A fiber grating detector based on additional lateral force loading

Abstract

A kind of fiber grating detector based on additional transverse force loading, base and hexagonal structure are integrated structure;Hexagonal structure top edge fixed mass block;Micro spiral column passes through the threaded hole of mass block and hexagonal top edge, and is threadedly connected;Hexagonal structure left and right sides are equipped with optical fiber hole;Fiber Bragg grating passes through the optical fiber hole of the left and right sides of hexagonal structure and is in contact with the bottom of micro spiral column;The detector takes hexagon as carrier, combines micro spiral column structure, when the detector is vibrated to generate axial strain, radial strain is loaded on fiber grating, the change of bragg grating center wavelength is detected in real time, which can realize the detection of external vibration signal by detector, and the advantage of transverse force can be maximized under the premise of special pre-tightening force loading.The detector can effectively widen the frequency band of the detector and improve the sensitivity by loading additional transverse force compared with not loading transverse force.The detector has the characteristics of simple structure, anti-electromagnetic interference, strong stability and the like.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber sensing, and particularly relates to a fiber grating detector based on additional transverse force loading. BACKGROUND

[0002] Petroleum and natural gas have become indispensable materials in the development of today's society. Energy supply shortage will become one of the great resistances of China's development, according to relevant experts. However, most of the oil and gas reservoirs with shallow burial and simple geological conditions have been discovered by exploration engineers. The current exploration task is to face some small structural oil and gas reservoirs with deep burial and complex geological conditions, which requires seismic data to have high rock resolution. Therefore, the difficulty of the exploration task increases rapidly. In the past half century, the most commonly used electric detector represented by the moving coil detector has essential shortcomings in detection sensitivity, response frequency band, multiplicity, spatial resolution, and anti-electromagnetic interference, which cannot meet the development of unconventional oil and gas reservoirs. Therefore, it is urgent to find a new detector in the field of seismic exploration.

[0003] The fiber Bragg grating type acceleration detector has unique advantages such as small size, light weight, high temperature resistance, corrosion resistance, and immunity to electromagnetic interference in the field of vibration testing. However, the increase of the resonance frequency will inevitably lead to the decrease of the sensitivity, which makes it a technical bottleneck in the development process. The resonance frequency of the currently reported medium and high frequency fiber grating type acceleration detector is about 1 kHz, and the sensitivity is 10-20 pm / g, which cannot meet the requirements of high-precision exploration in the well. Therefore, it is necessary to explore new methods and new structures to improve the sensitivity under the premise of wide frequency band. SUMMARY

[0004] In order to overcome the above-mentioned deficiencies of the prior art, the purpose of the present application is to provide a fiber grating detector based on additional transverse force loading, which overcomes the technical problems such as low sensitivity in wide frequency band, and adopts a special structure to maximize the advantage of transverse force under the premise of slight pre-tightening force, thereby improving the resonance frequency and sensitivity as much as possible.

[0005] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is:

[0006] A fiber grating detector based on additional transverse force loading, comprising a base (4), characterized in that the base (4) and the hexagonal structure (2) are a 3D printing integrated structure; the mass block (1) is fixed on the top edge of the hexagonal structure (2) through high-temperature resistant epoxy resin glue; the micro-spiral column body (5) penetrates the threaded hole of the mass block (1) and the top edge of the hexagonal structure (2) and is threadedly connected; the left and right sides of the hexagonal structure (2) are provided with fiber holes; the fiber Bragg grating (3) penetrates the fiber holes on the left and right sides of the hexagonal structure (2) and is in contact with the bottom of the micro-spiral column body (5); the fiber Bragg grating (3) is fixed on the two sides of the hexagonal structure (2) through epoxy resin glue; the left and right sides of the base (4) are provided with two threaded holes.

[0007] The diameter of the fiber hole of the hexagonal structure (2) is 0.1mm.

[0008] The inner angle of the left and right sides of the hexagonal structure (2) is 94 degrees.

[0009] The material of the hexagonal structure (2) is imported high-temperature-resistant black nylon (PA12).

[0010] The fiber ends need to be additionally provided with axial pre-tightening force to make the center wavelength drift range of the fiber grating between 0.01nm and 0.08nm.

[0011] The radius of the micro-spiral column body (5) is 1mm, the height is 8mm; the width of the mass block (1) is 11mm, the height is 3.9mm, the radius of the threaded hole is 1mm, and the depth is 3.9mm; the radius of the threaded hole penetrating the top edge of the hexagonal structure (2) is 1mm.

[0012] The micro-spiral column body (5) is in contact with the surface of the fiber stretched by weak axial pre-tightening force, so that the transverse force is more effectively loaded on the detector in the vibration process.

[0013] The length of the top edge of the hexagonal structure (2) is 11mm, the distance between the left and right top points is 20mm, the height is 10mm, the wall thickness is 0.87mm, and the overall thickness is 5mm.

[0014] The length of the base (4) is 30mm, the width is 5mm, the height is 3mm, and the two sides are fixed by bolts.

[0015] The beneficial effects of the present application are as follows:

[0016] The application uses special hexagonal transducer structure and micro spiral column structure to simultaneously load transverse force and radial force on the fiber grating when the detector is subjected to acceleration, and uses special hexagonal structure and prestress loading method to overcome the limitation of gravity inevitably bringing certain transverse prestress to the fiber grating when the traditional transverse force is loaded (depending on the weight of the fiber grating pendant), maximize the effect of transverse force, and further achieve the effect of widening the frequency band and improving the sensitivity.

[0017] The application uses hexagonal transducer structure to convert the transverse force of the mass block to the fiber grating into axial force to play the sensing mechanism, and uses micro spiral column to directly act the transverse force on the fiber grating, so that the strain of the FBG is effectively improved to further improve the sensitivity, and high sensitivity detection in the medium and high frequency band is more beneficial. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a structural schematic diagram of the application.

[0019] Figure 2 is a physical diagram of the application.

[0020] Figure 3 is a frequency measurement diagram of the fiber grating detector based on additional transverse force loading in examples 1 and 2.

[0021] Figure 4 is a sensitivity diagram of the fiber grating detector based on additional transverse force loading in example 1 in the flat area.

[0022] Figure 5 is a sensitivity diagram of the fiber grating detector based on additional transverse force loading in example 2 in the flat area. DETAILED DESCRIPTION

[0023] The application is further described in detail below in combination with the drawings and examples, but the application is not limited to the following embodiments.

[0024] As shown in Figure 1 , the detector is composed of a mass block (1), a hexagonal structure (2), a fiber Bragg grating (3), a base (4), and a micro spiral column (5). The mass block (1) has a threaded hole and is fixed to the top edge of the hexagonal structure (2) by high-temperature-resistant epoxy resin glue. The micro spiral column (5) passes through the threaded hole of the top edge of the hexagonal structure (2) and the threaded hole reserved in the mass block (1). The fiber Bragg grating (3) passes through the fiber holes reserved at the left and right vertices of the hexagonal structure (2), and is in contact with the micro spiral column (5) and is stretched by an axial pre-tightening force of 0.01-0.08 nm, and the left and right vertices of the hexagonal structure (1) are bonded by epoxy resin glue.

[0025] The working principle of the structure is as follows:

[0026] When the vibration signal acts on the detector, due to the connection of the top edge of the hexagon and the mass block and the micro-spiral column, the vibration signal will be transmitted to the hexagon and the micro-spiral column at the same time, the hexagon will be displaced up and down to bring the axial tension of the fiber Bragg grating, and the micro-spiral column will also extrude the FBG to bring the transverse tension, and the two tensions will combine to improve the strain of the FBG caused by the vibration, and the vibration signal can be detected by monitoring the center wavelength shift of the fiber Bragg grating.

[0027] The core problems of the method are: 1. The selected hexagonal structure should meet that the height of the micro-spiral column descending should be greater than the height of the axial tension of the left and right two points of the hexagon, so as to ensure the loading of the transverse force. 2. Due to the principle of energy conservation, the amplification coefficient of the transverse force should be as large as possible, and the sensitivity and resonance frequency of the detector as a whole should be maximized under the premise of ensuring the detection signal without distortion.

[0028] The detector takes a special hexagonal transducer structure as a carrier, and combines a micro-spiral column structure to load the radial strain on the fiber Bragg grating when the detector is subjected to axial strain caused by vibration, thereby improving the sensitivity. The hexagonal transducer structure provides axial tension during vibration, and the micro-spiral column provides transverse tension at the same time. The center wavelength change of the Bragg grating can be detected in real time to realize the detection of the external vibration signal by the detector. The special structure of the present application can maximize the advantage of the transverse force under the premise of special pre-tightening force. The comparison experiment proves that the additional loading of the transverse force on the detector can effectively widen the frequency band of the detector and improve the sensitivity compared with the case without loading the transverse force. The detector has the characteristics of simple structure, anti-electromagnetic interference, strong stability and the like.

[0029] Example 1

[0030] The hexagonal material is imported high-temperature-resistant black nylon PA12, and fiber holes are opened at the left and right top points of the hexagon. The fiber Bragg grating is inserted and in contact with the micro-spiral column, and the left and right sides of the hexagon are fixed by epoxy resin glue, so that the grating is on one side of the support. The grating area length of the grating written by femtosecond laser direct writing technology is 2mm, and the center wavelength is 1550nm. The fiber grating is stretched by a certain axial pre-tightening force while being fixed by epoxy resin glue, so that the wavelength drift is 0.06nm, that is, it is in a slightly stretched state. After the sensing unit is assembled, the whole is fixed on the vibration table by M2 bolts for testing.

[0031] Example 2

[0032] The hexagonal material is imported high-temperature-resistant black nylon PA12, optical fiber holes are opened at the left and right vertices of the hexagon, the fiber Bragg grating is inserted and fixed on the left and right sides of the hexagon by epoxy resin glue, the micro-spiral column is not added, the grating area length of the grating written by the femtosecond laser direct writing technology is 2 mm, the center wavelength is 1550 nm, the 3dB line width is 0.3 nm, the fiber grating is stretched by a certain axial pre-tightening force at the same time of being fixed by the epoxy resin glue, the wavelength drift is 1 nm, i.e. in the stretched state, the resonant frequency and the sensitivity under the loading of the axial force are tested, and the whole is fixed on the vibration table by M2 bolts for testing after the assembly of the sensing unit is completed.

[0033] Example 3

[0034] The hexagonal material is imported high-temperature-resistant black nylon PA12, optical fiber holes are opened at the left and right vertices of the hexagon, the fiber Bragg grating is inserted and fixed on the left and right sides of the hexagon by epoxy resin glue, the micro-spiral column is not added, the grating area length of the grating written by the femtosecond laser direct writing technology is 2 mm, the center wavelength is 1550 nm, the 3dB line width is 0.3 nm, the fiber grating is stretched by a certain axial pre-tightening force at the same time of being fixed by the epoxy resin glue, the wavelength drift is 1 nm, i.e. in the stretched state, the resonant frequency and the sensitivity under the loading of the axial force are tested, and the whole is fixed on the vibration table by M2 bolts for testing after the assembly of the sensing unit is completed.

[0035] In order to verify the beneficial effects of the present application, the inventors carried out the following tests on Example 1:

[0036] The fiber Bragg grating detector based on additional lateral force loading is fixed on the vibration table, the optical fiber is connected to the SM130 demodulator of Micro-Optics Company, and the amplitude-frequency characteristic and the sensitivity characteristic are measured. The frequency range of the amplitude-frequency characteristic test is 50-1250 Hz, the acceleration is 1g, the resonant frequency is 920 Hz when the lateral force and the radial force are loaded at the same time, and the resonant frequency is 750 Hz when only the axial force is loaded, and the results are as shown in Figure 3 ; the sensitivity test frequency is set to 300 Hz, 400 Hz and 500 Hz respectively, the acceleration is 5-15g, the highest sensitivity is 41.8pm / g, and the results are as shown in Figure 4 ; under the same conditions, the highest sensitivity in the flat area is 22.2pm / g when no lateral force is loaded, and the results are as shown in Figure 5 . The experimental phenomenon shows that the fiber Bragg grating detector based on additional lateral force loading has a wider frequency band and a sensitivity about 1 times higher than that of the single axial force loading.

[0037] Therefore, the present application can realize high-sensitivity detection of vibration signals, and through comparison experiments, it can be concluded that the method can effectively widen the frequency band and improve the sensitivity; moreover, the system has high reliability, and the sensing structure is simple, easy to cascade and reuse, and can be applied to the fields of oil and gas exploration, bridges, tunnels and the like.

Claims

1. A fiber Bragg grating detector based on additional lateral force loading, comprising a base (4), characterized in that: The base (4) is connected to the hexagonal structure (2); the mass block (1) is fixed on the top edge of the hexagonal structure (2); the micro-spiral column (5) passes through the threaded holes of the mass block (1) and the top edge of the hexagonal structure (2) and is threadedly connected; the left and right sides of the hexagonal structure (2) are provided with optical fiber holes; the fiber Bragg grating (3) passes through the optical fiber holes on the left and right sides of the hexagonal structure (2) and contacts the bottom of the micro-spiral column (5); the fiber Bragg grating (3) is fixed to the circular holes on both sides of the hexagonal structure (2) by epoxy resin glue; and two threaded holes are provided on the left and right sides of the base (4).

2. The fiber Bragg grating detector based on additional lateral force loading according to claim 1, characterized in that: The base (4) and the hexagonal structure (2) are a 3D printed integrated structure.

3. The fiber Bragg grating detector based on additional lateral force loading according to claim 1, characterized in that: The top edge of the hexagonal structure (2) is fixed to the mass block (1) via high-temperature resistant epoxy resin glue.

4. The fiber Bragg grating detector based on additional lateral force loading according to claim 1, characterized in that: The diameter of the optical fiber hole in the hexagonal structure (2) is 0.1 mm.

5. The fiber Bragg grating detector based on additional lateral force loading according to claim 1, characterized in that: The inner angles of the left and right sides of the hexagonal structure (2) are 94 degrees.

6. The fiber Bragg grating detector based on additional lateral force loading according to claim 1, characterized in that: The material of the hexagonal structure (2) is imported high-temperature resistant black nylon PA12.

7. The fiber Bragg grating detector based on additional lateral force loading according to claim 1, characterized in that: Additional axial preload is required at both ends of the optical fiber to keep the center wavelength of the fiber Bragg grating within the range of 0.01nm-0.08nm.

8. The fiber Bragg grating detector based on additional lateral force loading according to claim 1, characterized in that: The micro-spiral column (5) has a radius of 1 mm and a height of 8 mm; the mass block (1) has a width of 11 mm and a height of 3.9 mm, a threaded hole radius of 1 mm and a depth of 3.9 mm; and the threaded hole having a radius of 1 mm passing through the top edge of the hexagonal structure (2).

9. The fiber Bragg grating detector based on additional lateral force loading according to claim 1, characterized in that: The micro-helical column (5) contacts the surface of the optical fiber stretched with a weak axial pre-tightening force, so as to more effectively load the lateral force on the detector during the vibration process.

10. The fiber Bragg grating detector based on additional lateral force loading according to claim 1, characterized in that: The hexagonal structure (2) has an upper side length of 11 mm, a distance between left and right vertices of 20 mm, a height of 10 mm, a wall thickness of 0.87 mm, and an overall thickness of 5 mm; the base (4) has a length of 30 mm, a width of 5 mm, and a height of 3 mm, and is fixed on both sides by bolts.

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