Field of the invention

The present invention relates generally to the field of bridge based weigh-in-motion systems. In particular, the present invention relates to a weigh-in-motion system for sensing dynamic strain variations in a bridge. The present invention also relates to a bridge including the weigh- in-motion system and also to a method for using the system on a bridge.

Background

Weigh-in-motion (WIM) systems are designed to obtain vehicle information as vehicles drive over a measurement site. Such vehicle

infor ation for example could include vehicle weight, axle count, speed and further data. This information may thereby be obtained while vehicles are in motion, and thus do not require the vehicle to stop, making the weighing process more efficient.

Existing and traditional WIM systems require an installation on the top surface of the road, resulting in damage to asphalt, shortening of lifetime and expensive operation costs. More recently, fiber optic sensing has been implemented integrated in the asphalt as a way of recording vehicle information, including speed, axel count and weights. However, it has limitations on the hardware cost and installation requirements, such as the necessity of road closure during installation.

Summary of the invention

It is an object of the present invention to provide a weigh-in- motion system that obviates the abovementioned disadvantages of the prior art, which may be installed without interruption of service of the road and which is sufficiently accurate to capture a large range of various parameters providing vehicle data. With this in mind, according to a first aspect of the present invention, there may be provided a weigh-in-motion system for sensing dynamic strain variations in a bridge, comprising a plurality of strain sensors and a plurality of fixtures arranged as a plurality of sensing nodes; wherein each sensing node comprises: a first one of the fixtures comprising a first connector part and a first base attachable to a first location on the bridge; a second one of the fixtures comprising a second connector part and a second base attachable to a second location on the bridge; wherein the first location and the second location are separated by a distance B and define therebetween a measurement section; and one of the strain sensors that spans between and is fixed to the first and second connector parts by rigid fixings, separated by a distance A and defining therebetween a sensing section, so as to thereby form a spring connection between the first and second connector parts, the spring connection being extended or compressed to enable the strain sensor to provide an output signal dependent on the relative position of the first and second connector parts.

By attaching the sensing nodes to the bridge, rather than integrating the fiber optic sensing into the asphalt, the system of the present invention is more convenient to install.

The output signal of the sensing node is dependent on the relative position of the first and second connector parts. For this reason, preferably, the fixtures of the sensing nodes are attached at locations on the underside of the bridge deck. This is a preferred arrangement because the sensing nodes are well placed to sense the deformation of the bridge, causing the relative position of the first and second connector parts to change, as vehicles pass over the bridge. Other locations on the bridge are possible as long as, at such locations, the bridge experiences a deformation as vehicles pass over the bridge. The weigh-in-motion system of the present invention may apply a pre-tensed strain sensor that spans between the first and the second connector parts, and in particular having a sensing section in between the rigid fixings therewith. In some embodiments, said strain sensor can be an Fiber Bragg Grating FBG sensor. Of further note to the present invention is the fact that load exerted on the strain sensor, i.e. via the first and second connector parts, is conveyed substantially onto the strain sensor. By this, it is meant that there are no reinforcements or reinforcing structures present between the rigid fixing or between the first and second connector parts, in order to ensure that the load is fully (or at least most of it, e.g. at least more than 90% of the load) conveyed onto the sensing section of the strain sensor. This has multiple advantages. For example, the absence of such structures merely limits the distance between the first and second connector parts by the length of the sensing section of the strain sensor. The connector parts could be a fixed distance in embodiments where the rigid fixings retaining the strain sensor do not allow any coarse adjustments. Alternatively, some embodiments may include fixings that enable - for example in an unlocked position of at least one of the fixings - some form of coarse adjustment of the length of the strain sensor between the first and second connector part, whereas such fixings may further have a locked position wherein the fixing is rigid and does not allow the strain sensor to slide or move. Both types of fixings may be applied in accordance with the present invention, provided that the latter type of fixings enables a locked position wherein the fixing rigidly retains the strain sensor without allowing slipping or movement thereof relative to the fixing itself.

Another advantage is that the absence of further reinforcements or reinforcement structures causes the load exerted by the first and the second connector part to be fully conveyed onto the strain sensor.

Additional reinforcements typically divert part of the exerted load to be supported via those reinforcements, thereby reducing the intensity of the distance change recorded by the strain sensor which in turn reduces the sensitivity of the strain sensor. However, in the present invention even the slightest distance change amount may be detected by the strain sensor. When using FBG strain sensors, the of detection of strains might

have a magnitude of less than 10 microstrains or less, even with a precision of less than 0.01 microstrain.

The absence of reinforcements does not preclude the application of protective measures to protect the strain sensor from external influences.

According to some embodiments of the bridge based weight -in motion system, at least one of the first or the second connector parts are adjustable with respect to the bases. In a further embodiment, the connector parts are mounted to at least one of the first or second connector parts by means of an longitudinally adjustable connection. A further technical advantage of providing a difference in the distance between the first and second position (i.e. the length of the measurement section B) and the length of the strain sensor between the rigid fixings, i.e. the sensor gauge length, is that this allows for adjustability in the sensitivity of the strain sensor.

Any variation in the length of the measurement section caused by bending, twist, longitudinal expansion or compressing of bridge parts due to for example moving vehicles on the bridge, is detected by the strain sensor. In certain situations, it may be desirable to amplify or attenuate the sensitivity of the individual strain sensors, depending on the circumstances. In such situations, the above described longitudinal adjustable connection can be used. The ratio between the length of the measurement section and the strain gauge length can be referred to as transmission ratio.

In situations where the strain sensor is dimensioned such that the transmission ratio is greater than 1, an absolute extension of the strain sensor caused by the variation in the length of the measurement section results in a amplification of the sensitivity of the strain sensor. When using a fiber optic strain sensor, the stretching of an optical fiber per unit of length is larger when a certain absolute amount of extension in the bridge structure has to be distributed across a shorter sensor gauge length.

Therefore, the variation in the output signal from an intrinsic fiber optic sensor, e.g. the wavelength shift from the reflected and/or transmitted signal of a fiber Bragg grating, will thus be more intense at shorter gauge lengths. As a result, shortening of the sensor gauge length relative to the length of the measurement section results in a mechanical amplification of the strain level, thereby increasing the sensitivity of the sensor. The ratio between the length of the measurement section and the strain gauge length determines how much the variations of the installation distance caused by a moving vehicle on the bridge, are amplified to become measurable by the sensor. By using a relatively short sensor gauge length with a large enough measurement section, e.g. with a length of 1 meter and a strain sensor gauge length of 5 centimeter, the very small variations underneath a bridge which are caused by vehicles driving over the bridge become well

measurable. These even allow for detection and analysis of small vehicles on even rigid bridges, and even quantify the vehicle characteristics.

A further important advantage of using short gauge lengths, which is not yet discussed, is the fact that the resonance frequency of a biased spring - such as a biased strain sensor - is reversely dependent on the length thereof. Hence, at shorter sensor gauge lengths the resonance frequency is larger. Such larger resonance frequencies are beneficial for obtaining data at higher data rates, due to the fact that the response time of the system is shorter for higher resonance frequencies. Therefore, although for measuring static parameters this is not important, short gauge lengths in general are beneficial to sensor systems that are designed for measuring dynamic strain variations.

To provide some examples (which are meant to be explanatory rather than restrictive on the invention) in accordance with some

embodiments, the first and second connector parts and the strain sensor are dimensioned such that a ratio between the installation distance and the strain gauge length is 1.0 divided by X, wherein X is at least 2, preferably wherein X is larger than 4, more preferable larger than 5, and more preferable between 5 and 100. For example, in some embodiments, the installation distance is at least 0.3 meter, preferably between 0.5 and 5.0 meter, more preferable between 0.5 and 2.0 meter.

In some embodiments of the invention, connector parts are mounted to the at least one of the first or second connector part by means of a longitudinally adjustable connection. Without restricting this concept to certain embodiments, it is to be understood that other solutions that allow the extension length to be adjustable likewise fall within the scope of the present invention.

In accordance with some embodiments of the invention, the connector parts further include an anchoring mechanism for enabling said attachment to the first or second location, the anchoring mechanism including a tuning element for enabling adjustment of said location for attachment, for adjusting a pre-tensioning of the strain sensor at the sensing section. The tuning element could include a screw and bolt connection enabling fine tuning. It could also include a coarse adjuster, e.g. a sliding mechanism, to the benefit of ease of use during installation.

Moreover, the anchoring mechanism itself may be embodied such as to allow permanent or semi-permanent installation, e.g. by comprising fixing holes for a screw and plug connection for attachment to the bridge. Alternatively or additionally, the anchoring mechanism may comprise a surface

corresponding with an installation surface on the bridge, such as to allow the use of a glue or other adhesive. This enables easy removal in those situations wherein the installation is not permanent or where any structural modifications (e.g. drill holes) are not desired. In accordance with a further alternative, the anchoring mechanism includes a magnetic means or magnetic surface to enable direct attachment to steel, such as to a steel bridge. A particular embodiment thereof may even comprise a powered electromagnetic anchoring mechanism which can be switched on and off to enable very fast and easy temporary attachment to a steel bridge. The present invention according to the first aspect, by attaching a plurality of sensor nodes at multiple locations on the bridge, can record the timing difference between the subsequent measurement sections so as to be able to quantify the velocity of vehicles to a high accuracy. This, for example, may be apphed to obtain more sophisticated measurements, e.g. to allow detection of additional vehicle data such as wheel count or to enable prediction of a type of vehicle. For example, a car typically has a low weight, an axle count of two, a wheel count of four, and from the distance between the two axles and between the wheels on each axle, information may be derived about probable car models. The latter combined with weight and weight distribution may even allow an estimate of the number of passengers, or whether or not the car is too heavily loaded. A high weight vehicle with more than two axles and more than two wheels per axle, may typically indicate a truck. Additional axles following a first plurality of axles with the same velocity, but indicating a clearly different weight, may indicate a trailer behind the truck. Distribution of the weight per wheel may also provide information on whether the truck is correctly loaded.

Dynamic behavior of the weight per wheel may signal a defect or danger, or provide further interesting details.

In some embodiments, the strain sensor comprises an optical fiber, including an intrinsic fiber optic sensor. The intrinsic fiber optic sensors may be distinguishable, e.g. fiber Bragg gratings with

distinguishable reflection wavelengths. The specific grating wavelengths can be identified by monitoring the reflection or transmission spectra, using optical interrogators. In some embodiments, the strain sensor comprises an optical fiber and an optical sensor coupled to the optical fiber. In some embodiments, the strain gauge sensor comprises a cable having an electrical strain gauge coupled to the cable.

Preferably, the system comprises a controller. The controller may be arranged for sampling the output signal of the interrogator for enabling recording thereof at a sampling rate of at least 1000 samples per second, preferably at a sampling rate of at least 2000 samples per second. The invention is not limited to these data rates, and higher data rates may well be obtained by proper selection of the controller, and the proper selection of the other system components such as the interrogator, the predetermined sensor gauge length, the type of optical fiber and other circumstances.

According to a second aspect of the present invention, there may be provided a bridge including a WIM system according to the first aspect of the invention, wherein the bridge comprises at least one of a roadway, lane or railway track for enabling a vehicle to cross the bridge in a driving direction of the vehicle, and wherein the sensing nodes are attached to the bridge, preferably on the underside of the bridge deck, such that their respective strain sensors are aligned along the driving direction. This arrangement enables the monitoring of vehicle weights and high speed strain changes on the bridge, which can be correlated with each other due to the measurements being performed at multiple points and at high

resolution. The strain sensors being ahgned to the driving direction enable such measurements to be performed dynamically at a high data rate, such as to enable a time dependent monitoring while the vehicle crosses the bridge, e.g. by following the sensor responses caused by the vehicle.

According to a third aspect of the present invention, there may be provided a method for using the system of the first aspect on a bridge comprising analysing the output signals from the sensing nodes to

determine the characteristics of the vehicles passing over the bridge, wherein the characteristics include one or more of speed, weight, axle count, axle length, and load distribution. Based on data derived from the analysis step, metrics of bridge usage over time may be determined. Also, the data can be correlated for multiple purposes, such as to increase the accuracy thereof by increased statistics, to identify safety risks, or to obtain a larger range of directly measurable and derivable parameters and information about the vehicle. 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:

Figure 1 schematically illustrates a weigh-in-motion system in accordance with an embodiment of the invention;

Figure 2 schematically illustrates the weigh-in-motion system of figure 1 in cross section;

Figure 3 schematically illustrates an alternative embodiment of a weigh-in-motion system in accordance with the invention;

Figure 4 schematically illustrates a weigh-in-motion system in accordance with a further embodiment of the invention;

Figure 5 schematically illustrates a weigh-in-motion system in accordance with the invention deployed on a bridge;

Figure 6 schematically illustrates a weigh-in-motion system in accordance with the invention deployed on a bridge, showing an exemplary possible pattern of sensing nodes; and

Figure 7 schematically illustrates a further weigh-in-motion system in accordance with the invention deployed on a bridge, showing a further exemplary possible pattern of sensing nodes.

Detailed description

Figure 1 schematically illustrates an exemplary sensing node 100 of a weigh-in-motion system 1 in accordance with an embodiment of the present invention. The sensing node 100 comprises a first fixture 2a having a first base 3 and a second fixture 2b having a second base 5. The first and second bases 3 and 5 are respectively mounted or fixed to respective locations underneath a bridge. The bridge is schematically illustrated by surface 30 to which the bases 3 and 5 of the first and second fixtures 2a, 2b are fixed. The first and second fixtures 2a, 2b each include a case 28. A first connector part 20-1 is attached to first base 3 and a second connector part 20-2 is attached to the second base 5 via respective cases 28. The connector parts may have an externally threaded surface 27. Together, the threaded sections 27 and the cases 28 enable the distance between the connector parts 20-1, 20-2 to be adjusted.

Each of the fixtures 2a, 2b further comprises a signal connector 33 enabhng a signal cable 32 to be connected to the fixtures 2a, 2b,

respectively. The signal cable 32 may be fiber optic. A signal line7 in the form of an optical fiber runs from the fixture 2a to the fixture 2b, thereby spanning therebetween. The signal line 7 may comprise an intrinsic fiber optic sensor 6, such as the fiber Bragg grating (FBG) as a functional part of the strain sensor 1. Alternatively, the signal line may be an electrical lead connected to an electrical strain gauge.

Figure 2 shows a cross section of the strain sensor illustrated in figure 1. As illustrated in figure 2, the signal line 7 runs from a signal connector 33 through an internal channel 29 of the connector part 20-1 towards an internal channel 29 in the connector part 20-2 to a connector 33. The first fixture 2a comprises a rigid fixing 15 that clamps or fixes the signal line 7 to the connector part 20-1, and the second fixture 2b comprises a corresponding rigid fixing 16 that fixes the signal line 7 to the connector part 20-2. Internally in each of the connector parts 20, the signal line 7 stretches from each connector 33 to the respective rigid fixing 15, 16 of the respective connector parts 20-1, 20-2 through the respective channel 29. Therefore, being fixed on either side, e.g. for the first connector part 20-1 between connector 33 and the rigid fixing 15, the signal hne 7 is internally supported in the connector part 20 within the channel 29. In one

embodiment, no stretching takes place internally in each connector part 20. In between the first and second fixtures 2a, 2b, in particular in between the first rigid fixing 15 and the second rigid fixing 16, the signal line 7 spans the gap between the connector parts 20-1 and 20-2. The whole portion or region between the rigid fixings 15 and 16 is referred to as the sensing section 9 of the sensing node 100. The sensing section 9 contains the signal line 7 and the sensor 6.

As stated above, the first base 3 and the second base 5 are mounted to a supporting surface 30 of a bridge in respectively a first location 10 and a second location 11. Preferably, the first and second fixtures 2a, 2b may be installed underneath the bridge, underneath a road. In figure 2, the locations 10 and 11 are schematically indicated by dashed lines illustrating their mechanical centres which are located at the crossings of each dashed line and the surface 30. Further dashed lines in figure 2 indicate the relative locations of the rigid fixings 15 and 16 between which the signal line 7 is fixed, i.e. the end points of the sensing section 9.

It is important to notice that the sensing section 9 is provided by the signal line 7 which is spanned free of any further mechanical support between the rigid fixings 15 and 16. As a result, any movement of the bridge surface 30 that is imposed on the bases 3 and 5 which causes the first and second connector parts 20-1, 20-2 and rigid fixings 15 and 16 to move relative to each other, is conveyed unrestricted onto the spanned signal hne 7 between the rigid fixings 15 and 16. If any further mechanical support were to be present, a part of this load would be diverted via the additional support, thereby reducing the load on the signal line 7 in the sensing section 9. It is to be understood that the absence of any further mechanical support as indicated above, is intended to mean that a significant mechanical auxiliary support is absent. However, protective elements such as a sleeve, cladding or a casing of any kind may be present to protect sensing section 9.

As illustrated in figure 2, the length of the sensing section 9 is indicated by A, whereas the length of the measurement section between the bases 3 and 5 is indicated by B. The ratio between A and B is significant because this ratio enables the tuning of a mechanical amplification or attenuation factor that amplifies or attenuates the strain sensor sensitivity. This is explained as follows. Suppose movements were exerted on the first base 3 and the second base 5 such that these movements are directed in opposite directions. For example, the direction of these movements may be aligned with the direction of signal line 7, such that distance A increases. In principle, these movements would result in the locations of the rigid fixings 15 and 16 moving either closer together or further away from each other. As will be understood, moving the location of the first base 3, 0.1 mm apart from the second base 5 will result in the location of rigid fixing 15 to move 0.1 mm away from the rigid fixing 16 as well. The amplification of the signal can be understood as follows. The signal line 7 comprises the sensor 6. Moving location 10 0.1 mm away from location 11 results in a relative increase of the distance B equal to: 0.1 mm/B. Translating rigid fixing 15 over 0.1 mm in the same direction, on the other hand, results in a relative increase of the distance A of: 0.1 mm/A. If A < B, the relative extension of the sensing section 9 between rigid fixings 15 and 16 is much larger than the relative extension of the distance B between the mounting locations of the bases 3 and 5. In fact, the ratio B/A defines the

amplification factor that amplifies the sensitivity of the strain sensor, due to a larger relative extension of the sensor section 9 will result in a larger strain change. Because the weigh-in-motion system 1 illustrated in figure 2 uses a sensor signal indicative of differences in strain, the mechanical amplification provided by the amphfication factor B/A may advantageously be used to boost up the sensor signal and increase the sensitivity of the strain sensor overall.

Likewise, the absence of any further mechanical support means results in all the movement imposed on the bases 3 and 5 being imposed on the sensor section 9 between the rigid fixings 15 and 16 without

impediment. Thus, the sensing node 100 amplifies the signal by mechanical amplification provided by the difference between A and B, with

amplification factor B/A. Another embodiment of the present invention is illustrated in figure 3. In figure 3, the weigh-in-motion system is denoted by G, but otherwise the configuration and various elements of the weigh-in-motion system 1’ are similar to those of weigh-in-motion system 1 of figures 1 and 2, and corresponding elements are indicated by the same reference numerals.

A significant difference between the weigh-in-motion system 1 of figures 1 and 2 and the weigh-in-motion system G of figure 3, is that the rigid fixings 15 and 16, with respect to the location of the intrinsic strain sensor 6, are located behind the casings 28 in the vicinity of the connectors 33. As a result, the distance A will be larger than the distance B between the mounting locations 10 and 11. Therefore, in the weigh-in-motion system G of figure 3, the amplification factor B/A <1 (smaller than unity) thereby resulting in an attenuation of the sensor signal. The weigh-in-motion system 1’ may well be applied in situations wherein strain variations may become large enough to be detrimental to the integrity of the signal line 7 for example. In the example illustrated in figure 3, the bridge comprises bridge sections 30-1 and 30-2 having a flexible joint 31 connecting the bridge sections. Strain is measured across the flexible joint 31. However, the flexible joint 31 typically responds to seasonal effects strongly, and as a result, structural movements may result in a relatively large distance variation in the distance B between locations 10 and 11 of the first base 3 and the second base 5. The attenuation provided by the ratio B/A in the embodiment of figure 3 results in the absolute distance variation in the distance B to become relatively smaller across the sensing section 9.

An important feature of the embodiments of figures 1-3 is that various strain sensor sensitivity levels can be achieved at design and manufacturing phase, prior to installation, by changing the amphfication factor of the sensors via the B/A ratio. Furthermore, it allows for having weigh-in-motion systems with various sensitivities in one installation. Even further, this allows for customization for individual bridges and even sections. A further important feature of the embodiment of figures 1-3 is that the measurement range is adjustable via the pre-tension, tunable by the adjustable connection between the casing 28 and the threaded portion 27 on the connector parts 20. By using a threaded section 27, it is possible to accurately tune the pre-tension of the strain sensor. In case a rough and/or fine tuning is desired, different adjustable connections may possibly be applied. For example, a sliding connection between the longitudinal connection part 20 and the casing 28 may provide for this.

Figure 4 schematically illustrates a further embodiment of the weigh-in-motion system in accordance with the present invention. In the embodiment of figure 4, a first fixture 2 a, a second fixture 2b and a third fixture 2c are illustrated. The third fixture 2c, comprises a third base 40 and a connector part 20-3, and is located intermediate the first and second fixtures 2a, 2b. Sensing sections 9-1 and 9-2 are formed between each of the first and second fixtures 2a, 2b and the third fixture 2c. The fixtures 2a, 2b are provided with rigid fixings as in the other embodiments. The third fixture 2c is provided with rigid fixings at both ends of the connector part 20-3. In the embodiment illustrated in figure 4, a signal fine 7 extends between connector 33 of the first fixture 2 a and the connector 33 on the second fixture 2b. The signal line 7 goes all the way through the first connector part 20-1, the intermediate or third connector part 20-3 and the second connector part 20-2. The sensing section 9-1 contains a sensor 6-1 and the sensing section 9-2 contains a sensor 6-2, which may be both in the form of a fiber Bragg grating (FBG).

The intermediate or third connector part 20-3 is attached to the casing 28 by means of an adjustable connection. As may be appreciated, moving the longitudinal extension part of the connector part 20-3 to the left will shorten the sensing section 9-1 and will lengthen the sensing section 9- 2. Moving it towards the right in figure 4 has the opposite effect of lengthening the sensing section 9-1 and shortening the sensing section 9-2. In case both the sensing sections 9-1 and 9-2 are to be lengthened or shortened by an equal amount, this may be achieved by using the adjustable connections between the connector part 20-1 and casing 28 of the first fixture 2a and the connector part 20-2 and casing 28 of the second fixture 2b. As may be appreciated, the adjustable connection between casing 28 and intermediate or third connector 20-3 may be replaced by a fixed connection that is not adjustable. In that case, adjusting the lengths of the sensing sections 9-1 and 9-2 may only be achieved by adjusting the adjustable connections between the connector parts 20 and casings 28 of the first and second fixtures 2a, 2b, respectively.

Intermediate or third connector part 20-3 can be curved, such that to accommodate corners and/or structural features.

Figure 4 represents just one embodiment of a range of embodiments. As may be appreciated, the number of intermediate connector parts 20-3 may be much larger than just a single intermediate connector part 20-3. For example, a strain sensor arrangement may comprise a plurality of intermediate connector parts 20-3 (e.g. 2, 3, 4, 5, ....20, ....etc.). Alternatively, instead of using the intermediate connector part 20-3, an array of sensor nodes 100 may be obtained by using pairs of the sensor nodes 100 shown in figures 1-3 in a series arrangement

interconnected by signal cables 32.

In figure 5, the deployment of an array of sensor nodes 100 on a bridge 50 is schematically illustrated. Traffic is driving across the bridge 50, illustrated by the vehicles 52, 53 and 54. Underneath the bridge 50, a series of fixtures 55, which may include any combination of first fixtures 2a and second fixtures 2b (e.g. such as shown in figure 1) and intermediate fixtures 2c (e.g. such as shown in figure 4).

A signal line 57 spans between the various fixtures providing a plurality of sensing sections between mutual pairs of fixtures 55. A controller or data acquisition unit 59 is installed at the beginning of the signal line 57, and at the end of the signal hne 57, a terminator 60 may be placed. The distance between the fixtures 55 may be different. For example, the separation between fixture 55’ and 55” that is spanned by fiber section 57’ is much larger than that of the other spanned sections of the fiber 57.

The system in accordance with embodiments of the present invention may advantageously be installed underneath a bridge, such as is illustrated in figure 5. Conventional strain sensors, e.g. for measuring the weight of vehicles such as truck 54, are typically installed on or in the surface of the road. However, advantageously, the present system may more easily be installed without having to close down any road section. In addition, the system may also more easily be removed, and thereby allows to be temporarily installed as well. As may be appreciated, in certain situations, a disadvantage of installing a strain sensor underneath the bridge is that the signal to be measured is much weaker. However, as a result of the amplification obtainable by the present system, the strain signal is well measurable by the array of sensing nodes.

Figure 6 schematically illustrates a top view of a system of the present invention. In figure 6, a controller or data acquisition unit 80 connects via a first channel 81-1 to a first array of sensing nodes 71.

Moreover, the controller or data acquisition unit 80, via a second channel 81-2 connects to a second array of sensor nodes 76. The first array 71 is installed underneath the bridge 50, and underneath a first lane 61 of a road that passes the bridge 50. The second array 76 is located underneath the bridge 50 underneath a second lane 62 of the road. For example, the lanes 61 and 62 may convey traffic in different directions across the bridge 50. Figure 6 illustrates the relative locations of the sensors 55 of the array 71. The actual fixtures 55 of the array 71 are located underneath the bridge, and not on the bridge. Alternatively, the arrays can be attached on the side of the bridge, railings etc., based on the situation. Terminator portions 60-1 and 60-2 respectively end the array 71 and 76.

In a different embodiment comprising fiber optics strain sensors as illustrated in figure 7, a single interrogator channel 81 connects to a fiber forming a plurality of fiber strains that are arranged longitudinal with respect to the lanes 61 and 62 of the road. For example, three arrays 70, 71 and 72 are formed by running the optical fiber back and forth three times underneath road lane 61, thereby forming a sensor array 65 of arrays 70, 71 and 72. The fiber in section 82 interconnects towards strain 75 on the second road lane 62 and thereafter runs back and forth three times to form sensor arrays 75, 76 and 77. Sensor arrays 75, 76 and 77 together form sensor array 66 located underneath road lane 62. A terminator 60 is installed at the end of the fiber line.

In an alternative embodiment, electrical strain gauges with a respective data acquisition unit are used in a configuration, as shown in figure 7.

In the embodiments described hereinabove, to distinguish between signals from various sensor sections 9, the sensor 6 may be configured such as to each provide an identical sensor signal. For example, in case use is made of fiber Bragg gratings, each of the fiber Bragg gratings in each of the sensor sections 9 may have a different periodicity that enables to distinguish the reflected wavelength of that fiber Bragg grating from those of other fiber Bragg gratings in the sensor array.

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.