EP0155108A2

EP0155108A2 - Detection system

Info

Publication number: EP0155108A2
Application number: EP85301267A
Authority: EP
Inventors: Andrew George William Taylor
Original assignee: TROLL DETECTOR SYSTEMS Ltd
Current assignee: TROLL DETECTOR SYSTEMS LIMITED
Priority date: 1984-02-24
Filing date: 1985-02-25
Publication date: 1985-09-18
Also published as: GB8404838D0; EP0155108A3

Abstract

® A detection system for use in monitoring traffic flow along a roadway in which the detector (1) is buried in the roadway (3) and transmits the information by radio frequency to a receiver (4) located at a remote location, the detector incorporating a transmission coil and a detection coil, the detection coil being responsive to entry and exit of an object into and from the detection field produced by the transmission coil.

Description

This invention relates to a detection system, and particularly but not exclusively to a detection system for use in monitoring traffic flow along a roadway.
Vehicle detection systems are known which utilise induction loops buried in a roadway and arranged to be responsive to passage of a vehicle over the loop to signal such passage to a control station over a hardwired receiver link.
Such a system has the disadvantage that it requires considerable expenditure of time and money for installation of the loop, this also probably involving disruption to traffic flow on the roadway. Further, such a system will not work satisfactorily' on a metal reinforced roadway.
Other disadvantages of the use of a loop installed in the roadway itself are that generally the loop is of relatively large dimensions, being generally at least one metre or more across and this means that the field is such that more than one vehicle may be in the field at any one time and the speed of the vehicle may change dramatically as it passes through the field thereby giving spurious results. Furthermore the existence of a length of cable buried in the roadway creates problems when the roadway has to be resurfaced or any work has to take place.
In US-A-4302746 there is disclosed a system in which a two-axis magnetometer is used for vehicle detection in place of an induction loop. The magnetometer is incorporated in a self-powered detection unit which is implanted in a hole in the centre of a roadway, the unit detecting the presence of a vehicle by measuring its magnetic field, and then transmitting vehicle presence information to a roadside control unit by way of a radio frequency telemetry link.
It is a disadvantage of this system also that it will not work satisfactorily in a metal reinforced road. Other disadvantages are that vehicles to be detected must have a substantial mass, and that the sensitivity of the magnetometer decreases with use, and varies with changes in ambient temperature and with time.
According to this invention there is provided a detection system comprising a self powered detector having a transmission coil which is, in use, fed with a signal at a predetermined resonant frequency thereby to provide a detection field and a detection coil tuned to said resonant frequency and responsive to entry and exit of an object into and from said detection field; coding means operative to encode the response of the detection coil; a transmitter operative to transmit a signal including the output of said coding means; and a receiver responsive to receipt of a signal from the transmitter to receive information represented by the coded response of the detection coil.
A detection system will now be described by way of example with reference to the drawings in which:

Figure 1 is a pictorial illustration of the system installed in a roadway for monitoring traffic flow on the roadway;
Figure 2 is a block schematic diagram of the detector of the system;
Figure 3 is a vertical section of the installed system;
Figure 4 is a perspective view of parts of the apparatus when separated; and,
Figures 5 to 8 are traces of signals received by a system when various vehicles pass over the detector.

Referring to Figure 1 and 3, the system comprises a detector 1 buried in a hole 9 in the centre of one carriageway 2 of a roadway 3, and a receiver 4 mounted by the side of the roadway 3, possibly at a location remote (eg up to about 9.5 Km) from the detector 1 eg mounted to a traffic light. The detector 1 serves to detect the passage of vehicles over the detector, and to transmit certain information about a detected vehicle, for example its speed, length, number of axles, direction of travel, to an antenna 7 of the receiver 4 by way of a radio frequency telemetry link indicated at 5. The receiver 4 analyses the received information, and in response thereto functions to influence the traffic flow along the roadway 3 or elsewhere, for example by controlling the traffic lights 4 and/or other warning signs.
Although the detector can be mains powered, this requires cables to be laid in the roadway. This may be convenient in some situations, where the detectors are being installed before the roadway is built or in a car park where the width of the road is very restricted, but generally we prefer to provide the detector 1 as a self-powered unit which contains a battery 8 which can be recharged when required by a high energy pulsed signal transmitted from the receiver 4 to the detector 1, as will be described later.
The detector 1 is in the form of a hermetically sealed unit comprising a cylindrical plastics material housing containing the circuitry and batteries, preferably potted in a plastics material potting compound. A switch is provided by which the detector 1 can be activated when installed for use, this switch being in the form of a pair of coils adjacent an opening in the housing, and a magnet which is inserted into the opening to provide coupling between the coils thereby to activate the detector 1. For use, the detector is buried at a depth of about one inch (2.5cm) in a hole in a roadway, with its axis vertical, and covered with a sealing material of suitable strength and consistency, this after the detector 1 has been activated.
Referring now to Figures 3 and 4 the detector 1 is buried in the hole 9, the hole 9 being closed either by the surface dressing on the roadway or by an optional removable cover 50.
The detector 1 is mounted in a plastics container 51 having a closed upper end and the open lower end closed by a removable cover 52. The battery 8 is mounted at the lower end of the container 51 and above that there is mounted electronic parts of the detector 1. The electronic parts are mounted on a printed circuit board 53 mounted inside a tubular frame 54, the tubular frame 54 also incorporating adjacent its upper and lower ends plastics rings 56. The two upper plastics rings 56 each mount a copper coil, the upper copper coil comprising a transmission coil 10 and the lower copper coil a transmission coil 11. The lower plastics rings 56 mount a copper transmission coil 10. The two transmission coils 10 provide between them a field 6 and the receiver coil 11 receives a signal which varies depending upon the disturbances in that field 6, the disturbances being caused by motor vehicles. The upper space in the container 51 mounts a wire aerial (antenna) 22. '
Referring now to Figure 2, when the detector 1 is activated the transmission coils 10 are fed from a crystal controlled oscillator 12 at its resonant frequency of 150 kHz, the detection coil 11 being tuned to the same frequency. The transmission coils 10 radiate a three-dimesnional field 6 (of typical dimensions height 60-240cm; width 300-450cm; length along the road approximately 300cm) above the roadway, which field 6 is detected by the detector coil 11. When a vehicle enters the field 6 the inductive and capacitive coupling between the transmission coils 10 and the detection coil 11 is changed, the output voltage from the detection coil 11 being decreased.
At the same time the frequency of the output signal from the detection coil 11 is changed due to the change in the inductive coupling between the coils 10 and 11, the actual change being dependent upon the composition of the material entering the field of the transmission coils 10. This known phenomena can be used in detection systems other than vehicle detection systems as now being described, eg for material identification purposes.
The output signal from the detection coil 11 is fed to a voltage amplifier 13, the output of which is fed by way of a rectifier 14 to a voltage comparator 15 responsive to dynamic voltage changes in its input relative to a negative reference voltage.
The voltage amplifier 13 also controls a bistable device 16 which on entry of a vehicle into the field of the transmission coil 10 is switched to a state in which it effects energisation of the remaining circuitry of the detector 1 and which when the vehicle leaves the field is switched back to its original state such that the detector 1 is returned to its normal rest condition ready for detection of a further vehicle.
The output of the comparator 15 is fed to a central process 17 which serves to determine therefrom the speed of the detected vehicle, this being proportional to the rate of change in the output from the detection 11 as the vehicle enters the field of the transmission coils 10, and the length of the detected vehicle, this being determined from the time between the vehicle entering and leaving the field of the transmission coils 10 as detected by the detection coil 11, and the speed of the detected vehicle. This information is then fed by the processing device 17 in binary coded form, to a memory 18 where it is held for subsequent transmission by the detector unit 1.
Alternatively, the signal detected by the coil 11 may be transmitted and the processing carried out at the receiver 4. It would be simplest to digitise the signal from the detection coil 11 and pass this digitised signal to the receiver 4.
An advantage of the use of a detector which provides a limited detection field 6 (unlike the systems using loops which are buried in the ground and provide a very large field) is that the signal produced by the detection coil contains information which allows one in some circumstancers to deduce the type of vehicle detected. As already mentioned, we can detect the speed of the vehicle by the rise time of the pulse and from that speed and the width of the pulse we can detect the length of the vehicle. Furthermore, in the case of a commercial vehicle having substantial axle structures located below the remainder of the vehicle, these axle structures result in peaks in the output signal from the detection coil 11. By arranging for these peaks to be counted by the processing device 17 the number of axles on a detected vehicle can be determined, and this information stored in the memory 18 for subsequent transmission. Such information is useful in traffic monitoring and control systems of which the prsent detection system would form a part.
Figures 5 to 8 show signals produced in the course of experiments by various vehicles. The signals are in fact the output of the detection coil 11. Referring to Figure 5, there is shown the signal output for the passage of a motor car (a Ford Granada Estate) travelling at a first low speed. Figure 6 shows the signal output for the same motor vehicle travelling at a faster speed (in Figures 5 to 8 the right hand side of the signal is provided by the leading edge of the vehicle).
Figure 7 shows the same motor car towing a trailer and it can be seen that the waveform is similar to that shown in Figures 5 and 6 with the addition of a further peak to the left, the further peak being created by the trailer. From this it will be understood that it is easy to distinguish between vehicles which have and do not have trailers.
As is known, the Ford Granada does not incorporate large axles which are spaced from the body of the vehicle and so there is no particular evidence of the presence of axles. However in Figure 8 which is the signal in respect of the passage of a medium sized van (a Ford Transit) which incorporates a front axle and a rear axle it will be seen that the signal produces two distinct peaks P1 and P2 in respect of the two axles together with a third small peak P3 created by a tow bar attached to the rear of the vehicle.
On installation of the detector 1 in a roadway, it is given an identity by means of a unique identification number entered into the memory 18, for example by means of a keyboard temporarily coupled to the detector 1 prior to the unit being buried in the roadway. This unique identification number is retained in the memory, and is_Aread therefrom with each batch of information read therefrom for transmission, whereby a receiver of the information will be able to determine the source of the information, this enabling a single receiver to monitor a plurality of detectors 1 at different locations.
On detection of the vehicle leaving the field of the transmission coil 10, the information relevant to the vehicle is read from the memory 18 by an encoder device 19 in which it is used to modulate a radio frequency carrier which is then passed by way of a pre-amplifier 20 to a transmitter 21 for transmission from the aerial 22 incorporated in the detection unit 1, all this under the control of a clock signal from a clock signal generator 23 connected to the encoder device 19.
After transmission of the information from the memory 18, the memory is cleared, apart from the detector identification number, ready for receipt of information relating to a further detected vehicle.
Preferably the detector 1 includes only a single crystal controlled oscillator, the various signals required, namely the signal for the transmission coil 10, the clock signal, and the signal for modulation and transmission being derived by division from the single oscillator.
The detector 1 can have associated therewith a known temperature sensor device 100 operative to measure the temperature at the surface of the roadway, this information being supplied to the encoder device 19 for transmission with vehicle detection information or simply with the detection unit identification number. The sensor device 100 can also include humidity measuring means, this information being transmitted together with the temperature information whereby the possibility of ice on the roadway can be determined by the remote receiver. The humidity measuring means is simply a pair of electrodes of different metals capable of giving a voltage output in the presence of moisture at the surface and thus indicate the possibility of ice if the measured temperature is sufficiently low.
As previously mentioned, the detector 1 is powered by a battery 8 which, once the detector is activated and buried, serves for continual energisation of the transmission circuit for the transmission coil 10 and the reception circuit for the detection coil 11, and on detection of a vehicle serves for energisation of the remaining components of the detector by way of an electronic switch device 23 under the control of the bistable device 16.
The detector 1 can also include means to sense the voltage of the battery 8, and to respond to the sensed voltage falling below a predetermined level by causing transmission by the unit of a signal indicating this fact. The remote receiver can > respond to such an indication either by giving a warning that the detector identified requires replacement, or by transmitting a battery recharging signal, comprising bursts of high energy pulses, which is received by the detector 1 by way of an aerial, which can be the transmission aerial 22, and rectified by a rectifier device 24 before being used to recharge the battery 8.
The detector 1 is installed in a roadway to detect vehicles passing in a predetermined correct direction along the roadway. The detector 1 will however react to vehicles travelling in the wrong direction, the detection coil 11 giving an output signal of reversed configuration under such circumstances. This additional indication can be transmitted to the remote receiver to give a warning of the incorrect direction vehicle.
The remote receiver at which the signal from the detector 1 and the signals from a plurality of similar detectors are received, contains a plurality of channels all fed from a single antenna, the channels being of the phase-locked-loop type in order to obtain good performance on a norrow frequency band. On receipt of signal from a detector 1 an unoccupied channel is opened, and the received signal fed thereto. Each received signal is demodulated, decoded and stored in a memory for subsequent supply to a processing unit. As previously described, each received signal contains information identifying the detector transmitting the signal, and thus the processing unit can build up a picture of traffic flow at a plurality of different locations in a roadway network, and in dependence thereon operate to control the traffic as required, for example by controlling traffic lights and other traffic control devices.
Although the detector has been described primarily with respect to transmission of information using radio frequencies where the detector is installed before the roadway is built or where the roadway is very narrow, for example in a car park, it may be economically viable and sensible not only to power the detector by means of a cable laid under the roadway but also to pass the information from the detector to the receiver by means of a cable rather than by a radio frequency link.
We have described a system in which one detector is provided in a roadway. Although it is possible to detect the speed of an approaching vehicle by the rising time of the pulse, it is in many respects simpler to provide two detectors spaced longitudinally along the road at a short distance apart, for example, one meter apart and to utilise the difference in the time between the signals received by the two detectors to calculate the speed of the vehicle. Such an arrangement has the additional advantage that it provides two pulses which enables one to eliminate spurious background signals. Particularly with the use of a radio frequency link, there is the possibility of interference, for example from other radio frequency sources and indeed from the ignition systems of the vehicles themselves and the use of two detectors enables one to eliminate or reduce these background interference signals.
Although the use of two detectors increases the cost, it is still a matter of simply drilling two holes rather than cutting a long length of channel as is required in existing systems.
A single receiver 4 can be associated with a plurality of detectors 1, the information transmitted by a detector 1 including identification information enabling the receiver 4 to identify any transmitting detector 1.
The information transmitted by the detector 1 can include, in addition to vehicle information; information in relation to the state of the batteries powering the detector, and information about the roadway conditions, for example the temperature and humidity conditions on the roadway surface.

Claims

1. A detection system comprising a self-powered detector (1) having a transmission coil (10) which is, in use, fed with a signal at a predetermined resonant frequency thereby to provide a detection field (6) and a detection coil (11) tuned to said resonant frequency and responsive to entry and exit of an object into and from said detection field (6); coding means (19) operative to encode the response of the detection coil (11); a transmitter (21) operative to transmit a signal including the output of said coding means (19); and a receiver (4) responsive to receipt of a signal from the transmitter (21) to receive information represented by the coded response of the detection coil (11).

2. A system as claimed in claim 1 characterised in that said transmitter (21) comprises a radio frequency transmitter, the radio frequency transmitter in use being operative to transmit a radio frequency signal modulated by the output of said coding means (19).

3. A system as claimed in claim 1 or 2, characterised in that, for use in monitoring traffic flow along a roadway, the detector (1) is buried in the roadway (3) and the receiver (4) is located at a remote location.

4. A system as claimed in claim 3, characterised in that the signal transmitted by the detector (1) includes information indicating the speed of travel of vehicles passing through said detection field (6).

5. A system as claimed in claim 4, characterised in that said speed information is derived in the detector (1) from the rate of response of the detection coil (11) on entry of a vehicle into said detection field (6).

6. A system as claimed in any of claims 2 to 4, characterised in that a plurality of detectors (1) are buried in the roadway (3), and said plurality of detectors being spaced along the length of the roadway (3) so that a single vehicle will pass over more than one detector (1).

7. A system as claimed in claim 2 to 5, characterised in that in which the signal transmitted by the detector (1) includes information indicating the length of a vehicle passing through said detection field (6).

8. A system as claimed in claim 7, characterised in that said length information is derived in the detector (1) from the time of response of the detection coil (11) to a vehicle passing through said detection field (6).

9. A system as claimed in any of claims 2 to 8, characterised in that there is provided temperature sensing means (100) arranged to determine the temperature of the roadway in the vicinity of the detector (1), and coupled to the detector (1) such that the signal transmitted by the detector (1) can include information indicating the temperature of the roadway.

10. A system as claimed in any one of claims 2 to 9, characterised in that there is provided moisture sensing means (100) arranged to determine "the presence of moisture on the surface of the roadway in the vicinity of the detector (1), and coupled to the detector (1) such that the signal transmitted by the detector (1) can include information indicating the presence of moisture on the roadway.

11. A system as claimed in claim 10 characterised in that the moisture sensing means (100) comprises a pair of electrodes of different metals capable of giving a voltage output in the presence of moisture.

12. A system as claimed in any preceding claim, characterised in that the signal transmitted by the detector (1) includes information indicating the identity of the detector (1) whereby a plurality of detectors can transmit to a single receiver (4) unambiguously.

13. A system as claimed in any preceding claim, characterised in that the detector (1) is powered by a battery (8), and in which the signal transmitted by the detector (1) includes information indicating when output of the battery (8) falls below a predetermined level.

14. A system as claimed in claim 13, characterised in that in response to receipt of said battery output level information, the receiver (4) transmits a signal which after rectification is used at the detector (1) to recharge the battery (8).

15. A system as claimed in any preceding claim, characterised in that at the detector (1) only the transmission (10) and detection (11) coil circuitry is continually powered, all other parts of the detector (1) being powered in response to detection of an object entering said detection field.

16. A system as claimed in any preceding claim, characterised in that the receiver (4) includes a plurality of channels fed from a common antenna (7), an incoming signal from the detector (1) opening a vacant channel in which the received signal is demodulated and decoded prior to being stored in a central processing unit.