A device and method for detecting internal defects of absorbing coatings based on microwave thermal imaging

Abstract

The purpose of the present invention is to provide a device and method for detecting internal defects of absorbing coatings based on microwave thermal imaging technology, which belongs to the technical field of non-destructive detection of defects in absorbing materials. The device uses the electromagnetic signal radiated by the antenna to heat the absorbing coating, and uses an infrared thermal imager to measure the infrared signal of the internal defects of the coating. By changing the heating power, the temperature of the defective area on the surface of the coating reaches a dynamic balance. Combined with the conversion relationship between thermal energy and electromagnetic energy, a relationship between reflectivity, coating incident field strength and defective area temperature is established, and finally the relationship between reflectivity and defect depth is established. The method of the present invention does not need to consider the influence of cable loss, so that the defect depth calculation result is more accurate.

Description

Technical Field

[0001] The present invention belongs to the technical field of nondestructive detection of absorbing material defects, and in particular relates to a device and method for detecting internal defects of absorbing coatings based on microwave thermal imaging. Background Art

[0002] With the continuous development of the material field, absorbing coating materials are widely used in the field of electromagnetic technology, such as stealth aircraft, radar, etc. This material can greatly attenuate the energy of electromagnetic waves incident on the surface of the system and enhance its stealth effect. In addition, absorbing coating materials can also be used in the electromagnetic environment management of microwave equipment, through the precise restraint and shielding of electromagnetic waves, so as to better realize the utilization and propagation of signals. If there are internal defects in the absorbing coating, it will greatly affect its electromagnetic shielding performance. Therefore, controlling the quality of the absorbing coating through non-destructive defect detection is of great significance to the performance and safety of subsequent products.

[0003] Infrared thermal imaging is a non-contact, large-area imaging defect non-destructive detection technology. By heating the microwave coating and then measuring the temperature distribution, the defects of the absorbing coating can be quickly and clearly seen. At present, most infrared thermal imaging technologies use light-stimulated heating coatings. The principle is to use high-intensity light signals to exchange heat with the coating surface, and then transfer heat along the thickness of the coating according to solid heat conduction, thereby heating the entire coating. However, this method is not efficient and is difficult to be effective for targets that are too thick. Microwave excitation converts the electromagnetic energy absorbed by the coating into thermal energy, and as long as there is microwave loss inside the coating, heating can be achieved. This method is equivalent to every place inside the coating that interacts with microwaves being a heat source, so it is a kind of overall heating method. Combined with the strong absorption effect of absorbing coatings on microwaves and the characteristics of electromagnetic waves having a certain penetration depth into the coating, it can be known that microwave heating has higher heating efficiency and detection depth than general light excitation.

[0004] At present, the defect depth detection technology based on infrared thermal imaging mostly uses fitting. The paper "Research on Key Technologies for Nondestructive Detection of SiC Coating Defects by Pulsed Infrared Thermal Waves" proposes to achieve the purpose of detection by obtaining the relationship between defect depth and surface temperature through direct fitting or neural network methods. The disadvantage is that the process is complicated and the model needs to be trained through samples, and the final model cannot adapt to other test environments. Although there are related studies on accurately obtaining the defect depth by analyzing the microwave reflection relationship, such as the patent "A method for extracting defect depth information based on microwave heating (patent number: 202210171651.1)", this scheme only proposes a method for extracting defect depth information under microwave heating, and does not build and design a specific implementation device. At the same time, the extraction method does not take into account factors such as the coating surface electric field strength test, cable loss, and the influence of thermal convection at the defect, so the accuracy of defect detection is still not high enough.

[0005] Therefore, how to design a specific detection device scheme that can extract microwave coating defects based on microwave heating has become a research hotspot. Summary of the invention

[0006] In view of the problems existing in the background technology, the purpose of the present invention is to provide a device and method for detecting internal defects of absorbing coatings based on microwave thermal imaging. The device uses the electromagnetic signal radiated by the antenna to heat the absorbing coating, and uses an infrared thermal imager to measure the infrared signal of the internal defects of the coating. By changing the heating power, the temperature of the defective area on the surface of the coating reaches a dynamic balance. Combining the conversion relationship between thermal energy and electromagnetic energy, the relationship between reflectivity, coating incident field strength and defect area temperature is established, and finally the relationship between reflectivity and defect depth is established, thereby achieving accurate measurement of the defect depth value.

[0007] To achieve the above object, the technical solution of the present invention is as follows:

[0008] A microwave thermal imaging-based internal defect detection device for a microwave absorbing coating, comprising a signal source, an adjustable attenuator, a power amplifier, a directional coupler, a detector, a signal processing module, an infrared thermal imager, an antenna and a microwave dark box;

[0009] The signal source, the adjustable attenuator and the power amplifier are connected in sequence, the other end of the power amplifier is connected to the input end of the directional coupler, the output end of the directional coupler is connected to the antenna, the coupling end is connected to the input end of the detector, the output end of the detector is connected to the input end of the signal processing module, and the output end of the signal processing module is connected to the infrared thermal imager;

[0010] The infrared thermal imager, antenna and absorbing coating material to be tested are all arranged in a microwave dark box, and the microwave dark box is used to isolate electromagnetic interference; the absorbing coating material to be tested is arranged directly below the antenna, and the antenna emits electromagnetic waves vertically incident on the absorbing coating to heat the absorbing coating; the infrared thermal imager is arranged at a certain angle obliquely above the absorbing coating to measure the temperature distribution on the surface of the absorbing coating;

[0011] The signal source is used to generate a microwave signal; the adjustable attenuator is used to adjust the input power of the input end of the power amplifier to achieve adjustable output power of the power amplifier, so that the defect of the absorbing coating can reach a constant temperature state; the power amplifier is used to amplify the power of the input microwave signal, the detector is used to detect the output power of the coupling end of the directional coupler, and the signal processing module is used to receive temperature data of the thermal imager and output a temperature distribution map and calculate the defect depth.

[0012] Furthermore, the antenna is preferably a horn antenna, and the antenna operating power should be greater than 2W.

[0013] A method for detecting internal defects of an absorbing coating based on microwave thermal imaging comprises the following steps:

[0014] Step 1. The antenna transmits electromagnetic waves vertically incident on the absorbing coating to heat the absorbing coating and collect thermal images;

[0015] Step 2. Adjust the heating power of the antenna absorbing coating and combine it with the thermal image to stabilize the temperature of the defect area;

[0016] Step 3. Calculate the heating power of the antenna, and combine the temperature value when the temperature of the defective area reaches a stable state to calculate the reflectivity R of the coating defective area;

[0017] Step 4. Based on the multi-layer dielectric electromagnetic wave reflection model, the defect depth inside the coating is calculated according to the reflectivity R of the coating defect area.

[0018] Furthermore, the device of the present invention can also realize the extraction of the geometric features of the internal defects of the absorbing coating. The specific process is: adjust the heating power of the antenna absorbing coating, and combine the thermal image to make the temperature of the defect area tend to be stable; after a period of stable heating, stop heating the absorbing coating, obtain the thermal image with the best signal-to-noise ratio during the cooling process, and obtain the geometric features of the defects based on the thermal image using the edge detection algorithm.

[0019] Furthermore, the input power P of the antenna in step 3 in The specific calculation process is:

[0020]

[0021] Where P c is the measured power of the detector, C is the coupling degree of the directional coupler, and IL is the insertion loss parameter of the directional coupler.

[0022] Furthermore, the specific calculation process of the reflectivity R of the coating defect area in step 3 is:

[0023] When the temperature of the defect area reaches a dynamic equilibrium, the relationship between the power absorbed and dissipated by the microwave coating is as follows:

[0024] Q a =Q conv +Q rad =hΔS 1 (T obj -T amb )+εσΔS 1 [(T obj ) 4 -(T amb ) 4 ] (1)

[0025]

[0026] In the formula, Q a is the power absorbed by the microwave coating, Q conv is the power consumed by heat convection, Q rad is the power consumed by thermal radiation, h is the thermal convection coefficient, T obj is the absolute temperature of the defect area, T amp is the absolute temperature of the environment, ε is the infrared reflectivity, σ is the Boltzmann constant, ΔS 2 is the area of ​​the defect region, ΔS 1 =2ΔS 2 , η 0 is the wave impedance of air, R is the microwave reflectivity of the defect area, E i0 is the incident field intensity on the coating surface;

[0027] At the same time, the incident field intensity E on the coating surface is calculated based on the antenna size and position. i0 ,as follows:

[0028]

[0029] Where a is the long side of the antenna aperture, b is the short side of the antenna aperture, P in is the input power of the antenna, λ is the wavelength of the incident wave in free space, and r is the distance from the antenna aperture to the sample surface;

[0030] By combining the above equations, we can get the reflectivity R of the coating defect area, as follows:

[0031]

[0032] Furthermore, the heat convection coefficient h in indoor environment is generally 5W / (m 2 ·k); the infrared reflectivity ε is preferably 0.9.

[0033] The mechanism of the present invention is as follows: most of the signals at the input end of the directional coupler are transmitted to the output end, and the coupled end signal only comes from a small amount of the input end signal. Based on this, a detector is used to detect the signal power at the coupled end, and then the signal power at its input end is calculated according to the performance parameters of the directional coupler, and finally the power value at its output end is obtained, which is the accurate input power value of the antenna, and the input power of the antenna is crucial to the calculation accuracy of the microwave coating defect depth; at the same time, the incident field strength of the microwave coating surface can be accurately calculated according to the antenna size and position information, so the method of the present invention does not need to consider the influence of cable loss, so that the defect depth calculation result is more accurate.

[0034] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0035] The present invention establishes a relationship between reflectivity, coating incident field intensity and defect area temperature by changing the heating power of the microwave coating, and then calculates the depth value using the relationship between reflectivity and defect depth. The final defect depth value result is more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram of the structure of the detection device for internal defects of the absorbing coating of the present invention.

[0037] Figure 2 This is the thermal image of the absorbing coating sample to be tested at a certain moment in the heating process.

[0038] Figure 3 Schematic diagram of the electromagnetic wave reflection model in multi-layer media.

[0039] Figure 4 It is the temperature variation curve measured at the defect center and the defect-free position of the absorbing coating sample to be tested.

[0040] Figure 5 This is a thermal image of the absorbing coating sample to be tested at a certain moment in the cooling process.

[0041] Figure 6 is the calculation result of the edge detection algorithm.

[0042] In the figure, 1 is a signal source, 2 is an adjustable attenuator, 3 is a power amplifier, 4 is a directional coupler, 5 is a detector, 6 is a data processing end, 7 is an absorbing material, 8 is a horn antenna, 9 is an absorbing coating, 10 is an internal defect, 11 is a coating substrate, 12 is an infrared thermal imager, and 13 is a small microwave darkroom. DETAILED DESCRIPTION

[0043] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with the implementation modes and the accompanying drawings.

[0044] A device for detecting internal defects of absorbing coatings based on microwave thermal imaging technology, the structural diagram of which is shown in FIG. Figure 1 As shown, it includes a signal source 1, an adjustable attenuator 2, a power amplifier 3, a directional coupler 4, a detector 5, a signal processing module 6, a horn antenna 8, an infrared thermal imager 12 and a microwave dark box 13; the directional coupler 4 is a three-port device, which includes an input end 14, an output end 15, and a coupling end 16;

[0045] The signal source 1 is used to generate a microwave signal and transmit the microwave signal to the adjustable attenuator 2. The adjustable attenuator 2 is used to adjust the power of the microwave signal and transmit the microwave signal after the power is adjusted to the power amplifier 3. The power amplifier 3 is used to amplify the power of the microwave signal and transmit the amplified microwave signal to the input end 14 of the coupler 4. Most of the microwave signals input to the input end 14 are output to the horn antenna 8 through the output end 15, and a small part of the microwave signals are output to the detector 5 through the coupling end 16. Since the coupling end 16 and the output end 15 have a high degree of isolation, the power of the microwave signal at the coupling end 16 can be measured by the detector 5, and then the power of the input end 14 can be calculated according to the coupling degree of the directional coupler 4. The output end of the detector 5 is connected to the input end of the signal processing module 6, and the output end of the signal processing module 6 is connected to the infrared thermal imager 12. The signal processing module 6 is used to receive and analyze the microwave signal data transmitted by the detector 5.

[0046] The infrared thermal imager 12, the horn antenna 8 and the absorbing coating material to be tested are all arranged in a microwave dark box 13. The absorbing coating material to be tested includes a coating substrate 11 and an absorbing coating 9 on the surface of the coating substrate. The absorbing coating material to be tested is arranged directly below the horn antenna 8. The horn antenna 8 emits electromagnetic waves that are vertically incident on the absorbing coating 9 to heat the absorbing coating. The infrared thermal imager 12 is arranged at a certain angle obliquely above the absorbing coating 9 to measure the temperature distribution on the surface of the absorbing coating. The inner wall of the microwave dark box 13 is provided with absorbing material 7 to reduce the radiation effect of high-power microwaves on the human body.

[0047] Example 1

[0048] In this embodiment, the horn antenna 8 is a pyramidal horn antenna with an operating frequency of 5.8 GHz, a long side a=80 mm, a short side b=60 mm, facing the surface of the absorbing coating 9 during heating, and a vertical distance r=72 mm from the antenna aperture to the absorbing coating 9; the amplification gain of the power amplifier 3 is 43 dB, and the maximum output power is 20 W; the infrared thermal imager 12 has an image resolution of 384*288, and a temperature resolution of less than 50 mK; the absorbing coating 9 operates in the C band, has a thickness of 1.9 mm, and contains a square defect of 6 mm*6 mm and a depth of 0.5 mm at the bottom (the defect depth is defined as the distance from the upper surface of the coating to the defect, so the defect depth is 1.9-0.5=1.4 mm); the coating substrate is an aluminum plate.

[0049] A method for detecting internal defects of an absorbing coating based on microwave thermal imaging technology comprises the following steps:

[0050] Step 1. Start the signal source 1. After the adjustable attenuator 2 and the power amplifier 3 act, the generated high-power signal is output to the antenna by the directional coupler 4. The antenna transmits electromagnetic waves vertically incident on the absorbing coating, and the absorbing coating is quickly heated according to the microwave heating principle;

[0051] Step 2: Use the thermal imager 12 to track the highlighted area in the thermal image, which is the defect area; Figure 2 The thermal image during the heating process shows that there are defective areas and non-defective areas in the image. At this time, the temperature change of the defective area is monitored in real time, and the heating power is adjusted in conjunction with the adjustable attenuator 2 to stabilize the temperature of the defective area. Figure 4 is the temperature change process of the antenna heating process. It can be seen that when the temperature T obj =37℃, the temperature gradually stabilizes. At this time, the ambient temperature T amb =24℃;

[0052] Step 3. Use the directional coupler 4 and the detector 5 to measure the power P at the output end 15. in =2W, the specific calculation process is:

[0053]

[0054] Where P c is the measured power of the detector, C is the coupling degree of the directional coupler, and IL is the insertion loss parameter of the directional coupler;

[0055] Step 4. Combined with the temperature value when the defect area temperature reaches a stable state, calculate the reflectivity R of the coating defect area. The specific process is:

[0056] When the temperature of the defect area reaches a dynamic equilibrium, the relationship between the power absorbed and dissipated by the microwave coating is as follows:

[0057] Q a =Q conv +Q rad =hΔS 1 (T obj -T amb )+εσΔS 1 [(T obj ) 4 -(T amb ) 4 ] (2)

[0058]

[0059] In the formula, Q a is the power absorbed by the microwave coating, Q conv is the power consumed by heat convection, Q rad is the power consumed by thermal radiation, h is the thermal convection coefficient, Tobj is the absolute temperature of the defect area, T amp is the absolute temperature of the environment, ε is the infrared reflectivity, σ is the Boltzmann constant, ΔS 2 is the area of ​​the defect region, ΔS 1 =2ΔS 2 , η 0 is the wave impedance of air, R is the microwave reflectivity of the defect area, E i0 is the incident field intensity on the coating surface;

[0060] At the same time, the incident field intensity E on the coating surface is calculated based on the antenna size and position. i0 ,as follows:

[0061]

[0062] Where a is the long side of the antenna aperture, b is the short side of the antenna aperture, P in is the input power of the antenna, λ is the wavelength of the incident wave in free space, and r is the distance from the antenna aperture to the sample surface;

[0063] By combining the above equations, we can get the reflectivity R of the coating defect area, as follows:

[0064]

[0065] Step 5. Based on the multi-layer dielectric electromagnetic wave reflection model (refer to Section 2.3.1 of Chapter 2 of the doctoral thesis entitled "Research on Field Detection Technology of Microwave Reflectivity of Stealth Coatings"), the schematic diagram of the model structure is as follows: Figure 3 As shown, the defect depth inside the coating is calculated based on the reflectivity R of the coating defect area. The numerical result of the coating defect depth d1 can be calculated by the dichotomy method, and the steps are as follows:

[0066] Step 5.1. Set the solution interval and function to be d1∈[a,b], Midpoint p1 = (a + b) / 2, where R is the measured reflectivity, R m is the calculation formula for the reflectivity of multilayer materials, which satisfies the following in the interval [a, b]:

[0067] Step 5.2. If Then d1 = p1, terminate;

[0068] Step 5.3. If

[0069]

[0070]

[0071] Step 5.4. Repeat the above steps until the error is met and stop iterating;

[0072] The final defect depth is 1.3mm. Compared with the actual value of 1.4mm, the error of the defect depth is 0.1mm, which is highly accurate.

[0073] Turn off signal source 1 to cool down the coating. Since the thermal conductivity of the defect is low and heat transfer is blocked, the cooling rate of the defect area is significantly slower than that of the non-defect area. Therefore, the thermal image with the best signal-to-noise ratio can be obtained during the cooling process, such as Figure 5 As shown; finally, the edge detection algorithm can be used to obtain the geometric features of the defect from the thermal image, such as Figure 6 shown.

[0074] The above description is only a specific implementation mode of the present invention. Any feature disclosed in this specification, unless otherwise stated, can be replaced by other alternative features that are equivalent or have similar purposes; all the disclosed features, or all the steps in the methods or processes, except for mutually exclusive features and / or steps, can be combined in any way.

Claims

1. A method for detecting internal defects of an absorbing coating based on microwave thermal imaging, characterized in that: The following steps are involved: Step 1. The antenna transmits electromagnetic waves vertically incident on the absorbing coating to heat the absorbing coating and collect thermal images; Step 2. Adjust the heating power of the antenna absorbing coating and combine it with the thermal image to stabilize the temperature of the defect area; Step 3. Calculate the heating power of the antenna, and combine it with the temperature value when the temperature of the defective area reaches a stable state to calculate the reflectivity R of the coating defective area. The specific calculation process is: When the temperature of the defect area reaches a dynamic equilibrium, the relationship between the power absorbed and dissipated by the microwave coating is as follows: Q a =Q conv +Q rad =hΔS1(T obj -T amb )+εσΔS1[(T obj ) 4 -(T amb ) 4 ] (1) In the formula, Q a is the power absorbed by the microwave coating, Q conv is the power consumed by heat convection, Q rad is the power consumed by thermal radiation, h is the thermal convection coefficient, T obj is the absolute temperature of the defect area, T amp is the absolute temperature of the environment, ε is the infrared reflectivity, σ is the Boltzmann constant, ΔS2 is the area of ​​the defect area, ΔS1 = 2ΔS2, η0 is the wave impedance of the air, R is the microwave reflectivity of the defect area, E i0 is the incident field intensity on the coating surface; At the same time, the incident field intensity E on the coating surface is calculated based on the antenna size and position. i0 ,as follows: Where a is the long side of the antenna aperture, b is the short side of the antenna aperture, P in is the input power of the antenna, λ is the wavelength of the incident wave in free space, and r is the distance from the antenna aperture to the sample surface; By combining the above equations, we can get the reflectivity R of the coating defect area, as follows: Step 4. Based on the multi-layer dielectric electromagnetic wave reflection model, the defect depth inside the coating is calculated according to the reflectivity R of the coating defect area; The detection method is implemented based on the following detection device, which includes a signal source, an adjustable attenuator, a power amplifier, a directional coupler, a detector, a signal processing module, an infrared thermal imager, an antenna and a microwave dark box; The signal source, the adjustable attenuator and the power amplifier are connected in sequence, the other end of the power amplifier is connected to the input end of the directional coupler, the output end of the directional coupler is connected to the antenna, the coupling end is connected to the input end of the detector, the output end of the detector is connected to the input end of the signal processing module, and the output end of the signal processing module is connected to the infrared thermal imager; The infrared thermal imager, antenna and absorbing coating material to be tested are all arranged in a microwave dark box, and the microwave dark box is used to isolate electromagnetic interference; the absorbing coating material to be tested is arranged directly below the antenna, and the antenna emits electromagnetic waves vertically incident on the absorbing coating to heat the absorbing coating; the infrared thermal imager is arranged at a certain angle obliquely above the absorbing coating to measure the temperature distribution on the surface of the absorbing coating; The signal source is used to generate a microwave signal; the adjustable attenuator is used to adjust the input power of the input end of the power amplifier to achieve adjustable output power of the power amplifier, so that the defect of the absorbing coating can reach a constant temperature state; the power amplifier is used to amplify the power of the input microwave signal, the detector is used to detect the output power of the coupling end of the directional coupler, and the signal processing module is used to receive temperature data of the thermal imager and output a temperature distribution map and calculate the defect depth.

2. The method for detecting internal defects of an absorbing coating according to claim 1, characterized in that: The antenna is a horn antenna and the antenna working power should be greater than 2W.

3. The method for detecting internal defects of an absorbing coating according to claim 1, characterized in that: The specific process of extracting the geometric features of internal defects in the absorbing coating is as follows: adjusting the heating power of the antenna absorbing coating and combining it with the thermal image to stabilize the temperature of the defect area; after a period of stable heating, stopping the heating of the absorbing coating and obtaining a thermal image with the best signal-to-noise ratio during the cooling process, and obtaining the geometric features of the defects based on the thermal image using an edge detection algorithm.

4. The method for detecting internal defects of an absorbing coating according to claim 1, wherein: The input power P of the antenna in step 3 in The specific calculation process is: Where P c is the measured power of the detector, C is the coupling degree of the directional coupler, and IL is the insertion loss parameter of the directional coupler.

5. The method for detecting internal defects of an absorbing coating according to claim 1, wherein: The heat convection coefficient h in indoor environment is 5W / (m 2 ·k); infrared reflectivity ε is 0.9.

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