GB2060967A - Intruder alarm system - Google Patents

Abstract

An intruder alarm system comprising an intruder detection device A at least two alarm signal pulse length timers (SPX, SPY) and a coincidence detector D so interconnected electrically as to provide an alarm signal (Z') only when coincidence occurs between two or more signals (X''Y'') produced by said alarm signal pulse length timers (SPX, SPY) over a short period of time (800 ms-30 s). Two different intruder detection devices A, such as one operating on electromagnetic waves and one on ultrasonic waves, may be used (Figure 1). <IMAGE>

Description

SPECIFICATION Intruder alarm system This invention relates to an intruder alarm system.

It is known that an intruder detection device may give a false alarm signal attributable to interalia environmental disturbences, changes in the siting condition or incorrect siting.

i For example, intruder detection devices employing ultrasonic techniques may be sent into a spurious alarm condition by sound waves from telephone bells, door bells and other sources of high frequency sound, other ultrasonic detectors, air conditioning, spaced forced air heaters, moving drapes and even the movement of domestic animals.

ii Intruder detection devices employing microwave techniques may be sent into a spurious alarm condition by disturbance similar to those in i above. There is also an additional hazard in that if care is not taken in siting the alarm, microwaves may penetrate the fabric of the building and may be triggered from without.

iii Intruder detection devices employing electrical capacitance changes may be sent into a spurious alarm conditions by numerous means including changes in temperature and humidity.

iv Intruder detection devices using passive or active infra-red, although perhaps less prone to false alarms than the aforementioned devices, may also go into a spurious alarm condition the former due to heat radiation within the area being protected and the latter due to high ambient light such as sunlight shining directly into the optical area of the device.

Since such spurious alarm conditions represent a proportion of the alarm signals given during the operation of such intruder alarm systems and there appears to be no single intruder detection device that is completely free from this hazard, there is an urgent need for a reduction in faise alarm signals in intruder detection devices. To that end we provide according to the present invention an intruder alarm system comprising an intruder detection device at least two alarm signal pulse length timers and a coincidence detector so interconnected electrically as to provide an alarm signal only when coincidence occurs between two or more signals produced by said alarm signal pulse length timers cover a short length of time.

In a preferred system the intruder detection devices are connected with a signal processing device that includes (i) means for producing pulses of fixed length for presentation to a coincidence detector and (ii) a fault detector.

Conveniently an intruder alarm may comprise two different intruder detection devices electrically interconnected and provided with electrical signal processing such that both devices would essentially have to be sent into an alarm condition in a short time that is to say between 800 milliseconds and 30 seconds before an alarm was produced from the alarm, preferably 800 milliseconds to 5 seconds.

Consider different forms of intruder detection devices such as for example those operating from the energy in ultrasonic or acoustic waves, microwaves, passive and active infra-red light, let us call these different intruder detection devices A, B, C, D and E (n = 5) from which ten different combinational pairs (y 2) appear since: 5' 120 ~ 120 ~ 2' (5-2)' - 2x6 = 12 = 10 AB BC CD DE AC BD CE AD BE AE If we were to use five different devices (n = 5) and select any three (y = 3) then again ten different combinational triads appear since:: 5' = 120 = 120 ~ ~~~~~~ - - - 10 3' (5-3)' 6 x 2 12 ABC BCD CDE DEA EAC ABD BCE CDA DEB ABE and this applies mutatis mutandis with any number of different devices n such that where y is any integer the number of different combinations that can be formed is n' y' (n-y)' It may be that one has to install devices having a repetitive combination such as ABA, CAC an arrangement that will give diminished security. To extend such combinations above combinations of three is expensive and generally unnecessary.

The invention will be more clearly understood from the following description given by way of example only with reference to the Figures of the accompanying drawing in which Figures 1 and 2 are circuit diagrams and Figures 1A, 1 B; 2A, 2B, 2C a sequence of wave diagrams for different conditions in said circuits.

In Figure 1 two different intruder detection devices A and B provide respectively when tripped alarm signals X and Y. These signals each enter a signal process unit C including a pair of excess length alarm signal detectors ELx, ELY and alarm signal pulse length timers SPx, SPy each so connected as to receive the X and Y signals as designated by the suffix letters X and Y.

Detectors ELx, ELy allow a permanent alarm condition to generate a fault signal X' Y' to fault detector F and timers SPx, SPy produce a pulse of fixed length to give signals X" Y" respectively that enter a coincidence detector D providing, when coincidence occur, an alarm signal Z'.

Consider now the conditions shown in sequence Figures 1A. Intruder detection devices A and B have produced detection signals X' and Y' spaced in time by time t1. These signals respectively enter interalia alarm signal pulse length timers SPx, SPy where they each produce a fixed length pulses X" Y' of length in time respectively t2x, t2y giving coincidence over the time t3 which is recognised by the coincidence detector D to produce alarm signal Z'.

Excess length alarm signal detectors ELX ELy feed fault detector F with X' Y' to recognise any fault and generate a signal. In the conditions postulated for Figure 1A no fault signal is produced.

Consider now the conditions shown in sequence Figures 1 B. Intruder detection device A has gone into a permanent alarm condition producing continuous signal X which produces from ELx and F a fault signal Z and a fixed length pulse X" of length in time t2x. If now ata later time a detection signal Y is produced it enters SPy and produces a fixed length pulse Y" of length in time t2y but there is no coincidence of signals X" and Y and coincidence detector D does not produce an alarm signal; the fault is, however, announced from Ffor immediate attention to call for rectification of the alarm system.

Consider now one intruder alarm having but an intruder detection device alone.

The operation would be such that an alarm signal Z' would only result if the intruder detection device operated more than once with a preset short time period.

In Figure 2 the pulse length timer SPy is connected in a mode which will allow its output pulse to commence when the output from pulse length timer SPx returns to normal after an initial alarm condition.

In Figure 2A a single brief alarm signal from the intruder detector A produces pulse X' that initiates an output from XPx, X" this is presented to the coincidence detector D and to SPy.

When the signal from SPx resets, a pulse of predetermined length is generated by SPy, Y" there is no coincidence between Y" and X" and no alarm output Z' is generated.

In Figures 2B two signals from the detector at X operate in the same manner as Figure 2A except that when a second signal is received by SPx whilst a pulse is being generated by SPy, coincidence between X" Y" occurs at the cross hatched area and an alarm output from Z' will result.

In Figure 2C the detector A is in a permanent alarm condition, only one pulse will occur at X". This will not coincide with Y" and no alarm will be generated at Z'. However, the "excess length alarm signal detector" ELx will reveal this condition which will result in a signal X' and a fault output Z.

The use to which the output Z is put will depend on the particular circumstances. If detectors are employed that cannot normally generate a permanent alarm condition, e.g. microwave or ultrasonic detectors, this output X would be directed to a fault indication circuit only, but when used with detectors which would normally give a permanent alarm, if for instance they were permanently obstructed, e.g. beam interruption devices, it might be considered prudent to connect output Z to the intruder alarm circuit.

By using two or more intruder alarm systems in combination as separate units or separate circuits within one unit, a system could be made up that would provide alarm signals only if either two or more intruder detection devices operated within the set short period or if one only of the intruder detection devices operated two or more times.

Claims (7)

1. An intruder alarm system comprising an intruder detection device at least two alarm signal pulse length timers and a coincidence detector so interconnected electrically as to provide an alarm signal only when coincidence occurs between two or more signals produced by said alarm signal pulse length timers over a short period of time.

2. The intruder alarm system according to claim 1 wherein an excess length alarm signal detector is included to disclose a fault condition in the intruder detection device.

3. The intruder alarm system according to claim 1 or claim 2 wherein two or more intruder detection devices are provided each connected to its own alarm signal pulse length timer.

4. The intruder alarm system according to claim 3 wherein each intruder detection device is of a different form.

5. The intruder alarm system according to claim 4 wherein two different forms of intruder detection devices comprise one operating from electro-magnetic waves and one from ultrasonic waves.

6. An intruder alarm system constructed and arranged to operate substantially as hereinbefore described and as shown in Figures 1, 1A and 1 B.

7. An intruder alarm system constructed and arranged to operate substantially as hereinbefore described and as shown in Figures 2, 2A, 2B and 2C.