GB2165651A

GB2165651A - Inductive loop detector

Info

Publication number: GB2165651A
Application number: GB08523201A
Authority: GB
Inventors: David Arthur Buttemer
Original assignee: ELECTROMATIC
Current assignee: ELECTROMATIC
Priority date: 1984-09-24
Filing date: 1985-09-19
Publication date: 1986-04-16
Also published as: ZA8507319D; DE3533769A1; FR2570857A1; AU4761785A; AU585894B2; GB8424096D0; GB8523201D0

Abstract

In an inductive loop (36) detector used for detecting the presence of an object within an active area of the loop (36) a first signal dependent on the loop frequency is obtained at terminal (1) and compared in comparator (18) to a first reference signal which is dependent on the frequency of the loop without the object being within the active area of the loop. The signal produced by the comparison is compared at (22) to a threshold signal and an output signal is generated when the comparator (18) output crosses the threshold level. The reference signal is derived (using comparator (24), control (26) and memory capacitor (28)) from the first signal and is used to maintain an operating mode of the detector in the event of a power failure. <IMAGE>

Description

SPECIFICATION Inductive loop detector This invention relates to an inductive loop detector of the kind which is suitable for use in car park applications.

If the power supply to an inductive loop detector fails then information on the status of the detector, at the moment of power failure, is lost. For example if a vehicle is detected, and is within the detection area of the loop, immediately before the power failure and if the vehicle is still within the active area of the loop when power is restored, spurious operation of the detector is possible. If the detector is self-tuning it will automatically compensate for the presence of the vehicle and in fact indicate the absence of a vehicle. Thus, for example, a boom which controls access to a parking area will not be raised, despite the presence of the vehicle. Similar difficulties exist in fact in any situation where such a detector is installed and it is necessary to maintain the status of the loop during, or after, a power failure.

To avoid problems of this type it is desirable to include in a detector of this type some means for enabling the detector to recover from a short power failure, i.e. of the order of minutes, and to recommence operation in the mode which pertained immediately prior to the power failure.

The invention provides a method of operating an inductive loop detector which is used to detect the presence of an object within an active area of the loop, the loop being excited by means of an oscillator at a frequency which is dependent on the inductance of the loop, the method including the steps of generating a first signal which is dependent on the instantaneous frequency of the loop, generating a first reference signal which is dependent on the frequency of the loop without the object being within the active area of the loop, comparing the first signal to the first reference signal to produce a second signal, comparing the second signal to a threshold signal, and generating an output signal when the second signal crosses the threshold signal.

Preferably the second signal is dependent on the difference between the instantaneous loop frequency and the frequency of the loop without the object being within the active area of the loop.

The first reference signal may be stored as a voltage across a capacitor.

The method may include the further step of inhibiting output from the inductive loop detector while the said multiple of loop frequency is actually being varied.

The invention further extends to an inductive loop detector for detecting the presence of an object within an active area of the loop, the detector including oscillator means for exciting the loop, a controllable frequency divider which produces an output signal at a frequency which is a multiple of the oscillator frequency, a frequency to voltage converter which is connected to the frequency divider and which produces an intermediate output signal at a voltage which is a function of the frequency of the output signal of the frequency divider, means for deriving a signal from the intermediate signal, and means for comparing the derived signal to a threshold signal thereby to detect the presence of an object within the active area of the inductive loop.

Preferably the means for deriving the signal includes means for generating a first reference signal which is dependent on the oscillator frequency without the object being within the active area of the loop, and means for comparing the reference signal to the intermediate signal. The first reference signal may be variable to compensate for drift and other occurrences which can affect the operation of the inductive loop detector.

The intermediate signal may be compared to a second reference signal to produce a control signal used to control the operation of the frequency divider.

Means may be provided for inhibiting an output signal from the inductive loop detector while the divide ratio of the frequency divider is being varied.

The invention also provides an inductive loop detector which is used for detecting the presence of an object within an active area of the loop and which includes an oscillator for exciting the loop at a frequency which is dependent on the inductance of the loop, means for generating a first signal which is dependent on the instantaneous frequency of the loop, means for generating a first reference signal which is dependent on the frequency of the loop without the object being within the active area of the loop, means for comparing the first signal to the first reference signal to produce a second signal, means for comparing the second signal to a threshold signal, and means for generating an output signal when the second signal crosses the threshold signal.

The invention is further described by way of example with reference to the accompanying drawing which illustrates in schematic block diagram form a circuit of an inductive loop detector according to the invention.

The accompanying drawing illustrates, schematically, the circuit of an inductive loop detector according to the invention which includes an oscillator 10, a wave form generator 12, a programmable frequency divider 14, a frequency to voltage converter 16, a differential amplifier 18, a Schmitt trigger device 20, a comparator 22, a comparator 24, a control circuit 26 with a memory capacitor 28, an output logic circuit 30, an inhibiting circuit 32, and a loop fault detector circuit 34.

The oscillator 10 is used to energize an inductive loop 36. The oscillator 10 is free running with its frequency of oscillation being determined inter alia by the loop inductance.

This inductance together with a control capacitor 38 establishes the resonant frequency of fthe loop. Zener diodes 40 across the loop 36 provide lightning protection.

The oscillator 10 is of a kind such as a Colpitts oscillator which is configured to provide excitation with a sinusoidal wave form which is substantially pure. The wave form generator 12 converts the sinusoid into a square wave which is then applied to the frequency divider 14. The status of the frequency divider is maintained, in the event of short term power failures, by means of a capacitor 42 which is charged from a reference source 44. Thus if power failure should occur and then be restored the status of the frequency divider remains unaltered and in particular the divide ratio remains the same.

The detector circuit 34 is actuable by means of a reset switch 46. When the switch is closed the oscillator 10 is turned off and any external signals which are induced within the loop are applied to the detector 34. If such external signals in fact exist then a light emitting diode 48 is energized so that the presence of interfering signals can be indicated.

The frequency to voltage converter 16 produces an intermediate signal the voltage of which is proportional to the output frequency of the divider 14 and, consequently, is also proportional to the loop inductance. Thus a change in the loop inductance caused for example by the presence of a vehicle or other object on the loop manifests itself as a change in the voltage of the intermediate analogue signal output by the converter 16.

The analogue signal is applied to the differential amplifier 18. A signal derived from the control circuit 26 is applied to this amplifier and the output signal is applied to the comparator 22. The output signal is compared by the comparator to a threshold signal derived from a reference voltage dividing network 50.

The sensitivity of this network can be adjusted by means of a switch 52 which selects different contacts in a second voltage dividing network 54.

When the output signal of the differential amplifier 18 crosses the threshold reference signal the comparator switches and a signal is applied to the output logic circuit 30. This circuit contains a relay which is normally energized in the "undetected" mode. Thus when the comparator switches the relay is de-energized and a signal appears on contacts 56 controlled by the relay to indicate the presence of an object in the inductive loop. A light emitting diode 58, associated with the relay is energized to provide a visual indication of the presence of the object.

The intermediate signal applied to the differential amplifier 18 is also applied to the Schmitt trigger 20 which compares the intermediate signal to a reference signal. If the intermediate signal lies outside certain preset limits i.e. the voltage of the intermediate signal is either too high or too low then the Schmitt trigger switches and enables the clock in the frequency divider. The clock pulses are used to vary the divide ratio of the frequency divider so that the amplitude of the analogue signal is varied to bring it to within the predetermined limits. This self-tuning feature ensures that long term variations in the operating conditions of the detector are compensated for in such a way that the circuit can continue functioning effectively.

While the clock is being enabled i.e. while the divide ratio of the frequency divider is being actually altered the inhibit circuit 32 prevents operation of the output logic circuit 30 so that no spurious output signals are generated.

The differential amplifier 18 provides an output signal which is derived from the difference between the intermediate signal and a reference signal output by the control circuit 26.

This reference signal is derived from a voltage stored in the memory capacitor 28. The voltage across the capacitor is under the control inter alia of the logic circuit 30, the comparator 24 and the control signal output by the Schmitt trigger 20.

The comparator 24 effectively maintains the inputs to the amplifier 18 at a point where a state of balance is achieved, i.e. its output voltage is maintained below the threshold of the detect comparator 22. This is achieved by the comparator 24 causing the capacitor 28 to be charged or discharged accordingly.

In the first instance the signal applied to the comparator 22 reflects disturbances caused by drift or other effects of this kind. This signal is compared by the comparator 24 to a reference signal and the voltage across the capacitor 28 is varied accordingly to compensate for drift. In the second instance if the output signal of the comparator 22 undergoes a step change, indicating the presence of a vehicle or the like near the loop then the capacitor 28 is charged through a circuit with a long time constant so that it does not rapidly respond to large abrupt changes in the loop inductance. Thus the capacitor is made relatively insensitive to rapid changes in the loop inductance but is far more sensitive to small variations in the loop inductance which normally would not be caused by the presence of an object near or in the inductive loop. Finally the capacitor 28 responds to the control signal output by the Schmitt trigger 20. The control signal is indicative of the state of tune of the circuit and as this affects the intermediate signal applied to the differential amplifier 18 the other signal which is applied to the differential amplifier and which is under the control of the voltage across the capacitor is also varied by adjusting the voltage across the capacitor 28.

In effect therefore the voltage across the capacitor 28 reflects long term or gradual variations in the operating condition of the loop.

However the capacitor voltage does not reflect short term variations i.e. abrupt changes caused for example by the presence of a vehicle. The signal output by the amplifier effectively gives the difference between the two input signals i.e. the output signal is a reflection of short term variations only in the operating conditions of the loop. As the capacitor voltage can be maintained within acceptable limits substantially constant for an extended period, of the order at least of ten to fifteen minutes, then provided power is restored to the circuit within this period the signal output by the comparator 18 is directly related to the short term variations in the operating condition of the loop. Thus start up of the circuit in the condition which prevailed immediately prior to power failure is assured.

If use is not made of the capacitor 28 to provide a reference signal then, on start up of the detector after a power failure, the system has no means of detecting the presence of a vehicle which was in the active area of the loop before the power failure. Thus the system would start up in the wrong mode.

The voltage across the capacitor 28 thus serves as a memory which is indicative of the status of the inductive loop detector. The capacitor 28 has extremely low leakage and consequently should a power failure occur the voltage across it is maintained with a high degree of accuracy. Thus when the power is restored to the detector the circuit is able to assume the operating condition which prevailed immediately before the power failure.

This feature coupled with the fact that the capacitor 42 maintains the status quo of the frequency divider 14 means that the circuit has the facility to operate effectively, after a power failure, irrespective of whether or not a vehicle was over the inductive loop at the time of power failure or whether such vehicle has left the loop before restoration of the power.

Claims

1. A method of operating an inductive loop detector which is used to detect the presence of an object within an active area of the loop, the loop being excited by means of an oscillator at a frequency which is dependent on the inductance of the loop, the method including the steps of generating a first signal which is dependent on the instantaneous frequency of the loop, generating a first reference signal which is dependent on the frequency of the loop without the object being within the active area of the loop, comparing the first signal to the first reference signal to produce a second signal, comparing the second signal to a threshold signal, and generating an output signal when the second signal crosses the threshold signal.

2. A method according to claim 1 in which the second signal is dependent on the difference between the instantaneous loop frequency and the frequency of the loop without the object being within the active area of the loop.

3. A method according to claim 1 or 2 which includes the step of storing the first reference signal as a voltage across a capacitor.

4. A method according to any one of claims 1 to 3 wherein the first signal is derived from a multiple of the loop frequency produced by frequency divider means and which includes the step of comparing the first signal to a second reference signal to produce a control signal which is used to vary the said multiple of the loop frequency.

5. A method according to claim 4 which includes the steps of generating a third reference signal which is dependent on the said multiple, storing the third reference signal, and using at least the third reference signal to control the operation of the divider means.

6. A method according to claim 4 or 5 which includes the step of inhibiting output from the inductive loop detector while the said multiple of loop frequency is actually being varied.

7. An inductive loop detector for detecting the presence of an object within an active area of the loop, the detector including oscillator means for exciting the loop, a controllable frequency divider which produces an output signal at a frequency which is a multiple of the oscillator frequency, a frequency to voltage converter which is connected to the frequency divider and which produces an intermediate output signal at a voltage which is a function of the frequency of the output signal of the frequency divider, means for deriving a signal from the intermediate signal, and means for comparing the derived signal to a threshold signal thereby to detect the presence of an object within the active area of the inductive loop.

8. A detector according to claim 7 wherein the means for deriving the signal includes means for generating a first reference signal which is dependent on the oscillator frequency without the object being within the active area of the loop, and means for comparing the reference signal to the intermediate signal.

9. A detector according to claim 7 or 8 which includes means for comparing the intermediate signal to a second reference signal to produce a control signal which is used to control the operation of the frequency divider.

10. An inductive loop detector which is used for detecting the presence of an object within an active area of the loop and which includes an oscillator for exciting the loop at a frequency which is dependent on the inductance of the loop, means for generating a first signal which is dependent on the instantaneous frequency of the loop, means for generating a first reference signal which is dependent on the frequency of the loop without the object being within the active area of the loop, means for comparing the first signal to the first reference signal to produce a second signal, means for comparing the second signal to a threshold signal, and means for generating an output signal when the second signal crosses the threshold signal.

11. An inductive loop detector substantially as hereinbefore described with reference to the accompanying drawing.

12. A method of operating an inductive loop detector substantially as hereinbefore described with reference to the accompanying drawing.