Background

The present document relates to a load cell device, a sensor system and use of a load cell device. Such devices are generally applied to monitor infrastructures and to measure all kinds of mechanical variables. These sensors may be applied in buildings, for example to monitor motion thereof or any changes in load distribution. Other applications include the use of such devices in traffic monitoring systems, e.g. vehicle counters, speed monitors, distinguishing between different types of traffic. Yet other applications may be the use of such devices in weighing systems, such as truck weighing stations.

Although load cells are available various types providing different levels of accuracy , most of these are relatively difficult to install in large numbers, and are too expensive to provide be economically viable for a large field of applications. The abundance and increase of remote monitoring systems on the other hand asks for suitable solutions in this respect.

Summary of the invention

It is an object of the present invention to obviate the

abovementioned disadvantages and to provide an accurate load cell device and systems at relatively low cost that allow for rapid installation, where desired in large numbers.

To this end, there is provided herewith a load cell device for sensing a force or variations thereof, the load cell device including carrier made of an elastic material, the carrier comprising: at least one load receiving side for receiving a load action in a first direction, the load action being induced by the force exerted on the load cell; and at least one restriction side for restricting the carrier for counterbalancing the load action received via the load receiving side, wherein the at least one restriction side is located on a different side of the carrier with respect to load receiving side, such as to enable elastic deformation of the carrier dependent on the load action received via the load receiving side; wherein the load cell device further comprises at least one optical fiber including a fiber optic sensor, the at least one optical fiber being attached in at least two attachment locations to the carrier for detecting deformation thereof; and wherein the carrier is configured for deforming in a second direction upon receipt of a load action in the first direction, wherein the first and the second directions are mutually different directions; and wherein the optical fiber is attached to the carrier such that a strain of the optical fiber changes in response to a deformation of the carrier in the second direction.

A load cell device obtained in accordance with the above described principles provides for an accurate sensor for measuring load, while at the same time requiring a least number of parts. The elastic carrier is to be suitably shaped and modelled to provide the at least one load receiving side and restriction side, and converts and the load exerted thereon into a proportional action on the fiber optic sensor. By suitably matching the elastic properties of the elastic material, the load cell may be made either more sensitive or less sensitive dependent on the application wherein it may be used and the loads to be measured. The load cell created is therefore low cost, and can be made suitable for a large range of applications.

In addition, the elasticity of the carrier causes a deformation that changes the dimensions of the carrier in different directions. This is used by the invention measure the load by the fiber optic sensor in a different direction than wherein the load is exerted on the carrier. For example, applying a notional Cartesian coordinate system, a load exerted in the Z direction does not have to be measured in the Z direction, but due to the elastic carrier can also be measured by a fiber optic sensor in the X or Y direction. As a result, the principle may be applied to provide an

arrangement of multiple load cells in accordance with certain embodiments to be discussed further below. Such an arrangement may for example comprise a single optical fiber (or few optical fibers) that runs from load cell to load cell and which contains at least one fiber optic sensor for each load cell. It may be appreciated that, in accordance with some embodiments, the first and the second directions are mutually transverse directions. However, this is certainly not a requirement and in other embodiments the first and second direction may not be transverse.

In accordance with certain embodiments of the load cell, the optical fiber is aligned in the second direction for enabhng detection of a

deformation of the carrier in the second direction. The optical fiber may for example span between two (or more) attachment points with the fiber optic sensor located between the attachment points. A deformation of the carrier due to an exerted load causes the attachment point to change (for example, move further apart or move closer to each other) to thereby change a strain in the optical fiber that can be measure with the fiber optic sensor.

In accordance with other embodiments, the optical fiber is attached such as to be arranged around a circumference of the carrier for enabhng detection of a deformation of the carrier. In these embodiments, a load exerted on the carrier (e.g. in a longitudinal direction) compresses the carrier such that it’s diameter will increase. The optical fiber is thereby stretched, which is measurable by means of the fiber optic sensor.

In accordance with some embodiments of the invention, the elastic material of the carrier comprises a first stiffness, and the carrier further includes a deformation portion which is at least partially enclosed by the elastic material of the carrier. In these embodiments, the attachment locations for the at least one optical fiber are located on or near the deformation portion. The attachment location may for example be on or near the interface with the base elastic material of the carrier. This enables detection of a deformation of the deformation portion, for example upon a load being exerted on the load receiving side of the carrier. In some of these embodiments the deformation portion comprises a second stiffness which is smaller than the first stiffness. The deformation portion may thus easily deform when the elastic material of the carrier deforms. The second stiffness may also be proportionated to the first stiffness such as to achieve a certain sensitivity of the load cell with respect to the magnitude of the loads expected for a certain application. In some other of these

embodiments, the deformation portion may include a cut-out portion. The deformation portion may for example be completely formed of a cut-out portion (i.e. empty space) or a part of the deformation portion may be cut out. Other versions of this embodiment may for example include multiple cut-outs or an arrangement of cut-outs in a specific pattern such as to yield specific properties with respect to deforming or to reduce or tune the second stiffness relative to the first stiffness.

In accordance with some of these embodiments, the deformation portion is located on the carrier interposed between at least one of said at least one load receiving side, and one or more of said at least one restriction side or a further one of said at least one load receiving side, for enabhng deformation of the deformation portion dependent on the load action.

Some embodiments of the invention further include one or more load receptors for receiving the force exerted on the load cell and providing the load action to the at least one load receiving side. The load receptors, for example, may include dedicated or particularly shaped parts of the carrier, e.g. a projecting portion or elevation with respect to a side plane. The load receptors receive the load and convey it onto the at least one load receiving side of the carrier. In some of these embodiments, the load cell device comprises a base element including the one or more load receptors and the carrier as integrally formed parts, the one or more load receptors being formed by one or more load receptor sections of the base element and the carrier being formed by a carrier section of the base element, wherein the at least one load receiving side of the carrier is formed by a notional interface between each of the one or more load receptor sections and the carrier section of the integrally formed base element. Due to the fact that in these embodiments, the load receptors are integrally formed with the carrier to form the base element, the load receiving side of the carrier is only notionally present between the integrally formed elements. This, of course, is a matter of definition.

In accordance with some embodiments, the one or more load receptors include a first and second load receptor formed by a first and a second load receptor section of the base element, wherein the carrier section is interposed between the first and the second load receptor section, wherein the load receptor sections are parallel to the second direction, and wherein the carrier section in the second direction has a dimension smaller than the load receptor sections. Loads are exerted from both sides onto the carrier, to thereby exaggerate the deformation and increase the sensitivity. This is particularly advantageous for the invention due to the fact that the fiber optic sensor is aligned in a different direction with respect to the direction wherein the load is exerted. Deformation in the measurement direction is therefore a secondary effect of the deformation in the direction of the load, e.g. compressing the carrier in one direction causes, as a secondary effect, expansion of the carrier in a transverse direction. This secondary effect, dependent on the direction, may be smaller than the primary effect of the compression. Hence, by exerting the load on two load receptors, the effects on the deformation in the transverse direction may be doubled.

In accordance with some embodiments, the carrier comprises a plurality of restriction sides and at least one expansion side, the restriction sides being different sides of the carrier than the at least one expansion side, such as to enable an outward bending of the at least one expansion side due to the elastic deformation. A possible deformation in the direction of the exerted load may be restricted by a restriction side. Likewise, additional restrictions sides, not necessarily in the direction wherein the load is exerted, may further restrict deformation. As a result, deformation will be concentrated in the at least one expansion side. This may be used to improve sensitivity in the measurement direction, i.e. the direction wherein the optical fiber is aligned.

In accordance with some embodiments, the elastic material of the carrier comprises at least one of a group comprising: a rubber material, such as a natural rubber, a synthetic rubber, nitrile rubber, silicone rubber, latex rubber, urethane rubber, polether rubber, chloroprene rubber, ethylene vinyl acetate rubber; or an elastomer. As will be appreciated, the invention is not limited to these materials, and other materials may be applied that provide sufficient elasticity to the carrier to enable load detection for the application desired. In fact, elasticity may also be improved using additional modifications to a material, e.g. by adding holes or cut-outs as described above.

As referred to above, in accordance with some embodiments of the invention, the carrier further comprises a plurality of cut-out portions being at least partially enclosed by the elastic material for increasing

deformability of the carrier. A single fiber including multiple fiber optic sensors may be applied to run through the cut-out portions in a certain direction. In each cut-out portion, the fibers are attached in two attachment points having a fiber optic sensor arranged in between these attachments points for measuring deformation of the respective cut-out portion. In accordance with some specific embodiments, the load cell device comprises a plurality of optical fibers, each of said optical fibers including a fiber optic sensor, wherein each of the optical fibers is attached in at least two attachment locations to a respective one of the plurality of cut-out portions for detecting deformation of the carrier. The above described embodiments provide for arrangements of load cells, e.g. a load cell array, implemented as a single element. Whether a single fiber is ran through all cut-out portions, or each of a plurality of optical fibers is associated with a single cut-out ί portion may be determined by the skilled person. Also, multiple fibers may be associated with a single cut-out portion. This not only applies to the present embodiments including multiple cut-out portions, but also to the other embodiments including a single cut-out or deformation portion.

Additional fibers may reduce the probability of incorrect readings, and improve the accuracy of the result. The various fibers may be aligned in a same direction (improving reliability of a reading in that direction), or may be spanned in different directions (enabling detection of deformation in multiple directions, to thereby improve reliability and allow to distinguish between deformations having a different cause).

In some embodiments including multiple cut-out portions, the plurality of cut-out portions are arranged side-by-side in the second direction. In some embodiments, the at least one optical fiber or one or more of the plurality of optical fibers comprises a plurality of fiber optic sensors, wherein each of the fiber optic sensors is located between two attachments locations on a respective one of the side-by-side arranged cut-out portions to which the or each optical fiber is attached. Yet in other embodiments, the load cell device comprises a plurality of carriers arranged side-by-side, wherein the at least one optical fiber comprises a plurality of fiber optic sensors, wherein each of the fiber optic sensors is located between two attachments locations on a respective one of the side-by-side arranged carriers to which the or each optical fiber is attached. These all provide different embodiments of load cell arrangements in accordance with some embodiments of the invention.

In accordance with a particular class of embodiments of the present invention, the load cell device comprises a first and a second carrier, the first and second carrier being made of an elastic material, and wherein the first and the second carrier are interposed between a load receiving plate and a base plate, wherein the base plate is contiguous to at least one of the restriction sides of the first and second carrier, and wherein the load receiving plate is contiguous to at least one of the load receiving sides of the first and second carrier, wherein the load cell device further comprises at least a first and a second optical fiber including a fiber optic sensor, the first optical fiber being attached to and arranged around a circumference of the first carrier, and the second optical fiber being attached to and arranged around a circumference of the second carrier. In this class of embodiments, at least two carriers with optical fibers including fiber optic sensors are placed between a base plate and a load receiving plate. A load exerted on the load receiving plate causes the at least two carriers to compress in the longitudinal direction, and expand in the radial direction (outwards). Fiber optic sensors arranged around each carrier are thereby stretched, enabling the detection of the load and enabling to measure the magnitude of the load. By comparing the readings of the different carriers, it is possible to determine the location on the load receiving plate where the load was exerted.

In accordance with a second aspect, there is provided a sensor system for sensing a force or variations thereof, the sensor system including one or more load cell devices in accordance with any of the embodiments of the first aspect.

The present invention, in accordance with a third aspect, is further directed at a use of a load cell device according to any of the embodiments of the first aspect, wherein at least one of: the load cell device is embedded in a road underneath a road surface or in a rail for sensing traffic movements on the road or rail such as to identify one or more vehicles on the road or rail; or the load cell is installed on or embedded in a building structure such as to monitor force variations within the building structure. The invention may of course be applied in many different other situations, without departing from the claimed scope. Brief description of the drawings

The invention will further be elucidated by description of some specific embodiments thereof, making reference to the attached drawings. The detailed description provides examples of possible implementations of the invention, but is not to be regarded as describing the only embodiments falhng under the scope. The scope of the invention is defined in the claims, and the description is to be regarded as illustrative without being restrictive on the invention. In the drawings:

Figures la and lb schematically illustrates operation of an embodiment of the present invention;

Figures 2a, 2b and 2c schematically illustrates a top view, side view and cross section of an embodiment of the present invention

respectively;

Figure 3 schematically illustrates operation of an embodiment of the present invention;

Figure 4 illustrates a load cell array in accordance with an embodiment of the present invention;

Figure 5 schematically illustrates an application of a load cell array of the invention for traffic monitoring;

Figure 6 illustrates operation of the array of figure 5 embedded in a road;

Figure 7 illustrates operation of the load cell array of figures 5 and 6 embedded in a road;

Figure 8 is an exploded view of a further embodiment of the invention;

Figures 9 and 10 illustrate details of the embodiment of figure 8;

Figure 11 illustrates the embodiment of figure 8;

Figure 12 illustrates the application of a load cell array in accordance with the present invention for monitoring building integrity; Figure 13 illustrates a load cell array in accordance with an embodiment of the present invention;

Figure 14 illustrates a load cell array in accordance with an embodiment of the present invention;

Figure 15 schematically illustrates an application of a load cell array of figure 13 for traffic monitoring;

Figure 16 schematically illustrates a load cell sensor system embodied as a system mounted on a road surface;

Figures 17a/b, 18a/b and 19a/b illustrate various embodiments of carriers of a load cell device as for example depicted in figures 8-11.

Detailed description

Figures la and lb illustrate the operation of a load cell device in accordance with an embodiment of the present invention. The load cell device 1 illustrated in figures la and lb resembles a very basic version of the load cell device of the invention. The load device 1 comprises a carrier 2 which is made of an elastic material, such as suitable rubber or other material. Therefore the carrier 2 will deform in case an external force is exerted thereon. The carrier 2 comprises a load receiving side 4 onto which, in operation, the load will be exerted. Opposite the load receiving side 4, the carrier 2 comprises a restriction side 5. The restriction side 5 is intended to be restricted against a fixed object or underground such as to allow counterbalancing of the load exerted on the load receiving side 4. For example, the load receiving side 5 may be placed on the ground 3 as illustrated in figure la. In the embodiment of the figures la and lb, the restriction side 5 is on the opposite side with respect to the load receiving side 4. However this is not a requirement for the invention. The restriction side 5 may be located on one or more different sides of the carrier 2, as long as the load exerted on the load receiving side 4 is counterbalanced by either a reaction force or by friction. An optical fiber 7 is wound around the carrier 2, and attached or fixed to the carrier 2 in at least two location thereof. In between the two attachment locations, somewhere along the optical fiber 7, a fiber optic sensor 8 is present in the optical fiber 7. The fiber optic sensor 8 may for example include a fiber Bragg grating. A fiber Bragg grating (FBG) 8 is a periodic modification of the structure (e.g. density) of the fiber 7. Due to this periodic structure, light with a particular wavelength that matches the periodicity of the structure of the FBG 8 will be reflected back towards the input, while the remaining light in the fiber 7 is passed along towards the output 19 of the fiber. It may be appreciated though that different types of fiber optic sensors may be apphed instead or in addition, e.g. types of fiber optic sensors that allow to measure strain in the fiber 7 in any kind of manner.

The optical fiber 7 includes an optical source 15 at the input thereof. The optical source 15 lights the optical fiber 7 with a multi wavelength optical signal. Various different types of optical sources may be applied for this, and the invention is not limited to a particular type of optical source. At the other end of the optical fiber 7 an output 19 receives the optical signal as an optical output signal. In figure la, the optical source 15 provides an optical signal I _m. Because the fiber Bragg grating 8 reflects the wavelength li , at the output 19 of the optical fiber 7 the optical signal (I _m - li) no longer includes the wavelength li. This is illustrated in figure la.

In the embodiment illustrated in figure la, an fiber coupler 18 conveys the returning optical signal in the optical fiber 7 towards a second output 20. In the manner implemented in figure la, the fiber coupler 18 only passes the return signal from the optical fiber 7 towards the output 20.

Therefore, the optical signal received at the output 20 includes only the reflected wavelength li. The apphcation of a fiber coupler 18 in this example is not a requirement for the invention. Other optical elements may be applied that allow to lead the optical return signal to an optical analyzer, such as an optical circulator or an optical splitter. An optical circulator is an element forming an optical loop having in/outputs, wherein an optical signal incoming at an input leaves the loop again via the next downstream output. An arbitrary number of in/outputs may be provided. Further to the above, even the presence of the output 20 near the optical source 15 is not a requirement, because in accordance with the invention the output signal may also be obtained from output 19 providing the optical signal (I _m - li). Alternatively, even, also the output 19 may be absent and only the return signal could be analyzed from output 20 or a different kind of

implementation thereof. Many alternatives for implementation of the invention may become apparent to the skilled person, without departing from the scope as defined in the claims.

Turning to figure lb, a force 10 is exerted on the load receiving side 4 of the carrier 2. The carrier 2 is restricted on the restriction side 5 which is placed on the ground 3. Therefore, the carrier 2 will be compressed under the load 10 exerted thereon. This is illustrated in figure lb. As can be seen, the load 10 shortens the carrier 2 in the longitudinal direction, whereas in the radial direction the carrier 2 will expand as illustrated by the arrows 11.

As mentioned hereinabove, the optical fiber 7 is attached to the carrier 2 in at least two locations thereof. The fiber Bragg grating 8 is arranged inside the optical fiber 7 in between the two attachment locations. Due to the radial expanding of the carrier 2, the fiber 7 will be stretched. This results in a corresponding stretching of the fiber Bragg grating 8.

Therefore, the periodicity of the fiber Bragg grating 8 will change due to the stretching. Stretching of the fiber Bragg grating 8 will cause the reflected wavelength to become longer. As a result, the wavelength li that is reflected by the fiber Bragg grating 8 in figure lb will be longer than the wavelength l, that is reflected by the fiber Bragg grating 8 in the situation of figure la. This can be measured using an optical analysis system (not shown) that monitors the output signals from output 19 and 20. The difference in wavelength directly relates to the radio expansion of the carrier 2.

Therefore, from the difference in wavelength (Dl = l2 - li), the magnitude of the applied force 10 maybe determined. This principle can be used in many different ways, for example for traffic monitoring or for the monitoring for building integrity. For example, the outputs 19 and 20 of the load cell device 1 may be provided to an optical analyzer, e.g. a spectral analyzer or similar system, to monitor the reflected wavelength and any shifts therein over time.

Figures 2a-2c illustrates a load cell device 17 in accordance with a further embodiment of the present invention. The load cell device 17 comprises a base element 21 which includes two load receptors 24 and the carrier 2 as integrally formed parts. The base element 21 is therefore made of the elastic material that also forms the carrier 2. In figure 2b, the side view of the load cell device 19 shows that the carrier 2 is formed by a slightly thinner part of the base element 21. Each of the load receptors 24 includes a restriction side 5 that will in use be restricted by a fixed object or surface to hold it in place and provide a reaction force that counters an exerted load.

In figure 2a, it can be seen that an optical fiber 7 passes through the carrier 2 towards the other side of the carrier. The carrier 2 consists of a base carrier portion 25 and a deformation portion 26. The deformation portion 26 in the embodiment of figure 2a is made of a different elastic material. The elastic material of the deformation portion 26 is characterized by a second stiffness which is smaller than a first stiffness of the elastic material of the base portion 25 of the carrier 2. Alternatively, as will be described herein below, the deformation portion 26 may include a cut-out portion 30 where part or whole of the elastic material of the deformation portion 26 is absent. Figure 2c illustrates a cross section of the load cell device 17 across the line A-A. The cross section illustrates the load receptors 24, the base portion 25 of the carrier 2 and the deformation portion 26 including the fiber Bragg grating 8. The carrier 2 includes two load receiving sides 4 between the load receptors 24 and the base portion 25. However, due to the fact that the carrier 2 is integrally formed with the load receptors 24, the load receiving sides 4 of the carrier are resembled by the notional interfaces 4 (dotted lines) between base element 25 of the carrier and the load receptors 24. This is illustrated in figure 2c.

Figure 3 illustrates an embodiment of a load cell in accordance with the present invention which is more or less similar to the load cell illustrated in figures 2a to 2c. A difference between the load cell 17 of figures 2a through 2c and the load cell 27 of figure 3, is that the load cell 27 includes a cut-out portion 30 forming the deformation portion. Figure 3 illustrates a top view of the load cell 27. A force F may in use be exerted on the load receptors 24. The schematics of figure 3 illustrate that the force is directed perpendicular to the paper in the downward Z-direction onto the load receptor 24. Directions x, y and z are indicated by coordinate system 35 in figure 3.

Like figures 2a through 2c, the carrier 2 of the load cell devices 27 is slightly thinner than the load receptors 24. Exerting force F on to the load 24 causes each of the load receptors to deform. Like the embodiment of figures 2a to 2c, the load receptors 24 comprise a restriction side 5 (not visible in figure 3) on the back side thereof. In addition, further restriction sides 6 may be located on the sides of the load receptor 24 as illustrated in figure 3. Exerting force F on the load receptors 24, causes a deformation directed in the direction 31 illustrated in figure 3. By restricting

deformation at the restriction sides 5 and 6, such load induced deformation may only take place in unrestricted portions of the cell and may therefor intensify in these unrestricted portions. The cut-out portion 30 will be compressed due to the deformation and will become oval shaped. As a result, the carrier 2 will further deform in the directions indicated by arrows 32 in the side ways direction. This will cause the fiber 7 including the fiber Bragg grating 8 to be stretched as illustrated by arrows 33. As explained in relation to figures la and lb, this enables to measure the magnitude of the load F exerted on the load receptors 24. Figure 3 further illustrates the diameter D of the cut-out portion 30 when no force is exerted. The

dimensions hi and h2 and D are only exemplary and are not to be

considered as limiting on the invention.

Figure 4 illustrates a load cell array 38 in accordance with a further embodiment of the present invention. The load cell array 38 comprises two load receptors 24 similar to the embodiment illustrated in figure 3 and in figures 2a-2c. In between the load cell receptors 24, a plurality of carriers 40-1, 40-2, 40-3, 40-4 and 40-5 are integrally formed with the load receptors 24. Together, the load receptors 24 and the carriers 40-1 through 40-5 form the base element 21 of the load cell array 38. A single fiber 7 spans in the longitudinal direction of the array 38 through each of the carriers 40-1 through 40-5. Each carrier comprises a cut-out portion 43-1, 43-2, 43-3, 43-4 and 43-5 respectively. The optical fiber 7 comprises a plurality of fiber optic sensors 48-1, 48-2, 48-3, 48-4 and 48-5. Each of the fiber optic sensors 48-1 has a slightly different periodicity, such that each fiber optic sensor 48-1 through 48-5 reflects a different

wavelength back to the optical source. Moreover, the fiber 7 is attached to each cut-out portion 43-1, 43-2, 43-3, 43-4 and 43-5 in at least two respective sides of each fiber optic sensors 48-1, 48-2, 48-3, 48-4 and 48-5, such as to allow the fiber optic sensors 48-1 to 48-5 to measure deformations taking place in each of the carriers 40-1 to 40-5.

Suppose a load would be exerted anywhere on the load receptor 24, deformation of the carrier that is located closest to the exertion point of the load will be the greatest. As a result, one or more of the carriers 40-1 through 40-5 will deform with different magnitudes. Analysis of the wavelengths received at the outputs of the optical fiber 7 will allow to the determine where and when the load has been exerted on the load receptor 24, and what it’s magnitude was. This information may for example be applied to distinguish between different loads exerted, e.g. to evaluate their nature or, in the example of traffic monitoring, to determine a type of vehicle passing over the array 38.

Figure 5 schematically illustrates an application of a load cell array 38 used for traffic monitoring. Figure 5 schematically illustrates a part of a road 45 including a first road half 45-1 and second road half 45-2. Underneath each of the road halfs, as schematically illustrated in figure 5, a load cell array 38 has been installed across the full span of the road half.

The load cell array 38 comprises a plurality of carriers 40 and an optical fiber 7 (not visible) including fiber optic sensors 8 (not visible) spanned across the length of it through the carriers 40 (e.g. in the manner illustrated in figure 4). On the road 45, various vehicles such as cars 50, 55 and bicycle 60 drive on various road sections. Taking as an example car 50, the weight of the car 50 is exerted onto the pavement of the road 45 via the wheels 51. The car has a velocity 53 in the leftward direction. The weight of bicycle 60 is exerted via the wheels 61 onto the pavement 45. The velocity of the bicycle 60 is schematically indicated by arrow 63. As may be appreciated, the velocity 63 of the bicycle 60 is smaller than the velocity of 53 of the car 50. On the other road half 45-2, car 55 moves in the rightward direction with velocity 56. Its weight is exerted via wheels 57 onto the pavement of road 45. While driving, car 55 will drive over load cell array 38 installed in the road.

Upon passing the load cell array 38 on the first road half 45-1, the carriers 40 of load cell array 38 which are closest to the contact locations of the wheels 51 of car 50 will deform the most. Therefore, by analyzing the return signal coming from the load cell array 38 on the first road half 45-1, both the weight of car 55 and the location on the road 45 can be determined. More in particular, even the section of the first road half 45-1 where the car 50 drives can be determined with accuracy. In principle, in case the distance between the front wheels and back wheels 51 of the car 50 would be known, also the velocity of the car would be determinable. However, because this distance is different for each vehicle, without the use of any additional means for identification of the location of the wheels the signal coming from the load cell array 38 is in principle not suitable for determining the velocity of car 50 with sufficient accuracy. This of course could be different in case the model or make of the car could be determined or in case for example a camera would be additionally installed to determine the location of the wheels.

In case bicycle 60 drives over the load cell array 38, the passing of the bicycle will be detected by the load cell array 38. Likewise, the

particular section of the first road half 45-1 where the bicycle 60 drives, can also be determined. This is sufficient information for providing traffic statistics on road usage.

In a different application, load cell array 38 may even be used to monitor the weight a passing truck. For example, in case a heavy weight truck (too heavy for the road 45) passes the load cell array 38, its weight may immediately be determined upon passing, and if it exceeds a threshold an installed camera may identify the license plate of the vehicle.

Alternatively , the signal of load cell array may also be provided to a traffic fight or even an operable barrier that would allow to stop the truck that is too heavy. It may be appreciated, a combination of these applications is possible in a system of one or more load cell arrays including a monitoring system as described. For determining the total weight, the exerted load per wheel must be determined and summed, such as for a large truck with multiple axes (e.g. a large truck with 40 wheels distributed over 10 axes). Figures 6 and 7 schematically illustrate how the passing vehicle 50 will be detected by the load cell array 38. Figure 5 illustrates the first road half 45-1 having embedded therein the load cell 38. Upon passing of the wheel 51 of vehicle 50 a load 66 will be exerted via the load receptors 24 onto the load cell array 38. It may be appreciated, although the arrows 66 are illustrated in figure 6 as being of equal magnitude, the load will be the highest directly underneath the contact point of the wheel 5 Ion the pavement 45-1. This load will be distributed among the load receptors 24. The load 66 on the receptors 24 will cause the carrier 40 to deform as is illustrated by arrows 67. Carrier 40 comprises cut-out portion 43.

In figure 7, the load cell array 38 is illustrated in cross section through the carriers 40. Each carrier 40 comprises a cut-out portion 43. In figure 7, the carrier portions 40 having a cut-out portion 43 in between are illustrated. Each carrier portion 43 comprises a fiber optic sensor 48. The optical fiber 7 stands along the longitudinal direction of the array 38.

The situation in figure 7 is the same situation as illustrated in figure 6. The wheel 51 exerts a load onto the pavement 45-1, which causes a deformation of carrier 40. This deformation is schematically illustrated by the arrows 68. Due to the sides of the carrier 40 being pushed aside, the fiber optic sensor 48 is stretched resulting in a change of the reflected wavelength as explained herein above. This is proportional to the exerted load by the wheel 51. Also , the deformation 68 will primarily take place underneath the impact location of the wheel 51. Adjacent carriers 40 may also slightly deform, but the deformation will be different and maybe discriminated from the deformation of carrier 40 underneath wheel 51. This allows to identify the section of the first road half 45-1 where the wheel 51 has passed.

Figures 13 and 14 illustrate two further embodiments of load cell arrays 38’and 38” respectively, in accordance with the present invention. The load cell array 38’ consists of a plurality of interconnected load cells 100. The load cells 100 are similar to the load cell 1 of figure 3. Each load cell 100 of the array 38’ comprises two load receptors 24’ similar to the embodiment illustrated in figure 3 and in figures 2a-2c. In between the load cell receptors 24’ of each load cell, a carrier 40’ is integrally formed with the load receptors 24’. The various load cells 100 are interconnected via a frame 102, which may be integrally formed with the load cells 100. A single fiber 7’ follows a zigzagging path through the array 38’ through each of the carriers 40’. Each carrier 40’ comprises a cut-out portion. The optical fiber 7’ comprises a plurality of fiber optic sensors 48’ located in the cut-out portions of the various load cells 100. Each of the fiber optic sensors 48’ has a slightly different periodicity, such that each fiber optic sensor reflects a different wavelength back to the optical source. Moreover, the fiber 7’ is attached to each cut-out portion in at least two respective sides of each fiber optic sensors 48’, such as to allow the fiber optic sensors 48’ to measure

deformations taking place in each of the carriers 40’. Upon receiving a load, the receptors 24’ deform the cut-out portion by pushing its walls towards each other. In the spanning direction of the fiber 7’ through each cut-out portion, the fiber optic sensors 48’are thereby stretched.

Alternatively, as shown in figure 14, the load cells 102 may be arranged side-by-side in a same manner as in figure 13, although the fiber 7” is spanned in the longitudinal direction of the array. This prevents the necessity of making turns in the fiber to follow a zigzag path as in figure 13, because the fiber 7” follows a straight line. Although this provides an advantage, the disadvantage of this embodiment is that the fiber optic sensor 48” can only measure deformation of the carrier 40” by compressing the spanned fiber 7”. This is because a load exerted on the receptors 24” deforms the cut-out portion by pushing its walls towards each other.

Therefore, pre -stretching of the fiber 7” is required for enabling

measurement. This in turn may make the array 38” a bit prone to creep, i.e. release of fiber pre-stretching over time. As illustrated in figure 15, embedded in a road the array 38’ provides the advantage that a wheel of any of the vehicles 50, 55 or 60 will load the both receptors 24’ simultaneously. Application of a load cell array 38” as illustrated in figure 14 will provide the same advantage.

Figure 8 schematically illustrates an exploded view of a further embodiment of the present invention. In the embodiment illustrated in figure 8, a load receiving plate 74 and a base plate 75 are joined in a movable manner using tenons or pens 85. The pens 85 run through ring shaped spring members 86 that are located in between the base plate 75 and the load receiving plate 74. As a result, the load receiving plate 74 and the base plate 75 are kept at a small distance relative to each other, and this distance is variable dependent on the load exerted on the load receiving plate 74. A restriction side of the load cell 70 is located underneath the base plate 75 (not shown). A detail of the connection between the load receiving plate 74 and the base plate 75 via the pens 85 is illustrated in figure 10. Here, it can be seen that the ring shaped spring member 86 is in between the load receiving plate 74 and the base plate 75, creating variable distance 71 between elements 74 and 75. The pens 85 may simply be comprised of pens having a smooth surface, or may comprise a threaded section to fasten the pens 85 to the load receiving plate 74 (while providing a smooth pen section to penetrate the base plate 75 for allowing movement of the plates relative to each other). Alternatively, the pens 85 are inserted into the holes from underneath base plate 75, and the threaded sections may fix the pens 85 to the base plate 75. In that case the pens may have a smooth outer surface that extends into the holes in the load receiving plate 74. A load exerted on the load receiving plate 74 will deform the ring shaped spring member 86 by flattening it shghtly, thereby reducing the distance 71.

Back to figure 8, a first carrier 72-1 and a second carrier 72-2 are located between the load receiving plate 74 and the base plate 75. The first carrier 72-1 comprises a restriction side 76-1 and a load receiving side 78-1. The second carrier 72-2 comprises a restriction side 76-2 and a load receiving side 78-2. Moreover, a first fiber 80-1 runs through a fiber duct 79- 1 in the base plate 75 towards the first carrier 72-1. This fiber 80-1 is spun around the carrier 72-1, in the manner illustrated in figures la and lb. The fiber 80-1 is fixed in at least two attachment locations to the carrier 72-1. In between the attachment locations of the fiber 80-1, a fiber optic sensor (not shown) is located from measuring a deformation of carrier 72-1. In the same way, optical fiber 80-2 is passed along the fiber duct 79-2 in the base plate 75, and is spun around second carrier 72-2 in the same manner. Thus, the optic fiber 80-2 is fixed to the second carrier 72-2 in at least two attachment locations having the fiber optic sensor (not shown) therebetween.

The first carrier 72-1 and the second carrier 72-2 are kept in place in between the load receiving plate 74 and base plate 75 by means of screws 83. A detail of the connection is illustrated in figure 9 for the first carrier 72- 1. The screws 83 fix the first carrier 72-1 to both the load receiving plate 74 and the base plate 75. The first carrier 72-1 (like the second carrier 72-2) comprises a H-shaped cross section as can be seen in figure 9. The screw holes for screws 83 are arranged in the longitudinal direction on the load receiving sides 78-1 and the restriction side 76-1 of the first carrier 72-1. When a load is exerted onto the load receiving plate 74, the distance 71 between the base plate 75 and load receiving plate 74 will reduce (as explained) resulting in a deformation of the carrier 72-1 in between the screws 83. This results in a radial expansion of the carrier 72-1 as

illustrated in figures la and lb. As a result, the fiber 80-1 that is spun around the first carrier 72-1 will be stretched, and likewise the reflected wavelength reflected by the fiber optic sensor (not shown) will change and can be measured. An other view of the installed load cell 70 of this

embodiment can be seen in figure 11. In yet another embodiment, the screws 83 and the configuration of the plates is arranged such that the tightening of the screws results in a pre-load on carrier 72-1 and consequently results in a pretension on the fiber attached to 72-1 during assembly of the cell to ensure that the fiber is sufficiently tensioned to operate in its designed load and temperature range. The amount of tensioning introduced during the assembly can be monitored by having the fiber system connected to an interrogator unit (not shown) and tightening the screw until desired (target) Bragg wavelength is reached. To ensure that the tension remains stable after assembly, the screws 83 can be locked into position using various methods including but not limited to bonding by glue.

The load cell embodiment illustrated in figures 8 to 11 is depicted with two carriers 72-1 and 72-2. The skilled person may appreciate that a different number of carriers may be applied. The number of carriers 72-x that may be applied is only limited by the imagination of the skilled person, and in principle any number of carriers may be applied, starting from at least one carrier. A further embodiment, which is not shown, consists of four carriers, wherein a single carrier may be located near each corner, e.g.

where the pens 85 are located in figures 8 to 11. Such pens may in that case be located at elsewhere on the load cell. Also, the fiber ducts 79-1 and 79-2 may run in the longitudinal direction extending over the full length, and providing an input/output at the short sides of the load cell. This allows easy interconnection of an arbitrary number of load cells to span a certain length.

The carriers 72-1 and 72-2 of the load cells illustrated in figures 8 to 11 are not necessarily cylindrical, but could be formed differently than as depicted in the figures. Furthermore, the fibers 80-1 and 80-2, as indicated above, may be spun around the carriers 72-1 and 72-2 one or multiple times. Alternatively, the fibers 80-1 and 80-2 may be attached in a different manner to the carriers 72-1 and 72-1 to allow stretching of the fiber Bragg gratings (or other type of fiber optic sensor) in the fibers 80-1 and 80-2 under influence of deformation of the carriers 72-1 and 72-2. To gain sufficient sensitivity to deformations, the fibers 80-1 and 80-2 may be spun around the carriers 72-1 and 72-2 under some tension. This, in turn, may lead to constriction of the carriers 72-1 and 72-2 due to tensed lacing by the fibers 80-1 and 80-2, which is normally not desired. Furthermore, during the operation of the cell, radial forces which are stretching the fiber can result in a constriction of the rubber cell due to the material hardness differences, which can result in a loss of sensitivity, deviations from linear sensitivity and limitations in maximum detectable load. To resolve this, and additional protective element may be added to prevent undesired constriction, e.g. in the form of a protective layer or sheet of protective material between the fibers 80-1 and 80-2 and their carriers 72-1 and 72-2.

Various embodiments of the carriers 72-1 and 72-2 are depicted in figures 17, 18 and 19. Figures 17a and b illustrate an embodiment of a carrier 2 (useable as carrier 72-1 or 72-2 in figures 8-11 for example) which is largely similar to the embodiment of figure 1. The carrier 2 is placed underneath load receiving plate 74 and rests with its restriction side 5 (sometimes also referred to as load receiving side 5) on base plate 75. At the input, optical source 15 provides an optical signal Im, and at output 19 the optical signal I _m is received without wavelength li that is filtered out by the FBG 8. Hence I _ou t2 = (hn - li) at output 19. At output 20, via fiber coupler 18, an output signal I _outi = li is received, i.e. the reflected fraction of the input signal Ii _n . Fiber 7 is spun multiple times around the carrier 2 with no protective element in between to prevent constriction of carrier 2 due to lacing. Figure 17b illustrates a cross section of the carrier 2 transverse through its longitudinal axis. Figure 17b shows that the fiber 7 is spun around carrier 2 and is contiguous thereto without a protective element.

In figure 18a and 18b, a design of a carrier 2 is illustrated which is similar to that of figures 17a and 17b, with the difference that in figure 18a and 18b the carrier 2 comprises a protective sheet 120 that is

circumferentially arranged around the carrier 2. Preferably, to allow deformation, the carrier 2 is not constricted by the protective element 120 itself. This may be achieved e.g. by a cut through the protective element 120 from the top to the bottom as illustrated (i.e. from edge 121 to edge 122, e.g. in a longitudinal direction or obliquely), which prevents constriction and allows deformation of the carrier 2. If no such cut is provided in the protective element 121, dependent on its material of element 121 a measurable deformation may still be detected, though. To protect effectively against constriction, the protective element is made of a material having a material hardness that is larger than a material hardness of the elastic material of the carrier 2 and to provide sufficient sensitivity the wall thickness of element 121 is kept to a minimum.

In figures 19a and 19b, the carrier 2 is surrounded by a protective element 125 which is partly open. The protective element 125 in between the carrier and the optical fiber 7 provides at least two attachment elements on either side of a cut-through section. Edges 126-1 and 126-2 comprise a ledge or other structure that allows attachment of a fiber 7 between the edges 126-1 and 126-2. The fiber optic sensor 8 (e.g. an FBG) is located between the edges 126-1 and 126-2. Deformation of carrier 2 under a load will cause the edges 126-1 and 126-2 to move apart, thereby stretching the fiber 7 and fiber optic sensor 8. By choice of the designer, the fiber 7 may be attached as illustrated, or may be spun around the protective element 125 as well.

The load cells illustrated in the drawings, e.g. in figures 1 though 4 and figures 8 through 11, for traffic monitoring of any kind, may be embedded in a road. For example, the load cells or a sensor system including a plurality of load cells (e.g. as in figure 4, but not exclusively limited to this embodiment), may be installed in a trench in the road while providing a level surface for the traffic to drive over it. Alternatively, such a system may be completely embedded underneath one or more layers road paving (e.g. asphalt, gravel or clinker pavement). Alternatively, such load cells or a sensor system may be embodied as a system that may be mounted on a road surface. An example of such an embodiment is illustrated in figure 16. In the embodiment of figure 16, the system may comprise the load cell devices or the sensor system 38 (in this example a system as is illustrated in figure 4) arranged in between a ramp-up element 101 and a ramp-down element 102 that allow traffic to drive over. In the embodiment illustrated in figure 16, the ramp-up and ramp-down elements 101 and 102 are integrated into a support element 100. The sensor system 38 is covered with a protecting cover layer 103. Alternatively, the protective cover layer 103 may be absent, in which case the depth of the trench formed by the support element 100 for housing the sensor system 38 may be equal to the height of sensor system 38 to provide a leveled upper surface between the ramps 101 and 102. As may be appreciated, and arrangement of parts may not necessarily be integrated into a support element 100 as illustrated, but may consist of separate elements (e.g. separate ramp-up and ramp-down elements). The embodiment of figure 16, as well as other embodiments of systems that may be mounted on the road surface, allows easy installation of the sensor system without the necessity to open up the road pavement, e.g. as a temporary sensing facility. On the other hand, the advantages of an embedded system (e.g. as in figures 6 and 7) is that vehicles passing over it will not notice a bump or unevenness, i.e. it does not hinder the traffic in any way.

Figure 12 illustrates a further application of a load cell array 38 in accordance with the present invention. In figure 12, a load cell array 38 is schematically illustrated as being installed in the foundation of building 90. The foundation will be supported by the ground 93 as is illustrated by supporting reactive forces 95-1 and 95-2. A weight of the building is schematically illustrated by arrows 92-1 and 92-2. After installation, and in an normal static situation without any hidden risks or any hazards to the building, the output signals measured from the load cell array 38 will be static as well. In the situation illustrated in figure 12, a subsidence 98 has occurred underneath the building 90. Due to the subsidence, the building 90 may slightly fall in or be tilted. This tilting can be very subtle such that it is almost unnoticeable by the naked eye. However, the tilting causes a difference in the distribution of force 92-1, 92-2 exerted on the ground 93. This can be measured by monitoring the output signal from the load cell array 38 installed in the foundation of the building. This information can be used to monitor the integrity of the building, and to determine for example after an earthquake or other calamity, whether a building is still safe.

The present invention has been described in terms of some specific embodiments thereof. It will be appreciated that the embodiments shown in the drawings and described herein are intended for illustrated purposes only and are not by any manner or means intended to be

restrictive on the invention. It is believed that the operation and

construction of the present invention will be apparent from the foregoing description and drawings appended thereto. It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are possible which should be considered within the scope of the appended claims. Also kinematic inversions are considered inherently disclosed and to be within the scope of the invention. Moreover, any of the components and elements of the various embodiments disclosed may be combined or may be incorporated in other embodiments where considered necessary, desired or preferred, without departing from the scope of the invention as defined in the claims.

In the claims, any reference signs shall not be construed as limiting the claim. The term’comprising' and including’ when used in this description or the appended claims should not be construed in an exclusive or exhaustive sense but rather in an inclusive sense. Thus the expression ‘comprising’ as used herein does not exclude the presence of other elements or steps in addition to those listed in any claim. Furthermore, the words‘a’ and‘an’ shall not be construed as limited to‘only one’, but instead are used to mean‘at least one’, and do not exclude a plurality. Features that are not specifically or explicitly described or claimed may be additionally included in the structure of the invention within its scope. Expressions such as: "means for should be read as: "component configured for ..." or "member constructed to ..." and should be construed to include equivalents for the structures disclosed. The use of expressions like: "critical", "preferred", "especially preferred" etc. is not intended to limit the invention. Additions, deletions, and modifications within the purview of the skilled person may generally be made without departing from the spirit and scope of the invention, as is determined by the claims. The invention may be practiced otherwise then as specifically described herein, and is only limited by the appended claims.