CN113960328A - Sensing device and method for sensing two-dimensional flow velocity and two-dimensional acceleration by using same - Google Patents
Sensing device and method for sensing two-dimensional flow velocity and two-dimensional acceleration by using same Download PDFInfo
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- G—PHYSICS
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- G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
- G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
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Abstract
The invention discloses a sensing device and a method for sensing two-dimensional flow velocity and two-dimensional acceleration thereof. The sensing device has the advantages of simple structure, high sensitivity, self-compensation of temperature, electromagnetic interference resistance, no pollution, good durability, real-time monitoring and the like, and is particularly suitable for occasions with the requirements on long-time work, explosion prevention, water prevention, corrosion prevention, miniaturization, electromagnetic interference resistance and the like of the sensing device in the fields of ocean engineering construction, ocean structure health monitoring and the like.
Description
Technical Field
The invention belongs to the field of measurement, and particularly relates to a sensing device and a method for sensing two-dimensional flow velocity and two-dimensional acceleration by using the same.
Background
Ocean currents mainly take horizontal flow as a main part, and the load of a water body on a structure is closely related to the flow velocity of the water body. Under the action of long-term continuous horizontal water body load, the material of the marine structure is fatigued or even destroyed. In some special engineering fields, accurate acquisition of a water flow fine flow field plays a key role. Therefore, enhanced monitoring of the flow rate of a body of water is required to analyze the load characteristics experienced by a structure. For the case that the flow velocity of the water body exceeds the design range of the structure, the instant discovery and treatment are needed. Real-time monitoring of ocean flow rates is beneficial to improving the reasonability of ocean structure design, the safety of operation and the timeliness of problem finding and handling. In addition, the multi-point monitoring of the ocean flow velocity is also beneficial to analyzing the local ocean current characteristics, and provides a monitoring means for the basic scientific research related to the ocean current.
The existing flow velocity measurement methods mainly comprise three methods, namely an on-site monitoring method such as a drop-type section measuring instrument and the like, a remote sensing monitoring method such as a high-frequency ground wave radar and the like, and a traditional physical method such as a dyeing method and the like. The first one mainly adopts an electronic sensing device, generally needs strict waterproof and anticorrosion technologies, generates certain electrochemical noise and reduces the sensing precision; the second method mainly monitors the motion state of the ocean surface and cannot obtain the flow velocity distribution of the deep layer; the third method is time-consuming, labor-consuming and has great interference to the ecological environment.
Furthermore, in addition to flow rate, detailed knowledge of the state of relative motion also requires sensing of acceleration. The acceleration sensor mainly comprises a mass block, a damper, an elastic element, a sensitive element and the like. The principle is that the acceleration of the measured object is obtained according to the inertia force borne by the mass block and Newton's second law. The acceleration sensors in the market at present are classified according to the sensing elements, and include capacitance type, inductance type, strain type, piezoresistive type, piezoelectric type, and the like. The electronic sensing device can generate certain electrochemical noise under some special environments to cause signal distortion, so that the sensing precision of a measured signal is influenced, and the sensitivity has certain limitation.
Disclosure of Invention
The purpose of the invention is as follows: in order to overcome the defects of the prior art, the invention provides a sensing device which has a simple structure, can sense the motion state of a structure with high precision and does not pollute the environment.
It is another object of the present invention to provide a method for sensing a two-dimensional flow velocity by the sensing device.
It is a third object of the present invention to provide a method for sensing two-dimensional acceleration by the sensing device.
The technical scheme is as follows: the sensing device of the invention comprises:
the optical fiber probe comprises a flexible optical fiber sheath with a circular cross section and four single-mode optical fibers; the optical fiber sheath is provided with four blind holes extending inwards along the length direction from the end face of the optical fiber sheath, the four blind holes are symmetrically distributed around the central axis of the optical fiber sheath at equal intervals, the bottom surfaces of the blind holes form reflecting surfaces, and the four reflecting surfaces are all positioned on the same plane perpendicular to the central axis; the four single-mode optical fibers are coaxially inserted into the blind holes in a one-to-one correspondence manner, and gaps are reserved between the four single-mode optical fibers and the reflecting surfaces to form a Fabry-Perot resonant cavity;
the fixing part is connected with the optical fiber sheath and is used for fixing the optical fiber probe on a measured object;
and the spectrum analysis equipment emits broadband light to the four single-mode optical fibers, and receives and analyzes signals transmitted by the four single-mode optical fibers.
In one embodiment, the fixing portion is a protective sleeve, which is sleeved outside the four single-mode optical fibers and is hermetically connected to an end face of the optical fiber sheath, where the blind hole is formed; the pile casing is provided with scales for measuring water depth. The optical fiber sheath is connected with the protective cylinder in a sealing mode, on one hand, the optical fiber probe is protected, the phenomenon that water permeates into the device to affect use is avoided, on the other hand, the sensing depth can be fed back, and therefore the device is particularly suitable for measuring the flow velocity of a water body.
In one embodiment, the optical fiber probe further comprises a mass connected to a bottom of the optical fiber sheath. By adding a mass, sensing of two-dimensional acceleration can be achieved.
Preferably, the mass is detachably connected to the bottom of the optical fiber sheath. The mass can be used to sense two-dimensional flow velocity when detached and two-dimensional acceleration when attached.
In particular, the reflective surface is coated with a reflective coating having a reflectance of greater than 0.04 for light having a wavelength of 1550 nm.
Specifically, the wavelength of the broadband light is 200-1590 nm.
Corresponding to the sensing device, the invention provides a method for sensing two-dimensional flow velocity, which is characterized by comprising the following steps:
(1) placing the optical fiber probe in a water body;
(2) the water flow enables the optical fiber probe to be bent and deformed, so that the gap between the single-mode optical fiber and the reflecting surface is changed;
(3) analyzing the cavity length of each Fabry-Perot resonant cavity in real time through spectral analysis equipment to obtain a deformation state and the flowing direction of a water body;
(4) and obtaining the flow velocity of the water body relative to the optical fiber probe through the relation between the flow velocity and the deformation state of the optical fiber probe.
In the step (3), when the neutral plane of the device bending does not pass through the center of any single-mode optical fiber, two single-mode optical fibers are in a compression area, and the other two single-mode optical fibers are in a tension area; defining the direction of a central connecting line of the single-mode optical fiber which passes through the center of the cross section of the optical fiber sheath, points to the compression area and is parallel to the negative of the product of the length changes of the two adjacent cavities as a reference direction, and then the included angle alpha between the flowing direction of the water body and the reference direction is as follows:
wherein k is r1/r2，r1Is the distance, r, from the neutral plane to the center of the first single mode fiber within the region under tension, closer to the neutral plane2The distance between the center of a second single-mode fiber adjacent to the first single-mode fiber in the compression area and a neutral plane is obtained;
when the bent neutral surface of the device passes through the center of any single-mode optical fiber, the single-mode optical fiber with the cavity length being lengthened in the water body flowing direction points to the single-mode optical fiber with the cavity length being shortened;
in the step (4), the optical fiber probe is similar to an elastic rod with a uniform cross section, the load applied by the fluid is equivalent to a uniformly distributed load, and according to a Morrison equation, the influence of a horizontal inertia force is ignored to obtain an expression formula of the relationship between the flow velocity u and the cavity length change of the Fabry-Perot resonant cavity:
wherein lfiFor the length of the i-th single mode fiber inside the fiber jacket,/0iIs the corresponding cavity length, Deltal, of the ith single-mode fiber before deformationtiThe cavity length variation corresponding to the ith single-mode fiber is represented by i, which is the number of the single-mode fiber, and i is 1,2,3,4, riIs the distance between the center of the single mode fiber i and the neutral plane, E is the elastic modulus of the fiber probe, CDRho is the seawater density and d is the outer diameter of the optical fiber sheath, which is the drag force coefficient.
In addition, the invention provides a method for sensing two-dimensional acceleration, which comprises the following steps:
(1) firmly bonding the fiber probe to a measured object, and keeping the probe vertically suspended;
(2) the end mass block enables the optical fiber probe to be bent and deformed under the action of inertia force, so that the gap between the single-mode optical fiber and the reflecting surface is changed;
(3) analyzing the cavity length of each Fabry-Perot resonant cavity in real time through spectral analysis equipment to obtain a deformation state and an acceleration direction;
(4) and obtaining the acceleration of the measured object according to the relation between the acceleration of the measured object and the deformation state of the optical fiber probe.
In the step (3), when the neutral plane of the device bending does not pass through the center of any single-mode optical fiber, two single-mode optical fibers are in a compression area, and the other two single-mode optical fibers are in a tension area; defining the direction of a central connecting line of the single-mode optical fiber which passes through the center of the cross section of the optical fiber sheath, points to the compression area and is parallel to the negative number of the product of the length changes of two adjacent cavities as a reference direction, and then, the included angle alpha between the acceleration direction and the reference direction is as follows:
wherein k is r1/r2，r1Is the distance, r, from the neutral plane to the center of the first single mode fiber within the region under tension, closer to the neutral plane2The distance between the center of a second single-mode fiber adjacent to the first single-mode fiber in the compression area and a neutral plane is obtained;
when the bent neutral surface of the device passes through the center of any single-mode optical fiber, the single-mode optical fiber with the cavity length being lengthened in the acceleration direction points to the single-mode optical fiber with the cavity length being shortened;
in the step (4), the sensing device is similar to a gravity-free elastic rod with a uniform cross section, the end mass block generates an inertia force, and a cavity length change relation expression of the acceleration a of the measured object and the Fabry-Perot resonant cavity is obtained:
wherein lfiFor the length of the i-th single mode fiber inside the fiber jacket,/0iIs the corresponding cavity length, Deltal, of the ith single-mode fiber before deformationtiThe cavity length variation corresponding to the ith single-mode fiber is represented by i, which is the number of the single-mode fiber, and i is 1,2,3,4, riIs the distance between the center of the single mode fiber i and the neutral plane, E is the modulus of elasticity of the fiber probe, msIs the mass of the mass block.
Has the advantages that: compared with the prior art, the sensing device forms the Fabry-Perot resonant cavity through the matching of the optical fiber and the blind holes in the optical fiber sheath, and can realize real-time sensing of two-dimensional flow rate and the two-dimensional acceleration sensing through matching of the mass block based on the Fabry-Perot resonant cavity. The sensor has the advantages of simple structure, high sensitivity, self-compensation of temperature, electromagnetic interference resistance, no pollution, good durability, real-time monitoring and the like, and is particularly suitable for occasions with long-time working, explosion prevention, water prevention, corrosion prevention, miniaturization, electromagnetic interference resistance and other requirements on the sensor in the fields of ocean engineering construction, ocean structure health monitoring and the like.
Drawings
FIG. 1 is a schematic structural diagram of a sensing device according to embodiment 1 of the present invention;
FIG. 2 is a schematic structural view of the optical fiber probe of embodiment 1;
FIG. 3 is a schematic structural diagram of a sensing device according to embodiment 2 of the present invention;
FIG. 4 is a schematic structural view of the optical fiber probe of embodiment 2;
FIG. 5 is a top view of the fiber optic probe;
FIG. 6 is a schematic diagram of the reflected light and incident light propagation paths;
FIG. 7 is a schematic view of the optical fiber probe of embodiment 1 uniformly loaded;
FIG. 8 is a schematic view of the fiber optic probe of example 2;
FIG. 9 is a schematic diagram of the calculation of the bending direction of the fiber probe and the distance from each single mode fiber to the neutral plane.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings.
As shown in fig. 1 to 2, the sensing device of embodiment 1 is used for sensing a two-dimensional flow velocity, and mainly includes a fiber-optic probe 1, a fixing portion 2, and a spectral analysis apparatus 3.
The fiber probe 1 comprises a fiber sheath 11 and four single mode fibers 12. The optical fiber sheath 11 is made of polymer or gel, and is cylindrical, and has an outer diameter of 500 micrometers to 5 millimeters and a length of 5 millimeters to 10 centimeters. Four blind holes 13 are formed from the end face inwards along the length direction, the diameter is 130-200 micrometers, and the depth is determined according to the size of the optical fiber sheath 11. Referring to fig. 5, four blind holes 13 are symmetrically arranged around the central axis of the optical fiber sheath 11 at equal intervals, and the center of each blind hole 13 is located on a concentric circle, which is centered on the central axis of the optical fiber sheath 11. The bottom surfaces of the blind holes 13 are polished flat and smooth, and a reflective coating is applied to form the reflective surface 14, which is required to have a reflectance of light having a wavelength of 1550nm and its vicinity of 0.04 or more. The four light reflecting surfaces 14 are all in the same plane perpendicular to the central axis.
The diameters of the four single mode fibers 12 are slightly smaller than the diameters of the blind holes 13, the cut surfaces at the end parts of the four single mode fibers are polished flat and smooth, the four single mode fibers are respectively and coaxially inserted into the four blind holes 13, gaps between the four single mode fibers and the reflecting surface 14 are between 50 micrometers and 1 millimeter, and a cavity formed by the gaps is a Fabry-Perot resonant cavity.
The insertion opening of the blind hole 13 is subjected to bonding and sealing treatment by using epoxy resin or other adhesives having bonding and sealing functions.
In order to protect the optical fiber and ensure the sealing performance to avoid internal water seepage, the fixing portion 2 is a cylindrical steel casing in this embodiment, which is coaxially connected with the optical fiber sheath 11, the joint is also sealed by an adhesive, and the single-mode optical fiber 12 is inserted into the casing and enters the spectrum analyzing device 3. The surface of the pile casing is provided with length scales to monitor the water depth.
The spectrum analysis device 3 emits broadband light to the four single-mode optical fibers 12, and receives and analyzes signals transmitted by the four single-mode optical fibers 12. The wavelength of broadband light is 200-1590 nm, and signals are processed by adopting an analysis method based on the optical fiber Fabry-Perot resonant cavity principle.
The installation method of the sensing device comprises the following steps: the optical fiber probe 1 is connected to an ocean structure, a ship body or other structures through a protective cylinder and is vertically placed; the receiving ends of the four single-mode fibers 12 are initially connected with the spectral analysis device 3 for initial state calibration. And (3) disconnecting the optical fiber receiving end from the spectral analysis equipment 3, positioning through a protective cylinder with length scales, completely immersing the optical fiber probe 1 into the designated position of the seawater to be detected, and then fixing. The receiving ends of the four single-mode optical fibers 1 are connected with the spectral analysis equipment 3 to complete installation.
The sensing device monitors the cavity length of the Fabry-Perot resonant cavity, so that the deformation of the optical fiber probe 1 is monitored, and the flow velocity of a water body relative to the optical fiber probe 1 is monitored. Since each single mode optical fiber 12 is fixed only at the insertion end in the optical fiber sheath 11 and is not bonded to the inside of the optical fiber sheath 11, it can be deformed relatively freely, and therefore, when the optical fiber probe 1 is bent, the length of the resonance cavity corresponding to each optical fiber changes. Because the size and the rigidity of the optical fiber are smaller than those of the flexible optical fiber sheath 11 made of the polymer material, the optical fiber probe 1 is approximate to the solid polymer with the same outer diameter and height in the process of calculating the deformation when the fluid flows to the optical fiber probe 1, and the load applied when the fluid flows is approximately considered to be uniform load. And analyzing the deformation of the optical fiber probe 1 by using a cantilever beam model under uniformly distributed load. The single mode fiber and the fiber sheath are not bonded, and the length change of the fiber is ignored when the fiber probe 1 is bent. And (3) analyzing the cavity length corresponding to each optical fiber in real time to obtain the deformation curve and direction of the optical fiber probe. The flow velocity (including the size and the direction) of the seawater relative to the optical fiber probe, namely the flow velocity of the seawater relative to the structure, is obtained through the relationship between the flow velocity and the deformation state of the optical fiber probe.
Specifically, the sensing method comprises the following steps:
(1) vertically placing the optical fiber probe in a water body;
(2) the water flow enables the optical fiber probe to be bent and deformed, so that the gap between the single-mode optical fiber and the reflecting surface is changed;
(3) analyzing the cavity length of each Fabry-Perot resonant cavity in real time through spectral analysis equipment to obtain a deformation state and the flowing direction of a water body;
(4) and obtaining the flow velocity of the water body relative to the optical fiber probe through the relation between the flow velocity and the deformation state of the optical fiber probe.
As shown in fig. 6, the light (E) is detectedincThe light (E) is emitted from the core into the air in the cavity at the end of the single-mode fiber 12refl,1) Reflection at the core-air interface re-enters the core, and another part of the light (E)laun) The transmission into air from the core-air interface becomes transmitted light. The transmitted light continues to propagate in air (E)circ) The reflection occurs at the air-reflective coating interface, and the propagation direction of the reflection is opposite to that before the reflection (E)b-circ) The reflected light continues to travel to the air-core interface, a portion of which (E)RT) Emission occurs again in the direction of the reflective coating, another part of the light (E)back) Through the interface and into the core. Since the light reflected by the two interfaces is parallel to each other, the light propagates perpendicularly to the two interfaces, resulting in multiple reflections. Considering propagation equations and boundary conditions of light, and interference fringes between transmitted light transmitted from interface and reflected light reflected by interfaceAn element that reflects light having an electric field of:
wherein r isabIs the reflectivity of the interface of material a and material b, tabIs the transmission at the interface of material a and material b, wherein a is 1,2 or 3 and b is 1,2 or 3; subscripts 1,2,3 represent single mode fiber, air, reflective coating (reflective surface), respectively; j is an imaginary unit; phi 2 pi l0n0/λ0；l0Is the cavity length of the fabry-perot resonator; n is0Is the refractive index of air, and can be approximated as 1; lambda [ alpha ]0Is the wavelength of the probe light in vacuum.
From equation (1), the electric field intensity of the reflected light is a quasi-periodic function of the wavelength, and the derivation of equation (1) and making it equal to 0 can be obtained:
wherein λ0lmIs an extreme point in the reflected light electric field intensity distribution image along with the wavelength; m is the modulus of each extreme point. As can be seen from the formula (2), for the same modulus, the distribution of the intensity of the reflected light with the wavelength shows a linear relationship between each extreme point in the image and the cavity length, and the slope is 4n0And/m. Therefore, by monitoring the wavelength of each extreme point in the reflection spectrum corresponding to each optical fiber, the monitoring of each corresponding cavity length can be realized, and the sensitivity is 4n0/m。
Since the size and rigidity of the optical fiber are very small compared with those of optical fiber polymer or gel sleeves, the sensing device can be approximated to an elastic rod with a uniform cross section in the process of deformation when the fluid flows relative to the sensing device, and the load applied by the fluid is uniform load (fig. 7). According to the approximation, the deformation of the sensing device can be analyzed by utilizing a cantilever beam model under uniformly distributed load. Under the equipartition load, the distribution of the moment of flexure of cantilever beam along the axial does:
wherein x is the distance from the calculation point to the fixed end; q is the uniform load applied by the fluid on the sensing device; l is the length of the sensing device from the fixed end. Considering a sensing device with a circular cross-section, the strain profile is:
wherein r is the distance from the calculated point to the neutral plane; e is the modulus of elasticity of the sensing component; d is the outer diameter of the fiber optic polymer or gel sleeve. And (4) integrating the equation along the length of the rod to obtain the deformation of the sensing device along the axial direction. Because there is no bond between the optical fiber and the optical fiber sheath, the length change of the optical fiber is ignored when the sensing device is bent. Let the length of the ith optical fiber in the optical fiber sheath be lfiFrom the neutral plane riThe length of the cavity before deformation is l0iWherein the subscript i represents the number of the optical fiber. Variation Δ l from cavity lengthtiDeducing to obtain the size of the uniform load q as follows:
the formula for q above the unit column height at any height z of a vertical column according to the Morisen equation is:
the influence of the horizontal inertia force is neglected in equation (6). CDAnd p is the drag force coefficient, rho is the seawater density, and u is the seawater flow velocity.
The calculation formula of the seawater flow velocity is obtained as follows:
for the determination of the direction of flow of the fluid and the distance of each fiber from the neutral plane, the general case will first be discussed. As shown in fig. 9, when the neutral plane does not pass through the center of any single mode fiber, two fibers are necessarily in the compression zone and the other two fibers are in the tension zone because the four single mode fibers are arranged circumferentially symmetrically at equal intervals. The number 111 and 114 is the center of the four optical fibers, the solid line big circle is the outer contour of the optical fiber probe, and the broken line passing through the center of the circle is the neutral plane of the bending of the optical fiber probe. The optical fibers numbered 112 and 114 are in the compression area, and the cavity length is shortened; the fibers numbered 111 and 113 have longer cavity lengths in the tension zone. The cavity length changes of the four optical fibers can be obtained by analyzing the spectrum. Therefore, the direction parallel to the central connecting line of the two adjacent optical fibers with the product of the length change of the two cavities being negative is taken as a reference direction. The flow direction of the fluid can be known by obtaining the included angle between the neutral plane and the reference direction. The solid line arrow in fig. 9 indicates the reference direction in this example, and the solid line passing through the center of the circle is perpendicular to the reference direction. The dotted arrow is perpendicular to the neutral plane and is the deformation direction of the fiber probe, i.e. the flow direction of the fluid. The distances from the optical fibers with the numbers of 111-114 to the neutral plane are respectively recorded as r1～r4。
Note that the angle between the dashed arrow (the direction of flow of the fluid) and the solid arrow (the reference direction) is α.
Wherein k is r1/r2，r1Is the distance, r, from the neutral plane to the center of the first single mode fiber within the region under tension, closer to the neutral plane2The distance between the center of a second single-mode fiber adjacent to the first single-mode fiber in the compression area and a neutral plane is obtained; at this point, the direction of flow of the fluid relative to the sensing device has been derived.
Considering the special case, when the neutral plane passes through the center points of the two optical fibers, the number 111 and the number 113 optical fibers must be two non-adjacent optical fibers, and the central connecting line passes through the sensingThe center of the circle of the device. In this case, the two optical fibers are spaced from the neutral plane by a distance of 0, and the other two optical fibers are spaced from the neutral plane by a distance of
And alpha is 0, and the single-mode optical fiber with the longer cavity length in the water body flowing direction points to the single-mode optical fiber with the shorter cavity length.
For the general situation, the distances from four single-mode fibers to the neutral plane can be obtained, and the value from one fiber to the neutral plane is only needed for obtaining uniform load. Therefore, the values of the remaining three optical fibers are redundancy values, and the uniform loads obtained by theoretically substituting the four values should be consistent. However, in practical applications, the four obtained uniform loads should have slightly different values due to various errors and interferences. Therefore, in practical applications, the four obtained values can be averaged to reduce the error.
For special cases, since the two single-mode fibers pass through the neutral plane, the distance between the two single-mode fibers and the neutral plane and the cavity length change are both zero, and the molecular and distribution are both zero in the above formula. Therefore, the values of the two optical fibers cannot be used, the magnitude of the uniform load is solved by using the relevant values of the two optical fibers which do not pass through the neutral plane, and the average value of the result is obtained.
The sensing device of embodiment 2 is used for sensing two-dimensional acceleration, and as shown in fig. 3 and 4, the structure thereof is substantially the same as that of embodiment 1, except that: when underwater operation is not required, the fixing portion 2 can be directly mounted on the end portion of the optical fiber sheath 11 where the blind hole 13 is formed, by using a patch made of a metal or nonmetal sheet having a strong bending resistance and being not easily reacted with the environment.
A balancing weight 15 is connected to the bottom of optic fibre sheath 11, of course, can set balancing weight 15 to detachable structure, need measure when two-dimensional acceleration install on optic fibre sheath 11 can.
In addition, for measuring the two-dimensional acceleration, the gap between the single-mode optical fiber 12 and the reflecting surface 14 is set to be about 1mm, the outer diameter of the optical fiber sheath 11 is 5mm, the length is 10cm, and the diameter of the blind hole is preferably about 200 μm. Correspondingly, the height of the control mass block 15 is 5mm, the material is a material with high density and difficult reaction with the environment, and metals such as lead and the like or non-metals with high density can be generally selected.
The sensing method comprises the following steps:
(1) firmly bonding the fiber probe to a measured object, and keeping the probe vertically suspended;
(2) the end mass block enables the optical fiber probe to be bent and deformed under the action of inertia force, so that the gap between the single-mode optical fiber and the reflecting surface is changed;
(3) analyzing the cavity length of each Fabry-Perot resonant cavity in real time through spectral analysis equipment to obtain a deformation state and an acceleration direction;
(4) and obtaining the acceleration of the measured object according to the relation between the acceleration a of the measured object and the deformation state of the optical fiber probe.
As shown in fig. 6, the light (E) is detectedinc) The light (E) is emitted from the core into the air in the cavity at the end of the single-mode fiber 12refl,1) Reflection at the core-air interface re-enters the core, and another part of the light (E)laun) The transmission into air from the core-air interface becomes transmitted light. The transmitted light continues to propagate in air (E)circ) The reflection occurs at the air-reflective coating interface, and the propagation direction of the reflection is opposite to that before the reflection (E)b-circ). The reflected light continues to travel to the air-core interface, a portion of which (E)RT) Emission occurs again in the direction of the reflective coating, another part of the light (E)back) Through the interface and into the core. Since the light reflected by the two interfaces is parallel to each other, the light propagates perpendicularly to the two interfaces, resulting in multiple reflections. Considering the propagation equation and boundary conditions of light, and the interference condition between the transmitted light transmitted from the interface and the reflected light reflected by the interface, the electric field of the reflected light is:
wherein r isabIs the reflectivity of the interface of material a and material b, tabTransmittance of the interface of material a and material bWherein a is 1,2 or 3 and b is 1,2 or 3; subscripts 1,2,3 represent single mode fiber, air, reflective coating (reflective surface), respectively; j is an imaginary unit; phi 2 pi l0n0/λ0；l0Is the cavity length of the fabry-perot resonator; n is0Is the refractive index of air, and can be approximated as 1; lambda [ alpha ]0Is the wavelength of the probe light in vacuum. From equation (1), the electric field intensity of the reflected light is a quasi-periodic function of the wavelength, and the derivation of equation (1) and making it equal to 0 can be obtained:
wherein λ0lmIs an extreme point in the reflected light electric field intensity distribution image along with the wavelength; m is the modulus of each extreme point. As can be seen from the formula (2), for the same modulus, the distribution of the intensity of the reflected light with the wavelength shows a linear relationship between each extreme point in the image and the cavity length, and the slope is 4n0And/m. Therefore, by monitoring the wavelength of each extreme point in the reflection spectrum corresponding to each optical fiber, the monitoring of each corresponding cavity length can be realized, and the sensitivity is 4n0/m。
Since the size and rigidity of the optical fiber are very small compared with those of the optical fiber polymer or gel sleeve, the sensing device can be approximated to an elastic rod with a uniform section in the process of deformation relative to the sensing device, and the inertial force generated by the end mass block is equivalent to a concentrated load F (fig. 8). From the above approximation, the cantilever model under the action of F can be used to analyze the deformation of the sensing device. Under concentrated load, the distribution of the bending moment of the cantilever beam along the axial direction is as follows:
M(x)＝F(L-x) (3)
wherein x is the distance from the calculation point to the fixed end; f is the concentrated load applied on the sensing device; l is the length of the sensing device from the fixed end. Considering a sensing device with a circular cross-section, the strain profile is:
wherein r is the distance from the calculated point to the neutral plane; e is the modulus of elasticity of the sensing component; d is the outer diameter of the fiber optic polymer or gel sleeve. And (4) integrating the equation along the length of the rod to obtain the deformation of the sensing device along the axial direction. Because there is no bond between the optical fiber and the optical fiber sleeve, the length change of the optical fiber is ignored when the sensing device is bent. Let the length of the ith optical fiber in the optical fiber sleeve be lfiFrom the neutral plane riThe length of the cavity before deformation is l0iWherein the subscript i represents the number of the optical fiber. Variation Δ l from cavity lengthtiThe magnitude of the concentrated load F is derived as follows:
the acceleration calculation formula thus obtained is:
for determining the distance of each fiber from the neutral plane, the general case will first be discussed. As shown in fig. 9, when the neutral plane does not pass through the center of any single mode fiber, two fibers are necessarily in the compression zone and the other two fibers are in the tension zone because the four single mode fibers are arranged circumferentially symmetrically at equal intervals. The number 111 and 114 is the center of the four optical fibers, the solid line big circle is the outer contour of the optical fiber probe, and the broken line passing through the center of the circle is the neutral plane of the bending of the optical fiber probe. The optical fibers numbered 112 and 114 are in the compression area, and the cavity length is shortened; the fibers numbered 111 and 113 have longer cavity lengths in the tension zone. The cavity length changes of the four optical fibers can be obtained by analyzing the spectrum. Therefore, the negative direction parallel to the central line of two optical fibers with the adjacent cavity length changing is the reference direction. The acceleration direction of the measured object can be known by obtaining the included angle between the neutral plane and the reference direction. The solid line arrow in fig. 9 indicates the reference direction in this example, and the solid line passing through the center of the circle is perpendicular to the reference direction. The dotted arrow is perpendicular to the neutral plane and is the deformation direction of the fiber probeI.e. the direction of acceleration. The distances from the optical fibers with the numbers of 111-114 to the neutral plane are respectively recorded as r1～r4. Note that the angle between the dashed arrow (acceleration direction) and the solid arrow (reference direction) is α.
Wherein k is r1/r2，r1Is the distance, r, from the neutral plane to the center of the first single mode fiber within the region under tension, closer to the neutral plane2The distance between the center of a second single-mode fiber adjacent to the first single-mode fiber in the compression area and a neutral plane is obtained; the direction of acceleration is now derived.
Considering the special case, when the neutral plane passes through the center points of the two optical fibers, the number 111 and the number 113 optical fibers must be two non-adjacent optical fibers, and the central connecting line passes through the center of the circle of the sensing device. In this case, the two optical fibers are spaced from the neutral plane by a distance of 0, and the other two optical fibers are spaced from the neutral plane by a distance of
The single-mode optical fiber with the acceleration direction of the longer cavity length points to the single-mode optical fiber with the shorter cavity length, wherein alpha is 0.
For the general case, the distances of the four single-mode fibers from the neutral plane can be obtained, and the value of F from the neutral plane is only needed to be obtained. Therefore, the values of the remaining three optical fibers are redundant values, and F obtained by theoretically substituting the four values should match. However, in practical applications, the four concentrated loads obtained should have slightly different values due to various errors and interferences. Therefore, in practical applications, the four obtained values can be averaged to reduce the error.
For special cases, since the two single-mode fibers pass through the neutral plane, the distance between the two single-mode fibers and the neutral plane and the cavity length change are both zero, and the molecular and distribution are both zero in the above formula. Therefore, the values of the two optical fibers cannot be used, the magnitude of the uniform load is solved by using the relevant values of the two optical fibers which do not pass through the neutral plane, and the average value of the result is obtained.
Claims (10)
1. A sensing device, comprising:
the optical fiber probe (1) comprises a flexible optical fiber sheath (11) with a circular cross section and four single-mode optical fibers (12); the optical fiber sheath (11) is provided with four blind holes (13) extending inwards along the length direction from the end face of the optical fiber sheath, the four blind holes (13) are symmetrically distributed around the central axis of the optical fiber sheath (11) at equal intervals, the bottom surfaces of the blind holes (13) form reflecting surfaces (14), and the four reflecting surfaces (14) are all positioned on the same plane perpendicular to the central axis; the four single-mode optical fibers (12) are coaxially inserted into the blind holes (13) in a one-to-one correspondence manner, and gaps are reserved between the four single-mode optical fibers and the reflecting surface (14) to form a Fabry-Perot resonant cavity;
a fixing part (2) which is connected with the optical fiber sheath (11) and is used for fixing the optical fiber probe (1) on a measured object;
and the spectral analysis device (3) emits broadband light to the four single-mode optical fibers (12), and receives and analyzes signals transmitted by the four single-mode optical fibers (12).
2. The sensing device according to claim 1, wherein the fixing portion (2) is a casing which is sleeved outside the four single-mode optical fibers (12) and is hermetically connected to an end face of the optical fiber sheath (11) where the blind hole (13) is formed; the pile casing is provided with scales for measuring water depth.
3. The sensing device according to claim 1, wherein the fiber optic probe (1) further comprises a mass (15), the mass (15) being connected to the bottom of the fiber optic sheath (11).
4. A sensing device according to claim 3, wherein the mass is detachably connected to the bottom of the optical fibre sheath (11).
5. A sensing device according to claim 1, wherein the reflective surface (14) is coated with a reflective coating having a reflectivity of greater than 0.04 for light having a wavelength of 1550 nm.
6. The sensing device of claim 1, wherein the wavelength of the broadband light is 200-1590 nm.
7. A method of sensing a two-dimensional flow velocity using the sensing device of claim 1, comprising the steps of:
(1) placing the optical fiber probe in a water body;
(2) the water flow enables the optical fiber probe to be bent and deformed, so that the gap between the single-mode optical fiber and the reflecting surface is changed;
(3) analyzing the cavity length of each Fabry-Perot resonant cavity in real time through spectral analysis equipment to obtain a deformation state and the flowing direction of a water body;
(4) and obtaining the flow velocity of the water body relative to the optical fiber probe through the relation between the flow velocity and the deformation state of the optical fiber probe.
8. The method of sensing a two-dimensional flow velocity of claim 7, wherein in step (3), when the neutral plane of the device bend does not pass through the center of either single mode fiber, two single mode fibers are in the compression zone and the other two single mode fibers are in the tension zone; defining the direction of a central connecting line of the single-mode optical fiber which passes through the center of the cross section of the optical fiber sheath, points to the compression area and is parallel to the negative of the product of the length changes of the two adjacent cavities as a reference direction, and then the included angle alpha between the flowing direction of the water body and the reference direction is as follows:
wherein k is r1/r2，r1Is the distance, r, from the neutral plane to the center of the first single mode fiber within the region under tension, closer to the neutral plane2For second single mode light adjacent to the first single mode fiber in the compression zoneDistance between the center of the fiber and the neutral plane;
when the bent neutral surface of the device passes through the center of any single-mode optical fiber, the single-mode optical fiber with the cavity length being lengthened in the water body flowing direction points to the single-mode optical fiber with the cavity length being shortened;
in the step (4), the optical fiber probe is similar to an elastic rod with a uniform cross section, the load applied by the fluid is equivalent to a uniformly distributed load, and according to a Morrison equation, the influence of a horizontal inertia force is ignored to obtain an expression formula of the relationship between the flow velocity u and the cavity length change of the Fabry-Perot resonant cavity:
wherein lfiFor the length of the i-th single mode fiber inside the fiber jacket,/0iIs the corresponding cavity length, Deltal, of the ith single-mode fiber before deformationtiThe cavity length variation corresponding to the ith single-mode fiber is represented by i, which is the number of the single-mode fiber, and i is 1,2,3,4, riIs the distance between the center of the single mode fiber i and the neutral plane, E is the elastic modulus of the fiber probe, CDRho is the seawater density and d is the outer diameter of the optical fiber sheath, which is the drag force coefficient.
9. A method of sensing acceleration in two dimensions using the sensing device of claim 3, comprising the steps of:
(1) firmly bonding the fiber probe to a measured object, and keeping the probe vertically suspended;
(2) the end mass block enables the optical fiber probe to be bent and deformed under the action of inertia force, so that the gap between the single-mode optical fiber and the reflecting surface is changed;
(3) analyzing the cavity length of each Fabry-Perot resonant cavity in real time through spectral analysis equipment to obtain a deformation state and an acceleration direction;
(4) and obtaining the acceleration of the measured object according to the relation between the acceleration of the measured object and the deformation state of the optical fiber probe.
10. The method of sensing two-dimensional acceleration according to claim 9, characterized in that in step (3), when the neutral plane of the device bending does not pass through the center of either single mode fiber, two single mode fibers are in the compression zone and the other two single mode fibers are in the tension zone; defining the direction of a central connecting line of the single-mode optical fiber which passes through the center of the cross section of the optical fiber sheath, points to the compression area and is parallel to the negative number of the product of the length changes of two adjacent cavities as a reference direction, and then, the included angle alpha between the acceleration direction and the reference direction is as follows:
wherein k is r1/r2，r1Is the distance, r, from the neutral plane to the center of the first single mode fiber within the region under tension, closer to the neutral plane2The distance between the center of a second single-mode fiber adjacent to the first single-mode fiber in the compression area and a neutral plane is obtained;
when the bent neutral surface of the device passes through the center of any single-mode optical fiber, the single-mode optical fiber with the cavity length being lengthened in the acceleration direction points to the single-mode optical fiber with the cavity length being shortened;
in the step (4), the sensing device is similar to a gravity-free elastic rod with a uniform cross section, the end mass block generates an inertia force, and a cavity length change relation expression of the acceleration a of the measured object and the Fabry-Perot resonant cavity is obtained:
wherein lfiFor the length of the i-th single mode fiber inside the fiber jacket,/0iIs the corresponding cavity length, Deltal, of the ith single-mode fiber before deformationtiThe cavity length variation corresponding to the ith single-mode fiber is represented by i, which is the number of the single-mode fiber, and i is 1,2,3,4, riIs the distance between the center of the single mode fiber i and the neutral plane, E is the modulus of elasticity of the fiber probe, msIs the mass of the mass block.
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