JP2007333713A - Conversion mechanism for fbg and sensor applying the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mechanism that converts a measured physical quantity into an amount of expansion and contraction of an FBG by using an optical fiber FBG sensitive to strain, due to expansion and contraction, in view of the advantage of using light which will not be affected by electrical discharge, unlike conventional electric disaster prevention sensor which fails or malfunctions under the influence of electrical discharge due to lightening. <P>SOLUTION: A conversion mechanism structured, such that an FBG-mounted optical fiber is wound around the outer circumference of a ring spring is provided. The conversion mechanism can accurately convert, into expansion and contraction of the optical fiber, a measured physical quantity acting thereon so as to pressure crush the ring. The conversion mechanism uses an FBG obtained by writing a Bragg diffraction grating into the optical fiber. The conversion mechanism detects a detected amount converted into expansion and contraction of grating intervals. The value of expansion and contraction can be measured as a change in wavelength of Bragg reflected light. Since sensing and signal transmission are all carried out by using optical signals, and no electrical signals are used, the conversion mechanism has a feature of not being affected by ground leakages due to rainfalls, ground leakages due to meltwater from snowfalls, or lightning, even under inclement weather conditions underwater or outdoors. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention has been made to overcome the conventional drawbacks as a disaster prevention sensor, and relates to a disaster prevention sensor technology.

  As its means, it relates to the application of a pressure sensor using an optical fiber FBG, and in particular, when the pressure to be measured is wound around a ring-shaped spring at the part that converts the measured pressure into the amount of expansion and contraction of the FBG, The present invention relates to a technique that utilizes the occurrence of expansion and contraction in a wound FBG.

  The conventional disaster prevention sensor realized by the electric method has a background that malfunctions and failures occur frequently due to the influence of lightning discharge, and there is a demand for technological development to improve. On the other hand, the development of optical fiber technology has progressed, and there is a technical background that a new optical device called FBG has appeared, and it has been studied to realize various disaster prevention sensors using FBG as a sensor. It is also true that there are many problems. The present invention relates to a conversion technology that serves as a basis for using FBG as a disaster prevention sensor.

  Recently, floods, natural disasters such as earthquakes, and traffic disasters have occurred frequently in Japan. To reduce this damage as much as possible, it is important to predict the occurrence of disasters by placing sensors.

  For example, floods can be predicted to some extent by analyzing temporal changes in the water level distribution in upstream rivers and basins. Based on this information, water levels can be prevented by opening and closing sluice gates. It is possible to take measures to control disasters and prevent disasters.

  In this case, a sensor (water level meter) for measuring the water level is necessary, but most of the sensors that are generally used at present are based on electrical means. However, this is not effective in disaster prevention due to frequent occurrence of malfunctions and breakdowns due to lightning damage in various places.

The present invention is made in view of the great damage caused by such a lightning disaster prevention sensor, and is solved by a technique using an optical fiber type element called FBG as described below.

Thus, it is an object of the present invention to save human lives from natural disasters and man-made disasters.

Means to solve the problem

  The present invention is made in view of the current situation that an electrical disaster prevention sensor does not operate properly due to lightning discharge interference. By using light, an electromagnetic influence is avoided and an attempt is made to solve this problem. It is.

  Specifically, it uses a device (fiber Bragg grating: FBG) with a Bragg diffraction grating built in the optical fiber. This is because the Bragg reflection wavelength changes when it is expanded or contracted by an external force. However, if it can be converted into FBG expansion / contraction information, it can be detected by wavelength measurement.

  Therefore, what is needed is a conversion mechanism that converts the physical quantity to be measured into the expansion and contraction of the optical fiber.

However, there are various methods for converting the physical quantity to be measured into the expansion and contraction of the optical fiber. However, the optical fiber is glass and is mechanically fragile. There were problems such as being easily received, and there were problems with accuracy and stability. That is, in order to actually use the FBG, it is necessary to invent a new method for converting the physical quantity to be measured into the expansion and contraction of the optical fiber.

The present invention solves this problem by a mechanism in which an optical fiber is fixed around a ring-shaped spring. That is, when the ring-shaped spring is deformed by applying pressure from the physical quantity to be measured, the amount of deformation is converted into the expansion and contraction of the optical fiber fixed in a wound state. This conversion has no ambiguity compared to the conventional conversion method, and can be converted accurately.

The invention's effect

  The present invention relates to a mechanism for accurately converting a physical quantity to be measured into an expansion / contraction amount of the FBG by a method of winding and fixing the FBG around the outer periphery of a ring-shaped spring. By realizing a disaster prevention sensor using this mechanism, it is possible to provide a highly reliable disaster prevention sensor that could not be realized so far.

Since the ring-shaped spring used in the conversion mechanism according to the present invention is very simple as an analysis model, it is easy to analyze the elastic characteristics, response characteristics, etc., and the operation can be predicted and evaluated in advance.

Since the conversion mechanism according to the present invention can convert the physical quantity to be measured into the amount of expansion and contraction of the FBG more accurately than the conventional method, when applied to various measured physical quantity sensors, it is possible to realize a high-performance device. When applied to a disaster prevention sensor, a highly reliable sensor can be provided.

Details of the invention

(Principle of FBG)

The FBG is a Bragg diffraction grating written in the core of an optical fiber using ultraviolet light (Fig. 1), and reflects light of a specific wavelength determined by the grating spacing. The relationship between the grating interval and the wavelength is expressed by the following equation.
λ = n · D (1)
Here, λ is the reflection wavelength, D is the lattice spacing, and n is the average refractive index of the FBG.

Wavelength change is
The

It is known that an error will occur if it is based only on this.

It is also known that when temperature is T, both n and D are functions of temperature, and n is a function of wavelength.
n = n (T, λ) (3)
D = D (T) (4)
So in the linear region,
It becomes.

Here, s is a temperature coefficient, p is a wavelength-dependent function of refractive index, and the Sellmeier equation is known. ρ is a linear thermal expansion coefficient. Usually, the silica fiber is ^{ρ = 1 × 10 -6 /℃,s=9.5×10 -6} / ℃ about the size, the formula (5) Wavelength right side that uses the second term absorption band of It is disregarded in a region away from the region and having a small wavelength change.

When the FBG is fixed to another object, it is necessary to consider the thermal expansion of the object.

  Normally, two FBGs are used, the temperature dependence is common, and a sensor is configured so that stress is applied to only one of them, and an error due to temperature change is compensated from the difference in wavelength change.

  The present invention relates to a mechanism for converting a physical quantity to be measured into the amount of expansion / contraction of the FBG, and has been made in view of the problems in accuracy and stability in the conventional technology. The FBG is wound around a ring-shaped spring. The problem is solved by adopting a mechanism with a different structure.

In other words, in pressure detection, if the ring is subjected to a force that deforms to become an ellipse, the surface perpendicular to the major axis of the ellipse is curved with a small curvature, so that part of the attached optical fiber is the other part. Get bigger stretch. On the other hand, on the surface perpendicular to the minor axis, the curvature increases on the contrary, so compression occurs. Therefore, pressure information can be converted into expansion / contraction information of the FBG by arranging the FBG at one or both of these positions.

  At this time, if attention is paid to the fact that the force exerted on the optical fiber conflicts depending on the position where the FBG is arranged, the FBG can be used differentially, and the common error component such as the influence of temperature can be canceled. Is possible.

  In addition to the method of directly bonding the FBG to the ring-shaped spring, the method of fixing the FBG wound around the outer periphery of the ring-shaped spring includes a fiber at a 45-degree position when the FBG is arranged in the 0 degree direction or 90 degree direction. Can be bonded to the ring-shaped spring or fixed using mechanical pressure or a jig.

  Furthermore, by providing a groove equal to the thickness of the optical fiber on the outer surface of the ring, the FBG can be stably wound around the ring outer periphery. When two FBGs are used to obtain a differential operation, two grooves may be provided.

  The FBG conversion mechanism described above has many applications, and the present invention includes all sensors that incorporate the conversion mechanism. That is, a water level meter that measures the water level by applying water pressure to the FBG conversion mechanism, a seismometer that has a structure that supports the movement of the weight from six directions by the conversion mechanism, and a rain gauge that detects the water count signal by the FBG sensor Etc.

  An embodiment is shown in FIG. The state where the optical fiber 2 is wound around the outer periphery of the ring-shaped spring 1 is shown. One or two FBGs 3 are mounted. In the two cases, they are installed at positions of 0 degrees and 90 degrees as shown in the figure. The optical fiber is fixed at a position in the 45-degree direction so as to sandwich the FBG with the fixing jig 4.

  The partially enlarged view in the figure shows that one or a plurality of rows of grooves 6 are provided on the outer peripheral surface of the ring.

  The optical fiber 3 goes around the ring outer periphery, and the extra length is used as a lead fiber 5 for connection to an optical circuit. At this time, as shown in FIG. 2, when the lead 12 is taken out horizontally, the number of turns is 1 (A), and when it is aligned in one direction, it is 1.5 turns (B). In each case, the shape of the fixed position 13 and the base 14 and the perceivable pressure direction 15 can be selected.

The operation of the FBG conversion mechanism of the present invention will be described with reference to FIG.
Now, assuming that the pressure indicated by the arrow is applied from the bottom of the ring and the one that was a circle is deformed to be crushed into an ellipse, the outer periphery of the side part of the ring extends, and at the same time, the upper surface of the ring Since the force in the direction of contraction is received, if the FBG is disposed in this portion, the pressure can be accurately converted into the expansion and contraction of the FBG.

An application example 1 of the present invention is shown in FIG. The FBG conversion mechanism (B) 1 using the ring-shaped spring shown in FIG. 1 is housed in a cylindrical or rectangular outer sheath 22 and installed so as to be deformed by an external pressure 23. The outer jacket 22 has a space for storing the optical circuit 24, and is further connected to the outside by an external device and an optical cable 25 via a waterproof seal 26.

This water level gauge is installed in such a state that it hangs underwater with the conversion mechanism facing downward. The outside is connected to the light source device 28 and the wavelength analyzing device 29 via the optical cable 25 and the branch coupler 27. The water level is obtained from the wavelength analysis result and displayed on the display 30.

  A second embodiment is shown in FIG. This is a seismometer based on the ring type FBG conversion mechanism 31 of the present invention. The weight 32 is supported from six directions, and three-dimensional vibrations in the x-direction, the y-direction, and the z-direction can be detected. The container is doubled and includes an outer container 33, an inner container 34, and a pedestal 35. An optical fiber cable outlet 36 is provided on the base, and the cable is pulled out and connected to an external measuring unit.

  An embodiment of the optical wiring and signal processing system of the seismometer is shown in FIG. The fact that the FBG can be driven with such wiring is as described in the following patent application. The FBG 41 and the FBG 42 form a pair, and are incorporated in the conversion mechanisms that face each other in the x, y, and z directions.

The optical power from the light source 44 is distributed to the FBG via the branch coupler 43, the reflected light is detected by the photodiode 45, and the signal is given to the three-dimensional signal analyzer 46 and recorded by the recording device 47. On the other hand, the vibration state is displayed on the display 48.

The response characteristic of the ring type FBG conversion mechanism of the present invention is accompanied by an overshoot as shown in FIG. When this response signal is obtained as an analog signal as in the example of FIG. 6, the overshoot can be compensated by passing the output signal through an integration circuit and appropriately selecting the values of C and R.

  In the case of a digitized signal, overshoot can be eliminated by performing digital integration processing equivalent to that described above.

FIG. 2 is a diagram for explaining the present invention, and is composed of two diagrams. One is a side view and the other is represented by a sketch viewed from an oblique direction. Each of these shows a state in which an optical fiber mounted with FBG is wound around a ring-shaped spring. In addition, a state where both sides of the FBG are fixed to the ring and a state where a groove is formed in the ring so as not to be shifted after the optical fiber is wound are shown. Indicates that there are two variations (A) and (B) in the fixing method when the ring-shaped FBG conversion mechanism according to the present invention is actually used and the lead wire (optical fiber) drawing direction. . (A) is a type in which pressure is applied from above, and lead wires are taken out from both ends. (B) is to apply pressure from below by fixing at a position where it is suspended, and the lead wire is taken out upward. It is properly used depending on the application. Shows the operating principle of the ring-shaped FBG conversion mechanism according to the present invention. It is explained that when the ring is flattened as shown in the figure, a portion extending to the outer circumference (outer periphery) of the ring and a portion contracting are generated. By arranging the FBG so as to come to this portion, the force for crushing the ring can be detected by the FBG. These show the Example which applied the ring-shaped FBG conversion mechanism concerning this invention to the water pressure gauge. Since water pressure is proportional to the water depth in nature, it can also be applied as a water depth gauge or water level gauge. The block diagram in the figure shows the configuration and wiring of an optical system for driving the FBG. Describes an embodiment in which the ring-shaped FBG conversion mechanism of the present invention is applied to a seismometer. It is possible to measure vibrations in all directions by placing a weight in the center, placing a total of six ring-shaped FBG conversion mechanisms of the present invention on the front, rear, left, right, top and bottom, and measuring the load due to the acceleration applied to the weight. Is possible. FIG. 5 shows an optical wiring (optical system) for simultaneously detecting six high-speed vibrations by simultaneously driving the six ring-shaped FBG conversion mechanisms of the present invention shown in FIG. These show the response waveform with respect to the pulse input (impact load load) of FBG of the ring-shaped FBG conversion mechanism of this invention, and its correction circuit. Since the mechanism has a high response speed, it is shown that overshoot occurs in the waveform. Overshoot can usually be corrected by an integration circuit having the same time constant. This function can be realized not only as an actual electric circuit but also by digital processing.

Explanation of symbols

1 Ring-shaped spring: A basic material of the ring-shaped FBG conversion mechanism of the present invention. The following two optical fibers are wound around this.
2 Optical fiber (with FBG installed): An optical fiber in which FBG is written inside the core.
3 FBG part: FBG main body 4 written inside 2 above Fixing jig: It is necessary to fix the optical fiber of 2 above around the ring so as not to loosen, but a jig used for the fixing.
5 Optical fiber lead: An extended portion for sending light to the FBG at both ends of the optical fiber of 2 and extracting reflected light and transmitted light.
6 Groove: Attached to the ring 1 described above, prevents the wound optical fiber from shifting.
11 Ring type conversion mechanism (FIG. 1): Refers to the entire ring-shaped FBG conversion mechanism of the present invention.
12 Lead optical fiber: An extension required to send light to the FBG at both ends of the optical fiber of 2 and to extract reflected light and transmitted light.
13 Fixing screw: A screw for fixing the ring-shaped FBG converting mechanism of the present invention to the base.
14 Holding mechanism and fixed base: counterpart 15 when trying to fix the ring-shaped FBG conversion mechanism of the present invention Pressure direction: The ring-shaped FBG conversion mechanism of the present invention has a function of converting pressure into wavelength information. Indicates the direction of pressure that can be felt.
21 FBG conversion mechanism using ring spring ([FIG. 2] (B)): Application of the system of the embodiment [FIG. 2 (B)] is shown.
22 Mantle: A cylinder that protects the ring-shaped FBG conversion mechanism of the present invention. Or a box.
23 Water pressure direction: The ring-shaped FBG conversion mechanism of the present invention has a function of converting pressure into wavelength information, but indicates the direction of pressure that the mechanism can feel.
24 Optical circuit storage space: Space for storing an optical circuit (optical wiring) for driving the ring-shaped FBG conversion mechanism of the present invention.
25 Optical cable: In order to drive the ring-shaped FBG conversion mechanism of the present invention, it is necessary to introduce light source light into the FBG part. This is a cable incorporating an optical fiber for that purpose, and is water resistant in consideration of use in water. There is sex.
26 Waterproof seal: Installed in the cable inlet to prevent water from entering the water pressure gauge (water level gauge).
27 Light source device: A device for generating light that requires light source light in the FBG portion in order to drive the ring-shaped FBG conversion mechanism of the present invention.
28 Branch coupler: An optical element for separating light source light directed from the light source to the FBG and reflected light from the FBG.
29 Wavelength analyzer: The ring-shaped FBG conversion mechanism of the present invention converts pressure into wavelength information of reflected light, and is an optical device for measuring the wavelength information.
30 Water level display device: When the ring-shaped FBG conversion mechanism of the present invention is used as a water level meter, since the water pressure is converted into wavelength information, after the wavelength information measurement, it is converted again into a water level value and displayed. . A constant calibrated in advance is used for this conversion.
31 FBG conversion mechanism using ring-shaped spring (FIG. 1): The ring-shaped FBG conversion mechanism of the present invention itself. In this application, one FBG is placed in each mechanism.
32 weight: Used in the ring-shaped FBG conversion mechanism of the present invention to generate pressure by acceleration.
33 Outer container: A box for protecting an optical system including the ring-shaped FBG conversion mechanism of the present invention. The outer container is doubled in consideration of its robustness.
34 Inner container: Inside corresponding to 33 above.
35 Pedestal: A base for stable installation of the device.
36 Optical cable outlet: Hole for introducing an optical fiber into the container.
41 FBG: FBG attached to the ring-shaped FBG conversion mechanism of the present invention.
FBG paired with 42 41 FBG: In this application example, two rings are used facing each other. It refers to a pair with the 41 FBG.
43 Branch coupler: An optical element for separating light source light from the light source toward the FBG and reflected light from the FBG.
44 Light source: An apparatus for generating light that requires light source light in the FBG portion in order to drive the ring-shaped FBG conversion mechanism of the present invention.
45 Photodiode: A semiconductor element that measures the power of reflected light from the ring-shaped FBG conversion mechanism of the present invention.
46 Three-dimensional data processing device: The seismometer of this application example can detect vibrations in three directions (up, down, left and right, front and rear).
47 Recording device: A device for recording the value calculated in 46 above.
48 display device: a device for displaying the value calculated in 46 above.
101 FBG conversion mechanism using ring-shaped spring (FIG. 1): The ring-shaped FBG conversion mechanism of the present invention itself.
102 Signal waveform generated from 101: The horizontal axis is time, and the vertical axis is a waveform expressed by wavelength change. It shows the situation where overshoot occurs.
103 Signal processing circuit: A circuit for removing the overshoot appearing in the waveform 102. When the signal processing circuit is obtained as an analog signal, actual R and C are used. Further, when it is obtained as a digital signal, it can be realized by digital signal processing using equivalent R and C.

Claims (8)

  An optical fiber Bragg grating (hereinafter referred to as FBG) is a sensor device that responds to the expansion and contraction caused by external force by changing the reflection wavelength from the grating inside the optical fiber Bragg grating. Any physical quantity can be measured, but in order to convert the physical quantity to be measured into FBG expansion / contraction, a method of rounding a ribbon-shaped flat plate into a ring shape or cutting a cylindrical pipe into a predetermined width An FBG sensor having a conversion mechanism for an FBG, characterized in that the FBG portion is wound around and fixed to the flat ring-shaped spring created in step 1 while applying a predetermined tension in advance.   When an optical fiber is wound around and fixed to the outer periphery of the ring-shaped spring according to claim 1, a surface perpendicular to the major axis of the ellipse is curved with a small curvature when pressure is applied to collapse the ring into an elliptical state. The optical fiber in that part is stretched more than the other part, while the curvature perpendicular to the minor axis is oppositely large, so it is compressed, so one or both of these two cases An FBG sensor having a conversion mechanism for FBG, characterized in that pressure information is converted into expansion / contraction information of FBG by installing and fixing the FBG so that it is disposed at the position of FBG.   As described in claim 2, a portion that extends by utilizing the fact that the force acting on the FBG arranged around the force acting to crush the ring-shaped spring to be ovalized depends on the fixed position. FBG sensor with a conversion mechanism for FBGs, which uses differentially FBGs attached to the parts to be compressed.   With respect to the fixing method of the FBG wound around the ring-shaped spring described in claim 1, when the FBG is directly bonded to the ring-shaped spring or when the FBG is arranged in the 0 degree direction or the 90 degree direction, 45 degrees An FBG sensor having a conversion mechanism for FBG, characterized in that the fiber can be bonded to a ring-shaped spring at the position or fixed using mechanical pressure or a jig, and the stretching force can be transmitted to the FBG more accurately.   Regarding the fixing position of the FBG wound around the ring-shaped spring described in claim 4, when the FBG is arranged in the 0 degree direction or the 90 degree direction, the fiber is bonded to the ring spring at an angular position smaller than 45 degrees. Alternatively, an FBG sensor having a conversion mechanism for the FBG, characterized in that sensitivity and overshoot can be improved by fixing with mechanical pressure or a jig.   A ring-shaped spring for winding the FBG described in claim 1, wherein a groove is formed on the surface of the ring for winding the FBG so that the FBG can be wound stably. FBG sensor with a conversion mechanism.   The fixing position of the FBG described in claim 4 is set to two or more at one end, thereby reducing the tension difference between both sides of each fixing point, thereby reducing the fixing force and reducing the light transmission loss due to the fixing. An FBG sensor having a conversion mechanism for an FBG characterized by being able to do so.   All of the physical sensors characterized in that the FBG sensor described in claim 1 can be applied or built in, and a water level meter that measures the water level by applying a water pressure to the ring-shaped FBG sensor. It includes a seismometer based on the principle of detecting movement by the FBG sensor that supports movement from six directions, a rain gauge for detecting a water count signal by the FBG sensor, and the like.