Earthquake monitoring ultralow frequency acceleration sensor and design method thereof

Abstract

The invention discloses an earthquake monitoring ultralow frequency acceleration sensor and a design method thereof, the sensor comprises a cylindrical barrel, the upper surface of the cylindrical barrel is provided with an upper cover, and the lower surface of the cylindrical barrel is provided with a lower cover; the lower cover is connected with the support frame through a first support column; three single-component acceleration sensors are arranged in the cylindrical barrel; the supporting frame is divided into a first-layer tray, a second-layer tray and a third-layer tray from bottom to top, and the edges of the three trays are all connected with the second supporting column; a first single-component acceleration sensor is arranged on the first-layer tray, a second single-component acceleration sensor is arranged on the second-layer tray, and a third single-component acceleration sensor is arranged on the third-layer tray; the first single-component acceleration sensor is vertically installed on the first layer of tray, the second single-component acceleration sensor is horizontally installed on the second layer of tray, and the third single-component acceleration sensor is horizontally installed on the third layer of tray. The sensor is high in stability.

Description

Technical Field

[0001] The present invention relates to the technical field of earthquake monitoring, and in particular to an ultra-low frequency acceleration sensor for earthquake monitoring and a design method thereof. Background Art

[0002] Earthquake monitoring is crucial for revealing the internal structure of the Earth and for earthquake early warning, especially in the low-frequency range of 120 seconds (0.0083 Hz) to 20 Hz. Through the analysis of low-frequency seismic waves, scientists can gain a deep understanding of the structural characteristics of the Earth's interior and effectively identify early signals of earthquakes. This information is of great significance for improving the accuracy of earthquake prediction, formulating emergency response strategies, and mitigating the impact of earthquake disasters.

[0003] At present, the main sensors used for ultra-low frequency vibration monitoring are electrical accelerometers. However, traditional electrical accelerometers are susceptible to electromagnetic interference, corrosion, high cost, and wiring difficulties in actual projects. They are difficult to provide stable monitoring in complex environments, which brings bottlenecks to our ability to study the structure and dynamics of the earth and hinders us from obtaining rich geophysical and seismological information in remote and complex environmental areas. Overcoming this challenge is not only beneficial to basic research work, but also can monitor earthquakes in a timely manner and provide sufficient warnings for nearby areas.

[0004] Fiber optic sensors have the characteristics of anti-electromagnetic interference, high temperature and high pressure resistance, and corrosion resistance, and have received widespread attention. The current mainstream fiber optic sensors are distributed acoustic sensors (DAS), interferometric sensors, and fiber Bragg gratings (FBG). DAS can perform wide-area continuous monitoring and has high spatial resolution, which is suitable for real-time capture of seismic wave field changes, but the signal is complex and difficult to process effectively. Interferometric sensors are suitable for monitoring extremely weak vibration signals with extremely high precision, but their large-scale deployment is limited due to their high cost. In contrast, FBG sensors have shown special advantages in earthquake monitoring, such as easy networking, low deployment cost, and easy maintenance, and have gradually become a research hotspot.

[0005] In recent years, researchers have designed different types of FBG accelerometers. G. Gagliardi et al. used the most accurate H 13 C 14 Absolute wavelength calibration devices such as N standard gas cavities and optical frequency combs, combined with laser frequency-locked interrogation technology, can realize high-resolution low-frequency FBG resonant cavity wavelength measurement systems. However, in these systems, multiplexing of multi-component sensors is difficult. In addition, optical frequency combs are expensive, H 13 C 14N standard gas cell is affected by ambient temperature and is not suitable for multi-component low-frequency acceleration sensor design. Wang Zilin et al. proposed a single-component hollow triangular beam-assisted detector based on FBG, with an operating frequency band of 0.6-50.0Hz, which has a good linear relationship with the vibration signal frequency. The mechanical structure of the detector has good lateral anti-sensitivity, and the lateral sensitivity in the X and Y axis directions is 2.8% and 4.6%, but the sensitivity is low at only 120.1pm / g. Teng Yuntian et al. proposed a dual straight wing FBG accelerometer for low-frequency vibration measurement, with an operating frequency band of 5-22Hz and a sensitivity of 171.7pm / g, but the lateral sensitivity is relatively large at 9.5%. When measuring the specified direction signal, the sensor will be interfered by the lateral signal, thereby introducing errors and reducing the signal-to-noise ratio, making it more difficult to separate the actual measured low-frequency signal from the noise. Zhang Faxiang et al. designed a broadband high-sensitivity FBG acceleration sensor based on a double diaphragm H-shaped hinge, breaking through the limitation between sensitivity and resonant frequency. The working frequency band is 1-1000Hz, the sensitivity is 32dB re.pm / g, the lateral sensitivity is less than -26dB, and the acceleration resolution is 15μg / kHz. The structure is relatively complex and not easy to process and assemble. Although the above research results have promoted the development of the field of low-frequency vibration measurement to a certain extent. However, the current FBG accelerometer has the problems of low sensitivity and poor lateral anti-interference performance, which limits the pickup of low-frequency signals of FBG accelerometers. Summary of the invention

[0006] In order to solve the deficiencies of the prior art, the present invention provides an earthquake monitoring ultra-low frequency acceleration sensor and a design method thereof;

[0007] On the one hand, an ultra-low frequency acceleration sensor for earthquake monitoring is provided, comprising:

[0008] A cylindrical barrel, wherein an upper cover is disposed on the upper surface of the cylindrical barrel and a lower cover is disposed on the lower surface of the cylindrical barrel;

[0009] The lower cover is connected to the support frame through a first support column; three single-component acceleration sensors are arranged in the cylindrical barrel;

[0010] The support frame is divided into a first layer tray, a second layer tray and a third layer tray from bottom to top, and the edges of the three trays are connected to the second support column;

[0011] The first layer of trays is provided with a first single-component acceleration sensor, the second layer of trays is provided with a second single-component acceleration sensor, and the third layer of trays is provided with a third single-component acceleration sensor;

[0012] The first single-component acceleration sensor is vertically mounted on the first tray, the second single-component acceleration sensor is horizontally mounted on the second tray, and the third single-component acceleration sensor is horizontally mounted on the third tray;

[0013] The center line of the second single-component acceleration sensor in the length direction is perpendicular to the center line of the third single-component acceleration sensor in the length direction, the center line of the second single-component acceleration sensor in the length direction is perpendicular to the center line of the first single-component acceleration sensor in the length direction, and the center line of the first single-component acceleration sensor in the length direction is perpendicular to the center line of the third single-component acceleration sensor in the length direction.

[0014] On the other hand, a design method for an ultra-low frequency acceleration sensor for earthquake monitoring is provided, including: the ultra-low frequency acceleration sensor for earthquake monitoring is referred to as an acceleration sensor;

[0015] An equivalent model of the acceleration sensor is constructed, and formulas of the resonant frequency and sensitivity are derived, and several key parameters that affect the resonant frequency and sensitivity of the sensor are found from the formulas of the resonant frequency and sensitivity; the several key parameters include: the mass of the mass block, the length of each flexible beam, the width of each flexible beam, and the thickness of each flexible beam;

[0016] Based on the key parameters, a model to be optimized is established; the model to be optimized includes: an objective function and constraint conditions, and a genetic algorithm is used for optimization to obtain the optimal value of each key parameter;

[0017] Based on the optimal value of each key parameter, the acceleration sensor is simulated and analyzed to obtain the difference between the first-order modal frequency and the second-order modal frequency of the sensor. If the difference is greater than a set threshold, it means that the sensor can resist lateral interference.

[0018] A performance test experiment is performed on the acceleration sensor; wherein the performance test experiment parameters include: amplitude-frequency response, low-frequency output response, sensitivity, lateral sensitivity, sensitivity angle and dynamic range; if the values ​​of all experimental parameters meet the set requirements, it means that the test experiment of the acceleration sensor has passed.

[0019] The above technical solution has the following advantages or beneficial effects:

[0020] Based on the development of a variety of low-frequency FBG accelerometers, the present invention proposes a high-sensitivity ultra-low-frequency FBG accelerometer for earthquake monitoring. The sensor adopts a multi-flexible beam structure that can effectively realize the pickup of low-frequency signals, and the overall structure is simple and the integrated design and processing are used to reduce the impact of assembly, effectively improving the consistency and stability of the sensor. Glass powder packaging is used to improve the durability, reliability and stability of the sensor in harsh environments. The differential evolution genetic algorithm (DE) is used to achieve global optimization of the sensitivity of the structural dimensions under multiple constraints, significantly improving the sensitivity of the sensor. For the first time, the self-noise test based on the three-channel correlation method is discussed on the FBG sensor, and the FBG accelerometer of the present invention is compared with the mainstream commercial electrical seismic accelerometer G1B83 to verify the actual low-frequency performance of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0022] Figure 1 , Figure 2 and Figure 3 Schematic diagram of a single-component sensor according to Embodiment 1;

[0023] Figure 4 and Figure 5 Schematic diagram of a three-component sensor according to Embodiment 1;

[0024] Figure 6 Schematic diagram of the sensor structure equivalent model of embodiment 1;

[0025] Figure 7 This is a schematic diagram of the optimization results of the sensor structure parameters of Example 1;

[0026] FIG8( a ) is a schematic diagram of the first-order modal analysis results of Example 1;

[0027] FIG8( b ) is a schematic diagram of the second-order modal analysis results of Example 1;

[0028] FIG9( a) is a schematic diagram of a vibration table system of Example 1;

[0029] FIG9( b ) is a schematic diagram of an optical fiber sensing system according to Embodiment 1;

[0030] Fig.10 This is the overall waveform diagram of the X, Y and Z axes of Example 1;

[0031] Fig.11 is a schematic diagram of an amplitude-frequency response curve of Embodiment 1;

[0032] FIG12( a ) is the response of Example 1 at a frequency of 0.05 Hz;

[0033] FIG12( b ) is the response of Example 1 at a frequency of 0.5 Hz;

[0034] FIG12( c ) is the response of Example 1 at a frequency of 1 Hz;

[0035] FIG12( d ) is the response of Example 1 at a frequency of 10 Hz;

[0036] Fig.13 The sensor sensitivity and lateral sensitivity ratio of the first embodiment;

[0037] Fig.14 Angular dependence of wavelength drift of the fiber Bragg grating vibration sensor in the polar coordinate system of Example 1;

[0038] Fig.15 0.01-1 Hz self-noise power spectrum of Example 1;

[0039] Fig.16 This is a comparative observation experiment scene diagram of Example 1;

[0040] FIG17( a ) is a time domain diagram of the output response comparison experiment of Example 1;

[0041] FIG17( b ) is a frequency domain diagram of the output response comparison experiment diagram of Example 1;

[0042] Among them, 1, upper cover, 2, third single-component acceleration sensor, 3, second single-component acceleration sensor, 4, first single-component acceleration sensor, 5, second support column, 6, lower cover, 7, threaded hole, 8, optical fiber, 9, mass block, 10, first line segment, 11, second line segment, 12, third line segment, 13, fourth line segment, 14, fifth line segment, 15, sixth line segment, 16, seventh line segment, 17, eighth line segment, 18, ninth line segment, 19, tenth line segment, 20. the eleventh line segment, 21. the twelfth line segment, 22. the thirteenth line segment, 23. the fourteenth line segment, 24. the fifteenth line segment, 25. the sixteenth line segment, 26. the seventeenth line segment, 27. the eighteenth line segment, 28. the nineteenth line segment, 29. the twentieth line segment, 30. the twenty-first line segment, 31. the twenty-second line segment, 32. the first arc-shaped groove, 33. the first rectangular groove, 34. the second rectangular groove, 35. the third rectangular groove, 36. the second arc-shaped groove. DETAILED DESCRIPTION

[0043] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0044] Seismic monitoring is essential for revealing the internal structure of the earth, especially in the ultra-low frequency range of 120 seconds (0.0083 Hz) to 20 Hz. Although existing electrical instruments can effectively pick up these low-frequency signals, they face the challenges of high cost and difficult deployment in complex environments. In contrast, FBG accelerometers have a strong ability to adapt to complex environments, but have significant deficiencies in ultra-low frequency signal detection. The present invention designs a high-sensitivity ultra-low frequency FBG accelerometer based on a multi-flexible beam structure. First, an elastic body model of the sensor is established, and the formulas for sensitivity and resonant frequency are derived. Secondly, by applying the differential evolution genetic algorithm, the global optimization of sensitivity under multiple constraints is achieved, and modal simulation analysis is performed to verify the effectiveness of the model. Finally, based on the optimization results, a physical sensor is developed and a low-frequency vibration test system is built to test its performance. Experiments show that the FBG accelerometer has a flat response characteristic in the frequency range of 0.05 to 80 Hz, a sensitivity of 1025 pm / g, a lateral sensitivity ratio of less than 4.10%, and a dynamic range of 97 dB. In addition, the three-channel correlation method was used for the first time on the FBG accelerometer to test the seismograph self-noise, and compared with the mainstream seismic accelerometer to evaluate the actual performance of the sensor. The research results are expected to be used for large-scale low-frequency earthquake monitoring in complex environments.

[0045] Embodiment 1: This embodiment provides an ultra-low frequency acceleration sensor for earthquake monitoring;

[0046] like Figure 4 and Figure 5 As shown, the earthquake monitoring ultra-low frequency acceleration sensor comprises:

[0047] A cylindrical barrel, wherein an upper cover 1 is disposed on the upper surface of the cylindrical barrel, and a lower cover 6 is disposed on the lower surface of the cylindrical barrel;

[0048] The lower cover is connected to the support frame through a first support column; three single-component acceleration sensors are arranged in the cylindrical barrel;

[0049] The support frame is divided into a first layer tray, a second layer tray and a third layer tray from bottom to top, and the edges of the three trays are connected to the second support column 5;

[0050] The first layer of pallets is provided with a first single-component acceleration sensor 4, the second layer of pallets is provided with a second single-component acceleration sensor 3, and the third layer of pallets is provided with a third single-component acceleration sensor 2;

[0051] The first single-component acceleration sensor is vertically mounted on the first tray, the second single-component acceleration sensor is horizontally mounted on the second tray, and the third single-component acceleration sensor is horizontally mounted on the third tray;

[0052] The center line of the second single-component acceleration sensor in the length direction is perpendicular to the center line of the third single-component acceleration sensor in the length direction, the center line of the second single-component acceleration sensor in the length direction is perpendicular to the center line of the first single-component acceleration sensor in the length direction, and the center line of the first single-component acceleration sensor in the length direction is perpendicular to the center line of the third single-component acceleration sensor in the length direction.

[0053] Furthermore, if Figure 1 , Figure 2 and Figure 3 As shown, the internal structures of the first single-component acceleration sensor, the second single-component acceleration sensor and the third single-component acceleration sensor are consistent, and the first single-component acceleration sensor includes:

[0054] A metal block, wherein a plurality of etching lines and a plurality of threaded holes 7 are provided on the front side of the metal block;

[0055] The front side of the metal block is divided into the first area, the second area and the third area from top to bottom;

[0056] The first area is provided with a first etching line, a second etching line, a third etching line and a fourth etching line; a threaded hole is provided at the center of the first area;

[0057] The first etching line is arranged outside the second etching line, the first etching line is a gate shape, the second etching line is a gate shape, the third etching line is an L shape, and the fourth etching line is an L shape;

[0058] The first etching line comprises: a first line segment 10, a second line segment 11 and a third line segment 12 connected in sequence;

[0059] The second etching line includes: a fourth line segment 13, a fifth line segment 14 and a sixth line segment 15 connected in sequence;

[0060] The third etching line includes: a seventh line segment 16 and an eighth line segment 17 connected in sequence;

[0061] A fourth etching line, comprising: a ninth line segment 18 and a tenth line segment 19 connected in sequence;

[0062] The seventh line segment is arranged between the first line segment and the fourth line segment, the seventh line segment and the first line segment are parallel to each other, and the seventh line segment and the fourth line segment are parallel to each other;

[0063] The area between the first line segment and the seventh line segment is called the first flexible beam, and the area between the seventh line segment and the seventh line segment is called the second flexible beam;

[0064] The ninth line segment is set between the third line segment and the sixth line segment, the ninth line segment and the third line segment are parallel to each other, and the ninth line segment and the sixth line segment are parallel to each other;

[0065] The area between the third line segment and the ninth line segment is called a third flexible beam, and the area between the ninth line segment and the sixth line segment is called a fourth flexible beam.

[0066] The third region is provided with a fifth etching line, a sixth etching line, a seventh etching line and an eighth etching line;

[0067] The fifth etching line is arranged outside the sixth etching line, the fifth etching line is in the shape of an inverted gate, the sixth etching line is in the shape of an inverted gate, the seventh etching line is in the shape of an L, and the eighth etching line is in the shape of an L;

[0068] A fifth etching line, comprising: an eleventh line segment 20, a twelfth line segment 21 and a thirteenth line segment 22 connected in sequence;

[0069] The sixth etching line includes: a fourteenth line segment 23, a fifteenth line segment 24 and a sixteenth line segment 25 connected in sequence;

[0070] The seventh etching line includes: a seventeenth line segment 26 and an eighteenth line segment 27 connected in sequence;

[0071] An eighth etching line, comprising: a nineteenth line segment 28 and a twentieth line segment 29 connected in sequence;

[0072] The seventeenth line segment is disposed between the eleventh line segment and the fourteenth line segment, the seventeenth line segment and the eleventh line segment are parallel to each other, and the seventeenth line segment and the fourteenth line segment are parallel to each other;

[0073] The area between the eleventh line segment and the seventeenth line segment is called the fifth flexible beam, and the area between the seventeenth line segment and the fourteenth line segment is called the sixth flexible beam;

[0074] The nineteenth line segment is disposed between the thirteenth line segment and the sixteenth line segment, the nineteenth line segment and the thirteenth line segment are parallel to each other, and the nineteenth line segment and the sixteenth line segment are parallel to each other;

[0075] The area between the nineteenth line segment and the thirteenth line segment is called the seventh flexible beam, and the area between the nineteenth line segment and the sixteenth line segment is called the eighth flexible beam;

[0076] An end of the eighth line segment away from the seventh line segment is connected to an end of the eighteenth line segment away from the seventeenth line segment through the twenty-first line segment 30;

[0077] One end of the tenth line segment away from the ninth line segment is connected to one end of the twentieth line segment away from the nineteenth line segment through the twenty-second line segment 31 .

[0078] Further, the second region of the metal block is provided with a first arc-shaped groove 32, a first rectangular groove 33, a second rectangular groove 34, a third rectangular groove 35 and a second arc-shaped groove 36 which are connected in sequence;

[0079] The groove surface of the first arc-shaped groove, the groove surface of the second rectangular groove and the groove surface of the second arc-shaped groove are on the same plane;

[0080] The groove surface of the first rectangular groove and the groove surface of the third rectangular groove are on the same plane;

[0081] The groove surface of the first rectangular groove is lower than the groove surface of the first arc-shaped groove.

[0082] Furthermore, an optical fiber 8 is arranged on the second area of ​​the metal block, and the arrangement direction of the optical fiber is arranged along the direction of the first arc-shaped groove, the first rectangular groove, the second rectangular groove, the third rectangular groove and the second arc-shaped groove. The optical fiber is bonded with glass powder, and the optical fiber arranged in the second rectangular groove is provided with a grating.

[0083] The optical fiber is suspended on the first rectangular groove, and the optical fiber is suspended on the third rectangular groove. In other words, the optical fiber is not in contact with the surface of the first rectangular groove, and the optical fiber is not in contact with the surface of the third rectangular groove.

[0084] Further, the area enclosed by the first etching line, the third etching line, the fourth etching line, the fifth etching line, the seventh etching line, the eighth etching line, the twenty-first line segment and the twenty-second line segment of the metal block is the mass block 9.

[0085] Furthermore, both ends of the optical fiber are connected to a demodulator, and the demodulator is connected to a host computer.

[0086] When an earthquake occurs, the mass block shakes, and the optical fiber sends the captured motion signal to the demodulator, which then sends it to the host computer.

[0087] The low-frequency FBG acceleration sensor for earthquake monitoring consists of three mutually perpendicular single-component sensors. The single-component vibration sensor is mainly composed of a mass block, FBG, slots, and glass powder. Figure 1As shown. The sensor uses an inertial sensitive structure as an elastic sensitive element. By appropriately increasing the length, width and thickness, the sensor resonant frequency is reduced and the sensitivity is increased. The displacement amplitude of the mass block is controlled to prevent the FBG from being pulled apart, thereby avoiding excessive sensitivity of the structure. The design of the notch facilitates the pasting of high-temperature glass powder and the protection of optical fiber. The glass powder has good compatibility with the optical fiber material of the FBG and can form a solid bonding interface, thereby improving the durability and stability of the sensor in harsh environments. The overall structure is simple and the integrated design effectively improves the consistency, stability and reliability of the sensors by reducing the impact of assembly, making it more suitable for low-frequency earthquake monitoring in complex environments. A simple equivalent model of the vibration sensor structure is shown as Figure 2 shown.

[0088] Embodiment 2 provides a design method for an ultra-low frequency acceleration sensor for earthquake monitoring, including: the ultra-low frequency acceleration sensor for earthquake monitoring is referred to as an acceleration sensor;

[0089] S201: constructing an equivalent model of the acceleration sensor, and deriving formulas for the resonant frequency and sensitivity, and finding out several key parameters that affect the resonant frequency and sensitivity of the sensor from the formulas for the resonant frequency and sensitivity; the several key parameters include: the mass of the mass block, the length of each flexible beam, the width of each flexible beam, and the thickness of each flexible beam;

[0090] S202: Based on the key parameters, a model to be optimized is established; the model to be optimized includes: an objective function and constraint conditions, and a genetic algorithm is used for optimization to obtain the optimal value of each key parameter;

[0091] S203: Based on the optimal value of each key parameter, a simulation analysis is performed on the acceleration sensor to obtain a difference between a first-order modal frequency and a second-order modal frequency of the sensor. If the difference is greater than a set threshold, it indicates that the sensor can resist lateral interference.

[0092] S204: Perform a performance test experiment on the acceleration sensor; wherein the performance test experiment parameters include: amplitude-frequency response, low-frequency output response, sensitivity, lateral sensitivity, sensitivity angle and dynamic range; if the values ​​of all experimental parameters meet the set requirements, it means that the test experiment of the acceleration sensor has passed.

[0093] Furthermore, the S201: constructing an equivalent model of the acceleration sensor and deriving formulas for the resonance frequency and sensitivity include:

[0094] Depend on Figure 6It can be seen that the mass block is stimulated by external vibration with a vertical acceleration of a to produce longitudinal displacement and drive the fiber Bragg grating FBG to compress or stretch. Assuming that the displacement of the fiber Bragg grating FBG is the same as the displacement of the mass block, the length of the flexible beam is l, the width is w, and the thickness is t, then the equivalent stiffness K of the system is:

[0095]

[0096] The resonant frequency f is:

[0097]

[0098] The relative displacement of the mass block relative to the initial position is Δx, then Δx is:

[0099]

[0100] The strain Δε generated by the fiber Bragg grating FBG is:

[0101]

[0102] The fiber Bragg grating (FBG) generates strain, and its central wavelength changes accordingly. According to the coupled mode theory, if the temperature change can be ignored, the central wavelength change Δλ is:

[0103] Δλ=λ(1-p e )Δε (5)

[0104] λ is the center wavelength of the fiber Bragg grating FBG, P e is the effective elastic-optical coefficient of the optical fiber.

[0105] Then the sensor sensitivity S is:

[0106]

[0107] Furthermore, several key parameters affecting the resonant frequency and sensitivity of the sensor are found from the formula of resonant frequency and sensitivity; the several key parameters include: the mass of the mass block, the length distance of the flexible beam, the width of the flexible beam and the thickness of the flexible beam.

[0108] In order to measure low-frequency vibration signals, it is necessary to ensure that the sensor has a flat sensitivity response in the lower frequency band. From equations (2) and (6), it can be seen that the mass M of the sensor's mass block, the length of the flexible beam l, the width w, and the thickness t jointly determine the sensor's resonant frequency and sensitivity. Therefore, the above key parameters are optimized and the optimized model is simulated and analyzed.

[0109] In order to meet the requirements of high sensitivity and wide flat band of the sensor during monitoring, the sensor structure needs to be further optimized. Since the control variable method optimization is prone to fall into local optimality, the differential evolution DE algorithm is used to perform global optimization on the nonlinear programming problem of sensor size and sensitivity. It can be seen from equations (2) and (6) that the sensitivity S and the resonant frequency f depend on these key parameters, including the mass M, length l, width w, and thickness t of the mass block.

[0110] Further, S202: based on the key parameters, a model to be optimized is established; the model to be optimized includes: an objective function and constraint conditions, including:

[0111]

[0112]

[0113] The operation parameters were set before algorithm optimization, including the population size of 15, the scaling factor of 0.5, the crossover probability of 0.7, and the tolerance of 1×10 -6 . The optimization results of sensor structure parameters are as follows: Figure 7 shown.

[0114] Further, S202: using a genetic algorithm to optimize and obtain the optimal value of each key parameter, including:

[0115] The optimal value of the mass M of the mass block is 48.5 g, the optimal value of the length l is 21 mm, the optimal value of the width w is 1 mm, and the optimal value of the thickness t is 46 mm.

[0116] It can be seen from the figure that after 350 iterations, the convergence reached a maximum sensitivity of 1280 pm / g. The specific dimensions are shown in Table 1.

[0117] Table 1. Sensor parameters

[0118]

[0119]

[0120] Exemplarily, S203: based on the optimal value of each key parameter, simulation analysis is performed on the acceleration sensor to obtain the difference between the first-order modal frequency and the second-order modal frequency of the sensor. If the difference is greater than a set threshold, it indicates that the sensor can resist lateral interference, including:

[0121] The results of modal analysis of the sensor model are shown in Figure 8(a) and Figure 8(b) according to the relevant parameters in Table 1. As shown in Figure 8(a) and Figure 8(b), the first-order modal frequency of the sensor is 339.3 Hz, the second-order modal frequency is 1741.3 Hz, and the frequency difference between the first-order and second-order modes of the sensor is significantly increased, indicating that the structure can effectively reduce lateral interference.

[0122] Furthermore, S204: performing a performance test experiment on the acceleration sensor; wherein the performance test experiment parameters include: amplitude-frequency response, low-frequency output response, sensitivity, lateral sensitivity, sensitivity angle and dynamic range; if the values ​​of all experimental parameters meet the set requirements, it means that the test experiment of the acceleration sensor has passed.

[0123] Furthermore, the S204: performing a performance test experiment on the acceleration sensor includes: building a low-frequency test system, the low-frequency test system includes: a computer end is connected to a vibration table through a power amplifier, and a three-component fiber grating vibration sensor is placed on the vibration table; wherein the vibration table is composed of a horizontal and a vertical vibration table, and the minimum measurement frequency is 0.01 Hz; the three-component fiber grating vibration sensor is connected to a host computer through a demodulator.

[0124] In order to test the low-frequency performance of the sensor, a low-frequency vibration test system was built. The system mainly consists of a vibration table system and a fiber optic sensing system, as shown in Figure 9(a) and Figure 9(b). The vibration table system consists of a vibration table and a vibration control system. The vibration table consists of a horizontal and vertical vibration table from the Institute of Geophysics, China Earthquake Administration. The minimum measurement frequency is 0.01 Hz. The specific low-frequency vibration table performance indicators are shown in Table 2. The vibration table control system mainly consists of a vibration standard device measurement and control instrument, a standard vibration table power amplifier, a low-frequency vibration table motion tester, etc. The fiber optic sensing system consists of an improved MOI-si255 demodulator and a sensor.

[0125] Table 2 Performance indicators of low frequency vibration table

[0126] name Vertical vibration table Horizontal vibration table Frequency range 0.01-160 Hz 0.01-160 Hz Maximum displacement 0.15 m(pp) 0.15 m(pp) Maximum acceleration at full load <![CDATA[4.9 m / s 2 ]]> <![CDATA[4.9 m / s 2 ]]> Maximum load 18 kg 15 kg Distortion Displacement distortion ≤1% Displacement distortion ≤1% Acceleration lateral vibration ratio ≤3％ ≤3％

[0127] Furthermore, the amplitude-frequency response performance test experiment of the acceleration sensor is carried out, specifically including:

[0128] The amplitude-frequency characteristic reflects the response strength and stability of the sensor at different frequencies. The sensor is fixed on the horizontal and vertical vibration table, and the actual acceleration value of the vibration table is collected in real time by the laser absolute method. At the same time, the wavelength change of the FBG sensor collected by the FBG demodulator is recorded. The vibration table selects an acceleration value of 0.2g within the range of 2.0 to 100Hz. When it is in the range of 0.01 to 2.0Hz, the maximum amplitude of the low-frequency vibration table is limited, and the vibration table cannot provide an acceleration of 0.2g. After experimental testing on the vertical vibration table, the overall three-axis waveform diagram is obtained, as shown in the figure. Fig.10 shown.

[0129] The amplitude-frequency response of the sensor is obtained by calculating the overall amplitude of each X, Y, and Z axis, such as Fig.11 As shown in the figure, the sensor can output a relatively stable wavelength change within 0.05~80Hz, that is, 0.05~80Hz is the response frequency band of the sensor. The amplitude-frequency response of the X, Y, and Z axes in the sensor is consistent, indicating that the sensor has good directivity and frequency response characteristics, and can provide accurate and reliable three-dimensional measurement data in actual low-frequency signal measurement. Due to the limitation of the vibration table amplitude, the vibration table can only apply an acceleration of less than 10ug below 0.02Hz, and the sensor cannot accurately pick up this signal at present.

[0130] Furthermore, a low-frequency output response performance test experiment is conducted on the acceleration sensor, specifically including:

[0131] The frequency of seismic signals is concentrated below 20 Hz, so the monitoring capability of the sensor in this frequency band is crucial. From the time domain diagram of each frequency band below 20 Hz on the Z axis, it can be seen from Figure 12(a) and Figure 12(b) that the sensor can accurately capture signals with longer periods at the response frequencies of 0.05 Hz and 0.5 Hz, showing good low-frequency sensitivity. As the frequency increases, the 1 Hz signal in Figure 12(c) and the 10 Hz signal in Figure 12(d) show the stable response of the sensor to higher low-frequency signals, and the waveform maintains a clear sinusoidal shape, indicating that the sensor has excellent frequency response characteristics in the frequency band below 10 Hz. The results show that the accelerometer is suitable for signal detection within the seismic frequency range and can reliably capture and characterize the dynamic changes of low-frequency seismic signals.

[0132] Furthermore, the sensitivity and lateral sensitivity performance test experiments of the acceleration sensor are carried out, specifically including:

[0133] The sensors were fixed on the horizontal and vertical vibration tables respectively, and an excitation frequency of 2 Hz and an acceleration of 0.2 m / s were applied. 2 As the initial value, 0.2m / s 2Step size, excite the sine wave test sensor within the acceleration range, make it vibrate along the Z direction, and obtain the lateral sensitivity ratio of the optical fiber under the excitation component, such as Fig.13 shown.

[0134] Depend on Fig.13 It can be seen that when the applied frequency is 1Hz and the acceleration is 2m / s 2 When the vibration signal acts on the vibration sensor, the sensitivity of the sensor on the Z-axis component is 1025pm / g, which is consistent with the optimization result of 1280pm / g. The lateral sensitivity of the X and Y axes is 35pm / g and 42pm / g, and the X and Y lateral sensitivity ratios are 3.41% and 4.09% respectively, indicating that the sensor has a lower lateral sensitivity ratio while maintaining a high sensitivity.

[0135] Furthermore, the sensitivity angle performance test experiment of the acceleration sensor is carried out, specifically including:

[0136] Apply a horizontal excitation with a fixed frequency of 2Hz and an acceleration of 0.2g on a horizontal vibration table, and rotate the sensor angle in steps of 15° to test the angle dependence of the sensor sensitivity. Record the sensitivity at different angles in the X-axis and Y-axis directions, such as Fig.14 shown.

[0137] Since the acceleration provided by the vibration table remains constant in the vertical direction, assuming that the angle between the acceleration a and the two orthogonal axes of the optical fiber vibration sensor is θ, the equivalent acceleration applied to the x and y axes is ax = acosθ, ay = acosθ, that is, the "8" directionality in polar coordinates, which proves that the sensor has high sensitivity, anti-lateral interference characteristics, consistency and directional response.

[0138] Furthermore, a dynamic range performance test experiment is conducted on the acceleration sensor, specifically including:

[0139] The dynamic range usually reflects the sensor's ability to receive strong and weak signals. The dynamic range of the sensor and the acceleration dynamic range depend on the ratio of the maximum acceleration value to the minimum acceleration value, as shown in formula (9).

[0140]

[0141] To verify the minimum acceleration of the sensor, the sensor is placed in a cave. The temperature and humidity in the cave change little and the test environment can isolate unnecessary vibration and noise from the outside world, ensuring that the acceleration sensor only measures the sensor background noise. Three identical sensors are placed on the pendulum for testing. The test uses a three-channel correlation analysis method noise test to measure the minimum acceleration of the sensor.

[0142] At present, the test of self-noise is carried out with two identical seismographs with the same transfer function. However, the transfer function of FBG accelerometers is difficult to obtain. When the transfer function of the seismic sensor is unknown, three seismographs are placed at a distance less than the set distance and in the same direction, and it is assumed that the vibration amount at the same location is observed. The three-channel correlation analysis method is used to measure the time domain noise signal of the accelerometer.

[0143] Self-noise power spectrum N of FBG accelerometer ii for

[0144]

[0145] Where i, j, k = 1, 2, 3, and i≠j≠k, where P ii is the self-spectrum of the FBG acceleration sensor, numbered as P ik , P jk , P jk is the cross spectrum between two different FBG accelerometers, H i is the frequency response.

[0146] At the same time, in order to reduce the interference of the FBG acceleration sensor, the sensor is connected to a wavelength demodulator more than 20 meters away, and the demodulator sampling rate is set to 20Hz. 0.01-1Hz self-noise power spectrum, such as Fig.15 shown.

[0147] Depend on Fig.15 It can be seen that the value is better than 27μg / Hz in the range of 0.1~1Hz. 1 / 2 The experiment shows that the minimum wavelength change of the vibration signal is 27μg, and the maximum acceleration of the FBG central wavelength in the flat range is 2g. According to formula (9), the dynamic range of the FBG vibration sensor is calculated to be 97.47dB.

[0148] In order to demonstrate the low-frequency vibration measurement capability of the developed sensor, the performance values ​​of the sensor are compared with those of FBG vibration sensors described in the literature, as shown in Table 3.

[0149] Table 3. Comparison of FBG vibration sensors

[0150]

[0151] In order to verify the performance of our accelerometer in practical applications, we use FBG accelerometer and accelerometer G1B83 to measure external vibration. The performance indicators of G1B83 are shown in Table 4. Comparison of experimental scenarios, such as Fig.16Two acceleration sensors were placed on the observatory's pendulum for comparison testing. External vibrations were simulated by hitting them with a hammer outside the observatory and recording the conditions of the two acceleration sensors.

[0152] Table 4. Main performance indicators of G1B83

[0153]

[0154] The output response of the FBG accelerometer and the G1B83 sensor on the X, Y, and Z axes. As shown in Figure 17(a), the outputs of both sensors are maintained at a level close to zero from 0 to 15 seconds, indicating that there is no significant change in acceleration. In the time interval of 15 to 20 seconds, obvious acceleration changes occurred in all axes. The response trends of the two sensors are generally consistent, showing good synchronization, especially when the acceleration changes greatly. The frequency spectrum of the Z-axis acceleration of the two accelerometers is processed using fast Fourier transform (FFT), as shown in Figure 17(b). Both accelerometers show that the main frequency and amplitude are 17.59Hz, 0.001g, and the frequency spectra of the two accelerometers are basically consistent. The results show that the FBG and G1B83 sensors can effectively detect acceleration changes in different axes, verifying the reliability and consistency of the two in acceleration measurement.

[0155] In view of the problem that the current FBG accelerometers are difficult to effectively monitor ultra-low frequency vibration signals, the present invention proposes a high-sensitivity low-frequency FBG accelerometer with a multi-flexible beam structure. First, an elastic body model is established and the sensitivity and resonant frequency formulas are derived; secondly, a differential evolution genetic algorithm is used to achieve global optimization under multiple constraints, and modal simulation analysis is performed to verify the results; finally, a physical object is developed and a low-frequency vibration test system is built to test the performance of the sensor. Experimental results show that the flat range of the vibration sensor is 0.05-80Hz, and the sensitivity is 1025pm / g. The lateral sensitivity ratio is 4.1%, and the noise equivalent acceleration in the flat range is no more than 27μg / Hz 1 / 2 , dynamic range is better than 97dB, and has good sensitivity angle correlation characterization. In addition, the three-channel correlation method was applied to the FBG accelerometer for the first time to test the seismograph self-noise, and compared with the mainstream seismic accelerometer to evaluate the actual performance of the sensor. The research results are expected to be used for large-scale low-frequency earthquake monitoring in complex environments.

[0156] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. 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. Earthquake monitoring ultra-low frequency acceleration sensor, characterized by: include: A cylindrical barrel, wherein an upper cover is disposed on the upper surface of the cylindrical barrel and a lower cover is disposed on the lower surface of the cylindrical barrel; The lower cover is connected to the support frame through a first support column; three single-component acceleration sensors are arranged in the cylindrical barrel; The support frame is divided into a first layer tray, a second layer tray and a third layer tray from bottom to top, and the edges of the three trays are connected to the second support column; The first layer of trays is provided with a first single-component acceleration sensor, the second layer of trays is provided with a second single-component acceleration sensor, and the third layer of trays is provided with a third single-component acceleration sensor; The first single-component acceleration sensor is vertically mounted on the first tray, the second single-component acceleration sensor is horizontally mounted on the second tray, and the third single-component acceleration sensor is horizontally mounted on the third tray; The center line of the second single-component acceleration sensor in the length direction is perpendicular to the center line of the third single-component acceleration sensor in the length direction, the center line of the second single-component acceleration sensor in the length direction is perpendicular to the center line of the first single-component acceleration sensor in the length direction, and the center line of the first single-component acceleration sensor in the length direction is perpendicular to the center line of the third single-component acceleration sensor in the length direction.

2. The earthquake monitoring ultra-low frequency acceleration sensor according to claim 1, characterized in that: The internal structures of the first single-component acceleration sensor, the second single-component acceleration sensor and the third single-component acceleration sensor are consistent, and the first single-component acceleration sensor includes: A metal block, wherein a front surface of the metal block is provided with a plurality of etching lines and a plurality of threaded holes; The front side of the metal block is divided into the first area, the second area and the third area from top to bottom; The first area is provided with a first etching line, a second etching line, a third etching line and a fourth etching line; a threaded hole is provided at the center of the first area; The first etching line is arranged outside the second etching line, the first etching line is a gate shape, the second etching line is a gate shape, the third etching line is an L shape, and the fourth etching line is an L shape; The first etching line comprises: a first line segment, a second line segment and a third line segment connected in sequence; The second etching line includes: a fourth line segment, a fifth line segment and a sixth line segment connected in sequence; A third etching line, comprising: a seventh line segment and an eighth line segment connected in sequence; A fourth etching line, comprising: a ninth line segment and a tenth line segment connected in sequence; The seventh line segment is arranged between the first line segment and the fourth line segment, the seventh line segment and the first line segment are parallel to each other, and the seventh line segment and the fourth line segment are parallel to each other; The area between the first line segment and the seventh line segment is called the first flexible beam, and the area between the seventh line segment and the seventh line segment is called the second flexible beam; The ninth line segment is set between the third line segment and the sixth line segment, the ninth line segment and the third line segment are parallel to each other, and the ninth line segment and the sixth line segment are parallel to each other; The area between the third line segment and the ninth line segment is called a third flexible beam, and the area between the ninth line segment and the sixth line segment is called a fourth flexible beam.

3. The earthquake monitoring ultra-low frequency acceleration sensor as claimed in claim 2, characterized in that: The third region is provided with a fifth etching line, a sixth etching line, a seventh etching line and an eighth etching line; The fifth etching line is arranged outside the sixth etching line, the fifth etching line is in the shape of an inverted gate, the sixth etching line is in the shape of an inverted gate, the seventh etching line is in the shape of an L, and the eighth etching line is in the shape of an L; A fifth etching line, comprising: an eleventh line segment, a twelfth line segment and a thirteenth line segment connected in sequence; The sixth etching line includes: a fourteenth line segment, a fifteenth line segment, and a sixteenth line segment connected in sequence; The seventh etching line comprises: a seventeenth line segment and an eighteenth line segment connected in sequence; An eighth etching line, comprising: a nineteenth line segment and a twentieth line segment connected in sequence; The seventeenth line segment is disposed between the eleventh line segment and the fourteenth line segment, the seventeenth line segment and the eleventh line segment are parallel to each other, and the seventeenth line segment and the fourteenth line segment are parallel to each other; The area between the eleventh line segment and the seventeenth line segment is called the fifth flexible beam, and the area between the seventeenth line segment and the fourteenth line segment is called the sixth flexible beam; The nineteenth line segment is disposed between the thirteenth line segment and the sixteenth line segment, the nineteenth line segment and the thirteenth line segment are parallel to each other, and the nineteenth line segment and the sixteenth line segment are parallel to each other; The area between the nineteenth line segment and the thirteenth line segment is called the seventh flexible beam, and the area between the nineteenth line segment and the sixteenth line segment is called the eighth flexible beam; The end of the eighth line segment away from the seventh line segment is connected to the end of the eighteenth line segment away from the seventeenth line segment through the twenty-first line segment; An end of the tenth line segment farthest from the ninth line segment is connected to an end of the twentieth line segment farthest from the nineteenth line segment through the twenty-second line segment.

4. The earthquake monitoring ultra-low frequency acceleration sensor as claimed in claim 2, characterized in that: The second area of ​​the metal block is provided with a first arc-shaped groove, a first rectangular groove, a second rectangular groove, a third rectangular groove and a second arc-shaped groove connected in sequence; The groove surface of the first arc-shaped groove, the groove surface of the second rectangular groove and the groove surface of the second arc-shaped groove are on the same plane; The groove surface of the first rectangular groove and the groove surface of the third rectangular groove are on the same plane; The groove surface of the first rectangular groove is lower than the groove surface of the first arc-shaped groove; An optical fiber is arranged on the second area of ​​the metal block, and the arrangement direction of the optical fiber is arranged along the direction of the first arc-shaped groove, the first rectangular groove, the second rectangular groove, the third rectangular groove and the second arc-shaped groove. The optical fiber is bonded with glass powder, and the optical fiber arranged in the second rectangular groove is provided with a grating.

5. A design method for an ultra-low frequency acceleration sensor for earthquake monitoring, characterized in that: include: The earthquake monitoring ultra-low frequency acceleration sensor is referred to as the acceleration sensor; Construct an equivalent model of the acceleration sensor, derive the formulas of the resonant frequency and sensitivity, and find out several key parameters that affect the resonant frequency and sensitivity of the sensor from the formulas of the resonant frequency and sensitivity; The several key parameters include: the mass of the mass block, the length of each flexible beam, the width of each flexible beam and the thickness of each flexible beam; Based on the key parameters, a model to be optimized is established; the model to be optimized includes: an objective function and constraint conditions, and a genetic algorithm is used for optimization to obtain the optimal value of each key parameter; Based on the optimal value of each key parameter, the acceleration sensor is simulated and analyzed to obtain the difference between the first-order modal frequency and the second-order modal frequency of the sensor. If the difference is greater than a set threshold, it means that the sensor can resist lateral interference. A performance test experiment is performed on the acceleration sensor; wherein the performance test experiment parameters include: amplitude-frequency response, low-frequency output response, sensitivity, lateral sensitivity, sensitivity angle and dynamic range; if the values ​​of all experimental parameters meet the set requirements, it means that the test experiment of the acceleration sensor has passed.

6. The design method of the earthquake monitoring ultra-low frequency acceleration sensor as claimed in claim 5, characterized in that: Construct an equivalent model of the accelerometer and derive the formulas for the resonant frequency and sensitivity, including: The mass block is excited by external vibration with a vertical acceleration of a to produce longitudinal displacement and drive the fiber Bragg grating FBG to compress or stretch. Assuming that the displacement of the fiber Bragg grating FBG is the same as that of the mass block, the length of the flexible beam is l, the width is w, and the thickness is t, then the equivalent stiffness K of the system is: The resonant frequency f is: The relative displacement of the mass block relative to the initial position is Δx, then Δx is: The strain Δε generated by the fiber Bragg grating FBG is: The fiber Bragg grating (FBG) generates strain, and its central wavelength changes accordingly. According to the coupled mode theory, if the temperature change can be ignored, the central wavelength change Δλ is: Δλ=λ(1-p e )No (5) λ is the center wavelength of the fiber Bragg grating FBG, P e is the effective elastic-optical coefficient of the optical fiber; Then the sensor sensitivity S is:

7. The design method of the earthquake monitoring ultra-low frequency acceleration sensor as claimed in claim 5, characterized in that: Based on key parameters, establish the model to be optimized; The model to be optimized includes: an objective function and constraints, including: The operation parameters were set before algorithm optimization, including the population size of 15, the scaling factor of 0.5, the crossover probability of 0.7, and the tolerance of 1×10 -6 .

8. The design method of the earthquake monitoring ultra-low frequency acceleration sensor as claimed in claim 5, characterized in that: The sensitivity and lateral sensitivity performance test experiments of the acceleration sensor are carried out, including: The sensors were fixed on the horizontal and vertical vibration tables respectively, and an excitation frequency of 2 Hz and an acceleration of 0.2 m / s were applied. 2 As the initial value, 0.2m / s 2 Step length, excite the sine wave test sensor within the acceleration range, make it vibrate along the Z direction respectively, and obtain the lateral sensitivity ratio of the optical fiber under the excitation component; When the applied frequency is 1 Hz and the acceleration is 2 m / s 2 When the vibration signal acts on the vibration sensor, the sensitivity of the sensor on the Z-axis component is 1025pm / g, which is consistent with the optimization result of 1280pm / g; the lateral sensitivities of the X and Y axes are 35pm / g and 42pm / g, and the lateral sensitivity ratios of X and Y are 3.41% and 4.09% respectively, indicating that the sensor has a lower lateral sensitivity ratio while maintaining a high sensitivity.

9. The design method of the earthquake monitoring ultra-low frequency acceleration sensor as claimed in claim 5, characterized in that: The sensitivity angle performance test experiment of the acceleration sensor is carried out, including: Apply a horizontal excitation with a fixed frequency of 2 Hz and an acceleration of 0.2 g on a horizontal vibration table, and rotate the sensor angle in steps of 15° to test the angle dependence of the sensor sensitivity; record the sensitivity at different angles in the X-axis and Y-axis directions; Since the acceleration provided by the vibration table remains constant in the vertical direction, assuming that the angle between the acceleration a and the two orthogonal axes of the optical fiber vibration sensor is θ, the equivalent acceleration applied to the x and y axes is ax = acosθ and ay = acosθ.

10. The design method of the earthquake monitoring ultra-low frequency acceleration sensor according to claim 5, characterized in that: The dynamic range performance test experiment of the acceleration sensor is carried out, including: the dynamic range reflects the ability of the sensor to receive strong and weak signals. The dynamic range of the sensor and the dynamic range of the acceleration depend on the ratio of the maximum acceleration value to the minimum acceleration value.

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