CN111896095A - A Vibration Localization Method for Distributed Optical Fiber Double M-Z Interferometer Based on HHT Transform - Google Patents

Abstract

The invention discloses a vibration positioning method of a distributed optical fiber double M-Z interferometer based on HHT transformation. The specific steps are as follows: S1. Take a section of vibrating optical fiber signal S(t) and use a piecewise cubic Hermite difference polynomial as an empirical model Decompose EMD to obtain the eigenmode function IMFs; S2. Perform Hilbert transform on the decomposed IMFs to obtain the instantaneous frequency of the original signal S(t); S3. Superimpose the results of all eigenmode functions IMFs after HT, Obtain the Hilbert spectrum; S4. Extract the signal segment _Sp (t) containing the vibration information according to the peak position of the Hilbert spectrum; S5. Perform the cross-correlation operation and the positioning operation on the extracted signal segment _Sp (t) to obtain the vibration location information. The advantage is that the time delay can be obtained by extracting the signals in the area with vibration and performing the cross-correlation calculation, which is beneficial to improve the positioning accuracy and reduce the calculation time.

Description

A Vibration Localization Method for Distributed Optical Fiber Double M-Z Interferometer Based on HHT Transform

technical field

The invention relates to a vibration positioning method for double M-Z distributed optical fiber sensing, belonging to the technical field of distributed optical fiber sensing.

Background technique

Distributed optical fiber sensing system is a sensing system that uses optical fiber as a sensing sensitive element and a signal transmission medium, which can monitor and accurately locate the vibration behavior occurring on the optical fiber link in real time.

Distributed optical fiber sensing systems have a series of advantages, such as passive safety, long distance, high resolution, and low cost, and are widely used in building structure health monitoring, oil and gas pipeline safety early warning, perimeter security monitoring, and intelligent IoT applications. Prospects.

The distributed optical fiber sensing system based on double M-Z uses the interference principle of light. Due to the influence of noise, the vibration positioning accuracy will be reduced, and it is difficult to determine the vibration starting point from the signal. Need to find a way to accurately locate.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, the present invention provides a method for improving the vibration positioning of a dual M-Z distributed optical fiber sensing system, which can improve the positioning accuracy and reduce the data processing time. Its technical solution is,

A vibration positioning method of a distributed optical fiber double M-Z interferometer based on HHT transformation, the specific steps are:

S1. Take a section of vibrating optical fiber signal S(t) and perform empirical mode decomposition EMD according to the piecewise cubic Hermite difference polynomial to obtain the eigenmode function IMFs;

S2. Hilbert transform the decomposed IMFs to obtain the instantaneous frequency of the original signal S(t);

S3. Superimpose the results of all eigenmode functions IMFs after HT to obtain the Hilbert spectrum;

S4. Extract the signal segment _Sp (t) containing vibration information according to the peak position of the Hilbert spectrum;

S5. Perform cross-correlation operation and positioning operation on the extracted signal segment _Sp (t) to obtain vibration position information.

Further, the EMD process of empirical mode decomposition is to fit the maximum point of an original signal S(t) with the cubic Hermite difference method to fit the upper envelope, and obtain the lower envelope in the same way; Take the average of the envelope and the lower envelope to get the average envelope ml; subtract the average envelope ml from the original signal S(t) to get a new signal f(t), that is

S(t)-ml=f(t)

If the new signal f(t) still has negative local maxima and positive local minima, continue to decompose according to the above operation, thereby obtaining multiple eigenmode functions IMFs and aftermath.

Further, Hilbert spectrum analysis HSA performs Hilbert transform for multiple eigenmode functions IMFs decomposed by EMD, respectively, to obtain the instantaneous frequency, and converts all eigenmode functions IMFs to Hilbert The transformed results are combined to obtain the Hilbert spectrum, and the frequency variation of the original signal S(t) is obtained.

沿逆时针方向的信号段为

取高峰值处附近0.01s的数据做互相关运算，得出延时时间，进而得出振动位置信息。Further, the signal segment _Sp (t) containing vibration information is extracted according to the peak position of the Hilbert spectrum, wherein the signal segment along the clockwise direction is

The signal segment in the counterclockwise direction is

Take the data of 0.01s near the high peak for cross-correlation operation, obtain the delay time, and then obtain the vibration position information.

和沿逆时针方向的信号段为

到达两个平衡探测器的时间存在一定延迟τ，Further, the signal segment in the clockwise direction is

and the signal segment in the counterclockwise direction is

There is a certain delay τ in the time to reach the two balanced detectors,

τ=t _Lx +t _L -t _x ,

Vibration position calculation formula:

L is the length of the sensing fiber and the conducting fiber, τ is the time delay between the forward and reverse beams reaching the equilibrium detector, c is the speed of light in vacuum, n is the refractive index of the fiber, and x is the vibration position.

beneficial effect

Different from the traditional dual M-Z positioning method, the HHT is used here to determine the regional signal containing vibration, which can extract the effective signal clearly and intuitively, and extract the regional signal with vibration for cross-correlation calculation to obtain the time delay, which is beneficial to improve the positioning accuracy and reduce the calculation time. .

Description of drawings

Figure 1 is a schematic diagram of a dual M-Z distributed optical fiber sensing system;

Fig. 2 is the method flow chart of calculating the vibration position of distributed optical fiber sensing;

Fig. 3 is a section of original signal diagram;

Figure 4 is a Hilbert spectrogram of the original signal.

1-light source; 2-isolator; 3-coupler one; 4-coupler two; 5-coupler three; 6-coupler four; 7-sensing fiber; 8-conducting fiber; 9-first balance Detector; 10-Second Balance Detector; 11-Acquisition Processing System.

Detailed ways

The technology will be further described below in conjunction with the accompanying drawings 1-4 and specific embodiments to help understand the content of the present invention.

A vibration positioning method of a distributed optical fiber double M-Z interferometer based on HHT transformation, the specific steps are:

S1. Take a section of vibrating optical fiber signal S(t) and perform empirical mode decomposition EMD according to the piecewise cubic Hermite difference polynomial to obtain the eigenmode function IMFs;

Hilbert-Huang transform HHT includes empirical mode decomposition EMD and Hilbert spectral analysis HAS,

The basic idea of EMD is to convert a wave with irregular frequency changes into multiple single frequency waves + residual waves.

Among them, the EMD process of empirical mode decomposition is to fit the maximum point of an original signal S(t) with the cubic Hermite difference method to fit the upper envelope, and use the same method to obtain the lower envelope; Take the average of the line and the lower envelope to get the average envelope ml; subtract the average envelope ml from the original signal S(t) to get a new signal f(t), that is

S(t)-ml=f(t)

If the new signal n(t) still has negative local maxima and positive local minima, continue to decompose according to the above operation, thereby obtaining multiple eigenmode functions IMFs and aftermath.

S2. Hilbert Spectrum Analysis HSA performs Hilbert transforms on multiple eigenmode functions IMFs decomposed by EMD, respectively, thereby obtaining the instantaneous frequency of the original signal S(t);

When vibration occurs, the vibration will cause stress to the sensing fiber, and the frequency of the signal will become higher under the action of the stress, and the signal in the high frequency region contains the vibration signal.

S3. Superimpose the results of all eigenmode functions IMFs after HT to obtain the Hilbert spectrum, and obtain the frequency change of the original signal S(t).

S4. according to Hilbert spectrum peak position, extract the signal segment S _p (t) that contains vibration information; The extracted signal segment Sp ( _t ) is done cross-correlation operation and positioning operation to obtain vibration location information;

沿逆时针方向的信号段为

取高峰值处附近0.01s的数据做互相关运算，得出延时时间，进而得出振动位置信息。where the signal segment in the clockwise direction is

The signal segment in the counterclockwise direction is

Take the data of 0.01s near the high peak for cross-correlation operation, obtain the delay time, and then obtain the vibration position information.

Taking the data of 0.01s near the high peak value is specifically interpreted as: taking the data of 0.01s before and after the high peak value, for example, the high peak value is at 0.09s, and the data collection range is 0.085s-0.095s.

和沿逆时针方向的信号段为

到达两个平衡探测器的时间存在一定延迟τ，When there is vibration acting on point M, the signal segment in the clockwise direction is

and the signal segment in the counterclockwise direction is

There is a certain delay τ in the time to reach the two balanced detectors,

τ=t _Lx +t _L -t _x ,

Vibration position calculation formula:

L is the length of the sensing fiber and the conducting fiber, τ is the time delay between the forward and reverse light beams reaching the balanced detector, c is the speed of light in vacuum, n is the refractive index of the fiber, and x is the vibration position, that is, the point M to the coupling The distance of the second device.

The system parameters are as follows:

As shown in Figure 1, the light emitted by the light source 1 passes through the isolator 2 and then reaches the coupler-3 and is divided into two beams of 50:50, which propagate in the clockwise and counterclockwise directions respectively. The light propagating in the clockwise direction passes through the The coupler 24 is divided into two bundles and interferes at the other end of the sensing fiber 7 at the coupler 35, and is transmitted to the coupler 46 through the conducting fiber 8 to receive at the second balanced detector 10; The light is split by 50:50 at the coupler 3 5 after passing through the conducting fiber 8 , passes through the sensing fiber 7 in the counterclockwise direction, interferes at the coupler 2 4 , and is then received by the first balanced detector 9 . The first balance detector 9 and the second balance detector 10 convert the detected optical signals into electrical signals and then enter the acquisition and processing system 11 for positioning calculation.

The light source is a RIO narrow linewidth laser, the wavelength of light is 1550nm, the linewidth is less than 3kHz, and the power is 10mW;

The length of sensing fiber and conducting fiber is 2km;

The balanced detector uses Thorlabs PDB450C-AC, the selected gear is the gain of 10 ⁴ V/A, and the bandwidth is 45 MHz;

The split ratio of coupler one 3, coupler two 4, coupler three 5, coupler four 6 is 50:50;

Core^TM i5-7200U CPU@2.50GHz 2.71GHz，内存8G；The acquisition and processing system uses a computer with a built-in NI PCI-5114 acquisition card, and the computer processor uses

Core ^TM i5-7200U CPU@2.50GHz 2.71GHz, memory 8G;

The sampling rate is 10MHz/s, the sampling time is 0.1s, and the signal time in the vibration area is 0.01s;

Both the sensing fiber and the conducting fiber are double-core armored fiber optic cables;

The vibration position is 1800m away from the coupler 24, and the vibration is applied by stepping on the optical cable;

The function of the isolator is to only allow light to pass in one direction, preventing backscattered light from causing damage to the light source;

The collected signal is decomposed by EMD to obtain 10 layers of IMFs and aftermath, and then Hilbert transform is performed on each layer of IMF. After superposition, the Hilbert spectrum is obtained, which clearly and intuitively reflects the frequency change of the original signal; The frequency at the sampling point is the highest, and the regional signal at the sampling point 830,000 to 930,000 is selected as the effective signal of vibration, and the cross-correlation operation is performed to calculate the time delay of the two signals.

In order to illustrate the effectiveness of this method, the regional signals of different frequencies are respectively used for positioning operation, and the size of the positioning error is compared, as shown in the following table.

regional signal 0.05s～0.06s 0.06s～0.07s 0.07s～0.08s 0.08s～0.09s 0.09s～0.10s positioning error -80m 40m 20m 10m -20m

It can be seen from the table that the positioning error of the high-frequency signal is smaller than that of the low-frequency signal.

It can be seen from Figure 4 that the high peak is at 0.09s, and the data acquisition range is 0.08s-0.09s.

Of course, the above description does not limit the present technology, and the present technology is not limited to the above examples.

Changes, modifications, additions or substitutions made by a person of ordinary skill in the art within the essential scope of the present invention,

It also belongs to the protection scope of this technology.

Claims (5)

1. A vibration positioning method of a distributed optical fiber double M-Z interferometer based on HHT transformation is characterized by comprising the following specific steps,

s1, taking a section of vibration optical fiber signal S (t) to perform Empirical Mode Decomposition (EMD) according to a Hermite difference polynomial of three times in a segmented mode to obtain Intrinsic Mode Functions (IMFs);

s2, carrying out Hilbert transformation on the IMFs obtained by decomposition to obtain the instantaneous frequency of an original signal S (t);

s3, stacking results of all intrinsic mode functions IMFs after HT to obtain a Hilbert spectrum;

s4, extracting a signal segment S containing vibration information according to the peak position of the Hilbert spectrum_p(t)；

S5, extracting the signal segment S_pAnd (t) performing cross-correlation operation and positioning operation to obtain vibration position information.

2. The vibration localization method of a distributed fiber double M-Z interferometer based on HHT transformation as claimed in claim 1, wherein the EMD process is that fitting the maximum point of an original signal S (t) to the upper envelope line by using a three times Hermite difference mode, and obtaining the lower envelope line by using the same mode; taking the average value of the upper envelope line and the lower envelope line to obtain an average envelope line ml; subtracting the average envelope ml from the original signal S (t) to obtain a new signal f (t), i.e.

S(t)-ml＝f(t)

If there are also negative local maxima and positive local minima of the new signal f (t), the decomposition continues as above, thus yielding a plurality of eigenmode functions IMFs and a residual wave.

3. The vibration localization method of a distributed fiber double M-Z interferometer based on HHT transformation as claimed in claim 2, wherein Hilbert spectrum analysis HSA performs Hilbert transformation on a plurality of intrinsic mode functions IMFs decomposed by empirical mode decomposition EMD, respectively, to obtain instantaneous frequencies, and combines results of all intrinsic mode functions IMFs after Hilbert transformation to obtain Hilbert spectra, so as to obtain frequency variation of original signal s (t).

4. The method of claim 1, wherein the signal segment S containing vibration information is extracted according to Hilbert spectrum peak position_p(t) wherein the signal segment in the clockwise direction is

The signal segment in the counterclockwise direction is

And taking data of 0.01s near the high peak value to perform cross-correlation operation to obtain delay time and further obtain vibration position information.

5. The method of claim 4, wherein the signal segment in the clockwise direction is

And a signal segment in the counterclockwise direction of

There is a certain delay tau in the time of arrival at the two balanced detectors,

τ＝t_L-x+t_L-t_x，

vibration position calculation formula:

l is the length of the sensing optical fiber and the conducting optical fiber, tau is the time delay between the forward and backward beams of light reaching the balanced detector, c is the speed of light in vacuum, n is the refractive index of the optical fiber, and x is the vibration position.

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