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

Technical Field

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

Background Art

[0002] With the expansion of the scale of offshore wind power generation, inspections and maintenance of a large number of offshore wind turbines are required. Currently, inspections related to the inspection and maintenance of wind turbines rely on manual visual inspections and impact sound inspections, and low-cost and highly reliable inspection and maintenance are demanded. Here, investigations on offshore wind power generation in the Japanese coastal waters have been conducted under the condition of an offshore distance within 30 km. In order to correspond to the installation locations of future offshore wind power generation facilities in Japan, a measurement distance of 30 km is required (see, for example, Non-Patent Document 1).

[0003] Therefore, specifically, it is considered that one effective method for solving the above problems is to use ultrasonic measurement technology that can detect damage to wind turbine blades installed at positions more than 30 km away or loosening of bolts fixing the wind turbine blades, and enable constant monitoring of these.

[0004] By the way, the limit of the measurement speed of conventional DAS (DAS: distributed acoustic sensing) is due to the propagation speed of pulsed light in the optical fiber, and it is impossible to incident the next pulse until the incident pulsed light returns from the end of the optical fiber to the measurement device side (see, for example, Non-Patent Document 2). For example, since the time taken for pulsed light to travel back and forth through a 2-km optical fiber is about 20 μs, the upper limit of the sampling speed that can be measured is 50 Ksps (sps: samples / second) obtained by converting the 20-μs cycle into the sampling speed of sound waves, and it cannot follow the speed of ultrasonic inspection with a larger sampling speed.

[0005] Similarly, as the measurement distance of the optical fiber increases, the maximum measurable sampling rate also decreases. When the optical fiber length becomes 20 km, which is 10 times the normal size, the maximum measurable sampling rate also becomes 5 Ksps, which is 1 / 10th of the normal length, making ultrasonic measurement using optical fiber over long distances difficult.

[0006] Conventionally, as a means of monitoring and detecting damage to structures, FBG sensors (FBG: Fiber Bragg Gratings) have been used as optical fiber sensors. In some cases, a hybrid system of this FBG sensor and a piezoelectric element (PZT) has been incorporated into the structure to be measured, and elastic waves have been measured to identify the presence, location, or size of damage such as delamination in the structure (including the length of delamination). 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 provides a guideline for calculations using the velocity of elastic waves. If calculated using the elastic wave velocity Ve = 5 mm / μs in steel, the measurement position accuracy at f = 1 MHz is approximately 2.5 mm. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Nagai et al., "Expected recoverable yield of offshore wind power generation in coastal waters of Japan," Wind Energy, Vol. 34, No. 1, pp. 103-112, 2010. [Non-Patent Document 2] 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 [Non-Patent Document 3] Takeda et al., "Application of Optical Fiber Sensors to Aircraft Structural Health Monitoring Technology," Measurement and Control, Vol. 51, No. 3, pp. 234-240, 2012. [Non-Patent Document 4] https: / / www.kyowa-ei.com / jpn / product / sector / energy / appli-cation_150.html [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2014-194379 [Overview of the project] [Problems that the invention aims to solve]

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

[0010] The optical fiber ultrasonic distribution measurement apparatus disclosed herein is A LD, a chirp pulse light generation module that controls the frequency sweep range of the laser light emitted from the LD to generate a plurality of chirp pulse light beams, an optical fiber ring circuit having a predetermined ring length, which receives the chirp pulse light beams generated by the chirp pulse light generation module and outputs a chirp pulse light group, which is a collection of chirp pulse light beams with a defined sweep time interval, and a polarization diversity receiving module that receives the backscattered light from the optical fiber and the laser light from the LD when the chirp pulse light group emitted from the optical fiber ring circuit is incident on the optical fiber under test. A light source module having A ring chirp interval and delay control module that controls the sweep time interval of the chirp pulse light and controls the timing of the chirp pulse light group entering the optical fiber to be delayed by a certain period of time. Equipped with, The present invention is characterized by receiving and reconstructing the same number of backscattered light beams from the optical fiber as the number of chirp pulse beams generated when multiple chirp pulse beams with different sweep frequencies, generated by the chirp pulse beam generation module, are incident on the optical fiber, and by defining the frequency bandwidth of the received signal during the reconstruction process, each chirp pulse beam incident on the optical fiber is identified and received, and the distribution measurement of ultrasonic waves detected by the optical fiber is performed. [Effects of the Invention]

[0011] The optical fiber ultrasonic distribution measurement device disclosed in this application enables both long-distance measurement and high temporal resolution by combining two characteristic times: the interval time between adjacent chirp pulse light beams and the interval time between chirp pulse light beam groups. Furthermore, by controlling the time interval at which chirp pulse light beam groups are incident on the object to be measured, the device can be provided that allows for variable length of the object to be measured while maintaining the sampling rate of the incident light. [Brief explanation of the drawing]

[0012] [Figure 1] This is a model diagram showing an example of application of the optical fiber ultrasonic distribution measurement device according to Embodiment 1. [Figure 2] This figure shows a comparison table of wind turbine blade damage monitoring methods to which the optical fiber ultrasonic distribution measurement device according to Embodiment 1 is applied. [Figure 3] This diagram is a table that categorizes the measurement techniques used when monitoring wind turbine blades. [Figure 4] This diagram illustrates a method for detecting damage to an object under inspection using optical fibers. [Figure 5] This is a diagram for explaining the accelerated DAS technology used in the optical fiber ultrasonic distribution measurement device according to Embodiment 1. [Figure 6] This is a diagram for explaining a method of obtaining the backscattered light of a chirp pulse light used in the optical fiber ultrasonic distribution measurement device according to Embodiment 1. [Figure 7] This is a diagram for explaining the specifications of DAS used in the optical fiber ultrasonic distribution measurement device according to Embodiment 1. [Figure 8] This is a block diagram showing an example of the optical fiber ultrasonic distribution measurement device according to Embodiment 1. [Figure 9] This is a diagram showing a specific example of the optical fiber ultrasonic distribution measurement device according to Embodiment 1. [Figure 10] This is a diagram for explaining a specific example of the chirp pulse light incident on the object to be measured by the optical fiber ultrasonic distribution measurement device according to Embodiment 1, and the backscattered light of this chirp pulse light. [Figure 11] This is a diagram for explaining the signal processing when the beat signal after receiving the backscattered light of the chirp pulse light is divided into a plurality of bands by numerical simulation. [Figure 12] This is a diagram for explaining the signal processing of reconstructing on a shorter sampling period after correcting the distance shift of the signals divided into a plurality of bands. [Figure 13] This is a diagram for explaining the result when measuring a 10 KHz signal at 1 Ksps as a specific example of signal processing. [Figure 14] This is a diagram showing an example of the result of the vibration waveform obtained by analyzing the ultrasonic test data using a piezoelectric element and dividing it into eight bands used in the simulation. [Figure 15] This is a diagram showing the analysis result of the vibration waveform when analyzing the ultrasonic test data with the eight bands used in the simulation aligned. [Figure 16] This is a diagram showing the result when the vibration waveform, which is the ultrasonic test data, is passed through a low-pass filter to remove high-frequency noise. [Figure 17]This figure shows the vibration waveform with an enlarged time axis scale. [Modes for carrying out the invention]

[0013] This application primarily relates to a maintenance and inspection device using optical fibers, which is used in remote monitoring technology for the maintenance and inspection of wind turbines for offshore wind power generation. The contents of this application will be described below with a typical embodiment as an example.

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

[0015] Here, a sensing optical fiber cable 5, for example, using a single-mode optical fiber, is installed at a predetermined position on the blade 1 (see Figure 1B), which can continuously measure the temperature or strain distribution at a predetermined position on the blade 1 of the wind turbine 10. The sensor measurement signal from 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 Figure 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 temperature or strain at the predetermined position on the blade 1. Furthermore, the signals (wires) between the rotating and stationary parts of the wind turbine can be connected using an optical rotary joint. Although there is relative rotation between the nacelle and the tower, the rotation angle is within 180 degrees, so its effect can be absorbed by the flexibility of the cable. In this embodiment 1, an optical fiber ultrasonic distribution measurement device 30 (described in detail below) using ultrasonic measurement was applied as the optical fiber signal processing device.

[0016] Based on the above detection results, it is possible to detect damage to wind turbine blades located approximately 30 km away from the wind turbine's operation and management equipment, or loosening of bolts that secure blade 1 to hub 2. Conventionally, methods for monitoring damage to wind turbine blades (hereinafter also simply referred to as blades) are known to include manual visual inspection and measurement methods using FBG as an optical fiber (not continuous measurement) to measure deformation or damage to wind turbine blades (see, for example, Non-Patent Document 4).

[0017] Figure 2 is a table comparing the blade damage monitoring method described above with a conventional example. This table shows three representative monitoring methods for comparison. Conventional representative methods include manual inspection and the method using FBG as the optical fiber sensor (hereinafter referred to as the FBG method). Figure 2 shows a comparison of these methods with the present invention, categorized by monitoring content.

[0018] Conventional methods, firstly, rely on manual monitoring, making continuous monitoring difficult as it is limited to periodic checks. Furthermore, while ultrasonic testing is used for issues that cannot be visually confirmed, such as delamination of blades, inspecting the blades of offshore wind turbines presents cost challenges. Additionally, the FBG (Fiberglass Generator) method is insufficient for monitoring blade deformation and damage from a commercial standpoint, and measuring delamination and bolt loosening is difficult.

[0019] On the other hand, the method of this application can adequately handle any of the monitoring requirements, including continuous monitoring, monitoring of blade deformation and damage, and monitoring of delamination and bolt loosening. The following describes the measurement techniques used in the monitoring system of this application. In particular, regarding the system that uses optical fiber distributed measurement sensors to monitor blade deformation and damage, and interlaminar delamination and bolt loosening, we will focus on the ultrasonic DAS method used for monitoring interlaminar delamination and bolt loosening. According to this application, no electrical equipment is required for measurements on wind turbines or outdoors, and therefore it is not affected by lightning or bad weather at all, making monitoring possible even in cases where such climatic risks are high.

[0020] In describing the measurement techniques used in the monitoring system, we first organized the characteristics of the measurement techniques used when monitoring wind turbine blades using optical fiber distributed measurement sensors, categorized by measurement speed, and presented them as a measurement technique classification table in Figure 3.

[0021] In Figure 3, measurement speeds are divided into three categories: those with a measurement time of several seconds (hereinafter referred to as measurement speed category 1), those up to approximately 100 kHz (hereinafter referred to as measurement speed category 2), and those above 1 MHz (hereinafter referred to as measurement speed category 3). Typical measurement targets for measurement speed category 1 are static strains such as blade deflection, and the optical fiber distribution measurement used here is DTSS technology (resolution, accuracy, etc. are as shown in Figure 3). Typical measurement targets for measurement speed category 2 are dynamic vibrations and acoustics such as blade deflection, and the optical fiber distribution measurement used here is DAS technology (resolution, accuracy, etc. are as shown in Figure 3). Furthermore, typical measurement targets for measurement speed category 3 are ultrasonic waves used in ultrasonic flaw detection to detect delamination of blades. Conventionally, there are no practical examples of using optical fiber distribution measurement sensors to detect these ultrasonic waves. In this application, ultrasonic waves refer to those above 1 MHz in measurement speed category 3.

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

[0023] The left-hand diagram of Figure 4 shows a configuration diagram of an ultrasonic flaw detector equipped with a piezoelectric element 6 that generates and receives ultrasonic waves, a bolt 8a which is the object to be measured for internal damage 9 etc. by ultrasonic waves, a blade 13 and flange 14 fixed by the bolt 8a and nut 8b, and a coupling 15 for transmitting ultrasonic waves to the object to be measured without attenuation. An optical fiber 7 is installed inside the coupling 15 to detect the strain that occurs. Furthermore, typically, coupling materials such as silicon and thermoplastic resins are used, which have low attenuation when transmitting ultrasound (for example, those with an attenuation of less than 1 dB per 5 MHz). In the figure, the symbol F indicates the (main) propagation direction of ultrasound.

[0024] Furthermore, the right-hand diagram of Figure 4 schematically shows the signal intensity of the detected signal when damage 9 inside the bolt 8a is detected by the piezoelectric element 6 on the left above, receiving the reflected signal from the object under test of the ultrasonic waves generated by the piezoelectric element 6. The signal detection time at the position of the optical fiber 7 on the left above is used as the reference time (reference zero position of the time coordinate), and the signal intensity is shown in relation to the time elapsed since the ultrasonic waves were generated from the piezoelectric element 6. In this case, the ultrasonic waves propagate in the direction indicated by arrow F, and the optical fiber 7 placed inside the coupling material experiences strain in the longitudinal direction of the optical fiber due to the relationship of Poisson's ratio, thereby detecting the signal generated inside the object under test (on the time axis) by the ultrasonic waves. Note that in Figure 4, the direction perpendicular to the direction in which the ultrasonic waves propagate, indicated by arrow F above, corresponds to the longitudinal direction of the optical fiber.

[0025] Furthermore, the curves labeled Wa, Wb, and Wc in the right-hand diagram of Figure 4 schematically show the signals from the object under test detected by the optical fiber 7 due to the ultrasonic signal emitted by the piezoelectric element. The labels Wa, Wb, and Wc represent, in order, the signal detected when the signal first reaches the position (surface) of the object under test (in this case, the bolt 8a), the signal detected when it reaches the location of the internal damage 9 of the object under test, and the signal detected when it is reflected from the bottom surface of the object under test, respectively. Furthermore, as shown in Figure 4, the intensity levels of these three signals are in the relationship Wa > Wc > Wb. In parallel with the reception of the signal generated by the piezoelectric element from the object under test, the optical fiber 7 installed within the coupling 15 detects the signal corresponding to the reflected wave from the object under test due to the ultrasonic signal emitted by the piezoelectric element. The signal detected by the piezoelectric element due to damage to the object under test is mainly used as a verification signal for comparison when the optical fiber detects a signal due to damage to the object under test.

[0026] Here, from the perspective of minimizing the effect of attenuation of the ultrasonic waves used for measurement, the above-mentioned objects to be measured have an upper limit on their measurable thickness, depending on their material. For example, the thickness of the coupling 15 is about 6 mm, less than 60 mm if the object to be measured is made of CFRP, and less than 100 mm if it is made of steel. Details of the ultrasonic detection method using the optical fiber 7 will be explained below.

[0027] Furthermore, delamination of the blades can also be detected using the optical fiber 7. This detection method involves, for example, investigating the generation and propagation characteristics of elastic waves such as Lamb waves in the blade by comparing measured values ​​using the optical fiber with numerical analysis. Based on the results, the location and size of the delamination can be determined. For example, by using broadband Lamb waves, delamination of the layers forming the wind turbine blade can be detected, and the location and size of the delamination can be identified.

[0028] Furthermore, if the bolt is made of steel, the ultrasonic velocity inside the bolt is approximately 5000 m / s, and a frequency of 3.3 Msps (recognition speed of 0.3 μs) corresponds to a bolt length of 1.5 mm. In addition, for measuring damage to bolts longer than 1.5 mm, this can be addressed by delaying the incidence timing of the chirp pulse light. The above describes the detection of ultrasonic waves when there is damage to a bolt, but it is not limited to this. For example, loosening of the bolt's tightening of a blade can also be detected by optical fiber, as it is possible to detect this as a change in strain in the optical fiber based on the change in stress when the bolt tightening loosens.

[0029] Furthermore, as pointed out in the background technology section, measurements in the ultrasonic frequency band are necessary even over long distances of around 30 km, so it is necessary to develop a measurement method (measurement system) that satisfies these requirements. In other words, conventional DAS technology has limitations in measurement speed due to the propagation speed of laser pulsed light (hereinafter also simply referred to as pulsed light) in optical fibers (the upper limit sampling rate is 50 Ksps), and this sampling rate limit further decreases with long-distance measurements (when the optical fiber length becomes 20 km, which is 10 times the normal length, the upper limit sampling rate decreases to 5 Ksps, which is 1 / 10 of the above 50 Ksps), making ultrasonic measurement difficult. Therefore, it is necessary to overcome this problem.

[0030] Therefore, in order to realize an optical fiber ultrasonic distributed measurement device that enables the above ultrasonic measurement, it is necessary to increase the speed of conventional DAS technology. For this reason, in this development, a method was adopted that uses chirp pulsed light with 20 different frequency bands by rapidly and widely sweeping the frequency of the pulsed light incident on the optical fiber. The specific details of this will be explained below with reference to a diagram.

[0031] Figure 5A is a diagram illustrating conventional DAS technology. As shown in this diagram, in conventional DAS, it is necessary to wait until the reflection of the backscattered light of the pulsed light incident on the optical fiber has finished before the next pulsed light is incident on the optical fiber. Therefore, only one pulsed light incident on the optical fiber can be recognized at a time. For example, the time required for a pulsed light to travel back and forth through a 2km long optical fiber is approximately 20μs, so the upper limit of the measurable speed is limited to this value (see Figure 5A). In other words, in conventional DAS, the measurable speed is limited to the speed of the pulsed light traveling through the optical fiber (approximately 200,000 km / second).

[0032] Therefore, in this development, considering the need to measure ultrasound over long distances, the sampling rate was improved to approximately 66 times that of conventional DAS by using frequency-swept chirp pulse light (here, this refers to chirp light of one bandwidth; for example, a bandwidth of 150 MHz to 250 MHz. Details will be described later). In addition, the measurement was made possible over a long distance of 30 km by injecting the object under measurement with (time) intervals between groups of chirp pulse light, which are collections of several tens of the above chirp pulse light (here, each bandwidth is taken as an example). Specifically, the pulse light was injected into the optical fiber under measurement at intervals of approximately 300 μs (the time required for 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). Furthermore, the backscattered light returning from the optical fiber was individually received by a bandpass filter. However, if the signal is received with an interval of approximately 300 μs between each chirp pulse light group, a problem arises where the location on the optical fiber corresponding to the interval becomes a missing section. Therefore, the signal is acquired using the method shown in Figure 6.

[0033] Specifically, groups of chirp pulsed light, delayed at regular intervals (in this case, by a few microseconds, the duration of one ultrasonic cycle, as shown in Figure 6), were injected into the optical fiber (see the chirp pulsed light groups shown in the upper part of Figure 6). These were then received individually and combined. This method enabled continuous measurement without signal interruption. In other words, this method made it possible to acquire a continuous signal without any missing sections.

[0034] The key points of the developed long-range ultrasonic DAS technology are as described above. Below, we will explain some examples of specific numerical values ​​that will actually be used. When 20 different frequency bands of chirp pulsed light are incident on the object under test as pulsed light to be incident on the optical fiber, the backscattered light from the object under test is recovered and digitally processed. By setting the frequency width, each of the original incident signals is identified and received. By using a 2GHz receiver corresponding to the number of chirp signals in the 100MHz recovery band, the sampling rate can be increased to 3.3Msps, approximately 66 times that of 50Ksps.

[0035] Furthermore, by controlling the interval at which the chirp pulse light group, which is a collection of the chirp pulse light described above, is incident on the collection, the measurable optical fiber length can be varied while maintaining the sampling rate. Specifically, if the total length of the optical fiber is 30 km, the time required for the pulse light to travel this distance round trip in the optical fiber is 2 × 30 km / (200,000 km / s) ≈ 300 (μs), assuming the speed of light in the optical fiber is approximately 200,000 km / s. Therefore, the time interval at which the chirp pulse light group is incident on the optical fiber is controlled to approximately 300 (μs).

[0036] Based on the above, the characteristics of the developed long-range 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 that of conventional DAS. This value of 3.3 Msps corresponds to the identification speed of each optical pulse in the optical fiber ring circuit, which will be described later, which is 0.3 μs. However, in this embodiment 1, in addition to 0.3 μs, the (delay) control of the incident pulse light is also performed using 300 μs, which is the time interval between each chirp pulse light group. In addition, the optical fiber ultrasonic distribution measurement device of this application satisfies the following specifications, including the values ​​mentioned above. First, the spatial resolution is 1m to 2m, the sampling rate is 2Ksps to 3.3Msps, and the measurable range is 500m to 30km.

[0037] Next, the measurement device for realizing the specifications of the long-range ultrasonic DAS described above will be explained in detail using Figure 8. In Figure 8, arrows with arrows on both sides indicate that signals are transmitted and received in both directions, while arrows with an arrow on only one side indicate that signals are sent only in the direction indicated by the arrow. The thicker the arrow line, the faster the data transmission and reception is possible. In addition, the white arrows represent the transmission and reception between this measurement device and the object being measured.

[0038] Figure 8 is a block diagram showing an example of an optical fiber ultrasonic distributed measurement device 30, which is a measurement device that realizes the specifications of the long-distance ultrasonic DAS described above. As shown in Figure 8, the optical fiber ultrasonic distributed 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 pulses; a ring chirp interval and delay control module 33 for controlling the time axis control of the chirp pulse signal or the time delay of the received signal with high precision 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: abbreviation for Peripheral Component Interconnect Express; high-speed serial expansion bus standard) for high-speed data transmission between each of the above modules and the above processor.

[0039] Here, the light source module 31 is an LD311 (LD: Laser) that cannot be directly modulated and oscillates a laser with a narrow linewidth, for example, less than 1 kHz in its spectral linewidth (for example, a single-frequency laser). The system includes a diode (laser diode), a polarization diversity receiving module 317 that receives and processes only specific polarizations, an LD311, an optical control module 32, and an I / O circuit 312 that transmits and receives signals with the polarization diversity receiving module 317, etc., a chirp pulse light generation module 314 for generating chirp pulses, a beam splitter 313 (also called BS313 for short) that splits the optical path of the laser light emitted from the narrow-linewidth LD311 and sends it to the chirp pulse light generation module 314 and the polarization diversity receiving module 317, an optical fiber ring circuit 315 for making the laser light circulate in order to measure the change of a physical quantity in an optical fiber over a predetermined distance using chirp pulses, and an optical coupler 316 for causing chirp pulse light to be incident on the object to be measured and sending the backscattered light from the object to be measured to the polarization diversity receiving module 317.

[0040] Furthermore, by increasing the sampling rate to 3.3 Msps, it is possible to detect damage to bolts up to 3 mm in length using ultrasound (for example, an ultrasonic velocity of 5000 m / s = 5 mm / μs), as the continuous observation time is 6 μs, as described later. In addition, by using the ring chirp interval and delay control module 33 (with a delay timing control accuracy of 10 ns), the observation time of ultrasonic propagation is shifted by delaying the incidence timing of the chirp pulse light onto the object under measurement, making it possible to detect damage to bolts longer than 3 mm.

[0041] Next, Figure 9 shows a specific example of a measurement device that realizes the above-described optical fiber ultrasonic distribution measurement device 30. Here, in particular, specific examples of the two components that are the most distinctive components of this measurement device, namely the chirp pulse light generation module 314 and the optical fiber ring circuit 315 shown in Figure 8 above, will be explained below using Figure 9.

[0042] The laser beam emitted from the LD used for DAS measurement is split into two by a deflection beam splitter. One of these laser beams generates a chirp pulse and is directed towards the board on which the AOM (Acousto-Optics Modulator; the same applies hereafter) and drive circuit are mounted, as shown in Figure 9. However, the LD has a narrow linewidth and cannot be modulated directly. Therefore, the laser beam is modulated on the board on which the AOM and drive circuit are mounted, as described below.

[0043] The above AOM can generally control and modulate the intensity of light at a speed exceeding that of a mechanical shutter by repeatedly switching on and off at a specified frequency (here, for example, 100 MHz as shown in the figure), thereby continuously generating multiple pulses of light with a constant frequency difference. This control is performed by a Linear Frequency Modulation (LFM, i.e., Chirp) and drive circuit using a Direct Digital Synthesizer (DDS) as shown in Figure 9. The 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 pulse light group). The above is a description of a specific example of the chirp pulse light generation module 314.

[0044] Next, a specific example of the optical fiber ring circuit 315 will be described. Using the chirp pulse light generated by the control described above, multiple (for example, 20) 100 MHz chirp pulses are generated in the optical frequency circuit (hereinafter also called the optical fiber ring circuit) shown in the figure, delayed by a fixed time interval τ (here τ = 300 ns) to generate chirps. As shown in the figure, this optical frequency circuit uses an AOM similar to the chirp pulse light generation module 314 described above, as well as a polarizing beam splitter (P-BS) and an EDFA (Erbium-Doped Fiber Amplifier) ​​for signal flattening.

[0045] In this case, if the ring length of the optical frequency circuit is set to 60m to correspond to a predetermined distance (for example, 30km), changes in physical quantities such as strain corresponding to a position within the same optical fiber can be detected by the chirp pulse light at a discrimination speed of 0.3μs. This discrimination speed of 0.3μs translates to a sampling rate of 3.3Msps.

[0046] Next, using the above-described measuring device, we will explain an example of chirp pulse light incident on the object under measurement, and a specific example of a signal related to the backscattered light of this chirp pulse light, using 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 a 100MHz AOM output. The lower part of Figure 10 shows a total of 20 specific examples of signals that have been band-pass filtered (BPF) at 100ksps on the receiving side. In this case, the accuracy of the delay control was 10ns.

[0047] First, an example of the output pattern of the optical output (chirp pulse light incident on the object under measurement) will be specifically explained below based on Figure 10. The output pattern of this optical output is created by frequency sweeping with a 100MHz AOM. First, 150MHz is set as the first frequency, and the frequency is swept sequentially from 150MHz to 650MHz in increments of 100MHz, using six different frequencies (also called chirp frequencies). These chirp pulses are configured as one unit, and the final set consists of a total of 20 chirp pulses, ranging from 1650MHz to 2150MHz (this set is the chirp pulse light group explained earlier). The time interval to adjacent sweep frequencies (also simply called the sweep time interval) is set to 300ns. The time from the first chirp pulse light group (150MHz to 2150MHz) to the generation of the next chirp pulse light group is 300μs (the time it takes for pulse light to travel back and forth through a 30km long optical fiber).

[0048] Next, we will explain the signal pattern on the receiving side. On the receiving side, a 2GHz (=100MHz × 20) receiver is used, corresponding to the number of chirp signals in the 100MHz restoration band mentioned above. In this case, since a total of 20 chirp signals are observed with an identification speed of 0.3μs, the target signal can be observed within an observation time window of 6μs. Furthermore, the receiving side uses a band-pass filter (BPF) to individually identify the 20 chirp signals.

[0049] As described above, the optical fiber ultrasonic distribution measurement device of this embodiment 1, by combining two characteristic times—the interval time between adjacent chirp pulse light beams and the interval time between chirp pulse light beam groups—is capable of both long distance and high temporal resolution. Furthermore, by controlling the time interval at which chirp pulse light beam groups are incident on the object to be measured, it is possible to provide a device that can vary the length of the object to be measured while maintaining the sampling rate of the incident light. Furthermore, this optical fiber ultrasonic distribution measurement device can be used to provide a device or system for continuously monitoring damage to offshore wind turbine blades or bolts located tens of kilometers away.

[0050] <Verification through numerical simulation> The long-range ultrasonic DAS technology described above was verified through numerical simulations, confirming its feasibility. The details of this numerical simulation verification are explained below.

[0051] In this numerical simulation, we conducted a simulation of a method to resample a 10 kHz sine wave at a sampling rate of 1 Ksps using a chirp pulse with a time width of 2 μs to a rate of 4 Msps. This method will be explained in detail below using specific numerical values. Since the resampling period at 4 Msps is 0.25 μs, if this value can be confirmed in the simulation, it is considered to be a verification of the case where the discrimination speed of the optical pulse in this invention is 0.3 μs.

[0052] First, 50 consecutive measurements are performed at a measurement interval of 1 Ksps using a conventionally used chirp pulse with a time width of 2 μs (chirp frequency of 100 MHz) (Step S1). Next, the received beat signal is processed and passed through eight matching filters with equal bandwidths of 12.5 MHz, as shown below, to divide the 2 μs beat signal into the following eight bandwidths (step S2). • Band 1: 145.0MHz~157.5MHz Band 2: 157.5MHz~170.0MHz Band 3: 170.0MHz~182.5MHz • Band 4: 182.5MHz~195.0MHz • Band 5: 195.0MHz~207.5MHz Band 6: 207.5MHz~220.0MHz Band 7: 220.0MHz~232.5MHz Band 8: 232.5MHz~245.0MHz Here, the interval between adjacent bands in each divided band is 2μs / 8 = 0.25μs, which translates to a distance difference of 25m. For example, the distance difference between the divided signals of band 1 and band 2 is 25m, and the distance difference between the divided signals of band 1 and band 3 is 50m, which is this 25m plus the distance difference of 25m between the divided signals of band 2 and band 3. Next, the above 8 bandwidth division process is performed, that is, a distance shift is performed to correct the distance difference of each divided signal (step S3). The distance-shifted signal data described above consists of 50 measurements taken at a measurement speed of 1 Ksps, i.e., with a period T = 1002 μs interval. Therefore, to remove the time corresponding to this period difference so that all signal data are connected by signals at 0.25 μs intervals, the data is reconstructed on the time axis (step S4). The above signal processing enables resampling at a speed of 4 Msps (T=0.25 μs). Specifically, considering a case where 50 measurements are taken consecutively at a measurement speed of T=1002 μs, using a chirp pulse with a time width of 2 μs allows for resampling at intervals of 2 μs × 50 = 100 μs. In other words, pseudo-continuous measurement at 4 Msps (continuous measurement at 0.25 μs intervals) becomes possible. In this case, the spatial resolution is approximately 8 m (Δf=12.5 MHz). As a result, overlapping 10 kHz signals can be concatenated.

[0053] Next, the above methods 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 diagrams to explain the method of resampling a 10KHz sine wave at 4Msps with a sampling rate of 1Ksps (period T=1002μs). Figure 13 is a diagram that explains that by measuring a 10KHz sine wave for 50 periods at a measurement rate of 1Ksps using a 2μs chirp pulse and then resampling it at 4Msps, it is possible to measure one period of 10KHz.

[0054] First, let's explain Figures 11A to 12C. These figures illustrate the principle of the resampling method described above, based on the beat signal after reception. Figure 11A shows a beat signal measured using a conventional method. This figure illustrates a method using a chirp pulse with a pulse width of 2 μs, performed at a measurement speed of 1 Ksps (T = 1002 μs ≈ 1 ms), for a total of 50 measurements. In Figure 11A, N represents the number of measurements. Since the measurement period is 1002 μs, if the start time of the first measurement (N=1) is 0 seconds (s), the start time of the 50th measurement (N=50) will be 49.098 ms. Each of these beat signals has components in the frequency range of 145.0 MHz to 245.0 MHz.

[0055] Next, Figure 11B is a diagram showing an example of dividing the beat signal in the optical fiber ultrasonic distribution measurement device of this embodiment 1 into a constant bandwidth (12.5 MHz) (see step S2 above), and shows the results of dividing each of the 50 beat signals measured above into a total of 8 bandwidths with an interval of 0.25 μs between bandwidths. In Figure 11B, as shown by DN=m (where m is an integer from 1 to 50; the same applies hereafter), the beat signal is divided into eight bands between 145.0MHz and 245.0MHz by a matching filter, corresponding to the number of measurements shown as N=m in Figure 11A. In this case, within each DN shown as DN=m, a constant distance shift occurs in each band from band 2 to band 8 (the difference in distance shift between adjacent bands is constant at 25m).

[0056] Next, Figures 12A to 12C will be explained. Figure 12A is a reproduction of (part of) Figure 11B above. Figure 12B shows the result of performing a certain amount of distance shift processing for each band at each DN=m in order to eliminate the distance shift that occurred in each band in Figure 12A. From this figure, it can be seen that the amount of distance shift in each band follows the same pattern between adjacent DN=m (for example, between DN=1 and DN=2), and this is repeated with 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 the data is reconstructed (resampled) on the time axis. The result is shown in Figure 12C.

[0057] Next, Figure 13 shows the results of the above-described processing method when a 10kHz signal is measured at 1Ksps (T=1002μs) and then resampled. If we plot time on the horizontal axis and signal level (arbitrary scale) on the vertical axis, we can see that 50 measurement cycles are sufficient to measure one cycle of 10kHz. In this case, the sampling rate is 4Msps, corresponding to 0.25μs.

[0058] Next, an example of the results of analyzing 10kHz 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 analyzed (independently) for each of the above-mentioned bands. 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 499.8m. Here, the horizontal axis is time (unit: μs) and the vertical axis is distance (unit: m).

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

[0060] Next, Figure 15A shows the analysis results of the vibration waveform when the eight frequency bands are combined (hereinafter referred to as the raw data), rather than the measurement results for each frequency band as described above (the horizontal axis scale is the same as in Figure 14B). Overall, it can be seen that it shows a similar shape to the graph shown in Figure 14B. Furthermore, Figure 15B shows the result of calculating the spectrum (power spectral density) based on the data in Figure 15A (the horizontal axis is frequency). From Figure 15B, the vibration component of the piezoelectric element at 10 kHz can be confirmed.

[0061] Next, Figure 16B shows the results when the above raw data (see Figure 16A; identical to the data in 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 peak waveforms of the vibration is 100 μs, and it can be said that the vibration waveform at 10 kHz is present.

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

[0063] Based on the above, we were able to confirm through simulation the method of resampling a 10kHz sine wave at a sampling rate of 1Ksps using a chirp pulse with a time width of 2μs at a sampling rate of 4Msps.

[0064] Although this application describes exemplary embodiments, the various features, aspects, and functions described in the embodiments are not limited to the application of any particular embodiment, but can be applied individually or in various combinations to the embodiments. Accordingly, countless variations not illustrated are conceivable within the scope of the technology disclosed in this specification. These include, for example, modifications, additions, or omissions of at least one component. [Explanation of Symbols]

[0065] 1 Blade, 2 Hub, 3 Nacelle, 4 Tower, 5 Fiber Optic Cable, 6 Piezoelectric Element, 7 Fiber Optic, 8a Bolt, 8b Nut, 9 Damage, 10 Wind Turbine, 13 Blade, 14 Flange, 15 Coupling, 20 Fiber Optic Signal Processing Unit, 30 Fiber Optic Ultrasonic Distribution Measurement Unit, 31 Light Source Module, 32 Optical Control Module, 33 Ring Chirp Spacing 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 Fiber Optic Ring Circuit, 316 Optical Coupler, 317 Polarization Diversity Receiver Module

Claims

1. A laser diode (LD), a chirp pulse light generation module that controls the frequency sweep range of the laser light emitted from the LD to generate multiple chirp pulse light beams, an optical fiber ring circuit having a predetermined ring length, which receives the chirp pulse light beams generated by the chirp pulse light generation module and outputs a chirp pulse light group, which is a collection of chirp pulse light beams with a defined sweep time interval, and a polarization diversity receiving module that receives the backscattered light from the optical fiber and the laser light from the LD when the chirp pulse light group emitted from the optical fiber ring circuit is incident on the optical fiber under test. A light source module having A ring chirp interval and delay control module that controls the sweep time interval of the chirp pulse light and controls the timing of the chirp pulse light group entering the optical fiber to be delayed by a certain period of time. Equipped with, An optical fiber ultrasonic distribution measurement device characterized by receiving and reconstructing the same number of backscattered light from the optical fiber as the number of chirp pulses generated when multiple chirp pulses with different sweep frequencies generated by the chirp pulse light generation module are incident on the optical fiber, and by defining the frequency bandwidth of the received signal during the reconstruction process, thereby identifying and receiving each chirp pulse light incident on the optical fiber, and performing ultrasonic distribution measurement detected in the optical fiber.

2. The optical fiber ultrasonic distribution measuring device according to claim 1, characterized in that the sweep time interval of the chirp pulse light is determined in accordance with the frequency of the signal detected by the optical fiber, and a certain delay time that determines the incidence timing of the chirp pulse light group is determined by the length of the optical fiber.

3. The optical fiber ring circuit has an acousto-optic element for generating chirp pulses by delaying a plurality of chirp pulses of a predetermined frequency band at regular time intervals, and the ring length of the optical fiber ring circuit is set to a predetermined length corresponding to the identification speed of chirp pulses within the optical fiber, characterized in that the optical fiber ultrasonic distribution measuring device according to claim 1 or 2.

4. A method for measuring ultrasonic waves contained in backscattered light from an optical fiber, using an optical fiber ultrasonic wave distribution measuring device according to claim 1 or 2, wherein A method for measuring the ultrasonic distribution of an optical fiber, 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 sampling rate to generate multiple chirp pulse lights of different frequency bands, and when digitally processing the backscattered light of the generated chirp pulse light after it has been incident on the optical fiber, the frequency width during restoration is set to a predetermined frequency width to identify and receive each incident chirp pulse light.

5. A method for measuring ultrasonic waves contained in backscattered light from an optical fiber, using the optical fiber ultrasonic wave distribution measuring device described in Claim 3, A method for measuring the ultrasonic distribution of an optical fiber, 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 sampling rate to generate multiple chirp pulse lights of different frequency bands, and when digitally processing the backscattered light of the generated chirp pulse light after it has been incident on the optical fiber, the frequency width during restoration is set to a predetermined frequency width to identify and receive each incident chirp pulse light.

6. The optical fiber ultrasonic distribution measurement method according to claim 4, characterized in that the ring chirp interval and delay control module delay the incidence timing of the chirp pulse light into the optical fiber and form a continuous signal on the time axis with a period corresponding to the defined frequency width.

7. The optical fiber ultrasonic distribution measurement method according to claim 5, characterized in that the ring chirp interval and delay control module delay the incidence timing of the chirp pulse light into the optical fiber and form a continuous signal on the time axis with a period corresponding to the defined frequency width.

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