A liquid level measurement system and method based on a same-wavelength FBG array

Abstract

The application provides a liquid level measurement system and method based on a same-wavelength FBG array. The measurement system comprises a processing module, an FBG demodulator module, a temperature measurement fiber and a single-point reference grating; the temperature measurement fiber is an FBG temperature sensor array; the temperature measurement fiber and the reference grating are connected with the FBG demodulator module, and the FBG demodulator module is in communication connection with the processing module; the FBG demodulation system comprises a digital-analog conversion module, a light source, a circulator, a photodiode and a coupler; the light emitted by the light source enters the temperature measurement fiber and the reference grating to generate reflected light at the same time, the reflected light is transmitted to the photodiode for photoelectric conversion after passing through the circulator, the converted electrical signal is converted by the digital-analog conversion module to collect the light intensity signal, and the liquid level measurement value is calculated by the processing module. The application has high installation freedom, the installation frame is simple and convenient, the liquid level can be accurately measured in real time, high precision is achieved, and the safety and reliability coefficient is high.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of sensing, and particularly relates to a liquid level measurement system based on a same-wavelength FBG array and a mounting method. BACKGROUND

[0002] In today's era, the demand for accurate liquid level measurement in production and life and industrial applications is increasing. Traditional measurement techniques, such as capacitive liquid level meters, ultrasonic liquid level meters, radar liquid level meters, and float liquid level meters, can measure liquid levels, but are limited, especially in the fields of nuclear energy and chemical industry, where the environment is complex and harsh. These liquid level meters not only cannot measure the liquid level accurately in real time, but also cannot guarantee the safety of the entire system.

[0003] Optical fiber sensing technology has many advantages, such as being on the string, and many optical fiber liquid level sensors have emerged, such as the liquid level measurement method of FBG temperature sensor array, which has been proposed and verified. However, using wavelength to monitor liquid level is bound to face the problem of demodulation distortion when the grating wavelength is superimposed. In order to avoid this problem, in the process, multiple wavelengths of FBG are often selected and cascaded in order, which not only complicates the process, but also limits the number of measurement points to the wavelength range of the demodulation device, ultimately affecting the process difficulty and measurement accuracy. Therefore, intensity modulation type optical fiber sensors have also been applied to liquid level measurement in recent years, although the structure is simple, but intensity modulation is always difficult to break through the environmental restrictions, and is easily affected by external interference, and its accuracy and anti-interference ability are obviously weaker than that of wavelength modulation type optical fiber sensors.

[0004] In addition, in order to ensure the accuracy of measurement, the traditional measurement system usually needs to install the sensor vertically into the liquid, which undoubtedly increases the difficulty of construction. In order to fundamentally solve these problems and implement them in the fields of production and life and industrial applications, it is very meaningful to use some new technologies and methods to accurately measure the liquid level. SUMMARY

[0005] In view of the problems existing in the prior art, the present application provides a liquid level measurement system based on a same-wavelength FBG array and a mounting method, which can accurately measure the liquid level in harsh environments without damaging the structure of the container.

[0006] In order to achieve the above technical purpose, the application provides a liquid level measurement system based on a same-wavelength FBG array, which comprises a processing module, an FBG demodulator module, a temperature measurement fiber and a reference grating; the temperature measurement fiber is an FBG temperature sensor array, the reference grating is a single-measurement-point FBG with a center wavelength consistent with the temperature measurement fiber, the temperature measurement fiber and the reference grating are both inserted into a liquid storage container after being packaged with a metal sheath and a connector, and the head inserted into the liquid is completely sealed; the reference grating is placed at the bottom of the liquid storage container, and the measurement point position is at the same height as the first measurement point position of the temperature measurement fiber inserted into the bottom of the liquid storage container; the signal output tail ends of the temperature measurement fiber and the reference grating are connected with optical cables through flanges, the optical cables are connected to the FBG demodulator module through FC interfaces, and the FBG demodulator module is in communication connection with the processing module.

[0007] The FBG demodulation system comprises a digital-analog conversion module, a light source, a circulator, a photodiode and a coupler; the light emitted by the light source enters the temperature measurement fiber and the reference grating to generate reflected light, the reflected light is transmitted to the photodiode for photoelectric conversion after passing through the circulator, the converted electrical signal is converted by the digital-analog conversion module to collect the light intensity signal, and then is transmitted to the processing module for processing and analysis to calculate the liquid level measurement value in the liquid storage container.

[0008] The application further provides a technical scheme that the processing module calculates the liquid level measurement value in the liquid storage container as follows: when there is liquid in the container, the reflected light intensity I R :

[0009] I R =[(1-R2) m-n IR2+…+(1-R2) m-1 IR2]R1(n≥1); ①

[0010] In the formula, R1 is the reflectivity of the reference grating, and R2 is the reflectivity of the measurement grating;

[0011] d is the grating spacing in the single temperature measurement fiber FBG array;

[0012] m is the total number of grating points in the single temperature measurement fiber FBG array;

[0013] n is the number of grating points of the single temperature measurement fiber immersed in the liquid;

[0014] I is the light intensity of the broadband light source;

[0015] The temperature resolution of the temperature measurement fiber and the fiber temperature monitoring system reaches 0.1℃;

[0016] When there is no liquid in the container, the reflected light intensity I R= I (n = 0), at this time, the liquid level of the container does not need to be measured; since the reflectivity of the measurement grating is small compared with the reference grating, (1-R2) is approximately 1; and n is not 0, the reflection light intensity calculation formula 1 can be simplified as:

[0017] I R = nIR1R2;

[0018] According to formula 1, the reflection light intensity I R is obtained, and then the liquid level measurement value in the liquid storage tank is calculated as:

[0019]

[0020] Wherein L L is the liquid level measurement value in the liquid storage tank;

[0021] d is the grating spacing in a single temperature measurement fiber FBG array;

[0022] Alpha is the angle between the temperature measurement grating and the horizontal line, when the angle is pi / 2, it is vertical installation; when the light intensity output value is I, that is, n = 0, at this time, the liquid level is 0.

[0023] The preferred technical scheme of the present application: the processing module is a PC, and a terminal software is installed, and the FBG demodulator module is connected with the processing module through a signal line.

[0024] The preferred technical scheme of the present application: the temperature measurement fiber is a quasi-distributed temperature measurement fiber with a plurality of temperature measurement gratings engraved along the path, and the reflectivity R1 is lower than 0.1%; the reflectivity R2 of the reference grating is higher than 99%; the wavelength of all the measurement gratings of the temperature measurement fiber and the initial wavelength of the reference grating are all lambda1 at 25 DEG C.

[0025] The preferred technical scheme of the present application: the liquid storage container is a circular, square or rectangular sealed container, and a through hole corresponding to the insertion of the temperature measurement fiber and the reference grating is formed in the top of the container, and the temperature measurement fiber and the reference grating are inserted into the container and sealed and fixed at the through hole in the top of the container through a flange.

[0026] The preferred technical scheme of the present application: the temperature measurement fiber and the reference grating are directly inserted into the liquid storage container, and the bottom is flush with the bottom surface of the container; one reference grating and one or more temperature measurement fibers are inserted into each liquid storage container; the plurality of temperature measurement fibers do not need to be vertically installed in the liquid storage container, and the plurality of measurement points on the plurality of temperature measurement fibers are arranged in parallel or intersected.

[0027] The preferred technical scheme of the present application: the FBG demodulator module further comprises a beam splitter and an optical switch, and a plurality of circulators and couplers are arranged, the beam splitter divides the light source into multiple paths, and simultaneously can provide light sources for the temperature measuring optical fibers and reference optical fibers in multiple different liquid storage tanks, the reflected light of each path is transmitted to a photodiode for photoelectric conversion after passing through a circulator and a coupler, the converted electrical signal is converted by a digital-to-analog conversion module to collect the light intensity signal, and then is transmitted to a processing module for processing and analysis; and the optical switch is used to ensure that there is no cross-influence between channels, so that simultaneous monitoring of multiple channels of a sensing system in multiple different environments is realized.

[0028] The present application also provides a liquid level measurement method based on the same-wavelength FBG array, characterized in that the measurement steps are as follows:

[0029] S1. A reference grating and one or more temperature measuring optical fibers are installed in a liquid storage tank, the temperature measuring optical fiber is a quasi-distributed temperature measuring optical fiber with multiple temperature measuring gratings marked along the path, and the reflectivity R1 of the temperature measuring optical fiber is lower than 0.1%; the reference grating is a single-measurement-point FBG with the same center wavelength as the temperature measuring optical fiber, and the reflectivity R2 of the reference grating is higher than 99%; the measurement point position of the reference grating is the same as the first measurement point position of the temperature measuring optical fiber inserted into the bottom of the liquid storage container.

[0030] S2. The signal output ends of the reference grating and the temperature measuring optical fiber are connected to the FBG demodulator module through an optical cable, the FBG demodulator module is in communication connection with a processing module; the FBG demodulation system comprises a digital-to-analog conversion module, a light source, a circulator, a photodiode and a coupler.

[0031] S3. The light emitted by the light source simultaneously enters the temperature measuring optical fiber and the reference grating to generate reflected light, the reflected light is transmitted to the photodiode for photoelectric conversion after passing through the circulator, the converted electrical signal is converted by the digital-to-analog conversion module to collect the light intensity signal, and then is transmitted to the processing module for processing and analysis, and the liquid level measurement value in the liquid storage container is calculated; the specific calculation process is as follows:

[0032] When there is liquid in the container, the reflected light intensity I R :

[0033] I R =[(1-R2) m-n IR2+…+(1-R2) m-1 IR2]R1(n≥1); ①

[0034] In the above formula, R1 is the reflectivity of the reference grating, and R2 is the reflectivity of the measurement grating;

[0035] d is the grating spacing in the FBG array of a single temperature measuring optical fiber.

[0036] m is the total number of grating points of the single temperature measuring fiber FBG array;

[0037] n is the number of grating points of the single temperature measuring fiber immersed in the liquid;

[0038] I is the light intensity emitted by the broadband light source;

[0039] The temperature resolution of the temperature measuring fiber and the fiber temperature monitoring system reaches 0.1℃;

[0040] When there is no liquid in the container, the reflected light intensity I R =I(n=0), at this time, the liquid level of the container does not need to be measured; since the reflectivity of the measurement grating is much smaller than that of the reference grating, (1-R2) is approximately 1; and n is not 0, the reflected light intensity calculation formula ① can be simplified as: I R =nIR1R2;

[0041] According to formula ①, the reflected light intensity I R is obtained, and then the liquid level measurement value in the liquid storage tank is calculated by formula ② as:

[0042]

[0043] where L L is the liquid level measurement value in the liquid storage tank;

[0044] d is the distance between two grating points of a single temperature grating;

[0045] α is the angle between the temperature measuring grating and the horizontal line, when the angle is π / 2, it is vertical installation; when the light intensity output value is I, that is, n=0, at this time, the liquid level is 0.

[0046] The preferred technical scheme of the present application: when multiple temperature measuring fibers are arranged in the S1 step, multiple measuring points on the multiple temperature measuring fibers are arranged in parallel or intersected; each temperature measuring fiber and the reference grating are packaged with a metal sheath and a connector and then inserted into the liquid storage container, and the head inserted into the liquid is completely sealed, and the reference grating is placed at the bottom of the liquid storage container; the temperature measuring fiber and the reference grating are both installed on the liquid storage tank through flanges.

[0047] The preferred technical scheme of the present application is that the processing module and the FBG demodulator module can simultaneously adjust and measure the liquid level height of multiple different liquid storage tanks, and realize multi-channel and multiple different environment measurement; specifically, the FBG demodulator module is integrated into a demodulation system, multiple circulators and couplers are arranged, a beam splitter and an optical switch are added, the light source is split into multiple paths by the beam splitter, and the light source is provided for the temperature measuring optical fibers and reference optical fibers in multiple different liquid storage tanks, the reflected light of each path is transmitted to a photodiode for photoelectric conversion through a circulator and a coupler, and the optical switch is used to ensure that there is no cross influence between channels, and the simultaneous monitoring of a sensing system in multiple channels and multiple different environments is realized.

[0048] The reflectivity R1 of the measurement grating is lower than 0.1%, and the reflectivity R2 of the reference grating is higher than 99%, so that only the light with the same wavelength as the reference grating can be reflected. Since the liquid temperature T 液 There is a certain difference between the gas temperature T 气 above the liquid surface and the FBG wavelength immersed in water, the wavelength of the FBG immersed in water becomes λ2, and the wavelength of the FBG placed in air is λ3. Due to the high reflectivity of the reference grating, only the light with the wavelength λ2 can be reflected. These lights are superimposed on each other, so that the reflected light intensity increases. By monitoring the size of the reflected light intensity, the position of the gas-liquid interface can be calculated. In other words, by monitoring the size of the reflected light intensity, the liquid level value in the container can be known. It is worth noting that the intensity of the reflected light does not change with the installation angle. This makes the installation degree of freedom high, and reduces the difficulty of construction.

[0049] The temperature measuring optical fiber and the reference grating in the present application are packaged with metal sheath and connector, and the head inserted into the liquid needs to be completely closed to prevent water pressure from causing cross interference of grating temperature measurement and improve the accuracy of liquid level measurement. The temperature measuring optical fiber is a quasi-distributed temperature measuring optical fiber with multiple temperature measuring gratings engraved along the path, and the rear-end temperature monitoring system can realize high-precision temperature measurement along the path after adjustment, and the reflectivity R1 is lower than 0.1%. The reference grating is a single-measurement-point FBG, the center wavelength is consistent with the temperature measuring optical fiber, the position is flush with the first measurement point at the tail of the temperature measuring optical fiber, and is placed at the bottom of the container, and the reflectivity R2 is higher than 99%. The light intensity monitoring and demodulation system is composed of a light source, a circulator, a coupler, a photodiode, a digital-to-analog converter and an upper computer, the measurement optical fiber and the reference grating are connected with the light source and the photodiode through the circulator and the coupler, and the photodiode is connected with the upper computer for signal processing and output. When the light source emitted passes through the FBG array, each FBG will reflect a characteristic spectrum signal, and since the reference grating is a strong reflection grating, only the light with the same wavelength as the reference grating can be reflected. The reflected light is transmitted to the photodiode PD for photoelectric conversion after passing through the circulator, and then the light intensity signal is collected through the digital-to-analog conversion, and finally the information is transmitted to the upper computer for signal processing and analysis through the signal transmission line.

[0050] The present application has the following advantages and beneficial effects:

[0051] (1) The present application uses the same wavelength FBG array based on intensity modulation for liquid level measurement, compared with the traditional liquid level measurement method, has many advantages such as small size, light weight, high sensitivity, anti-electromagnetic interference, corrosion resistance, etc., can realize high-precision measurement without affecting the measured system; compared with the wavelength modulation type FBG liquid level measurement sensor, not only solves the problem of wavelength superposition and greatly simplifies the demodulation system, improves the integration; compared with the intensity modulation type liquid level sensor, avoids its poor stability, while retaining its simple demodulation system.

[0052] (2) The acquisition module of the present application only needs to collect the intensity of reflected light, the response time and measurement error of the system can be guaranteed; and the intensity of reflected light will not change with the installation angle, so the measurement optical fiber is not limited by the installation angle, and the degree of freedom is high when installing the measurement optical fiber, which reduces the construction difficulty and improves the practicability.

[0053] (3) The present application uses a plurality of gratings staggered installation method to reduce the spacing between the gratings, more accurately calculates the liquid level of the container, and the calculation method is the same as that of a single measurement optical fiber grating, and the measurement optical fiber grating is equivalent to a FBG temperature sensing array, then the liquid level value can be calculated by using the liquid level measurement formula of a single optical fiber.

[0054] (4) The temperature measurement optical fiber in the present application does not need to consider the wavelength crossing problem between FBGs and the wavelength range problem of the demodulator, and the grating wavelength is completely unified, which means that any number of measurement points can be written on a grating, the denser the measurement points, the more accurate the position of the gas-liquid interface, and the more accurate the liquid level value, the spacing between the grating points can be adjusted according to the actual demand to improve the measurement accuracy, and various installation methods can be selected during installation to improve the measurement accuracy and resolution.

[0055] (5) The present application can highly integrate the demodulation module into a demodulation system, and on the basis of the above measurement system, the light source is divided into multiple paths by using a beam splitter, and the liquid levels of multiple containers can be measured simultaneously; the optical switch is used to ensure that there is no cross-influence between channels, realizing simultaneous monitoring of multiple measurement points by one sensing system, which is very efficient and meaningful. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 is a schematic diagram of the measurement system in the present application;

[0057] Figure 2 is a schematic diagram of the measurement points of multiple measurement optical fibers being parallel to each other;

[0058] Figure 3 This is a schematic diagram showing the cross-layout of measurement points on multiple measuring optical fibers;

[0059] Figure 4 This is a schematic diagram in the embodiment where multiple measurement optical fibers are equivalent to a single measurement optical fiber;

[0060] Figure 5 This is a schematic diagram illustrating the simultaneous measurement of liquid levels in multiple different tanks in this invention.

[0061] In the diagram: 1—Processing module, 2—Signal line, 3—Digital-to-analog conversion module, 4—Light source, 5—Circulator, 6—Photodiode, 7—Coupler, 8—Optical cable, 9—Flange, 10—Reference fiber, 11—Temperature measuring fiber, 12—Bundle splitter, 13—Optical switch, 14—Liquid storage container. Detailed Implementation

[0062] To make the objectives, technical solutions, and advantages of this invention clearer, the following detailed description and various testing environments and installation methods are provided. The illustrative embodiments and descriptions of this invention are for explaining the invention only and are not intended to limit the invention.

[0063] In this embodiment, the temperature-sensing fiber 11 is an FBG temperature sensor array, and the reference grating 10 is a single-point FBG with its center wavelength consistent with that of the temperature-sensing fiber 11; Figures 1 to 5 As shown, both the temperature-sensing optical fiber 11 and the reference grating 10 are encapsulated in a metal sheath and connector before being inserted into the liquid storage container 14, with the end inserted into the liquid completely sealed. The reference grating 10 is positioned at the bottom of the liquid storage container 14, and its measurement point is at the same height as the first measurement point at the bottom of the liquid storage container 14 where the temperature-sensing optical fiber 11 is inserted. In this embodiment, the processing module 1 is a PC with terminal software installed, and the FBG demodulator module is connected to the processing module 1 via signal line 2. The temperature-sensing optical fiber 11 is a quasi-distributed temperature-sensing optical fiber with multiple temperature-sensing gratings etched along its length, and its reflectivity R1 is less than 0.1%. The reflectivity R2 of the reference grating is greater than 99%. The wavelengths of all the measurement gratings of the temperature-sensing optical fiber 11 and the initial wavelength of the reference grating 10 are both λ1 at 25°C.

[0064] The liquid storage container 14 in the embodiment is a sealed container of various shapes such as round, square or rectangular. It has a through hole on the top for inserting the corresponding temperature measuring optical fiber 11 and reference grating 10. After the temperature measuring optical fiber 11 and reference grating 10 are inserted into the liquid storage container 14, they are sealed and fixed at the through hole on the top of the liquid storage container 14 by the flange 9.

[0065] Example 1 provides a liquid level measurement system based on a co-wavelength FBG array, such as... Figure 1As shown, the measurement system includes a single temperature measurement optical fiber 11 for measuring the liquid level in a single liquid storage container 14, and includes a processing module 1, an FBG demodulator module, a temperature measurement optical fiber 11, and a reference grating 10. The temperature measurement optical fiber 11 is directly inserted into the liquid storage container 14, with the bottom flush with the bottom surface of the liquid storage container 14. The signal output tail ends of the temperature measurement optical fiber 11 and the reference grating 10 are connected by flanges 9 and optical cables 8, respectively. The optical cables 8 are connected to the FBG demodulator module through FC interfaces. The FBG demodulator module is in communication connection with the processing module 1. The FBG demodulation system includes a digital-to-analog conversion module 3, a light source 4, a circulator 5, a photodiode 6, and a coupler 7. The light emitted by the light source 4 enters the temperature measurement optical fiber 11 and the reference grating 10 to generate reflected light. The reflected light is transmitted to the photodiode 6 for photoelectric conversion after passing through the circulator 5. The converted electrical signal is converted by the digital-to-analog conversion module 3 for light intensity signal acquisition, and then transmitted to the processing module 1 for processing and analysis to calculate the liquid level measurement value in the liquid storage container 14.

[0066] In the second embodiment, a liquid level measurement system based on a same-wavelength FBG array is provided. The processing module and the FBG demodulator module divide the signal into multiple paths through a beam splitter, each path can be connected to a different liquid storage container 14 to measure the liquid level height, and multiple different environments can be measured. Moreover, the form of the liquid storage container 14 is not limited. As shown in the following figure, Figure 5 The FBG demodulator module is in communication connection with the processing module 1. The FBG demodulation system includes a digital-to-analog conversion module 3, a light source 4, a photodiode 6, a beam splitter 12, an optical switch 13, multiple circulators 5, and multiple couplers 7. The beam splitter 12 divides the light source 4 into multiple paths, and can provide light sources for the temperature measurement optical fibers and reference fibers in multiple different liquid tanks. Each reflected light is transmitted to the photodiode 6 for photoelectric conversion after passing through a circulator 5 and a coupler 7. The converted electrical signal is converted by the digital-to-analog conversion module 3 for light intensity signal acquisition, and then transmitted to the processing module 1 for processing and analysis. The optical switch 13 is used to ensure that there is no cross-influence between channels, and to realize simultaneous monitoring of multiple different environments in a sensing system.

[0067] In the above embodiments, the temperature measurement optical fiber 11 in each liquid storage container 14 can be single or multiple. The temperature measurement optical fiber 11 can also have multiple temperature measurement optical fibers 11. The multiple temperature measurement optical fibers 11 do not need to be vertically installed in the liquid storage container 14, and multiple measurement points on the multiple temperature measurement optical fibers 11 can be arranged in parallel (as shown in the following figure, Figure 2 or arranged in cross (as shown in the following figure, Figure 3 ).

[0068] The FBG temperature sensor array 11 and the reference grating 6 are directly placed in the liquid in the embodiment, without vertical installation, the bottom is flush with the water bottom, to ensure the accuracy of the measurement. The above-mentioned signal output and receiving part can be integrated in the FBG demodulator, the FBG temperature sensor array 11 and the reference grating are connected with the optical cable 8 through the flange 9, and the optical cable 8 is connected with the FBG demodulator module through the FC interface. The FBG demodulator module is connected with the processing module 1 through the network cable, and the network cable plays a role in signal transmission.

[0069] In addition to the angle, the distance between the grids can be adjusted. We can also improve the resolution and accuracy of the sensor by adjusting the installation method. For example, multiple gratings are installed in parallel, and the intensity of the reflected light of multiple gratings is superimposed and detected, which can improve the minimum resolution of the whole system.

[0070] Because there is an inherent distance d between the two gratings in the array. Therefore, there is a certain error in the system, in order to realize more accurate measurement, we adopt the staggered installation of multiple gratings (as shown in Figure 3 ), that is, the grid points are staggered, which can improve the accuracy of the system. When calculating the staggered installation of multiple gratings, the staggered installation of optical fibers can be equivalent to a FBG temperature sensor array, and then the liquid level measurement formula of a single optical fiber is used to calculate the liquid level value. Taking three measurement gratings as an example, as shown in Figure 4 , if it is a single measurement grating, the grating distance is d, and if the liquid level is between the two grating points, the liquid level measurement has a large error. If the three optical fibers are staggered, and the grid points of the second and third optical fibers are located between the two grid points of the first optical fiber, the distance between the two grid points can be reduced to d / 3, and when calculating the liquid level, it can be equivalent to a single measurement grating with a grating point distance of d / 3, which improves the measurement accuracy of the whole system.

[0071] In the embodiment, the process of calculating the liquid level measurement value in the liquid storage container by the processing module is as follows: when there is liquid in the container, the reflected light intensity I R :

[0072] I R =(1-R2) m-n IR2+…+(1-R2) m-1 IR2]R1(n≥1); ①

[0073] In the formula, R1 is the reflectivity of the reference grating, R2 is the reflectivity of the measurement grating, d is the grating distance in the single temperature measurement optical fiber FBG array, m is the total number of grating points in the single temperature measurement optical fiber FBG array, and n is the number of grating points of the single temperature measurement optical fiber immersed in the liquid.

[0074]

[0075]

[0076] n is the number of grating points of the single temperature measurement optical fiber immersed in the liquid.​​

[0077] I is the light intensity emitted by a broadband light source;

[0078] The temperature resolution of the temperature measuring optical fiber and the optical fiber temperature monitoring system reaches 0.1 DEG C.

[0079] When there is no liquid in the container, the reflected light intensity I R = I (n = 0), at this time, the liquid level of the container does not need to be measured; since the reflectivity of the measurement grating is much smaller than that of the reference grating, (1-R2) is approximately 1; and n is not 0, the reflected light intensity calculation formula 1 can be simplified as:

[0080] I R = nIR1R2;

[0081] According to formula 1, the reflected light intensity I R is obtained, and then the liquid level measurement value in the liquid storage tank is calculated by installing formula 2 as:

[0082]

[0083] Where L L is the liquid level measurement value in the liquid storage tank;

[0084] d is the grating spacing in a single temperature measuring optical fiber FBG array;

[0085] Alpha is the angle between the temperature measuring grating and the horizontal line, when the angle is pi / 2, it is vertical installation; when the light intensity output value is I, that is, n = 0, at this time, the liquid level is 0.

[0086] The principle of the application is that there is an inherent difference between the temperature of the liquid and the gas above the liquid surface, and the optical fiber sensor laid along the way can analyze the gas-liquid interface. Compared with the traditional liquid level measurement method, the application has the characteristics of corrosion resistance, high temperature and high pressure resistance, small size, high safety, remote transmission and measurement, etc. A single system can monitor multiple to-be-measured points, greatly increasing the system integration, and compared with the wavelength modulation type FBG liquid level measurement sensor, the application not only solves the problem of wavelength superposition and greatly simplifies the demodulation system, and improves the integration. Compared with the intensity modulation type liquid level sensor, the application avoids the poor stability of the intensity modulation type liquid level sensor, while retaining the simple demodulation system. Since the acquisition module only needs to acquire the reflected light intensity, the response time and measurement error of the system can be guaranteed.

[0087] The system in the application is not limited by installation angle, has high installation degree of freedom, can control measurement precision by adjusting FBG temperature sensor array grid point spacing, and can effectively improve system precision and resolution by using two special installation modes to ensure measurement accuracy. In the calculation process, the linear inversion relationship between reflected light intensity and liquid level value is fully utilized, and through software algorithm processing flow, liquid level accurate measurement can be realized quickly, in real time and with high precision. In addition, the overall structure has good environmental adaptability, mature installation structure, high safety and reliability coefficient, is far away from the harsh environment, and is very suitable for application in the field of liquid level measurement in extreme environments such as storage tanks in the chemical production process.

[0088] The above specific embodiments further illustrate the purpose, technical solutions and beneficial effects of the application. It should be understood that the above description is only a specific embodiment of the application and is not used to limit the protection scope of the application. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the application should be included in the protection scope of the application.

Claims

1. A liquid level measurement system based on a same-wavelength FBG array, characterized in that: The measurement system comprises a processing module (1), an FBG demodulator module, a temperature measuring fiber (11) and a reference grating (10); the temperature measuring fiber (11) is an FBG temperature sensor array, the reference grating (10) is a single-point FBG with a center wavelength consistent with the temperature measuring fiber (11), and the temperature measuring fiber (11) and the reference grating (10) are both packaged with a metal sheath and a connector and then inserted into a liquid storage container (14), with the head inserted into the liquid being completely sealed, the reference grating (10) being placed at the bottom of the liquid storage container (14) and the measuring point position being the same as the first measuring point position of the temperature measuring fiber (11) inserted into the bottom of the liquid storage container (14); the signal output tail ends of the temperature measuring fiber (11) and the reference grating (10) are connected through flanges (9) and optical cables (8) respectively, the optical cables (8) are connected to the FBG demodulator module through FC interfaces, and the FBG demodulator module is in communication connection with the processing module (1). The FBG demodulator module comprises a digital-analog conversion module (3), a light source (4), a circulator (5), a photodiode (6) and a coupler (7); the light emitted by the light source (4) enters the temperature measuring fiber (11) and the reference grating (10) to generate reflected light, the reflected light is transmitted to the photodiode (6) for photoelectric conversion after passing through the circulator (5), the converted electrical signal is converted by the digital-analog conversion module (3) to collect the light intensity signal, and then the light intensity signal is transmitted to the processing module (1) for processing and analysis to calculate the liquid level measurement value in the liquid storage container (14); The process of the processing module to calculate the liquid level measurement in the reservoir is as follows: when there is liquid in the container, it reads the reflected light intensity I R : I R = [(1 - R2) m-n IR2+…+(1 - R2) m-1 IR2]R1, n > 1 ① In the formula, R1 is the reflectivity of the reference grating, and R2 is the reflectivity of the measurement grating. d is the grating spacing in the single temperature measuring fiber FBG array; m is the total number of written grating points in the single temperature measuring fiber FBG array; n is the number of grating points of the single temperature measuring fiber immersed in the liquid; I is the light intensity of the broadband light source; The temperature resolution of the temperature measuring fiber and the optical fiber temperature monitoring system reaches 0.1℃. When there is no liquid in the container, n = 0, the reflected light intensity I R = I, at this time, there is no need to measure the liquid level of the container; since the reflectivity of the measurement grating is much smaller than that of the reference grating, (1-R2) is approximately 1; and n is not 0, the reflected light intensity calculation formula ① can be simplified as: I R = nIR1R2; According to formula ①, the reflected light intensity I R is obtained, and then the liquid level measurement value in the liquid storage tank is calculated according to formula ②: wherein L L is the liquid level measurement in the storage tank; d is the grating spacing in the single temperature measuring fiber FBG array; α is the included angle between the temperature measuring fiber and the vertical line of the tank, when the included angle is π / 2, it is vertical installation; when the light intensity output value is I, n=0, and the liquid level height is 0.

2. The liquid level measurement system based on the same-wavelength FBG array according to claim 1, characterized in that: The processing module (1) is a PC and is provided with terminal software, and the FBG demodulator module is in signal connection with the processing module (1) through a signal line (2).

3. The liquid level measurement system based on the same-wavelength FBG array according to claim 1, characterized in that: The temperature measuring fiber (11) is a quasi-distributed temperature measuring fiber with multiple temperature measuring gratings written along the path, and the reflectivity R1 is lower than 0.1%; the reflectivity R2 of the reference grating is higher than 99%; the wavelengths of all the measurement gratings of the temperature measuring fiber (11) and the initial wavelength of the reference grating (10) are all λ1 at 25℃.

4. The liquid level measurement system based on the same-wavelength FBG array according to claim 1, characterized in that: The liquid storage container (14) is a circular, square or rectangular sealed container, and a through hole corresponding to the insertion of the temperature measuring fiber (11) and the reference grating (10) is formed at the top of the liquid storage container (14); after the temperature measuring fiber (11) and the reference grating (10) are inserted into the liquid storage container (14), they are sealed and fixed at the through hole at the top of the liquid storage container (14) through the flanges (9).

5. The liquid level measurement system based on the same-wavelength FBG array according to claim 1, characterized in that: The temperature measuring fiber (11) and the reference grating (10) are directly inserted into the liquid storage container (14), and the bottom is flush with the bottom surface of the liquid storage container (14); one reference grating (10) and one or more temperature measuring fibers (11) are inserted into each liquid storage container (14); the multiple temperature measuring fibers (11) do not need to be vertically installed in the liquid storage container (14), and multiple measuring points on the multiple temperature measuring fibers (11) are arranged in parallel or intersected.

6. A liquid level measurement system based on a same-wavelength FBG array according to claim 5, characterized in that: The FBG demodulator module further comprises a beam splitter (12) and an optical switch (13), and a plurality of circulators (5) and couplers (7) are arranged, the beam splitter (12) divides the light source (4) into multiple paths, and can simultaneously provide light sources for the temperature measuring fibers and the reference fibers in multiple different liquid storage tanks, the reflected light of each path is transmitted to the photodiode (6) for photoelectric conversion after passing through a circulator (5) and a coupler (7), the converted electrical signal is converted by the digital-to-analog conversion module (3) for light intensity signal acquisition, and then is transmitted to the processing module (1) for processing and analysis; and the optical switch (13) is used to ensure that there is no cross-influence between channels, so that multiple channels of one sensing system can simultaneously monitor multiple different environments.

7. A method for liquid level measurement based on a same-wavelength FBG array, characterized in that, The measurement steps are as follows: S1. One reference grating and one or more temperature measuring fibers are installed in the liquid storage tank, the temperature measuring fiber is a quasi-distributed temperature measuring fiber with multiple temperature gratings marked along the path, and the reflectivity R1 is less than 0.1%; the reference grating is a single-measuring-point FBG, the center wavelength is consistent with the temperature measuring fiber, and the reflectivity R2 of the reference grating is higher than 99%; the measuring point position of the reference grating is the same as the first measuring point position of the temperature measuring fiber inserted into the bottom of the liquid storage container; S2. The signal output ends of the reference grating and the temperature measuring fiber are connected to the FBG demodulator module through an optical cable, the FBG demodulator module is in communication connection with the processing module; the FBG demodulator module comprises a digital-to-analog conversion module, a light source, a circulator, a photodiode and a coupler; S3. The light emitted by the light source simultaneously enters the temperature measuring fiber and the reference grating to generate reflected light, the reflected light is transmitted to the photodiode for photoelectric conversion after passing through the circulator, the converted electrical signal is converted by the digital-to-analog conversion module for light intensity signal acquisition, and then is transmitted to the processing module for processing and analysis, and the liquid level measurement value in the liquid storage container is calculated; the specific calculation process is as follows: When there is liquid in the container, the reflected light intensity I read by it R : I R = [(1 - R2) m-n IR2+…+(1 - R2) m-1 IR2]R1, n ≥ 1 ① In the formula, R1 is the reflectivity of the reference grating, and R2 is the reflectivity of the measuring grating; d is the grating spacing in the FBG array of a single temperature measuring fiber; m is the total number of grating points in the FBG array of a single temperature measuring fiber; n is the number of grating points of a single temperature measuring fiber immersed in the liquid; I is the light intensity of the broadband light source; The temperature resolution of the temperature measuring fiber and the fiber temperature monitoring system reaches 0.1℃; When there is no liquid in the container, n = 0, the reflected light intensity I R = I, at this time, there is no need to measure the liquid level of the container; since the reflectivity of the measurement grating is much smaller than that of the reference grating, (1-R2) is approximately 1; and n is not 0, the reflected light intensity calculation formula ① can be simplified as: I R = nIR1R2; According to formula ①, the reflected light intensity I is obtained R Then, the liquid level measurement value in the liquid storage tank is calculated according to formula ② as wherein L L is the liquid level measurement in the storage tank; d is the distance between two grating points of a single temperature grating; α is the included angle between the temperature measuring fiber and the vertical line of the tank, when the included angle is π / 2, it is vertical installation; when the light intensity output value is I, n=0, and the liquid surface height is 0.

8. The liquid level measurement method based on the same-wavelength FBG array according to claim 7, characterized in that: In the S1 step, the multiple temperature measuring optical fibers are arranged in parallel or cross arrangement; each temperature measuring optical fiber and the reference grating are packaged with a metal sheath and a connector and then inserted into the liquid storage container, and the head inserted into the liquid is completely closed, and the reference grating is arranged at the bottom of the liquid storage container; the temperature measuring optical fiber and the reference grating are installed on the liquid storage tank through flanges.

9. The liquid level measurement method based on the same-wavelength FBG array according to claim 7, characterized in that: The processing module and the FBG demodulator module can simultaneously adjust the liquid level of multiple different liquid storage tanks to realize the measurement of multiple different environments in multiple channels; specifically, the FBG demodulator module is integrated into a demodulation system, multiple circulators and couplers are arranged, a beam splitter and an optical switch are added, the light source is divided into multiple paths by the beam splitter, and the temperature measuring optical fibers and the reference optical fibers in multiple different liquid storage tanks are provided with light sources, respectively, each reflected light is transmitted to a photodiode for photoelectric conversion through a circulator and a coupler, and the optical switch is used to ensure that there is no cross influence between channels, thereby realizing the simultaneous monitoring of multiple different environments in multiple channels of a sensing system.

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