KR101611792B1 - FBG Strain Sensor Probe for Temperature Compensation and Method for Sensing thereof - Google Patents

FBG Strain Sensor Probe for Temperature Compensation and Method for Sensing thereof Download PDF

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KR101611792B1
KR101611792B1 KR1020150051791A KR20150051791A KR101611792B1 KR 101611792 B1 KR101611792 B1 KR 101611792B1 KR 1020150051791 A KR1020150051791 A KR 1020150051791A KR 20150051791 A KR20150051791 A KR 20150051791A KR 101611792 B1 KR101611792 B1 KR 101611792B1
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optical fiber
bimetal
equation
temperature
metal
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권일범
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한국표준과학연구원
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
    • G01B11/165—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by means of a grating deformed by the object
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
    • G01B11/18—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge using photoelastic elements

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  • General Physics & Mathematics (AREA)
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  • Length Measuring Devices By Optical Means (AREA)

Abstract

The present invention relates to a fiber Bragg grating (FBG) strain sensor probe which uses a bimetal beam to perform temperature compensation by an FBG probe. According to an embodiment of the present invention, the FBG strain sensor probe capable of temperature compensation comprises: a fixed bracket whose lower end is fixed on a target measurement object; a bimetal beam separated from the fixed bracket by a predetermined first distance, whose lower end is fixed on the target measurement object; and an optical fiber wherein one side thereof is installed on a free end of the fixed bracket, the other side thereof is installed on a free end of the bimetal beam, light propagated thereinto, and an FBG is arranged between the one side and the other side thereof to reflect a portion of the light corresponding to a Bragg wavelength. If a temperature changes, the bimetal is deformed, and deformation of the bimetal can prevent the Bragg wavelength from changing according to a change of the temperature.

Description

FIELD OF THE INVENTION [0001] The present invention relates to an FBG strain sensor, and more particularly,

The present invention relates to an FBG strain sensor probe capable of temperature compensation, and more particularly, to an FBG strain sensor probe configured to be temperature-compensated by a FBG probe itself using a bimetallic beam.

The optical fiber can be used as a sensor for various physical parameters because the change of the inherent characteristic is sensitive to the change of the external environment. In addition, due to its characteristics, it can be densely installed inside the structure by using a long length, which is advantageous for distribution type measurement and can be used for real-time monitoring of facilities such as bridges, tunnels and buildings.

Fig. 1 shows the general structure of such an optical fiber. As shown in FIG. 1, the optical fiber generally comprises a core portion which is the center of the optical fiber, a cladding portion which protects the center, and a cover portion. The main component of the core and the cladding is made of glass, and the cladding surface is coated with a polymer or an acrylate to protect the core and the cladding which are the main constituents.

In the optical fiber core, a germanium (Ge) material is usually added to increase the refractive index of the cladding, which may cause structural defects in the process of placing the material on the silica glass. In this case, when the optical fiber core is irradiated with strong ultraviolet rays, the refractive index of the optical fiber is changed while the bonding structure of Ge is deformed.

The fiber Bragg grating refers to a periodic change in the refractive index of the optical fiber core using this phenomenon. This grating reflects only the wavelengths satisfying the Bragg condition and transmits the other wavelengths as they are.

When the ambient temperature of the grating is changed or an axial load is applied to the grating, the refractive index or the length of the optical fiber changes, so that the wavelength of the reflected light changes. Therefore, by measuring the wavelength of the light reflected from the fiber Bragg grating, temperature, tensile, pressure or bending can be detected, and the sensor can be applied.

The fiber Bragg grating sensor is a fiber optic device that periodically modulates the refractive index of the core according to the energy distribution of the interference fringe and reflects light of a specific wavelength (Bragg wavelength).

2 schematically shows the structure of a conventional optical fiber Bragg grating sensor and the grating portion of a probe. The fiber Bragg grating has the structure and operating characteristics as shown in Fig. The periodic change in refractive index of the core serves as a Bragg grating.

When a broadband light is incident on the Bragg grating, the light of a wavelength corresponding to the Bragg condition as shown in the following Equation 1 causes a constructive interference to be reflected at the Bragg grating portion, and the light of the remaining wavelength is transmitted.

Figure 112015035796280-pat00001

here,

Figure 112015035796280-pat00002
Is the Bragg wavelength,
Figure 112015035796280-pat00003
Is a core effective reffractive index, which represents the average refractive index when light travels in one cycle of the Bragg grating,
Figure 112015035796280-pat00004
Represents the period of the Bragg grating engraved in the core.

As can be seen from the above equation (1), the Bragg wavelength of light reflected from the lattice

Figure 112015035796280-pat00005
) Is the effective refractive index (
Figure 112015035796280-pat00006
) And the grating period (
Figure 112015035796280-pat00007
) ≪ / RTI > Since the effective refractive index and the period of the grating are a function of the temperature and the strain, the Bragg wavelength is changed when disturbance such as temperature or strain is applied to the fiber Bragg grating.

The following equation (2) can be obtained by taking an entire differential value of Bragg wavelength in the Bragg condition, and then substituting the equation of temperature, strain, lattice spacing, and effective refractive index.

Figure 112015035796280-pat00008

here,

Figure 112015035796280-pat00009
Is the thermal expansion coefficient of the optical fiber,
Figure 112015035796280-pat00010
Is a thermodynamic number indicating the refractive index change of the optical fiber due to temperature,
Figure 112015035796280-pat00011
Is a photoelastic constant and has a value of approximately 0.22.

If the changed Bragg wavelength is precisely measured, the temperature or strain applied to the optical fiber grating can be calculated through Equation (2). This is the principle that a fiber Bragg grating can be used as a sensor.

Assuming that there is no change in the temperature applied to the sensor in Equation (2)

Figure 112015035796280-pat00012
), Equation (2) can be simply expressed as Equation (3) below.

Figure 112015035796280-pat00013

Using Equation (3), FGB can be used as a strain sensor. As shown in Equation (3), this strain can be obtained by accurately measuring the amount of change in wavelength.

The FBG sensor is suitable for long-term measurement because it is easy to multiplex the sensor regardless of electromagnetic interference, and has excellent corrosion resistance. Recently, it has been applied to various fields such as structural integrity monitoring, slope monitoring, and hull stress monitoring of civil structures such as bridges and tunnels.

2, the conventional FGB probe comprises a light source 2, a connection portion 4, a wavelength detector 6, and the like, which are connected by an optical fiber.

As can be seen from the enlarged drawing, the Bragg grating sensor portion of the optical fiber is engraved with a Bragg grating by a predetermined length. The Bragg wavelength of the reflected light reflected from the Bragg grating is measured in the light emitted from the light source 2 through the optical fiber and the degree of deformation of the measured object (for example, a large structure such as a bridge or a building) .

Such a fiber Bragg grating sensor is small in size, has no influence on an electromagnetic field, is excellent in stability against chemicals, and is attracting attention as a sensor for monitoring industrial equipment.

However, in practice, since the change in temperature in Equation (2) does not become 0

Figure 112015035796280-pat00014
), There is a problem that the strain can not be accurately derived from the above-mentioned equation (3) even if the change of the Bragg wavelength is accurately measured. Thus, there is a limit to accurately measuring the strain, particularly in areas where precise measurement is required.

Conventionally, an FBG for measuring temperature is separately installed and a correction method is used from strain FBG data. However, this is disadvantageous in terms of cost and complexity, and there is a problem that the usability is somewhat deteriorated.

Accordingly, there is a need to develop a technology for automatically correcting the temperature in the FBG probe itself.

Korean Patent Registration No. 10-1314848 Korean Patent Registration No. 10-1280922 Korean Patent Publication No. 10-0943710

It is an object of the present invention to provide a FBG strain sensor probe for a user to be able to perform temperature correction at the FBG probe itself by using a bimetal beam.

More specifically, in the present invention, an optical fiber is installed in a state in which a slight initial deformation is induced, and when deformation is induced in the bimetal beam according to the temperature change, deformation of the bimetal beam is configured to prevent the Bragg wavelength from being changed, To provide users with FBG strain sensor probes that can accurately measure the strain without receiving the strain.

Another object of the present invention is to provide a FBG strain sensor probe to a user, which can simplify the signal processing amount without requiring additional temperature correction, reduce complexity, and perform strain measurement more quickly.

It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are not intended to limit the invention to the precise form disclosed. It can be understood.

An FBG strain sensor probe for measuring a strain of an object to be measured, the FBG strain sensor probe capable of temperature correction related to an example of the present invention for realizing the above-mentioned object includes a fixed bracket having a lower end fixed to the object to be measured; A bimetal beam fixed at the lower end to the measured object and spaced apart from the fixed bracket by a predetermined first distance; And an optical fiber bragg grating for reflecting light corresponding to a Bragg wavelength of the light, wherein the optical waveguide is installed at a free end of the fixing bracket and the other end is installed at a free end of the bimetal beam, Wherein the bimetal beam is deformed when a change in temperature occurs, and the deformation of the bimetallic beam is changed according to the change of the temperature, It is possible to prevent the Bragg wavelength from being changed.

The optical fiber is installed at the free end of the fixing bracket and the free end of the bimetal beam in a state of being pulled by a predetermined initial tensile length, and initial strain is generated in the optical fiber installed in the pulled state.

Further, the initial strain generated in the optical fiber is determined according to the following equation (A).

Equation A

Figure 112015035796280-pat00015

In the above equation (A)

Figure 112015126427507-pat00016
Is an initial strain of the optical fiber,
Figure 112015126427507-pat00017
Is a modulus of elasticity of the bimetallic beam,
Figure 112015126427507-pat00018
Is the moment of inertia of the bimetal beam,
Figure 112015126427507-pat00019
Is an elastic modulus of the optical fiber,
Figure 112015126427507-pat00020
Is a sectional area of the optical fiber,
Figure 112015126427507-pat00021
Is the distance between the lower end of the bimetal beam and the free end of the bimetallic beam on the other side of the optical fiber,
Figure 112015126427507-pat00022
Is the first distance,
Figure 112015126427507-pat00023
Is the initial tensile length.

Also, the bimetallic view may include a first metal; And a second metal coupled to the first metal and having a thermal expansion coefficient different from the thermal expansion coefficient of the first metal.

Also, the deformation of the bimetallic beam caused by the change of the temperature is a deflection of the free end of the bimetallic beam.

Also, the deformation of the bimetallic beam caused by the change of the temperature is induced by the thermal expansion coefficient of the first metal having a different value and the thermal expansion coefficient of the second metal.

Further, the deformation of the bimetallic beam according to the change of the temperature is determined according to the following expression (B).

Equation B

Figure 112015035796280-pat00024

In the above equation (B)

Figure 112015126427507-pat00025
Is the distance between the lower end of the bimetal beam and the free end of the bimetallic beam on the other side of the optical fiber,
Figure 112015126427507-pat00026
Is the temperature change,
Figure 112015126427507-pat00027
Is a deformation of the bimetal beam according to the change of the temperature,
Figure 112015126427507-pat00028
Is defined by the following equation (C).

Equation C

Figure 112015035796280-pat00029

In the above equation (C)

Figure 112015126427507-pat00030
Is a modulus of elasticity of the first metal,
Figure 112015126427507-pat00031
Is an elastic modulus of the second metal,
Figure 112015126427507-pat00032
Is the thickness of the first metal,
Figure 112015126427507-pat00033
Is the thickness of the second metal,
Figure 112015126427507-pat00034
Is a difference between the thermal expansion coefficient of the first metal and the thermal expansion coefficient of the second metal.

Further, in order to prevent the Bragg wavelength from changing in accordance with the temperature change,

Figure 112015126427507-pat00035
The bimetal beam and the optical fiber are used. In the above equation (C)
Figure 112015126427507-pat00036
Is the first distance,
Figure 112015126427507-pat00037
Is the optical temperature coefficient of the optical fiber,
Figure 112015126427507-pat00038
Is the photoelastic coefficient of the optical fiber,
Figure 112015126427507-pat00039
Is the thermal expansion coefficient of the optical fiber.

The apparatus may further include a measurement unit that measures a strain of the object using light reflected from the optical fiber Bragg grating, and the measurement unit may measure a strain of the object using Equation (D) .

delete

Equation D

Figure 112015035796280-pat00040

In the above equation (D)

Figure 112015126427507-pat00041
Is a strain of the object to be measured,
Figure 112015126427507-pat00042
Is the Bragg wavelength,
Figure 112015126427507-pat00043
Is the modulus of elasticity of the bimetallic beam,
Figure 112015126427507-pat00044
Is the moment of inertia of the bimetal beam,
Figure 112015126427507-pat00045
Is an elastic modulus of the optical fiber,
Figure 112015126427507-pat00046
Is the cross-sectional area of the optical fiber.

In another aspect of the present invention, there is provided a sensing method of a FBG strain sensor probe capable of temperature compensation, comprising: a step of generating a temperature change; Generating a deformation in the bimetal beam corresponding to the change of the temperature; Preventing a change in the Bragg wavelength according to the temperature change by deformation of the bimetal beam; And measuring the strain of the object using light reflected from the optical fiber Bragg grating, wherein a lower end of the fixing bracket and a lower end of the bimetal beam are fixed to the object to be measured, and the fixing bracket and the bimetal And the other end of the optical fiber is installed at the free end of the bimetal beam, and the light propagates into the optical fiber, It is possible to prevent an optical fiber Bragg grating (FBG) reflecting light corresponding to a Bragg wavelength from being disposed between one side of the optical fiber and the other side of the optical fiber.

The optical fiber is installed at the free end of the fixing bracket and the free end of the bimetal beam in a state of being pulled by a predetermined initial tensile length, and initial strain is generated in the optical fiber installed in the pulled state.

According to another aspect of the present invention, there is provided a method of installing an FBG strain sensor probe capable of temperature correction, the method comprising: fixing a lower end of a fixing bracket to a measured object; A step of making one side pull the optical fiber installed at the free end of the fixing bracket by a predetermined initial tension length; And fixing the lower end of the bimetal beam provided on the other end of the optical fiber to the object to be measured at a free end, wherein the fixed bracket and the bimetal are spaced apart from each other by a predetermined first distance, And an optical fiber Bragg grating (FBG) is disposed between one side of the optical fiber and the other side of the optical fiber to reflect light corresponding to a Bragg wavelength of light traveling to the inside of the optical fiber, The deformation of the bimetal beam can be prevented and the deformation of the bimetal beam can be prevented from changing the Bragg wavelength according to the change of the temperature.

The FBG strain sensor probe according to one embodiment of the present invention for realizing the above-mentioned problems is characterized in that, in a program in which instructions that can be executed by the digital processing apparatus are implemented tangibly to perform the sensing method of the FBG strain sensor probe, A sensing step of sensing a temperature change; Generating a deformation in the bimetal beam corresponding to the change of the temperature; Preventing a change in the Bragg wavelength according to the temperature change by deformation of the bimetal beam; And measuring the strain of the object using light reflected from the optical fiber Bragg grating, wherein a lower end of the fixing bracket and a lower end of the bimetal beam are fixed to the object to be measured, and the fixing bracket and the bimetal And the other end of the optical fiber is installed at the free end of the bimetal beam, and the light propagates into the optical fiber, It is possible to prevent an optical fiber Bragg grating (FBG) reflecting light corresponding to a Bragg wavelength from being disposed between one side of the optical fiber and the other side of the optical fiber.

The present invention can provide a user with an FBG strain sensor sensor that is configured to perform temperature correction at the FBG probe itself by using a bimetallic beam.

More specifically, in the present invention, an optical fiber is installed in a state in which a slight initial deformation is induced, and when deformation is induced in the bimetal beam according to the temperature change, deformation of the bimetal beam is configured to prevent the Bragg wavelength from being changed, The FBG strain sensor transducer can be provided to the user to accurately measure the strain without receiving the strain.

Further, the present invention can provide a user with an FBG strain sensor sensor capable of simplifying the signal processing amount, reducing the complexity, and performing the strain measurement more quickly since a separate temperature correction is unnecessary.

It should be understood, however, that the effects obtained by the present invention are not limited to the above-mentioned effects, and other effects not mentioned may be clearly understood by those skilled in the art to which the present invention belongs It will be possible.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate a preferred embodiment of the invention and, together with the description, serve to provide a further understanding of the technical idea of the invention, It should not be construed as limited.
1 shows a general structure of an optical fiber according to the present invention.
2 schematically shows the structure of a conventional optical fiber Bragg grating sensor and the grating portion of a probe.
Figure 3 shows an example of an FBG strain sensor probe that may be implemented in accordance with the present invention.
4 is an embodiment of a bimetallic beam applicable to the FBG strain sensor probe of the present invention.
5 is a flow chart related to an example of a sensing method of an FBG strain sensor probe according to the present invention.
6A and 6B schematically illustrate deflection of a bimetal beam according to a change in temperature.
FIG. 7 shows experimental results of the sensitivity of the FBG strain sensor probe of the present invention depending on the thickness and width of the bimetallic beam.
8 shows experimental results on the initial deflection of the bimetal beam according to the thickness and width of the bimetal beam.

Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings. In addition, the embodiment described below does not unduly limit the contents of the present invention described in the claims, and the entire configuration described in this embodiment is not necessarily essential as the solution means of the present invention.

< FBG  Strain sensor Transducer  Configuration>

Hereinafter, the configuration of the FBG strain sensor probe to be proposed by the present invention will be described in detail with reference to FIGS. 3 and 4. FIG. Fig. 3 shows an example of an FBG strain sensor probe that can be implemented according to the present invention, and Fig. 4 shows an embodiment of a bimetallic beam applicable to the FBG strain sensor probe of the present invention.

3, the FBG strain sensor probe 100 of the present invention includes a fixing bracket 20 installed on a measured object 10, a bimetal beam 30, an optical fiber 40, a measuring unit (not shown) . However, the components shown in FIG. 3 are not essential, so an FBG strain sensor probe 100 having more or fewer components may be implemented.

The fixing bracket 20 and the bimetalic beam 30 are spaced apart from each other by the first distance L. [ An optical fiber 40 is installed at the free end of the fixing bracket 20 and the free end of the bimetallic beam 30 and the optical fiber 40 is installed at a distance l from the lower end of the bimetal beam 30. [ The lower end of the fixing bracket 20 and the lower end of the bimetal beam 30 are fixed to the object 10 to be measured.

Referring to FIG. 4, the bimetal beam 30 is formed by coupling the first metal 32 and the second metal 34 together. The thermal expansion coefficient of the first metal 32 and the thermal expansion coefficient of the second metal 34 are different from each other and the difference between the thermal expansion coefficient of the first metal 32 and the thermal expansion coefficient of the second metal 34 The

Figure 112015035796280-pat00047
.

The thickness of the first metal 32 is

Figure 112015035796280-pat00048
And the thickness of the second metal 34 is expressed by
Figure 112015035796280-pat00049
And both can be the same. The first metal 32 and the second metal 34 have a width of W. [

Meanwhile, in order to measure the strain, the optical fiber 40 should be kept in a tight state. Therefore, the optical fiber 40 is installed in a state of being pulled by a predetermined initial tension length. If the optical fiber 40 is not in a pulled state, the optical fiber 40 may become loose, which may make it difficult to measure the strain.

Specifically, the optical fiber 40 has an initial tensile length

Figure 112015035796280-pat00050
At the free end of the fixed bracket 20 and at the free end of the bimetallic beam 30. [ In this initial state, the fixing bracket 20 is not deformed, but the free end of the bimetalic beam 30 is deflected by the pulling force of the optical fiber 40
Figure 112015035796280-pat00051
Can occur. The initial strain of the optical fiber 40 in the initial state is expressed by Equation (4) below.

Figure 112015035796280-pat00052

Here, the free end of the bimetallic beam 30 is a concentrated load

Figure 112015035796280-pat00053
Lt; / RTI &gt;
Figure 112015035796280-pat00054
Is the modulus of elasticity of the optical fiber 40,
Figure 112015035796280-pat00055
Is the cross-sectional area of the optical fiber (40).

Concentrated load

Figure 112015035796280-pat00056
The initial deflection of the bimetallic beam (30)
Figure 112015035796280-pat00057
Can be expressed by Equation (5) below.

Figure 112015035796280-pat00058

here,

Figure 112015035796280-pat00059
Is the modulus of elasticity of the bimetallic beam 30,
Figure 112015035796280-pat00060
Sectional secondary moment of the bimetallic beam 30. [0053]

(4) and (5), the initial strain of the optical fiber 40 can be obtained as shown in Equation (6) below.

Figure 112015035796280-pat00061

In this state, a force is applied to the measured object 10

Figure 112015035796280-pat00062
The bimetallic beam 30 undergoes bending deformation. At this time, the strain of the optical fiber 40
Figure 112015035796280-pat00063
The strain of the object 10 to be measured
Figure 112015035796280-pat00064
And the free end deflection of the bimetal beam (30)
Figure 112015035796280-pat00065
The strain of the optical fiber 40
Figure 112015035796280-pat00066
The following equation (7) can be used.

Figure 112015035796280-pat00067

The free end deflection of the bimetal beam (30)

Figure 112015035796280-pat00068
The strain of the optical fiber 40
Figure 112015035796280-pat00069
Is expressed by Equation (8) below, and the tensile strain of the optical fiber (40) is expressed by Equation (9) below.

Figure 112015035796280-pat00070

Figure 112015035796280-pat00071

The above equations (7) to (9)

Figure 112015035796280-pat00072
The following equation (10) can be obtained.

Figure 112015035796280-pat00073

Meanwhile, consideration will be given to the case where a temperature change is given to the FBG strain sensor probe 100 of the present invention. The bimetal beam (30) is free from deflection of the free end

Figure 112015035796280-pat00074
, Which is shown in Equation (11) below.

Figure 112015035796280-pat00075

The detailed derivation of Equation (11) is shown in Equation (12) below. In the case of using a bimetal beam 30 having a constant width, the radius of curvature according to the temperature change is expressed by Equation (12).

Figure 112015035796280-pat00076

Figure 112015035796280-pat00077

Figure 112015035796280-pat00078

here,

Figure 112015035796280-pat00079
Is the modulus of elasticity of the first metal 32,
Figure 112015035796280-pat00080
Is the modulus of elasticity of the second metal 34,
Figure 112015035796280-pat00081
Is the thickness of the first metal 32,
Figure 112015035796280-pat00082
Is the thickness of the second metal (34).

Therefore, the strain of the optical fiber 40 due to the deflection of the free end of the bimetallic beam 30 according to the temperature change can be expressed by the following Equation (13).

Figure 112015035796280-pat00083

The strain of the optical fiber 40 due to the temperature expansion is expressed by Equation (14) below.

Figure 112015035796280-pat00084

here,

Figure 112015035796280-pat00085
Is the coefficient of thermal expansion of the optical fiber (40).

As described above, the mechanical strain and the strain-induced strain changes affecting the FBG strain sensor probe 100 were examined. This strain change and the temperature change ultimately lead to a change in the Bragg wavelength, which is expressed by the following equation (15). That is,

Figure 112015035796280-pat00086
And temperature change
Figure 112015035796280-pat00087
The change in Bragg wavelength
Figure 112015035796280-pat00088
Is expressed by the following equation (15).

Figure 112015035796280-pat00089

Figure 112015035796280-pat00090

here,

Figure 112015035796280-pat00091
Is the optical temperature coefficient of the optical fiber 40,
Figure 112015035796280-pat00092
Is the photoelastic coefficient of the optical fiber (40).

Equation (6), Equation (10), Equation (13), and Equation (14) are substituted into Equation (15) and the following Equation (16) can be obtained.

Figure 112015035796280-pat00093

In order to prevent the Bragg wavelength change from occurring with respect to the temperature change in Equation (16), the following Equation (17) must be satisfied.

Figure 112015035796280-pat00094

In Equation 17,

Figure 112015035796280-pat00095
Is a parameter related to the physical properties of the bimetallic beam 30, and the right side of the equation (17) is a parameter relating to the physical properties of the optical fiber 40. [

Accordingly, the strain of the object 10 to be measured,

Figure 112015035796280-pat00096
And wavelength change
Figure 112015035796280-pat00097
The sensitivity of the bimetallic beam 30 is changed according to the following equation (18).

Figure 112015035796280-pat00098

As described above, the FBG strain sensor probe 10 using the bimetallic beam 30 has a difference between the actual strain to be measured and the FBG response strain and the above equation. Therefore, by properly adjusting this amount, it is possible to design the FBG strain sensor transducer (100) that maximally increases the measurement range while maintaining an acceptable resolution.

< FBG  Strain sensor Transducer  Sensing method>

Hereinafter, a sensing method of a FBG strain sensor probe to be proposed by the present invention will be described in detail with reference to the drawings.

5 is a flow chart related to an example of a sensing method of an FBG strain sensor probe according to the present invention.

Referring to FIG. 5, when a temperature change occurs in the FBG strain sensor probe 100 (S10), the bimetallic beam 30 is deformed in response to a temperature change (S12). Since the bimetal beam 30 is composed of the first metal 32 and the second metal 34 having different thermal expansion coefficients, deformation of the bimetalic beam 30 may occur due to a change in temperature.

Here, Figs. 6A and 6B schematically show deflection of a bimetal beam according to a temperature change. For example, when the temperature rises, the free end of the bimetallic beam 30 may be deformed to the left as shown in FIG. 6A. On the contrary, when the temperature is lowered, the bimetallic beam 30 30) may be deformed to the right side.

Subsequently, the bimetal beam 30 is deformed to prevent the Bragg wavelength from changing in accordance with the temperature change (S14). As the temperature rises

Figure 112015035796280-pat00099
And the bimetallic beam 30 is deformed in the direction of canceling it. When the temperature falls
Figure 112015035796280-pat00100
Becomes negative, and the bimetallic beam 30 is deformed in the direction of canceling it.

Next, the measuring unit measures the strain of the measured object 10 using light reflected from the optical fiber Bragg grating 42 (S16). In step S16, the measuring unit measures the strain using Equation (18).

7 and 8 show actual experimental results using the FBG strain sensor probe 100 of the present invention. FIG. 7 shows experimental results of the sensitivity of the FBG strain sensor probe of the present invention depending on the thickness and width of the bimetallic beam, and FIG. 8 shows experimental results of the initial deflection of the bimetal beam according to the thickness and width of the bimetal beam.

For the automatic temperature correction, the bimetal beam 30 and the gauge length are selected so as to satisfy Equation (17). Also, the initial strain of Equation (6) will have the widest operating range when designed to have a size of about 5000 microns.

In this experiment, the characteristic values of the optical fiber were set as follows.

Figure 112015035796280-pat00101
,
Figure 112015035796280-pat00102
,
Figure 112015035796280-pat00103
,
Figure 112015035796280-pat00104
,
Figure 112015035796280-pat00105

The characteristic values of the bimetallic beam 30 are set as follows.

Figure 112015035796280-pat00106
,
Figure 112015035796280-pat00107
,
Figure 112015035796280-pat00108
,
Figure 112015035796280-pat00109
,
Figure 112015035796280-pat00110

Using this characteristic value, the strain sensitivity change value as shown in FIG. 7 and the initial deflection amount as shown in FIG. 8 can be obtained.

 The present invention proposes a probe having a vertical bimetal beam for automatic temperature compensation of an FBG sensor. The sensitivity of the transducer is defined as the sensitivity of the wavelength change of the FBG according to the strain of the material to be measured. This sensitivity is changed according to the size of the bimetallic beam. Also, it can be seen that the initial displacement must be set differently according to the size of the bimetallic beam. After this initial displacement setting, it will operate as an FBG probe with an automatic temperature compensation with a strain measurement range of ± 5000 microns.

The present invention can also be embodied as computer-readable codes on a computer-readable recording medium. A computer-readable recording medium includes all kinds of recording apparatuses in which data that can be read by a computer system is stored. Examples of the computer-readable recording medium include a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, an optical data storage device, and the like, and may be implemented in the form of a carrier wave (for example, transmission via the Internet) . The computer readable recording medium may also be distributed over a networked computer system so that computer readable code can be stored and executed in a distributed manner. In addition, functional programs, codes, and code segments for implementing the present invention can be easily inferred by programmers of the technical field to which the present invention belongs.

It should be understood that the above-described apparatus and method are not limited to the configuration and method of the embodiments described above, but the embodiments may be modified so that all or some of the embodiments are selectively combined .

10: Measured object
20: Retaining bracket
30: Bimetal Bo
32: 1st metal
34: 2nd metal
40: Optical fiber
42: Fiber Bragg Grating
100: FBG strain sensor probe

Claims (14)

An FBG strain sensor probe for measuring a strain of a workpiece,
A fixing bracket having a lower end fixed to the object to be measured;
A bimetal beam fixed at the lower end to the measured object and spaced apart from the fixed bracket by a predetermined first distance; And
The other end of the optical fiber bragg grating is provided at the free end of the fixing bracket and the other end is installed at the free end of the bimetal beam. The optical fiber Bragg grating reflects light corresponding to the Bragg wavelength of the light. And an optical fiber (FBG) disposed between the one side and the other side,
When a change in temperature occurs, deformation of the bimetal beam occurs, and deformation of the bimetal beam prevents the Bragg wavelength from being changed according to the change of the temperature,
The optical fiber includes:
A free end of the fixed bracket and a free end of the bimetal beam in a state of being pulled by a predetermined initial tension length,
An initial strain is generated in the optical fiber installed in the pulled state,
Wherein the initial strain generated in the optical fiber is determined according to the following equation.
Equation
Figure 112015126427507-pat00111

In the above equation,
Figure 112015126427507-pat00112
Is an initial strain of the optical fiber,
Figure 112015126427507-pat00113
Is a modulus of elasticity of the bimetallic beam,
Figure 112015126427507-pat00114
Is the moment of inertia of the bimetal beam,
Figure 112015126427507-pat00115
Is an elastic modulus of the optical fiber,
Figure 112015126427507-pat00116
Is a sectional area of the optical fiber,
Figure 112015126427507-pat00117
Is the distance between the lower end of the bimetal beam and the free end of the bimetallic beam on the other side of the optical fiber,
Figure 112015126427507-pat00118
Is the first distance,
Figure 112015126427507-pat00119
Is the initial tensile length. delete delete The method according to claim 1,
The bimetal-
A first metal; And
And a second metal coupled to the first metal and having a thermal expansion coefficient different from the thermal expansion coefficient of the first metal. 5. The method of claim 4,
The deformation of the bimetal beam, which is generated in accordance with the change of the temperature,
Wherein the bimetal beam is deflected at the free end of the bimetallic beam. 5. The method of claim 4,
The deformation of the bimetal beam, which is generated in accordance with the change of the temperature,
Wherein the temperature-compensated FBG strain sensor probe is induced by a thermal expansion coefficient of the first metal and a thermal expansion coefficient of the second metal having different values. The method according to claim 6,
Wherein the deformation of the bimetallic beam according to the temperature change is determined according to the following equation (1).
Equation 1
Figure 112015035796280-pat00120

In the above equation (1)
Figure 112015035796280-pat00121
Is the distance between the lower end of the bimetal beam and the free end of the bimetallic beam on the other side of the optical fiber,
Figure 112015035796280-pat00122
Is the temperature change,
Figure 112015035796280-pat00123
Is a deformation of the bimetal beam according to the change of the temperature,
Figure 112015035796280-pat00124
Is defined by the following equation (2).
Equation 2
Figure 112015035796280-pat00125

In Equation (2)
Figure 112015035796280-pat00126
Is a modulus of elasticity of the first metal,
Figure 112015035796280-pat00127
Is an elastic modulus of the second metal,
Figure 112015035796280-pat00128
Is the thickness of the first metal,
Figure 112015035796280-pat00129
Is the thickness of the second metal,
Figure 112015035796280-pat00130
Is a difference between the thermal expansion coefficient of the first metal and the thermal expansion coefficient of the second metal. 8. The method of claim 7,
Wherein the bimetal beam and the optical fiber satisfying the following formula are used to prevent the Bragg wavelength from changing according to the temperature change.
Equation
Figure 112015035796280-pat00131

In the above equation,
Figure 112015035796280-pat00132
Is the first distance,
Figure 112015035796280-pat00133
Is the optical temperature coefficient of the optical fiber,
Figure 112015035796280-pat00134
Is the photoelastic coefficient of the optical fiber,
Figure 112015035796280-pat00135
Is the thermal expansion coefficient of the optical fiber. 9. The method of claim 8,
And a measuring unit for measuring a strain of the measured object using light reflected from the optical fiber Bragg grating. 10. The method of claim 9,
Wherein the measuring unit measures a strain of the object to be measured by using the following equation.
Equation
Figure 112015035796280-pat00136

In the above equation,
Figure 112015035796280-pat00137
Is a strain of the object to be measured,
Figure 112015035796280-pat00138
Is the Bragg wavelength,
Figure 112015035796280-pat00139
Is the modulus of elasticity of the bimetallic beam,
Figure 112015035796280-pat00140
Is the moment of inertia of the bimetal beam,
Figure 112015035796280-pat00141
Is an elastic modulus of the optical fiber,
Figure 112015035796280-pat00142
Is the cross-sectional area of the optical fiber. delete delete delete delete
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