CN111879969A - Medium-high frequency elliptical hinge double-fiber grating acceleration sensor and measurement method - Google Patents

Abstract

The present disclosure proposes a medium-high frequency elliptical hinge double fiber grating acceleration sensor and a measurement method, including: an elliptical hinge, a fixed bracket and a mass block; one end of the elliptical hinge is fixedly connected to the fixed bracket, and the other end of the elliptical hinge is connected to to the mass block; the upper and lower surfaces of the fixed bracket and the mass block are respectively on the same plane, and fiber gratings are respectively pasted between the fixed bracket and the mass block on the same plane and located directly above and below the elliptical hinge. The medium and high frequency dual FBG acceleration sensor based on elliptical hinge, on the premise of ensuring high precision, takes advantage ^of the large range of motion of the elliptical hinge to improve the sensitivity of the sensor, and uses the differential method to demodulate the reflected wavelength to double the sensitivity the goal of.

Description

A medium and high frequency elliptical hinge double fiber grating acceleration sensor and measurement method

technical field

The present disclosure belongs to the technical field of acceleration sensors, and in particular relates to a medium and high frequency elliptical hinge double fiber grating acceleration sensor and a measurement method.

Background technique

The statements in this section merely provide background information related to the present disclosure and do not necessarily constitute prior art.

Infrastructure construction such as high-speed railways and viaducts is gradually advancing, but high-frequency vibrations in railways and bridges will seriously affect the structural health. Acceleration is one of the parameters used to describe the vibration of objects, and the vibration of large structures can be reflected by measuring acceleration. Fiber Bragg Grating (FBG) accelerometer, as a passive device, has the advantages of high measurement sensitivity, electromagnetic insensitivity, and distributed measurement compared with traditional electromechanical accelerometers. It is very suitable for monitoring large-scale Medium and high frequency vibration of the structure.

Medium and high frequency vibration is the main reason for the deterioration of large structures such as bridges and tunnels. Using optical sensors to obtain the vibration frequency of objects is an important means to monitor the high frequency vibration of large structures. In recent years, with the continuous development of FBG sensing technology, FBG accelerometers have become a new research direction, and are widely used in seismic monitoring, petrochemical, national defense, health monitoring of large-scale projects and infrastructure. Wang Hongliang et al. developed a fiber grating acceleration sensor with equal intensity double cantilever beams, using ANSYS to determine the optimal parameters of the sensor, and realize real-time monitoring of low-frequency signals below 50Hz, with a sensitivity of 20.85pm/ms ^-2 . Jia Zhenan et al. developed a cantilever fiber grating vibration sensor, the resonant frequency of the sensor is 90Hz, the flat region is 10-50Hz, and the sensor sensitivity is 121pm/g ² . OmPrakash et al. designed a new double-L cantilever fiber Bragg grating accelerometer. Compared with the single-L cantilever fiber Bragg grating accelerometer, the design not only increases the sensitivity, but also realizes temperature self-compensation. The sensitivity of the sensor is 406.7pm/g. Gutiérrez N et al. reported a fiber grating accelerometer based on a hexagonal hollow cylinder. The sensor is characterized by miniaturization and light weight, but this design reduces the sensitivity of the sensor. The experimental verification The sensitivity of the sensor is only 19.65pm /g.

In addition, one of the characteristics of medium and high frequency sensors is that the frequency and sensitivity are inversely proportional. Due to the above objective conditions and the lack of accuracy of the demodulation equipment at the time, the early sensors did not measure the specific sensitivity. In recent years, the medium and high frequency fiber grating acceleration sensors reported The sensitivity is generally not high. The research on fiber grating accelerometers is mostly concentrated in the mid-low frequency range, and there are relatively few studies on mid- and high-frequency fiber grating accelerometers.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned deficiencies of the prior art, the present disclosure provides a medium-high frequency elliptical hinge double fiber grating acceleration sensor, which improves the sensitivity of the sensor by taking advantage of the large movement range of the elliptical hinge on the premise of ensuring high precision.

To achieve the above purpose, one or more embodiments of the present disclosure provide the following technical solutions:

In a first aspect, a medium-high frequency elliptical hinge double fiber grating acceleration sensor is disclosed, comprising: an elliptical hinge, a fixed bracket and a mass block;

One end of the elliptical hinge is fixedly connected with the fixing bracket, and the other end of the elliptical hinge is connected to the mass;

The upper and lower surfaces of the fixing bracket and the mass block are respectively on the same plane, and fiber gratings are respectively pasted between the fixing bracket and the mass block on the same plane and located directly above and below the elliptical hinge.

The above-mentioned elliptical hinge, fixed bracket and mass block are directly cut from the same piece of metal material. The positional relationship is shown in Figure 1. The three materials are the same, all of which are spring steel. The fixing bracket acts as a fixed sensor and sticks the optical fiber, and the elliptical hinge acts as an elastic element. The mass is used to provide the inertia of Newton's third law.

In a second aspect, a method for measuring a medium and high frequency elliptical hinge double fiber grating acceleration sensor is disclosed, including:

When vibrating, the mass block rotates around the mass point of the elliptical hinge under the action of inertial force, and at the same time drives the upper and lower fiber gratings to produce telescopic deformation, thereby causing the reflection wavelength of the fiber grating to drift;

Among them, at the same time, one fiber grating stretches and the other fiber grating shrinks, and the wavelength shift is obtained by taking the difference between the reflected wavelengths of the two fiber gratings;

By demodulating the variation of the central wavelength, the relationship between the variation of the central wavelength, that is, the wavelength drift, and the acceleration is obtained.

Specifically, the sensor measures the wavelength shift generated by the FBG under the action of the vibration signal. Measurement steps: Fix the sensor on the vibrating table, and connect the upper and lower FBGs to the wavelength demodulator and data acquisition at the same time. When the vibration table vibrates, the wavelength demodulator demodulates the wavelength at each moment, and then the data is input into the computer through data acquisition.

Data processing: First, preprocess the collected data to remove gross errors. Then use MATLAB software to calculate the peak-to-peak value of the wavelength shift to obtain the sensitivity of a single FBG. Finally, the same method is used to find the wavelength shift under the double FBG, and the sensitivity of the double FBG is calculated.

One or more of the above technical solutions have the following beneficial effects:

The technical solution of the present disclosure is based on a medium and high frequency double FBG acceleration sensor based on an elliptical hinge. On the premise of ensuring high precision, the sensitivity of the sensor is improved by taking advantage of the large movement range of the elliptical hinge, and the differential method is used to demodulate the reflected wavelength to achieve the sensitivity. multiplication purpose.

Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will become apparent from the description which follows, or may be learned by practice of the invention.

Description of drawings

The accompanying drawings that constitute a part of the present disclosure are used to provide further understanding of the present disclosure, and the exemplary embodiments of the present disclosure and their descriptions are used to explain the present disclosure and do not constitute an improper limitation of the present disclosure.

FIG. 1 is a schematic structural diagram of a sensor according to an embodiment of the present disclosure;

FIG. 2 is a structural vibration model of a sensor according to an embodiment of the present disclosure;

Fig. 3 is the influence of the semi-major axis b and semi-minus c of the elliptical hinge on the sensitivity and the resonant frequency according to the embodiment of the disclosure;

Figure 4. The static stress simulation analysis diagram of the simulation example;

Fig. 5 modal simulation analysis diagram of simulation example;

Fig. 6 simulation example harmonic response simulation analysis diagram;

Figure 7 is a diagram of the sensor sensitivity calibration experiment system;

Fig. 8 Time-domain response curve of the acceleration sensor;

Figure 9 Sensitivity linear fit plot.

Detailed ways

It should be noted that the following detailed description is exemplary and intended to provide further explanation of the present disclosure. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

It should be noted that the terminology used herein is for the purpose of describing specific embodiments only, and is not intended to limit the exemplary embodiments according to the present disclosure. As used herein, unless the context clearly dictates otherwise, the singular is intended to include the plural as well, furthermore, it is to be understood that when the terms "comprising" and/or "including" are used in this specification, it indicates that There are features, steps, operations, devices, components and/or combinations thereof.

The embodiments of this disclosure and features of the embodiments may be combined with each other without conflict.

This embodiment discloses a medium and high frequency double FBG acceleration sensor based on an elliptical hinge. On the premise of ensuring high precision, the sensitivity of the sensor is improved by taking advantage of the large movement range of the elliptical hinge, and the differential method is used to demodulate the reflected wavelength. , to achieve the purpose of doubling the sensitivity. The sensitivity and resonant frequency of the sensor are analyzed by the theoretical model of the sensor, and the structural parameters of the sensor are optimized and simulated by MATLAB and ANSYS software to obtain the structural parameters of the sensor. Develop sensors, and verify and analyze the experimental results through sensor calibration experiments.

Specifically, this embodiment discloses a mid-high frequency elliptical hinge double fiber grating acceleration sensor. Referring to FIG. 1 , the fiber grating acceleration sensor is composed of an elliptical hinge, two FBGs, a mass block, and a fixed bracket.

Among them, the FBG is pasted up and down between the mass block and the fixed bracket, and the two ends of the FBG are applied with equal prestress during pasting to prevent the chirp effect.

One end of the elliptical hinge is fixedly connected with the fixing bracket, and the other end of the elliptical hinge is connected to the mass;

The upper and lower surfaces of the fixed bracket and the mass block are respectively on the same plane, and the fiber grating is respectively pasted between the fixed bracket and the mass block on the same plane and located directly above and below the elliptical hinge.

The drift of the central wavelength of the FBG, that is, the amount of wavelength drift is closely related to the strain, and the relationship between the two can be expressed as

where P _e is the effective elastic-optic coefficient, usually 0.15-0.22, Δλ _B is the drift of the center wavelength of the fiber Bragg grating under strain, and ε is the axial strain generated by the fiber Bragg grating.

The structure size and mechanical model of the sensor are shown in Figure 2. Take a FBG as an example to analyze the theoretical sensitivity of the sensor. When an acceleration of magnitude g is applied in the vertical direction of the sensor and the direction is upward, it can be known from Newton's third law of motion that the mass block vibrates with the hinge as the center.

From the moment balance equation, we can get

where m is the mass of the mass block, d is the distance from the mass center of the mass block to the hinge center, k is the elastic coefficient of the fiber, l is the elongation of the fiber, h is the height of the mass block, K is the rotational stiffness of the hinge, θ is the rotation angle of the hinge.

The rotational stiffness of the hinge is

in

In the formula, E is the elastic modulus of the material, w is the thickness of the hinge, s=c/t, c is the short semi-axis of the elliptical hinge, and t is the minimum thickness between the hinges. The sensor sensitivity S is the ratio of the center wavelength change Δλ of the fiber grating to the acceleration a, namely

where P _e is the elastic-optic coefficient, λ _B is the center wavelength of the grating, ε _f is the fiber strain, and Δl is the fiber elongation. The above is the sensitivity analysis of a single FBG, because the double FBG is symmetrically pasted one above the other. At the same time, one FBG stretches and the other FBG shrinks. The changes of the reflection wavelengths of the two FBGs are opposite. Therefore, the wavelength shift is obtained by taking the difference of the reflection wavelengths of the two FBGs, and the sensitivity of the sensor is twice the sensitivity of the single FBG acceleration sensor.

Sensor resonance frequency analysis:

Another important parameter of the sensor is the resonant frequency F, which is closely related to the frequency range that the sensor can measure. Generally speaking, the higher the resonant frequency of the sensor, the larger the frequency range that the sensor can measure. Let the moment of inertia of the mass block rotating around the hinge center be J, and the rotation angle of the elliptical hinge be θ.

The resonant frequency of the system is

where the moment of inertia J is

Influence of structural parameters on sensor performance:

It can be seen from equations (5) and (6) that the grating length l of the two-point-stick FBG acceleration sensor is only related to the sensitivity, and has nothing to do with the resonant frequency, and the shorter the fiber grating, the higher the sensitivity. In addition, the sensitivity and resonant frequency of the sensor are closely related to the dimensions of the major semi-axis b, minor semi-axis c and thickness t of the elliptical hinge. The sensor is made of spring steel with a density of 7800kg/m ³ , an elastic modulus of 2.1E11 Pa, a sensor thickness of 12mm, an elastic modulus of the optical fiber of 7.2E10 Pa, an effective elastic-optic coefficient of 0.22, and the center wavelength of the fiber grating is 1553nm, the length l is 5mm.

Discuss the variation of sensitivity and resonant frequency with long semi-axis b and short semi-axis c under different hinge thickness t. Let the mass of the mass m=18.72g, the unit of b, c∈(0,10) is mm, when the thickness t of the elliptical hinge is 0.2mm, 0.6mm, and 1mm, respectively, the obtained sensitivity and resonance frequency change as shown in the figure 3 shown.

It can be seen from Figure 3 that when the parameters of the elliptical hinge change, the sensitivity and resonant frequency of the sensor both change. When the long semi-axis b of the elliptical hinge increases, the sensitivity of the sensor increases and the resonant frequency decreases; when the short semi-axis c of the elliptical hinge increases, the sensitivity of the sensor decreases and the resonant frequency increases; When the thickness t increases, the sensitivity of the sensor increases and the resonant frequency decreases. Therefore, it is necessary to further optimize the structural parameters of the elliptical hinge to obtain the optimal sensitivity and resonance frequency.

Structural parameter optimization:

In order to obtain the optimal sensitivity and resonant frequency, the optimization toolbox of MATLAB software is used to optimize the long semi-axis b, short semi-axis c and thickness t of the sensor's elliptical hinge. Firstly, u in the stiffness equation (3) of the hinge is simplified. When s∈(1,7) in equation (4), the parameter s is polynomially fitted with MATLAB, and we can get:

F(x)=0.0003574x ⁴ -0.007678x ³ +0.06342x ² -0.2615x+0.7126 (8)

The fitting determination coefficient and mean square error of formula (8) are 0.9997 and 0.00124, respectively. These two values fully illustrate the rationality of data fitting. The formula (8) is brought into formula (4), and then MATLAB is used to carry out the parameters. optimization. Set the sensitivity and resonant frequency of the sensor as the target parameters, take the long semi-axis b, short semi-axis c and thickness t of the elliptical hinge as constraints, and establish an optimization model combined with the MATLAB optimization toolbox, taking into account the size and size of the sensor and The possibility of machining, the optimized model is obtained as

MAX S (9)

It can be seen from the optimization results that when the semi-major axis b of the ellipse hinge is 2.578mm, the semi-axis c is 2.081mm, and the thickness t of the ellipse hinge is 0.7919mm, the optimal sensitivity and resonance frequency can be obtained. Taking into account the machining accuracy, the final values of b, c and t are 2.6mm, 2.1mm and 0.8mm, respectively.

ANSYS Simulation

The sensor is modeled with the structural parameters obtained from the above analysis and optimization, and imported into ANSYS for simulation analysis. First, import the model into the ANSYS static stress simulation tool, impose fixed constraints on the fixed support of the sensor model, and apply an external load of standard earth gravitational acceleration g (g=9.8m/s ² ) to the whole sensor, and the strain cloud diagram of the model is obtained as shown in the figure 4 shown. It can be seen that the deformation at the free end of the sensor is the largest, and gradually decreases towards the fixed end, and the maximum deformation at the free end is 0.56 μm.

According to the results of static stress analysis, the sensor model is modal analysis, as shown in Figure 5, the obtained resonant frequencies of the first-order mode and the second-order mode are 764.82Hz and 5101.4Hz, respectively. As shown in Fig. 5(a), the first-order mode is a simple harmonic vibration, indicating that the sensor model vibrates along the Y-axis under the action of external excitation. As shown in Fig. 5(b), the second-order mode is a rotational mode shape, indicating that the sensor model rotates along the X-axis under the action of external excitation. By comparing the modal data of each order, it is found that the resonant frequency of the first-order modal is quite different from the resonant frequency of the second-order modal, which indicates that the cross-coupling of the structural sensor is small, which can effectively reduce the cross-interference.

Finally, the harmonic response of the model is analyzed, and fixed constraints are imposed on the fixed bracket of the sensor model to analyze the system dynamic response of the sensor model under the action of sinusoidal loads of different frequencies. The frequency change is set to 0-900Hz, the step size is 10Hz, and the sinusoidal load size is 2g. The obtained dynamic response of the sensor is shown in Figure 6.

It can be seen from Figure 6 that the resonant frequency of the sensor is about 780Hz, and the curve is relatively flat below 500Hz, which is conducive to the realization of medium and high frequency measurements.

Sensor calibration experiment:

In order to ensure that the sensor has good performance below 500Hz, it is necessary to perform a sensitivity calibration experiment on the acceleration sensor. The sensor sensitivity calibration experimental system is mainly composed of two parts, the vibration test system and the signal demodulation system, as shown in Figure 7. The vibration test system includes a vibration table, a signal generator, a signal amplifier, etc.; the signal demodulation system includes a wavelength demodulator, a computer, etc.; the designed sensor base and the vibration table are closely connected by bolts. The reflected light wave of the FBG is inserted into the circulator through the transmission fiber, and finally sent to the demodulator to demodulate the information carried by the wavelength change of the light wave. The experimental environment temperature is 25 °C.

Analysis of response characteristics: In order to test the response characteristics of the sensor, the output frequency of the shaking table is set to 325Hz, and the output amplitude of the acceleration is set to 1g, and the time domain curve of the response of the output frequency corresponding to the fiber grating acceleration sensor is obtained, as shown in Figure 8.

It can be seen from Figure 8 that the sensor has a good output frequency response. At each moment, the central wavelength changes of the upper and lower two fibers of the fiber grating acceleration sensor are equal in magnitude and opposite in direction.

Sensor sensitivity linear analysis:

In the sensor sensitivity linear test, 160Hz and 325Hz are selected to apply a sinusoidal excitation signal to the sensor, and the test range of acceleration is increased from 0.1g to 2.0g, and the step size is 0.1g. The wavelength drift of the sensor under the same excitation frequency and different accelerations is measured. By fitting the linear relationship between the wavelength drift and the acceleration, the sensitivity fitting curve of the sensor is obtained, as shown in Figure 9.

It can be seen from Figure 9 that when the vibration frequency is 160 Hz, the single-fiber sensitivity is 40.83pm/g, the fitting determination coefficient R ² =0.9987, the dual-fiber differential sensitivity is 90.91pm/g, and the fitting determination coefficient R ² =0.9988. When the vibration frequency is 325Hz, the single-fiber sensitivity is 59.22pm/g, the fitting determination coefficient R ² =0.9956, the dual-fiber differential sensitivity is 132.53pm/g, and the fitting determination coefficient R ² =0.9962.

Analysis of experimental results: Comparing the results of the sensor sensitivity calibration experiment with the theoretical sensitivity value, it is found that there is an error of 3%, but it can meet the needs of engineering applications. The reason for the error is speculated: (1) There is an error in the linear fitting of equation (4) using MATLAB software in the theoretical analysis part, and the data selected in the theoretical calculation are all approximate values; Insufficient precision of equipment and uncontrollable temperature during processing lead to processing errors of elliptical hinges.

The embodiment of the present disclosure proposes a medium and high frequency acceleration sensor based on an elliptical hinge, and analyzes and verifies the optimized result through a sensor sensitivity calibration experiment. The experimental results show that the sensitivity is about 132pm/g, the measurable frequency range is 80-495Hz, and the resonant frequency of the sensor is about 780Hz. In addition, the experiment further verifies that the dual-fiber differential demodulation method can double the sensitivity of the sensor, and proposes a new idea for monitoring the medium and high frequency vibration of large structures.

When the sensor is at rest, the upper and lower fiber gratings are not stressed, so the center wavelength of the fiber does not change. When the sensor vibrates, the mass block rotates around the mass point of the elliptical hinge under the action of inertial force, and at the same time drives the upper and lower FBGs to produce telescopic deformation, which in turn causes the reflection wavelength of the FBG to drift. Finally, by demodulating the variation of the central wavelength, the relationship between the variation of the central wavelength, that is, the wavelength drift, and the acceleration is obtained.

The above descriptions are only preferred embodiments of the present disclosure, and are not intended to limit the present disclosure. For those skilled in the art, the present disclosure may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present disclosure shall be included within the protection scope of the present disclosure.

Although the specific embodiments of the present disclosure have been described above in conjunction with the accompanying drawings, they do not limit the protection scope of the present disclosure. Those skilled in the art should understand that on the basis of the technical solutions of the present disclosure, those skilled in the art do not need to pay creative efforts. Various modifications or variations that can be made are still within the protection scope of the present disclosure.

Claims (10)

1. a medium and high frequency elliptical hinge double fiber grating acceleration sensor, is characterized in that, comprises: elliptical hinge, fixed bracket and mass block; One end of the elliptical hinge is fixedly connected with the fixing bracket, and the other end of the elliptical hinge is connected to the mass; The upper and lower surfaces of the fixing bracket and the mass block are respectively on the same plane, and fiber gratings are respectively pasted between the fixing bracket and the mass block on the same plane and located directly above and below the elliptical hinge. 2 . The mid-high frequency elliptical hinge double fiber grating acceleration sensor according to claim 1 , wherein the two ends of the fiber grating apply equal prestressing force when pasting, so as to prevent the chirp effect from occurring. 3 . 3. The mid-high frequency elliptical hinge double fiber grating acceleration sensor according to claim 1, wherein in the fiber grating, at the same time, one fiber grating stretches, and the other fiber grating shrinks, and the The difference of the reflection wavelengths of the two fiber gratings is used to obtain the wavelength shift. 4 . The mid-high frequency ellipse hinge double fiber grating acceleration sensor according to claim 1 , wherein the wavelength shift of the fiber grating is proportional to the axial strain generated by the fiber grating. 5 . 5 . The mid-high frequency elliptical hinge double fiber grating acceleration sensor according to claim 1 , wherein the sensitivity of the sensor is the ratio of the center wavelength change of the fiber grating to the acceleration. 6 . 6. a kind of medium and high frequency elliptical hinge double fiber grating acceleration sensor as claimed in claim 1, is characterized in that, described sensor is made material selects spring steel, selects its density, elastic modulus, sensor thickness, optical fiber based on measurement accuracy. Elastic modulus, effective elastic-optic coefficient, center wavelength of fiber grating and fiber grating length. 7. a kind of medium and high frequency elliptical hinge double fiber grating acceleration sensor as claimed in claim 1, is characterized in that, the sensitivity of setting sensor and resonant frequency are target parameters, with the major semi-axis, minor semi-axis and thickness of the elliptical hinge The value of is a constraint condition, an optimization model is obtained, and the structural parameters of the optimized sensor are obtained based on the optimization model. 8. A kind of medium and high frequency ellipse hinge double fiber grating acceleration sensor as claimed in claim 7, it is characterized in that, the sensor is modeled based on the structural parameters of the optimized sensor, a fixed constraint is imposed on the fixed support of the sensor model, and the sensor is obtained. The system dynamic response of the model under the action of sinusoidal loads of different frequencies is obtained to obtain the resonant frequency of the sensor that is beneficial to the realization of medium and high frequency measurements. 9. The measurement method based on the arbitrary described a kind of middle and high frequency elliptical hinge double fiber grating acceleration sensor of claim 1-8, is characterized in that, comprises: When vibrating, the mass block rotates around the mass point of the elliptical hinge under the action of inertial force, and at the same time drives the upper and lower fiber gratings to produce telescopic deformation, thereby causing the reflection wavelength of the fiber grating to drift; Among them, at the same time, one fiber grating stretches and the other fiber grating shrinks, and the wavelength shift is obtained by taking the difference between the reflected wavelengths of the two fiber gratings; By demodulating the variation of the central wavelength, the relationship between the variation of the central wavelength, that is, the wavelength drift, and the acceleration is obtained. 10. A sensor sensitivity calibration experiment system, characterized in that it includes a vibration test system and a signal demodulation system; Described vibration test system comprises shaking table, signal generator, signal amplifier; Between the fixed support base of any described acceleration sensor of claim 1-8 and shaking table are closely connected by bolts, and described signal generator will produce After the signal is amplified by the signal amplifier, the vibration table drives the acceleration sensor to vibrate; The signal demodulation system includes a circulator and a wavelength demodulator; the reflected light wave of the fiber grating of the acceleration sensor is inserted into the circulator through the transmission fiber, and finally sent to the wavelength demodulator to demodulate the information carried by the wavelength change of the light wave.

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