GB2230609A - "Air pressure change intruder detection systems" - Google Patents

Abstract

It is often desirable to detect and signal the presence of an intruder in a defined area. The present invention suggests a novel form of infra-sonic detection system in which the selectivity of the detector device is improved by using differential amplification techniques. It proposes detection and alarm apparatus comprising a sub-sonic pressure wave transducer 42 whose signal is fed to first filter means 43 (to discriminate between sonic and sub-sonic pressure waves) and then amplified, the output then going to second filter means driving a differential amplifier 44 the output of which is a "difference" signal relating only to the sub-sonic frequencies of interest, which signal is then fed to a control unit 45 and an alarm device 46 triggerable thereby. A full component circuit diagram is given. The desired response is obtained by adjusting a resistive element in one of the filters at the input to the differential amplifier 44. <IMAGE>

Description

Detection Svstems This invention concerns detection systems, and relates in particular to such systems, useful in the construction of burglar alarms and the like, that slmploy sub-sonic detectors.

For many purposes it may be desirable to detect and signal the presence of an intruder in a defined area typically, for example, a burglar breaking into a house, or an unauthorised person entering a security area.

Present-day detection systems are mostly of the infrared or ultra-sonic variety; the former react to the heat (infra-red) emissions of the intruder, whilst the latter are rather like radar devices, and emit sound (ultra sound) of a frequency above the normal human hearing range (30 to 20,000 Hertz) and look for the change in the reflected signal to be expected when something moves into the emitted beam. All these systems suffer, however, from various disadvantages. For example, they tend to be "line-of-sight" devices0 and only react to what they can actually "see" in their rather narrow field of view.Another disadvantage is that the detectors are relatively short-range devices, and a large number of them are needed to cover a large area, especially if that area is broken up into a number of smaller areas (as a house is made up of rooms).

It has previously been suggested that these, and other, problems be overcome by using a quite different type of detector device, namely a sub-sonic (infra- sonic) device that responds to very slow changes in the pressure of the ambient air - pressure changes that are so slow they correspond to sound waves in the region below 20 Hz, which is well below the normal human hearing range. Some technical success has been achieved with these infra-sound devices, despite the problem of producing an actual detector that responds only to such low frequencies, but none seem to have worked well enough to have attained any commercial success.The present invention suggests a novel form of infra-sonic detection system that is technically better designed to do what is required, in which the selectivity of the detector device is improved by employing differential amplification techniques to identify those inputs in the frequency range of interest.

In one aspect, therefore, the invention provides detection and alarm apparatus comprising: a sub-sonic pressure wave detector providing a useable output signal; first filter means, to discriminate between sonic and sub-sonic pressure waves; amplifier means, to amplify the detector's output signal; second filter means, being a bandpass filter, to select the required frequency band of the amplified signal; and an alarm device triggerable by the thus-filtered signal; and wherein the second filter means includes a differential amplifier the input of which is the detector's amplified and then filtered output and the output of which is a "difference" signal relating only to the frequencies of interest.

The pressure transducer can in general be of any type, including conventional microphones, provided it is indeed responsive to sub-sonic pressure waves (that is, waves in the region below 20 Hz, and especially in the region between about 0.5 Hz and 10 Hz) - and also provided that it has sufficient response or signal-tonoise ratio within the sub-sonic band of interest. A typical type is an electric condenser - or "electret" microphone.

The detector is associated with first filter means to enable the system to discriminate between the subsonic pressure waves to which it has to respond and the sonic (audible) and ultra-sonic waves which it must in effect ignore. The filter is a lowpass filter in that it filters out - removes - the unwanted higher frequency pressure waves, so passing on to the transducer only the required sub-sonic pressure waves.

The purposes of the amplifier and (electronic) bandpass filter are firstly to amplify the output from the pressure transducer to a sufficient level, and secondly to provide selection of only the sub-sonic pressure waves of interest so that only these result in the switching-on (triggering) of the subsequent alarm circuit. The level of amplification (desirably about 100 to 1000 times) and the selection of the frequency of interest (conveniently passing signals in roughly the 0.5 to 5.0 Hz range, and filtering out signals from 10 Hz upwards) is made to allow the system to discriminate against ambient pressure events that do not correspond to the opening of a door or window, say, or to movements of people or objects.Studies have indicated that it is possible to discriminate with a very low false alarm rate and high reliability between the events of interest - i.e. doors or windows being opened or forced - and ambient pressure changes caused by atmospheric events such as the wind or the movements of vehicles.

The detection and selection of the required frequencies can be attained, as in earlier apparatus, by the use of a comparator which uses a fixed reference (voltage) against which the signal from the amplifier is compared, and thus which effectively controls the sensitivity of the apparatus. However, a more efficient way of discriminating between the required and non-required frequencies, employed in the apparatus of the present invention, is by the use of a differential amplifier preceded by two slightly different low-pass filters.The detector's amplified output is fed to these two filters, and the slightly different outputs therefrom (because of the filters' different charActeristics) are fed to a differential amplifier that then produces an amplified "difference" signal relating only to the frequencies of interest, and thus useable to trigger the subsequent circuitry (specifically, the alarm). Choosing the appropriate filter pair, once the desired frequency selection range has been decided, is a relatively simple mathematical matter of calculating the necessary R/C values of the filters. For example, for the output of the differential amplifier to be signals only in the range 0.5 to 5.0 Hertz the R/C values of each filter (as in the accompanying Drawings, discussed further hereinafter) should be about 10 kilohms and 4.5 microfarads.

With this differential amplifier method, the actual sensitivity and selectiveness of the system is determined by the difference between the characteristics of the two filters. In the particular example given above, this difference may be arranged by having one of the R elements in the form of a variable resistor, which can in use be adjusted up or down to give the system the desired response.

The purpose of the alarm is to provide an indication that something has happened and been detected - for example, that an intruder has entered, or attempted entry to, the protected area. The alarm can be of any suitable type - for example, any audible or visible alarm, such as a siren, sonar alert, flashing beacon or other conventional alarm-indicating device and for the purpose of this invention includes devices that will transmit signals to remote indicators or to a remote monitoring unit. The alarm is triggered by the output of the amplifier and filter unit. Depending on the application the alarm can naturally be configured to remain on for a minimum time interval regardless of the duration of the trigger.

It will normally be desirable to provide, in addition to the detector, pre-filter, amplifier, postfilter and alarm, some sort of control unit to provide various optional/essential facilities, depending on the application. These may include a power supply (such as batteries or a mains-powered low voltage supply), test facilities, low battery indication, entry and exit delay, key switched "on", "off" and "test", and a disable function to prevent aelf-triggering by the alarm device. The control unit can be implemented in any conventional micro-elestronics, such as a CMOS logic gate or a single chip microprocessor.

The purpose of the system of the invention is to detect the sub-sonic (below the audible frequency range; i. e. less than 1OHz) pressure waves produced by the P.e.ning rk wor. or windows, or by the movements of people or objects within a room, building or any suitable space, and then use the detection to switch on an alarm. The alarm would therefore give an indication of an intruder's presence within the room, or of an attempted entry to the room.

The advantages of the invention are that the detector does not need line of sight access to the areas to be protected, is unaffected by obstructions, and is totally passive. In addition, by selecting particular sub-sonic frequencies or frequency bands it is possible to discriminate between ambient pressure changes caused by non-intruders and pressure changes caused by intruders or intrusion attempts. It is further possible to optimise the discrimination for a particular application such as a car or household dwelling.

An embodiment of the invention is now described, though by way of illustration only, with reference to the accompanying Drawings in which: Figure 1 shows a schematic block diagram of a detection system of the invention; and Figure 2 shows a circuit diagram of one such detection system.

As shown in Figure 1, the essential elements of the second embodiment of sub-sonic detector/alarm are five units interconnected as shown. The units are as follows: - A pressure transducer (42) receiving and transducing low frequency pressure waves; an amplifier and electronic bandpass filter (43); a double low-pass filter and differential amplifier (44); a control unit (45) to provide various optional and essential facilities including a power supply and test facilities; and an alarm (40).

The operation of this system is described with reference to Figure 2.

The signal output from the pressure transducer (42) la passed to an operational amplifier (43) configured as a bandpass filter between 0.33Hz and 3.3Hz. The output of the operational amplifier is fed to a combination of twin filter and differential amplifier (44), and thence to a pair of nand gates configured as a monostable (46).

The monostable produces pulses of approximately 100 seconds duration independently of the duration of the trigger signal from the filter and differential amplifier unit 44.

The output of the monostable is fed, via a delay circuit composed of a third nand gate (47), resistor (48) and capacitor (49), to the input of an FET (50), and the output of this final FET is used to switch in sonic alarm (51).

The signal from the differential amplifier is also connected to another nand gate (52), which drives a light emitting diode (53), so the diode will light whenever the alarm is triggered. This is used for test purposes.

A resistor and capacitor (55, 54) produce a low signal when the unit is first switched on. This low signal is connected to the monostable (46), and disables the monostable for half a minute or so. This allows the User to switch on the unit and leave the area without triggering the alarm.

The delay circuit 48, 49 and 47 produces a delay before the alarm is triggered. This allows the User to enter the area to turn off the alarm without the alarm being sounded.

The switch (66) is used to provide a test function that does not sound the sonic alarm.

via a resistor (60), capacitor (61) and FET (62) there may be disabled the signal to the alarm when the alarm triggered, so preventing self-triggering of the unit.

When first set up, the sensitivity and selectivity of the system is adjusted in situ in a building using the variable resistor that is part of the differential amplifier's filter pair feed. The apparatus is switched on (in test mode, with an LED signifying detection), and the sensitivity etc is gradually adjusted until general background "noise" has no effect. A small window (or similar object) in the building is then repeatedly opened and shut, with the adjustment continuing until that, too, just has no effect. The adjustment is then reversed until opening the window just does have an effect, whereupon the system should now be ready capable of detecting any opening door or window, but not responding to wind or passing traffic.

Claims (7)

1. Detection and alarm apparatus comprising: a sub-sonic pressure wave detector providing a useable output signal; first filter means, to discriminate between sonic and sub-sonic pressure waves; amplifier means, to amplify the detector's output signal; second filter means, being a bandpass filter, to select the required frequency band of the amplified signal; and an alarm device triggerable by the thus-filtered signal; and wherein the second filter means includes a differential amplifier the input of which is the detector's amplified and then filtered output and the output of which is a "difference" signal relating only to the frequencies of interest.

2. Apparatus as claimed in Claim 1, wherein the pressure transducer is an electric condenser - or "electret" - microphone.

3. Apparatus as claimed in either of the preceding Claims, wherein the level of amplification is from 100 to 1000 times.

4. Apparatus as claimed in any of the preceding Claims, wherein the selection by filtration of the frequency of interest is such as to passing signals in roughly the 0.5 to 5.0 Hz range, filtering out signals from 10 Hz upwards.

5. Apparatus as claimed in any of the preceding Claims, wherein the detection and selection of the required frequencies is attained by the use of a differential amplifier preceded by two slightly different low-pass filters, the detector's amplified output being fed to these two filters, and the slightly different outputs therefrom being fed to a differential amplifier that then produces an amplified "difference" signal relating only to the frequencies of interest.

6. Apparatus as claimed in Claim 5, wherein the actual sensitivity and selectiveness of the system is determined by the difference between the characteristics of the two filters, and this difference is arranged by having one of the filter resistive elements in the form of a variable resistor which can in use be adjusted up or down to give the system the desired response.

7. Detection and alarm apparatus as claimed in any of the preceding Claims and substantially as described hereinbefore.