JP2000097786A - Mechanical force sensor - Google Patents

Abstract

(57) [Problem] To use a single FBG (optical fiber Bragg diffraction grating) with a uniform grating interval so that a single FBG can detect a mechanical force without being affected by a temperature change. I do. SOLUTION: An FB having a uniform lattice spacing in a part of the length direction.
An optical fiber 1 having G4 and a tension member 11 having a portion 12 which generates non-uniform distortion when a tensile force is applied. The FBG 4 portion of the optical fiber 1 generates non-uniform distortion of the tension member 11. It was adhesively fixed to the part 12. When a tensile force is applied, the lattice spacing of the FBG 4 becomes non-uniform, and the bandwidth of the reflected wave is widened. Therefore, the tensile force can be measured by changing the bandwidth.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mechanical force sensor for detecting a mechanical force such as a tensile force, a compressive force, and a pressure using an FBG (optical fiber Bragg diffraction grating).

[0002]

2. Description of the Related Art An FBG is an optical fiber having a function of reflecting light of a specific wavelength called a Bragg wavelength among light of various wavelengths propagating in an optical fiber.
Utilizing this property, FBGs are used in sensing elements such as pressure sensors, tensile force sensors, and temperature sensors, or in optical communication circuit elements such as optical filters.

FIG. 9 schematically shows an optical fiber having an FBG. Reference numeral 1 denotes an optical fiber, which is composed of a core 2 and a clad 3, and an FBG 4 is formed in a part of the optical fiber in the length direction. FBG4 has a high refractive index region N _H and a low refractive index region N _H
L has a structure formed alternately in the length direction of the optical fiber. The light transmission characteristics of the FBG are represented by two parameters, a Bragg wavelength (reflection wavelength) and a reflection wavelength range.

[0004] In the FBG, light within a certain wavelength range centered on the Bragg wavelength is reflected among the light propagating in the optical fiber. The Bragg wavelength λ _B is determined by the refractive index _ne and the lattice spacing d as in the following equation.

[0005]

[Number 1] λ _B = 2n _e · d

The refractive index _ne is called an effective refractive index and is an amount determined by the structure of the optical fiber, but is substantially equal to the refractive index of the core material. Further, the wavelength range of light reflected by the FBG, that is, the bandwidth is determined by the number of gratings, and the more the number of gratings, the narrower the bandwidth.

When a tensile force or a change in temperature is applied to the optical fiber 1, the Bragg wavelength (reflection wavelength) of the FBG 4 changes. This is because the refractive index _ne and the lattice spacing d change due to a change in tensile force or temperature. However, since the number of gratings does not change, the bandwidth of the reflected wave does not change.

When such an FBG is applied to a pressure sensor, a tensile force sensor, or the like, a method of detecting a change in Bragg wavelength has been conventionally used. However, in this method, it is not possible to distinguish whether the change in the Bragg wavelength is due to strain or a change in temperature. Therefore, it is necessary to employ some kind of temperature compensation. Conventional representative temperature compensation methods include a method using two FBGs,
This is a method using an FBG having a structure called chirped grating.

FIGS. 10A and 10B show a temperature compensation method using two FBGs. In the figure, 1a is the first F
1st optical fiber having BG4a, 1b is second FB
A second optical fiber having G4b, 5 is a disk-shaped diaphragm that bends by pressure, 6 is a pressure vessel that supports the periphery of the diaphragm 5 in a sealed state, and 7 is a pressurized fluid introduction unit. The first FBG 4a is for detecting distortion, and is fixed to the center of the diaphragm 5 in a diametrical direction by means such as bonding. This is because the center of the diaphragm 5 has the highest distortion detection sensitivity. Pressure vessel 6
When the internal pressure increases and the diaphragm 5 bends, the first FBG 4a is distorted and the Bragg wavelength changes. On the other hand, the second FBG 4b is for temperature compensation, and is fixed to a position where the distortion of the diaphragm 5 is not detected (pressure vessel fixing portion) by means such as bonding.

With the above configuration, the change in the Bragg wavelength due to the temperature change is calibrated in advance for the second FBG 4b for temperature compensation. Also, the first F for distortion detection
Regarding the BG 4a, a change in Bragg wavelength due to a change in pressure (strain) and temperature is calibrated in advance. By performing such a procedure, when the change in the Bragg wavelength is measured by the first FBG 4a, the change in the Bragg wavelength due to the temperature change measured by the second FBG 4b can be corrected. The exact pressure given can be measured. However, this method uses the first FBG 4a and the second FBG 4a.
It is assumed that the temperatures of 4b are equal.

FIG. 11 shows the F of the chirped grating structure.
BG is shown. In the figure, 1 is an optical fiber, 2 is a core,
3 is clad, 8 is F with chirped grating structure
BG. This FBG 8 is manufactured as follows. First, the cladding 3 where the FBG is to be formed
Is etched with hydrofluoric acid or the like to be processed into a tapered shape.
Next, a constant tensile force is applied to the optical fiber 1 to maintain the state. When a tensile force is applied, a thin portion expands more than a thick portion in the tapered region. In this state,
An ordinary FBG having a constant lattice spacing is formed. After that, when the tensile force of the optical fiber is removed, the thin part shrinks more than the thick part in the tapered region, and as a result,
The FB of the chirped grating structure in which the lattice interval gradually narrows from the thick part to the thin part as shown in FIG.
G8 will be obtained.

The FBG 8 having such a chirped grating structure corresponds to an FBG having a different lattice spacing, that is, an FBG having a different Bragg wavelength which is continuously connected, and thus reflects light of various wavelengths. That is, the bandwidth of the reflected wave is widened. When a tensile force is applied to the FBG 8 having the chirped grating structure, the FBG changes from the chirped grating structure to the FBG having a uniform lattice spacing, and accordingly, the bandwidth of the reflected wave is reduced. At this time, if a temperature change is applied, the temperature change only changes the Bragg wavelength and has no relation to the change in the bandwidth. Therefore, if the change in the bandwidth is measured using the FBG having the chirped grating structure, the strain (tensile force) can be measured regardless of the temperature change.

[0013]

However, the above-mentioned FBG
There are the following problems in the method of using two of them. -Since two FBGs are required, the cost increases. -Connection work between two FBGs is required, which increases the number of assembly steps and increases costs. -In terms of the configuration of the sensor system, two FBGs are connected and inserted into the transmission path, so that transmission loss increases. If the transmission loss increases, the detection light becomes weaker, so that the SN ratio deteriorates and the measurement accuracy deteriorates. -In the case of multi-point measurement, if the transmission loss increases, the number of sensors that can be installed is limited. Therefore, the number of measurement points is reduced, and the monitoring range is narrowed. -Calibration is required for two FBGs in advance. In practice, the two FBGs do not have exactly the same temperature, but have an error in temperature compensation.

The method using the chirped grating FBG has the following problems. -It is necessary to apply tension to the optical fiber when forming the FBG, which makes the production troublesome and the device becomes complicated. A step of forming a tapered portion in the optical fiber is required, and it is not easy to precisely form the tapered portion. Also, the cost of the taper processing is increased. -Since the tapered portion is thin and intense defects due to the tapering process are inevitably generated on the glass surface, the strength of the optical fiber is significantly reduced, and handling becomes difficult.

An object of the present invention is to provide a mechanical force sensor that solves the above problems.

[0016]

SUMMARY OF THE INVENTION A mechanical force sensor according to the present invention has an optical fiber having an FBG having a uniform lattice spacing in a part of its length, and generates a non-uniform distortion when a mechanical force is applied. The optical fiber has a FBG portion fixed to a portion of the strain generating member that generates non-uniform strain (claim 1).

In the dynamic force sensor of the present invention, the portion of the distortion generating member that generates the non-uniform distortion can be constituted by changing the cross-sectional area in the direction in which the mechanical force is applied. ).

When the mechanical force sensor according to the present invention is a pressure sensor, the F sensor having a uniform lattice spacing in a part of the length direction.
The optical fiber may include an optical fiber having a BG and a diaphragm for detecting pressure, and the FBG portion of the optical fiber may be fixed to a portion of the diaphragm that generates uneven distortion.

In the case where the mechanical force sensor according to the present invention is a pressure sensor, the mechanical force sensor comprises an optical fiber having an FBG having a uniform lattice spacing in a part of the length direction, and a pressure detecting diaphragm. It is more preferable that the FBG portion of the optical fiber is fixed so as to extend over a portion of the diaphragm that generates extensional strain and a portion of the diaphragm that generates contraction strain (claim 4).

Further, the mechanical force sensor according to the present invention has an optical fiber having an FBG having a uniform lattice spacing in a part of the length direction, wherein a sectional area of the section having the FBG changes in the longitudinal direction; A strain generating member having a portion that generates a uniform strain when a mechanical force is applied, and generates a uniform strain of the strain generating member on both sides of a portion where the cross-sectional area of the optical fiber changes in the longitudinal direction. It is also possible to adopt a configuration fixed to a portion to be formed (claim 5).

[0021]

Embodiments of the present invention will be described below in detail with reference to the drawings. [Embodiment 1] FIG. 1 shows an embodiment of the present invention. This embodiment is a tension sensor. In the figure, reference numeral 1 denotes an optical fiber having FBGs 4 having a uniform lattice spacing in a part of the length direction, and reference numeral 11 denotes a tension member (strain generating member) made of a metal plate. The tension member 11 has a portion 12 whose width is gradually narrowed at an intermediate portion in the length direction. This portion 12 generates uneven elongation strain when the tensile member 11 receives a tensile force. In other words, a portion with a narrow width generates a larger elongation strain than a portion with a wide width. The portion of the FBG 4 of the optical fiber 1 is fixed to a portion 12 where the width of the tension member 11 is gradually narrowed by means such as bonding with the length direction thereof being directed to the length direction of the tension member 11.

Since the tensile force sensor is configured as described above, when a tensile force is applied to the tensile member 11, a portion 12 having a gradually narrowing width has a larger elongation strain as the width decreases. Also, the FBG 4 fixed thereto has the same elongation strain. As a result, FBG4
Means that the lattice spacing changes from a uniform state to a non-uniform state (chirped grating structure), and the bandwidth of the reflected wave is wider after applying a tensile force than before applying a tensile force as shown in FIG. Therefore, the magnitude of the tensile force can be measured by measuring the bandwidth. Since the change in the bandwidth is independent of the change in temperature, the tensile force can be measured without being affected by the change in temperature by using this tensile force sensor.

A prototype of a tensile force sensor having the following structure was manufactured. The thickness of the tension member 11 is 100 μm. The maximum width of the portion 12 where the width is gradually narrowed = 10 mm The minimum width of the portion 12 where the width is gradually narrowed = 5 mm Material: stainless steel FBG4, Bragg wavelength: 1.55 μm Bandwidth : 0.2 nm As a result of applying a tensile force to the tensile force sensor and changing the tensile force in the range of 0 to 20 kg, it was found that the bandwidth of the reflected wave was changed. The temperature was changed from 0 ° C to 40 ° C, but the bandwidth did not change.

[Embodiment 2] FIG. 3 shows another embodiment of the present invention. This embodiment is also a tensile force sensor. In the figure, reference numeral 1 denotes an optical fiber having the same FBG 4 as in the first embodiment, and reference numeral 13 denotes a tensile member (strain generating member) made of a metal round bar. The tension member 13 has a lengthwise hole at the center, and a tapered portion whose outer diameter gradually decreases in the middle of the lengthwise direction.
Has 14 The tapered portion 14 generates uneven elongation strain when the tension member 13 receives a tensile force. That is, a portion having a small outer diameter generates a larger elongation strain than a portion having a large outer diameter. The optical fiber 1 is inserted through a hole in the center of the tension member 13 so that the portion of the FBG 4 is located in the tapered portion 14, and is fixed to the inner surface of the tension member 13 by means such as bonding. Such a tensile force sensor can detect the tensile force as in the first embodiment.

When the above-described tensile force sensor is manufactured, it is easy to manufacture by vertically splitting the tensile member 13 and bonding the optical fiber 1 therebetween with an adhesive. The tensile force sensor of the type shown in FIG. 3 can also be used as a compressive force sensor if a compressive force is applied to the tensile member 13 from both ends.

[Embodiment 3] FIG. 4 shows still another embodiment of the present invention. This embodiment is a pressure sensor. In the figure, 1 is an optical fiber having the same FBG 4 as in the first embodiment, 5 is a disk-shaped diaphragm (strain generating member) that bends by pressure, 6 is a pressure vessel that supports the periphery of the diaphragm 5 in a sealed state, and 7 is a pressure vessel. It is a pressure fluid introduction part. In this embodiment, the FBG 4 is fixed at a position deviated from the center of the diaphragm 5 in the radial direction by means such as bonding as shown in FIGS. 4 (a) and 4 (b).

FIG. 4C shows the magnitude of the distortion in each part in the radial direction when pressure is applied to the diaphragm 5. Since the diaphragm 5 has a certain degree of rigidity and its peripheral portion is completely fixed to the edge of the pressure vessel 6, distortion of the surface of the diaphragm 5 when pressure is applied to the diaphragm 5 is limited to a certain position in the radial direction. After T,
On the other hand, elongation strain occurs near the center and shrinkage strain occurs near the outer periphery. In the case of a disk-shaped diaphragm, the position T at which the surface distortion changes from elongation distortion to contraction distortion is about 0.6 times the radius from the center.

Therefore, if the FBG 4 having a uniform lattice spacing is fixed to the diaphragm 5 so as to straddle the above-described position T, the FBG 4 may be pressed when pressure is applied to the diaphragm 5.
Is extended at one end side (center side of the diaphragm) and contracted at the other end side (outer peripheral side of the diaphragm). As a result, F
Since the lattice spacing of the BG4 increases at one end and decreases at the other end, the FBG4 changes to a chirped grating structure. Therefore, by measuring the bandwidth of the reflected wave, it is possible to detect the magnitude of the pressure independently of the temperature change.

A prototype pressure sensor having the following structure was manufactured. The thickness of the diaphragm 5 is 0.15 mm, the diameter is 25 mm, and the material is stainless steel. The Bragg wavelength of the FBG4 is 1.55 μm. The bandwidth is 0.2 nm. Pressure is applied to this pressure sensor, and the pressure is reduced from 0 to 0.3 atm. ) After the change, the bandwidth of the reflected wave became about 2 nm. The temperature was changed from room temperature by about 40 ° C., but the bandwidth was not changed.

[Embodiment 4] FIG. 5 shows still another embodiment of the present invention. This embodiment can be used both as a tensile force sensor and as a bending stress sensor. In the figure, reference numeral 1 denotes an optical fiber having the same FBG 4 as in the first embodiment, and reference numeral 15 denotes a tensile member (strain generating member) made of a metal square bar. The tension member 15 has a groove 16 formed in a middle portion in the length direction in a direction orthogonal to the length direction to reduce the cross-sectional area of the portion. Therefore, when the tensile member 11 receives a tensile force, the portion of the groove 16 generates more elongation strain than the other portions. The optical fiber 1 is arranged such that half of the FBG 4 is in the opening section of the groove 16 and the other half is on the tension member 15. The optical fiber 1 is fixed to the tension member 15 on both sides of the groove 16 by means such as bonding.

With such a configuration, when a tensile force is applied to the tension member 15, the portion of the FBG 4 which is fixed to the tension member 15 hardly expands, and the portion within the opening section of the groove 16 becomes large. extend. Therefore, the Bragg wavelength of the FBG 4 is divided into two as shown in FIG. 6, but the tensile force can be detected as the expansion of the bandwidth as in the above-described embodiment. Note that the sensor of this embodiment can also be used as a bending stress sensor when a force is applied to the tension member 15 in the directions of arrows P and Q in FIG. 5B.

[Embodiment 5] FIG. 7 shows still another embodiment of the present invention. In this embodiment, a groove 16 is formed in the longitudinal middle portion of the tension member 15, and the FBG 4 is disposed so as to extend over both sides of the groove 16, and both ends of the FBG 4 are bonded and fixed to the tension member 15 on both sides of the groove 16. It was done. Even with such a configuration, the same effect as in the fourth embodiment can be obtained.

[Embodiment 6] FIG. 8 shows still another embodiment of the present invention. This embodiment is a tension sensor.
In the figure, 1 is an optical fiber, 2 is a core, 3 is a clad, and 4 is an FBG 4 formed at a part in the length direction and having a uniform lattice spacing. The clad 3 in the section including the FBG 4 is etched into a tapered shape by hydrofluoric acid or the like. Reference numeral 17 denotes a tension member (strain generating member) made of a metal bar or a metal plate having a uniform cross-sectional area in the length direction. The optical fiber 1 has both sides of the tapered portion 18 fixed to the tension member 17 by means such as bonding.

With such a configuration, when a tensile force is applied to the tension member 17, a uniform elongation strain is generated in the tension member 17 in the length direction, but the tapered portion 18 of the optical fiber 1.
In this case, a larger elongation strain is generated as the outer diameter becomes smaller. For this reason, the FBG 4 changes from a state where the lattice spacing is uniform to a state where the lattice spacing is non-uniform (chirped grating structure), and the magnitude of the tensile force can be measured by measuring the bandwidth.

[0035]

As described above, according to the present invention, 1
The mechanical force can be detected by the book FBG without being affected by the temperature change, and an FBG having a uniform lattice spacing can be used. Therefore, it is possible to solve the problem in the case of using two conventional FBGs and the problem in the case of using the FBG having the chirped grating structure.

[Brief description of the drawings]

1A and 1B show one embodiment of a tensile force sensor according to the present invention, wherein FIG. 1A is a plan view and FIG.

FIG. 2 is a graph showing a waveform of a reflected wave of an FBG of the tensile force sensor of FIG.

FIG. 3 is a perspective view showing another embodiment of the tensile force sensor according to the present invention.

4A and 4B show an embodiment of the pressure sensor according to the present invention, wherein FIG. 4A is a plan view, FIG. 4B is a cross-sectional view taken along line bb of FIG. 4A, and FIG. 4C is a graph of diaphragm distortion.

5A and 5B show still another embodiment of the tensile force sensor according to the present invention, wherein FIG. 5A is a plan view and FIG. 5B is bb in FIG.
Line sectional view.

FIG. 6 is a graph showing a waveform of a reflected wave of the FBG of the tensile force sensor of FIG. 5;

7A and 7B show still another embodiment of the tensile force sensor according to the present invention, wherein FIG. 7A is a plan view and FIG. 7B is bb in FIG.
Line sectional view.

FIG. 8 is a sectional view showing still another embodiment of the tensile force sensor according to the present invention.

FIG. 9 is a cross-sectional view illustrating the structure of an FBG.

FIG. 10 shows a pressure sensor using a conventional FBG.
(A) is a top view, (b) is a bb line sectional view of (a).

FIG. 11 is a sectional view showing an FBG having a chirped grating structure.

[Explanation of symbols]

 1: Optical fiber 4: FBG (optical fiber Bragg diffraction grating) 5: Diaphragm 6: Pressure vessel 11: Plate-like tension member 12: Part that gradually narrows 13: Round bar-like tension member 14: Tapered part 15: Square rod-shaped tension member 16: groove 17: tension member with uniform cross-sectional area

Claims (5)

[Claims] An FBG having a uniform lattice spacing in a part of the length direction.
(Fiber Bragg Diffraction Grating) and a strain generating member having a portion that generates non-uniform strain when a mechanical force is applied.
A mechanical force sensor wherein a portion G is fixed to a portion of the distortion generating member that generates uneven distortion. 2. The mechanical force sensor according to claim 1, wherein the portion of the distortion generating member that generates the non-uniform distortion has a sectional area changed in a direction in which a mechanical force is applied. 3. An FBG having a uniform lattice spacing in a part of the length direction.
A pressure sensor comprising: an optical fiber having the following formula: and a diaphragm for detecting pressure, wherein an FBG portion of the optical fiber is fixed to a portion of the diaphragm that generates uneven distortion. 4. An FBG having a uniform lattice spacing in a part of the length direction.
And a diaphragm for pressure detection, wherein the FBG portion of the optical fiber is fixed so as to extend over a portion of the diaphragm that generates elongation distortion and a portion that generates contraction distortion. Pressure sensor. 5. An FBG having a uniform lattice spacing in a part of the length direction.
An optical fiber having a section having an FBG in which the cross-sectional area changes in the longitudinal direction; and a strain generating member having a portion that generates uniform strain when a mechanical force is applied. A mechanical force sensor wherein both sides of a portion where the area changes in the longitudinal direction are fixed to portions of the strain generating member which generate uniform strain.