CN103335668B - A kind of distributed fiber optic temperature strain measurement method - Google Patents

Abstract

The present invention relates to the measuring method of distributed fiberoptic sensor, be specifically related to a kind of distributed fiber optic temperature strain measurement method, comprise the steps: that (1) lays temperature sensing optical fiber and strain sensing optical fiber; (2) temperature profile results and Brillouin shift distribution results is received; (3) described temperature profile results is alignd and interpolation with Brillouin shift distribution results; (4) judge whether temperature profile results exists temperature peaks, and determine the Strain Distribution of temperature province inner fiber.The present invention proposes to substitute direct detection by the mode of image data post-processed, effectively can overcome the problem that Measuring Time increases.Software carries out consecutive number strong point progressive mean to the data of high spatial resolution, effectively can not only reduce spatial resolution, and counts can to realize fast and DTS spatial resolution exact matching by optimizing progressive mean.

Description

A Distributed Optical Fiber Temperature and Strain Measurement Method

technical field

The invention relates to a measurement method of a distributed optical fiber sensor, in particular to a distributed optical fiber temperature and strain measurement method.

Background technique

Optical fiber sensing technology is a new type of sensing technology, which has the advantages of high measurement accuracy, anti-electromagnetic interference, intrinsic safety, distributed measurement, etc., and is widely used in fields such as electric power, petrochemical, structure, and fire protection. The distributed optical fiber sensor based on Brillouin scattering is a fiber optic sensor that has developed rapidly in recent years. The measurement of various parameters, such as the measurement of the temperature and strain of the cable, especially the Brillouin Optical Time Domain Analyzer (Brillouin Optical Domain Analysis, BOTDA) based on the stimulated Brillouin scattering effect, uses the optical path structure of the loop fiber, due to the The laser scattering signal is amplified, and the measurement distance can reach tens of kilometers, which is much higher than other types of distributed optical fiber sensors. It is the most promising type of distributed optical fiber sensors at present. However, since the Brillouin frequency shift is sensitive to temperature and strain at the same time, and the two are linearly related, there will be a problem of cross-sensitivity between temperature and strain in the actual application process, so it is difficult to obtain from the final Brillouin frequency shift. The variation caused by temperature and stress is separated from the frequency shift, which seriously hinders the application and promotion of this type of sensor.

The related solutions that can measure temperature and strain at the same time have been reported as follows:

1. Dual-parameter method of scattered light intensity and frequency shift (J.Smithetal, "Simultaneous distributed strain and temperature measurement," Appl. Opt, 38: 5372-5377, 1999): This method measures backscattered light intensity and frequency shift at the same time. The cubic equation is solved for temperature and stress variations. However, the measurement of Brillouin light intensity limits the detection distance of the sensor, and the light intensity detection is easily affected by external disturbances, light source output power jitter, and polarization state drift.

2. Landau-Placzek ratio method (P.C.WaitandT.P.Newson, "Landau-Placzekratioappliedtodistributedfibresensing," OpticsCommunications, 122(4-6):141-146, 1996), this method simultaneously measures the Brillouin scattered light intensity and Strain-insensitive Rayleigh scattered light intensity, the temperature change is extracted by calculating the ratio of the two (Landau-Placzek ratio), but the measurement of Rayleigh scattered light limits the detection length and spatial resolution of the sensor, and the system complexity is significantly increased .

In order to essentially solve the problem of cross-sensitivity between temperature and strain in distributed optical fibers, methods combining other physical effects, such as the combined effects of Raman scattering and Brillouin scattering, have been proposed at home and abroad. The authorized announcement number CN201955173U is an improvement scheme proposed for this principle. Since Raman scattering is only sensitive to temperature, Brillouin scattering frequency shift is sensitive to both temperature and strain. Therefore, the temperature measured by the Raman scattering technique is used to demodulate the strain component contained in the Brillouin scattering frequency shift, and the following formula is used to solve the strain distribution:

$\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&Delta;v</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ;</span>$

Where: k _T is the Brillouin frequency shift temperature coefficient, k _ε is the Brillouin frequency shift strain coefficient, and T(z) is the temperature distribution.

Applying this scheme to strain demodulation still has the following problems:

1. Optical fibers cannot be multiplexed, resulting in misalignment of the length coordinates. The scheme structure of joint demodulation strain based on Raman-type distributed optical fiber temperature sensor (DTS) and Brillouin optical time domain analyzer BOTDA is shown in Figure 1. DTS is relatively mature for distributed temperature measurement technology, and generally uses multimode optical fiber 31 as a sensing element. The BOTDA uses a common single-mode communication optical fiber 32 . Therefore, the optical fibers used by the multi-mode DTS and BOTDA cannot be multiplexed, and the single-mode optical fiber 32 and the multi-mode optical fiber 31 need to be laid in parallel. Due to differences in optical fibers, the excess length settings of single-mode and multi-mode optical fibers cannot be completely consistent, and the length coordinates measured by DTS and BOTDA may be misaligned in physical space.

2. The sampling rate is inconsistent. Due to the difference in technical solutions between BOTDA and DTS, as well as differences in instrument configurations, the sampling rates of the two devices are often inconsistent, which will result in different numbers of sampling points for the two devices for the same fiber distance.

3. Spatial resolution difference. In order to comprehensively optimize the performance of long-distance temperature measurement, the spatial resolution of DTS is generally not high (greater than 1m, typically 2-5m). BOTDA has a strong long-distance measurement advantage, and can achieve high-precision measurement with a spatial resolution of 0.5m. For areas with temperature peaks, due to the inconsistent spatial resolution of DTS and BOTDA, there will be differences in the slopes of the rising and falling edges of the temperature peaks measured by the two, as shown in Figure 2. A strong oscillating curve (see the black circle in the figure) will be left on the strain curve after demodulation, resulting in wrong strain results.

Contents of the invention

Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a distributed optical fiber temperature and strain measurement method, which combines the principles of Raman scattering and Brillouin scattering to achieve high-resolution demodulation of strain in the stable temperature region, and Strain correct demodulation is achieved in regions of temperature variation.

The object of the present invention is to adopt following technical scheme to realize:

A distributed optical fiber temperature and strain measurement method, the improvement is that the method includes the following steps:

(1) laying temperature sensing optical fiber and strain sensing optical fiber;

(2) Receive temperature distribution results and Brillouin frequency shift distribution results;

(3) Align and interpolate the temperature distribution result with the Brillouin frequency shift distribution result;

(4) Judging whether there is a temperature peak in the temperature distribution result, and determining the strain distribution of the optical fiber in the temperature region.

Wherein, in the step (1), the sensing optical fiber for measuring temperature and the sensing optical fiber for measuring the Brillouin frequency shift caused by strain are arranged in parallel.

Wherein, in the step (2), the matching alignment unit receives the low spatial resolution temperature distribution result T _D (z) measured by the distributed Raman scattering temperature sensor DTS and the high temperature distribution result T D (z) measured by the distributed Brillouin time domain analysis BOTDA. Spatial resolution of Brillouin frequency shift distribution results Δv _B (z).

Wherein, in the step (3), the data volume of the Brillouin frequency shift distribution result Δv _B (z) and the temperature distribution result T _D (z) are the same, and the positions of the data points correspond to each other.

Wherein, in the step (4), the judgment of the area of the temperature peak is to judge whether there is a rising and falling edge; if the temperature monotonous change range in the continuous area exceeds the threshold (determined according to the actual working conditions), it is considered that there is a temperature rise and fall in this area Along, there is a temperature peak;

Determining the strain distribution of the fiber in the temperature region involves:

a. For the stable temperature region without rising and falling edges, the strain distribution ε(z) of the optical fiber:

$\mathrm{\&epsiv;}\left(z\right)=\frac{\mathrm{\&Delta;}{v}_B\left(z\right)-k_T{T}_{\mathrm{D.}}\left(z\right)}{k_{\mathrm{\&epsiv;}}}$ ①;

Where: k _T is the Brillouin frequency shift temperature coefficient, k _ε is the Brillouin frequency shift strain coefficient;

b. For the temperature change area with rising and falling edges, the Brillouin frequency shift distribution results Δv _B (z) are spatially accumulated to obtain the average value Δv _L (z), so that the spatial resolution of the Brillouin frequency shift distribution results is consistent with the temperature The distribution result T _D (z) matches; determine the fiber strain distribution ε(z) in this temperature region:

$\mathrm{\&epsiv;}\left(z\right)=\frac{\mathrm{\&Delta;}{v}_L\left(z\right)-k_T{T}_{\mathrm{D.}}\left(z\right)}{k_{\mathrm{\&epsiv;}}}$ ②.

Wherein, in b, the Brillouin frequency shift distribution result Δv _B (z) adjacent data points are accumulated and averaged to realize the spatial resolution of distributed Brillouin time domain analysis BOTDA and distributed Raman scattering temperature sensor DTS automatic rate matching.

Wherein, in the step (4), the method for judging the rising and falling edges of the temperature is applicable to the demodulation of the temperature and strain in the temperature stable region of the distributed Raman scattering temperature sensor DTS and the high spatial resolution of the strain; the distributed Raman scattering temperature Sensor DTS temperature sudden change area, the demodulation of temperature strain without strain distortion.

Compared with prior art, the beneficial effect that the present invention reaches is:

1. The present invention uses DTS test data to realize the separation of distributed Brillouin strain measurement results, which can solve the problem of temperature-strain coupling in the application of Brillouin technology, and promote the engineering application of strain monitoring technology with great economic value.

2. The present invention proposes to replace the direct detection by post-processing the collected data, which can effectively overcome the problems of long measurement time, high cost of direct measurement technology, and difficulty in engineering application.

3. The present invention adopts the method of feature point matching, data interpolation (extraction), and data alignment in the data processing method, which has the characteristics of fast processing speed, good curve fitting effect, and little influence of temperature on the extracted stress data, which meets the requirements of the strain monitoring system. requirements for real-time operation.

4. Under the condition of different acquisition technologies, the algorithm performs the accumulation and averaging of adjacent data points on the data with high spatial resolution, which can not only effectively reduce the spatial resolution, but also quickly achieve the same spatial resolution as DTS by optimizing the number of accumulated average points. exact match.

5. The technical algorithm involved in the present invention has strong adaptability, and solves the reality of matching difficulties caused by various factors such as inconsistency of sensing channels, different sampling data volumes, and different spatial resolutions in practical applications. The promotion and application of technology has laid a solid foundation.

Description of drawings

Figure 1 is a structural diagram of DTS-assisted BOTDA for strain demodulation;

Figure 2 is a schematic diagram of strain demodulation errors caused by spatial resolution differences;

Fig. 3 is the basic flowchart of the distributed optical fiber strain measurement method provided by the present invention;

Fig. 4 is the original data schematic diagram of DTS temperature measurement result and BOTDA frequency shift measurement result provided by the present invention;

Fig. 5 is a schematic diagram of the results after alignment and interpolation of the DTS temperature measurement results provided by the present invention and the BOTDA frequency shift measurement results;

Fig. 6 is a schematic diagram of accumulating and averaging adjacent data points provided by the present invention to realize spatial resolution matching;

Fig. 7 is a graph of strain distribution after demodulation provided by the present invention.

detailed description

The specific implementation manners of the present invention will be further described in detail below in conjunction with the accompanying drawings.

The basic process of the distributed optical fiber strain measurement method provided by the present invention is shown in Figure 3, and it comprises the following steps:

(1) Laying temperature sensing optical fiber and strain sensing optical fiber: the optical fiber used to measure temperature and the optical fiber used to measure the Brillouin frequency shift caused by strain are arranged in parallel.

(2) Receive the low spatial resolution temperature distribution T(z) measured by the distributed Raman scattering temperature sensor (DTS) and the high spatial resolution Brillouin measured by the distributed Brillouin time domain analysis (BOTDA) Frequency shift distribution result Δv(z).

(3) Align and interpolate (or sample) the temperature measurement results and frequency shift measurement results to realize the data of the Brillouin frequency shift distribution Δv _B (z) of BOTDA and the temperature distribution T _D (z) of DTS The quantities are the same, and the positions of the data points correspond to each other.

(4) Judging the rising and falling edges of T _D (z), and then judging whether there is a temperature peak in the temperature region: if the temperature monotonous change in the continuous region exceeds the threshold, it is considered that there is a rising and falling edge, that is, there is a temperature peak. Determining the strain distribution of the temperature fiber in the temperature region includes:

a) For the stable temperature region without rising and falling edges, the strain distribution ε(z) of the optical cable:

$\mathrm{\&epsiv;}\left(z\right)=\frac{\mathrm{\&Delta;}{v}_B\left(z\right)-k_T{T}_{\mathrm{D.}}\left(z\right)}{k_{\mathrm{\&epsiv;}}}$ ①;

Among them: k _T is the Brillouin frequency shift temperature coefficient, k _ε is the Brillouin frequency shift strain coefficient;

b) For the temperature change area with rising and falling edges, the Brillouin frequency shift distribution Δv _B (z) is spatially accumulated and averaged to obtain Δv _L (z), so that its spatial resolution matches T _D (z) . Simultaneous equations calculate the strain ε(z) in this region:

$\mathrm{\&epsiv;}\left(z\right)=\frac{\mathrm{\&Delta;}{v}_L\left(z\right)-k_T{T}_{\mathrm{D.}}\left(z\right)}{k_{\mathrm{\&epsiv;}}}$ ②.

In the present invention, before performing strain demodulation, it is necessary to determine whether there is a temperature peak in the DTS temperature data. The method of judging the region of the temperature peak is mainly to find the rising and falling edges. If the continuous rising or falling temperature change value of multiple connected data points exceeds the threshold, it is considered that there is a temperature rising and falling edge in this area, that is, there is a temperature peak.

The present invention proposes that for a region with stable temperature, the Brillouin frequency shift data used for calculating the strain adopts the data measured under the high spatial resolution of the BOTDA. For the region where there is a temperature peak, the Brillouin translation data with a low spatial resolution that matches the DTS data after spatial accumulation and averaging is used to calculate the strain. In this way, not only the stable temperature region is realized, the strain can be demodulated with high resolution; but also the strain can be correctly demodulated in the temperature sudden change region.

In the presence of temperature peaks, in order to avoid erroneous strain results during strain demodulation, it is necessary to obtain Brillouin frequency shift data with low spatial resolution matching DTS, so that the rising and falling edges of the two coincide. Generally, the means to reduce the spatial resolution of BOTDA is to increase the pulse width of the pump light or reduce the bandwidth of the photodetection circuit. But changing the bandwidth of the photodetection circuit is cumbersome and impractical. Changing the pulse width of the pump light can achieve a spatial resolution similar to that of DTS, but it is limited by the adjustment step of the optical pulse width, and it is generally difficult to achieve an accurate matching of the spatial resolution. In addition, after changing the pulse width of the pump light, it is necessary to adjust the parameter configuration of other optoelectronic devices to obtain better measurement performance. Therefore, it is more complicated to realize a spatial resolution adjustment. Even if two spatial resolution parameter configurations are completed, completing a strain demodulation BOTDA needs to switch between the two parameter configurations, which will double the measurement time.

Example

In this embodiment, the distributed Brillouin time-domain analysis BOTDA has a sampling rate of 1000M and a spatial resolution of 1m; the distributed Raman scattering temperature sensor DTS has a sampling rate of 200M and a spatial resolution of 4m.

1. Distributed optical fiber temperature and strain measurement structure:

The sensing fiber of DTS1 is a multimode fiber 31 and the sensing fiber of BOTDA2 is a single mode fiber 32 as a typical structure of the measuring part.

BOTDA is a loop structure, the end of the fiber needs to be fused, and the front end is connected to two ports of BOTDA. The multi-mode optical fiber 31 and the single-mode optical fiber 32 are arranged in parallel along with the power transmission cable.

DTS and BOTDA receive the scattering signal and calculate the temperature T(z) and Brillouin frequency shift Δv(z) data to be measured. As shown in Figure 4, the lengths of DTS and BOTDA are misaligned.

2. Calibration alignment of measurement data:

Observe the curve characteristics and find the calibration points. Taking Figure 4 as an example, BOTDA selects A ₁ and A ₂ as calibration points, and DTS selects B ₁ and B ₂ as calibration points; physically, A ₁ and B ₁ are at the same position, and A ₂ and B ₂ are The same location; with BOTDA as the benchmark, the DTS curve passes through the calibration algorithm, so that the data in B ₁ B ₂ is in one-to-one correspondence with the data in A ₁ A ₂ .

Assuming that the optical fiber is evenly distributed, with BOTDA as the benchmark, the distance between the two calibration points is L=A ₂ -A ₁ ; the data points included in B ₁ to B ₂ are N; obtained by the following formula:

$B\left[\mathrm{no}\right]=A_1+\mathrm{no}\&CenterDot;\frac{L}{N-1},\mathrm{no}=\mathrm{0,1,2}...N-1$ ③;

The converted B[0] is located at the same position as A1, B[N _{- 1} ] is located at the same position as A2, and the length values between them are evenly distributed.

If the curve contains multiple calibration points, the distance between every two adjacent calibration points can also be calibrated by the above method. In actual situations, it is not so easy to determine the position of the calibration temperature peak, and it is often necessary to artificially set several temperature heating points in order to determine the alignment calibration point.

3. Interpolation and sampling:

After the distance calibration, the data of BOTDA and DTS are consistent in length. But they still have a difference in sampling rate. Between A ₁ and A ₂ , the distance data of BOTDA is represented by an array P[M], corresponding to the frequency shift V[M]; the distance data of DTS is represented by an array Q[N], and the temperature data is T[N]; if BOTDA The sampling rate is 1000M, while the DTS sampling rate is 200M, then M is 5 times of N. Therefore, it is necessary to sample the BOTDA data or interpolate the DTS data. In order to obtain a greater amount of information, it is more appropriate to choose the latter. General one-dimensional interpolation algorithms include: linear interpolation, polynomial interpolation and cubic spline interpolation. Select the cubic spline interpolation method in this column. After calibration, alignment and interpolation of the DTS temperature data, the results are shown in Figure 5.

4. Judgment of the temperature peak area of strain demodulation, and event search for the DTS temperature peak:

<1> Search the entire DTS curve and record the area where the temperature rises 10 points continuously or drops 10 points continuously.

<2> Determine the range of temperature change in the area. If it exceeds three times the noise, it is considered that there is a temperature peak. Continue to the following steps to solve the strain. Otherwise, the temperature in this area is considered to be stable, and the distributed strain information is directly solved by formula ① as ε(z).

<3> Expand the searched area back and forth, and record the spatial range of the maximum monotonous change of each area. If it exceeds the threshold, this range is considered to be the rising or falling edge of the temperature.

<4> In the spatial area where there are rising and falling edges of temperature, BOTDA spatial accumulation and averaging can effectively reduce the spatial resolution. Optimize the implementation of cumulative average points to improve the matching accuracy of spatial resolution. After the BOTDA data in this example are accumulated and averaged at 41 adjacent points, the rising and falling edges of the BOTDA temperature peak almost coincide with the DTS temperature peak, as shown in Figure 6. In the temperature peak area, formula ① is used for strain demodulation, but the Brillouin frequency shift used is the data after spatial accumulation and averaging.

5. The finally demodulated strain is shown in Figure 7, and there is no wrong strain information in the temperature peak region. In the stable temperature region, the Brillouin frequency shift required to calculate the strain has not been accumulated and averaged, and the resolution is high.

The distributed optical fiber strain measurement method provided by the present invention is based on the principle of joint Raman scattering and Brillouin scattering, and realizes strain high-resolution demodulation in the temperature stable region, and realizes correct strain demodulation in the temperature changing region, which can be used for transmission lines The online monitoring provides a new type of condition monitoring means.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: the present invention can still be Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the scope of the claims of the present invention.

Claims (1)

1. A distributed optical fiber temperature strain measurement method, characterized in that, the method comprises the steps: (1) laying temperature sensing optical fiber and strain sensing optical fiber; (2) Receive temperature distribution results and Brillouin frequency shift distribution results; (3) aligning and interpolating the temperature distribution result and the Brillouin frequency shift distribution result; (4) Judging whether there is a temperature peak in the temperature distribution result, and determining the strain distribution of the optical fiber in the temperature region; In the step (1), the sensing fiber used to measure temperature is arranged in parallel with the sensing fiber used to measure the Brillouin frequency shift caused by strain; In the step (2), the matching alignment unit receives the temperature distribution result T _D (z) of the low spatial resolution measured by the distributed Raman scattering temperature sensor DTS and the high spatial resolution measured by the distributed Brillouin time domain analysis BOTDA Brillouin frequency shift distribution result Δv _B (z) of the rate; In the step (3), the data volume of the Brillouin frequency shift distribution result Δv _B (z) and the temperature distribution result T _D (z) are the same, and the positions of the data points correspond to each other; In the step (4), the judgment of the area of the temperature peak is to judge whether there is a rising and falling edge; if the temperature monotonous change range exceeds the threshold in the continuous area, it is considered that there is a temperature rising and falling edge in this area, that is, there is a temperature peak; Determining the strain distribution of the fiber in the temperature region involves: a. For the stable temperature region without rising and falling edges, the strain distribution ε(z) of the optical fiber: $\mathrm{\&epsiv;}\left(z\right)=\frac{\mathrm{\&Delta;}{v}_B\left(z\right)-k_T{T}_{\mathrm{D.}}\left(z\right)}{k_{\mathrm{\&epsiv;}}}$ ①; Among them: k _T is the Brillouin frequency shift temperature coefficient, k _ε is the Brillouin frequency shift strain coefficient; b. For the temperature change area with rising and falling edges, the Brillouin frequency shift distribution results Δv _B (z) are spatially accumulated to obtain the average value Δv _L (z), so that the spatial resolution of the Brillouin frequency shift distribution results is consistent with the temperature The distribution result T _D (z) matches; determine the fiber strain distribution ε(z) in this temperature region: $\mathrm{\&epsiv;}\left(z\right)=\frac{\mathrm{\&Delta;}{v}_L\left(z\right)-k_T{T}_{\mathrm{D.}}\left(z\right)}{k_{\mathrm{\&epsiv;}}}$ ②; In the above b, the Brillouin frequency shift distribution result Δv _B (z) is accumulated and averaged at adjacent data points to realize the spatial resolution of the distributed Brillouin time domain analysis BOTDA and the distributed Raman scattering temperature sensor DTS automatic matching; In the step (4), the method for judging the rising and falling edges of the temperature is applicable to the demodulation of the temperature strain in the temperature stable zone of the distributed Raman scattering temperature sensor DTS and the high spatial resolution of the strain; the distributed Raman scattering temperature sensor DTS Temperature abrupt region, the demodulation of temperature strain without strain distortion.