EP2128837A1

EP2128837A1 - Device for sensing at least one property of a surface-bound vehicle

Info

Publication number: EP2128837A1
Application number: EP08009965A
Authority: EP
Inventors: Reinhold Pieper; Richard Hunter Brown
Original assignee: Meas Deutschland GmbH
Current assignee: TE Connectivity Sensors Germany GmbH
Priority date: 2008-05-30
Filing date: 2008-05-30
Publication date: 2009-12-02
Also published as: WO2009144029A1

Abstract

Device for sensing at least one property of a surface-bound vehicle comprising:
- a pressure sensitive sensor with at least one sensor-element, whereby said pressure sensitive sensor is designed to produce a signal based on a pressure induced change of a property of the sensor-element, and
- a magnetic sensor that is designed to detect local changes in the earth's magnetic field and/or the magnetic field generated by the vehicle.

Description

Field of the Invention

The invention relates to a device for sensing at least one property of a surface-bound vehicle. The invention especially relates to the detection and classification of road vehicles, including detection of vehicle presence, speed, length, weight, and inter-vehicle separation.

Backround of the Invention

As road usage increases, there is a growing need to monitor road traffic and collect data relating to road usage. There is also a need to enforce local or national laws regarding maximum limits of speed, weight or vehicle separation. Many different sensor technologies already exist for detecting the passage of road traffic, with some being particularly suited to detecting presence, others to detecting speed, and others to detecting axle weight.
Inductive loops are well known as vehicle detectors (see for example US 3,164,802 (Kleist et al )). Such a detector is formed by embedding a few turns of wire in a large planar loop, typically around 2 x 2 m, into slots cut into the roadway. As a vehicle passes over the loop, the inductance of the loop is altered. The loop usually forms one part of a network that is set into electrical oscillation, so that the inductance change causes a change in oscillation frequency. In the case of a simple large square loop, the detection range is around 2/3 of the edge dimension, and so the entire chassis (even of trucks) influences the inductance of the loop. Smaller loops can be configured so that individual axles can be detected, although these may then not have sufficient detection range to detect the chassis of a high vehicle, and therefore may not detect accurately the true length of such a vehicle.
A conventional inductive loop requires generally at least 4 slots for the main perimeter of the loop, plus usually 4 diagonal cuts at the corners of the main loop to avoid 90 degree bends in the conductors, plus at least one additional lead-in slot to allow the conductors to be brought from the detecting loop to roadside conduit. Although the slots cut for loop installation are generally narrower than those required for an axle sensor, the time actually required to mark out and cut the many loop slots is significant. After laying the turns of wire into the slots, the slots must be sealed with some form of sealant material. Any movement of the wire within the slots can create false signals. Ingress of water can change the inductance of the loop. Each slot cut introduces a degree of damage to the road surface and increases the vulnerability of the surface to erosion by frost, snowploughs, and traffic. Therefore it is advantageous to introduce the minimum number of slot cuts into the road surface.
Vertically-oriented "blade" loops, such as described in US 6, 417,784 (Hilliard et al ), are also known. In this reference, some effort has been made to extract wheel or axle count from the detection system, rather than simply detecting a single vehicle.
Magnetic sensors are also well known as vehicle detectors. Devices such as flux-gate magnetometers and magnetoresistive (MR) sensors have been described (see US 3,249,915 (Koerner ), US 5,491,475 (Rouse et al )).
Axle sensors based upon the piezoelectric principle ( US 4,712,423 (Siffert et al )) or using fiber-optic sensing ( US 5,926,584 (Motzko et al )) generate output pulses corresponding to pressure applied by the tire as it traverses the sensor, which is normally deployed across the full lane width, transverse to the direction of traffic flow.

Summary of the invention

Given this background, the problem to be solved by the invention is to propose a device for sensing at least one property of a surface-bound vehicle that is more compact and potentially allows for sensing a variety of properties of a surface-bound vehicle.
This problem is solved by claim 1 and 16. Preferred embodiments are described in the sub-ordinate claims 2 to 15.
The basic concept of the invention is to make use of magnetic sensors which allow the sensing of non-weight related measurements, such as vehicle classifications, in combination with a pressure sensitive sensor that can - in a preferred embodiment - be used as a trigger or point of reference for the evaluation of the signal from the magnetic sensor.
The device according to the invention can be used for sensing properties of any kind of surface-bound vehicle. In an especially preferred embodiment, the device according to the invention is used to sensor at least one property of a car or lorry. Further examples of use (without limitation) are the sensing of properties of planes on an airstrip, railway carriages or even bicycles.
According to the invention, the device has at least one pressure sensitive sensor and at least one magnetic sensor. In preferred embodiments, the device has a plurality of such sensors.
A pressure sensitive sensor according to the invention has at least one sensor element, whereby said pressure sensitive sensor is designed to produce a signal based on a pressure induced change of a property of the sensor element. Possible embodiments of such pressure sensitive sensors are known in the art. In many cases, they are designed as sensors using the piezoelectric principle. The passage of a tire of a surface-bound vehicle applies pressure to such a sensor and generates a corresponding electrical charge or voltage signal. In many cases, these piezoelectric elements are embedded in a casing, e.g. embedded in the tarmac of the road. The pressure that is to be sensed by the piezoelectric element is often transferred by a resin that is situated between the actual sensor element and the top surface, which is contacted by the vehicle.
Other pressure sensitive sensors can be based on a fibre-optic sensor element. Fibre-optic sensors are often designed in the form of a loop, such that light sent into one end of the fibre is attenuated or otherwise modulated by the pressure of the tire as it crosses transversely to the sensor. In such designs the modulated light is detected at the other end of the fibre.
When two axle sensors are deployed at a fixed and known distance apart, the fast pulses from each sensor can act as "start" and "stop" triggers for a speed measurement. Having short pulses with fast risetime gives good accuracy. Alternate methods of calculating speed from two axle sensors include calculating a cross-correlation function and finding the time delay that gives peak value in the result - here again, short pulses with fast risetimes give the greatest accuracy. The axle sensors may also provide amplitude information relating to the peak force applied, which in turn allows the weight of each axle to be calculated as it crosses the sensor. Utilizing the signals from two (or more) such sensors allows an average to be taken, which helps to minimize random errors or errors due to oscillation in the instantaneous weight due to bouncing of the vehicle suspension.
Pressure sensitive sensors can provide a very short pulse as a tire footprint crosses the sensor. Having short pulses with fast risetime gives a good accuracy. Such pressure sensitive sensors may provide amplitude information relating to the peak force applied, which in turn allows the weight of each axle to be calculated as it crosses the sensor.
Magnetic sensors may detect either the local changes in the Earth's field caused by the passage of a vehicle, or the magnetic field generated by the vehicle itself (i.e. engine and components), or a combination of both of these effects. Generally, magnetic sensors as understood herein are capable of detecting a magnetic field change in the direction of at least one magnetic axis, preferably in two or three orthogonal magnetic axis. Magnetic sensors as understood herein can be based on a variety of detection principles, including but not limited to the Hall effect, magnetoresistive effects (AMR, GMR, CMR, TMR), or may be designed as fluxgates. Magnetic sensors can be "point" sensors - they have relatively small geometry, and therefore may give signals that vary according to the relative position of the passing vehicle within the lane. Therefore it would be advantageous to deploy a number of such point sensors to form a "line array" in order to provide more uniform detection capability.
The number of nodes (magnetic sensors) in an array of magnetic sensors may be varied at will, for example to cater for differing lane widths or to improve detection accuracy. The magnetic sensor array may for example have three 3-axis magnetic sensors, or may for example have five such sensors. It may be advantageous to provide more sensor nodes, in order to improve detection of vehicles such as motorbikes with relatively weak magnetic impact, to make the magnetic sensing less dependent upon the exact location of the vehicle within the lane width, and/or to improve the rejection of false triggers cause by vehicle passage in adjacent lanes. It should be noted that the spacing of the sensor nodes need not be uniform, and also that the relative signal contribution from each node may be weighted according to its position within the array. For example, - in arrangements where the magnetic sensors are in specific arrangement to the lane-markings of a road - it may be advantageous to have the contribution from nodes near the centerline of the lane biased stronger than the nodes further out towards the lane boundaries. This mechanical arrangement allows easy installation, and the number and spacing of the individual nodes may be varied in accordance with the target lane width. It may be advantageous, for example, to offer an array with 3 nodes spaced at 0.75 m apart for use in a lane width of 3.0 m, but 4 nodes spaced at 0.75 m apart for a lane width of 3.5 m. Since commonly-supplied axle detectors are offered with sensing lengths from 1.8 to 4.5 m in many increments, the ability to fabricate arrays with a similar range of total length and selectable node density is most convenient and advantageous. The mechanical arrangement allows for the nodes to be interconnected.
If the device according to the invention is provided with two magnetic sensor arrays, these two magnetic sensor arrays can, but need not be identical magnetic sensor arrays.
Specific advantages are achieved with a preferred embodiment, wherein the pressure sensitive sensor and the magnet sensor are arranged in proximity to one another. The invention is however not limited to placing pressure sensitive sensor and the magnetic sensor in immediate vicinity to one onother. The pressure sensitive sensor and the magnetic sensor may even be distanced apart as far as 10 m or even more. Depending on the property to be sensed, the upper-limit to a distance between the pressure sensitive sensor and the magnetic sensor may be that distance where the vehicles are not expected to change their speed of travel. On straight road conditions, these distances may be substantial. Especially preferred, the pressure sensitive sensor and the magnetic sensor are distanced apart by not more than 2,5 m, especially preferred by not more than 1 m. In a preferred embodiment, however the pressure sensitive sensor and the magnetic sensor are co-located in the direction of travel of the vehicle, for example with the magnetic sensor or the array of magnetic sensors lying directly under a pressure sensitive sensor.
In a preferred embodiment, the device has a basic body. The basic body of the device can be a section of the road and can, e.g., be made of tarmac or concrete. The basic body does not necessarily have to be separate from a road and is used within this description simply for the purpose of reference to elements that may surround the pressure sensitive sensor and the magnetic sensor. If so desired, the basic body may, however, be a clearly defined body, for example in cases where the pressure sensitive sensor and the magnetic sensor are to be arranged in very specific correlation to one another and where this arrangement is best achieved in specific manufacturing sites. In these cases, the pressure sensitive sensor and the magnetic sensor will be arranged in a basic body and will be transported with the basic body to the location of final installation, where the basic body is placed into or onto the surface. Such a device with a transportable basic body may also be used in situations, where the road is of non-tarmac-like or non-concrete-like substance, e.g. a gravel road. Also, in specific applications, e.g. for the use in sensing the properties of railway carriages, it might become necessary to provide the device with a specific design of a basic body.
In a preferred embodiment, the device according to the invention has a basic body and at least one slot in the basic body, whereby the pressure sensitive sensor is located in the slot. Generally, piezoelectric elements or fibre-optic sensor used in road-applications of devices for sensing at least one property of a surface-bound vehicle are placed into at least one slot that is introduced into the road. To build up a device according to the invention, one can make use of a pressure sensitive sensor already located in the road. To build the device according to the invention, one can simply introduce a further slot into the road and locate the magnetic sensor in this further slot. One can even make use of the one slot that is already provided for the pressure sensitive sensor and can locate the magnetic sensor in the same slot as the pressure sensitive sensor. This provides advantage, because it reduces the number of slots that have to be introduced into the road. Possibly, the slot will be made deeper than as is done today for today's placement of pressure sensitive sensors. Where today pressure sensitive sensors are placed a few mm below the surfaces, for example 9 mm below the surfaces, the slot according to this preferred embodiment of the invention can be made deeper such that the magnetic sensor is placed below a pressure sensitive sensor. For example, the distance between the top of the magnetic sensor and the bottom of the pressure sensitive sensor could be around 25 mm. A resin or a different material can be placed between the magnetic sensor and the pressure sensitive sensor. This material is preferably smooth and continuous in order to allow good operation of the pressure sensitive sensor.
In a preferred embodiment, a slot with a pressure sensitive sensor located therein is filled with resin or other comparable material to close the slot. The means for closing such slots are, however, well known in the art.
In a preferred embodiment, the device has an array of magnetic sensors. Using an array of magnetic sensors increases the area of coverage of the device.
In a preferred embodiment, the device according to the invention has a basic body and a first slot in the basic body and a second slot in the basic body, whereby a first pressure sensitive sensor is located in the first slot and a second pressure sensitive sensor is located in the second slot. Using a first pressure sensitive sensor and a second pressure sensitive sensor allows these sensors to act as "start" and "stop" triggers for a speed measurement. For this purpose, the two pressure sensitive sensors can be deployed at fixed and known distances apart. It is also possible to use signals from two pressure sensitive sensors in order to take an average, e.g. of a weight measurement, which helps to minimize random error due to oscillation in the instantaneous weight due to bouncing of the vehicle suspension.
In a preferred embodiment, a magnetic sensor is located in the first slot and/or a magnetic sensor is located in the second slot. Using two magnetic sensors or two arrays of magnetic sensors allows for the signals from the magnetic sensors to be summed (or otherwise compared) in order to provide an improved signal to noise ratio, for example by providing better rejection of "common mode" signals (signals which affect both arrays quasi-simultaneously, such as significant magnetic disturbances occurring at some distance perpendicular to the arrays).
The electrical arrangement may be that each sensor node is individually brought to the remote detection system using multi-conductor cables, but preferably the sensor modes may be connected together to provide just a single signal output representing some combination of the individual signals. In the case where each sensor node provides an output representing the RMS or sum of 3 individual magnetic axes, configured in the form of a current output, then the several nodes within the array may simply be wired together so that the final output is the linear sum of the individual currents. Each of the sensor nodes may also provide an output current that is pre-scaled according to its position within the array so that the final sum has weighted contributions from individual nodes.
An alternative electrical arrangement is one where the sensor nodes are under digital control. For example, the sensor nodes can report the instantaneous value of their magnetic signals on command from a trigger signal, either as an analog or a digital value. In this case, the interconnecting cables and the extended feeder cable should be understood to represent a communication bus.
In a preferred embodiment, the magnetic sensor is a three-axis-magnetometer.
The device according to the invention finds use in a road with such a device. In embodiments, where an array of magnetic sensors is used, the array preferably is arranged perpendicular to the general direction of travel on that road. In a preferred embodiment, the array is arranged to cover only half of a lane on the road. Such arrays would then only receive pressure from only one tire or group of tires per axle. Depending on what type of information is to be gathered, it might be sufficient to only obtain information on one tire or group of tires per axle.

Detailed description of the Figures

Hereafter, the invention is described making use of drawings that only show preferred embodiments of the invention and do not limit the invention to what is shown in these Figures or described with reference to these Figures.

Fig. 1: outline plan view of an installation according to the prior art using two axle detectors and an inductive loop, showing eight saw cuts required for the loop;
Fig. 2: outline plan view of an embodiment of the current invention, showing three 3-axis magnetic sensor nodes mounted within the same slot as the second axle detector of a pair of axle detectors, with feeder cables for both the magnetic sensor array and the 2^nd axle detector brought out along same channel;
Fig. 3: outline plan view of an alternative embodiment of the current invention, similar to Fig. 2, but here deploying two identical magnetic sensor arrays, one in each of the two slots of the two axle detectors;
Fig. 4: outline plan view of another alternative embodiment of the current invention, similar to Fig. 2, but where the single magnetic sensor array contains five rather then 3 nodes;
Fig. 5: a view of the proposed linear array according to the invention, containing three 3-axis magnetic sensor nodes, linked together with interconnecting cable and communicating with a remote detector box (system) by means of a single extended feeder cable;
Fig. 6a: a cross-sectional view of a "prior art" installation of a piezoelectric axle detector, located within spring clips pushed into a slot cut into the pavement surface,
Fig. 6b: an installation similar to the one shown in Fig. 6a but with a deeper slot, with the magnetic sensor nodes according to the invention mounted below the level of the axle detector and
Fig. 7: an outline elevation view of the installation of Fig. 6a, 6b, also showing the deeper slot region cut to accept the additional magnetic sensor array according to the invention.

Fig 1 shows a typical conventional installation where two axle detectors 1, 2 are spaced apart at a fixed and known distance apart (for example, 2.4 m), with a substantially rectangular inductive loop 3 (for example, 2 x 2 m) formed using 3 or 4 turns of wire lying between the two axle detectors. Because it is not recommended to have the wire bent at 90 degrees at the intersection of perpendicular saw cuts, it is standard practice to "break the corners" with diagonal cuts, so that the final shape of the installed loop is actually octagonal. At least 8 separate saw cuts have to be marked out and cut in order to install just the loop itself, with a further two cuts for the axle detectors. As can be seen from Fig. 1, installations like this typically are installed in a specific relationship to markings 4 made on the surface of the road. These markings 4 define a lane, along which the traffic is to travel in the traveling direction depicted with the arrow A. The installation is arranged in such a relation to the lane-markings 4 that ensures that traffic traveling along the lane can be detected.
Fig 2 shows how an embodiment of the present invention. Fig. 2 shows a device for sensing at least one property of a surface-bound vehicle. The device has two pressure sensitive sensors designed as axle detectors 10, 11. The axle detectors 10, 11 each may have a plurality of sensor-elements arranged in an array or be formed in continuous fashion from extrduded or wound piezoelectric material, or using continuous fiber-optic cable. The axle detectors 10, 11 are designed to produce a signal based on a pressure induced change of a property of the sensor-element, in this case the signal created by the respective piezoelectric element as pressure is exerted on this piezoelectric element.
The device shown in Fig. 2 furthermore has a magnetic sensor made up of a magnetic sensor array 12 made up of three 3-axis magnetic sensors 13, 14, 15. The magnetic sensor is designed to detect local changes in the earth's magnetic field and/or the magnetic field generated by the vehicle. As with the conventional installation shown in Fig. 1, the device shown in Fig. 2 is arranged in a specific relationship relative to the lane-markings 4.
The device shown in Fig. 2 is installed using just two saw cuts. In this example, a linear array 12 of three 3-axis magnetic sensors 13, 14, 15 is embedded in the roadway beneath the second (downstream) axle detector 10. It should be noted that the array might also be installed under the first axle detector 11. In this case, the timing information from the pair of axle detectors 10,11 "lags" the magnetic signal, whereas with the magnetic sensor array 12 installed under the second axle detector 10, the timing information from the axle detectors 10,11 "leads" the magnetic signal. With co-located magnetic sensor array 12 and axle detector 10, it is perfectly practical to lead the feeder cables of both sensor types out of the same slot. The slot may need to be cut somewhat deeper to accommodate the additional cabling, but this is not difficult to achieve.
Fig 3 shows a further embodiment of the current invention, where two similar or identical magnetic sensor arrays 12, 22 are deployed, each lying directly under an axle detector 10, 11. The magnetic sensor array 22 has three 3-axis magnetic sensors 23, 24, 25. In this case, the signals from the two magnetic sensor arrays 12, 22 may be combined, or compared, in order to improve signal/noise ratio by providing better rejection of "common mode" signals (signals which affect both arrays quasi-simultaneously, such as significant magnetic disturbances occurring at some distance perpendicular to the arrays).
Fig 4 shows a device according to the invention with a denser array of sensor nodes ( magnetic sensors 13, 14, 15, 16, 17) within a single magnetic sensor linear array 18. It should be noted that the spacing of the sensor nodes need not be uniform as shown.
Fig 5 shows a proposed embodiment where individual 3-axis magnetic sensor nodes 13, 14, 15 are housed within sealed housings, linked with interconnecting cables 30, 31, and a feeder cable 32, in the form of a single linear array 10.
Fig 6a shows a cross-sectional view of a conventional installation of a typical piezoelectric axle detector (Type "Roadtrax BL™" from Measurement Specialties, Inc). The axle detector 40 is first located into a spring plastic clip 41, which is then pushed down into a slot 42 cut into the surface of the pavement. After inserting the entire length of the sensor, the slot is filled with a special encapsulation resin, which is poured so that the level is flush or slightly proud of the original road surface level (not shown in Fig. 6a). If necessary, excess resin can be ground off after the resin has cured, to leave a perfectly flush surface.
Fig. 6b shows a cross-sectional view of a device according to the invention. As can be seen, an almost identical arrangement to the one shown in Fig. 6a can be used to accommodate the linear array of magnetic sensors 12 described above, requiring only that the slot 42 be cut somewhat deeper. This extra depth, with some clearance provided between the underside of the axle detector spacer clips and the upper surface of the magnetic sensor nodes, is readily filled with the same encapsulation resin (not shown in Fig. 6b). According to the convention of this application document, Fig. 6b shows the device according to the invention to have a basic body, namely the pavement 43, with a slot 42 in the basic body 43.
Fig 7 shows a cross-sectional view along the length of the slot 42, using an installation method similar to that illustrated in Fig. 6b. It should be appreciated that the depth of the slot need not be uniform underneath the magnetic sensor array as shown, but could have deeper regions for each sensor node formed by plunging the saw at these points, with shallower regions to accommodate just the interconnecting cable between. Cutting a deeper slot is not difficult, since the sawblades used for typical slot-cutting operations allow much greater depth than normally required for a simple axle detector. Arrow B points to the level of the road surface.

Claims

Device for sensing at least one property of a surface-bound vehicle comprising:
- a pressure sensitive sensor with at least one sensor-element, whereby said pressure sensitive sensor is designed to produce a signal based on a pressure induced change of a property of the sensor-element,
and

- a magnetic sensor that is designed to detect local changes in the earth's magnetic field and/or the magnetic field generated by the vehicle.
Device according to claim 1, characterized in that the pressure sensitive sensor has at least one piezoelectric element.
Device according to claim 1 or 2, characterized in that the pressure sensitive sensor has at least one fiber-optic sensor.
Device according to any one of claims 1 to 3, characterized in that the pressure sensitive sensor and the magnetic sensor are distanced apart less than 10 m with respect to the direction of travel.
Device according to claim 4, characterized in that the pressure sensitive sensor and the magnetic sensor are co-located with respect to the direction of travel of the vehicle.
Device according to any one of claims 1 to 5, characterized by a basic body and at least one slot in the basic body, whereby the pressure sensitive sensor is located in the slot.
Device according to any one of claims 1 to 6, characterized by a basic body and at least one slot in the basic body, whereby the magnetic sensor is located in the slot.
Device according to claim 7, characterized in that the magnetic sensor and the pressure sensitive sensor are located in the same slot.
Device according to any one of claims 7 to 8, characterized by a basic body and a first slot in the basic body and a second slot in the basic body, whereby a first pressure sensitive sensor is located in the first slot and a second pressure sensitive sensor is located in the second slot.
Device according to claim 9, characterized in that a magnetic sensor is located in the first slot and/or a magnetic sensor is located in the second slot.
Device according to any one of claims 1 to 10, characterized by a magnetic sensor that is a three-axis-magnetometer.
Device according to any one of claims 1 to 12, characterized by at least two magnetic sensors forming an array of magnetic sensors.
Device according to claim 12, characterized in that each magnetic sensor has an output that is directly connected with the output of the other magnetic sensors in the array of magnetic sensors in order to provide a summation of the respective outputs.
Device according to claim 12 or 13, characterized in that the output of each magnetic sensor can be weighted according to its position along the array.
Device according to any one of claims 12 to 14, characterized in that each magnetic sensor can be remotely interrogated using a communication bus to provide an analog or a digital value corresponding to its locally-detected signal strength.
Road with a device according to any one of claims 1 to 15.