An optical fiber current sensor system and method for underground spaces such as coal mines

Abstract

The present invention provides an optical fiber current sensor system for underground spaces such as coal mines. The magnetic circuit unit of the sensor system is composed of a wire, a fiber Bragg grating, a magnetostrictive composite material, a magnetic conductor, a coil, and a DC power supply. The acquisition unit is composed of a fiber Bragg grating demodulator and a computer. Among them, the magnetostrictive composite material is arranged in a symmetric stepped beam structure, one end of the magnetic conductor is arranged in a stepped structure, and the other end is arranged in a planar structure; the coil is connected to the DC power supply, and the output current of the DC power supply passes through the coil to provide a bias magnetic field for the magnetostrictive composite material. Compared with traditional optical fiber current sensors, the sensor of the present invention realizes magnetic concentration through the stepped design of the magnetostrictive material and the magnetic conductor, significantly improving the sensitivity of the sensor. By applying different bias magnetic fields through an adjustable coil, the sensor has linearity in different measurement ranges, broadening the linear range.

Description

Technical Field

[0001] The present invention belongs to the field of underground space current measurement, and particularly relates to an optical fiber current sensor system and method for underground spaces such as coal mines. Background Art

[0002] Currently, the underground rail transit system adopts a DC traction power supply system. During the return process of the traction current, since the running rail and the ground cannot be completely insulated, part of the current will leak from the running rail to the surrounding medium. The leakage current will cause serious electrochemical corrosion to the structural steel bars and buried metal pipelines in the underground space. Especially in the auxiliary transportation system of coal mines, when the battery locomotive travels on the track, once there is current leakage, it is easy to induce risks such as mine gas and coal dust explosion accidents. Therefore, it is crucial to monitor the current leakage in the underground space. Summary of the Invention

[0003] Object of the Invention: Aiming at the above problems, the present invention proposes an optical fiber current sensor system for underground spaces such as coal mines. The sensor system has good magnetic focusing effect, adjustable bias magnetic field, and low cost.

[0004] Technical Solution: To achieve the object of the present invention, the technical solution adopted by the present invention is: An optical fiber current sensor system for underground spaces such as coal mines, the sensor system includes a magnetic circuit unit and a collection unit. Among them, the magnetic circuit unit includes a wire (1), a magnetic conductor (2), a coil (3), a DC regulated power supply (4), a magnetostrictive composite material magnetic induction element (5), and an optical fiber grating (6); the collection unit includes an optical fiber grating demodulator (7) and a computer (8); the magnetostrictive composite material magnetic induction element (5) includes a first magnetostrictive composite material magnetic induction element (5-1) and a second magnetostrictive composite material magnetic induction element (5-2), and the first magnetostrictive composite material magnetic induction element (5-1) and the second magnetostrictive composite material magnetic induction element (5-2) are of a stepped structure with 3 steps. The stepped ends with the smallest tenon bottom area of the step tenons of the first magnetostrictive composite material magnetic induction element (5-1) and the second magnetostrictive composite material magnetic induction element (5-2) are integrally formed and connected.

[0005] The wire (1) is located at the exact center of the inner ring of the magnetic conductor (2). The magnetic conductor (2) is composed of two symmetrically distributed circular-ring-shaped electromagnetic pure iron pieces (21) and one rectangular parallelepiped electromagnetic pure iron piece (22). One end of each circular-ring-shaped electromagnetic pure iron piece (21) is a stepped magnetic circuit structure with 3 steps, and the other end is a planar magnetic circuit structure. The two ends of the rectangular parallelepiped electromagnetic pure iron piece (22) are connected to the planar magnetic circuit structures of the two circular-ring-shaped electromagnetic pure iron pieces (21). A multi-turn coil (3) is wound around the rectangular parallelepiped electromagnetic pure iron piece (22). One end of the coil (3) is connected to the positive pole of the DC regulated power supply (4), and the other end is connected to the negative pole of the DC regulated power supply (4). The stepped end with the largest tenon bottom area of the stepped tenon of the first magnetostrictive composite magnetic induction element (5-1) is connected to the stepped end with the smallest tenon bottom area of the stepped tenon of one of the circular-ring-shaped electromagnetic pure iron pieces (21). The stepped end with the largest tenon bottom area of the stepped tenon of the second magnetostrictive composite magnetic induction element (5-2) is connected to the stepped end with the smallest tenon bottom area of the stepped tenon of the other circular-ring-shaped electromagnetic pure iron piece (21).

[0006] One end of the fiber grating (6) is pasted at the central position of the surface of the stepped end where the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) are connected, and the other end is connected to the fiber grating demodulator (7). The fiber grating demodulator (7) is connected to the computer (8) through a data connection line.

[0007] Preferably, the total length of the integral molding connection of the stepped ends with the smallest tenon bottom area of the stepped tenons of the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) is 10 mm, the total thickness is 1 mm, and the total width is 10 mm.

[0008] Preferably, the circular-ring-shaped electromagnetic pure iron pieces (21) and the rectangular parallelepiped electromagnetic pure iron piece (22) are made of DT4C material. The contact area between the stepped end with the largest tenon bottom area of the stepped tenon of the first magnetostrictive composite magnetic induction element (5-1) and the stepped end with the smallest tenon bottom area of the stepped tenon of one of the circular-ring-shaped electromagnetic pure iron pieces (21) is 100 mm 2 ², and the contact area between the stepped end with the largest tenon bottom area of the stepped tenon of the second magnetostrictive composite magnetic induction element (5-2) and the stepped end with the smallest tenon bottom area of the stepped tenon of the other circular-ring-shaped electromagnetic pure iron piece (21) is 100 mm 2 ². The cross-sectional area of the contact between the planar magnetic circuit structure at the other end of the circular-ring-shaped electromagnetic pure iron piece (21) and the rectangular parallelepiped electromagnetic pure iron piece (22) is 180 mm 2 .

[0009] Preferably, by energizing the coil (3), a magnetic field is generated in the coil (3) to provide a bias magnetic field for the magnetostrictive composite (5) magnetic induction element.

[0010] Preferably, the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) are made by uniformly mixing Terfenol-D powder, epoxy resin, curing agent and coupling agent in proportion. Among them, the ratio of Terfenol-D powder to epoxy resin is 5:1, the ratio of epoxy resin to curing agent is 3:1, and the coupling agent accounts for 2% of the whole mixture.

[0011] In addition, the present invention also proposes a sensing signal demodulation method for a fiber optic current sensor system in underground spaces such as coal mines according to the above, and this method includes the following steps:

[0012] (1): The power supply supplies power to the wire (1). After the wire (1) is powered on, an annular magnetic field is formed around the wire (1). The magnetic conductor (2) aggregates the annular magnetic field and transmits it to the magnetostrictive composite magnetic induction element (5). The sensor is modeled according to Ampere's circuital law, and the magnetic field H1 on the step with the smallest bottom area of the step tenon of the first magnetostrictive composite magnetic induction element (5-1) or the second magnetostrictive composite magnetic induction element (5-2) is obtained. Specifically:

[0013]

[0014] In the formula, μ1 is the magnetic permeability of the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2), and μ2 is the magnetic permeability of the magnetic conductor (2);

[0015] The step tenons of the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) are third-order tenons. The first-order tenon, the second-order tenon and the third-order tenon are arranged in ascending order of the bottom area of the tenon. l1 is the height of the first-order tenon, S1 is the bottom area of the first-order tenon, l2 is the height of the second-order tenon and the third-order tenon, S2 is the bottom area of the second-order tenon, and S3 is the bottom area of the third-order tenon;

[0016] The step tenons of the circular electromagnetic pure iron (21) are third-order tenons. The first-order tenon, the second-order tenon and the third-order tenon are arranged in ascending order of the bottom area of the tenon. The heights of the first-order tenon, the second-order tenon and the third-order tenon of the circular electromagnetic pure iron (21) are equal to l3. S4, S5, and S6 are respectively the bottom area of the first-order tenon, the bottom area of the second-order tenon, and the bottom area of the third-order tenon;

[0017] l4 is half of the length of the cuboid electromagnetic pure iron (22);

[0018] l is the annular length of each annular piece of electromagnetic pure iron (21);

[0019] I1 is the current of the wire (1);

[0020] (2): The DC regulated power supply (4) supplies power to the coil (3). After the coil (3) is powered on, a magnetic field is generated. The magnetic field generated by the coil (3) is transmitted along the magnetic conductor (2) to the magnetostrictive composite magnetic induction element (5), providing a bias magnetic field for the magnetostrictive composite magnetic induction element (5). The bias magnetic field provided by the coil (3) is calculated by the following formula:

[0021]

[0022] In the formula, N is the number of turns of the coil, I2 is the current for powering the coil, and L e is the length of the cuboid electromagnetic pure iron (22);

[0023] (3): The magnetostrictive composite magnetic induction element (5) generates strain under the action of two magnetic fields. The strain generated by the magnetostrictive composite magnetic induction element (5) causes a change in the central wavelength of the fiber Bragg grating (6) attached to the surface. The wavelength change amount △λ B is shown in the following formula:

[0024]

[0025] In the formula, λ B1 is the central wavelength length without the magnetic field, and λ B2 is the central wavelength length under the action of two magnetic fields. n eff is the effective refractive index of the fiber Bragg grating, Λ is the grating period, P e is the effective photoelastic coefficient of the optical fiber, L is the effective length of the fiber Bragg grating, and k is the magnetostrictive coefficient of the magnetostrictive composite magnetic induction element (5);

[0026] (4): The fiber Bragg grating demodulator (7) amplifies and collects the data of the signal returned by the fiber Bragg grating (6). The collected data is sent to the computer (8) for signal processing and calculation, and finally the signal of the measured sensor is demodulated to realize the measurement of the current of the wire (1).

[0027] Advantageous effects: Compared with the prior art, the technical solution of the present invention has the following advantageous technical effects:

[0028] The technical solution disclosed by the present invention can realize adjustable bias magnetic field, and at the same time, the magnetic flux density at the middle position of the magnetostrictive composite material is more concentrated. Moreover, the technical solution of the present invention not only improves the sensitivity of the sensor but also reduces the cost of the sensor. Description of the Drawings

[0029] Figure 1 It is a schematic diagram of the sensor system;

[0030] Figure 2 It is a structural diagram of the sensor;

[0031] Figure 3 It is a schematic diagram of the total length, total width and total thickness of the magnetostrictive composite material;

[0032] Figure 4 It is a schematic diagram of the length and cross-sectional area of the steps of the magnetostrictive composite material steps;

[0033] Figure 5 It is a schematic diagram of the length and cross-sectional area of the steps of the stepped magnetic circuit structure at one end of each circular ring-shaped electromagnetic pure iron;

[0034] In the figure: 1. Wire, 2. Magnetoconductor, 3. Coil, 4. DC regulated power supply, 5. Magnetostrictive composite material magnetic induction element, 6. Fiber Bragg grating, 7. Fiber Bragg grating demodulator, 8. Computer, 21. Circular ring-shaped electromagnetic pure iron, 22. Cuboid electromagnetic pure iron, 5-1. First magnetostrictive composite material magnetic induction element, 5-2. Second magnetostrictive composite material magnetic induction element. Specific implementation manner

[0035] The following further describes the specific implementation manner of this patent in conjunction with the accompanying drawings.

[0036] Figure 1 It is a schematic diagram of the sensor system. As Figure 1 shown, the present invention proposes an underground space optical fiber current sensor system such as in coal mines. This sensor system includes a magnetic circuit unit and a collection unit. Among them, the magnetic circuit unit includes a wire (1), a magnetoconductor (2), a coil (3), a DC regulated power supply (4), a magnetostrictive composite material magnetic induction element (5) and a fiber Bragg grating (6); the collection unit includes a fiber Bragg grating demodulator (7) and a computer (8); the magnetostrictive composite material magnetic induction element (5) includes a first magnetostrictive composite material magnetic induction element (5-1) and a second magnetostrictive composite material magnetic induction element (5-2), and the first magnetostrictive composite material magnetic induction element (5-1) and the second magnetostrictive composite material magnetic induction element (5-2) are of a stepped structure with 3 steps, and the stepped ends with the smallest tenon bottom area of the step tenons of the first magnetostrictive composite material magnetic induction element (5-1) and the second magnetostrictive composite material magnetic induction element (5-2) are integrally formed and connected;

[0037] The wire (1) is located at the exact center of the inner ring of the magnetic conductor (2). The magnetic conductor (2) is composed of two symmetrically distributed circular-ring-shaped electromagnetic pure iron pieces (21) and one rectangular parallelepiped electromagnetic pure iron piece (22). One end of each circular-ring-shaped electromagnetic pure iron piece (21) is a stepped magnetic circuit structure with 3 steps, and the other end is a planar magnetic circuit structure. The two ends of the rectangular parallelepiped electromagnetic pure iron piece (22) are connected to the planar magnetic circuit structures of the two circular-ring-shaped electromagnetic pure iron pieces (21). Multiple turns of a coil (3) are wound around the rectangular parallelepiped electromagnetic pure iron piece (22). One end of the coil (3) is connected to the positive pole of a DC regulated power supply (4), and the other end is connected to the negative pole of the DC regulated power supply (4); The stepped end with the largest tenon bottom area of the stepped tenon of the first magnetostrictive composite material magnetic induction element (5-1) is connected to the stepped end with the smallest tenon bottom area of the stepped tenon of one of the circular-ring-shaped electromagnetic pure iron pieces (21). The stepped end with the largest tenon bottom area of the stepped tenon of the second magnetostrictive composite material magnetic induction element (5-2) is connected to the stepped end with the smallest tenon bottom area of the stepped tenon of the other circular-ring-shaped electromagnetic pure iron piece (21);

[0038] One end of the fiber Bragg grating (6) is pasted at the central position of the surface of the stepped end where the first magnetostrictive composite material magnetic induction element (5-1) and the second magnetostrictive composite material magnetic induction element (5-2) are connected, and the other end is connected to a fiber Bragg grating demodulator (7). The fiber Bragg grating demodulator (7) is connected to a computer (8) through a data connection line.

[0039] The total length of the integrally formed connection of the stepped ends with the smallest tenon bottom areas of the stepped tenons of the first magnetostrictive composite material magnetic induction element (5-1) and the second magnetostrictive composite material magnetic induction element (5-2) is 10 mm, the total thickness is 1 mm, and the total width is 10 mm.

[0040] The circular-ring-shaped electromagnetic pure iron pieces (21) and the rectangular parallelepiped electromagnetic pure iron piece (22) are made of DT4C material. The connection contact area between the stepped end with the largest tenon bottom area of the stepped tenon of the first magnetostrictive composite material magnetic induction element (5-1) and the stepped end with the smallest tenon bottom area of the stepped tenon of one of the circular-ring-shaped electromagnetic pure iron pieces (21) is 100 mm 2 ; The connection contact area between the stepped end with the largest tenon bottom area of the stepped tenon of the second magnetostrictive composite material magnetic induction element (5-2) and the stepped end with the smallest tenon bottom area of the stepped tenon of the other circular-ring-shaped electromagnetic pure iron piece (21) is 100 mm 2 ; The cross-sectional area of the contact between the planar magnetic circuit structure at the other end of the circular-ring-shaped electromagnetic pure iron piece (21) and the rectangular parallelepiped electromagnetic pure iron piece (22) is 180 mm 2 .

[0041] By energizing the coil (3), a magnetic field is generated in the coil (3) to provide a bias magnetic field for the magnetostrictive composite material (5) magnetic induction element.

[0042] The first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) are made by uniformly mixing Terfenol-D powder, epoxy resin, curing agent and coupling agent in proportion. Among them, the ratio of Terfenol-D powder to epoxy resin is 5:1, the ratio of epoxy resin to curing agent is 3:1, and the coupling agent accounts for 2% of the whole mixture.

[0043] In addition, the present invention also proposes a method for demodulating the sensing signal of a fiber optic current sensor system in underground spaces such as coal mines according to the above, and the method includes the following steps:

[0044] (1): The power supply supplies power to the wire (1). After the wire (1) is powered on, an annular magnetic field is formed around the wire (1). The magnetic conductor (2) aggregates the annular magnetic field and transmits it to the magnetostrictive composite magnetic induction element (5). The sensor is modeled according to Ampere's circuital law, and the magnetic field H1 on the step with the smallest bottom area of the step convex tenon of the first magnetostrictive composite magnetic induction element (5-1) or the second magnetostrictive composite magnetic induction element (5-2) is obtained. Specifically:

[0045]

[0046] In the formula, μ1 is the magnetic permeability of the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2), and μ2 is the magnetic permeability of the magnetic conductor (2);

[0047] The step convex tenons of the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) are third-order convex tenons, and are successively the first-order convex tenon, the second-order convex tenon and the third-order convex tenon according to the bottom area of the convex tenon from small to large; l1 is the height of the first-order convex tenon, S1 is the bottom area of the first-order convex tenon, l2 is the height of the second-order convex tenon and the third-order convex tenon, S2 is the bottom area of the second-order convex tenon, and S3 is the bottom area of the third-order convex tenon;

[0048] The step convex tenons of the circular electromagnetic pure iron (21) are third-order convex tenons, and are successively the first-order convex tenon, the second-order convex tenon and the third-order convex tenon according to the bottom area of the convex tenon from small to large. The heights of the first-order convex tenon, the second-order convex tenon and the third-order convex tenon of the circular electromagnetic pure iron (21) are equal to l3. The S4, S5, and S6 are respectively the bottom area of the first-order convex tenon, the bottom area of the second-order convex tenon, and the bottom area of the third-order convex tenon;

[0049] l4 is half of the length of the cuboid electromagnetic pure iron (22);

[0050] l is the annular length of the ring of each circular electromagnetic pure iron (21);

[0051] I1 is the current of the wire (1);

[0052] (2): The DC regulated power supply (4) supplies power to the coil (3). After the coil (3) is powered on, a magnetic field is generated. The magnetic field generated by the coil (3) is transmitted along the magnetoconductive body (2) to the magnetostrictive composite magnetic induction element (5), providing a bias magnetic field to the magnetostrictive composite magnetic induction element (5). The bias magnetic field provided by the coil (3) is calculated by the following formula:

[0053]

[0054] In the formula, N is the number of turns of the coil, I2 is the current for powering on the coil, and L e is the length of the rectangular electromagnetic pure iron (22);

[0055] (3): The magnetostrictive composite magnetic induction element (5) generates strain under the action of two magnetic fields. The strain generated by the magnetostrictive composite magnetic induction element (5) causes the central wavelength of the fiber Bragg grating (6) attached to the surface to change. The wavelength change amount △λ B is shown in the following formula:

[0056]

[0057] In the formula, λ B1 is the central wavelength length without magnetic field, λ B2 is the central wavelength length under the action of two magnetic fields, n eff is the effective refractive index of the fiber Bragg grating, Λ is the grating period, P e is the effective photoelastic coefficient of the optical fiber, L is the effective length of the fiber Bragg grating, and k is the magnetostrictive coefficient of the magnetostrictive composite magnetic induction element (5);

[0058] (4): The fiber Bragg grating demodulator (7) amplifies and collects the data of the signal returned by the fiber Bragg grating (6). The collected data is sent to the computer (8) for signal processing and calculation, and finally the signal of the measured sensor is demodulated to realize the measurement of the current of the wire (1).

[0059] As Figure 2 shown, the magnetostrictive composite magnetic induction element (5) is of a stepped design. The total length of the material of the magnetostrictive composite magnetic induction element (5) is 10 mm, the total thickness of the material is 1 mm, and the total height of the material is 10 mm. The stepped shape can concentrate the magnetic lines of force at the central position of the magnetostrictive composite magnetic induction element (5), improve the magnetic field strength at the central position of the magnetostrictive composite magnetic induction element (5), and thus improve the sensitivity of the fiber optic current sensor.

[0060] The magnetostrictive composite magnetic induction element (5) is the sensing head of the fiber optic current sensor. By changing the structural parameters of the magnetostrictive composite magnetic induction element (5), the magnetic field strength at the center of the magnetostrictive composite magnetic induction element (5) can be increased. For example, increasing the number of steps of the magnetostrictive composite material (5) or reducing the material thickness, material length, and material height of the magnetostrictive composite magnetic induction element (5).

[0061] The circular ring-shaped electromagnetic pure iron (21) and the cuboid electromagnetic pure iron (22) are made of DT4C material. One end of the circular ring-shaped electromagnetic pure iron (21) is a stepped magnetic circuit structure with 3 steps, and the cross-sectional area in contact with the magnetostrictive composite magnetic induction element (5) is 100 mm 2 , and the stepped magnetic circuit structure can concentrate the magnetic field. The other end of the circular ring-shaped electromagnetic pure iron (21) is a planar magnetic circuit structure, and the cross-sectional area in contact with the cuboid electromagnetic pure iron (22) is 180 mm 2 . Therefore, placing the magnetostrictive composite magnetic induction element (5) in the middle of the stepped structures of the two circular ring-shaped electromagnetic pure irons (21) can increase the magnetic flux density of the magnetostrictive composite magnetic induction element (5).

[0062] The magnetostrictive composite magnetic induction element (5) is made by uniformly mixing Terfenol-D powder, epoxy resin, curing agent, and coupling agent in proportion. The ratio of Terfenol-D powder to epoxy resin is 5:1, the ratio of epoxy resin to curing agent is 3:1, and the coupling agent accounts for 2% of the whole mixture. The role of epoxy resin is to bond Terfenol-D powder particles, and the role of the coupling agent is to enhance the bonding force between Terfenol-D powder particles and epoxy resin. The coupling agent treats Terfenol-D powder by the overall mixing method, which is simpler and easier to operate than the surface treatment method.

Claims

1. A fiber optic current sensor system for underground spaces such as coal mines, characterized in that, The sensor system includes a magnetic circuit unit and a collection unit. Among them, the magnetic circuit unit includes a wire (1), a magnetic conductor (2), a coil (3), a DC regulated power supply (4), a magnetostrictive composite magnetic induction element (5), and a fiber Bragg grating (6); the collection unit includes a fiber Bragg grating demodulator (7) and a computer (8); the magnetostrictive composite magnetic induction element (5) includes a first magnetostrictive composite magnetic induction element (5-1) and a second magnetostrictive composite magnetic induction element (5-2), and the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) are of a stepped structure with 3 steps. The stepped ends with the smallest tenon bottom area of the stepped tenons of the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) are integrally formed and connected. The wire (1) is located at the exact center of the inner ring of the magnetic conductor (2). The magnetic conductor (2) is composed of two symmetrically distributed circular electromagnetic pure irons (21) and a rectangular electromagnetic pure iron (22). One end of each circular electromagnetic pure iron (21) is a stepped magnetic circuit structure with 3 steps, and the other end is a planar magnetic circuit structure. The two ends of the rectangular electromagnetic pure iron (22) are connected to the planar magnetic circuit structures of the two circular electromagnetic pure irons (21). Multiple turns of the coil (3) are wound around the rectangular electromagnetic pure iron (22). One end of the coil (3) is connected to the positive pole of the DC regulated power supply (4), and the other end is connected to the negative pole of the DC regulated power supply (4); the stepped end with the largest tenon bottom area of the stepped tenon of the first magnetostrictive composite magnetic induction element (5-1) is connected to the stepped end with the smallest tenon bottom area of the stepped tenon of one of the circular electromagnetic pure irons (21), and the stepped end with the largest tenon bottom area of the stepped tenon of the second magnetostrictive composite magnetic induction element (5-2) is connected to the stepped end with the smallest tenon bottom area of the stepped tenon of the other circular electromagnetic pure iron (21). One end of the fiber Bragg grating (6) is pasted at the central position of the surface of the stepped end where the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) are connected, and the other end is connected to the fiber Bragg grating demodulator (7). The fiber Bragg grating demodulator (7) is connected to the computer (8) through a data connection line.

2. The fiber optic current sensor system for underground spaces such as coal mines according to claim 1, wherein The total length of the integral formation connection of the stepped ends with the smallest tenon bottom area of the stepped tenons of the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) is 10 mm, the total thickness is 1 mm, and the total width is 10 mm.

3. The fiber optic current sensor system for underground spaces such as coal mines according to claim 1, wherein The circular-ring-shaped electromagnetic pure iron (21) and the cuboid-shaped electromagnetic pure iron (22) are made of DT4C material. The step end with the largest tenon bottom area of the step tenon of the first magnetostrictive composite magnetic induction element (5-1) is connected to the step end with the smallest tenon bottom area of the step tenon of one of the circular-ring-shaped electromagnetic pure irons (21), and the contact area is 100 mm 2 , the step end with the largest tenon bottom area of the step tenon of the second magnetostrictive composite magnetic induction element (5-2) is connected to the step end with the smallest tenon bottom area of the step tenon of the other circular-ring-shaped electromagnetic pure iron (21), and the contact area is 100 mm 2 , the cross-sectional area of the planar magnetic circuit structure at the other end of the circular-ring-shaped electromagnetic pure iron (21) in contact with the cuboid-shaped electromagnetic pure iron (22) is 180 mm 2 .

4. The fiber optic current sensor system for underground spaces such as coal mines according to claim 1, wherein By energizing the coil (3), the coil (3) generates a magnetic field to provide a bias magnetic field for the magnetostrictive composite magnetic induction element (5).

5. The fiber optic current sensor system for underground spaces such as coal mines according to claim 1, characterized in that, The first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) are made by uniformly mixing Terfenol-D powder, epoxy resin, curing agent and coupling agent in proportion. Among them, the ratio of Terfenol-D powder to epoxy resin is 5:1, the ratio of epoxy resin to curing agent is 3:1, and the coupling agent accounts for 2% of the whole mixture.

6. A demodulation method for sensing signals of an optical fiber current sensor system in underground spaces such as coal mines according to any one of claims 1-5, characterized in that, The method includes the following steps: (1): The power supply supplies power to the wire (1). After the wire (1) is powered on, an annular magnetic field is formed around the wire (1). The magnetic conductor (2) gathers the annular magnetic field and transmits it to the magnetostrictive composite magnetic induction element (5). According to Ampere's circuital law, the sensor is modeled to obtain the magnetic field H1 on the step with the smallest bottom area of the step tenon of the first magnetostrictive composite magnetic induction element (5-1) or the second magnetostrictive composite magnetic induction element (5-2). Specifically: In the formula, μ1 is the magnetic permeability of the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2), and μ2 is the magnetic permeability of the magnetic conductor (2); The step tenons of the first magnetostrictive composite magnetic induction element (5-1) and the second magnetostrictive composite magnetic induction element (5-2) are three-order tenons. The first-order tenon, the second-order tenon and the third-order tenon are arranged in ascending order of the bottom area of the tenon. l1 is the height of the first-order tenon, S1 is the bottom area of the first-order tenon, l2 is the height of the second-order tenon and the third-order tenon, S2 is the bottom area of the second-order tenon, and S3 is the bottom area of the third-order tenon; The step tenons of the circular electromagnetic pure iron (21) are three-order tenons. The first-order tenon, the second-order tenon and the third-order tenon are arranged in ascending order of the bottom area of the tenon. The heights of the first-order tenon, the second-order tenon and the third-order tenon of the circular electromagnetic pure iron (21) are equal to l3. S4, S5 and S6 are the bottom areas of the first-order tenon, the second-order tenon and the third-order tenon respectively; l4 is half of the length of the cuboid electromagnetic pure iron (22); l is the annular length of each circular electromagnetic pure iron (21) ring; I1 is the current of the wire (1); (2): The DC regulated power supply (4) supplies power to the coil (3). After the coil (3) is powered on, a magnetic field is generated. The magnetic field generated by the coil (3) is transmitted along the magnetic conductor (2) to the magnetostrictive composite magnetic induction element (5) to provide a bias magnetic field for the magnetostrictive composite magnetic induction element (5). The bias magnetic field provided by the coil (3) is obtained by calculation; (3): The magnetostrictive composite magnetic induction element (5) generates strain under the action of the two magnetic fields. The strain generated by the magnetostrictive composite magnetic induction element (5) causes the central wavelength of the fiber grating (6) attached to the surface to change; (4): The fiber Bragg grating demodulator (7) amplifies and acquires the data of the signal returned by the fiber Bragg grating (6), and the acquired data is sent to the computer (8) for signal processing and calculation, and finally the signal of the measured sensor is demodulated to realize the measurement of the current of the wire (1).

7. The sensing signal demodulation method according to claim 6, characterized in that, The bias magnetic field provided by the coil (3) in step (2) is calculated by the following formula: Where N is the number of turns of the coil, I2 is the current passing through the coil, and L e is the length of the rectangular electromagnetic pure iron (22).

8. The demodulation method of the sensing signal according to claim 7, wherein Wavelength change amount Δλ B As shown in the following formula: Where λ B1 is the central wavelength length without magnetic field, λ B2 is the central wavelength length under the action of two magnetic fields, n eff is the effective refractive index of the fiber grating, Λ is the grating period, P e is the effective photoelastic coefficient of the optical fiber, L is the effective length of the fiber grating, and k is the magnetostrictive coefficient of the magnetostrictive composite magnetic induction element (5).

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