CN103398800B - A kind of for large structure quasi-distributed fiber grating temperature strain measuring system - Google Patents

Abstract

The invention provides a quasi-distributed optical fiber grating temperature strain measurement system for large structures, including: low-light Sm125 optical fiber grating demodulator, single-mode optical fiber jumper, optical switch, Bragg optical fiber grating sensor, Ethernet line, industrial control computer The present invention is mainly used for strain monitoring of large complex structures, and has high sensitivity, fast response speed, wide coverage and strong anti-electromagnetic interference capability. The invention makes full use of the characteristics of fiber grating wavelength division multiplexing and time division multiplexing to form a large network; the temperature resolution of the invention is 0.1°C, and the strain resolution is 1με, which has strong practicability.

Description

A quasi-distributed fiber grating temperature and strain measurement system for large structures

technical field

The invention belongs to the technical field of optical fiber measurement, in particular to a temperature and strain measurement system for quasi-distributed optical fiber gratings used in large structures.

Background technique

Temperature and strain measurement are active areas of development in fiber optic sensing technology. Traditional resistance strain gauges and thermocouples have disadvantages such as difficult installation, wiring, and maintenance, and the measurement range is small, the cable layout is complicated, susceptible to electromagnetic interference, and the system reliability is low. The use of quasi-distributed fiber gratings is an effective method. The optical signal propagates in the optical fiber and is inherently uncharged. Four cables complete signal transmission, while optical fiber sensing only needs a single-mode optical fiber to carry dozens or even hundreds of sensors.

The sensing process of fiber grating is to obtain sensing information through the modulation of the fiber Bragg wavelength by external physical parameters, and it is a wavelength modulation fiber sensor. At present, this sensing technology has been widely used in aerospace, chemical medicine, water conservancy and hydropower and other fields.

Existing fiber grating temperature and strain sensors focus on the characteristics of a single sensor itself, but do not form a large network to comprehensively detect the structure.

Contents of the invention

The object of the present invention is to address the deficiencies of the prior art and provide a quasi-distributed optical fiber grating temperature and strain measurement system for large-scale structures, which can effectively detect temperature changes at various points of large-scale complex structures and locations where strains are concentrated.

The technical solution adopted in the present invention is: a quasi-distributed optical fiber grating temperature and strain measurement system for large structures, the measurement system includes: low-light Sm125 optical fiber grating demodulator, single-mode optical fiber jumper, optical switch, Bragg Fiber Bragg grating sensor, Ethernet cable and industrial computer; among them, the single-mode fiber jumper connects the four parallel output channels of the Sm125 fiber grating demodulator and the input channel of the optical switch to transmit optical signals; each output port of the optical switch Multiple fiber Bragg grating sensors with different central wavelengths are connected in series; the Ethernet cable is connected to the Sm125 fiber grating demodulator and the industrial computer, and the electrical signal is transmitted from the Sm125 fiber grating demodulator to the industrial computer, and finally calculated by the industrial computer Demodulate the temperature and strain and display the results; the laser light emitted by the narrow-band scanning light source in the low-light Sm125 fiber grating demodulator is transmitted to the input end of the optical switch through the single-mode fiber jumper, and each input port of the optical switch Corresponding to four output ports, the laser is switched between these four ports, and transmitted from an output port of the optical switch to the fiber Bragg grating sensor in series, each fiber Bragg grating sensor will reflect back the light of a specific wavelength and send it back to the original path It returns to the Microlight Sm125 fiber grating demodulator and is converted into an electrical signal by the photodetector. The electrical signal is then transmitted to the industrial control computer by the Ethernet cable. Finally, the industrial control computer completes the calculation and demodulation work, and obtains and displays temperature and strain information.

Preferably, the low-light Sm125 fiber grating demodulator scans four channels in parallel, the scanning laser range is 1510nm to 1590nm, the bandwidth is 80nm, and the demodulation accuracy is 1pm.

Preferably, the optical switch is a 4×16 optical switch, the connector is APC, the insertion loss is less than or equal to 1.0 dB, and the repeatability is less than or equal to ±0.05 dB.

Preferably, the output demodulation range of each channel of the optical switch is 1510nm to 1590nm, separated by 2nm, each channel can carry 40 fiber Bragg grating sensors, and one system can carry 640 sensors.

Preferably, the Fiber Bragg Grating sensor has a wavelength variation range of 2nm, a detection temperature variation range of 200°C, and a detection strain variation range of 1500με.

Principle of the present invention is:

In conjunction with Fig. 1, the principle of a quasi-distributed optical fiber grating temperature and strain measurement system for large-scale structures of the present invention is illustrated. , Fiber Bragg grating sensor 4, Ethernet cable 5, industrial computer 6; wherein there are four single-mode fiber jumper wires 2, which are respectively connected to the four channels of the Microlight Sm125 fiber Bragg grating demodulator 1 and the four input ports of the optical switch 3 , the connectors are all APC, and each input interface of the optical switch 3 is divided into four optical channels, and the optical switch 3 is controlled by the program to switch to the channel to be used. The optical switch 3 has sixteen output channels in total, each of which is connected to the A plurality of fiber Bragg grating sensors 4 are connected, and the electrical signal demodulated by the low-light Sm125 fiber grating demodulator 1 is transmitted to the industrial computer 6 through the Ethernet line 5, and the temperature and strain are calculated and displayed by the industrial computer 6; the low-light Sm125 The laser output from the light source of the fiber grating demodulator 1 is transmitted to the input end of the optical switch 3 through the single-mode fiber jumper, and the optical switch 3 selects the working output channel to transmit the laser through this channel, so that the laser can be transmitted to the fiber Bragg grating sensor 4. A specific fiber Bragg grating sensor 4 will reflect laser light of a specific wavelength, and the reflected light will return to the optical switch 3 through the original route and then back to the low-light Sm125 fiber grating demodulator 1, and the low-light Sm125 fiber grating demodulator 1 The photodetectors in the circuit receive and convert electrical signals, which are transmitted to the industrial control computer 6 via the Ethernet cable 5, and finally the industrial control computer 6 calculates and gives the results of temperature and strain. The specification of the optical switch 3 is 4×16, the four channels of the Sm125 fiber grating demodulator 1 at the four input ports are connected, and each of the sixteen output ports can be connected with a fiber Bragg grating sensor string. The characteristics of time division multiplexing. The center wavelength of the fiber Bragg grating sensor 4 is set every 2nm between 1510nm and 1590nm, and the bandwidth range of 80nm can be connected into as many as forty fiber Bragg grating sensor strings, which utilizes the characteristic of wavelength division multiplexing .

The beneficial effect of the present invention compared with prior art is:

(1), the present invention uses a 4×16 optical switch to expand the four channels of Sm125 to sixteen channels through time division multiplexing, and uses wavelength division multiplexing to reasonably allocate the center wavelength of the fiber Bragg grating sensor, so that the entire system can be carried There are as many as 640 sensors, so that it can conduct comprehensive detection of large structures, and has strong practicability.

(2) The structure of the present invention is simple, and the use is very flexible. Users can arrange to use any one or several optical switches according to different needs. The number of sensors can also be adjusted under the condition that the central wavelength is between 1510nm and 1590nm without duplication. Choose arbitrarily.

(3) The temperature resolution of the present invention can reach 0.1°C, the strain resolution can reach 1με, the measurement accuracy is much higher than that of traditional electric sensors, the response speed is fast, and the anti-electromagnetic interference ability is strong.

Description of drawings

Fig. 1 is the schematic diagram of the quasi-distributed fiber grating temperature and strain measurement system for large structures of the present invention;

Fig. 2 is a schematic diagram of sensor 1 linear fitting;

FIG. 3 is a schematic diagram of linear fitting of the sensor 2 .

In the figure: 1. Low-light Sm125 fiber grating demodulator, 2. Single-mode fiber jumper, 3. Optical switch, 4. Bragg fiber grating sensor, 5. Ethernet cable, 6. Industrial computer.

detailed description

Specific embodiments of the present invention will be described below in conjunction with the accompanying drawings, so as to better understand the present invention. It should be noted that in the following description, when detailed descriptions of known functions and designs may dilute the main content of the present invention, these descriptions will be omitted here.

As shown in Figure 1, the bidirectional four-channel coupled distributed optical fiber Raman temperature measurement system of the present invention includes: low-light Sm125 fiber Bragg grating demodulator 1, single-mode fiber jumper 2, optical switch 3, fiber Bragg grating Sensor 4, Ethernet line 5, industrial control computer 6; Wherein, one end of four single-mode optical fiber jumpers 2 is connected to the four channels of the low-light Sm125 fiber grating demodulator 1, and the other end is connected to four input ends of the optical switch 3; Sixteen output terminals of the switch 3 are connected to a fiber grating string composed of a plurality of fiber Bragg grating sensors 4 ;

The laser light emitted by the micro-light Sm125 fiber grating demodulator 1 reaches the input end of the optical switch 3 through the optical fiber jumper 2, and the optical switch 3 is artificially designated to select one or more output channels, and the laser is transmitted to Prague through the optical switch 3 Fiber Bragg grating sensors 4, these sensors can reflect the laser of a specific wavelength back to the optical switch 3 and then to the micro-light Sm125 fiber Bragg grating demodulator 1, which is received and converted into an electrical signal by the detection circuit in the micro-light Sm125 fiber Bragg grating demodulator 1 , transmitted to the industrial control computer 6 through the Ethernet line 5, and then the industrial control computer 6 processes and displays the temperature and strain information.

In ordinary optical fibers, the uniform fiber grating with the simplest structure is formed by letting the core refractive index vary with the period, which is the fiber Bragg grating (FBG) in the present invention. The sensing principle is that the light propagating in the fiber core Scattering will occur at each grating surface. If the Bragg condition cannot be satisfied, the phases of light reflected by the sequentially arranged grating planes will gradually become different until they cancel each other out; if the Bragg condition can be satisfied, the light reflected by each grating plane will Accumulate step by step, and finally form a reflection peak in the reverse direction, and the central wavelength is determined by the fiber parameters. That is to say, FBG is essentially a narrow-band filter, which reflects light in a very narrow frequency band back (the reflectivity can reach more than 90%), and transmits light in other frequency bands.

In a periodic fiber Bragg grating (FBG), the reflected Bragg wavelength can be expressed by the refractive index and period:

λ _B =n _eff Λ(1)

(1) In the formula, λ _B is the reflection wavelength of the FBG center, n _eff is the effective refractive index of the FBG grid area, and Λ is the grid pitch of the FBG.

When a beam of broadband light is incident on a Bragg grating, only the narrowband spectrum that meets the above grating resonance conditions will be reflected back. When the fiber grating is subjected to external effects (temperature, stress, etc.), the effective refractive index n _eff and the grating pitch Λ will be affected and changed, so that the Bragg wavelength λ _B will shift. If the change of this offset is detected, the external action information that affects the change can be known, which is the basic principle of FBG sensing.

Specifically, when the temperature changes and the central wavelength shifts:

T=1000Δλ/K _t (2)

(2) In the formula, T is the temperature, Δλ is the drift of the center wavelength, and K _t is the temperature coefficient of the fiber grating.

When the strain change causes the central wavelength to shift:

E=1000Δλ/ _Kε (3)

(3) In the formula, ε is the strain, Δλ is the drift of the center wavelength, and K _ε is the gauge factor of the fiber Bragg grating.

Generally speaking, when the change of FBG center wavelength is not large, the change of FBG wavelength caused by temperature change of 1°C is about 10pm. At the same time, the wavelength change caused by a strain change of 1με is about 1.2pm. Different fiber gratings will have different temperature sensitivity coefficients due to the use of different processes to write gratings or the use of different optical fibers and different annealing processes, especially for packaged fiber gratings. To a certain extent, the temperature sensing characteristics of fiber Bragg gratings are imaged, so different FBGs must be calibrated before they can be measured in practice.

Take the sensor calibrated below as an example. The temperature control range is from 20°C to 60°C, and the temperature control accuracy is 0.1°C. A round of data is collected every 5°C. After reaching a temperature and being constant, we collect data at this temperature for about 2 minutes at a sampling rate of 2Hz. About 240 sets of data, each set of data includes center wavelength and power. What we care about is the information of the center wavelength, and take the average value of these 240 center wavelengths as the value of the temperature calibration point. The calibration data in Table 1 are obtained:

Table 1 Calibration data of two temperature sensors

Figure 2 and Figure 3 are obtained by linear fitting of sensor 1 and sensor 2 respectively:

It can be seen from the fitting equation that the temperature sensitivity coefficient of sensor 1 is 24pm/°C, and that of sensor 2 is 28pm/°C. The linearity is good in this temperature range.

The parts not disclosed in detail in the present invention belong to the known technology in the art.

Although the illustrative specific embodiments of the present invention have been described above, so that those skilled in the art can understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, As long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concept of the present invention are included in the protection list.

Claims (1)

1. A quasi-distributed fiber grating temperature strain measurement system for large-scale structural bodies is characterized in that: the measurement system includes: the system comprises a low-light level Sm125 fiber grating demodulator (1), a single-mode fiber jumper (2), an optical switch (3), a Bragg fiber grating sensor (4), an Ethernet cable (5) and an industrial control computer (6); wherein,

the single-mode optical fiber jumper (2) is connected with four parallel output channels of the glimmer Sm125 fiber grating demodulator (1) and an input channel of the optical switch (3) and is used for transmitting optical signals;

each output end of the optical switch (3) is connected with a plurality of Bragg fiber grating sensors (4) with different central wavelengths in series;

the Ethernet line (5) is connected with the glimmer Sm125 fiber grating demodulator (1) and the industrial control computer (6), the electric signal is transmitted from the glimmer Sm125 fiber grating demodulator (1) to the industrial control computer (6), and finally the industrial control computer (6) calculates and demodulates the temperature and the strain to display the result;

laser emitted by a self-contained narrow-band scanning light source in the glimmer Sm125 fiber grating demodulator (1) is transmitted to the input end of the optical switch (3) through a single-mode fiber jumper (2), each input port of the optical switch (3) corresponds to four output ports, the laser is switched among the four ports and is transmitted from a certain output port of the optical switch (3) to the Bragg fiber grating sensors (4) which are connected in series, each Bragg fiber grating sensor (4) can reflect light with a specific wavelength and return to the glimmer Sm125 fiber grating demodulator (1) in the original circuit and is received and converted into an electrical signal by the photoelectric detector, the electrical signal is transmitted to the industrial control computer (6) through the Ethernet cable (5), and finally the industrial control computer (6) completes calculation and demodulation work to obtain and display temperature and strain information;

the micro-light Sm125 fiber grating demodulator (1) performs four-channel parallel scanning, the scanning laser range is 1510nm to 1590nm, the bandwidth is 80nm, and the demodulation precision is 1 pm;

the optical switch (3) is a 4X 16 optical switch, the joint is APC, the insertion loss is less than or equal to 1.0dB, and the repeatability is within the range of +/-0.05 dB;

each path of output demodulation range of the optical switch (3) is 1510nm to 1590nm, the demodulation ranges are separated by taking 2nm as a unit, each path can carry forty Bragg fiber grating sensors (4), and one set of system can carry sixty-hundred-forty sensors;

the Bragg fiber grating sensor (4) has the wavelength variation range of 2nm, the detection temperature variation range of 200 ℃ and the detection strain variation range of 1500 mu;

the quasi-distributed fiber grating temperature strain measurement system for the large-scale structural body uses the following calibrated fiber bragg grating sensors, specifically, the temperature control range is from 20 ℃ to 60 ℃, the temperature control precision is 0.1 ℃, data is acquired at intervals of 5 ℃, after each temperature is reached and is constant, data of about 2 minutes and about 240 groups of data are acquired at the temperature at the sampling rate of 2Hz, each group of data comprises central wavelength and power, the average value of the 240 central wavelengths is taken as the value of a temperature calibration point, and the calibration data of the table 1 are obtained:

TABLE 1 calibration data of two temperature sensors

Respectively carrying out linear fitting on the fiber bragg grating sensor 1 and the fiber bragg grating sensor 2 to obtain a fitting equation, wherein the fitting equation shows that the temperature sensitivity coefficient of the fiber bragg grating sensor 1 is 24 pm/DEG C, the temperature sensitivity coefficient of the fiber bragg grating sensor 2 is 28 pm/DEG C, and the linearity is better in the temperature interval range;

the quasi-distributed fiber grating temperature strain measurement system for the large structure body adopts a 4 x 16 optical switch, four channels of a dim light Sm125 fiber grating demodulator are expanded into sixteen channels through time division multiplexing, and the central wavelength of a Bragg fiber grating sensor is reasonably distributed by utilizing the wavelength division multiplexing, so that the whole system can carry up to six hundred and forty sensors, and further the large structure body can be comprehensively detected;

the quasi-distributed fiber grating temperature strain measurement system for the large structure body can arrange and use any one or more optical switches according to different requirements, and the number of the sensors can be selected randomly under the condition that the central wavelength is 1510nm to 1590nm and is not repeated;

the temperature resolution of the quasi-distributed fiber bragg grating temperature strain measurement system for the large-scale structural body can reach 0.1 ℃, and the strain resolution can reach 1 mu.