GB2129123A - Intruder detector and method - Google Patents

Abstract

A method of detecting the presence or the passage of an intruder in a region under surveillance comprises: transmitting an infrared beam from a transmitter; receiving the beam in a receiver such that the region under surveillance is determined by the path followed by the beam between the transmitter and the receiver; and raising an alarm indicative of the presence or passage of an intruder whenever there is a significant change in the characteristics of the beam as received by the receiver. The infrared beam is transmitted in the form of pulses (Figure 1A), each comprising a packet of oscillations (Figure 1C) at a carrier wave frequency (Figure 1B), and the receiver is tuned to be sensitive to infrared radiation whose amplitude varies at said carrier wave frequency. This provides considerable immunity from interference by such natural phenomena as the sun and fog, and can also be made highly tamper-proof. <IMAGE>

Description

SPECIFICATION Intruder detector and method The present invention relates to a method of detecting intruders, and to an intruder detector implementing the method.

In security systems there is often a need to detect the presence of an intruder in a region under surveillance, or to detect the passage of an intruder through such a region. Such intruders are generally people, often people attempting to break in somewhere, but the term "intruder" is used in a wider sense to cover other objects capable of intruding, e.g. motor vehicles or animals. The present invention relates in particular to an intruder detector and method that make use an "infrared barrier".

An infrared barrier generally comprises a separate transmitter and receiver, and the region under surveillance may be a perimeter or a volume which may be indoors or out of doors. The main distinctions between indoor and outdoor equipment lies in whether the receiver needs protecting from solar radiation, and whether the transmitter and the receiver need to be lodged in housings which are weatherproof, and perhaps thermostatically heated. Apart from such differences due to the intended operating environment, infrared barriers all operate on much the same principle, namely; infrared radiation, usually modulated in some way, is transmitted by the transmitter, and is then received by the receiver. An intrusion causes some kind of change to the radiation as received compared with the radiation as received under normal circumstances.

The transmitter often makes use of an infrared emitting diode operating in the near infrared. To save energy and to avoid overheating the transmitting diode, it is common practice to transmit a short pulse lasting a millisecond or so once every hundred or so milliseconds, giving a repetition rate of around ten hertz.

Infrared barriers of this type operate satisfactorily indoors, but they are not completely reliable outdoors, if only because the barrier can be "blinded" by the sun or by fog. There is the further problem that both indoors and outdoors, such barriers are liable to tampering, e.g. by using a portable transmitter to "dazzle" the receiver and prevent it from detecting intruders.

Preferred implementations of the present invention provide a method of detecting intruders and a device employing the method, said preferred implementations being substantially immune to the problems outlined above.

In one aspect the present invention provides a method of detecting the presence or the passage of an intruder in a region under surveillance, the method comprising: transmitting an infrared beam from a transmitter; receiving the beam in a receiver such that the region under surveillance is determined by the path followed by the beam between the transmitter and the receiver; and raising an alarm indicative of the presence or passage of an intruder whenever there is a significant change in the characteristics of the beam as received by the receiver; wherein the infrared beam is transmitted in the form of pulses, each comprising a packet of oscillations at a carrier wave frequency, and wherein the receiver is tuned to be sensitive to infrared radiation whose amplitude varies at said carrier wave frequency.

In another aspect, the present invention provides an intruder detector implementing the above-defined method, the detector comprising a transmitter of infrared radiation, and a receiver of infrared radiation, the transmitter comprising means for transmitting pulses of infrared radiation in the form of packets of oscillations at a carrier frequency, and the receiver comprising an infrared radiation sensitive device connected to tuned means responsive to a signal whose amplitude varies at said carrier frequency, a phase locked loop detector for extracting a useful signal from the tuned means, useful signal processing means connected to the detector, and alarm means connected to the processing means.

An embodiment of the invention is described by way of example with reference to the accompanying drawings, in which: Figures 1 A, 1 B, and 1 C are waveform diagrams respectively showing: infrared signals as transmitted by a conventional infrared barrier; a steady high frequency oscillation; and infrared signals as transmitted in accordance with the present invention; Figure 2 is a block diagram of an infrared transmitter in accordance with the invention; Figure 3 is a block diagram of an infrared receiver in accordance with the invention; and Figures 4A to 4F are waveform diagrams showing different voltages in the receiver of Figure 3 as a function of time.

Reference is made initially to Figures 1 A to 1 C.

A low frequency pulse waveform 1 A is multiplied by a high frequency oscillation 1 B. This provides a signal in the form of a series of packets of oscillations 1 C. For the sake of clarity, only a few cycles are shown in each packet, but a typical number might be sixteen oscillations per packet.

Figure 2 is a block diagram of an infrared transmitter showing how such signal multiplication of the kind outlined above is applied to control the driving current of an infrared emitting diode. An unregulated DC power supply 1, represented in the drawing by a 1 2 volt battery, feeds a regulator 2. Regulated DC at less than 12 volts powers a carrier wave oscillator circuit 3, and a pulse wave generator circuit 4. The circuits 3 and 4 output signals whose waveforms are shown in Figures 1 B and 1 A respectively. These two signals are applied to respective inputs to a multiplier circuit 5 whose output provides a waveform as shown in Figure 1 C. This modulated waveform is applied to the input of a power amplifier 6 connected to drive an output infrared emitting diode 7.Infrared radiation whose amplitude is modulated by the Figure 1 C waveform leaves the diode 7 via an optical system 8, represented in the drawing by a single converging lens. In a working system, the resulting modulated infrared beam is directed in such a manner as to be detected by an infrared receiver.

Figure 3 is a block diagram of a receiver suitable for use with the transmitter shown in Figure 2. It comprises a receiver optical system 11 for focusing received infrared radiation onto an infrared detector diode 12. A sourcedyne impedance converter 13 matches the diode's output impedance to the input impedance of a cascode connected amplifier 14. The amplified signal is applied to a high input impedance bandpass filter and amplifier 1 5 whose output is limited by a limiter 16 before being applied to a phase locked loop (PPL) detector 1 7. The circuit described thus far serves to re-create as accurately as possible the transmitted pulsed carrier waveform as shown in Figure 1 C.

The main problem up to this point is obtaining an adequate signal-to-noise ratio. The remaining receiver circuits downstream from the phase locked loop detector 17 work on the basis that an adequate signal-to-noise ratio has been achieved, and they serve to distinguish certain kinds of false alarms from genuine alarms, and also to detect certain kinds of tampering.

In the embodiment shown in Figure 3, said downstream receiver circuits are arranged in two branches. A first branch includes a local oscillator 1 9 capable of locking on to the low frequency component (waveform 1 A) in the received infrared pulses. The local generator 1 9 serves as a local standard representative of the timing of said low frequency component as received. A threshold circuit 1 8 blanks out the pauses between pulses in the received signal, but during the packets of received high frequency oscillations, it forwards a pulse to a comparator 20 which also receives the locally produced pulses from the generator 19.So long as both inputs to the comparator remain the same it produces a zero output, but if there is a difference, a difference-indicating output signal is produced and is applied to an integrator 21. The integrator 21 is followed by a threshold detector 22, a monostable 23 and an alarm-controlling relay 24. The threshold used by the detector 22 and the integrator's natural rate of decay are chosen such that some minimum number of consecutive pulse differences must occur before an alarm is raised, and so that the effect of occasional isolated pulse differences is not cumulative.

The operation of the first branch of the downstream receiver circuits can be better understood from the waveforms in Figures 4A to 4F. Figure 4A shows an idealised waveform as might be expected from the output of the limiter 1 6. One packet of pulses is missing, for example because a bird has flown through the region under surveillance. In this example it is assumed that bird-sized intruders are to be ignored, but that man-sized intruders should raise the alarm. Figure 4B shows a more realistic waveform where the amplitude of the signal is not very much greater than the amplitude of the ndise which accompanies it. Such a waveform is typical of a distance of 150 metres between the transmitter and the receiver.Figure 4C shows the corresponding output from the phase locked loop detector 17, as cleaned up if necessary, by the threshold circuit 1 8. An output is used for the detector 17 which reproduces the low frequency pulse signal which is modulating the carrier frequency signal. Figure 4D shows the corresponding low frequency signal output by the local generator 1 9. The two signals differ for the duration of the missing pulse, and this results in a rise in the output voltage from the integrator 21 (Figure 4E). The integrator output then decays over the next few pulses, and no alarm is raised, as indicated by the waveform at the output of the comparator 22 which is shown in Figure 4F.

However, since the integrator output takes several pulses to decay, a sequence of consecutive missing pulses would be cumulative, and the component values could be arranged to raise an alarm after, say, five consecutive pulses have been missed.

A second downstream receiver branch is shown in simplified form in Figure 3. This branch comprises an amplifier 25, a threshold comparator 26 and a second alarm-controlling relay 27. The threshold used by the comparator 26 is set to be slightly higher than the highest level of signal received from the genuine transmitter in some particular location relative to the receiver, The second branch is intended to provide a special "tampering" alarm in the event that an attempt is made to "dazzle" the receiver with pulses generated by a transmitter carried by an intruder.

To have any effect at all, the intruder's transmitter would need to be transmitting a carrier wave at substantially the same frequency as the genuine transmitter, and even then it would be unlikely to be transmitting pulses in time with the genuine pulses. So an intruder with a second transmitter would almost certainly raise an alarm via the first branch. However, the second branch provides added security, particularly against an intruder who manages to displace the genuine transmitter nearer to the receiver in order to give himself free space in which to move behind the displaced transmitter.

The use of a frequency carrier wave at a "high" frequency, where the high frequency would typically be around 1 6 kHz, provides considerable protection against interference from the sun or from fog. The receiver circuit is insensitive to unvarying infrared signal levels, even at high intensity, while a component varying at the chosen carrier frequency is readily extracted from a much higher general background level of infrared radiation. Thus, to be effective, interference must not only be constituted by infrared radiation in that part of the spectrum to which the detector 12 is sensitive, but it must also be in the form of a carrier wave at the system frequency, having substantially the same amplitude at the receiver as the genuine signal, and modulated by pulses that are synchronised with the genuine pulses.

A single infrared transmitter may be used with a plurality of receivers, in which case means should be provided to split the transmitter beam into beams directed to each receiver. One way of doing this is to place the transmitter diode away from the focus of the optical system 8 so as to obtain a divergent beam from the transmitter.

Various improvements can be devised within the context of the invention for further increasing security. The pulses need not be transmitted regularly or at a fixed frequency, but could be transmitted in a coded sequence; in which case the receiver's local generator 1 9 would need to be locked into the right place in the sequence. If transmission conditions are such as to ensure relatively noise-free transmission (e.g. indoors or over relatively short distances), the downstream receiver circuits could be arranged to check the phase of the carrier frequency, or simply to count the number of cycles of carrier that are received for each pulse. Sequences similar to those suggested for the pulses could thus be encoded in the number of carrier wave cycles transmitted in each pulse.

Digital circuits would be almost essential for implementing such improvements, and could well be used in place of some of the analog circuits described above, e.g. the integrator 21 could be implemented as an up/down counter.

Claims (12)

1. A method of detecting the presence or the passage of an intruder in a region under surveillance, the method comprising: transmitting an infrared beam from a transmitter; receiving the beam in a receiver such that the region under surveillance is determined by the path followed by the beam between the transmitter and the receiver; and raising an alarm indicative of the presence or passage of an intruder whenever there is a significant change in the characteristics of the beam as received by the receiver; wherein the infrared beam is transmitted in the form of pulses, each comprising a packet of oscillations at a carrier wave frequency, and wherein the receiver is tuned to be sensitive to infrared radiation whose amplitude varies at said carrier wave frequency.

2. A method according to claim 1, wherein an alarm is raised after a predetermined number of consecutive pulses are absent in the received radiation.

3. A method according to claim 1 or 2, wherein an alarm is raised if the amplitude of the received infrared radiation exceeds a threshold value.

4. A method according to claim 3, wherein an alarm indicative of intruder tampering is raised if the amplitude of the received radiation exceeds said threshold value.

5. A method of detecting the presence or passage of an intruder substantially as herein described with reference to the accompanying drawings.

6. An intruder detector implementing the method of any preceding claim, the detector comprising a transmitter of infrared radiation, and a receiver of infrared radiation, the transmitter comprising means for transmitting pulses of infrared radiation in the form of packets of oscillations at a carrier frequency, and the receiver comprising an infrared radiation sensitive device connected to tuned means responsive to a signal whose amplitude varies at said carrier frequency, a phase locked loop detector for extracting a useful signal from the tuned means, useful signal processing means connected to the detector, and alarm means connected to the processing means.

7. An intruder detector according to claim 6, wherein the transmitter comprises an oscillator for generating said carrier frequency, a pulse generator for generating pulses at a low frequency relative to the carrier frequency, a multiplier for multiplying the carrier frequency by the pulses, and an infrared emitting device controlled to emit infrared radiation in response to a multiplication output signal from the multiplier.

8. An intruder detector according to claim 6 or 7, wherein the receiver signal processing means includes integrator means connected to raise an alarm when a predetermined number of consecutive pulses is missing from the received infrared radiation.

9. An intruder detector according to claim 6, 7 or 8, wherein the receiver signal processing means includes signal level responsive means connected to raise an alarm when the received infrared signals exceeds a predetermined threshold amplitude.

10. An intruder detector according to.claim 9, wherein said signal level responsive means is connected to raise an alarm indicative of tampering by an intruder.

11. An intruder detector according to claim 6, 7, 8, 9 or 10, comprising a single transmitter used in conjunection with a plurality of receivers.

12. An intruder detector substantially as herein described, with reference to the accompanying drawings.