GB2182474A - Intruder detector signal processing - Google Patents

Abstract

A genuine human intrusion through an electric or electromagnetic field barrier gives rise to a characteristic form of signal including relatively high and relatively sharp positive and/or negative peaks (inflections). To detect this accurately, the signal received from the barrier is applied to detector means 1, which gives a first output signal whenever there is, selectively, either an increase (or decrease) in signal level exceeding a predetermined amplitude, and also to circuit means 2, 3, 6 which holds the value of the maximum (or, respectively, the minimum) level attained, compares this maximum (or minimum) value with the subsequent level, so as to detect a fall (or rise) in signal level, and gives a second output signal when the difference between the maximum (or minimum) value and the subsequent level exceeds a predetermined amount, an alarm signal being transmitted whenever the first and second output signals occur during the course of a predetermined time period. <IMAGE>

Description

SPECIFICATION Signal processing This invention relates to a method of processing signals from intrusion detection devices of the kind which are used to form a perimeter detection system around a site to be protected with a view to determining when an intrusion or an attempted intrusion takes place.

Intrusion detection devices of this kind operate by creating an electric or electromagnetic field along a suitable perimeter line between transmitter and receiver units. Intrusion of the field by a human results in a detectable perturbation- of that field which usually involves either an increase or a decrease in the level of output signal from the receiver unit compared to the standard unperturbed signal level. Accordingly, most previously proposed detection circuitry has operated by generating an alarm signal whenever the signal level has increased or decreased beyond a predetermined threshold. Often a timing element is included in the circuitry too so that an alarm signal is generated only when the signal level has exceeded the relevant threshold for a predetermined time period.

The setting of an appropriate threshold level and of an appropriate time period avoids false alarm signals being generated by low-level or very short lived perturbations as may be caused by wildlife entering the field or by certain environmental changes. However, some environmental changes can give rise to large and fairly sudden increases or decreases in signal level which will, with these known systems, generate false alarms.

The signal change brought about as a result of an intrusion, for example by a human being fully traversing the field, has a characteristic form consisting of successive signal changes of opposite polarity, and environmental changes, however severe, would not normally simulate this particular form.

It is an object of the present invention to provide a method of identifying the aforesaid characteristic form of signal change and of generating an alarm signal only on the occurrence of such a change thereby avoiding the problems inherent in previous method of processing signals from intrusion detection devices.

With this object in view, the present invention provides a method of processing a received signal from an intrusion detection device comprising applying the signal to detector means which gives a first output signal when there is selectively, either an increase (or a decrease) in signal level exceeding a predetermined amplitude and also applying the signal to circuit means which holds the value of the maximum (or, respectively, the minimum) signal level attained, compares this maximum (or minimum) value with the subsequent level so as to detect a fall (or rise) in signal level, and gives a second output signal when the difference between the maximum (or minimum) value and the subsequent level exceeds a predetermined amount, an alarm signal being transmitted whenever the first and second output signals occur during the course of a predetermined time period.

It will be appreciated that the aforesaid circuitry is able to detect successive signal changes in opposite directions, namely either a positive signal excursion followed by a reduction in signal level (a positive signal inflection), or, depending on the chosen detector means and circuit means, a negative signal excursion followed by a rise in signal level (a negative signal inflection). Of course, both the increase and subsequent reduction of signal level associated with a positive inflection must exceed a predetermined value and occur within a predetermined time to give rise to an alarm. Similarly, if a negative signal inflection is concerned, both the decrease and subsequent rise in signal level must exceed a predetermined value and occur within a predetermined time to give rise to an alarm.

Preferably the received signal is applied simultaneously to respective detector means and circuit means, the first of which detect an increase and subsequent fall in signal level and give rise to an alarm signal when their threshold and time criteria are met, and the second of which detect a decrease and subsequent rise in signal level and give rise to an alarm signal when their threshold and time criteria are met.

At least one inflection in the signal level of sufficient magnitude and rapidity should occur when the field of an intrusion detection device is traversed by a human intruder. However, such an inflection is most unlikely to be caused by environmental phenomena, such as storms, which would only be likely to cause a large and sudden increase or decrease in signal level and only a very gradual change back to the initial quiescent level.

Preferably the first output signal of the respective detector means, which is provided when its threshold amplitude is exceeded, serves to trigger a timer which enables an AND-gate, the AND-gate only generating an alarm signal if the second output signal is applied to it before the timer times out its predetermined period.

The circuit means providing the second output signal may conveniently include a track and hold circuit to hold the value of the maximum (or minimum) value attained, and a comparator to compare the maximum (or minimum) value with the actual level. If no alarm signal is generated within the predetermined period the timer operates to re-set the track and hold circuit.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram illustrating one half of a simple circuit which can be used to carry out the method of signal processing in accordance with the invention; and Figure 2 is a diagram illustrating one half of a more complex circuit which can be used to carry out the method of the invention.

Figure 3 illustrates two examples of the form of signal changes which are detected and which give rise to alarm signals when the method of the invention is employed.

Referring to Fig. 1 and assuming that an intrusion of a detection field will result in an initial positive going signal excursion, the signal is applied to three elements, namely a positive threshold detector 1, a positive track and hold circuit 2 and a differential amplifier 3.

When the signal excursion exceeds a threshold level preset in the detector 1, the latter provides an output signal which triggers a timer 4. The timer 4, in turn, passes an enabling signal to an AND-gate 5. The track and hold circuit 2 follows the input signal and holds the maximum value thereof so that when the input signal starts to decline (at the positive inflection point) the differential amplifier 3 provides a positive output to an alarm comparator 6. When the comparator 6 input exceeds its threshold level it provides an output to the AND-gate 5 and providing this output occurs before the expiration of the timing period determined by the timer 4 alarm output is generated by the AND-gate 5.However, if the timer 4 times out before the input signal declines sufficiently for the comparator 6 to provide an output to the AND-gate 5, the AND-gate 5 is disabled and the track and hold circuit 2 is reset by the timer 4 so the circuitry effectively returns to its initial condition.

The foregoing describes the operation for a positive going signal excursion only, but as previously mentioned, it is a preferred feature of this invention that both positive and negative going signal inflections should be catered for and therefore a second mirror image circuit is provided, as indicated in Fig. 1, which responds in like manner to that previously described but responsive to negative going signal excursions. Thus provided that the respective threshold and timing criteria are met an alarm output will be given by the AND-gate 5 in the event of a positive signal inflection and by the corresponding AND-gate 5a in the event of a negative signal inflection. The outputs of either of the two AND-gates 5, 5a being transmitted via an OR-gate 7.

A refined version of the circuitry is illustrated in Fig. 2. Elements which serve the same function as in Fig. 1 have been allocated the same reference numerals and it will be seen that three addition elements 8, 9, 10 have been incorporated into the part of the circuit relevant to detection of a positive signal inflection. An inflection detector 8 receives the input signal directly and the output from the track and hold circuit 2 and gives an output in the event of a decrease in the input signal immediately following an increasing signal input. The outputs from this inflection detector 8 and from the positive threshold detector 1 are fed to an AND-gate 9 such that a trigger signal is only passed to the timer 4 if detection of a positive signal excursion is immediately followed by a decrease in signal level.This ensures that in the event of a rapid signal increase such as caused by environmental changes e.g. sudden onset of a rain storm followed by a slow decline in signal level due to the time constants of the preceding circuitry (not shown) the inflection detector 8 will not operate before the timer 4 has timed out and therefore an alarm state is inhibited.

Finally, a buffer element 10 is incorporated prior to the differential amplifier 3, which element 10 functions to pass only positive going signals to the differential amplifier 3, thus preventing spurious triggering of the alarm comparator 6 due to negative going input signals.

As in the previous case of Fig. 1 a separate channel is provided for the processing of negative going signals. It will be appreciated that the band pass characteristics of the previous circuitry will affect the overall performance, and are tailored accordingly. In addition, the time constants of the described circuitry will also affect the circuitry performance, and are defined according to the detailed performance requirements.

Claims (7)

1. A method of processing a signal received from an instrusion detection device comprising applying the signal to detector means which gives a first output signal when there is, selectively, either an increase (or a decrease) in signal level exceeding a predetermined amplitude and also applying the signal to circuit means which holds the value of the maximum (or, respectively, the minimum) signal level attained, compares this maximum (or minimum) value with the subsequent level, so as to detect a fail (or rise) in signal level, and gives a second output signal when the difference between the maximum (or minimum) value and the subsequent level exceeds a predetermined amount, an alarm signal being transmitted whenever the first and second output signals occur during the course of a predetermined time period.

2. A method as claimed in claim 1 wherein the received signal is applied simultaneously to respective detector means and circuit means, the first of which detect an increase and subsequent fall in signal level and give rise to an alarm signal when their threshold and time criteria are met and the second of which detect a decrease and subsequent rise in signal level and give rise to an alarm signal when their threshold and time criteria are met.

3. A method as claimed in claim 1 or 2 wherein the first output signal, emanating from the detector means or the respective detector means when its threshold amplitude is exceeded, serves to trigger a timer which enables an AND-gate, the AND-gate only generating an alarm signal if the second output signal is applied to it before the timer times out its predetermined period.

4. A method as claimed in claim 3 wherein the circuit means providing the second output signal includes a track and hold circuit to hold the value of the maximum (or minimum) value attained, and a comparator to compare the maximum (or minimum) value with the actual level.

5. A method as claimed in claim 4 wherein the timer operates to re-set the track and hold circuit if no alarm signal is generated within the predetermined period.

6. A method as claimed in claim 4 or 5 wherein the received signal is also applied to an inflection detector which compares same with the maximum (or minimum) value held by the track and hold circuit and only permits triggering of the timer in the event of a sufficiently sharp positive (or negative) signal inflection.

7. A circuit arrangement for carrying out the method claimed in any preceding claim and substantially as hereinbefore described with reference to and as illustrated in Fig. 1 or Fig. 2 of the accompanying drawings.