A high-precision liquid level measurement device and method based on swept frequency interferometry

Abstract

The present invention discloses a high-precision liquid level measurement device and method based on frequency sweeping interferometry, belonging to the field of optomechanical and electrical integration measurement technology, and is used for high-precision liquid level measurement. The optical path consists of two parts: a frequency sweeping correction optical path and a measurement calibration optical path; the components include a frequency sweeping light source, an optical fiber splitter, a high-speed acquisition card, a computer, an optical fiber circulator, an optical fiber reflector, a Fabry-Perot (FP) etalon, a balanced photodetector (BPD), and a liquid level measurement probe. The FP etalon implements the frequency sweeping correction function, and the measurement calibration optical path switches channels through a one-to-two photoelectric switch, and high-speed signal acquisition is performed through the BPD and a high-speed acquisition card. The measurement process of the present invention does not involve mechanical movement and has the advantage of being maintenance-free. The introduction of the FP etalon for nonlinear frequency sweeping calibration improves measurement accuracy, and under relatively favorable experimental conditions, micron-level measurement accuracy can be achieved. This non-contact measurement method can meet the needs of high-precision liquid level measurement in fields such as chemistry and chemical engineering.

Description

Technical Field

[0001] The invention discloses a high-precision liquid level measuring device and method based on swept frequency interferometry, belonging to the technical field of optomechanical and electrical integration measurement. Background Art

[0002] Liquid level measurement is a crucial metric in the chemical and chemical industry. High-precision level measurement is crucial for efficient production in industries such as chemical and pharmaceuticals. Current level sensor methods primarily include float-type level sensors, sight glass-based level gauges, fiber Bragg grating (FBG)-based level sensors, and ultrasonic-based level sensors. Float-type sensors utilize a lever-like structure consisting of a float and a connecting rod. When the liquid level rises, the float rises, mechanically locking the float and the connecting rod, which in turn generates a corresponding displacement due to the applied force. Float-type level sensors are simple in structure, lack electrical components, and are widely used, but their accuracy is limited. Sight glass-based level gauges use a transparent window and scale lines on the outside of the container to read the liquid level. This method generally provides measurement accuracy in the millimeter range, with a more pronounced upper limit.

[0003] FBG-based liquid level sensors can convert liquid level changes into digital outputs. They are digital sensors whose basic operating principle is based on FBG stress measurement. They generally consist of a light source, an FBG sheet, and a mechanical structure. The mechanical structure is similar to that of a float-type sensor, but primarily converts the liquid level into a stress-strain output. This output acts on the FBG sheet, and the stress change is determined by the change in the FBG's return wavelength, which is then converted into a liquid level change. FBG liquid level sensors offer high accuracy, dynamic measurement capabilities, and no safety risks associated with electrical use. However, they have a limited range and are susceptible to vibration, temperature fluctuations, and other factors. Their measurement resolution is extremely high, but their repeatability is relatively poor. Furthermore, their contact-based measurement method is prone to contamination of the liquid.

[0004] Ultrasonic liquid level sensors utilize the reflection properties of ultrasonic waves at the interface between two media. If the time interval between the emission of an ultrasonic pulse and the reception of the transmitted wave by the receiving transducer is known, the position of the interface can be determined, thereby enabling measurement of the object. Sensors are categorized as single-transducer or dual-transducer, depending on the functions of the transmitting and receiving transducers. Generally, the propagation speed of ultrasonic waves in air is primarily dependent on temperature. Therefore, when the temperature is known, the ultrasonic speed is known, and the measured distance can be determined by simply recording the time from emission to reception. This sensor has a measurement range of 60 mm to 1000 mm, an accuracy of 0.3%, and a linearity of ±0.05% (dependent on the measured length). This sensor is simple to operate, inexpensive, maintains high accuracy even in harsh environments, and is easy to install and maintain. However, the speed of sound is directly related to the density and humidity of the airborne medium, making this method susceptible to temperature and liquid evaporation.

[0005] Swept-frequency interferometry is commonly used in optical coherence tomography (OCT) for medical examinations. It offers the advantages of high resolution, high speed, and non-contact measurement. This invention is the first to utilize this method for high-precision liquid level measurement and incorporates innovative design optimizations in the liquid level measurement probe design and improved repeatability. Summary of the Invention

[0006] The purpose of the present invention is to provide a high-precision liquid level measurement device and method based on swept frequency interferometry to solve the problem of non-contact high-precision measurement (micrometer level) in the field of chemistry and chemical engineering in the prior art.

[0007] A high-precision liquid level measurement device based on swept frequency interferometry includes an optical path and components. The optical path includes a swept frequency correction optical path and a measurement calibration optical path. The components include a swept frequency light source, two optical fiber splitters, a one-to-two photoelectric switch, two optical fiber circulators, an optical fiber reflector, a liquid level measurement probe, a high-speed acquisition card, and a computer.

[0008] The swept frequency correction optical path includes, in sequence, a first fiber circulator, a first fiber reflector, an FP etalon, and a first BPD. The swept frequency light source is connected to the input end of the first fiber circulator via a first fiber splitter. The output end of the first fiber circulator is connected to the first fiber reflector via a connecting optical fiber. The return port of the first fiber circulator is connected in sequence to the FP etalon, the first BPD, and the first port of the high-speed acquisition card.

[0009] The measurement and calibration optical path sequentially includes a second fiber circulator, a one-to-two photoelectric switch, a calibration optical path, a measurement optical path, and a second BPD; the calibration optical path includes a second fiber splitter, a second fiber reflector, and a fixed-length high-precision fiber delay line; the measurement optical path consists of a coated fiber collimator and a liquid level reflection surface. The coated fiber collimator serves as a liquid level measurement probe and is fixed on an electric optical adjustment mount to control light scanning across the liquid surface.

[0010] The swept-frequency light source is connected to the input of the second fiber circulator through the first fiber splitter. When the one-to-two photoelectric switch is set to the measurement light path, the output of the second fiber circulator is connected to the liquid level measurement probe through a long guide fiber. After the light is emitted from the liquid level measurement probe to the liquid surface, the return light returns to the second fiber circulator along the original path through the coated fiber collimator and is connected to the second BPD. Finally, it is connected to the second port of the high-speed acquisition card.

[0011] When the one-to-two photoelectric switch is set to the calibration optical path, the second optical fiber splitter of the calibration optical path splits the optical path into two. One optical path is connected to the second optical fiber reflector and then returns, and the other optical path passes through the fixed-length high-precision optical fiber delay line and then passes through the third optical fiber reflector and returns. The return light of the two optical paths returns to the second optical fiber circulator and then connects to the second BPD.

[0012] A high-precision liquid level measurement method based on swept frequency interferometry, using the high-precision liquid level measurement device based on swept frequency interferometry, the measurement method steps include:

[0013] S1. Adjust the liquid level measuring probe perpendicular to the liquid surface, set the one-to-two photoelectric switch to the measuring light path, and adjust the electric optical adjustment bracket. When the signal intensity amplitude measured by the second BPD is the largest, it indicates that the reflected light intensity is the strongest. At this time, the liquid level measuring probe is perpendicular to the liquid surface.

[0014] S2. Perform calibration. Set the one-to-two photoelectric switch to the calibration optical path. Use a fixed-length, high-precision optical fiber delay line to read the signal data of the frequency sweep correction optical path and the measurement optical path. The high-speed acquisition card simultaneously collects the frequency sweep correction optical path signal and the calibration optical path signal, performs synchronous correction on the measurement signal, and establishes a mapping relationship between the measured calibration optical path frequency and the calibration optical path measurement length:

[0015] ;

[0016] Where, is the frequency, is the proportionality coefficient, is the length of the fixed-length high-precision optical fiber delay line used for calibration;

[0017] S3, measure, turn the one-to-two photoelectric switch to the measuring optical path, the high-speed acquisition card collects the sweep correction optical path signal and the calibration optical path signal at the same time, and performs synchronous correction on the measurement signal. According to the proportional coefficient obtained in S2 And the frequency data obtained by measuring the optical path Determine the distance from the liquid surface to the coated surface of the coated fiber collimator , Equivalent to liquid level information:

[0018] ;

[0019] Complete the measurement process;

[0020] S4. The computer performs calculations and analysis based on the obtained electrical signal, and takes the average value of the liquid level information after multiple measurements;

[0021] S5. The computer automatically stores the liquid level information and displays it on the computer interface in real time.

[0022] Design the frequency sweep correction optical path of the FP etalon and high-speed acquisition card, establish equal frequency interval resampling of the measurement optical path signal through cubic spline interpolation and perform frequency domain analysis;

[0023] A first-order linear fitting relationship between the frequency and time of the extracted swept-frequency light source signal is established and introduced into the frequency domain analysis results to realize the nonlinear correction of the swept-frequency interferometry measurement.

[0024] A self-coherent optical path is designed between the end face of the coated optical fiber collimator and the liquid surface. The length of the self-coherent optical path is less than the coherent length of the swept-frequency light source. At this time, the coherent length is the maximum measurable range of the liquid level measurement.

[0025] Use the electric optical adjustment frame to adjust the vertical return of light at the liquid surface. When the light is injected from the coated fiber collimator perpendicular to the liquid surface and is reflected back vertically, the return light signal intensity is the largest.

[0026] A calibration optical path is designed by combining a one-to-two photoelectric switch and a fixed-length high-precision optical fiber delay line. The frequency signal and calibration length are calibrated in the calibration optical path. Then, the one-to-two photoelectric switch is used to switch to the measurement optical path, realizing calibration and rapid measurement of the measurement process.

[0027] Compared with the existing technology, the present invention has the following beneficial effects: the measurement process has no mechanical moving parts and is maintenance-free, and the non-contact measurement meets the needs of micron liquid level measurement in the chemical and chemical industry; linear frequency sweeping is achieved to ensure measurement stability and measurement accuracy, and rapid calibration of the optical path is achieved; the non-contact optical measurement method does not pollute the liquid surface and will not be corroded by the liquid, avoiding contact with toxic and harmful liquids, and has good environmental adaptability, good portability, and suitability for engineering. It is a better measurement method for liquid level measurement with high precision requirements in chemical reactions such as chemical industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0029] Figure 2 This is a schematic diagram of the swept frequency interferometry measurement principle used in the present invention;

[0030] Figure 3 It is a schematic diagram of the structure of the liquid level measurement probe;

[0031] Figure 4 is the time domain diagram of the measured interference signal;

[0032] Figure 5 It is the beat frequency signal to solve the distance diagram to be measured;

[0033] The accompanying drawings include: 1-sweep frequency light source and first fiber optic splitter, 2-first fiber optic circulator, 3-first fiber optic reflector, 4-FP standard tool, 5-first BPD, 6-one-to-two photoelectric switch, 7-second fiber optic splitter, 8-second fiber optic reflector, 9-fixed-length high-precision fiber optic delay line, 10-third fiber optic reflector, 11-fiber optic collimator, 12-liquid level reflection surface, 13-second fiber optic circulator, 14-second BPD, 15-amplification structure of liquid level measurement probe, 16-high-speed acquisition card, 17-single-mode optical fiber, 18-electric optical adjustment frame. DETAILED DESCRIPTION

[0034] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention are described clearly and completely below. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0035] A high-precision liquid level measurement device based on swept frequency interferometry, such as Figure 1 , including a swept frequency light source and a first optical fiber splitter 1, a second optical fiber splitter 7, a one-to-two photoelectric switch 6, two optical fiber circulators, a swept frequency correction optical path, a measurement and calibration optical path, a high-speed acquisition card 16 and a computer;

[0036] The swept frequency correction optical path includes, in sequence, a first fiber circulator 2, a first fiber reflector 3, an FP etalon 4, and a first BPD 5. The swept frequency light source is connected to the input end of the first fiber circulator 2 via a first fiber splitter. The output end of the first fiber circulator 2 is connected to the first fiber reflector 3 via a connecting optical fiber. The return port of the first fiber circulator 2 is connected in sequence to the FP etalon 4, the first BPD 5, and the first port of the high-speed acquisition card 16.

[0037] The measurement and calibration optical path sequentially includes a second fiber circulator 13, a one-to-two photoelectric switch 6, a calibration optical path, a measurement optical path, and a second BPD 14; the calibration optical path includes a second fiber splitter 7, a second fiber reflector 8, and a fixed-length high-precision fiber delay line 9; the measurement optical path includes a coated fiber collimator 11 and a liquid level reflection surface 12. The coated fiber collimator 11 serves as a liquid level measurement probe and is fixed on an electric optical adjustment mount 18 to control light scanning on the liquid surface.

[0038] The swept-frequency light source is connected to the input end of the second fiber circulator 13 through the first fiber splitter. When the one-to-two photoelectric switch 6 is set to the measurement light path, the output end of the second fiber circulator 13 is connected to the liquid level measurement probe through a long guide fiber. After the light is emitted from the liquid level measurement probe to the liquid surface, the return light returns to the second fiber circulator 13 along the original path through the coated fiber collimator 11, and then enters the second BPD 14, and finally enters the second port of the high-speed acquisition card 16;

[0039] When the one-to-two photoelectric switch 6 is set to the calibration optical path, the second optical fiber splitter 7 of the calibration optical path splits the optical path into two. One optical path is connected to the second optical fiber reflector 8 and then returns. The other optical path passes through the fixed-length high-precision optical fiber delay line 9 and then passes through the third optical fiber reflector 10 and returns. The return light of the two optical paths returns to the second optical fiber circulator 13 and then connects to the second BPD14.

[0040] A high-precision liquid level measurement method based on swept frequency interferometry, using the high-precision liquid level measurement device based on swept frequency interferometry, the measurement method steps include:

[0041] S1. Adjust the liquid level measuring probe perpendicular to the liquid surface, set the one-to-two photoelectric switch 6 to the measuring light path, and adjust the electric optical adjustment bracket 18. When the signal intensity amplitude measured by the second BPD 14 is the largest, it indicates that the reflected light intensity is the strongest. At this time, the liquid level measuring probe is perpendicular to the liquid surface.

[0042] S2. Perform calibration by setting the one-to-two photoelectric switch 6 to the calibration optical path. The signal data of the frequency sweep correction optical path and the measurement optical path are read through the fixed-length high-precision optical fiber delay line 9. The high-speed acquisition card 16 simultaneously collects the frequency sweep correction optical path signal and the calibration optical path signal, performs synchronous correction on the measurement signal, and establishes a mapping relationship between the measured calibration optical path frequency and the calibration optical path measurement length:

[0043] ;

[0044] Where, is the frequency, is the proportionality coefficient, is the length of the fixed-length high-precision optical fiber delay line 9 used for calibration;

[0045] S3, measure, turn the one-to-two photoelectric switch 6 to the measuring optical path, the high-speed acquisition card 16 simultaneously collects the sweep correction optical path signal and the calibration optical path signal, and performs synchronous correction on the measurement signal. According to the proportional coefficient obtained in S2 And the frequency data obtained by measuring the optical path Determine the distance from the liquid surface to the coated surface of the coated optical fiber collimator 11 , Equivalent to liquid level information:

[0046] ;

[0047] Complete the measurement process;

[0048] S4. The computer performs calculations and analysis based on the obtained electrical signal, and takes the average value of the liquid level information after multiple measurements;

[0049] S5. The computer automatically stores the liquid level information and displays it on the computer interface in real time.

[0050] Design the frequency sweep correction optical path of the FP etalon 4 and the high-speed acquisition card 16, establish equal frequency interval resampling of the measurement optical path signal through cubic spline interpolation and perform frequency domain analysis;

[0051] A first-order linear fitting relationship between the frequency and time of the extracted swept-frequency light source signal is established and introduced into the frequency domain analysis results to realize the nonlinear correction of the swept-frequency interferometry measurement.

[0052] The fiber collimator 11 is coated on its end face to realize the self-coherent optical path design between the fiber collimator 11 end face and the liquid surface (that is, the optical path difference between the light reflected from the liquid surface and the light reflected from the fiber coating is less than the coherence length, and the light from the two reflection surfaces will generate optical coherence by itself). The self-coherent optical path length is less than the coherence length of the swept laser. At this time, the coherence length is the maximum measurable range of the liquid level measurement.

[0053] The electric optical adjustment frame 18 is used to adjust the vertical return of light at the liquid surface. When light is injected from the fiber collimator 11 perpendicular to the liquid surface and is reflected back vertically, the return light signal intensity is the largest.

[0054] A calibration optical path is designed by using a one-to-two photoelectric switch 6 and a fixed-length high-precision optical fiber delay line 9. The frequency signal and the calibration length are calibrated in the calibration optical path. Then, the one-to-two photoelectric switch 6 is switched to the measurement optical path to realize calibration and rapid measurement of the measurement process.

[0055] The FP etalon 4 and the high-speed acquisition card 16 collect the process of frequency changing with time, record the relationship between time and frequency, and perform linear correction and time resampling to achieve linear correspondence between frequency and time.

[0056] Specifically, the time resampling is equal optical frequency interval sampling, and the peak value of the transmission spectrum information of the FP etalon 4 is detected to obtain the time interval sequence of adjacent resonance peaks. Assuming that the frequency corresponding to the first transmission spectrum peak point is , No. The peak point corresponds to the frequency , FSR is the free spectral range parameter of FP etalon 4:

[0057] ;

[0058] The time-frequency mapped signal is resampled using spline interpolation with equal frequency intervals to obtain a time series corresponding to the equal frequency intervals. The time series is resampled to the measured signal to achieve linear frequency sweep correction.

[0059] The measurement frequency in the calibration optical path is ,according to Get the proportional coefficient , after resampling, the frequency is obtained , combined with Calculate the distance from the liquid surface to the coating surface of the optical fiber collimator 11 , , completing the liquid level measurement process.

[0060] The liquid level measuring probe of the present invention is installed above the liquid surface, and the amplifying structure 15 of the liquid level measuring probe is as shown in FIG. Figure 3 The fiber collimator 11 is combined with an electric optical adjustment mount 18, which enables automated vertical adjustment of the optical path at the liquid surface scanning position. A reflection-enhancing film is applied to the fiber collimator 11, forming a self-coherent optical path between the film and the liquid level reflective surface 12. The liquid level measurement probe is fixed to the top of the liquid container. The electric optical adjustment mount 18 in the probe enables two-dimensional scanning of light on the liquid surface, ensuring that the light strikes the liquid vertically. The installation requirements for the liquid level measurement probe are to ensure both vertical stability of the probe and horizontal scanning of the laser.

[0061] The present invention performs swept frequency interferometry, such as Figure 2 As shown in Figure 1, two beams of swept-frequency light interfere with each other. is the starting frequency of the swept light source, b is the sweep rate, T is the sweep period, B is the sweep range, is the time delay between the two swept beams, It is the frequency difference between the two sweeping lights. When two beams of light interfere with each other, a difference frequency will be generated. , this frequency and the time delay between the two signals The time delay is positively correlated with the optical path difference, so the distance can be inferred from the frequency difference. Measuring the liquid level involves measuring the distance between the liquid surface and the surface of the fiber collimator 11 coated with an enhanced reflection coating. Using the principle of swept-frequency interferometry, the liquid level can be measured by analyzing the spectrum of the measurement optical path. However, the nonlinearity of the swept-frequency light source can affect the measurement results. The FP etalon 4 is introduced here to complete the nonlinear calibration of the light source. The FP etalon 4 primarily implements the nonlinear calibration of the swept-frequency light source. The coherence length of the swept-frequency light source directly determines the measurable range of the measurement.

[0062] After completing the nonlinear calibration and measurement of the swept-frequency light source using a dual-channel high-speed acquisition card 16 or an oscilloscope, the measured data is subjected to frequency domain analysis and demodulation to infer the liquid level height. The correlation coefficient between frequency and measured distance is related to the swept-frequency light source acquisition equipment and can be obtained through a calibration process. Therefore, the present invention incorporates a calibration optical path selected by the one-to-two photoelectric switch 6. A calibration optical path with a known optical path difference is established using a one-to-two optical path splitter, the one-to-two photoelectric switch 6, and a standard fixed-length optical fiber delay line. This establishes the coefficient of the linear correlation between the optical path difference and the measured frequency signal. By switching the one-to-two photoelectric switch 6 to the measurement optical path, a self-coherent optical path is formed between the liquid level reflective surface 12 and the coating of the fiber collimator 11, and this distance is within the coherence range, resulting in swept-frequency interferometry. By analyzing the measured frequency signal of the swept-frequency interferometry signal in combination with the correlation coefficient, the measured distance can be calculated. The above-mentioned swept-frequency interferometry measurement is performed under ideal swept-frequency interferometry, but the frequency sweeping process of the swept-frequency light source is not an ideal linear sweeping process. The swept-frequency correction optical path of the FP etalon 4 and high-speed acquisition card 16 is designed. Because a swept-frequency light source undergoes a continuous, non-jumpy frequency change, the FP etalon 4 enables equal-frequency sampling of the swept-frequency light source. The measurement optical path signal is then resampled to equal frequency intervals using cubic spline interpolation, and frequency domain analysis is performed. A first-order linear fit relationship between the frequency and time of the extracted swept-frequency light source signal is established and incorporated into the frequency domain analysis results to achieve nonlinear correction for the swept-frequency interferometry measurement.

[0063] After the swept frequency light source passes through the input end of the second fiber circulator 13, the output end of the second fiber circulator 13 is directed to the calibration optical path through the one-to-two photoelectric switch 6; the light in the calibration optical path is split into two by the second fiber splitter 7, one end is connected to the second fiber reflector 8, and the other end passes through the fixed-length high-precision fiber delay line 9 and then returns through the third fiber reflector 10; the two paths of light re-enter the second fiber circulator 13 through the one-to-two photoelectric switch 6 and are converted into analog signals by the second BPD 14, and then received by the high-speed acquisition card 16 and converted into digital signals and transmitted to the computer; they are transmitted and processed simultaneously with the swept frequency correction signal, and after completing the nonlinear swept frequency correction, their measured frequency is calculated. , so we can get , and obtain the proportional coefficient .

[0064] Then, the one-to-two photoelectric switch 6 is switched to the measurement optical path. First, the vertical calibration of the optical path is completed. By debugging the electric optical adjustment frame 18, the return light signal is made the strongest, that is, the signal amplitude collected by the acquisition card is the largest. At this time, the optical path enters the liquid surface vertically and returns vertically. When the optical path is adjusted, the measurement signal result is collected and transmitted and processed simultaneously with the sweep frequency correction signal. After completing the nonlinear sweep frequency correction, the frequency information is obtained. Then, combined with the proportional coefficient Calculate the distance from the liquid surface to the coating surface of the optical fiber collimator 11 , , completing the measurement process.

[0065] In the specific processing of the embodiment, the test process of the swept frequency interferometry liquid level measurement system includes the following steps:

[0066] S1, optical path vertical calibration. Move the one-to-two photoelectric switch 6 to the measurement optical path, and adjust the motorized optical adjustment bracket 18 to maximize the signal strength of the high-speed acquisition card 16, i.e., the optical path returns vertically, completing the optical path vertical calibration.

[0067] S2, calibration and frequency sweep correction. Switch the one-to-two photoelectric switch 6 to the calibration optical path, receiving signals from both optical paths simultaneously. The light source is split into two paths, one entering the frequency sweep correction optical path and the other entering the calibration optical path.

[0068] After the swept frequency light source is connected to the input end of the first fiber circulator 2 via an optical fiber, the output end of the first fiber circulator 2 is reflected back to the first fiber circulator 2 by the fiber reflector through the swept frequency correction optical path; then, the signal is emitted from the first fiber circulator 2 and connected to the FP etalon 4. After that, the signal is received by the first BPD 5 and converted into an analog electrical signal. After being input into the high-speed acquisition card 16, it is converted into a digital electrical signal and input into the computer for processing;

[0069] After the swept frequency light source is connected to the input end of the second fiber circulator 13 through the optical fiber, the output end of the second fiber circulator 13 is directed to the calibration optical path through the one-to-two photoelectric switch 6; the light in the calibration optical path is split into two by the second fiber splitter 7, one end is connected to the second fiber reflector 8, and the other end passes through the fixed-length high-precision fiber delay line 9, and then returns through the third fiber reflector 10; the two paths of light re-enter the second fiber circulator 13 through the one-to-two photoelectric switch 6, return from the output port, and are converted into analog signals by the second BPD 14, and then received by the high-speed acquisition card 16 and converted into digital signals and transmitted to the computer; they are transmitted and processed simultaneously with the swept frequency correction signal, and after completing the nonlinear swept frequency correction, their measured frequency is calculated. , so we can get , and obtain the proportional coefficient .

[0070] S3, measurement and frequency sweep calibration. The light source is split into two: one path enters the frequency sweep calibration optical path, following the same process as in S2; the other path enters the calibration optical path. After the frequency sweep light source is connected to the input end of the first fiber circulator 2 via an optical fiber, the output end of the first fiber circulator 2 is reflected back to the first fiber circulator 2 via the frequency sweep calibration optical path by the fiber reflector. The light is then emitted from the first fiber circulator 2 and connected to the FP etalon 4. The signal is then received by the first BPD 5 and converted into an analog electrical signal. This signal is then input to the high-speed acquisition card 16, converted into a digital electrical signal, and input into a computer for processing.

[0071] After the swept frequency light source is connected to the input end of the second fiber optic circulator 13 through the optical fiber, the output end of the second fiber optic circulator 13 is directed to the measuring optical path through the one-to-two photoelectric switch 6; the light in the measuring optical path passes through the optical fiber collimator 11 of the liquid level measuring probe and is vertically emitted to the liquid level and reflected back. The light re-enters the second fiber optic circulator 13 through the one-to-two photoelectric switch 6 and returns from the output port. It is converted into an analog signal by the second BPD 14 and then received by the high-speed acquisition card 16 and converted into a digital signal and transmitted to the computer; it is transmitted and processed simultaneously with the swept frequency correction signal. After completing the nonlinear swept frequency correction, its measurement frequency is calculated. , combined with the proportional coefficient in S2 , the measured distance can be obtained .

[0072] The wavelength scanning range of the tunable laser of the embodiment of the present invention is 1270nm~1350nm, the sweep frequency is 100kHz, that is, the sweep period is 10μs, and the wavelength tuning rate is 8mm / s. Given a sampling rate of 4GSa / s, the number of sampling points obtained in a single sweep period is 40,000. The measurement interference results after nonlinear calibration of the FP standard 4 are shown as follows: Figure 4 shown.

[0073] According to the calibration The measured interference result is analyzed by fast Fourier transform to obtain the spectrum of the measured interference signal, namely the beat frequency interference signal. The peak point of the spectrum is extracted and the distance value to be measured can be further calculated. The beat signal frequency is proportional to the distance to be measured. To enhance the intuitiveness and readability of the picture, the horizontal axis of the spectrum is processed as the distance value to be measured, such as Figure 5 As shown in the figure, the amplitude is the result of the fast Fourier transform and is a dimensionless quantity. The calculated measurement value is 9.9877mm. Compared with the standard measurement result of 10.0000mm given by the laser interferometer, the error is 12.3μm, which proves that this measurement method has good measurement results in a static working environment.

[0074] In this invention, a self-coherent optical path design is achieved by connecting the liquid level measurement probe via a single-mode optical fiber 17 and using a coating to increase reflection. The measured distance is calculated from the liquid surface to the coated surface of the fiber collimator 11, representing the distance from the liquid level to the sensor. When the liquid level changes, the distance from the liquid level to the reflective surface also changes, completing the liquid level measurement.

[0075] S4. The computer performs calculations and analysis based on the obtained electrical signal, and takes the average value of the liquid level information after multiple measurements;

[0076] S5. The computer automatically stores the liquid level information and displays it on the computer interface in real time.

[0077] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents, and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high-precision liquid level measuring device based on swept frequency interferometry, characterized in that: It includes optical paths and devices. The optical paths include frequency sweep correction optical paths and measurement calibration optical paths. The devices include frequency sweep light sources, two optical fiber splitters, a one-to-two photoelectric switch, two optical fiber circulators, optical fiber reflectors, a liquid level measurement probe, a high-speed acquisition card, and a computer. The swept frequency correction optical path includes, in sequence, a first fiber circulator, a first fiber reflector, an FP etalon, and a first BPD. The swept frequency light source is connected to the input end of the first fiber circulator via a first fiber splitter. The output end of the first fiber circulator is connected to the first fiber reflector via a connecting optical fiber. The return port of the first fiber circulator is connected in sequence to the FP etalon, the first BPD, and the first port of the high-speed acquisition card. The measurement and calibration optical path sequentially includes a second fiber circulator, a one-to-two photoelectric switch, a calibration optical path, a measurement optical path, and a second BPD; the calibration optical path includes a second fiber splitter, a second fiber reflector, and a fixed-length high-precision fiber delay line; the measurement optical path consists of a coated fiber collimator and a liquid level reflection surface. The coated fiber collimator serves as a liquid level measurement probe and is fixed on an electric optical adjustment mount to control light scanning across the liquid surface. The swept-frequency light source is connected to the input of the second fiber circulator through the first fiber splitter. When the one-to-two photoelectric switch is set to the measurement light path, the output of the second fiber circulator is connected to the liquid level measurement probe through a long guide fiber. After the light is emitted from the liquid level measurement probe to the liquid surface, the return light returns to the second fiber circulator along the original path through the coated fiber collimator and is connected to the second BPD. Finally, it is connected to the second port of the high-speed acquisition card. When the one-to-two photoelectric switch is set to the calibration optical path, the second optical fiber splitter of the calibration optical path splits the optical path into two. One optical path is connected to the second optical fiber reflector and then returns, and the other optical path passes through the fixed-length high-precision optical fiber delay line and then passes through the third optical fiber reflector and returns. The return light of the two optical paths returns to the second optical fiber circulator and then connects to the second BPD.

2. A high-precision liquid level measurement method based on swept frequency interferometry, characterized in that: Using the high-precision liquid level measuring device based on swept frequency interferometry as claimed in claim 1, the measuring method comprises the following steps: S1. Adjust the liquid level measuring probe perpendicular to the liquid surface, set the one-to-two photoelectric switch to the measuring light path, and adjust the electric optical adjustment bracket. When the signal intensity amplitude measured by the second BPD is the largest, it indicates that the reflected light intensity is the strongest. At this time, the liquid level measuring probe is perpendicular to the liquid surface. S2. Perform calibration. Set the one-to-two photoelectric switch to the calibration optical path. Use a fixed-length, high-precision optical fiber delay line to read the signal data of the frequency sweep correction optical path and the calibration optical path. The high-speed acquisition card simultaneously collects the frequency sweep correction optical path signal and the calibration optical path signal, performs synchronous correction on the measurement signal, and establishes a mapping relationship between the measured calibration optical path frequency and the calibration optical path measurement length: ； Where, is the frequency, is the proportionality coefficient, is the length of the fixed-length high-precision optical fiber delay line used for calibration; S3, to measure, turn the one-to-two photoelectric switch to the measuring optical path, the high-speed acquisition card simultaneously collects the sweep correction optical path signal and the measuring optical path signal, and performs synchronous correction on the measuring signal according to the proportional coefficient obtained in S2. And the frequency data obtained by measuring the optical path Determine the distance from the liquid surface to the coated surface of the coated fiber collimator , Equivalent to liquid level information: ； Complete the measurement process; S4. The computer performs calculations and analysis based on the obtained electrical signal, and takes the average value of the liquid level information after multiple measurements; S5. The computer automatically stores the liquid level information and displays it on the computer interface in real time.

3. The high-precision liquid level measurement method based on swept frequency interferometry according to claim 2, characterized in that: Design the frequency sweep correction optical path of the FP etalon and high-speed acquisition card, establish equal frequency interval resampling of the measurement optical path signal through cubic spline interpolation and perform frequency domain analysis; A first-order linear fitting relationship between the frequency and time of the extracted swept-frequency light source signal is established and introduced into the frequency domain analysis results to realize the nonlinear correction of the swept-frequency interferometry measurement.

4. The high-precision liquid level measurement method based on swept frequency interferometry according to claim 3, characterized in that: A self-coherent optical path is designed between the end face of the coated optical fiber collimator and the liquid surface. The length of the self-coherent optical path is less than the coherent length of the swept-frequency light source. At this time, the coherent length is the maximum measurable range of the liquid level measurement.

5. The high-precision liquid level measurement method based on swept frequency interferometry according to claim 4, characterized in that: Use the electric optical adjustment frame to adjust the vertical return of light at the liquid surface. When the light is injected from the coated fiber collimator perpendicular to the liquid surface and is reflected back vertically, the return light signal intensity is the largest.

6. The high-precision liquid level measurement method based on swept frequency interferometry according to claim 5, characterized in that: A calibration optical path is designed by combining a one-to-two photoelectric switch and a fixed-length high-precision optical fiber delay line. The frequency signal and calibration length are calibrated in the calibration optical path. Then, the one-to-two photoelectric switch is used to switch to the measurement optical path, realizing calibration and rapid measurement of the measurement process.

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