CN110608675A

CN110608675A - Multi-point displacement testing method based on fiber grating sensing technology

Info

Publication number: CN110608675A
Application number: CN201910756158.4A
Authority: CN
Inventors: 王群敏; 陈文华; 张文君; 孙浩; 周德春
Original assignee: Zhejiang East China Engineering Safety Technology Co Ltd; PowerChina Huadong Engineering Corp Ltd
Current assignee: Zhejiang Huadong Mapping And Engineering Safety Technology Co ltd; PowerChina Huadong Engineering Corp Ltd
Priority date: 2019-08-16
Filing date: 2019-08-16
Publication date: 2019-12-24
Anticipated expiration: 2039-08-16
Also published as: CN110608675B

Abstract

The invention relates to a multipoint displacement testing method based on a fiber grating sensing technology. The invention aims to provide a multipoint displacement testing method based on the fiber bragg grating sensing technology, which is simple to implement, high in measurement accuracy and capable of realizing on-line monitoring. The technical scheme of the invention is as follows: a multipoint displacement testing method based on fiber grating sensing technology is characterized in that: the fiber grating optical system comprises a carrier, an inclinometer, a fiber grating string I and a fiber grating string II; mounting an inclinometer at one end of a carrier to be used as a displacement 0 point; the fiber grating string I and the fiber grating string II are symmetrically arranged on two sides of the carrier about the central line of the carrier; measuring the central wavelength lambda of the fiber grating string I and the fiber grating string II_Bi0、λ’_Bi0As an initial value; obtaining an initial angle θ of an inclinometer₀. The invention is suitable for the fields of fiber grating sensing technology and engineering measurement technology.

Description

Multi-point displacement testing method based on fiber grating sensing technology

Technical Field

The invention relates to a multipoint displacement testing method based on a fiber grating sensing technology. The method is suitable for the fields of fiber grating sensing technology and engineering measurement technology.

Background

In engineering excavation processes of dams, tunnels, underground roadways, foundation pits, side slopes and the like, engineering technicians often need to know the horizontal displacement of deep parts, the settlement of the adjacent ground and the spatial and temporal evolution rules of the deep parts to determine supporting modes and parameters so as to ensure the safety of engineering construction.

At present, a multipoint displacement measuring device for deep parts at home and abroad mainly comprises an inclinometer for measuring horizontal displacement and a multipoint displacement meter for testing axial deformation, and multipoint strain sensors are connected in series to calculate deflection, so that a displacement boundary test of two points needs to be added, while a deep horizontal displacement test can only measure the horizontal displacement of an orifice point (a single point), if light depends on the displacement test, the displacement of each point cannot be directly calculated, but the multipoint settlement measuring device for linear distribution is still blank.

Disclosure of Invention

The technical problem to be solved by the invention is as follows: aiming at the existing problems, the multipoint displacement testing method based on the fiber bragg grating sensing technology is simple to implement, high in measuring accuracy and capable of realizing on-line monitoring.

The technical scheme adopted by the invention is as follows: a multipoint displacement testing method based on fiber grating sensing technology is characterized in that: the fiber grating optical system comprises a carrier, an inclinometer, a fiber grating string I and a fiber grating string II;

mounting an inclinometer at one end of a carrier to be used as a displacement 0 point;

the fiber grating string I and the fiber grating string II are symmetrically arranged on two sides of the carrier about the central line of the carrier;

measuring the central wavelength lambda of the fiber grating string I and the fiber grating string II_Bi0、λ’_Bi0As an initial value; obtaining an initial angle θ of an inclinometer₀；

When the carrier is deformed synchronously with the measured object, the central wavelength lambda of the fiber grating string I and the fiber grating string II is measured_Bi1、λ’_Bi1As test values; obtaining a test angle theta of a caster₁；

Comparing the test value with the initial value, and calculating the strain variation delta epsilon of each point according to the sensing principle and the temperature compensation principle of the fiber Bragg grating_zi：

In the formula, Pe is the elasto-optic coefficient;

according to the strain variation of each point and the theory of mechanics of materials, the displacement y of each point is obtained_iComprises the following steps:

Δθ₀＝θ₁-θ₀；

in the formula,. DELTA.x_iThe distance between the upper and lower measuring sections along the length direction of the measured object; r is half of the distance between two symmetrical strain measuring points on the section of the measured object.

The fiber grating can not only sense strain, but also transmit signals, and a plurality of fiber grating sensors are connected in series to form a fiber grating string for distributed strain observation. The optical fiber sensing technology has the outstanding advantages of light weight, small diameter, strong electromagnetic interference resistance, corrosion resistance, high temperature resistance, small signal attenuation, integration of information sensing and information transmission and the like, and can realize strain measurement on a measured object.

The invention has the beneficial effects that: the invention installs an inclinometer at one end of the object to be measured, sets two groups of fiber grating strings which are linearly distributed and are symmetrically arranged by taking the central line as the axis, and measures the compressive strain and the tensile strain of each interface in real time, thereby calculating the deflection of the object to be measured, realizing the real-time monitoring of the stability of the object to be measured and ensuring the safety of engineering.

The invention utilizes the fiber grating sensing technology and combines the goniometer to measure the multi-point displacement, overcomes the defect that the traditional displacement measuring method needs to waste a large amount of labor, and has convenient installation, high measuring precision and lower cost.

Drawings

FIG. 1 is a schematic structural diagram of an initial state of the embodiment.

FIG. 2 is a schematic cross-sectional view of an embodiment.

Fig. 3 is a schematic structural diagram of a modified embodiment.

Detailed Description

The embodiment is a multipoint displacement testing method based on a fiber grating sensing technology, which comprises a carrier, an inclinometer 1, a fiber grating string I3 and a fiber grating string II 4, and comprises the following specific steps:

s01, mounting the inclinometer 1 at one end of the carrier (shown in figure 1) as a displacement 0 point;

s02, symmetrically installing the fiber grating string I3 and the fiber grating string II 4 on two sides of the carrier (see figure 2) relative to the central line of the carrier;

s03, acquiring the initial angle theta of the declination angle gauge 1 in the initial state₀(ii) a Measuring the initial central wavelength of the fiber grating string I3 in the initial state as lambda_B10，λ_B20，λ_B30，……，λ_Bi0，……，λ_Bm0The initial center wavelength of the fiber grating string II 4 in the initial state is lambda'_B10，λ’_B20，λ’_B30，……，λ’_Bi0，……，λ’_Bm0；

S04, obtaining the testing angle theta of the caster 1 after the carrier synchronously deforms with the object 2 to be tested₁(ii) a Measuring the central wavelengths of the deformed fiber grating strings I3 as lambda respectively_B11，λ_B21，λ_B31，……，λ_Bi1，……，λ_Bm1The center wavelengths of the fiber grating strings II 4 are respectively lambda'_B11，λ’_B21，λ’_B31，……，λ’_Bi1，……，λ’_Bm1；

S05, comparing the test value with the initial value according to the optical fiberCalculating the strain variation delta epsilon of each point according to the Bragg grating sensing principle and the temperature compensation principle_zi：

1. Fiber bragg grating sensing principle

The fiber Bragg grating sensor is one functional fiber sensor with fiber Bragg grating as sensitive element. According to the optical fiber coupling mode theory, when a beam of broadband light passes through the fiber grating, mode coupling is generated, when the Bragg condition is met, the grating plays the role of a reflector and reflects a narrow-band light wave (the rest wavelengths are transmitted continuously through the fiber grating), and the central wavelength of the narrow-band light wave is the Bragg wavelength lambda of the grating_B。

λ_B＝2n_effΛ (1)

When the grating is subjected to the action of external temperature, strain and the like, the lambada and the n_effThe consequent changes Δ Λ and Δ n_effResulting in a shift of the reflection wavelength by Δ λ in accordance with the Bragg condition_B。

Δλ_B＝2Δn_effΛ+2n_effΔΛ (2)

The above formula is the mechanism of grating sensing monitoring, and the displacement delta lambda of the central wavelength of the reflected wave meeting the Bragg condition is monitored_BThe external detected signals such as temperature, strain, stress and the like acting on the fiber bragg grating are detected.

The shift generated by the central wavelength of the fiber grating is as follows:

in the formula ∈_ZFor axial strain, P_eIs the coefficient of elasticity, alpha_∧Is the coefficient of thermal expansion, alpha, of the optical fiber_nIndicating the thermo-optic coefficient, and Δ T is the amount of change in temperature.

2. Principle of temperature compensation

The multipoint displacement testing device adopts a design of two fiber bragg gratings, the FBGs 1 and the FBG2 are symmetrically arranged on two sides of the same section of the carrier respectively, and the temperature self-compensation of the FBG1 by the FBG2 can be realized. The thermal expansion coefficient and thermo-optic coefficient of FBGs 1 and 2 are the same, and therefore the temperature sensitivity coefficients of both are also the same. It can be seen from the structural design that the shift of the center wavelength is independent of temperature and only changes with the change of strain. The difference of the central wavelength drifts of the two fiber gratings is as follows:

from the above, the amount of change in strain Δ ∈ at each point in the present embodiment_ziCan be expressed by the following formula:

s06, obtaining the displacement y of each point according to the strain variation of each point and the mechanics theory of materials_iComprises the following steps:

from the theory of material mechanics, it can be known that:

angle of rotation theta of a certain cross section of the object 2 relative to an adjacent cross section_iStrain difference delta epsilon of two symmetrical points on the section_iThe relationship of (1) is:

in the formula,. DELTA.x_iThe distance between the upper and lower measuring sections along the length direction of the object 2 to be measured; r is half of the distance between two symmetrical strain measuring points on the corresponding section of the object 2 to be measured.

Combining the equations (3) and (7), assuming that the change amount of the inclination angle measured by the inclinometer 1 is Δ θ₀And the displacement of the bottom is 0, then the displacement y of a certain section on the measured object 2 is obtained_iComprises the following steps:

the displacement yi of each point on the object 2 to be measured in this embodiment is represented by the following formula:

Δθ₀＝θ₁-θ₀； (10)

the carrier in this embodiment is a flexible material with a certain rigidity, the deformation modulus of which is slightly smaller than that of soil, and the carrier can be a tubular substance made of PVC material, such as a PVC inclinometer pipe. When the displacement of the deep part is tested, the fiber grating can not be directly arranged on the tested object, the fiber grating is fixedly arranged on the carrier firstly through a carrier, and then the carrier provided with the fiber grating is wrapped with the tested object, so that the tested object and the carrier synchronously deform, the strain on the carrier is the strain of the tested object, and the strain of the carrier is equal to the strain of the tested object in the fiber grating test.

Claims

1. A multipoint displacement testing method based on fiber grating sensing technology is characterized in that: the device comprises a carrier, an inclinometer (1), a fiber grating string I (3) and a fiber grating string II (4);

mounting an inclinometer (1) at one end of a carrier as a displacement 0 point;

the fiber grating string I (3) and the fiber grating string II (4) are symmetrically arranged on two sides of the carrier about the central line of the carrier;

measuring the central wavelength lambda of the fiber grating string I (3) and the fiber grating string II (4)_Bi0、λ’_Bi0As an initial value; obtaining an initial angle theta of an inclinometer (1)₀；

When the carrier is deformed synchronously with the object (2), the central wavelength λ of the fiber grating string I (3) and the fiber grating string II (4) is measured_Bi1、λ’_Bi1As test values; obtaining a test angle theta of the caster (1) after deformation₁；

In the formula, P_eIs the elasto-optic coefficient;

according to the strain variation of each point and the theory of mechanics of materials, the displacement yi of each point is obtained as follows:

Δθ₀＝θ₁-θ₀；

in the formula,. DELTA.x_iThe distance between the upper and lower measuring sections along the length of the measured object (2); r is half of the distance between two symmetrical strain measuring points on the section of the measured object (2).