A hoop type fiber grating sensor and a calibration method thereof

Abstract

The application relates to a hoop type fiber grating sensor and a calibration method thereof, which comprises a packaging piece, a fiber grating assembly, a connecting assembly and a protection assembly; the packaging piece comprises a first packaging piece and a second packaging piece; the fiber grating assembly comprises a plurality of sensor assemblies and a temperature compensation assembly, the sensor assembly comprises a first strain grating and a second strain grating, and the two strain gratings are symmetrically arranged in the middle parts of the two packaging pieces. The application has the advantages that the strain measuring units are symmetrically arranged in the middle parts of the ring-shaped packaging pieces composed of two half rings, so that the two strain gratings are symmetrically distributed in the ring direction. When the measured steel bars are bent due to factors such as installation eccentricity and steel bar non-circularity, the bending strain component directions sensed by the two strain gratings are opposite, the eccentric bending strain can be offset through subsequent fusion calculation, so that the average ring strain of the whole steel bar can be obtained, and the influence of eccentricity on the measurement result is reduced.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic grating sensing and structural health monitoring technology, specifically relating to a clamp-type fiber optic grating sensor and its calibration method. Background Technology

[0002] In engineering structures such as concrete structures and underground tunnel segments, reinforcing bars are the main load-bearing components. Their stress-strain state directly reflects the safety and service performance of the structure. Therefore, long-term, in-situ monitoring of reinforcing bar stress and strain is of significant engineering importance. Fiber Bragg grating (FBG) sensors, due to their advantages such as resistance to electromagnetic interference, corrosion resistance, and suitability for long-term burial, have been widely used in structural health monitoring. In existing technologies, several fiber Bragg grating-based sensing structures have been proposed for strain measurement of load-bearing components. For example, patent CN116336954A achieves strain measurement by arranging optical fibers axially and applying pretension; another example is patent CN111141227A, which proposes a fiber Bragg grating sensor encapsulation structure applied to reinforcing bars. This structure encapsulates the fiber Bragg grating within a metal or composite material structure and fixes it to the surface of the reinforcing bar to monitor its axial strain. These technical solutions are applicable to linear components or those where axial tension can be directly applied. However, their fiber optic arrangements are primarily linear, and the sensor measurement results are highly dependent on the installation direction and consistency. In cases where the actual stress state of the reinforcing steel is complex, installation space is limited, or construction eccentricity exists, additional bending strain or measurement deviations can easily be introduced, and the overall circumferential or radial stress characteristics of the reinforcing steel are difficult to reflect. On the other hand, for circumferential force or radial pressure measurement scenarios, patent CN119984620B has already implemented bolt tightening force monitoring through a spherical contact structure and circumferential deformation measurement, and proposed a corresponding loading and unloading measurement model. This type of pressure ring sensor has good eccentricity compensation capability in localized pressure scenarios such as bolt preload monitoring. However, it is mainly designed for measuring pressure or force under external loads. The sensor structure is usually set up as an independent measurement unit, which makes it difficult to directly adapt to components with large slenderness ratios, such as steel bars, which need to be embedded in concrete. At the same time, its measurement objects are mainly local pressure or loads, and it has not been specifically designed for the transmission mechanism of the transformation of radial deformation of steel bars into circumferential strain. It also has not solved the problems of fiber optic grating arrangement, loss control and strain decoupling under ring packaging conditions.

[0003] A review of existing technologies reveals that current rebar strain sensors based on linearly arranged fiber optic gratings are prone to measurement errors caused by installation eccentricity, rebar non-circularity, or uneven local contact. Their strain sensing primarily reflects local axial strain, failing to accurately characterize the overall consistency of the rebar's stress. While existing ring or pressure ring sensors possess some circumferential deformation measurement capabilities, they primarily focus on external loads or short-term testing scenarios. Their structural design, installation methods, and calibration methods for long-term collaboration with rebar are not systematically designed for the long-term monitoring needs of heavy reinforced concrete underground structural components. Furthermore, for heavy and complex reinforced concrete underground structural components such as underground tunnel segments, their internal loading conditions are complex, and their service environment is harsh, making long-term monitoring and assessment of structural safety extremely difficult. Any anomalies often have a wide-ranging impact and serious consequences. In practical applications across multiple subway and underground engineering projects, the inventors have found that directly deploying traditional linear or locally bonded fiber optic sensor components in these structural components is highly susceptible to damage during segment production, transportation, assembly, and service. Moreover, the stability and accuracy of measurement results are significantly affected by construction deviations and contact conditions. Based on long-term research and engineering practice experience on the structural characteristics, processing technology and actual use conditions of heavy and complex underground structural components such as shield tunnel segments and pipe jacking segments in Wuxi, Hefei, Chongqing and other places, this paper summarizes the problems and shortcomings of existing technologies and proposes a new fiber optic grating sensor structure. Summary of the Invention

[0004] The present invention aims to solve the measurement error problem caused by existing linear rebar strain sensors due to installation eccentricity, rebar non-roundness, or uneven local contact.

[0005] The present invention solves the above-mentioned technical problems through the following technical means:

[0006] A clamp-type fiber Bragg grating sensor includes a package, a fiber Bragg grating assembly, a connecting assembly, and a protective assembly. The package includes a first package and a second package, both of which are hollow semi-circular ring-shaped strip components. The fiber Bragg grating assembly is located inside the package. The protective assembly is located in the fiber extension section, and the connecting assembly is located at the connection point of the two packages. The fiber Bragg grating assembly includes multiple sensor components and a temperature compensation component. The sensor components include a first strain grating and a second strain grating, symmetrically located in the middle of the two packages. A detachable mounting port is provided on the inner side of the package at the location of the first strain grating and the second strain grating.

[0007] This invention symmetrically arranges fiber optic strain measurement units in the center of a ring-shaped encapsulation structure composed of two semi-rings, so that the two strain gratings are symmetrically distributed in the circumferential direction. When the tested reinforcing bar experiences bending strain due to factors such as installation eccentricity, non-roundness of the reinforcing bar, or uneven concrete constraint, the bending strain components sensed by the two gratings are in opposite directions. Through subsequent fusion calculation, the eccentric bending strains can be mutually canceled at the physical level, thereby obtaining average circumferential strain information that reflects the overall stress state of the reinforcing bar.

[0008] Preferably, the connecting assembly includes multiple connecting rods and multiple connectors. The connecting rods and the connectors are connected by a detachable or fixed connection method, which includes a slotted snap-fit ​​connection, a threaded connection, a clamp connection, a plug-in connection, or other combined connection methods.

[0009] Preferably, the protection component includes a locking element, a backstop, a capillary tube, and a capillary tube connecting device. The backstop is located at the junction of the optical fiber lead-out end and the encapsulation component. The capillary tube is sleeved over the lead-out optical fiber to protect it. The locking element is located between the backstop and the capillary tube. The capillary tube connecting device is located between the capillary tube and the locking element to fix the capillary tube to the locking element.

[0010] Preferably, the temperature compensation component is located inside the connection of the package and specifically includes a temperature compensation grating and a temperature compensation sleeve. The temperature compensation grating is fixed to the package through the temperature compensation sleeve.

[0011] This invention incorporates a temperature-compensating grating in the connection area of ​​the annular package, ensuring its thermal consistency with the overall sensor structure while mechanically isolating it from the circumferential strain. By acquiring the temperature-compensating grating signal and establishing a decoupling relationship between the strain grating and the temperature-compensating grating, the influence of ambient temperature variations on the circumferential strain measurement results can be effectively eliminated, improving the sensor's measurement accuracy under complex environmental conditions.

[0012] Preferably, the sensor assembly includes a strain grating, a strain grating groove, and a cover; the strain grating groove is located on the inner wall of the package, the strain grating is embedded and fixed in the strain grating groove, and the cover wraps around the strain grating to protect it.

[0013] Preferably, the method also includes a calibration method for the clamp-type fiber Bragg grating sensor, the calibration method comprising: firstly, confirming the sensor assembly status and performing a self-check of the optical path integrity; then, calibrating the temperature characteristics, radial and circumferential strain transfer, decoupling and encapsulating the calibration parameters; and finally, performing consistency verification to determine whether the consistency requirements are met. If they are met, on-site monitoring and strain back calculation are performed; if they are not met, readjustment and recalibration are performed.

[0014] Because the strain gratings are arranged along the annular encapsulation structure, the optical fiber is in a continuously bent state within the closed curved path. This alters the optical signal transmission characteristics and strain-wavelength response compared to a straight arrangement. If the traditional calibration method for straight fiber gratings is still used, systematic measurement errors are easily introduced. Therefore, based on the aforementioned annular encapsulation structure, this invention further constructs a dedicated calibration method and solution model that matches its stress and transmission characteristics, ensuring the accuracy and consistency of strain measurement results under annular conditions.

[0015] Preferably, the specific process of temperature characteristic calibration includes: zero strain holding: controlling the variable diameter loading module to maintain a constant outer diameter to ensure that the sensor is not affected by changes in mechanical strain; step temperature rise: under the condition of zero strain holding, the ambient temperature is adjusted in stages and set. n Temperature points After stabilizing at each temperature point, the center reflection wavelength of the temperature compensation grating is collected. Data processing: using wavelength variation To represent the temperature response of the grating, using the formula Calculate the wavelength variation and corresponding temperature difference at different temperatures; verify the independence of temperature compensation. Whether it is consistent with the standard bare grating, ensuring that the temperature compensation structure at the connection is not subjected to thermal stress compression; where Take 1, 2 and , representing the first strain grating, the second strain grating, and the temperature compensation grating, respectively. These are the temperature sensitivity coefficients of the three gratings. This is the initial bias term for the temperature-compensated grating. Indicates temperature. This indicates the change in wavelength in response to different gratings.

[0016] Preferably, the specific process of the radial and circumferential strain transfer calibration includes: constant temperature environment setting: fixing the ambient temperature at standard room temperature. Eliminate thermal drift interference; graded variable diameter loading: control the variable diameter simulation loading module to expand the outer diameter in stages, assuming the number of loading stages is... j , No. j The increment of the stage diameter is Standard circumferential strain calculation: According to the geometric equations of elasticity, the standard circumferential strain reference value applied to the sensor by the diameter-changing device is defined as follows: Sensitivity calculation: using the formula and These represent the strain sensitivity coefficients of the first strain grating and the second strain grating, respectively; where... Indicates standard room temperature. Indicates the outer diameter of the variable diameter loading module. Indicates the first The increment of the loading stage diameter. express The standard circumferential strain reference value under the corresponding loading level, This represents the wavelength shift of the first strain grating at the corresponding order. This represents the wavelength shift of the second strain grating at the corresponding order. This represents the strain sensitivity coefficient of the first strain grating. This represents the strain sensitivity coefficient of the second strain grating.

[0017] Preferably, the specific process of decoupling and encapsulating the calibration parameters includes: cross-sensitivity coefficient calculation: using the formula and Calculate the temperature response ratio of each strain grating relative to the temperature-compensated grating; key parameters obtained: generate a grating containing... The parameter set; where and These represent the cross-sensitivity coefficients of the first strain grating and the second strain grating relative to the temperature compensation grating, respectively.

[0018] Preferably, the specific process of on-site monitoring and strain back-calculation includes: benchmark value acquisition: recording the center wavelengths of the first strain grating, the second strain grating, and the temperature compensation grating at the initial moment. Real-time wavelength detection: at the monitoring time t Acquire the wavelengths corresponding to the three gratings in real time. Temperature decoupling: Utilizing the change in temperature-compensated grating, combined with the cross-sensitivity coefficient obtained from calibration. and Remove the heat output component, and then use the formula and Calculate the net strain wavelength drift of the first and second strain gratings; calculate the unilateral circumferential strain: then use the formula... and formula Back-calculation of the local strain of each of the two semi-circular ring packages; dual-grating fusion and eccentricity error elimination: using the formula To compensate for the bending component error in the opposite direction introduced by installation eccentricity; final stress calculation: using the formula Calculate the final circumferential stress; where This represents the initial wavelength of the first strain grating, the second strain grating, and the temperature compensation grating. This indicates that the first strain grating, the second strain grating, and the temperature compensation grating are in... Time wavelength, and This represents the net strain wavelength shift of the first and second strain gratings. and This represents the local strain of the first and second encapsulation components, respectively. This represents the true mean circumferential strain. This indicates the elastic modulus of the steel reinforcement being measured. This represents the final circumferential stress.

[0019] The advantages of this invention are: (1) This invention arranges fiber optic grating strain measurement units symmetrically in the middle of a ring-shaped encapsulation structure composed of two semi-rings, so that the two strain gratings are symmetrically distributed in the circumferential direction. When the measured steel bar undergoes bending strain due to factors such as installation eccentricity, steel bar non-circularity, or uneven concrete constraint, the bending strain components sensed by the two gratings are in opposite directions. Through subsequent fusion calculation, the eccentric bending strain can be mutually canceled at the physical level, thereby obtaining the average circumferential strain information reflecting the overall stress state of the component. Based on the above stable and eccentric strain measurement results, this invention can not only be used for long-term health monitoring of underground structural components, but also provide reliable basic data support for component stress evolution analysis, attitude adjustment parameter inversion, and construction control during the construction of underground engineering such as pipe jacking and shield tunneling, thus expanding the application value of sensors in underground engineering structure monitoring and construction auxiliary decision-making. (2) Compared with the problem that traditional linear fiber Bragg gratings can only sense local uniaxial strain, the present invention adopts a ring-shaped encapsulation structure, which makes the fiber Bragg grating form a continuous circumferential strain sensing path along the outer periphery of the steel bar, and can more realistically reflect the overall circumferential strain state of the steel bar under axial force or radial expansion. Through the structural cooperation between the semi-ring encapsulation body, the strain transfer cover and the grating slot, the strain can be stably transferred from the steel bar to the working section of the grating, avoiding local slippage or strain concentration, and improving the consistency and repeatability of strain measurement; (3) In this invention, a temperature compensation grating is set in the connection area of ​​the annular packaging structure to keep it thermally consistent with the overall sensor and mechanically isolated from the circumferential strain. By acquiring the temperature compensation grating signal and establishing the decoupling relationship between the strain grating and the temperature compensation grating, the influence of ambient temperature changes on the circumferential strain measurement results can be effectively eliminated, thereby improving the measurement accuracy of the sensor under complex environmental conditions; (4) Given that the ring-shaped encapsulation structure inevitably places the fiber grating in a bent optical path under working conditions, changes in the bending radius may cause changes in the optical signal transmission characteristics, thereby affecting the wavelength demodulation stability and measurement accuracy, this invention constructs a dedicated calibration system and calibration method that matches the ring structure to address the above problems. This calibration system simulates the actual stress and deformation state of steel bars in actual engineering by using radial variable diameter loading, so that the optical fiber is under the same bending and stress conditions as in engineering applications during the calibration stage. In the calibration process, the optical transmission effect introduced by bending is characterized as a whole and the parameters are solidified, avoiding the systematic errors introduced by using linear tension or non-circular loading methods to calibrate the ring sensor, thereby ensuring the applicability and reliability of the calibration parameters under actual ring working conditions; (5) This invention integrates the ring-shaped encapsulation structure, the dedicated calibration system and the corresponding strain back calculation method to ensure that the sensor is consistent in terms of structural form, stress mechanism and data processing method. The parameters obtained from calibration can be directly applied to the strain back calculation process in the engineering field, avoiding measurement deviations caused by mismatch between structural form and calibration method. It is particularly suitable for long-term stress and strain monitoring of steel bars inside concrete structures or underground structure segments. Attached Figure Description

[0020] Figure 1 This is a schematic diagram showing the position of the clamp-type fiber Bragg grating sensor according to the first embodiment of the present invention; Figure 2 This is a schematic diagram of the clamp-type fiber optic grating sensor structure according to the first embodiment of the present invention; Figure 3 This is a perspective view of the clamp-type fiber Bragg grating sensor according to the first embodiment of the present invention; Figure 4 This is a schematic diagram of the strain grating structure inside the clamp-type fiber optic grating sensor according to the first embodiment of the present invention; Figure 5 This is a schematic diagram of the temperature compensation grating structure inside the clamp-type fiber Bragg grating sensor according to the first embodiment of the present invention; Figure 6 This is a schematic diagram of the clamp-type fiber Bragg grating sensor connector structure according to the first embodiment of the present invention; Figure 7 This is a schematic diagram of the structure of the clamp-type fiber Bragg grating sensor protection assembly according to the first embodiment of the present invention; Figure 8 This is the calibration system corresponding to the clamp-type fiber Bragg grating sensor calibration method of the second embodiment of the present invention; Figure 9 This is a flowchart of the calibration method for the clamp-type fiber Bragg grating sensor according to the second embodiment of the present invention.

[0021] Numbering on the map: 1. Package component; 11. First package component; 12. Second package component; 2. Fiber Bragg grating assembly; 21. Optical fiber; 22. Strain grating assembly; 221. First strain grating; 222. Second strain grating; 223. Strain grating groove; 224. Cover; 23. Temperature compensation assembly; 231. Temperature compensation grating; 232. Temperature compensation sleeve; 3. Connecting components; 31. Connecting rod; 32. Connector; 321. Cylinder; 322. Wedge; 323. Spring; 33. Connecting buckle; 4. Protective components; 41. Anti-reverse device; 42. Capillary tube; 43. Locking element; 44. Capillary tube connection device. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1: Combination Figure 1 , Figure 2 and Figure 3 As shown, a clamp-type fiber Bragg grating sensor includes a package 1, a fiber Bragg grating assembly 2, a connecting assembly 3, and a protection assembly 4; wherein the fiber Bragg grating assembly 2 is located inside the package 1, the connecting assembly 3 is located at the connection point of the two packages 1, and is used to connect the two packages 1, and the protection assembly 4 is located at the junction where the fiber Bragg grating assembly 2 extends out of the package 1, and is used to protect the fiber Bragg grating assembly 2.

[0024] Figure 1 is a schematic diagram of the installation position of the sensor on the steel bar being measured.

[0025] For details, please refer to [link / reference]. Figure 2 The package 1 includes a first package 11 and a second package 12, which are two hollow semi-circular ring-shaped strip components. The first package 11 and the second package 12 are connected end to end to form a ring.

[0026] See Figure 3 , Figure 4 and Figure 5The fiber optic grating assembly 2 includes an optical fiber 21, a strain grating assembly 22, and a temperature compensation assembly 23. The optical fiber 21 comprises one or more fibers, located inside the package 1, capable of moving within a certain range inside the package 1, and extending from the lower wall surface of the same side end of the first package 11 and the second package 12. The strain grating assembly 22 includes a first strain grating 221, a second strain grating 222, a strain grating groove 223, and a cover 224. The first strain grating 221 is located in the middle of the first package 11, and the second strain grating 222 is located in the middle of the second package 12. A strain grating groove 223 is pre-formed on the inner side of the middle portion of the first package 11 and the second package 12. The size of the strain grating groove 223 is consistent with that of the first strain grating 221 and the second strain grating 222, allowing the first strain grating 221 and the second strain grating 222 to be fixed inside the package 1 by embedding into the strain grating groove 223. Cover 224 is a box-shaped structure located inside encapsulation 1, primarily used to shield the strain gauge assembly 22, while also serving to transmit strain. Cover 224 can be removed during sensor installation and grating inspection to facilitate grating inspection and maintenance. (See also...) Figure 2 and Figure 4 Once the inspection is complete, the cover 224 is secured to the square hole in the encapsulation 1 via a specific snap-fit ​​mechanism, ensuring that the fiber Bragg grating assembly 2 is unaffected by external environment or pressure, and preventing displacement or damage to the fiber Bragg grating assembly 2 during installation or use. Furthermore, in its connected state, the cover 224 forms a mechanical connection with the strain grating assembly 22, thereby transferring strain to the strain grating assembly 22 and realizing the strain measurement function. The arrangement of the strain grating assembly 22 allows the sensor to synchronously respond to structural deformation under strain. Specifically, the fiber optic cable 21 and the strain grating assembly 22 are arranged along a ring structure. Compared to the traditional linear arrangement, this design ensures that the sensor can uniformly sense strain changes in all directions of the structure, improving measurement accuracy and reliability. (See also...) Figure 5 The temperature compensation component 23 includes a temperature compensation grating 231 and a temperature compensation sleeve 232. The temperature compensation component 23 is located at the tail of the first package 11, near the connection point with the second package 12. The temperature compensation sleeve 232 is fixedly installed to the first package 11, and the temperature compensation grating 231 is fixed to the temperature compensation sleeve 232, thus achieving a fixed connection with the first package 11. In this way, the temperature compensation grating 231 is thermally consistent with the package 11, but is unaffected by structural strain. The function of the temperature compensation grating 231 is to isolate the influence of temperature on the strain signal. By acquiring the signal from the temperature compensation grating 231, temperature correction can be performed on the structural strain signal, thereby achieving decoupling between strain and temperature.

[0027] See Figure 3 and Figure 6The connecting component 3 includes connecting rods 31, connectors 32, and connecting clips 33 (the connecting component 3 includes multiple connecting rods 31 and multiple connectors 32, wherein the connecting rods 31 and connectors 32 are connected by a detachable or fixed connection method, including oblique groove snap-fit ​​connection, threaded connection, clamp connection, plug-in connection, or other combined connection methods; this embodiment uses oblique groove snap-fit ​​as an example). Specifically, the connecting rods 31 are located on both ends of the second encapsulation component 12, with two on each side (at least two on each side), arranged symmetrically vertically. The connecting rods 31 are generally cylindrical, with one end connected and fixed to the second encapsulation component 12, and the other end cut with an oblique cut. An oblique groove is also provided in the middle of the cylinder; the oblique groove and oblique cut of the connecting rods 31 located at the top face upwards, and the oblique groove and oblique cut of the connecting rods 31 located at the bottom face downwards. Connectors 32 are located on both ends of the first package 11, corresponding in number and position to the connecting rods 31, and include a cylinder 321, a wedge 322, and a spring 323. The cylinder 321 is precisely matched with the connecting rod 31, allowing the connecting rod 31 to be inserted into the cylinder 321. A circular hole is pre-drilled at the center of the inner wall of the cylinder 321, through which the wedge 322 is connected to the spring 323. The wedge 322 is cylindrical, with one end connected to the spring 323 and the other end cut into a wedge shape. One end of the spring 323 is fixed to the inner wall of the first package 11, and the other end is connected to the wedge 322. The wedge 322 can move up and down along the pre-drilled circular hole in the cylinder 321 via the spring 323. The wedge 322 of the upper connector 32 is located on the upper wall of the cylinder 321, and the wedge 322 of the lower connector 32 is located on the lower wall of the cylinder 321. When the first package 11 is connected to the second package 12, the connecting rod 31 is inserted into the connector 32. At this time, the cut connecting rod 31 and the wedge 322 press against each other, compressing the spring 323. As the connecting rod 31 continues to penetrate, the wedge 322 will engage with the inclined groove of the connecting rod 31, thereby achieving a tight connection between the first package 11 and the second package 12. The connecting buckle 33 is located on the side of the first package 11 away from the optical fiber 21 lead-out section. The side of the second package 12 away from the optical fiber 21 lead-out section has a pre-reserved slot that matches the connecting buckle. The connecting buckle 33 connects with the matching slot to fix the first package 11 and the second package 12.

[0028] See Figure 7The protective component 4 includes a check valve 41, a capillary tube 42, a locking element 43, and a capillary tube connecting device 44. The check valve 41 is located at the junction of the encapsulation component 1 and the fiber optic cable 21 lead-out section, closely adhering to the outer wall of the encapsulation component 1. The fiber optic cable 21 passes through the check valve 41, preventing movement of the fiber optic cable 21 during installation and effectively preventing loosening or stretching of the fiber optic cable 21 during long-term use. The capillary tube 42 is sleeved on the outside of the fiber optic cable 21 lead-out section, providing protection for the fiber optic cable 21 and preventing damage from external forces during construction. The locking element 43 is located between the check valve 41 and the capillary tube 42, and is ring-shaped. The locking element 43 not only applies pre-tension to the fiber optic cable 21 to ensure stable operation of the fiber optic cable 21 and the strain gauge assembly 22 in the sensor, but also adjusts the pre-tension of the fiber optic cable 21, keeping it stable during construction and subsequent use, preventing loosening or displacement of the fiber optic cable 21, and ensuring the reliability of the measurement results. The capillary connecting device 44 is annular and located between the capillary 42 and the locking member 43. It is used to fix the capillary 42 to the locking member 43, thereby achieving the fixation of the capillary 42.

[0029] Through the above structural design, this sensor achieves multiple functions such as reliable structural fixation, controllable temperature influence, stable fiber optic stress, and long-term sealed protection while ensuring the accuracy of strain measurement. It can work stably for a long time in harsh environments.

[0030] Example 2: This embodiment, based on Embodiment 1, places the first strain grating 221 and the second strain grating 222 inside the locations of the two strain gratings, i.e., as shown in the figure. Figure 2 The inner side of the annular package shown has a detachable mounting port, the size of which is slightly larger than the size of the first strain gauge 221 and the second strain gauge 222. The first strain gauge 221 and the second strain gauge 222 can be inspected, replaced, or otherwise operated through this detachable port.

[0031] Example 3: Because this invention employs a ring-shaped encapsulation structure composed of two semi-rings, the fiber grating is arranged circumferentially along the outer periphery of the reinforcing bar. Under sensor operation, the optical fiber is inevitably in a bent optical path. Compared to a straight-lined fiber, the ring arrangement causes changes in the fiber's bending radius during stress or deformation, leading to additional losses, altered transmission characteristics, and reduced demodulation stability during signal transmission, thus affecting the accuracy of fiber grating wavelength measurement. If the traditional tensile calibration method, suitable for straight fiber structures, is used to calibrate this type of ring-shaped sensor, it is difficult to accurately reflect the sensor's stress state and optical transmission conditions in actual engineering applications, easily introducing systematic errors and resulting in inconsistencies between calibration parameters and actual engineering usage conditions.

[0032] To address the aforementioned issues, this invention does not simply follow the existing calibration approach for fiber Bragg grating sensors. Instead, it combines the stress characteristics of the ring-shaped encapsulation structure with the transmission characteristics of optical fibers under bending conditions to specifically construct a calibration method and system that matches the clamp-type fiber Bragg grating sensor. By simulating the actual stress conditions of reinforcing steel bars during the calibration stage, the optical fiber is placed under circumferential stress and bending optical path conditions consistent with engineering applications during the calibration process. This allows for the overall characterization and parameter solidification of the transmission characteristic changes introduced by the ring structure, thereby ensuring that the obtained calibration parameters accurately reflect the strain response relationship of the sensor under actual use conditions.

[0033] Based on this, the present invention proposes the following calibration method for clamp-type fiber Bragg grating sensors, see reference. Figure 8 This application's calibration method is based on a clamp-type fiber Bragg grating sensor and its matching calibration system. The calibration system specifically includes a constant temperature control module, a high-precision variable diameter simulation loading module, a multi-channel fiber demodulation module, a data processing and fusion decoupling calibration module, and a parameter storage and output module. The constant temperature control module provides a controllable and stable calibration temperature environment, enabling multi-temperature point loading. The high-precision variable diameter simulation loading module preferably uses a precision mechanical expansion shaft or a hydraulically splitting cylinder. This device has a standard cylindrical outer surface, and its outer diameter D can achieve uniform radial expansion under the controller's drive, with a displacement control accuracy better than 1. The coaxiality error is less than 0.02mm. This is the hardware foundation that distinguishes this calibration system from existing technologies, used to provide standardized and controllable radial displacement input to simulate the radial expansion process of steel bars or pipe sections under stress; the multi-channel fiber optic demodulation module has at least 3 synchronous acquisition channels for real-time acquisition of the sensor's first strain grating 221 ( ), second strain grating 222 ( ) and temperature compensation grating 231 ( The system features a central reflection wavelength and optical path loss (optical power) monitoring function. The data processing and fusion decoupling calibration module, as the core of the system's calculations, incorporates the invention's unique "radial-circumferential strain transfer model" and "dual-grating symmetric fusion algorithm" to complete sensitivity coefficient fitting, eccentricity error correction, and final calibration parameter generation. The parameter storage and output module generates electronic archives of the calculated sensor-specific parameter sets or writes them into the sensor's built-in chip for use by the engineering site monitoring system. These subsystems work collaboratively through data and control interfaces to complete the multi-stage calibration of the clamp-type fiber Bragg grating sensor. Each step in the calibration method—"physical quantity acquisition + parameter solution"—must be completed by the corresponding module in the calibration system.

[0034] See Figure 9 The specific steps of the calibration method based on the clamp-type fiber Bragg grating sensor include: S1. Sensor assembly and optical path integrity self-test; The specific process of S1 is as follows: First, the clamp-type sensor to be calibrated is placed on the initial cylindrical surface of the variable diameter simulation loading module, and the initial simulation diameter is set. The diameter should be equal to the nominal diameter of the sensor (e.g., Φ20mm, or any other diameter); after initial installation, simultaneously tighten the locking pieces 43 at both ends of the sensor, and observe the reading on the demodulation module until... and The center wavelength of each fiber optic grating generates a preset initial stretching drift (preferably 2-3 nm, which ensures the fiber grating remains within its linear operating range); the optical power value of each channel is read through the demodulation module. ; where, if any channel's If the macrobending loss is below a preset threshold (e.g., -25dBm, below which the macrobending loss exceeds the sensing allowable range), it is determined that the ring packaging causes excessive macrobending loss (macrobending loss: optical signal transmission loss caused by macro bending of optical fiber), and the packaging process needs to be readjusted or the defective product needs to be rejected.

[0035] S2. Temperature Characteristic Calibration: S2a. Zero Strain Holding; S2b. Step Temperature Rise; S2c. Coefficient Fitting; The specific process of S2 is as follows: Under the condition that the mechanical loading module maintains zero load, the variable diameter loading module is controlled to maintain the outer diameter. Constant temperature ensures the sensor is unaffected by changes in mechanical strain; a temperature sequence is set using an environmental temperature control module. ( =1,…,n), covering the sensor's operating temperature range (-20℃~60℃, the service environment temperature of underground engineering tunnel segments), and maintaining a constant temperature at each point for at least 30 minutes; recording the change in reflected wavelength at each temperature point when the sensor is placed at different temperatures; selecting a reference temperature (usually 20°C) as the starting point, and using this temperature as a reference to measure the sensor's changes at other temperatures; for and The wavelength changes at different temperatures are recorded. Each grating exhibits a different wavelength drift depending on its degree of temperature influence, and these wavelength changes are used to record the wavelength shift. The temperature response of the grating is represented by the following formula: For each grating, the least squares linear fitting formula is used: ;verify Whether it is consistent with the standard bare grating, and ensure that the temperature compensation structure at the connection point is not subjected to thermal stress compression; among which Represent (First strain grating) (Second strain grating) (Temperature compensation grating) express , and Temperature sensitivity coefficient, Indicates temperature. Indicates the outer diameter and intercept of the variable diameter loading module. This is a system bias term used in the temperature calibration process to correct for zero-point errors introduced by the initial assembly state and the testing system. This indicates the change in wavelength in response to different gratings.

[0036] S3, Radial-Circumferential Strain Transfer Calibration: S3a, Isothermal Environment Setting; S3b, Staged Variable Diameter Loading; S3c, Standard Circumferential Strain Calculation; S3d, Sensitivity Solution; The specific process of S3 is as follows: Fix the ambient temperature at the standard room temperature. (e.g., 20℃) to eliminate thermal drift interference; control the variable diameter simulation loading module to expand the outer diameter in stages. Let the number of loading stages be... , No. The increment of the stage diameter is According to the geometric equations of elasticity, the standard circumferential strain reference value applied to the sensor by the diameter-changing device is defined as: At each loading level, the response difference between the two strain gratings 221 and 222 needs to be established through linear regression to determine the relationship between wavelength and strain. The wavelength drift of the first strain grating 221 and the second strain grating 222 at the corresponding loading level is recorded as follows: and The relationship between wavelength and standard circumferential strain was established through linear regression to obtain the system-level strain sensitivity coefficient. and In the formula Indicates standard room temperature. Indicates the first The increment of the loading stage diameter. express The standard circumferential strain reference value under the corresponding loading level, This represents the wavelength shift of the first strain grating 221 at the corresponding order. This represents the wavelength shift of the second strain grating 222 at the corresponding order. This represents the strain sensitivity coefficient of the first strain grating 221. This represents the strain sensitivity coefficient of the second strain grating 222.

[0037] S4. Calibration Parameter Decoupling and Encapsulation: S4a. Cross-sensitivity Coefficient Calculation; S4b. Key Parameter Acquisition; The specific process of S4 is as follows: using the formula... and Calculate the temperature response ratio of strain grating assembly 22 relative to temperature compensation assembly 23 for subsequent direct decoupling without temperature measurement; generate a generator containing... The calibration parameter set; where and These represent the cross-sensitivity coefficients of the first strain grating 221 and the second strain grating 222 relative to the temperature compensation grating 231, respectively.

[0038] S5. Consistency Verification; The specific steps of S5 are: perform "expansion-contraction" full-scale cyclic loading on the sensor 3 times to verify whether the hysteresis error is less than 1% of the maximum full scale, ensuring... The repetitiveness.

[0039] S6. Judgment; The specific process of S6 is as follows: determine whether the consistency of each parameter in step S5 is within the set range. If the requirements are met, proceed to step S8; otherwise, proceed to step S7.

[0040] S7. Readjust and repeat calibration; The specific steps of S7 are as follows: If the relevant parameters in S5 do not meet the consistency verification after judgment, the locking part 43 is readjusted, and after adjustment, it returns to step S1.

[0041] S8. On-site monitoring and strain back calculation method; S8a. Reference value acquisition; S8b. Real-time wavelength monitoring; S8c. Temperature decoupling; S8d. Single-sided circumferential strain calculation; S8e. Dual-grating fusion and eccentricity error elimination; S8f. Final stress calculation; The specific steps of S8 are: record the center wavelengths of the first strain grating 221, the second strain grating 222, and the temperature compensation grating 231 at the initial moment. During the monitoring period The wavelengths corresponding to the first strain grating 221, the second strain grating 222, and the temperature compensation grating 231 are collected. The change in temperature-compensated grating is used in conjunction with the cross-sensitivity coefficient obtained from the calibration in step S4. Remove the heat output component and use the formula and The net strain wavelength shift of the first strain grating 221 and the second strain grating 222 is obtained; then the formula is used... and formula The local strain of the first encapsulation component 11 and the second encapsulation component 12 is calculated in reverse; for the uneven stress caused by installation eccentricity or non-roundness of the reinforcing bars in the annular clamp structure, the formula is used. The weighted average fusion algorithm is used to calculate the true average circumferential strain of the reinforcing steel; finally, based on the elastic modulus of the tested reinforcing steel... Using the formula Calculate the final circumferential stress; where The initial wavelengths of the first strain grating 221, the second strain grating 222, and the temperature compensation grating 231 are indicated. This indicates that the first strain grating 221, the second strain grating 222, and the temperature compensation grating 231 are in... Time wavelength, and This represents the net strain wavelength shift of the first strain grating 221 and the second strain grating 222. and This indicates the local strain of the first package 11 and the second package 12, This represents the true mean circumferential strain. This indicates the elastic modulus of the steel reinforcement being measured. This represents the final circumferential stress.

[0042] This application arranges the strain grating assembly 22 symmetrically in the middle of a ring-shaped encapsulation structure composed of two semi-rings, so that the two strain gratings are symmetrically distributed in the circumferential direction. When the measured rebar undergoes bending strain due to factors such as installation eccentricity, rebar non-circularity, or uneven concrete constraint, the bending strain components sensed by the two gratings are in opposite directions. Through subsequent fusion calculation, the eccentric bending strain can be mutually canceled at the physical level, thereby obtaining the true average circumferential strain of the entire rebar and significantly reducing the influence of eccentricity on the measurement results. Compared with the problem that traditional linear fiber gratings can only sense local unidirectional strain, this invention adopts a ring-shaped encapsulation structure, so that the fiber grating forms a circumferential strain sensing path along the outer periphery of the rebar, which can more realistically reflect the overall circumferential strain state of the rebar under axial force or radial expansion conditions. Through the structural cooperation between the encapsulation component 1, the cover component 224, and the strain grating groove 223, the strain can be stably transferred from the rebar to the working section of the grating, avoiding local slippage or strain concentration, and improving the consistency and repeatability of strain measurement. This invention incorporates a temperature compensation component 23 in the connection area of ​​the encapsulation 1, ensuring thermal consistency with the overall sensor structure while mechanically isolating it from circumferential strain. By acquiring the signal from the temperature compensation grating 231 and establishing a decoupling relationship between the strain grating component 22 and the temperature compensation component 23, the influence of ambient temperature changes on circumferential strain measurement results can be effectively eliminated, improving the sensor's measurement accuracy under complex environmental conditions. Addressing the force characteristics of the clamp-type fiber Bragg grating sensor, which transforms radial deformation into circumferential strain, this invention also constructs a matching dedicated calibration system. This system simulates the stress state of reinforcing steel in actual engineering through radial diameter variation loading. This calibration method accurately reflects the force mechanism of the clamp-type fiber Bragg grating sensor during the calibration stage, avoiding the systematic errors introduced by using linear tension calibration methods for circumferential sensors, thereby improving the physical consistency and engineering applicability of calibration parameters. This invention achieves overall consistency between the sensor's structural form, force mechanism, and data processing methods through the integrated design of the annular encapsulation structure, dedicated calibration system, and corresponding strain back-calculation method. The parameters obtained from calibration can be directly applied to the strain back-calculation process on the engineering site, avoiding the problem of mismatch between different structural forms and calibration methods. It is particularly suitable for long-term stress and strain monitoring of steel bars inside concrete structures or underground structural segments.

[0043] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Terms such as "upper," "lower," "left," "right," "front," and "rear" used in the invention are merely for clarity of description and are not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0044] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A clamp-type fiber Bragg grating sensor, characterized in that, The system includes a package, a fiber Bragg grating assembly, a connecting assembly, and a protective assembly. The package includes a first package and a second package, both of which are hollow semi-circular ring-shaped strip components. The fiber Bragg grating assembly is located inside the package. The protective assembly is located in the fiber extension section, and the connecting assembly is located at the connection point of the two packages. The fiber Bragg grating assembly includes multiple sensor components and a temperature compensation component. The sensor components include a first strain grating and a second strain grating, symmetrically located in the middle of the two packages. A detachable mounting port is provided on the inner side of the package at the location of the first strain grating and the second strain grating.

2. The clamp-type fiber Bragg grating sensor according to claim 1, characterized in that, The connecting assembly includes multiple connecting rods and multiple connectors. The connecting rods and the connectors are connected by a detachable or fixed connection method, including a slotted snap-fit ​​connection, a threaded connection, a clamp connection, a plug-in connection, or other combined connection methods.

3. A clamp-type fiber Bragg grating sensor according to claim 1, characterized in that, The protection assembly includes a locking element, a backstop, a capillary tube, and a capillary tube connecting device. The backstop is located at the junction of the optical fiber lead-out end and the encapsulation component. The capillary tube is sleeved over the lead-out optical fiber to protect it. The locking element is located between the backstop and the capillary tube. The capillary tube connecting device is located between the capillary tube and the locking element to fix the capillary tube to the locking element.

4. A clamp-type fiber Bragg grating sensor according to claim 1, characterized in that, The temperature compensation component is located inside the connection of the package and includes a temperature compensation grating and a temperature compensation sleeve. The temperature compensation grating is fixed to the package through the temperature compensation sleeve.

5. A clamp-type fiber Bragg grating sensor according to claim 1, characterized in that, The sensor assembly includes a strain grating, a strain grating groove, and a cover; the strain grating groove is located on the inner wall of the package, the strain grating is embedded and fixed in the strain grating groove, and the cover wraps around the strain grating to protect it.

6. A calibration method for a clamp-type fiber Bragg grating sensor according to any one of claims 1 to 5, characterized in that, The calibration method includes: first, confirming the sensor assembly status and performing a self-check of the optical path integrity; then, calibrating the temperature characteristics, radial and circumferential strain transfer, decoupling and encapsulating the calibration parameters; finally, performing consistency verification to determine whether the consistency requirements are met. If they are met, on-site monitoring and strain back calculation are performed; if they are not met, the calibration is readjusted and repeated.

7. The calibration method for a clamp-type fiber Bragg grating sensor according to claim 6, characterized in that, The specific process of temperature characteristic calibration includes: zero strain holding: controlling the variable diameter loading module to maintain a constant outer diameter to ensure that the sensor is not affected by changes in mechanical strain; step temperature rise: under the condition of zero strain holding, the ambient temperature is adjusted in stages and set. n Temperature points After stabilizing at each temperature point, the center reflection wavelength of the temperature compensation grating is collected. Data processing: using wavelength variation To represent the temperature response of the grating, using the formula Calculate the wavelength variation and corresponding temperature difference at different temperatures; verify the independence of temperature compensation. Whether it is consistent with the standard bare grating, ensuring that the temperature compensation structure at the connection is not subjected to thermal stress compression; where Take 1, 2 and , representing the first strain grating, the second strain grating, and the temperature compensation grating, respectively. These are the temperature sensitivity coefficients of the first strain grating, the second strain grating, and the temperature compensation grating, respectively. This is the initial bias term for the temperature-compensated grating. Indicates temperature. This indicates the change in wavelength in response to different gratings.

8. The calibration method for a clamp-type fiber Bragg grating sensor according to claim 7, characterized in that, The specific process for radial and circumferential strain transfer calibration includes: constant temperature environment setting: fixing the ambient temperature at standard room temperature. Eliminate thermal drift interference; graded variable diameter loading: control the variable diameter simulation loading module to expand the outer diameter in stages, assuming the number of loading stages is... j , No. j The increment of the stage diameter is Standard circumferential strain calculation: According to the geometric equations of elasticity, the standard circumferential strain reference value applied to the sensor by the diameter-changing device is defined as follows: Sensitivity calculation: using the formula and These represent the strain sensitivity coefficients of the first strain grating and the second strain grating, respectively; where... Indicates standard room temperature. Indicates the outer diameter of the variable diameter loading module. Indicates the first Stage loading stage diameter increment, express The standard circumferential strain reference value under the corresponding loading level, This represents the wavelength shift of the first strain grating at the corresponding order. This represents the wavelength shift of the second strain grating at the corresponding order. This represents the strain sensitivity coefficient of the first strain grating. This represents the strain sensitivity coefficient of the second strain grating.

9. A calibration method for a clamp-type fiber Bragg grating sensor according to claim 8, characterized in that, The specific process of decoupling and encapsulating the calibration parameters includes: cross-sensitivity coefficient calculation: using the formula... and Calculate the temperature response ratio of each strain grating relative to the temperature-compensated grating; key parameters obtained: generate a grating containing... The parameter set; where and These represent the cross-sensitivity coefficients of the first strain grating and the second strain grating relative to the temperature compensation grating, respectively.

10. A calibration method for a clamp-type fiber Bragg grating sensor according to claim 9, characterized in that, The specific process of on-site monitoring and strain back calculation includes: baseline value acquisition: recording the center wavelengths of the first strain grating, the second strain grating, and the temperature compensation grating at the initial moment. Real-time wavelength detection: at the monitoring time t Acquire the wavelengths corresponding to the three gratings in real time. Temperature decoupling: Utilizing the change in temperature-compensated grating, combined with the cross-sensitivity coefficient obtained from calibration. and Remove the heat output component, and then use the formula and Calculate the net strain wavelength drift of the first and second strain gratings; calculate the unilateral circumferential strain: then use the formula... and formula Back-calculation of the local strain of each of the two semi-circular ring packages; dual-grating fusion and eccentricity error elimination: using the formula To compensate for the bending component error in the opposite direction introduced by installation eccentricity; final stress calculation: using the formula Calculate the final circumferential stress; where This represents the initial wavelength of the first strain grating, the second strain grating, and the temperature compensation grating. This indicates that the first strain grating, the second strain grating, and the temperature compensation grating are in... Time wavelength, and This represents the net strain wavelength shift of the first and second strain gratings. and This represents the local strain of the first and second encapsulation components, respectively. This represents the true mean circumferential strain. This indicates the elastic modulus of the steel reinforcement being measured. This represents the final circumferential stress.

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