CN100353167C - Sensing device for optical fiber optical grating flow speed - Google Patents

Abstract

The invention relates to a sensing device capable of high-precision real-time monitoring and measurement of parameters such as flow velocity and flow rate of various fluids, and can be widely used in many fields such as building structures, aerospace, marine exploration and scientific research. The sensing mechanism is mainly composed of a closed telescopic tube, a fiber grating, a split ring, and an optical fiber. The characteristics are: the designed flow rate-pressure tube has two different flow rates, which can be used to convert the flow rate into pressure for sensing and monitoring. When the fluid passes through the flow rate-pressure tube, there is a pressure difference inside and outside the airtight telescopic tube, which will elongate or shorten in the axial direction, and then drive the split ring to compress or stretch, and finally lead to the drift of the center wavelength of the fiber grating. Under a constant working environment temperature, according to the drift of the center wavelength, use the corresponding formula to obtain high-sensitivity sensing and monitoring of parameters such as flow velocity and flow rate. The device uses a fiber grating as a sensing element, has a simple structure, a novel method and high measurement accuracy.

Description

Fiber Bragg Grating Flow Velocity Sensing Device

technical field

The invention relates to a sensing device, capable of high-precision real-time monitoring and measurement of parameters such as flow velocity and flow rate of various fluids, and can be widely used in many fields such as building structures, aerospace, marine exploration and scientific research, and belongs to a Novel non-destructive testing technology.

Background technique

Fiber Bragg Grating is a new photonic device that has appeared recently, which is a spatially periodic refractive index distribution formed in the fiber core. This periodic structure can change and control the propagation behavior of light waves in optical fibers. The fiber grating sensor is to embed the fiber grating inside the substrate material and structure or paste it on its surface to make it sensitive to the parameter to be measured, and sense the size and direction of the parameter to be measured through the change of the wavelength and bandwidth of the fiber grating. Combining fiber grating sensor arrays with wavelength division multiplexing and time division multiplexing systems can realize multi-point monitoring of material properties (such as temperature, strain, pressure, displacement, velocity, acceleration, etc.). Fiber Bragg grating sensing technology has been widely used in many fields such as building structure, aerospace, marine exploration and scientific research, and belongs to a novel non-destructive testing technology.

The invention is a fiber grating sensing device capable of high-precision sensing of flow velocity and flow rate. The search results show that there is no patent report on the use of fiber gratings to realize flow velocity and flow sensing.

Contents of the invention

The object of the present invention is to disclose a structure of a fiber grating sensing device, which can sense parameters such as flow velocity and flow rate of fluid with high precision. The invention uses the optical fiber grating as the sensing element, and has the advantages of high precision, anti-electromagnetic interference, remote and multi-point distributed measurement and monitoring, and the like.

Sensing principle of the invention

First, use the flow velocity-pressure tube, through which the flow velocity in the tube can be converted into pressure; then, design the corresponding fiber grating pressure sensing mechanism to sense the pressure; calculate the flow velocity value in the tube according to the measured pressure. Its main feature is that the flow rate is converted into pressure by using the flow rate-pressure tube, and then sensed and monitored by the fiber grating pressure sensing mechanism.

Technical solution of the present invention: fiber grating flow velocity sensing device, which includes a flow velocity-pressure tube, characterized in that: the conduit 7 connects the two through the air hole a above the flow velocity-pressure tube 1 and the air hole c of the airtight cylinder 3 Up, the conduit 8 connects the two through the air hole b above the flow rate-pressure tube and the air hole d in the center of the closed cylinder, and communicates with the closed telescopic tube 2 in the center of the closed cylinder. The inverted split ring 5 is in the airtight cylinder The inner right side is connected and fixed to the airtight telescopic tube and the airtight cylinder through the connecting rods 9 and 10 respectively. The fiber grating 4 is written on the optical fiber 6 and pasted on the side of the split ring. One end of the optical fiber 6 is suspended, and the other end passes through the airtight cylinder.

According to the Bernoulli equation, the pressure inside the pipe is related to the flow velocity, the greater the flow velocity, the lower the pressure. Flow Velocity-Pressure There are two different flow velocities V ₁ and V ₂ in the tube, and the pressures P ₁ and P ₂ in these two places are also different. There is a pressure difference ΔP, that is, the pressure difference ΔP can be used to characterize the flow velocity. From the fluid continuity equation and the Bernoulli equation, the pressure difference ΔP between the flow velocity V ₁ and the flow velocity V ₂ can be expressed as:

$\mathrm{\&Delta;P}=P_1-{\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="C20051001589700071.png"\; state="\{\{state\}\}">P</figure-callout>}}_2=\mathrm{\&kappa;\&rho;}{\mathrm{<figure-callout\; id="1"\; label="V"\; filenames="C20051001589700071.png"\; state="\{\{state\}\}">V</figure-callout>}}_1^2+C=\mathrm{\&kappa;\&rho;}{V}^2+C$ (1)

In the formula, κ is the coefficient related to the flow velocity-pressure tube structure, ρ is the density of the fluid, C is a constant, and V=V ₁ is the flow velocity of the fluid. From formula (1), it can be seen that the pressure difference ΔP between the flow velocity V ₁ and the flow velocity V ₂ is proportional to the square of the flow velocity V. That is, the measurement of the flow velocity V can be converted into the measurement of the pressure difference ΔP, and the pressure difference ΔP can be measured by the fiber Bragg grating pressure sensing mechanism.

The fiber grating pressure sensing mechanism is mainly composed of a closed telescopic tube, a closed cylinder, a fiber grating, a split ring, and an optical fiber. The elastic coefficient of the airtight expansion tube is very small, and it can only expand and contract in the axial direction, and one end of the airtight expansion tube is pasted on one end of the airtight cylinder. The split ring is placed flat in the airtight cylinder, and the semicircular arches of the split ring are respectively connected and fixed with the airtight expansion tube and the airtight cylinder through connecting rods. The fiber grating is pasted on the side of the split ring, and the axis of the grating is parallel to the side of the split ring. One end of the optical fiber is suspended in the air, and the other end passes through the closed cylinder. When there is a pressure difference between the inside and outside of the closed telescopic tube, the telescopic tube will elongate or shorten along the axial direction, so that the semi-circular arch of the open ring tube will be subjected to pressure or tension, and then the fiber grating will be stretched or compressed, passing through the center wavelength of the grating. The pressure difference between the inside and outside of the closed expansion tube can be measured. One conduit connects the flow rate-pressure tube and the closed cylinder respectively through the air hole a above the flow rate-pressure tube and the air hole c of the closed cylinder, and the other one passes through the air hole b above the flow rate-pressure tube and the center of the closed cylinder respectively The air hole d connects the two and communicates with the closed expansion tube 2 in the center of the closed cylinder. At this time, the pressure difference between the axial direction of the closed expansion tube and the inside and outside is equal to the pressure difference ΔP between the flow velocity V ₁ and the flow velocity V ₂ , and ΔP corresponds to the flow velocity, so the flow velocity can be measured by the change of the center wavelength of the grating.

At a constant temperature, the central wavelength of the fiber Bragg grating will drift to the short wave under the action of the tension F of the split ring

$\mathrm{\&Delta;\&lambda;}=\frac{12{\mathrm{\&mu;}}_1{\mathrm{\&lambda;}}_0(1-p_e)d}{\mathrm{E.}{a}^{3}b}\&CenterDot;(\frac{a}{2}-\frac{{\mathrm{<figure-callout\; id="2"\; label="a"\; filenames="C20051001589700071.png"\; state="\{\{state\}\}">a</figure-callout>}}^2}{12d})f$ (2)

Where a is the width of the open ring straight beam, b is the thickness of the straight beam, the radius of the semi-circular arch of the open ring is r, the moment action distance d=r+a/2 is the distance between the external force F and the centerline of the straight beam, and pe _is the effective optical fiber The elastic-optical coefficient is about 0.22, E is the Young's modulus of the split ring material, λ ₀ is the center wavelength of the fiber grating, and μ ₁ is the strain transfer factor of the split ring, which is related to factors such as the substrate material and pasting conditions. And F=ΔPS, ΔP is the pressure difference inside and outside the closed expansion tube, and S is the cross-sectional area of the expansion tube. at this time

$\mathrm{\&Delta;\&lambda;}=\frac{12{\mathrm{\&mu;}}_1{\mathrm{\&lambda;}}_0(1-p_e)d}{\mathrm{E.}{a}^{3}b}\&CenterDot;(\frac{a}{2}-\frac{{\mathrm{<figure-callout\; id="2"\; label="a"\; filenames="C20051001589700071.png"\; state="\{\{state\}\}">a</figure-callout>}}^2}{12d})\mathrm{\&Delta;PS}=\frac{12\mathrm{\&kappa;}{\mathrm{\&mu;}}_1{\mathrm{\&mu;}}_2{\mathrm{\&lambda;}}_0(1-p_e)\mathrm{\&rho;dS}}{\mathrm{E.}{a}^{3}b}(\frac{a}{2}-\frac{{\mathrm{<figure-callout\; id="2"\; label="a"\; filenames="C20051001589700071.png"\; state="\{\{state\}\}">a</figure-callout>}}^2}{12d}){\mathrm{<figure-callout\; id="2"\; label="V"\; filenames="C20051001589700071.png"\; state="\{\{state\}\}">V</figure-callout>}}^2+\mathrm{\&Omega;}$ (3)

in $\mathrm{\&Omega;}=\frac{12{\mathrm{\&mu;}}_1{\mathrm{\&lambda;}}_0(1-p_e)\mathrm{wxya}}{\mathrm{E.}{a}^{3}B}-(\frac{a}{2}-\frac{a^2}{12d}),$ μ ₂ is the correction coefficient of the sensing device.

From formula (3), it is easy to know that the center wavelength shift Δλ of the fiber grating is linearly related to the square of the flow velocity, so we can use the grating wavelength encoding technology to measure the flow velocity of the fluid. In addition, it can be seen from the above formula that the wavelength shift of the fiber grating Δλ is related to the torque action distance d, the straight beam width a, thickness b, and the structure of the flow velocity-pressure tube. Improving the parameters of flow rate-pressure tube and split ring can further improve the sensing sensitivity.

The flow W is the product of the flow velocity V and the flow velocity-pressure tube cross-sectional area s, that is, W=sV.

The beneficial effect of the invention is that the optical fiber grating is used as the sensing element, the structure is simple, the method is novel, and the measurement precision is high. Based on the advantages of fiber grating itself, this sensor also has the advantages of anti-electromagnetic interference, corrosion resistance, suitable for working in harsh environments such as flammable, explosive, high temperature and high pressure, and remote and multi-point distributed measurement.

Description of drawings

Fig. 1 is a structural schematic diagram of a fiber grating flow velocity sensing device.

Fig. 2 is a diagram of the measuring device of the present invention.

Fig. 3 is a typical spectrogram of the present invention within a certain flow rate range.

Fig. 4 is a graph showing the relationship between center wavelength shift and corresponding flow velocity in the present invention.

Among them: 1 flow rate-pressure tube, 2 airtight telescopic tube, 3 airtight cylinder, 4 fiber grating, 5 split ring, 6 optical fiber, 7 conduit, 8 conduit, 9 connecting rod, 10 connecting rod, 11 fluid inlet, 12 fluid outlet , 13 fiber optic coupler, 14 light source, 15 light detector, V ₁ , V ₂ flow rate—the flow rate of the fluid in the pressure tube, a. air hole, b. air hole, c. air hole, d. air hole.

Detailed ways

Embodiments of the present invention are further specifically described below in conjunction with the accompanying drawings:

Fig. 1 is a schematic structural diagram of a fiber grating flow velocity sensing device, and Fig. 2 is a diagram of a measuring device.

Technical solution of the present invention: fiber grating flow velocity sensing device, which includes a flow velocity-pressure tube, characterized in that: the conduit 7 connects the two through the air hole a above the flow velocity-pressure tube 1 and the air hole c of the airtight cylinder 3 Up, the conduit 8 connects the two through the air hole b above the flow rate-pressure tube and the air hole d in the center of the closed cylinder, and communicates with the closed telescopic tube 2 in the center of the closed cylinder. The inverted split ring 5 is in the airtight cylinder The right side of the interior is connected and fixed to the airtight telescopic tube and the airtight cylinder through connecting rods 9 and 10 respectively. The fiber grating 4 is written on the optical fiber 6 and pasted on the side of the split ring. One end of the optical fiber 6 is suspended in the air, and the other end passes through the airtight cylinder. Place the flow rate-pressure tube 1 and the closed cylinder 3 horizontally. The fiber grating 4 is pasted on the side of the split ring 5, and the connection method between the split ring 5 and the conduit is as described above. The light emitted by the light source 14 reaches the fiber grating 4 through the fiber coupler 13 and is reflected.

The above-mentioned V ₁ and V ₂ are different flow velocities in the flow rate-pressure tube, and the flow rate-pressure tube is made of glass, polymer, metal, alloy and other materials.

The airtight expansion tube can only expand and contract in the axial direction, and it is made of organic polymer and aluminum foil materials.

The fiber grating is glass, plastic fiber Bragg grating or long period fiber grating.

The split ring is made of organic polymer or metal material with medium Young's modulus of 1000MPa-4000MPa.

The optical fiber is a single-mode optical fiber.

The working environment temperature of the fiber grating flow velocity sensing device is between -20°C and 70°C.

Example

The velocity-pressure tube is made of glass.

The airtight telescopic tube is made of aluminum foil tube.

The airtight cylinder and the split ring are made of plexiglass (Young's modulus is about 2500Mpa).

Fiber Bragg grating is a fiber Bragg grating written by using quartz single-mode fiber, using ultraviolet exposure technology and phase mask method, or a long-period fiber grating written by using amplitude mask method. The fiber grating is pasted on the side of the split ring straight beam. The fiber is a quartz single-mode fiber; the fiber coupler is a 2×2 or 1×2 fiber coupler.

The light source is a broadband light source or a tunable fiber laser; the optical detector is a fiber optic spectrometer.

The actual flow rate—the inner diameter of the pressure tube is 26.5mm, and the inner diameter of the throat is 11.0mm. The effective cross-sectional area of the telescopic tube is S=9.5×10 ^-3 m ² , and the fiber Bragg grating used is a fiber Bragg grating with a length of about 12mm, a central wavelength of about 1562nm, and a peak value greater than 7dB. The split ring is made of plexiglass (Young's modulus is about 2500Mpa), its radius is r=60.0mm, straight beam length L=40.0mm, straight beam width a=4.42mm, thickness b=2.60mm, moment action distance d = 62.2 mm.

At a constant room temperature, the sensing device was tested for flow velocity sensing, and the schematic diagram of the measuring device is shown in Figure 3 . Figure 4 is a typical spectrum of the sensing device in a certain range of flow rates. Multiple experiments have shown that the sensing device has a flow velocity sensing dynamic range of 0.051-0.147m/s, and can at least distinguish a flow velocity of 0.0003m/s. To the best of our knowledge, this is the optimal value reported so far. Improving the parameters of the flow velocity-pressure tube and the fiber grating pressure sensing mechanism can further improve the sensitivity of flow velocity sensing.

As mentioned above, through the corresponding formula, high-precision sensing of fluid flow can also be performed. In practical applications, the optical signal is converted into an electrical signal through a photoelectric converter, and then the computer is used for data processing, which can sense and monitor the flow rate and flow rate in real time.

Claims (7)

1, a kind of sensing device for optical fiber optical grating flow speed, it comprises flow velocity-pressure pipe, it is characterized in that: upper conduit (7) couples together flow velocity-pressure pipe (1) and airtight cylinder (3) by the flow velocity-pore a of pressure pipe (1) top and the pore c of airtight cylinder (3) respectively, downcomer (8) couples together flow velocity-pressure pipe (1) and airtight cylinder (3) by the flow velocity-pore b of pressure pipe top and the pore d of airtight cylindrical center respectively, and the airtight draw-tube (2) of downcomer (8) and airtight cylindrical center communicates, inverted split ring (5) is on airtight cylinder interior right side, be connected and fixed with airtight draw-tube and airtight cylinder by left connecting rod (9) and right connecting rod (10) respectively, fiber grating (4) writes on the optical fiber (6), and be pasted on the upper side of split ring, one end of optical fiber (6) is unsettled, and the other end passes airtight cylinder.

2, sensing device for optical fiber optical grating flow speed according to claim 1 is characterized in that: flow velocity V with pore a corresponding position flows through in flow velocity-pressure pipe ₁With the flow velocity V that flows through with pore b corresponding position ₂Be different flow velocity in flow velocity-pressure pipe pipe, flow velocity-pressure pipe is by glass, polymkeric substance, metal or alloy material.

3, sensing device for optical fiber optical grating flow speed according to claim 1 is characterized in that: described airtight draw-tube can only stretch vertically, and it is made by organic polymer or aluminum foil material.

4, sensing device for optical fiber optical grating flow speed according to claim 1 is characterized in that: described fiber grating is the Fiber Bragg Grating FBG or the long period fiber grating of glass or plastics.

5, sensing device for optical fiber optical grating flow speed according to claim 1 is characterized in that: described split ring is made by organic polymer or the metal material of medium Young modulus 1000Mpa～4000MPa.

6, sensing device for optical fiber optical grating flow speed according to claim 1 is characterized in that: described optical fiber is single-mode fiber.

7, sensing device for optical fiber optical grating flow speed according to claim 1 is characterized in that: the operating ambient temperature of described sensing device is between-20 ℃～70 ℃.

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