A multi-parameter optical fiber monitoring device for loosening of power transmission tower bolts

Abstract

This application provides a multi-parameter fiber optic monitoring device for bolt loosening on transmission towers, belonging to the field of bolt loosening monitoring. It aims to solve the problems of existing bolt loosening monitoring schemes, such as susceptibility to strong electromagnetic interference, low integration, inability to cover the entire loosening cycle, and high false alarm rate. This device includes an integrated fiber optic grating-Fabry-Perot cavity sensor head and signal demodulation system, employing a coaxial push rod-mass block integrated linkage structure to achieve synchronous acquisition and mechanical decoupling of bolt axial displacement and triaxial vibration. Through a dual-band optical path architecture of 1310nm O-band and 1550nm C-band, combined with a dual-window wavelength division multiplexer, complete optical decoupling of the two parameters is achieved. It is equipped with dual-link independent wavelength calibration, hardware full-link synchronous triggering, and a dual-parameter mutual verification mechanism. This application is inherently resistant to strong electromagnetic interference and can achieve highly reliable monitoring of bolt loosening throughout its entire lifecycle.

Description

Technical Field

[0001] This application relates to the field of bolt loosening monitoring, and in particular to a multi-parameter fiber optic monitoring device for bolt loosening on power transmission towers. Background Technology

[0002] As the core supporting structure of high-voltage transmission lines, transmission towers rely on high-strength bolts for fixing at all connection points. The preload of these bolts directly determines the stability of the tower structure and the safe operation of the transmission line. Bolt loosening is one of the most prominent potential faults in the long-term operation of transmission towers. Under the influence of outdoor temperature changes, wind, rain, vibration, corrosion, and aging, the bolt preload gradually decreases, progressing from early micrometer-level gap displacement to abnormal structural vibration. Ultimately, this can lead to major power grid accidents such as tower connection failure, tower collapse, and line breakage, causing huge economic losses and adverse social impacts.

[0003] With the increasing maturity of fiber optic sensing technology, its passive operation, resistance to electromagnetic interference, high sensitivity, and tolerance to harsh environments have garnered widespread attention in the power industry. However, currently, there is a lack of efficient monitoring solutions for monitoring bolt loosening on transmission towers that can simultaneously capture early minute displacements and later vibration anomalies, feature integrated optical path design, and adapt to strong outdoor electromagnetic and wide-temperature environments. Therefore, developing a multi-parameter complementary, compact, and interference-resistant fiber optic monitoring device to achieve early and accurate warning and real-time monitoring of bolt loosening is an urgent need to improve the operation and maintenance level of transmission lines and ensure the safe and stable operation of the power grid. Summary of the Invention

[0004] To address the problems of existing transmission tower bolt loosening monitoring schemes, such as the susceptibility of electrical sensors to strong electromagnetic interference, low integration of existing fiber optic sensing schemes, inability to cover the entire bolt loosening cycle, and high false alarm rate, this application proposes a multi-parameter fiber optic monitoring device for transmission tower bolt loosening. By constructing an integrated sensing architecture, the axial micro-displacement of the bolt and the loosening vibration are synchronously acquired through the same transmission component, and dual-parameter composite modulation is completed within the same optical fiber. This avoids the design defects of splicing dual sensing systems and achieves accurate and highly reliable monitoring of the entire bolt loosening cycle.

[0005] The technical solution adopted in this application is: a multi-parameter fiber optic monitoring device for loose bolts on transmission towers, including a fiber optic grating-Fabry-Perot cavity integrated sensor head and a signal demodulation system. The fiber optic grating-Fabry-Perot cavity integrated sensor head includes a closed integrated sensor head shell, and the integrated sensor head shell is coaxially nested and fixed with the bolt being measured. Inside the integrated sensor head housing, along the central axis from the end of the bolt being measured to the end of the optical fiber outlet, a rigid coaxial push rod, a Fabry-Perot interferometer moving end mirror, a hollow cylindrical inertial mass block, and a single-mode sensing fiber are arranged coaxially in sequence. The rigid coaxial push rod has a stepped coaxial structure, including a thick shaft section and a thin shaft section. One end face of the thick shaft section of the rigid coaxial push rod is rigidly fixed to the end face of the bolt being measured, and the Fabry-Perot interferometer moving end mirror is rigidly fixed coaxially to the end face of the thin shaft section of the rigid coaxial push rod; the other end face of the thick shaft section of the rigid coaxial push rod is rigidly fixed coaxially to the end face of the hollow cylindrical inertial mass block. Four elastic cylindrical rods are orthogonally and symmetrically distributed on the outer periphery of a hollow cylindrical inertial mass block. The inner end of each elastic cylindrical rod is fixed to the outer wall of the hollow cylindrical inertial mass block, and the outer end of each elastic cylindrical rod is fixed to the inner wall of the integrated sensor head shell through a corresponding fixed base. Vibration-sensitive fiber gratings corresponding to the radial X-axis and Y-axis are attached and fixed at the midpoint of each elastic cylindrical rod. The two fiber gratings on the X-axis and Y-axis form push-pull differential measurement pairs respectively. One end face of the single-mode sensing fiber is coated with a high-reflectivity film to serve as the fixed-end reflector of the Fabry-Perot interferometer cavity. This forms an eigentype parallel-plane Fabry-Perot interferometer cavity with the moving-end reflector of the Fabry-Perot interferometer cavity. One side of the fixed-end reflector of the Fabry-Perot interferometer cavity is a gratingless suspended section that coaxially passes through the central through-hole of the hollow cylindrical inertial mass block. The right side of the gratingless suspended section is a Z-axis axial vibration sensitive fiber grating. One end of the Z-axis axial vibration sensitive fiber grating is rigidly fixed to the center of the end face of the hollow cylindrical inertial mass block, and the other end of the Z-axis axial vibration sensitive fiber grating is rigidly fixed to the inner wall of the integrated sensing head housing. The single-mode sensing fiber is inscribed with a Z-axis axial vibration sensitive fiber grating, two fiber gratings on the X-axis and two fiber gratings on the Y-axis. The end of the single-mode sensing fiber passes through the fixed connector of the integrated sensing head housing and is connected to the signal demodulation system.

[0006] Furthermore, both the moving end mirror of the Fabry-Perot cavity and the fixed end mirror of the Fabry-Perot interferometer cavity are coated with a dielectric film that is semi-reflective and semi-transparent in the O-band at 1310nm and highly transparent in the C-band at 1550nm, forming stable multi-beam interference only in the 1310nm band and without interference modulation on the 1550nm band light.

[0007] Furthermore, on the outer periphery of the hollow cylindrical inertial mass block, four elastic cylindrical rods are orthogonally and symmetrically distributed at 90° angles along the positive and negative X-axis and the positive and negative Y-axis; among them, the positive X-axis elastic cylindrical rod and the negative X-axis elastic cylindrical rod are symmetrically arranged along the X-axis, and the positive Y-axis elastic cylindrical rod and the negative Y-axis elastic cylindrical rod are symmetrically arranged along the Y-axis; a positive X-axis sensitive fiber grating is attached and fixed at the midpoint of the positive X-axis elastic cylindrical rod, a negative X-axis sensitive fiber grating is attached and fixed at the midpoint of the negative X-axis elastic cylindrical rod, a positive Y-axis sensitive fiber grating is attached and fixed at the midpoint of the positive Y-axis elastic cylindrical rod, and a negative Y-axis sensitive fiber grating is attached and fixed at the midpoint of the negative Y-axis elastic cylindrical rod. The five fiber gratings—Z-axis axial vibration-sensitive fiber grating, X-axis positive sensitive fiber grating, X-axis negative sensitive fiber grating, Y-axis positive sensitive fiber grating, and Y-axis negative sensitive fiber grating—are equally spaced in the C-band range of 1525–1565 nm. In operation, the reflection spectra of any two fiber gratings do not overlap or crosstalk. The five fiber gratings allow high transmission of 1310 nm O-band light without modulation.

[0008] Furthermore, the thin shaft section of the rigid coaxial push rod passes coaxially through the coaxial guide tube fixed to the inner wall of the integrated sensor head housing. The thin shaft section of the rigid coaxial push rod forms a precise sliding fit with the inner hole of the coaxial guide tube. One end of the coaxial guide tube extends to the front end of the moving end mirror of the Fabry-Perot cavity to ensure the long-term parallelism of the two mirrors of the Fabry-Perot interference cavity.

[0009] Furthermore, the gratingless suspended section of the fixed-end reflector of the Fabry-Perot cavity coaxially passes through the central through-hole of the hollow cylindrical inertial mass block, and is suspended without contact with the inner wall of the through-hole, in order to avoid interference with displacement measurement caused by fiber bending.

[0010] Furthermore, the Z-axis axial vibration sensitive fiber Bragg grating is pre-stretched during packaging and is arranged coaxially with the central axis of the fiber Bragg grating-Faber cavity integrated sensor head without bending, in order to ensure the linearity and sensitivity of axial vibration measurement.

[0011] Furthermore, the signal demodulation system includes an O-band displacement measurement module, a C-band acceleration measurement module, and a signal demodulation and processing module. The O-band displacement measurement module includes a tunable laser, an optical isolator, a first 1×2 fiber coupler, a first fiber circulator, a 1310nm O-band fiber Fabry-Perot etalon, a first reference photodetector, and a first photodetector. The C-band acceleration measurement module includes a broadband light source, a second fiber circulator, a tunable Fabry-Perot filter, a second 1×2 fiber coupler, and a 1550nm C-band fiber optic Fabry-Perot etalon. The optical signals output from the fiber optic Fabry-Perot etalon, the second reference photodetector, the second photodetector, the O-band displacement measurement module, and the C-band acceleration measurement module are respectively connected to the two input terminals of the wavelength division multiplexer. The wavelength division multiplexer is a 1310nm / 1550nm dual-window wavelength division multiplexer. Its port a corresponds to the 1310nm O-band channel, port b corresponds to the 1550nm C-band channel, and port c, the common port, is connected to the fiber optic grating-Fabry-Perot cavity integrated sensing head to realize the beam combining transmission of dual-band light and reverse wavelength division demodulation. The electrical signals output by the O-band displacement measurement module and the C-band acceleration measurement module are both connected to the signal demodulation and processing module to complete the real-time demodulation, data storage, and anomaly warning of the dual-parameter signals.

[0012] Furthermore, the output of the tunable laser is connected to the input of the optical isolator, the output of the optical isolator is connected to the a input of the first 1×2 fiber coupler, the b output of the first 1×2 fiber coupler is connected to the a input of the first fiber circulator, the c output of the first 1×2 fiber coupler is connected to the input of the 1310nm O-band fiber Fabry-Perot etalon; the b bidirectional port of the first fiber circulator is connected to the a port of the wavelength division multiplexer, the c output of the first fiber circulator is connected to the input of the first photodetector; and the output of the 1310nm O-band fiber Fabry-Perot etalon is connected to the input of the first reference photodetector.

[0013] Furthermore, the output of the broadband light source is connected to input a of the second fiber optic circulator; the bidirectional b port of the second fiber optic circulator is connected to port b of the wavelength division multiplexer; the c output of the second fiber optic circulator is connected to input a of the tunable Fabry-Perot filter; the b output of the tunable Fabry-Perot filter is connected to input a of the second 1×2 fiber optic coupler; the c output of the second 1×2 fiber optic coupler is connected to input a of the second photodetector; the b output of the second 1×2 fiber optic coupler is connected to input a 1550nm C-band fiber optic Fabry-Perot etalon; and the output of the 1550nm C-band fiber optic Fabry-Perot etalon is connected to input a second reference photodetector.

[0014] Furthermore, the outputs of the first photodetector, the first reference photodetector, the second photodetector, and the second reference photodetector are connected to the a, b, c, and d input ports of the data acquisition card, respectively. The digital trigger output e of the data acquisition card is connected to the external trigger input of the tunable laser and the external trigger input c of the tunable Fabry-Perot filter, respectively, serving as the main trigger source of the system to achieve full-link hardware synchronization. The f output of the data acquisition card is connected to the signal demodulation and processing module, and the output of the signal demodulation and processing module is connected to the input of the computer, completing the real-time demodulation, data storage, and anomaly warning of the dual-parameter signal.

[0015] The advantages of this application over the prior art are as follows: I. This application utilizes an integrated optical path design, integrating the fixed-end reflector of the Fabry-Perot interferometer cavity and the fiber grating onto the same optical fiber to form an inseparable optical structure. Furthermore, the bolt loosening displacement and triaxial vibration acceleration are simultaneously acquired through the same rigid coaxial push rod, overcoming the low integration defects of existing solutions. Simultaneously, the dual-parameter monitoring adopts a dual-band segmented design of O-band and C-band, combined with the multiplexing and demultiplexing architecture of a dual-window high-isolation wavelength division multiplexer. Within the same sensing fiber and the same coaxial integrated sensing head, the displacement and vibration dual parameters are decoupled, suppressing crosstalk noise at the source, eliminating mutual modulation crosstalk between the two parameters, improving the sensing signal-to-noise ratio, and achieving highly stable synchronous monitoring of dual parameters.

[0016] Second, this application achieves synchronous high-precision measurement of displacement and dynamic vibration through multi-parameter complementary monitoring and precise matching of dual links and sensing units, enabling accurate identification of loosening throughout the entire cycle. The dual-parameter mutual verification effectively filters false loosening signals caused by temperature changes and wind loads, avoiding the limitations of single-parameter monitoring, reducing the false alarm rate of outdoor field monitoring, and realizing full-cycle monitoring from early warning to accurate judgment. At the same time, this application configures fiber optic Fabry-Perot etalons and independent calibration branches for the displacement and vibration demodulation links respectively, which can correct the sweep frequency nonlinearity, temperature drift, and wavelength jump of the tunable laser in real time, as well as the inherent errors of the tunable Fabry-Perot filter such as piezoelectric hysteresis and scanning hysteresis, adapting to the needs of long-term online monitoring of transmission lines.

[0017] Third, this application adopts a coaxial push rod-mass block integrated linkage structure to achieve co-source transmission and mechanical decoupling of axial displacement and triaxial vibration acceleration. By rigidly connecting the rigid coaxial push rod and the inertial mass block to form an integrated linkage component, the two measured physical quantities are synchronously transmitted along the same mechanical path, ensuring the temporal co-source and spatial synchronization of the two parameters. Specifically, the axial displacement directly changes the Fabry-Perot cavity length through the push rod, and the vibration generates tensile and compressive strains on the triaxial fiber grating region through the inertial force of the mass block. The two physical quantities coexist on the same mechanical path, and are separated and do not interfere with each other through different sensing mechanisms, achieving mechanical decoupling. The device as a whole is a fully sealed integrated structure, naturally resistant to strong electromagnetic interference, and fully adaptable to the outdoor environment of high-voltage and strong electromagnetic fields of transmission towers. The structural dimensions can be fully adapted to the end face monitoring requirements of transmission tower bolts, making installation and maintenance convenient and suitable for long-term online monitoring of transmission lines. Attached Figure Description

[0018] The following description, in conjunction with the accompanying drawings, further illustrates this application: Figure 1 A schematic diagram of the axial longitudinal section of the fiber optic grating-Faber cavity integrated sensor head of the multi-parameter fiber optic monitoring device for loosening transmission tower bolts provided in this embodiment of the application; Figure 2 for Figure 1 The right view; Figure 3 A schematic diagram of the signal demodulation system structure of the multi-parameter fiber optic monitoring device for loose bolts on power transmission towers provided in this application; In the diagram: 1 is a rigid coaxial push rod; 2 is a Fabry-Perot cavity moving end reflector; 3 is a Fabry-Perot cavity fixed end reflector; 4 is a coaxial guide tube; 5 is an integrated sensor head housing; 6 is a Z-axis axial vibration-sensitive fiber grating; 7 is a single-mode sensing fiber; 8 is a hollow cylindrical inertial mass block; 9 is a positive X-axis elastic rod fixing base; 10 is a negative X-axis elastic rod fixing base; 11 is a positive Y-axis elastic rod fixing base; 12 is a negative Y-axis elastic rod fixing base; 13 is a positive X-axis elastic cylindrical rod; 14 is a negative X-axis elastic cylindrical rod; 15 is a positive Y-axis elastic cylindrical rod; 16 is a negative Y-axis elastic cylindrical rod; 17 is a positive X-axis sensitive fiber grating; 18 is a negative X-axis sensitive fiber grating; 19 is a positive Y-axis sensitive fiber grating; 20 is a Y-axis... 21 is a negative-sensitive fiber Bragg grating, 22 is a tunable laser, 23 is an optical isolator, 24 is a first 1×2 fiber coupler, 25 is a first fiber circulator, 26 is a broadband light source, 27 is a second fiber circulator, 28 is a wavelength division multiplexer, 29 is a fiber Bragg grating-Fabry-Perot cavity integrated sensing head, 30 is a tunable Fabry-Perot filter, 31 is a second 1×2 fiber coupler, 32 is a 1550nm C-band fiber Fabry-Perot etalon, 33 is a second-channel reference photodetector, 34 is a 1310nm O-band fiber Fabry-Perot etalon, 35 is a first-channel reference photodetector, 36 is a first-channel photodetector, 37 is a data acquisition card, 38 is a signal demodulation and processing module, and 39 is a computer. Detailed Implementation

[0019] like Figures 1 to 3 As shown, this application provides a multi-parameter fiber optic monitoring device for loose bolts on transmission towers, comprising a fiber optic grating-Fabry-Perot cavity integrated sensor head 28 and a signal demodulation system. The integrated fiber optic grating-Fabry-Perot cavity sensor head 28 includes a closed integrated sensor head housing 5. Inside the integrated sensor head housing 5 are a rigid coaxial push rod 1, a Fabry-Perot cavity moving end reflector 2, a Fabry-Perot cavity fixed end reflector 3, a coaxial guide tube 4, a Z-axis axial vibration-sensitive fiber optic grating 6, and a single-mode sensing fiber. 7. Hollow cylindrical inertial mass block; 8. X-axis positive elastic rod fixing base; 9. X-axis negative elastic rod fixing base; 10. Y-axis positive elastic rod fixing base; 11. Y-axis negative elastic rod fixing base; 12. X-axis positive elastic cylindrical rod; 13. X-axis negative elastic cylindrical rod; 14. Y-axis positive elastic cylindrical rod; 15. Y-axis negative elastic cylindrical rod; 16. X-axis positive sensitive fiber optic grating; 17. X-axis negative sensitive fiber optic grating; 18. Y-axis positive sensitive fiber optic grating; 19. Y-axis negative sensitive fiber optic grating; 20.

[0020] The fiber optic grating-Fabry-Perot cavity integrated sensor head 28 is a closed shell structure. The left end face of the integrated sensor head shell 5 has an internal hexagonal countersunk platform that matches the bolt being tested, used to achieve coaxial nesting positioning between the integrated sensor head shell 5 and the bolt. Inside the integrated sensor head shell 5, along the central axis from the bolt end to the fiber optic output end, a rigid coaxial push rod 1, a Fabry-Perot cavity moving end reflector 2, a hollow cylindrical inertial mass block 8, and a single-mode sensing fiber 7 are arranged coaxially in sequence. The rigid coaxial push rod 1 has a stepped coaxial structure, consisting of a thick shaft section and a thin shaft section from left to right. The left end of the rigid coaxial push rod 1, i.e., the thick shaft section, is rigidly fixed to the end face of the bolt being tested, and is completely coaxially aligned with the bolt. The right end of the rigid coaxial push rod 1, i.e., the rightmost end face of the thin shaft section, has the Fabry-Perot cavity moving end reflector 2 rigidly fixed coaxially, which moves axially synchronously with the rigid coaxial push rod 1. The thin shaft section coaxially passes through the coaxial guide tube 4, which is fixed to the inner wall of the front end of the integrated sensor head housing 5, ensuring the parallelism of the two mirrors of the Fabry-Perot interferometer cavity. The rightmost end of the thick shaft section of the rigid coaxial push rod 1 is rigidly and coaxially fixed to the left end face of the hollow cylindrical inertial mass block 8. The hollow cylindrical inertial mass block 8 moves rigidly and synchronously with the rigid coaxial push rod 1 and the bolt being measured, providing inertial force for the triaxial acceleration sensing.

[0021] The leftmost end face of the single-mode sensing fiber 7 is coated with a high-reflectivity film, serving as the fixed-end reflector 3 of the Fabry-Perot interferometer cavity. This forms a non-intrinsic parallel-plane Fabry-Perot interferometer cavity with the moving-end reflector 2. Both the moving-end reflector 2 and the fixed-end reflector 3 are coated with a dielectric film that provides semi-reflection and semi-transparency in the O-band (1310nm) and high transmittance in the C-band (1550nm), forming stable multi-beam interference only in the 1310nm band, with no interference modulation in the 1550nm band. The right side of the fixed-end reflector 3 is a gratingless suspended section, coaxially passing through the central through-hole of the hollow cylindrical inertial mass block 8, without contact with the inner wall of the through-hole, without bending, and suspended in mid-air. The right side of the grating-free suspended section is a Z-axis axial vibration sensitive fiber optic grating 6. The left end of the grating is rigidly fixed to the center of the right end face of the hollow cylindrical inertial mass block 8 through a metal capillary sleeve, and the right end is rigidly fixed to the inner wall of the rear end of the integrated sensor head shell 5 through a metal capillary sleeve. The grating is pre-stretched during packaging, coaxial with the central axis and without bending, and is used to measure the axial vibration acceleration of the bolt.

[0022] Four elastic cylindrical rods are orthogonally and symmetrically distributed at 90° along the positive and negative X-axis and the positive and negative Y-axis on the outer periphery of the hollow cylindrical inertial mass block 8. Among them, the elastic cylindrical rod 13 and the elastic cylindrical rod 14 along the positive X-axis are symmetrically arranged along the X-axis, and the elastic cylindrical rod 15 and the elastic cylindrical rod 16 along the positive Y-axis are symmetrically arranged along the Y-axis. The inner end of each elastic cylindrical rod is rigidly fixed to the outer wall of the hollow cylindrical inertial mass block 8, and the outer end of each elastic cylindrical rod is rigidly fixed to the inner wall of the integrated sensor head shell 5 through a corresponding fixed base. After passing through the Z-axis grating segment, the single-mode sensing fiber 7 travels along a preset path to four elastic cylindrical rods. A corresponding radial vibration-sensitive fiber grating is fixedly attached to the midpoint of each elastic cylindrical rod. Specifically, the X-axis positive sensitive fiber grating 17 and the X-axis negative sensitive fiber grating 18 are fixed to the two elastic rods along the X-axis, forming an X-axis push-pull differential measurement pair. Similarly, the Y-axis positive sensitive fiber grating 19 and the Y-axis negative sensitive fiber grating 20 are fixed to the two elastic rods along the Y-axis, forming a Y-axis push-pull differential measurement pair, used to measure the radial vibration acceleration of the bolt. The differential measurement pair enables temperature self-compensation and simultaneously improves the radial acceleration measurement sensitivity, serving as the core sensitive unit for measuring the radial loosening acceleration of the bolt. All fiber Bragg gratings are cascaded onto the same single-mode sensing fiber 7, sharing the same optical path with the Fabry-Perot interferometer cavity to form a composite modulation spectrum. The center wavelengths of all fiber Bragg gratings are evenly spaced within the C-band range of 1525–1565 nm, and the reflection spectra of any two fiber Bragg gratings do not overlap or crosstalk. All fiber Bragg gratings exhibit high transmittance and unmodulated passage of 1310 nm O-band light, without interfering with the displacement measurement of the Fabry-Perot cavity. The end of the single-mode sensing fiber 7 is a transmission fiber segment, extending rearward to the outlet housing on the right side of the integrated sensing head housing 5. A sealed fixing connector is installed at the fiber outlet on the right side, through which the fiber passes. The connector, housing, and fiber are all sealed.

[0023] The signal demodulation system includes a tunable laser 21, an optical isolator 22, a first 1×2 fiber coupler 23, a first fiber circulator 24, a broadband light source 25, a second fiber circulator 26, a wavelength division multiplexer 27, a fiber optic grating-Fabry-Perot cavity integrated sensor head 28, a tunable Fabry-Perot filter 29, a second 1×2 fiber coupler 30, a 1550nm C-band fiber Fabry-Perot etalon 31, a second reference photodetector 32, a second photodetector 33, a 1310nm O-band fiber Fabry-Perot etalon 34, a first reference photodetector 35, a first photodetector 36, a data acquisition card 37, a signal demodulation and processing module 38, and a computer 39.

[0024] The tunable laser 21 is a 1310nm O-band displacement measurement-specific narrow-linewidth tunable laser, outputting linearly swept light with a wavelength range of 1280~1360nm. Its output end is connected to the input end of the optical isolator 22, which allows the light source to pass through in one direction and blocks backflow of reflected light into the tunable laser 21. The output end of the optical isolator 22 is connected to the a input end of the first 1×2 fiber coupler 23, which splits the input light into two paths with a 90:10 ratio. 90% of the main path light is used for displacement sensing measurement, and 10% of the branch path light is used for wavelength calibration. The 90% main path b output end of the first 1×2 fiber coupler 23 is connected to the a input end of the first fiber circulator 24, and the 10% calibration branch c of the first 1×2 fiber coupler 23 is connected to the other two paths. The output is connected to the input of a 1310nm O-band fiber Fabry-Perot etalon 34; the first fiber circulator 24 is a 1310nm O-band dedicated three-port circulator used to separate the forward incident light from the reverse reflected light; the 1310nm O-band fiber Fabry-Perot etalon 34 provides a fixed wavelength reference for real-time calibration of the frequency sweeping process of the tunable laser 21; the b-port of the first fiber circulator 24 is connected to the a-port of the wavelength division multiplexer 27, and the c-output of the first fiber circulator 24 is connected to the input of the first photodetector 36; the output of the 1310nm O-band fiber Fabry-Perot etalon 34 is connected to the input of the first reference photodetector 35 for real-time calibration of the frequency sweeping wavelength of the tunable laser 21. The first reference photodetector 35 converts the calibration optical signal into an electrical signal, providing a data basis for wavelength calibration.

[0025] The broadband light source 25 is a dedicated broadband light source for C-band acceleration measurement at 1550nm, outputting continuous broadband light with a wavelength range of 1525~1565nm. Its output end is connected to the a input end of the second fiber optic circulator 26. The second fiber optic circulator 26 is a dedicated three-port circulator for C-band at 1550nm, used to separate the forward incident light from the reverse reflected light. The b bidirectional port of the second fiber optic circulator 26 is connected to the b port of the wavelength division multiplexer 27, and the c output end of the second fiber optic circulator 26 is connected to the a input end of the tunable Fabry-Perot filter 29. The tunable Fabry-Perot filter 29 is used to perform high-speed wavelength scanning on the returned broadband light to demodulate the fiber grating reflection spectrum. The b output end of the tunable Fabry-Perot filter 29 is connected to the a input end of the second 1×2 fiber optic coupler 30. Fiber optic coupler 30 splits the input light into two 50:50 paths, one for vibration sensing measurement and the other for wavelength calibration. The output (c) of the second 1×2 fiber optic coupler 30 is connected to the input of the second photodetector 33 to acquire the vibration sensing signal of the fiber optic grating. The output (b) of the second 1×2 fiber optic coupler 30 is connected to the input of a 1550nm C-band fiber Fabry-Perot etalon 31. The 1550nm C-band fiber Fabry-Perot etalon 31 provides a fixed wavelength reference for real-time calibration of the tunable Fabry-Perot filter 29 during scanning. The output of the 1550nm C-band fiber Fabry-Perot etalon 31 is connected to the input of the second reference photodetector 32 for real-time calibration of the scanning wavelength of the tunable Fabry-Perot filter 29. The second reference photodetector 32 converts the calibration optical signal into an electrical signal, providing a data basis for wavelength calibration.

[0026] The wavelength division multiplexer 27 is a 1310nm / 1550nm dual-window wavelength division multiplexer. Its port a corresponds to the 1310nm O-band channel, port b corresponds to the 1550nm C-band channel, and port c (common port) is connected to the fiber optic grating-Fabry-Perot cavity integrated sensor head 28 to realize the beam combining and reverse wavelength division demodulation of the dual-band light. The output terminals of the first photodetector 36, the first reference photodetector 35, the second photodetector 33, and the second reference photodetector 32 are respectively connected to ports a, b, and c of the data acquisition card 37. The c and d input ports are connected; the data acquisition card 37 is a 4-channel synchronous sampling data acquisition card, and its digital trigger e output terminal is connected to the external trigger input terminal of the tunable laser 21 and the external trigger input terminal c of the tunable Fabry-Perot filter 29, respectively, to achieve full-link hardware synchronization as the main trigger source of the system; the f output terminal of the data acquisition card 37 is connected to the signal demodulation and processing module 38, and the output terminal of the signal demodulation and processing module 38 is connected to the input terminal of the computer 39 to complete the real-time demodulation, data storage and abnormal early warning of the dual-parameter signal.

[0027] The principle of bolt loosening displacement detection in this application is as follows: When the bolt undergoes axial loosening and displacement, the loosening displacement is synchronously transmitted to the Fabry-Perot interferometer cavity through the rigid coaxial push rod 1. The axial micro-displacement of the bolt is obtained by demodulating the change in cavity length, achieving high-sensitivity capture of early micro-loosening of the bolt. The characteristic vibration caused by bolt loosening is synchronously transmitted through the same rigid coaxial push rod 1, and the triaxial vibration acceleration of the bolt is obtained by demodulation through the fiber optic grating array. The signal demodulation and processing module 38 obtains the bolt displacement and triaxial vibration acceleration dual parameters collected by the same rigid coaxial push rod 1 through synchronous demodulation. Relying on the dual parameter mutual verification mechanism, the accurate determination of real loosening is achieved, distinguishing between permanent bolt loosening and temporary displacement caused by temperature change and wind load, and reducing the false alarm rate of on-site monitoring.

[0028] When the tested bolt is secure and the system is operating normally, the tunable laser 21 outputs a 1310nm linearly swept beam. After being split by the optical isolator 22 and the first 1×2 fiber coupler 23, 90% of the main beam enters the dedicated 1310nm channel of the wavelength division multiplexer 27 via the first fiber circulator 24. After being combined by the wavelength division multiplexer 27, the beam enters the fiber grating-Fabry-Perot cavity integrated sensing head 28. The 1310nm probe beam generates stable multi-beam interference only within the Fabry-Perot interferometer cavity. The interference-reflected light carrying the initial cavity length information returns along the original path, and after being reverse-splittered by the wavelength division multiplexer 27, it enters the first photodetector 36 through the c output terminal of the first fiber circulator 24, completing the conversion of optical signal to electrical signal; the 10% calibration branch light split off from the first 1×2 fiber coupler 23 enters the first reference photodetector 35 through the 1310nm O-band fiber Fabry-Perot etalon 34, providing real-time wavelength calibration for the frequency sweeping process of the tunable laser 21. Meanwhile, the broadband light source 25 outputs 1550nm broadband light, which enters the dedicated 1550nm channel of the wavelength division multiplexer 27 via the second fiber optic circulator 26. After being bundled by the wavelength division multiplexer 27, it enters the fiber grating-Fabry-Perot cavity integrated sensing head 28. The 1550nm probe light passes through the Fabry-Perot interferometer cavity without interference modulation and is only reflected by the series-connected triaxial fiber grating. The reflected light carrying the initial strain information returns along the original path, and after being reverse-splittered by the wavelength division multiplexer 27, it enters the tunable Fabry-Perot filter 29 through the C-output terminal of the second fiber optic circulator 26. After high-speed wavelength scanning, it is split by the second 1×2 fiber coupler 30. One beam enters the second photodetector 33 to complete the conversion of the fiber grating vibration sensing optical signal to an electrical signal. The other beam enters the second reference photodetector 32 via the 1550nm C-band fiber Fabry-Perot etalon 31 to provide real-time wavelength calibration for the scanning process of the tunable Fabry-Perot filter 29. The data acquisition card 37 serves as the system's sole main trigger source, synchronously triggering the frequency sweeping action of the tunable laser 21 and the scanning action of the tunable Fabry-Perot filter 29. Simultaneously, it synchronously samples the four input signals to ensure that the time axes of the displacement sensing signal, vibration sensing signal, and corresponding wavelength calibration signal are 100% aligned. The acquired electrical signals are transmitted to the signal demodulation and processing module 38, which calculates the initial cavity length of the bolt and the initial center wavelength of the grating, storing them as reference values ​​in the computer 39.

[0029] When the tested bolt loosens axially and causes displacement, the rigid coaxial push rod 1 moves axially synchronously with the tested bolt, causing the moving end reflector 2 of the Fabry-Perot interferometer cavity to undergo coaxial displacement, directly changing the cavity length of the eigentype Fabry-Perot interferometer cavity, and generating phase modulation on the 1310nm O-band probe light; the interference reflected light carrying the loosening displacement information returns along the original path, and after being reversed by the wavelength division multiplexer 27, it enters the first photodetector 36 through the c-output terminal of the first fiber optic circulator 24, completing the conversion of optical signal to electrical signal; One photodetector 36 transmits the converted electrical signal to the input port a of the data acquisition card 37. The data acquisition card 37 then sends the acquired signal to the signal demodulation and processing module 38. The signal demodulation and processing module 38, in conjunction with the calibration signal synchronously acquired by the first reference photodetector 35, corrects the sweep frequency nonlinearity, temperature drift, and wavelength jump error of the tunable laser 21, accurately calculates the real-time cavity length change of the Fabry-Perot cavity, and finally obtains the axial micro-displacement of the bolt loosening, achieving nanometer-level resolution capture of early bolt micro-loosening.

[0030] When the tested bolt loosens, it generates characteristic vibrations under wind loads, which are synchronously transmitted to the hollow cylindrical inertial mass block 8 via the rigid coaxial push rod 1. Under inertial force, the hollow cylindrical inertial mass block 8 undergoes triaxial vibration: axial vibration drives the hollow cylindrical inertial mass block 8 to reciprocate axially, generating axial tensile and compressive strains on the Z-axis axial vibration-sensitive fiber grating 6 fixed at both ends, thus modulating the wavelength of the 1550nm C-band probe light; radial vibration drives the hollow cylindrical inertial mass block 8 to reciprocate radially, causing bending deformation of the corresponding elastic cylindrical rod, which in turn causes regular strains in the X-axis and Y-axis push-pull differential fiber grating pairs, generating differential wavelength modulation of the 1550nm C-band probe light; the fiber grating reflected light carrying vibration acceleration information returns along the original path and is then reverse-splitted by the wavelength division multiplexer 27. The light enters the tunable Fabry-Perot filter 29 through the output port C of the second fiber optic circulator 26, and after high-speed wavelength scanning, it is split by the second 1×2 fiber optic coupler 30. One light enters the second photodetector 33 to complete the conversion of the optical signal carrying vibration information into an electrical signal. The second photodetector 33 transmits the converted electrical signal to the input port C of the data acquisition card 37. The data acquisition card 37 synchronously sends the acquired signal to the signal demodulation and processing module 38. The signal demodulation and processing module 38, combined with the calibration signal synchronously acquired by the second reference photodetector 32, corrects the piezoelectric hysteresis, scanning nonlinearity, and temperature drift error of the tunable Fabry-Perot filter 29, accurately extracts the real-time center wavelength drift of each fiber grating, eliminates temperature common-mode interference through differential calculation, and finally obtains the triaxial vibration acceleration of the bolt in the axial and X / Y radial directions.

[0031] The signal demodulation and processing module 38 acquires the bolt displacement and triaxial vibration acceleration dual parameters collected by the same rigid coaxial push rod 1 through synchronous demodulation, and transmits the loosening data synchronously to the computer 39 to complete data storage and visualization.

[0032] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A multi-parameter fiber optic monitoring device for loose bolts on power transmission towers, characterized in that: The system includes a fiber grating-Faber cavity integrated sensor head (28) and a signal demodulation system. The fiber grating-Faber cavity integrated sensor head (28) includes a closed integrated sensor head housing (5), which is coaxially nested and fixed with the bolt being measured. Inside the integrated sensor head housing (5), along the central axis from the end of the bolt being measured to the end of the optical fiber output, a rigid coaxial push rod (1), a Fabry-Perot interferometer moving end reflector (2), a hollow cylindrical inertial mass block (8), and a single-mode sensing fiber (7) are arranged coaxially in sequence. The rigid coaxial push rod (1) is a stepped coaxial structure, including a thick shaft section and a thin shaft section. One end face of the thick shaft section of the rigid coaxial push rod (1) is rigidly fixed to the end face of the bolt being measured. The end face of the thin shaft section of the rigid coaxial push rod (1) is rigidly fixed to the Fabry-Perot interferometer moving end reflector (2) coaxially. The other end face of the thick shaft section of the rigid coaxial push rod (1) is rigidly fixed to the end face of the hollow cylindrical inertial mass block (8) coaxially. Four elastic cylindrical rods are orthogonally and symmetrically distributed on the outer periphery of the hollow cylindrical inertial mass block (8). The inner end of each elastic cylindrical rod is fixed to the outer wall of the hollow cylindrical inertial mass block (8), and the outer end of each elastic cylindrical rod is fixed to the inner wall of the integrated sensor head shell (5) through a corresponding fixed base. The vibration-sensitive fiber gratings of the corresponding radial X-axis and Y-axis are attached and fixed at the midpoint of each elastic cylindrical rod. The two fiber gratings of the X-axis and Y-axis respectively form a push-pull differential measurement pair. One end face of the single-mode sensing fiber (7) is coated with a high-reflection film to serve as the fixed end mirror (3) of the Fabry-Perot interferometer cavity, forming an eigentype parallel plane Fabry-Perot interferometer cavity with the moving end mirror (2) of the Fabry-Perot interferometer cavity. One side of the fixed end mirror (3) of the Fabry-Perot interferometer cavity is a gratingless suspended section, which coaxially passes through the central through hole of the hollow cylindrical inertial mass block (8). The right side of the gratingless suspended section is a Z-axis axial vibration sensitive fiber grating (6). One end of the Z-axis axial vibration sensitive fiber grating (6) is rigidly fixed to the center of the end face of the hollow cylindrical inertial mass block (8), and the other end of the Z-axis axial vibration sensitive fiber grating (6) is rigidly fixed to the inner wall of the integrated sensing head shell (5). The single-mode sensing fiber (7) is inscribed with Z-axis axial vibration sensitive fiber grating (6), two fiber gratings on the X-axis and two fiber gratings on the Y-axis. The end of the single-mode sensing fiber (7) passes through the fixed connector of the integrated sensing head shell (5) and is connected to the signal demodulation system.

2. The multi-parameter fiber optic monitoring device for loose bolts on transmission towers according to claim 1, characterized in that: Both the moving end mirror (2) of the Fabry-Perot cavity and the fixed end mirror (3) of the Fabry-Perot interference cavity are coated with a dielectric film with a 1310nm O-band semi-reflective and 1550nm C-band high transmittance. Stable multi-beam interference is formed only in the 1310nm band, and there is no interference modulation on the 1550nm band light.

3. The multi-parameter fiber optic monitoring device for loose bolts on transmission towers according to claim 2, characterized in that: Four elastic cylindrical rods are orthogonally and symmetrically distributed at 90° on the outer periphery of the hollow cylindrical inertial mass block (8) in the positive and negative X-axis and positive and negative Y-axis directions; among them, the positive X-axis elastic cylindrical rod (13) and the negative X-axis elastic cylindrical rod (14) are symmetrically arranged on the X-axis, and the positive Y-axis elastic cylindrical rod (15) and the negative Y-axis elastic cylindrical rod (16) are symmetrically arranged on the Y-axis; the positive X-axis elastic cylindrical rod (13) is attached to the midpoint of the rod body of the positive X-axis elastic cylindrical rod (13) and the negative X-axis elastic cylindrical rod (14) is attached to the midpoint of the rod body of the negative X-axis elastic cylindrical rod (14) and the positive Y-axis elastic cylindrical rod (15) is attached to the midpoint of the rod body of the positive Y-axis elastic cylindrical rod (15) and the negative Y-axis elastic cylindrical rod (16) is attached to the midpoint of the rod body of the negative Y-axis elastic cylindrical rod (16) and the negative Y-axis elastic cylindrical rod (16) is attached to the midpoint of the rod body of the positive Y-axis elastic cylindrical rod (16) and the negative Y-axis elastic cylindrical rod (16) is attached to the midpoint of the rod body of the positive Y-axis elastic cylindrical rod (13) and the negative Y-axis elastic cylindrical rod (14) is attached to the midpoint of the rod body of the positive Y-axis elastic cylindrical rod (15) and the negative Y-axis elastic cylindrical rod (16) is attached to the midpoint of the rod body of the positive Y-axis elastic cylindrical rod (16) and the negative Y-axis elastic cylindrical rod (15 ... The five fiber gratings (6), (17), (18), (19), and (20) are arranged at equal intervals in the C-band range of 1525~1565nm. Under working conditions, the reflection spectra of any two fiber gratings do not overlap or crosstalk. The five fiber gratings allow high transmission of 1310nm O-band light without modulation.

4. The multi-parameter fiber optic monitoring device for loose bolts on transmission towers according to claim 1, characterized in that: The thin shaft section of the rigid coaxial push rod (1) passes coaxially through the coaxial guide tube (4) fixed to the inner wall of the integrated sensor head housing (5). The thin shaft section of the rigid coaxial push rod (1) forms a precise sliding fit with the inner hole of the coaxial guide tube (4). One end of the coaxial guide tube (4) extends to the front end of the Fabry-Perot cavity moving end reflector (2) to ensure the long-term parallelism of the two mirrors of the Fabry-Perot interference cavity.

5. The multi-parameter fiber optic monitoring device for loose bolts on transmission towers according to claim 1, characterized in that: The gratingless suspended section of the fixed end reflector (3) of the Fabry-Perot cavity coaxially passes through the central through hole of the hollow cylindrical inertial mass block (8), and is suspended without contact with the inner wall of the through hole, in order to avoid the interference of fiber bending on displacement measurement.

6. The multi-parameter fiber optic monitoring device for loose bolts on transmission towers according to claim 1, characterized in that: The Z-axis axial vibration sensitive fiber grating (6) is pre-stretched during packaging and is arranged coaxially with the central axis of the fiber grating-Faber cavity integrated sensor head (28) without bending, in order to ensure the linearity and sensitivity of axial vibration measurement.

7. A multi-parameter fiber optic monitoring device for loose bolts on transmission towers according to claim 2, characterized in that: The signal demodulation system includes an O-band displacement measurement module, a C-band acceleration measurement module, and a signal demodulation and processing module (38). The O-band displacement measurement module includes a tunable laser (21), an optical isolator (22), a first 1×2 fiber coupler (23), a first fiber circulator (24), a 1310nm O-band fiber Fabry-Perot etalon (34), a first reference photodetector (35), and a first photodetector (36). The C-band acceleration measurement module includes a broadband light source (25), a second fiber circulator (26), a tunable Fabry-Perot filter (29), and a second 1×2 fiber coupler (30). A 1550nm C-band fiber Fabry-Perot etalon (31), a second reference photodetector (32), and a second photodetector (33) are provided. The optical signals output by the O-band displacement measurement module and the C-band acceleration measurement module are respectively connected to the two input terminals of the wavelength division multiplexer (27). The wavelength division multiplexer (27) is a 1310nm / 1550nm dual-window wavelength division multiplexer. Its port a corresponds to the 1310nm O-band channel, port b corresponds to the 1550nm C-band channel, and port c is connected to the fiber grating-Fabry-Perot cavity integrated sensor head (28) to realize the beam combining transmission and reverse wavelength division demodulation of dual-band light. The electrical signals output by the O-band displacement measurement module and the C-band acceleration measurement module are both connected to the signal demodulation and processing module (38) to complete the real-time demodulation, data storage and abnormal early warning of the dual-parameter signals.

8. A multi-parameter fiber optic monitoring device for loose bolts on transmission towers according to claim 7, characterized in that: The output of the tunable laser (21) is connected to the input of the optical isolator (22). The output of the optical isolator (22) is connected to the a input of the first 1×2 fiber coupler (23). The b output of the first 1×2 fiber coupler (23) is connected to the a input of the first fiber circulator (24). The c output of the first 1×2 fiber coupler (23) is connected to the input of the 1310nm O-band fiber Fabry-Perot etalon (34). The b bidirectional port of the first fiber circulator (24) is connected to the a port of the wavelength division multiplexer (27). The c output of the first fiber circulator (24) is connected to the input of the first photodetector (36). The output of the 1310nm O-band fiber Fabry-Perot etalon (34) is connected to the input of the first reference photodetector (35).

9. A multi-parameter fiber optic monitoring device for loose bolts on a transmission tower according to claim 7, characterized in that: The output of the broadband light source (25) is connected to the a input of the second fiber optic circulator (26); the b bidirectional port of the second fiber optic circulator (26) is connected to the b port of the wavelength division multiplexer (27); the c output of the second fiber optic circulator (26) is connected to the a input of the tunable Fabry-Perot filter (29); the b output of the tunable Fabry-Perot filter (29) is connected to the a input of the second 1×2 fiber optic coupler (30); the c output of the second 1×2 fiber optic coupler (30) is connected to the input of the second photodetector (33); the b output of the second 1×2 fiber optic coupler (30) is connected to the input of the 1550nm C-band fiber Fabry-Perot etalon (31); and the output of the 1550nm C-band fiber Fabry-Perot etalon (31) is connected to the input of the second reference photodetector (32).

10. A multi-parameter fiber optic monitoring device for loose bolts on a transmission tower according to claim 7, characterized in that: The outputs of the first photodetector (36), the first reference photodetector (35), the second photodetector (33), and the second reference photodetector (32) are connected to the a, b, c, and d input ports of the data acquisition card (37), respectively. The digital trigger e output of the data acquisition card (37) is connected to the external trigger input of the tunable laser (21) and the external trigger input c of the tunable Fabry-Perot filter (29), respectively, to achieve full-link hardware synchronization as the main trigger source of the system. The f output of the data acquisition card (37) is connected to the signal demodulation and processing module (38), and the output of the signal demodulation and processing module (38) is connected to the input of the computer (39) to complete the real-time demodulation, data storage, and abnormal early warning of the dual-parameter signal.

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