JP3571936B2 - Pressure measuring device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to pressure measurement apparatus using an optical fiber.
[0002]
[Prior art]
As means for detecting the pressure of a fluid using an optical fiber, the following is conventionally known.
(1) A light shielding member disposed between the end faces of an optical fiber is moved up and down by pressure, and a change in pressure is detected as a change in optical transmission loss (Japanese Patent Application Laid-Open No. 8-152372).
{Circle around (2)} A method in which the distance between the end faces of an optical fiber is changed by pressure, and the change in pressure is detected as a change in optical transmission loss (Japanese Patent Application Laid-Open No. 62-251632).
(3) A method in which the curvature of an optical fiber is changed by pressure, and the change in pressure is detected as a change in optical transmission loss (Japanese Patent Laid-Open No. 3-181831).
{Circle around (4)} A method in which a magnetic field applied to a magneto-optical element is increased or decreased by pressure, and a change in pressure is detected as a change in a rotation angle of a polarization plane (Japanese Patent Laid-Open No. 63-205534).
(5) A method in which the stress on the photoelastic material is changed by pressure, and the change in pressure is detected as a change in the rotation angle of the plane of polarization (Japanese Patent Laid-Open No. 1-88224).
[0003]
[Problems to be solved by the invention]
However, these methods have the following problems.
a. In the methods of (1) to (3), it is necessary to manufacture a light-shielding member having a fixed position, an interval between the end faces of the optical fiber, or an initial state of the curvature of the optical fiber. Is required, and the cost increases. Also, the initial state may change during transportation.
b. Since the methods (1) to (3) detect the pressure as the magnitude of the light transmission loss, a plurality of such sensors are connected in series to one optical fiber to measure the pressure at a plurality of points. When performing a batch operation (one light source and one photodetector) at one end of the base station, if a large transmission loss occurs in the sensor closer to the base station, the detection accuracy of the sensor farther away is significantly reduced. There is a problem of doing. In order to avoid this, it is conceivable to reduce the maximum value of the transmission loss of each sensor. However, if this is done, the dynamic range of each sensor will be reduced, and the detection accuracy will be reduced.
[0004]
c. In the methods (4) and (5), the rotation of the plane of polarization occurs in the optical fiber between the light source and the sensor, and errors tend to occur in the measurement results.
d. In the methods (4) and (5), a plurality of pressures cannot be collectively measured by connecting a plurality of optical fibers in series to one optical fiber.
[0005]
SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to detect pressure with high accuracy and stability, and to connect a plurality of sensors in series to one optical fiber so that a plurality of pressures can be detected. An object of the present invention is to enable detection to be performed collectively.
[0006]
To achieve this object, the pressure measuring device of the present invention
A plurality of pressure sensors each including an optical fiber having an FBG (optical fiber Bragg diffraction grating) formed in a part of the length direction thereof and means for converting a change in pressure into a change in elongation strain of the FBG ;
The FBGs of each pressure sensor are connected in series, and the center wavelength in the absence of pressure is shifted by a predetermined interval,
Further, there is provided means for inputting light to the optical fiber including the plurality of FBGs connected in series and measuring a wavelength shift amount of the Bragg reflected light,
It is characterized by the following.
The pressure sensor used in this device detects pressure by converting a change in pressure into elongation strain of FBG. If the elongational strain of the FBG changes due to the change in pressure, the wavelength of the Bragg reflected light of the FBG when light (continuous light) is incident on the optical fiber including the FBG changes. Can be measured.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[Embodiment 1]
FIG. 1 shows an embodiment of the pressure sensor used in the present invention. In the figure, 1 is a pressure sensor, 3 is a container into which a pressure fluid is introduced, 5 is a diaphragm for closing a circular opening of the container 3, 7 is a pressure transmission rod that rises vertically from the center of the diaphragm 5 to outside the container 3, 9 Is a rigid disk fixed vertically to the tip of the pressure transmitting rod 7, 11 is a tensile strength line connecting the rigid disk 9 and the portion of the container 3 around the diaphragm 5 without slack, and 13 is a single mode light. The fiber 15 is an FBG (optical fiber Bragg diffraction grating) formed in a part of the optical fiber 13 in the longitudinal direction.
[0011]
A plurality of tensile wires 11 are provided at regular intervals in the circumferential direction. The FBG 15 is fixed to one of the tensile wires 11 in the same direction as the tensile wire 11 with an adhesive or the like.
[0012]
In the pressure sensor 1, when the pressure of the fluid in the container 3 increases, the diaphragm 5 is displaced outward, and the pressure transmission rod 7 tries to push up the rigid disk 9. Since it is retained in the container 3, a tension corresponding to the magnitude of the pressure is applied to the tensile strength line 11. As a result, tensile strain is generated in the tensile strength line 11, and tensile strain is also generated in the FBG 15 integrated with the tensile strength line 11. When elongation strain occurs in the FBG 15, the wavelength of the Bragg reflected light of the FBG 15 changes. Therefore, if the change is measured by means described later, the pressure of the fluid in the container 3 can be measured.
[0013]
[Embodiment 2]
FIG. 2 shows another embodiment of the pressure sensor used in the present invention. This pressure sensor 1 is of a type in which the inside of the container 3 is at atmospheric pressure and measures the pressure of the fluid outside the container 3. Therefore, the pressure transmission rod 7 rises vertically from the center of the diaphragm 5 into the container 3, and the rigid disk 9, the tensile strength wire 11, the optical fiber 13, and the FBG 15 are provided in the container 3 in the same manner as in the first embodiment. ing. In this case, since the container 3 is immersed in the pressure fluid, the optical fiber 13 is led out to the outside atmosphere through a pipe 17 that connects the inside of the container 3 and the outside air. The operation of the pressure sensor 1 is the same as that of the first embodiment.
[0014]
FIG. 3 shows an embodiment of a pressure measuring device using the pressure sensor of FIG. In this example, three pressure sensors 1 are connected in series by an optical fiber 19 of a transmission line. Reference numeral 21 denotes a connection box for housing a connection between the optical fiber 13 of the pressure sensor 1 and the optical fiber 19 of the transmission line. The base end of the optical fiber 19 of the transmission line is connected to the measuring device 23. The measuring device 23 includes a light source 25, a coupler 29, an optical spectrum analyzer 31, and the like. It is preferable that the measuring device 23 is combined with a computer (not shown) for processing the analysis result of the optical spectrum analyzer 31. A non-reflection terminal 33 is provided at the end of the optical fiber 13 of the pressure sensor 1 farthest from the measuring device 23.
[0015]
When light (continuous light) is sent from the light source 25 through the coupler 29, Bragg reflected light generated by the FBG 15 of each pressure sensor 1 returns to the optical spectrum analyzer 31. The center wavelength of the FBG 15 attached to each pressure sensor 1 when there is no pressure is shifted by a predetermined interval in advance. This interval is determined in consideration of the center wavelength fluctuation width of the Bragg reflected light according to the measurement range of each pressure sensor 1. Therefore, it can be distinguished from which pressure sensor 1 the signal is from which range of the central wavelength of the Bragg reflected light.
When a pressure is applied to the diaphragm 5 of the pressure sensor 1 and an elongation distortion occurs in the FBG 15, the wavelength of the Bragg reflected light changes with a clear correlation with the elongation distortion as shown in FIG.
[0016]
Therefore, if the wavelength of the Bragg reflected light returned from each FBG 15 is measured by the optical spectrum analyzer 31, the pressure applied to each pressure sensor 1 can be measured. In this measurement, since the elongational strain of the FBG and the wavelength of the Bragg reflected light are clearly correlated, accurate pressure measurement can be performed, and the measurement value of one FBG affects the measurement value of the other FBG. Since no pressure is given, pressure measurement at a plurality of locations can be performed collectively.
[0017]
[Embodiment 3]
FIG. 5 shows still another embodiment of the pressure sensor used in the present invention. In the figure, the same parts as those in FIG. 1 or FIG. 2 are denoted by the same reference numerals. This pressure sensor 1 differs from the pressure sensor shown in FIG. 1 or FIG. 2 in that a bellows 35 is used instead of the diaphragm. The bellows 35 is arranged so as to surround one end of the pressure transmitting rod 7 penetrating the wall of the container 3, and one end of the bellows 35 is connected to the container 3, and the other end is connected to the pressure transmitting rod 7. It is joined to one end of the flange portion 37 while maintaining airtightness or liquid tightness. The case where the container 3 surrounds the bellows 35 side is the same as the embodiment of FIG. 1, and the case where the container 3 surrounds the rigid disk 9 side is the same as the embodiment of FIG. The method of use is the same as that of FIG. 1 or FIG.
[0018]
[Embodiment 4]
FIG. 6 shows still another embodiment of the pressure sensor used in the present invention. In the figure, the same parts as those in FIG. 2 are denoted by the same reference numerals. This pressure sensor 1 is different from the pressure sensor of FIG. 2 in that a bourdon tube 39 is used instead of the diaphragm, and a tension line 11 is provided between the tip of the bourdon tube 39 and a fixed support point 41 (integral with the container 3). And the FBG 15 is attached to the tensile strength wire 11. When the internal pressure increases, the Bourdon tube 39 tends to spread, but since the tip is retained by the tensile strength line 11, the force is applied to the tensile strength line 11 and is converted into the elongation strain of the FBG 15. Therefore, pressure measurement can be performed similarly to the embodiment of FIG.
[0019]
[Embodiment 5]
FIG. 7 shows still another embodiment of the pressure sensor used in the present invention. In the figure, the same parts as those in FIG. 2 are denoted by the same reference numerals. This pressure sensor 1 is different from the pressure sensor of FIG. 2 in that the FBG 15 is directly attached to the diaphragm 5 and the pressure transmission rod and the tensile strength line are omitted. The FBG 15 is desirably attached to the center of the diaphragm 5 in the radial direction as shown.
[0020]
In the case of this embodiment, when pressure is applied to the diaphragm 5, the diaphragm 5 is elastically deformed, whereby the FBG 15 is stretched and strained. Therefore, the pressure can be measured similarly to the embodiment of FIG. Since the pressure sensor of this embodiment is structurally very simple, there is an advantage that it can be easily manufactured and the cost can be reduced. If the container 3 is formed on the opposite side of the diaphragm 5, the pressure in the container 3 can be detected similarly to the pressure sensor of the first embodiment.
[0021]
[Embodiment 6]
FIG. 8 shows still another embodiment of the pressure sensor used in the present invention. This pressure sensor has the FBG 15 fixed directly to the diaphragm 5 as in the fifth embodiment, but differs from the fifth embodiment in that the FBG 15 is fixed to the diaphragm 5 with tension applied. Reference numeral 47 denotes an adhesive for fixing the FBG 15 to the diaphragm 5, and the FBG 15 is embedded in the adhesive 47.
[0022]
Since the diaphragm 5 is generally made of stainless steel, if the FBG 15 is fixed to the diaphragm 5 without applying tension, if the diaphragm 5 contracts due to a temperature drop, the FBG 15 sags, and a dead zone is formed in a region where the measurement pressure is low as shown in FIG. 9B. However, if the FBG 15 is fixed by applying tension, the FBG 15 does not sag, so that the dead zone can be eliminated as shown in FIG. 9A.
[0023]
In order to fix the FBG 15 to the diaphragm 5 by applying tension, a method as shown in FIG. 10 may be adopted. First, as shown in (a), the FBG 15 is disposed at a predetermined position on the diaphragm 5 (in this case, in the radial direction at the center), and the optical fiber 13 on one side of the FBG 15 is fixed to the diaphragm 5 with an adhesive tape 49A. Next, tension is applied to the optical fiber 13 by sandwiching the optical fiber 13 on the other side of the FBG 15 with the clip 51. At this time, a tension meter 53 such as a spring balance is connected to the clip 51 so that the magnitude of the tension can be recognized. The tension applied to the optical fiber 13 is preferably about 0.05 kgf as described later.
[0024]
With a predetermined tension applied to the optical fiber 13 as described above, the optical fiber 13 on the other side of the FBG 15 is fixed to the diaphragm 5 with an adhesive tape 49B as shown in FIG. In this state, the FBG 15 is kept under tension. Next, as shown in (c), the FBG 15 is fixed to the diaphragm 5 with an adhesive 47, and the optical fiber 13 is bent at a reasonable curvature (having a radius of 30 mm or more), so that the pedestal 5A of the diaphragm 5 (corresponding to the bottom frame of the container). ) Is fixed with an adhesive 47. As the adhesive 47, a two-component mixed epoxy adhesive can be used. After the adhesive 47 has hardened, the adhesive tapes 49A and 49B are peeled off, as shown in (d), and the FBG 15 is fixed to the diaphragm 5 under tension. The reason why the optical fiber 13 is bonded and fixed to the pedestal 5A is to prevent a tension other than the tension based on the distortion of the diaphragm 5 from being applied to the FBG 15.
[0025]
When the FBG 15 is bonded and fixed to the diaphragm 5, the tension of the FBG 15 is preferably set to about 0.05 kgf. The reason is as follows. First, the strain amount of the FBG when tension is applied to the optical fiber including the FBG is obtained. The outer diameter of the optical fiber is 0.125 mm, and the stress in the elongation direction f = F / S = 40.0 × 10 ⁶ (N / m ² ) From the Young's modulus of silica glass of 7.31 × 10 ¹⁰ (N / m ² ), the strain amount ε = 547 με (0.05%). This value is sufficiently smaller than the allowable distortion (about 0.2%) when the optical fiber is used. Further, since the material of the diaphragm is generally stainless steel, the FBG fixed to the diaphragm expands and contracts at the linear expansion coefficient of stainless steel. The linear expansion coefficient of stainless steel is 15 με / ° C., which is 30 times larger than the linear expansion coefficient of quartz glass of 0.5 με / ° C. Therefore, if a tension equivalent to 500 με is given in advance, even if the temperature decreases by about 30 ° C., the FBG does not sag due to the contraction of the diaphragm, and the dead zone can be eliminated.
[0026]
When the FBG 15 is fixed to the diaphragm 5 with an adhesive, a variation in the Bragg reflection wavelength (variation in the initial wavelength) is likely to occur in a finished product stage due to a variation in the cured state of the adhesive. By doing so, the initial wavelength variation can be made sufficiently small.
[0027]
The pressure sensor of FIG. 8 shows a case where the diaphragm 5 is integrated with a pedestal 5A (corresponding to the bottom frame of the container 3). That is, the diaphragm 5 is formed by cutting one surface of a disk having the same thickness as the pedestal 3A to a required thickness. The material is stainless steel (SUS630). Since the diaphragm 5 formed by shaving is integrated with the pedestal 3A, there is an advantage that the diaphragm 5 has high strength, has no risk of peeling, and has high reliability. However, the diaphragm 5 may be fixed to the pedestal by welding or pressure welding (see FIG. 7).
[0028]
In the pressure sensor shown in FIG. 8, an optical fiber is introduced into the container 3 in the form of an optical fiber cord 55. The container 3 holds the tensile strength member (such as aramid fiber) of the optical fiber cord 55. A portion 57 (a rod-shaped body in the illustrated example) is provided. If the optical fiber cord 55 is retained in the retaining portion 57, even if a tension is applied to the optical fiber cord 55, the tension does not affect the FBG 15. The optical fiber cord 55 is housed in a flexible tube 58 connected to the container 3 to ensure handleability and strength. When the pressure sensors are connected in series as shown in FIG. 3, two optical fiber cords on the inlet side and the outlet side are introduced into the container 3 (see FIG. 7).
[0029]
Further, an extra-length storage section 59 for an optical fiber is provided in the container 3. The optical fiber 13 on the FBG 15 side and the optical fiber on the optical fiber cord 55 side are fusion-spliced, and the extra connection length is stored in the extra length storage section 59. With such a structure, assembling of the pressure sensor is facilitated, and productivity is improved.
[0030]
In order to correct the measurement error based on the temperature change of the diaphragm 5, an FBG 61 for detecting the temperature of the diaphragm 5 is fixed to a position not affected by the distortion of the diaphragm 5 with an adhesive 47 as shown in FIG. The temperature of the diaphragm 5 may be detected by the FBG 61, and the temperature of the detection value of the pressure detection FBG 15 may be compensated based on the detected temperature.
[0035]
【The invention's effect】
As described above, according to the present invention, the pressure of a fluid can be stably measured with high accuracy using an optical fiber. Further, since a plurality of pressure sensors can be connected in series to one optical fiber to measure pressures at a plurality of points collectively, it is very economical.
[Brief description of the drawings]
FIG. 1 is a sectional view showing an embodiment of a pressure sensor used in the present invention.
FIG. 2 is a sectional view showing another embodiment of the pressure sensor used in the present invention.
FIG. 3 is an explanatory view showing one embodiment of a pressure measuring device according to the present invention.
FIG. 4 is a bluff showing the relationship between the FBG stretching strain and the wavelength of Bragg reflected light.
FIG. 5 is a sectional view showing still another embodiment of the pressure sensor used in the present invention.
FIG. 6 is a sectional view showing still another embodiment of the pressure sensor used in the present invention.
7A and 7B show still another embodiment of the pressure sensor used in the present invention. FIG. 7A is a longitudinal sectional view, and FIG. 7B is a transverse sectional view taken along line BB of FIG.
FIG. 8 is a half perspective view showing still another embodiment of the pressure sensor used in the present invention.
FIG. 9 is a graph showing a difference in sensitivity of detecting the amount of distortion between a case where the FBG is fixed by applying tension to the diaphragm and a case where the FBG is fixed without applying tension.
FIGS. 10A to 10D are explanatory views showing a method of fixing an FBG by applying tension to a diaphragm in the order of steps.
FIG. 11 is a plan view showing a state in which an FBG for temperature compensation is fixed to a diaphragm.
[Explanation of symbols]
1: pressure sensor 3: container 5: diaphragm 7: pressure transmission rod 9: rigid disk 11: tensile strength line 13: optical fiber 15: FBG (optical fiber Bragg diffraction grating)
19: Transmission line optical fiber 23: Measuring instrument 25: Light source 29: Coupler 31: Optical spectrum analyzer 35: Bellows 39: Bourdon tube

Claims (4)

A plurality of pressure sensors each including an optical fiber having an FBG (optical fiber Bragg diffraction grating) formed in a part of the length direction thereof and means for converting a change in pressure into a change in elongation strain of the FBG ;
The FBGs of each pressure sensor are connected in series, and the center wavelength in the absence of pressure is shifted by a predetermined interval,
Further, there is provided means for inputting light to the optical fiber including the plurality of FBGs connected in series and measuring a wavelength shift amount of the Bragg reflected light,
A pressure measuring device, characterized in that: 2. The pressure measuring device according to claim 1, wherein the FBG is fixed in a state where tension is applied to means for converting a change in pressure into a change in elongation strain of the FBG. 3. The pressure measuring device according to claim 2, wherein the means for converting a change in pressure into a change in elongation strain of the FBG is a diaphragm. The pressure measuring device according to claim 3, wherein the optical fiber near the FBG is fixed to a pedestal of the diaphragm.

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