An internal fixed-point ultra-weak fiber Bragg grating strain cable

Abstract

An internally fixed-point ultra-weak fiber Bragg grating (FBG) strain cable comprises an ultra-weak fiber Bragg grating (UFBG) array located at a center and internally engraved with multiple ultra-weak Bragg grating (UFBG) sensing units; a metal spiral armor helically sheathed around the UFBG array, with adhesive injected between the UFBG array and the inner wall of the metal spiral armor, which solidifies to form bonding points; a metal mesh cross-woven along the axial direction of the metal spiral armor; a sheath tightly wrapped around the metal spiral armor and the metal mesh; and sensor markers coated on the sheath at locations corresponding to the UFBG sensing units. The present invention pre-bonds the grating array and the metal spiral armor at selected points, then processes them into a linear optical cable. The elastic deformation properties of the metal spiral armor are utilized to achieve distributed sensing, ensuring reliable transmission of stress and strain from the measured object to the sensing optical fiber. This completely avoids the chirping problem during the cabling process, resulting in a simple cabling process and convenient engineering installation, thus possessing promising application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of strain optical cables, and in particular to an internal fixed-point ultra-weak optical fiber Bragg grating strain optical cable. Background Art

[0002] When monitoring large structures and infrastructure such as bridges, tunnels, and roadbeds, traditional point-type fiber Bragg grating (FBG) sensors struggle to fully monitor the entire structure. Furthermore, as the number of deployed sensors increases, the complexity and cost of the FBG system increase dramatically. Fiber optic sensors based on Brillouin scattering (SBRS) enable long-distance distributed sensing and are widely popular in the field of engineering structure monitoring. However, due to the limitations of stimulated Brillouin scattering (SBRS), the strain monitoring accuracy at a single point is limited to tens of microstrains. Furthermore, the system is complex and suffers from poor real-time performance, making it difficult to meet the needs of demanding monitoring applications. Ultra-weak fiber Bragg gratings (UFBRs) have extremely low reflectivity. Through time-division / wavelength-division multiplexing (TD / WDM), the number of multiplexed sensing units and spatial resolution can be greatly increased, enabling dense, point-type sensing of large strains. However, this approach still cannot completely overcome the challenges of quasi-distributed sensing. Improving UFBR sensing technology to achieve fully distributed strain sensing over long distances remains a major challenge facing UFBR sensing.

[0003] Ultra-weak fiber Bragg grating arrays (hereinafter referred to as "grating arrays") often require multiple layers of protection to facilitate transportation and construction, such as spiral armoring of the optical cable, metal wire braiding, longitudinal Kevlar fiber laying, and the addition of an outer sheath. The above processing improves the survivability of the grating array, but the more coating material, the less direct contact the sensing unit can have with the object being measured, which can easily cause direct or indirect strain transmission distortion. In addition, the grating array is very sensitive to processing stress and microbending during cabling. The cabling process of traditional strain optical cables will encounter obvious problems when transplanted to ultra-weak gratings. For example, the Chinese patent "Metal-based Cable-like Distributed Fiber Optic Sensor" (Patent No.: ZL: 201110212087.5) proposes the use of an external steel wire bundle that is twisted in a spiral form around a sensing fiber Bragg grating string at the center. During the actual cabling process, the grating area will cause serious spectral chirp or distortion due to uneven coating, excessive length of the grating area, and uneven stress caused by local twisting.

[0004] The Chinese patent "A stress-strain optical cable based on an ultra-short weak grating array" (application number: 201811184852.5) ​​proposes to effectively reduce the probability of cabling chirp or distortion by shortening the grating length, but this places high demands on the grating preparation and cabling process, and the problem of grating chirp cannot be completely avoided. Summary of the Invention

[0005] In response to the problems of quasi-distributed sensing and grating chirp in existing ultra-weak grating optical cables, the present invention proposes an internally fixed-point ultra-weak fiber Bragg grating strain optical cable with a simple manufacturing process, good strain transfer effect, and easy laying. The grating array and metal spiral armor are pre-internal bonded at selected points and then processed into a linear optical cable. The elastic deformation characteristics of the metal spiral armor are utilized to achieve reliable transmission of stress and strain from the measured object to the optical fiber, completely avoiding the chirp problem in the cabling process. In addition, the cabling process is simple, the engineering laying is convenient, and it has good application prospects.

[0006] The technical solution adopted by the present invention is:

[0007] An internal fixed-point ultra-weak fiber Bragg grating strain optical cable, comprising:

[0008] An ultra-weak fiber Bragg grating array located in the center, with multiple ultra-weak Bragg grating sensing units engraved inside;

[0009] The ultra-weak fiber Bragg grating array is spirally sleeved with a metal spiral armor, and an adhesive is injected between the ultra-weak fiber Bragg grating array and the inner wall of the metal spiral armor, and solidified to form a bonding point;

[0010] A metal mesh woven crosswise along the axis of the metal spiral armor;

[0011] A sheath tightly wrapped around the metal spiral armor and the outer periphery of the metal mesh;

[0012] A sensor mark is coated on the sheath at a position corresponding to the ultra-weak Bragg grating sensing unit.

[0013] The ultra-weak fiber grating array further includes a core, a cladding, a first tight coating layer, and a second tight coating layer;

[0014] The fiber core is inscribed with an ultra-weak Bragg grating sensor unit with a reflectivity of 0.001% to 0.1%. The fiber core and cladding are drawn from a G.657 fiber preform. The first tight coating layer coats and surrounds the cladding, and the second tight coating layer coats and surrounds the first tight coating. The fiber diameter of the first tight coating is 0.17 to 0.20 mm, while the fiber diameter of the second tight coating is 0.90 mm.

[0015] The metal spiral armor is formed by winding a stainless steel strip with an inner diameter greater than 2.0 mm, a width of 1 mm and a thickness of 0.3 mm.

[0016] The bonding point is formed by enlarging the spiral gap of the metal spiral armor at a selected position, injecting adhesive into the inner hole of the metal spiral armor and then curing it, and is used to transfer the stress of the metal spiral armor to the ultra-weak fiber grating array.

[0017] The sheath is made of engineering plastics such as PE and has a thickness greater than 0.5 mm.

[0018] According to the requirements of the construction site, an internal fixed-point ultra-weak fiber Bragg grating strain optical cable is laid to ensure that the entire cable is in a pre-tensioned state and that the cable is tightly fitted to the object being measured. One end of the strain optical cable is connected to an ultra-weak fiber Bragg grating strain analyzer to read out the positions and initial wavelengths of all ultra-weak Bragg grating sensing units on the cable and save them as the initial strain state values. The sampling time interval is set according to the on-site monitoring requirements, and the wavelength changes of the ultra-weak Bragg grating sensing units at different times are read. Based on the calibrated wavelength-strain coefficient, the strain changes of each ultra-weak Bragg grating sensing unit relative to the initial state are calculated to display the stress distribution at the cable laying position.

[0019] The present invention provides an internal fixed-point ultra-weak fiber Bragg grating strain optical cable, which has the following beneficial effects:

[0020] 1) Distributed stress / strain monitoring is realized:

[0021] Through structural innovation, the sheath and spiral armor are used to transfer stress / strain, and the stress / strain of the object to be measured is transmitted to the grating array in segments. By connecting multiple segments, a fully distributed sensor in the length direction is formed, which solves the problem that traditional ultra-weak gratings can only perform point-based sensing.

[0022] 2) Simple processing and stable performance:

[0023] This optical cable is designed to meet the needs of ultra-weak grating stress / strain monitoring by improving traditional field optical cables. The cable's armoring, braiding, and jacketing are all automated, with minimal manual dispensing required to achieve the desired design. The manufacturing process is simple. The outer sheath and metal spiral armor are tightly integrated, ensuring efficient stress / strain transmission, minimal creep, and stable cable performance.

[0024] 3) Easy to lay:

[0025] The entire optical cable has the same appearance as ordinary optical cables, allowing for direct installation using the same laying process as ordinary optical cables, simplifying on-site construction. By marking the grating position, the impact of local bends on the sensing point can be effectively avoided, effectively avoiding blind spots. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the cross-sectional structure of the ultra-weak grating optical cable of the present invention;

[0027] Among them: 1 is an ultra-weak fiber Bragg grating array; 2 is a metal spiral armor; 3 is a bonding point; 4 is a metal wire mesh; 5 is a sheath; and 6 is a sensor marker.

[0028] Figure 2 Schematic diagram of the structure of the ultra-weak strain fiber Bragg grating array of the present invention;

[0029] Wherein: 11 is an ultra-weak fiber Bragg grating; 12 is a fiber core; 13 is a fiber cladding; 14 is a first tight coating layer; 15 is a second tight coating layer.

[0030] Figure 3 Schematic diagram of the internal fixed-point ultra-weak fiber Bragg grating strain cable sensing system applied to mountain landslide monitoring. DETAILED DESCRIPTION

[0031] An internal fixed-point ultra-weak fiber Bragg grating strain optical cable, comprising:

[0032] An ultra-weak fiber Bragg grating array 1 located in the center and having multiple ultra-weak Bragg grating sensing units 11 engraved therein;

[0033] The ultra-weak fiber Bragg grating array 1 is loosely spirally sleeved with a metal spiral armor 2, and an adhesive is injected between the ultra-weak fiber Bragg grating array 1 and the inner wall of the metal spiral armor 2, and solidified to form a bonding point 3;

[0034] A metal mesh 4 cross-woven along the axial direction of the metal spiral armor 2;

[0035] A sheath 5 tightly wrapped around the metal spiral armor 2 and the metal mesh 4;

[0036] A sensor mark 6 is coated on the sheath 5 at a position corresponding to the ultra-weak Bragg grating sensor unit 11 .

[0037] The ultra-weak fiber Bragg grating array 1 also includes a core 12, a cladding 13, a first tight coating layer 14, and a second tight coating layer 15; the core 12 is engraved with an ultra-weak Bragg grating sensing unit with a reflectivity of 0.001% to 0.1%, the core 12 and the cladding 13 are drawn from a G.657 type optical fiber preform rod, the first tight coating layer 14 coats and wraps the cladding 13, and the second tight coating layer 15 coats and wraps the first tight coating layer 14.

[0038] The fiber diameter of the first tight coating layer 14 is 0.17-0.20 mm, and the fiber diameter of the second tight coating layer 15 is 0.90 mm. To ensure effective stress / strain transmission, the slip layer design of traditional optical fibers is eliminated. Through the double-layer tight coating, the strain range of the ultra-weak grating array can reach over 50,000 με, fully leveraging the large strain advantage of the ultra-weak grating and better meeting the needs of monitoring different strain ranges.

[0039] The central wavelengths of the multiple ultra-weak Bragg grating sensing units 11 can be the same or different, and the spacing between the sensing units can be customized. Ultra-weak gratings can multiplex tens of thousands of sensing units on a single optical fiber. This design fully utilizes the large capacity of ultra-weak gratings and, through wavelength / time division multiplexing, can further improve the spatial resolution and capacity of the grating array.

[0040] The metal spiral armor 2 is formed by winding a stainless steel strip with an inner diameter greater than 2.0mm, a width of 1mm, and a thickness of 0.3mm. This design ensures a good fit between the metal spiral armor 2 and the 0.9mm diameter grating array, preventing additional losses due to bending or squeezing. The width and thickness of the stainless steel strip also provide the optical cable with a certain degree of recovery, enabling bidirectional monitoring of stress and strain changes.

[0041] The bonding point 3 is formed by expanding the spiral gap of the metal spiral armor 2 at a selected position, injecting adhesive into the inner hole of the metal spiral armor 2 and then curing it, and is used to transfer the stress of the metal spiral armor 2 to the ultra-weak fiber grating array 1.

[0042] The weaving density of the metal mesh 4 is adjusted according to the tensile strength of the optical cable. The higher the tensile strength, the higher the weaving density.

[0043] The sheath 5 is made of engineering plastics such as PE and has a thickness greater than 0.5 mm. The sheath 5 has a certain thickness, which can not only improve the protection of the internal optical fiber, but also effectively prevent damage caused by external factors such as friction, and is waterproof and moisture-proof.

[0044] An internally fixed ultra-weak fiber Bragg grating (FBG) strain gauge cable was laid according to the construction site's requirements, ensuring that the entire cable was pre-tensioned and that the cable was in close contact with the object being measured. One end of the strain gauge cable was connected to an ultra-weak fiber Bragg grating (FBG) strain analyzer (model RS-HFBGA-04, manufacturer: Yichang Ruichuan Optoelectronics Technology Co., Ltd.). The position and initial wavelength of all ultra-weak Bragg grating (FBG) sensing units on the cable were read and saved as the initial strain state value. The sampling interval was set according to the site's monitoring requirements, and the wavelength change of the FBG sensing units at different times was read. Based on the calibrated wavelength-gauge coefficient, the strain change of each FBG sensing unit relative to the initial state was calculated. This displayed the stress distribution at the cable's installation location.

[0045] The present invention proposes an internally fixed-point ultra-weak fiber Bragg grating strain optical cable with a simple manufacturing process, good strain transfer effect, and easy laying. The grating array and metal spiral armor are pre-bonded internally at selected points and then processed into a linear optical cable. The elastic deformation characteristics of the metal spiral armor are used to achieve distributed sensing, ensuring the reliable transmission of stress and strain from the measured object to the sensing optical fiber, completely avoiding the chirp problem in the cabling process, and having a simple cabling process and convenient engineering laying, which has good application prospects.

Claims

1. A method for monitoring stress of an internally fixed-point ultra-weak fiber Bragg grating strain gauge cable, characterized by: The invention comprises an internal fixed-point ultra-weak fiber Bragg grating strain optical cable, the fiber Bragg grating strain optical cable comprising: An ultra-weak fiber Bragg grating array (1) located in the center and having a plurality of ultra-weak Bragg grating sensing units (11) engraved therein; The ultra-weak fiber grating array (1) is spirally sleeved with a metal spiral armor (2), and an adhesive is injected between the ultra-weak fiber grating array (1) and the inner wall of the metal spiral armor (2), and solidified to form a bonding point (3); A metal mesh (4) cross-woven along the axial direction of the metal spiral armor (2); A sheath (5) tightly wrapped around the metal spiral armor (2) and the outer periphery of the metal mesh (4); A sensor mark (6) is coated on the sheath (5) at a position corresponding to the ultra-weak Bragg grating sensing unit (11); The bonding point (3) is formed by enlarging the spiral gap of the metal spiral armor (2) at a selected position, injecting an adhesive into the inner hole of the metal spiral armor (2), and then curing the adhesive, and is used to transfer the stress of the metal spiral armor (2) to the ultra-weak fiber grating array (1); According to the requirements of the construction site, the internal fixed-point ultra-weak fiber Bragg grating strain optical cable is laid to ensure that the entire cable is in a pre-tensioned state and that the cable fits tightly to the object being measured; Connect one end of the strain optical cable to an ultra-weak fiber Bragg grating strain analyzer, read out the positions and initial wavelengths of all ultra-weak Bragg grating sensing units (11) on the optical cable, and save them as initial strain state values; The sampling time interval is set according to the on-site monitoring requirements, the wavelength change of the ultra-weak Bragg grating sensor unit (11) at different times is read, and the strain change of each ultra-weak Bragg grating sensor unit (11) relative to the initial state is calculated based on the calibrated wavelength-strain coefficient, and the stress distribution at the optical cable laying position is displayed.

2. The method for monitoring stress of an internally fixed-point ultra-weak fiber Bragg grating strain cable according to claim 1, characterized in that: The ultra-weak fiber grating array (1) further comprises a fiber core (12), a cladding (13), a first tight coating layer (14), and a second tight coating layer (15); The fiber core (12) is engraved with an ultra-weak Bragg grating sensing unit with a reflectivity of 0.001% to 0.1%. The fiber core (12) and the cladding (13) are drawn from a G.657 type optical fiber preform rod. The first tight coating layer (14) coats and wraps the cladding (13), and the second tight coating layer (15) coats and wraps the first tight coating layer (14).

3. The stress monitoring method for an internal fixed-point ultra-weak fiber Bragg grating strain cable according to claim 2, characterized in that: The diameter of the first tight coating layer (14) is 0.17-0.20 mm, and the diameter of the second tight coating layer (14) is 0.90 mm.

4. The stress monitoring method for an internal fixed-point ultra-weak fiber Bragg grating strain cable according to claim 1, characterized in that: The metal spiral armor (2) is formed by winding a stainless steel strip with an inner diameter greater than 2.0 mm, a width of 1 mm, and a thickness of 0.3 mm.

5. The stress monitoring method for an internal fixed-point ultra-weak fiber Bragg grating strain cable according to claim 1, characterized in that: The sheath (5) is made of engineering plastics such as PE and has a thickness greater than 0.5 mm.

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