WO2006004419A1

WO2006004419A1 - Dynamic pressure sensor with optical fiber and interferometer

Info

Publication number: WO2006004419A1
Application number: PCT/NO2005/000236
Authority: WO
Inventors: Karianne Pran; Geir Sagvolden; Øyvind LUND-JOHANSEN
Original assignee: Light Structures As
Priority date: 2004-07-02
Filing date: 2005-06-28
Publication date: 2006-01-12
Also published as: NO20042812D0

Abstract

The invention relates to a pressure sensor for measuring dynamic pressure changes, and a system comprising this sensor, wherein the pressure has a defined main direction relative to the sensor. The sensor comprising a plate being adapted to be positioned with one side toward the pressure direction, and the other side toward a supporting structure, and an optical waveguide stretching along the circumference of said plate, said optical waveguide being coupled to an optical interferometer providing an interference signal corresponding to changes in the length of said optical waveguide, and being adapted for coupling to an interrogation means for calculating change in waveguide length and thus an indication of the applied pressure in the plate, said interrogation means also including a light source with a chose coherence length for transmitting an optical signal into said waveguide.

Description

DYNAMIC PRESSURE SENSOR WITH OPTICAL FIBER AND INTERFEROMETER

PRESSURE SENSOR

The invention relates to the field of measuring pressure and/or force on a surface. More specifically it relates to a fiber optic sensor for sensing the impact pressure of liquids such as petroleum products or water, due to sloshing of the liquid in the cargo and ballast tanks of ships.

BACKGROUND OF THE INVENTION There is a trend in the ship building industry to build ever larger classes of ships with ever larger cargo tanks. Lately there have been proposed classes of liquid natural gas carriers (LNGC) that have tanks where the condensed gas has a large free surface where waves may form as the ship moves in the ocean waves. These in-tank waves will then impact on the walls of the tanks and potentially cause high pressures which may damage the structure.

The cargo tanks of such LNGCs are lined with an elaborate containment system to contain the volatile liquid and insulate the cooled liquid from the surrounding environment. Traditional methods for measuring pressure requires the sensor to be in physical contact with the liquid, which means penetrating all the barriers of the containment system, which in turn reduces the safety of the arrangement.

To our knowledge all conventional, electrical pressure sensors require the sensitive part of the transducer to be in contact with the liquid. These are typically based on measuring the effects of deflecting a membrane made from e.g. piezoelectric materials that generate a measurable current or voltage when subject to a change in pressure against the membrane. The diameter of the membrane is in general small on the scale of ship cargo tanks, and pressures over larger areas are mapped by arranging banks of several pressure sensors in a grid pattern.

Classical balloon-spring pressure sensors work by transferring the pressure to the inside of a coiled up tube, which tube will uncoil in a predictable manner as the internal pressure increases. For applications where the pressure medium is highly corrosive or in other ways incompatible with the spring, a highly flexible membrane may be used to transfer the pressure to an incompressible, non-corrosive fluid which in turn acts to uncoil the spring. Such a highly flexible membrane implies a modification of the primary containment membrane and is thus unattractive for the sloshing monitoring application.

The present invention provides a means for measuring high, dynamic (sloshing) pressures without penetrating the primary membrane of the containment system, and furthermore a means based on fiber optic sensing principles that are inherently explosion safe as there are no risks of sparks being formed should the system cabling be compromised (no electrical currents/voltage, and low optical effects). The chosen fiber optic sensing principle employs a fiber optic interferometer, and functions by transferring a pressure against a surface of the sensor transducer to an elongation of the sensing portion of a fiber interferometer. The mechanism is such that an equally good measurement of the impact pressure may be made with the pressure being transferred by e.g. the primary membrane of a LNG containment system, as with the transducer placed in a house immersed in the LNG. The sensing principle is similarly suited for sensing pressure in other liquids or against other membrane-like surfaces, e.g. thin-walled containers backed by a support structure or a sensor house embedded in a road surface to perform weigh-in-motion.

The sensor has no moving parts and does not rely on fluids which may change mechanical properties as the temperature is lowered to LNG temperatures (-161.5 degrees C), or leak in connection with sensor failure.

Some fiber optic means for measuring pressure or force have also been published. In US5,926,584 Motzko et al describes a fiber optic load sensor based on attenuation of light propagating in a fiber through bending of the fiber. The attenuation is induced by wrapping the fiber around a rod of a similar diameter to the fiber and pressing the fiber against the rod. In US5,308,973 Odoni et al. proposes a force transducer in which a fiber is trapped between two surfaces, which surfaces will transfer a pressure to the fiber and cause a change in birefringence of the fiber. This birefringence is possible to characterize by observing the polarization state on the output end of the fiber. A similar mechanism is used in GB 2303445 in which Mazerolle et al suggests a hydrophone based on winding the fiber around the thin walled cylinder and the applied pressure resulting in an anisotropic stress in the cross section of the fiber causing birefringence in the fiber. The pressure may thus be characterized by studying the polarization state of the light.

The present invention proposes to use interferometry to sense length change in a fiber caused by pressure or force. An alternative method for measuring the length change, but not with a sufficient resolution for this application, is shown in US 4269506, where Johnson et al uses transit time of optical pulses in a fiber to measure the fiber length and possible length changes.

A person versed in the art will know that there are a large number of classical interferometers that have fiber optic equivalents, such as Sagnac, Fabry-Perot, Michelson and Mach-Zehnder interferometers, and that all these are suited for different measuring tasks, see e.g. [I]. A number of these types of interferometers have been employed for measuring acoustic pressures in hydrophone and microphone systems, see [2] and references therein.

In one configuration, described in US6,233,374 [3], the sensing arm of an interferometer is wound around a thin-walled cylinder or mandrel, and the deflection of the cylinder wall works to stretch or compress the optical fiber, thus generating the optical phase shift which is the signal in an interferometer.

In another configuration, described in US5,218,197 [4], the internal pressure in a pipe is measured by wrapping the sensing arm of an interferometer around the circumference of the pipe cross-section, and the bulging of the pipe in response to increases in pressure is thus transferred to the interferometer arm. A third configuration, described in [2], has the signal arm arranged in a spiral fashion and attached to a flexural disk, or diaphragm, which distends under pressure, thus stretching the fiber in the signal arm.

Several methods have been devised for interpreting, or demodulating, the interferometer output [5-6], but an elegant and passive method is by using a 3x3 coupler as an exit coupler in the interferometer [7-8]. This solves the problem of signal fading. The two interferometer beams, the signal and reference beams, interfere in the 3x3 coupler, and three output signals are generated which are 120 degrees out of phase with each other. Thus at least two out of the three outputs show a significant power level and signal slope at all times.

US5,313,266 Keolian et al. proposes a demodulation scheme based on a 3x3 coupler in which both arms of the interferometer are exposed to the signal.

OBJECT OF THE INVENTION

The object of the invention is to provide a simple and resilient means for monitoring and characterizing the sloshing pressures against tank linings on cargo ships without the need to penetrate the primary membrane containing the cargo in the tank. An additional object is to do so in a way that maintains the explosion safety level of the cargo system.

THE INVENTION

According to the present invention a method for measuring pressure is provided in which a pressure responsive element, e.g. an elastic plate or circular disk, is incorporated in the lining of a cargo tank, which element is in physical contact with a rigid means of transferring the pressure against the primary membrane of the lining to the disk, and the disk being supported by the tank wall, the disk further being equipped with a means to measure the deformation of the disk as a response to the pressure being monitored. The invention is characterized as stated in the independent claims. A multitude of such pressure sensitive elements may be positioned at different locations around the tank to form a tank monitoring system. In some applications it may also be interesting to place sensors not only in physical contact with the primary membrane of a containment system, but also farther away from the cargo, closer to the tank wall itself, to monitor how the sloshing pressure forces are absorbed and distributed by the structure of the containment system. Furthermore, one or all tanks on a cargo vessel may be equipped with such tank monitoring systems to form a shipwide sloshing pressure monitoring system.

In a preferred embodiment the means for measuring the deformation of the pressure responsive element is an optical fiber, which fiber is stressed by the deformation of the pressure responsive element, and which stress is remotely quantifiable by a fiber optic sensing technique. Such techniques include, but are not restricted to, interferometry and fiber Bragg gratings.

According to a further aspect of the current invention the pressure sensitive element is a disk with a circumferential groove in which the sensing fiber may be fixed, and in which the fiber is somewhat protected from the environment. The disk can be made of different materials, e.g. metals, polymers or reinforced polymers, and choosing a material with a higher modulus of elasticity will render the sensor less responsive to pressure, but also less sensitive to disk bending, which will be a noise source in this type of system.

The disk is further equipped with an in-plane rigid attachment featuring a void in which excess fiber may be coiled up, and further featuring a means for anchoring a cable which transports the optical signals to and from the transducer disk.

In a further preferred embodiment the fiber fixed to the circumference of the disk forms part of the signal arm of a fiber Mach-Zehnder interferometer, while the input coupler, output coupler and reference arm are contained in the attached void space and coupled to the fiber optic cable. After the reference fiber and couplers are placed in the void, the entrances to the void should be sealed to avoid the accumulation of moisture in the vicinity of the fiber. Likewise the signal arm should be coated with a suitable barrier material to avoid moisture in the optical fiber.

The Mach-Zehnder interferometer is preferably formed using a 3x3 coupler for the output coupler. It is an important aspect of the invention that the sensitivity of the pressure sensor can be increased by wrapping the signal arm of the interferometer several times around the disk. It is also possible to temperature compensate the pressure measurement by attaching the reference arm to a material with the same or similar thermal expansion as the disk forming the pressure transducer.

In a possible embodiment an additional temperature sensor may be placed in the void before it is sealed, to provide a temperature measurement and reference. This temperature sensor is preferably fiber optic.

A sloshing sensor system would consist of one or several sensing elements connected to a central processing unit via a grid of cables. The processing unit as a minimum includes a light source, optical detectors and electronics to interpret the signals. Typically the processing unit will also contain some computer resource to carry out more advanced signal processing in real-time, and means for saving data for post processing. Realtime data output should be available for presentation of the results to the user.

The invention will be described more in detail below with reference to the accompanying drawings, illustrating the invention by way of examples. Figure 1. A fiber optic device for interferometric measurement of pressure on an elastic plate Figure 2. An elastic, pressure sensitive element with a means for measuring deformation in physical contact with a membrane and supported by a plane structure Figure 3. Cross-section of a membrane type LNG tank with a pressure sensor placed behind the primary membrane, supported by the insulation Figure 4. A possible configuration of a tank sloshing monitoring system with pressure sensors placed behind both primary and secondary membranes of the containment system

Figure 5. A plate with optical fiber mounted in a groove in the rim to measure the circumferential expansion due to a pressure applied normal to the surface

Figure 6A. A fiber optic Mach-Zehnder interferometer

Figure 6B. A fiber optic Mach-Zehnder interferometer with a 3x3 output coupler Figure 7. A Mach-Zehnder interferometer with the signal arm wrapped around a pressure sensitive circular disk, and with a 3x3 output coupler, attached to a laser light source, optical detectors and a demodulation system

Figure 8. A temperature compensated Mach-Zehnder interferometer with the reference arm wrapped around a thermally responsive material Figure 9. A pressure sensor incorporating an additional temperature sensor

DETAILED DESCRIPTION OF THE INVENTION

In figure 1 a preferred embodiment of the invention is illustrated in which the pressure sensor is constituted by a circular plate 24 with an optical fiber being wound around its circumference. As is illustrated in figures 7 and 8 the plate will expand radially when being subject to a pressure having a direction along the plate normal. Thus the length of the optical fiber 15 will vary with the applied pressure. This length variation may be measured using an interferometer, as will be described more in detail below.

Figure 2 illustrates a practical use of the pressure sensor wherein a dynamic liquid pressure may be measured without the use of pressure transmitting liquids or moving parts, and without direct contact with the liquid exerting the pressure. The technique relies on placing an elastic element 10, e.g. a plate of a suitable material, in physical contact with a membrane 11, i.e. a plate that is thin in relation to its in-plane dimensions. The elastic element 10 is further supported by a backing structure 12 that provides a plane against which the element rests. The backing structure encloses the elastic element on all sides but the side in physical contact with the membrane, and serves to support the membrane where it is not supported by the elastic element. There should, however, be a certain open space 13 around the edges of the elastic element, in order that it may expand in the transverse direction. When a dynamic liquid pressure is applied to the membrane 11, the force exerted on the membrane 11 is transferred to the elastic element 10, causing it to compress in the thickness direction and expand in the orthogonal direction in accordance with the material's elastic modulus and Poisson's ratio. By fixing a means of measuring either the compression or the transverse expansion to the elastic element, it is possible to characterize the incident pressure without being in direct contact with the liquid exerting the pressure.

The means for measuring distortion may be electrical or optical in nature. It is in the nature of the method that the spatial resolution of the measurement may be scaled by varying the size of the elastic element, and the sensitivity may be varied somewhat through the choice of material for the elastic element. Possible materials are metals such as aluminum, brass or invar, or softer materials such as polymers or reinforced polymer composites, or combinations thereof such as a soft composite core enclosed by a metal skin. The arrangement is very resilient, and is suited for use in hazardous environments, as there is little risk of breaching the membrane in cases where it forms a barrier.

One application for this method is in measuring liquid sloshing pressure in ship cargo- or ballast tanks. In particular the method is suited for use in LNG carriers of the membrane type where the liquid natural gas 2 is carried in insulated tanks with metal barriers to contain the liquid in the tank, see figure 3. A cross-section the hull 7 of the LNGC has ballast tanks 8 with a tank wall 5 separating the ballast tank 8 from the cargo tank. The tank wall is lined with insulating material 6 and a primary barrier 3 and a secondary barrier 4. By placing the elastic element 10 in a recess in the insulation 6 in physical contact with the primary membrane 3, it is possible to monitor the forces exerted on the containment system without breaching the primary barrier, thus maintaining the safety level of the containment system.

The method will work equally well to monitor similar sloshing pressures in ordinary cargo- or ballast tanks, either by placing the elastic element against the tank wall and let the liquid exert the force directly, or by enclosing the sensor in a house with a stiff backing structure and a membrane to separate the elastic element from the liquid. The method will also be applicable to other types of dynamic pressure applications, such as measuring the dynamic pressure exerted by vehicles on a road body (weigh-in-motion).

In a preferred embodiment the elastic element 10 takes the form of a plate where a groove 14 has been machined around the rim to accommodate an optical fiber 15 to act as the means to register the deformation of the plate, see figure 5. In this case the method utilizes the transverse expansion rather than the thickness compression to characterize the pressure exerted on the elastic element. The plate may take any shape that has a rim smooth enough to avoid bending losses or breakage of the optical fiber, i.e. there should be no radius smaller than approximately 2cm to ensure longevity of the sensor. Due to fabrication considerations circular, or in some cases, rounded rectangular plates as illustrated in figure 5, will usually be preferred.

The fiber optic means for measuring the transverse expansion may be a fiber Bragg grating or an interferometer. In a preferred embodiment a fiber optic interferometer is used for the measurement, by fixing the signal arm to the transducer plate, hi this case the elastic plate is preferably a circular disk, as this shape will provide a more uniform sensitivity along the fiber in the signal arm. If one were to apply a pressure to the rim using an implement of a fixed area and a fixed force, this would cause the same interferometric phase change to be generated at all points along the rim, as there will always be the same length of signal arm exposed to the pressure. For other shapes of plate, the exposed length of fiber would vary with the position along the rim.

A number of fiber optic interferometers have been published for use in fiber optic sensors, such as Michelson, Mach-Zehnder and Sagnac interferometers. In the present invention we prefer to use a fiber optic Mach-Zehnder interferometer to measure the deformation of the elastic element, see figure 6 A and 6B. This interferometer consists of an input fiber 17 which is one entry fiber of a first fiber optic coupler 18, which coupler splits the light in two paths, a signal path 19 and a reference path 20. The two paths are recombined and the light allowed to interfere in a second fiber optic coupler 21, and the intensity of light is distributed on the exit ports 22 of the second optical coupler in accordance with the relative phase of the two light beams. An elongation of the signal arm relative to the reference arm will cause the path length for the signal beam to lengthen, and the phase of this beam to increase in relation to the reference phase. The two intensities on an ordinary 2x2 optical coupler is

I₂ = I_o smφ + π

where φ is the phase difference between the two beams which is given by the change in optical path difference nΔl and the wavelength λ of the light φ = 2mAl/ λ n is the refractive index of the fiber. The signal is independent of the initial path difference between the two paths, allthough as a person versed in the art will know, it is necessary that the coherence length of the light is longer than the path difference in order to get interference.

We see that in cases where φ is close to an integer multiple of n one of the intensities will be very small while the other is close to I₀. Moreover, with this arrangement it is impossible to determine if φ is increasing or decreasing in a simple manner.

These problems are elegantly solved by replacing the second coupler by a 3x3 optical coupler 23 see figure 6B, in which case there are three output intensities

J₁ = J₀ sin φ

I₂ = Z_o sinø> + 2;τ/3

J₃ = J₀ sinζZ> + 4;τ/3

Using a Mach-Zehnder interferometer with a 3x3 output coupler we ensure that at least two of the three output intensities is of a significant size and is easily measured. In addition it is possible from the three signals to determine if the phase is increasing or decreasing in a simple manner.

The sensitivity of the interferometer depends on the material properties of the transducer disk as this determines the change in circumference Δs for a given pressure P Δs = 2πΔr =: 2πrvP/E

where E is the elastic modulus of the disk and vis Poisson's ratio. The change in circumference is transferred to a corresponding change in path Al. It is possible to scale the sensitivity of the interferometer by winding several turns of signal fiber around the transducer disk, in which case the sensitivity is a function also of the number of turns JV: φ = 2mNAs I λ = 4π²rnNvP /(AE)

A schematic of a sensor arrangement is shown in figure 7 where a circular transducer disk 24 is exposed to a pressure P, and is equipped with an optical fiber 15 around the rim, which fiber is the main portion of the signal arm 19 of a Mach-Zehnder interferometer. In order to function, the interferometer has a coherent light source 25 coupled to the input fiber 17 on the first optical coupler 18, and each output fiber 22 is coupled to an optical detector 26. The intensities measured by the detectors are then passed to demodulation electronics 27 that recovers the phase from the three intensities. A number of demodulation techniques have been published and are available to a person versed in the art.

It is evident that the sensing arrangement will be sensitive to thermal expansion of the transducer disk. A simple means for temperature compensating the interferometer sketched in figure 8 is to attach the reference arm 20 of the interferometer to a pressure isolated element 28 made of a material with a thermal expension that yields a similar thermally induced elongation of the reference arm 20 as the transducer disk 24 exerts on the signal arm 19. In this case the simple solution is to have a small path difference between the signal and reference arms, and use the same material for the temperature reference as the transducer disk.

In a practical embodiment the interferometer less the signal arm must be protected, and a sturdy optical cable used for transporting the light to and from the interferometer and the optical detectors, light source and electronics. Figure 1 shows how this may be solved by attaching a block 33 with a cavity 29 to the transducer disk 24 in which there is room for the couplers, the reference arm and fiber optic splices. In addition this attachment has an entry point 30 for an optical cable 31. The cavity is handily formed by machining a recess in two plates 34 and 35, one of which is fixed to the transducer disk 35, the other of which can be attached to the former as a lid 34 with adhesives or screws or both.

For cases where it is of interest to characterize the temperature at the pressure sensor location, a temperature sensor 32 may be placed in the attached cavity 29 along with the interferometer, see figure 9. This temperature sensor is preferably fiber optic, and can be e.g. a fiber Bragg grating mounted in a way that isolates it from stresses while providing thermal contact with the environment. A number of temperature sensor arrangements have been published, e.g. WO03076887 and EP0892250. The temperature sensor may be provided with a separate fiber in the optical cable or be multiplexed onto one of the fibers used for the interferometer using e.g. wavelength division multiplexing.

The physical dimensions of a transducer disk should be determined by evaluating the physical dimensions of the environment in which it is to be used, and the desired spatial resolution of the measurements. In testing the method and sensor embodiment for use in LNG tankers, we have chosen a transducer disk of Aluminium of 30cm diameter and a thickness of 12mm. The thickness was chosen as a readily available plate thickness for Aluminium, and the diameter was chosen as it is a practical size and coincides with a geometrical periodicity that is found in some LNG containment systems. An added benefit of the relative thickness of the disk is that the disk becomes stiffer and less prone to bending, which will generate a false signal. With these parameters and a laser light source with a wavelength of approximately 1550nm we found a sensitivity of 2.9 rad/bar for a single turn of fiber on the transducer disk, and an increased sensitivity of 11 rad/bar for four turns of fiber.

Adding a threaded hole in the center of the transducer disk to allow a fixation to the support structure did not affect the performance of the sensors. According to an alternative embodiment of the invention when other characteristics, such as different sensitivity and/or limited physical dimensions, are required, the disk size may be limited to e.g. 5cm. In this case the disk may be positioned inside the cavity or compartment 29 and under the lid 34 which may act as the membrane separating the sensor from the liquid in a tank.

For practical purposes it is interesting to form comprehensive monitoring systems by placing a number of sensors 1 at different positions around the tank lining, as depicted in figure 4. A number of such pressure sensors may be combined in a measurement system for tank- wide sloshing monitoring. Of particular interest is the foremost tank of a cargo carrier, as this is the location in the ship that experiences the larger motions. However, monitoring of all tanks may be interesting for ships traveling in rough seas. In addition the sloshing monitoring may be combined with hull stress monitoring to gather even more comprehensive knowledge of conditions.

For summing up the invention provides a device and a system for measuring dynamic pressure, especially related to liquid transportation characterized by placing a pressure responsive element in physical contact with a membrane, which membrane is separating the pressure responsive element from a liquid exerting dynamic pressure, which element is backed and supported by a planar support structure, which element is deformed by pressure transferred to the element surface by liquid impact on the membrane, and which element is equipped with a means for measuring the pressure induced deformation of the element. In the preferred application of the system the membrane is part of a ship cargo containment system and the support structure is part of the tank wall.

The pressure responsive element is constituted by a pressure responsive element in the form of a plate, which plate will expand circumferentially when exposed to a pressure normal to the plate surface, which plate has an optical fiber attached to the perimeter of the plate, which optical fiber forms the signal arm of an optical fiber interferometer. The plate is preferably a circular disk and the optical fiber is placed in a protective groove around the circumference. For measuring the expansion of the fiber positioned around the perimeter of the plate the optical fiber constitutes a part of an interferometer, preferably a Mach-Zehnder interferometer, which interferometer is comprised of an input fiber, a first fiber optic coupler that splits the light in two paths, the first path following the signal path around the disk, the second path being a reference path following a second optical fiber, which second optical fiber is insulated from the pressure-induced deformation, which interferometer is further comprised of a second fiber optic coupler that combines the light traveling along the signal and reference paths respectively, which second coupler allows the two beams to interfere and an interference pattern to form on the output arm(s) of said second coupler. The output coupler will in most cases be a 3x3 fiber optic coupler.

The plate may be equipped with a physical attachment, which attachment has a hollowed out trough, which trough may be covered by a lid to form a cavity, which cavity holds the interferometer save the signal arm in a non-deforming environment, which attachment additionally has an entry point for and stress relief for a sturdy optical cable, which cable is spliced to the input fiber of the interferometer and at least one output fiber of said interferometer, which splices are similarly protected in the cavity of said attachment.

Also the interferometer is temperature compensated by attaching the reference arm to a material with the same thermal expansion as the sensor disk in order that the reference arm experiences a similar thermally induced elongation as the signal arm.

In addition or alternatively the system may be equipped with one or more fiber optic temperature sensors to characterize the temperature at the pressure sensor locations.

The optical fiber is coupled to a coherent light source on the input fiber and a interferometer demodulation system on at least one output fiber, which demodulation system comprises optical detector(s) and electronics to recover the pressure-induced interferometer phase signal. When being used for monitoring pressure conditions in a tank several pressure sensors are positioned at chosen parts, e.g. the part being most sensitive to strain and stress in the material, to monitor the pressure exerted on a larger portion of the tank wall, or, if applicable, in a number of tanks, e.g. to form a shipwide monitoring system.

Although the invention has been described here mainly in relation to tanks, especially tanks on LNG ships, a person skilled in the art will realize that it may also be used in similar applications, especially for measuring pressure variations in harsh environments. An alternative use could e.g. be measuring the pressure fluctuations on a surface from passing cars for detecting and possibly characterizing the passing vehicles.

REFERENCES 1. J.P. Dakin and B. Culshaw, Eds. Optical Fiber Sensors. Principles and

Components, VoIs. 1-2, Artech House, Boston/London 1988. 2. S. Knudsen Fiber-optic Acoustic sensors based on the Michelson and Sagnac interferometers: Responsivity and noise properties, PhD thesis 1996:18, Norwegian Institute of Technology, Trondheim 1996. 3. US6,233,374, May 15 2001, Ogle et al, Mandrel-wound fiber optic pressure sensor.

4. US5,218,197, June 8 1993, Carroll, Method and apparatus for the non-invasive measurement of pressure inside pipes using a fiber optic interferometer sensor

5. A. Dandridge, A.B. Tveten, T.G. Giallorenzi, Homodyne demodulation scheme for fiber optic sensors using phase generated carrier, IEEE J. Quantum Electron vol QE-18, pp. 1647-1653, (1982).

6. J.H. Cole, B.A. Danver, J.A. Bucaro, Synthetic heterodyne interferometric detection, IEEE J Quantum Electron, vol QE-18, pp. 694-697, (1982).

7. S.K. Sheem, T.G. Giallorenzi, K. Koo, Optical techniques to solve the signal fading problem in fiber interferometers, Appl.Opt. vol 21, pp. 689-693, (1982).

8. K.P.K00, A.B.Tveten and A. Dandridge, "Passive stabilization scheme for fiber interferometers using (3x3) fiber directional couplers", Appl. Phys. Lett., vol 41 no. 7, pp. 616-618, (1982). 9. US5,313,266 May 17, 1994, Keolian et al.. Demodulators for optical fiber interferometers with [3x3] outputs.

10. US5,926,584 July 20, 1999, Motzko et al., Fiber optic load sensor

11. US5,308,973 May 3, 1994, Odoni et al., Method and device for the measurement of force by a fiber optics system by evaluating phase shift of light waves.

12. WO03076887 Sept 18, 2003, Pran et al. Fiber optic sensor package.

13. EP0892250 Jan 20, 1999, Foote P.D and Haran F.M., A strain isolated optical fibre Bragg grating sensor.

14. GB2303445 JuI 20, 1995, Mazerolle et al., Pressure sensor with wound optical fibre.

15. US4,269,506 May 26, 1981, Johnson et al., Apparatus for measuring the influence of physical parameters on the length of a path

Claims

C l a i m s

1. Pressure sensor for measuring dynamic pressure changes, the pressure having a defined main direction, comprising a plate or disk being adapted to be positioned with one side toward the pressure direction, and the other side toward a supporting structure so that a pressure in said main direction forces a lateral expansion toward the circumference of the plate or disk, and an optical waveguide stretching along the circumference, said optical waveguide being coupled to an optical interferometer providing an interference signal corresponding to changes in the length of said optical waveguide, and being adapted for coupling to an interrogation means for calculating change in waveguide length and thus an indication of the applied pressure in the plate, said interrogation means also including a light source with a chosen coherence length for transmitting an optical signal into said waveguide.

2. Pressure sensor according to claim 1, wherein the interferometer is constituted by an optical Mach-Zehnder waveguide interferometer and said waveguide stretching along the circumference of the disk thus constituting one of the two optical paths of the Mach-Zehnder interferometer.

3. Pressure sensor according to claim 1, wherein the interferometer comprises a 3x3 coupler as an exit coupler being coupled to said interrogation means.

4. Pressure sensor according to claim 1, wherein the plate is an essentially circular disk.

5. Pressure sensor according to claim I₅ wherein the optical waveguide is an optical fibre being positioned in a groove along the outer edge of the disc, and the optical fiber is wound a chosen number of times around the disc.

6. Pressure sensor according to claim 1, also comprising a temperature sensor being adapted to be coupled to said interrogation means for measuring the temperature at the sensor.

7. Pressure sensor according to claim 1, comprising a protective housing, containing said interferometer.

8. Pressure sensor according to claim 1, wherein said interferometer comprises a reference waveguide constituted by a second waveguide in said optical interferometer, the reference waveguide being positioned around a second plate being similar to the first plate, the interferometer thus comprising two thermally corresponding optical paths, in which the second plate being positioned without contact with said pressure changes thus constitutes a temperature compensation to the output of ' the interferometer.

9. Pressure sensor system for measuring pressure fluctuations in a tank, especially a LNG tank, the tank having an inner membrane being in contact with the content of the tank, comprising at least one pressure sensor according to any one of claims 1-8, and an interrogation means coupled to the output of said interferometer at each of said at least one sensor, wherein the at least one pressure sensor is positioned in a recess behind the membrane, with one of the side surfaces of the sensor plate is in contact with the membrane, and said other side is in contact with the inner wall of said recess.

10. Pressure sensor system according to claim 9, comprising a number of temperature sensors for monitoring the temperature in chosen positions around the tank