A liquid level sensor

Abstract

The present application relates to the technical field of liquid level measurement, and particularly relates to a liquid level sensor, which comprises a first diaphragm, a second diaphragm and a tubular shell, the first diaphragm and the second diaphragm are respectively sealedly connected with two open ends of the shell, the gas pressure of an internal space surrounded by the first diaphragm, the second diaphragm and the shell is the same as the atmospheric pressure; the shell is a rigid member, the first diaphragm and the second diaphragm have the same stress-strain property; the first diaphragm and the second diaphragm are both provided with fiber Bragg gratings on the surface of the internal space, the fiber Bragg gratings are connected with a measuring instrument through an optical fiber, and the liquid level is measured by using the sensitivity of the first diaphragm and the second diaphragm to the internal pressure change of the liquid and the sensitivity of the fiber Bragg gratings to the strain change. The sensor has good reliability and maintainability.

Description

Technical Field

[0001] This invention relates to the field of liquid level measurement technology, and more specifically to a liquid level sensor. Background Technology

[0002] The aircraft fuel measurement system is a key component of the aircraft fuel system. Through this system, the aircraft obtains real-time information on the remaining fuel levels in its fuel tanks and relays this information to the computer and pilot. Based on the remaining fuel levels in each tank, the system performs tasks such as engine fuel supply, active center of gravity control, and fuel loading / unloading management. Accurate fuel level measurement is crucial for precisely calculating aircraft range, improving flight quality, and enhancing flight economy. Currently, the most widely used and technologically mature technology in this field is the capacitive fuel level sensor.

[0003] After long-term development, various fuel level measurement methods have emerged, including float-type measurement technology, magnetostrictive measurement technology, and capacitive measurement technology. Each measurement technology has its own advantages and disadvantages: float-type sensors have a simple measurement structure, but they have a large number of moving parts, resulting in poor stability and maintainability; magnetostrictive sensors have high measurement accuracy and good reliability, but the movable magnetic float may cause jamming, and they are greatly affected by electromagnetic interference; capacitive sensors are currently the most widely used and the most mature technology, with high measurement accuracy, good stability, and good maintainability, but they have poor resistance to electromagnetic interference, and since capacitors are electromagnetic energy storage elements, they cannot be intrinsically safe for measurement in flammable and explosive fuel systems, and condensation from fuel between the two plates can cause false readings or even short circuits.

[0004] To address the aforementioned problems in traditional solutions, efforts should be made to develop fuel level sensors based on new principles. Summary of the Invention

[0005] To address the aforementioned shortcomings of existing technologies, this invention provides a liquid level sensor with good reliability and maintainability.

[0006] Technical solution

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This invention provides a liquid level sensor for measuring the liquid level. The liquid level sensor includes a first diaphragm, a tubular housing, and a second diaphragm arranged sequentially from top to bottom. The first diaphragm and the second diaphragm are respectively connected to both ends of the housing. The first diaphragm, the second diaphragm, and the housing enclose an internal space, the gas pressure of which is the same as the gas pressure of atmospheric space. The housing is a rigid component, and the first diaphragm and the second diaphragm have the same stress-strain properties. Fiber Bragg gratings are respectively disposed at at least one corresponding position on the opposing surfaces of the first diaphragm and the second diaphragm, and the fiber Bragg gratings at the two corresponding positions are used to measure the strain value of the corresponding diaphragm. The liquid level sensor determines the liquid level based on the change in the strain value of the first diaphragm and the second diaphragm.

[0009] Furthermore, the liquid level sensor also includes a solver, wherein the first center wavelength drift of the fiber optic grating at corresponding positions of the first diaphragm and the second diaphragm has two first changes, and the solver is used to calculate the density value of the liquid based on the two first changes of the first diaphragm and the second diaphragm, and to calculate the liquid level value based on the linear relationship between the first center wavelength drift of the second diaphragm and the liquid level.

[0010] Furthermore, the liquid level sensor also includes a temperature compensation plate identical to the second diaphragm. The temperature compensation plate has a reference grating disposed at the same position as the fiber grating of the second diaphragm. The temperature compensation plate is placed at the same temperature as the second diaphragm. The temperature compensation plate is used to obtain a second change in the second center wavelength drift of the reference grating between the first temperature and the second temperature when the strain value is zero.

[0011] Furthermore, the solver is used to calculate a third change in the third center wavelength shift of the second diaphragm between the first temperature and the second temperature based on the second change.

[0012] Furthermore, the solver is also used to obtain a fourth change in the fourth center wavelength shift of the second diaphragm between the first strain and the second strain under the same temperature conditions, and the solver calculates the liquid level value based on the third change and the fourth change.

[0013] Furthermore, the diaphragm is circular, and when there is only one fiber grating, the fiber grating is located at the center point of the corresponding diaphragm;

[0014] When there are multiple fiber Bragg gratings, the fiber Bragg gratings are disposed on both sides of the center point of the corresponding diaphragm, and / or at the edge of the corresponding diaphragm.

[0015] Furthermore, the diaphragm is fixedly connected to or detachably connected to the housing, and both the diaphragm and the housing are made of metal.

[0016] Furthermore, when the diaphragm is detachably connected to the housing, each diaphragm is sealed to the end face of the housing via a clamping device;

[0017] Each of the clamping devices includes a clamping ring and a limiting ring. The end of the housing has a threaded section and a limiting section with at least one key block on its outer surface. The first end of the limiting ring has an annular flange on its inner side and a keyway that mates with the key block at its second end. The flange is used to abut the corresponding diaphragm against the end face of the limiting section.

[0018] Each of the clamping rings has a threaded portion and a force-applying portion for applying preload to the flange of the limiting ring. The threaded portion is connected to a threaded section at the end of the housing via an internal thread. The inner diameter of the force-applying portion is the same as the inner diameter of the flange.

[0019] Furthermore, the force-applying part has at least one guide groove on the end face away from the corresponding diaphragm for the liquid to pass through.

[0020] Furthermore, the sidewall of the housing has two through holes for optical fibers to pass through, and the through holes are also used to communicate with the atmospheric space.

[0021] Beneficial effects

[0022] This invention provides an oil level sensor that, compared with traditional fuel level sensors, has stronger electromagnetic interference resistance in principle, achieves intrinsically safe measurement, is suitable for extreme environments such as high temperature and high pressure, has no moving parts, and has good reliability and maintainability. In addition, this aircraft fuel level sensor also integrates density measurement and temperature compensation functions, improving its practicality. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0024] Figure 1 This is a schematic diagram of the fiber Bragg grating principle;

[0025] Figure 2 This is a schematic diagram of an oil level sensor provided in an embodiment of the present invention;

[0026] Figure 3This is a schematic diagram of the upper clamping ring in an oil level sensor provided in an embodiment of the present invention;

[0027] Figure 4 This is a schematic diagram of the lower clamping ring in an oil level sensor provided in an embodiment of the present invention;

[0028] Figure 5 This is a schematic diagram of the metal housing in an oil level sensor according to an embodiment of the present invention;

[0029] Figure 6 This is a schematic diagram of a limiting ring in an oil level sensor according to an embodiment of the present invention;

[0030] Figure 7 This is a schematic diagram of the fiber optic grating arrangement in an oil level sensor according to an embodiment of the present invention;

[0031] Figure 8 This is a schematic diagram of the fiber optic cable routing and splicing of an oil level sensor according to an embodiment of the present invention;

[0032] Figure 9 This is a schematic diagram of an experiment using an oil level sensor according to an embodiment of the present invention. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0034] First, the measurement principle used in this invention will be explained.

[0035] Fiber Bragg gratings utilize the photosensitivity of optical fiber materials to create a spatial phase grating within the fiber core. Essentially, they form a narrowband filter or mirror within the fiber core. When a broadband beam of light passes through the fiber Bragg grating, wavelengths that satisfy the grating's Bragg condition are reflected, while the remaining wavelengths continue to propagate through the grating. Figure 1 As shown. The fiber gratings studied in this technical solution are all uniformly periodic fiber gratings, and the relationship between the center wavelength of their reflected light and the changes in fiber grating temperature and strain is approximately as follows:

[0036]

[0037] In the formula: α f ξ is the thermal expansion coefficient of the optical fiber; ξ is the thermo-optic coefficient of the optical fiber material; P eLet be the elastic-optical coefficient of the optical fiber material. Therefore, if changes in the external measured quantity cause changes in the temperature and strain of the fiber Bragg grating, the center wavelength of the reflected light will also change accordingly. Utilizing this property, fiber Bragg gratings can be fabricated into pressure-sensitive fuel level sensors. The pressure at the bottom of the liquid is converted into strain acting on the fiber Bragg grating by a pressure-sensitive element for measurement, thereby obtaining the liquid level information.

[0038] See Figure 2-5 An embodiment of the present invention provides a liquid level sensor for measuring the liquid level. The liquid level sensor includes a first diaphragm, a tubular housing 3, and a second diaphragm arranged sequentially from top to bottom. The first and second diaphragms are respectively connected to both ends of the housing 3. The first diaphragm, the second diaphragm, and the housing 3 enclose an internal space, the gas pressure of which is the same as the gas pressure of atmospheric space. The housing 3 is a rigid component, and the first and second diaphragms have the same stress-strain properties. Fiber gratings are respectively arranged at at least one corresponding position on the opposing surfaces of the first and second diaphragms, and the fiber gratings at the two corresponding positions are used to measure the strain value of the corresponding diaphragm. The liquid level sensor determines the liquid level based on the change in the strain value of the first and second diaphragms, and the specific measurement principle is as described above.

[0039] In this embodiment, the liquid level sensor further includes a solver. The first center wavelength drift of the fiber optic grating at corresponding positions on the first and second diaphragms each has two first variations. The solver is used to calculate the density value of the liquid based on the two first variations of the first and second diaphragms, and to calculate the liquid level value based on the linear relationship between the first center wavelength drift of the second diaphragm and the liquid level. The solver enables the present invention to have a density measurement function. Specifically, the solver can calculate the center wavelength shift of the first and second diaphragms caused only by strain changes, expressed as:

[0040]

[0041] Assuming the measuring point, i.e., the location where the fiber grating is attached, is the center point of the diaphragm (r = 0), substituting this into the above formula yields the center wavelength shift Δλ at that measuring point. B0 for

[0042]

[0043] Let p0 represent the gas pressure in the oil-free space inside the fuel tank, p a This indicates the gas pressure inside the sensor.

[0044]

[0045] Considering the sensor's ventilation design, the above equation can be simplified to:

[0046]

[0047] Ultimately, we can obtain:

[0048]

[0049] As can be seen from the above formula, when the parameters of the circular flat diaphragm and the fiber grating are determined, the fuel level value is linearly related to the change in the center wavelength of the reflected light from the fiber grating.

[0050] Finally, since the upper and lower bottom surfaces of the sensor are made of identical diaphragms (first and second diaphragms), the loads p1 and p2 at their respective locations can be simultaneously acquired. According to the principle of hydrostatic pressure, p1 and p2 can be expressed as: p1 = ρgh1, p2 = ρgh2, and the fuel density is... After completing the temperature compensation calculation, it is assumed that p1 and p2 are proportional to the center wavelength shift of the fiber gratings on the two diaphragms, i.e., p1 = K1Δλ B1 p2=K2Δλ B2 When the diaphragm parameters and fiber grating parameters are known, the values ​​of coefficients K1 and K2 are also determined. By combining the height difference between the upper and lower diaphragms and the local acceleration, the fuel density can be calculated by measuring the change in the center wavelength at the corresponding measuring points of the upper and lower diaphragms, thus achieving compensated measurement of fuel density.

[0051] In this embodiment, the solver is used to calculate the third change of the third center wavelength drift of the second diaphragm between the first temperature and the second temperature based on the second change, and to achieve temperature compensation measurement through the third change.

[0052] In this embodiment, the solver is also used to obtain a fourth change in the fourth center wavelength shift of the second diaphragm between the first strain and the second strain under the same temperature conditions. The solver calculates the liquid level value based on the third change and the fourth change. Thus, the measurement of liquid level, temperature, and density is completed.

[0053] In this embodiment, the diaphragm is circular. When there is one fiber Bragg grating, the fiber Bragg grating is located at the center point of the corresponding diaphragm; when there are multiple fiber Bragg gratings, the fiber Bragg gratings are located on both sides of the center point of the corresponding diaphragm, and / or at the edge of the corresponding diaphragm. See also... Figure 7 Both the first and second diaphragms have fiber Bragg gratings at their centers and edges, and the fiber Bragg gratings on the first and second diaphragms are arranged correspondingly. Both the first and second diaphragms have two fiber Bragg gratings at their center and two at their edges, and all the fiber Bragg gratings on each diaphragm are arranged in a straight line.

[0054] Specifically, the first and second diaphragms, acting as pressure-sensing elements, are used for pressure sensing and gas-liquid isolation, converting the pressure outside the sensor into strain on the first and second diaphragms. Assuming the working radius of the circular flat diaphragm is R and its thickness is δ, under the action of a uniform pressure difference Δp on both sides, points at different radial positions of the diaphragm will produce different displacements and strains. The following assumptions are made regarding this circular flat diaphragm:

[0055] ① The overall thickness of the flat diaphragm will not change, and the deformation of the entire diaphragm is symmetrical.

[0056] ②The central axis position of the circular flat diaphragm remains unchanged.

[0057] ③ The strain of the diaphragm in its axial direction can be ignored.

[0058] According to the principles of mechanics of materials, for a circularly fixed diaphragm, the radial strain ε of the diaphragm surface at a distance r from the center point along the radial direction is... r The relationship between the pressure difference and the pressure on the diaphragm is as follows:

[0059]

[0060] In the formula: E is the elastic modulus of the circular flat diaphragm (Pa), μ is the Poisson's ratio of the circular flat diaphragm, R is the working radius of the circular flat diaphragm (m), δ is the thickness of the circular flat diaphragm (m), and r is the radius of the measuring point (m).

[0061] Furthermore, the working diameter of the diaphragm was selected to be 80 mm, the diaphragm thickness to be 0.1 mm, and 304 stainless steel was chosen as the processing material for the experimental circular flat diaphragm, with an elastic modulus of 190 GPa and a Poisson's ratio of 0.3. Based on the theoretical analysis and Abaqus finite element simulation of the strain sensitivity of the first and second diaphragms, it can be seen that the radial strain sensitivity is greatest at the center point and edge point of both the first and second diaphragms. Figure 7 In this design, the fiber Bragg gratings are mounted using four measuring points on each sheet, with two points at the center and two at the edge. The fiber Bragg gratings are arranged in a straight line. Furthermore, to address the placement of the splice rod within the sensor's internal space, the optical fiber is led out from inside the sensor through a vent, and the splicing is performed externally. A schematic diagram of the fiber routing and splicing process for one set of fiber Bragg gratings is shown below. Figure 8 As shown, every two fiber Bragg gratings are fused to one fiber, and a total of four fusion splices are performed.

[0062] In this embodiment, the diaphragm is fixedly or detachably connected to the housing, and both the diaphragm and housing are made of metal. When the diaphragm is detachably connected to the housing, each diaphragm is sealed to the end face of the housing via a clamping device. Each clamping device includes a clamping ring and a limiting ring. The end of the housing has a threaded section and a limiting section with at least one key on its outer surface. The first end of the limiting ring has an annular flange on its inner side, and the second end has a keyway that mates with the key. The flange is used to abut the corresponding diaphragm against the end face of the limiting section. Each clamping ring has a threaded portion and a force-applying portion for applying preload to the flange of the limiting ring. The threaded portion is connected to the threaded section of the housing end via an internal thread, and the inner diameter of the force-applying portion is the same as the inner diameter of the flange. (See also...) Figure 2 and Figure 6 The sensor also includes two annular limiting rings 4, so that the upper clamping ring 1 and the lower clamping ring 2 are respectively fixed to the housing 3 through the limiting rings 4. Small holes are opened on the side wall of the metal housing 3 for optical fiber routing. Figure 5 and Figure 6 The sensor consists of a metal housing 3 and a limiting ring 4, which are fitted together using a keyway structure. Two small holes are symmetrically located on the side wall of the metal housing 3 for ventilation and fiber optic cable routing. The distance between opposite sides of the metal housing 3 is 101 mm, and its height is 40 mm; the limiting ring has an outer diameter of 93.2 mm, an inner diameter of 80 mm, and a height of 5.3 mm.

[0063] In this embodiment, the side wall of the housing 3 has two symmetrical small holes, which are respectively connected to a straight connector and a T-connector. One branch of the T-connector is used for fiber optic cable connection, and the other branch is used for ventilation between the sensor's internal space and the external atmosphere. Further, the other branch of the T-connector connects to a vent connector, which, in conjunction with a plastic hose, connects the internal gas space of the aircraft fuel level sensor to the external atmosphere, ensuring that the internal gas pressure of the aircraft fuel level sensor is consistent with the gas pressure in the empty space of the fuel tank. Specifically, the metal housing 3 is designed with through holes. One through hole uses a straight connector to connect the fiber optic cable; the other through hole uses a T-connector, one branch of which is used for fiber optic cable connection, and the other branch is used for ventilation between the sensor's internal space and the external atmosphere. The pagoda-shaped vent connector used in this technical solution, combined with a plastic hose, connects the internal gas space of the sensor to the external atmosphere, ensuring that the internal gas pressure of the sensor is consistent with the gas pressure in the empty space of the fuel tank. This ensures that the pressure difference information sensed by the sensor is only related to the fuel level, thus achieving pressure compensation for the sensor.

[0064] In this embodiment, the end face of the force-applying part opposite to the corresponding diaphragm has at least one guide groove for liquid to pass through. See also... Figure 3 and Figure 4Multiple guide grooves are evenly distributed on the upper clamping ring 1 and the lower clamping ring 2. By opening evenly distributed guide grooves on the upper clamping ring 1 and the lower clamping ring 2, fuel passages are provided to prevent fuel from remaining in the groove of the sensor.

[0065] To better illustrate the invention, the inventors also designed a calibration and measurement experimental system to match the sensor, and used this system to complete the static calibration experiment of the sensor, the liquid level measurement experiment, and the temperature compensation experiment in combination with the Optigrating simulation software.

[0066] First, the calibration and measurement experimental system of this technical solution will be described. The main components of the experimental system include: sensors, water tank, standard water level gauge, water pump, fiber optic demodulator, network cable, and host computer. Its principle is as follows: Figure 9 As shown. The main measuring instrument used in this platform is the DI522-8 fiber optic demodulator, with a minimum resolution of 1 μm. The standard water level gauge is a hydrostatic split-type liquid level sensor with a measurement accuracy of 0.2%, responsible for indicating the actual water level value in the tank during the measurement process. The sensor is connected to the fiber optic demodulator via a fiber optic patch cord and a fiber optic FC connector. The fiber optic demodulator communicates with the host computer using the UDP communication protocol, and the two are connected via a network cable using the network port.

[0067] The technical effects of this design scheme in static calibration experiments, liquid level measurement experiments, density calculation and temperature compensation experiments will be described in detail below.

[0068] 1. Static calibration experiment

[0069] The static calibration experiment aimed to determine the correspondence between the center wavelength of the sensor at each measuring point and the water level value, with the measurement range being 0-500 mm. During the experiment, the water level was measured starting from 0 mm, using the reading of a standard sensor as the standard liquid level value. The experiment used 50 mm intervals as measurement segments, recording the center wavelength results of each fiber optic grating of the fiber optic sensor when the standard sensor reading was an integer multiple of 50 mm. Throughout the static calibration process, the sensor diaphragm remained in the experimental water tank, and its reading fluctuated between 1546.227 nm and 1546.233 nm. Since the theoretical temperature sensitivity of the fiber optic grating is 10.3 pm / ℃, and the temperature change does not exceed 0.6℃, the influence of temperature change can be ignored during data processing.

[0070] According to the sensor performance testing standards, this experiment conducted three sets of forward and reverse stroke calibration experiments on each sensor, using the average of the three center wavelengths at the liquid level measurement point as the output result for fitting. The linear relationships obtained from the forward and reverse stroke fitting for measurement points 2 and 3 are as follows:

[0071] λ P2=0.002057*h+1532.134 R 2 =0.9986

[0072] λ R2 =0.002046*h+1532.151 R 2 =0.9979

[0073] λ P3 =0.002035*h+1534.336 R 2 =0.9984

[0074] λ R3 =0.002023*h+1534.356 R 2 =0.9973

[0075] Based on the fitting results, the actual water level measurement sensitivities at the center points (measuring points 2, 3, 6, and 7) of the improved sensor are approximately 2.05 pm / mm, 2.03 pm / mm, 1.97 pm / mm, and 1.96 pm / mm, respectively, while the actual water level measurement sensitivities at the edge points (measuring points 1, 4, 5, and 8) are 2.77 pm / mm, 2.81 pm / mm, 2.67 pm / mm, and 2.57 pm / mm, respectively. Using calibration data from the center points of the lower diaphragm (measuring points 2 and 3), the sensor's characteristics, including linearity, hysteresis, repeatability, and overall error, were calculated. The results show that the linearity of the two measuring points in both forward and reverse strokes is 0.69%, 1.68%, 1.09%, and 3.42%, respectively; the repeatability errors are 1.59% and 1.71%, respectively; the hysteresis errors are 1.18% and 1.14%, respectively; and the overall errors are 4.10% and 4.84%, respectively.

[0076] 2. Liquid level measurement experiment

[0077] The purpose of the liquid level measurement experiment with the sensor is to obtain the sensor's liquid level measurement accuracy. In this technical solution, the liquid level measurement experiment is conducted under the same environment as the liquid level calibration experiment. The liquid level measurement experiment starts from a liquid level of 0 mm, and a measurement point is selected every 80 mm, that is, measurements are taken at six points: 0.0 mm, 80.0 mm, 160.0 mm, 240.0 mm, 320.0 mm, 400.0 mm, and 480 mm. The measured value at each point is calculated using the relationship obtained in the calibration experiment. By comparing the calculated measured value with the standard liquid level value, the sensor's measurement accuracy is calculated. The results are shown in the table below.

[0078]

[0079]

[0080] Using the ratio of measurement error to measurement range as the relative error, it can be seen that the maximum relative errors of sensor measuring points 2 and 3 are 0.72% and 0.83%, respectively.

[0081] 3. Density Calculation and Temperature Compensation Experiment

[0082] The density calculation and temperature compensation experiments were conducted to verify the various compensation functions of the sensor.

[0083] Data from measuring points 2 and 6 on the upper and lower diaphragms at a liquid level of 400 mm were used for density compensation calculation. The center wavelength shifts at these two points relative to the 0 mm liquid level were 0.819 nm and 0.723 nm, respectively. The calculated medium density at this point was 0.962 g / cm³. 3 With the actual density of water being 1 g / cm³ 3 The calculation shows a relative error of 3.8%, which indicates that the sensor can effectively achieve density compensation.

[0084] Optigrating is an integrated fiber grating design software that enables grating analysis. In this design scheme, this software is used to perform temperature compensation simulation verification. The specific process is as follows:

[0085] (1) Fiber optic setup

[0086] A single-mode fiber was created as a reference fiber, and its parameters were adjusted to match those of the compensation diaphragm fiber. Another single-mode fiber was created as the sensing fiber, and its parameters were adjusted to match those of the fiber at measuring point 2 in the liquid level measurement experiment. The initial strain value was set to zero, and the reference temperature to 25℃. The measured center wavelengths of the two fibers were 1546.184 nm and 1532.144 nm, respectively.

[0087] (2) Temperature calibration and curve fitting

[0088] Starting from 100℃, a test temperature was set every 5℃ downwards, with a minimum of 25℃. The strain value was kept at zero. The center wavelengths of the reference fiber and the sensitive fiber were tested, and the simulation results were fitted using the least squares method to obtain a quadratic function relationship between the center wavelength drift of the two fibers and the temperature change.

[0089] (3) Liquid level measurement

[0090] The test temperatures were set to 35℃ (△T=10℃), 45℃ (△T=20℃), 55℃ (△T=30℃), 65℃ (△T=40℃), and 75℃ (△T=50℃). At each temperature, the liquid level (strain value) was changed, and the center wavelength of the two fiber gratings was measured. First, based on the center wavelength drift of the reference fiber, the △T and the center wavelength drift of the sensitive fiber caused by △T were calculated using the fitted curve. Then, the center wavelength drift of the sensitive fiber caused by strain was calculated. Finally, the fitted curve of measuring point 2 in the static calibration experiment was substituted to obtain the measured liquid level, and the relative error was calculated. After compensation, the maximum positive relative error was 0.57%, and the maximum negative relative error was -0.51%.

[0091] In summary, this invention proposes an oil level sensor that, compared to traditional fuel level sensors, possesses stronger electromagnetic interference resistance in principle, achieves intrinsically safe measurement, is suitable for extreme environments such as high temperature and high pressure, and has no moving parts, exhibiting good reliability and maintainability. Furthermore, the sensor integrates density measurement and temperature compensation functions, enhancing its practicality. Simultaneously, this invention established a calibration and measurement experimental platform for the sensor, completing static calibration experiments and liquid level measurement experiments. Temperature compensation experiments were also conducted using Optigrating simulation software. Experimental results demonstrate that the sensor design is reasonable, has good accuracy, and possesses practical application value.

[0092] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions will not cause the essence of the corresponding technical solutions to deviate from the protection scope of the technical solutions of the embodiments of the present invention.

Claims

1. A liquid level sensor, characterized in that, The liquid level sensor is used to measure the liquid level. It includes a first diaphragm, a tubular housing, and a second diaphragm arranged sequentially from top to bottom. The first diaphragm and the second diaphragm are respectively connected to both ends of the housing. The first diaphragm, the second diaphragm, and the housing enclose an internal space. The gas pressure in the internal space is the same as the gas pressure in the atmosphere. The housing is a rigid component, and the first diaphragm and the second diaphragm have the same stress-strain properties. A fiber grating is disposed at at least one corresponding position on the opposing surfaces of the first diaphragm and the second diaphragm, and the fiber gratings at the two corresponding positions are used to measure the strain value of the corresponding diaphragm; The liquid level sensor determines the liquid level based on the change in strain values ​​of the first diaphragm and the second diaphragm. Both the first diaphragm and the second diaphragm are circular metal diaphragms; Two fiber gratings are set at the center point of the first diaphragm and two fiber gratings are set at the edge of the second diaphragm, and all fiber gratings are arranged in a straight line. The liquid level sensor also includes a solver. The first center wavelength drift of the fiber optic grating at the corresponding positions of the first diaphragm and the second diaphragm has two first changes. The solver is used to calculate the density value of the liquid based on the two first changes of the first diaphragm and the second diaphragm, and to calculate the liquid level value based on the linear relationship between the first center wavelength drift of the second diaphragm and the liquid level. The liquid level sensor also includes a temperature compensation plate identical to the second diaphragm. The temperature compensation plate has a reference grating set at the same position as the fiber grating of the second diaphragm. The temperature compensation plate is placed at the same temperature as the second diaphragm. The temperature compensation plate is used to obtain a second change in the second center wavelength drift of the reference grating between the first temperature and the second temperature when the strain value is zero. The solver is used to calculate, based on the second change, a third change in the third center wavelength shift of the second diaphragm between the first temperature and the second temperature; The solver is also used to obtain a fourth change in the fourth center wavelength shift of the second diaphragm between the first strain and the second strain under the same temperature conditions, and the solver calculates the liquid level value based on the third change and the fourth change.

2. The liquid level sensor according to claim 1, characterized in that, When there is only one fiber grating, the fiber grating is located at the center point of the corresponding diaphragm; When there are multiple fiber Bragg gratings, the fiber Bragg gratings are disposed on both sides of the center point of the corresponding diaphragm, and / or at the edge of the corresponding diaphragm.

3. The liquid level sensor according to claim 2, characterized in that, The diaphragm is fixedly connected to or detachably connected to the housing, and both the diaphragm and the housing are made of metal.

4. The liquid level sensor according to claim 3, characterized in that, When the diaphragm is detachably connected to the housing, each diaphragm is sealed to the end face of the housing via a clamping device; Each of the clamping devices includes a clamping ring and a limiting ring. The end of the housing has a threaded section and a limiting section with at least one key block on its outer surface. The first end of the limiting ring has an annular flange on its inner side and a keyway that mates with the key block at its second end. The flange is used to abut the corresponding diaphragm against the end face of the limiting section. Each of the clamping rings has a threaded portion and a force-applying portion for applying preload to the flange of the limiting ring. The threaded portion is connected to a threaded section at the end of the housing via an internal thread. The inner diameter of the force-applying portion is the same as the inner diameter of the flange.

5. The liquid level sensor according to claim 4, characterized in that, The force-applying part has at least one flow channel on the end face away from the corresponding diaphragm for the liquid to pass through.

6. The liquid level sensor according to claim 1, characterized in that, The sidewall of the housing has two through holes for optical fibers to pass through, and the through holes are also used to communicate with the atmospheric space.

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