WO2020188888A1 - Capteur à fibre optique de détection de vibration et procédé de détection de vibration - Google Patents

Capteur à fibre optique de détection de vibration et procédé de détection de vibration Download PDF

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WO2020188888A1
WO2020188888A1 PCT/JP2019/045789 JP2019045789W WO2020188888A1 WO 2020188888 A1 WO2020188888 A1 WO 2020188888A1 JP 2019045789 W JP2019045789 W JP 2019045789W WO 2020188888 A1 WO2020188888 A1 WO 2020188888A1
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waveform
moving average
otdr
light
abnormality
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PCT/JP2019/045789
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English (en)
Japanese (ja)
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祥宏 神田
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沖電気工業株式会社
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
    • G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means

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  • the present disclosure is a vibration detection optical fiber sensor and vibration that can be used to detect illegal intrusion of people in large facilities such as power plants and factories, and to detect cracks in large civil engineering structures such as bridges and roads. Regarding the detection method.
  • vibration transmitted to an optical fiber can be detected simply by replacing the laser used to generate an optical pulse as input light with a laser having a small frequency drift and a narrow line width (for example, J.C. Juarez, See W. Maier, K. N. Choi, and H. F. Taylor, “Distributed Fiber-Optic Intrusion Sensor System,” IEEE JLT, vol. 23, No. 6, June 2005, pp. 2081-2087).
  • a laser having a small frequency drift and a narrow line width for example, J.C. Juarez, See W. Maier, K. N. Choi, and H. F. Taylor, “Distributed Fiber-Optic Intrusion Sensor System,” IEEE JLT, vol. 23, No. 6, June 2005, pp. 2081-2087).
  • the waveform of the light intensity of the backscattered light is the strong interference of the coherent backscattered light generated at multiple scattering centers in the optical fiber while the optical pulse propagates through the optical fiber. Observed as a result.
  • the refractive index and birefringence of the optical fiber change only at that position.
  • the relative phase difference between the plurality of coherent backscattered lights from the plurality of scattering centers at the position where the vibration is applied changes.
  • the waveform changes when the relative phase difference between the waveforms changes. Therefore, in the observed OTDR waveform, only the waveform at the time corresponding to the position where the vibration is applied changes. By utilizing this phenomenon, the occurrence and position of vibration can be detected by calculating the difference between the observed OTDR waveform and the OTDR waveform observed at a previous time.
  • phase sensitive OTDR This method is called phase sensitive OTDR or ⁇ -OTDR.
  • the phase sensitive OTDR the difference from the OTDR waveform at the previous time is obtained. At this time, even if there is no abnormal vibration, if the amplitude fluctuation of the OTDR waveform is large, a large noise will be generated. This causes the presence or absence of abnormal vibration to be overlooked or false detection for the purpose of detecting abnormal vibration.
  • methods called moving average (moving averaging) and moving differential (moving differential) are known (for example, Y. Lu, T. Zhu, L. Chen, and X. Bao, “ Distributed Vibration Sensor Based on Coherent Detection of Phase-OTDR, ”IEEE JLT, vol. 28, No. 22, Nov. 15, 2010, pp.3243-3249).
  • the moving average is a method of reducing noise when the difference is calculated by obtaining the moving average for each position of a plurality of OTDR waveforms observed with the passage of time.
  • the movement difference is the difference from the OTDR waveform observed at a slightly distant time, instead of calculating the difference from the OTDR waveforms adjacent to each other on the time axis when calculating the difference between the OTDR waveforms observed at different times. Is a method of calculating. When the duration of the vibration is sufficiently long compared to the time interval for acquiring the OTDR waveform, a large difference value can be obtained as compared with the method of calculating the difference between the OTDR waveforms adjacent to each other on the time axis.
  • the ratio of the values with and without abnormal vibration is about several times.
  • the magnitude of the difference when there is abnormal vibration is, in principle, limited to the difference between the maximum value and the minimum value of the possible amplitude of the OTDR waveform at the maximum.
  • the operation of the vibration detection optical fiber sensor is unstable.
  • the phase-sensitive OTDR can detect even a slight vibration, the degree of noise depends on the environment in which the optical fiber is laid. Therefore, it is difficult to distinguish the presence or absence of an abnormality in an environment affected by wind or the like.
  • the present disclosure statistically evaluates changes in the OTDR waveform by utilizing the nature of noise in addition to the use of moving averages and moving differences in OTDRs, such as phase-sensitive OTDRs, to determine the presence or absence of abnormal vibrations.
  • a vibration detection optical fiber sensor and a vibration detection method for clearly discriminating.
  • the first aspect of the present disclosure is a vibration detection optical fiber sensor, which comprises a light source unit, an optical fiber, a light intensity acquisition unit, an OTDR waveform acquisition unit, a moving average acquisition unit, a difference waveform acquisition unit, and the like. It includes a position-specific statistical information acquisition unit, an abnormality degree acquisition unit, and an abnormality determination unit.
  • the light source unit generates an optical pulse as probe light.
  • the probe light is input to the optical fiber, and the light intensity acquisition unit acquires the intensity of the output light in which the probe light is backscattered by the optical fiber.
  • the OTDR waveform acquisition unit sequentially acquires and stores the OTDR waveform, which is the waveform of the output light for each optical pulse sent from the light intensity acquisition unit.
  • the moving average acquisition unit acquires the kth moving average waveform by averaging the temporally continuous OTDR waveforms of the kth (k is an integer of 1 or more) to the k + M-1 (M is an integer of 2 or more).
  • M is an integer of 2 or more.
  • the difference waveform acquisition unit acquires the moving average waveform of the k + N-1 (N is an integer of 2 or more) and the difference waveform p'(x) of the kth moving average waveform.
  • the position-specific statistical information acquisition unit acquires the sample mean ⁇ (x) and the standard deviation ⁇ (x) of each position for the difference waveforms of the kth to k + i-1 (i is an integer of 2 or more).
  • the abnormality degree acquisition unit acquires the abnormality degree waveform indicating the abnormality degree by calculating the abnormality degree a (p') of each position using the following equation (1).
  • the abnormality determination unit acquires the presence or absence of abnormal vibration and the position of the abnormal vibration from the value of the degree of abnormality.
  • the second aspect of the present disclosure is an exemplary embodiment in the above aspect, and the light source unit may be configured to include a narrow line width laser, a function generator, and an intensity modulator.
  • the narrow line width laser is a light source that generates a laser beam, and the line width may be 10 kHz or less. Even if the function generator generates an electrical pulse at a constant frequency.
  • the intensity modulator may generate an optical pulse by converting the laser beam into an optical pulse with an electric pulse.
  • a third aspect of the present disclosure is a vibration detection method, which includes an optical pulse generation process, a light intensity acquisition process, an OTDR waveform acquisition process, a moving average acquisition process, a difference waveform acquisition process, and a position-based method. It includes a statistical information acquisition process, an abnormality degree acquisition process, and an abnormality determination process.
  • an optical pulse is generated as probe light.
  • the intensity of the output light in which the probe light is backscattered by the optical fiber is acquired.
  • the OTDR waveform acquisition process the OTDR waveform, which is the waveform of the output light for each optical pulse acquired in the light intensity acquisition process, is sequentially acquired and stored.
  • the moving average acquisition process the kth moving average waveform is acquired by averaging the temporally continuous kth (k is an integer of 1 or more) to k + M-1 (M is an integer of 2 or more) OTDR waveforms. And remember.
  • the moving average waveform of the k + N-1 (N is an integer of 2 or more) and the difference waveform p'(x) of the kth moving average waveform are acquired.
  • the sample mean ⁇ (x) and standard deviation ⁇ (x) of each position are acquired for the difference waveforms of the kth to k + i-1 (i is an integer of 2 or more).
  • an abnormality degree waveform indicating the abnormality degree is acquired by calculating the abnormality degree a (p') at each position using the above equation (1).
  • the abnormality determination process the presence or absence of abnormal vibration and the position of the abnormal vibration, if any, are acquired from the value of the degree of abnormality.
  • a fourth aspect of the present disclosure is an exemplary embodiment in the third aspect, wherein the optical pulse generation process includes a process of generating a laser beam using a narrow line width laser having a line width of 10 kHz or less and a constant frequency.
  • a process of generating an electric pulse and a process of generating an optical pulse by converting a laser beam into an electric pulse with an electric pulse may be provided.
  • the vibration detection optical fiber sensor and the vibration detection method of the present disclosure not only take the difference waveform between the OTDR waveform and the moving average waveform, but also obtain the sample average and standard deviation of each position of the difference waveform.
  • the degree of abnormality is obtained by using, and the presence or absence of abnormal vibration is determined from the value of this degree of abnormality.
  • the ratio of the peak values when there is an abnormality and when there is no abnormality becomes larger than the ratio of the peak values in the difference waveform, and the presence or absence of the abnormality becomes clearer. Furthermore, it is possible to clearly judge the detection of distributed abnormal vibrations that suppress the dependence of sensitivity on the distance from the input end of the optical fiber.
  • FIG. 1 is a block diagram showing a schematic configuration of a vibration detection optical fiber sensor.
  • This vibration detection optical fiber sensor is configured to include a light source unit 10, an optical fiber 20, an optical circulator 30, and a measurement unit 40. This vibration detection optical fiber sensor is used for OTDR.
  • the light source unit 10 periodically generates an optical pulse as probe light.
  • the spatial resolution of the vibration detection optical fiber sensor depends on the width of this optical pulse. Further, the measurement distance of the vibration detection optical fiber sensor depends on the frequency of the optical pulse.
  • the optical pulse takes 5 ns to propagate 1 m through the optical fiber 20. When observing backscattered light, it takes a round trip time of forward propagation and reverse propagation, so a delay of 10 ns per 1 m occurs. For example, when the pulse width is 100 ns and the frequency is 5 kHz, the spatial resolution is 10 m and the maximum measurement distance is 20 km.
  • the light source unit 10 includes, for example, a laser light source 12, an intensity modulator 14, a function generator 16, and an optical amplifier 18.
  • the laser light source 12 generates laser light as continuous light in the communication wavelength band.
  • the laser light source 12 it is preferable to use a so-called narrow line width laser having a line width of 10 kHz or less.
  • this vibration detection optical fiber sensor can be used for a phase sensitive OTDR.
  • the wavelength of the laser beam may be arbitrary, but it is preferable to use a standard single-mode optical fiber with a low loss of 1550 nm.
  • the laser beam generated by the laser light source 12 is sent to the intensity modulator 14.
  • the function generator 16 generates a rectangular electric pulse. This electric pulse is sent to the intensity modulator 14.
  • the electric pulse generated by the function generator 16 has, for example, a pulse width of 100 nsec and a repetition frequency of 5 kHz.
  • the output of the function generator 16 is also sent to the analog-to-digital (A / D) converter 44, which will be described later, and is used as a trigger signal.
  • the intensity modulator 14 converts the laser beam into an optical pulse with an electric pulse to generate an optical pulse.
  • This optical pulse is sent to the optical amplifier 18.
  • the pulse width and frequency of the optical pulse generated by the intensity modulator 14 are both the same as the electric pulse generated by the function generator 16.
  • the optical pulse has a pulse width of 100 nsec and a repetition frequency of 5 kHz.
  • the optical pulse generated by the intensity modulator 14 is amplified by the optical amplifier 18 and then sent to the optical fiber 20 as probe light via the optical circulator 30.
  • the probe light sent to the optical fiber 20 propagates in the optical fiber 20, and backscattered light is generated along with the propagation of the probe light. This backscattered light is sent to the measuring unit 40 as output light via the optical circulator 30.
  • the measurement unit 40 includes a photodetector 42 and an A / D converter 44 as a light intensity acquisition unit, and a calculation unit 50.
  • the output light input from the optical fiber 20 to the measuring unit 40 is sent to the photodetector 42.
  • the photodetector 42 can be composed of, for example, a photodiode (PD).
  • PD photodiode
  • the photodetector 42 converts the output light into an electric signal by square detection and sends it to the A / D converter 44.
  • the light intensity of the backscattered light is small. Therefore, it is desirable that the photodetector 42 has a sensitivity to receive light even at about -30 dBm.
  • the A / D converter 44 converts the electric signal received from the photodetector 42 into a digital signal.
  • the sampling frequency of the A / D converter 44 may be large enough to sample the OTDR waveform, and about 200 MHz is sufficient.
  • the digital signal obtained by the A / D converter 44 is sent to the arithmetic unit 50.
  • the arithmetic unit 50 statistically evaluates changes in the OTDR waveform using a digital signal, detects the presence or absence of abnormal vibration, and identifies the position of vibration.
  • arithmetic unit 50 for example, a commercially available personal computer (PC) in which a program for detecting the presence or absence of abnormal vibration and specifying the position of vibration is installed can be used.
  • the arithmetic unit 50 will be described as being configured to include a CPU (Central Processing Unit) 60, a RAM (Random Access Memory) 52, a ROM (Read Only Memory) 54, and a storage unit 56.
  • the CPU 60 realizes each functional unit described later by executing a program stored in the ROM 54.
  • the processing result of each functional unit is temporarily stored in the RAM 52.
  • the OTDR waveform is periodically sent from the A / D converter 44 to the arithmetic unit 50.
  • the OTDR waveform is represented by, for example, taking x indicating the distance from the input end of the optical fiber 20 on the horizontal axis and taking the signal strength p (x, k) at the distance x on the vertical axis.
  • k is an integer of 1 or more and corresponds to the number of the optical pulse input to the optical fiber 20.
  • the number k of this optical pulse indicates the elapsed time.
  • the end of the optical fiber 20 on the side connected to the optical circulator 30 is used as the input end.
  • the OTDR waveform acquisition unit 62 sequentially stores, that is, stores the OTDR waveforms of the kth to k + M-1, which are OTDR waveforms for M pieces (M is an integer of 2 or more) that are continuous in time, in the storage unit 56. After M OTDR waveforms are stored, the oldest OTDR waveform is deleted and a new OTDR waveform is stored in the storage unit 56.
  • the moving average acquisition unit 64 receives the OTDR waveform from the A / D converter 44, the moving average of the kth moving average is obtained by averaging the M OTDR waveforms of the kth to k + M-1 that are continuous in time. Get the waveform.
  • the kth moving average waveform is stored in the storage unit 56.
  • the time-consecutive N moving average waveforms (N is an integer of 2 or more) acquired by the moving average acquisition unit 64 are sequentially stored in the storage unit 56. After N moving average waveforms are stored, the oldest moving average waveform is deleted and a new moving average waveform is stored in the storage unit 56.
  • the difference waveform acquisition unit 66 obtains the kth difference waveform p'(x) by taking the difference between the k + N-1 moving average waveform and the kth moving average waveform stored in the storage unit 56. get. In addition, this difference may be referred to as a moving difference.
  • the kth moving average waveform is stored in the storage unit 56.
  • the time-continuous i moving average waveforms (i is an integer of 2 or more) acquired by the moving average acquisition unit 64 are sequentially stored in the storage unit 56. After i moving average waveforms are stored, the oldest moving average waveform is deleted and a new moving average waveform is stored in the storage unit 56.
  • the position-specific statistical information acquisition unit 68 acquires the sample average ⁇ (x) and the standard deviation ⁇ (x) of each position for the difference waveforms of the kth to k + i-1.
  • the sample average ⁇ (x) and the standard deviation ⁇ (x) can be obtained, for example, at intervals of about the spatial resolution of this vibration detection optical fiber sensor. Alternatively, it may be the interval between measurement points adjacent to each other in the longitudinal direction of the optical fiber 20 when viewed from the digital data acquired by the A / D converter 44.
  • the abnormality degree acquisition unit 70 acquires a waveform indicating the abnormality degree (hereinafter, also referred to as an abnormality degree waveform) by calculating the abnormality degree at each position using the above equation (1).
  • the abnormality determination unit 72 determines the presence or absence of abnormal vibration from the degree of abnormality.
  • the threshold value used to determine the presence or absence of the degree of abnormality may be set to an appropriate value by monitoring the influence of the environment of the installation location.
  • the number M of waveforms when calculating the moving average is determined, for example, based on the line width of the laser light source 12.
  • the line width of the laser light source 12 is 500 Hz
  • M can be set to 20.
  • the moving average waveform becomes the average for a time of 4 msec.
  • the position-specific statistical information acquisition unit 68 acquires the sample average ⁇ (x) and the standard deviation ⁇ (x), it is preferable to perform the calculation without abnormal vibration. Therefore, for example, when the abnormality determination unit 72 determines that there is an abnormality vibration, the position-specific statistical information acquisition unit 68 excludes the sample corresponding to the position, and the sample average ⁇ (x) and the standard deviation. It can be configured to acquire ⁇ (x). Further, when the abnormality determination unit 72 determines that there is an abnormal vibration, the moving average acquisition unit 64 may be configured to acquire the moving average waveform excluding the corresponding OTDR waveform.
  • the time difference between the OTDR waveform and the moving average waveform when acquiring the difference waveform should be set to be equal to or longer than the period of abnormal vibration.
  • the frequency of vibration generated by hitting the optical fiber is about several hundred Hz.
  • the time difference is preferable to set to about 4 msec.
  • the number M of waveforms when calculating the moving average is determined, for example, based on the line width of the laser light source 12.
  • the line width of the laser light source 12 is 500 Hz
  • M can be set to 20.
  • the moving average waveform becomes the average for a time of 4 msec.
  • FIG. 2 is a schematic diagram for explaining the concept of determining abnormal vibration.
  • FIG. 2 (1) shows the difference waveform
  • FIG. 2 (2) shows the degree of abnormality obtained based on the difference waveform shown in FIG. 2 (1).
  • i 4 and four difference waveforms from the first to the fourth are shown.
  • the three axes (1) in FIG. 2 show the distance from the input end of the optical fiber 20, the elapsed time, and the light intensity of the difference waveform. Further, in FIG. 2 (2), the horizontal axis shows the distance from the input end of the optical fiber, and the vertical axis shows the degree of abnormality.
  • the position-specific statistical information acquisition unit 68 acquires the sample average ⁇ (x) and standard deviation ⁇ (x) of each position, and the abnormality determination unit 72 causes an abnormality. Judge the presence or absence of abnormal vibration from the degree. By normalizing with the sample mean ⁇ (x) and standard deviation ⁇ (x) at each position in this way, as shown in FIG. 2 (2), the degree of anomaly in the section where the disturbance is always applied is small. And its impact is underestimated. Therefore, the position dependence of the disturbance can be suppressed.
  • the characteristic test of this vibration detection optical fiber sensor will be described.
  • the length of the optical fiber 20 is set to 18.2 km.
  • the optical fiber at a position of 15.6 to 15.8 km from the input end side of the optical fiber 20 was vibrated and abnormal vibration was applied.
  • the laser light source 12 was a narrow line width laser having a line width of 500 Hz.
  • the pulse width and frequency of the optical pulse were set to 100 nsec and 5 kHz, respectively.
  • the number M of the OTDR waveforms used when calculating the moving average was set to 20, and the time difference when acquiring the difference waveform was set to 4 msec. This time difference of 4 msec is equivalent to 20 light pulses when converted to the number N of optical pulses.
  • the number i of the difference waveforms when acquiring the sample mean ⁇ (x) and the standard deviation ⁇ (x) was set to 20.
  • the number M of waveforms used when calculating the moving average, the number N of optical pulses converted from the time difference when acquiring the difference waveform, the sample mean ⁇ (x) and the standard deviation ⁇ is equal to 20.
  • (1) and (2) of FIG. 3 are diagrams showing the results of the characteristic test.
  • (1) of FIG. 3 is a diagram showing a movement difference, in which the horizontal axis shows the distance [unit: km] from the input end of the optical fiber 20, and the vertical axis shows the signal level [unit: mV]. It is shown.
  • FIG. 3 (2) is a diagram showing the degree of abnormality, in which the horizontal axis shows the distance [unit: km] from the input end of the optical fiber 20 and the vertical axis shows the degree of abnormality.
  • the signal level of the movement difference is the same as that no abnormal vibration is applied.
  • the signal level varies with distance.
  • the degree of abnormality is calculated using the above equation (1), and as a result, the range is 0 to 15.6 km. In the range of 15.8 to 18.2 km, the degree of anomaly is almost 0, and it can be clearly distinguished from the degree of anomaly in the section of 15.6 to 15.8 km to which the abnormal vibration is applied.
  • this vibration detection optical fiber sensor statistically evaluates the fluctuation of the OTDR waveform at each position in the vicinity of each time with the passage of time. Therefore, the detection of the distributed abnormal vibration that suppresses the dependence of the sensitivity on the distance from the input end of the optical fiber 20 can be clearly determined. Furthermore, even if the external environment changes slowly, the effect of suppressing the influence on the sensitivity of vibration detection can be expected.
  • Coherent detection may be performed by interfering the backscattered light with the laser light generated by the laser light source, as described in Nov. 15, 2010, pp.3243-3249.
  • a 90-degree light hybrid receiver may be used to observe the phase change of the backscattered light.
  • Heterodyne detection may also be performed on the backscattered light.

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Abstract

Dans la présente invention, une unité d'acquisition d'intensité de lumière acquiert l'intensité de la lumière de sortie résultant de la rétrodiffusion de la lumière de sonde dans une fibre optique. Un moyen d'acquisition de forme d'onde OTDR acquiert et mémorise séquentiellement des formes d'onde OTDR qui sont les formes d'onde de la lumière de sortie pour chaque impulsion optique envoyée par l'unité d'acquisition d'intensité lumineuse. Un moyen d'acquisition de moyenne mobile acquiert et mémorise une kième forme d'onde moyenne mobile par moyennage de la kième à la (k + M - 1)ième forme d'onde OTDR temporellement continue. Un moyen d'acquisition de forme d'onde différentielle acquiert une forme d'onde différentielle p'(x) pour une (k + M + N)ième forme d'onde OTDR et la kième forme d'onde moyenne mobile. Un moyen d'acquisition d'informations statistiques spécifiques à la position acquiert une moyenne d'échantillon μ(x) et un écart-type σ(x) pour chaque position de la kième à la (k + i - 1)ième (où i est un nombre entier supérieur ou égal à 2) des formes d'onde différentielles. Un moyen d'acquisition de degré d'anomalie calcule les degrés d'anomalie à chaque position. Un moyen de détermination d'anomalie détermine la présence d'une vibration anormale sur la base des valeurs des degrés d'anomalie, et lorsque c'est le cas, acquiert la position à laquelle s'est produite la vibration anormale.
PCT/JP2019/045789 2019-03-18 2019-11-22 Capteur à fibre optique de détection de vibration et procédé de détection de vibration WO2020188888A1 (fr)

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