GB2164185A - Inductive loop detector - Google Patents

Description

SPECIFICATION Loop detector This invention relates generallyto a loop detector of the type used for inductively sensing the presence or passage of a vehicle on a roadway.

Inductive loop vehicle detectors are being used to an increasing extentto detectthe passage or presence of road going vehicles of all kinds. However an ancilliary detection requirement, which is becoming of increasing importance, is the ability not only to detect a specific class of vehicle such as buses and emergency vehicles within a given area butalsoto detectspecificindividual vehicles. Forexample it is highly desirable to detect an emergency vehicle such as an ambulance in orderto achievetraffic priorityfor the vehicle by favourably controlling the traffic lights at intersections th rough which the vehicle must pass.

It is also necessary, for example to afford to a specific vehicle access to an area, to be a ble to identify vehicles on an individual basis. This ability may, in addition, be used to bill the responsible person forthe use of a toll road, for law enforcement reasons, orfor a variety of other purposes.

Specific vehicles of the kind mentioned are equipped with transmitters and inductive loops, buried in the roadways, act as sensors which are responsive to the transmitted signals. The advantage of this system, over other radio-based systems, is that the transmitter must pass overthe loop for detection to occur and when this happensthe precise location ofthevehicle is determinable.

Standard vehicle detection systems currently use one or more buried loops at predetermined location, e. g. at intersections, or atthe approachesto such locations, and the number of loops increases with the complexity ofthe installation location. It is expensive to install a set of loops and it should be noted that each set of loops requires additional maintenance and care toensureitscontinued operation. Itshould be borne in mind thatthe loops and feeders constitute the weakest linkwithin a detection system.

It may also not be possible to install an additional loop at a desired location, for sensing a transmitted signal, as an existing detector loop may occupy a desired position. Underthese circumstances a compromise installation mustbe arrived at.

Yetanotherproblem isthatthefrequency band allocatedforthetransmitted signals generally falls within the frequency band which has been allocated to vehicle detectors. The detector loopfunctions as a transmitting element and this can cause interference problems with the receiver loop.

It is an object of the present invention to provide a solution to the aforementioned problems.

The invention provides a method of operating a loopdetectorwhich includesthestepsofenergizing a loopwith a firstsignal at a firstfrequency and monitoring the loop to detect a second signal received bythe loop at a second frequencywhich is different fromthefirstfrequency.

The loop is energized, forconventional vehicle detection, at a first frequency which is maintained substantially constant or alternatively which is contained within specified frequency limits. The second signal, which is transmitted by a suitably equipped vehicle, is detected by means of a receiver which is selectively responsive to the second frequency.

If the signal atthe secondfrequency is not continual lybeingtransmitted byavehiclethenthe method of the invention may include the steps, upon detecting the presence of a vehicle, oftransmitting an enabling signal by modulating the first signal, detecting the enabling signal on the vehicle, and activating a transmitter on the vehicle to transmitthe second signal. Alternatively the detector loop carrierfrequen- cy only enables the vehicle transmitter. In general terms therefore a signal is transmitted to activate the vehicle's transmitter.

The invention also provides an inductive loop detectorwhich inclues receiver meanswhich is responsive to a transmitted signal, at a predetermined frequency, detected bythe loop.

The receiver means may include a band-pass filter which passes the predetermined frequency. The band-pass filter may include a parallel tuned circuit and preferably is transformer coupled to the loop.

The loop detector operates at a distinct frequency, or alternatively within a given frequency range, which in electromagneticterms is remotefrom the predetermined frequency.

According to one variation of the invention the loop detector, upon detecting the presence of a vehicle, transmits an enabling signaltothevehiclecausing it to transmitthe signal atthe said predetermined frequency. In its simplestform the enabling signal is the carrierfrequency which is detected by suitable circuitry on the vehicle which then enables a transmitter on the vehicle.

The invention isfurtherdescribed bywayof example with reference to the accompanying drawing which illustratesthe principles ofthe invention.

The accompanying drawing illustrates an inductive loop 10 which in practice is buried within a roadway, a front end 12 of a conventional inductive loop detector circuit 13, not shown in detail, and a front end 14 of a receiver circuit 15. Thefull receiver circuitry is also not shown.

Thefront end 12 of the detector illustrates schematically an oscillatororexciting source 16which energizes a parallel tuned circuit 18 consisting of an inductor 20 connected in parallel to a capacitor 22. The circuit 18 is coupled by means of a transformer 24to the inductive loop 10.

Thefront end 14 of the receiver includes an amplifier26which is connected acrosstheterminals of a second parallel tuned circuit 28 consisting of an inductor30 and a tuning capacitor32. The circuit 28 is coupled by means of a transformer34tothe inductive loop 10.

The loop detectorfunctions in a normal mannerand therefore is not described in detail in this specification.

The source 16 energizesthe loop 10 ata frequency determined bythe circuit 18. Ideallythisfrequencyis maintained constant, within practical limits, oralternatively keptwithin a tightly prescribedfrequency band. A vehicle which traversesthe loop 10 orwhich otherwise comes within its sphere of electromagnetic influence attemptsto alterthe frequency of oscillation and underthese conditions, in accordance with conventional practice, a control signal is generated which restores the energized frequency of the loop to its desired value. The control signal is then a measure or an indicator of the presence of the vehicle.

In accordance with the invention the circuit 28 of the receiver is tuned to the frequency which is allocated in the particular instance to transmitters carried on board emergency orotherselected vehicle types or specific individual vehicles. This frequency is remote, in electromagneticterms, from the energizing signal which is applied to the inductive loop 10. As the circuit 28 is parallel tuned it has a high impedance at its resonant frequency but a low impedance at other frequencies. Thus the impedance of the receiver input to the inductive loop has a negligible effect on the energizing signal applied to the inductive loop.

In similarwaythe parallel tuned circuit 18 inputs an impedance to the inductive loop which has only a minor effect on a transmitted signal detected by the inductive loop when the frequency of the transmitted signal is remotefrom the resonantfrequency of the circuit 18.

Itfollowsthereforethatthedetectorcanfunction in the normal mannerto energizethe loop 10 and detect the presence or passage of vehicles overthe loop.

Transmitted signals received bythe loop have no significant effect on the detector. On the other hand such signals are efficiently coupled to the receiver because of the band-pass filter effect of the circuit 28 butthe loop energizing signal is effectively rejected by the circuit.

The invention thus makes it possible for a single loop to be used in a general vehicle detector, and to form the basis for detecting specific transmitterequipped vehicles.

In certain situations it is not permitted to operate a transmitter on a vehicle continuously. Underthese conditionsthe presence of the vehicle can be detected in the normal manner by means of the detector and, upon such detection, the energizing source 16 can be modulated to transmit an enabling signal to detector means on the vehicle. For example the source 16 can be pulse-code modulated so thatthe loop 10 is energized for a succession of shorttime intervals.

Upon detection ofthis signal by a detector on the vehicle a transmitter on the vehicle is enabled and the transmitted signal is received by the receiver in the manner described. Appropriate action can then be initiated bythe receiver. In avariation ofthe invention the loop detector carrierfrequency alone can be used to enable the transmitter i. e. without resorting to modulating the source 16.

It is reiterated thatthe system of the invention is particularlysuitable for use with a fixed frequency loop detector. However it operates effectively with loop detectors which have free running front ends provided the loopfrequency can be contained within specified limits.

Claims (10)

1. CLAIMS 1. A method of operating a loop detectorwhich includes the steps of energizing a loop with a first signal atafirstfrequencyand monitoringthe loopto detect a second signal received by the loop at a second frequency which is differentfromthefirstfrequency.
2. 2. A method according to claim 1 which inclues the step of maintaining the first frequency within specified frequency limits.
3. 3. A method according to claim 2 wherein the first frequency is maintained substantially constant.
4. 4. Amethodaccordingtoclaim1, 2or3which includes the step of detecting the second signal by means of a receiverwhich is selectively responsiveto the second frequency.
5. 5. A method according to any one of claims 1 to 4 which includesthestep, upon detecting the presence of vehicle by means of the loop detector, oftransmit- ting a signal to activate a transmitter on the vehicle, the transmittertransmitting the second signal atthe second frequency.
6. 6. An inductive loop detectorwhich includes means for energizing the loop at a first frequency, and includes receiver meanswhich is responsiveto a transmitted signal, at a predetermined frequency, detected bythe loop, the predetermined frequency being differentfrom the firstfrequency.
7. 7. A detector according to claim 6 wherein the receiver means inclues a band-passfilterwhich passes the predetermined frequency.
8. 8. An inductive loop detector which includes an inductive loop, means for energizing the loop at afirst frequency, a first parallel tuned circuitwhich is resonant atthe first frequency and which couples the energizing means to the loop, a receiver, and a second parallel tuned circuit which is resonant at a second frequency which is differentfrom the firstfrequency and which coupled the receiverto the loop.
9. 9. An inductive loop detector substantially as hereinbefore described with reference to the accompanying drawing.
10. 10. A method of operating a loop detector sub stantially as hereinbefore described with reference to the accompanying drawing.