Three-dimensional fiber grating seismic sensor

Abstract

A three-dimensional fiber grating seismic detector, a U-shaped mounting seat has a fiber fixed in the vertical direction of the inner side of one side wall, the fiber is in a suspended state, the fiber is engraved with a first grating and a second grating at a certain distance, a first hollow hexagonal beam, a second hollow hexagonal beam and a mass are sequentially arranged between the other side wall of the U-shaped mounting seat and the fiber, the mass is flush with the fiber on the side close to the fiber and is fixed between the first grating and the second grating through glue, and the center line of the second hollow hexagonal beam is orthogonal to the center line of the first hollow hexagonal beam. The two hollow hexagonal beams with orthogonal center lines can collect three-dimensional seismic information, the packaging mode of two gratings engraved on the same fiber is adopted, the sensitivity of the detector is effectively improved, the temperature compensation in the Z-axis direction is realized, and compared with common vibration sensors, the detection of three-dimensional vibration signals is realized through only two fiber gratings and a mass.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology, specifically relating to a three-dimensional fiber optic grating seismic detector. Background Technology

[0002] Vibration measurement technology plays a vital role in earthquake monitoring. Most existing seismic exploration techniques rely on electrical detectors, but these detectors suffer from drawbacks such as susceptibility to electromagnetic interference and poor high-temperature resistance. In recent years, fiber optic gratings (FBGs) have been widely applied in numerous fields due to their advantages, including immunity to electromagnetic interference, high-temperature resistance, and ease of networking. Applying FBG detectors to seismic wave monitoring can compensate for the shortcomings of traditional detection methods; therefore, researching novel FBG seismic detectors is of great significance to the development of seismic exploration technology.

[0003] A fiber optic grating accelerometer consists of three parts: inertial mass, elastic element, and damping system. Since this accelerometer achieves its measurement purpose by changing the wavelength, when the fiber optic grating is subjected to an excitation signal, the grating deforms, generating strain, and the original optical signal changes. The demodulation system converts the changed optical signal into the drift of the FBG's center wavelength. By comparing the change in the center wavelength, it can detect external vibration signals.

[0004] With the deepening research on FBG sensing and the increasing demand for vibration testing in recent years, FBG vibration sensing technology has been extensively studied by numerous scholars. Currently, fiber grating accelerometer seismic detectors mainly have four structures: beam type, hinge type, diaphragm type, and elastic column type. Among them, the beam type structure has strong anti-interference ability, simple processing, and good stability, but it is limited to the acquisition of single-component or two-component seismic signals, making it difficult to meet the needs of acquiring multi-directional seismic information. To obtain richer seismic wave information, three-dimensional fiber grating seismic detectors have become the main research focus. Therefore, it is necessary to design small-volume fiber grating seismic detectors based on three-dimensional elastic structures to acquire multi-directional seismic information and achieve self-temperature compensation. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a three-dimensional fiber optic grating seismic detector that is simple in structure, small in size, highly sensitive, and capable of detecting X, Y and Z directions.

[0006] The technical solution adopted to solve the above technical problems is: a three-dimensional fiber optic grating seismic detector, wherein an optical fiber is vertically fixed on the inner side of one side wall of a U-shaped mounting base, the optical fiber is suspended, and a first grating and a second grating are engraved on the optical fiber at a certain distance. A first hollow hexagonal beam, a second hollow hexagonal beam, and a mass block are arranged sequentially between the other side wall of the U-shaped mounting base and the optical fiber, the mass block is flush with the optical fiber and fixed between the first grating and the second grating by adhesive, and the center line of the second hollow hexagonal beam is orthogonal to the center line of the first hollow hexagonal beam.

[0007] As a preferred technical solution, the first hollow hexagonal beam and the second hollow hexagonal beam have the same structure.

[0008] As a preferred technical solution, in the first hollow hexagonal beam, a set of two parallel and equal beams are used as fixed beams, and the remaining four beams are used as free beams. The lengths of the four free beams are equal, and the length of the fixed beam is less than the length of the free beam.

[0009] As a preferred technical solution, the included angle between the two connected free beams is an acute angle.

[0010] As a preferred technical solution, the center wavelengths of the first grating and the second grating are different, while the grating region lengths are the same.

[0011] As a preferred technical solution, the optical fiber is subjected to a certain prestress before being fixed.

[0012] As a preferred technical solution, the sensitivity of the detector in the X-axis direction is:

[0013]

[0014] In the formula, Pe is the effective optical elasticity coefficient of the optical fiber, λ1 is the center wavelength of the first grating, M is the weight of the mass block, and ε x E represents the initial strain of the first or second grating in the X-axis direction. f Let A be the elastic modulus of the optical fiber. f This represents the cross-sectional area of ​​the optical fiber. Let l be the structural stiffness of the detector in the X-axis direction, and l be the length between the fixed point of the optical fiber on the mounting base and the fixed point on the mass block.

[0015] The sensitivity of the detector in the Y-axis direction is:

[0016]

[0017] In the formula, ε y Let be the initial strain of the first or second grating in the Y-axis direction. The structural stiffness of the detector in the Y-axis direction;

[0018] The sensitivity of the detector in the Z-axis direction is:

[0019]

[0020] In the formula, K sz K represents the structural stiffness of the detector in the Z-axis direction. f This refers to the elastic stiffness of the optical fiber.

[0021] The beneficial effects of this invention are as follows:

[0022] This invention utilizes two orthogonally aligned, hollowed-out hexagonal beams to acquire three-dimensional seismic information. By employing an encapsulation method where two fiber optic gratings are inscribed on the same optical fiber, it effectively improves the detector's sensitivity while achieving temperature compensation in the Z-axis direction. Compared to ordinary vibration sensors, it achieves three-dimensional vibration signal detection using only two fiber optic gratings and a mass block. The invention achieves a resonant frequency of 492.78 Hz and a sensitivity of 7.5 pm / g in the X-axis direction; a resonant frequency of 129.27 Hz and a sensitivity of 102.2 pm / g in the Y-axis direction; and a resonant frequency of 93.05 Hz and a sensitivity of 387.5 pm / g in the Z-axis direction. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of the three-dimensional fiber optic grating seismic detector of the present invention.

[0024] Figure 2 This is a schematic diagram of the structure of the first hollowed-out hexagonal beam 2 of the present invention.

[0025] Figure 3 This is a simplified diagram of vibration along the X-axis.

[0026] Figure 4 This is a simplified diagram of vibration in the Y-axis direction.

[0027] Figure 5 This is a simplified diagram of vibration in the Z-axis direction.

[0028] Figure 6 This is a vibration simulation diagram of the present invention in the X-axis direction.

[0029] Figure 7 This is a vibration simulation diagram of the present invention in the Y-axis direction.

[0030] Figure 8 This is a vibration simulation diagram of the present invention in the Z-axis direction. Detailed Implementation

[0031] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the present invention is not limited to the following embodiments.

[0032] exist Figure 1 In this embodiment, a three-dimensional fiber optic grating seismic detector includes a U-shaped mounting base 1, an optical fiber 5, a first grating 4, a second grating 7, a first hollow hexagonal beam 2, a second hollow hexagonal beam 3, and a mass block 6.

[0033] The inner side of the right wall of the U-shaped mounting base 1 has a groove machined horizontally. Fiber optic cables 5 are vertically fixed at both ends of the groove. Before fixing, the fiber optic cables 5 are subjected to a certain prestress and are suspended. The fiber optic cables 5 are engraved with a first grating 4 and a second grating 7 at a certain distance. Between the left wall of the U-shaped mounting base 1 and the fiber optic cables 5, a first hollow hexagonal beam 2, a second hollow hexagonal beam 3, and a mass block 6 are arranged in sequence, suspended and connected as one piece. The side of the mass block 6 close to the fiber optic cables 5 is flush with the fiber optic cables 5 and fixed between the first grating 4 and the second grating 7 with adhesive. The first hollow hexagonal beam 2 is horizontally arranged and is used to collect vibration signals in the Z-axis and X-axis directions. The center line of the second hollow hexagonal beam 3 is orthogonal to the center line of the first hollow hexagonal beam 2 and is used to collect vibration signals in the Y-axis and X-axis directions.

[0034] exist Figure 2 In this embodiment, in the first hollow hexagonal beam 2, two pairs of opposite and parallel beams are used as fixed beams 2-2, and the remaining four beams are used as free beams 2-1. The distance L between the two fixed beams 2-2 is 13mm, and the lengths of the four free beams 2-1 are equal. The length of the fixed beam 2-2 is less than the length of the free beam 2-1. The included angle between two adjacent free beams 2-1 is an acute angle, which is 85.8° in this embodiment. The second hollow hexagonal beam 3 has the same structure as the first hollow hexagonal beam 2.

[0035] In this embodiment, the center wavelengths of the first grating 4 and the second grating 7 are 1544.145nm and 1552.362nm, respectively, and the grating length is 3mm for each. Using two gratings doubles the detector's sensitivity and simultaneously achieves temperature compensation in the Z-axis direction. The two gratings, combined with the first hollowed-out hexagonal beam 2 and the second hollowed-out hexagonal beam 3, form a sensitive sensing structure. Inertial forces cause displacement of the sensitive sensing structure, resulting in axial strains of the same magnitude and opposite directions for both gratings. This axial strain is converted into a drift in the center wavelength of the two gratings, achieving modulation of the grating center wavelength by acceleration.

[0036] An external acceleration signal causes the mass block 6 to generate an inertial force, which in turn causes strain in the sensitive structures of the first hollow hexagonal beam 2 and the second hollow hexagonal beam 3. This strain causes axial strain in the fiber optic grating, resulting in a drift in the center wavelength of the fiber optic grating. The amount of the center wavelength drift is obtained through a demodulation system, thereby enabling the detection of the acceleration signal.

[0037] The sensitivity of the detector in the X-axis direction in this embodiment is:

[0038]

[0039] In the formula, Pe is the effective elastic-optical coefficient of fiber 5, λ1 is the center wavelength of the first grating 4, M is the weight of mass block 6, and ε x E represents the initial strain of either the first grating 4 or the second grating 7 in the X-axis direction. f Let A be the elastic modulus of fiber 5. f Let be the cross-sectional area of ​​fiber 5. denoted as the structural stiffness of the detector in the X-axis direction, and l is the length between the fixed point of the optical fiber 5 on the mounting base and the fixed point on the mass block 6.

[0040] like Figure 3 Analysis of the structural stiffness of the detector in the X-axis direction. In a static state, under the influence of gravity, the displacement of the mass block relative to its original equilibrium position is x0, and the angle between the optical fiber and its original equilibrium position is... When a certain acceleration signal is applied to the detector, the mass block 6 further generates a displacement x1 under the vibration signal applied externally. At this time, the effective length of the optical fiber 5 is L1. Force analysis is performed on it:

[0041]

[0042]

[0043] Thus, the structural stiffness of the detector in the X-axis direction is obtained. for:

[0044]

[0045] The sensitivity of the detector in the Y-axis direction in this embodiment is:

[0046]

[0047] In the formula, ε y Let be the initial strain of the first grating 4 or the second grating 7 in the Y-axis direction. The structural stiffness of the detector in the Y-axis direction;

[0048] like Figure 4 Analysis of the structural stiffness of the detector in the Y-axis direction: In the static state, under the action of gravity, the displacement of the mass block relative to its original equilibrium position is y0, and the angle between the optical fiber and its original equilibrium position is α. When a certain acceleration signal is applied to the detector, the mass block 6 further generates a displacement y1 under the vibration signal applied externally. At this time, the effective length of the optical fiber 5 is L1. Force analysis is performed on it:

[0049] 2K f ΔLsinα+Ksy y0-Mg=0

[0050] Thus, the structural stiffness of the detector in the Y-axis direction is obtained. for:

[0051]

[0052] The detector's sensitivity in the Z-axis direction is:

[0053]

[0054] In the formula, K sz K represents the structural stiffness of the detector in the Z-axis direction. f The elastic stiffness of fiber 5.

[0055] like Figure 5 Analysis of the structural stiffness of the detector in the Z-axis direction: When the mass vibrates in the Z-axis, the first grating is in a stretched state and the second grating is in a relaxed state, or the first grating is in a relaxed state and the second grating is in a stretched state. Then, according to Newton's second law, we get:

[0056]

[0057] In the formula, z is the displacement of the mass block in the z-direction, and M... z Let be the equivalent mass of the mass block, and 'a' be the acceleration in the Z direction. Then, the structural stiffness of the detector in the Z-axis direction is:

[0058]

[0059] To verify the beneficial effects of the present invention, the inventors conducted a simulation experiment on the three-dimensional fiber optic grating seismic detector of Embodiment 1. Figures 6-8 It can be seen that the resonant frequency of the detector in the X-axis direction is 492.78Hz; the resonant frequency of the detector in the Y-axis direction is 129.27Hz; and the resonant frequency of the detector in the Z-axis direction is 93.05Hz.

Claims

1. A three-dimensional fiber optic grating seismic detector, characterized in that: An optical fiber is vertically fixed to the inner side of one side wall of the U-shaped mounting base, and the optical fiber is suspended. The optical fiber is engraved with a first grating and a second grating at a certain distance. Between the other side wall of the U-shaped mounting base and the optical fiber, a first hollow hexagonal beam, a second hollow hexagonal beam, and a mass block are arranged in sequence, suspended and connected as one piece. The side of the mass block near the optical fiber is flush with the optical fiber and is fixed between the first grating and the second grating with glue. The center line of the second hollow hexagonal beam is orthogonal to the center line of the first hollow hexagonal beam.

2. The three-dimensional fiber optic grating seismic detector according to claim 1, characterized in that: The first hollow hexagonal beam has the same structure as the second hollow hexagonal beam.

3. The three-dimensional fiber optic grating seismic detector according to claim 1 or 2, characterized in that: In the first hollow hexagonal beam, a set of two parallel and equal beams are used as fixed beams, and the remaining four beams are used as free beams. The lengths of the four free beams are equal, and the length of the fixed beam is less than the length of the free beam.

4. The three-dimensional fiber optic grating seismic detector according to claim 3, characterized in that: The included angle between any two connected free beams among the four free beams is an acute angle.

5. The three-dimensional fiber optic grating seismic detector according to claim 1, characterized in that: The first grating and the second grating have different center wavelengths, but the grating region lengths are the same.

6. The three-dimensional fiber optic grating seismic detector according to claim 1, characterized in that: The optical fiber is subjected to a certain prestress before being fixed.

7. The three-dimensional fiber optic grating seismic detector according to claim 1, characterized in that: The sensitivity of the detector in the X-axis direction is: In the formula, Pe is the effective elastic-optical coefficient of the optical fiber. The center wavelength of the first grating Let be the weight of the mass block. Let be the initial strain of the first or second grating in the X-axis direction. The elastic modulus of the optical fiber. This represents the cross-sectional area of ​​the optical fiber. Let l be the structural stiffness of the detector in the X-axis direction, and l be the length between the fixed point of the optical fiber on the mounting base and the fixed point on the mass block. The sensitivity of the detector in the Y-axis direction is: In the formula, Let be the initial strain of the first or second grating in the Y-axis direction. The structural stiffness of the detector in the Y-axis direction; The sensitivity of the detector in the Z-axis direction is: In the formula, The structural stiffness of the detector in the Z-axis direction. This refers to the elastic stiffness of the optical fiber.

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