Experimental study on the effect of measurement system characteristics on pressure-sensitive coating calibration

The pressure measurement technology of optical pressure-sensitive coating (Pressure sensitivePaint, PSP) is a new method of surface pressure measurement in wind tunnel test, which has the characteristics of no insertion measurement, small ts flow field interference and high spatial resolution. Since the 1980s, PSP measurement technology has been developing and perfecting, and has played an increasingly important role in the research of fluid dynamics, aerodynamics and aerospace. The paper describes NASA's use of optical pressure-sensitive measurement technology to measure the surface pressure of the leaf gate wind tunnel and the blades of the turbine, and the german application of this technology to the measurement of the surface pressure of the aircraft model. At present, Europe, the United States, Japan and other countries have been widely used in optical pressure measurement technology in a variety of pneumatic experimental measurement. Since 2000, the relevant research units in China have also started the research work of this measurement technology. The China Aerospace Industry Aerodynamics Research Institute has completed the validation test of PSP technology on model models in high-speed wind tunnels, and Northwestern University of Technology has completed the measurement of impermancing mechanical still leaf blades. The pressure calibration experiment is one of the important links in PSP pressure measurement technology, and the resulting calibration curve is the key factor to determine the accuracy of the measurement results. Based on the self-established optical pressure measurement calibration system and domestic pressure-sensitive coatings, this paper studies the effect of camera aperture and excitation light intensity on calibration experiments through a large number of calibration experiments and image data processing, analyzes the factors affecting the characteristics of pressure-sensitive coatings, and proposes corresponding solutions and measures to provide reference for further research on optical pressure measurement technology.
1 Measurement System
's self-built pressure-sensitive measurement calibration system consists of an excitation light source, a pressure calibration system, an image acquisition system, and image post-processing software, as shown in Figure 1.figure
Figure 1 pressure-sensitive coating measurement system schematic
excitation light source system is the core part of the measurement system, the emission of a certain amount of ultraviolet light is used to excite the light-sensitive molecules in the coating to produce energy-level jump, thus emitting a different fluorescence than the excitation wavelength of light. The excitation light source consists of an ultraviolet light source and a filter. The ultraviolet light source used has 200W and 400W strong and weak two-speed power, the wavelength range of ultraviolet light emitted is 200 to 750nm. The coating used is a pressure-sensitive coating developed by the Chinese Academy of Sciences, which has an optimum excitation wavelength of 320 to 340nm, so a filter is added after the light source. The pressure calibration system is used to provide any constant pressure within 10kPa to 4.5 MPa. Its basic form is a sealed pressure chamber, the front lysophore is used to show the pressure by exciting light and fluorescence, the back has two interfaces, one for an air compressor or vacuum pump, and the other for a pressure meter or vacuum meter. The calibration sheet is a square metal block, first coated with white epoxy primer, and then with a spray gun with a thickness of 20 to 40 m of pressure-sensitive paint, placed in the center of the pressure calibration chamber. The coating used belongs to classpolymer organic matter, in the range of 0 to 60 degrees C photosensitive properties are stable, the temperature of this experiment in this range, so the design of the calibration module does not take into account the temperature impact.
image acquisition system is used to capture fluorescent images emitted by ultraviolet light at certain pressures. The image collector used in the experiment is a 10-bit gray-scale science-grade CCD camera with a resolution of 1600 pixels×1200 pixels, which can be automatically collected by its own software and save the pictures on the computer. The picture is TIFF format, which has the advantages of extensity, convenience, modifiability and so on, and is convenient for post-processing work. However, the setting of camera parameters such as camera lens back plate and aperture will have some effect on the brightness and shooting angle of the captured fluorescent image. The captured images are processed and analyzed using Afix2, a professional PSP image processing software. The software is powerful, can make noise culling and smooth processing of a single image, can also set the display range of the image and the display of pseudo-color, extract the grayscale value of any pixel in the figure, and the average grayscale value of multiple pixels around the pixel point, perform basic operations between images, modify the grayscale value, and also complete the restoration of image deformation caused by changes in camera angle.
2 Experimental principle and experimental process
the exposure of the excitable light to the pressure-sensitive coating will emit different intensity fluorescence according to the ambient pressure, when the ambient pressure becomes greater, the oxygen annihilation effect of the coating increases, so that the luminous intensity of the coating weakens, so that a certain pressure corresponds to a certain fluorescent intensity image. In order to establish a more accurate relationship between pressure and fluorescence intensity, the measurement system and the environment will have an effect on the captured image, and the relative values of pressure and light intensity are usually used to represent the relationship between the two, i.e. the
calibration experiment is shown in Figure 2. The arrangement of the measuring system should take into account the effects of light angle, light uniformity, image acquisition, shooting angle, reflective light, and ambient light. In order to obtain a better image quality, the light emitted by the light source should be directed vertically to the surface of the calibration film, and the camera lens plane should be parallel to the surface of the experimental part to minimize imaging deformation. However, due to the limitations of the camera and light source size, it is not possible to fully meet the above requirements, but should be as close as possible, at the same time, the camera should be placed to avoid the calibration module lysoglass reflection of excitation light, in order to ensure better experimental results, the experimental environment as far as possible to maintain a black environment to avoid the results of the experiment light Pollution; the experiment first turns on the excitation light source, waits half an hour for the excitation light source to stabilize before conducting the experiment, and because the excitation light source generates large heat, in order to avoid the effect of rising temperature on the laser light-emitting mass, so the experiment uses a blower to cool the excitation light source.The range of
pressure calibration should include all the pressures measured in the actual experiment, so a large pressure range calibration experiment to obtain a pressure-sensitive coating calibration curve with a full pressure range ensures that all pressure changes in the actual experiment are included, the accuracy of the optical pressure measurement is guaranteed, and the pressure suitability and pressure sensitivity of the pressure-sensitive coating used in the experiment can be detected. In the calibration experiment, the pressure range is 27.4 to 217.4kPa, which changes every 10kPa pressure, and when the pressure is adjusted, the pressure in the pressure calibration chamber should be stabilized before the group image is collected. Because the intensity of the excitation and the size of the camera aperture will affect the brightness of the captured fluorescent image, and then affect the accuracy of optical pressure measurement, this calibration experiment studies the effect of excitation light intensity and camera aperture value on the pressure calibration curve. The calibration experiments were conducted at five different camera aperture values and two different intensity excitation lights: 5 aperture F values of 2.8, 4.0, 5.6, 8.0 and 11.0, respectively; In the experiment, the CCD camera itself has the existence of dark current, which will produce noise and other effects, and the input and output of CCD camera itself is nonlinear, which will also affect the experimental results. In order to reduce the system error, 20 images were averaged under each operating condition, and each image was collected at 0.2s. The image processing results show that in 20 images under the same pressure, the grayscaness value of each image is only 0.6% different from the average.
3 gives an image taken in an experiment with a pseudo-color display. It can be seen from the figure that the fluorescence intensity emitted by the calibration sheet pressure-sensitive coating at the same pressure is not completely consistent, and the image has strong and low light areas, which is due to the uniformity of light, the transmission and reflectivity of lyphosphorescent, the uneven distribution of photosensitive molecules in coatings, the arrangement of experimental parts and the influence of some experimental conditions such as cameras. In order to eliminate the effect of uneven luminescence under the same pressure, the background light and dark current are first removed from the collected fluorescent image, and then, with the local atmospheric state as the reference, the fluorescent image obtained is compared with the fluorescent image of the reference state, and the resulting image is obtained. Figure 4 is a processed image displayed in pseudo-color, visible from the figure, after the calculation of uneven lighting
effect has been basically eliminated, to obtain a fluorescent image under relative pressure.
application in the processing of the resulting image to take a point around a point of grayscale mean i.e. (1) in the grayscale ratio I0/I, with the pressure p at this time than the local atmospheric pressure to obtain p/p0 in the (1) to obtain a pair of values of the sampling point. The calibration curve of the optical pressure-sensitive coating is obtained by fitting all sampling points with least two-way. According to the processing method and type (1) of the image, the calibration curve is represented by the pressure relative value and the light strength relative value, so theoretically the setting of the measurement system should not affect the results of the calibration curve, but after a lot of experiments in this paper, the excitation light and camera aperture are the main factors affecting the calibration curve in the test system.
3 Experimental results analyze the effect of
3.1 excitation light intensity on calibration
Figures 5 and 6 give images captured at typical pressures of 200W and 400W for excitation light source power, respectively, and the camera aperture value of the two experiments is F2.8. As shown in Figure 5, the fluorescence emitted by all pressure-sensitive coatings is dim, and the fluorescence intensity of the image changes with the pressure, but the change is very small. This is mainly because the weak excitation light intensity is not enough to stimulate the large energy leap of the photosensitive molecules, so that the fluorescence of the photosensitive molecules is also weak. When the intensity of the light source increased to 400W (see Figure 6), the image of the same pressure collected increased significantly compared with Figure 5, and the brightness of the image gradually weakened with the increase of pressure, and the intensity of fluorescent light from the pressure-sensitive coating changed significantly with the pressure. This is just like the properties of pressure-sensitive coatings, the greater the pressure, the stronger the oxygen annihilation effect, and thus the darker the fluorescent image. Comparing Figures 5 and 6, it is learned that when the power of the excitation light source is 400 W, the pressure-sensitive coating is more sensitive to pressure.
6 excitation light source 400W
Figure 7 is the calibration curve obtained when two excitation lights are used at different aperture values. As shown in Figure 7, the pressure ratio increases with the increase of the grayscale ratio, and the calibration curve obtained when the intensity of the excitation light source is 400W is steeper, the sampling point distribution is more regular, satisfied (1), which indicates that when the power of the excitation light source is 400W, the excitation light emitted has better excitation to the pressure-sensitive coating, and the fluorescence intensity of the coating is more sensitive to environmental pressure. When the light source is excited 200W, the distribution of sampling points is scattered, and the pattern (1) can no longer reflect the distribution law of sampling points. More obviously, when the calibration curve reaches a peak with an excitation light power of 200W when the F=2.8 (see Figure 7(a)), the pressure ratio begins to decrease with the grayscale ratio, and it is clear that the light strength obtained in this range? The pressure calibration curve is not monotonous and can result in incorrect measurements when applied to actual pressure measurements.
Figure 7 calibration curve when different excitation of light
the optical pressure-sensitive coating measurement technology and measurement principle analysis shows that when the excitation light is too weak, the outer electrons of the photosensitive molecules in the coating are not easily stimulated to produce energy-producing leaps and fluorescence, even if a weak fluorescence will be a small amount of oxygen annihilation, so that when the ambient pressure changes, grayscale changes are not obvious, plus the image at this time the signal noise is relatively small, resulting in irregular sampling point distribution. Therefore, in the formation of optical pressure measurement system, attention should be paid to the selection of the intensity of the excitation light source.
effect of 3.2 aperture on calibration
it can also be found in Figure 7 that the setting of the camera aperture value also has an effect on the calibration curve. Therefore, this paper also carried out a study on the impact of aperture on calibration curves. That is, when the light source is 400W, experiments were conducted under 5 sets of apertures. Due to the space limit and combined with Figure 6 (F-2.8), the fluorescent images taken at the aperture values of 5.6 (see Figure 8) and 11.0 (see Figure 9) are given here. As can be seen from the figure, under the same aperture, the fluorescent image gradually darkens as the ambient pressure increases, which is consistent with the trend in Figure 6. By comparing the fluorescent images collected at 2.8, 5.6 and 11.0, the larger the aperture value, the darker the fluorescent images collected, and the weaker the change with the pressure. This indicates that the setting of the camera aperture has some effect on the fluorescence intensity of the captured fluorescent image, mainly because when the aperture value increases, less light enters the camera photosenses, so the darker the resulting image.
10 gives a calibration curve with different aperture values when the excitation light source is 400W. In the figure, the calibration line obtained according to the measurement sampling point at each aperture value is monotonous in the ambient pressure range of this paper, and the light intensity increases with the increase of pressure. At negative pressure (p/p0<1.0), the slope of the calibration curve is large, and at positive pressure (p/p0>1.0), the slope of the calibration curve decreases gradually. This is because under certain light intensity, when the pressure gradually increases, the image darkens and the signal-to-noise ratio decreases, resulting in a decrease in the curve slope in the high-pressure zone. In addition, looking at all the curves in Figure 10, it is found that the calibration curves at F=2.8 and F=4.0 are very close, but as the aperture increases, the slope of the calibration curve increases, and the gap between adjacent curves is increasing, and the pressure sensitivity characteristics reflected in the calibration curve are more obvious. According to type (1), the calibration curve of the pressure-sensitive coating reflects the relationship between the pressure ratio and the light-force ratio, and the resulting calibration curve is independent of the light and dark of the captured image. This is because when the aperture becomes larger, the amount of light through the lens will be reduced, the image is darker, but the faint interference light can not enter the camera photosensic device, resulting in an increased signal-to-noise ratio of the image, that is, a single fluorescent image is collected, so its fitted calibration curve slope is larger.
4 Conclusion
optical pressure measurement technology is a new type of pressure measurement technology, with high spatial resolution, no interference to the current field and other advantages. In this paper, a large number of calibration experiments have been carried out on domestic pressure-sensitive coatings, focusing on the influence of excitation light strength and CCD camera aperture value in pressure-sensitive coating calibration. The experimental results show
(1) the effect of uneven system and lighting on the luminescence of pressure-sensitive coatings can be eliminated by ratio calculation.
(2) the excitation of light needs to have a certain intensity, the intensity of the excitation of light should be strong enough to make the photosensitive molecules in the coating to produce energy-grade jump generation.
(3) camera's aperture value, the greater the resulting image signal-to-noise ratio, and the greater the slope of the resulting pressure-sensitive coating calibration curve.
in order to further investigate the effect of image acquisition system on the calibration curve of pressure-sensitive coatings, the photomedration tube will be used as an image acquisition system to replace the CCD camera in future studies.
To the Chinese
, Jin Yugao and Chen Liusheng of the National Key Laboratory of Polymer Physic Chemistry of the Institute of Chemistry of the Chinese Academy of Sciences provided it free of charge