FR2628205A1 - Enclosed corrosion resistant fibre=optic pressure gauge - has isotropic and weakly refringent optical fibre enclosed between ribbons made from corrosion resistant metal - Google Patents

Abstract

The pressure gauge consists of an optical fibre (1) which is enclosed and gripped by two metal ribbons (3) (4). The optical fibre is made of isotropic glass and is weakly birefringent. The metal ribbons are made of steel or steel alloys which are resistant to corrosion and other hostile influences. The birefringence of the optical fibre is such that light vibrating parallel to the direction of any pressure applied is displaced in phase from light which is vibrating in an orthogonal direction. This phase displacement is measured and used to determine the external pressure. The space between the optical fibre and the ribbons may be in vacuum in which case the absolute pressure is measured, or the space may be filled with liquid or gas in which case the difference between the external and internal pressures is measured. ADVANTAGE - Gauge is sensitive and is easy to apply to difficult practical environments.

Description

FRENCH REPUBLIC Publication no .: 2,628,205 (to be used only for

  the NATIONAL INSTITUTE reproduction orders)

INDUSTRIAL PROPERTY

  D National registration number 88 02765 PARIS

  IntCI: GO1 L 11 / OO.

Q APPLICATION FOR A PATENT FOR INVENTION Ai

  Filing date March 4, 1988. Applicant (s): COMPAGNIE GENERALÈ D'ELECTRI-

CITE, Société anonyme. - FR.

  (Priority: ('Inventor (s): André Tardy; Michel Jurczyszyn: Roger Boislaigue. N Date of public availability of the

  application: BOPI "Patents" no 36 of September 8, 1989.

  6 References to other national documents appearing

rentés: rentés Holder (s):

  (Agent (s): Danièle Laroche, Sospi.

  Fiber optic pressure gauge. Pressure gauge constituted by an optical fiber 1 2 wedged between two metal strips 2, 3. When a force is applied in a direction of the fiber, the phase difference between the light vibrations polarized in this direction and the light vibrations polarized according to the direction is measured. the orthogonal direction. s d0 U r At fmHAI'ILI.EI NATIONALE. 27. rue de le Convention - 75732 PARIS CEDEX 15 Fiber optic pressure gauge The present invention relates to a fiber optic pressure gauge. We know a pressure gauge where we measure the variations of the phase of the light wave propagated by an optical fiber placed in an isotropic pressure field, and whose index is modified by this pressure. This principle is described in the article by G.B. HOCKER "Fiber-optic acoustic-sensors with increased sensitivity by use of

  composite structures "OPTICS LETTERS vol. 4, n 10 / October 1979.

  In order to increase the sensitivity of the measurement, the glass fiber is coated with a silicone elastomer, or any highly compressible plastic material. For a wavelength equal to 1 micron, the variation of

  measured phase is equal to 660 radians / MegaPascal / meter of fiber.

  Such a structure suffers from phenomena of non-linearity and inability to operate under a high static load, due to the

  inelastic behavior of elastomers.

  We also know a gauge comprising a complex glass fiber having a glass core and two parallel channels, located

  on either side of the heart and filled with a gas or liquid.

  Such a fiber is described in the article by H.M. XIE et al "Side-hole fiber for fiber-optic pressure sensing" OPTICS LETTERS, vol 11, n 5 /

May 1986.

  When the gas or the liquid is at a pressure P, anisotropic stresses develop in the core of the fiber and induce a birefringence by elasto-optical effect. This birefringence leads to a phase shift between the orthogonally polarized light vibrations which propagate in the fiber. For a wavelength of 1 micron, the phase shift is equal to 60 radians / Megapascal / meter of fiber. The sensitivity of this gauge is insufficient. Furthermore its manufacture

  involves the production of an extremely expensive special fiber.

  A third type of fiber optic pressure gauge is known, described by A. BERTHOLDS and R. DANDLIKER "High resolution photoelastic pressure sensor using low birefringence fiber" APPLIED OPTICS / Vol 25, n

February 3/1986.

  According to this principle, an optical fiber with low intrinsic birefringence

: 2628205

  - 2 - is arranged between two plates parallel to each other, on which pressure constraints are applied. We study the phase shift between the light vibration located in the plane of application of the force and a

  light vibration located in the orthogonal plane.

  Such an arrangement presents great difficulties of practical use. The object of the present invention is to avoid these drawbacks and to produce a more sensitive pressure gauge than the preceding gauges.

  and easier to implement in a wide variety of environments.

  The subject of the present invention is a fiber optic pressure gauge with low intrinsic birefringence, comprising means for measuring the phase shift between first polarized light vibrations parallel to the direction of a force applied to the fiber and second orthogonally polarized light vibrations. to this direction, characterized in that said fiber is clamped, so as to be wedged, between two ribbons of an elastic material whose width is of the order of 0.5 to 2 centimeters and whose edges

are joined together two by two.

  The material of the ribbons is preferably chosen from metals such as steel, and alloys of different grades resistant to corrosion, to heat or to controlled expansion; the choice depends in particular on the media and the fluids where the gauge must be located and whose

  the physical and chemical characteristics are very varied.

  A gauge is thus obtained in which all the materials have a

  elastic behavior in a wide range of loads.

  The thickness of each ribbon can be of the order of 25 to 250 m.

  According to one embodiment, said ribbons are wound with

so as to form a helix.

  Optionally the conduits defined inside the ribbons can be filled with liquid or gas under pressure. In particular, the creation of a vacuum inside the tube leads to an absolute pressure sensor (pressure outside the tube). More generally, the pressure measured, either internal or external, can represent a difference, of any value, between the two internal pressures and

outside.

  Other characteristics and advantages of the present invention

  will appear during the following description of embodiments

  given by way of illustration but in no way limitative.

  In the attached drawing: - Figure 1 very schematically shows in section a pressure gauge

according to the invention.

  - Figure 2 is a diagram used as a basis for calculating the phase shift

  optics inherent in the force exerted by the ribbons on the stuck fiber.

  - Figure 3 schematically illustrates an alternative embodiment of

gauge according to the invention.

  We see in Figure 1 an isotropic glass optical fiber 1, of radius r equal for example to 50 microns, sandwiched between two ribbons

  metallic 2 and 3, for example steel. These ribbons are welded edge to edge.

  edge in 5 and 6. Their width is preferably 1 centimeter, but

can range from 0.5 to 2 cm.

  The fiber 1 must be wedged between the two ribbons which therefore create on it two diametrically opposite forces whose value is a function

of the bending of the ribbons.

  In order to define these initial forces, the diagram in FIG. 2 is used, where a blade 10 is considered to be a beam bent by a concentrated force F, applied to the center of the blade. The arrow in the center is equivalent to the radius r of the fiber. The blade 10 is fixed at 11 and 12. The length 1, corresponding to the distance between the two welds of the tapes excluding the welded portions, is visible on the

  Figure 1 and shown in Figure 2.

  It is customary to consider in this case that the support conditions are intermediate between those of simple installation and

those of simple support.

  The relationship between the force F and the length of the fiber L is therefore defined by the double inequality: 4E. r (h / 1) 3 <F / L <16 E r (h / l) with (l- é) (1 -_F) h: thickness of the blade 10 E: modulus of elasticity of the material of the blade 10 -4-: Poisson's ratio of the material of the blade 10 E h / 12 (1 -ô) represents the bending stiffness of a blade of

unit length.

  The value of the preload per unit of length, obtained experimentally-

  ment, is significantly above the median value, i.e.: F / L * 13 Er (h / l) 3

(1- 2)

  Numerical example: For steel: E = 2.10 MPa = 0.3 With 1 = 10 mm, r = 50 m, the preload is at h = 100 m F / L = 143 N / m

h = 50 m F / L = 18 N / m.

  The preload applied to the fiber therefore leads to a phase shift between the orthogonal light vibrations which propagate in the fiber, by: A: = o104. C E (h / l) 3. (L />) / (1- 2)

  which does not depend on the diameter of the fiber.

  With the numerical values of the previous example and for = lm: If h = 100 1m LÄ / L = 69 rads / m

If h = 50 m d / L = 8 rads / m.

  In the presence of an external pressure Pe 'the fiber is stressed in compression by additional forces concentrated in the middle of the ribbons. If we call X these forces and if we use the hypothesis of built-in supports:

X / L = 0.5 P 1

  e In the case of the simple support hypothesis, the result is little different:

X / L = 0.62 P 1

  e The blade 10 is considered as a beam undergoing a uniformly distributed load P L. To this uniform load corresponds an e -

  new reaction X to the support constituted by the fiber.

  The optical phase shift linked to the pressure P is then: = 4 C Pe L l / (r) e This does not depend on the thickness of the blades.

  C is the relative photoelastic constant of silica glass.

  If we return from the schematic model of Figure 2 to the embodiment of Figure 1, we can say that the sensitivity per unit length of the gauge according to the invention is: 1 / LA / A Pe = 4 C 1 / ( Nr) The sensitivity is all the more important as the ratio 1 / r is large. At the wavelength of 1 m, a gauge width 1 = 10 mm and a fiber radius of 50 m, lead to a sensitivity of

  2640 rads.m NMPa-1 or 264 rads.m -1 bar -

  This sensitivity is much higher than that obtained by the

  implementation of the gauges of the prior art.

  If we have a gauge of 1 meter long we can detect: for 1 milliradian: 0.4 Pa = 0.004 mbar

for 1 microradian: 0.4 mPa.

  The phase shift measurement devices are similar to those described in the aforementioned articles using the properties of

birefringence of the fiber.

  The variation of the birefringence following the pressure applied to the gauge is measured by observing the interference which takes place between the

  parallel components transmitted by a polarization analyzer.

  In the presence of an internal pressure created by a liquid or a gas, the sensitivity obtained is the same, provided that the pressure

  does not cancel the preload applied to the fiber.

  When you need a fairly long length of tape, the size of the gauge can be a problem. The compactness can be improved by making a helical winding with turns 20 visible in FIG. 3. The pitch of the turns is noted p, and the radius of the winding - 6 is R. The curvature of the bonded fiber and the length of fiber per revolution S, are: / e = 42R / (p2 +4 2R2 S = (4I2R2 + p2) 1/2 If the turns are contiguous, p is about 1, width of the ribbons. With 1 = 10 mm and R = 27 mm, the length per revolution is 17 cm and six turns are necessary to have 1 meter of interaction length. The size of the sensor is then of the order of x 6 cm The curvature of the fiber induces a linear birefringence of value

2 2

  An = 0.5 C E r: 9 C: relative photoelastic constant of the silica glass E: elastic modulus of the silica glass: 7.10 MPa The corresponding phase shift 2e An.L / X is, in the example considered, of

  2.5 rads / m for a fiber of radius r = 50 m.

  The helical winding implies, in addition to the curvature of the fiber, a rotation W of the plane of curvature per unit of fiber length equal to

2 22 1/2

  W = 2 M / S.p / (p 2 + 41 2R2) 1/2 This optical activity can however be ignored if the system is operating in the reflection mode. In this mode one end of the fiber is treated so as to reflect light. The rotation resulting from the forward propagation is canceled due to an equal rotation

  but in the opposite direction of the wave propagating in the opposite direction.

  This operating mode also leads to doubling the sensitivity or, for an identical sensitivity, to halving the

sensor volume.

  The pressure gauge according to the invention, in addition to its superior sensitivity, has other advantages with respect to the sensors of the prior art. Thus, it is not disturbed by thermal phenomena such as fiber-elastomer composite gauges. In addition, in the structure according to the invention, the fiber is very well protected against the media.

aggressive exteriors.

  E The pressure gauge according to the invention finds multiple applications: detection of hydrostatic pressures, unidirectional stresses inside materials such as concrete for example, detection of low frequency sound waves in the air or

in water....

  Of course, the invention is not limited to the example described. Without departing from the scope of the invention, any means may be replaced by a

equivalent means.

-8-

Claims (1)

CLAIMS 1 / Optical fiber pressure gauge with low intrinsic birefringence comprising means for measuring the phase shift between first light-polarized vibrations parallel to the direction of a force applied to the fiber and second light vibrations polarized orthogonally to this direction, characterized by the fact that said fiber is clamped, so as to be wedged, between two ribbons of an elastic material whose width is of the order of 0.5 to 2 centimeters and whose edges are secured two by two. 2 / pressure gauge according to claim 1, characterized in that said elastic material is chosen from metals and their alloys. 3 / pressure gauge according to one of the preceding claims, characterized in that the thickness of each strip is between 25 and 250 m. 4 / pressure gauge according to one of the preceding claims characterized in that said ribbons are wound so as to produce a helix. / Pressure gauge according to one of the preceding claims, characterized in that it is filled with liquid or gas under a pressure such that the preload applied to the fiber by said ribbons is not canceled. 1/1 FIG. 1 ',: FIG.2 Il10_ FI..3 ..