CN114235135A - Amplitude demodulation vibration positioning detection method based on double differential step length - Google Patents
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Abstract
The invention discloses an amplitude demodulation vibration positioning detection method based on double differential step length, which introduces two different differential step length values on the basis of the traditional differential accumulation algorithm, accurately designs a first differential step length and a second differential step length, fully utilizes the relation among the vibration signal frequency, the total number of backward Rayleigh scattering signals and the differential step length, effectively improves the calculation efficiency and the signal-to-noise ratio of a system, reduces the strong dependence of the traditional amplitude differential accumulation algorithm on the detection signal frequency, and reduces the probability of missing detection of the vibration signal.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to an amplitude demodulation vibration positioning detection method based on double differential step lengths.
Background
The phase-sensitive time domain reflectometer (phi-OTDR) is based on the backscattered Rayleigh light of a highly coherent detection pulse, has high sensitivity, and can detect weak vibration events on a sensing optical fiber of dozens of kilometers. Therefore, phi-OTDR has attracted more and more attention in various fields such as structural health monitoring, traffic monitoring, intrusion detection, and pipeline monitoring. In all applications of phi-OTDR, detecting and locating vibrations with high reliability and low false alarm probability is a fundamental work. In the absence of vibration of the fiber, there is no significant change in the phase and amplitude of each spatial sampling point RBS (backward rayleigh scattering) on the sensing fiber. In contrast, when the sensing fiber is subjected to external vibrations, the refractive index fluctuations at this point cause the phase of the optical waves to change locally, which in turn causes the phase and amplitude of the RBS to change. Therefore, the detection signal at the vibration position is significantly different from elsewhere, and both phase demodulation and amplitude demodulation methods can be used for vibration detection. The phase demodulation method measures the optical phase of the RBS, performs a difference operation between two adjacent points, and detects vibration from a change in the phase. Amplitude demodulation-based perturbation localization method that detects the presence of vibrations by measuring the variation between successive RBS traces.
Where accurate quantitative detection is not required, amplitude demodulation schemes are considered one of the most straightforward and commonly used ways of detecting vibration events because of their ease of implementation. For amplitude demodulation phi-OTDR (AD-phi-OTDR), a differential accumulation method is generally adopted to position vibration, wherein continuous Rayleigh scattering traces with a certain pulse number are measured, every two Rayleigh scattering traces with a distance of k are subtracted to generate a differential signal (k is a differential step length), the differential signal is accumulated to obtain the signal-to-noise ratio of a detection signal, and then the vibration is positioned through the signal-to-noise ratio of the detection signal. Because the frequencies of different vibration signals are different, in order to demodulate a correct vibration signal, the traditional amplitude Accumulation Difference Method (ADM) needs to continuously and manually adjust the total number of collected RBS traces according to actual conditions, and the difference step value is set according to experience.
Disclosure of Invention
The invention provides an amplitude demodulation vibration positioning detection method based on double differential step length, aiming at solving the problem of misjudgment probability of the traditional amplitude accumulation difference method.
In order to solve the problems, the invention is realized by the following technical scheme:
an amplitude demodulation vibration positioning detection method based on double differential step lengths comprises the following steps:
Step 4, calculating a first detection signal-to-noise ratio vector by using the first detection signal vector according to a signal-to-noise ratio calculation formula, namely obtaining a first detection signal-to-noise ratio at each optical fiber sampling point; similarly, according to a signal-to-noise ratio calculation formula, calculating a second detection signal-to-noise ratio vector by using a second detection signal vector to obtain a second detection signal-to-noise ratio at each optical fiber sampling point;
wherein k is1Is the first difference step, k2Is a second difference step; mu is a first step length coefficient, mu is more than 0 and less than 1; sigma is a second step length coefficient, and sigma is more than 0 and less than 1; i is0As backward Rayleigh scatteringThe trace samples a constant.
In the scheme, the sampling constant I of the backward Rayleigh scattering trace0Comprises the following steps:
I0＝1/(fs×Tp)
wherein f issFor vibration signal frequency, TpIs the sampling period.
In the above scheme, the first step-length coefficient μ and the second step-length coefficient σ satisfy the following relationship:
where μ is the first step size coefficient and σ is the second step size coefficient.
In the above scheme, the jth element of the first detection signal-to-noise ratio vector, i.e. the first detection signal-to-noise ratio SNR at the jth fiber sampling point1(j) Comprises the following steps:
in the above scheme, the jth element of the second detection signal-to-noise ratio vector, i.e. the second detection signal-to-noise ratio SNR at the jth fiber sampling point2(j) Comprises the following steps:
wherein S is1(j) The j element of the first detection signal vector is the first detection signal at the j optical fiber sampling point; max (S)1(j-Δι)：S1(j + Δ ι)) represents taking the maximum value of the j- Δ ι th to j + Δ ι th elements of the first detection signal vector, namely, the maximum value of the first detection signal in the interval from the j- Δ ι th to the j + Δ ι th optical fiber sampling points; s2(j) The j element of the second detection signal vector is the second detection signal at the j optical fiber sampling point; max (S)2(j-Δι)：S2(j + Δ ι)) representsTaking the maximum value of the j-delta iota element to the j + delta iota element of the second detection signal vector, namely the maximum value of the second detection signal in the interval from the j-delta iota optical fiber sampling point to the j + delta iota optical fiber sampling point; j is 1,2, J is the number of sampling points of the optical fiber vibration, and Δ ι is a positive integer.
In the above scheme, the positive integer Δ ι is set as:
where c is the speed of light propagation in vacuum, TMKTo detect the pulse width of the pulse, neffIn order to be the effective refractive index of the optical fiber,
indicating a rounding down.
Compared with the prior art, the method introduces two different differential step values on the basis of the traditional differential accumulation algorithm, accurately designs the first differential step and the second differential step, fully utilizes the relation among the vibration signal frequency, the total number of RBSs and the differential step, effectively improves the calculation efficiency and the signal-to-noise ratio of the system, reduces the strong dependence of ADM on the detection signal frequency, and reduces the probability of missing detection of the vibration signal.
Drawings
FIG. 1 is a schematic diagram of an amplitude demodulation vibration localization detection system based on double differential step sizes.
FIG. 2 is a schematic diagram of a method for amplitude demodulation vibration localization detection based on double differential step sizes.
FIG. 3 is a waveform diagram of experiment 1, (a) detection signal S0(b) a first detection signal S1(c) a second detection signal S2。
FIG. 4 is a waveform diagram of experiment 2, (a) a first detection signal S1(b) a second detection signal S2。
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to specific examples.
Referring to fig. 1, an amplitude demodulation vibration positioning detection system based on double differential step length mainly comprises a laser, a first coupler, an acousto-optic modulator, a signal generator, a circulator, a sensing optical fiber, a polarization modulator, a second coupler, a balance detector and an upper computer. The output end of the laser is connected with the input end of the first coupler, one output end of the first coupler is connected with one input end of the acousto-optic modulator, and the other output end of the first coupler is connected with one input end of the second coupler. The output end of the signal generator is connected with the other input end of the acousto-optic modulator, the output end of the acousto-optic modulator is connected with the first port of the circulator, the second port of the circulator is connected with the sensing optical fiber, and the third port of the circulator is connected with the other input end of the second coupler. The output end of the second coupler is connected with the input end of the balance detector, and the output end of the balance detector is connected with the upper computer.
The laser is a high-power narrow-linewidth laser, the spectral linewidth of the laser is 3kHz, the central wavelength is 1550.15nm, the maximum output power is 100mW, and the power instability is less than 1%. The splitting ratio of the first coupler is different from that of the second coupler, wherein the splitting ratio of the first coupler is 9:1, and the splitting ratio of the second coupler is 5: 5. The bandwidth of the balanced detector is 200MHz, and the detection responsivity is 0.9A/W.
Based on the above system, an amplitude demodulation vibration positioning detection method based on double differential step length is realized, as shown in fig. 2, and specifically includes the following steps:
Wherein d isi,jThe amplitude at the jth optical fiber sampling point at the ith sampling time is represented, I is 1, 2. DeltaLThe system sample resolution.
First difference step k1Comprises the following steps:
k1＝μI0
second difference step k2Comprises the following steps:
k2＝σ×k1
wherein mu is a first step length coefficient, and mu is more than 0 and less than 1; sigma is a second step length coefficient, and sigma is more than 0 and less than 1; the first step size coefficient mu and the second step size coefficient sigma satisfy the following relationship:
I0sampling constants for the back Rayleigh scattering traces, I0＝1/(fs×Tp)，fsFor vibration signal frequency, TpIs a sampling period; the sampling constant is expressed at 1/fsIn the signal period of (2) with TpThe vibration signal is sampled at a sampling period of (a). If the invention is applied to the pipeline safety field, the frequency monitored according to the known early warning event is between 160 and 640Hz, f is at the momentsAnd selecting the lower limit frequency of the early warning event monitoring as 160 Hz.
In the process of positioning vibration by using the AD-phi-OTDR, the upper computer acquires data by using a continuous sampling mode, which causes the total number of RBS traces to be greatly increased to cause the reduction of the operating efficiency of the system. Therefore, the invention researches the problem that the positioning effect is optimal due to the total number of RBS tracks (sampling constant) required in vibration detection. The invention limits the sampling constant by using the vibration frequency and the sampling period, thereby greatly reducing the time required by the vibration positioning of the system and improving the operating efficiency of the system. The k value in the conventional ADM algorithm is generally selected by taking a plurality of artificial measurements. The method of artificially obtaining the k value needs a lot of time to test to reduce the missing rate, and the selection mode of the k value has no definite basis, so that the k value at this time cannot be proved to be the optimal value. In addition, the traditional ADM algorithm is highly dependent on the vibration frequency, so that the traditional ADM algorithm has strong dependence on the difference step k and the total number of rayleigh scattering. Improper total number of k and Rayleigh scattering can result in reduced system SNR and even increased system false drop rate. The method firstly utilizes the pushed sampling constant to limit the first differential step length, selects two different differential step length values, and limits the selection of the second differential step length and the first differential step length to a certain extent, thereby avoiding the increase of the system omission factor caused by single selection of the differential step length.
Row r and column j of the first differential amplitude matrix, i.e. the first differential amplitude Δ d at sampling point of the j optical fiber at sampling time r1(r, j) is:
the jth element of the first probe signal vector, i.e. the first probe signal S at the jth fiber sampling point1(j) Comprises the following steps:
row r and column j elements of the second differential amplitude matrix, i.e. the second differential amplitude Δ d at sampling point of the j optical fiber at sampling time r2(r, j) is:
the jth element of the second probe signal vector, i.e. the second probe signal S at the jth fiber sampling point2(j) Comprises the following steps:
wherein,
step 4, calculating a first detection signal-to-noise ratio vector by using the first detection signal vector according to a signal-to-noise ratio calculation formula, namely obtaining a first detection signal-to-noise ratio at each optical fiber sampling point; and similarly, according to a signal-to-noise ratio calculation formula, calculating a second detection signal-to-noise ratio vector by using a second detection signal vector, namely obtaining a second detection signal-to-noise ratio at each optical fiber sampling point.
The jth element of the first detection SNR vector, i.e., the first detection SNR at the jth fiber sampling point1(j) Comprises the following steps:
the jth element of the second detection SNR vector, i.e., the second detection SNR at the jth fiber sampling point2(j) Comprises the following steps:
wherein S is1(j) Is firstThe jth element of the probe vector (i.e., the first probe at the jth fiber sampling point), max (S)1(j-Δι)：S1(j + Δ ι)) represents taking the maximum value of the j- Δ ι th to j + Δ ι th elements of the first detection signal vector, namely the maximum value of the first detection signal in the interval from the j- Δ ι th to the j + Δ ι th sampling points of the optical fiber; s2(j) Is the jth element of the second probe signal vector (i.e., the second probe signal at the jth fiber sampling point), max (S)2(j-Δι)：S2(j + Δ ι)) represents taking the maximum value of the j- Δ ι th to j + Δ ι th elements of the second detection signal vector, namely the maximum value of the second detection signal in the interval from the j- Δ ι th to the j + Δ ι th sampling points of the optical fiber; j1, 2, J, Δ ι are positive integers. J1, 2, J, Δ ι are positive integers. In the present invention, in the case of the present invention,
c is the speed of propagation of light in vacuum, TMKTo detect the pulse width of the pulse, neffFor the effective refractive index of the fiber, | · | means rounding down. If the optical fibre used is a single-mode fibre, the effective refractive index n of whicheff1.45, pulse width T of the probe pulseMK100ns, and a propagation velocity c of light in vacuum of 3X 108m/s, then
if the first detection signal-to-noise ratio or the second detection signal-to-noise ratio at the optical fiber sampling point is greater than the signal-to-noise ratio threshold value, judging that a vibration event exists at the optical fiber sampling point;
otherwise, judging that no vibration event exists at the optical fiber sampling point.
The performance of the invention is illustrated by the following experiments:
in order to better realize the discrimination, the experiment is used for the first detection signal S1And a second detection signal S2Performing binarization processing on the first detection signal S1The binarized signal is denoted S3(ii) a Second detection signal S2The binarized signal is denoted S4. The decision signal S for determining whether the jth optical fiber sampling point on the sensing link of the sensing optical fiber has a vibration event is defined as follows:
S(j)＝S3(j)∪S4(j)
the specific decision rule is shown in table 1:
TABLE 1 based on the double-k method
Vibration identification and decision
When S (j) at the jth spatial sampling point on the sensing link is '1', namely the first digital detection signal S3Or the second digital detection signal S4A logic of "1" indicates that there is a vibration event at that location; otherwise, when s (j) is "0", it indicates that there is no vibration event at the position. Through the vibration judgment signal S, the vibration misjudgment probability caused by various uncertain factors and the dependence of the conventional amplitude difference accumulation algorithm on the curve difference interval can be reduced, and the accuracy of vibration detection is improved.
Experiment 1: the sensing optical fiber is a 1.4km standard single-mode optical fiber, the acousto-optic modulator is driven by an arbitrary waveform generator, the bandwidth of the acousto-optic modulator is 200MHz, the repetition period of the generated detection pulse is 14 mus, and the pulse width is 100 ns. The vibration event is excited by a piezoelectric ceramic extensor (PZT) located at the optical fiber 530 m. When a PZT vibration signal is driven as a sine wave signal having a frequency of 1kHz, a signal S is detected0First detection signal S1And a second detection signal S2The waveforms of (a) are shown in fig. 3(a), 3(b) and 3(c), respectively. When the difference step is equal to the sampling constant I071, the probe signal S generated by the conventional ADM algorithm0Signal to noise ratio at vibration location is less than 2dBIt is impossible to determine whether or not vibration exists. When the first step coefficient mu and the second step coefficient are both 0.5, the first difference step k1Take 35, second difference step k2Taking 17, the first detection signal S generated by the double-k method1And a second detection signal S2The signal-to-noise ratio at the vibration location was 6.2dB and 5.9dB, respectively, both greater than 2 dB. Thus, the first detection signal is binarized into a signal S3Signal S binarized with second detection signal4And all are '1', and the vibration decision signal S is '1', so that the vibration can be effectively located.
Experiment 2: the optical fiber sensing is a standard single-mode optical fiber of 3.2km, the acousto-optic modulator is driven by an arbitrary waveform generator, the bandwidth is 200MHz, the repetition period of the generated detection pulse is 32 mus, and the pulse width is 100 ns. Two vibration events are respectively positioned at z11080m piezoelectric ceramic actuator (PZT) and a piezoelectric actuator located at z21340m speaker. The frequency of the driving signal for the PZT is 500Hz, and the frequency of the driving signal for the speaker is 200 Hz.
The number of RBS traces collected is determined by the lower frequency of 160Hz for early warning event monitoring in pipeline security applications, i.e., I0200, first difference step k1Take 100, second difference step k2And taking 50. By a first difference step k1The generated first detection signal S1(FIG. 4(a)) at z1Signal-to-noise ratio at 1080m PZT vibration location is 3.2dB (greater than 2dB), second difference step k2The generated second detection signal S2(fig. 4(b)) the signal-to-noise ratio at this position is 2.8dB (greater than 2 dB). Signal S after binarization of the first detection signal3To "1", the second detection signal is binarized into a signal S4And is "1", and the vibration decision signal S is "1" at this time, the PZT vibration can be effectively localized. By a first difference step k1The generated first detection signal S1(FIG. 4(a)) at z21340m loudspeaker vibration position the signal to noise ratio is 2.7dB (greater than 2dB), the second difference step k2The generated second detection signal S2(fig. 4(b)) the signal-to-noise ratio at this position was 3.6dB (greater than 2 dB). Signal S after binarization of the first detection signal3To "1", the second detection signal is binarized into a signal S4And is "1", and the vibration decision signal S is "1" at this time, the speaker vibration can be effectively localized.
The results of 2 experiments show that the double-k method can reliably detect two vibration events which are located at different positions and have different frequencies.
The invention provides an amplitude demodulation vibration positioning detection method (double-k method for short) based on double differential step length, which can effectively reduce the system vibration positioning missing detection rate and reduce the frequency dependence on detection signals. Meanwhile, the method can also reduce the time required by data processing and improve the signal-to-noise ratio of the system.
It should be noted that, although the above-mentioned embodiments of the present invention are illustrative, the present invention is not limited thereto, and thus the present invention is not limited to the above-mentioned embodiments. Other embodiments, which can be made by those skilled in the art in light of the teachings of the present invention, are considered to be within the scope of the present invention without departing from its principles.
Claims (5)
1. An amplitude demodulation vibration positioning detection method based on double differential step lengths is characterized by comprising the following steps:
step 1, carrying out quadrature demodulation on an intermediate frequency signal of a vibration event collected by a phase-sensitive time domain reflectometer to obtain an amplitude matrix of the vibration event, wherein an element d of an ith row and a jth column of the amplitude matrixi,jRepresenting the amplitude at the jth fiber sample point at the ith sample time;
step 2, every k1Performing differential operation on the amplitude matrix by rows to generate a first differential amplitude matrix, and accumulating the first differential amplitude matrix to generate a first detection signal vector;
step 3, every k2Performing differential operation on the amplitude matrix by rows to generate a second differential amplitude matrix, and accumulating the second differential amplitude matrix to generate a second detection signal vector;
step 4, calculating a first detection signal-to-noise ratio vector by using the first detection signal vector according to a signal-to-noise ratio calculation formula, namely obtaining a first detection signal-to-noise ratio at each optical fiber sampling point; similarly, according to a signal-to-noise ratio calculation formula, calculating a second detection signal-to-noise ratio vector by using a second detection signal vector to obtain a second detection signal-to-noise ratio at each optical fiber sampling point;
step 5, comparing the first detection signal-to-noise ratio and the second detection signal-to-noise ratio at each optical fiber sampling point with a preset signal-to-noise ratio threshold respectively: if the first detection signal-to-noise ratio or the second detection signal-to-noise ratio at the optical fiber sampling point is greater than the signal-to-noise ratio threshold value, judging that a vibration event exists at the optical fiber sampling point; otherwise, judging that no vibration event exists at the optical fiber sampling point;
wherein k is1Is the first difference step, k1＝μI0；k2Is the second difference step, k2＝σ×k1(ii) a Mu is a first step length coefficient, mu is more than 0 and less than 1; sigma is a second step length coefficient, and sigma is more than 0 and less than 1; i is0The back rayleigh scatter trace is sampled constant.
2. The method of claim 1, wherein the backward Rayleigh scattering trace sampling constant I is a double differential step size based amplitude demodulation vibration localization detection method0Comprises the following steps:
I0＝1/(fs×Tp)
wherein f issFor vibration signal frequency, TpIs the sampling period.
3. The amplitude demodulation vibration positioning detection method based on the double differential step size as claimed in claim 1, wherein the first step size coefficient μ and the second step size coefficient σ satisfy the following relationship:
where μ is the first step size coefficient and σ is the second step size coefficient.
4. The method as claimed in claim 1, wherein the amplitude demodulation vibration positioning detection method based on double differential step size,
the jth element of the first detection SNR vector, i.e., the first detection SNR at the jth fiber sampling point1(j) Comprises the following steps:
the jth element of the second detection SNR vector, i.e., the second detection SNR at the jth fiber sampling point2(j) Comprises the following steps:
wherein S is1(j) The j element of the first detection signal vector is the first detection signal at the j optical fiber sampling point; max (S)1(j-Δι)：S1(j + Δ ι)) represents taking the maximum value of the j- Δ ι th to j + Δ ι th elements of the first detection signal vector, namely, the maximum value of the first detection signal in the interval from the j- Δ ι th to the j + Δ ι th optical fiber sampling points; s2(j) The j element of the second detection signal vector is the second detection signal at the j optical fiber sampling point; max (S)2(j-Δι)：S2(j + Δ ι)) represents taking the maximum value of the j- Δ ι th to j + Δ ι th elements of the second detection signal vector, namely the maximum value of the second detection signal in the interval from the j- Δ ι th to the j + Δ ι th sampling points of the optical fiber; j is 1,2, J is the number of sampling points of the optical fiber vibration, and Δ ι is a positive integer.
5. The amplitude demodulation vibration positioning detection method based on the double difference step sizes as claimed in claim 4, wherein the set positive integer Δ ι is:
where c is the speed of light propagation in vacuum, TMKTo detect the pulse width of the pulse, neffIn order to be the effective refractive index of the optical fiber,
indicating a rounding down.
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| AU2020100890A4 (en) * | 2020-05-29 | 2020-07-16 | Guilin Univ. Electr. Techol. | A method and device for improving sensitivity of vibration sensing for vehicles |
| CN111811637A (en) * | 2020-08-12 | 2020-10-23 | 桂林电子科技大学 | Vehicle vibration identification device with multi-path information fusion and judgment method |
| CN112747815A (en) * | 2021-01-06 | 2021-05-04 | 苏州光格科技股份有限公司 | Coherent fading noise suppression method in distributed optical fiber acoustic wave sensing system |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN105356881A (en) * | 2015-11-27 | 2016-02-24 | 上海新跃仪表厂 | Metal vibration gyro differential signal high-resolution detection circuit and detection signal selection method thereof |
| US20200182685A1 (en) * | 2016-08-25 | 2020-06-11 | Shanghai Jiaotong University | Distributed fibre sensing system and vibration detection and positioning method therefor |
| AU2020100890A4 (en) * | 2020-05-29 | 2020-07-16 | Guilin Univ. Electr. Techol. | A method and device for improving sensitivity of vibration sensing for vehicles |
| CN111811637A (en) * | 2020-08-12 | 2020-10-23 | 桂林电子科技大学 | Vehicle vibration identification device with multi-path information fusion and judgment method |
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