CN102901840A - High-sensitivity FBG (Fiber Bragg Grating) acceleration sensor - Google Patents

Abstract

The invention relates to a high-sensitivity FBG (Fiber Bragg Grating) acceleration sensor; and the high-sensitivity FBG acceleration sensor comprises a base and an upper cover, wherein the base is connected with the upper cover by a bolt; a cantilever beam is arranged in a space formed between the base and the upper cover; one end of the cantilever beam is connected with the base by a thread; the other end of the cantilever beam is connected with a mass block by a thread; the mass block is in an impending state; two supporting blocks are arranged on the cantilever beam; one supporting block is arranged at the end of the cantilever beam connected with the base; the supporting block II is arranged between the supporting block I and the mass block; optical cable joints are arranged on two sides of the base; an optical fiber penetrates through the two optical cable joints and is fixed on the supporting block I and the supporting block II; and an optical grating area is formed between the supporting block I and the supporting block II. The high-sensitivity FBG acceleration sensor provided by the invention has the advantages as follows: as two ends of the optical grating area are fixed between the cantilever beam and the supporting blocks, the sensitivity of the sensor is enhanced, chirp or multimodal phenomenon is avoided, a mechanical model of design calculation is deduced, and overload safety protection design, optical grating prestretching design and so on are carried out on the sensor; and through finite element analysis computation, strain distribution situation and 6-order fixed frequency of the cantilever beam are obtained, and it is proved that an analysis result is consistent with design computation.

Description

High-sensitivity FBG acceleration sensor

Technical Field

The invention relates to a fiber bragg grating sensor, in particular to a high-sensitivity FBG acceleration sensor.

Background

The fiber grating sensor belongs to one type of fiber sensors, and the sensing process based on the fiber grating obtains sensing information by modulating fiber Bragg wavelength through external physical parameters, and is a wavelength modulation type fiber sensor. The existing fiber grating acceleration sensor is easy to generate chirp or multi-peak phenomenon when in use, and the sensitivity of the existing fiber grating acceleration sensor is a problem. And when the external too large impact acts, the too large acceleration can generate great axial strain on the grating so that the grating is broken.

Disclosure of Invention

The present invention is to solve the above-mentioned drawbacks of the prior art and to provide a durable FBG acceleration sensor with high sensitivity.

The invention adopts the technical scheme for solving the technical problems that: the high-sensitivity FBG acceleration sensor comprises a base, an upper cover, a base and the upper cover are connected through screws, a cantilever beam is arranged in a space formed by the base and the upper cover, one end of the cantilever beam is in threaded connection with the base, the other end of the cantilever beam is in threaded connection with a mass block, the mass block is in a suspended state, two support blocks are arranged on the cantilever beam, a first support block is arranged at a connecting end of the cantilever beam and the base, a second support block is arranged between a first support block and the mass block, optical cable connectors are arranged on two sides of the base, optical fibers are fixed on the first support block and the second support block through the two optical cable connectors, and a grating area is.

Preferably, the grating is provided with a pre-tension.

Preferably, the bottom of the base is provided with a limit screw, the center position of the limit screw is collinear with the concentrated load of the mass block, and a gap is formed between the end face of the limit screw and the mass block.

Preferably, the base is made of aluminum alloy.

Preferably, a thread sealing glue is arranged at the threaded connection position in the space formed by the base and the upper cover.

Preferably, a lubricating oil damping fluid is provided in a space formed by the base and the upper cover.

The invention has the beneficial effects that: the sensitivity of the sensor can be obviously improved by adjusting the size of the support block, and meanwhile, the grating area part adopts a suspension mode and is not directly adhered to the cantilever beam, so that the phenomenon of center wavelength chirp or broadening can be effectively avoided; the specific structural size of the novel acceleration sensor is obtained, and meanwhile, structural design is carried out on pretensioning, overload damage prevention and temperature compensation; finite element simulation analysis is carried out on a sensing unit of the sensor, and the result is in good agreement with the previous design theory result; therefore, the fiber bragg grating acceleration sensing structure provides a novel design thought and method for the design of a novel acceleration sensor, and has great practical significance and value.

Drawings

FIG. 1 is a schematic structural view of the present invention;

FIG. 2 is a diagram of a mechanical model of a sensing unit;

FIG. 3 is a geometric relationship diagram of a pure bending beam bending deformation surface;

FIG. 4 is a cloud of strain profiles;

FIG. 5 is a first order modal stress distribution cloud;

description of reference numerals: the device comprises a base 1, an upper cover 2, a mass block 3, a cantilever beam 4, a support block I5, a support block II 6, an optical cable connector 7, an optical fiber 8, a grating area 9 and a limiting screw 10.

Detailed Description

The invention will be further described with reference to the accompanying drawings in which:

example (b): as shown in fig. 1, a high sensitivity FBG acceleration sensor, including base 1, upper cover 2, pass through screwed connection between base 1 and the upper cover 2, be equipped with cantilever beam 4 in the space that base 1 and upper cover 2 formed, cantilever beam 4 one end and base 1 threaded connection, the other end and quality piece 3 threaded connection, quality piece 3 is in unsettled state, be equipped with two piece on the cantilever beam 4, wherein a 5 link of locating cantilever beam 4 and base 1, a two 6 piece locate between a 5 piece and the quality piece 3, base 1 both sides are equipped with optical cable joint 7, optic fibre 8 just is fixed in on a 5 piece and a two 6 piece through two optical cable joint 7, grating district 9 is located between a 5 piece and a two 6 piece. As the sensor is used for acceleration vibration measurement, all the threaded joints in the base are required to be subjected to anti-loosening treatment, namely, the threaded sealant is coated to prevent loosening, the tail end optical cable joint 7 and the optical cable are connected in a conventional mode (compressing and glue spreading), and lubricating oil damping liquid is filled in the whole base 1.

The key of the design is to establish a mechanical model between the acceleration and the frequency information of the external excitation and the fiber bragg grating, the mechanical model of the cantilever beam 4 and the mass block 3 is adopted during the design, the inertial force generated by the external excitation acceleration is converted into the strain on the surface of the cantilever beam 4, and the fiber bragg grating is fixed on the surface of the cantilever beam 4. Therefore, the central wavelength of the grating is changed due to the surface strain, and a linear relation between the central wavelength change and the external excitation acceleration can be established by combining the strain characteristic of the grating and the knowledge of the mechanics theory, so that the external excitation acceleration can be converted by detecting the wavelength variation of the grating, and meanwhile, the working frequency range for detecting the external excitation signal is determined by the design inherent frequency of the sensor. And performing FFT (fast Fourier transform) on the grating time domain wavelength change signal to obtain a frequency characteristic curve of the external excitation signal, so as to detect the frequency component of the external excitation signal.

Two important criteria in the design of an acceleration sensor are: dynamic range (operating frequency) and sensitivity. When the sensor is used in the field of structure detection (bridges, dams and the like), the sensor is generally required to have good sensitivity in a certain dynamic range, so that the micro-vibration condition of a structural body can be monitored. The vertical acceleration of railways and highway bridges is usually lower than 0.35g, the lateral acceleration is lower than 0.1g, the vibration of large-scale structures such as highways, railway bridges and high-rise buildings is low-frequency vibration, and the natural frequency is generally lower than 10 Hz. Here, a cantilever beam and a mass mechanical model are adopted as the acceleration sensor sensing unit. The simplified sensing unit structure is shown in fig. 2, and can be simplified into a single-degree-of-freedom second-order system consisting of an inertial mass m, an elastic element k and a damping C, and the influence of the optical fiber rigidity is ignored during calculation because the optical fiber rigidity is far less than that of the elastic cantilever beam.

For the above mechanical model, the natural frequency can be obtained as:

$f_n=\frac{\mathrm{\&omega;}}{2\mathrm{\&pi;}}=\frac{1}{2\mathrm{\&pi;}}\sqrt{\frac{{\mathrm{Ebh}}^3}{4(m+m_c){\mathrm{<figure-callout\; id="3"\; label="L"\; filenames="HDA00002287956000011.png"\; state="\{\{state\}\}">L</figure-callout>}}^3}}$

the structural form can be obtained by combining the mechanics of materials, the relation between the wavelength change and the strain of the fiber bragg grating and the bending deformation theorem:

$S=\frac{\mathrm{\&Delta;\&lambda;}}{a}=0.78\frac{t+2h}{t}\frac{6m}{{\mathrm{Ebh}}^2}(L-x){\mathrm{\&lambda;}}_0$

wherein,

f_n: the natural frequency of the cantilever beam; s: an acceleration sensitivity coefficient; Δ λ: a wavelength variation amount;

a: acceleration; lambda [ alpha ]₀: an original wavelength; m: mass of the mass block; m is_c: cantilever beam mass;

e: cantilever beam elastic modulus; b: cantilever beam width; l: cantilever beam length;

t: cantilever beam thickness; h: the thickness of the supporting block (the thickness of the supporting block I and the supporting block II is considered as equal);

meanwhile, boundary size constraint is designed by combining the structure, and the geometrical size of the cantilever beam is L =45, m is obtained by adopting a numerical optimization method_cAnd when the unit is mm, the sensitivity and the natural frequency are higher, wherein 65Mn material is selected for the cantilever beam, the elastic modulus E =200GPa, and h =7.5mm are substituted to obtain the sensor sensitivity of 800pm/g and the natural frequency of 50 HZ. At present, other manufacturers in the industry adopt cantilever beam structure acceleration sensors, under the condition of equivalent geometric dimension, the sensitivity of the acceleration sensors is far lower than the value (for example, Philotoid reaches 360pm/g, and 300pm/g of a purple sweater), and the sensitivity of the acceleration sensors can be adjusted by changing h.

Since the acceleration sensor is designed with a measuring range of +/-1 g, the grating is required to be axially stretched and axially compressed. It is important to pre-stretch the grating to ensure that the variation of the central wavelength of the grating is still detected when the grating is axially compressed. According to the mechanical principle of cantilever beam materials, the following relationship exists between the surface strain and external force:

$\mathrm{\&epsiv;}=\frac{6F(L-x)}{{\mathrm{Ebh}}^2}$

in combination with the structural size of the sensor unit designed in the foregoing, when the external excitation acceleration is 1g, the maximum strain on the surface of the cantilever beam is ∈ =40.81 μ ∈, and the geometric relationship between the deformation of the pure bending beam in material mechanics is shown in fig. 3, where the surface strain of the cantilever beam and the strain at the surface height h have the following relationship:

$\frac{\mathrm{\&epsiv;}}{\mathrm{\&epsiv;}1}=\frac{t/2}{t/2+h}$

$\mathrm{\&epsiv;}1=\frac{t+2h}{t}\mathrm{\&epsiv;}$

according to the relationship between the wavelength of the bare grating and the strain, namely k =1.2 pm/mu epsilon, the strain of the cantilever beam to the grating can be converted into the change quantity delta lambda of the central wavelength of the grating, which is: Δ λ = k ∈ 1=783.55 pm.

Therefore, to meet the requirement of the acceleration sensor with the measuring range of +/-1 g, the grating needs to be pre-stretched to be not less than 783.55pm, and the grating is designed to be packaged according to the pre-stretching of 1000 pm.

When the sensor is under the action of external overlarge impact (the value is far greater than the designed range of 1 g), the grating generates great axial strain due to overlarge acceleration, so that the grating is broken, and according to the material characteristics of the optical fiber, when the optical fiber is stretched by external force, the maximum stretching rate before breaking is about 1% of the initial length, namely 10000 mu epsilon. Therefore, it is important to design the optical grating for overload protection. Combining the above calculation formula, the generation of 10000 μ ∈ by the grating is equivalent to the generation of 625 μ ∈ on the cantilever surface, and then the free end inertial force is:

$F=\frac{{\mathrm{\&epsiv;<figure-callout\; id="2"\; label="Ebh"\; filenames="HDA00002287956000011.png"\; state="\{\{state\}\}">Ebh</figure-callout>}}^2}{6(L-x)}=13.65N$

when the free end of the cantilever beam is under the action of the inertial force F =13.65N, the maximum deflection of the free end is as follows:

$r=\frac{{4\mathrm{<figure-callout\; id="3"\; label="FL"\; filenames="HDA00002287956000011.png"\; state="\{\{state\}\}">FL</figure-callout>}}^3}{{\mathrm{<figure-callout\; id="3"\; label="Ebh"\; filenames="HDA00002287956000011.png"\; state="\{\{state\}\}">Ebh</figure-callout>}}^3}=1.44\mathrm{mm}$

i.e. when the free end of the cantilever beam 4 is displaced by 1.44mm, the grating will be pulled apart. In order to avoid the phenomenon, a limit screw 10 is arranged on the sensor base, the center position of the limit screw is collinear with the concentrated load of the free end of the cantilever beam 4, and the end face of the limit screw is designed to be displaced to 1mm (smaller than 1.44mm limit size) from the mass block 3 when the limit screw 10 is completely screwed into a specified position.

The two ends of the grating are fixed on the cantilever beam made of the same material, and meanwhile, the grating is in a stretching state, and when the external temperature changes, the central wavelength of the grating also changes. The internal temperature compensation is not designed, and the zone temperature compensation is carried out by means of an external temperature sensor. Because sensor base 1 has adopted the aluminum alloy material, it has very high thermal conductivity, can transmit inside the sensor fast to ambient temperature's change, and inside grating is in exposing simultaneously, can the quick response ambient temperature change. The effect of temperature on the wavelength can be well rejected by means of external ambient compensation.

The strain and natural frequency calculation analysis is carried out on the structure of the acceleration sensor sensing unit from the aspect of theoretical calculation, the influence of local changes (such as hole opening and cushion height) of the structure is not considered in the previous analysis, a three-dimensional model is established after the size and the quality of each structural part of the sensing unit are obtained according to the previous theoretical calculation, and the designed three-dimensional model is subjected to simulation analysis so that the previous theoretical calculation can be well verified.

And (3) static analysis of the sensing unit, establishing related constraint conditions during analysis, applying fixed support constraint at the left end of the cantilever beam, defining the connection surface of the cantilever beam and the mass block as complete binding support constraint, and adding a concentrated load of 0.9N at the free end of the cantilever beam. The cantilever beam material is 65Mn, and the mass block material is brass. The strain distribution cloud chart obtained by the calculation and the solution of the meshing is shown in FIG. 4, and it can be seen from the diagram that the maximum strain occurs at the side of the cantilever beam close to the fixed end, and the strain value at the joint of the support block and the surface of the cantilever beam is between 31.34 and 39.17 mu epsilon, which is closer to the surface strain value of 40.81 mu epsilon calculated in the previous step. The strain value of the upper surface of the support block is very small, mainly because the structural material at the position is relatively thick, the strain between the inner parts of the materials is very small, and the main deformation is caused by the displacement of the support block integrally due to the bending of the cantilever beam.

And (3) performing modal analysis on the sensing unit, establishing related constraint conditions during analysis, applying fixed support constraint at the left end of the cantilever beam, defining the connection surface of the cantilever beam and the mass block as complete binding support constraint, and setting the cantilever beam to be 65Mn and the mass block to be brass. The first 6 natural frequencies are extracted by a meshing calculation solution, as shown in the following table.

The first order natural frequency is 52Hz, which is very close to the 48.42Hz structural natural frequency calculated by us above. The remaining higher order natural frequencies are not meaningful for structural studies. Therefore, a structural strain distribution cloud chart of the sensing unit at the first-order natural frequency of 52Hz is calculated and is shown in FIG. 5, and it can be seen from the cloud chart that the maximum strain occurs on the surface of the cantilever beam close to the fixed end, and the maximum stress is 16562MPa >500MPa (tensile strength of stainless steel material of the cantilever beam), so the fracture phenomenon of the cantilever beam occurs at the frequency. According to the combination of the simulation calculation and the numerical calculation, the obtained maximum equivalent strain is almost similar to the first-order inherent frequency value, so that the result obtained by the analysis can be judged to be credible.

The two ends of the grating area of the invention are fixed between the cantilever beam and the support block, thus improving the sensitivity of the sensor, avoiding the phenomenon of chirp or multiple peaks, deducing a mechanical model for designing and calculating, and carrying out overload safety protection design, grating pre-stretching design and the like on the sensor. The strain distribution condition and the 6 th order natural frequency of the cantilever beam are obtained by adopting finite element analysis and calculation, and the analysis result is proved to be consistent with the design calculation.

In addition to the above embodiments, the present invention may have other embodiments. All technical solutions formed by adopting equivalent substitutions or equivalent transformations fall within the protection scope of the claims of the present invention.

Claims (6)

1. The utility model provides a high sensitivity FBG acceleration sensor, includes base (1), upper cover (2), through screwed connection, characterized by between base (1) and upper cover (2): be equipped with cantilever beam (4) in the space that base (1) and upper cover (2) formed, cantilever beam (4) one end and base (1) threaded connection, the other end and quality piece (3) threaded connection, quality piece (3) are in unsettled state, be equipped with two piece on cantilever beam (4), wherein the link of cantilever beam (4) and base (1) is located to a piece (5), a piece two (6) are located between a piece (5) and quality piece (3), base (1) both sides are equipped with optical cable joint (7), optic fibre (8) just are fixed in on a piece (5) and a piece two (6) through two optical cable joint (7), grating district (9) are located between a piece (5) and a piece two (6).

2. The high sensitivity FBG acceleration sensor according to claim 1, characterized in that: the grating is provided with a pre-stretch.

3. The high sensitivity FBG acceleration sensor according to claim 1, characterized in that: the base (1) bottom is equipped with a stop screw (10), and the central point of stop screw (10) puts and the concentrated load collineation of quality piece (3), is equipped with the clearance between the terminal surface of stop screw (10) and quality piece (3).

4. The high sensitivity FBG acceleration sensor according to claim 1, characterized in that: the base (1) is made of aluminum alloy.

5. The high sensitivity FBG acceleration sensor according to claim 1, characterized in that: and a thread sealing glue is arranged at the threaded connection part in the space formed by the base (1) and the upper cover (2).

6. The high sensitivity FBG acceleration sensor according to claim 1, characterized in that: and lubricating oil damping liquid is arranged in a space formed by the base (1) and the upper cover (2).

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