JP2009300221A - Vibration sensor using optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a vibration sensor capable of covering a wide band, especially a band on the low-frequency side. <P>SOLUTION: An optical fiber 1 includes an input end 11, a circulation part 12, and an output end 13. Coherent light is input into the input end 11. The circulation part 12 is arranged between the input end 11 and the output end 13. The circulation part 12 is constituted by circulating the optical fiber 1 in the laminated state. Light input from the input end 11 passes the circulation part 12. Light passing the circulation part 12 is output from the output end 13. A detection part 2 detects a frequency change as light between the light input into the input end 11 and the light output from the output end 13. Hereby, a vibration applied to the circulation part 12 can be detected. A holey fiber is used as the optical fiber 1. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a sensor capable of detecting vibration using a change in wavelength of light passing through an optical fiber.

  Patent Document 1 listed below describes a vibration measurement device using an optical fiber. In this apparatus, a bending portion is formed in the optical fiber by bending the optical fiber. After attaching the curved portion to the measurement site, coherent light is input to the input end of the optical fiber. Input light to the optical fiber is output from the output end of the optical fiber through the curved portion. The frequency of light passing through the bending portion (frequency as light) changes corresponding to vibration applied to the bending portion. Therefore, the vibration applied to the bending portion can be measured by detecting the frequency change between the input light and the output light. According to this method, there is an advantage that minute vibrations can be measured over a wide frequency band. In this method, the S / N ratio in vibration measurement can be improved by increasing the length of the bending portion.

  As an application example of vibration measurement using such a principle, there is vibration measurement of rock in the ground. By measuring minute vibrations in the rock mass, it becomes possible to detect the displacement and abnormalities of the rock mass. Such a technique is considered to be applicable, for example, for evaluation of cavity stability at the time of construction of a storage tank for LPG (Liquefied Petroleum Gas) constructed in bedrock, maintenance during operation, and monitoring.

In this case, using a hole formed by boring, the bending portion is embedded in the rock and the vibration applied to the bending portion is measured. In such an application example, a high S / N ratio is required to measure minute vibrations in the ground. Moreover, since the diameter of a boring hole is small, it is necessary to make a curved part small.
WO2003 / 2956

  By the way, data indicating that the frequency band to be detected as minute vibrations in the bedrock is approximately 30 kHz to 120 kHz during cavity excavation in granite. It is thought that micro vibrations in the bedrock are generated as a breaking sound from the bedrock that has been partially destroyed by factors such as the occurrence of displacement and microcracks. It is considered that the band of the destructive sound is in the above-described range.

  However, the conventional vibration measuring apparatus using an optical fiber has a problem that it is difficult to cover the band.

  Further, in an optical fiber used for general optical communication or the like, the core / clad relative refractive index difference in the optical fiber is small. For this reason, if the optical fiber is bent with a bending radius of 10 mm, the light transmission loss value increases by about 1 dB in one turn. Therefore, when the conventional optical fiber is used, it is difficult to reduce the size of the vibration sensor.

  In order to reduce the bending loss value of the optical fiber, it is effective to increase the core / clad relative refractive index difference in the optical fiber. However, when an optical fiber having such a structure is connected to an optical fiber used for general optical communication, there is a problem that the connection loss value increases and the output optical signal detected by the vibration sensor decreases. there were.

  The present invention has been made in view of the above situation. The present invention intends to provide a vibration sensor that can cover a band necessary for vibration measurement of a rock mass.

  Another object of the present invention is to provide a vibration sensor that is small and has a small optical loss value.

  The present invention can be expressed as the contents described in the following items.

(Item 1)
An optical fiber and a detector,
The optical fiber includes an input end, a circulation portion, and an output end,
The input end is configured to receive coherent light,
The circuit portion is disposed between the input end and the output end,
And the said circumference part is constituted by laminating and circulating the above-mentioned optical fiber,
Furthermore, it is comprised so that the light input from the said input end may pass in the said circumference | surroundings part,
From the output end, the light that has passed through the circulating portion is output,
The detection unit detects vibration applied to the circulation unit by detecting a frequency change as light between the light input to the input end and the light output from the output end. It is composed,
A vibration sensor, wherein a holey fiber is used as an optical fiber constituting the circulating portion.

  Here, the holey fiber includes a core, a clad, holes formed around the core, and a coating layer provided outside the clad.

  By providing such a configuration, the frequency band of vibration that can be measured can be expanded.

(Item 2)
Item 2. The vibration sensor according to Item 1, wherein the light input to the input end of the optical fiber is coherent light.

  The coherent light is light having substantially the same phase, and is emitted from, for example, a laser light source. The wavelength of light is not particularly limited, but is preferably in the range of 1300 nm to 1600 nm in order to extend the transmission distance of light.

(Item 3)
Item 3. The vibration sensor according to item 1 or 2, wherein the circular portion is formed in a substantially cylindrical shape.

(Item 4)
Item 4. The vibration sensor according to Item 3, wherein the vicinity of the axis of the rotating portion is hollow.

(Item 5)
The vibration detection method of detecting a vibration using the vibration sensor of any one of the items 1-4.

  According to the present invention, it is possible to provide a vibration sensor capable of measuring vibrations in a wide band.

  Further, since the holey fiber is used, the vibration sensor can be made small, and furthermore, a vibration sensor with a small optical loss value can be provided.

  Hereinafter, an embodiment of a vibration sensor according to the present invention will be described with reference to the accompanying drawings.

(Configuration of Vibration Sensor According to Embodiment)
The vibration sensor of this embodiment includes an optical fiber 1 and a detection unit 2 (see FIG. 1). This basic configuration is the same as the technique described in Patent Document 1 described above.

  The optical fiber 1 includes an input end 11, a revolving part 12, and an output end 13. The input end 11 is configured to receive coherent light from the detection unit 2. Here, coherent light refers to light that has the same phase as required for vibration measurement. The coherent light can be emitted from, for example, a laser light source.

  The circulating portion 12 is disposed between the input end 11 and the output end 13 (see FIG. 1). Moreover, the circulation part 12 is comprised by laminating | stacking and circulating the optical fiber 1 in the thickness direction (up-down direction in FIG. 3) of the circulation part 12 (refer FIGS. 2-4). Here, the thickness direction of the rotating portion 12 means the axial direction of the rotating portion 12. By forming the revolving part 12, a curved part (curved part) is formed in the optical fiber 1. The circuit 12 is configured to allow light input from the input end 11 to pass therethrough. In addition, an adhesive (for example, an epoxy-based adhesive) is applied around the circulating portion 12 so that the circulating state can be maintained.

  Moreover, the circumference part 12 of this embodiment is formed in the substantially column shape. Furthermore, the vicinity of the axial center of the rotating portion 12 is hollow. The inner diameter of the hollow portion in the optical fiber 1 is preferably about 10 mm. If the inner diameter is less than this, optical transmission may be difficult in view of the mechanical strength (strength against bending) of the optical fiber 1. Moreover, in order to make the circumference part 12 small, it is desirable to make the internal diameter of a hollow part small. Therefore, it is considered preferable to make the inner diameter of the hollow portion as small as possible within a range that does not hinder optical transmission.

  From the output end 13 of the optical fiber 1, light that has passed through the revolving part 12 is output.

  The detector 2 detects a change in frequency as light between the light input to the input end 11 of the optical fiber 1 and the light output from the output end 13. Thereby, the detection unit 2 is configured to detect vibration applied to the circulation unit 12. The principle of this detection is the same as the technique described in Patent Document 1 described above.

  In addition, a holey fiber is used as the optical fiber 1 according to the present embodiment (see FIG. 5). Thereby, in this embodiment, the circulation part 12 is comprised with the holey fiber.

  The holey fiber has a so-called hole assist structure. The optical fiber 1 according to the present embodiment includes a core 15, a clad 16, a plurality of holes 17 formed around the core 15, and a coating layer 18 provided outside the clad. The core 15 has a higher refractive index than the clad 16. Further, the refractive index of the hole 17 is approximately 1.

  In a general optical fiber, optical transmission characteristics can be obtained by the difference in refractive index between the core and the cladding. On the other hand, in the case of a holey fiber, the effective refractive index of the clad 16 around the core 15 is smaller than the refractive index of the material of the clad 16 itself due to the presence of the holes 17. For this reason, a core / cladding relative refractive index difference is increased, and a structure in which optical loss does not increase even when the fiber is bent can be obtained.

  In addition, the core / cladding structure dimension of the holey fiber can be the same as that of a general optical fiber core / cladding structure. For this reason, according to this embodiment, even if optical fibers are connected, the connection loss value can be reduced.

  In this embodiment, an ultraviolet curable acrylic resin is used as the material of the covering layer 18.

  In the above example, the entire optical fiber 1 is configured with a holey fiber, but it is also possible to configure only the loop portion 12 with a holey fiber and the other part with a normal optical fiber. Since the means for connecting the optical fibers is already well known, description thereof is omitted.

(Operation of vibration sensor)
When using the vibration sensor of the present embodiment, the orbiting portion 12 is disposed at a location where vibration measurement is to be performed. At this time, it is preferable that both are fixed by appropriate fixing means (for example, an adhesive, an adhesive tape, grease, etc.) so that vibration is transmitted from the vibration measurement site to the rotating portion 12. In this state, when light is input from the input end 11 of the optical fiber 1, the light passes through the circulating unit 12 while circulating around the optical fiber 1. Thereafter, this light is output from the output end 13 of the optical fiber 1.

  When vibration is transmitted to the revolving part 12, the frequency (or wavelength) of the light passing through the revolving part 12 changes corresponding to the vibration. By detecting this change in frequency by the detection unit 2, it is possible to measure the vibration applied to the circulation unit 12.

  In this measurement, since frequency variation of light is used, there is an advantage that minute vibration can be measured with higher accuracy than a vibration sensor using intensity change of light passing through an optical fiber. Compared to a vibration sensor using a piezo element, the vibration sensor of this embodiment has an advantage that high-precision vibration measurement in a wide band is possible.

  Moreover, in this embodiment, since the holey fiber was used as the optical fiber 1 which comprises the circulation part 12, there exists an advantage that the frequency band (especially low frequency side band) of the measurable vibration can be expanded. Specific examples will be described later.

(Configuration example of protective case for vibration sensor)
Next, the protective case 3 that houses the rotating portion 12 in the vibration sensor of the present embodiment will be described. The protective case includes a main body 31, a lid 32, and a protective tube 33 (see FIGS. 6 and 7).

  The main body 31 is formed with a recess 311 for accommodating the revolving part 12 therein. Further, a hole 312 is formed on the side surface of the main body 31 for allowing the optical fiber 1 to pass from the circulating portion 12 to the input end 11 and the output end 13.

  The lid 32 is configured to block the opening surface of the recess 311. The lid 32 and the main body 31 are fixed by appropriate fixing means such as bolts. The lid 32 and the main body 31 are preferably sealed with a sealing member such as an O-ring.

  The protective tube 33 is connected to the hole 312 of the main body 31 and accommodates the optical fiber 1 extending from the hole 312 to the outside.

  In order to attach the circulating portion 12 to the inner surface of the recess 311, it is preferable to fix both of them with a fixing means such as an adhesive. When the protective case 3 is used, the vibration transmitted to the rotating part 12 by the protective case 3 can be measured.

(Experimental example)
Next, an experimental example using the vibration sensor according to the present embodiment will be described.

  First, the experimental apparatus will be described with reference to FIG. Here, an iron block (size: 80 cm × 80 cm × 40 cm) is used as the test piece 4. The test piece 4 is attached with the rotating portion 12 described in the above embodiment. As an attachment state, it attaches so that the end surface of the circulation part 12 may become in parallel with the surface of the test piece 4. FIG. As an attachment method, adhesion with an adhesive is preferable, but not particularly limited. The vibration detected by the detector 2 can be displayed on the digital oscillograph 5.

  A predetermined vibration is applied to the test piece 4 by an oscillator 6. Vibration applied from the oscillator 6 can also be displayed on the digital oscillograph 5.

  In this experimental example, the piezoelectric element (PZT) 7 was attached to the test piece 4 and vibration measurement was performed for verification. The output of the piezo element 7 is sent to the digital oscillograph 5 via the amplifier 8. This piezo element 7 has a resonance point at approximately 30 kHz.

  Furthermore, as a comparative example, vibration measurement was performed using a vibration sensor using a conventional optical fiber. Details of the comparative example will be described later.

The results are shown in FIG. In this graph, the horizontal axis represents vibration frequency (kHz), and the vertical axis represents received voltage sensitivity (dB). The meaning of the lines in this graph is as follows.
-Solid line with ■: Oscillation intensity of vibration obtained with AE sensor using piezo element (PZT),
Broken line: sensitivity (dB) by vibration sensor of comparative example,
Solid line: Sensitivity (dB) by the vibration sensor of the experimental example using this embodiment.

  Here, the configurations of the comparative example and the experimental example are as follows.

(Configuration of comparative example)
Optical fiber: Use normal fiber, not holey fiber,
Fiber coating: Polyimide resin,
Fiber length: 65m,
The outer diameter of the covering part is 150 μm,
Circumference shape: inner diameter 8 mm, outer diameter 19.2 mm, thickness 6 mm,
Other configurations in the comparative example were the same as those in the experimental example.

(Configuration of experimental example)
Optical fiber: Uses holey fiber,
Fiber coating: UV curable acrylic resin,
Fiber length: 80m
The outer diameter of the covering part is 210 μm,
Circumferential shape: inner diameter 10 mm, outer diameter 28.9 mm, thickness 6 mm.

  As can be seen from the results shown in FIG. 9, when the vibration sensor of the experimental example (solid line in the figure) is used, the sensitivity is greatly improved particularly in the band of 30 kHz to 80 kHz as compared with the comparative example. .

  In addition, since the vibration sensor according to the present embodiment uses a holey fiber, there is an advantage that bending loss in the rotating portion 12 is small.

  Furthermore, since the vibration sensor of the present embodiment uses a holey fiber, there is an advantage that low-loss optical transmission is possible in a wide band.

  The vibration sensor according to the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the gist of the present invention.

  For example, in the said embodiment, although the surrounding part 12 shall be accommodated in the protective case 3, it is not restricted to this, The protective case 3 can be abbreviate | omitted depending on a use. For example, the circulating part 12 may be directly attached to the measurement site.

  In the above-described embodiment, an example in which vibrations in the ground are measured has been described. However, vibrations other than the ground can be measured, such as by measuring vibrations of mechanical parts and bridges.

It is explanatory drawing which shows the whole structure of the vibration sensor in one Embodiment of this invention. FIG. 2 is an enlarged plan view of a circulation part shown in FIG. 1. It is sectional drawing which follows the AA line of FIG. It is a perspective view of the circumference | surroundings part shown in FIG. FIG. 2 is an enlarged cross-sectional view of the optical fiber shown in FIG. It is a schematic principal part sectional drawing which shows an example of a protective case, Comprising: It is a figure of the state which opened the lid | cover. FIG. 7 is a schematic plan view of FIG. 6 in a state where a lid is omitted. It is a block diagram for demonstrating the experimental apparatus used by the experiment example. It is a graph which shows the result obtained by the experiment example, Comprising: The horizontal axis shows the frequency (kHz) of a vibration, and the vertical axis | shaft has shown receiving voltage sensitivity (dB).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber 11 Input end 12 Circulation part 13 Output end 15 Core 16 Cladding 17 Hole 18 Cover layer 2 Detection part 3 Protective case 31 Main body 311 Recess 312 Hole 32 Lid 33 Protective tube 4 Test piece 5 Digital oscillograph 6 Oscillator 7 Piezo Element 8 Amplifier

Claims (5)

An optical fiber and a detector,
The optical fiber includes an input end, a circulation portion, and an output end,
The input end is configured to receive light,
The circuit portion is disposed between the input end and the output end,
And the said circumference part is constituted by laminating and circulating the above-mentioned optical fiber,
Furthermore, it is comprised so that the light input from the said input end may pass in the said circumference | surroundings part,
From the output end, the light that has passed through the circulating portion is output,
The detection unit detects vibration applied to the circulation unit by detecting a frequency change as light between the light input to the input end and the light output from the output end. It is composed,
A vibration sensor, wherein a holey fiber is used as an optical fiber constituting the circulating portion.   2. The vibration sensor according to claim 1, wherein the light input to the input end of the optical fiber is coherent light.   The vibration sensor according to claim 1, wherein the circular portion is formed in a substantially cylindrical shape.   The vibration sensor according to claim 3, wherein the vicinity of the axis of the rotating portion is hollow.   The vibration detection method which detects a vibration using the vibration sensor of any one of Claims 1-4.

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