WO2024105845A1

WO2024105845A1 - Optical fiber ultrasonic distribution measurement device and optical fiber ultrasonic distribution measurement method

Info

Publication number: WO2024105845A1
Application number: PCT/JP2022/042685
Authority: WO
Inventors: 哲賢李; 欣増岸田; 大治東
Original assignee: ニューブレクス株式会社
Priority date: 2022-11-17
Filing date: 2022-11-17
Publication date: 2024-05-23

Abstract

This optical fiber ultrasonic distribution measurement device (30) is provided with: a light source module (31) that has an LD (311), a chirped pulse light generation module (314), an optical fiber ring circuit (315) which has a predetermined ring length, into which chirped pulse light is input, and which outputs a chirped pulse light group at a prescribed sweep time interval, and a polarized wave diversity reception module (317) that receives back scattered light and laser light from the LD (311); and a ring chirp interval and delay control module (33) that controls the sweep time interval for chirped pulse light and delays the timing at which a chirped pulse light group enters a measurement object, wherein a plurality of pulses of chirped pulse light with different sweep frequency bands are caused to enter the measurement object, the same number of rays of back scattered light as the number of pulses of the chirped pulse light are received and restored, and in the restoration, the frequency band widths of the received signals are defined so as to receive the pulses of the chirped pulse light in such a manner that each chirped pulse light that has entered the measurement object is identified.

Description

Optical fiber ultrasonic distribution measuring device and optical fiber ultrasonic distribution measuring method

This application relates to an optical fiber ultrasonic distribution measurement device and an optical fiber ultrasonic distribution measurement method.

As offshore wind power generation expands, inspection and maintenance of many offshore wind turbines is required.Currently, inspection and maintenance of wind turbines relies on manual visual inspection and hammering inspection, so there is a need for low-cost and reliable inspection and maintenance.
Here, surveys on offshore wind power generation near Japan have been conducted within a distance of 30 km from the shore, and a measurement distance of 30 km is required to accommodate the installation locations of offshore wind power generation facilities planned in Japan in the future (see, for example, non-patent document 1).

Therefore, one effective way to solve the above problems is to realize constant remote monitoring of a large number of offshore wind turbines using ultrasonic measurement technology that can detect damage to wind turbine blades installed more than 30 km away, or loose bolts that secure the wind turbine blades, enabling constant monitoring.

The measurement speed limit of conventional DAS (distributed acoustic sensing) is due to the travel speed of the pulsed light in the optical fiber, and the next pulse cannot be input until the input pulsed light returns from the end of the optical fiber to the measuring device (see, for example, Non-Patent Document 2). For example, since it takes about 20 μs for the pulsed light to travel back and forth through a 2 km optical fiber, the maximum measurable sampling speed is 50 Ksps (sps: samples per second), which is the 20 μs period converted into the acoustic sampling speed, and it cannot keep up with the speed of ultrasonic testing, which has a higher sampling speed.

Similarly, if the measurement distance of the optical fiber is extended, the maximum measurable sampling speed also decreases. If the optical fiber length is 20 km, 10 times that of the normal size, the maximum measurable sampling speed also becomes 1/10 of the normal size, at 5 Ksps, making it difficult to measure ultrasound over long distances using optical fiber.

　In the past, as a means of monitoring and detecting damage to structures, FBG sensors (Fiber Bragg Gratings) have been used as optical fiber sensors, and a hybrid system of this FBG sensor and a piezoelectric element (PZT) has been incorporated into the structure to be measured to measure elastic waves, thereby identifying the presence or absence of damage such as peeling of the structure (including the peeling length), as well as the location of the damage, or the size of the damage. However, the upper limit of the elastic waves used in this elastic wave measurement has been reported to be around 1 MHz (see, for example, Non-Patent Document 3 and Patent Document 1). The principle of ultrasonic detection is a guideline for calculations using the speed of elastic waves, and when calculated with an elastic wave speed in steel, Ve = 5 mm/μs, the measurement position accuracy at f = 1 MHz is approximately 2.5 mm.

JP 2014-194379 A

This application discloses technology to solve the problems described above, and aims to realize a device that enables both long-distance measurements and high time resolution by combining two characteristic times: the interval between adjacent chirp pulse light and the interval between chirp pulse light groups. It also aims to provide a device that can vary the measurable length of a measured object while maintaining the sampling rate of the incident light by controlling the time interval at which the chirp pulse light groups are incident on the measured object.

The optical fiber ultrasonic distribution measurement device disclosed in the present application is
an LD, a chirp pulse light generating module which controls the frequency sweep range of the laser light emitted from the LD to generate a plurality of chirp pulse lights, an optical fiber ring circuit having a predetermined ring length, receiving the chirp pulse light generated by the chirp pulse light generating module and outputting a chirp pulse light group which is a collection of chirp pulse lights having a prescribed sweep time interval, and a polarization diversity receiving module which receives backscattered light from an optical fiber which is a measured object when the chirp pulse light group emitted from the optical fiber ring circuit is input into the optical fiber, and receives the laser light from the LD.
A light source module having
a ring chirp interval and delay control module for controlling a sweep time interval of the chirp pulse light and for controlling a timing at which the chirp pulse light group is input to the optical fiber to be delayed by a certain time;
Equipped with
The present invention is characterized in that a number of backscattered lights from the optical fiber are received and restored, the number of which is equal to the number of chirp pulse lights generated when a plurality of chirp pulse lights having different sweep frequencies generated by the chirp pulse light generating module are incident on the optical fiber, and by specifying the frequency bandwidth of the received signal during the restoration process, each chirp pulse light incident on the optical fiber is identified and received, and a distribution measurement of the ultrasound detected by the optical fiber is performed.

The optical fiber ultrasonic distribution measurement device disclosed in this application combines two characteristic times, the interval between adjacent chirp pulse light and the interval between chirp pulse light groups, to realize a device that is capable of both long-distance measurement and high time resolution, and by controlling the time interval at which the chirp pulse light groups are incident on the object to be measured, it is possible to provide a device that can vary the measurable length of the object to be measured while maintaining the sampling rate of the incident light.

1 is a model diagram showing an application example of an optical fiber ultrasonic distribution measurement device according to a first embodiment. 1 is a table comparing methods for monitoring damage to wind turbine blades to which the optical fiber ultrasonic distribution measurement device of the first embodiment is applied. FIG. This is a table categorizing the measurement technologies used in monitoring wind turbine blades. 1A to 1C are diagrams for explaining a method for detecting damage or the like that has occurred in an object to be inspected using optical fibers. 1 is a diagram for explaining a high-speed DAS technique used in an optical fiber ultrasonic distribution measurement device according to a first embodiment. FIG. 4A to 4C are diagrams for explaining a method of acquiring backscattered light of chirp pulse light used in the optical fiber ultrasonic distribution measurement device according to the first embodiment. 2 is a diagram for explaining the specifications of a DAS used in the optical fiber ultrasonic distribution measurement apparatus according to the first embodiment. FIG. 1 is a block diagram showing an example of an optical fiber ultrasonic distribution measurement device according to a first embodiment; 1 is a diagram showing a specific example of an optical fiber ultrasonic distribution measurement device according to a first embodiment; 3A to 3C are diagrams for explaining specific examples of chirp pulse light incident on a measurement object by the optical fiber ultrasonic distribution measurement device according to the first embodiment, and backscattered light of this chirp pulse light. 11 is a diagram for explaining signal processing in the case where a beat signal after receiving backscattered light of chirp pulse light is divided into a plurality of bands by a numerical simulation. FIG. 13 is a diagram for explaining signal processing in which signals divided into multiple bands are corrected for distance deviations and then reconstructed on a time basis with a shortened sampling period. FIG. FIG. 11 is a diagram for explaining the results when a 10 KHz signal is measured at 1 Ksps as a specific example of signal processing. FIG. 13 is a diagram showing an example of vibration waveform results obtained by analyzing ultrasonic test data divided into eight bands used in a simulation using a piezoelectric element. FIG. 13 is a diagram showing an analysis result of a vibration waveform when ultrasonic test data is analyzed with the eight bands used in the simulation aligned. FIG. 13 is a diagram showing the results when a vibration waveform, which is ultrasonic test data, is passed through a low-pass filter to remove high-frequency noise. FIG. 2 is a diagram showing an enlarged time axis scale of a vibration waveform.

This application primarily relates to a maintenance and inspection device using optical fiber for remote monitoring technology related to the maintenance and inspection of wind turbines for offshore wind power generation. The contents of this application will be explained below using a representative embodiment as an example.

Embodiment 1.
An application example of the optical fiber ultrasonic distribution measurement device of the first embodiment will be described with reference to Figures 1A to 1C. Figure 1A is a model diagram of a wind turbine 10 which is an example to which the optical fiber ultrasonic distribution measurement device according to the first embodiment is applied. Also, Figure 1B is an enlarged view of a portion P in Figure 1A. Furthermore, Figure 1C is a diagram showing an overview of a processing device that processes signals from an optical fiber in the optical fiber ultrasonic distribution measurement device.
1A, a number of blades 1 (wings 1) are fixedly attached to a hub 2, and rotate together with the wind force. This hub 2 constitutes a part of the wind turbine body called a nacelle 3, and the nacelle 3 is supported by a tower 4.

Here, a sensing optical fiber cable 5 using, for example, a single mode optical fiber is installed at a predetermined position on the blade 1 of the wind turbine 10 (see FIG. 1B), which can continuously measure the temperature or strain distribution at a predetermined position on the blade 1. A sensor measurement signal at the predetermined position on the blade 1 measured by this optical fiber cable 5 is transmitted to an optical fiber signal processing device 20 (see FIG. 1C) via a signal communication optical fiber cable (not shown). The optical fiber signal processing device 20 processes the sensor measurement signal to remotely detect the continuous distribution state of the temperature or strain at the predetermined position on the blade 1.
The signal (line) between the rotating part and the fixed part of the wind turbine can be connected using an optical rotary joint. Although there is a relative rotation between the nacelle and the tower, the rotation angle is within 180 degrees, so the effect of this can be absorbed by the flexibility of the cable. In the first embodiment, an optical fiber ultrasonic distribution measurement device 30 (described in detail below) using ultrasonic measurement is applied as the optical fiber signal processing device.

The above detection results make it possible to detect damage to the blades of a wind turbine approximately 30 km away from the wind turbine's operation and management facility, or loosening of the bolts that secure the blade 1 to the hub 2. Conventionally, methods for monitoring damage to wind turbine blades (hereinafter simply referred to as blades) include manual visual inspection, and examples of measuring deformation or damage to wind turbine blades using a measurement method (not continuous measurement) that uses FBG as an optical fiber (see, for example, Non-Patent Document 4).

Figure 2 is a table comparing the above-mentioned blade damage monitoring method with conventional examples. In this table, three typical monitoring methods are shown for comparison. Typical conventional methods include a manual inspection method and the above-mentioned method using FBG as an optical fiber sensor (hereinafter referred to as the FBG method). Figure 2 shows a comparison of these methods with the method of the present application by monitoring content.

Firstly, conventional methods require manual monitoring, which means that constant monitoring is difficult. Also, ultrasonic inspection is used to check for issues that cannot be visually confirmed, such as delamination of the blades, but inspecting the blades of offshore wind turbines poses cost issues. Also, the FBG method is insufficient in terms of commercial viability when it comes to monitoring deformation and damage to the blades, and it is difficult to measure delamination and loose bolts.

On the other hand, the method of the present invention can adequately handle any monitoring needs, such as constant monitoring, monitoring for deformation or damage to the blade, and monitoring for delamination or loose bolts.
The measurement technology used in the monitoring method of the present application will be described below. In particular, the method of monitoring deformation and damage of blades, and delamination and loose bolts using optical fiber distributed measurement sensors will be described, with emphasis on the ultrasonic DAS monitoring method for monitoring delamination and loose bolts. According to the present application, no electrical equipment is required for measurements at wind turbines or outdoors, and lightning and bad weather are not a factor, making monitoring possible even in high weather conditions.

In discussing the measurement technologies used in the monitoring method, we first classify the measurement technologies used to monitor wind turbine blades using optical fiber distributed measurement sensors according to measurement speed, and present the characteristics of each category in the measurement technology category table in Figure 3.

In FIG. 3, the measurement speed is divided into a measurement time of several seconds (hereinafter referred to as measurement speed category 1), up to about 100 KHz (hereinafter referred to as measurement speed category 2), and 1 MHz or higher (hereinafter referred to as measurement speed category 3). A typical measurement target for measurement speed category 1 is static strain such as blade deflection, and the optical fiber distribution measurement used here is DTSS technology (resolution, accuracy, etc. are as shown in FIG. 3). A typical measurement target for measurement speed category 2 is dynamic vibration/sound such as blade deflection, and the optical fiber distribution measurement used here is DAS technology (resolution, accuracy, etc. are as shown in FIG. 3). A typical measurement target for measurement speed category 3 is ultrasonic waves for ultrasonic flaw detection used to detect delamination of blades. Conventionally, there has been no practical example of detection using this ultrasonic wave as the measurement target using an optical fiber distribution measurement sensor. In this application, ultrasonic waves refer to 1 MHz or higher in the above measurement speed category 3.

Then, next, using the model diagram shown in Figure 4, a detection method using optical fiber to detect damage or loosening of the bolts that secure the blade, or delamination of the blade, which are related to the above measurement speed category 3, will be explained below, taking as an example a method using an ultrasonic flaw detector (piezoelectric element).

The diagram on the left side of Fig. 4 is a configuration diagram showing an ultrasonic flaw detector equipped with a piezoelectric element 6 that generates and receives ultrasonic waves, a bolt 8a which is a measurement object in which internal damage 9 is measured by ultrasonic waves, a blade 13 and a flange 14 which are fixed by this bolt 8a and a nut 8b, and a coupling 15 for transmitting ultrasonic waves to this measurement object without attenuation. An optical fiber 7 is installed in the coupling 15 to detect generated strain, etc.
Furthermore, the coupling material is usually made of a material that has a small attenuation when ultrasonic waves are transmitted (for example, an attenuation of less than 1 dB per 5 MHz), such as silicone, thermoplastic resin, etc. The symbol F in the figure indicates the (main) propagation direction of ultrasonic waves.

The diagram on the right side of Figure 4 shows a schematic diagram of the signal strength of the detected signal in response to the passage of time from the generation of the ultrasonic wave from the piezoelectric element 6, with the signal detection time at the position of the optical fiber 7 on the left side being the reference time (reference zero position of the time coordinate) in the case where the piezoelectric element 6 on the left side receives a reflected signal from the measured object of the ultrasonic wave generated by this piezoelectric element, and damage 9 inside the bolt 8a is detected. In this case, the ultrasonic wave propagates in the direction indicated by the arrow F, and the optical fiber 7 placed inside the coupling material detects (on the time axis) the signal generated inside the measured object by the ultrasonic wave, as a result of strain occurring in the longitudinal direction of the optical fiber due to the (relationship with) Poisson's ratio. In Figure 4, the direction perpendicular to the direction in which the ultrasonic wave propagates indicated by the arrow F corresponds to the longitudinal direction of the optical fiber.

Furthermore, the curves labeled Wa, Wb, and Wc in the diagram on the right side of Figure 4 are schematic representations of signals from the object to be measured detected by optical fiber 7 using ultrasonic signals emitted by the piezoelectric element, and the symbols Wa, Wb, and Wc respectively represent the signal detected when the ultrasonic signal first reaches the position (surface) of the object to be measured (bolt 8a in this case), the signal detected when the signal reaches the position of damage 9 inside the object to be measured, and the signal detected when reflected off the bottom surface of the object to be measured.
The intensity levels of these three signals have the relationship Wa>Wc>Wb, as shown in Fig. 4. In parallel with the reception of the signal generated in the object to be measured by the piezoelectric element, a signal corresponding to the reflected wave from the object to be measured due to the ultrasonic signal emitted by the piezoelectric element is detected by the optical fiber 7 installed in the coupling 15. The signal due to damage or the like of the object to be measured detected by the piezoelectric element is mainly used as a verification signal to be compared when the optical fiber detects a signal due to damage or the like of the object to be measured.

Here, in order to reduce the effects of attenuation of the ultrasonic waves used for measurement, there is an upper limit to the measurable thickness of the object being measured, depending on the material. For example, the thickness of the coupling 15 is about 6 mm, and if the object being measured is made of CFRP, it is less than 60 mm, and if it is made of steel, it is less than 100 mm. Details of the method of detecting ultrasonic waves using the optical fiber 7 are described below.

Furthermore, delamination of the blades can also be detected using the optical fiber 7. For example, this detection method involves investigating the generation and propagation characteristics of elastic waves such as Lamb waves in the blades by comparing measurements made using optical fiber with numerical analysis, and based on the results, it is possible to determine the location and size of the delamination. For example, broadband Lamb waves can be used to detect delamination of the layers that make up the wind turbine blades, and the location and size of the delamination can be identified.

When the bolt is made of steel, the speed of ultrasonic waves inside the bolt is approximately 5000 m/s, and a frequency of 3.3 Msps (with a discrimination speed of 0.3 μs) corresponds to a bolt length of 1.5 mm. Damage measurement in bolts longer than 1.5 mm can be achieved by delaying the timing of incidence of the chirp pulse light. The above describes the detection of ultrasonic waves when the bolt is damaged, but this is not the only case. For example, loosening of the bolt's fastening of the blade can also be detected by optical fiber, since it is possible to detect this as a change in strain in the optical fiber based on the change in stress when the bolt is loosened.

Furthermore, as pointed out in the background art, measurements in the ultrasonic band will be necessary even over long distances of about 30 km, so it is necessary to develop a measurement method (measurement system) that satisfies these requirements.
That is, in conventional DAS technology, there is a problem that ultrasonic measurement becomes difficult due to the measurement speed limit (upper limit sampling speed is 50 Ksps) caused by the traveling speed of laser pulse light (hereinafter also simply referred to as pulse light) in optical fiber, and this sampling speed limit is further reduced by long-distance measurement (when the optical fiber length becomes 20 km, which is 10 times longer than usual, the upper limit sampling speed decreases to 5 Ksps, which is 1/10 of the above 50 Ksps), and so this problem needs to be overcome.

In order to realize an optical fiber ultrasonic distributed measurement device that can perform the above ultrasonic measurements, it is necessary to increase the speed of conventional DAS technology. For this reason, in this development, a method was adopted that uses chirped pulse light in 20 different frequency bands by sweeping the frequency of the pulse light entering the optical fiber quickly and over a wide range. The specific details of this method are explained below using figures.

Figure 5A is a diagram for explaining conventional DAS technology. As shown in this figure, in conventional DAS, the next pulse of light must be input after the backscattered light of the pulse of light input to the optical fiber has finished reflecting, so only one pulse of light input to the optical fiber can be recognized. For example, the time it takes for a pulse of light to travel back and forth through a 2 km long optical fiber is approximately 20 μs, so the upper limit of the measurable speed is limited to this value (see Figure 5A). In other words, in conventional DAS, the measurement speed is limited to the speed of the pulse of light traveling through the optical fiber (approximately 200,000 km/sec).

In this development, taking into consideration the need to measure ultrasonic waves over long distances, the sampling speed was improved to about 66 times that of conventional DAS by using frequency-swept chirp pulse light (here, chirp pulse light of one bandwidth is referred to. For example, a bandwidth of 150 MHz to 250 MHz. Details will be described later), and the chirp pulse light group, which is a collection of chirp pulse light consisting of several tens of chirp pulse lights (here, each bandwidth is constant, for example), is incident on the object to be measured with a (time) interval between them, making it possible to measure long distances of 30 km. In other words, the chirp pulse light is incident on the optical fiber to be measured with an interval of about 300 μs (the time it takes for the pulse light to go round trip through a 30 km long optical fiber), which is the time equivalent to a distance of 30 km, and the backscattered light returning from this optical fiber is received (see Figure 5B). In addition, at this time, the backscattered light returning from the optical fiber is individually received by a bandpass filter. However, if the chirp pulse light groups are received with an interval of about 300 μs between each other, the location on the optical fiber that corresponds to the time interval will be a missing section, which is a problem. Therefore, we decided to acquire the signal using the method shown in Figure 6.

In other words, a group of chirp pulse light delayed at regular intervals (here, by a few microseconds, which is the time for one cycle of ultrasound, as shown in Figure 6) is injected into the optical fiber (see the group of chirp pulse light shown in the upper part of Figure 6), and after receiving them individually, they are all combined. In this way, it is possible to perform measurements that are continuous in time without signal interruption. In other words, this makes it possible to obtain a continuous signal without any missing intervals.

The main points of the long-distance ultrasonic DAS technology that we have developed have been described above. Below, we will explain examples of specific values that are actually used.
When 20 different frequency bands of chirp pulse light are incident on the test object as pulse light to be input into the optical fiber, the backscattered light from the test object is generated. By setting the frequency width when restoring and digitally processing the backscattered light from the test object, each of the original incident signals can be identified and received, and by using a 2 GHz receiver corresponding to the number of chirp signals in the 100 MHz restoration band, the sampling speed can be increased to 3.3 Msps, approximately 66 times faster than 50 Ksps.

Then, by controlling the interval at which the chirp pulse light group, which is an aggregate of the above chirp pulse light, is input to the aggregate, the measurable optical fiber length can be varied while maintaining the sampling speed. Specifically, when the total length of the optical fiber is 30 km, the time it takes for the pulse light to travel this distance back and forth in the optical fiber is 2 x 30 km/(200,000 km/s) ≈ 300 (μs), assuming that the speed of light in the optical fiber is approximately 200,000 km/s, so the time interval at which the chirp pulse light group is input to the optical fiber is controlled to approximately 300 (μs).

From the above, the features of the developed long-distance ultrasonic DAS technology compared to other DAS technologies can be summarized as shown in Figure 7. In conclusion, the measurable optical fiber length is 30 km, and the maximum sampling speed is 3.3 Msps, which is 66 times faster than conventional DAS. This value of 3.3 Msps corresponds to 0.3 μs, which is the discrimination speed of each optical pulse in the optical fiber ring circuit described later. However, in this embodiment 1, the (delay) control of the incident pulse light also uses 300 μs, which is the time interval between each chirp pulse light group, in addition to 0.3 μs.
Including the above values, the optical fiber ultrasonic distribution measurement device of the present application satisfies the following specifications: spatial resolution is 1 m to 2 m, sampling speed is 2 Ksps to 3.3 Msps, and measurable range is 500 m to 30 km.

Next, the measurement device for achieving the specifications of the long-distance ultrasonic DAS will be described in detail with reference to Figure 8. In Figure 8, an arrow with arrows on both sides indicates that signals are sent and received in both directions, while an arrow with arrows on only one side indicates that signals are sent only in the direction of the arrow. Note that the thicker the arrow, the faster data can be sent and received. Also, the open arrows represent transmission and reception between the measurement device and the object being measured.

FIG. 8 is a block diagram showing an example of an optical fiber ultrasonic distribution measurement device 30 that is a measurement device that realizes the specifications of the long-distance ultrasonic DAS. As shown in FIG. 8, the optical fiber ultrasonic distribution measurement device 30 includes a light source module 31 for generating chirp pulse light, transmitting it to the object to be measured, and receiving backscattered light from the object to be measured, an optical control module 32 for controlling the frequency of the generated chirp pulse, etc., a ring chirp interval and delay control module 33 for controlling the chirp pulse signal on the time axis or controlling the time delay of the received signal with a high accuracy of about 10 ns, an A/D converter 34 for converting the received analog signal into a digital signal for processing by a computer (CPU module 35 having a processor, memory, etc., not shown), and a PCIe bus 36 (PCIe: an abbreviation for Peripheral Component Interconnect Express, a high-speed serial expansion bus standard) for transmitting data at high speed between each of the above modules and the above processor.

Here, the light source module 31 includes an LD 311 (LD: Laser Diode) that oscillates a narrow-linewidth laser (e.g., a single-frequency laser) that cannot be directly modulated and has a spectral linewidth of, for example, less than 1 kHz, a polarization diversity receiving module 317 that receives and processes only a specific polarization, an I/O circuit 312 that transmits and receives signals to and from the LD 311, an optical control module 32, and the polarization diversity receiving module 317, a chirp pulse light generating module 314 for generating chirp pulses, and a narrow-linewidth LD 311. It is equipped with a beam splitter 313 (also called BS313 for simplicity) for splitting the optical path of the laser light emitted from 11 and sending it to the chirp pulse light generating module 314 and the polarization diversity receiving module 317, an optical fiber ring circuit 315 for circulating the laser light to measure changes in physical quantities in an optical fiber of a specified distance using chirp pulses, and an optical coupler 316 for injecting the chirp pulse light into the object to be measured and sending the backscattered light from the object to the polarization diversity receiving module 317.

In addition, by increasing the sampling speed to 3.3 Msps, it is possible to detect damage to bolts up to 3 mm in length using ultrasonic waves (for example, ultrasonic speed 5000 m/s = 5 mm/μs), since the continuous observation time is 6 μs as described below. Furthermore, by using the ring chirp interval and delay control module 33 (with a control accuracy of 10 ns for the delay timing) to delay the timing of incidence of the chirp pulse light on the object being measured, the observation time of ultrasonic propagation is shifted, making it possible to detect damage to bolts of a length greater than 3 mm.

Next, FIG. 9 shows a specific example of a measurement device that realizes the above-mentioned optical fiber ultrasonic distribution measurement device 30. Here, specific examples of the two components, the chirp pulse light generating module 314 in FIG. 8 above, and the optical fiber ring circuit 315, which are the most characteristic components of this measurement device, will be described below with reference to FIG. 9.

The laser light emitted from the LD used for DAS measurement is split into two by a polarizing beam splitter, and one of these laser lights is directed toward a board on which an AOM (Acousto-Optics Modulator, hereafter the same) and driver circuit are mounted, as shown in Figure 9, to generate a chirp pulse. However, the LD mentioned above has a narrow linewidth and cannot be directly modulated. Therefore, the laser light is modulated by a board on which an AOM and driver circuit are mounted, as explained below.

The AOM can generally control and modulate the intensity of light at a speed faster than a mechanical shutter by repeatedly turning it on and off at a specified frequency (here, for example, 100 MHz as shown in the figure), and can continuously generate multiple pulsed lights with a certain frequency difference. This control is performed by an LFM (Linear Frequency Modulate, or Chirp) using a DDS (Direct Digital Synthesizer) and a driving circuit as shown in Figure 9. Specific values used here are, for example, a certain frequency difference Δf = 100 MHz and a time interval of 300 μs (the interval between each chirp pulse light group). This concludes the explanation of a specific example of the chirp pulse light generating module 314.

Next, a specific example of the optical fiber ring circuit 315 will be described. Using the chirp pulse light generated through the control described above, the optical frequency loop circuit (hereinafter also referred to as the optical fiber ring circuit) shown in the figure generates chirps by delaying multiple (e.g. 20) 100 MHz chirp pulse lights at a constant time interval τ (here τ = 300 ns). As shown in the figure, this optical frequency loop circuit uses an AOM similar to the chirp pulse light generating module 314 described above, as well as a polarizing beam splitter (P-BS) and an EDFA (Erbium-Doped Fiber Amplifier) for signal flattening.

In this case, if the ring length of the optical frequency loop circuit is set to 60 m to correspond to a given distance (e.g., 30 km), the chirped pulse light can detect changes in physical quantities such as distortion corresponding to positions within the same optical fiber at an identification speed of 0.3 μs. This identification speed of 0.3 μs is equivalent to a sampling speed of 3.3 Msps.

Next, an example of chirp pulse light incident on the object to be measured, which is realized using the above-mentioned measuring device, and a specific example of a signal related to the backscattered light of this chirp pulse light will be explained with reference to Figure 10. The upper part of Figure 10 shows an example of a signal whose frequency sweep range (also called the chirp frequency sweep range) is controlled by the 100 MHz AOM output. The lower part of Figure 10 shows a specific example of a total of 20 signals that have been BPF (band pass filtered) at 100 ksps on the receiving side. The accuracy of the delay control in this case was 10 ns.

First, an example of the output pattern of the optical output (chirp pulse light incident on the measured object) will be specifically described below with reference to FIG. 10. The output pattern of this optical output is swept by a 100 MHz AOM, with 150 MHz set as the first frequency, and then swept in increasing order at six different frequencies (also called chirp frequencies) from 150 MHz to 650 MHz in increments of 100 MHz. This chirp pulse light is configured as one set of units, and the final set is a set of 20 chirp pulse lights consisting of a set from 1650 MHz to 2150 MHz (this set is the chirp pulse light group described above). The time interval between adjacent sweep frequencies (also called the sweep time interval for simplicity) is set to 300 ns. The time from the first chirp pulse light group (from 150 MHz to 2150 MHz) to the generation of the next chirp pulse light group is 300 μs (the time it takes for the pulse light to travel back and forth through a 30 km long optical fiber).

Next, the signal pattern on the receiving side will be explained. On the receiving side, a 2 GHz (= 100 MHz x 20) receiver is used, which corresponds to the number of chirp signals in the 100 MHz recovery band mentioned above. In this case, a total of 20 chirp signals are observed at a discrimination speed of 0.3 μs, so the target signal can be observed in an observation time window of 6 μs. Also, on the receiving side, a bandpass filter is used to individually discriminate the 20 chirp signals.

As described above, according to the optical fiber ultrasonic distribution measurement device of the first embodiment, by combining two characteristic times, namely, the interval time between adjacent chirp pulse light and the interval time between chirp pulse light groups, it is possible to realize a device that enables both long distance and high time resolution, and by controlling the time interval at which the chirp pulse light groups are incident on the object to be measured, it is possible to provide a device that can vary the measurable length of the object to be measured while maintaining the sampling speed of the incident light.
Furthermore, this optical fiber ultrasonic distribution measurement device can be used to provide a device or system that constantly monitors the blades or bolts of offshore wind turbines installed several tens of kilometers away to detect damage, etc.

<Verification by Numerical Simulation>
The long-distance ultrasonic DAS technology described above was verified by a numerical simulation, and it was confirmed that it is feasible. The details of the verification by the numerical simulation will be described in detail below.

In this numerical simulation, a method of resampling a 10 KHz sine wave at 4 Msps with a sampling rate of 1 Ksps using a chirp pulse with a time width of 2 μs was simulated. This method will be explained in detail below using specific numerical values. Note that the resampling period at 4 Msps is 0.25 μs, so if this value can be used to confirm the simulation, it is believed that this will serve as a verification of the case where the discrimination speed of the optical pulse of this application is 0.3 μs.

First, 50 consecutive measurements are performed at measurement intervals of 1 Ksps using a conventional chirp pulse with a time width of 2 μs (chirp frequency of 100 MHz) (step S1).
Next, the received beat signal is subjected to signal processing and passed through eight matched filters having equal bandwidths of 12.5 MHz, as shown below, to divide the 2 μs beat signal into the following eight bands (step S2).
Band 1: 145.0MHz to 157.5MHz
Band 2: 157.5MHz to 170.0MHz
Band 3: 170.0MHz to 182.5MHz
Band 4: 182.5MHz to 195.0MHz
Band 5: 195.0MHz to 207.5MHz
Band 6: 207.5MHz to 220.0MHz
Band 7: 220.0MHz to 232.5MHz
Band 8: 232.5MHz to 245.0MHz
Here, the interval between adjacent divided bands is 2 μs/8=0.25 μs, which translates into a distance difference of 25 m. For example, the distance difference between the divided signals of band 1 and band 2 is 25 m, and the distance difference between the divided signals of band 1 and band 3 is 50 m, calculated by adding the distance difference between the divided signals of band 2 and band 3, 25 m, to this 25 m.
Next, the signal is divided into the eight bands, that is, a distance shift is performed to correct the distance deviation of each divided signal (step S3).
The signal data subjected to the distance shift processing has a measurement speed of 1 Ksps, i.e., 50 data sets with a period T of 1002 μs. Therefore, the data is reconstructed on the time axis to remove the time corresponding to this period difference so that all signal data are connected with signals with a 0.25 μs interval (step S4).
The above signal processing enables resampling at a speed of 4 Msps (T = 0.25 μs). Specifically, when considering a case where 50 consecutive measurements are performed at a measurement speed of T = 1002 μs, resampling at intervals of 2 μs x 50 times = 100 μs is possible by using a chirp pulse with a time width of 2 μs. Specifically, pseudo-continuous measurements at 4 Msps (continuous measurements at 0.25 μs intervals) are possible. The spatial resolution in this case is about 8 m (Δf = 12.5 MHz). As a result, overlapping 10 kHz signals can be connected.

The above method will now be described in detail with reference to Figures 11A to 17C. Of these figures, Figures 11A to 12C show the signal processing carried out on the received beat signal for each processing step described above, and are intended to explain a method of resampling a 10 KHz sine wave at 4 Msps with a sampling speed of 1 Ksps (period T = 1002 μs). Figure 13 explains that by using a 2 μs chirp pulse to measure 50 periods of a 10 KHz sine wave at a measurement speed of 1 Ksps and resampling at 4 Msps, it is possible to measure one period of 10 KHz.

11A to 12C are diagrams illustrating the principle of the above-mentioned resampling method based on the received beat signal.
Fig. 11A shows a beat signal measured by a conventional method. Here, a chirp pulse with a pulse (time) width of 2 μs is used, and a total of 50 measurements are performed at a measurement speed of 1 Ksps (T = 1002 μs ≒ 1 ms). N shown in Fig. 11A is the number of measurements. Since the measurement period is 1002 μs, if the start time of the first measurement (N = 1) is 0 (zero) seconds (s), the start time of the 50th measurement (N = 50) is 49.098 ms. Each of these beat signals has frequency components from 145.0 MHz to 245.0 MHz.

Next, FIG. 11B is a diagram showing an example in which the beat signal in the optical fiber ultrasonic distribution measurement device of the first embodiment is divided at a constant bandwidth (12.5 MHz) (see step S2 above), and shows the results of dividing each of the beat signals measured 50 times into a total of eight bands with an interval of 0.25 μs between bands.
As shown in Fig. 11B as DN=m (m is an integer from 1 to 50, the same applies below), the beat signal is divided into eight bands between 145.0 MHz and 245.0 MHz by a matched filter, corresponding to the number of measurements shown as N=m in Fig. 11A. In this case, in each band from band 2 to band 8 shown in each DN=m, a certain distance shift occurs depending on each band (the difference in distance shift between adjacent bands is a constant 25 m).

Next, we will explain Figures 12A to 12C. Figure 12A is a reprint of Figure 11B (part of it). Figure 12B is a diagram showing the results of performing a fixed amount of distance shift processing for each band at DN=m in order to eliminate the distance deviation that occurred in each band in Figure 12A. From this diagram, it can be seen that the distance shift amount in each band between adjacent DN=m (for example, between DN=1 and DN=2) has the same pattern, and this is repeated in a period of 1002 μs. In other words, the time difference between adjacent DN=m is 1002 μs. Therefore, in order to eliminate the time difference related to this period, the period is changed from 1002 μs to 0.25 μs at each DN=m, and a process of reconstructing (resampling) on the time axis is performed. The result is shown in Figure 12C.

Next, Figure 13 shows the results of the above processing method when a 10 KHz signal is measured and resampled at 1 Ksps (T = 1002 μs). If the horizontal axis represents time and the vertical axis represents signal level (arbitrary scale), it can be seen that one period of 10 KHz can be measured by measuring 50 periods. The sampling speed in this case is 4 Msps, which corresponds to 0.25 μs.

Next, an example of the results of analyzing 10 KHz test data using a PZT (piezoelectric element) with the above simulation method will be described below. Figures 14A and 14B show an example of the results when each of the above bands is analyzed (independently). Figure 14A is a waterfall graph showing the measurement results of test data in band 1 as an example. The PZT signal can be seen at a point of 499.8 m. Here, the horizontal axis is time (unit: μs) and the vertical axis is distance (unit: m).

The analysis results of the distortion rate (unit: nε/μs) of the measured object due to the vibration of the piezoelectric element in the above eight bands (band 1 to band 8), including the above results, are shown in Figure 14B. The horizontal axis is time (unit: μs), the same as in Figure 14A.

Next, Fig. 15A shows the analysis results (hereafter referred to as the original data) of the vibration waveform when all eight bands are aligned (combined) rather than the measurement results for each of the above bands (the scale of the horizontal axis is the same as in Fig. 14B). Looking at the whole picture, it can be seen that it shows a similar shape to the graph shown in Fig. 14B. Fig. 15B also shows the results of calculating the spectrum (power spectral density) based on the data in Fig. 15A (the horizontal axis is frequency). From Fig. 15B, the 10 KHz vibration component of the piezoelectric element can be confirmed.

Next, Figure 16B shows the results when the above raw data (see Figure 16A; the same data as Figure 15A) is passed through an LPF (Low pass filter) with a cutoff frequency of 15 kHz to remove high frequency noise. As shown in the figure, the time difference between the vibration peak waveforms is 100 μs, which can be said to represent the above 10 kHz vibration waveform.

Furthermore, based on the 10 kHz vibration waveform in FIG. 16B (here, the same vibration waveform as in FIG. 16B is shown as FIG. 17A), FIGS. 17B and 17C show the vibration waveform with the scale of the time axis enlarged. The time scale shown in FIG. 17B is about 10 times that of FIG. 17A, and the time scale shown in FIG. 17C is about 400 times that of FIG. 17A. When the time scale is enlarged to FIG. 17C, it can be seen that the analyzed point of 10 kHz (0.25 μs, which corresponds to the above-mentioned discrimination speed), indicated by a circle in the graph, which was not clear in FIGS. 17A and 17B, is clearly shown.

The above enabled us to verify through simulation a method of resampling a 10 KHz sine wave at 4 Msps with a sampling rate of 1 Ksps using a chirp pulse with a time width of 2 μs.

Although exemplary embodiments are described herein, the various features, aspects, and functions described in the embodiments are not limited to application to a particular embodiment, but may be applied to the embodiments alone or in various combinations.
Therefore, countless modifications not illustrated are expected within the scope of the technology disclosed in the present specification, including, for example, modifying, adding, or omitting at least one component.

1 Blade, 2 Hub, 3 Nacelle, 4 Tower, 5 Optical fiber cable, 6 Piezoelectric element, 7 Optical fiber, 8a Bolt, 8b Nut, 9 Damage, 10 Wind turbine, 13 Blade, 14 Flange, 15 Coupling, 20 Optical fiber signal processing device, 30 Optical fiber ultrasonic distribution measurement device, 31 Light source module, 32 Optical control module, 33 Ring chirp interval and delay control module, 34 A/D converter, 35 CPU module, 36 PCIe bus, 311 LD, 312 I/O circuit, 313 Beam splitter, 314 Chirp pulse light generation module, 315 Optical fiber ring circuit, 316 Optical coupler, 317 Polarization diversity receiving module

Claims

an LD, a chirp pulse light generating module which controls the frequency sweep range of the laser light emitted from the LD to generate a plurality of chirp pulse lights, an optical fiber ring circuit having a predetermined ring length, receiving the chirp pulse light generated by the chirp pulse light generating module and outputting a chirp pulse light group which is a collection of chirp pulse lights having a prescribed sweep time interval, and a polarization diversity receiving module which receives backscattered light from an optical fiber which is a measured object when the chirp pulse light group emitted from the optical fiber ring circuit is input into the optical fiber, and receives the laser light from the LD.
A light source module having
a ring chirp interval and delay control module for controlling a sweep time interval of the chirp pulse light and for controlling a timing at which the chirp pulse light group is input to the optical fiber to be delayed by a certain time;
Equipped with
An optical fiber ultrasonic distribution measurement device, characterized in that the optical fiber receives and restores the same number of backscattered lights from the optical fiber as the number of chirp pulse lights generated when a plurality of chirp pulse lights having different sweep frequencies generated by the chirp pulse light generating module are incident on the optical fiber, and by specifying the frequency bandwidth of the received signal during the restoration process, the optical fiber receives and identifies each chirp pulse light incident on the optical fiber, and performs distribution measurement of the ultrasonic waves detected by the optical fiber.
The optical fiber ultrasonic distribution measurement device according to claim 1, characterized in that the sweep time interval of the chirped pulse light is determined according to the frequency of the signal detected by the optical fiber, and the constant delay time that determines the timing of incidence of the chirped pulse light group is determined according to the length of the optical fiber.
The optical fiber ring circuit has an acousto-optical element for delaying multiple chirp pulse lights of a predetermined frequency band at regular time intervals to generate chirp pulses, and the ring length of the optical fiber ring circuit is set to a predetermined length corresponding to the discrimination speed of the chirp pulse light in the optical fiber. The optical fiber ultrasonic distribution measurement device described in claim 1 or 2 is characterized in that:
4. An optical fiber ultrasonic distribution measurement method for measuring ultrasonic waves contained in backscattered light from an optical fiber by using the optical fiber ultrasonic distribution measurement device according to claim 1, comprising:
The optical fiber ultrasonic distribution measurement method is characterized in that the frequency of the chirp pulse light incident on the optical fiber is swept within a predetermined high-speed and wide range of sampling speeds to generate chirp pulse light of a plurality of different frequency bands, and when the generated chirp pulse light is incident on the optical fiber and the backscattered light of the chirp pulse light is restored and digitally processed, a frequency width at the time of restoration is set to a specified frequency width, thereby identifying and receiving each incident chirp pulse light.
The optical fiber ultrasonic distribution measurement method according to claim 4, characterized in that the ring chirp interval and delay control module delays the timing of incidence of the chirp pulse light into the optical fiber and forms a continuous signal on the time axis with a period corresponding to the specified frequency width.