CN107167217B - Acousto-optic coupled optical fiber liquid level sensor system - Google Patents
Acousto-optic coupled optical fiber liquid level sensor system Download PDFInfo
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- G—PHYSICS
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- 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
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- Measurement Of Levels Of Liquids Or Fluent Solid Materials (AREA)
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Abstract
The invention discloses an optical fiber liquid level sensor which is characterized in that an optical fiber extending into liquid to be measured is used as a measuring body; a structure for coupling light waves and sound waves into the optical fiber at the same time is designed at one end of the optical fiber; the other end of the optical fiber is a free end which is suspended in the liquid to be measured; the surface of the optical fiber is covered with a coating layer which is not compatible with the liquid to be measured. The invention has the beneficial effects that: ultra-small size; continuously measuring in a large measuring range; non-metallic, non-conductive properties; corrosion resistance; high and low temperature resistance and low cost. The sensor has great advantages over the conventional liquid level sensor when detecting liquid levels in extreme environments, such as liquid hydrogen, liquid oxygen, liquid helium, nuclear reactor water level and conductive liquid.
Description
Technical Field
The invention belongs to the crossing field of optical fiber sensing technology, material science, photoelectronic technology and acoustic waveguide technology, relates to device packaging and photoelectric detection technology, and particularly relates to an optical fiber liquid level sensor system for realizing wide-range liquid level measurement by adopting an acousto-optic coupling mode.
Background
The liquid level sensor is used for detecting the measuring instrument of liquid height, is generally used in liquid production, storage, transfer and use, and is especially to several kinds of high-risk liquid such as extremely low temperature, high temperature, strong corrosivity, hypertoxic, volatile and flammable, and the sensing control to the liquid level is essential almost, and accurate liquid level monitoring helps safety in production to in time report to the police when having the leakage risk.
Among the level sensors, fiber optic level sensors are commonly used for level measurement in special liquid environments, for example, level measurement is implemented using Fiber Bragg Gratings (FBGs). The optical fiber liquid level sensor has obvious advantages compared with the traditional mechanical and electrical sensors, such as the characteristics of extremely low temperature resistance and relative high temperature resistance, generally, the main component of the optical fiber material is made of silicon dioxide, and due to the temperature resistance of the silicon dioxide, the traditional optical fiber liquid level sensor can measure the temperature as low as liquid helium, and the wide temperature range as high as 800 ℃, which is far beyond the use temperature range of the traditional mechanical and electrical sensors; secondly, it has high corrosion resistance, and in general, strong acid or strong base solution can not corrode the optical fiber except for strong acid such as hydrofluoric acid. In addition, the optical fiber liquid level sensor also has non-conductivity, so the optical fiber liquid level sensor can also be used for measuring conductive liquid.
The basic principle of the optical fiber sensor for liquid level measurement is that the change of liquid level is measured by an optical fiber measuring means, and then liquid level information is converted into signals of electricity, light, pressure or other information according to a certain rule and is output, so that the liquid level measurement meeting the requirements of information transmission, processing, storage, feedback control and the like is realized.
In the prior art, level sensors associated with optical fibers (also referred to as fiber optic level sensors) comprise four broad categories. The first type is a mechanical fiber liquid level sensor, in which the fiber is simply used as a "scale mark" with small diameter, light weight, high and low temperature resistance, and corrosion resistance. One end of the optical fiber is connected to the floating body structure (the liquid level sensor main body), and the other end is connected to the mechanical telescopic mechanism. As the position of the level float structure changes, it will result in a change in the length of the optical fiber. By recording the physical length of the fiber, level measurement can be achieved. The second type is a grating optical fiber liquid level sensor, in which the optical fiber itself is used as a single-point liquid level sensor of a sensor probe, which is similar to a liquid level switch, and generally uses a fiber grating (as described in patent document CN 101194160 a), a fiber fabry perot interference cavity, or an optical fiber with a microstructure engraved on the surface to detect the liquid level, and when the liquid level reaches different grating or microstructure positions, the optical characteristics are changed, so as to measure the change of the liquid level. In the third type, a transmission-type optical fiber liquid level sensor, in which an optical fiber is used only as a carrier of an optical signal, is configured such that a microstructure probe, such as a pressure probe, a refractive index probe, a temperature probe, or the like, is mounted or machined on an end face of the optical fiber, and the change in liquid level is detected by the probe, and the detected liquid level information is converted into an optical signal and conducted to a signal demodulation system through the optical fiber, thereby achieving liquid level measurement (for example, as described in patent document CN 103918288A). A fourth type of distributed fiber optic liquid level sensor is characterized by a fiber grating or distributed fiber optic vibration sensor (described in, for example, WO2015/128680 a 1) that is continuously processed on an optical fiber at high density, wherein the liquid level is confirmed by sensing different physical states inside and outside the liquid level by the grating using a fiber grating scheme and by searching the spectrum of the grating to demodulate the fiber grating and finding a jump point. In the scheme, the position of the fiber grating is fixed, the dependence on the distribution density (or grating spacing) of the fiber grating is large, and the fiber grating is a quasi-distributed measurement scheme.
However, there is no fiber optic liquid level sensor solution that can perform a fully distributed measurement of the liquid level in the true sense, in other words, there is no fiber optic liquid level sensor solution that can perform the measurement of the filling level and the remaining level using only one optical fiber. In the prior art, in order to solve this problem, two different liquid level sensors are generally required to be used for solving the problem. In addition, the prior art provides few universal solutions for use in extreme environments, which can measure liquid hydrogen, liquid oxygen, liquid helium, conductive liquid, high temperature nuclear reactor water level, and the like.
Meanwhile, an optical fiber liquid level sensor introducing an acousto-optic fiber coupling mode does not exist in the prior art. There are no relevant reports on consulting global patent literature and authoritative periodicals. In principle, when ultrasonic waves (when a high-frequency voltage is applied to the piezoelectric transducer, the vibration of the piezoelectric transducer generates ultrasonic waves in a medium) propagate in a transparent medium, the refractive index of the medium changes periodically in space, and the phenomenon that light passing through the medium changes is called acousto-optic effect. When the acoustic frequency is increased and the beam width is much greater than the acoustic wavelength, this periodic variation of the refractive index acts as a grating, causing the incident beam to undergo acousto-optic diffraction, and after the beam passes through the acoustic field, one side of the outgoing beam exhibits a strong first order diffracted light, referred to as acousto-optic bragg diffraction. When ultrasonic waves pass through a solid medium, the electric dipole moment of the medium molecules is changed, so that the refractive index of the medium is periodically changed to form a refractive index grating (i.e. an ultrasonic grating).
The invention provides an optical fiber liquid level sensor based on an acousto-optic fiber coupling mode, namely an optical fiber liquid level sensor solution which can carry out full-distributed measurement on liquid level in the real sense, and is a universal solution used in an extreme environment.
Disclosure of Invention
In view of the above technical problems, an object of the present invention is to provide a wide-range optical fiber liquid level sensor system capable of performing full-distributed liquid level measurement, performing filling level and residual liquid level measurement, and measuring liquid hydrogen, liquid oxygen, liquid helium, conductive liquid, high temperature nuclear reactor water level, etc. with only one optical fiber, which can be used in extreme environments.
Specifically, the following technical scheme is adopted for solving the technical problem:
the acousto-optic coupled optical fiber liquid level sensor system is characterized by comprising an optical fiber extending into liquid to be detected, a coupling structure arranged at one end of the optical fiber and used for coupling light waves and sound waves into the optical fiber, a coating layer covering the surface of the optical fiber and not in relative relationship with the liquid to be detected, and an acousto-optic signal demodulation system, wherein the other end of the optical fiber is a free end and is suspended in the liquid to be detected.
The acousto-optically coupled optical fiber liquid level sensor system is characterized in that sound waves and light waves are coupled into the system from one end of the optical fiber, and the other end of the optical fiber is a free end and is suspended in liquid.
The acousto-optically coupled optical fiber liquid level sensor system is characterized in that a sound wave coupling sleeve is arranged outside the optical fiber at the joint where the sound wave is coupled into the optical fiber. The acoustic coupling sleeve and the optical fiber are bonded by curable glue or connected by laser welding, so that the relative position of the acoustic coupling sleeve and the optical fiber is fixed. The acoustic coupling sleeve is shaped or polished to a particular shape, preferably a shape that is adapted to the optical fiber and to the mounting of the piezoelectric ceramic, thereby contributing to an increase in acoustic coupling efficiency.
The acousto-optic coupled optical fiber liquid level sensor system is characterized in that one or more piezoelectric ceramic plates are further arranged outside the acoustic coupling sleeve. Preferably, two pieces of piezoelectric ceramic mounted opposite each other are used to achieve various modes of acoustic coupling into the fiber. One or more piezoelectric ceramic plates arranged outside the sound wave coupling sleeve are used as sound wave generating devices to generate sound waves.
According to the invention, in order to avoid the influence of liquid drops hung outside the optical fiber on the liquid level, the outer surface of the optical fiber is preferably covered with a coating layer which is not compatible with the liquid to be measured.
According to the invention, the device also comprises an acousto-optic signal demodulation system which is used for demodulating the acoustic wave and light wave signals coupled into the optical fiber. The acousto-optic signal demodulation system mainly comprises the optical circulator and a wavelength adjusting system. When an acoustic grating propagates light forward in the fiber, it couples with light propagating from a broad spectrum light source and reflects only light waves of a specific wavelength that satisfies the grating bragg diffraction condition. The reflected light with the specific wavelength enters a wavelength demodulation system through an optical circulator for demodulation. When a long-period grating formed by sound waves is transmitted to a liquid-gas interface, the transmission speed of the sound waves is changed, and the grating pitch of the long-period acoustic grating is further changed, so that the reflected light wavelength is shown to be suddenly changed on the optical characteristic, the phenomenon is captured by a continuously monitored wavelength demodulation system, and the liquid level height can be converted by calculating the time interval between the initial sound wave generation and the sudden wavelength change.
According to the invention, in order to ensure a relatively stable position of the optical fibre in the liquid under certain special conditions, such as lateral acceleration of the tank or strong vibrations of the liquid, it is possible to load the freely hanging end of the optical fibre with a weight or to connect it to the bottom of the tank. In other words, the free suspension end of the optical fiber is loaded with a heavy object or is connected with the bottom of the liquid storage tank of the liquid to be measured, so that the optical fiber is prevented from moving laterally under the condition of transverse acceleration or strong external disturbance.
As described above, the fiber optic liquid level sensor system provided by the present invention is based on the principle of acousto-optic coupling, and uses piezoelectric ceramic to couple sound waves into an optical fiber and form an acoustic grating in the optical fiber, which propagates along the optical fiber. Among other things, the production of acoustic gratings is based on ultrasound induced changes in the refractive index of solids. Because the optical fiber body is an acoustic waveguide, an acoustic long-period fiber grating can be formed in the optical fiber by selecting proper ultrasonic frequency. After the long-period fiber grating is optically coupled with the wide spectrum, the light reflection of specific wavelength can be realized. When the acoustic wave propagates to the liquid-gas interface, a discontinuous acoustic wave mode is formed, resulting in the effect that the coupling effect of the sound and the light is slightly changed, which is reflected in that the wavelength of the reflected light is changed on the optical characteristic. The liquid level height can be calculated by detecting the reflection time of the sudden change of the optical wavelength and according to the propagation speed of the sound wave in the optical fiber. According to the invention, the defect that the grating needs to be continuously written in the optical fiber by using a physical method in the common distributed fiber grating liquid level sensing is avoided, and the defect that the measuring range is too small and is only limited in the length of the fiber grating when the common long-period fiber grating scheme is used is also overcome. On the other hand, the invention only generates a dynamic-propagation acoustic wave grating in the optical fiber by using the acoustic wave, and the cost can be greatly reduced according to the invention without reforming the optical fiber. In addition, the liquid level test range of the invention can be expanded to the effective propagation distance of the acoustic grating, which can reach about tens of meters. Meanwhile, according to the invention, because the sound velocity is far lower than the light velocity, the liquid level is calculated by using the sound wave propagation time interval instead of the light wave propagation time interval, and the precision requirement of time measurement is reduced by five orders of magnitude, thereby greatly reducing the cost of hardware design. According to the invention, a sufficiently accurate time measurement is given, i.e. a highly accurate distributed measurement can be achieved.
The optical fiber liquid level sensor system can realize ultra-small size, has the characteristics of nonmetal, non-conductivity, corrosion resistance and high and low temperature resistance, can realize large-range continuous measurement, and has low cost. According to the invention, the optical fiber liquid level sensor has great advantages compared with the conventional liquid level sensor when being used for detecting liquid level occasions under extreme environments, such as liquid hydrogen, liquid oxygen, liquid helium, nuclear reactor water level and conductive liquid. In addition, according to the present invention, a contact measurement technique can be provided, which can avoid the influence on the measurement due to the environmental change above the liquid level, such as the influence of factors such as the absorption or scattering of the vapor, the disturbance of the gas flow, the powder layer, etc., compared with the non-contact measurement such as the acoustic or optical measurement, and thus has high reliability.
Drawings
FIG. 1 is a schematic diagram of a 45 degree angle appearance of an acousto-optic coupling structure used in a fiber optic liquid level sensor system in accordance with an embodiment of the present invention.
FIG. 2 is a schematic cross-sectional view perpendicular to the central axis of an acousto-optic coupling structure for use in a fiber optic liquid level sensor system in accordance with an embodiment of the present invention.
FIGS. 3 a-3 c are schematic diagrams of the use of piezoelectric ceramics to generate various acoustic wave modes in a fiber optic liquid level sensor system according to embodiments of the present invention.
FIG. 4 is a schematic structural diagram of an acousto-optic signal demodulation system of an optical fiber liquid level sensor system according to an embodiment of the present invention.
Detailed Description
The present invention will be described in detail with reference to the embodiments with reference to the drawings, and it will be understood by those skilled in the art that the description is illustrative and various modifications and changes can be made to the present invention, and the present invention is not limited to the embodiments.
FIG. 1 is a schematic diagram of a 45 degree angle appearance of an acousto-optic coupling structure used in a fiber optic liquid level sensor system in accordance with an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view perpendicular to the central axis of an acousto-optic coupling structure for use in a fiber optic liquid level sensor system in accordance with an embodiment of the present invention.
As shown in fig. 1 and fig. 2, the diagrams are all state diagrams of a structure in which sound waves are coupled into an optical fiber, and the diagrams show that the optical fiber liquid level sensor system includes an optical fiber 1, an optical fiber outer sleeve 2 (a sound wave coupling sleeve), a piezoelectric ceramic 3, an ultraviolet curable adhesive 4, and other kinds of adhesive 5. In the specific embodiment of the present invention, the optical fiber liquid level sensor system is an acousto-optic coupled optical fiber liquid level sensor system, the optical fiber 1 extends into the liquid to be measured, and an acousto-optic coupling structure is arranged at one end of the optical fiber 1 outside the liquid level, and the acousto-optic coupling structure includes a sound wave coupling structure and an optical coupling structure (not shown) which are composed of components including an optical fiber outer sleeve 2 and a piezoelectric ceramic 3 and are used for coupling light waves and sound waves into the optical fiber at the same time. In the present embodiment, the relative positions of three members, i.e., the optical fiber 1, the optical fiber outer tube (acoustic wave coupling tube) 2, and the piezoelectric ceramic 3, are fixed by using the uv curable adhesive 4 and other types of adhesives 5. The coupling structure shown in fig. 1 and 2 is mounted at the position shown by reference numeral 3 (piezoelectric ceramic) in fig. 4, and forms the acousto-optically coupled optical fiber liquid level sensor system of the invention together with other components. The outer surface of the optical fiber 1 is coated with a coating layer (not shown) that is not compatible with the liquid to be measured, and in addition, the optical fiber liquid level sensor system further comprises an acoustic-optical signal demodulation system (as shown in fig. 4, which will be described in detail later) for demodulating the generated acoustic and optical signals.
As described above, in the embodiment of the present invention, the optical fiber 1 is provided at one end outside the liquid surface with the acoustic wave coupling structure and the optical coupling structure composed of the optical fiber outer sleeve 2 and the piezoelectric ceramic 3, for coupling the optical wave and the acoustic wave into the optical fiber at the same time. The light coupling structure may employ a conventional light coupling device, which is located as shown at reference numeral 6 in fig. 4, where reference numeral 6 is illustrated as a broad spectrum light source in fig. 4. The simplest optical coupling structures are such as those employing lasers with fiber pigtailed outputs. In addition to this, lenses may be used to focus the spatial laser light into the fiber core, or optical splitters with fiber pigtail outputs, etc. may be used. The acoustic wave coupling structure adopts the structure that the optical fiber outer sleeve 2 is matched with the piezoelectric ceramic 3. As shown in fig. 4, both the acoustic wave coupling structure (the position shown by reference numeral 3) and the optical coupling structure (the position shown by reference numeral 6) are arranged outside the liquid surface of the optical fiber and on the same optical fiber. In actual operation, the two structures simultaneously couple acoustic and optical signals into the optical fiber. Generally, the optical coupling structure and the acoustic wave coupling structure are not completely co-located. Because the speed of light in the optical fiber is far greater than the speed of sound, and data acquisition is based on the propagation time of sound waves, the time error caused by the position deviation of the two structures is negligible. In this sense, in the present embodiment, the coupling structure for coupling the optical wave and the acoustic wave into the optical fiber at the same time may be regarded as an integrated structure formed by the optical coupling structure and the acoustic wave coupling structure.
In the present embodiment, at the junction where the acoustic wave is coupled into the optical fiber 1, an acoustic wave coupling sleeve 2 is provided outside the optical fiber. The sound wave coupling sleeve 2 (namely the optical fiber outer sleeve 2) is connected with the optical fiber 1 by curable glue 4 or laser welding. The acoustic coupling sleeve 2 is shaped or polished to a specific shape, preferably a shape adapted to the optical fiber 1 and to the piezoelectric ceramic 3, thereby contributing to an improvement in acoustic coupling efficiency.
In the specific implementation mode of the invention, both the sound wave and the light wave are coupled into the system from one end of the optical fiber 1, and the other end of the optical fiber 1 is a free end and is suspended in the liquid to be measured.
In the present embodiment, the optical fiber outer sleeve 2 (i.e. the acoustic wave coupling sleeve) has an inner diameter slightly larger than the outer diameter of the optical fiber 1. In order to achieve the optimal acoustic coupling effect, after the acoustic wave coupling outer sleeve is placed to a preset position, the gap between the sleeve and the optical fiber is filled with ultraviolet curable glue 4 in a liquid form, and then the ultraviolet curable glue is cured. This gap may also be filled with a curable liquid such as low temperature glass.
In the embodiment of the present invention, one or more piezoelectric ceramic plates 3 may be mounted outside the acoustic coupling sleeve. Preferably, two pieces of piezoelectric ceramic 3 mounted opposite each other are used to achieve various modes of acoustic coupling into the optical fiber.
According to the invention, in order to avoid the influence of the liquid drops suspended outside the optical fiber 1 on the liquid level, the outer surface of the optical fiber 1 is preferably covered with a coating layer which is not compatible with the liquid to be measured. As such a coating layer, there are many materials in the prior art, and any material that can achieve incompatibility with the liquid to be measured can be selected as the coating layer material according to the application, and it is popular to use a nano coating material, which is a nano coating that can promote any liquid to bounce off the surface of an object, and when the optical fiber is immersed in liquids such as oil, organic alkali solvent, liquid hydrogen, liquid oxygen, liquid helium, nuclear reactor water level, and conductive liquid, the coating layer coated on the surface of the optical fiber repels the liquids, so that the liquid drops of the liquid to be measured will not be attached to the coating layer.
In accordance with the present invention, in order to ensure that the optical fibers are in a relatively stable position in the fluid under certain special conditions, such as when the fluid reservoir is subjected to lateral acceleration or when the fluid exhibits strong vibration, a weight (not shown) may be loaded onto the free hanging ends of the optical fibers or attached to the bottom of the fluid reservoir.
According to the invention, as the ultraviolet curable adhesive, the common ultraviolet curable adhesive sold in the market can be adopted, usually IPDI, HEA and oligomer dihydric alcohol are synthesized to form urethane acrylate prepolymers with different structures, and a proper amount of auxiliary agents are added to prepare the ultraviolet curable adhesive, the ultraviolet curable adhesive mainly comprises a photosensitizer, an active diluent and a prepolymer, and the photosensitizer is rapidly decomposed into free radicals or cations under the irradiation of ultraviolet rays with proper wavelength and light intensity to further initiate unsaturated bond polymerization so as to cure the material. In the specific embodiment of the invention, the ultraviolet curable adhesive in liquid form is filled in the gap between the acoustic wave coupling outer sleeve and the optical fiber, and then curing is performed, so that the sleeve and the optical fiber are connected in a curing manner.
As the other type of the adhesive 5, various adhesives other than the ultraviolet curing adhesive, for example, a general-purpose adhesive, a room temperature curing type epoxy adhesive, and excellent adhesive strength for sealing and bonding between metal, ceramic, wood, glass, fiber products, and hard plastics can be used. In the embodiment of the present invention, as shown in fig. 1, in order to make the piezoelectric ceramic 3 and the optical fiber outer tube 2 contact well, a flat surface is partially polished on the outer peripheral surface of the outer tube 2 so as to facilitate the fixing and connection of the two by the adhesive 5.
FIGS. 3a 3c are schematic structural diagrams illustrating various acoustic wave modes generated by using the piezoelectric ceramic 3 in the optical fiber liquid level sensor system according to the embodiment of the present invention. As shown in fig. 3a to 3c, the structure described in the figure is an example in which only two pieces of piezoelectric ceramics 3 are pasted on the acoustic wave coupling outer tube 2. The number of the piezoelectric ceramics 3 may be 1 or more, and for convenience of explanation, two pieces are illustrated in the drawings as an example. By controlling the vibration direction of the piezoelectric ceramic 3, various modes of acoustic coupling into the optical fiber can be realized. Wherein the arrows in fig. 3a to 3c indicate the vibration direction of the piezoelectric ceramic at a certain time. In fig. 3a, the two piezoelectric ceramics 3 vibrate in the same direction, and the vibration generates transverse waves along the direction perpendicular to the optical fiber axis; in FIG. 3b, the two piezoelectric ceramics 3 vibrate in opposite directions, but in a direction perpendicular to the fiber axis, and the vibration generates a rotating wave; in fig. 3c, both piezoelectric ceramics 3 vibrate in the same direction and along the axial direction of the optical fiber, and the vibration generates a longitudinal wave. All three acoustic waves can create an acoustic grating in the fiber. However, the propagation speeds of the acoustic waves in different modes in the optical fiber are different, and the generation frequency of the acoustic waves needs to be adjusted according to the corresponding acoustic wave modes, so that the acoustic wave grating meets the requirements of the long-period grating, and the smooth coupling with the optical wave is realized.
The structure as described in fig. 3 is a mode of relatively mounting 2 pieces of piezoelectric ceramics, and has a simple structure and a coupling mode which is easy to control. In practical application, 1 or more pieces of piezoelectric ceramics can be used, and various mounting modes are adopted. For example, if 3 pieces of piezoelectric ceramics are used to generate longitudinal waves, they can be installed in an equilateral triangle. By the installation mode, the acoustic wave grating can meet the requirement of a long-period grating, and smooth coupling with light waves is realized.
FIG. 4 is a schematic structural diagram of an acousto-optic signal demodulation system of an optical fiber liquid level sensor system according to an embodiment of the present invention. As shown in fig. 4, 3 is a piezoelectric ceramic, 6 is a light source, 7 is an optical circulator, 8 is a piezoelectric driver, 9 is a wavelength adjustment system, and arrows indicate signal directions. The optical circulator 7 and the wavelength adjusting system 9 mainly form an acousto-optic signal demodulation system. The light source 6 is a wide-spectrum light source, and the wide-spectrum light source 6 continuously inputs light rays to the optical fiber through the optical circulator 7. On the other hand, the piezoelectric ceramic 3 generates a train of short periodic acoustic pulses under the drive of the piezoelectric driver 8, the train of pulses forming an acoustic grating, and the timing is started at the same time. In order to achieve a high positional resolution, the length of the acoustic wave train is typically only a few millimeters. When an acoustic grating propagates light forward in the fiber, it couples with light propagating from a broad spectrum light source and reflects only light waves of a specific wavelength that satisfies the grating bragg diffraction condition. The reflected light having the specific wavelength enters the wavelength demodulation system 9 through the optical circulator 7 to be demodulated. When the long-period grating formed by the acoustic wave is transmitted to the liquid-gas interface, the transmission speed of the acoustic wave is changed, and the grating pitch of the long-period acoustic grating is changed, so that the reflected light wavelength is shown to be suddenly changed in optical characteristics, and the phenomenon is captured by the continuously monitored wavelength demodulation system 9, so that the timing is finished at the same time. The liquid level height can be converted by calculating the time interval between the occurrence of the initial sound wave and the sudden change of the wavelength.
As mentioned above, in order to avoid the influence of the liquid drop hanging on the surface of the optical fiber on the test result, the surface of the optical fiber is uniformly covered with a coating layer which is not relative to the liquid to be tested.
According to the invention, the system can adopt cheap piezoelectric ceramics and light-emitting diodes as the sound generator and the wide-spectrum light source respectively, so that the cost of the sensor system can be greatly reduced. According to the sensor, the diameter of the used optical fiber can reach about 125 micrometers, the optical fiber can be made of inorganic non-metallic materials, the physical and chemical properties are very stable, the sensor can resist extreme environments and extremely low temperatures, and the sensor can be used for detecting the water level of a reactor of a nuclear power station. On the other hand, the non-conducting property of the electrolyte is compatible with the measurement requirement of the liquid level in the electrolytic cell, so the electrolyte has important application value.
From the foregoing, it will be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims.
Claims (6)
1. The utility model provides an optic fibre level sensor system, its characterized in that, including stretch into the inside optic fibre of the liquid that awaits measuring, set up in optic fibre one end and be used for getting into the coupling structure of optic fibre with the simultaneous coupling of sound wave and cover the coating that the optic fibre surface is not hydrophilic mutually with the liquid that awaits measuring, wherein, the sound wave all gets into the system from the one end coupling of optic fibre with the light wave, and the other end of optic fibre hangs in liquid for the free end, gets into in the sound wave coupling the junction of optic fibre the optic fibre outside is provided with sound wave coupling sleeve pipe, this sound wave coupling sleeve pipe with but use the bonding of curable glue or adopt laser fusion to connect between the optic fibre to fixed its.
2. The fiber optic liquid level sensor system of claim 1 further comprising an acousto-optic signal demodulation system for demodulating the acoustic and light wave signals coupled into the fiber.
3. The fiber optic liquid level sensor system of claim 1, wherein the acoustic coupling sleeve is contoured or polished to a particular shape to enhance acoustic coupling efficiency.
4. The fiber optic liquid level sensor system of claim 1, wherein one or more piezoceramic wafers are mounted externally of the acoustic coupling sleeve as an acoustic wave generating device.
5. The fiber optic liquid level sensor system of claim 4 wherein the oppositely mounted piezoceramic wafers are two pieces.
6. The fiber optic liquid level sensor system of claim 1, wherein the free floating end of the fiber optic is loaded with a weight or is connected to the bottom of a liquid storage tank for the liquid to be measured to prevent the fiber optic from moving laterally in the presence of lateral acceleration or strong external disturbance.
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| CN112649069B (en) * | 2020-12-15 | 2022-07-29 | 北京航天控制仪器研究所 | Liquid level detection system based on strain optical fiber sensor |
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