A the present moment ensuring of machines and aggregates operation not on its resource, but on its actual condition is of great importance. It is especially important at operation of energetic plants, on chemical enterprises, transport, in a gas and oil extracting and producing complex. The operation on an actual condition is possible by complex monitoring of objects. The optical and visual inspection and its contemporary branch, optical-television inspection, is significant integral part of complex monitoring, that gives essential part of the information on a current status of object. The value of the information about object obtained with optical-television monitoring systems is essentially increased when gaging functions are added to it.
In the given paper, the PC-based measuring optical-television defectoscopic system DX 3 DCM (Deep explorer 3, Double Channel, Measuring), is described. The system is intended for the inspection of pipes, high pressure vessels, and other heavy duty long size items.
Fig 1: Block diagram of PC-based optic television measuring defectoscopic system
DX 3 DCM
The block diagram of DX 3 DCM is shown on Fig.1. The system consists of TV camera head (TVH) with lightsources, connection cable (length up to 100 m), power and control unit (PCU), PC and a printer. In future, it is supposed to equip DX 3 DCM with UV light sources for the detection of lumenescent soilings and/or defects detected by means of fluorescent penetrant of magnetic particle inspection.
Fig 2: Optical diagram of TVH (See details in text.)|
The optical diagram of TVH is given on Fig.2. TVH includes panoramic view TV camera with wide angle objective 1, frontal light sources 3, lateral view TV camera 2 with objective 4, slot light sources 5 located at the right angle to the symmetry plain of TV camera 3, and light sources 6.
The design of the TVH is made on a modular approach, which allows quick adjust of the system for different defectoscopic tasks. Basic model of TVH is intended for the operation in normal conditions. TVH for heavy duty operation (gaseous environment, water vapour, fire hazard, etc.) available on request.
LED light sources 3 with adjustable brightness are intended for object illumination by panoramic view. The special feature of lateral view subsystem is that it contains two mutually orthogonal slot light sources. These sources form light lines, projected at 45E on the internal surface of the object for measuring of defects, oriented in a different way, using the light section method. General object illumination by the operation of lateral view camera is carried out by light sources 6.
By means of line light sources and lateral view camera the operator can measure basic dimensions of a defect (width, length and depth), calculate dependent values (square, form-factor) and, as well, distance to the defect by means of triangulation method. In particular, this allows to increase the exactitude of measuring of defects dimensions due to operating record-keeping of the current image scale.
TVH is moved inside the object under inspection by means of a transport (telescopic bar, or wheel vehicle). The transport is equipped with coordinate unit, which allows to record geometric location of the defect.
Coordinate unit transmits directly to PC, or indicates on its own display current horizontal coordinate, and, on request, the angle of TVH rotation, for its subsequent documenting.
The image digitizer processes Y/C PAL or CCIR TV signal.
In case of colour TV cameras, small-size halogen lamps (colour temperature 3200 K) are used instead of LED.
For calibration and metrologic certification of DX 3 DCM linear scales, tip length measures, photometric step test, set of neutral filters, digital luxmeter and projector of test patterns.
To estimate the defect dimension, the operator marks with cursor points x1, x2, y1, y2 coressponding to maximum size of the object on its length and width. By this, coordinates of these points are displayed on the screen, as well as differences of these coordinates. True size of a defect is determined by multiplication of number of pixels corresponding to object size, on the current conversion factor m, preset for particular object by the calibration of the system. Finally, defect dimensions H and B are determined from equations:
H = Dx × m, B = D y × m
The calibration of DX 3 DCM system is carried out with measuring standards certified by Rostest. The measuring standard represents flexible metal rulers acording to GOST 3570-86. The unit value c = 1 mm. The ruler is applied to the internal surface of a tube of given diameter on its generatrix, and after on a circle.
The operator observes the measuring standard image on a display. Relocating cursor, the operator determines the number of pixels n corresponding to N number of units on the scale. Then, the conversion factor is calculated:
It is essential that the conversion factor m = const in radial direction only, i.e. in cross-section of a pipe perpendicular to the optic axis of TV camera objective and coinciding to the plane normal to this axis in the point of its interception with centre of the mirror located before TV camera at the angle of 45E to its axis (see Fig.3).
| Fig 3: a - general principle of scale positioning; b - scale is located on pipe generatrix; c - scale is located on the circle|
At the same time, in adjacent sections located along the pipe the scale is sharply changing by removing from cross-section B-B mentioned above, due to the alteration of the distance between the objective and defect. By this, the defect image keeps its sharpness within the limits of sharpness depth. This should be taken into account by measuring of length of extended defects corresponding to view field angles " ³ 10E.
To eliminate the error of measuring of extended defects' length, the special methodics is proposed. The methodics is based on moving of the TV camera along the tube axis by means of the transport such as a toothed or telescopic bar with longitudinal scale disposed in parallel with camera optic axis, or with coordinate device based of a circuit breaker.
The operator sequentially mates the images of extremities of defect with center of a screen and reads corresponding readings on the bar scale, or on the display of the coordinate counter. The defect size is estimated directly in units of scale or coordinate display. By the given methodics the system is invariant to the scale of optical image, so:
The given methodics is less efficient as contrasted to above-stated, however it has significant advantages in metrology.
The methodics can be distributed also on gauging of the size of located on a circle of a tube, i.e. gauging of its angular size by means of turn of a bar, on which the camera is installed, by adjustment of images of defect edges with center of a screen and reading the corresponding readouts g1and g2 from angle gauge display. By this, Dg=g1-g2, and defect linear size equals to H = Dg ×CE, where CE (mm / grade) - scale interval of angle measuring scale on the bar for a particular diameter of a tube. The given methodics excludes the influence of objective distortion errors to measuring error, however it requires individual calibration of the system for any particular diameter.
For measuring of defects depth, the light section method is used in DX 3 DCM system.
In case the defect size is determined on the monitor screen, for example, height (depth) Hscr, this dimension is related with actual corresponding size of defect with the ratio:
where % is the scale of image conversion in object space, V - image scale of the television observation system which equals to:
where V0 is the scale of the optic system, Vtv is the scale of video channel which equals to the ratio of corresponding raster values on the display and CCD chip. Profile conversion factor equals to:
i.e. to the ratio of size of the observed defect profile in the object space to its size in the normal cross-section. From the analysis it is seen that for polished surfaces B = 2 × sinb, it doesn't depend on the projection angle a, and grows with growing of the observation angle, that leads to increase of method sensitivity. However it is desirable that a » b to eliminate losses of image brightness due to incomplete use of input aperture of receiving objective a<<b. for dissipating surfaces, and only for a = b the equation is correct. Thus, at equality of projection and observation angles the profile conversion scale doesn't depend on a surface reflection factor. Also, it is essential that in case (a+b) = 90E the observation profile also lies in the plane parallel to the subject plane. Because of this it is useful to establish a = b = 45E provided there are no design limitations.
The profile conversion scale error is characterized by the value of the difference Dh = ht - hn where ht is an actual height of the observed defect profile, hn is the profile height for nominal (calculated) angles an, bn and gH = aH + bH. By targeting of TVH onto the object surface the deviations of these angles for values of Da, Db and Dg correspondingly, are possible. After the conversion we get the equation for relative error:
The analysis displayed that by Da » Db » Dg » 3E relative error does not exceed 3%.
The given conclusion is correct, provided there is no image defocussing due to modification of the internal diameter of the object, no alteration of distance between TVH and object internal surface, etc. This causes additional error related to smoothening of the image of measuring light line, and, correspondingly, with inaccuracy of targeting the cursor on its border when measuring on halftone image.
The depth of sharpness depends on the image scale, diameter of objective inlet opening, and on an angular value of permissible diffusion spot, expressed in a radian measure, in the plane of CCD chip, which equals to e £2a/t, where a is the diagonal of CCD pixel, t is the focal distance of camera objective.
Procedure and methodics of calibration of DX 3 DCM system
Calibration and metrological certification of DX 3 DCM system for measuring of plain defect dimensions on the item surface is carried out using measurement standards.
Procedures of calibration and certification of DX 3 DCM system for measuring of defects depth (height) using the light section method were carried out according to especially developed methodics. The methodics is based on usage of measurement standards of step type built from Ioganson plates, i.e. from the set of tip length measures.
Calibration measures of different length formed steps of height D from 0,05 to 10 mm (Fig.4.b).
|Fig 4: Calibration test bodies for DX 3 DCM system: a - slot type, b - step type, c - gauging diagram.|
The measures were reseated to the basis (gauged flat massive plate made from quartz). The characteristic sizes of measures A = 10 ¸40 mms; B = 10 mm.
The reference object was disposed in front of THV so that the profile of light section is placed on center of steps, and it would be orthogonal to the plane (Fig.5.).
Fig 5: The work window of GaugePro module, with loaded image.|
The inaccuracy of steps value was determined by inaccuracy of linear measures (0,001mm), i.e. this corresponds to the relative error of 1 % for the minimum size A=0,1 mm.
The calibration of DX 3 DCM system consists in cursor targeting on upper and lower borders of light section accordingly, and gauging of its coordinates in pixels of a display field, i.e.Dx = x1 - x2 (fig.6.). Then the conversion scale to actual dimension from pixel coordinates M = DH/Dx, mm / pixel, is determined.
The scale M is determined as average value of a number of gaugings:
where is n - number of gaugings, M - i-th value M.
The calibration was made for both channels of a system, by this the test body turned on 90° in a plane of the basis.
To research the influence of cross sectional dimensions (width) of volumetric defect to the measuring error of its depth (height) we offered the special "vertical" pattern based, as in the first case, on tip length standards. However, the standards are not placed closely to each other, but with fixed clearance which is defined by thickness Dt of a gasket between. (Fig.7).
Varying width of the gasket Dt, we estimate capability of system research metrology performances in two-parameter space (DH and Dt), i.e. functionDH =f (Dt).
The width of gaskets was changed from 0,1 up to 5 mm.
The researches was made for characteristic parameters of items defects, for diagnostic of which the system DX 3 was intended, i.e.Dt» 0,1¸
0,3 mm, DH» 0,5¸1,0 mm, by TV-camera optical objective magnification of b0» 0,1x(that corresponds to standard distance between objective and surface l» 100 mm and representative value of focal distance of a lens f' = 10 mm).
As established, for these conditions the defect width Dt³1 mm practically has no influence on a measuring error of its depth (height). By this, the relative error of defect profile elements gauging does not exceed . At decreasing of defect opening (up to 0,2¸0,3 mm) the error increases up to value DH»1,0 %.
The material of gaskets was taking into account maximum approximation of optical and performances of its surface (reflection factor, dispersion index, chromaticity etc.) to the applicable performances of a material of substantial object to fulfill the principle of a similarity of physical performances of object and measurement standard, adopted in a metrology. For metallic machine-building items being inspected by DX 3 DCM system, the steel polished plates with a surface roughness Ra = 0,5 microns appeared to be most suitable. For dielectric items, the gaskets made of organic glass with a different roughness of a working surface were used.
The measurement standards described above are recommended for metrology certification of systems such as DX 3 on its operation; the standards successfully have passed approbation under production conditions and are included in a package of metrology maintenance means for DX 3 DCM.
The software and process of monitoring on DX 3 DCM.
The software of DX 3 DCM works under Windows ' 98 and consists of three parts: FrameCapturer - the control module for image digitizer setup and control, and image capturing; FrameEnhancer - the module for static image processing and GaugePro - the measuring module. In all modules the number "controls" - screen push buttons and sliders - is minimized, each of them is supplied with the hint, so even unexperienced stuff could also work with the software.
The operation in the environment of Windows ' 98 allows to have all three modules loaded simultaneously without deboosting of system operation, and the modules could be switched easily by means of taskbar.
Before starting the system should be calibrated. In the sharpness zone of the gaging camera (TVH is still outside of the object) the test-object of known dimensions, or with the marked reference points with known spacing interval between them, is placed. The image of the test-object is captured and according to the procedure described below, the spacing interval between reference points in pixels is gauged. Then system requests for the actual spacing interval between points in mm, and thus the scaling ratio, used in further gaugings, is calculated.
During the inspection the operator observes the dynamic video image from TVH moving inside the object, in a window of FrameCapturer, having switched to the direct view camera. When the defect is detected on the wall of internal cavity of the object, the operator stops the vehicle and, having switched to gaging camera, brings TVH to defect so that the defect would be in the crosshair of gaging beams. The image is captured transmitted into Windows clipboard.
Then, if needed, the operator could switch onto FrameEnhancer and load the image from the clipboard for processing. Using the FrameEnhancer, the operator could digitally adjust brightness, contrast and sharpness of the image, remove noise and emphasize borders of the defect, which essentially eases gauging. After finishing processing with FrameEnhancer, the image is put into the clipboard again and/or could be saved.
Then, the user switches onto the module GaugePRO and loads the image from clipboard into the working window. The exterior of a screen at this stage is given on Fig.5.
Using the mouse, the operator marks proper points on the defect image, between which the distance should be calculated. At bracing the second point the size is placed on record, which one can be printed out after any quantity of gaugings or upon termination of monitoring object.
Optic-television gaging defectoscopic system DX 3 was successfully istalled on several Russian enterprises for gas and oil extraction equipment, and also in power machine construction industry. The practice has shown, that the system could be applied in combination with other methods of inspection, for example, with fluorescent penetrant testing and fluorescent magnetic particle testing. For this purpose in next generation of DX 3 systems it is planned to equip the TVH with zoom objectives and servo-system for adjusting of gaging rays for the inspection on objects of a variable internal diameter.
|TVH dimensions (dia.Hlength), mm
|TV camera view field angles:
||panoramic 180E, lateral 70E
|CCD chips, pixels
|Spectral range of TV camera,Fm
||b/w: 0,4)1,2; color: 0,38 ) 0,78
|Dynamic range of TV camera, dB
|Minimal illumination intensity on the object, lux
|Object dimensions measuring error
||on surface 1%, in depth 3%
||PAL composite and Y/C, 1 Vpp
|Digital video image size, bit
|Supported video formats
||TIFF (LZW, nLZW), PCX, JPEG(by compression factor 3¸ 5)
|PC Type / OS
||Pentium III 500 MHz / Win98
|Table : General technical data of basic set of DX 3 DCM|