Distributed optical fiber sensing performance optimization method for hydraulic structure vibration response

Abstract

The invention discloses a distributed optical fiber sensing performance optimization method for the vibration response of a hydraulic structure, and the method comprises the steps: firstly constructing a distributed optical fiber vibration sensing system with adjustable pulse width, and building a system performance index checking platform, so as to obtain distributed vibration signals under different system parameters; analyzing the spatial and temporal distribution characteristics of measured values, establishing a mathematical relationship model of measured value noise and spatial resolution, and quantifying the influence of the noise on the measurement precision. According to the method, distributed dynamic strain values under different spatial resolutions are calculated in combination with a simulation model of a target hydraulic structure, and optimal system parameter setting is optimized by integrating the relationship among measured value noise, measured truth values and the spatial resolutions, so that the system perception performance is improved. The reliability and engineering applicability of the distributed optical fiber vibration sensing system in hydraulic structure health monitoring can be effectively improved, high-precision and low-noise monitoring data are provided for long-term vibration monitoring of projects such as dams, bridges and tunnels, and the method has high engineering application value.

Description

Technical Field

[0001] The present invention relates to the technical field of safe operation and maintenance of water conservancy projects, and in particular to a distributed optical fiber sensing performance optimization method for vibration response of a hydraulic structure. Background Art

[0002] Distributed fiber optic vibration sensing technology has been an important research direction in the field of structural health monitoring in recent years, especially in the dynamic response monitoring of hydraulic structures such as dams, bridges, and tunnels. This technology uses optical fiber as a sensing medium and realizes the perception of vibration signals along the entire optical fiber through optical effects such as Brillouin scattering or Rayleigh scattering. Compared with traditional point sensing technology, distributed fiber optic vibration sensing systems have the advantages of long-distance coverage, anti-electromagnetic interference, high spatial resolution and continuous monitoring, and can provide high-density monitoring data for the safety assessment of hydraulic structures. However, this technology still faces many challenges in practical applications, affecting its measurement accuracy and engineering applicability. First, the measurement accuracy of distributed fiber optic vibration sensing systems is affected by factors such as optical signal demodulation method, optical fiber layout method, and environmental noise. Especially in complex hydraulic environments, the measured signal-to-noise ratio is low, resulting in reduced availability of measurement data. Secondly, spatial resolution is a key parameter affecting measurement accuracy. Higher spatial resolution can improve signal quality, but at the same time increase measurement noise, making the measurement results more disturbed. Although lower spatial resolution can reduce noise, it may cause vibration characteristics to be blurred, affecting the monitoring effect. Therefore, how to optimize the spatial resolution and achieve the best balance between measurement accuracy and signal-to-noise ratio is a technical problem that needs to be solved in the field of distributed optical fiber vibration monitoring of hydraulic structures. In addition, existing research focuses on improving the limit of a single performance indicator of the system, while ignoring the relationship between various performance indicators, especially the influence of spatial resolution on measurement noise and strain resolution, which still needs to be further studied. Summary of the invention

[0003] Purpose of the invention: In response to the problems in the background technology, the present invention proposes a distributed fiber optic sensing performance optimization method for the vibration response of hydraulic structures, which can effectively overcome the limitations of existing distributed fiber optic vibration sensing systems in large-scale hydraulic structure monitoring, improve data quality and measurement stability, provide more reliable and accurate technical support for long-term health monitoring of hydraulic structures, and further expand the application prospects of distributed fiber optic sensing technology in the fields of civil engineering and water conservancy engineering.

[0004] Technical solution: The present invention discloses a distributed optical fiber sensing performance optimization method for hydraulic structure vibration response, comprising the following steps:

[0005] Step 1: Construct an adjustable pulse-width distributed fiber optic vibration sensing system. Select a standard hydraulic test structure to set the distributed fiber optic vibration sensing area, connect it to the adjustable pulse-width distributed fiber optic vibration sensing system for distributed fiber optic vibration sensing tests, obtain the corresponding fiber static strain distribution under different system parameter settings, and convert it into distributed fiber optic vibration sensing measurement values;

[0006] Step 2: Perform spatio-temporal distribution characteristic analysis on the fiber static strain distribution under different system parameter settings, including static strain distribution analysis to determine the specific position of the fiber sensing section, vibration signal time-domain baseline calibration and analysis to obtain the calibrated distributed fiber optic vibration sensing measurement values, and vibration signal frequency-domain characteristic analysis to reflect the vibration characteristics of the structure under test;

[0007] Step 3: Use the distributed vibration sensing measurement values when the standard hydraulic test structure is stationary as measurement noise. Based on the data set of spatial resolution and measurement noise under all spatial resolution settings, fit the relationship model between the measurement noise and spatial resolution of the adjustable pulse-width distributed fiber optic vibration sensing system, and determine the system strain resolution and strain measurement range;

[0008] Step 4: Establish a simulation model of the target hydraulic structure, and simulate and calculate the distributed dynamic strain values corresponding to each spatial resolution of the target hydraulic test structure as measurement true values;

[0009] Step 5: Synthesize the relationship between measurement noise, measurement true value and spatial resolution, and select the optimal system spatial resolution of the distributed fiber optic sensing system for the vibration response of the target hydraulic structure.

[0010] Furthermore, the adjustable pulse-width distributed fiber optic vibration sensing system in Step 1 specifically includes a narrow linewidth laser, an optical splitter, an MZM, an erbium-doped fiber amplifier EDFA, a photodetector, a low-pass filter, and an FPGA acquisition module. Among them, the MZM is used to modulate the pulse width to change the system spatial resolution; a single narrow linewidth laser is used as the light source, and the light is divided into two paths by the optical splitter. One path is used as the pump light, and the other path is used as the probe light; the pump light is modulated by the MZM to form an adjustable pulse of 1 ns - 15 ns, and then enters the fiber after the power is enhanced by the erbium-doped fiber amplifier EDFA. The probe light is modulated into a linear frequency-swept signal by the Chirp generator, and then modulated by the MZM and amplified by the EDFA; the two paths of light undergo Brillouin scattering in the fiber. After passing through the optical combiner, the probe light enters the photodetector, and the signal is processed by the low-pass filter and the frequency shift is extracted by the FPGA acquisition module to finally obtain the Brillouin frequency shift distribution for strain measurement.

[0011] Furthermore, the specific process of obtaining the corresponding distributed fiber optic vibration sensing measurement values under different system parameter settings in Step 1 is as follows:

[0012] Step 1.1: Select the material for manufacturing the standard beam, design the size and fixing method of the beam, fabricate and install the beam model as the standard hydraulic test structure;

[0013] Step 1.2: Determine the distributed fiber optic vibration sensing area on the beam, paste and fix the optical fiber, and connect the optical fiber to the optical signal demodulation device;

[0014] Step 1.3: Set the initial system parameters, including spatial resolution, sampling frequency, measurement point interval, and sensing distance, measure the static strain distribution of the optical fiber corresponding to the initial system parameters, change the spatial resolution, and obtain the static strain distributions of the optical fiber corresponding to different spatial resolutions;

[0015] Step 1.4: According to the principle of distributed fiber optic vibration sensing, taking the Brillouin gain spectrum BGS of the static strain distribution of the optical fiber as a reference, set the measurement optical frequency, capture the Brillouin gain change corresponding to the measurement optical, and obtain the distributed fiber optic vibration sensing measurement value in the strain format through conversion of the Brillouin gain change.

[0016] Furthermore, in step 2, perform spatio-temporal distribution characteristic analysis on the distributed fiber optic vibration sensing measurement values under different system parameter settings. The specific process is as follows:

[0017] Step 2.1: Analyze the static strain distributions of the optical fiber under different spatial resolutions obtained in step 1, and determine the exact position of the fiber sensing section according to the average range of the strain measurement values of the system sampling points characterized by the following formula:

[0018]

[0019] where s(z) is the measurement value at the z coordinate of the optical fiber, is the average strain within the spatial resolution R at the z coordinate of the optical fiber, and R is the spatial resolution; when the measurement point coordinate is transformed from the fiber connection section to the fiber sensing section, due to the difference in strain between the fiber sensing section and the connection section, the starting and ending coordinates of the fiber sensing section can be determined. Affected by the strain averaging effect, the measurement value of the distributed fiber optic vibration sensing system is reduced or increased compared to the actual strain of the optical fiber. When it is reduced, it is from the pump end side entering the fiber sensing section, and when it is increased, it is from the continuous section side entering the fiber connection section;

[0020] Step 2.2: Analyze the distributed fiber optic vibration sensing measurement values obtained in step 1 within the fiber sensing section determined in step 2.1, and perform baseline calibration through determining the vibration equilibrium position and recursive filtering. The calculation formula is as follows:

[0021]

[0022] where: ε0 represents the measured value of distributed fiber optic vibration sensing with spatio-temporal distribution; ε′ is the measured value of distributed fiber optic vibration sensing after baseline calibration in space; z is the measuring point coordinate; t is the time; n is the number of sampling points from the measured value time t0 to t n .

[0023] Step 2.3: Analyze the time-frequency domain feature distribution of the signal, and judge the integrity of the measured values at each measuring point. If the measured value is offset or missing, check the instrument parameter settings and re-measure. If the measured value is complete, continue with the operation process; where the missing measured value means that the measured value exceeds the sensing range of the distributed fiber optic vibration demodulation system.

[0024] Furthermore, in the step 3, the relationship model between the measured value noise and the spatial resolution of the fitted adjustable pulse width distributed fiber optic vibration sensing system is as follows:

[0025] Step 3.1: Keep the standard hydraulic test structure stationary for vibration measurement, and use the obtained distributed vibration sensing measured value at this time as the measured value noise under the current spatial resolution setting.

[0026] Step 3.2: Change the spatial resolution to obtain the data set of spatial resolution and measured value noise under all spatial resolution settings.

[0027] Step 3.3: Based on the data set, fit the coefficients a and b in the following formula to obtain the relationship model between the measured value noise and the spatial resolution:

[0028]

[0029] where, E noise is the measurement noise energy, R is the spatial resolution, and a and b are the relationship model coefficients.

[0030] Step 3.4: Based on the relationship model in step 3.3, calculate the system strain resolution and strain measurement range corresponding to the spatial resolution. Among them, the strain resolution ε min and the strain measurement range ε max are:

[0031]

[0032] where, ε is the fiber strain, C ε is the Brillouin frequency shift conversion coefficient of the fiber strain, v slope is the frequency offset value between the position of the maximum value of the Brillouin gain spectrum BGS slope and the central axis of the Brillouin gain spectrum BGS, and v CW is the range of the Brillouin gain spectrum BGS moving in the positive direction of the frequency axis.

[0033] Furthermore, in the step 4, calculate the distributed dynamic strain values corresponding to each spatial resolution, and the specific process is as follows:

[0034] Step 4.1: Establish a simulation model of the target hydraulic structure through finite element vibration simulation;

[0035] Step 4.2: Conduct dynamic simulation with reference to the standard response spectrum, site response spectrum or measured excitation time series;

[0036] Step 4.3: Calculate the average distributed dynamic strain value at the fiber layout positions under each spatial resolution setting of the simulation model of the target hydraulic structure as the simulation measurement value of the fiber optic sensing demodulation system, that is, the measurement true value.

[0037] Furthermore, the process of step 5 for selecting the optimal system spatial resolution of the distributed fiber optic sensing system for the vibration response of the target hydraulic structure is as follows:

[0038] Step 5.1: According to the simulation calculation results of step 4, select measurement points such that the amplitude of the measurement true value sequence is greater than the system strain resolution calculated in step 3 and less than the strain measurement range;

[0039] Step 5.2: Consider the simulation sensing section that meets the requirements in step 5.1 as the effective range, and its signal-to-noise ratio of the vibration measurement value is:

[0040]

[0041] where V i is the amplitude of the fiber optic measurement value of the vibration at the i-th characteristic frequency of the measurement point, max represents the situation where the vibrations corresponding to each characteristic frequency reach the peak value in the same direction simultaneously, and at this time the superposition amplitude value is the largest, V max (R) represents the maximum amplitude of the fiber optic measurement value of the vibration at the measurement point, N is the noise amplitude, R is the spatial resolution, and n represents the total number of characteristic frequencies considered;

[0042] Step 5.3: Calculate the comprehensive signal-to-noise ratio value SNR multi as:

[0043]

[0044] where: n L is the total number of measurement points; V i,j (R) is the amplitude of the fiber optic measurement value of the vibration at the i-th characteristic frequency with the spatial resolution R at the j-th measurement point; is the maximum amplitude after the superposition of each order frequency component corresponding to the spatial resolution R at the j-th measurement point;

[0045] Step 5.4: Analyze the variation law of the signal-to-noise ratio with the spatial resolution according to the comprehensive signal-to-noise ratio value of multiple measurement points, and determine the optimal system spatial resolution.

[0046] Beneficial effects:

[0047] 1. A method for optimizing the distributed optical fiber sensing performance of the vibration response of hydraulic structures provided by the present invention dynamically changes the spatial resolution of the system by adjusting the modulation pulse width of the MZM (Mach-Zehnder modulator), and combines measured value noise modeling, finite element simulation calculation, and multi-measurement point signal-to-noise ratio analysis to optimize the parameter configuration of the system to improve the measurement accuracy and signal quality. First, by establishing a mathematical model between the measured value noise and the spatial resolution, analyzing the measurement error under different spatial resolution settings, and combining the simulation calculation of the vibration response of the target hydraulic structure, the distributed dynamic strain values corresponding to different spatial resolutions are obtained. Secondly, comprehensively considering the relationship between the true measurement value, the measured value noise, and the spatial resolution, a multi-measurement point signal-to-noise ratio optimization method is adopted to analyze the trend of the signal-to-noise ratio changing with the spatial resolution, and the optimal spatial resolution setting is selected to ensure that the vibration signal can maintain the vibration characteristics to the greatest extent while ensuring a high signal-to-noise ratio. By dynamically adjusting the spatial resolution of the system through adjustable pulse width modulation (MZM), the problem of insufficient measurement accuracy caused by the fixed spatial resolution in the traditional distributed optical fiber vibration sensing system is solved. The optimized spatial resolution can accurately capture the key vibration characteristics under different monitoring requirements, improve the adaptability of the monitoring system, provide more reliable and accurate technical support for the long-term health monitoring of hydraulic structures, and further expand the application prospects of distributed optical fiber sensing technology in the fields of civil engineering and water conservancy engineering.

[0048] 2. The present invention fits the relationship between the measured value noise and the spatial resolution, establishes a mathematical model, and theoretically analyzes and optimizes the measured value signal-to-noise ratio. Compared with the traditional method, this optimization strategy can effectively reduce the measurement noise, improve the signal-to-noise ratio of the vibration signal, and enable the system to obtain high-quality vibration data even in a complex environment.

[0049] 3. The present invention predicts the distributed dynamic strain values under different spatial resolutions by simulating and calculating the vibration response of the target hydraulic structure, and based on the mutual relationship between the true measurement value, the measured value noise, and the spatial resolution, optimally selects the system parameters most suitable for the monitoring environment, thereby enhancing the applicability of the system under different working conditions and improving the stability and reliability of long-term monitoring.

[0050] 4. The present invention is not only applicable to the dynamic monitoring of hydraulic structures such as dams, bridges, and tunnels, but also can be extended to other complex engineering fields such as ocean engineering and earthquake monitoring. The optimized system can provide higher-quality fully distributed vibration data, provide more accurate and stable technical support for the health monitoring of hydraulic structures, and thus improve the scientific and intelligent level of engineering safety management. Description of the Drawings

[0051] Figure 1 It is a technical implementation process for improving the distributed optical fiber sensing performance of the vibration response of a hydraulic structure;

[0052] Figure 2 It is a schematic diagram of the standard model;

[0053] Figure 3 It is a schematic diagram for determining the initial measured value distribution, i.e., the sensing position;

[0054] Figure 4 It is a schematic diagram for calibrating the spatio-temporal baseline of the measured values;

[0055] Figure 5 It is a schematic diagram for optimizing the parameters of the distributed optical fiber vibration sensing system for beam structures;

[0056] Figure 6 It is a schematic diagram of the finite element dynamic simulation model of a concrete gravity dam;

[0057] Figure 7 It is a schematic diagram of the response spectrum based on which the seismic excitation input time history curve is obtained. Specific implementation manners

[0058] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, and cannot be used to limit the protection scope of the present invention.

[0059] The present invention provides a technology for improving the distributed optical fiber sensing performance of the vibration response of hydraulic structures. By optimizing the pulse width, the spatial resolution of the distributed optical fiber vibration sensing system is optimized, and the signal-to-noise ratio and measurement accuracy of the measured values are improved. This method combines noise modeling, simulation calculation, and signal-to-noise ratio optimization analysis to determine the optimal system parameter settings, thereby improving the accuracy and reliability of the monitoring data. This technology can be widely applied to the long-term health monitoring of hydraulic structures such as dams, bridges, and tunnels, providing high-precision and low-noise distributed vibration data, and enhancing the scientificity and practicality of structural safety assessment.

[0060] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0061] Refer to Figure 1 , Figure 1 It is a schematic diagram of the implementation process of a technology for improving the distributed optical fiber sensing performance of the vibration response of hydraulic structures provided by an embodiment of the present invention. As shown in reference Figure 1 shown, the implementation process of this technology includes the following steps:

[0062] Step 1: Construct an adjustable pulse-width distributed fiber optic vibration sensing system. Select a standard hydraulic test structure to set the distributed fiber optic vibration sensing area, connect it to the adjustable pulse-width distributed fiber optic vibration sensing system for distributed fiber optic vibration sensing tests, obtain the corresponding fiber static strain distribution under different system parameter settings, and convert it into distributed fiber optic vibration sensing measurement values.

[0063] The adjustable pulse-width distributed fiber optic vibration sensing system specifically includes a narrow linewidth laser, an optical splitter, an MZM, an EDFA, a photodetector, a low-pass filter, and an FPGA acquisition module. Among them, the MZM is used to modulate the pulse width to change the system spatial resolution. This system uses a single narrow linewidth laser (1550 nm) as the light source. The light is divided into two paths by a 50 / 50 optical splitter. One path is used as the pump light, and the other path is used as the probe light. The pump light is modulated by a Mach-Zehnder Modulator (MZM) to form an adjustable pulse of 1 ns - 15 ns, and then enters the optical fiber after the power is enhanced by an erbium-doped fiber amplifier (EDFA). The probe light is modulated into a linear frequency sweep signal (±100 MHz) by a Chirp generator, then modulated by the MZM, and amplified by the EDFA. Brillouin scattering occurs when the two paths of light propagate in the optical fiber, causing the probe light to be affected by Brillouin gain or attenuation. After passing through an optical coupler, the probe light enters a photodetector (APD / PIN). The signal is processed by a low-pass filter and then the frequency shift is extracted by an FPGA or MATLAB. Finally, the Brillouin frequency shift distribution is obtained for strain measurement. This system has an adjustable spatial resolution (10 cm - 150 cm), and combines the slope-assisted Brillouin optical time domain analysis technology (SA-BOTDA) to improve the measurement speed and sensitivity.

[0064] Build a platform for verifying the actual application performance indicators of the system, and obtain the strain-format distributed vibration signals corresponding to different system parameter settings. The specific process is as follows:

[0065] Step 1.1: Select appropriate materials for the standard beam, design the size and fixing method of the beam, and fabricate and install the beam model. In this embodiment, refer to Figure 2 , and use a cantilever beam for distributed fiber optic vibration sensing tests. This model is a homogeneous aluminum beam, and its size is as Figure 2As shown, the fixed end to the free end of the beam is 2 m long, 0.15 m wide, and 0.012 m thick. The sides where the length and width are located form a vertical plane (subsequently referred to as the length-width plane). The cantilever beam is fixed using an iron frame base and a fixture. The fixture is stable enough to ensure that the vibration signal only reflects the dynamic characteristics of the beam itself and is not affected by the fixture and the base. During the vibration monitoring experiment, a displacement step excitation is applied to the free end of the cantilever beam, and the same initial offset is maintained to make the structural true dynamic responses consistent under different spatial resolution settings.

[0066] Step 1.2: Determine the distributed fiber optic vibration sensing area on the beam, paste and fix the optical fiber, and connect the optical fiber to the system optical signal demodulation device. The optical fiber selected is SMF-28e ordinary single-mode optical fiber, which is led out from the system pump end. The optical fiber sensing section is pasted on the surface of the cantilever beam using silicone sealant, and then connected back to the instrument detection optical emission end to form the optical fiber loop of the system.

[0067] Step 1.3: Set the initial system parameters (spatial resolution, sampling frequency, measurement point interval, sensing distance, etc.), measure the static strain distribution of the optical fiber corresponding to the initial system parameters to ensure the quality of subsequent measured values, change the spatial resolution, and obtain the static strain distributions of the optical fiber corresponding to different spatial resolutions.

[0068] Step 1.4: According to the principle of distributed fiber optic vibration sensing, take the Brillouin gain spectrum (BGS) of the initial static measurement value as a reference, set the measurement optical frequency according to the following formula, and the system captures the Brillouin gain change ΔI CW (z) of the measurement optical frequency corresponding to it, and obtains the distributed vibration signal in the strain format by performing conversion on ΔI CW (z).

[0069] Step 2: Perform spatio-temporal distribution characteristic analysis on the distributed fiber optic vibration sensing measurement values under different system parameter settings, including static strain distribution analysis to determine the specific sensing position, vibration signal time-domain baseline calibration and analysis to ensure the quality of the measured values and obtain the signal-to-noise ratio, and vibration signal frequency-domain characteristic analysis to reflect the vibration characteristics of the measured structure.

[0070] Step 2.1: Analyze the initial static strain distributions of the optical fiber under different spatial resolutions obtained in Step 1.3, and determine the exact position of the optical fiber sensing section according to the average range of the strain measurement values of the system sampling points characterized by the following formula. When the measurement point coordinates are converted from the optical fiber connection section to the optical fiber sensing section, due to the difference in strain between the optical fiber sensing section and the connection section, the starting point and ending point coordinates of the optical fiber sensing section can be judged. Affected by the strain averaging effect, the measured value of the distributed fiber optic vibration sensing system is smaller (entering the optical fiber sensing section from the pump end side) or larger (entering the optical fiber connection section from the continuous section side) than the actual strain of the optical fiber. Figure 3 It shows that the measured value of the measurement point strain reflects the average of the true strain of the structure within the spatial resolution range from this point to the coordinate origin direction;

[0071]

[0072] Among them, s(z) is the measured value at the z - coordinate of the optical fiber, is the average strain within the spatial resolution R at the z - coordinate of the optical fiber, and R is the spatial resolution.

[0073] Step 2.2: Analyze the distributed vibration signal measured in Step 1 within the optical fiber sensing section determined in Step 2.1, and perform baseline calibration through determining the vibration equilibrium position and recursive filtering. The calculation formula is as follows. From Figure 4 It can be seen that before and after the baseline calibration process, the offsets of the three - dimensional spatio - temporal measured values in time and space are effectively removed, and the process of the vibration of the cantilever beam gradually decaying after being subjected to a step excitation can also be observed;

[0074]

[0075] Among them: ε0 represents the vibration measured value of the spatio - temporal distribution; ε′ is the vibration measured value after baseline calibration in space; z is the measuring point coordinate; t is the time; n is the number of sampling points from the measured value time t0 to t n of the measured value.

[0076] Step 2.3: Analyze the time - frequency domain feature distribution of the signal, and judge the integrity of the measured values at each measuring point. If the degree of deviation and missing of the measured values is relatively high, where the measured value deviation is an obvious trend of change in the measured value affected by electronic devices, which is significantly different from the fact that most actual vibrations revolve around a certain equilibrium position, and the measured value missing is that the measured value exceeds the sensing range of the distributed optical fiber vibration demodulation system. Usually, the measured value is a long sequence of 0, the system upper limit value, or NAN, then check the instrument parameter settings and re - measure. If the measured values are complete and the frequency - domain features are relatively obvious, continue with the operation process.

[0077] Step 3: Fit the relationship model between the measured value noise and the spatial resolution of the tunable pulse - width distributed optical fiber vibration sensing system.

[0078] Step 3.1: Keep the test beam stationary for vibration measurement, and use the obtained distributed vibration sensing signal at this time as the measured value noise under the current spatial resolution setting;

[0079] Step 3.2: Change the spatial resolution to obtain a data set of the spatial resolution and the measured value noise intensity under all spatial resolution settings;

[0080] Step 3.3: Based on the above data set, fit the coefficients in the following formula to obtain the relationship model between the measured value noise and the spatial resolution;

[0081]

[0082] Among them, E noiseTo measure the noise energy, R is the spatial resolution, and a and b are the coefficients of the relationship model.

[0083] Step 3.4: Based on the relationship model described in Step 3.3, calculate the system strain resolution and strain measurement range corresponding to the spatial resolution. Among them, the strain resolution ε min and the strain measurement range ε max are:

[0084]

[0085] where ε is the fiber strain, C ε is the Brillouin frequency shift conversion coefficient of the fiber strain, v slope is the frequency offset value between the position of the maximum BGS slope and the central axis of the BGS, v CW is the range of the BGS moving in the positive direction of the frequency axis.

[0086] Extract the vibration measurement values of each simulation measurement point under the change of spatial resolution, introduce the relationship model between the system measurement noise and the spatial resolution, and then calculate the relationship between the signal-to-noise ratio of each simulation measurement point (with an interval of 10 cm) and the spatial resolution, and obtain the relationship between the comprehensive signal-to-noise ratio value of multiple measurement points and the spatial resolution as Figure 5 shown. According to the maximum value position shown by the dotted line, the optimal spatial resolution of the distributed fiber optic vibration sensing system for this cantilever beam should be set to 60 cm.

[0087] Step 4: Simulate and calculate the vibration response of the target hydraulic structure, and calculate the distributed dynamic strain values corresponding to each spatial resolution.

[0088] Step 4.1: Establish a simulation model of the target hydraulic structure. In this embodiment, the cross-section of the concrete gravity dam is used as the research object for dynamic analysis, and the massless foundation method is adopted to avoid artificial amplification. In addition, the dam foundation extends 1 time the dam height in the upstream and downstream and depth directions, and the seismic load adopts the standard response spectrum recommended by the "Code for Seismic Design of Hydraulic Structures" (GB 51247-2018), as Figure 7 shown;

[0089] Step 4.2: Conduct dynamic simulation with reference to the standard response spectrum, the site response spectrum or the measured excitation time series;

[0090] Step 4.3: Calculate the average dynamic strain value at the fiber layout position under each spatial resolution setting of the finite element model as the simulation measurement value of the fiber optic sensing demodulation system, that is, the measurement true value.

[0091] Step 5: Synthesize the relationship between the measurement noise, the measurement true value and the spatial resolution, and implement the optimization of the spatial resolution of the distributed fiber optic sensing system for the vibration response of the target hydraulic structure to improve the system perception performance.

[0092] Step 5.1: According to the simulation calculation results, select measurement points such that the amplitude of the true value sequence of their measurements is greater than the system strain resolution and less than the strain measurement range. The calculation formulas for the system strain resolution and the strain measurement range are shown in Step 3.4.

[0093] Step 5.2: Consider the simulated sensing section that meets the requirements described in Step 5.1 as the effective range, and its vibration measurement signal-to-noise ratio is:

[0094]

[0095] where V i is the amplitude of the vibration optical fiber measurement at the i-th characteristic frequency of the measurement point, max represents the situation where the vibrations corresponding to each characteristic frequency reach the peak in the same direction simultaneously, at this time the superimposed amplitude value is the largest, N is the noise amplitude, R is the spatial resolution, and n represents the total number of characteristic frequencies considered.

[0096] Step 5.3: Calculate the comprehensive signal-to-noise ratio value of multiple measurement points as

[0097]

[0098] where: n L is the total number of measurement points; V i,j (R) is the amplitude of the vibration optical fiber measurement corresponding to the spatial resolution R at the i-th characteristic frequency of the j-th measurement point; is the largest amplitude after the superposition of each order frequency component corresponding to the spatial resolution R of the j-th measurement point.

[0099] Step 5.4: Analyze the variation law of the signal-to-noise ratio with the spatial resolution according to the comprehensive signal-to-noise ratio value of multiple measurement points, and determine the optimal system spatial resolution.

[0100] The above embodiments are only for explaining the technical concept and characteristics of the present invention, and the purpose is to enable those who are familiar with this technology to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. All equivalent transformations or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A distributed optical fiber sensing performance optimization method for hydraulic structure vibration response, characterized in that: The following steps are involved: Step 1: Construct an adjustable pulse width distributed optical fiber vibration sensing system, select a standard hydraulic test structure to set up a distributed optical fiber vibration sensing area, connect it to the adjustable pulse width distributed optical fiber vibration sensing system to conduct a distributed optical fiber vibration sensing test, obtain the corresponding optical fiber static strain distribution under different system parameter settings and convert it into a distributed optical fiber vibration sensing value; Step 2: Perform time-space distribution characteristic analysis on the static strain distribution of the optical fiber under different system parameter settings, including static strain distribution analysis to determine the specific location of the optical fiber sensing segment, vibration signal time domain baseline calibration and analysis to obtain the calibrated distributed optical fiber vibration sensor sense value, and vibration signal frequency domain characteristic analysis to reflect the vibration characteristics of the measured structure; Step 3: Taking the measured value of the distributed vibration sensor when the standard hydraulic test structure is stationary as the measured noise, based on the data set of spatial resolution and measured noise under all spatial resolution settings, fit the relationship model between the measured noise and spatial resolution of the adjustable pulse width distributed optical fiber vibration sensing system, and determine the system strain resolution and strain measurement range; Step 4: Establish a simulation model of the target hydraulic structure, and simulate and calculate the distributed dynamic strain values ​​corresponding to each spatial resolution of the target hydraulic test structure as the measured true value; Step 5: Considering the relationship between measurement noise, true value and spatial resolution, the optimal system spatial resolution of the distributed fiber optic sensing system for the vibration response of the target hydraulic structure is selected.

2. The distributed optical fiber sensing performance optimization method for hydraulic structure vibration response according to claim 1 is characterized in that: The adjustable pulse width distributed optical fiber vibration sensing system in step 1 specifically includes a narrow line width laser, an optical beam splitter, an MZM, an erbium-doped fiber amplifier EDFA, a photodetector, a low-pass filter, and an FPGA acquisition module, wherein the MZM is used to modulate the pulse width to change the spatial resolution of the system; A single narrow linewidth laser is used as the light source, and the light is split into two paths through an optical beam splitter, one for pump light and the other for detection light; The pump light is modulated by MZM to form a 1ns-15ns adjustable pulse, and then enters the optical fiber after the power is enhanced by the erbium-doped fiber amplifier EDFA. The detection light is modulated by the Chirp generator into a linear sweep signal, and then modulated by the MZM and amplified by the EDFA. The two beams of light undergo Brillouin scattering in the optical fiber, pass through the photosynthesizer, and the detection light enters the photodetector. After the signal is processed by a low-pass filter, the frequency shift is extracted through the FPGA acquisition module, and finally the Brillouin frequency shift distribution is obtained for strain measurement.

3. The distributed optical fiber sensing performance optimization method for hydraulic structure vibration response according to claim 1 is characterized in that: In step 1, the corresponding distributed optical fiber vibration sensor values ​​under different system parameter settings are obtained, and the specific process is as follows: Step 1.1: Select standard beam manufacturing materials, design the beam size and fixing method, and manufacture and install the beam model as a standard hydraulic test structure; Step 1.2: Determine the distributed optical fiber vibration sensing area on the beam, stick and fix the optical fiber, and connect the optical fiber to the optical signal demodulation device; Step 1.3: Set the initial system parameters, including spatial resolution, sampling frequency, measurement point interval, and sensing distance, measure the static strain distribution of the optical fiber corresponding to the initial system parameters, change the spatial resolution, and obtain the static strain distribution of the optical fiber corresponding to different spatial resolutions; Step 1.4: Based on the principle of distributed optical fiber vibration sensing, the Brillouin gain spectrum BGS of the static strain distribution of the optical fiber is used as a reference, the measurement light frequency is set, the Brillouin gain change corresponding to the measurement light is captured, and the distributed optical fiber vibration sensing sensor value in strain format is obtained by converting the Brillouin gain change.

4. The distributed optical fiber sensing performance optimization method for hydraulic structure vibration response according to claim 1 is characterized in that: In step 2, the temporal and spatial distribution characteristics of the distributed optical fiber vibration sensor values ​​under different system parameter settings are analyzed, and the specific process is as follows: Step 2.1: Analyze the static strain distribution of the optical fiber at different spatial resolutions obtained in step 1, and determine the precise position of the optical fiber sensing segment according to the average range of strain measurements at the system sampling points represented by the following formula: Where s(z) is the measured value at the optical fiber z coordinate, is the average strain within the spatial resolution R at the optical fiber z coordinate, where R is the spatial resolution. When the measuring point coordinates are transformed from the optical fiber connection segment to the optical fiber sensing segment, the starting point and end point coordinates of the optical fiber sensing segment can be determined due to the difference between the strain of the optical fiber sensing segment and the strain of the connection segment. Affected by the strain averaging effect, the measured value of the distributed optical fiber vibration sensing system is reduced or increased compared with the actual strain of the optical fiber. When it is reduced, it is when the pump end side enters the optical fiber sensing segment, and when it is increased, it is when the continuous segment side enters the optical fiber connection segment. Step 2.2: Analyze the distributed optical fiber vibration sensor sense value obtained based on the measurement in step 1 in the optical fiber sensing segment determined in step 2.1, and perform baseline calibration by determining the vibration balance position and recursive filtering. The calculation formula is as follows: Where: ε0 represents the distributed optical fiber vibration sensor value distributed in time and space; ε′ is the distributed optical fiber vibration sensor value after baseline calibration in space; z is the coordinate of the measuring point; t is the time; n is the measurement time from t0 to t n The number of sampling points; Step 2.3: Analyze the signal's time-frequency domain characteristic distribution and determine the integrity of the measured values ​​at each measuring point. If the measured value is offset or missing, check the instrument parameter settings and re-measure. If the measured value is complete, continue the operation process. Missing measured values ​​means that the measured value exceeds the sensing range of the distributed optical fiber vibration demodulation system.

5. The distributed optical fiber sensing performance optimization method for hydraulic structure vibration response according to claim 1 is characterized in that: The relationship model between the measured noise and spatial resolution of the adjustable pulse width distributed optical fiber vibration sensing system is fitted in step 3, and the specific process is as follows: Step 3.1: Keep the standard hydraulic test structure stationary to perform vibration measurement, and use the distributed vibration sensor value obtained at this time as the measurement noise under the spatial resolution setting of this time; Step 3.2: Change the spatial resolution to obtain the data set of spatial resolution and measurement noise under all spatial resolution settings; Step 3.3: Fit the coefficients a and b in the following formula based on the data set to obtain the relationship model between the measurement noise and spatial resolution: Among them, E noise is the measurement of noise energy, R is the spatial resolution, a and b are the coefficients of the relationship model; Step 3.4: Based on the relationship model in step 3.3, calculate the system strain resolution and strain measurement range corresponding to the spatial resolution, where the strain resolution ε min and strain measurement range ε max for: Where ε is the fiber strain, C ε is the Brillouin frequency shift conversion factor of optical fiber strain, v slope is the frequency offset between the maximum position of the Brillouin gain spectrum BGS slope and the central axis of the Brillouin gain spectrum BGS, v CW It is the range in which the Brillouin gain spectrum BGS moves positively to the frequency axis.

6. The distributed optical fiber sensing performance optimization method for hydraulic structure vibration response according to claim 1 is characterized in that: The step 4 calculates the distributed dynamic strain value corresponding to each spatial resolution, and the specific process is: Step 4.1: Establish a simulation model of the target hydraulic structure through finite element vibration simulation; Step 4.2: Perform dynamic simulation with reference to the standard response spectrum, site response spectrum or measured excitation time series; Step 4.3: Calculate the average distributed dynamic strain value of the optical fiber layout position under each spatial resolution setting of the simulation model of the target hydraulic structure as the simulation measurement value of the optical fiber sensing demodulation system, that is, the measurement true value.

7. The distributed optical fiber sensing performance optimization method for hydraulic structure vibration response according to claim 1 is characterized in that: The step 5 selects the optimal system spatial resolution of the distributed optical fiber sensing system for the vibration response of the target hydraulic structure, and the specific process is as follows: Step 5.1: According to the simulation calculation results of step 4, select the measuring points so that the amplitude of the measured true value sequence is greater than the system strain resolution calculated in step 3 and smaller than the strain measurement range; Step 5.2: The simulated sensing segment that meets the requirements in step 5.1 is regarded as the effective range, and its vibration measurement signal-to-noise ratio is: Among them, V i is the measured amplitude of the vibrating optical fiber at the i-th characteristic frequency of the measuring point. max means that the vibrations corresponding to each characteristic frequency reach the peak value in the same direction at the same time. At this time, the superimposed amplitude value is the largest. V max (R) represents the maximum amplitude of the vibrating optical fiber measured at the measuring point, N is the noise amplitude, R is the spatial resolution, and n represents the total order of the characteristic frequencies considered; Step 5.3: Calculate the comprehensive value of the signal-to-noise ratio (SNR) of multiple measurement points multi for: Where: n L is the total number of measuring points; V i,j (R) is the optical fiber measurement amplitude corresponding to the spatial resolution R of the vibration of the i-th order characteristic frequency at the j measurement point; is the maximum amplitude after superposition of frequency components of various orders corresponding to the spatial resolution R of the j measuring point; Step 5.4: Based on the comprehensive value of the signal-to-noise ratio at multiple measurement points, analyze the variation of the signal-to-noise ratio with the spatial resolution and determine the optimal system spatial resolution.

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