WO2004113830A1

WO2004113830A1 - Strain and ae measurement device using optical fiber sensor

Info

Publication number: WO2004113830A1
Application number: PCT/JP2004/008315
Authority: WO
Inventors: Hiroshi Tsuda
Original assignee: National Institute Of Advanced Industrial Science And Technology
Priority date: 2003-06-17
Filing date: 2004-06-14
Publication date: 2004-12-29
Also published as: JP3944578B2; JP2005009937A

Abstract

A strain and AE measurement device includes an FBG (Fiber Bragg Grating) sensor, a wide band light source, a coupler for branching reflected light transmitted from the FBG sensor, a strain measurement filter for reflecting or transmitting the reflected light branched by the coupler, and a filter for detecting AE (acoustic emission, elastic wave emission). The strain measurement filter and the AE detection filter have different transmittances corresponding to two types of wavelength. The light passing through or reflected by the strain measurement filter and the AE detection filter changes its intensity according to the change of the Bragg wavelength. The intensity change is converted into an electric signal by a photo-electric converter so as to detect a strain change as well as AE accompanying generation of a fine damage in the material/structure.

Description

Specification

Strain and AE measurement device using optical fiber sensor

Technical field

[0001] The present invention uses a fiber Bragg grating (hereinafter referred to as "FBG") sensor to detect a distortion change and to emit an elastic wave (acoustic emission) accompanying the occurrence of microscopic damage to a material structure. Hereinafter, it is referred to as “AE”.

[0002] The present invention can be applied to a case where a sound wave is generated using a piezoelectric element to evaluate the soundness of a structure, and furthermore, a case where a high-speed strain change due to an impact load is detected. .

[0003] That is, the present invention can be applied to the case where a single FBG sensor simultaneously measures strain for checking the load of a material or a structure and a damage state and AE caused by microscopic destruction. The present invention is expected to be used for soundness evaluation of automobiles, aircraft, bridges, buildings, and the like.

Background art

Conventionally, AE has been detected by using a piezoelectric element, and shock load has been detected by using a strain gauge.

[0005] Also, a technique has been proposed in the United States in which a reflected wave from an FBG sensor is passed through an FBG having a Bragg wavelength substantially equal to the Bragg wavelength of the FBG sensor, and AE is detected from the transmitted light (Non-Patent Document) 1).

[0006] Further, regarding the change in the Bragg wavelength of the FBG sensor, conventionally, the distortion is measured by measuring the wavelength of the reflected wave from the FBG sensor using an optical spectrum analyzer.

[0007] Non-Patent Document 1: 1. Perez, H.-Shii Cui and E. Udd, 2001 SPIE, Vol. 4328, p.209-215 Disclosure of the Invention

Problems the invention is trying to solve

[0008] However, the conventional technique using a piezoelectric element for AE detection has a drawback that it is affected by electromagnetic interference because measurement parameters are directly converted into electric signals for measurement.

[0009] FBG sensors do not suffer from electromagnetic interference because they convert measurement parameters into optical signals. The detected waveform cannot always reproduce the original AE waveform, and the waveform may appear distorted.

[0010] Furthermore, the technique of measuring the Bragg wavelength change of an FBG sensor using an optical spectrum analyzer is based on the fact that the sampling rate of the optical spectrum analyzer is usually about one sampling per second, so that a high-speed strain change or AE It is not possible to follow up and detect minute distortion changes with frequency characteristics of several hundred kHz. For this reason, there is a problem that a high-speed strain change cannot be detected in a following manner.

An object of the present invention is to solve such a conventional problem, and an object of the present invention is to realize an optical fiber strain sensor having the following features.

(1) Accurate strain change can be detected when FBG sensor detects high-speed strain change due to AE or impact load

(2) By changing the reflection characteristics of the filter, the FBG sensor can detect a wide range of strain changes from AE, which is a small strain change, to an impact load that causes a large strain change.

(3) The FBG sensor converts measurement parameters into optical signals and is not affected by electromagnetic interference.

(4) Measures both strain and AE with one FBG sensor, and reduces the number of sensors in strain and AE measurement, which are important when evaluating the integrity of materials and structures.

Means for solving the problem

[0012] In order to solve the above problems, the present invention provides an FBG sensor made of an optical fiber in which an FBG is written and attached to a subject, a broadband light source for making broadband wavelength light incident on the FBG sensor, and the FBG sensor. An optical fiber sensor comprising a coupler that branches reflected light transmitted from the sensor, a strain measurement filter that reflects or transmits the reflected light branched by the coupler, and an AE detection filter, respectively. The strain and AE measuring device, the strain measurement filter and the AE detection filter have different transmittances corresponding to the two types of wavelengths, respectively. The intensity of the transmitted or reflected light of the AE detection filter changes due to the change in the Bragg wavelength, which is converted to an electrical signal by a photoelectric converter to simultaneously change the distortion and AE. An optical fiber strain and AE measuring device characterized by detecting at the same time is provided. [0013] Based on the information on the change in distortion obtained by converting the signal into an electric signal by the photoelectric converter, a wavelength band in which the transmittance of the AE detection filter for AE detection changes is controlled. It is possible to measure the transmitted light intensity, the reflected light intensity, or the difference AE between the transmitted light intensity and the reflected light intensity of the AE detection filter.

The invention's effect

According to the strain and AE measurement device using the optical fiber sensor having the above configuration,

Using an FBG sensor, both strain and AE can be measured simultaneously with a single sensor.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram for explaining the principle of FBG.

FIG. 2 is a diagram for explaining the relationship between Bragg wavelength and distortion.

FIG. 3 is a diagram illustrating Example 1 of the present invention.

FIG. 4 is a diagram for explaining the operation of the first embodiment.

FIG. 5 is a diagram for explaining the operation of the first embodiment.

FIG. 6 is a diagram for explaining the operation of the first embodiment.

FIG. 7 is a view for explaining Example 2 of the present invention.

FIG. 8 is a diagram illustrating a third embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of a strain and AE measuring device using an optical fiber sensor according to the present invention will be described below with reference to the drawings based on an embodiment.

[0017] (FBG operating principle)

Before describing the present invention, the principle of the FBG, which is the basis of the present invention, will be described with reference to FIG. Light from the broadband light source is incident from the terminal (1) of the optical circulator to the FBG sensor (optical fiber with FBG) connected to terminal (2).

[0018] From the FBG sensor, a narrow band having a center wavelength (herein referred to as "the Ragg wavelength") is given as λ で given by twice the product of the refractive index n and the interval の of the refractive index change. Is reflected, and the other light components pass through the FBG sensor. The optical circulator sends the reflected light from the FBG sensor connected to terminal (2) to terminal (3) as shown in the figure. FIG. 2 is a diagram showing the relationship between the Bragg wavelength and the strain received by the FBG sensor. When the FBG sensor receives a strain, the refractive index changes the interval and the refractive index. Now, when the FBG sensor receives a strain of ε in the fiber axis direction, it is known that the change Δ λ Β of the Bragg wavelength λ 与え is given by the following equation 1 under a constant temperature condition.

[0020] [number 1]

Here, pe is a photoelastic constant (= 0.213), and ε is an optical fiber axial strain applied to the FBG. Therefore, the Bragg wavelength shifts to the long wavelength side when the FBG sensor is subjected to tensile strain, and shifts to the short wavelength side when subjected to compressive strain. For example, when subjected to a strain change of FBG sensor force X 10-6 with a Bragg wavelength of 1550 nm, the Bragg wavelength changes (shifts) by 1.2 pm. In short, the center wavelength of the reflected wave from the FBG sensor fluctuates in proportion to the change in strain applied to the FBG.

(Features of the present invention)

The features of the present invention will be described below. In the present invention, in order to measure the change in the Bragg wavelength at high speed, reflected light from the FBG sensor is passed through a filter having different transmittance according to the wavelength, and the change in the Bragg wavelength is converted into a change in light intensity.

[0024] Then, the reflected light from the FBG sensor is transmitted and branched to two fibers by a 1 X 2 coupler via an optical circulator, and a strain measurement filter having different transmittances for two wavelengths and an AE detection filter, respectively. Filter. The transmitted light and the reflected light of both filters change in intensity due to the change in Bragg wavelength. By detecting these with a photoelectric converter, strain change and AE can be detected simultaneously.

[0025] It is assumed that the strain measurement filter has a characteristic that the transmittance changes over a wide wavelength range as compared with the AE detection filter. An FBG sensor with a Bragg wavelength of 1550 nm without distortion produces a wavelength shift of 1.2 pm per 1 x 10-6 strain, so if it is assumed that the object under test will be subjected to a maximum of ± 1% strain. The distortion results in a Bragg wavelength change of ± 12 nm. Therefore, the change in strain can be measured by a filter whose transmittance varies in the wavelength range of 1538 to 1562 nm. A configuration for detecting AE from this measurement system using FBG as an AE detection filter has already been proposed by the present inventors in Japanese Patent Application No. 2002-340197. In order to detect AE, it is necessary that the power of the reflection wavelength band from the FBG sensor be within the transmittance change wavelength band of the SAE detection filter.

[0027] Since the change in the transmittance of the AE detection filter is limited to a very narrow wavelength range of about 0.4 band or less, if a large change in distortion occurs, the change in the light reflected from the sensor is blocked. The wavelength may fluctuate significantly and deviate from the wavelength range of the transmittance change of the AE detection filter. For this reason, it is preferable that the AE detection filter is a tunable filter whose transmittance change wavelength band changes according to the strain applied to the test object.

[0028] The transmitted light and the reflected light of the strain measurement filter are converted into electric signals by a photoelectric converter, and the strain is measured. Based on this distortion information, the operating wavelength range (wavelength band where the transmittance changes) of the tunable filter for AE detection is controlled. The AE can be measured from the transmitted light or reflected light intensity of the tunable filter, or the difference between the transmitted light intensity and the reflected light intensity.

[0029] With this system, strain is measured from the signal that has passed through the strain measurement filter, and AE is measured from the signal that has passed through the AE detection filter. Hereinafter, Embodiments 13 of the present invention will be described with reference to the drawings.

Example 1

FIG. 3 is a diagram illustrating a strain and AE measuring device using the optical fiber sensor according to the present invention. In FIG. 3, light from a broadband light source is incident on an FBG sensor via an optical circulator. The FBG sensor is fixed to an object to be measured.

[0031] The reflected light from the FBG sensor is passed through an IX2 coupler via an optical circulator. The I X 2 coupler splits the reflected light from the FBG sensor into two optical fibers. One optical fiber is connected to a filter for strain measurement, and the other is connected to a tunable filter for AE detection.

[0032] Transmitted light and reflected light of each filter are connected to a photoelectric converter, and each light intensity is converted into an electric signal. The reflected light of the filter can be extracted by attaching an optical circulator in front of the filter. Transmitted light intensity of strain measurement filter The strain can be measured from the degree and the reflected light intensity.

[0033] The photoelectric converter Sst is connected to a tunable filter control unit. The tunable filter control unit evaluates the Bragg wavelength that has moved due to the distortion change, and sends a signal for controlling the tunable filter's operating wavelength range (the wavelength range in which the transmittance changes) to the tunable filter.

FIG. 4 is a diagram showing the principle of strain measurement using a strain measurement filter. In FIG. 4, when the FBG sensor receives a strain, the Bragg wavelength changes. The transmitted light and reflected light intensity obtained by passing the reflected light from the FBG sensor through a filter whose transmittance changes with wavelength vary with the position of the Bragg wavelength.

For example, as shown in the upper diagram of FIG. 4, when the reflected light from the FBG sensor passes through a filter whose transmittance decreases as the wavelength becomes longer, when the FBG sensor receives compressive strain, (The Bragg wavelength shifts to the shorter wavelength side), and the transmitted light intensity of the filter increases.

As shown in the lower diagram of FIG. 4, when a tensile strain is applied (the Bragg wavelength shifts to the longer wavelength side), the transmitted light intensity of the filter decreases. By converting the transmitted light intensity change into an electric signal by a photoelectric converter, the Bragg wavelength change can be evaluated as the electric signal intensity.

With respect to the intensity of the reflected light, the signal intensity changes with the Bragg wavelength change according to the same principle from the relationship of reflectance = 1-transmittance. That is, the transmitted light intensity and the reflected light intensity of the filter change due to the distortion. However, the intensity of the light received by the photoelectric converter changes each time the optical fiber connector is connected. This is caused by misalignment of the connector connection. For this reason, it is not possible to quantitatively evaluate distortion using transmitted light or reflected light intensity alone. The distortion can be quantitatively evaluated from the value obtained by dividing the difference between the transmitted light intensity and the reflected light intensity by the sum of the two intensities.

[0038] The upper diagram of FIG. 5 is a diagram illustrating the principle of AE detection by the FBG sensor. Since the change in strain due to AE is very small, a filter with a narrow wavelength range where the transmittance is changing compared to strain measurement is required to detect AE with the FBG sensor. The Bragg wavelength of the light reflected from the FBG sensor is affected by AE, albeit minutely.

By passing this Bragg wavelength change through a filter having a narrow band transmittance change, Convert to change. For example, as shown in FIG. 5, when there is no AE, the reflected light of the Bragg wavelength s returns from the FBG sensor, and the center wavelength of the transmittance change of the AE detection filter is F. FBG sensor force The Bragg wavelength changes due to the strain change due to SAE. The Bragg wavelength changes in compression and tensile strain respectively; I s ′ and s ′ ′.

[0040] The transmitted light intensity of the filter changes in proportion to the area indicated by oblique lines due to a change in distortion due to AE. Therefore, the output of the photoelectric converter that converts the transmitted light intensity of the filter into an electric signal by the change in distortion due to AE is as shown in the lower diagram of Fig. Regarding the reflected light intensity, the reflected light intensity changes when AE is detected in the same manner from the relationship of reflectance = 1-transmittance. Also, the difference between the transmitted light and the reflected light signal intensity is changed by AE according to the same principle.

As a filter for detecting AE, there is a dielectric multilayer filter and an FBG force. In the example of this figure, a low-pass, no-pass, or bypass filter using a band-pass filter as the AE detection filter may be used.

FIG. 6 is a diagram showing the movement of the tunable filter operating wavelength band with a change in distortion. The filter for detecting AE has a wavelength range in which the transmittance changes about 0.4 nm.

[0043] For this reason, when the Bragg wavelength shifts rapidly due to a large distortion change, the wavelength range of the reflected light of the FBG sensor deviates from the wavelength range in which the transmittance of the AE detection filter changes. Become. Therefore, for AE measurement, it is necessary to change the operating wavelength range of the AE measurement filter according to the strain change measured by the strain measurement filter.

[0044] The tunable filter can change the operating wavelength range by an external control signal. By using a tunable filter as the filter for AE measurement, the operating wavelength range of the AE measurement filter is controlled according to the strain received by the FBG sensor. At this time, the operating wavelength range of the tunable filter is controlled by using the strain information evaluated from the value obtained by dividing the difference between the transmitted light intensity and the reflected light intensity of the strain measurement filter by the sum of the two intensities. Example 2

FIG. 7 shows a second embodiment of the present invention, in which simultaneous multi-point strain by a plurality of FBG sensors, an AE meter It is a configuration that enables measurement. This shows a device that measures strain and AE at multiple points simultaneously by arranging FBG sensors with different Bragg wavelengths in series. The reflected light from the FBG sensor array is separated into signals from each FBG sensor by the optical demultiplexer and output. In FIG. 7, the optical circulator in front of the filter and the extraction of reflected light from the filter are not shown for simplification of the drawing.

FIG. 8 shows Embodiment 3 of the present invention, and shows a configuration in which a plurality of FBG sensors can measure strain and AE at a specific location. This shows a device that arranges FBG sensors with different Bragg wavelengths in series and measures the strain and AE at the location where a specific FBG sensor is attached.

[0047] The reflected light from the FBG sensor array from the optical circulator is passed through a tunable filter to extract only the desired reflected light component from the FBG sensor. In addition, the operating wavelength band of the strain measurement filter is changed in conjunction with it. Note that in FIG. 8, the optical circulator in front of the filter and the extraction of reflected light from the filter are omitted for simplification of the drawing.

As described above, the embodiment of the strain and AE measuring device using the optical fiber sensor according to the present invention has been described based on the embodiment, but the patent is not limited to such embodiment. It goes without saying that there are various embodiments within the scope of the technical matters described in the claims.

Industrial applicability

[0049] As described above, the strain and AE measuring device using the optical fiber sensor according to the present invention can measure both strain and AE simultaneously using a single sensor using the FBG sensor. The present invention can be applied to generation of elastic waves using a piezoelectric element to evaluate the soundness of a structure, and to detection of a high-speed strain change due to an impact load.

Therefore, the present invention can be applied to simultaneous measurement of strain and AE due to the occurrence of microscopic destruction for examining the load of materials and structures and the damage state with one FBG sensor. INDUSTRIAL APPLICABILITY The present invention is expected to be used for soundness evaluation of automobiles, aircraft, bridges, buildings, and the like.

Claims

The scope of the claims

[1] An FBG sensor, which is composed of an optical fiber on which FBG is written, and is attached to a subject, a broadband light source for inputting broadband wavelength light to the FBG sensor, and a coupler for branching the reflected light transmitted by the FBG sensor. A strain measurement filter and an AE detection filter that respectively reflect or transmit the reflected light branched by the coupler, and a strain and AE measurement device using an optical fiber sensor,

The strain measurement filter and the AE detection filter have different transmittances for the two wavelengths, respectively.

The intensity of the transmitted light or reflected light of the strain measurement filter and the AE detection filter changes due to the change in the Bragg wavelength, and these are converted to electrical signals by a photoelectric converter to detect the change in strain and AE simultaneously. An optical fiber strain and AE measuring device characterized by

[2] The wavelength band in which the transmittance of the AE detection filter for AE detection changes is controlled based on the information on the change in distortion obtained by converting the signal into an electric signal by the photoelectric converter, and The strain using the optical fiber sensor according to claim 1, wherein the transmitted light intensity, the reflected light intensity, or the differential force AE between the transmitted light intensity and the reflected light intensity of the AE detection filter can be measured. AE measurement device.