, especially in HS with small cells.
The PC-based television defectoscopic system DX 3 SCH for the inspection of soldering quality of HS, was developed in MSIA "SPECTRUM" to provide the persistence of geometrical and optical defectoscopy processes. The block diagram of the system is shown no Fig.1. The system includes the optoelectronic accembly, consisting of CCD camera 4 and two lightsources 3 and 5, intended for wide-angle and local illumination of honeycombs, accordingly. Video digitizer 7 transmits video image into the PC 8. The operator 9 observes the honeycomb images on the display 8. If needed, the frame could be frozen and printed on the printer 10. The system also includes mains adaptors (not shown on the Fig.1 ), and the object positioning device.
Fig 1: Block diagram of the system for the inspection of quality of honeycomb seals soldering DX 3 SCH. 1- object; 2 - object positioning device; 3 - wide angle lightsource; 4 - CCD camera; 5 - laser lightsource for local lighting; 6 - image digitizer; 7 - PC; 8 - diaplay; 9 - operator; 10 - printer.|
Optical diagram of the optoelectronic assembly is given on the Fig.2. Irradiation of a light-emitting diode 1 with the narrow directional diagram ("<100) is collimated additionally by the lens 2 up to the value of beam divergence about 10- 20, and through the beam splitter (translucent mirror) 3 it is being directed to the object under inspection - the honeycomb 4 at the right angle to its surface. The deflection of beam axis from the normal to HS surface within "10 is provided by the angle adjusting of beam splitter 3.
Fig 2: Optic diagram of the electronic module of DX 3 SCH system. 1 - light diode; 2 - collimator; 3 - beam splitter; 4 - object; 5 - light filter; 6 - objective; 7 - CCD; |
The observation of honeycomb cells is carried out using the method of light field; by which light dazzle from cells gives the characteristic reflection, schematically given on Fig.3. By quality soldering, smooth surface of welding alloy fillet works as small spheric mirror and concentrates light along the axis of the cell. In this case the cell could be modelled with prismatic hexagonal lightguide, which symmetrizes beams reflected from HS cell bottom.
Fig 3: The diagram of forming of light dazzles from different segments of honeycombs for quality soldering (a), violation of the thermal mode (poor spreading of the soldering alloy)(b), and for the case of complete or partial absence of soldering alloy (c)|
By bad soldering fillets are absent, and rough surface of welding alloy works as a quasidiffuse (Lambert) reflector giving comparatively full picture of light dazzles of rather random structure.
By total absence (missing) of welding alloy, in the reflected light the cell bottom looks like bright hexagonal dazzle repeating cell shape. Corresponding drawings of cells and its brightness profiles are also shown on Fig.3.
By poor adjoining of HS to the base due to bad soldering, the effect of light percolating (tunnelling) from the lightguide inserted into the cell to adjacent cells through small gaps between HS and base, works as additional criteria of a defect (Fig.4). Usually this, laborious enough, operation is made only in doubtful segments of HS, detected with TV method before.
Fig 4: Method of forming of the inspection image.|
In DX 3 SCH system a method of spectral selection is applied to exclude the influence of outer lightsources. For this, light filter 5 with passband coinciding spectral irradiation characteristic of a light-emitting diode 1, is installed before objective 6 of TV camera 7.
Besides the illumination of the object with collimated beam of light, the system also illuminates the object with low divergence beams from optical lightguides 8, located symmetrically in a ring zone around the lens 6 to provide the uniform illuminance of the object.
The aperture of ray beam at the tips of lightguides is about 10E. This allows to observe HS with small-size cells (#1 mm) disposed on cylindrical surfaces of major curvature (R#l00 mm), because the illumination with parallel beam in this case is precluded by shading (shielding) of honeycombs disposed outside of optic axis of illumination, as well as by disturbance of orthogonality of a falling of beams on the bottom of honeycombs.
The input tips of lightguides 8 are lighted by the halogen bulb KGM 10 (50 Wt, 12 V) with reflective ellipsoid 11 through heat filter 9. The selectivity of the radiation at the output of lightguides is provided by filtering with light filter.
The local lighting of honeycombs is made by the irradiation of laser 14 (8 = 0,63 mm, W = 2 mWt) through polymer optic lightguide 12 dia.0,6 mm. The laser irradiation is transmitted to the lightguide by means of focussing gradan 13.
The monitoring of input of the optical waveguide in the cell and observation of light infiltration is made by means of TV-camera 7. To increase the sensitivity and noise protection the laser modulation with frequency of 5¸10 Hz is stipulated by means of obturator 15 and the motor of DPM type 16.
The setup of video system is made using test objects with several types of defects, agreed with customer.
Thr methodics of operation consists of HS consecutive inspection (for example, by the rotation of the object with low linear velocity of approx 1 mm/s), saving coordinates, sizes and types of defects, and comparing these data with acceptable values.
At the present moment, the system operation in the interactive mode is intended. The operator sets up the optimal modes of object lighting, inspects regularity of its installation in the reposition mechanism, controls the image quality by analog methods (objective irising and image scaling by means of adjusting the objective focal distance) as well as using digital ones (histogram equalisation, sharpening, applying S-shape brightness transfer function, etc.)
If needed, the image could be either printed, or stored in image databse.
As far as statistic data of defect images is being accumulated, by combining the results of TV-inspection with the results of metallographic testing, the system could be upgraded to the expert one operating in defect recognition mode. Thus, the system will reject the items measuring thr defects and using existing identification criteria.
On Fig.5, images of poorly (a) and good- (b) soldered HS are shown. As it is seen from Fig.5, DX 3 SCH system allows to distinguish defect areas reliably.
Fig 5: HS soldering quality: poor (a), and good (b)|
The prospective of system development is the automation of poor quality soldering rejection, using either live image, or video recording.
|General technical data of DX 3 SCH system
|Field of view, mm
||20´20 and 10´10
|TV-block size (W´H´D), mm
|Inspection velocity, mm/s
|Honeycomb cells size (diameter entered in hexahedral cell), mm
|Honeycomb height, mm
|HS rings diameter, mm
|Honeycomb width, mm
|Lightguide wavelength, mm
|Helium-neone laser wavelength, mm
|CCD camera technical data:
|Chip size (diagonal), inch
|CCD matrix format, pixels
|Light sensitivity, lux
|Spectral range, mm
|Image digitizer technical data:
|Digital frame size, pixels
|Color depth, bits
|One frame grabbing duration, ms
|Compression factor by digitizing the dynamic image (min)