JP2005283469A - Vibration measuring apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a vibration measuring apparatus that controls the directivity in vibration measurement. <P>SOLUTION: The apparatus comprises a light source, a waveguide 2 and a detector. The light source is for inputting input light to the waveguide 2. The waveguide 2 is constituted of, for example, an optical fiber. The waveguide 2 comprises an ellipse 120 to which vibration to be measured is applied. The ellipse 120 is formed so that the input light passes through. A detector detects changes in frequency between an output light from the waveguide 2 that passes through the ellipse 120 and the input light. The ellipse 120 comprises an arc 121 and a straight line 122. Directivity in the vibration measurement can be controlled, by adjusting the ratio of l/R between the radius r of the curvature of the arc 121 and the length l of the straight-line l. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an apparatus for measuring vibration.

Conventionally, a vibration sensor using a piezoelectric element such as a piezo has been used to measure an elastic wave (elastic vibration) propagating in an object and a vibration of a mechanical structure. However, this technique has a problem that the measurable frequency band is limited, the additional mass affects the object to be measured, and the gauge length cannot be increased.

  It has also been proposed to apply a laser Doppler optical fiber sensor to a vibration sensor. The principle of this method is roughly as follows. First, a light source is connected to one end of the optical fiber. At the other end of the fiber, a reflecting mirror that reflects input light and returns it to the fiber is attached. When vibration is applied to the optical fiber, the optical path length in the fiber changes as the fiber expands and contracts. When the time change of the optical path length is dL / dt, the frequency of light reflected from the fiber end changes in proportion to dL / dt due to the Doppler effect. Therefore, vibration can be measured by measuring the frequency change between the reflected light from the fiber end and the input light. Such a sensor has a solution problem of low sensitivity although it has a wide band.

  By the way, the inventor has bent the optical fiber and applied vibration to the bent portion, and observed the frequency change between the input light and the output light that passed through the fiber. As a result, the knowledge that the frequency change corresponding to the minute vibration has already occurred in the curved portion has been obtained.

Based on this knowledge, the present inventor has filed the following patent application.
International Publication WO03 / 002956

  An outline of the technique described in this document will be described with reference to FIG. This vibration measuring apparatus includes an input unit 1, an optical fiber 2, a detection unit 3, and an AOM (Acoustic Optical Modulator) 4.

  The input unit 1 inputs input light to the optical fiber 2. Specifically, the input unit 1 is a laser using a semiconductor or gas. The input unit 1 is connected to the optical fiber 2 via a coupler 21. A half mirror 11 for sending a part of the input light to the AOM 4 is disposed between the input unit 1 and the coupler 21.

  The optical fiber 2 has a curved portion 20 to which vibration to be measured is applied. The bending portion 20 is formed by circling the optical fiber 2. The bending part 20 is arrange | positioned in the location which should measure a vibration. For example, the bending portion 20 is fixed to the measurement site by a fixing means such as an adhesive or an adhesive tape.

  The detection unit 3 detects a frequency change between the output light from the optical fiber 2 and the input light from the input unit 1 that has passed through the bending unit 20. Specifically, the beat of the input light sent through the half mirror 11, the AOM 4 (described later) and the half mirror 31 and the output light from the optical fiber 2 can be taken to detect a change in beat frequency. ing. Thereby, a frequency change between input and output light is detected. The detection unit 3 is connected to the fiber 2 via the coupler 22.

The AOM 4 can change the input optical frequency f ₀ to f ₀ + f _M (f _M includes positive and negative). Such AOM configurations are well known.

The vibration measurement method using this apparatus is as follows.
First, the bending portion 20 of the fiber 2 is arranged at a measurement location using an arbitrary fixing means (for example, an adhesive). On the other hand, input light is sent from the input unit 1 to the fiber 2. When vibration (elastic wave) is applied to the bending portion 20 in this state, the frequency of light passing through the bending portion 20 changes according to the vibration. That is, the frequency of the output light changes. This frequency change is detected by the detector 3. Thereby, the vibration applied to the bending part 20 is detectable as a frequency change.

  By the way, in addition to the above knowledge, the present inventor has obtained knowledge about the shape of the arc portion that can control the directivity in vibration measurement.

  The present invention has been made based on this finding, and an object thereof is to provide a vibration measuring device capable of controlling directivity in vibration measurement.

  The vibration measuring apparatus according to the present invention includes an input unit, a waveguide, and a detection unit. The input unit inputs input light to the waveguide. The waveguide has an oval part to which vibration to be measured is applied. The ellipse part allows the input light to pass through. The detection unit detects a change in frequency between the output light from the waveguide that has passed through the ellipse and the input light.

  The oval portion may include two arc portions and two straight portions connecting the opening portions of the two arc portions.

  The waveguide is, for example, an optical fiber.

  The ellipse can also be formed by circling the optical fiber.

  The vibration measuring device may have directivity in vibration measurement.

  The vibration measuring method according to the present invention is configured to control directivity in vibration measurement by adjusting a ratio between the radius of curvature R of the arc portion and the length l of the linear portion in the vibration measuring device. Yes.

  ADVANTAGE OF THE INVENTION According to this invention, the vibration measuring device which can control the directivity in vibration measurement can be provided.

  A vibration measuring apparatus and measuring method according to an embodiment of the present invention will be described below with reference to FIG. In the description of this embodiment, the same components as those in the apparatus shown in FIG.

  In this vibration measuring apparatus, an oval part 120 is used instead of the bending part 20 of FIG. The oval part 120 is configured by rotating the optical fiber 2 as one waveguide in an oval shape. The oval portion 120 includes two arc portions 121 and two straight portions 122 that connect the opening portions of the two arc portions 121. The straight portions 122 are substantially parallel in this embodiment.

  The straight part 122 may have a shape having a radius of curvature that is significantly larger than the arc part 121 (that is, close to a straight line). Further, the curvature of the arc portion 121 may not be constant. Therefore, the shape of the oval part 120 may be a flat ellipse.

  The type of the optical fiber 2 is not particularly limited, and appropriate types such as a GI type, an SI type, a single mode type, and a multimode type can be used. Further, instead of the optical fiber 2, other types of waveguides (for example, a hollow metal pipe or an optical path micro-processed on a glass substrate) can be used.

  As the material of the optical fiber 2 or an alternative waveguide, a material that reversibly deforms, that is, elastically deforms, is preferable. The elastic body includes almost all industrial materials such as metal materials such as steel, glass, ceramics, and plastics.

Next, an example of a vibration measurement method using the apparatus will be described.
First, the oval part 120 of the fiber 2 is arranged at a measurement location using an arbitrary fixing means (for example, an adhesive or an adhesive tape). The fixing means is optional. Further, as long as vibration can be transmitted to the oval part 120, the oval part 120 may not be fixed.

  On the other hand, input light is sent from the input unit 1 to the fiber 2. In this state, when vibration (elastic wave) is applied to the ellipse 120, the frequency of light passing through the ellipse 120 changes according to the vibration. That is, the frequency of the output light changes. This frequency change is detected by the detector 3. Thereby, the vibration applied to the oval part 20 can be detected as a frequency change. The vibration that can be detected in the oval part 120 is considered to be a vibration having a vector component in the radial direction of the arc part 121 in principle.

  Moreover, the frequency of the input light input into the detection part 3 can be changed by using AOM4. By changing the amount of frequency change in the AOM 4, it is possible to know whether the frequency change of the light that has passed through the oval part 120 is in the positive direction or in the negative direction.

  The operation up to this point is the same as the technique described in Patent Document 1. However, in the apparatus of this embodiment, directivity in vibration measurement can be obtained for the following reason. Moreover, this directivity can be controlled by changing the ratio between the radius of curvature R of the arc portion 121 and the length l of the linear portion 122. The reason is described below.

When light is incident from one end A (light source) of an arbitrary waveguide Γ that can expand and contract and bend as schematically shown in FIG. 3 and observed at the other end B (observation point), the Doppler effect of light The frequency change f _D of the generated light is expressed as follows:

Where f ₀ and c ₀ are the frequency of the light source and the speed of light in vacuum, n _eq is the equivalent refractive index of the optical waveguide, vector v is the displacement velocity vector of the line element ds, and κ is the curvature of the line element ds , Vector r and vector n are the unit direction vector and unit normal vector of the line element ds, respectively.

  It is assumed that a light source (input unit) and a detection unit are attached to the ends A and B of the waveguide Γ in FIG. In this case, both ends do not move (or move together with the waveguide), so the first term in the above equation is zero. When a part of the waveguide is attached to the object to be measured, if deformation / extension in the waveguide other than the curved part can be ignored, both ends A and B are approximately the ends of the attached curved part and Can be considered.

せん断ひずみ速度を

とする。 In the following, the sensitivity is calculated for several waveguide shapes. However, it is assumed that the strain rate distribution is uniform in the measurement target region (integration region), and the x- and y-direction axial strain rates are

Shear strain rate

And

(1) Case of Circular Shape The theoretical formula of the relationship between strain rate and frequency shift in the case of a perfect circular loop waveguide as shown in FIG. 4 is as follows. Here, λ is an arbitrary proportionality constant. However, physically, λ corresponds to the wavelength of light in the waveguide.

When the loop has N windings, f _D is as follows, where Ka is the strain transfer coefficient (calibration coefficient) in the coating layer or adhesive layer existing between the curved portion and the vibration generating portion.

However, R _av is an average value of the radius of the loop wound N times. In volume N, the sensitivity is N times. In the case of a perfect circle shape, the so-called principal strain rate is measured regardless of the coordinate system. In other words, there is no directivity.

(2) In the case of U-shaped The theoretical formula in the case of a U-shaped waveguide consisting of a linear portion having a length l and a semicircular portion having a radius R as shown in FIG.

となり、l/Rの比が大きいほど、指向性と長手方向の感度はともに高くなる。 Strain sensitivity ratio S _xy in the longitudinal direction (x direction) and lateral direction (y direction) is

Thus, the larger the ratio of l / R, the higher the directivity and the longitudinal sensitivity.

(3) In the case of an oval shape As shown in FIG. 2, the theoretical formula in the case of an oval shape with both ends having a radius R and the length of the parallel portion is as follows.

The sensitivity ratio S _xy can be expressed as follows.

  Even in the case of an oval shape, the directivity and sensitivity increase in proportion to l / R, which is the same as the U-shape. In the case of N windings, the overall sensitivity is N times the same as in the case of a perfect circle. The shear strain rate component does not affect the Doppler frequency shift.

  Therefore, according to the apparatus of this embodiment, there is an advantage that the directivity in vibration measurement can be controlled by adjusting the ratio of 1 / R. Other configurations and advantages are the same as those of the technique described in Patent Document 1.

  The description of the embodiment is merely an example, and does not indicate a configuration essential to the present invention. The configuration of each part is not limited to the above as long as the gist of the present invention can be achieved.

It is explanatory drawing for demonstrating the outline of the conventional vibration measuring device. It is explanatory drawing which shows the shape of the ellipse part in the vibration measuring device which concerns on one Embodiment of this invention. It is explanatory drawing for demonstrating the directivity control based on a waveguide shape. It is explanatory drawing for demonstrating the directivity control based on a waveguide shape. It is explanatory drawing for demonstrating the directivity control based on a waveguide shape.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Input part 2 Optical fiber 20 Bending part 3 Detection part 120 Oval part 121 Arc part 122 Straight line part

Claims (6)

An input unit, a waveguide, and a detection unit;
The input unit inputs input light to the waveguide,
The waveguide has an oval part to which vibration to be measured is applied,
The oval part is one through which the input light passes,
The vibration measuring apparatus, wherein the detection unit detects a frequency change between output light from the waveguide that has passed through the ellipse and input light.   The vibration measuring apparatus according to claim 1, wherein the ellipse portion includes two arc portions and two straight portions connecting the opening portions of the two arc portions.   The vibration measuring apparatus according to claim 1, wherein the waveguide is an optical fiber.   The vibration measuring apparatus according to claim 3, wherein the oval part is formed by circling the optical fiber.   The vibration measuring apparatus according to claim 1, wherein the vibration measuring apparatus has directivity in vibration measurement. The vibration measurement method using the vibration measurement device according to claim 2, wherein the directivity in vibration measurement is controlled by adjusting a ratio between a radius of curvature R of the arc portion and a length l of the straight portion. A vibration measurement method characterized by that.

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