BACKGROUND

The present invention pertains to a sensor module for load monitoring.

The present invention further pertains to an optic sensor system

comprising such a sensor module.

The present invention still further pertains to an application wherein the optic sensor system is used to monitor loads exerted on an artifact.

Use of fiber optic sensors is known for purpose of load monitoring in artifacts, such as infrastructures are known as such. For example EP2372322 notes that a reduction in the amount of cabling can be achieved by employing optic sensors for example, designed as Fiber Bragg Gratings disposed in a single optic fiber. The optic sensors are, for example, designed as Fibre Bragg Gratings. The cited document proposes to add to the support one or more temperature sensors for measuring temperature, and to use the temperature measurements to increase the accuracy of measurement results. It is a disadvantage that separate temperature sensor are required, which complicates the design.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a sensor module for load monitoring in an artifact that enables a compensation for temperatures while obviating separate sensors for measuring the temperature. In order to meet this object, according to a first aspect of the invention, a sensor module is provided as claimed in claim 1. The sensor module as claimed therein comprises an assembly of a carrier of an elastic material and at least a first and a second longitudinal optic fiber section with a first and a second fiber optic sensor element, and being mounted to the carrier in a first and a second mutually different directions at a respective pair of mutually opposite ends, the sensor module being configured to receive a load on a first load receiving side and to be restrained on at least a second side and the carrier being deformable by the received load in said a first and a second mutually different directions, resulting in a mutually different deformation of said at least a first and a second longitudinal optic fiber section so as to enable a computation of a temperature compensated magnitude of said load action based on respective corresponding optic response signals from said a first and a second fiber optic sensor element.

When a mechanical load is imposed on a surface of an artifact wherein the sensor module is embedded, a deformation of the carrier occurs or is changed.

This deformation is measurable by the fiber optic sensor elements as an optic response signal indicative for a longitudinal strain in case the fiber optic sensor element is a strain sensor element and as an optic response signal indicative for a longitudinal deformation in case the fiber optic sensor element is a deformation sensor element. In the sequel, it will be presumed that the fiber optic sensor element is an optical strain sensor element, the disclosure is however equally apphcable for a sensor module using optical deformation sensor elements. Hence the carrier in the assembly serves as a mechanical conversion means that converts the load into a respective longitudinal strain and/or longitudinal deformation of each of the longitudinal optic fiber sections. Moreover, the effect of the deformation of the carrier on the first and the second longitudinal optic fiber sections is different as they are arranged at mutually different angles in the carrier. Upon receiving a mechanical load, the carrier will typically tend to be compressed in a direction determined by the load and the restriction(s) and to expand in directions wherein the carrier is unrestricted. Also temperature variations will typically result in deformations of the carrier and be measurable by the fiber optic sensor elements in the optic response signals as a (change of) longitudinal strain and or longitudinal deformation. However, contrary to the case of a deformation caused by a mechanical load, deformation in the carrier due to temperature variations will be more, though not necessarily fully, uniform deformation. Therewith, changes in strain in the first and the second

longitudinal optic fiber section due to temperature variations will be mutually related in a manner different from changes in strain in the first and the second longitudinal optic fiber section due load actions.

Therewith an estimated value of a load exerted on the sensor module which is compensated for temperature variations can be obtained from a combination of the first and second optic response signals. Alternatively or additionally, a load compensated estimation of the temperature at the location of the sensor module can be obtained from another a combination of the first and second optic response signals.

Furthermore, the differential approach allows for the

compensation/balancing of any longer time scale creep and changes in the sensor structure and materials to ensure long term stability of the sensor calibrations.

Preferably one of the first and the second mutually different directions is selected as the direction wherein a load action causes a maximum compression of the carrier and the other one of the first and the second mutually different directions is selected as the direction wherein a load action causes a maximum expansion of the carrier. Typically these directions are substantially orthogonal with respect to each other. Therewith an estimated value for an exerted load may be obtained for example as a difference of the strain values indicated by the optic response signals and an estimated value for a temperature may be obtained for example as a sum of the strain values indicated by the optic response signals.

However also a smaller difference in angle, e.g. when the directions differ by 10 or 20 degrees may be sufficient to obtain optic response signals suitable for calculation of a temperature compensated estimation of the load and/or a load compensated estimation of the temperature. Such a small angle may for example be contemplated if priority is given to other design characteristics.

A mode of mechanical conversion of a load exerted onto the loadable surface into a (change of magnitude of the) strain can be further determined in a way in which the carrier is restrained by the artifact in which it is embedded or by other means. A load exerted on the elastic material causes the material to reduce in size in a direction of the load and to expand in directions wherein the carrier is left unrestrained. The at least a first and a second longitudinal section, optionally arranged with space inside the carrier are protected by the latter. The carrier may for example be manufactured of a rubber. An elastic modulus of the rubber can be set at a desired value for a particular application by a proper composition of components and additives. For example in an application where the expected load is relatively low, e.g. a floor in a building the elastic modulus may be relatively low as compared to an application wherein the expected load is relatively high, e.g. a road provided for heavy traffic.

It is noted that the at least a first and a second longitudinal optic fiber sections may be provided with a preset strain in the sensor module. In this way it can be avoided that a load needs to exceed a threshold before a strain occurs therein.

In an embodiment the at least a first and a second longitudinal optic fiber sections are formed by a common optic fiber. In that case optic measurement data indicative for a longitudinal strain in the at least a first and a second

longitudinal optic fiber section becomes available through that common optic fiber. Alternatively it may be contemplated to provide the at least a first and a second longitudinal optic fiber sections as sections of respective optic fibers.

In an embodiment the carrier of the sensor module comprises a central portion in which the first and the second longitudinal optic fiber section are arranged at their respective pair of mutually opposite ends and the carrier additionally comprises at least one end portion having a size in a third direction transverse to the first and the second directions that is larger than a size of the central portion in the third direction. This embodiment of the sensor module is advantageous in that it can be accommodated in various ways in the artifact to be monitored. In particular its construction makes it suitable to be

accommodated in an artifact in a manner aligned with a loadable surface of that artifact, that is to say in a plane defined by the first and second direction that is parallel to the loadable surface. Upon loading the surface of the artifact, the at least one end portion of the sensor module aligned in this manner will be compressed in said third direction. As an result the end portion will expand in directions transverse to this third direction. In particular, it will expand in a direction in the plane defined by the first and the second direction and therewith deform the central portion in which the first and the second longitudinal optic fiber section are arranged.

In a preferred embodiment of this embodiment the at least one end portion is provided at a side of the central portion near one of the mutually opposite ends of one of the respective pairs. In this embodiment a difference between the effects of a deformation of the central portion on the at least a first and on the at least a second longitudinal optic fiber section are maximized. Nevertheless, a difference measurement would still be possible in other configurations, provided that a sufficient asymmetry is present between the orientation of the at least a first and the at least a second longitudinal optic fiber section with respect to the end portion.

Optimally, the carrier further comprises a second end portion at a side of the central portion opposite the first end portion having a size of its dimension in the third direction that is larger than the size of the central portion in the third direction.

Both the sensor module comprising the thickened end portion(s) as described above and the sensor module not having one or two thickened end portions are applicable when they are arranged with their plane defined by the first and the second longitudinal optic fiber section transverse to a loadable surface of an artifact wherein they are embedded. Regardless the presence of having one or two thickened end portions, a load exerted on the loadable surface will induce mutually different deformations in the carrier in a direction

perpendicular to the loadable surface and in directions aligned with the surface.

The sensor module may be provided as a separate element, for example as one of a plurality of separate elements.

Alternatively, a plurality of sensor modules may be integrated in a mat or strip. In such an arrangement a set of sensor modules may share a common optic fiber, for example in that the first and the second longitudinal optic fiber section of each of the sensor modules in the set of sensor modules are formed by that each are part of that common optic fiber. Alternatively in such a set, a first common optic fiber may include the first longitudinal optic fiber section of each of the sensor modules and a second common optic fiber may include the second longitudinal optic fiber section of each of the sensor modules in the set.

In an embodiment a plurality of sensor modules are arranged in a grid in a plane parallel to that of a loadable surface of an application. In such an

arrangement a first set of optic fibers may be arranged at distance from each other parallel to a first main direction in said plane, and a second set of optic fibers may be arranged at distance from each other parallel to a second main direction, transverse to said first main direction in said plane. Therein the optic fibers of the first set may include the at least one longitudinal sections of the sensor modules and the optic fibers of the second set may include the at least a second longitudinal sections of the sensor modules.

In an embodiment of the sensor module the carrier comprises an outer part and an inner part arranged within a central portion of the outer part, the inner part being of a material having a relatively high modulus of elasticity in comparison to that of the outer part, and providing for mechanical support of end portions of the at least a first and a second longitudinal optic fiber section. The inner part may for example be of a polymer or a metal to provide for the relatively high modulus of elasticity. Alternatively a rubber having a relatively high modulus of elasticity may be used for this purpose. This assembly of an inner part and an outer part enables a higher overall flexibility while

maintaining a proper protection of the at least a first and a second longitudinal optic fiber section.

In an embodiment of this embodiment, the inner part provides for mechanical support of the at least a first and a second optic fiber section in that the at least a first and a second optic fiber section are included in a respective at least a first and a second sensor component that are supported by the inner part of the carrier. This is advantageous in that in a manufacturing stage the sensor module can be assembled from a carrier and a first and a second sensor

component. Preferably the only difference between the first and the second sensor component is the orientation with which they are assembled in the carrier. Therewith only a single type of sensor component needs to be

manufactured, and errors in the selection of sensor components to be assembled is avoided. The use of mutually separate, optionally identical sensor components, leaves open the option to manufacture other sensor modules that include only one such a sensor component, for example only one sensor component arranged as a first sensor component or as a second sensor component.

In an embodiment at least one of said at least a first sensor component and a second sensor component comprises a frame that is fixed within said inner part and that has a first and a second mutually opposite sides, wherein the first and second mutually opposite sides are elastically connected by means of a resilient element. The resilient element is provided to exert a longitudinal tension on the longitudinal optic fiber section of the sensor component and may be a pre tensioned resilient element so that a threshold in the response of the sensor module is avoided.

In an embodiment of the sensor module a first and a second resilient element mounting sections are provided extending in a direction from the first and second mutually opposite sides wherein at least one of the first and second resilient element mounting sections is movable with respect to the other resilient element mounting section.

Various options are available for fixing the resilient element. According to one of these options the resilient element is fixed between the first and second mutually opposite sides. According to another one of these options the resilient element is fixed between the first and second resilient element mounting sections. According to again another one of these options the resilient element is fixed between the first or second mutually opposite sides and the first or second resilient element mounting sections.

In an embodiment of the sensor module having the frame, as referred to above, comprises a ring-shaped portion integral with, and having arranged therein said first and second resilient element mounting sections.

The ring shaped portion integral with, and having arranged therein the mounting sections provides for a robust, but flexible construction. In particular the frame with the ring shaped portion can be easily mounted within the support component, while still being capable to accurately follow its deformations, therewith allowing accurate measurements to be obtained with the optic strain sensor element.

The inner part of the sensor module may be left open in order to facilitate its deformation and therewith a detection of changes of load and/or temperature of the artifact wherein it is arranged. A space left open in the inner part of the sensor module also facilitates an assembly of the carrier with the sensor components. A remaining space inside the carrier may be filled with an inert fluid (such as a gel). Therewith ingress of potentially harmful substances in the substrate of the application can be avoided.

In an embodiment the frame may form a support component being of a material having a relatively high modulus of elasticity in comparison to that of the outer part. In a preferred embodiment the frame is molded from a polymer, and is mounted within such a support component. This is advantageous in that the support component can be of a relatively simple shape.

As the support component provides for a relatively high modulus of elasticity, the frame, being arranged within the support component can be of a relatively flexible, lightweight material. In particular the frame may be formed as a molded polymer object, which can be manufactured efficiently.

According to a second aspect of the invention, an optic sensor system including the optic sensor module is provided as claimed in claim 14.

According to a third aspect of the invention an application of the optic sensor system is provided as claimed in claim 17.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects are described in more detail with reference to the following drawings. Therein:

FIG. 1, 1A schematically show an embodiment of a sensor module according to the first aspect of the invention, therein FIG. 1A shows a cross- section according to IA-IA in FIG. 1, FIG. IB illustrates some dimensions of the sensor module in said cross- section,

FIG. 2, 2 A schematically illustrate an embodiment of an optic sensor system according to the second aspect of the invention as wells as an embodiment of an application according to the third aspect of the invention, therein FIG. 2A shows a cross-section according to IIA-IIA in FIG. 2,

FIG. 3, 3A illustrate aspects pertaining to a use of a sensor module according to the first aspect, therein FIG. 3 illustrates some mechanical aspects, and FIG. 3A illustrates exemplary signals obtained from the sensor module,

FIG. 4 schematically shows an interrogator forming part of an embodiment of an optic sensor system according to the second aspect of the invention,

FIG. 5 illustrates aspects pertaining to an alternative use of a sensor module according to the first aspect,

FIG. 6A, 6B show another embodiment of a sensor module according to the first aspect of the invention, therein FIG. 6A shows the sensor module as an assembly and FIG. 6B shows an exploded view of the assembly,

FIG. 7 A, 7B shows a sensor component of the sensor module of FIG. 6A, 6B according to a first and a second perspective view,

FIG. 8A, 8B show a part of the carrier of the sensor module in a

perspective view and a top view respectively,

FIG. 9A, 9B show the sensor component of FIG. 7 A, 7B in more detail, therein FIG. 9A is a top view and FIG. 9B is a cross-section according to IXB-IXB of FIG. 9 A,

FIG. 10A, 10B show other aspects of the sensor component of FIG. 7 A, 7B in more detail, therein FIG. 10A is a top view and FIG. 10B is a cross-section according to XB-XB of FIG. 10A,

FIG. 11A, 11B, 11C show an example of an integrated set of sensor modules, therein FIG. 11A shows a pair of sensor modules in the integrated set of sensor modules, FIG. 11B shows the full integrated set of sensor modules and a further assembly element and FIG. 11C shows an example of a final product including the integrated set. DESCRIPTION OF EMBODIMENTS

FIG. 1, 1A schematically show an embodiment of a sensor module 1 according to the present invention. Therein FIG. 1A shows a cross-section according to IA-IA in FIG. 1. The sensor module 1 comprises an assembly of a carrier 20 and at least a first and a second longitudinal optic fiber section 11a, l ib. The a first and a second longitudinal optic fiber section 11a, l ib each have a proper fiber optic sensor element, indicated as first fiber optic sensor element 12a in the first longitudinal optic fiber section 11a second fiber optic sensor element 12b in the second longitudinal optic fiber section l ib. The first longitudinal optic fiber section 11a is arranged inside the carrier 20 in a first direction x and mounted at mutually opposite ends 13al, 13a2 to the carrier, for example with an adhesive or using clamping means. Similarly, but in a different direction y, the second longitudinal optic fiber section l ib is arranged inside the carrier 20 and mounted at mutually opposite ends 13b 1, 13b2 to the carrier. The fiber optic sensor elements 12a, 12b may be an optic strain-sensor element 12a, such as a fiber bragg grating (FBG) or another type of optic sensor element, e.g. an optic deformation sensor element, e.g. a Fabry-Perot type optic sensor element. In an embodiment one or more additional optic strain-sensor elements may be provided within a same longitudinal section, for example to provide redundant sensor readings, for example to determine an average value of such readings that is more reliable than individual sensor readings. Alternatively one or more additional optic strain-sensor elements may serve as a back-up in the inadvertent case that the optic strain-sensor element 12a does not function properly. The carrier 20 is deformable in a first and a second direction corresponding to the longitudinal directions of the longitudinal optic fiber sections 11a, l ib.

In the embodiment of FIG. 1, 1A, the longitudinal sections 11a and l ib are both longitudinal sections of a common optic fiber 10. In the embodiment shown the optic fiber is guided outside the carrier 20 for example to a next sensor module. Alternatively, in the absence of a next sensor module, the optic fiber may end at the second end 13b2 of the further longitudinal section lib. Alternatively, the longitudinal sections 11a and l ib may be part of mutually different optic fibers.

The sensor module 1 is configured to receive a load action (F, see FIG. 3) on a first load receiving side, for example on a side formed by end portion 211. The sensor module 1 is further configured to be restrained on at least a second side for example formed by end portion 212, differing from the first side such as to enable a deformation of the carrier 20 by the load action in the first and the second mutually different directions x, y, resulting in a mutually different deformation of the at least a first and a second longitudinal optic fiber section 11a, l ib. These mutually different deformations enable a computation of a temperature compensated magnitude of said load action based on respective corresponding optic response signals from the first and second fiber optic sensor element.

The carrier 20 comprises the end portions 211, 212 in mutually opposite directions along the at least a second longitudinal optic fiber section l ib. FIG. IB schematically shows a cross-section Cl transverse to the longitudinal section 11a according to IB-IB in FIG. 1. For comparison FIG. IB also schematically shows a cross-section C2 of the central portion 213 according to IA-IA in FIG. 1. As becomes apparent from FIG. IB, a circumference of the cross-section Cl is larger than a circumference of cross-section C2. The cross-section Cl has a

circumference of 2*(wl+dl), whereas cross-section C2 has a circumference of 2*(w2+d2). By way of example the width wl, w2 of the end-portions 211, 212 and of the central portion 213 may be 15.5 cm and 9 cm respectively. In particular the difference in circumference is provided by a difference in thickness. By way of example the thickness of the thickness dl, d2 of the end-portions 211, 212 and of the central portion 213 may be 3 and 2 cm respectively. Whereas the presence of the end portions 211 and 212 is optional they enable a different mode of use, as described below with reference to FIG. 5.

In the embodiment of the sensor module shown in FIG. 1, 1A, IB, the carrier 20 comprises an outer part 210 and an inner part 220. The latter is arranged within a central portion 213 of the outer part and is of a material that has a relatively high modulus of elasticity in comparison to that of the outer part. The inner part 220 provides for mechanical support of end portions of the longitudinal sections 11a, l ib. In this embodiment the inner part comprises a hollow cylindrical portion 221 definin an axis in a third direction z, transverse to directions x, y, a first pair of mutually opposite protrusions 222al, 222a2, and a second pair 222bl, 222b2 of mutually opposite protrusions. The first and the second pair of protrusions point into a space defined by the hollow cylindrical portion 221. Longitudinal section 11a of the optic fiber extends from a first one 13al of its mutually opposite ends through a first one 222al of the first pair of mutually opposite protrusions towards a second one 222a2 of the first pair of mutually opposite protrusions and through said second one to a second one 13a2 of its mutually opposite ends. Longitudinal section l ib extends from a first one 1 lb 1 of its mutually opposite ends through a first one 222b 1 of the second pair of mutually opposite protrusions towards a second one 222b2 of the second pair of mutually opposite protrusions and through said second one to a second one l lb2 of its mutually opposite ends. By way of example the inner part 220 may have a diameter in the range of a few cm to 10 or 20 cm, for example 8 cm, and the protrusions 222al, a2 may leave a space between them in the range of 1 mm to a few cm.

In the embodiment of FIG. 1, the space inside the carrier is filled with an inert liquid 240. The inert liquid is for example a gel, that on the one hand allows a movement of the longitudinal sections 11a, l ib within the space, but that on the other hand dampens motions for protection thereof. The gel further mitigates digression of harmful substances into the space.

By way of non-limiting example, a sensor module 1 of FIG. 1 may have di ensions as specified in FIG. IB and FIG. 3. As illustrated therein the sensor module has a total height h (FIG. 3) a maximum width wl and a maximum thickness dl. The total height h and the maximum width wl may be of the same order of magnitude, e.g. in the range of 10 to 20 cm. In this example the total height h and the maximum width wl respectively are 16 cm and 15.5 cm. The maximum thickness dl may for example be in the order of a few cm, for example 3 cm. FIG. 2, 2A shows an optic sensor system comprising an interrogator 40 and a sensor module 1 as presented in FIG. 1, 1A and IB. FIG. 2 A shows a cross- section according to IIA-IIA in FIG. 2. The optic fiber 10 of the sensor module is coupled to the interrogator 40. The interrogator 40 is configured to transmit an optical interrogation signal into the optic fiber, and to receive a response optical signal that has been modulated by the fiber optic sensor element 12a of the longitudinal optic fiber section 11a and by the further fiber optic sensor element 12b of the longitudinal optic fiber section 11a of the optic fiber 10. As further illustrated in FIG. 2, the sensor module may be part of a series of sensor modules, one of which is illustrated by way of example as 1A in FIG. 2, 2 A. Each sensor module may include a plurality of sensor elements in respective

longitudinal sections of the optic fiber 10. The various sensor elements in the optic fiber 10 may have mutually different optical characteristics, for example mutually different characteristic wavelengths. Accordingly a change in strain of a particular longitudinal optic fiber section is detectable as a change in its associated unique characteristic wavelength. Also other optical properties may be used to modify the optical interrogation signal into a response signal, for example an amount of absorption. For example the optic strain-sensor elements could have a same characteristic wavelength, but change the absorption of light in response to strain to a different extent. As shown in FIG. 2A, 2B the interrogator 40 may be coupled to additional optic fibers 10a, .. , 10n that may have similar sensor modules or other optic sensor modules. Also the interrogator 40 may receive input signals from other types of sensors, such as cameras, acceleration sensors, acoustic sensors and the like. In the embodiment shown the interrogator 40 forwards output data to a wireless transmitter 70. Alternatively a wired connection may be used for this purpose, or the interrogator 40 may provide a storage space or a display means that enables an operator to access the output data.

In the embodiment shown in FIG. 2, 2A, the optic sensor system 1 is part of an application 500 wherein the sensor module is arranged within a

constructive part of an artifact. The artifact is a civil engineering asset, in this example the civil engineering asset is a road 50 having a loadable surface 51 to support conveyance elements 92 of a vehicle 90. Reference number 52 indicates a longitudinal direction of the road corresponding to direction z. Other examples of civil engineering asset are a railway support, a bridge and a tunnel.

As schematically shown in FIG. 2, 2 A, the sensor module 1 is arranged in an orientation wherein its third direction (z) is a direction in a plane that is (at least) substantially parallel (e.g. with a deviation of at most 10 degrees ) to a plane defined by the loadable surface 51. The longitudinal section 11a is arranged in a direction that is at least substantially transverse to the loadable surface 51.

Referring again to FIG. 1, it can be seen that in this embodiment the carrier 20 of the sensor module 1 is provided with a reinforcement layer 230 at a side facing the loadable surface 51. The reinforcement layer, for example of a metal, e.g. stainless steel, may have a higher elasticity modulus than that of the first part 210 of the carrier 20 and serves to provide for a homogeneous load distribution.

FIG. 3 schematically shows how a load F imposed on the sensor module 1 causes a deformation of the carrier 20 and changes an amount of strain in the longitudinal sections 11a, l ib as schematically illustrated by the solid arrows and is detectable in detected response signals. If a load F is imposed on the sensor module 1, for example indirectly via the substrate of a road and a reinforcement layer 230, the strain in the longitudinal section 11a is reduced, due to a deformation of the carrier 20. This reduction in strain is detectable by optic strain-sensor element 12a. For example if the optic strain-sensor element 12a is an FBG element, its characteristic wavelength l _; will decrease as is

schematically illustrated in FIG. 3A. Further, due to the deformation of the carrier 20, the strain in the longitudinal section l ib is increased, which is detectable in this embodiment by an increase of the characteristic wavelength li, of optic strain sensor element 12b. Accordingly any of the optical response signals obtained from the optic strain sensor elements 12a, 12b is suitable to estimate a load F imposed on a loadable surface.

Also a change of temperature T may cause a change in strain values occurring in the longitudinal sections 11a, l ib due to a thermal expansion of the substrate of the road or of the carrier 20. These are schematically illustrated by the dashed arrows T. It has been found that an increase of temperature T tends to increase the strain in both longitudinal sections 11a, l ib, which is also detectable by the optical response signals obtained from the optic strain sensor elements 12a, 12b. As schematically illustrated by the dashed arrows in FIG. 3A, an increase in temperature T tends to result in an increase of the characteristic wavelengths Xa, b of both optic strain sensor elements 12a, 12b.

In order to estimate a load exerted on artifact, while mitigating deviations due to temperature variations the optic sensor system may include an

interrogator 40 as illustrated in FIG. 4. The interrogator 40 shown therein includes a conversion module 41 for converting response optic signals from the respective fiber optic sensor elements 12a, 12b into respective magnitude signals S t2a, Si 2b indicative for a deformation or strain in the fiber optic sensor elements 12a, 12b. The interrogator 40 further includes a first estim tion module 42 to estimate a load F imposed upon the loadable surface 51 from a difference between the first and the second magnitude as indicated by the magnitude signals S t2a, S i2t>. The first estimation module 42 issues an output signal SF, indicative for the estimated value of the load F. Therewith deviations in the estimated load F due to temperature variations are at least mitigated.

In the embodiment of FIG. 4, the interrogator 40 further includes a second estimation module 43 to estimate a temperature prevailing within the sensor module 20 from a sum of the first and the second magnitude as indicated by the magnitude signals S i2a, Sia. In a further refinement of the system, the

estimations of load and temperature can be improved by a calibration matrix that correlates the mean of the wavelengths lhi = a + lΐ and the difference of wavelengths d = Xa - b to temperature and load via multiple coefficients to ensure cross-talk between lhi to load and d to temperature is also compensated. The calibration matrix can also be, in a further refinement, nonlinear.

As also schematically shown in FIG. 2, 2A, a sensor module 1A may be provided wherein the longitudinal sections 11a, l ib are both at least

substantially arranged in a plane defined by a loadable surface of a civil engineering asset. Accordingly a third direction transverse to the directions of the longitudinal sections at least substantially (e.g. deviating by at most 10 degrees) coincides with a normal vector of the loadable surface, e.g. a surface 51 of a road 50.

As indicated above, this alternative mode of use is made possible by the presence of one or more of the end portions 211, 212 as is now further illustrated with reference to FIG. 5.

Also in this arrangement, a load imposed upon the carrier 20 causes it to deform, due to its elastic properties, which becomes datable as a change in optic response signals received for optic strain-sensor elements 12a, 12b. When a load F is imposed on the sensor module 20, the end portions 211, 212 are compressed, as illustrated by the arrows pointing inward in portions 211, 212. Therewith the portions 211, 212 expand in transverse directions x, as indicated by the arrows pointing outward from portions 211, 212. Therewith a strain in longitudinal section 11a is reduced. The portions 211, 212 also expand in transverse directions z (not shown). As a result of this expansion, the strain in longitudinal section l ib is increased. Accordingly, in the same way as demonstrated for the arrangement of FIG. 3, a temperature compensated estimation of the load F can be obtained by subtracting the sensor signals, for example with the circuitry of FIG. 4. Likewise this circuitry can be used to estimate a temperature from a summation of the signals. Although optimal results are achieved with the carrier 20 having a pair of such thickened end portions 211, 212 a single such end portion would be sufficient, in particular if additional mechanical restrictions are provided.

A shown in FIG. 5, the sensor module 1A is configured to receive a load action on a first load receiving side, formed by the top faces of the end portions 211, 212 and to be restrained on at least a second side by the bottom faces of the end portions. This enables a deformation of the carrier 20 by the load action F in the first and the second mutually different directions (x, y). If for example only the left thickened end portion 211 is present, the carrier may be mechanically restrained at its right side to enable the deformation of the carrier 20 by the load action F in the first and the second mutually different directions (x, y). FIG. 6 A shows an alternative embodiment of the sensor module 1, which is assembled from a carrier 20 and a first and a second component 250, 260 as schematically illustrated in the exploded view of FIG. 6B.

The carrier 20 comprises an outer part 210 and an inner part 220 arranged within the outer part. The inner part 220 (See FIG. 8A, 8B) is manufactured of a material having a relatively high modulus of elasticity in comparison to that of the outer part 210 and provides for mechanical support of the end portions of the at least a first and a second optic fiber section. This may for example be achieved in that the outer part 210 is provided from rubber, and the inner part is provided from a metal, e.g. stainless steel.

The inner part provides for mechanical support of the at least a first and a second optic fiber section in that the at least a first and a second optic fiber section 11a, l ib are included in a respective at least a first and a second sensor component 250, 260, that are supported by the inner part of the carrier. The first sensor component 250 with a first one of the longitudinal fiber optic sections and the second sensor component 260 with a second one of the longitudinal fiber optic sections can be readily assembled with the carrier 20. The sensor components 250, 260 can be identical apart from their orientation in the assembly.

FIG. 7 A, 7A shows one of the optic sensor components, here component 250 in more detail from two different views. As shown therein the optic sensor component 250 comprises a frame 251having a first and a second mutually opposite sides 252a, 252b. The first and second mutually opposite sides 252a,

252b are elastically connected by means of a resilient element 256. The sensor component 250 and likewise, the sensor component 260 snugly fit into the inner part 220 of the carrier 20.

An embodiment of a sensor component 250, as shown in FIG. 7 A, 7B, is now described in more detail with reference to FIG. 9A, 9B. Therein FIG. 9A shows a top-view, and FIG. 9B shows a cross-section according to IXB-IXB in FIG. 9A. As shown therein the sensor component, here to be used as the sensor component 250 including the first longitudinal optic fiber section 11a with fiber optic sensor 12a, comprises a frame 251 having a first and a second mutually opposite sides 252a, 252b. The frame 251 can be easily and reliably assembled within the inner part 220 of the carrier, as illustrated in FIG. 6A.

FIG. 9A, 9B further show that the sensor component 250 includes a sensor mounting section 253, a pre-tensioned resilient element 256 and a resilient element mounting section 257 inside the frame 251. The sensor mounting section 253 extends in a direction from the first side 252a to the second side 252b. The sensor mounting section 253 has a first mounting portion 254 facing the first side 252a to mount a first end 13a 1 of the first longitudinal optic fiber section 11a, and a second mounting portion 255 facing the second side 252b to mount the second end 13a2 of the first longitudinal optic fiber section, opposite the first end 13al. The second mounting portion 255 is movable with respect to the first mounting portion 254 and the resilient element 256 is fixed in a pre-tensioned manner between the second mounting portion 255 and the resilient element mounting section 257 at the second side 252b. Therewith the resilient element 256 provides for a mechanical coupling while exerting a pretension to first longitudinal optic fiber section 11a.

The pre-tensioned resilient element provides for a way to define a strength of a mechanical coupling between the longitudinal optic fiber section to the carrier. The mechanical coupling is determined by a stiffness of the resilient element k _s on the one hand and a joint stiffness k _t0t of the optic fiber and a stiffness determined by the movabihty of the second mounting portion 255 with respect to the first mounting portion 254 on the other hand. The mechanical coupling M defined by these parameters is

The mechanical coupling may for example be selected in a range between 0.001 and 0.1. If the mechanical coupling is substantially stronger than 0.1, e.g. more than 0.5 this involves the risk of damage to the optic fiber. If the

mechanical coupling is substantially weaker than 0.001, e.g. less than 0.0001, the sensitivity is relatively low, involving the risk of an increased noise in

measurements. In the embodiment shown the second mounting portion 255 is U-shaped, having first and second legs 2551, 2552 facing the first and the second side respectively and protruding from the sensor mounting section 253. The second mounting portion 255 has a mounting layer 2553 supported by the first and second legs. The second end 13a2 of the first longitudinal optic fiber section and one end 2561 of the resilient element 256 are fixed to the mounting layer 2553. The other end 2562 of the resilient element 256 is fixed to the resilient element mounting section 257. The U-shaped constitution of the second mounting portion 255 provides for a relatively high flexibility along a longitudinal direction defined by said first and second side, while providing for a high stability in other directions.

As shown in FIG. 10A, 10B, in the embodiment the U-shaped second mounting portion 255 is arranged within side walls 2592-2594 of a tray shaped portion 259 of the sensor mounting section 253, and the first and second legs 2551, 2552 of the U-shaped second mounting portion 255 extend from a bottom 2591 of the tray shaped portion. In this arrangement the U-shaped second mounting portion 255 has a very stable support, while requiring a modest amount of material. In particular a simple molding process, requiring only a single pair of mold parts is enabled in that the bottom of the tray shaped portion is open below said mounting layer.

FIG. 11A - 11C illustrate a possible way of integrating sensor modules 1A, IB, etc into a strip 300. FIG. 11A shows a portion of a strip with sensor modules 1A, IB, the first sensor module 1A comprising carrier 20A with sensor

components 250A, 260A and the carrier 20B with sensor components 250A, 260A. The sensor modules 1A, IB, ... are integrated in the strip, but are mechanically decoupled by flexible bridge portions 310AB, that allow the sensor modules 1A,

IB to move in a manner relatively independently with respect to each other. As shown in FIG. 11B an arbitrary number of sensor modules 1A, IB, ... , 1H may be integrated in this manner. The strip 300 may be provided with a protection plate 320 and bottom and top cover elements (FIG. 11C).

In the embodiment shown a common bottom cover element 340 is provided and a respective top cover element 330A, 330B, ..., 330H for each of the sensor modules 1A, IB, , 1H. The mutually separate top cover elements 330A, 330B, ..., 330H enable the sensor modules to individually receive a load action on their first load receiving side. The common bottom cover element 340 restrains all sensor modules 1A, IB, ... , 1H, in the array 300 on their second side such as to enable a deformation of the carrier 20 by the load action in both directions x, y. In this case the strip is configured to be arranged with the x-axis in a direction aligned with a loadable surface of an artifact (road, wall of a building etc) and with the y-direction transverse to the loadable surface. Therewith the sensor modules 1A, IB, ..., 1H in the strip 300 are arranged to receive a load action in the y-direction.

Alternatively the strip 300 may be packaged otherwise in order to enable its use when accommodated in a plane aligned with the loadable surface. For example, in that case the strip 300 may be provided with a separate front cover element (not shown) for each of the sensor modules 1A, IB, ... , 1H. In that embodiment the front cover elements are applied against the thickened end- portions 211A, 212A ... of each of the sensor modules 1A, IB, ... , 1H. In that embodiment the mutually separate front cover elements enable the sensor modules to individually receive a load action in the direction of the z-axis

(transverse to the x and the y axis) on the front surface of their thickened end portions 211A, 212A, which in that embodiment is their load receiving side. As in FIG. 11B, a common back cover plate 320 may be provided to restrain the sensor modules on the opposite side.

Upon application of a load on a cover plate, the load is transferred to the thickened end portions 211A, 212A of a sensor and causes a compression of these end portions in the z-direction. Due to this compression the end portions expand in directions x, y, transverse to the z direction, therewith also causing a deformation of the carrier 20 by the load action in both directions x, y. It is noted that in the embodiment shown in FIG. 11A-11C, with the strip arranged with its y-direction transverse to the loadable surface, the thickened portions 211A, 212A are optional. However, the presence of the thickened portions 211A, 212A enables the strip 300 to be further used in an application in a manner aligned with the loadable surface of the artifact. Therewith a same product is suitable in both cases.

In summary, the sensor module disclosed herein comprises an assembly of a carrier of an elastic material and at least a first and a second longitudinal optic fiber section with a first and a second fiber optic sensor element. The at least a first and a second longitudinal optic fiber section are mounted to the carrier in a first and a second mutually different directions at a respective pair of mutually opposite ends. The sensor module is configured to receive a load on a first load receiving side and to be restrained on at least a second side. When the carrier is deformed by a received load, mutually different deformations occur in the first and the second longitudinal optic fiber section. A magnitude of said load action which is compensated for temperature can be computed from the corresponding optic response signals from the first and a second fiber optic sensor element.

As noted rubber can be suitable in certain applications as an elastic material for use in the carrier, as its elastic modulus can easily be set at a desired value for a particular application by a proper composition of components and additives. For example in an application where the expected load is relatively low, e.g. a floor in a building the elastic modulus may be relatively low. In alternative applications the expected load is relatively high, e.g. a road provided for heavy traffic. Nevertheless, various other elastic materials such as wood, polymers, and metals are applicable for this purpose while achieving the same effect. For example the carrier 20 and all the attached parts can be machined from various combinations of metals and polymers, with different thicknesses to achieve different rigidities as well as rehable operation in a wider range of temperatures. As a further alternative, combinations of elastic materials may be applied. In an exemplary embodiment, the carrier is provided with an outer part of rubber, and with an inner part of a polymer or of a metal, such as stainless steel or visa versa.

It is further noted that the sensor modules may be integrated in any other manner, for example in a mat comprising a two-dimensional array of sensor modules. Likewise the sensor modules may be integrated through mechanical decoupling elements that allow relative movements between the sensor elements and the mat may be reinforced with cover plates. Separate cover plates may be provided for each sensor element on their load receiving side.