F-P displacement sensor, underground pipe gallery and joint displacement detection method

Abstract

The invention relates to the technical field of displacement measurement, and provides an F-P displacement sensor and a method for detecting displacement of an underground pipe gallery and a seam, and the F-P displacement sensor comprises two sections of single-mode optical fibers, a capillary tube, a sealing layer, a packaging film and an adhesive tape. The two sections of single-mode optical fibers are coaxially arranged, and the axes of the single-mode optical fibers coincide with the axis of the F-P displacement sensor; the axis of the capillary tube coincides with the axis of the F-P displacement sensor, the capillary tube surrounds the two sections of single-mode optical fibers in the circumferential direction of the F-P displacement sensor, and the two sections of single-mode optical fibers are longer than the capillary tube and extend outwards from the two ends of the capillary tube; the inner wall of the capillary tube is spaced from the outer wall of the single-mode fiber by a certain distance; the sealing layers are arranged at two ends of the capillary; the packaging film surrounds the capillary tube in the circumferential direction of the F-P displacement sensor. The packaging film is surrounded by the adhesive tape; and the single-mode optical fiber extends outwards from the two ends of the adhesive tape. According to the scheme, the micrometric displacement monitoring sensitivity and precision are improved, the long-term durability of the pipe gallery in a severe environment is improved, the production cost is reduced, and the yield is improved.

Description

Technical Field

[0001] This invention relates to the field of displacement measurement technology, and in particular to an FP displacement sensor and a method for detecting displacement at underground pipe corridors and joints. Background Technology

[0002] As a key infrastructure to ensure the normal operation of cities, the structural health monitoring of urban underground utility tunnels is of great significance. The minute displacements (typically 10-20 μm) between tunnel joints and segments are the core parameters reflecting structural stability.

[0003] To effectively apply FP sensors to the monitoring of micro-displacement in underground utility tunnels and achieve large-scale deployment, three core issues need to be overcome: Most existing FP sensors have not been specifically optimized for micro-displacement monitoring scenarios in terms of structural parameters, making it difficult to meet the requirements for displacement monitoring sensitivity; Traditional sensors lack fault-tolerant design. To ensure performance consistency, they rely on high-precision assembly processes during production. This not only results in low yield and poor efficiency during mass production, but also significantly increases costs, making it difficult to meet the actual needs of kilometer-scale deployment in utility tunnels. The long-term packaging reliability of sensors in the high humidity and corrosive environment of pipe racks and the problem of temperature cross-interference need to be solved.

[0004] Chinese invention patent CN120467220A discloses a Fabry-Perot cavity-based pipe gallery joint deformation monitoring system. The core sensing unit of this system is an intrinsic FP strain sensor, composed of two single-mode optical fibers and a capillary tube, and encapsulated using a metal welding process. This patent focuses primarily on the system's networking and integration, leaving gaps in the design and performance optimization of the sensor itself. The sensor's design is positioned for strain monitoring, without in-depth optimization for high-precision monitoring of the crucial "micro-displacement" physical quantity in pipe gallery monitoring.

[0005] The accuracy of micro-displacement monitoring is difficult to meet the standards, and the displacement sensitivity is too low, making it impossible to accurately and stably capture micro-displacements in the order of 10-20μm. Large-scale deployment is costly, and the yield rate and fault tolerance of sensors are low during mass production. (3) There are hidden dangers in the long service life. The use of only a single metal welding encapsulation is insufficient to protect against the corrosive environment inside the pipe gallery. (4) It is impossible to effectively suppress temperature cross-interference by relying on a single sensor. The difference in thermal expansion coefficient of materials and temperature compensation are not considered. According to the temperature sensitivity formula (the ratio of wavelength drift caused by temperature change to the amount of temperature change), the temperature fluctuation of ±15℃ in the pipe gallery will introduce a displacement error of more than 5μm, which will have an adverse effect on the long-term accuracy of displacement measurement data.

[0006] Therefore, there is a need to provide an FP displacement sensor and a method for detecting displacement at underground utility tunnels and joints, to improve the sensitivity and accuracy of micro-displacement monitoring, enhance the long-term durability of utility tunnels in harsh environments, reduce production costs, and increase the yield rate.

[0007] The information disclosed in the background section is only intended to enhance the understanding of the background of this application, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0008] The main purpose of this invention is to overcome the problems of low sensitivity and accuracy, high cost, low yield and poor durability of existing FP sensors, and to provide an FP displacement sensor and a method for detecting displacement at underground pipe corridors and joints, thereby improving the sensitivity and accuracy of micro-displacement monitoring, improving the long-term durability of pipe corridors in harsh environments, reducing production costs and increasing the yield.

[0009] To achieve the above objectives, the first aspect of the present invention provides an FP displacement sensor, comprising: two single-mode optical fibers, a capillary tube, a sealing layer, an encapsulation film, and an adhesive tape; The two single-mode optical fibers are coaxially arranged and spaced a certain distance apart. This gap forms a Fabry-Perot interferometer cavity, and the axis of the single-mode optical fiber coincides with the axis of the FP displacement sensor. The axis of the capillary coincides with the axis of the FP displacement sensor. The capillary surrounds the two single-mode optical fibers along the circumference of the FP displacement sensor. The length of the two single-mode optical fibers is longer than that of the capillary and extends outward from both ends of the capillary. The inner wall of the capillary is at a certain distance from the outer wall of the single-mode optical fiber. The sealing layer is disposed at both ends of the capillary tube; The encapsulation film surrounds the capillary along the circumferential direction of the FP displacement sensor; The tape surrounds the encapsulating film; the single-mode optical fiber extends outward from both ends of the tape.

[0010] According to an exemplary embodiment of the present invention, the capillary is made of silicon dioxide, the sealing layer is made of metal welding, and the encapsulation film is made of polytetrafluoroethylene; The length of the interference cavity formed by the two single-mode optical fibers is 200-400 μm; the length of the capillary is 8-16 mm; the thickness of the capillary wall is 190-250 μm; and the gap between the inner wall of the capillary and the outer wall of the single-mode optical fiber is filled with epoxy resin.

[0011] As a second aspect of the present invention, the present invention provides an underground utility tunnel, including the aforementioned FP displacement sensor and a plurality of pipes connected end to end; a plurality of FP displacement sensors are provided at the joint of every two pipes.

[0012] According to an exemplary embodiment of the present invention, the pipe is a cuboid structure, with four connecting planes connecting every two pipes. Each of the three connecting planes is provided with three parallel FP displacement sensors. The axis of the FP displacement sensor is parallel to the direction of the pipe length, the joint is perpendicular to the axis of the FP displacement sensor, and intersects with the capillary tube.

[0013] As a third aspect of the present invention, the present invention provides a method for detecting displacement at joints in underground utility tunnels, comprising the following steps: The aforementioned underground utility tunnel is adopted; Install FP displacement sensor; Continuously acquire data from the FP displacement sensor; Compensate the data from the FP displacement sensor; Calculate the axial deformation characteristics of the underground utility tunnel.

[0014] According to an exemplary embodiment of the present invention, the mounting of the FP displacement sensor includes: Determine the assembly tolerance zone and initial cavity length of the FP displacement sensor, compare it with the FP displacement sensor of the standard template, and find the end face positions of the two single-mode optical fibers. Determine the effective working range of the FP displacement sensor so that the joint treatment is within this effective working range.

[0015] According to an exemplary embodiment of the present invention, the compensation of the data from the FP displacement sensor includes: using a dual-parameter linear compensation algorithm to compensate for the wavelength shift caused by temperature.

[0016] According to an exemplary embodiment of the present invention, the compensation for the wavelength shift caused by temperature using a dual-parameter linear compensation algorithm includes: The uncompensated wavelength shift is calculated using a dual-parameter linear compensation algorithm based on the temperature change. The compensated displacement is obtained based on the uncompensated wavelength offset and data from the FP displacement sensor.

[0017] According to an exemplary embodiment of the present invention, the calculation of the axial deformation characteristics of the underground utility tunnel includes: The plane parameters of the plane containing the joint are obtained by using the least squares method from multiple measurement data. The axial deformation characteristics of the underground utility tunnel are obtained through this planar parameter; The axial deformation characteristics of this underground utility tunnel include axial expansion and contraction, tilting, and uneven deformation.

[0018] According to an exemplary embodiment of the present invention, the step of calculating the plane parameters of the plane containing the joint using the least squares method from multiple measurement data includes: Formulas for determining the plane containing the joint; The plane formula is: z i =ax i +by i +c, where (x i y i , z i () represents the coordinates of the measurement points in the measurement data; The plane parameters of the plane containing the joint are obtained by using the least squares method. The plane parameters are a, b, and c.

[0019] The advantages of this invention are: This solution optimizes and combines the structural parameters of sensors for pipe gallery displacement monitoring, enabling the fabrication of displacement sensors with different ranges and application scenarios. It defines the effective working range of the fixed interference cavity and achieves a fault-tolerant assembly design based on the position independence of the interference cavity. A composite encapsulation structure is realized through a combination of metal welding seals and a PTFE film. Systematic temperature drift is suppressed using homogeneous silica material and a temperature compensation algorithm. This results in comprehensive improvements in four aspects: micro-displacement monitoring performance, economies of scale in large-scale production, environmental durability, and measurement accuracy. Sensitivity reaches 78.72 pm / μm; an assembly tolerance of ±2mm increases the yield to over 95% and reduces assembly time by over 60%; the composite encapsulation ensures a service life of over 5 years; and the system error is controlled within ±3.12μm, with a temperature drift error of <±0.5μm. Attached Figure Description

[0020] The above and other objects, features, and advantages of this application will become more apparent from the detailed description of exemplary embodiments with reference to the accompanying drawings. The drawings described below are merely some embodiments of this application, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0021] Figure 1 The diagram illustrates the structure of the underground utility tunnel.

[0022] Figure 2 A schematic diagram of the FP displacement sensor is shown.

[0023] Figure 3 The schematic diagram illustrates the principle of FP interferometry.

[0024] Figure 4 The diagram illustrates the displacement characteristic curves of the large displacement transformation scheme.

[0025] Figure 5The diagram schematically illustrates the verification of the position independence of the interference cavity.

[0026] Figure 6 The temperature sensitivity relationship is illustrated schematically. Detailed Implementation

[0027] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, they are provided so that this application will be thorough and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions of them will be omitted.

[0028] Furthermore, the described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. Numerous specific details are provided in the following description to give a thorough understanding of embodiments of this application. However, those skilled in the art will recognize that the technical solutions of this application can be practiced without one or more of the specific details, or other methods, components, apparatuses, steps, etc., can be employed. In other instances, well-known methods, apparatuses, implementations, or operations are not shown or described in detail to avoid obscuring various aspects of this application.

[0029] The block diagrams shown in the accompanying drawings are merely functional entities and do not necessarily correspond to physically independent entities. That is, these functional entities can be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor devices and / or microcontroller devices.

[0030] The flowcharts shown in the accompanying drawings are merely illustrative and do not necessarily include all content and operations / steps, nor do they necessarily have to be performed in the described order. For example, some operations / steps can be broken down, while others can be combined or partially combined; therefore, the actual execution order may change depending on the specific circumstances.

[0031] It should be understood that although the terms first, second, third, etc., may be used herein to describe various components, these components should not be limited by these terms. These terms are used to distinguish one component from another. Therefore, the first component discussed below may be referred to as the second component without departing from the teachings of this application. As used herein, the term "and / or" includes all combinations of any one and more of the associated listed items.

[0032] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of exemplary embodiments, and the modules or processes in the drawings are not necessarily essential for implementing this application, and therefore cannot be used to limit the scope of protection of this application.

[0033] According to a first specific embodiment of the present invention, the present invention provides an underground utility tunnel, such as... Figure 1 As shown, it includes FP displacement sensors and multiple pipes connected end to end; multiple FP displacement sensors are installed at the joint of every two pipes.

[0034] The pipeline has a cuboid structure, with four connecting planes between every two pipelines. A nine-point matrix layout is used, where three parallel FP displacement sensors are installed on each of the three connecting planes. The axis of each FP displacement sensor is parallel to the direction of the pipeline length, and the distance between every two FP displacement sensors is equal. The joint is perpendicular to the axis of the FP displacement sensor and intersects with the capillary tube 2 of the FP displacement sensor. For example... Figure 1 As shown, three sensors are arranged on each of the three connecting planes on the two sides and the top of the joint, for a total of nine FP displacement sensors, forming a spatial displacement monitoring matrix.

[0035] like Figure 2 As shown, the FP displacement sensor is a high-sensitivity, high-robustness, corrosion-resistant intrinsic fiber optic FP displacement sensor specifically designed for micro-displacement monitoring in underground utility tunnels. It includes: two single-mode optical fibers 1, a capillary tube 2, a sealing layer 3, an encapsulation film 4, and adhesive tape 5.

[0036] Two single-mode fiber segments 1 are coaxially arranged, with a certain distance between them. This gap constitutes a Fabry-Perot interferometer cavity. Specifically, the two single-mode fiber segments 1 are spaced apart along the sensor axis, and their opposite end faces form the Fabry-Perot interferometer cavity. The FP resonant cavity, also known as a planar parallel cavity, is a type of optical resonant cavity composed of two parallel plane mirrors and is commonly used in semiconductor lasers. This is the earliest optical resonant cavity proposed in the history of laser technology and is the foundation of the Fabry-Perot interferometer. The end faces of the two single-mode fiber segments 1 are polished and placed parallel to each other, with the axis of the single-mode fiber 1 coinciding with the axis of the FP displacement sensor. The length of the interference cavity formed by the two single-mode fiber segments 1 (initial cavity length d) is 200-400 μm. Figure 2 As shown, the length of the interference cavity is the distance between the two single-mode fiber segments 1. Figure 2 The initial cavity length is marked as D, located directly above the mark A. It is the initial length of the Fabry-Perot interferometer cavity formed between the two single-mode fiber end faces. Based on the negative correlation between FSR (Free Spectral Range) and cavity length... , Taking 1550nm, n=1, where n is the refractive index of the interference cavity medium), the corresponding FSR in this range is 3-6nm, which can be accurately identified by a conventional demodulator with a wavelength resolution of 0.02nm; among which 240μm is the optimal value.

[0037] like Figure 3As shown, the core principle of fiber optic FP sensing technology is based on the phenomenon of multi-beam interference. When a parallel beam I0 is incident on an interference cavity formed by two parallel reflecting surfaces, multiple reflections and transmissions occur, forming a coherently superimposed reflected light I. R With transmitted light I T Its interference characteristics can be expressed by the phase difference formula. (in For phase difference, Let λ be the incident light wavelength, n be the refractive index of the interference cavity medium, d be the cavity length, and β be the refraction angle of the beam between the reflecting surfaces. Changes in the cavity length directly lead to changes in the phase difference and light intensity signal, which is the theoretical basis for displacement monitoring. To mitigate the impact of temperature changes on monitoring accuracy from the source, a coefficient of thermal expansion of 0.55 × 10⁻⁶ is selected. -6 The capillary tube 2 and single-mode optical fiber 1 are made of silica material at a temperature of / ℃, which can effectively reduce the structural thermal stress caused by temperature fluctuations and avoid monitoring deviations caused by thermal expansion and contraction of materials.

[0038] The axis of capillary tube 2 coincides with the axis of the FP displacement sensor. Capillary tube 2 surrounds the two single-mode optical fibers 1 circumferentially around the FP displacement sensor. The two single-mode optical fibers 1 are longer than capillary tube 2 and extend outwards from both ends of capillary tube 2. The inner wall of capillary tube 2 is spaced apart from the outer wall of single-mode optical fiber 1. The space between the inner wall of capillary tube 2 and the outer wall of single-mode optical fiber 1 is filled with epoxy resin. Capillary tube 2 is a hollow structure used to provide stable physical support and environmental isolation. Capillary tube 2 is made of silicon dioxide.

[0039] The length L of capillary tube 2 is 8-16 mm. ANSYS Workbench finite element simulation verified that the displacement sensitivity within the length L range of capillary tube 2 is positively correlated with the length (8 mm corresponds to a sensitivity of 13.55 pm / μm, and 16 mm corresponds to a sensitivity of 77.9 pm / μm). Selecting 12 mm as the optimal value ensures high sensitivity (above 32.71 pm / μm) while accommodating narrow installation spaces of 5-20 mm in pipe rack joints. The capillary wall thickness is 190-250 μm, corresponding to an outer diameter of 340-400 μm. Simulation data shows a negative correlation between wall thickness and cavity length change (190 μm wall thickness corresponds to a cavity length change of 11.911 μm under a displacement of 100 μm, and 250 μm wall thickness corresponds to a change of 8.2 μm). Determining 190 μm as the optimal value satisfies the sensor's required mechanical strength (compressive strength ≥50 MPa) while maximizing deformation transmission efficiency.

[0040] The core sensing structure of this scheme includes two single-mode optical fibers 1, a silica capillary tube 2, and a metal-welded sealing layer 3. One of its core contributions lies in determining the optimal combination and range of structural parameters necessary for achieving high-precision monitoring of micro-displacement in the utility tunnel through system simulation analysis and experimental verification. Please refer to Table 1, which shows the correspondence between the simulated structure and performance of the core structural parameters.

[0041]

[0042] Table 1 As shown in Table 1, the initial cavity length d is set to 200-400 μm. Within this range, the displacement sensitivity is negatively correlated with the cavity length, with 240 μm being the optimal value. Experiments show that the displacement sensitivity can reach 78.72 pm / μm at this value. The capillary length L is set to a range of 8-16 mm. Within this range, the displacement sensitivity is positively correlated with the length. 12 mm is selected as the optimal value, which can fully adapt to the installation requirements of the narrow space of the pipe gallery while ensuring high sensitivity. The capillary wall thickness t is set to 190-250 μm. Within this range, the displacement sensitivity is negatively correlated with the wall thickness. 190 μm is determined as the optimal value, which can meet the mechanical strength required by the sensor and maximize the deformation transmission efficiency.

[0043] Experiments verified that the optimal parameter combination is d=240μm, L=12mm, t=190μm.

[0044] To address the potential for larger displacement monitoring scenarios within utility tunnels, if pipe settlement exceeds 20 μm, the initial cavity length (d) of the sensor is adjusted to 300-400 μm, while the capillary length (L) is extended to 16 mm, maintaining an optimal wall thickness of 190 μm. Experimental data shows that with this parameter combination, the sensor's displacement sensitivity can reach levels of 51.07 pm / μm to 77.9 pm / μm. When d = 300 μm and L = 16 mm, the sensitivity is 51.07 pm / μm, with a wavelength drift of 5.107 nm corresponding to a 100 μm displacement; when d = 400 μm and L = 16 mm, the sensitivity is 77.9 pm / μm, with a wavelength drift of 7.79 nm corresponding to a 100 μm displacement. This design maintains excellent sensitivity while broadening the sensor's effective measurement range, achieving a balance between accuracy and range in large displacement scenarios.

[0045] The alternative structural parameter schemes and their performance for large displacement scenarios are shown in Table 2 below:

[0046] The displacement characteristic curve of the large displacement transformation scheme (d=400μm, L=16mm) is as follows: Figure 4As shown in the diagram, the black curve represents the case with an initial cavity length of 480 μm. As the length of capillary 2 increases from 8 mm to 16 mm, the displacement sensitivity gradually increases from approximately 15 μm to around 30 μm, showing a slow overall growth trend. The red curve corresponds to the case with an initial cavity length of 400 μm. With the increase in capillary 2 length, the displacement sensitivity rapidly increases from approximately 15 μm to nearly 80 μm, a significantly greater increase than that with an initial cavity length of 480 μm. At the same capillary length, the displacement sensitivity is higher with an initial cavity length of 400 μm; and as the capillary length increases, the difference in displacement sensitivity between the two initial cavity lengths gradually widens.

[0047] like Figure 2 As shown, the range from 6.1 mm to 10.1 mm from either end along the axial direction of capillary 2 is defined as the effective working range A. A model with capillary 2 length L = 8 mm and initial cavity length d = 200 μm was constructed using finite element simulation. The cavity position was changed to 6.1 mm / 7.1 mm / 8.1 mm / 9.1 mm / 10.1 mm for verification. Within this range, the sensor's displacement sensitivity remained stable—the cavity length change was consistently 5.833-5.834 μm for a 100 μm displacement, with a maximum deviation of less than 0.001 μm. The position of the interference cavity did not affect accuracy, providing a tolerance of ±2 mm for assembly. This tolerance is based on the results of finite element simulation. The ±2 mm tolerance refers to the actual installation position of the interference cavity along the capillary axial direction, which can fluctuate within the range of 6.1 mm to 10.1 mm, with a fluctuation range length of 4 mm, corresponding to a tolerance range of ±2 mm. Figure 2 The part A marked with a dotted line.

[0048] Through finite element simulation analysis of the system, it was found that the displacement sensitivity of the fiber optic FP displacement sensor has significant stability when the interference cavity is in a specific range along the capillary axis.

[0049] A sensor model with a capillary length L of 8 mm and an initial cavity length d of 200 μm was constructed. Then, the axial position of the FP interferometer cavity inside the capillary was systematically changed, and simulations were performed for five different cavity positions with a distance of 6.1 mm, 7.1 mm, 8.1 mm, 9.1 mm, and 10.1 mm from one end of the capillary.

[0050] like Figure 5The simulation results show that, under the five different cavity positions, when a displacement of 0-100 μm is applied to the sensor, the cavity length change (Δd) curves basically coincide. Specifically, under a displacement of 100 μm, the cavity length change at all positions is between 5.833 μm and 5.834 μm, with a maximum deviation of less than 0.001 μm. This simulation result strongly proves that within this range, the sensor's displacement sensitivity performance remains highly stable, and the degree of change is negligible. The range of 6.1 mm to 10.1 mm from either end of capillary 2 is clearly defined as the "effective working range." Finite element simulation verification shows that, based on the principle of mechanical homogeneity of the capillary structure, as long as it is within this "effective working range," it is position-independent.

[0051] Based on this, this solution creates an assembly tolerance zone of ±2mm. During the actual assembly process, operators do not need to perform precise microscopic optical adjustments or strict cavity centering operations. They only need to fix the two fiber end faces forming the FP cavity within this tolerance zone to ensure that all produced sensors have highly consistent displacement sensitivity performance.

[0052] Sealing layer 3 is disposed at both ends of capillary tube 2. Sealing layer 3 is made of metal welding and is used to fix single-mode optical fiber 1 and achieve the first moisture-proof seal.

[0053] The encapsulation film 4 surrounds the capillary 2 circumferentially along the outer wall of the capillary 2. The encapsulation film 4 is made of polytetrafluoroethylene (PTFE) as a second chemical corrosion resistant barrier, together forming a long-lasting corrosion-resistant encapsulation system.

[0054] The tape 5 surrounds the outer surface of the encapsulation film 4, securing the encapsulation film 4 to the capillary tube 2. The single-mode optical fiber 1 extends outward from both ends of the tape 5. The tape 5 is a high-temperature resistant tape, with a temperature tolerance of 260℃.

[0055] To address the complex environment of humid and corrosive gases within the utility tunnel, this solution employs a composite long-lasting anti-corrosion encapsulation method combining metal welding and PTFE film. High-purity tin wire is used for metal welding and sealing, forming the first robust and moisture-proof barrier that effectively prevents external moisture from intruding into the sensor and avoids damage to core components due to moisture. Simultaneously, a PTFE film is wrapped around the outer wall of the capillary and secured with high-temperature resistant tape, forming the second chemical corrosion-resistant barrier to protect the sensor from the corrosive gases within the utility tunnel.

[0056] As a preferred embodiment, to cope with the highly corrosive environment of the special pipe gallery, this solution adds additional enhanced anti-corrosion measures to the core metal welded sealing layer 3. Specifically, an additional epoxy resin coating with a thickness of 10-15 μm is sprayed onto the outside of the metal welded sealing layer 3. This organic polymer coating forms a composite protective structure with the metal sealing layer, which can more effectively block the erosion of corrosive media. Experimental verification shows that the sensor using this enhanced encapsulation solution can still guarantee a service life of more than 5 years in an environment with a hydrogen sulfide concentration as high as 30 ppm, significantly improving its environmental tolerance under extremely harsh working conditions.

[0057] According to a second specific embodiment of the present invention, the present invention provides a method for detecting displacement at joints in underground utility tunnels, comprising the following steps: The underground utility tunnel described in the first specific embodiment is adopted.

[0058] S1: Install the FP displacement sensor.

[0059] Installing an FP displacement sensor includes: The assembly tolerance zone and initial cavity length of the FP displacement sensor are determined, and compared with a standard template FP displacement sensor to locate the end face positions of the two single-mode fiber segments. Specifically, one or more sensors with known initial cavity lengths (e.g., d=240μm) and excellent performance are prefabricated in a laboratory environment as "standard templates". During on-site assembly, operators compare the reflection spectrum of the sensor to be assembled with the reflection spectrum waveform of the "standard template" in real time. By observing and matching the waveform of the interference spectrum with the free spectral range (FSR), the end face positions of the two single-mode fiber segments can be quickly and accurately located and fixed, keeping the initial cavity length of the sensor within the allowable error range (e.g., ±5μm). This method fully utilizes the design advantage of "interference cavity position independence," eliminating the need for complex calibration using a precision optical interferometer to control the initial cavity length of the sensor within an allowable error range. This further reduces equipment dependence during on-site assembly and is suitable for emergency repair scenarios without power or specialized instruments. The assembly time for a single fiber can be reduced to less than 10 minutes.

[0060] Then, the effective working range of the FP displacement sensor is determined so that the joint is disposed within this effective working range.

[0061] The advantage of this method is that it makes full use of the design advantage of "interference cavity position independence", eliminating the need for complex calibration using expensive and precise optical interferometers. This greatly reduces the technical threshold and equipment cost for on-site deployment, shortens the assembly time of a single sensor from 40 minutes to 15 minutes, and significantly improves the efficiency and convenience of on-site construction.

[0062] S2: Continuously acquire data from the FP displacement sensor.

[0063] The fiber optic FP displacement sensor utilizes a Fabry-Perot air cavity formed by the end faces of two single-mode optical fibers as the sensing element. In the demodulator, the ASE broadband light source (1528–1568 nm) is coupled into the sensor via optical fiber, and its optical path is as follows: Incident light: Broadband light propagates along a single-mode fiber and is incident on the first end face at 0° (the same angle as the axis), with a reflectivity of approximately 3.6%.

[0064] Partial reflection: The incident light produces a reflected beam R1 at the first end face.

[0065] Transmission into the cavity: The remaining light passes through the end face into the air cavity and propagates back and forth between the two parallel end faces at a refraction angle β.

[0066] Multiple reflections and refractions: The light beam undergoes multiple reflections and transmissions between the two end faces, forming multi-beam interference.

[0067] Coherent superposition: All reflected light superimposes in the optical fiber to form an interference reflection spectrum, the intensity of which is expressed as: ; ; Cavity length variation causes wavelength shift: When the tube gallery undergoes a slight displacement, the cavity length d changes by Δd, which causes the reflection spectrum to shift along the wavelength direction. .

[0068] The displacement can be calculated based on the demodulated wavelength shift. ; Where Δd represents displacement. This represents the wavelength shift, and S is the displacement sensitivity obtained from experimental calibration.

[0069] Experimental tests show that as the loading increments from 0–100 μm, the reflection spectrum continuously shifts towards longer wavelengths, and the peak shift is highly linear (R²>0.99).

[0070] S3: Compensate the data from the FP displacement sensor.

[0071] At the algorithm optimization and experimental verification level, by integrating a temperature compensation algorithm into the monitoring system, real-time dynamic correction can be achieved for displacement measurement errors that may be induced by temperature changes. Experimental results show that the sensor's temperature sensitivity is -11.68 pm / ℃, and the linear fit goodness of fit R² reaches 0.98679; Figure 6As shown, further calculations revealed that the sensor's displacement-temperature cross-sensitivity was only 0.357 pm / ℃. This extremely low cross-sensitivity indicates that within the typical ±15℃ ambient temperature fluctuation range of the utility tunnel, the displacement measurement error introduced by temperature drift was significantly suppressed. Through a systematic combination of sensor structural design and algorithm optimization, the displacement measurement error caused by temperature drift was successfully controlled within ±0.5μm, effectively ensuring the accuracy and reliability of long-term monitoring data.

[0072] Compensating for the data from the FP displacement sensor includes using a dual-parameter linear compensation algorithm to compensate for wavelength shifts caused by temperature.

[0073] The dual-parameter linear compensation algorithm is used to compensate for the wavelength shift caused by temperature, including: The uncompensated wavelength shift is calculated using a dual-parameter linear compensation algorithm based on the temperature change. The compensated displacement is obtained based on the uncompensated wavelength offset and data from the FP displacement sensor.

[0074] Specifically, although this scheme uses silicon dioxide as the material for capillary 2 and optical fiber to minimize the impact of structural thermal expansion, experiments show that the sensor still has a temperature sensitivity of approximately -11.68 pm / °C. To eliminate the wavelength shift caused by temperature, a dual-parameter linear compensation algorithm is employed. ; in, This represents the wavelength shift after temperature compensation. The uncompensated wavelength shift is represented by ΔT; the temperature change is represented by K. T Temperature sensitivity (calibrated value is -11.68 pm / ℃).

[0075] The displacement after compensation is: ; Where Δd is the compensated displacement. S represents the wavelength shift after temperature compensation, and S represents the displacement sensitivity.

[0076] Experimental verification shows that, under temperature disturbance conditions of ±15°C, the displacement error caused by temperature drift after compensation is reduced from the original 5–6 μm to <0.5 μm.

[0077] S4: Calculate the axial deformation characteristics of the underground utility tunnel.

[0078] The calculation of the axial deformation characteristics of underground utility tunnels involves first calculating the axial relative displacement Δd of the underground utility tunnel, and then monitoring the three-dimensional spatial attitude of the joints, including axial expansion, tilting, and uneven deformation.

[0079] S41: Calculate the plane parameters of the plane where the joint is located by using the least squares method from multiple measurement data.

[0080] The plane parameters of the plane containing the joint are obtained by using the least squares method from multiple measurement data, including: Formulas for determining the plane containing the joint; The plane formula is: z i =ax i +by i +c, where (x i y i , z i (z) represents the coordinates of the measurement point in the measurement data. i This refers to the axial relative displacement Δd measured by the FP displacement sensor at that measuring point. i .

[0081] The plane parameters of the plane containing the joint are obtained by using the least squares method. The plane parameters are a, b, and c.

[0082] Each FP displacement sensor outputs the relative displacement Δd between two single-mode fiber segments 1, which is obtained through a displacement demodulation algorithm: ; Where Δd represents the relative displacement between the two single-mode fiber segments 1. S represents the wavelength shift, and S represents the displacement sensitivity (sensitivity of sensors with different parameters ranges from 32.71 to 78.72 pm / μm).

[0083] The coordinates of the measuring point are set as (x i y i , z i The plane of the joint is approximated as: z i =ax i +by i +c, after collecting z-data from nine points, use the least squares method. Find a, b, and c.

[0084] S42: The axial deformation characteristics of the underground utility tunnel are obtained through this planar parameter. The axial deformation characteristics of the underground utility tunnel include: axial expansion and contraction, tilting and uneven deformation.

[0085] The plane constant represents the reference displacement in the z direction; the axial deformation (i.e., axial expansion and contraction) is the tensile or compressive value of a single point in the z direction, and the overall axial deformation is the axial expansion and contraction value of the entire joint surface; x is the transverse direction of the pipe gallery, y is the vertical direction, and z is the axial direction of the pipe gallery.

[0086] Based on the plane parameters, the overall axial deformation is calculated as Δc = c. t -c t0The plane constants at the current time (t) and the reference period (t0) are c, respectively. t and c t0 △c>0 represents tension, and △c<0 represents compression; the axial tilt angle is characterized by the plane fitting parameters a and b, where a is the lateral tilt parameter, corresponding to the small rotation of the joint about the y-axis; and b is the vertical tilt parameter, corresponding to the small rotation of the joint about the x-axis.

[0087] Uneven axial deformation (i.e., non-uniform deformation) = the maximum value of the z-displacement difference between any two points among the nine points. Using the nine-point data, the overall axial deformation of the joint and the uneven axial deformation, i.e., the structural attitude change, can be obtained simultaneously.

[0088] The deformation of pipe gallery joints exhibits three-dimensional coupling characteristics, and single-point or two-point methods cannot distinguish between different types of structural deformation, such as axial expansion, tilting, and torsion. This proposed nine-point layout method can construct a spatial attitude model of the joint, simultaneously identifying complex working conditions such as axial deformation, rotation, and torsion. Even when a single point is damaged or abnormal, the deformation surface can still be accurately calculated using the remaining points.

[0089] To verify the actual measurement performance of the fiber optic FP displacement sensor proposed in the micro-displacement monitoring scenario of underground utility tunnels, a systematic verification experiment was conducted on sensors with various combinations of structural parameters. The experiment was carried out in a simulated underground utility tunnel environment, using a precision micro-displacement platform to apply displacement, and a dial indicator as the benchmark verification instrument. The overall measurement accuracy was evaluated by comparing the displacement calculated by the sensor with the actual applied displacement. The experimental results are as follows: (1) High sensitivity and high linearity: The sensor with the optimal parameter combination (initial test cavity length d=240μm, capillary length L=12mm, wall thickness t=190μm) was used for testing. The displacement sensitivity was obtained as 78.72pm / μm through linear fitting.

[0090] (2) Repeatable experiments: Three independent repeated tests were conducted under the same loading conditions. The test curves almost overlapped, and the standard deviation of the 100μm displacement point did not exceed ±0.21μm, indicating that the sensor has good repeatability and stability.

[0091] Verification of the measured accuracy of sensors with multiple parameters: To comprehensively evaluate performance, accuracy verification experiments were conducted on sensors with three different parameter combinations. Specific data are shown in the following three tables:

[0092] Table 3 shows the sensor with L=8mm and d=400μm, and a measurement error of ±4.85μm.

[0093]

[0094] Table 4 shows the measurement error of a sensor with L=12mm and d=240μm: ±4.25μm.

[0095] Table 5 shows the measurement error of a sensor with L=16mm and d=480μm: ±3.65μm.

[0096] (4) Overall measurement accuracy The experiments described above covered various typical configurations, including short-cavity high sensitivity, optimal parameters, and long-cavity large range. Experimental results show that the sensor's measurement error remains low under different parameter combinations. Based on all test results, the total uncertainty of the fiber optic FP displacement sensor in this design for displacement measurement in a pipe gallery environment can be controlled within ±5.0 μm.

[0097] This solution optimizes and combines the structural parameters of sensors for pipe gallery displacement monitoring, enabling the fabrication of displacement sensors with different ranges and application scenarios. The initial cavity length is 200-400μm (optimal 240μm), capillary length is 8-16mm (optimal 12mm), and wall thickness is 190-250μm (optimal 190μm). The effective working range is defined by fixing the interference cavity, achieving a fault-tolerant assembly design based on the position independence of the interference cavity. The interference cavity is fixed within the "effective working range" of 6.1mm to 10.1mm axial distance from either end of the capillary. A composite encapsulation structure is achieved through a combination of metal welding seal and a polytetrafluoroethylene (PTFE) film, based on homogeneous silica material (thermal expansion coefficient 0.55×10⁻⁶). -6 The system, with its temperature compensation algorithm (±℃) suppressing systematic temperature drift, achieves comprehensive improvements in four aspects: micro-displacement monitoring performance, economies of scale in large-scale production, environmental durability, and measurement accuracy. Sensitivity reaches 78.72 pm / μm; an assembly tolerance of ±2mm increases the yield to over 95% and reduces assembly time by over 60%; composite packaging ensures a service life of over 5 years; and system errors are controlled within ±3.12μm, with temperature drift error <±0.5μm.

[0098] Exemplary embodiments of the present invention have been specifically shown and described above. It should be understood that the present invention is not limited to the detailed structures, arrangements, or implementations described herein; rather, the present invention is intended to cover various modifications and equivalent arrangements contained within the spirit and scope of the appended claims.

Claims

1. An FP displacement sensor, characterized in that, include: Two single-mode optical fibers, capillary tube, sealing layer, encapsulation film, and tape; The two single-mode optical fibers are coaxially arranged and spaced a certain distance apart. This gap forms a Fabry-Perot interferometer cavity, and the axis of the single-mode optical fiber coincides with the axis of the FP displacement sensor. The axis of the capillary coincides with the axis of the FP displacement sensor. The capillary surrounds the two single-mode optical fibers along the circumference of the FP displacement sensor. The length of the two single-mode optical fibers is longer than that of the capillary and extends outward from both ends of the capillary. The inner wall of the capillary is at a certain distance from the outer wall of the single-mode optical fiber. The sealing layer is disposed at both ends of the capillary tube; The encapsulation film surrounds the capillary along the circumferential direction of the FP displacement sensor; The tape surrounds the encapsulating film; the single-mode optical fiber extends outward from both ends of the tape.

2. The FP displacement sensor according to claim 1, characterized in that, The capillary tube is made of silicon dioxide, the sealing layer is made of metal welding, and the encapsulation film is made of polytetrafluoroethylene. The length of the interference cavity formed by the two single-mode optical fibers is 200-400 μm; the length of the capillary is 8-16 mm; the thickness of the capillary wall is 190-250 μm; and the gap between the inner wall of the capillary and the outer wall of the single-mode optical fiber is filled with epoxy resin.

3. An underground utility tunnel, characterized in that, It includes multiple FP displacement sensors as described in claim 1 or 2, and multiple pipes connected end to end; multiple FP displacement sensors are installed at the joint of every two pipes.

4. The underground utility tunnel according to claim 3, characterized in that, The pipe has a cuboid structure, with four connecting planes connecting every two pipes. Three parallel FP displacement sensors are installed on each of the three connecting planes. The axis of the FP displacement sensor is parallel to the length direction of the pipe, the joint is perpendicular to the axis of the FP displacement sensor, and intersects with the capillary tube.

5. A method for detecting displacement at joints in underground utility tunnels, characterized in that, Includes the following steps: The underground utility tunnel described in claim 3 or 4 is adopted; Install FP displacement sensor; Continuously acquire data from the FP displacement sensor; Compensate the data from the FP displacement sensor; Calculate the axial deformation characteristics of the underground utility tunnel.

6. The method for detecting displacement at joints in underground utility tunnels according to claim 5, characterized in that, The installation of the FP displacement sensor includes: Determine the assembly tolerance zone and initial cavity length of the FP displacement sensor, compare it with the FP displacement sensor of the standard template, and find the end face positions of the two single-mode optical fibers. Determine the effective working range of the FP displacement sensor so that the joint treatment is within this effective working range.

7. The method for detecting displacement at joints in underground utility tunnels according to claim 5, characterized in that, The compensation of the FP displacement sensor data includes: using a dual-parameter linear compensation algorithm to compensate for the wavelength shift caused by temperature.

8. The method for detecting displacement at joints in underground utility tunnels according to claim 7, characterized in that, The method of using a dual-parameter linear compensation algorithm to compensate for wavelength shift caused by temperature includes: The uncompensated wavelength shift is calculated using a dual-parameter linear compensation algorithm based on the temperature change. The compensated displacement is obtained based on the uncompensated wavelength offset and data from the FP displacement sensor.

9. The method for detecting displacement at joints of underground utility tunnels according to claim 5, characterized in that, The calculation of the axial deformation characteristics of the underground utility tunnel includes: The plane parameters of the plane containing the joint are obtained by using the least squares method from multiple measurement data. The axial deformation characteristics of the underground utility tunnel are obtained through this planar parameter; The axial deformation characteristics of this underground utility tunnel include axial expansion and contraction, tilting, and uneven deformation.

10. The method for detecting displacement at joints of underground utility tunnels according to claim 9, characterized in that, The process of determining the plane parameters of the plane containing the joint using the least squares method from multiple measurement data includes: Formulas for determining the plane where the joint is located; The plane formula is: z i =ax i +by i +c, where (x i y i , z i () represents the coordinates of the measurement points in the measurement data; The plane parameters of the plane containing the joint are obtained by using the least squares method. The plane parameters are a, b, and c.

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