A multi-core FRP ultra-weak fiber Bragg grating shape sensing optical cable and sensing system

Abstract

A multi-core FRP ultra-weak fiber grating shape sensing optical cable and sensing system, the sensing optical cable includes a plurality of optical fibers containing ultra-weak fiber grating arrays evenly distributed along the central axis, and the plurality of optical fibers are wrapped with an outer cladding containing fiber reinforced plastic FRP. The outer cladding is formed by high-temperature curing of glass fiber and resin material. The optical fiber adopts bend-resistant optical fiber, and an ultra-weak fiber grating array with equal spacing is engraved inside, and each grating unit of the ultra-weak fiber grating array is located in the same radial section. The utility model not only realizes the precise positioning of multiple optical fibers, but also eliminates the need for special multi-core preform rod drawing and production, which greatly simplifies the requirements for special equipment for multi-core optical fiber production. It can also effectively achieve high-strength protection for fragile optical fibers, and has the dual functions of force bearing and perception sensing.

Description

Technical Field

[0001] The utility model relates to the technical field of sensing optical fibers, in particular to a multi-core FRP ultra-weak optical fiber grating shape sensing optical cable and a sensing system. Background Art

[0002] Fiber optic shape sensing technology is an emerging technology developed in recent years. Fiber optic shape sensors are usually composed of several optical fibers with a specific spatial arrangement that are closely arranged together. Fiber optic shape sensing uses the differential strain response of each optical fiber under deformation to measure the position and shape of the fiber optic shape sensor itself or the object connected to it. In recent years, this technology has made progress in the tracking of catheter positions in minimally invasive medical interventional surgeries, the measurement of key structural morphology in the aerospace field, and the deformation monitoring of long-distance pipelines and cables.

[0003] The current mainstream fiber optic shape sensing technologies mainly include multi-core fiber Bragg grating (FBG) sensing technology, optical frequency domain reflectometry (OFDR) technology, and Brillouin scattering technology (BOTDR) technology. Multi-core FBG shape sensing technology is limited by the number of multiplexed FBGs on a single optical fiber, and the deformation sensing distance is only at the meter level. It is currently mainly used for positioning during surgery. Optical frequency domain reflectometry technology has higher spatial resolution and is more suitable for 3D shape sensing, but is limited by the bandwidth of the light source. The detection length is only about 100 meters, and the narrow linewidth continuous frequency sweeping light source and multi-core Bragg grating preparation of the core components cannot be domestically produced. The high price limits its application in the field of flexible inclinometers. Brillouin scattering technology (BOTDR) can increase the detection distance to the km level, but the lower positioning

[0004] (meter level) and strain accuracy (10με) will lead to large reconstruction errors.

[0005] All of the above technologies require pulling out multi-core optical fibers through multi-core preform rods. This process requires the production of special optical fiber preform rods, and dedicated devices are needed to couple and extract optical signals from dense fiber cores. The system cost is high, the application cases are limited, and there are few reports on large-scale spatial shape perception.

[0006] Ultra-weak fiber Bragg grating technology is a fiber sensing technology based on grating structure. Ultra-weak fiber Bragg grating is similar to making tens of thousands of low-reflectivity microstructures in the fiber core. It can "selectively" reflect the light signal passing through it. When the period of these microstructures changes, its reflection spectrum also changes accordingly, thereby reflecting the changes in physical quantities around the optical fiber, such as temperature, strain, displacement, etc. The reflectivity of ultra-weak fiber Bragg grating is usually less than 0.1%. It inherits the characteristics of traditional fiber Bragg grating (FBG) sensors, and also has the advantages of large multiplexing capacity, good mechanical properties, and easy cabling. It can accommodate up to tens of thousands of sensor units on a single optical fiber, effectively alleviating the demand for capacity of the sensor network. It is highly favored in the field of structural monitoring, especially suitable for structural health monitoring of large buildings (such as tunnels, dams, high-rise buildings, bridges, etc.). At present, the resolution of ultra-weak fiber Bragg grating sensing systems has reached 0.1m, and the sensing distance exceeds tens of kilometers. The use of multi-core ultra-weak fiber Bragg gratings for shape sensing has the advantages of both sensing distance and resolution. However, there are huge challenges in making large-scale ultra-weak grating arrays over long distances in multi-core optical fibers. Summary of the invention

[0007] In order to solve the technical problem that traditional optical fiber sensors are difficult to install and easy to break at the construction site, the utility model provides a multi-core FRP ultra-weak fiber grating shape sensing optical cable and its preparation method, which treats multiple 0.25mm standard bend-resistant optical fibers engraved with ultra-weak grating arrays to withstand temperature so that they can adapt to 200℃ thermal curing molding, and then uses mold hot pultrusion to make multi-core FRP optical cables. This design not only achieves precise positioning of multiple optical fibers, but also eliminates the need for special multi-core preform rod drawing, greatly simplifying the requirements for special equipment for multi-core optical fiber production. It can also effectively achieve high-strength protection for fragile optical fibers, and has the dual functions of force bearing and perception sensing.

[0008] The technical solution adopted by the utility model is:

[0009] A multi-core FRP ultra-weak fiber grating shape sensing optical cable, comprising:

[0010] Multiple optical fibers containing ultra-weak fiber grating arrays are evenly distributed along the central axis.

[0011] Multiple optical fibers are wrapped in an outer cladding containing fiber reinforced plastic (FRP).

[0012] The outer cladding is formed by high temperature curing of glass fiber and resin material.

[0013] The optical fiber is a bend-resistant optical fiber, and an ultra-weak fiber grating array with equal spacing is engraved inside, and each grating unit of the ultra-weak fiber grating array is located in the same radial section.

[0014] The grating reflectivity of the ultra-weak fiber grating array is about 0.001% to 0.1%. The ultra-weak fiber grating array is a grating with the same wavelength, or gratings with different wavelengths are inserted between gratings with the same wavelength.

[0015] There are three optical fibers. When viewed from the cross-section of the three optical fibers, the three fiber cores are distributed on the same circumferential surface at an angle of 120°, forming a three-core FRP optical cable.

[0016] The optical fiber is formed by uniformly coating a high-temperature resistant coating twice on the outside of the bend-resistant optical fiber coating.

[0017] The high temperature resistant coating can withstand a temperature greater than or equal to 190° C. and has a diameter greater than 0.35 mm.

[0018] The optical fiber is a G.657A2 bend-resistant optical fiber.

[0019] A method for preparing a multi-core FRP ultra-weak fiber grating shape sensing optical cable comprises: subjecting a plurality of standard bend-resistant optical fibers engraved with ultra-weak fiber grating arrays to temperature resistance treatment, filling them with glass fibers and resin materials, positioning them through a mold, and then hot pultruding them to obtain a multi-core FRP optical cable.

[0020] The multi-core FRP ultra-weak fiber Bragg grating shape sensing optical cable is embedded in the object to be measured and connected to the ultra-weak fiber Bragg grating demodulator to form an ultra-weak fiber Bragg grating sensing system for strain field monitoring.

[0021] A multi-core FRP ultra-weak fiber Bragg grating shape sensing optical cable and its preparation method, the technical effects are as follows:

[0022] 1) It avoids the complicated preparation process of multi-core fiber grating array and has high cost performance:

[0023] This process does not require the preparation of multi-core fiber preforms, nor does it require the simultaneous writing of grating arrays on multi-core fibers, and avoids the problem of multi-core fiber coupling output. The cost of the optical cable is about 1 / 10 of that of traditional multi-core FBG fibers, and the composite process has a high yield rate, which is convenient for improving the sensitivity of the optical cable through structural design, and is cost-effective.

[0024] 2) The optical cable has high sensitivity, good strength and convenient construction:

[0025] By optimizing the spacing of the three-core fiber gratings in the optical cable, the sensitivity of the optical cable can be flexibly designed. Compared with 0.25mm multi-core optical fiber, the sensitivity is increased by hundreds of times. At the same time, fiber reinforced plastic FRP is a composite material composed of a polymer matrix and fiber reinforcement. The fiber can be made of glass, carbon, aramid or basalt and can be flexibly replaced. FRP also has the advantages of light weight, corrosion resistance and durability. The sensor cable made by FRP process can be directly wound, and the strength is increased by hundreds of times. There is no need for secondary packaging into cables, and the construction is simple and convenient.

[0026] 3) The demodulation technology is mature, the deformation sensing system is cheap, and the application prospects are broad:

[0027] The cost of the ultra-weak fiber Bragg grating demodulator is only 1 / 3 to 1 / 4 of that of BOTDR / A, but it has high demodulation accuracy, good real-time performance, and mature technology. Combined with low-cost multi-core ultra-weak fiber Bragg grating cables, the price of the entire system is relatively low. It has obvious cost advantages in deformation monitoring and has broad application prospects.

[0028] 4) Due to the low cost, high sensitivity and easy construction of the utility model, it has a wide range of application potentials in structural health monitoring, deep ground deformation monitoring and many other occasions. In particular, in large-scale engineering projects that require long-term and continuous monitoring, the application of the utility model can greatly improve the monitoring efficiency and accuracy.

[0029] In summary, the utility model is superior to traditional sensors, and fundamentally solves the shortcomings of traditional external sensors such as poor monitoring accuracy, high cost, easy damage, and poor durability. It not only strengthens the structure, but also can monitor the stress state of the structure for a long time. The optical fiber and the monitored object are fully coupled, and the excellent sensing characteristics of the optical fiber sensor are fully inherited, which greatly improves the accuracy and durability of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a schematic diagram of the structure of the multi-core FRP ultra-weak fiber Bragg grating shape sensing optical cable of the utility model.

[0031] Figure 2 This is a cross-sectional view of the three-core FRP optical cable described in an embodiment of the present utility model.

[0032] Figure 3 This is a cross-sectional view of the three-core FRP optical cable described in an embodiment of the present utility model.

[0033] Figure 4 Schematic diagram of curvature calibration of three-core FRP optical cable on the calibration board.

[0034] Figure 5 Schematic diagram of temperature calibration of three-core FRP optical cable in a constant temperature and humidity chamber.

[0035] Figure 6 This is the fitting curve of the three-core FRP optical cable at 1528nm.

[0036] Figure 7 This is the fitting curve of the three-core FRP optical cable at 1540nm.

[0037] Figure 8 This is the fitting curve of the three-core FRP optical cable at 1552nm.

[0038] Fig. 9 This is the curvature calibration result diagram for wavelength 1552.

[0039] Fig.10 This is the curvature calibration result diagram for wavelength 1542.

[0040] Fig.11 This is the curvature calibration result diagram for wavelength 1529.

[0041] Among them: 1-optical fiber, 2-outer cladding, 3-ultra-weak fiber grating array, 4-grating unit, 5-calibration disk, 6-curvature line slot, 7-constant temperature box, 8-air outlet, 9-temperature and humidity sensor probe, 10-temperature and humidity recorder, 11-ultra-weak fiber grating demodulator, 12-computer, 13-measuring hole, 14-high temperature resistant coating, A-three-core FRP optical cable. DETAILED DESCRIPTION

[0042] like Figure 1 , Figure 3 As shown, a multi-core FRP ultra-weak fiber Bragg grating shape sensing optical cable comprises:

[0043] A plurality of heat-resistant pretreated optical fibers 1 are evenly distributed along the central axis.

[0044] A plurality of optical fibers 1 are covered with an outer cladding 2 made of fiber reinforced plastic FRP.

[0045] The outer cladding 2 is formed by high temperature curing of glass fiber and resin material;

[0046] Glass fiber and resin materials are combined to form glass fiber reinforced composite plastics. By axially filling glass fiber and resin materials, not only can the mechanical properties of fiber reinforced plastics FRP be improved, but also the performance of the material can be optimized to meet specific application requirements by adjusting the arrangement and content of the fibers.

[0047] The optical fiber 1 is a bend-resistant optical fiber, and an ultra-weak fiber grating array 3 with equal spacing is engraved inside, and each grating unit 4 of the ultra-weak fiber grating array 3 is located in the same radial section. Ensure that multiple fiber cores are aligned, and the stress values ​​measured each time are on the same plane. The position error of multiple ultra-weak fiber gratings in the same radial section is less than 1mm.

[0048] The optical fiber 1 is formed by uniformly coating the anti-bend optical fiber coating layer with a high temperature resistant coating 14 for a second time. The high temperature resistant coating 14 can withstand a temperature greater than or equal to 190° C. and has a diameter greater than 0.35 mm.

[0049] The grating reflectivity of the ultra-weak fiber grating array 3 is about 0.001% to 0.1%. The ultra-weak fiber grating array 3 can be a grating of the same wavelength, or different wavelength gratings can be inserted between the gratings of the same wavelength to increase the density of the sensor, thereby improving the spatial resolution of the sensor.

[0050] like Figure 2 As shown, there are three optical fibers 1. From the cut surface of the three optical fibers, the three fiber cores are distributed on the same circumference at an angle of 120°, forming a three-core FRP optical cable A. The temperature sensitivity coefficient of the three-core FRP optical cable A is 18.9 pm / °C, and the high-temperature resistant coating 14 still has high accuracy in a high-temperature environment of 180°C. The high-temperature resistant coating 14 is made of modified acrylate or organic ceramic coating with secondary UV coating.

[0051] The optical fiber 1 is a G.657A2 bend-resistant optical fiber, which can suppress the microbend loss caused by the secondary coating. For example, after using G.657 optical fiber, the loss is greatly reduced, and kilometer-level ultra-weak fiber Bragg grating sensing optical fiber can be produced to reduce the additional loss caused by stress during thermal curing molding.

[0052] A method for preparing a multi-core FRP ultra-weak fiber grating shape sensing optical cable comprises the following steps: subjecting a plurality of 0.25 mm standard bend-resistant optical fibers engraved with an ultra-weak fiber grating array 4 to temperature resistance treatment so that they can adapt to 200°C thermal curing molding; and filling the fibers with glass fiber, resin and resin materials and then positioning the fibers through a mold and then hot pultruding molding to obtain a multi-core FRP optical cable.

[0053] The glass fiber reinforced composite plastic of the outer cladding 2 is formed by high temperature curing of resin, glass fiber or carbon fiber, etc., with 70% glass fiber and 30% resin, and the curing temperature is 160°C.

[0054] The ultra-weak fiber Bragg grating including the high temperature resistant coating 14 maintains a good morphology during the entire carbon fiber manufacturing process. The high temperature resistant coating 14 effectively reduces the influence of the shrinkage of the glass fiber reinforced composite plastic on the grating area during the thermal curing molding process.

[0055] The diameter of the multi-core FRP ultra-weak fiber Bragg grating sensing cable is only 2.0 mm, and it has the advantages of high strength, large strain range, good repeatability, and small size, making it easy to integrate with large structures.

[0056] The multi-core FRP ultra-weak fiber Bragg grating shape sensing optical cable is embedded in the object to be measured, such as large structures, landslides, bridges, tunnels, etc., and connected to the ultra-weak fiber Bragg grating demodulator to form an ultra-weak fiber Bragg grating sensing system for strain field monitoring. The strain sensing accuracy of the system can reach 1με. The temperature accuracy is ±0.5℃, and the optical fiber batch preparation process is mature. It will become a new type of strain sensor and be put into a wide range of scenarios.

[0057] When the multi-core FRP ultra-weak fiber grating shape sensing cable bends and deforms in space, the multi-core FRP cable structure and ultra-weak fiber grating array 3 inside the cable can accurately respond to the shape change of the cable. Each ultra-weak fiber grating unit inside the multi-core FRP cable can be regarded as an independent strain sensing unit, which can capture the tiny strain changes generated by the cable during the bending process. These strain changes have obvious directionality and continuity, which means that they can not only characterize the local deformation of the cable, but also reflect the overall shape change trend of the cable.

[0058] When a multi-core FRP ultra-weak fiber Bragg grating shape sensing cable is bent, the fiber Bragg gratings at different positions will experience different degrees of stretching or compression, resulting in changes in the reflected wavelength of the grating. By monitoring the changes in these wavelengths, the bending degree and bending direction of the cable can be captured in real time. Combined with advanced strain-deformation inversion algorithms, the complete shape of the cable in space can be inverted from these local strain information, including the bending curvature, bending angle, and bending direction of the cable.

[0059] (I) The working principle of FRP optical cable curvature calibration is:

[0060] like Figure 4 As shown, first connect the connecting wires of the three-core FRP optical cable A and lay it flat on a horizontal plane and let it stand. After the wavelength is stable, measure the initial center wavelength data of the three optical fibers 1 respectively, and then put them into different curvature line grooves 6 in the calibration plate 5 for calibration experiments. After each placement, control the six degrees of freedom of the head and tail sections to be stable, and fix them with soft glue to ensure that they will not be affected by other stresses. After the wavelength data is stable, take out the data, repeat five groups of experiments for comparison, place five different curvature line grooves 6 in each group, take the wavelength data of three optical fibers 1 for each curvature, and finally use the three-dimensional algorithm to reconstruct the three-dimensional curve according to its wavelength change.

[0061] Performance testing and test result analysis:

[0062] (II) Temperature calibration:

[0063] like Figure 5As shown, firstly, the multi-core FRP ultra-weak fiber Bragg grating shape sensing optical cable is coiled into a coil and fixed with tape and put into the thermostat 7. The thermostat 7 is provided with an air outlet 8, a temperature and humidity sensor probe 9, the temperature and humidity sensor probe 9 is connected to the temperature and humidity recorder 10, the multi-core FRP optical cable is connected to the ultra-weak fiber Bragg grating demodulator 11, and the ultra-weak fiber Bragg grating demodulator 11 is connected to the computer 12. Then the connecting line is led out from the measuring hole 13 of the thermostat 7 and then sealed with a plug to ensure the internal temperature of the thermostat 7 is stable. Finally, the test is started, and the program is set to the range of 20°C to 60°C. A set of wavelength data is taken every 5°C, and a cooling experiment is carried out after reaching 60°C. The method of taking temperature data is the same as above, and each set of temperature is taken after the temperature and wavelength are stable, so as to avoid interference from other factors to the greatest extent.

[0064] First, 14 grating points with grating numbers 3#-16# in the middle section of the multi-core FRP optical cable are selected for heating / cooling temperature data analysis. The analysis results are shown in Tables 1 and 2 below:

[0065] Table 1 Temperature sensitivity coefficient of grid points in FRP constant temperature box and its average value

[0066]

[0067] Table 2 Wavelength variation difference of heating / cooling at the same grating point

[0068]

[0069] (III) Conclusion: According to the test, the difference of FRP grid points is the largest when it is 20-35 degrees. Further analysis and the above table show that the difference is the largest when it is 25 degrees. Among them, FBG6, FBG8, FBG9, and FBG10 change relatively stably, and FBG9 changes the least. The study found that the FBG11# grid point may be at the position of the air outlet 8 inside the constant temperature box 7 during temperature calibration. Therefore, the wavelength change is affected by wind vibration, resulting in inaccurate results.

[0070] Figure 6 This is the fitting curve of the three-core FRP optical cable at 1528nm. Figure 7 This is the fitting curve of the three-core FRP optical cable at 1540nm. Figure 8 This is the fitting curve of the three-core FRP optical cable at 1552nm. In the experiment of studying the temperature sensing characteristics at room temperature, the temperature of the three-core FRP reflection spectrum was recorded from 20℃ to 60℃ with a step size of 5℃, and then the data was analyzed and fitted using software. Figures 6 to 8It can be seen that the wavelength drift of FBGs of different wavelengths is relatively stable during temperature changes within the normal temperature range. When the temperature rises, the spectrum drifts toward the long-wave direction, and the temperature sensitivity coefficients are 18.5, 18.4, and 18.9 pm / ℃ respectively; when the temperature drops, the spectrum drifts toward the short-wave direction, and the temperature sensitivity coefficients are 18.7, 18.9, and 18.9 pm / ℃ respectively. There is no obvious difference in the drift of the FBG central wavelength in the same temperature interval. The average linearity of the three wavelengths is 0.999 regardless of heating or cooling, and the linearity is good.

[0071] (IV) Curvature calibration: The wavelength data of different curvature calibrations in the calibration disk 5 are plotted, and the results are as follows:

[0072] Fig. 9 This is the curvature calibration result diagram for wavelength 1552. Fig.10 This is the curvature calibration result diagram for wavelength 1542. Fig.11 This is the result of the curvature calibration at wavelength 1529. In the experiment of studying the change of standard curvature, the curvature of the three-core FRP reflection spectrum was recorded from 2.2m -1 Up to 3.0m -1 The curvature changes of different wavelengths show a certain linear trend and conform to the law of two decreases and one increase. There is no obvious difference in the drift of the FBG central wavelength. The average linearity of the three wavelengths in the curvature calibration is 0.999, and the linearity is good. This result shows that the three-core FRP can be well used in the wavelength-strain-deformation three-dimensional curve reconstruction algorithm for curve reconstruction.

[0073] (V) Analysis of curvature calibration results:

[0074] The reason why two of the three optical fibers drop and one rise is that the multi-core optical fiber to be tested is bending. The three fiber cores are evenly distributed, and the central angle between each two fiber cores is 120°. When the optical fiber is bent, along the tangent direction of the fiber bending point, the outer core of the multi-core optical fiber will produce tensile stress due to deformation, and the inner core of the multi-core optical fiber will produce compressive stress due to shortening deformation, thereby causing the cross-correlation peak of the Rayleigh scattering spectrum to shift. The essential reason for this phenomenon is still due to stress changes.

[0075] From the above description about the utility model, it can be known that the multi-core FRP optical cable has good flexibility and high strain transfer coefficient, can effectively sense the spatial deformation of large structures, avoids the use of expensive multi-core optical fibers and special couplers, does not require complicated wiring and connection of multiple sensors, can greatly reduce the difficulty of deployment, and can be used in many occasions where electrical sensing methods cannot be used.

[0076] The utility model utilizes the stress distribution characteristics of ultra-weak fiber Bragg gratings, combines high-precision wavelength demodulation technology and strain-deformation inversion methods, and can not only provide accurate stress conditions and deformation information of optical cables, but also achieve high-precision measurement and monitoring of the spatial shape of optical cables, thus showing broad application prospects in the fields of optical cable monitoring and management, structural health monitoring, etc.

Claims

1. A multi-core FRP ultra-weak fiber Bragg grating shape sensing optical cable, characterized in that : It comprises a plurality of optical fibers (1) containing ultra-weak fiber grating arrays (3) uniformly distributed along a central axis, the plurality of optical fibers (1) being wrapped by an outer cladding (2) containing fiber reinforced plastic, and the optical fiber (1) being formed by uniformly coating a high-temperature resistant coating (14) on the outside of a bend-resistant optical fiber coating for a second time.

2. An ultra-weak fiber Bragg grating sensing system, characterized in that: It comprises the multi-core FRP ultra-weak fiber Bragg grating shape sensing optical cable as claimed in claim 1, wherein the multi-core FRP ultra-weak fiber Bragg grating shape sensing optical cable is embedded in the object to be measured and connected to the ultra-weak fiber Bragg grating demodulator (11) for strain field monitoring.

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