WO2000047967A1

WO2000047967A1 - Optical fibers sensor and optical device for detecting stress and/or strain

Info

Publication number: WO2000047967A1
Application number: PCT/IT1999/000025
Authority: WO
Inventors: Sergio Leali
Original assignee: Pllb Elettronica S.P.A.
Priority date: 1999-02-08
Filing date: 1999-02-08
Publication date: 2000-08-17
Also published as: AU2545399A

Abstract

A fibre optics sensor for detecting and measuring physical stresses and/or strains in a flat and/or tubular and/or irregularly shaped surface (2), being of a type which comprises at least one sensing optical fibre and a back-up member for said at least one fibre (3), is characterized by said at least one sensing optical fibre being laid into a determined geometric pattern, the sensor comprising a means of detecting an attenuation in reflected optical power through said at least one fibre, and a calculating means for re-constructing stresses and/or strains in the surface (2) from the value of the optical power attenuation through said at least one optical fibre.

Description

OPTICAL FIBERS SENSOR AND OPTICAL DEVICE FOR DETECTING STRESS AND/OR STRAIN

* * * * *

DESCRIPTION

Field of the Invention

This invention relates to a fibre optics sensor and an optical apparatus for detecting and measuring physical stresses and/or strains in a flat and/or tubular and/or irregularly shaped surface.

Speci ically, the invention relates to a fibre optics sensor for detecting and measuring physical stresses and/or strains in a flat and/or tubular and/or irregularly shaped surface, being of a type which comprises at least one sensing optical fibre and a back-up member for said at least one fibre.

The invention further relates to an optical apparatus for detecting and measuring mechanical stresses and/or strains in flat or tubular surfaces, being of a type which comprises at least one fibre optics sensor according to the invention, and a plurality of transmitters and receivers connected according to the following:

each sensor fibre is supplied a constant optical power from an transmitter;

a receiver is connected to each fibre, on the opposite side from the transmitter side thereof.

The invention relates, particularly but not exclusively, to a fibre optics sensor for detecting and measuring mechanical stresses and/or strains in flat or tubular surfaces, and the following description is given with reference to this field of application for convenience of explanation only. Prior Art

As is well known, the use of optical fibres as sensors of mechanical stresses and strains in civil constructions is a long standing subject of investigation.

Sensors employing optical fibres have been developed which have specialized structures according to application, and can detect and locate mechanical stresses, or stresses of another nature (e.g., temperature).

Existing fibre optics sensors (FOS) are based on two types of technologies, namely:

Fabry-Perot technology (FOS-FP) ;

Fibre Bragg Grating technology (FOS-FBG) .

Both technologies employ very short optical fibre sections having peculiar geometric configurations. These technologies differ from each other by the structure of the fibre used.

Known sensors of this type are essentially local sensors arranged to detect physical stresses or mechanical loads at given spots on a structure corresponding to the positions where the sensors have been placed.

A major drawback with such sensors resides in the limited spatial spread of the stresses that they are able to detect .

The underlying technical problem of this invention is to provide a fibre optics sensor adapted to detect mechanical stresses, even when these are spread over a large area, the sensor having such structural and functional features as to overcome the limitations of similar sensors provided in the prior art .

Summary of the Invention The concept behind this invention provides for the use, as the sensors, of a plurality of optical fibres having a determined geometric structure, and for the fibre optics sensor to include a plastic back-up member, in particular a tape, wherein the fibres are embedded.

Based on this concept, the technical problem is solved by a fibre optics sensor as previously indicated and further defined in the characterizing portion of Claim 1.

The problem is also solved by an optical apparatus for detecting and measuring mechanical stresses and/or strains in flat or tubular surfaces, as previously indicated and further defined in the characterizing portion of Claim 17.

Furthermore, the problem is solved by a method of locating a strained spot on a surface or structure subjected to mechanical or thermal stresses, as previ-ously indicated and further defined in the characterizing portion of Claim 26.

The features and advantages of a fibre optics sensor and an optical apparatus according to the invention will be apparent from the following description of embodiments thereof, given by way of non-limitative examples with reference to the accompanying drawings .

Brief Description of the Drawings

In the drawings :

Figure 1 shows schematically a possible embodiment of a fibre optics sensor according to the invention;

Figure 2 shows an embodiment of the sensor of Figure 1;

Figure 3 shows a second embodiment of the fibre optics sensor according to the invention;

Figure 4 shows a further embodiment of the sensor of Figure 1; Figure 5 schematically illustrates the phenomenon of the light power of an optical fibre becoming attenuated through bends in the optical fibre;

Figure 6 is an exemplary plot of the qualitative evolution of the light power attenuation through an optical fibre versus its radius of curvature;

Figures 7A and 7B are respective schematic views of an optical fibre, under normal conditions and in the presence of mechanical strain;

Figures 8A and 8B are respective schematic views of an optical fibre, under normal conditions and in the presence of mechanical strain due to its component parts having different coefficients of thermal expansion;

Figure 9 is a mathematical schematic of the sensor of this invention as formed of two bundles of optical fibres;

Figure 10 is a three-dimensional view of a surface to which a sensor according to the invention can be mounted;

Figure 11 shows schematically an optical apparatus for measuring strain which incorporates a sensor according to the invention;

Figure 12 illustrates an application of the strain measuring optical apparatus of Figure 11 to a tubular structure;

Figure 13 illustrates an application of the strain measuring optical apparatus of Figure 11 to a tubular structure;

Figure 14 illustrates an application to a flat structure with scan-laid optical fibres of the strain measuring optical apparatus shown in Figure 11;

Figure 15 illustrates an application to a flat structure, with spiral-laid optical fibres of the strain measuring optical apparatus shown in Figure 11.

Detailed Description

Referring to the drawing views, a fibre optics sensor according to this invention is generally shown schematically at 1.

For simplicity of illustration, reference will be made hereinafter to the instance of a flat surface which may either be subjected to mechanical or thermal strains, or be intruded upon and attacked from the outside.

Advantageously in this invention, the presence of physical stresses or strains can be detected by measuring the attenuation in light power through the optical fibres of the sensor, while the points of application of such stresses/strains can be located in two ways, namely:

by an appropriate geometric layout of two sets of fibres over the surface or inside the structure to be monitored;

by measuring the reflected power in fibres laid along a single direction.

Sensors can be considered which comprise a plurality of optical fibres in a layout shaped as a grid, solenoid, twisting, spiral or another type of geometry.

In the example of Figure 1, the optical fibres used in the making of this sensor are laid over a bi-dimensional surface and divided basically into two sets, as follows:

1st set: fibres extending along a first direction;

2nd set: fibres extending along a second direction across the first.

Both fibre sets lie close to the surface, so that the fibres will follow any deformations produced in the surface by mechanical stresses.

In particular, the fibre optics sensor 1 comprises, as shown schematically in Figure 1, N_H fibres laid along a first direction (FO) , and N_v fibres laid along a second direction (FV) across the first, above a surface 2 which may be of very large area.

One embodiment of a fibre optics sensor 1 according to the invention is shown in Figure 2. It comprises a "tape" 3 formed from a very thin, e.g. about 1 mm thick, pliable plastic material having plural optical fibres embedded therein which extend in the longitudinal direction of the tape (corresponding to the FO fibres) and in the transverse direction of the tape (corresponding to the FV fibres) .

Advantageously in this invention, the sensor 1 uses optical fibres which are evenly distributed over a large surface. For example, single mode fibres may be used, such as those normally employed for telecommunications and hence available at reasonable prices.

For fibres lying along the second transverse direction FV within the plastic tape 3, however, a solution must be provided to the problem of measuring the attenuation through each fibre section. This problem can be solved, for example, by having two sets of auxiliary fibres FS laid along the tape edges to pick up the optical power from each section of the fibres FV, as depicted in Figure 3.

Thus, the auxiliary fibres FA can be terminated at one end of the tape 3 for measuring the optical power attenuation therethrough.

It should be further noted that, inside the plastic tape 3, the fibres may be laid in a straight-line pattern, as has been assumed above, or a sinusoidal pattern, as shown in Figure 4 for the fibre section 4, or other more complex patterns . In this way, the responsiveness of the sensor 1 to straining forces in the surface 2, or mere pressure forces acting on the surface 2, is improved.

It should be reminded that the attenuation of optical power, or optical attenuation, through an optical fibre is tied to external physical actions, such as mechanical stresses, temperature, pressure, etc., to different extents according to the composition of the optical material and the geometric structure of the fibre.

For simplicity of calculation, the dependence of optical attenuation on mechanical straining forces will be considered herein below.

When a fibre F is subjected to mechanical deformation, its optical attenuation increases as a function of the radius R of curvature of the deformation. The phenomenon is known by the term "bending", and categorized as micro-bending and macro-bending according to whether the radius of curvature is comparable with the core diameter of the fibre.

Figure 5 illustrates how the attenuation increase originates in consequence of the deformation undergone by a fibre F, and highlights points A, B and C where a light loss is experienced. Losses can be of two types, namely: a loss by reflection, PR, at the transition point of the curvature, and a loss by absorption, PA (bending absorption) in the region where the fibre is subject to deformation.

These combined effects produce an increase in the overall attenuation whose qualitative evolution is shown in Figure 6.

The change in optical attenuation as the fibre deformation varies can be evaluated by means of appropriate algorithms, a most widely used of such algorithms being that known as FD-BPM (Finite Difference Beam Propagation Method) and described, for example, in Yevick, D. and Hermansson, B, "Spli-Step Finite Difference Analysis of Rib Waveguides", Electronic Letters, No. 7, 1989.

Advantageously, according to the invention, it has been thought of applying this same algorithm, but working in the reverse direction to derive the geometric configuration of the fibre F along its axis from the variations in overall attenuation through the fibre, thereby to derive the presence of either spread or concentrated mechanical stresses or strains.

A simplified mathematic model of an optical fibre F has been used for the purpose .

In this model, the optical attenuation, as expressed in dB's, of a fibre is a linear function of the fibre length and can increase, all the other conditions being the same, depending on deformations coming under the "bending" headin .

The variations in attenuation of a fibre extending along a longitudinal axis can be regarded to be a function of the geometrical configuration of the fibre itself. Figure 7A shows a fibre F laid parallel to the abscissa axis, while Figure 7B shows the same fibre F in a deformed configuration represented by the following function:

y = F(s) (1)

The variation in optical attenuation, with respect to the original configuration, will be a function of F(s), as follows :

ΔA_ldB = 3 [F(s)] (2)

where A^ is the overall attenuation of the fibre F, considered to have a constant length.

From a formal standpoint, the inverse function may also be considered: F ( s) = 3^'1 [ΔA_|dB] ( 3 )

This inverse function 3^"1 [AA^] is not, however, a univocal function.

Advantageously in this invention, a most probable value is picked from the aggregate of the values provided by the inverse function 3^" , once a value for the variable AA^ is given

The above-described bending phenomenon would occur, for example, in a fibre F which is subjected to thermal variations and has different coefficients of thermal expansion between its core 5 and the core-enclosing space or cladding 6, as shown in Figures 8A and 8B.

In particular, the fibre F shown in these Figures has a cylindrical configuration with a core, having a diamater Deo, which is surrounded by a cladding, having a diameter Del and an outer protective layer 7.

Shown schematically in Figure 8B is a possible deformation induced in the fibre F as a result of a temperature gradient T1-T2 being applied to the fibre, in particular with T1>T2.

If in normal conditions the fibre F has an optical attenuation of Adb, then this attenuation will increase by ΔAdb in the presence of a deformation, as defined - for example - by a function F(S) .

The increase in optical attenuation due to bending can be calculated with reference to a coefficient of attenuation given as :

where :

A,,, is the coefficient of attenuation due to bending;

D_co is the core diameter;

NA is a numerical aperture;

R is the bending radius of curvature.

The coefficient of attenuation, A-., can be used to calculate the loss along the longitudinal axis, s, by the following expression:

dP/ds = A.-P (5)

where, P is the optical power within the fibre, at an abscissa s.

The expressions (4) and (5) above ^"allow the overall attenuation of the fibre F to be obtained by integration for a given bending characterized by the function F(s) .

Consider now an area of the plane (x,y) which is confined between:

Yl < y < Y2 (6)

XI < x < X2 (7)

and two sets, one horizontal and one vertical, of fibres inside this area, as shown in Figure 9; then there will be:

a set of fibres: Fibre HI,..., Fibre Hn, arranged horizontally;

a set of fibres: Fibre VI,..., Fibre VM, arranged vertically.

Thus, for each fibre, an overall optical attenuation can be determined in accordance with the following formal correspondence : Fibre Hi: corresponding attenuation A_Hi;

Fibre Vj : corresponding attenuation A_vj .

Suppose that the fibres are bonded to stay parallel to the axis X and the axis Y, respectively; in the presence of a stress acting parallel to the z axis, each fibre will undergo bending, assuming a geometric configuration defined by a function F_Hi(z)^' or F_vj (z) , according to whether the fibre is horizontal or vertical.

Possible deformations of the surface in the plane (x,y) are illustrated qualitatively by Figure 10.

The original rectangle, defined by the limits (6) and (7) in the plane (x,y) , will be changed by a deformation into a surface:

z = S(x,y) (8)

and, therefore, there will be variations in attenuations which can be represented in the following manner:

for horizontal fibres: ΔA_Hi = 3 [F_hi (x) ] (9)

with i=l, ... ,H_N

for vertical f ibres : ΔA_vj = 3 [F_vj (x) ] ( 10 )

with j =l , . . . , V_M

From the attenuation variations, the surface deformations are given by the following relationships:

for horizontal fibres: F_Hi (x) = 3^"1 [ΔA_hi] (ll)

with i=l, ... ,H_N

for vertical fibres: F_vj (x) = S^DA^] (12)

with j=l, ... ,V_M

taking in due account that, as mentioned before, the function 3^" [ . ] is not a univocal function and requires, therefore, a suitable algorithm to determine the actual corresponding function F_Hi (x) or F_vj (x) . This algorithm can be based upon relations existing between the optical attenuation measurements made on the whole of the vertical FV and horizontal FO fibres.

In general, it can be stated that, for a set of measured values

[ΔA_Hi, ΔA_vj] (13)

there will be a corresponding set of surfaces S(x,y) . Of this set, the most probable value should be determined by mathematic and experimental considerations.

For example, if the surface z=F(x,y) behaves variably over time, but still conforms with a "similarity" law, the surface at a time t+τ will be:

S(x,y, t+τ) = K x S(x,y,t) (14)

where K is a proportionality constant. In this case, the actual deformation can be found fairly easily by making attenuation measurements continually over time.

Another instance of the strained surface being immediate to determine is that of a concentrated strain for, in this case, variations in attenuation will only occur at some of the vertical fibres and some of the horizontal fibres.

Figure 11 illustrates schematically an embodiment of an optical apparatus for measuring strains in a flat surface by a sensor according to the invention, and using transmitters and receivers connected in the following manner :

each fibre is supplied a constant optical power, from an transmitter TX connected through a transmit system 8 which comprises a splitter and plural distribution fibres; on the far side from the transmission side, a receiver RX is connected to each fibre through a receive system 9 which comprises essentially a plurality of pick-up fibres;

the optical power values measured by the receivers RX are reported to a central optical system (OS) which will calculate the attenuation through each fibre, considering that the transmitted powers are constant .

A second embodiment of the optical apparatus for strain measurements according to the invention, shown in Figure 12, concerns the application of this sensor for controlling stresses and strains occurring in a tubular structure 11, e.g. a gas pipeline. In this case, the horizontal fibres FO of the sensor would be laid along the tubular structure 11, and the vertical fibres FV would be wound ring-like around the tubular structure 11. A raceway 12 placed on top of the tubular structure 11, for example, contains an optical fibre 13 adapted to distribute the optical power to the individual vertical fibres, and a bus 14 for supplying the receivers and picking up measurement data of the power received on each wound fibre (vertical fibre FV) .

From a functional standpoint, the arrangement is equivalent to that shown in Figure 11, but for a different physical positioning of the transmitters TX and the receivers RX on the ends of the vertical fibres FV.

Another possible form of this optical apparatus for measuring strain and/or intrusion can be embodied to either monitor a flat surface or a pipeline 15, using a single sensing optical fibre 16, which may include sets of horizontal fibres FO, for example, and measuring the reflected power by means of an OTDR 17.

In this case, the strains can be located from anomalies detected by the OTDR 17.

This embodiment of the optical apparatus for strain measurements is useful especially where the strains do not vary sharply over time. In fact, the measurements then can be made cyclically by the OTDR 17, and the OTDR can be shared by several sensing optical fibres 16.

To check for the presence of mechanical stresses all along a laid pipeline 15, a sensing optical fibre 16 may be wound spirally around the structure and connected through an optical switch 18 at one end of the OTDR 17, as shown in Figure 13. The spiral-wound optical fibre 16 may comprise a single fibre, or alternatively a set of fibres embedded, for example, in a plastic tape, to provide a substantially solenoid-like configuration.

The reflected power measurement made by the OTDR 17 connected to the spiral-wound sensing optical fibre 16 via the optical switch 18 enables the detection of mechanical stresses, producing a concentrated deformation in the pipeline 15, to be detected even a distance away from the point where the measurement is taken.

The strain-measuring optical apparatus of this invention may further comprise a sensing optical fibre 19 laid scan- like over a flat surface 20 to be monitored, as shown in Figure 14, into a substantially serpentine con iguration. Here again, a measurement of reflected optical power would be made by the OTDR 17.

Finally, a modification of the above application is illustrated by Figure 15, wherein a sensing optical fibre 21 is laid into a concentric coil over the flat surface 20 to be monitored, and connected to one end of the OTDR 17.

Advantageously in this invention, the fibre optics sensor and optical apparatus for measuring strains can be used in many situations where the occurrence of mechanical stresses, abnormal displacements, pressure from intrusions, etc. in a structure however long is to be monitored.

Claims

1. A fibre optics sensor for detecting and measuring physical stresses and/or strains in a flat and/or tubular and/or irregularly shaped surface (2) , being of a type which comprises at least one sensing optical fibre and a back-up member for said at least one fibre (3) , characterized in that said at least one sensing optical fibre is laid into a given geometric pattern, and in that it comprises a means of detecting an attenuation in reflected optical power through said at least one fibre, and comprises a calculating means for re-constructing stresses and/or strains in the surface (2) from the value of the optical power attenuation through said at least one optical fibre.

2. A fibre optics sensor according to Claim 1, characterized in that it comprises at least first (FO) and second (FV) sets of sensing optical fibres.

3. A fibre optics sensor according to Claim 2, characterized in that said first set (FO) of optical fibres is laid over said surface along a first direction and said second fibre set (FV) is laid along a substantially transverse direction to the first, in a substantially gridlike geometric pattern.

4. A fibre optics sensor according to Claim 3 , characterized in that it additionally comprises auxiliary optical fibres (FA) for picking up the optical power output by each section of said second fibre set (FV) .

5. A fibre optics sensor according to Claim 2, characterized in that said first set (FO) of optical fibres is laid lengthwise over a tubular structure (11) and said second set (FV) of optical fibres is wound ring-like around said tubular structure (11) in a substantially solenoidlike pattern.

6. A fibre optics sensor according to Claim 5, characterized in that it further comprises an optical power-distributing optical fibre (13) connected to said second set (FV) of optical fibres.

7. A fibre optics sensor according to Claim 1, characterized in that said at least one sensing optical fibre (16) is laid spirally around a tubular structure (15) .

8. A fibre optics sensor according to Claim 1, characterized in that said at least one sensing optical fibre (21) is laid scan-like over a flat surface (20) to be monitored in a substantially twisting pattern.

9. A fibre optics sensor according to Claim 1, characterized in that said at least one sensing optical fibre (19) is laid in a concentric coil pattern over a flat surface (20) to be monitored.

10. A fibre optics sensor according to Claim 1, characterized in that said calculating means effects a reconstruction of the strains in the surface (2) by applying, in the reverse direction, an algorithm for evaluating the optical attenuation through a deformed optical fibre.

11. A fibre optics sensor according to Claim 10, characterized in that said calculating means applies an algorithm known as an FD-BPM (Finite Difference Beam Propagation Method) algorithm.

12. A fibre optics sensor according to Claim 2, characterized in that said bundle of optical fibres are spread evenly across said surface.

13. A fibre optics sensor according to Claim 2, characterized in that said bundle of optical fibres are embedded in a thin back-up member made from a pliable plastic material (3) .

14. A fibre optics sensor according to any of Claims 1 to 13, characterized in that single-mode optical fibres are employed therein.

15. A fibre optics sensor according to any of Claims 1 to

14, characterized in that said optical fibres are laid in a straight-line pattern.

16. A fibre optics sensor according to any of Claims 1 to 14, characterized in that said optical fibres are laid in a sinusoidal pattern to improve the sensor responsiveness.

17. An optical apparatus for detecting and measuring mechanical stresses and/or strains in a surface, being of a type which comprises at least one fibre optics sensor as claimed in any of Claims 1 to 16, and comprises a plurality of transmitters (TX) and receivers (RX) connected to the fibres such that :

a constant optical power is transmitted to each sensor fibre from a respective one of the transmitters (TX) ;

each fibre is connected to a respective one of the receivers (RX) , on the opposite side from the transmitter side thereof;

characterized in that it further comprises a central optical system (OS) arranged to calculate the attenuation through each fibre, taking into account constant transmitted powers, from the optical power values measured by said receivers (RX) .

18. An optical apparatus according to Claim 17, characterized in that said transmitters (TX) are each connected to a corresponding optical fibre through a transmit system (8) comprising a splitter and a plurality of distribution fibres, and that said receivers (RX) are each connected to said fibre through a receive system (9) comprising essentially a plurality of pick-up fibres.

19. An optical apparatus according to Claim 17, characterized in that said sensor comprises a first set (FO) of optical fibres laid along a tubular structure (11) and a second set (FV) of optical fibres wound ring-like around said tubular structure (11) , said sensor further comprising an optical power-distributing optical fibre (13) connected to said second set (FV) of optical fibres, and a data supply/pick-up bus (14) connected to the receivers (RX) •

20. An optical apparatus according to Claim 17, characterized in that said sensor includes a sensing optical fibre (16) wound ring-like around a tubular structure (15) and connected to a reflected power measuring means (17) .

21. An optical apparatus according to Claim 20, characterized in that said sensing optical fibre (16) is wound spirally around the tubular structure (15) .

22. An optical apparatus according to Claim 20, characterized in that said sensing optical fibre (16) comprises a plurality of optical fibres embedded in an elastic enclosing medium and connected to said reflected power measuring means (17) via an optical switch (18) .

23. An optical apparatus according to Claim 17, characterized in that said sensor includes a sensing optical fibre (19) laid scan-like over a flat surface (20) to be monitored and connected to a reflected power measuring means (17) .

24. An optical apparatus according to Claim 17, characterized in that said sensor includes a sensing optical fibre (21) laid in a concentric coil over a flat surface (20) to be monitored and connected to a reflected power measuring means (17) .

25. An optical apparatus according to Claims 20, 23 and 24, characterized in that said reflected power measuring means

(17) comprises essentially an OTDR.

26. A method of locating a strained spot on a surface or a structure subjected to mechanical or thermal stresses, characterized by that a measurement of reflected optical power is made inside a set of optical fibres functioning as sensors, said set of optical fibres being laid in a suitable geometric pattern of first (FO) and second (FV) fibre sets over the surface or inside the structure to be monitored.

27. A method of locating a strained spot according to Claim 26, characterized in that an inverse strain evaluation is performed on the basis of an optical attenuation through an optical fibre subjected to deformation, using an inverse evaluation algorithm effective to derive a geometric configuration of said fibre, along an axis thereof, from variations in the overall optical attenuation through said fibre, thereby to detect the presence of distributed or concentrated mechanical stresses or strains.

28. A method of locating a strained spot according to Claim 27, characterized in that it applies an algorithm known as an FD-BPM (Finite Difference Beam Propagation Method) algorithm.