Displacement and liquid level simultaneous measurement method based on optical fiber ring-down cavity spectrum analysis

Abstract

A displacement and liquid level simultaneous measurement method based on optical fiber ring-down cavity spectral analysis comprises the steps that the pulse width and the center wavelength of a pulse light signal are modulated, and the cavity length of a first optical fiber ring-down cavity and the cavity length of a second optical fiber ring-down cavity are set; a pulse signal enters the first optical fiber ring-down cavity and the second optical fiber ring-down cavity through the optical fiber circulator and is converted into a ring-down curve, and then the ring-down curve is received by the optical fiber circulator; all peak values and peak value positions on the ring-down curve are found through a peak searching function; smoothing the curve at the peak value to obtain a smoothed ring-down curve at the peak value; performing fast Fourier transform on the ring-down curve, and extracting nodes corresponding to all peak values; linear fitting is conducted on nodes at the same interval, and a first fitting signal of the displacement amount and a first fitting signal of the liquid level amount are obtained; according to the variable quantity of the first fitting signal and the variable quantity of the second fitting signal, the displacement variable quantity and the liquid level variable quantity are obtained through calculation, and high-sensitivity simultaneous measurement of the displacement and the liquid level is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber sensing, and in particular to a method for simultaneously measuring displacement and liquid level based on optical fiber ring-down cavity spectrum analysis. Background Art

[0002] Traditional electronic fiber optic displacement sensors measure displacement based on changes in capacitance. They are generally composed of fixed electrodes and moving electrodes. When the moving electrode moves, the capacitance value changes, and the displacement is calculated by measuring the change in capacitance value. They usually require electronic components and precision manufacturing, so the cost is high, and the operating temperature range is limited. They cannot work in extreme high or low temperature environments. Traditional electronic fiber optic liquid level sensors use optical principles to measure the liquid level of liquids or media. Their sensitivity is affected by the attenuation of light signals and the fluctuation of light sources. The Mach-Zehnder structure is a sensor system that uses the principle of optical interference to measure displacement or liquid level changes. Its principle is based on the interference effect of light. It measures the displacement or liquid level of the measured body through the change in the interference pattern caused by the displacement in the optical path. It has high requirements for environmental conditions such as temperature, mechanical vibration, and stability of the optical path length. Summary of the invention

[0003] In view of the problems in the prior art, the present invention provides a method for simultaneous measurement of displacement and liquid level based on fiber ring-down cavity spectrum analysis, aiming to improve the accurate measurement of displacement and level and avoid the influence of light source fluctuation and system noise.

[0004] A method for simultaneously measuring displacement and liquid level based on fiber ring-down cavity spectrum analysis comprises a fiber circulator, wherein the output end of the fiber circulator is connected to the input end of the circulator through a first optical coupler, a first optical fiber extension line, a displacement sensing unit and a second optical coupler in sequence, and the first optical coupler is also connected to the second optical coupler; the first optical coupler is also connected to the second optical coupler through a third optical coupler, a second optical fiber extension line, a liquid level sensing unit and a fourth optical coupler, and the fourth optical coupler is also connected to the third optical coupler, wherein the first optical coupler, the first optical fiber extension line, the displacement sensing unit and the second optical coupler form a first fiber ring-down cavity, the third optical coupler, the second optical fiber extension line, the liquid level sensing unit and the fourth optical coupler form a second fiber ring-down cavity, and the cavity length of the first fiber ring-down cavity is less than the cavity length of the second fiber ring-down cavity; the method for simultaneously measuring displacement and liquid level comprises the following steps:

[0005] Step 1: modulate the pulse width and central wavelength of the pulsed optical signal, and set the cavity lengths of the first fiber ring-down cavity and the second fiber ring-down cavity;

[0006] Step 2: The pulse signal enters the first fiber ring-down cavity and the second fiber ring-down cavity through the fiber circulator and is converted into a ring-down curve, which is then received by the fiber circulator;

[0007] Step 3: Find all peaks and peak positions on the ring-down curve through the peak finding function;

[0008] Step 4: After smoothing the curve at the peak, a smoothed ring-down curve at the peak is obtained;

[0009] Step 5: Perform fast Fourier transformation on the ring-down curve in step 4 and extract the nodes corresponding to all peak values;

[0010] Step 6: Perform linear fitting on the nodes at the same interval to obtain a first fitting signal of the displacement and a first fitting signal of the liquid level;

[0011] Step 7: According to the change amount of the first fitting signal and the change amount of the second fitting signal, the displacement change amount and the liquid level change amount are calculated.

[0012] Further, the optical pulse signal emitted by the pulse light source module enters the first optical fiber coupler after passing through the 1000m optical fiber extension line and the optical fiber circulator, 1% of the output of the first optical fiber coupler enters the second optical fiber coupler through the 65m first optical fiber extension line and the displacement sensing unit in sequence, 99% of the output of the second optical fiber coupler enters the first optical coupler, and 1% of the output of the second optical fiber coupler is output through the optical fiber circulator; 99% of the output of the first optical coupler enters the third optical coupler, 99% of the output of the third optical coupler is connected to the air, 1% of the output of the third optical fiber coupler enters the fourth optical fiber coupler after passing through the 125m second optical fiber extension line and the liquid level sensing unit in sequence, 99% of the output of the fourth optical fiber coupler returns to the third optical fiber coupler, and 1% of the output of the fourth optical fiber coupler is sent to the second optical fiber coupler.

[0013] Further: the displacement sensing unit includes a first single-mode optical fiber and a displacement testing device for causing the first single-mode optical fiber to deform, the displacement testing device includes a top plate and a bottom plate arranged in parallel up and down, a vertical guide rod is arranged between the top plate and the bottom plate, the vertical guide rod is fixedly connected to the bottom plate, the top plate is slidably arranged on the vertical guide rod and is lifted and lowered along the vertical guide rod, a return spring is movably sleeved on the vertical guide rod, and the two ends of the return spring are respectively abutted against the top plate and the bottom plate; the first single-mode optical fiber is passed through the top plate and the bottom plate, and pressure plates are arranged between the top plate and the first single-mode optical fiber and between the first single-mode optical fiber and the bottom plate, the pressure plates are fixedly connected to the corresponding top plate and the bottom plate, the pressure plates are arranged at intervals along the long direction of the first single-mode optical fiber and are used to abut against the first single-mode optical fiber, and when the distance between the top plate and the bottom plate is reduced, the pressure plates are interlaced and squeezed to squeeze the first single-mode optical fiber and make it wavy.

[0014] Further: the liquid level sensing unit includes a second single-mode optical fiber and a liquid level testing device for deforming the second single-mode optical fiber, the liquid level testing device includes a base for sinking to the bottom of the water, two foam blocks are fixedly connected to the base, a gap is left between the two foam blocks, the second single-mode optical fiber is provided with a tapered area, the tapered area is located in the gap between the two foam blocks and is suspended, and the second single-mode optical fiber is fixedly connected to the two foam blocks near the two ends of the tapered area.

[0015] Further, the pulse width of the pulse light signal is 100ns and is a square wave, and the central wavelength of the pulse light signal is 1550nm; the cavity length of the first fiber ring-down cavity is set to 65m, and the cavity length of the second fiber ring-down cavity is set to 125m.

[0016] Beneficial effects of the present invention: The present invention adopts a fiber ring-down system and a frequency domain spectrum algorithm to achieve high-sensitivity simultaneous measurement of displacement and liquid level, and the collected digital signals are convenient for remote measurement and intelligent real-time monitoring. The fiber optic displacement sensor and fiber optic liquid level sensor designed by the present invention have low loss, high sensitivity, simple manufacturing process, high mechanical strength and good repeatability. The present invention adopts a spectrum analysis algorithm to achieve the resolution of different cavity lengths, without adding fiber extension lines between cavities for resolving multiple physical quantities, solving the problems of time domain signal aliasing and crosstalk, further simplifying the system and reducing losses. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a flow chart of the present invention;

[0018] Figure 2 A schematic diagram of the connection structure of the first fiber ring-down cavity and the second fiber ring-down cavity in the present invention;

[0019] Figure 3 It is a structural schematic diagram of the displacement sensing unit in the present invention;

[0020] Figure 4 It is a structural schematic diagram of the liquid level sensing unit in the present invention;

[0021] Figure 5 It is a schematic diagram of the structure near the tapered region on the second single-mode optical fiber in the present invention. DETAILED DESCRIPTION

[0022] The present invention is described in detail below in conjunction with the accompanying drawings. The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be interpreted as limiting the present invention. The directional terms such as left, middle, right, top, and bottom in the embodiments of the present invention are only relative concepts or are based on the normal use state of the product, and should not be considered as restrictive.

[0023] A method for simultaneous measurement of displacement and liquid level based on fiber ring-down cavity spectrum analysis, such as Figure 2 As shown, it includes a fiber circulator CIR, the output end of the fiber circulator CIR is connected to the input end of the circulator CIR through the first optical coupler OC1, the first optical fiber extension line SMF1, the displacement sensing unit and the second optical coupler OC2 in sequence, and the first optical coupler OC1 is also connected to the second optical coupler OC2; the first optical coupler OC1 is also connected to the second optical coupler OC2 through the third optical coupler OC3, the second optical fiber extension line SMF2, the liquid level sensing unit and the fourth optical coupler OC4, and the fourth optical coupler OC4 is also connected to the third optical coupler OC3, wherein the first optical coupler OC1, the first optical fiber extension line SMF1, the displacement sensing unit and the second optical coupler OC2 form a first fiber ring-down cavity FLRD1, the third optical coupler OC3, the second optical fiber extension line SMF2, the liquid level sensing unit and the fourth optical coupler OC4 form a second fiber ring-down cavity FLRD2, and the cavity length of the first fiber ring-down cavity FLRD1 is less than the cavity length of the second fiber ring-down cavity FLRD2; combined Figure 1 As shown, the method for simultaneous displacement and liquid level measurement includes the following steps:

[0024] Step 1: modulate the pulse width and center wavelength of the pulse light signal, and set the cavity length of the first fiber ring-down cavity and the second fiber ring-down cavity; to ensure the integrity and rapidity of the ring-down waveform and the data acquisition rate; during the measurement process, it was found that too long cavity length and too short pulse width would cause distortion of the ring-down curve, thereby affecting the detection result, so the pulse light signal for transmission was modulated, and the pulse width was set to 100ns and set to a square wave with a center wavelength of 1550nm; the cavity length of the first fiber ring-down cavity was set to 65m, and the cavity length of the second fiber ring-down cavity was set to 125m;

[0025] Step 2: The pulse signal enters the first fiber ring-down cavity and the second fiber ring-down cavity through the fiber circulator and is converted into a ring-down curve, which is then received by the fiber circulator; after the optical pulse signal emitted by the pulse light source module enters the first fiber coupler through the 1000m fiber extension line and the fiber circulator, 1% of the output of the first fiber coupler enters the second fiber coupler through the 65m first fiber extension line and the displacement sensing unit, 99% of the output of the second fiber coupler enters the first optical coupler, and 1% of the output of the second fiber coupler is output through the fiber circulator; 99% of the output of the first optical coupler enters the third optical coupler, 99% of the output of the third optical coupler is connected to the air, 1% of the output of the third fiber coupler enters the fourth fiber coupler after passing through the 125m second fiber extension line and the liquid level sensing unit, 99% of the output of the fourth fiber coupler returns to the third fiber coupler, and 1% of the output of the fourth fiber coupler is sent to the second fiber coupler;

[0026] Step 3: Find all peaks and peak positions on the ring-down curve through the peak finding function;

[0027] Step 4: After smoothing the curve at the peak, a smoothed ring-down curve at the peak is obtained;

[0028] Use linear interpolation to smooth around the peak. The linear interpolation is as follows:

[0029] Between two data points (x1, y1)(x1, y1) and (x2, y2)(x2, y2), estimate the y value corresponding to a point x between these two points. The linear interpolation value is completed by the following steps: Determine the interpolation range: Make sure that the interpolation point x falls between x1 and x2; Calculate the interpolation formula: The formula for linear interpolation is as follows:

[0030]

[0031] Where y is the estimated value of the point. Check the adjusted curve to ensure that all peaks reach the target height and the curve transition is smooth.

[0032] Step 5: Perform fast Fourier transformation on the ring-down curve in step 4 and extract the nodes corresponding to all peak values;

[0033] Step 6: Perform linear fitting on the nodes at the same interval to obtain a first fitting signal of the displacement and a first fitting signal of the liquid level;

[0034] Step 7: According to the change amount of the first fitting signal and the change amount of the second fitting signal, the displacement change amount and the liquid level change amount are calculated.

[0035] Among them, when setting the displacement sensing unit and the liquid level unit, the beam propagation method (BPM) is used to simulate the light field distribution of the bent optical fiber. The simplified simulation model focuses on the side section of the ring optical fiber sensor because it has the largest bending and uniform curvature radius. The model parameters are set as follows: the refractive index (RI) of the single-mode optical fiber (SMF) is 1.45205 / 1.44681, the diameter is 8.3 / 125μm, and the simulation length of the single-mode optical fiber is set to 35mm. The light field distribution simulation results of the ring structure under different bending radii, where (a)-(f) correspond to bending radii of 12mm, 10mm, 8mm, 6mm, 4mm and 2mm, respectively. When the incident wavelength is 1550nm, as the bending radius decreases, the energy in the core of the single-mode optical fiber gradually leaks into the cladding, prompting the interaction between the cladding mode and the core mode. In the present invention, the optical fiber displacement sensor decreases as the displacement applied by the displacement stage increases. In the process of gradually decreasing the lumbar diameter, although the area of ​​the embellishment field gradually decreases, the intensity of the embellishment field gradually increases, and the transmission loss of the optical fiber becomes larger and larger. We finally determined that the lumbar diameter of 50μm is the appropriate size of the optical fiber liquid level sensor. At this time, the embellishment field intensity and transmission loss are both in the most ideal state. When the input pulse light passes through the optical fiber sensor, as the bending radius decreases and the intensity of the embellishment field increases, more core energy leaks into the cladding, increasing the intracavity loss. The relationship between the displacement and the liquid level and the intracavity loss is determined to achieve the detection of displacement and liquid level.

[0036] like Figure 3 As shown, the displacement sensing unit includes a first single-mode optical fiber 12 and a displacement testing device for deforming the first single-mode optical fiber 12. The displacement testing device includes a top plate 11 and a bottom plate 16 arranged in parallel up and down. A vertical guide rod 14 is arranged between the top plate 11 and the bottom plate 16. The vertical guide rod 14 is fixedly connected to the bottom plate 16. The top plate 11 is slidably arranged on the vertical guide rod 14 and is lifted and lowered along the vertical guide rod 14. A return spring 13 is movably sleeved on the vertical guide rod 14. The two ends of the return spring 13 are respectively abutted against the top plate 11. 1 and the bottom plate 16; the first single-mode optical fiber 12 is arranged between the top plate 11 and the bottom plate 16, and a pressing plate 15 is arranged between the top plate 11 and the first single-mode optical fiber 12 and between the first single-mode optical fiber 12 and the bottom plate 16. The pressing plates 15 are fixedly connected to the corresponding top plate 11 and the bottom plate 16. The pressing plates 15 are arranged at intervals along the long direction of the first single-mode optical fiber 12 and are used to abut on the first single-mode optical fiber 12. When the distance between the top plate 11 and the bottom plate 16 is reduced, the pressing plates 15 are interlaced and squeezed to squeeze the first single-mode optical fiber 15 and make it wavy.

[0037] Among them, combined Figure 4 and Figure 5As shown, the liquid level sensing unit includes a second single-mode optical fiber 21 and a liquid level testing device for deforming the second single-mode optical fiber 21. The liquid level testing device includes a base 23 for sinking to the bottom of water. Two foam blocks 22 are fixedly connected to the base 23. A gap is left between the two foam blocks 22. The second single-mode optical fiber 21 is provided with a tapered area 211. The tapered area 211 is located in the gap between the two foam blocks 22 and is suspended in the air so that it is suspended and not affected by external forces except water pressure. The force generated by the water pressure can cause micro-deformation of the tapered area 211. The second single-mode optical fiber 21 is fixedly connected to the two foam blocks 22 at positions close to both ends of the tapered area 211. The tapered area 211 will cause more energy leakage as the water pressure increases, thereby causing an increase in system loss.

[0038] The top plate of the displacement test device measures the peak slope change within the displacement range of 0-12mm with a step length of 1mm. The liquid level sensor unit is placed in a beaker and water is added to the beaker with a step length of 1cm to measure the peak slope change within the liquid level range of 0-30cm. Each measuring point is tested for 5 minutes and two optical fiber sensors are placed in a temperature box at the same time to measure the change of the ring-down curve at 0-100℃ to eliminate the interference of temperature. When the downward displacement of the top plate becomes larger and larger, the bending radius of the first single-mode optical fiber As the water pressure increases, the loss in the system will increase, and the peak slope of the ring-down pulse will change accordingly; the second single-mode optical fiber will also cause more energy leakage as the water pressure increases, causing the system loss to increase. Finally, the resolved slope and the corresponding parameters are linearly fitted, and the corresponding formula between the displacement X1 and the displacement ring-down envelope slope y1 is y=-52.62X1+125.32; the corresponding formula between the liquid level X2 and the liquid level ring-down envelope slope y2 is: y2=-25.22X2-79.63.

[0039] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for simultaneous measurement of displacement and liquid level based on fiber ring-down cavity spectrum analysis, characterized in that: The invention comprises an optical fiber circulator, wherein the output end of the optical fiber circulator is connected to the input end of the circulator through a first optical coupler, a first optical fiber extension line, a displacement sensing unit and a second optical coupler in sequence, and the first optical coupler is also connected to the second optical coupler; the first optical coupler is also connected to the second optical coupler through a third optical coupler, a second optical fiber extension line, a liquid level sensing unit and a fourth optical coupler, and the fourth optical coupler is also connected to the third optical coupler, wherein the first optical coupler, the first optical fiber extension line, the displacement sensing unit and the second optical coupler form a first optical fiber ring-down cavity, the third optical coupler, the second optical fiber extension line, the liquid level sensing unit and the fourth optical coupler form a second optical fiber ring-down cavity, and the cavity length of the first optical fiber ring-down cavity is smaller than the cavity length of the second optical fiber ring-down cavity; the method for simultaneously measuring displacement and liquid level comprises the following steps: Step 1: modulate the pulse width and central wavelength of the pulsed optical signal, and set the cavity lengths of the first fiber ring-down cavity and the second fiber ring-down cavity; Step 2: The pulse signal enters the first fiber ring-down cavity and the second fiber ring-down cavity through the fiber circulator and is converted into a ring-down curve, which is then received by the fiber circulator; Step 3: Find all peaks and peak positions on the ring-down curve through the peak finding function; Step 4: After smoothing the curve at the peak, a smoothed ring-down curve at the peak is obtained; Step 5: Perform fast Fourier transformation on the ring-down curve in step 4 and extract the nodes corresponding to all peak values; Step 6: Perform linear fitting on the nodes at the same interval to obtain a first fitting signal of the displacement and a first fitting signal of the liquid level; Step 7: According to the change amount of the first fitting signal and the change amount of the second fitting signal, the displacement change amount and the liquid level change amount are calculated.

2. The method for simultaneous measurement of displacement and liquid level based on fiber ring-down cavity spectrum analysis according to claim 1 is characterized in that: After the optical pulse signal emitted by the pulse light source module enters the first optical fiber coupler through the 1000m optical fiber extension line and the optical fiber circulator, 1% of the output of the first optical fiber coupler enters the second optical fiber coupler through the 65m first optical fiber extension line and the displacement sensing unit in turn, 99% of the output of the second optical fiber coupler enters the first optical coupler, and 1% of the output of the second optical fiber coupler is output through the optical fiber circulator; 99% of the output of the first optical coupler enters the third optical coupler, 99% of the output of the third optical coupler is connected to the air, and 1% of the output of the third optical fiber coupler enters the fourth optical fiber coupler after passing through the 125m second optical fiber extension line and the liquid level sensing unit in turn, 99% of the output of the fourth optical fiber coupler returns to the third optical fiber coupler, and 1% of the output of the fourth optical fiber coupler is sent to the second optical fiber coupler.

3. The method for simultaneous displacement and liquid level measurement based on fiber ring-down cavity spectrum analysis according to claim 1 is characterized in that: The displacement sensing unit includes a first single-mode optical fiber and a displacement testing device for deforming the first single-mode optical fiber. The displacement testing device includes a top plate and a bottom plate which are arranged in parallel up and down. A vertical guide rod is arranged between the top plate and the bottom plate. The vertical guide rod is fixedly connected to the bottom plate. The top plate is slidably arranged on the vertical guide rod and is lifted and lowered along the vertical guide rod. A return spring is movably sleeved on the vertical guide rod. The two ends of the return spring abut against the top plate and the bottom plate respectively. The first single-mode optical fiber is passed through the top plate and the bottom plate. A pressure plate is arranged between the top plate and the first single-mode optical fiber and between the first single-mode optical fiber and the bottom plate. The pressure plate is fixedly connected to the corresponding top plate and the bottom plate. The pressure plates are arranged at intervals along the long direction of the first single-mode optical fiber and are used to abut against the first single-mode optical fiber. When the distance between the top plate and the bottom plate is reduced, the pressure plates are interlaced and squeezed to squeeze the first single-mode optical fiber and make it wavy.

4. The method for simultaneous measurement of displacement and liquid level based on fiber ring-down cavity spectrum analysis according to claim 1 is characterized in that: The liquid level sensing unit includes a second single-mode optical fiber and a liquid level testing device for deforming the second single-mode optical fiber. The liquid level testing device includes a base for sinking to the bottom of water. Two foam blocks are fixedly connected to the base, and a gap is left between the two foam blocks. The second single-mode optical fiber is provided with a tapered area, and the tapered area is located in the gap between the two foam blocks and is suspended in the air. The second single-mode optical fiber is fixedly connected to the two foam blocks near the two ends of the tapered area.

5. The method for simultaneous displacement and liquid level measurement based on fiber ring-down cavity spectrum analysis according to claim 1 is characterized in that: The pulse width of the pulse light signal is 100 ns and is a square wave, and the central wavelength of the pulse light signal is 1550 nm; the cavity length of the first fiber ring-down cavity is set to 65 m, and the cavity length of the second fiber ring-down cavity is set to 125 m.

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