GB2026169A - Loop detector arrangements - Google Patents

Abstract

A loop detector arrangement for detecting metal objects (e.g. motor vehicles) comprises an oscillator (1) and programmable frequency divider (2) operative on energisation of the arrangement to progressively produce a range of different frequencies in turn and to supply them to a parallel resonant loop circuit (9) via a resistor (8). A phase comparator (11) is connected across the resistor (8) such that its inputs will go out of phase when the loop detects a metal object, the output of the comparator thus operating a relay (15) via an amplifier (13) and driver (14). A state of tune sensor (12) connected to the output of the phase comparator (11) effects automatic tuning in that it stops the above-mentioned frequency progression when it detects that correct tuning has been effected. <IMAGE>

Description

SPECIFICATION Loop detector arrangement This invention relates to loop detectors widely used for detecting bodies of metal and in particularly, but not exclusively, motor vehicles.

Conventional loop detectors generally have the loop itself, which in the case of motor vehicles is embodied in, or applied to, the upper surface of a roadway or the like, forming the tank circuit of an oscillator adapted to oscillate at a resonant frequency determined by itself and the tuning capacitors of the circuit. The presence of a body of metal such as a vehicle in the loop creates a variaton in the loop inductance with a resultant frequency change. The frequency change is detected and utilized to effect the desired switching or other function required of the detector.

Such existing loop detectors require, at the time of installation, complicated procedures in order to determine a satisfactory frequency of operation since interference by nearby separate loops of other loop detectors, or an installation of loop detectors, can cause cross-talk, thereby resulting in spurious and erratic operation of the detector. Also, long term environmental loop drift can cause the two frequencies of operation of adjacent loops to approach one another with the same consequent results. Unless precise loop frequencies are in fact known, this problem can be extremely difficult to eliminate.

In the case of multi-loop installations, and with a view to obviating cross-talk and the associated difficulties, an installation has been proposed which operates on a digital basis and wherein each of a number of loops is activated and electronically examined in turn. However, due to the fact that the digital basis of operation of such apparatus and the fact that small variations may have to be detected, an entire cycle of examination of all the loops takes an appreciable length of time, of the order of seconds. This gives rise to the difficulty that a metal body can be present momentarily in a loop and yet not be detected if that loop is inactive for the duration of the presence of the metal body in the loop.

To avoid the difficulties associated with detecting frequency changes in the loop it has been proposed to control the oscillations in the loop by means of a crystal oscillator. In such a case it is possible to detect, over a resistor in series with the loop, a phase shift which will occur when a metal body is present in the loop. However, tuning of such loop detector circuits proved to be difficult in that complicated variable capacitor circuits had to be provided in order to vary the frequency of operation, and also a large range of crystal oscillators having different frequencies of oscillation had to be available for selection when an installation was being tuned. Due to the practical difficulties this type of loop detector arrangement has, as far as applicant is aware, been withdrawn from the market as a result of disfavour.

It is the object of the present invention to provide a loop detector arrangement which will enable fast detection of metal bodies to take place and also which will obviate great difficulties in tuning the tank circuits of which the loops form a part.

In accordance with this invention there is provided a loop detector arrangement comprising an oscillator adapted to oscillate at a predetermined frequency and connected to the input of a frequency divider adapted to provide a range of different frequency outputs according to selection thereof, the output from the frequncy divider being connected to a detector circuit embodying a phase comparator connected across a suitable resistance connected directly in series with a parallel resonant loop circuit and a state of tune sensor connected to the phase comparator.

Further features of the invention provide for the output from the frequency divider to be passed through a waveform squarer; for the frequency divider to be a programmable frequency divider connected to a binary counter to effect programming thereof, for the binary counter to be driven by a countenable oscillator which is connected to the state of tune sensor such that the countenable oscillator is inhibited when the state of tune sensor senses a predetermined analogue voltage level from the output of the phase comparator, for the binary counter to also be connected to a reset switch and for the frequency divider to be adapted to provide about 256 different output frequencies in decreasing steps of frequency, in which case an eight stage binary counter is utilized.

It will be understood that in the preferred form embodying the further features just described the tuning of a tank circuit is effected automatically since the binary counter will progressively alter the frequency on the output side of the programmable frequency divider automatically until the counter is inhibited as a result of the state of tune sensor sensing the desired analog voltage output from the phase comparator. Thus in this preferred form automatic tuning is provided and therefore the probiems normally associated with tuning are not experienced at all.

In order that the invention may be more fully understood a preferred embodiment thereof will now be described with reference to the accompanying block diagram of the circuit of a loop detector arrangement.

In this case the oscillator adapted to provide a constant frequency output is a crystal oscillator 1. A crystal oscillator is chosen since it is, at the present time, probably the most reliable source of a constant frequency of oscillation. In this particular instance the crystal oscillator is a commercially available oscillator having an output frequency of about 4,43 megahertz. The output from the crystal oscillator is fed to a programmable frequency divider 2 adapted to divide the frequency of the crystal oscillator down in equal steps to produce, in this case, 265 different frequencies thereby providing a range of output frequencies between about 300 kilohertz and 20 kilohertz. The programmable frequency divider is connected to an eight stage binary counter 3 adapted to programme the frequency divider.A count enable oscillator 4 activates the binary counter so that it automatically proceeds to programme the frequecy divider to output frequencies progressively in steps from one frequency to the next whilst the count enable oscillator is activated. The binary counter is also connected to a reset switch 5 so that the programmed series of output frequencies from the frequency divider can be re-started at any required time.

The output from the frequency divider is fed to a logic block 6 which simultaneously divides the frequency by two and squares the waveform provided by the frequency divider. The squarer feeds the square wave signals to a buffer 7 and thence through a resistor 8 to the parallel resonant loop circuit 9 of the detector.

It will be understood that the output from the buffer is a square wave whilst the signal on the loop side of the resistor 8, as a result of the inductance of the loop, is an approximate sine wave. Thus the signal from the loop side of the resistor is fed through an amplifier and waveform squarer unit 10 to a phase comparator 11 whereas the output from the buffer is fed directly to the comparator.

It will be understood that whilst no metal object is detected in the loop the two square waves received by the phase comparator will coincide but that when a metal object is sensed by the loop it will tend to pull the two phases out of synchronisation. This is detected by the phase comparator which provides an analogue output ranging say between 0 volts and 10 volts according to the condition. A nominal output voltage of say 4 volts is provided when the phases are in synchronisation.

The output from the phase comparator is fed to a state of tune sensor 12 which is adapted to inhibit the count enable oscillator 4 when the nominal voltage output from the comparator is attained.

The remainder of the circuit is of a known type and includes an amplifier 13 connected to the output from the phase comparator and the output from the amplifier is fed to a relay driver and memory unit 14 which in turn operates a relay 15 in the usual way.

It will be understood that, in use when the loop detector is switched on, the programmable frequency divider and binary counter will co-operate to progress through the range of frequencies provided at the output from the logic block 6 and when the correct frequency is attained the state of tune sensor will inhibit the count enable oscillator which in turn will stop the progression of output frequencies at the required one. The loop will then detect the presence of metal objects in known manner: the phase shift will be sensed by the phase comparator whose output will change accordingly and cause the relay driver and memory unit to be operated as required.

The state of tune sensor is, of course, arranged not to activate the count enable oscillator during such phase change detection.

It will be understood that many variations may be made to the above described embodiment of the invention without departing from the scope hereof.

In particular, the programmable frequency divider may be arranged differently and it could even be arranged to be programmed manually until such time as an indicator associated with the state of tune sensor indicates that the system is in tune. Also the crystal oscillator could be replaced by any other suitable oscillator which will provide a constant frequency output.

It will also be understood by those skilled in the art that the above described loop detector arrangement is particularly well suited for use in multi-loop installations of the type in which each loop is electronically examined in rotation so that interference between adjacent loops is totally eliminated. In such an installation, the state of each programmable frequency divider is held in its programmed condition for the periods of time during which other loops are being examined electronically. The use of the above system enables a multi-loop installation to operate in a highly effective manner since it requires an extremely short length of time to examine the analog voltage outputs from the phase comparator in respect of each loop detector circuit.It will be understood that only one phase comparator is required but apart from that the circuit described above with reference to the block diagram will generally be repeated for each loop. It is generally considered to be suitable to make installations which can treat two, three, four or five loops in the above described manner, but since the operation of the above described circuit is extremely fast it would, in this case, now be possible to multiplex up to ten four-channel units thereby servicing 40 channels and loops from a central control station, for example.

A rough comparison of times required for examining a four channel installation are that by using the old digital method based upon the frequency change in a loop one or more seconds would be required to examine four channels whereas with the above described analog system four channels could be examined in the order of a few milliseconds and even possibly one millisecond.

The invention therefore provides a highly useful loop detector arrangement which can be utilized in a multi-channel system effectively and wherein, as mentioned above, no cross talk will occur between the loops of the installation as a result of the fact that each is only activated in turn.

Claims (12)

1. A loop detector arrangement comprising an oscillator adapted to oscillate at a pre-determined frequency and connected to the input of a frequency divider adapted to provide a range of different frequency outputs according to selection thereof, the output from the frequency divider being connected to a detector circuit embodying a phase comparator connected across a suitable resistance connected directly in series with a parallel resonant loop circuit, and a state of tune sensor connected to the phase comparator.

2. A loop detector arrangement as claimed in claim 1 in which the frequency divider is programmable to provide a range of different frequency outputs according to the content of the programme input.

3. A loop detector arrangement as claimed in claim 2 in which there is included programming means for the frequency divider comprising a binary counter driven by a count enable oscillator, the binary counter output in use causing the frequency divider to output a predetermined range of different frequencies during counting of the oscillator input.

4. A loop detector arrangement as claimed in claim 3 in which the count enable oscillator is inhibited when the state of tune sensor senses a predetermined analogue voltage level from the output of the phase comparator.

5. A loop detector arrangement as claimed in claim 4 in which the binary counter is provided with a reset switch.

6. A loop detector arrangement as claimed in claim 3 in which the frequency divider is adapted to provide the different frequencies in the range in equal linear steps of frequency.

7. A loop detector arrangement as claimed in any one of claims 3,4 or 5 in which the range of frequencies includes 256 different frequencies and an eight stage binary counter is used to produce same.

8. A loop detector arrangement as claimed in any one of the preceding claims in which the output from the frequency divider is passed through a waveform squarer and thence to the detector circuit.

9. A loop detector arrangement as claimed in claim 8 in which the phase comparator input on the loop side of the resistance has a waveform squarer in series therewith.

10. A loop detector arrangement as claimed in claim 9 in which the waveform squarer in the comparator input is integral with a voltage amplifier.

11. A loop detector arrangement as claimed in any one of the preceding claims in which a plurality of parallel resonant loop circuits are adapted to be successively interrogated.

12. A loop detector arrangement as claimed in claim 1 and substantially as herein described with reference to the accompanying drawings.