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

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 measurement device and optical fiber ultrasonic distribution measurement method

[0001] The present application relates to an optical fiber ultrasonic distribution measurement device and an optical fiber ultrasonic distribution measurement method.

[0002] As the scale of 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, and low-cost, reliable inspection and maintenance is required. Here, surveys of offshore wind power generation in the waters off Japan have been conducted within a distance of 30 km from 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).

[0003] Therefore, one effective way to solve the above problems is to use ultrasonic measurement technology, which can detect damage to wind turbine blades installed more than 30 km away or loose bolts that secure the wind turbine blades, and enable constant monitoring of these things, to realize constant remote monitoring of a large number of offshore wind turbines.

[0004] Incidentally, the measurement speed limit of conventional DAS (distributed acoustic sensing) is due to the traveling speed of pulsed light in optical fiber, and the next pulse cannot be input until the input pulsed light returns from the end of the optical fiber to the measurement device (see, for example, Non-Patent Document 2). For example, since the time it takes for pulsed light to travel back and forth through a 2 km optical fiber is approximately 20 μs, the upper limit of the measurable sampling speed is 50 Ksps (sps: samples per second), which is the 20 μs period converted into the sampling speed of sound waves, and this cannot keep up with the speed of ultrasonic testing, which has a higher sampling speed than this.

[0005] Similarly, if the measurement distance of the optical fiber is increased, the maximum measurable sampling speed also decreases. If the optical fiber length is 20 km, which is 10 times longer than 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.

[0006] Conventionally, as a means for monitoring and detecting damage to structures, an FBG sensor (Fiber Bragg Gratings (FBG)) has been used as an optical fiber sensor, 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, location, or size of damage such as peeling of the structure (including the length of the peeling), but the upper limit of the elastic waves reported for this elastic wave measurement is about 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 velocity of the elastic wave, and when calculated with an elastic wave velocity in steel, Ve = 5 mm / μs, the measurement position accuracy at f = 1 MHz is about 2.5 mm.

[0007] Nagai et al., "Expected Recoverable Capacity of Offshore Wind Power in Japan's Coastal Waters," Wind Energy, Vol. 34, No. 1, pp. 103-112, 2010; K. Kishida et al., "Development of Real-Time Time Gated Digital (TGD) OFDR Method and Its Performance Verification," Sensors, 2021, 21, 4865. pp. 1-16; Takeda et al., "Application of Optical Fiber Sensors to Aircraft Structural Health Monitoring," Instrumentation and Control, Vol. 51, No. 3, pp. 234-240, 2012. https: / / www.kyowa-ei.com / jpn / product / sector / energy / appli-cation_150.html

[0008] JP 2014-194379 A

[0009] The present application discloses technology for solving the above-mentioned problems, and aims to realize an apparatus that enables both long-distance measurement and high time resolution by combining two characteristic times: the interval time between adjacent chirp pulse light and the interval time between chirp pulse light groups. It also aims to provide an apparatus that can vary the measurable length of an object to be measured while maintaining the sampling rate of the incident light by controlling the time interval at which chirp pulse light groups are incident on the object to be measured.

[0010] The optical fiber ultrasonic distribution measurement device disclosed in the present application comprises a light source module having: an LD; a chirp pulse light generation module that generates a plurality of chirp pulse lights by controlling the frequency sweep range of laser light emitted from the LD; an optical fiber ring circuit having a predetermined ring length, to which the chirp pulse light generated by the chirp pulse light generation module is input and which outputs a chirp pulse light group that is a collection of chirp pulse lights with a defined sweep time interval; and a polarization diversity receiving module that receives backscattered light from an optical fiber that is a measurement target when the chirp pulse light group emitted from the optical fiber ring circuit is input into the optical fiber, and the laser light from the LD; and a ring chirp interval and delay control module that controls the sweep time interval of the chirp pulse light and performs control to delay the timing at which the chirp pulse light group is input into the optical fiber by a certain time, The chirp pulse light generating module generates a plurality of chirp pulse lights with different sweep frequencies, and the chirp pulse light generating module receives and restores the same number of backscattered lights from the optical fiber as the number of chirp pulse lights generated when the chirp pulse lights are incident on the optical fiber. In addition, by specifying the frequency bandwidth of the received signal during the restoration process, the chirp pulse lights incident on the optical fiber are identified and received, and the distribution of the ultrasound detected by the optical fiber is measured.

[0011] According to the optical fiber ultrasonic distribution measurement device disclosed in the present application, by combining two characteristic times, namely, the interval time between adjacent chirp pulse light beams and the interval time between chirp pulse light groups, it is possible 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.

[0012] 1 is a model diagram showing an application example of the optical fiber ultrasonic distribution measurement device according to the first embodiment. FIG. 2 is a table comparing methods for monitoring damage to wind turbine blades to which the optical fiber ultrasonic distribution measurement device according to the first embodiment is applied. FIG. 3 is a table classifying measurement technologies used when monitoring wind turbine blades. FIG. 4 is a diagram explaining a method for detecting damage, etc., that has occurred in an object to be inspected using optical fiber. FIG. 5 is a diagram explaining a high-speed DAS technology used in the optical fiber ultrasonic distribution measurement device according to the first embodiment. FIG. 6 is a diagram explaining a method for acquiring backscattered light of chirp pulse light used in the optical fiber ultrasonic distribution measurement device according to the first embodiment. FIG. 7 is a diagram explaining specifications of the DAS used in the optical fiber ultrasonic distribution measurement device according to the first embodiment. FIG. 8 is a block diagram showing an example of the optical fiber ultrasonic distribution measurement device according to the first embodiment. FIG. 9 is a diagram showing a specific example of the optical fiber ultrasonic distribution measurement device according to the first embodiment. FIG. 10 is a diagram explaining specific examples of chirp pulse light that is incident on an object to be measured by the optical fiber ultrasonic distribution measurement device according to the first embodiment, and the backscattered light of this chirp pulse light. FIG. 11 is a diagram explaining signal processing when a beat signal after receiving backscattered light of chirp pulse light is divided into multiple bands by numerical simulation. 1 is a diagram for explaining signal processing in which distance deviations of signals divided into multiple bands are corrected and then the signals are reconstructed on a time basis with a shortened sampling period. FIG. 1 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. 1 shows an example of the results of vibration waveforms obtained by analyzing ultrasonic test data divided into eight bands used in a simulation using a piezoelectric element. FIG. 2 is a diagram showing the analysis results of vibration waveforms obtained by analyzing ultrasonic test data with the eight bands used in the simulation aligned. FIG. 3 is a diagram showing the results when high-frequency noise is removed by applying a low-pass filter to the vibration waveform, which is ultrasonic test data. FIG. 4 is a diagram showing an enlarged time axis scale of the vibration waveform.

[0013] The present application relates to a maintenance and inspection device using optical fiber, which is used for remote monitoring technology related to the maintenance and inspection of wind turbines for offshore wind power generation. The contents of the present application will be described below using representative embodiments as examples.

[0014] Embodiment 1. An application example of the optical fiber ultrasonic distribution measurement device of embodiment 1 will be described using Figures 1A to 1C. Figure 1A is a model diagram of a wind turbine 10, which is an example of an application of the optical fiber ultrasonic distribution measurement device of embodiment 1. Figure 1B is an enlarged view of part P in Figure 1A. Figure 1C is a diagram showing an overview of a processing device that processes signals from optical fibers in the optical fiber ultrasonic distribution measurement device. In Figure 1A above, multiple blades 1 (wings 1) are fixedly attached to a hub 2, and rotate together due to wind force. This hub 2 forms part of the wind turbine body called a nacelle 3, and the nacelle 3 is supported by a tower 4.

[0015] 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 ). The 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 processor 20 (see FIG. 1C ) via a signal communication optical fiber cable (not shown). The optical fiber signal processor 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. Note that the signal (line) between the rotating and fixed parts of the wind turbine can be connected using an optical rotary joint. Although there is also relative rotation between the nacelle and the tower, the rotation angle is within 180 degrees, so the flexibility of the cable can absorb the effect. In the first embodiment, an optical fiber ultrasonic distribution measurement device 30 (described in detail below) using ultrasonic measurement is used as the optical fiber signal processor.

[0016] From the above detection results, it is possible to detect damage to the blades of a wind turbine approximately 30 km away from the wind turbine operation and management facility, or loosening of the bolts that secure the blades 1 to the hub 2. Conventionally, as a method for monitoring damage to wind turbine blades (hereinafter simply referred to as blades), examples have been known in which deformation or damage to wind turbine blades is measured by manual visual inspection or by a measurement method that uses FBG as an optical fiber (not a continuous measurement) (see, for example, Non-Patent Document 4).

[0017] Figure 2 is a table comparing the above-mentioned blade damage monitoring method with conventional examples. This table shows three representative monitoring methods for comparison. Representative conventional methods include a manual inspection method and the above-mentioned method using FBGs as optical fiber sensors (hereinafter referred to as the FBG method). Figure 2 compares these methods with the method of the present application by type of monitoring.

[0018] Conventional methods rely on manual labor, so monitoring must be done periodically, making constant monitoring difficult. Ultrasonic flaw detection is used to check for issues that cannot be visually confirmed, such as delamination of blades, but inspecting the blades of offshore wind turbines is costly. Furthermore, the FBG method is insufficient for commercial use in monitoring blade deformation and damage, and it is difficult to measure delamination and loose bolts.

[0019] On the other hand, the method of the present application can adequately handle any monitoring needs, such as continuous monitoring, monitoring for blade deformation and damage, and monitoring for delamination and loose bolts. The following describes the measurement technology used in the monitoring method of the present application. In particular, the method of monitoring for blade deformation and damage, and for delamination and loose bolts using optical fiber distributed measurement sensors will be described, with an emphasis on the ultrasonic DAS monitoring method for monitoring for delamination and loose bolts. According to the present application, measurements at wind turbines or outdoors do not require any electrical equipment, and are therefore not affected by lightning or bad weather, making monitoring possible even in cases where such weather risks are high.

[0020] In describing the measurement technologies used in the monitoring method, we first classify the measurement technologies used when monitoring wind turbine blades using optical fiber distributed measurement sensors according to measurement speed, and summarize the characteristics of each category. This is shown in the measurement technology classification table in Figure 3.

[0021] In FIG. 3 , the measurement speed is divided into three categories: a measurement time of several seconds (hereinafter referred to as measurement speed category 1), up to approximately 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 distributed 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 distributed 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 used in ultrasonic flaw detection testing to detect delamination in blades. Conventionally, there have been no practical examples of using an optical fiber distributed measurement sensor to detect ultrasonic waves as a measurement target. In this application, ultrasonic waves refer to those of measurement speed category 3 above, which are 1 MHz or higher.

[0022] Next, using the model diagram shown in Figure 4, a detection method using optical fiber to detect damage or loosening of bolts that secure the blade or delamination of the blade, which is related to the above-mentioned measurement speed category 3, will be explained below, taking as an example a method using an ultrasonic flaw detector (piezoelectric element).

[0023] The left side of Figure 4 is a schematic diagram showing an ultrasonic flaw detector equipped with a piezoelectric element 6 that generates and receives ultrasonic waves, a bolt 8a that is a test object in which internal damage 9 is measured using ultrasonic waves, a blade 13 and a flange 14 that are fixed by the bolt 8a and nut 8b, and a coupling 15 for transmitting ultrasonic waves to the test object without attenuation. An optical fiber 7 is installed within the coupling 15 to detect generated strain, etc. Typically, the coupling material is a material that has low attenuation when ultrasonic waves are transmitted (e.g., less than 1 dB attenuation per 5 MHz), such as silicone or thermoplastic resin. The symbol F in the figure indicates the (main) propagation direction of the ultrasonic waves.

[0024] The diagram on the right side of Figure 4 shows a schematic diagram of the signal strength of the detected signal relative to the passage of time from the generation of the ultrasonic wave from the piezoelectric element 6, with the signal detection time at the left optical fiber 7 as the reference time (the reference zero position on the time coordinate) when the piezoelectric element 6 receives a reflected signal of the ultrasonic wave generated by the piezoelectric element 6 and detects internal damage 9 in the bolt 8a. In this case, the ultrasonic wave propagates in the direction indicated by arrow F, and the optical fiber 7 mounted inside the coupling material detects (on the time axis) the signal generated inside the object by the ultrasonic wave due to strain generated in the longitudinal direction of the optical fiber due to Poisson's ratio. In Figure 4, the direction perpendicular to the propagation direction of the ultrasonic wave indicated by arrow F corresponds to the longitudinal direction of the optical fiber.

[0025] The curves labeled Wa, Wb, and Wc on the right side of Figure 4 are schematic representations of signals from the object being measured detected by the optical fiber 7 in response to the ultrasonic signal emitted by the piezoelectric element. The Wa, Wb, and Wc represent, in order, the signal detected when the ultrasonic wave first reaches the surface of the object being measured (here, the bolt 8a), the signal detected when the ultrasonic wave reaches the location of the damage 9 inside the object being measured, and the signal detected when the ultrasonic wave is reflected from the bottom surface of the object being measured. The intensity levels of these three signals have the relationship Wa > Wc > Wb, as shown in Figure 4. In parallel with the piezoelectric element receiving the signal generated by the object being measured, the optical fiber 7 installed in the coupling 15 detects a signal corresponding to the reflected wave from the object being measured due to the ultrasonic signal emitted by the piezoelectric element. The signal detected by the piezoelectric element due to damage or the like in the object being measured is primarily used as a verification signal to be compared with when the optical fiber detects a signal due to damage or the like in the object being measured.

[0026] Here, in order to reduce the influence of attenuation of the ultrasonic waves used for measurement, the object to be measured has an upper limit on the measurable thickness depending on the material. For example, the thickness of the coupling 15 is about 6 mm, and if the object to be measured is made of CFRP, it is less than 60 mm, and if the object is made of steel, it is less than 100 mm. Details of the method for detecting ultrasonic waves using the optical fiber 7 will be described below.

[0027] 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 in the layers that make up the wind turbine blades, and the location and size of the delamination can be identified.

[0028] If the bolt is made of steel, the ultrasonic wave speed inside the bolt is approximately 5,000 m / s, and a frequency of 3.3 Msps (discrimination speed of 0.3 μs) corresponds to a bolt length of 1.5 mm. Furthermore, measuring damage to bolts longer than 1.5 mm can be achieved by delaying the timing of the chirped pulse light incidence. While the above describes the detection of ultrasonic waves when there is damage to the bolt, this is not limiting. For example, loosening of the bolt fastening a blade can also be detected using optical fiber, since it can be detected as a change in strain in the optical fiber based on the change in stress caused by loosening the bolt.

[0029] Furthermore, as pointed out in the background art, measurements in the ultrasonic band are required even over long distances of about 30 km, and so it is necessary to develop a measurement method (measurement system) that satisfies these requirements. That is, in conventional DAS technology, there is a limit to the measurement speed (the upper limit sampling rate is 50 Ksps) due to the traveling speed of laser pulse light (hereinafter also simply referred to as pulse light) in optical fiber, and this limit in sampling rate further decreases with long-distance measurements (when the optical fiber length reaches 20 km, which is 10 times longer than usual, the upper limit sampling rate decreases to 5 Ksps, which is 1 / 10 of the above 50 Ksps), which makes ultrasonic measurement difficult. This problem needs to be overcome.

[0030] Therefore, 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, we adopted a method that uses chirped pulsed light in 20 different frequency bands by sweeping the frequency of the pulsed light incident on the optical fiber quickly and over a wide range. The specific details of this method are explained below using diagrams.

[0031] FIG. 5A is a diagram illustrating conventional DAS technology. As shown in this figure, conventional DAS requires that the next pulse of light be injected into the optical fiber after the backscattered light of the pulsed light injected into the optical fiber has finished reflecting, and therefore can only recognize one pulse of light injected into the optical fiber. For example, the time required for pulsed 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 FIG. 5A). In other words, conventional DAS limits the measurement speed to the speed of the pulsed light traveling through the optical fiber (approximately 200,000 km / s).

[0032] Therefore, in this development, taking into consideration the need to measure ultrasound over long distances, frequency-swept chirp pulse light (here, chirp pulse light of a single bandwidth, e.g., 150 MHz to 250 MHz; details will be described later) was used to improve the sampling speed by approximately 66 times that of conventional DAS. Furthermore, chirp pulse light groups, which are collections of several tens of chirp pulse light beams (each of which has a constant bandwidth), were incident on the object to be measured with a time interval between them, thereby enabling long-distance measurements of 30 km. Specifically, the chirp pulse light beams were incident on the optical fiber to be measured with an interval of approximately 300 μs (the time required for the pulse light to travel back and forth through a 30-km-long optical fiber), which corresponds to a distance of 30 km, and the backscattered light returning from this optical fiber was received (see Figure 5B). In addition, the backscattered light returning from the optical fiber was individually received using a bandpass filter. However, if the chirped pulse light groups are received with an interval of about 300 μs between them, the problem arises that the locations on the optical fiber corresponding to the intervals will be missing sections. Therefore, we decided to acquire signals using the method shown in Figure 6.

[0033] That is, a group of chirped pulsed light delayed at regular intervals (here, as shown in Figure 6, by a few microseconds, which is the time for one cycle of ultrasound) is injected into the optical fiber (see the chirped pulsed light group shown in the upper part of Figure 6), and after receiving them individually, all of them are combined. This makes it possible to perform continuous measurements in time without signal interruptions. In other words, this makes it possible to obtain a continuous signal without any missing intervals.

[0034] The key points of the long-distance ultrasonic DAS technology that we have developed have been described above. Below, we will explain an example of specific values ​​that are actually used. When chirp pulse light in 20 different frequency bands is incident on an object to be measured as pulse light to be input into an optical fiber, the backscattered light from the object to be measured is restored and digitally processed. By setting the frequency width when this backscattered light from the object to be measured is restored, each of the original incident signals can be identified and received. 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.

[0035] Then, by controlling the interval at which the chirped pulse light group, which is an aggregate of the chirped pulse light, is targeted, 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 required for the pulse light to travel back and forth over this distance in the optical fiber is 2×30 km / (200,000 km / s)≈300 (μs), assuming that the speed of light in the optical fiber is approximately 200,000 km / s, and therefore the time interval at which the chirped pulse light group is injected into the optical fiber is controlled to approximately 300 (μs).

[0036] Based on the above, the characteristics 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 the 0.3 μs, which is the discrimination speed of each optical pulse in the optical fiber ring circuit described below. However, in this first embodiment, the (delay) control of the incident pulsed light uses not only 0.3 μs but also 300 μs, which is the time interval between each chirp pulse light group. 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.

[0037] Next, a measurement device for realizing the specifications of the above-mentioned long-distance ultrasonic DAS will be described in detail using Figure 8. In Figure 8, an arrow with arrows on both sides indicates that signals are transmitted 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 transmitted and received. Furthermore, the open arrow indicates transmission and reception between this measurement device and the object to be measured.

[0038] 8 is a block diagram showing an example of an optical fiber ultrasonic distribution measurement apparatus 30 that is a measurement apparatus that realizes the specifications of the long-distance ultrasonic DAS. As shown in Fig. 8, the optical fiber ultrasonic distribution measurement apparatus 30 includes a light source module 31 for generating chirp pulse light, transmitting it to a measurement object, and receiving backscattered light from the measurement object, 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 (a CPU module 35 having a processor, memory, etc., not shown), and a PCIe bus 36 (PCIe: Peripheral Component Interconnect Express, a high-speed serial expansion bus standard) for transmitting data at high speed between the above modules and the processor.

[0039] The light source module 31 includes an LD 311 (LD: Laser) that oscillates a laser with a narrow linewidth (e.g., a single-frequency laser) that cannot be directly modulated and has a spectral linewidth of, for example, less than 1 kHz. the optical control module 32, an I / O circuit 312 for transmitting and receiving signals to and from the polarization diversity receiving module 317, an LD 311, an optical control module 32, a chirp pulse light generating module 314 for generating chirp pulses, a beam splitter 313 (also referred to simply as BS 313) for splitting the optical path of the laser light emitted from the narrow linewidth LD 311 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 in order to measure changes in physical quantities in an optical fiber of a predetermined distance using chirp pulses, and an optical coupler 316 for irradiating the chirp pulse light to the object under test and sending backscattered light from the object under test to the polarization diversity receiving module 317.

[0040] Furthermore, by increasing the sampling speed to 3.3 Msps, it is possible to detect damage in bolts up to 3 mm in length using ultrasonic waves (for example, ultrasonic velocity 5000 m / s = 5 mm / μs) with a continuous observation time of 6 μs, as described below. Furthermore, by using the ring chirp interval and delay control module 33 (with a control precision of 10 ns for delay timing) to delay the timing of incidence of the chirp pulse light on the object to be measured, the observation time of ultrasonic propagation is shifted, making it possible to detect damage in bolts of a length greater than 3 mm.

[0041] Next, a specific example of a measurement device that realizes the above-mentioned optical fiber ultrasonic distribution measurement device 30 is shown in Fig. 9. Here, specific examples of the two most distinctive components of this measurement device, the chirped pulse light generating module 314 and the optical fiber ring circuit 315 in Fig. 8 described above, will be described below using Fig. 9.

[0042] 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 generates a chirp pulse and is directed toward a board on which an AOM (Acousto-Optics Modulator; the same applies below) and a driver circuit are mounted, as shown in Figure 9. However, the LD has a narrow linewidth and cannot be directly modulated. Therefore, the laser light is modulated by a board on which an AOM and a driver circuit are mounted, as explained below.

[0043] The AOM generally controls and modulates the intensity of light at a speed faster than a mechanical shutter by repeatedly turning it on and off at a specified frequency (e.g., 100 MHz shown in the figure). This allows it to continuously generate multiple pulsed beams with a constant frequency difference. This control is performed by a Linear Frequency Modulator (LFM, or Chirp) using a Direct Digital Synthesizer (DDS) and a driver circuit, as shown in FIG. 9. Specific values ​​used here are, for example, a constant frequency difference Δf = 100 MHz and a time interval of 300 μs (the interval between each chirp pulsed beam group). This concludes the description of a specific example of the chirp pulsed beam generating module 314.

[0044] Next, a specific example of the optical fiber ring circuit 315 will be described. Using the chirped pulse light generated under 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 chirped pulse lights at a fixed time interval τ (here, τ = 300 ns) in order to generate chirps. As shown in the figure, this optical frequency loop circuit includes an AOM similar to that used in the chirped pulse light generating module 314, as well as a polarization beam splitter (P-BS) and an erbium-doped fiber amplifier (EDFA) for signal flattening.

[0045] In this case, if the ring length of the optical frequency loop circuit is set to 60 m corresponding to a predetermined distance (e.g., 30 km), changes in physical quantities such as strain corresponding to positions within the same optical fiber can be detected using the chirped pulse light at a discrimination speed of 0.3 μs, which corresponds to a sampling speed of 3.3 Msps.

[0046] Next, an example of chirp pulse light incident on a measured object using the above-described measurement device and a specific example of a signal related to the backscattered light of this chirp pulse light will be described with reference to FIG. 10. The upper part of FIG. 10 shows an example of a signal whose frequency sweep range (also called the chirp frequency sweep range) is controlled by a 100 MHz AOM output. The lower part of FIG. 10 shows a specific example of a total of 20 signals that have been bandpass filtered at 100 ksps on the receiving side. Note that the accuracy of delay control in this case was 10 ns.

[0047] First, an example of an output pattern of optical output (chirped pulse light incident on a measured object) will be specifically described below with reference to Fig. 10. This optical output pattern is achieved by sweeping the frequency using a 100 MHz AOM, with 150 MHz set as the first frequency, and then sweeping in increasing order through six different frequencies (also called chirp frequencies) from 150 MHz to 650 MHz in 100 MHz increments. Each chirped pulse light constitutes a set, and the final set is a set ranging from 1650 MHz to 2150 MHz, resulting in a total of 20 chirped pulse light sets (this set is the chirped 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 chirped light pulse group (from 150 MHz to 2150 MHz) to the next chirped light pulse group is 300 μs (the time it takes for the pulse light to travel back and forth through a 30 km long optical fiber).

[0048] Next, the signal pattern on the receiving side will be described. A 2 GHz (= 100 MHz x 20) receiver is used on the receiving side, corresponding to the number of chirp signals in the 100 MHz restoration band. In this case, a total of 20 chirp signals are observed at a discrimination speed of 0.3 μs, meaning that the target signal can be observed within a 6 μs observation time window. Furthermore, the receiving side uses a band-pass filter to individually discriminate the 20 chirp signals.

[0049] As described above, according to the optical fiber ultrasonic distributed measurement device of the first embodiment, by combining two characteristic times, namely, the interval time between adjacent chirp pulse light beams and the interval time between chirp pulse light groups, it is possible to realize a device that can achieve both long distances 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. Furthermore, by using this optical fiber ultrasonic distributed measurement device, it is possible to provide a device or system that can constantly monitor damage to the blades or bolts of offshore wind turbines installed several tens of kilometers away.

[0050] <Verification by Numerical Simulation> The above-mentioned long-distance ultrasonic DAS technology was verified by numerical simulation, and it was confirmed that it is feasible. The verification by this numerical simulation will be described in detail below.

[0051] 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 performed. 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 considered that this will be a verification of the case where the discrimination speed of the optical pulse of this application is 0.3 μs.

[0052] First, 50 consecutive measurements are performed at a measurement interval of 1 Ksps using a conventional chirp pulse with a duration of 2 μs (chirp frequency of 100 MHz) (step S1). Next, the received beat signal is processed and passed through eight matched filters with 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 (band1): 145.0 MHz to 157.5 MHz ・Band 2 (band2): 157.5 MHz to 170.0 MHz ・Band 3 (band3): 170.0 MHz to 182.5 MHz ・Band 4 (band4): 182.5 MHz to 195.0 MHz ・Band 5 (band5): 195.0 MHz to 207.5 MHz ・Band 6 (band6): 207.5 MHz to 220.0 MHz ・Band 7 (band7): 220.0 MHz to 232.5 MHz ・Band 8 (band8): 232.5 MHz to 245.0 MHz Here, the interval between adjacent bands in each divided band is 2 μs / 8 = 0.25 μs, which translates to a difference in distance 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, which is the sum of this 25 m and the distance difference between the divided signals of Band 2 and Band 3 (25 m). Next, the division process for the eight bands is performed, i.e., a distance shift is performed to correct the distance difference for each divided signal (step S3). The signal data after the distance shift process is measured at a speed of 1 Ksps, i.e., 50 times with a period T = 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 at 0.25 μs intervals (step S4). The above signal processing enables resampling at a speed of 4 Msps (T = 0.25 μs). Specifically, when considering 50 consecutive measurements at a measurement speed of T = 1002 μs, using a chirp pulse with a time width of 2 μs enables resampling at intervals of 2 μs × 50 = 100 μs, which makes possible pseudo-continuous measurements of 4 Msps (successive measurements at 0.25 μs intervals).In this case, the spatial resolution is about 8 m (Δf=12.5 MHz), which allows overlapping 10 kHz signals to be connected.

[0053] Next, the above method will be explained in detail below using Figures 11A to 17C. Of these figures, Figures 11A to 12C show the signal processing performed on the received beat signal for each processing step described above, and are used to explain a method of resampling a 10 kHz sine wave at 4 Msps with a sampling rate 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 rate of 1 Ksps and resampling at 4 Msps, it is possible to measure one 10 kHz period.

[0054] First, let us consider Figures 11A to 12C. These figures explain the principle of the above-mentioned resampling method based on the received beat signal. Figure 11A shows a beat signal measured using 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 in Figure 11A represents 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 contains frequency components between 145.0 MHz and 245.0 MHz.

[0055] Next, Figure 11B shows an example of dividing the beat signal in the optical fiber ultrasonic distribution measurement device of the first embodiment at a constant bandwidth (12.5 MHz) (see step S2 above), showing the results of dividing each of the 50 beat signals measured above into a total of eight bands with an interval of 0.25 μs between bands. As shown in Figure 11B with 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 indicated by N = m in Figure 11A. In this case, a constant distance shift occurs in each band from band 2 to band 8 shown within each DN = m (the difference in distance shift between adjacent bands is a constant 25 m).

[0056] Next, we will explain Figures 12A to 12C. Figure 12A is a reprint of Figure 11B (part thereof). Figure 12B shows the results of performing a fixed distance shift process for each DN=m band in order to eliminate the distance deviation that occurred in each band of Figure 12A. From this figure, it can be seen that the distance shift amount in each band between adjacent DN=m (for example, between DN=1 and DN=2) follows the same pattern, and this is repeated in a cycle 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 cycle, the cycle is changed from 1002 μs to 0.25 μs for each DN=m, and a process of reconstruction (resampling) is performed on the time axis. The results are shown in Figure 12C.

[0057] 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). With time on the horizontal axis and signal level (arbitrary scale) on the vertical axis, it can be seen that one 10 kHz period can be measured in 50 periods. The sampling rate in this case is 4 Msps, which corresponds to 0.25 μs.

[0058] Next, we will explain an example of the results of analyzing 10 kHz test data using the above simulation method using a PZT (piezoelectric element). 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 is observed at a point of 499.8 m. Here, the horizontal axis represents time (unit: μs) and the vertical axis represents distance (unit: m).

[0059] Fig. 14B shows the analysis results of the strain rate (unit: nε / μs) of the object under test due to the vibration of the piezoelectric element in the eight bands (bands 1 to 8), including the results described above. The horizontal axis represents time (unit: μs), as in Fig. 14A.

[0060] Next, Fig. 15A shows the analysis results (hereinafter referred to as "raw data") of the vibration waveform when all eight bands are aligned (combined) rather than the measurement results for each band (the horizontal axis scale is the same as in Fig. 14B). Looking at the overall shape, 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). Fig. 15B confirms the 10 kHz vibration component of the piezoelectric element.

[0061] Next, the raw data (see FIG. 16A, the same as the data in FIG. 15A) was passed through a low-pass filter (LPF) with a cutoff frequency of 15 kHz to remove high-frequency noise. The result is shown in FIG. 16B. As shown in the figure, the time difference between the vibration peak waveforms is 100 μs, and it can be said that the 10 kHz vibration waveform is expressed.

[0062] Furthermore, based on the 10 kHz vibration waveform of Figure 16B (here, the same vibration waveform as Figure 16B is shown as Figure 17A), Figures 17B and 17C show the vibration waveform with the scale of the time axis enlarged. The time axis scale shown in Figure 17B is about 10 times that of Figure 17A, and the time axis scale shown in Figure 17C is about 400 times that of Figure 17A. When the time axis scale is enlarged to Figure 17C, it can be seen that the 10 kHz analyzed point (0.25 μs, corresponding to the above-mentioned identification speed), indicated by a circle in the graph, which was not clear in Figures 17A and 17B, is clearly shown.

[0063] As described above, 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 confirmed by simulation.

[0064] Although exemplary embodiments are described in this application, the various features, aspects, and functions described in the embodiments are not limited to the application of a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless variations not illustrated are anticipated within the scope of the technology disclosed in this specification. For example, variations in, addition to, or omission of at least one component are included.

[0065] 1 Blade (wing), 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

1. An LD, a chirped pulse light generation module that controls the frequency sweep range of the laser light emitted from the LD to generate a plurality of chirped pulse lights, a ring length of a predetermined length, and the chirped pulse light generated by the chirped pulse light generation module is input, and a fiber optic ring circuit that outputs a chirped pulse light group that is an aggregate of chirped pulse lights with a prescribed sweep time interval, and a polarization diversity reception module that receives the backward scattered light from the optical fiber generated when the chirped pulse light group emitted from the fiber optic ring circuit is incident on the optical fiber as the object to be measured, and the laser light from the LD, A light source module having, A ring chirp interval and delay control module that controls the sweep time interval of the chirped pulse light and performs control to delay the timing at which the chirped pulse light group is incident on the optical fiber by a fixed time, Comprising, A plurality of chirped pulse lights having different sweep frequencies generated by the chirped pulse light generation module are received and restored from the same number of backward scattered lights from the optical fiber generated when incident on the optical fiber, and when performing restoration processing, by defining the frequency bandwidth of the received signal, each chirped pulse light incident on the optical fiber is identified and received, and an optical fiber ultrasonic distribution measurement device characterized by performing distribution measurement of the ultrasonic waves detected by the optical fiber.

2. The sweep time interval of the chirped pulse light is determined corresponding to the frequency of the signal detected by the optical fiber, and the fixed delay time for determining the incident timing of the chirped pulse light group is determined by the length of the optical fiber. The optical fiber ultrasonic distribution measurement device according to Claim 1.

3. The fiber optic ring circuit has an acousto-optic element for generating chirped pulses by delaying a plurality of chirped pulse lights in a predetermined frequency band at regular time intervals, and the ring length of the fiber optic ring circuit is set to a predetermined length corresponding to the discrimination speed of the chirped pulse light in the optical fiber. The optical fiber ultrasonic distribution measurement device according to Claim 1 or Claim 2.

4. A method for measuring the distribution of ultrasonic waves in an optical fiber using the optical fiber ultrasonic wave distribution measuring apparatus according to claim 1 or 2, which measures ultrasonic waves contained in the backward scattered light from the optical fiber, wherein the frequency of the chirp pulse light incident on the optical fiber is swept within a range of a predetermined high-speed and wide-range sampling speed to generate different chirp pulse lights in a plurality of frequency bands, and when restoring and digitally processing the backward scattered light of the chirp pulse light after the generated chirp pulse light is incident on the optical fiber, the frequency width at the time of restoration is set to a specified frequency width, thereby identifying and receiving each of the incident chirp pulse lights. A method for measuring the distribution of ultrasonic waves in an optical fiber is characterized by this.

5. A method for measuring the distribution of ultrasonic waves in an optical fiber using the optical fiber ultrasonic wave distribution measuring apparatus according to claim 3, which measures ultrasonic waves contained in the backward scattered light from the optical fiber, wherein the frequency of the chirp pulse light incident on the optical fiber is swept within a range of a predetermined high-speed and wide-range sampling speed to generate different chirp pulse lights in a plurality of frequency bands, and when restoring and digitally processing the backward scattered light of the chirp pulse light after the generated chirp pulse light is incident on the optical fiber, the frequency width at the time of restoration is set to a specified frequency width, thereby identifying and receiving each of the incident chirp pulse lights. A method for measuring the distribution of ultrasonic waves in an optical fiber is characterized by this.

6. The method for measuring the distribution of ultrasonic waves in an optical fiber according to claim 4, characterized in that the incident timing of the chirp pulse light on the optical fiber is delayed by the ring chirp interval and delay control module, and a continuous signal is formed on the time axis at a period corresponding to the specified frequency width.

7. The method for measuring the distribution of ultrasonic waves in an optical fiber according to claim 5, characterized in that the incident timing of the chirp pulse light on the optical fiber is delayed by the ring chirp interval and delay control module, and a continuous signal is formed on the time axis at a period corresponding to the specified frequency width.