Abstract:
The present invention provides an optical fiber sensor for traffic monitoring, which comprises a former consisting of an elongate plate, and an optical fiber wound onto at least one surface of the elongate plate. The elongate plate is flexible in a direction transverse to the at least one surface such that passage. Traffic over the optical fiber sensor is arranged to cause a variation in at least one predetermined property of an optical signal transmitted through the optical fiber sensor. The resulting sensor has a reduced depth which makes it easier to locate within the surface of a traffic route, has increased flexibility to enable it to confirm to the surface of the traffic route, and has good cross axis sensitivity rejection.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to traffic monitoring techniques, and in particular to sensors used in traffic monitoring systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    It will be appreciated by those skilled in the art that traffic may come in many different forms. For example, considering solely traffic on land, such traffic can take a variety of forms, including but not limited to vehicles on roads, bicycles on paths, trains on rails, people on paths, aircraft on runways, etc.  
           [0003]    There are several reasons why information regarding traffic on a particular section of a traffic route (for example a road, a path, a railway line, etc) may be collected. One of these may be for the effective management of traffic, where information regarding the speed and volume of traffic is useful. This enables alternative routes to be planned in response to accidents or route closures and to attempt to relieve congestion perhaps by altering speed limits.  
           [0004]    Considering the example of roads, many new roads are built with a sacrificial top layer which is designed to wear out and be replaced. The significant costs associated with road repairs and road building, in addition to the disruption caused by such works, requires that repairs are carried out only when needed. The sacrificial layer should neither be replaced too soon, leading to unnecessary costs, nor too late, risking more serious damage to the underlying structure of the road. An accurate determination of the volume of traffic on a particular road section is therefore essential.  
           [0005]    A further reason why traffic information is required is for the enforcement of regulations and laws. There are regulations relating to maximum allowable weights for heavy goods vehicles (HGVs) which are borne out of concerns for safety and also to lessen the damage that overladen vehicles may do to the road structure. A measure of dynamic vehicle weight helps to ensure that such regulations are adhered to. This also applies to other forms of traffic route, for example to the issue of overweight trains on particular railway routes. Particular lines may be approved for use by trains up to a certain weight maximum, and the rail operators should adhere to these weight restrictions. Again, the ability to measure dynamic weight would help to ensure that such regulations are adhered to.  
           [0006]    Simple information regarding vehicle speed may be used to monitor and enforce speed limits.  
           [0007]    There may also be a requirement to collect information regarding the types of vehicle using a particular section of road. This may be to prevent unsuitable vehicles such as HGVs from using rural roads or to plan future road building schemes. Classification of vehicle type may be achieved from a determination of dynamic vehicle weight and axle count.  
           [0008]    It is clear that information regarding the speed, weight, volume and type of traffic can all be used to help with an effective traffic management programme. There are several methods in use to obtain this information, however these have associated problems.  
           [0009]    Many sections of road are overseen by video cameras. The images from these cameras are fed to central points to be analysed to provide information regarding vehicle speed and type and traffic volume. However, due to the complexity of the images, it is not always possible to reliably automate the analysis of the data received, meaning that they must be studied visually. There is a limit to how many images can be analysed in this way. Furthermore, the quality of the images collected may be influenced by weather conditions. Fog or rain can obscure the field of view of the cameras, as can high vehicles, and high winds can cause the cameras to vibrate. In many countries, camera systems are operated by law enforcement agencies, so there is often an added complication in making the information collected available to the agencies involved with traffic management. It is also not possible to determine the weight of a vehicle from a video image. The commissioning costs of video camera systems for traffic monitoring can also be high.  
           [0010]    The vast majority of new roads and large numbers of existing roads are provided with inductive sensors. These are wire loops which are placed below the road surface. As a vehicle passes over the sensor, the metal parts of the vehicle, i.e. the engine and the chasis, change the frequency of a tuned circuit of which the loop is an integral part. This signal change can be detected and interpreted to give a measure of the length of a passing vehicle. By placing two loops in close proximity to one another, it is also possible to determine the vehicle&#39;s speed. The quality of the data collected by inductive loop sensors is not always high and is further compromised by the fact that the trend in many modern vehicles is to have fewer metal parts. This leads to a smaller signal change which is more difficult to interpret. In addition, because of a tendency for road builders to make greater use of steel in highway construction, there is an increasing problem of interference affecting the accuracy of readings.  
           [0011]    Although cheap to produce, inductive sensors are large and as such their placement, particularly in existing roads, causes significant disruption. This has associated costs. A major drawback with the use of inductive loops for traffic management is that they are not amenable to multiplexing. Each sensor site requires its own data collection system, power supply and data communication unit. This increases the cost of the complete sensor significantly, which results in the majority of installed inductive loops not being connected, and therefore incapable of collecting data. Furthermore, although inductive loops can be used to count vehicles and, if deployed in pairs, to determine vehicle speed, they cannot be used to measure dynamic vehicle weight. Vehicle classification is thus not possible.  
           [0012]    Two methods for determining the weight of vehicles, in particular HGVs are in common use. Vehicle weight can be measured using a weigh-bridge. This is very accurate but requires the vehicle to leave the highway to a specific location where the measurement can take place. An alternative method is to attempt to measure the weight of the vehicle as it is in transit. Commonly, piezo-electric cables are placed under the surface of the road, which produce a signal proportional to the weight of the vehicle as it passes over. This method is more convenient but less accurate than a weigh-bridge. As with inductive loop sensors, piezo-electric sensors are not amenable to multiplexing so each requires a similar data collection system, power supply and data communication unit. The sensors are also more expensive and less robust than inductive loop sensors.  
           [0013]    In order to obtain the maximum amount of information regarding traffic on a particular section of road, piezo-electric sensors are often deployed in tandem with inductive loops.  
           [0014]    GB-A-2,214,771 describes a pressure or weight transducer that comprises an optical fibre located on a flat rigid surface. A constant light signal is fed into one end of the fibre from a control unit, and the control unit includes a receiver at the other end of the fibre which detects changes in the light transmission level and produces an output signal which is either calibrated to be proportional to the changes in pressure or weight applied to the fibre or to change from a first to second state in response to that pressure exceeding a predetermined threshold.  
           [0015]    Optical fibre sensors can also be used to detect pressure using a different technique. When a length of optical fibre is subjected to an external pressure the fibre is subjected to a strain. This strain impos s a change in property (e.g. phase) in an optical signal propagating through the optical fibre due to a combination of the physical length change and the stress-optic effect, and this change in property can b detected. As it is possible to analyse for very small changes in property such as phase, such optical fibre sensors are extremely sensitive to applied pressure. This high sensitivity allows optical fibre sensors to be used, for example, in acoustic hydrophones where sound waves with intensities equivalent to a pressure of 10 −4  Pa are routinely detectable. Such high sensitivity can however also cause problems. Such optical fibre sensors are not ideally suited for use in applications where a low sensitivity is required, for example for detecting gross pressure differences in an environment with high background noise. However, such optical fibre sensors have the advantage that they can be multiplexed without recourse to local electronics.  
           [0016]    It would be desirable to provide a sensor for traffic monitoring which would be easily deployed, would provide the required accuracy and could be multiplexed with other sensors to simplify data collection, data communication and power supply.  
         SUMMARY OF THE INVENTION  
         [0017]    Viewed from a first aspect, the present invention provides an optical fibre sensor for traffic monitoring, comprising: a former comprising an elongate plate; and an optical fibre wound onto at least one surface of the elongate plate, the elongate plate being flexible in a direction transverse to the at least one surface such that passage of traffic over the optical fibre sensor is arranged to cause a variation in at least one predetermined property of an optical signal transmitted through the optical fibre sensor; wherein the variation in at least one predetermined property of an optical signal transmitted through the optical fibre comprises a variation in phase, which is detectable by an interferometric interrogation system.  
           [0018]    In accordance with the present invention, an optical fibre is wound onto at least one surface of an elongate plate, such that when traffic passes over the sensor this causes a variation in phase of an optical signal transmitted through the fibre, which can then be detected by an appropriate interrogation system.  
           [0019]    Since an elongate plate is used to hold the optical fibre, the optical fibre sensor can be made sufficiently thin that it can be readily deployed in a traffic route without having to dig a substantial groove in the traffic route to accommodate the sensor. Further, due to the flexibility of the elongate plate, the sensor can readily be made to adopt the shape of the traffic route surface, and hence can for example adopt the shape of the camber of a highway surface, thus making it simple to ensure that the sensor is at a uniform depth below the surface. This helps to improve the uniformity of response along the length of the sensor, and hence improve the accuracy available from the sensor. Furthermore, since the sensor is based on phase-based optical fibre technology, it is possible to multiplex a plurality of such optical fibre sensors together to thereby enable a simplification in the data collection, data communication and power supply systems. In addition, this type of sensor is easy to store and deploy. It may be wound onto a spool for storage and transportation, and then unwound as required.  
           [0020]    In preferred embodiments, the elongate plate is provided with a pair of curved elements which protrude from the at least one surface and are spaced from each other along the elongate axis, wherein the optical fibre is wound longitudinally between the curved elements. Hence, in such preferred embodiments, the optical fibre traverses up and down the elongate axis of the elongate plate, and at the end of each traversal passes around the curved element prior to traversing back along the elongate axis in the opposite direction. When an optical fibre is bent, there is a tendency for light to be lost from the optical fibre. By ensuring that the optical fibre passes around a curved element of appropriate dimensions, this ensures that excessive light loss does not occur as the optical fibre is bent in order to enable it to traverse in opposite directions the elongate plate. It will be appreciated by those skilled in the art that the appropriate dimensions for the curved surface, for example its radius, will depend on the optical fibre being used. In preferred embodiments, an optical fibre with a high Numerical Aperture (NA) is chosen, thereby increasing the amount by which the optical fibre can be bent before excessive light loss starts to occur. In such embodiments, it has been found that providing curved elements with a diameter of 8 mm is sufficient to allow the optical fibre to the passed up and down the length of the elongate plate without excessive light loss as the optical fibre is bent to change direction.  
           [0021]    It will be appreciated that the curved elements may be moulded as part of the elongate plate itself, or alternatively could be provided as separate elements for fitting in an appropriate manner to the elongate plate. In embodiments where the curved elements are separate elements to be fitted to the elongate plate, it will be appreciated that they may be fitted in a way in which they are fixed, or alternatively may be attached to the elongate plates so that they are rotatable. In one embodiment, each curved element is rotatable about an axis transverse to the at least one surface of the elongate plate. By allowing the curved element to rotate this enables the strain on the various lengths of optical fibre passing between the curved elements to be equalised.  
           [0022]    In preferred embodiments, each curved element comprises a spindle, which is preferably attached to the elongate plate such that it protrudes from the at least one surface in a direction substantially perpendicular to the surface. In one embodiment, the spindle is rotatable. However, in preferred embodiments, the spindle is fixed, and the curved element further comprises a wheel rotatably mounted on the spindle.  
           [0023]    For ease of handling and deployment, it is desirable that the spindles are short in comparison to the length of the strip. An example sensor may have a length of approximately 3.5 m, with a depth of approximately 5 mm (the spindles therefore being less than or equal to 5 mm in length). This is sufficient to wind the required length of optical fibre, yet results in a sensor which is thin enough to remain flexible.  
           [0024]    It will be appreciated that the curved elements may be located at any appropriate point along the surface of the elongate plate. However, the sensitivity of the optical fibre sensor is generally increased the longer the length of optical fibre used with the sensor. Accordingly, to make best use of the length of the elongate plate, it is preferable that the pair of curved elements are located towards opposing ends of the elongate plate.  
           [0025]    In preferred embodiments, the optical fibre sensor further comprises a pair of termination plates provided towards opposing ends of the elongate plate, each termination plate being coupled to a corresponding one of the curved elements so as to guide the optical fibre to and from that curved element. In preferred embodiments, each termination plate is arranged to receive the curved element and is shaped so as to guide the optical fibre to and from that curved element. In one preferred embodiment, the optical fibre is arranged to follow a predetermined path on the at least one surface between the pair of curved elements, and the termination plate serves to provide a smooth transition for the optical fibre between that predetermined path and the outer periphery of the curved element.  
           [0026]    It will be appreciated that the elongate fibres may be left to follow their natural route along the at least one surface between the pair of curved elements. However, in preferred embodiments the optical fibre sensor further comprises one or more guide members protruding from the at least one surface of the elongate plate and positioned between the pair of curved elements, the guide members being arranged to guide the optical fibre along a predetermined path on the at least one surface between the pair of curved elements. This ensures that the fibres can be kept away from an edge region of the at least one surface, and accordingly reduces the risk of damage to the optical fibres. Preferably, the predetermined path is a central path along the elongate axis of the elongate plate.  
           [0027]    As an alternative to embodiments which include spindles, and/or wheels on the elongate plate, the optical fibre can instead be wound longitudinally around the long axis of the elongate plate so as to pass along both surfaces of the elongate plate. In yet another alternative design, the optical fibre is wound helically around the short axis of the elongate plate.  
           [0028]    Preferably, the optical fibre sensor further comprises a coating provided over the elongate plate and optical fibre. This coating may serve to protect the optical fibre from damage, and may for example be formed of a material such as epoxy, polyurethane of Butyl rubber. In preferred embodiments, the coating is provided not merely to protect the optical fibre from damage, but also to de-sensitise it. Accordingly, in such preferred embodiments, the coating comprises a compliant compound for reducing the sensitivity of the optical fibre sensor. In this embodiment, the compliant material effectively absorbs a proportion of any applied force, thereby enabling the sensor to be used to detect larger forces and pressures than would ordinarily by possible with optical fibre sensors. The choice of compliant compound may vary from a highly compliant material, such as grease, to a less compliant material such as epoxy, polyurethane or Butyl rubber.  
           [0029]    As mentioned above, dependent on the choice of coating applied to the elongate plate and optical fibre, the coating itself may provide appropriate protection for the optical fibre. However, in preferred embodiments, the optical fibre sensor further comprises an additional elongate plate, the coating being sandwiched between the elongate plate and the additional elongate plate. This arrangement not only provides additional protection for the optical fibre sandwiched between the two elongate plates, but also provides the sensor with symmetry, such that the optical fibre passes generally through the centre of the optical fibre sensor.  
           [0030]    In preferred embodiments, the elongate plate comprises a metal strip. Examples of suitable metals include steel, brass, tin alloys, aluminium alloys, etc. Alternatively, the elongate plate comprises a non-metal strip, for example a plastic such as Perspex and high density polyethylene, or alternatively nylon or some composite materials.  
           [0031]    The elongate strip may preferably be of any suitable dimensions provided that it remains sufficiently flexible to be able to adopt the shape of the traffic route surface. A typical example may have a long axis of 3 to 3.5 m, a short axis of 1 to 2 cm and a thickness of 0.5 to 1 mm.  
           [0032]    Preferably, the optical fibre sensor further comprises a semi-reflective element coupled to at least one end of the optical fibre. For a single, isolated sensor a semi-reflective end is used at either end of the sensor. However, more commonly a number of sensors are connected in series so that each individual sensor need have only one semi-reflective element. In this case, each semi-refl ctive elem nt acts as the first semi-reflective element for one sensor and also as the second semi-reflective element for the preceding sensor. The exception to this is the last sensor in a series, which requires an additional, terminal semi-reflective element.  
           [0033]    Suitably, the semi-reflective element is either a fibre optic X-coupler with on port mirrored, or a Bragg grating.  
           [0034]    In accordance with a second aspect of the present invention a traffic monitoring system comprises: at least one sensor station; and an interferometric interrogation system; wherein the at least one sensor station comprises at least one optical fibre sensor in accordance with the first aspect of the present invention, the at least one optical fibre sensor being deployable in a traffic route; wherein the interferometric interrogation system is adapted to respond to the variation in said at least one predetermined property produced in the at least one optical fibre sensor due to a force applied by a unit of traffic passing the at least one sensor station.  
           [0035]    This provides a low cost, reliable traffic monitoring system which can be highly multiplexed. Remote interrogation is possible so neither local electronics nor local electrical power are required.  
           [0036]    In preferred embodiments, the interferometric interrogation system comprises a reflectometric interferometric interrogation system, more preferably the interferometric interrogation system comprises a pulsed reflectometric interferometric interrogation system or architecture.  
           [0037]    In a system where time division multiplexing is used to distinguish individual sensors, reflectometric and particularly, pulsed reflectometric interferometry allow for a very efficient multiplexing architecture that can be used with distributed sensors.  
           [0038]    Alternatively, the interferometric interrogation system comprises a Rayleigh backscatter interferometric interrogation system, with a pulsed Rayleigh backscatter interferometric interrogation system being particularly preferred.  
           [0039]    A non-Rayleigh backscattering reflectometric system relies upon discrete reflectors between sensors. These are comparatively expensive components, which may add to the cost of the overall system. In contrast, Rayleigh backscattering relies on reflection of light from inhomogeneities in the optical fibre. This removes the need for discrete reflectors, reducing the overall cost of the system. However, the data collected from such a system requires more complex analysis than a reflectometric interrogation system.  
           [0040]    Preferably, the system comprises a plurality of sensor stations, wherein adjacent stations are connected together by a length of optical fibre.  
           [0041]    The length of optical fibre connecting adjacent sensor stations defines the optical path length between adjacent sensor stations. Commonly, the connecting optical fibre is extended, and as such the optical path length between adjacent sensor stations is substantially equal to their physical separation. However, the connecting optical fibre need not be fully extended, in which case the physical separation of adjacent sensor stations may be any distance up to that of the length of the optical fibre used to connect adjacent sensor stations.  
           [0042]    Conveniently, the length of optical fibre connecting adjacent sensor stations is between 100 m and 500 m.  
           [0043]    Preferably, each sensor station comprises a plurality of fibre optic sensors, more preferably, each sensor station comprises at least one fibre optic sensor per lane of the traffic route.  
           [0044]    Most preferably, each sensor station comprises at least two optical fibre sensors, separated from each other by a known distance, per lane of the traffic route. Separated pairs of sensors can be used to determine traffic speed.  
           [0045]    Suitably, the known distance is between 0.5 m and 5 m. The known distance refers to the physical separation of the fibre optic sensors and not to the optical path length of the optical fibre between each sensor.  
           [0046]    This provides a traffic monitoring system which can be employed to monitor traffic on any type of traffic route, from a single lane road, railway line, path, etc, to a multi-lane motorway. The sensor stations may be sited at intervals along the entire length of the traffic route or only on sections where traffic monitoring is crucial, for example at known congestion sites or accident blackspots.  
           [0047]    Considering the example of a highway as the traffic route, ensuring that each lane of the highway has at least one fibre optic sensor means that some traffic information can be collected irrespective of the part of the highway on which traffic is flowing. This simplest system for a single lane highway would have two sensors, one for each direction of traffic. Although this would give information regarding vehicle weight, traffic volume and axle count, it could not be used to give a measure of vehicle speed. Vehicle speed may however be determined by placing two sensors, separated by a known, short distance, per lane of the highway. It may be desirable to place more than two sensors per lane of the highway, for example three sensors placed in close proximity to each other may be used to give a measure of vehicle acceleration. Such a measurement may be of use at road junctions, roundabouts or traffic lights.  
           [0048]    Preferably, each sensor is deployed so that its longest dimension is substantially in the plane of the traffic route and substantially perpendicular to the direction of traffic flow on the traffic route.  
           [0049]    Preferably, the longest dimension of each sensor is substantially equal to the lane width of the traffic route.  
           [0050]    This helps to ensure that the passage of any vehicle on any part of the highway is registered by the system.  
           [0051]    Considering the example of a highway as the traffic route, in the UK the width of a lane of highway may range from around 2.5 m for a minor road up to around 3.65 m for a motorway. Other parts of the world may have road systems of differing lane widths.  
           [0052]    Preferably, each sensor is deployed beneath the surface of the traffic route.  
           [0053]    As an example, for deployment in an existing road, a thin channel or groove can be cut in the road to accommodate each sensor. The groove may then be refilled and the surface of the road made good again. Clearly, in the case of a new road the sensors can simply be incorporated into the structure of the road during construction.  
           [0054]    It is possible, but less preferred to deploy the sensors so that they are attached to the surface of the highway rather than embedded in it. This may be useful if the system is to be used for a short time in a particular location before being moved. Clearly, in this instance the sensors employed may need to be protected or be strong enough to be able to withstand the greater forces associated with vehicles passing directly over them.  
           [0055]    Preferably, the optical fibre sensor comprises a sensing fibre coupled to a dummy fibre; wherein the optical path length of the sensing fibre is such that the sensitivity of the sensor is low; and wherein the optical path length of the dummy fibre is greater than that of the sensing fibre such that the combined optical path length of the sensing fibre and the dummy fibre is sufficient to allow the sensor to be interrogated by an interferometric interrogation system, such as a pulsed interferometric interrogation system.  
           [0056]    Preferably, the optical path length of the dummy fibre is at least 2 times greater than that of the sensing fibre. However, it will be appreciated that the dummy fibre does not necessarily have to be at least twice the length of the sensing fibre. It could be longer or shorter as needed providing the sensor fibre plus dummy fibre length is long enough to be interrogated by the width of the smallest interrogation pulse generated by the switches in the pulse reflectometric architecture.  
           [0057]    The sensitivity of an optical fibre sensor is substantially proportional to the length of the optical fibre it contains. The length of the sensing section is preferably short in order to reduce the sensitivity of the sensor to a level where a reliable measurement of the large forces associated with vehicle traffic is possible. However, a short section of optical fibre cannot easily be interrogated using a pulsed interferrometric system. This is because the minimum pulse length is limited by optical switch performance. By using a dummy fibre, the total optical path length of the sensor is increased so that pulsed interferometric interrogation is made simpler.  
           [0058]    Preferably, the sensing fibre is substantially straight  
           [0059]    Preferably, the sensing fibre and the dummy fibre comprise sections of a single optical fibre. This simplifies the construction of the sensor. Alternatively, the sensing fibre and the dummy fibre may be spliced together or joined by any other suitable means.  
           [0060]    Preferably, the sensor further comprises a casing substantially surrounding at least one of the sensing fibre and the dummy fibre.  
           [0061]    In the case of the optical fibre sensor comprising a sensing section and a dummy section, if a semi-reflective element is included, then preferably that semi-reflective element is located on the dummy section of the optical fibre sensor.  
           [0062]    In accordance with a third aspect of the present invention, a method for monitoring traffic comprises providing a plurality of sensor stations on a traffic route; deploying a plurality of optical fibre sensors in accordance with the first aspect of the present invention at each sensor station; interfacing each optical fibre sensor to an interferometric interrogation system; employing time division multiplexing such that the interrogation system is adapted to monitor an output of each optical fibre sensor substantially simultaneously; and using the output of each optical fibre sensor to derive data relating to the traffic passing each sensor station.  
           [0063]    Preferably, the method further employs wavelength division multiplexing such that the number of optical fibre sensors which the interrogation system is adapted to monitor is increased.  
           [0064]    Preferably, the method further employs spatial division multiplexing such that the number of optical fibre sensors which the interrogation system is adapted to monitor is increased.  
           [0065]    Preferably, the data derived relates to at least one of vehicle speed, vehicle weight, traffic volume, axle separation and vehicle classification. The weight is determined by calculating the area under the axle response (phase change) curve. Amplitude and width (freq) of this curve are determined by the vehicle speed as well as the weight. Calibrated weights are calculated by multiplying the area under the curve by the speed times a scale factor, with the speed being determined by the peak signal separation time between each of the two sensors in a pair. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0066]    The invention will now be described by way of example only with reference to the following drawings in which:  
         [0067]    [0067]FIG. 1 shows example of a section of a traffic monitoring system according to an embodiment of the present invention in place on a two lane highway;  
         [0068]    [0068]FIG. 2 shows an extended section of a traffic monitoring system according to an embodiment of the present invention;  
         [0069]    [0069]FIG. 3 shows a single sensor station suitable for a traffic monitoring system according to an embodiment of the present invention in place on a six lane highway;  
         [0070]    [0070]FIG. 4 shows an example of an optical fibre sensor suitable for use in a road traffic monitoring system according to an embodiment of the present invention;  
         [0071]    [0071]FIGS. 5  a - d  show four further examples of optical fibre sensors suitable for use in a road traffic monitoring system according to an embodiment of the present invention;  
         [0072]    [0072]FIG. 6 shows a perspective view of an example of an optical fibre sensor suitable for use in a road traffic monitoring system;  
         [0073]    [0073]FIG. 7 shows a cross section of the sensor of FIG. 6 taken along the line A-A;  
         [0074]    [0074]FIG. 8 shows a cross section of an alternative shaped casing suitable for the sensor of FIG. 6.  
         [0075]    [0075]FIG. 9 shows a graphical representation of a typical response of a piezo electric sensor as a vehicle passes over it.  
         [0076]    [0076]FIG. 9 a  shows a schematic diagram of three sensors connected in series;  
         [0077]    [0077]FIG. 10 shows a schematic diagram of an interferometric interrogation system suitable for use in a traffic monitoring system according to an embodiment of the present invention.  
         [0078]    [0078]FIG. 11 shows a representation of the spatial arrangement of a set of sensor groups which may be interrogated by the system of FIG. 10;  
         [0079]    [0079]FIG. 12 shows the derivation of the optical signal timings for the set of sensor groups of FIG. 11;  
         [0080]    [0080]FIG. 13 shows a perspective view of a sensor of the type shown in FIG. 6, deployed beneath the surface of a highway;  
         [0081]    [0081]FIGS. 14 a - e , illustrates how a sensor may be deployed beneath the surface of a highway; and,  
         [0082]    [0082]FIGS. 15 a - b  show the signals recorded from a car and an HGV passing over a sensor of the type shown in FIG. 6; and  
         [0083]    [0083]FIGS. 16A to  16 K illustrate an optical fibre sensor suitable for use in a road traffic monitoring system according to a preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0084]    As mentioned previously, traffic may come in many different forms. For example, considering solely traffic on land, such traffic can take a variety of forms, including but not limited to vehicles on roads, bicycles on paths, trains on rails, people on paths, aircraft on runways, etc. For the purpose of illustrating embodiments of the present invention, traffic consisting of vehicles on a highway will be considered.  
         [0085]    [0085]FIG. 1 shows a section of a traffic monitoring system in place on a two lane highway  1 . Two sensor stations  2  are shown connected by a length of optical fibre  3 . In FIGS. 1 and 2 the optical fibre  3  is shown extended and hence the physical separation of the sensor stations, indicated by distance  4  is substantially equal to the optical path length of the optical fibre  3 . Optical fibre  3  need not be fully extended, in which case the physical separation of the sensor stations, distance  4 , may be less than the optical path length of the optical fibre  3 . A more extended section of the system showing five sensor stations  2  is shown in FIG. 2.  
         [0086]    Each sensor station  2  comprises four fibre optic sensors  5 , connected to one another in series and to optical fibre  3  by optical fibre  6 . At each sensor station  2  the sensors  5  are deployed in the highway  1  such that there are two sensors, separated as indicated by distance  7 , per lane of the highway. Arrows  8  represent the direction of travel of traffic on each lane of the highway. Each sensor is arranged such that its longest dimension is perpendicular to the direction of traffic flow  8 , and substantially equal to the width of a lane of the highway. This ensures that a vehicle passing a given sensor station  2  will elicit a response from at least one fibre optic sensor  5 , irrespective of its direction of travel or positioning on the lane of the highway. A knowledge of the physical separation of the sensors  7  within each sensor station allows a determination of vehicle speed to be made. All sensor stations are connected by optical fibre  3  to an interferometric interrogation system  9 .  
         [0087]    In FIG. 3 a single sensor station  2  is shown in place as part of a traffic monitoring system for a multi-lane highway  10  for example a motorway. In this case twelve sensors  5  are deployed in order to ensure that a vehicle passing the sensor station on any of the six lanes  11  of the highway elicits a response irrespective of its direction of travel  8  or its choice of lane  11 .  
         [0088]    A schematic illustration of a sensor design of embodiments of the present invention is shown in FIG. 4. The sensor  12  comprises a sensing fibre  13  and a dummy fibre  14 . In this example the dummy fibre is shown coiled inside a casing  15 . A semi-reflective element  16  is coupled to the dummy fibre. This arrangement allows a large length of dummy fibre to be contained in a small volume, thereby reducing the overall size of the sensor. Other arrangements are clearly possible, the dummy fibre may be wound on a reel or former or, if the overall size of the sensor is unimportant, simply left extended. In FIG. 4, a sheath  17  is shown around the sensing fibre  13 . This may be separate to, or integral with, the dummy fibre casing  15 . The sheath  17  serves to protect the sensing fibre from damage. It may for example, comprise a metal or a plastic. The cross sectional shape of the sheath is preferably chosen such that is provides the sensor with lateral rigidity. In FIG. 4, the sheath is merely illustrated conceptually. Further details of the sheath employed in embodiments of the present invention to protect and support the sensing optical fibre will be provided later with reference to FIGS. 5 and 16.  
         [0089]    It is possible, but less preferred, to omit either or both of the casing  15  and the sheath  17 . This reduces the cost and complexity of the sensor, but results in a less robust sensor which may be damaged easily.  
         [0090]    In use, the sensor is deployed in such a way that the sensing fibre  13  extends across the width of the highway lane to be interrogated. The force exerted by a vehicle passing over the sensing fibre produces a signal which can be detected by the interrogation system. The length of the sensing fibre, typically around 24 m, means that the sensitivity of the sensor is suitable for detecting the large forces associated with the passage of vehicles. The dummy fibre  14  is positioned such that it is not affected by the passage of vehicles. This may be achieved by arranging for the dummy fibre to be at the edge of the highway or between lanes of the highway. The packaging of the dummy fibre may be arranged to insulate the fibre from vibrations.  
         [0091]    More details of sensor designs of embodiments of the present invention are shown in FIG. 5. These designs may be used with or without the dummy fibre illustrated schematically in FIG. 4, as appropriate. This design of sensor is based around a thin strip  18 , which is commonly a metal strip. The optical fibre  19  is attached to the strip to form the sensor. In FIG. 5 a , the optical fibre is wound around two spindles  20  attached to each end of the strip. FIGS. 5 b ,  5   c  and  5   d  omit the spindles and have the fibre wound around the strip itself. The fibre may be wound longitudinally, FIG. 5 b , or helically around the short axis of the strip, FIGS. 5 c    5   d . In FIG. 5 d , small indents  21  are made into the edges of the strip  18 . These are useful in locating the optical fibre as it is wound. In each example, the fibre may be protected by applying a thin overlayer of epoxy or polyurethane (not shown). The use of a thin strip as a former provides sensors which are flexible. This enables them to adopt the camber of the highway, into which they are deployed and also allows them to be wound onto a drum for ease of storage and deployment. Clearly, modifications to the design of the sensors shown in FIG. 5 may be made without departing from the scope of the present invention. Indeed, a preferred implementation of the embodiment illustrated schematically in FIG. 5 a  , will be discussed later with reference to FIGS.  16 A and to  16 K. Semi-reflective elements have been omitted from FIG. 5 for clarity.  
         [0092]    A further example of a sensor  22  shown in FIGS. 6 and 7, comprises an optical fibre  23  which, instead of being wound onto an elongate plate, is wound round a steel bar  24  and placed into a casing  25 . In this example the optical fibre  23  is a 50 m length of double coated, high numerical aperture fibre with an outside diameter of 170 μm (FibreCore SM1500—6.4/80), although other lengths and specifications of optical fibre may equally be used. The steel bar  24  is a 3 m length of M12 threaded bar and the optical fibre is wound in co-operation with the thread. This makes it simple to wind the optical fibre evenly along the length of the bar. A 10 mm diameter unthreaded bar can be used in place of the M12 bar, although this makes it more difficult to ensure that the fibre is wound evenly. Alternatively, a more widely spaced, machined helical groove may be used instead of a thread. Clearly, the dimensions of the bar can be altered to provide a sensor of the appropriate size for a desired application. Furthermore, the bar need not comprises a metal bar, suitable alternative materials may include plastics, such as polyurethane and composite materials. A semi-reflective element  16  is coupled to one end of the fibre. If the sensor is to be used in isolation, or if it forms the terminal sensor in a series of sensors, then an additional semi-reflective element is coupled to the other end of the sensor.  
         [0093]    In order to reduce the sensitivity of the sensor so that it is suitable for detecting large forces and pressures, a compliant material  26  is provided intermediate the steel bar  24  and the casing  25 . This material is able to absorb the majority of any external force applied to the sensor. Unlike traditional optical fibre sensors where high sensitivity is often paramount, this sensor design is deliberately de-sensitised by choosing a compliant material which effectively absorbs the majority of any applied force. This means that a sensor comprising a highly compliant material, such as a grease, may be used to detect larger forces and pressures than would ordinarily be possible with existing optical fibre sensors. During manufacture, it is convenient to partially fill the casing  25  with the compliant material  26  and then place the bar  24  and optical fibre  23  on top. The bar is then overfilled with more of the compliant material. As shown in FIG. 7, this results in the bar being completely surrounded by the compliant material. An optional cap  27  may be provided to protect the sensor. This is useful if the compliant material  26  is chosen to be a soft material such as a grease. It may be possible to omit the cap  27 , if the compliant material is one which is designed to set, for example, an epoxy resin.  
         [0094]    The casing  25  is made from sheet steel, but can be made from any suitable material, such as aluminium, and is conveniently slightly longer than the steel bar  24 . FIGS. 6 and 7 show a casing with a substantially rectangular cross section. This shape adds lateral rigidity to the sensor and helps to eliminate a type of signal ambiguity which is often encountered with piezo-electric sensors. This signal ambiguity is illustrated in FIG. 9. The curve  28  of signal strength against time, represents a typical response due to a vehicle passing over a piezo-electric sensor. If consists of two peaks  29 ,  30 . The main peak  29  is produced as the vehicle passes directly over the sensor. It is this part of the signal which is of use. The second smaller peak  30 , produced prior to the main peak, is due to the surface of the road being pushed up by the weight of the vehicle as it travels along. This produces what is sometimes referred to as a bow wave which travels ahead of the vehicle. The lateral rigidity afforded by the box shaped cross section of the casing in the present example reduces the effect of the ‘bow wave’, giving a signal which is representative of a vehicle as it passes directly over the sensor.  
         [0095]    An alternatively shaped casing which also provides lateral rigidity and hence reduces the ‘bow wave’ effect is shown in FIG. 8.  
         [0096]    Other alternatively shaped casings may be used, for example the casing may comprise a cylindrical tube with an internal diameter slightly larger that the outer diameter of the bar  24 . In this case the annular void formed between the bar and the casing would be filled with a compliant material.  
         [0097]    In accordance with preferred embodiments of the present invention, rather than using an optical fibre sensor in which the optical fibre is wound around a cylindrical bar, an optical fibre sensor is instead employed of the type described earlier with reference to FIG. 5, in which the optical fibre is wound on an elongate plate. In preferred embodiments, this optical fibre sensor is constructed as shown in FIGS. 16A to  16 K.  
         [0098]    As shown in FIG. 16A, the optical fibre sensor has a former consisting of an elongate plate  100  upon which are located a number of guide members  110 , and a pair of termination plates  120 . The guide members  110  and termination plates  120  are merely illustrated schematically in FIG. 16A, with their preferred shape and configuration being discussed later with reference to FIGS. 16C to  16 G. In preferred embodiments, the elongate plate  100  has holes provided therein towards opposing ends of the elongate plate, and each termination plate has a corresponding hole provided through it, such that each termination plate is located towards a corresponding end of the elongate plate with the hole in the termination plate being aligned with the hole in the elongate plate. With regard to the guide members, these are spaced along the length of the elongate plate  100 , and serve to guide the optical fibre between the two termination plates. The exact number of guide members utilised is a matter of design choice, but in preferred embodiments the guide members are spaced equidistantly between the termination plates.  
         [0099]    Each termination plate  120  is configured such that it is arranged to receive a wheel  130 , each wheel having a hole therein which is aligned with the hole in the corresponding termination plate  120 . As will be described later with reference to FIGS. 16H and 16I, the wheel preferably includes a groove in its circumferential edge which is arranged to receive the optical fibre  140 .  
         [0100]    A pair of spindles  150  are provided, each being passed through the holes in a corresponding wheel  130 , termination plate  120 , and end of the elongate plate  100 . This spindle serves to locate the various elements in position, and also provides an axis about which the corresponding wheel  130  may rotate.  
         [0101]    In accordance with preferred embodiments of the present invention, an optical sensing fibre is passed up and down the length of the elongate plate  100  passing round the circumference of the relevant wheel  130  at the end of each traversal of the elongate plate. The optical fibre  140  is located within the guide members  110  as it traverses the elongate plate to ensure that the optical fibres pass along a predetermined path, preferably this path being along the central axis of the elongate plate. As will be discussed later, the shape of each termination plate  120  is such that it serves to guide the optical fibre from the central axis to the outer circumference of the corresponding wheel  130 , and then back towards the central axis of the elongate plate. By providing wheels which are free to rotate whilst the optical fibre is wound thereon, this enables the strain on the various lengths of optical fibre passing between the wheels to be equalised.  
         [0102]    Once the optical fibre has been wound between the wheels  130  as described above, then in preferred embodiments the optical fibre is then provided with a coating to both protect the optical fibre and/or desensitise it. In preferred embodiments, the coating is obtained by potting the optical fibre in a compliant potting compound in order to reduce the sensitivity of the optical fibre sensor. The compliant compound may be a highly compliant material, such as grease, or alternatively can be a material which is harder and designed to set, for example, an epoxy resin. In preferred embodiments, polyurethane is used as the compliant compound, which is applied as a liquid and then polymerised.  
         [0103]    In a preferred embodiments, during manufacture, the elongate plate  100  is placed within a channel to be used as the mould for the resin, preferably this channel being machined out of a metal bar. The termination plates  120  and guide members  110  are then positioned on the elongate plate, as are the wheels  130  and spindles  150 , after which the optical fibre  140  is wound between the wheels as described earlier. At this stage, the potting compound is then applied to the components of the optical fibre sensor present in the channel, for example by pouring the potting compound into the channel in the example of an epoxy resin or polyurethane. Typically the potting compound is applied to a level where it will form a flat upper surface for the optical fibre sensor.  
         [0104]    Depending on the choice of potting compound, the potting compound itself may be hard enough once set to provide sufficient protection for the optical fibre sensor. However, in preferred embodiments, a second elongate plate  160  is located on top of the potting compound to form an upper surface of the optical fibre sensor. In preferred embodiments, this elongate plate  160  has two holes provided therein to enable the elongate plate to be located on the spindles  150 . This arrangement not only provides additional protection for the optical fibre sandwiched between the two elongate plates, but also provides the sensor with symmetry, such that the optical fibre passes generally through the centre of the optical fibre sensor.  
         [0105]    When providing the sensor with a second elongate plate, this is preferably applied during manufacture prior to setting of the compliant potting compound, and serves to form a composite “sandwich” with the fibre in the middle suspended in potting compound between the two elongate plates  100 ,  160 . This composite structure is then compressed while the potting compound (e.g. Polyurethane) sets, preferably by attaching a lid to the mould, which then serves as a compression jig. Once cured, the composite structure is removed from the compression jig and is ready for use.  
         [0106]    [0106]FIG. 16B is an illustration of the optical fibre sensor of FIG. 16A from a top plan view, with the second elongate plate  160  omitted. As can be seen, termination plates  120  are provided at each end of the elongate plate  100  and are arranged to accommodate respective wheels  130 . The optical fibre is then passed up and down the length of the elongate plate  100 , at each end passing around the circumference of the wheel  130  within a groove provided in the circumferential edge of the wheel  130 . The termination plates  120  then serve to guide the optical fibre  140  back towards the central axis of the elongate plate  100 , with further guide members  110  being positioned along the length of the elongate plate to guide the optical fibre  140  along the central axis.  
         [0107]    [0107]FIGS. 16J and 16K provide details of dimensions of the elongate plate in accordance with preferred embodiments, FIG. 16J providing a plan view and FIG. 16K providing a side view. In preferred embodiments, the elongate plate is formed of a metal strip, for example steel, brass, tin alloys, aluminium alloys, etc. Alternatively, the elongate plate comprises a non-metal strip, for example nylon, polyurethane, etc. As can be seen from FIG. 16J, the elongate plate of preferred embodiments is 3.3 m long with two holes being machined therein 15 mm from each end. In preferred embodiments, the elongate plate is 10 mm wide and 0.5 mm thick.  
         [0108]    [0108]FIG. 16C illustrates a plan view of the guide member of preferred embodiments, whilst FIG. 16D provides an end view of the preferred guide member. As shown in FIG. 16C, the guide member preferably comprises two raised portions  200  raised above a lower surface  230 , each raised portion  200  being provided with a curved edge  210  at each end to serve to align the optical fibre with a groove  240  provided along the length of the guiding member. In preferred embodiments, the guide member is 20 mm long, 10 mm wide, and 2.5 mm deep, with the raised portions  200  being raised 1.5 mm above the lower surface  230 .  
         [0109]    [0109]FIG. 16E illustrates a top plan view of the termination plate  120  of preferred embodiments, whilst FIG. 16F provides a corresponding side view and FIG. 16G provides a corresponding end view. As with the guide members  110 , the termination plate has a base  300  with a number of raised portions  310  being provided thereon. A hole  320  is provided within the base  300  to align with the corresponding hole in the elongate plate  100 , and arranged to receive a corresponding spindle  150 . Each of the raised portions  310  is provided with a shaped edge  330 ,  340 , which serves to guide the optical fibre between the central path  240  and the circumference of a wheel  130  which is centred around the hole  320 . A number of dimensions are illustrated on the drawing, all of these dimensions being expressed in millimetres. However, in essence, the termination plate is preferably 40 mm long, 10 mm wide and 2.5 mm deep, with the raised portions  310  being 1.5 mm above the base  300 .  
         [0110]    [0110]FIG. 16H illustrates the wheel of preferred embodiments which is located within the recess  300  of a corresponding termination plate, whilst FIG. 16I illustrates an end view of that wheel. As can be seen from FIG. 16H, the wheel preferably has a diameter of 10 mm, with a circumferential groove  410  of approximately 0.1 mm depth being provided within the circumferential edge. Through the centre of the wheel, a hole  400  is drilled which has the same dimensions as the hole drilled through the base  300  of the termination plate, and again allows the spindle to pass therethrough.  
         [0111]    In preferred embodiments, approximately 24 m of high NA fibre is laid along the length of the elongate plate  100  and bent around the 8 mm diameter groove of the wheels  130 , thus accommodating approximately 6.5 passes of the fibre along the length of the elongate plate. A reinforced cable and semi-reflective coupler is in preferred embodiments spliced to the optical fibre in a know manner, and potted at one end of the elongate plate, while at the other end the optical fibre is spliced in a known manner into a reinforced cable before being potted.  
         [0112]    It will be appreciated that the various example dimensions provided above when describing FIGS. 16A to  16 K are merely provided for sake of illustration and could readily be altered without departing from the scope of the present invention.  
         [0113]    The sensor design illustrated in FIGS. 16A to  16 K has been found to offer a number of technical advantages over the sensor design illustrated earlier with reference to FIG. 6. Firstly, the overall sensor has a depth of approximately 5 mm, which allows a significantly shallower groove to be cut in the surface of the traffic route, and simplifies the positioning of the sensor accurately below the road surface. Furthermore the flexibility of the elongate plates  100 ,  160  ensures that the sensor is flexible enough to allow it to conform to the contours of the traffic route, for example the camber of a road, and accordingly this design avoids some of the rigidity problems of the design of FIG. 6. Furthermore, whilst the FIG. 6 design did reduce the effect of the “bow wave” due to its lateral rigidity, the design of FIGS. 16A to  16 K offers a significantly increased lateral rigidity which significantly further reduces the relative “bow wave” response of the sensor. This is because the horizontal stiffness of the strip is much higher than the vertical stiffness.  
         [0114]    It has been found that the sensor design of FIGS. 16A to  16 K has all the advantages of size, flexibility and cross axis sensitivity rejection of the best conventional piezoelectric Weigh In Motion (WIM) sensors coupled with the advantages of using a fibre optic sensor. These include the ability to multiplex many sensors together on a single fibre, the ability to interrogate sensors over very large distances, and the increase in reliability due to the removal of all electrical components for the sensor.  
         [0115]    In FIG. 9 a , three sensors  12 ,  12 ′ and  12 ″ are shown connected in series. In preferred embodiments, each sensor is constructed as shown in FIGS. 16A to  16 K. However, any of the other described sensor designs may also be employed. Sensors  12  and  12 ′ each have one semi-reflective element  16  and  16 ″ respectively, coupled to the optical fibre  13 . In use, sensor  12  employs both semi-reflective elements  16  and  16 ′. Similarly, sensor  12 ′ is defined by semi-reflective elements  16 ′ and  16 ″. Sensor  12 ″ is a terminal sensor, hence it has two semi-reflective elements coupled to the fibre  16 ″ and  16 ′″.  
         [0116]    [0116]FIG. 10 shows an example of an interferometric interrogation system. The architecture of FIG. 10 is based upon a reflectometric time division multiplexed architecture incorporating some additional wavelength and spatial division multiplexing. The light from n lasers  31 , for example n distributed feedback (DFB) semiconductor lasers or DFB fibre lasers, is combined using a dense wavelength division multiplexer (DWDM)  32  before passing through an interferometer  33 . The interferometer  33  comprises two acousto-optic modulators (AOM) which are also known as Bragg cells  34  and a delay coil  35 . Pulses of slightly different frequency drive the Bragg cells  34  so that the light pulses diffracted also have this frequency difference. The output from the interferometer is in the form of two separate interrogation pulses. These are amplified by an erbium doped fibre amplifier (EDFA)  36 , and then separated into n different fibres  37  by a second DWDM  38 . Each fibre  37  feeds into a 1×N coupler  39 . Each coupler  39  splits the input into N fibres  40 . In FIG. 10 each coupler  39  is shown as having four output fibres  40  that is N=4. N may be greater or less than this as required. It is also not necessary that all 1×N couplers  39  have the same value for N. Each fibre  40  terminates in a sensor, a group of sensors or a number of groups of sensors  41 . It is clear that the number of individual sensors which can be interrogated by the architecture of FIG. 8 may be large. A typical system may have n=8 and N=4 with 5 groups of 8 sensors connected to each output fibre  40 . This provides a system where 1280 individual sensors may be interrogated. The maximum number of sensors is limited by the optical power budget, but may be up to several thousand or more.  
         [0117]    The return light from the sensors is passed to individual photo-receivers  42  via return fibres  43 . The photo-receivers can incorporate an additional polarisation diversity receiver which is used to overcome the problem of low frequency signal fluctuations caused by polarisation fading. This is a problem common to reflectometric time division architecture. Electrical signals are carried from the photo-receivers to a computer  44  which incorporates an analogue to digital converter  45 , a digital demultiplexer  46 , a digital demodulator  47  and a timing card  48 . After digital signal processing within the computer the signal may be extracted as formatted data for display or storage or converted back to an electrical signal via a digital to analogue converter (not shown).  
         [0118]    The success of the architecture of FIG. 10 is critically dependent upon the correct timing of the optical signals. This is achieved by using specific lengths of optical fibre within each sensor, between each sensor within a group of sensors and between each group of sensors. An example arrangement is shown in FIG. 11, where five groups  49  of sensors, each group comprising eight individual sensors  50 , are shown separated by a distance of 1 km. Each sensor  50  comprises a total of 50 m of optical fibre so each group  49  has an optical path length of 400 m.  
         [0119]    On first inspection it may seem to be necessary to deploy groups of sensors at exactly known and measured intervals for example every 1 km. This is not the case as delay coils may be used to allow sensor groups to be deployed closer together. If a sensor group cannot be deployed within a set distance then a dummy sensor group consisting of a 400 m coil of fibre could be used and the next group of sensors then deployed on the carriageway. Altering the timing of the interrogation pulses will also allow for various group spacings, for example 500 m, 1 km, 5 km as required.  
         [0120]    Using the specific fibre lengths defined in FIG. 11, it is possible to define the optical signal timings. This is shown in FIG. 12. This allows that a sampling rate of approximately 41 kHz should be possible for each group of sensors. This results in a high dynamic range over a measurement bandwidth of several kHz at each sensor.  
         [0121]    The pulse train to the sensors consists of a series of pulse pairs, where the pulses are of slightly different frequencies. At each end of each sensor is a semi-reflector. The pulse separation between the pulses is such that it is equal to the two-way transit time of the light through the fibre between these semi-reflectors. When these semi-reflectors reflect pulse pairs, the reflection of the second pulse overlaps in time with the reflection from the first pulse from the next semi-reflector along the fibre. The pulse train reflected from the sensor array consists of a series of pulses each containing a carrier signal being the difference frequency between the two optical frequencies. The detection process at the photodiode results in a series of time-division-multiplexed (TDM) heterodyne pulses, each of which corresponds to a particular sensor in the array. When a pressure signal impinges on a sensor it causes a phase modulation of the carrier in the reflected pulse corresponding to that sensor.  
         [0122]    To implement the scheme of FIGS. 11 and 12 there is a requirement to generate accurate timing pulses as well as a reasonably sophisticated demultiplexing and demodulation process. By using a computer equipped with analogue to digital converters and able to perform digital signal processing, it is possible to do all of the necessary processing in the digital domain. This improves bandwidth and dynamic range when compared to more conventional analogue approaches.  
         [0123]    [0123]FIGS. 13 and 14 show one example of how sensors may be deployed beneath the surface of a highway. Whilst FIGS. 13 and 14 show the sensor of FIGS. 6 and 7, it will be appreciated that the same basic deployment technique can also be used for the sensor designs of FIGS. 5 and 16. A slot or groove  51  is cut into the surface of a highway  52  using a disk cutter. The groove, which is usually slightly longer than the sensor, includes a thinner section  53  used as a channel to accommodate a lead out optical fibre  54 . FIG. 13 shows only a lead out groove from one end of the sensor, clearly a similar groove would be cut at the other end of the sensor to enable two sensors to be connected together. Stand off blocks  55  are placed at intervals along the base of the groove, suitably every 0.5 m or so. The sensor  56  is then deployed on top of the stand off blocks  55 . The stand off blocks ensure that the sensor is not directly in contact with the base of the groove thereby helping to insulate it from vibrations. Once the sensor is in place, a potting resin  57  is poured into the groove so that the sensor is completely encapsulated. The stand off blocks allow the potting resin to flow beneath the sensor. Preferably, the groove is slightly overfilled with potting resin as shown in FIG. 14 d . After a final operation to grind the surface of the resin flush with the surface of the highway, the sensor is suitable for use.  
         [0124]    When deploying “strip” sensors of the type illustrated in FIGS. 5 and 16, it may be more appropriate to use clips that would support the sensors flush with or just below the traffic route surface, instead of using stand off blocks  55 , since this is envisaged to be a better and easier solution than using stand off blocks.  
       EXAMPLE 1  
       [0125]    A single sensor of the type shown in FIG. 6, was deployed in a highway as described in FIGS. 13 and 14. FIG. 15 a  shows the response of the sensor as a car is driven over it at three different speeds, 15 mph, 30 mph and 55 mph shown by data curves  58 ,  59  and  60  respectively. Each curve comprises two peaks which correspond to the two axles of the car. The distance between the peaks is representative of the axle separation and the axle weight can be derived as a function of the integrated area bounded by each peak and the vehicle speed. In this example the vehicle weight can be derived as the speed of the vehicle is known. As described previously, at least two sensors, separated by a known distance, are required to measure the speed of a passing vehicle.  
       EXAMPLE 2  
       [0126]    [0126]FIG. 15 b  shows the data collected as an anticulated vehicle was driven over the sensor used in example 1 above. Data curves  61  and  62  represent a laden vehicle and an unladen vehicle respectively. Each curve comprises four peaks, corresponding to the four axles of the vehicle. Again the axle weight is derived from a knowledge of the vehicle speed and the area bounded by the peaks. In this example, however, as the speed of the vehicle was the same for both the laden test and the unladen test, the numerical difference between the areas bounded by the peaks gives a direct indication of the weight difference of the vehicle. This weight difference is equivalent to the weight of the load carried by the vehicle.  
         [0127]    Although a particular embodiment of the invention has been described herein, it will be apparent that the invention is not limited thereto, and that many modifications and additions may be made within the scope of the invention. For example, various combinations of the features of the following dependent claims could be made with the features of the independent claims without departing from the scope of the present invention.