CN109374089B - Optical fiber sensing system for simultaneously measuring liquid level and liquid temperature and measuring method thereof - Google Patents
Optical fiber sensing system for simultaneously measuring liquid level and liquid temperature and measuring method thereof Download PDFInfo
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- 239000013307 optical fiber Substances 0.000 title claims abstract description 73
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
- G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
- G01F23/28—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring the variations of parameters of electromagnetic or acoustic waves applied directly to the liquid or fluent solid material
- G01F23/284—Electromagnetic waves
- G01F23/292—Light, e.g. infrared or ultraviolet
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
- G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
- G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
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Abstract
The invention discloses an optical fiber sensing system for simultaneously measuring liquid level and liquid temperature and a measuring method thereof, wherein the optical fiber sensing system comprises a threaded packaging pipe, a microstructure optical fiber and a coherent phase demodulating device, wherein the microstructure optical fiber is embedded into a threaded groove of the threaded packaging pipe, one end of the microstructure optical fiber is connected into the coherent phase demodulating device, and the other end of the microstructure optical fiber is connected into measured liquid; the surface of the threaded packaging tube is coated with a layer of soft glue for packaging and protecting the microstructure optical fiber; the coherent phase demodulation device is used for demodulating distributed sensing signals in the microstructure optical fiber, measuring the temperature response of each independent sensing section in the microstructure optical fiber and calculating the liquid level and the liquid temperature. The invention roughly positions the liquid level height according to the sensing section at the position of the liquid level temperature sudden change, and precisely positions the liquid level height according to the relation between the phase change of the sensing section and the liquid level height, thereby realizing fully distributed sensing, and having large system response dynamic range, high measurement precision, fast response and wide application range.
Description
Technical Field
The invention relates to the field of optical fiber sensing, in particular to an optical fiber sensing system for simultaneously measuring liquid level and liquid temperature and a measuring method thereof.
Background
Liquid level and temperature monitoring are very important in the fields of petroleum transportation and storage, chemical processing, flood early warning, wastewater treatment and the like. For example, in an oil storage tank, the accurate measurement of the liquid level and the oil storage temperature can know the production and storage conditions in real time, ensure safe and stable production and storage, and greatly influence inventory management and economic operation. The level sensors currently in the mainstream are based on electrical sensing. For example, existing capacitive sensors require metal capacitive plates and metal wiring to be placed within the can. Such electrical wiring requires careful shielding, bonding, and grounding to minimize parasitic capacitance, and further requires periodic maintenance to ensure the integrity of the electrical contacts. Secondly, the capacitance probe also needs temperature sensor input for measuring, so as to improve the liquid level measuring accuracy, and safety accidents are easily caused once an electric sensor fails in a flammable and explosive environment.
The optical fiber sensor can be normally used in severe environment due to the characteristics of small volume, corrosion resistance, electromagnetic interference resistance, easy networking and multiplexing and the like, and is more and more emphasized in the field of sensors. The existing optical fiber liquid level sensor is mainly based on an optical interference structure or an optical fiber grating and the like, has the problems of small measurement range and easy influence of external environment on measurement precision, and generally needs temperature compensation to improve the accuracy of liquid level measurement.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides an optical fiber sensing system for simultaneously measuring liquid level and liquid temperature and a measuring method thereof, aiming at solving the problems of small measuring range, easy influence of external environment on measuring precision and low measuring precision of the existing optical fiber liquid level sensor.
In order to achieve the purpose, the invention provides an optical fiber sensing system for simultaneously measuring liquid level and liquid temperature, which comprises a threaded packaging tube, a microstructure optical fiber and a coherent phase demodulation device. The microstructure optical fiber is embedded into a thread groove of the thread packaging tube, one end of the microstructure optical fiber is connected into the coherent phase demodulation device, and the other end of the microstructure optical fiber is connected into liquid to be detected; the surface of the thread packaging tube after the micro-structure optical fiber is embedded is coated with a layer of soft glue for packaging and protecting the micro-structure optical fiber, and the thread packaging tube is also used for improving the spatial resolution and the sensitivity of liquid level and temperature measurement; the coherent phase demodulation device is used for demodulating distributed sensing signals in the microstructure optical fiber, measuring the temperature response of each independent sensing section in the microstructure optical fiber and calculating the liquid level and the liquid temperature.
The invention uses a coherent phase demodulation method to demodulate distributed sensing signals in the microstructure optical fiber, measures the temperature response of each independent sensing section in the microstructure optical fiber, and calculates the liquid level and liquid temperature information through a temperature demodulation algorithm. In liquid level monitoring, due to the liquid evaporation effect and the difference of specific heat capacity of liquid and gas, the temperature of gas phase and liquid phase on the gas-liquid contact surface is suddenly changed. The quasi-distributed liquid level monitoring can be realized by finding out the corresponding sensing section by utilizing the position of the optical fiber temperature sensor for measuring the temperature mutation. Furthermore, the sensing section at the liquid level can be divided into two parts in gas and liquid, the temperature fields of the two parts can be regarded as uniform within a short distance, the liquid level can be further accurately positioned according to the linear relation between the immersion depth of the optical fiber in the liquid and the phase value measured by the sensing section, and the full-distributed liquid level monitoring is realized. In the temperature measurement, because the microstructure optical fiber comprises a plurality of independent sensing sections which are closely connected, the sensing sections in the liquid can realize the distributed liquid temperature measurement at different depths. Furthermore, the characteristics of high sensitivity and ultra-long sensing distance of the microstructure optical fiber are utilized, and the large-range high-precision liquid level and liquid temperature monitoring can be realized.
Preferably, the microstructure fiber is provided with a plurality of scattering enhancement points for enhancing a backscattering signal of the sensing fiber and improving a signal-to-noise ratio of the system, the connecting fiber between the scattering enhancement points is used as a sensing area, and the microstructure fiber is divided into a plurality of independent sensing sections by the scattering enhancement points.
Preferably, the scattering enhancing points are generated on the bending-resistant single mode fiber by ultraviolet exposure, all the scattering enhancing points are arranged at equal intervals, and the reflectivity of the scattering enhancing points is-50 dB to-40 dB.
Preferably, the threaded packaging tube is made of a hard material with thread grooves, and the distance and the depth of the thread grooves are larger than the diameter of the microstructure optical fiber.
Preferably, the coherent phase demodulation device comprises a light source module, an optical modulation module, a coherent receiving module and a digital signal processing module; the light source module is used for generating continuous narrow linewidth laser; the input end of the light modulation module is connected with the output end of the light source module and is used for carrying out frequency shift and modulation processing on the continuous narrow linewidth laser and outputting a short pulse sequence; one input end of the coherent receiving module is connected with the output end of the light source module, and the other input end of the coherent receiving module is connected with the output end of the micro-structured optical fiber, and the coherent receiving module is used for enabling the reference light and the back scattering light pulse subsequence to generate interference to form a beat frequency optical signal subsequence, converting the beat frequency optical signal subsequence into an electric signal and outputting the beat frequency subsequence; the input end of the digital signal processing module is connected with the output end of the coherent receiving module and is used for carrying out phase demodulation on the beat frequency sub-sequence.
In another aspect of the present invention, the present invention provides a method for measuring a liquid level and a liquid temperature simultaneously in an optical fiber sensing system, in which probe light is frequency shifted and modulated into a plurality of short pulse sequences, each short pulse is reflected by a plurality of scattering enhancing points in a micro-structured optical fiber to form a short pulse subsequence, the returned back-scattered light pulse subsequence and reference light enter a coherent receiving module to interfere to form a beat frequency light signal subsequence, the intensity of the beat frequency light signal subsequence is obtained through coherent detection, then phase change between adjacent scattering enhancing points is demodulated through a cross-correlation method, phase information on a corresponding sensing segment is restored through phase unwrapping, and finally, the liquid level and the liquid temperature are obtained simultaneously. The cross-correlation method is to process the waveforms of adjacent subsequences in the reflected light signal by a cross-correlation algorithm to obtain the phase information of the sensing segment between the scattering enhancement points corresponding to the subsequences.
Preferably, each short pulse in the sequence of backscattered light pulses has a duration of
Interval of two adjacent short pulses
Wherein N is the refractive index of the microstructure fiber, L is the distance between the scattering enhancing points in the microstructure fiber, N is the number of the scattering enhancing points in the microstructure fiber, and c is the speed of light. Each back scattering optical signal subsequence is not overlapped, so that each back scattering optical signal subsequence is not overlapped with a beat frequency optical signal subsequence generated by continuous narrow-line-width laser, the demodulation rate of the digital signal processing module to the beat frequency electric signal is improved, and the response frequency range of the optical fiber sensing system is enlarged.
The measuring method of the optical fiber sensing system for simultaneously measuring the liquid level and the liquid temperature also comprises the steps of finding the sensing section with the largest phase variation due to the temperature mutation at the liquid level interface, and extracting the phase signal of the sensing section
And roughly positioning the liquid level height h according to the position of the sensing section on the threaded packaging pipe; extracting a phase signal of a first sensor section located below the liquid level interface
Extracting phase signals of a first sensor section above a liquid level interface
Obtaining the accurate liquid level by formula
Wherein l is the center distance between adjacent sensing sections of the microstructure fiber packaged on the thread packaging tube. Before measurement is started, a phase value corresponding to the phase signal of each sensing section and the temperature is found according to the currently known liquid temperature, and as the phase variation on each sensing section is in a linear relation with the temperature variation, the temperature variation information of the liquid can be obtained according to the phase signal of the sensing section completely immersed in the liquid during temperature measurement, and the temperature variation information of the gas can be obtained according to the phase signal of the sensing section completely immersed in the gas.
Compared with the prior art, the technical scheme of the invention has the following technical effects:
1. the liquid level height is roughly positioned according to the sensing section at the position of the liquid level temperature sudden change, and further, the liquid level height is accurately positioned according to the relation between the phase change of the sensing section and the liquid level height, so that the full-distributed sensing is realized.
2. Because the microstructure optical fiber comprises a plurality of independent sensing sections, the sensor can realize distributed liquid level temperature measurement at different heights.
3. The adoption of the ultra-weak reflectivity scattering enhancement point can realize the cascade connection of thousands of sensing sections, thereby greatly improving the liquid level detection range.
4. Because the coherent detection method and the cross-correlation method are adopted to demodulate the phase, the real-time frequency response range can reach several kHz, and low-frequency quasi-static signals can be detected, the system response dynamic range is large, and the liquid level measurement of fast change and quasi-static change can be realized.
Drawings
FIG. 1 is a schematic view of the microstructure fiber distribution of a fiber sensing system for simultaneous measurement of liquid level and liquid temperature provided by the present invention;
FIG. 2 is a schematic structural diagram of an optical fiber sensing system for simultaneously measuring liquid level and liquid temperature provided by the present invention;
FIG. 3 is a schematic structural diagram of a coherent phase demodulation device in an optical fiber sensing system for simultaneously measuring liquid level and liquid temperature according to the present invention;
fig. 4 is a schematic structural diagram of a light source module, an optical modulation module and a coherent receiving module in the coherent phase demodulation apparatus provided in the present invention;
FIG. 5 is a schematic view of the results of liquid level measurements in an embodiment of the present invention;
FIG. 6 is a diagram illustrating the measurement result of the liquid temperature in the embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The invention relates to an optical fiber sensing system for simultaneously measuring liquid level and liquid temperature, as shown in fig. 1, the embodiment comprises: a threaded packaging tube, a microstructure fiber 5 and a coherent phase demodulation device. A 248nm ultraviolet laser is utilized to write scattering enhancement points on the bending-resistant single-mode fiber after light beam focusing to manufacture the microstructure fiber 5, the distance between adjacent scattering enhancement points is 2m, and the reflectivity is-50 dB; the threaded packaging tube is made of an aluminum metal tube with the outer diameter of 20mm, thread grooves are engraved on the surface of the aluminum metal tube by using a numerical control machine, the depth of each thread groove is 0.3mm, and the distance between every two thread grooves is 0.3 mm; the microstructure optical fiber 5 is embedded into a thread groove of the thread packaging tube, one end of the microstructure optical fiber is connected into the coherent phase demodulation device, and the other end of the microstructure optical fiber is immersed into liquid to be detected; the surface of the thread packaging tube after the micro-structure optical fiber is embedded is coated with a layer of soft glue for packaging and protecting the micro-structure optical fiber, and the thread packaging tube is also used for improving the spatial resolution and the sensitivity of liquid level and temperature measurement; the coherent phase demodulation device is used for demodulating distributed sensing signals in the microstructure optical fiber, measuring the temperature response of each independent sensing section in the microstructure optical fiber and calculating the liquid level and the liquid temperature.
As shown in fig. 2-4, the coherent phase demodulation apparatus includes a light source module 1, an optical modulation module 2, a coherent receiving module 3 and a digital signal processing module 4; the light source module 1 comprises a laser light source 11 and a first optical fiber coupler 12; the optical modulation module 2 comprises an acousto-optic modulator 21, an erbium-doped fiber amplifier 22, a narrow linewidth band-pass filter 23 and a circulator 24, wherein the acousto-optic modulator 21 shifts frequency by 200MHz, has short pulse duration of 10ns and pulse interval of 20 mu s; the erbium-doped fiber amplifier 22 is used for amplifying the detection light, and the narrow linewidth filter 23 is used for filtering noise of a waveband other than the detection light; the coherent reception module 3 includes a second fiber coupler 31 and a balanced detector 32. The laser light source 11 emits continuous narrow line width laser with the wavelength of 1549.7nm, and the ratio of light splitting is 99: the first optical fiber coupler 12 of 1 is divided into reference light and probe light, the probe light is modulated by the light modulation module, then is input through an a port of an optical circulator 24, and is output to the microstructure optical fiber from a b port of the optical circulator 24, a backscattered light pulse subsequence returned by each scattering enhancement point on the microstructure optical fiber 5 is input through the b port of the optical circulator 24, and is output from a c port of the optical circulator 24, and the splitting ratio is 50: 50, the second optical fiber coupler 31 combines the backscattered light pulse subsequence with the reference light beam and inputs the combined signal to the balance detector 32 to form beat frequency and output an electric signal, the digital signal processing module 4 obtains phase information of a sensing section between adjacent scattering enhancement points by using a cross-correlation and difference algorithm for the obtained electric signal, and then restores the waveform of the sensing signal by using a phase unwrapping algorithm, and the position of the sensing signal is obtained by the sequence of the received short pulse subsequences. The position information of each sensing segment can be corresponding to the corresponding sensing segment by the sequence of the subsequences of each short pulse reflected back by different scattering enhancement points, wherein the positions of the scattering enhancement points are determined in the manufacturing process of the microstructure optical fiber.
In specific application, because of temperature jump at the liquid level interface, a sensing section with the largest phase variation is found, and a phase signal of the sensing section is extracted
And roughly positioning the liquid level height h according to the position of the sensing section on the threaded packaging pipe; extracting a phase signal of a first sensor section located below the liquid level interface
Extracting phase signals of a first sensor section above a liquid level interface
Obtaining the accurate liquid level by formula
Wherein l is the center distance between adjacent sensing sections of the microstructure fiber packaged on the thread packaging tube. As shown in fig. 5, the liquid level rises at a constant speed of 0.4mm/s, sequentially passes through 5 sensor segments in a time period of 100s to 225s, and after calculation by the demodulation method and synthesis of liquid level measurement data of the 5 sensor segments, the relationship between liquid level height change and phase change within a range of 50mm is obtained, and the liquid level measurement sensitivity is 26.9 rad/mm. In addition, theBefore measurement is started, a phase value corresponding to the phase signal of each sensing section and the temperature is found according to the currently known liquid temperature, and as the phase variation on each sensing section is in a linear relation with the temperature variation, the temperature variation information of the liquid can be obtained according to the phase signal of the sensing section completely immersed in the liquid during temperature measurement, and the temperature variation information of the gas can be obtained according to the phase signal of the sensing section completely immersed in the gas.
The invention relates to a liquid temperature demodulation example of an optical fiber sensing system for simultaneously measuring liquid level and liquid temperature, as shown in figure 6, a sensing section in liquid has a phase change and temperature change curve graph and the temperature sensitivity is 29.7 rad/DEG C.
Finally, it should be noted that the above-mentioned embodiments are only for illustrating the technical solutions of the present invention and not for limiting, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, which should be covered by the claims of the present invention.
Claims (7)
1. Optical fiber sensing system of liquid level and liquid temperature simultaneous measurement, its characterized in that includes: the device comprises a threaded packaging tube, a microstructure optical fiber and a coherent phase demodulation device;
the microstructure optical fiber is embedded into a thread groove of the thread packaging tube, one end of the microstructure optical fiber is connected into the coherent phase demodulation device, and the other end of the microstructure optical fiber is connected into liquid to be detected; the microstructure optical fiber is provided with a plurality of scattering enhancement points for enhancing a backscattering signal of the sensing optical fiber, the connecting optical fiber between the scattering enhancement points is used as a sensing area, and the microstructure optical fiber is divided into a plurality of independent sensing sections by the scattering enhancement points;
the surface of the threaded packaging tube after the microstructure optical fiber is embedded is coated with a layer of soft glue for packaging and protecting the microstructure optical fiber;
the coherent phase demodulation device is used for demodulating the distributed sensing signals in the microstructure optical fiber, measuring the temperature response of each independent sensing section in the microstructure optical fiber and calculating the liquid level and the liquid temperature.
2. The fiber optic sensing system of claim 1, wherein the scattering enhancement points are produced on the bend-resistant single mode fiber by uv exposure, all of the scattering enhancement points being equally spaced, the scattering enhancement points having a reflectivity of-50 dB to-40 dB.
3. The fiber optic sensing system of claim 1, wherein the threaded packaging tube is fabricated from hard material with thread grooves machined therein, the pitch and depth of the thread grooves being greater than the diameter of the microstructured optical fiber.
4. The fiber optic sensing system of claim 1, wherein the coherent phase demodulation means comprises a light source module, an optical modulation module, a coherent receiving module and a digital signal processing module;
the light source module is used for generating continuous narrow linewidth laser;
the input end of the light modulation module is connected with the output end of the light source module and is used for carrying out frequency shift and modulation processing on continuous narrow linewidth laser and outputting a short pulse sequence;
one input end of the coherent receiving module is connected with the output end of the light source module, and the other input end of the coherent receiving module is connected with the output end of the microstructure optical fiber, and the coherent receiving module is used for enabling reference light and a back scattering light pulse subsequence to generate interference to form a beat frequency optical signal subsequence, converting the beat frequency optical signal subsequence into an electric signal and outputting the beat frequency subsequence;
and the input end of the digital signal processing module is connected with the output end of the coherent receiving module and is used for carrying out phase demodulation on the beat frequency subsequence.
5. A measurement method based on the optical fiber sensing system of claim 1, characterized in that the probe light is frequency shifted and modulated into a plurality of short pulse sequences, each short pulse in the microstructured optical fiberReflecting by a plurality of scattering enhancement points to form a short pulse subsequence, allowing the returned back scattering light pulse subsequence and reference light to enter a coherent receiving module to interfere to form a beat frequency light signal subsequence, obtaining the intensity of the beat frequency light signal subsequence through coherent detection, demodulating the phase change between adjacent scattering enhancement points through a cross-correlation method, recovering the phase information on a corresponding sensing section through phase unwrapping, and finally obtaining the liquid level and the liquid temperature at the same time; each short pulse in the backscattered light pulse train has a duration of
Interval of two adjacent short pulses
Wherein N is the refractive index of the microstructure fiber, L is the distance between the scattering enhancing points in the microstructure fiber, N is the number of the scattering enhancing points in the microstructure fiber, and c is the speed of light.
6. The method of claim 5, further comprising extracting the phase signal of the sensing segment having the largest amount of phase change at the liquid level interface
Phase signal of the first sensor section below the liquid level boundary
And the phase signal of the first sensor section above the liquid level interface
And the liquid level height h is roughly positioned according to the position of the sensing section on the threaded packaging pipe to obtain the accurate liquid level height
Wherein l is the center distance between adjacent sensing sections of the microstructure fiber packaged on the thread packaging tube.
7. The measurement method according to claim 5, further comprising finding a phase value of the phase signal of each sensing segment corresponding to the temperature according to the currently known liquid temperature, obtaining information on the temperature change of the liquid from the phase signal of the sensing segment completely immersed in the liquid, and obtaining information on the temperature change of the gas from the phase signal of the sensing segment completely immersed in the gas.
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| CN113578231B (en) * | 2021-07-07 | 2022-08-23 | 浙江世纪康大医疗科技股份有限公司 | Mechanism and method for preparing liquid preparation containing magnetic liquid particles |
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