FR3135324A1 - Method, system and sensor for monitoring a structure by optical fiber - Google Patents

Abstract

Method and system for monitoring a structure using an optical fiber having a longitudinal extent passing through i) at least one first zone (Z1) capable of dimensional variation in one direction, by arranging in the at least one first zone a developed length of fiber which is a multiple of a dimension of the zone in this direction, and by ii) at least a second zone (Z2) along which the optical fiber (1) ensures continuity between an injection point signal entering the fiber and the at least one first zone, the optical fiber being placed in the second zone (Z2) in mechanical connection with the structure so that the fiber is active for detection. This ensures both distributed surveillance in the second zones and surveillance with high sensitivity and excellent spatial resolution in the first zones (Z1). Figure for abstract: Fig. 1

Description

Method, system and sensor for monitoring a structure by optical fiber

The present invention relates to a method for monitoring a structure, and more particularly its deformations or any other parameter of interest which is the cause or consequence of its deformations, by injecting an optical signal into an optical fiber arranged on or in the structure to be to watch.

The present invention also relates to a system for implementing the method.

The present invention also relates to a sensor for implementing the method or being part of the system.

State of the prior art

The deformations can be of varied nature. This may involve ensuring the solidity of the structure. It may also involve identifying crossings of limits (borders, protected perimeters) by people, animals, vehicles, etc., based on the pressures exerted on a ground or boundary structure, detecting vibrations, for example to identify presences based on acoustic waves, etc. The structure can for example be a civil engineering work, a pipeline, soil, an aquatic environment, a road, a mobile telephone tower, a nuclear reactor, a wind turbine, etc.

It is known to monitor the deformations of a structure, or a parameter of interest associated with these deformations, by attaching a sensitive optical fiber to the structure into which an optical signal is injected. The deformations suffered by the structure are transmitted to the optical fiber. In the event of deformation, this is detected according to the variation of a parameter relating to an optical output signal provided by the optical fiber in response to the incoming signal.

According to certain methods which do not make it possible to localize the deformation, the parameter is the optical power of the output signal. This parameter only provides an averaged indication of the extent of the monitored area. A reassuring measure can conceal a local phenomenon which would have merited particular interest, an alert, an intervention or other appropriate decision/reaction. In the event of an alarming or significant result of an event, this type of process does not provide information on the affected site.

According to other methods, the output signal corresponds to reflections and/or backscattering of the incoming signal, created intentionally in the optical fiber or due to random variations in certain physical parameters of the fiber. These reflections and/or backscattering are altered by the deformations undergone by the optical fiber. Due to the speed of propagation of optical signals in the fiber, a time interval elapses between the injection of the signal in the form of a pulse and the instant of capture of a reflection or backscatter. The alteration of this response provides information on the existence and location of a deformation or variation in deformation along the fiber. This process is known as OTDR (“Optical Time Domain Reflectometry”). In particular, we know the OTDR process exploiting Rayleigh backscattering due to the interaction of photons with the local physical particularities of the optical fiber. Even with the OTDR technique, the localization of a phenomenon occurring along the fiber is subject to limits in terms of spatial resolution, linked to the duration of the input pulse, which cannot be infinitely small.

For small structures, in particular of a size less than the spatial resolution of the detection method, it is known to arrange the fiber according to a geometry forming curves so as to equip the structure with a fiber length greater than the dimension of the monitored site. This increases the sensitivity of detection without degrading the quality of localization. With this in mind, document US 2005/02 05 718 A1 describes individual detectors equipping sites located along a railway line.

US patent 7,274,441 B2 recommends limiting detection to localized sites, spaced apart. At these sites, the fiber is arranged in a spatially concentrated manner, notably wound in contiguous helical turns around a hollow mandrel. Here again, limiting monitoring to small sites equipped with a long length of fiber increases sensitivity. Between the monitored sites, the fiber only serves as an essentially passive optical connection, while generating time intervals between the signals coming from the different monitored sites which eliminate any problem relating to spatial resolution.

Patent EP 1 360 672 B1 concerns the monitoring of road traffic at various points on a route. Here again, an optical fiber comprises active zones exposed to the phenomenon to be detected, namely the passage of a vehicle, and at least one essentially passive connection zone. However, this involves limiting the sensitivity of the active zones, to filter out parasitic phenomena. For this purpose, the fiber is straight in the active zones. It is also a question of increasing the developed length of the fiber in the connection zone between active zones. For this, in its connection zones, the fiber is arranged helically inside a support mandrel, or around a cylindrical core, or in oblong turns around a flat core or around two studs of end.

For structures comprising relatively small and closely spaced areas to be monitored, it is known from document US 2008/01 66 120 A1 to arrange the fiber alternately in the areas to be monitored, where the fiber is associated with the structure, and in connection zones where the fiber is separated from the structure. Here again, the output signals from the connection zones do not indicate any deformation. To facilitate the identification of a monitored site which is undergoing deformation, it is proposed to increase the fiber length in the connection zones, or between the injection point and the first monitored site, by comparison with the distance between these sites. As part of an OTDR process, the time domains corresponding to each of the sites, respectively, are thus more distinctly separated.

It has been found according to the invention that certain structures, in particular heterogeneous, raise a particular problem for efficient detection. Certain areas of the structure are exposed to certain types of deformation and other areas to other types of deformation. In particular, there may be initial zones susceptible to concentrated deformation, other zones along which distributed deformation is expected. There may be an area requiring special attention, for example the area of passage through an enclosure. There may also be a desire to collect quasi-punctual measurements without giving up distributed measurements.

For example, a bridge is susceptible to localized deformation in the area of the supporting pillars -or piers-, and to deformation distributed between the piers. In all these cases, and others of the same kind, the risk is to underestimate a localized deformation which will have been too strongly averaged due to the insufficient spatial resolution of the process implemented, even in OTDR technology.

Furthermore, when the size of the structure to be monitored is large, for example several kilometers or tens of kilometers, it is necessary to increase the duration of the pulses of the incoming signal from an OTDR process. This results in a degradation of spatial resolution.

The aim of the invention is thus to propose a method which makes it possible to take into account in a specifically different way deformations according to their probable typology and/or their origin and/or probable consequence depending on their site of occurrence, and/or the desired nature and/or precision for the parameter(s) of interest at different sites.

According to the invention, the method for monitoring a natural or man-made structure in relation to at least one parameter of interest, this method comprising the following steps:
– arrange in mechanical connection with the structure at least one sensitive optical fiber at a detection site having a longitudinal extent passing through i) at least a first zone capable of dimensional variation in at least one direction of dimensional variation, by arranging in the at least a first zone a developed length of fiber which is a multiple of a dimension of the zone according to the direction of deformation, and by ii) at least a second zone along which the optical fiber ensures continuity between a point of injection of optical signal into the fiber and the at least one first zone;
– inject an incoming optical signal into the optical fiber, through the injection point;
capturing an optical output signal, which is provided at least indirectly by the optical fiber after traveling at least part of the length of the optical fiber;
– obtain information on the deformation of the structure by processing the optical output signal;
is characterized in that in at least part of the at least one second zone, the fiber is mechanically linked to the structure so as to be active there for detection.

Thus, according to the invention, the same optical fiber ensures differentiated detection depending on the area of the structure concerned. In a first zone of relatively small dimension, a relatively large developed length of optical fiber is arranged. The sensitivity of the optical fiber to deformation in this first zone is therefore amplified. The method thus makes it possible to effectively detect significant, if not dangerous, local deformations, whereas such deformations could go unnoticed, or be misinterpreted, if they were only detected as a non-overweighted element of an average, which This is the average of a method operating by amplitude detection or the average over a distance corresponding to the spatial resolution of an OTDR process.

At the same time, as part of said differentiated detection, the fiber is coupled to the structure in at least part of the at least one second zone, providing therein a detection precision which is typically, for example, that of a fiber arranged longitudinally relative to the detection range.

In the context of a method operating by amplitude detection, the signal indicative of the overall deformation benefits from an overweighting of the responses to the deformations of the first zones. As part of an OTDR process, the detected deformations can be assigned to the first zones and respectively to the second zones since these correspond to respective time domains of the output signal.

Typically, the at least one first zone is a zone essentially susceptible to deformation, or concentrated dimensional variation, and the at least one second zone is a zone essentially susceptible to distributed deformation, or dimensional variation. Each zone then benefits from the most effective detection sensitivity with regard to its typology.

In certain applications, the length of fiber installed in at least part of the at least one second zone is substantially equal to 1m per meter of longitudinal extent. This arrangement makes it possible in particular to detect deformations on sites with a very large longitudinal extent, for example several tens of km, while minimizing, in relation to this extent, the power required for the source supplying the incoming signal.

Preferably, in the at least one first zone, the optical fiber is arranged in several round trips parallel to an expected direction of local dimensional variation. Thus the dimensional variation is applied to the fiber multiplied by the sum of the numbers of outward journeys and the number of returns. For example, the optical fiber can be arranged in the form of oblong turns.

According to an advantageous version, in at least one first zone, the fiber is arranged in a spiral, in particular with oblong turns as explained above.

In the at least one first zone, the optical fiber can be arranged on a support comprising for the positioning of the optical fiber a channel configured in turns, two turns of the channel communicating with each other to allow one of the two aforementioned turns of the channel to receive at least two turns of optical fiber.

Non-limitatively, in the at least one first zone, the optical fiber is arranged in a plane, in particular a plane parallel to a surface of the structure against which the sensitive fiber is arranged. Thus the size of the system is optimized in the first zone. Furthermore, the entire extent of the fiber is in a well-defined relationship with the surface of the structure to be monitored.

In another version, in the at least one first zone, the optical fiber is wound in the form of a spool. This version is particularly advantageous when the sensor must be embedded in a constituent mass (for example concrete, polymer, carbon fibers, etc.) of the structure.

In the at least one first zone, the optical fiber can be arranged on a support comprising means for positioning the fiber according to a predetermined geometry, the support being in mechanical connection with the structure and sufficiently deformable to transmit the variations to the optical fiber dimensions of the structure according to the direction of dimensional variation when said support is mechanically linked to the structure. Thanks to the predetermined geometry given to the optical fiber, a dimensional variation of the structure in the first zone influences the optical fiber in a predictable manner.

In a particular mode of implementing the method, the at least one first zone comprises a multidirectional zone where the fiber is arranged in at least two directions forming an angle between them. The optical fiber is thus made sensitive to dimensional variations in said at least two directions.

As already mentioned above, in an improved mode of implementation of the method, the output signal is composed of reflections – or backscattering – of the incoming signal generated by interaction of the incoming signal with the material of the optical fiber, the sites are identified reflection according to a time domain of capture of each backscatter, and a deformation is assigned to a backscatter site according to a variation of at least one physical parameter of the output signal in the time domain of capture associated with this site backscatter (“OTDR” reflectometry).

Preferably, the developed length of optical fiber arranged in the at least one first zone is at least substantially equal to a spatial resolution of the optical reflectometry processing. Thus, in a time domain of the output signal, the output signal only describes the dimensional variations of the first corresponding zone, without mixing with the influence of the second adjacent zone(s).

Typically, the dimension of the first zone measured parallel to the direction of dimensional variation to which the optical fiber is sensitive in this zone is less than or substantially equal to 50 cm. In other words, with regard to the dimensions, often very large, of the structure to be monitored, the method according to the invention provides a measurement which can be described as punctual or quasi-punctual in at least one first zone.

In one embodiment, the optical fiber is embedded in a constituent mass of the structure and to place the optical fiber in mechanical connection with the structure in the first zone the support comprises anchoring means which link the support to the constituent mass at least for dimensional variations parallel to the direction of local sensitivity of the optical fiber. When the mass undergoes dimensional variations, the anchoring means transmit said variations to the support, which makes them subject to the sensitive optical fiber with the multiplication factor specific to the arrangement of the optical fiber on the support.

In implementation modes where the optical fiber is embedded in a constituent mass of the structure, to put the optical fiber in mechanical connection with the structure in the first zone, the support comprises anchoring means which bind the support to the constituent mass, at least for dimensional variations parallel to the direction of local sensitivity of the optical fiber.

In certain applications, it is also possible to fix the support to a solid element before embedding this solid element and said support equipped with the sensitive optical fiber in the mass of the structure. The solid element can for example be a metal frame placed then embedded in concrete to form a reinforced concrete structure.

In a preferred example, the structure is an overhead wire line comprising cables supported by line supports such as poles or pylons, the at least one first zone is adjacent to a line support, the at least one second zone is extends along a path on the line supports, and the optical fiber comprises connection zones, constituting essentially passive third zones for detection, which extend between the line supports.

In another preferred example, the structure is a bridge comprising at least one deck supported by piers, the at least one second zone extends along the deck between the piers, and the at least one first zone is adjacent to at least one battery.

The structure may also be a structure exposed to vandalism, in which case the at least one first zone corresponds to at least one element exposed to vandalism, and the at least one second zone corresponds to at least one structural element of the structure.

The structure may also be a nuclear reactor. The structure can also be a pipeline such as a pipeline, a gas pipeline, a water pipe or the like, and the at least one first zone comprises all or part of inspection manholes, connections, geological sites at risk, sites susceptible to vandalism.

In one embodiment, the structure is a wind turbine. The sensitive optical fiber is arranged on the mast of the wind turbine, and a failure of a rotating part of the wind turbine is detected when an alteration of a vibration mode of the mast is detected. Preferably, the wind turbine mast is equipped with the sensitive optical fiber according to the invention, alternating first and second zones.

More generally, the at least one first zone is typically adjacent to a site of fragility of the structure.

According to a second aspect of the invention, the system for implementing the method according to the first aspect supplemented where appropriate by all or part of its improvements, this system comprising:
– at least one sensitive single-mode optical fiber which in service is mechanically linked to a structure to be monitored;
a high coherence laser;
– means for injecting coherent optical pulses into the optical detection fiber from the laser;
– a processing circuit for capturing and heterodyning a backscatter signal provided at least indirectly by the sensitive optical fiber in response to each pulse;
– means for detecting the phase of the heterodyne signal;
is characterized in that the sensitive optical fiber is placed in mechanical connection with the structure in first zones and in second zones, the developed length of fiber being greater in the first zones than in the second zones with respect to a respective dimension of each zone according to a direction of dimensional variation to which the optical fiber is sensitive.

Preferably the developed length of optical fiber arranged in each first zone is at least substantially equal to a spatial resolution of the optical reflectometry processing.

In at least one first zone, the optical fiber is preferably arranged in accordance with one of the provisions explained above concerning the method.

According to a third aspect of the invention, the sensor for implementing the method according to the first aspect of the invention supplemented where appropriate by all or part of its improvements, or form part of the system according to the second aspect of the invention. invention supplemented where appropriate by all or part of its improvements, is characterized in that it comprises a support on which an optical fiber is arranged, the support comprising positioning means defining for the optical fiber a geometry having a direction of elongation according to which the optical fiber performs several round trips, the support and the optical fiber being joined so that a dimensional variation of the support according to the direction of elongation is transmitted to the optical fiber with a multiplying coefficient corresponding substantially to the sum of the number of outward journeys and the number of returns, the support comprising securing means to be made integral with a structure to be monitored at least with regard to dimensional variations according to the direction of elongation.

In one embodiment, the support is integrated into a box which encloses the optical fiber in the first zone. Preferably the inside of the box contains a filling mass, in which the sensitive optical fiber is embedded.

Particularly when the sensor is intended to be embedded in a constituent mass of the structure to be monitored, the securing means may include anchoring conformations in the mass of the structure, such as concrete, polymer, carbon fibers, etc.

In certain embodiments, the positioning means define an oblong spiral geometry for the optical fiber.

In other embodiments, the positioning means comprise an oblong core defining for the optical fiber a flattened coil geometry to define the direction of elongation.

In some applications the sensitive optical fiber is mounted in a sheath, preferably without the possibility of slipping. This sheath protects the optical fiber, particularly if the sensor must be embedded in a mass constituting the structure without including a housing or other overall protection for the fiber.

In one version the sensor comprises an optical coupler, or optical connector, at each end of the section of the optical fiber belonging to the sensor, to allow the insertion of the sensor into an optical circuit, in particular the connection to second zones as defined in the framework of the process. Such a sensor forms a ready-to-use component which greatly facilitates the installation of a system such as that of the second aspect of the invention.

Other advantages and characteristics of the invention will appear on reading the detailed, non-limiting description which follows.

The detailed description which follows refers to the appended drawings in which:

is a diagram of a system for implementing the method according to the invention according to the OTDR technique with heterodyning and phase detection;

is a plan view of a spiral fiber coil sensor for implementing the method according to the invention before installation of a mass embedding the optical fiber and installation of a sensor cover;

is a view of a detail, on an enlarged scale, of the sensor of the ;

is a sectional view according to IV-IV of the , showing the sensor with its cover;

is a partial elevation view of a bridge equipped with the system according to the invention;

is a partial perspective view of a beam equipped with a bidirectional sensor assembly;

is a partial perspective view of a beam equipped with a tri-directional sensor assembly;

is a schematic elevation view of a nuclear reactor equipped for implementing the invention, the reactor enclosure being seen in section;

is a top view of the reactor of the , the tank being again shown in section;

is a partial view of an underground pipeline equipped for the implementation of the invention;

is an elevation view of a mobile telephone tower equipped for implementing the invention;

is a partial elevation view of an electrical line equipped for the implementation of the invention;

,

And

are views (partial view with regard to the ) in perspective of three embodiments for a sensor according to the invention intended to be embedded in a mass constituting the structure to be monitored; And

is a partial perspective view of a wind turbine equipped for the implementation of the invention.

Detailed description of embodiments

It is specified that the present description, above and below, is understood to cover any particularity likely to confer patentability on a combination, even if this particularity is not described separately or described in the context of the combination. , and even if it is part of a sentence or paragraph evoking particularities absent from the combination.

In the non-limiting example shown in , the system according to the invention is of the OTDR type with heterodyning and phase detection. It comprises a sensitive optical fiber 1 intended to be mechanically connected to the structure to be monitored (not shown in Figure 1). ) so that variations in the parameter of interest which affect the structure result in corresponding dimensional variations of the optical fiber. For example in the case of a solid structure exposed to dimensional variations in at least a certain direction, the optical fiber can be secured to the structure, for example glued to the structure, so as to undergo dimensional variations corresponding to said dimensional variations of the structure. In the case where the structure is not solid, for example an aquatic environment or even land whose parameter of interest is the pressure or a parameter whose variations cause the pressure to vary (for example a passage over the land or a vibration in water), or even a terrain whose parameter of interest is relative to the movements of this terrain (telluric movements, collapses etc.), the fiber is placed in contact with the structure so that variations in pressure or depending on the case, the movements result in variations in length, possibly vibration, of the optical fiber.

At its proximal end 2, the optical fiber 1 is connected to an operating device comprising an injection circuit 3, a reception circuit 4 and an acquisition circuit 6.

The injection circuit comprises a high coherence laser 7 whose continuous laser beam is transmitted by an optical link 8 to an acousto-optic modulator AOM electrically connected to an acoustic frequency generator AWG. In a known manner, the AOM modulator transforms the continuous laser beam into rectangular optical pulses of very short duration whose pulse frequency is that of the AWG generator. Each pulse contains a certain number of optical periods. The optical frequency in these pulses is equal to the sum of the optical frequency of the laser and the much lower acoustic frequency provided by the AWG generator.

The optical pulses supplied by the AOM modulator are amplified by an optical amplifier 11, for example with a fiber doped with erbium ions (EDFA: Erbium Doped Fiber Amplifier), then filtered by a band-pass optical filter 12, before being transmitted to the sensitive optical fiber 1 through an optical circulator 13.

The Rayleigh backscatters are received by the optical circulator 13 which directs them towards the reception circuit 4 via an optical fiber 14 input to the reception circuit 4.

A coupler 18 having a division ratio 1:99 (example) makes it possible to take the signal from source 7 (1% of its power in the example) on an optical fiber 17 to carry out heterodyning. Heterodyning is carried out between the signal from fiber 17 and that coming from fiber 14 at the photodiode of a detector 23 after recombination, for example 50:50, of said signals by an optical coupler 19.

However, the frequencies of these two signals are slightly different: one is the frequency of the laser and the other the frequency of the backscatter signal, namely the slightly increased frequency produced by the acousto-optic modulator AOM. As a result, the photodiode of detector 23 provides a beat signal whose frequency is the acoustic frequency (as the difference between the two mixed frequencies), and the phase is exactly the phase of the backscattered signal. In other words, when the phase of the backscattered signal varies by a certain angle, the phase of the heterodyne signal varies by the same angle since the phase of the signal coming from the laser is stable. And thanks to the acoustic frequency much lower than the optical frequency of the laser, this phase can be measured by known means.

The OTDR technique with heterodyning and phase detection is preferred because it provides much greater precision than that which would be provided, for example, by a simple measurement of time shift of the backscattered signal, which in addition would be more complex to implement. artwork.

The optoelectronic detector or converter 23 supplies its electrical signal to an electronic preprocessing module 24 whose output is transmitted to the acquisition circuit 6 which includes an analog/digital converter 26 and a processing device 27 which extracts the phase of the signal at each moment. At a given moment, the phase thus determined corresponds to a site determined along the sensitive optical fiber 1 as a function of the time elapsed since the last injected pulse. The system is therefore capable of providing information on the parameter of interest for each region along the fiber.

The structure described so far, in particular for producing the pulses, generating a slight frequency difference between the optical output signal and an optical reference signal with a view to heterodyning, and processing the heterodyne signal to extract the phase n This is only one example among other possible ones, some examples of which are given in the prior art mentioned in the introduction.

The spatial resolution of the system as described so far is limited by the duration of the pulses provided by the AOM modulator, which cannot be infinitely small. A backscattered signal can, at a given time, come from the start of a pulse having encountered a variation in the refractive index relatively far in the fiber or from the end of a pulse having encountered a variation in the refractive index less far in the fiber. The longer the sensitive fiber 1 is (up to several tens of kilometers for certain applications), the more the power of the laser must be increased. This leads to increasing the duration of the pulses to maintain the parasitic effects of the start and end of a pulse at an acceptable level, and therefore leads to degrading the spatial resolution.

In addition, the sensitivity of the system as described so far is limited by the amplitude of the deformation of the fiber depending on the nature of the parameter of interest and depending on the amplitude of the variations to be detected.

Furthermore, in certain applications, the parameter of interest can be distributed by nature, such as for example a bending deformation of a structure, in certain zones, and localized in other zones, for example in a site of potential fragility . The parameter of interest may even be different from one area to another. In certain other or the same applications, we may want a global measurement for certain areas and a precise measurement at certain specific sites, even if these specific sites are not functionally different from the rest of the structure.

To respond to this multiple problem, the invention provides for defining on the structure at least two types of zones, namely first zones Z1, quasi-point, intended for detection offering great sensitivity and excellent spatial resolution, and second zones Z2 extending between the zones Z1 and along which the detection will operate with lower sensitivity and spatial resolution.

In the second zones Z2 the fiber is mechanically connected to the structure in a configuration which can be conventional, typically at the rate of one meter of fiber for 1 meter of structure measured according to the direction of variation in length of the fiber as a result of the variations of the parameter of interest.

The same sensitive optical fiber 1 runs through at least a first zone Z1 and a second zone Z2, typically, successively and alternately, several first zones Z1 alternating with several second zones Z2. This does not exclude that the system is multi-channel, with for example a common laser supplying several sensitive optical fibers, at least one of which would pass through first zones Z1 and second zones Z2 as described.

The first zones Z1 preferably have, parallel to the direction of the variation in length to be detected, a length much lower than the spatial resolution of the system. In the first zones Z1, the sensitive optical fiber 1 forms loops so that the developed length of the fiber 1 placed in a zone Z1 is a multiple of the length of the zone Z1.

Preferably, we make sure to maximize, among this developed length, the proportion of this length which is arranged parallel to said direction. To do this, for example, we ensure that the fiber moves back and forth parallel to said direction.

In an advantageous embodiment, the developed length of the fiber in a first zone Z1 is at least equal to the spatial resolution of the system. For example, in a first zone Z1 having, depending on the direction of dimensional variation, a length of approximately 40 cm, the developed length of the sensitive fiber can be approximately 4 meters.

In each of the first zones, the deformation of the fiber is amplified due to the multiplication of the deformation of the fiber relative to the deformation of the medium in the zone. This means an increase in sensitivity in the first areas. On the other hand, at certain given times counted from the pulse, the received backscatter signal comes largely or entirely from a first respective zone Z1, so that the measurement of the parameter of interest can without uncertainty be assigned to this area.

In an embodiment shown in more detail in Figures 2 to 4, in each first zone Z1 the optical fiber is configured in a spiral with elongated turns parallel to the direction of the deformations to be captured.

It is also planned, in an embodiment also illustrated by Figures 2 to 4, that the system includes sensors 28 intended to equip the first zones Z1. Each sensor 28 includes means for positioning the optical fiber according to the planned configuration, for example the spiral configuration mentioned above. In the example shown, the positioning means comprise a support plate 29 on which guides 31 are formed defining a channel for the fiber. The guides 31 are formed on one of the two opposite main faces of the plate. The spiral extends in a plane substantially parallel to this main face.

Preferably, the guides 31 are bars separated by interruptions which allow at least one turn of the fiber to close at least once on itself so that at least one channel turn contains at least two turns of fiber, see the details of the .

In one embodiment, the support plate constitutes the bottom of a box 32. After placing the fiber 1 on the support plate 29, the box is closed with a cover 33. Once closed, the box 32 defines between plate and cover two opposite orifices 34 for the optical fiber.

Preferably, once the fiber has been placed in the planned configuration, it is embedded in a mass 36 which can for example be a polymer or a tar. In the embodiment with box 32 (Figures 2 to 4), this mass is put in place before closing the cover, or after closing by an injection orifice not shown. This orifice as well as the orifices 34 for the fiber are sealed by overflow of the mass at the end of injection.

In service, the support plate is attached to the structure to be monitored. A second main face of the support plate, opposite the main face receiving the optical fiber, is applied against a surface of the structure to be monitored. In the example shown, the plate has holes 37 for fixing the plate to the structure using screws, nails, rivets or other appropriate means depending on the nature of the structure to be monitored. It is still possible to glue the plate to the structure if this is compatible with the nature of the structure to be monitored.

In each first zone Z1 the fiber is mechanically connected to the structure so that any dimensional variation of the structure according to the direction of variation to be detected is effectively transmitted to the fiber with the planned multiplicative factor, without being able, for example, to be partially transmitted to an adjacent region of the fiber, equipping an adjacent zone Z2. It is also necessary to prevent the means of positioning the fiber in the first zones Z1 from preventing the fiber from deforming freely under the action of dimensional variations of the structure in zone Z1.

For this, in the embodiment shown, the support, more particularly the support plate, has a high deformability in the direction of detection, in comparison with the structure to be monitored. This is obtained by a judicious choice of the material of the plate and the area of its section measured perpendicular to the direction of detection. A suitable material may be an elastomer. When a cover 33 and/or a filling mass 36 are provided, they must also be sufficiently deformable so as not to hinder the transmission of deformations from the structure to the optical fiber.

In one embodiment, the optical fiber is prevented from passing by sliding from one zone to an adjacent zone, so that the deformations of the fiber relating to each zone respectively faithfully reflect the respective dimensional variations of these zones. To do this, it is possible to fix the fiber to the structure at both ends of zone Z1. For example, in the case of using a support such as the support plate 29, it is possible to subject the optical fiber to a certain tightening at the two ends of the zone Z1, for example in the two orifices 34, by example by a certain tightening between the plate 29 and the cover 33.

Alternatively or in addition, the fiber can be fixed to a support, for example the support plate 29 over all or part of the extent of the region of the fiber associated with zone Z1. The mass 36 can ensure such connection. Bonding the fiber to the support, particularly before placing the mass, can also provide this function, replacing or complementing the adhesive effect of the mass.

In the zone Z1 equipped with the sensor 28, a deformation undergone by the structure parallel to the direction of elongation of the turns of the sensor generates in the region of the optical fiber which is housed in the sensor a variation in length which is n times that undergone by the structure, being equal to twice the number of turns of the sensor in this region. The sensitivity of the system with regard to deformations in zone Z1 is therefore multiplied by n. In the example shown there are 6.5 turns so the sensitivity is multiplied by 13.

Preferably, the fiber section 1 which is part of the sensor 28 has two connection ends, on either side of the sensor 28, intended to be connected, by any means such as welding or optical connector, with two other constituent sections of the two adjacent zones, such as two second zones Z2.

In the example of the , the structure to be monitored is a bridge comprising a deck 41 resting on piers 42. The parameter of interest is the deformation of the deck in bending around transverse horizontal axes. The sensitive optical fiber 1 is arranged longitudinally on a side face of the apron, at a distance from the neutral fiber of the apron illustrated by the dotted line 43. Thus any variation in the bending of the apron under the effect of a load such as a vehicle crossing the bridge generates variations in length (extensions and compressions depending on the locations) of the deck along the optical fiber 1. We chose as first zones Z1 measurement sites located above the piers 42. The second zones Z2 extend along the spans between the piers 42.

There schematically illustrates the implementation of the invention to obtain a bidirectional measurement in a first zone Z1 in the example of a beam 51. Generally speaking, and more particularly in the second zones Z2, the sensitive optical fiber 1 is fixed longitudinally on a face chosen according to the parameter of interest, here a lateral face 52.

In the first zone Z1 we have placed an individual sensor 28x oriented to be sensitive to dimensional variations of the beam 51 according to the longitudinal direction of the fiber 1 and the beam, and a sensor 28y oriented to be sensitive to dimensional variations of the beam 51 in a transverse direction of the fiber face 52 of the beam. The sensitive optical fiber 1 passes successively through the two individual sensors, forming a 90° turn between them. On the side of the individual sensor 28y opposite the individual sensor 28x, the sensitive optical fiber forms a 180° turn to return to the general axis of the fiber 1 then a 90° turn to constitute the region of the fiber 1 associated with the second neighboring zone Z2.

In service, the system provides for the first zone Z1 equipped with the bidirectional sensor 28xy information on local deformations in the two directions each corresponding to the orientation of a respective individual sensor. Thanks to the invention, the information will benefit from excellent sensitivity. So that the information relating to each of the two directions is even more clearly distinguished from that relating to the other direction, thanks to the excellent spatial resolution of the sensors according to the invention, it can be provided that the length of fiber between the two individual sensors 28x and 28y are at least equal to the spatial resolution outside the sensor. This length of interlayer fiber is preferably mechanically decoupled from the structure to be monitored and more generally from any source of variable deformation.

More generally, by a successive arrangement of the two individual sensors along the sensitive optical fiber 1, the backscattering associated with each of the two directions of dimensional variation are clearly separated in time. Preferably the developed length of the optical fiber in each of the two individual sensors is at least equal to the spatial resolution of the system.

As shown in the in an example which will only be described for its differences with that of the , the invention also makes it possible to produce a 28xyz three-dimensional sensor. The latter comprises a third individual sensor 28z secured to the structure to be monitored so as to be further sensitive to dimensional variations of the structure in a third direction which is transverse, preferably perpendicular to the detection directions of the individual sensors 28x and 28y. The sensitive optical fiber passes successively through the three individual sensors. In this example the third individual sensor 28z is attached to a face 53 of the beam 51 which is adjacent to the face 52 receiving the two other individual sensors 28x and 28y. The three individual sensors 28x, 28y and 28z are for example identical to that described as sensor 28 with reference to Figures 2 to 4.

In service, the system provides for the first zone Z1 equipped with the 28xyz tri-directional sensor information on local deformations according to the three directions each corresponding to the orientation of a respective individual sensor. Thanks to the invention, the information relating to each direction will benefit from excellent sensitivity. Analogous to what has been described with regard to the bidirectional sensor 28xy, the three individual sensors 28x, 28y and 28z are preferably connected to each other by two intermediate sections of appropriate length and mechanically decoupled from the structure to be monitored or any other source deformation.

In an embodiment not shown, the bidirectional sensor comprises two individual sensors in the x and z directions, arranged on two different faces of the structure to be monitored, for example the two faces 52 and 53 of the .

In the example shown in Figures 8 and 9, the system according to the invention equips a nuclear reactor 61 comprising a vessel 62 of the reactor and an enclosure 63 enclosing the vessel 62. The vessel 62 and the enclosure 63 are of generally cylindrical shape with a vertical axis. Various sensors 28, for example as described with reference to Figures 2 to 4, are attached to the exterior surface of the tank 62 and to the exterior surface of the enclosure 63. These sensors are here placed so as to be sensitive to dimensional variations parallel to the common axis of the tank and the enclosure. A sensitive optical fiber 1 passes successively through the sensors 28 of the tank 62. The same fiber, or another, passes successively through the sensors 28 of the enclosure 63. If there are two fibers they can be powered by the same laser source and connected to the same reception circuit. They each have their own acousto-optic modulator which emits pulses alternately so that during processing the backscattering can be processed separately and assigned without error to one or the other of the two optical fibers.

The possibility of a system with several sensitive fibers is not limited to the example of application in Figures 8 and 9, nor more generally to the application to a nuclear reactor.

In the example of the , the system according to the invention equips a pipe 71, for example a pipeline. The fiber 1 is generally attached to the pipe, parallel to the longitudinal direction of the pipe. The first zones Z1 correspond to particularities such as connections 72, manholes 73, equipped or not with valves 74, etc. At least one sensor such as 28 of Figures 2 to 4 is fixed against the pipe 71 or the body of the valve 73 in zones Z1. Not shown, other sensors such as 28 could be fixed on equipment other than directly part of the pipeline, the fiber 1 then making one or more detours to pass through this or these sensors.

In the example shown in , the system according to the invention equips a mobile telephone tower 81 comprising a mast 82 supporting antennas 83, and comprising a technical box 84 at the foot of the mast 82. The box 84 is connected by cabling 86 to the antennas 83 and other devices 87 installed at the top of the mast. This type of equipment runs a risk of mechanical failure of the mast 82, and a risk of vandalism, towards the box 84 or the cabling 86 or, by climbing the mast, towards the antennas 83 and other devices 87 placed at height, in generally accessible by a ladder (not shown) fitted inside the mast.

In this example, fiber 1 is arranged in a round-trip path to the top of the mast. In one sense the fiber is attached to the mast parallel to its vertical direction, and passes through a certain number of sensors 28a. At the top of the mast the fiber passes through at least one sensor 28b mounted on the devices 87 to be sensitive to malicious action against them. In the other direction, the fiber 1 is subject to the cabling 86, passing through at least one sensor 28c intended to be sensitive to vandalism forces exerted on the cabling 86, then through a sensor 28d on the box 84, there again to be sensitive to efforts resulting from vandalism.

There represents a part of an electrical line 91 equipped with a system according to the invention. A pylon 92 supports a certain number of cables 93 via insulators 94. Bad weather such as wind, frost, etc. can overload the pylons and lead to breakage. It is important to detect the beginnings of a break which can be repaired before destroying the pylon and breaking the electrical connection. The system according to the invention makes it possible both to locate the faulty pylon and to locate the weakened site on the faulty pylon.

To do this, fiber 1 passes successively through successive pylons. Between two pylons the fiber extends freely in a third zone Z3 which is passive for detection. On each pylon the fiber extends along a path passing through different sites of probable fragility, at each of which a sensor 28 has been placed. we have only shown the fiber on the left half of the pylon so that the figure is better readable in comparison with the right half, but it must be understood that it is expected that the right half will generally also be instrumented, for example as the symmetrical of the left half.

Figures 13 to 15 represent examples of sensors according to the invention adapted to be embedded in a mass such as concrete, a synthetic material, or even a carbon fiber body.

In the embodiment of the , the sensor plate, instead of having holes 37 for fixing on a structural surface to be monitored, has at each of its two opposite ends relative to the direction of deformation to which said sensor is sensitive, an anchoring collar 101 projecting transversely to said direction. The interior of the sensor is, for example, similar to that of the sensor 28, particularly with regard to the spiral arrangement of the sensitive fiber. The anchoring collars 101 have the function of positively securing the sensor with the surrounding mass, particularly with regard to movements having a component parallel to the direction of sensitivity of the sensor.

During the manufacture of the structure, the sensor is inserted into the still malleable mass. After hardening of the mass, any variation in dimension of the mass parallel to the direction of sensitivity of the sensor produces a variation in distance between the two flanges 101 and therefore a corresponding dimensional variation of each outward and each return of the portion of sensitive fiber arranged in the sensor.

The method of carrying out the is similar to that of the except that the flanges are replaced, at each end of the sensor, by anchor teeth 102 projecting transversely to the direction of sensitivity of the sensor. The teeth 102 are formed by cutting and folding a metal plate 103 on which the sensor such as 28 of Figures 2 to 4 is fixed. The plate 103, while being very inexpensive, allows the same model of sensor to be used , for example such as 28, for surface fixing of a structure and for installation in the mass of a structure. Rivets 104 attach the sensor 28 to the plate 103.

In the example of the , the plate is replaced by a coil 106 having an axis 107 transverse to the direction of sensitivity. A core 108 of the coil has, seen along its axis 107, an oblong profile, elongated in the direction of sensitivity. The core is bordered at each end by a cheek 109 projecting radially outwards. The core 108 has a passage 111, through or not, along its axis. The fiber 1 is wound in several turns around the radially outer face of the core 108, between the two cheeks 109. Typically the coil 106 is metallic, for example made of sheet metal. For example, we can start from a section of metal tube which we flatten radially to give it the aforementioned oblong profile, then we plastically deform the two ends of the tube to form the two cheeks 109. We have also shown schematically in the an optical connector 105 at one of the two ends of the fiber section 1 belonging to the sensor.

This realization has the double advantage of being very economical and of ensuring excellent connection with the surrounding mass which, before solidification, is housed between the cheeks 109 and in the passage 111.

In the implementation shown in , the invention is applied to a wind turbine 112 comprising a mast 113, a pivoting nacelle 114 at the top of the mast 113, and a rotor 116 supported in rotation by the nacelle 114. The sensitive optical fiber 1 is arranged on the mast 113 forming back and forth up and down the mast 113, with an alternation of first and second zones Z1 and Z2 respectively. Although the moving parts 114 and 116 are not equipped with sensitive fiber, their failures are detectable via the sensitive fiber 1 due to the alteration of the vibration modes of the mast.

Of course, the invention is not limited to the examples described and represented. The optical fiber could be arranged for example in a zigzag and not in a spiral in sensors such as 28. The method can use a heterodyning technique other than that described, and an acquisition method different from that described. The method according to the invention is applicable to structures other than those described, for example on or in a wall, for example a surrounding wall, on a fence, in an aquatic environment to detect acoustic waves, in land, etc. .

Claims (32)

Method for monitoring a natural or man-made structure relative to at least one parameter of interest, this method comprising the following steps:
– arrange in mechanical connection with the structure at least one sensitive optical fiber (1) in a detection site having a longitudinal extent passing through i) at least a first zone (Z1) capable of dimensional variation according to a direction of dimensional variation, in arranging in the at least one first zone a developed length of fiber which is a multiple of a dimension of the zone (Z1) according to the direction of dimensional variation, and by ii) at least one second zone (Z2) along which the optical fiber (1) ensures continuity between an optical signal injection point entering the fiber and the at least one first zone (Z1);
– inject an incoming optical signal into the optical fiber, through the injection point;
– capture an optical output signal, which is provided at least indirectly by the optical fiber after having traveled at least part of the length of the optical fiber;
– obtain information on the parameter of interest by processing the optical output signal;
characterized in that in at least part of the at least one second zone (Z2), the fiber (1) is mechanically linked to the structure so as to be active for detection. Method according to claim 1, characterized in that the at least one first zone (Z1) is a zone susceptible to concentrated dimensional variation and the at least one second zone (Z2) is a zone susceptible to distributed dimensional variation. Method according to claim 1 or 2, characterized in that in at least part of the at least one second zone (Z2) the length of fiber installed is substantially equal to 1m per meter of longitudinal extent. Method according to one of Claims 1 to 3, characterized in that in at least one first zone the optical fiber is arranged in several round trips parallel to an expected direction of local dimensional variation. Method according to one of claims 1 to 4, characterized in that in the at least one first zone the optical fiber is arranged in a spiral. Method according to one of claims 1 to 5, characterized in that in at least one first zone the optical fiber is arranged on a support (29) comprising, for positioning the optical fiber, a channel configured in turns, two turns of the channel communicating with each other to allow one of the two aforementioned turns of the channel to receive at least two turns of optical fiber. Method according to one of claims 1 to 6, characterized in that in the at least one first zone (Z1), the optical fiber (1) is arranged along a plane, in particular a plane parallel to a surface of the structure against which the sensitive fiber is arranged. Method according to one of claims 1 to 4, characterized in that in the at least one first zone the optical fiber is wound in the form of a spool. Method according to one of claims 1 to 8, characterized in that in at least one first zone the optical fiber is arranged on a support (29, 106) comprising means (31, 108, 109) for positioning the fiber according to a predetermined geometry, the support being in mechanical connection with the structure and sufficiently deformable to transmit to the optical fiber the dimensional variations of the structure according to the direction of dimensional variation when it is in mechanical connection with the structure. Method according to claim 9, characterized in that the optical fiber is embedded in a mass constituting the structure and in that to put the optical fiber in mechanical connection with the structure in the first zone the support comprises anchoring means (101, 102, 109, 111) which link the support to the constituent mass at least for dimensional variations parallel to the direction of local sensitivity of the optical fiber (1). Method according to one of claims 1 to 10, characterized in that the at least one first zone comprises a multidirectional zone where the fiber is arranged in at least two detection directions, forming an angle between them. Method according to one of claims 1 to 11, characterized in that the dimension of the first zone measured parallel to the direction of dimensional variation to which the optical fiber is sensitive in this zone is less than or substantially equal to 50cm. Method according to one of claims 1 to 12, characterized in that the output signal is composed of backscatter generated by interaction of the incoming signal with the material of the optical fiber, in that the backscatter sites are identified according to a time domain of capture of each backscatter, and in that a deformation is assigned to a backscatter site according to a variation of at least one physical parameter of the output signal in the time domain of capture associated with this site of capture backscatter (“OTDR” reflectometry). Method according to claim 13, characterized in that the developed length of optical fiber arranged in the at least one first zone (Z1) is at least substantially equal to a spatial resolution of the optical reflectometry processing. Method according to one of claims 1 to 14, characterized in that the structure is an overhead wire line (91) comprising cables (93) supported by line supports such as poles or pylons (92), in that the 'at least a first zone (Z1) is adjacent to a line support, in that the at least one second zone (Z2) extends along a path on the line supports, and in that the optical fiber comprises connection zones (Z3) between the line supports (92). Method according to one of claims 1 to 14, characterized in that the structure is a bridge comprising at least one deck (41) supported by piles (42), in that the at least one second zone (Z2) is 'extends along the apron (41) between the piles, and in that the at least one first zone (Z1) is adjacent to at least one pile. Method according to one of claims 1 to 14, characterized in that the structure is a structure (81) exposed to vandalism, in that the at least one first zone (Z1) corresponds to at least one element (84, 86 , 87) exposed to vandalism, and in that the at least one second zone (Z2) corresponds to at least one structural element of the structure. Method according to one of claims 1 to 14, characterized in that the structure is a nuclear reactor (61). Method according to one of claims 1 to 14, characterized in that the structure is a pipeline (71) such as a pipeline, a gas pipeline, a water pipe or the like, and in that the at least one first zone (Z1) includes all or part of inspection manholes (73), connections (72), geological sites at risk, sites sensitive to vandalism. Method according to one of claims 1 to 14, characterized in that the structure is a wind turbine (112), in that the sensitive optical fiber is arranged on the mast (113) of the wind turbine, and in that a failure of a rotating part (114, 116) of the wind turbine is detected when an alteration of a vibration mode of the mast is detected. Method according to one of claims 1 to 20, characterized in that the at least one first zone comprises a first zone adjacent to a site of fragility of the structure. System for implementing the method according to one of claims 1 to 21, comprising:
– at least one sensitive single-mode optical fiber (1) which in service is mechanically linked to a structure to be monitored;
– a high coherence laser (7);
– means (AOM) for injecting coherent optical pulses into the optical detection fiber from the laser;
– a processing circuit for capturing and heterodyning a backscatter signal provided at least indirectly by the sensitive optical fiber in response to each pulse;
– means for detecting the phase of the heterodyne signal;
characterized in that the sensitive optical fiber (1) is secured to the structure over a detection range in first zones (Z1) and in second zones (Z2), the developed fiber length being greater in the first zones ( Z1) only in the second zones (Z2) with respect to a respective dimension of each zone in a direction of dimensional variation to which the optical fiber is sensitive. System according to claim 22, characterized in that the developed length of optical fiber arranged in each first zone (Z1) is at least substantially equal to a spatial resolution of the optical reflectometry processing. System according to claim 22 or 23, characterized in that in the at least one first zone (Z1) the sensitive optical fiber (1) is arranged in accordance with one of claims 4 to 12. Sensor to be used during the implementation of the method according to one of claims 1 to 21 or to form part of the system according to one of claims 22 to 24, characterized in that it comprises a support (29, 106) on which an optical fiber (1) is arranged, the support comprising positioning means (31, 108, 109) defining for the optical fiber a geometry having a direction of elongation according to which the optical fiber makes several trips back and forth, the support and the optical fiber being joined so that a dimensional variation of the support according to the direction of elongation is transmitted to the optical fiber with a multiplying coefficient corresponding substantially to the sum of the number of outward journeys and the number of returns, the support comprising securing means (37, 101, 102, 111) to be secured to a structure to be monitored at least with regard to dimensional variations according to the direction of elongation. Sensor according to claim 25, characterized in that the support (29) is integrated into a box (32) which encloses the optical fiber (1) in a first zone (Z1). Sensor according to claim 26, characterized in that the interior of the box contains a filling mass (36), in which the sensitive optical fiber (1) is embedded. Sensor according to one of claims 25 to 27, characterized in that the securing means comprise anchoring conformations (101, 102, 111) for anchoring the support in a constituent mass of the structure. Sensor according to one of claims 25 to 28, characterized in that the positioning means (36, 108, 109) define an oblong spiral geometry for the optical fiber (1). Sensor according to one of claims 25 to 28, characterized in that the positioning means comprise an oblong core (108) defining for the optical fiber a coil geometry flattened in the direction of elongation. Sensor according to one of claims 25 to 30, characterized in that the sensitive optical fiber is mounted in a sheath without the possibility of sliding. Sensor according to one of claims 25 to 31, characterized in that it comprises an optical connector (105) at each end of the section of the optical fiber belonging to the sensor, to allow the insertion of the sensor into an optical circuit.