A microwave coaxial sensor and sensing system for detecting defects in polyethylene pipes

Abstract

The invention discloses a microwave coaxial sensor and a sensing system for detecting defects in polyethylene pipelines, comprising a microwave coaxial sensor, a microwave network analyzer and a computer. The microwave coaxial sensor comprises a coaxial line portion and a circular waveguide portion for fitting to a pipe wall, wherein the circular waveguide portion is detachably mounted at an opening of the coaxial line portion; the coaxial line portion comprises an outer conductor and an inner conductor, wherein the inner conductor is mounted in the outer conductor, and a filling material is used to fill the space between the outer conductor and the inner conductor, and an adapter is mounted on the outer conductor for inputting and outputting microwave signals, wherein the adapter adopts a coupling ring to perform magnetic field excitation, and the circular waveguide portion is fitted to the surface of the pipeline, and the inside of the pipeline is detected by utilizing an edge electric field region, thus having a larger detection area, being able to fully cover the thickness of the pipeline, and having a simple structure, and adopting a resonant frequency to analyze signal changes caused by defects, being able to qualitatively and quantitatively analyze defect size information, and simplifying the signal processing process.

Description

Technical Field

[0001] The invention relates to the technical field of material defect detection, and in particular to a microwave coaxial sensor and a sensing system for detecting defects in polyethylene pipes. Background Art

[0002] Compared with traditional metal pipes, polyethylene pipes have the advantages of corrosion resistance, long service life, light weight, easy installation and transportation, etc., and have been widely used in industries such as petroleum, chemical industry, natural gas, and urban water supply and drainage. During the processing, installation and service, the pipes may have defects such as internal material unevenness, pores, inclusions, unfused, and pressure loss. Therefore, in order to improve the safety of polyethylene pipes, it is very important to adopt a reliable damage detection method.

[0003] At present, the commonly used methods for detecting polyethylene pipes include infrared thermal imaging, X-ray, terahertz and ultrasound, etc. Each technology has advantages and limitations. For example, infrared detection devices are not convenient and are not easy to detect on site. X-ray detection is costly, slow, and has ionizing radiation hazards to the human body. Terahertz instruments are expensive and the information processing process is complicated; the defect detection rate of conventional ultrasonic technology is greatly affected by human factors. In addition, compared with metals, polyethylene can absorb more sound wave energy, which greatly increases the difficulty of detection.

[0004] In recent years, microwave-based detection technology has received increasing attention; microwave methods have the advantages of low power consumption (milliwatt level), low cost, no need for coupling agents, easy portability and experimental setup, simple signal analysis, good repeatability, and no ionizing radiation.

[0005] Most of the existing microwave methods use the open waveguide method: the open waveguide is placed at a certain height from the outer surface of the pipe (also called the lift-off distance), and the existence of defects is judged based on the obtained frequency domain or time domain information; the change in scanning height will affect the received signal value, so this puts high demands on the stability of the scanning system where the probe is placed; at the same time, the sensing area is limited to the near field range of the probe and cannot cover the pipe wall thickness well; resulting in low detection efficiency, so there is an urgent need for a microwave coaxial sensor and sensing system for detecting polyethylene pipe defects to solve the above problems. Summary of the invention

[0006] The present invention provides a microwave coaxial sensor and a sensing system which can detect the inside of a pipeline through an edge electric field region and can detect defects in a polyethylene pipeline by fully covering the thickness of the pipeline, so as to solve the problems existing in the above-mentioned prior art.

[0007] To achieve the above object, the present invention provides the following technical solution: a microwave coaxial sensor for detecting defects in polyethylene pipes, comprising a coaxial line portion and a circular waveguide portion for fitting the pipe wall, wherein the circular waveguide portion is detachably installed at the opening of the coaxial line portion;

[0008] Among them, the coaxial line part includes an outer conductor and an inner conductor, the inner conductor is installed in the outer conductor, and the outer conductor and the inner conductor are filled with filling material, and an adapter is installed on the outer conductor for inputting and outputting microwave signals, and the adapter adopts a coupling ring to perform magnetic field excitation.

[0009] Preferably, the length of the inner conductor is one quarter of the wavelength corresponding to the designed operating frequency, and the wavelength λ of the microwave signal in the coaxial transmission line is g The calculation formula is: Among them, ε′ r is the dielectric constant of the filling material between the inner conductor and the outer conductor of the coaxial line, c is the speed of light in a vacuum, and f is the operating frequency.

[0010] Preferably, the filling material is Teflon material.

[0011] Preferably, the propagation mode of the wave in the coaxial part is a TEM mode, wherein the designed operating frequency is less than the cut-off frequency of the TEM mode, that is, Where a and b are the inner conductor radius and the outer conductor inner diameter.

[0012] Preferably, the maximum value of the designed operating frequency is not greater than the cutoff frequency of the fundamental mode TEM mode of the circular waveguide, that is, Where g is the radius of the opening of the circular waveguide.

[0013] Preferably, the length of the circular waveguide portion is determined based on the definition of the circular waveguide attenuation factor and the set attenuation degree, wherein the circular waveguide attenuation factor α is calculated as:

[0014] Preferably, the adapter is a radio frequency SMA type adapter, wherein the radio frequency SMA type adapter is symmetrically installed on both sides of the inner conductor.

[0015] Preferably, the adapter is a radio frequency N-type adapter, and a transition region for achieving impedance matching is provided between the coaxial line portion and the radio frequency N-type adapter, wherein the impedance corresponding to the cross section of the transition region must meet the design requirement of 50 ohms, and the impedance formula is:

[0016] A sensing system of a microwave coaxial sensor for detecting defects in polyethylene pipes, comprising a microwave coaxial sensor, a microwave network analyzer and a computer, wherein the microwave network analyzer is connected to an adapter of the microwave coaxial sensor via a flexible coaxial radio frequency cable to collect signals, and the computer is connected to the microwave network analyzer via a network cable to receive the collected signal data, and the signal data is fitted by programming to obtain an accurate resonant frequency value and quality factor;

[0017] The relationship between the change in the local dielectric constant distribution caused by defects and the change in the resonant frequency and quality factor can be expressed as:

[0018]

[0019]

[0020]

[0021] Among them, ω1, f1 and Q1 are the complex angular frequency, resonant frequency and unloaded quality factor in the defect-free case, ω2, f2 and Q2 are the complex angular frequency, resonant frequency and unloaded quality factor in the defect-containing case, ε and μ are the dielectric constant and permeability distribution of the filling material in the resonance region, respectively. and is the electric field and magnetic field distribution in the resonant region without defects; V C and V m are the volumes of the resonance area and the defect, respectively; Δε is the change in dielectric constant at the defect location.

[0022] Preferably, the computer reads the transmission coefficient S from the network analyzer through the programmable instrument standard command SCPI language code 21 Data, S 21 The value of and the frequency f satisfy the Lorentz equation: Where T = |S 21 | 2 To transfer energy, Q L is the loaded quality factor, and the unloaded quality factor Q U for:

[0023] Compared with the prior art, the present invention has the following beneficial effects: the present invention utilizes the fringe electric field region to detect the inside of the pipeline by bonding the circular waveguide part with the surface of the pipeline, has a larger detection area, can fully cover the thickness of the pipeline, has a simple structure, can be operated by hand, and uses the resonant frequency to analyze the signal changes caused by the defect, can qualitatively and quantitatively analyze the defect size information, simplifies the signal processing process, and, by recording the scanning trajectory of the probe at the same time, can obtain the position of the defect in the circumferential direction of the pipeline, is flexible and convenient to operate, and improves the detection efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0025] In the attached picture:

[0026] Figure 1 It is a schematic structural diagram of the microwave coaxial sensor of the present invention;

[0027] Figure 2 It is a structural schematic diagram of the sensing system of the microwave coaxial sensor of the present invention;

[0028] Figure 3 is a graph of the sensor resonance response signal corresponding to different through hole radii r on the inner wall of the tube of the present invention;

[0029] Figure 4 is the resonant frequency f of the sensor of the present invention r The relationship between the radius r of the through hole on the inner wall of the tube;

[0030] Figure 5 It is a structural schematic diagram of the radio frequency N-type adapter sensor of the present invention;

[0031] Figure 6 It is a schematic diagram of the structure of the circular waveguide part for detecting the inner wall of the tube of the present invention;

[0032] Numbers in the figure: 1. Microwave coaxial sensor; 11. Outer conductor; 12. Inner conductor; 13. Filling material; 14. RF SMA adapter; 15. Circular waveguide part; 16. RF N-type adapter; 17. Transition area; 2. Microwave network analyzer; 3. Computer; 4. Flexible coaxial RF cable; 5. Network cable; 6. Pipeline; 7. Defect. DETAILED DESCRIPTION

[0033] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0034] Example: Figure 1 As shown, a microwave coaxial sensor for detecting defects in polyethylene pipes comprises a coaxial line portion and a circular waveguide portion 15 for fitting the pipe wall, wherein the circular waveguide portion 15 is detachably mounted at the opening of the coaxial line portion, wherein the opening where the circular waveguide portion 15 fits the pipe 6 is adapted according to different pipe wall sizes;

[0035] The coaxial line portion includes an outer conductor 11 and an inner conductor 12. The inner conductor 12 is installed in the outer conductor 11, and the outer conductor 11 and the inner conductor 12 are filled with a filling material 13. In order to fix the inner conductor 12 of the coaxial line and prevent foreign objects from entering the sensor, in this embodiment, the filling material 13 is Teflon material; and an adapter installed on the outer conductor 11 for inputting and outputting microwave signals, and the adapter adopts a coupling ring to perform magnetic field excitation.

[0036] The length of the inner conductor 12 is one quarter of the wavelength corresponding to the designed operating frequency, and the wavelength λ of the microwave signal in the coaxial transmission line is g The calculation formula is: ε′ r is the dielectric constant of the filling material 13 between the inner conductor 12 and the outer conductor 11 of the coaxial line, c is the speed of light in a vacuum, and f is the operating frequency.

[0037] Among them, the propagation mode of the wave in the coaxial part is TEM mode. In order to avoid the interference of the high-order TEM mode of the coaxial line on the detection signal, the operating frequency is designed to be less than the cutoff frequency of the TEM mode, that is, Wherein, a and b are the radius of the inner conductor 12 and the inner diameter of the outer conductor 11; in this embodiment, a 5% safety margin is considered when setting the operating frequency; in order to make the resonant structure have a higher quality factor, if the space between the inner and outer conductors 11 of the sensor is filled with air, the ratio of the outer diameter to the inner diameter must be close to 3.6;

[0038] In order to prevent signal leakage, the maximum operating frequency is designed to be no greater than the cutoff frequency of the fundamental mode TEM mode of the circular waveguide, that is, Wherein, g is the radius of the opening of the circular waveguide portion 15, and a safety margin of 5% is also considered;

[0039] The length of the circular waveguide portion 15 is determined based on the definition of the circular waveguide attenuation factor and the set attenuation degree, wherein the circular waveguide attenuation factor α (in dB / m) is calculated as follows: The unit of frequency is MHz, and the length of the circular waveguide portion 15 cannot be too long, otherwise the microwave signal attenuation degree is too large (such as greater than 10 dB), resulting in a weak electric field intensity when the microwave propagates to the opening.

[0040] In this embodiment, the outer conductor 11 and the inner conductor 12 are made of aluminum alloy or copper and are obtained by CNC machine processing; if a higher quality factor is required, silver or gold with higher conductivity can be used for inner surface plating; the adapter is a RF SMA type adapter 14 for input and output, and a coupling ring is used for magnetic field excitation. The RF SMA type adapter 14 is symmetrically installed on both sides of the inner conductor 12.

[0041] refer to Figure 2As shown, a sensing system based on the microwave coaxial sensor 1 for detecting defects 7 in polyethylene pipes 6 includes a microwave coaxial sensor 1, a microwave network analyzer 2 and a computer 3. The microwave network analyzer 2 is connected to the adapter of the microwave coaxial sensor 1 through a flexible coaxial radio frequency cable 4 to collect signals. The computer is connected to the microwave network analyzer 2 through a network cable 5 to receive the collected signal data, and fits the signal data through programming to obtain accurate resonant frequency values ​​and quality factors. The signal power is set within 1 mW, and the computer reads the transmission coefficient S from the network analyzer through the programmable instrument standard command SCPI language code. 21 Data, S 21 The value of and the frequency f satisfy the Lorentz equation: Where T = |S 21 | 2 To transfer energy, Q L is the loaded quality factor, and the unloaded quality factor Q U for:

[0042] Evaluation of defect 7 size: According to the material perturbation theory, the influence of tiny defect 7 of polyethylene pipe 6 on the resonance signal is analyzed. The relationship between the change of local dielectric constant distribution caused by defect 7 and the change of resonance frequency and quality factor can be expressed as:

[0043]

[0044]

[0045]

[0046] Where ω1, f1 and Q1 are the complex angular frequency, resonant frequency and unloaded quality factor in the case of no defect 7, ω2, f2 and Q2 are the complex angular frequency, resonant frequency and unloaded quality factor in the case of defects, ε and μ are the dielectric constant and permeability distribution of the filling material in the resonance region, respectively. and is the electric field and magnetic field distribution in the resonant region without defects; V C and V mare the resonance area and the volume of the defect, respectively; Δε is the change in dielectric constant at the defect position. It can be seen that the sensor is sensitive to both the dielectric constant value and volume of defect 7; the greater the difference between the dielectric constant value of defect 7 and that of polyethylene, or the larger the volume of defect 7, the greater the resonance frequency offset will be; for example, for pore defect 7, since the change in dielectric constant is less than zero, the resonance frequency will increase; the larger the pore volume, the greater the offset caused; by measuring different prefabricated sizes of a specific defect 7 type, the relationship between the defect 7 size and the frequency change is obtained, and the calibration of the detection system is realized, thereby achieving the purpose of quantitative analysis of the measured defect 7.

[0047] In a specific embodiment, a sensor model for detecting a 2-inch pipe (63 mm in diameter and 5.8 mm in wall thickness) was established in the CST simulation software; the sensor inner diameter was 10 mm, the outer diameter was 36 mm, and the maximum length was 31 mm. A through-hole model was established at the half-thickness of the inner wall of the pipe to simulate defect 7, and the effect of different through-hole radii on the resonant frequency was analyzed; Figure 3 As shown, there is a good monotonic relationship between the resonant frequency and the radius, and the resonant frequency increases with the increase of the radius, such as Figure 4 As shown, the quantitative relationship between the two is obtained by fitting, which is helpful for evaluating the size of defect 7.

[0048] In another embodiment, referring to Figure 5 As shown, the adapter is a radio frequency N-type adapter 16, and a transition area 17 for achieving impedance matching is provided between the coaxial line portion and the radio frequency N-type adapter 16, transitioning from the smaller coaxial structure at the adapter to the larger coaxial structure of the above-mentioned sensor, wherein the impedance corresponding to the cross section of the transition area 17 must meet the design requirement of 50 ohms, and the length of the transition section can be designed to be half of the maximum diameter of the coaxial structure, and the impedance formula is:

[0049] The design also considers the restrictions on the operating frequency of the coaxial line high-order mode, the circular waveguide fundamental mode of the extended section of the outer conductor 11, and the attenuation degree of the extended section, which is the same as above. Unlike the above-mentioned sensor, the sensor in this embodiment can operate in a wider frequency range, and the lowest frequency can be DC. Through single-port measurement, the reflection coefficient S 11 Analyze the frequency domain reflection coefficient S of defect 7 11 The change of amplitude is analyzed by calibration to determine the size of the fixed defect 7 causing S 11 The frequency point corresponding to the maximum change is obtained as the optimal detection frequency. 11The relationship between; The defect 7 size evaluation method of the perturbation method is also applicable to the sensor in this embodiment. By measuring the prefabricated size defect 7, the changes in the amplitude and phase of the detection signal at different frequencies are obtained. The working frequency and detection index that can provide the highest sensitivity are studied and analyzed, which is convenient for subsequent actual detection signal analysis.

[0050] In another embodiment, a position sensor can be installed on the outer surface of the sensor to obtain corresponding real-time position information. The handheld sensor rotates and scans around the outer surface of the pipe 6 to monitor the abnormal resonance signal and determine the position of the defect 7 in combination with the scanning trajectory.

[0051] In another embodiment, referring to Figure 6 As shown, when the inner wall of the pipe needs to be inspected, the curved surface at the sensor opening can be adjusted accordingly to meet the requirement of fitting with the inner surface of the pipe 6.

[0052] Finally, it should be noted that the above description is only a preferred example of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A microwave coaxial sensor for detecting defects in polyethylene pipes, characterized in that: It comprises a coaxial line part and a circular waveguide part for fitting the pipe wall, wherein the circular waveguide part is detachably installed at the opening of the coaxial line part; Among them, the coaxial line part includes an outer conductor and an inner conductor, the inner conductor is installed in the outer conductor, and the outer conductor and the inner conductor are filled with filling material, and an adapter is installed on the outer conductor for inputting and outputting microwave signals, and the adapter adopts a coupling ring to perform magnetic field excitation.

2. The microwave coaxial sensor for detecting defects in polyethylene pipes according to claim 1, characterized in that: The length of the inner conductor is one quarter of the wavelength corresponding to the designed operating frequency, and the wavelength λ of the microwave signal in the coaxial transmission line g The calculation formula is: Among them, ε′ r is the dielectric constant of the filling material between the inner conductor and the outer conductor of the coaxial line, c is the speed of light in a vacuum, and f is the operating frequency.

3. A microwave coaxial sensor for detecting defects in polyethylene pipes according to claim 2, characterized in that: The filling material is Teflon material.

4. The microwave coaxial sensor for detecting defects in polyethylene pipes according to claim 2, characterized in that: The propagation mode of the wave in the coaxial part is the TEM mode, wherein the designed operating frequency is less than the cut-off frequency of the TEM mode, that is, Where a and b are the inner conductor radius and the outer conductor inner diameter.

5. The microwave coaxial sensor for detecting defects in polyethylene pipes according to claim 4, characterized in that: The maximum value of the designed operating frequency is not greater than the cutoff frequency of the circular waveguide fundamental mode TEM mode, that is, Where g is the radius of the opening of the circular waveguide.

6. The microwave coaxial sensor for detecting defects in polyethylene pipes according to claim 5, characterized in that: The length of the circular waveguide portion is determined based on the definition of the circular waveguide attenuation factor and the set attenuation degree, wherein the circular waveguide attenuation factor α is calculated as:

7. The microwave coaxial sensor for detecting defects in polyethylene pipes according to claim 6, characterized in that: The adapter is a radio frequency SMA type adapter, wherein the radio frequency SMA type adapter is symmetrically installed on both sides of the inner conductor.

8. The microwave coaxial sensor for detecting defects in polyethylene pipes according to claim 6, characterized in that: The adapter is a radio frequency N-type adapter, and a transition region for achieving impedance matching is provided between the coaxial line portion and the radio frequency N-type adapter, wherein the impedance corresponding to the cross section of the transition region must meet the design requirement of 50 ohms, and the impedance formula is:

9. A sensing system for detecting defects in polyethylene pipes based on the microwave coaxial sensor according to any one of claims 1 to 7, characterized in that: It includes a microwave coaxial sensor, a microwave network analyzer and a computer. The microwave network analyzer is connected to the adapter of the microwave coaxial sensor through a flexible coaxial radio frequency cable to collect signals. The computer is connected to the microwave network analyzer through a network cable to receive the collected signal data, and the signal data is fitted through programming to obtain accurate resonant frequency value and quality factor. The relationship between the change in the local dielectric constant distribution caused by defects and the change in the resonant frequency and quality factor can be expressed as: Among them, ω1, f1 and Q1 are the complex angular frequency, resonant frequency and unloaded quality factor in the defect-free case, ω2, f2 and Q2 are the complex angular frequency, resonant frequency and unloaded quality factor in the defect-containing case, ε and μ are the dielectric constant and permeability distribution of the filling material in the resonance region, respectively. and is the electric field and magnetic field distribution in the resonant region without defects; V C and V m are the volumes of the resonance area and the defect, respectively; Δε is the change in dielectric constant at the defect location.

10. The sensing system of a microwave coaxial sensor for detecting defects in polyethylene pipes according to claim 9, characterized in that: in, The computer reads the transmission coefficient S from the network analyzer through the SCPI language code of the programmable instrument standard command 21 Data, S 21 The value of and the frequency f satisfy the Lorentz equation: Where T = |S 21 | 2 To transfer energy, Q L is the loaded quality factor, and the unloaded quality factor Q U for:

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