GB1598077A

GB1598077A - Vehicle detector systems

Info

Publication number: GB1598077A
Application number: GB2382077A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1977-06-04
Filing date: 1977-06-04
Publication date: 1981-09-16

Description

(54) IMPROVEMENTS IN OR RELATING TO VEHICLE DETECTOR SYSTEMS (71) We, THE PLESSEY COMPANY LIM ITED, a British Company, of Vicarage Lane, Ilford, Essex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to vehicle detector systems of the kind comprising a detector loop laid on or in a roadway traffic lane or the like.

According to the present invention a vehicle detector system comprises a vehicle detector loop which in use is laid on or in a roadway traffic lane or the like, a controlled oscillator including a resonance circuit of which the detector loop forms a part, a reference oscillator, and a phase detector responsive to a signal derived from the controlled oscillator and to a signal from the reference oscillator for providing an error signal in the presence of a phase difference between the signals supplied thereto, which error signal is fed back to the controlled oscillator to nullify the said phase difference.

Thus in a system according to the present invention there is no resetting-up from time to time of the oscillator associated with the loop as would normally be required with changes in environmental conditions since its frequency is phase locked to the reference oscillator.

The time constants of the system are arranged so that short term inductance changes normally produced by a vehicle passing over the loop are not effective to cause the frequency of the controlled oscillator to be reset, whereas long term changes due for example to changing environmental conditions do cause the oscillator frequency to be reset so that it corresponds with the reference frequency.

The system may include timing means responsive to a signal indicative of the presence of a vehicle parked over the loop for a predetermined period for initiating a retuning cycle under these conditions.

The reference oscillator may be crystal controlled.

The resonance circuit of the controlled oscillator may include a capacitor bank switched under the control of a successive approximation register thereby coarsely to set the frequency of the controlled oscillator in dependence upon an output signal derived from the phase detector, fine tuning being effected in dependence upon the application of an output signal derived from the phase detector and applied to a voltage variable capacitor or varicap device. The successive approximation registor may be fed from the phase detector via an integrator and a comparator arrangement.

A plurality of comparators may be provided responsive to the integrator and each having a different predetermined threshold reference level applied thereto whereby output signals from one or other of the comparators afforded when the input signal to the comparator exceeds its threshold reference level may be utilised to reset the successive approximation register or initiate some other function in accordance with the amplitude of the input signal which derives from the integrator.

At least one comparator may be arranged to feed a gating circuit operatively associated with counter means whereby the presence of a signal output from the integrator above a predetermined threshold level for a predetermined period may be detected so that signals appertaining to the presence of a vehicle parked on or in the vicinity of the loop may be recognised and the oscillator re-tuned accordingly.

All embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a generally schematic block diagram of a loop detector system; Figure 2 is a generally schematic block diagram of part of the system shown in Figure 1 and wherein Figure 3 is a further block diagram showing part of the system of Figure 1.

Referring now to Figure 1, a loop 1 which is normally buried in a roadway or a traffic lane and its feeder 2 comprise the inductive arm of a Collpitt's oscillator 3. Capacitive arms of the oscillator are formed by capacitor banks 4, capacitors from which are selected by a control register 5 for coarse tuning, fine tuning being effected by varicap diodes 6.

An output from an amplifier 7 which forms a part of the oscillator 3 is squared in a squarer 8 and fed into one input of a phase comparator 9. A crystal controlled oscillator 10 which operates at a frequency of between 1360KHz to 1488KHz is divided by 16 in divider 11 to produce an output signal which lies in the frequency range between 85 and 93K Hz and which is fed into the other input of the phase comparator 9.

The output of the phase comparator 9 is fed directly to the varicap diodes 6 of the oscillator 3 and also to an integrator 12 whose time constant is 10 milliseconds. The characteristics of the phase comparator are such that the integrator output which varies positively or negatively about 50% of the supply voltage will produce an output voltage linearly dependent on the phase difference which obtains between the limits of + 180', after which the output saturates.

This permits the status of the phase locked loop to be monitored by applying the integrator output to a comparator unit 13 having reference level thresholds defined as a percentage of the supply voltage. The comparator unit 13 comprises four comparator subunits 13a, 13b, 13c and 13d which have thresholds of 8%, 20%, 50% and 80% of supply voltage respectively. When the phase locked loop comprising the oscillator 3 is phase locked, the potential on the output of the integrator 12 expressed as a percentage of the supply voltage will be nominally 50So but may lie anywhere within the range 25% to 75%. The presence of a vehicle causes this voltage at the output of the integrator 12 to drop to near 0%.The output of the comparators 13a, 13b, 13c and 13d are fed to the control logic 14 which produces a re-tune command to the oscillator coarse tuning register 5 when required and a binary signal on line 14a indicating the presence of a vehicle in the loop. Comparator 13 and control logic 14 will now be described in more detail with reference to Figures 2 and 3 which bear where appropriate the same numerical designations as used in Figure 1.

When the loop oscillator 3 is locked to the reference oscillator 11 and no vehicle is present, then the output from the integrator 12 will lie between 25% and 75% of the supply voltage. If environmental changes cause this voltage to shift above 80% such that the integrator output voltage on line 15 exceeds 80iso of the supply voltage corresponding to the reference voltage applied to line 16 of the comparator 13d, then an immediate re-tune operation takes place so that the voltage obtaining on line 15 is constrained to return within the range 25% to 75%.

If however environmental changes cause the voltage on line 15 from the integrator to drop below 20% of the supply voltage such that it falls to below the reference voltage on line 20 of comparator 13b, then after a three minute delay introduced by the counter 17 arranged to count clock pulses on line 18 from a clock pulse generator 19, a re-tune operation will take place. This three minute delay is necessary since the presence of a vehicle causes the controller to drop to near 0% but unless the vehicle is parked it will only be in the loop for a minute at most, taking account of traffic lights, and as soon as it leaves the loop the counter 19 is reset.

Under the foregoing environmental changes, the system is able to detect a vehicle whilst waiting the three minute period to re-tune because the vehicle present comparator 13a is set to operate at 8% of the supply voltage.

The comparator 13a as well as the comparator 1 3b is arranged to feed the counter 17, the link between the comparators and the counter being effected via an OR gate 17a. Thus if a parked vehicle is present for more than three minutes the oscillator will be re-tuned.

The output of the counter 17 is gated via an OR gate 21. If comparator 13dor the counter 17 provides an output signal and provided no re-tuning operation is taking place then the successive approximation register 5 is commanded to re-tune by means of a signal on line 22. During re-tuning the vehicle present output signal on line 23 is forced low by a logic circuit (not shown) forming part of the logic circuit 14 to prevent false vehicle indications. In the re-tuning process the contents of the successive approximation register 5 which is operated under control of the clock generator 19 is cleared hence the circuit will oscillate at a reeasonably high frequency. As the next clock pulse is applied the largest capacitor 24 of the switched capacitors will be introduced into the capacitive arms 4 of the oscillator. This will produce an output frequency which is higher or lower than the reference frequency. If it is higher, then the capacitor 24 will be retained in circuit at the next positive going edge of the clock generator 19 is cleared hence the circuit will oscillate at a reasonably high edge when the next most significant capacitor 23 is switched in. Operation of successive approximation registers and the logic for controlling such registers are well known and so further explanations applied to the present invention are believed to be unnecessary.

As was previously mentioned the output of the phase comparator 9 is indicative of the sense of the phase difference between the inputs applied thereto and hence by connecting the comparator 13c to the output of integrator 12 on line 15, (the comparator threshold as applied on line 25 being 50% of the supply voltage), a signal is made available on line 26 which is fed to the register 5 to inform it if the oscillator 3 frequency is higher or lower than the reference frequency.

This process of capacitive switching is carried down eleven stages of switching (not all of which are shown) of decreasing capacitance and by the eleventh stage the frequency of the loop oscillator is very close to the reference frequency the final phase lock being effected automatically by control of the varicap diodes 6.

Vehicle detection is accomplished due to the fact that when a vehicle is above a loop, the inductance of that loop will change and hence the frequency of the loop oscillator will vary. This causes a reduction of voltage on the integrator output and hence the 8% threshold comparator 13a is triggered to indicate the presence of a vehicle and this results in a signal on line 23 at the output of comparator 13a. Should the vehicle remain in the loop for more than three minutes, the tuning is re-adjusted although the sensor may be capable 'of detecting a vehicle driving over any unoccupied part of the loop in spite of the fact that a vehicle is parked in its vicinity.

WHAT WE CLAIM IS: 1. A vehicle detector system comprising a vehicle detector loop which in use is laid on or in a roadway traffic lane or the like, a controlled oscillator including a resonance circuit of which the detector loop forms a part, a reference oscillator, and a phase detector response to a signal derived from the controlled oscillator and to a signal from the reference oscillator for providing an error signal in the presence of a phase difference between the signals supplied thereto, which error signal is fed back to the controlled oscillator to nullify the said phase difference.

2. A vehicle detector system according to claim 1 including timing means responsive to a signal indicative of the presence of a vehicle parked over the loop for a predetermined period for initiating a re-tuning cycle under these conditions.

3. A vehicle detector system according to claim 1 or claim 2, in which the reference oscillator is crystal controlled.

4. A vehicle detector system according to any one of the preceding claims in which the resonance circuit of the controlled oscillator includes a capacitor bank switched under the control of a successive approximation register thereby coarsely to set the frequency of the controlled oscillator in dependence upon an output signal derived from the phase detector, fine tuning being effected in dependence upon the application of an output signal derived from the phase detector and applied to a voltage variable capacitor or varicap device.

5. A vehicle detector system according to claim 4 in which the successive approximation register is fed from the phase detector via an integrator and a comparator arrangement.

6. A vehicle detector system according to claim 5, including a plurality of comparators responsive to the integrator and each having a different predetermined threshold reference level applied thereto whereby output signals from one or other of the comparators afforded when the input signal to the comparator exceed its threshold reference level may be utilised to reset the successive approximation register or initiate some other function in accordance with the amplitude of the input signal which derives from the integrator.

7. A vehicle detector system according to claim 6, in which at least one comparator is arranged to feed a gating circuit operatively associated with counter means whereby the presence of a signal output from the integrator above a predetermined threshold level for a predetermined period may be detected so that signals appertaining to the presence of a vehicle parked on or in the vicinity of the loop may be recognised and the oscillator retuned accordingly.

8. A vehicle detector system substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. approximation registers and the logic for controlling such registers are well known and so further explanations applied to the present invention are believed to be unnecessary. As was previously mentioned the output of the phase comparator 9 is indicative of the sense of the phase difference between the inputs applied thereto and hence by connecting the comparator 13c to the output of integrator 12 on line 15, (the comparator threshold as applied on line 25 being 50% of the supply voltage), a signal is made available on line 26 which is fed to the register 5 to inform it if the oscillator 3 frequency is higher or lower than the reference frequency. This process of capacitive switching is carried down eleven stages of switching (not all of which are shown) of decreasing capacitance and by the eleventh stage the frequency of the loop oscillator is very close to the reference frequency the final phase lock being effected automatically by control of the varicap diodes 6. Vehicle detection is accomplished due to the fact that when a vehicle is above a loop, the inductance of that loop will change and hence the frequency of the loop oscillator will vary. This causes a reduction of voltage on the integrator output and hence the 8% threshold comparator 13a is triggered to indicate the presence of a vehicle and this results in a signal on line 23 at the output of comparator 13a. Should the vehicle remain in the loop for more than three minutes, the tuning is re-adjusted although the sensor may be capable 'of detecting a vehicle driving over any unoccupied part of the loop in spite of the fact that a vehicle is parked in its vicinity. WHAT WE CLAIM IS:

1. A vehicle detector system comprising a vehicle detector loop which in use is laid on or in a roadway traffic lane or the like, a controlled oscillator including a resonance circuit of which the detector loop forms a part, a reference oscillator, and a phase detector response to a signal derived from the controlled oscillator and to a signal from the reference oscillator for providing an error signal in the presence of a phase difference between the signals supplied thereto, which error signal is fed back to the controlled oscillator to nullify the said phase difference.