GB2302406A - Apparatus for detecting the breaching of a closed environment - Google Patents

Abstract

A sensor 1 is provided to sense movement of air resulting from the breaching of a closed environment within which the apparatus is located. The sensor 1 has a transducer 2 for generating a signal in response to air movement within the environment. The transducer 2 comprises a rigid diaphragm 4 mounted within a housing 3 by a mounting member 5 allowing the whole diaphragm 4 to move as a body relative to the housing 3 with the mounting member 5 sealing the diaphragm 4 to the housing 3 so as to define behind the diaphragm 4 a closed chamber 8 which is unaffected by movements of air outside the housing 3, the diaphragm 4 thereby being operative to move in response to air movements resulting from the breaching of the closed environment, and a processing arrangement responsive to the movement of the diaphragm 4 for generating an alarm.

Description

APPARATUS FOR DETECTING THE BREACHING OF A CLOSED ENVIRONMENT This invention relates to apparatus for detecting the breaching of a closed environment by detecting movement of air or changes in air pressure (sometimes called sub-sound or infra-sound because of the very low frequencies, typically about 1.75Hz, which are involved) resulting from the breaching of the environment. The closed environment may be, for example, a closed and empty vehicle such as an aircraft or spacecraft or submarine or even a road vehicle or a closed room within a building, for example a secure laboratory or a bank vault.Breaching of the closed environment may occur when, for example, a person or animal tries to gain access to or tries to exit the closed environment or when a normally secure door, window or hatch fails or the closed environment is otherwise breached by, for example, failure of a seal or a fault.

Conventionally, when an aircraft is parked at an airfield or is otherwise closed and empty (for example when the aircraft is being towed), the hatches or doors, although closed, are usually unlocked and are simply sealed by tape so that an intrusion can only be detected by detecting removal of the tape.

In the interests of improved security, efforts have been made to devise apparatus which can be used on a parked (or otherwise closed and empty) aircraft to provide a reliable indication that the closed environment has been breached by, for example, an intrusion. Most attempts to devise such apparatus rely on the use of microphones to detect sub-sound or infra-sound resulting from the attempt by an intruder to open an aircraft hatch. Microphones generally use a tensioned diaphragm which is fixedly mounted at its periphery to the microphone housing. In the case of an electrostatic or electret microphone, the diaphragm may be mounted between two conductive plates so that movement or vibration of the diaphragm in response to air movements are detected as changes in capacitance. The sensitivity and characteristics of such microphones tend, however, to vary.Moreover, the tension within the diaphragm is liable to change with the ambient conditions, for example with changes in temperature, so resulting in undesired changes in the response characteristics of the microphone.

It is an aim of the present invention to provide an apparatus for detecting the breaching of a closed environment which should be capable of producing more reproducible and/or sensitive results than previously proposed apparatus.

In one aspect, the present invention provides apparatus having a diaphragm movable as a whole in response to charges in air pressure or air movements resulting from the breaching of a closed environment.

In another aspect, the present invention provides apparatus having separate diaphragms responsive to changes in air pressure or air movements and to vibrations and/or noise acting upon the closed environment for enabling an alarm to be provided when a breach of the closed environment occurs. The separate diaphragms may independently sense changes in air pressure or air movement and vibration or may provide outputs which are subsequently processed to provide respective signals indicating changes in air pressure or air movement and vibrations.

Allowing the diaphragm to move as a whole in response to air movements and so should be unaffected by changes such as temperature, etc. The diaphragm may be sealed to a housing to define a closed chamber behind the diaphragm which should enable a good response at the frequencies desired to be detected which are very low and in the region of a few Hz, for example 1.75Hz.

An embodiment of the present invention provides an apparatus for detecting the breaching of a closed environment comprising sensing means for sensing movement of air resulting from the breaching of a closed environment within which the apparatus is located, the sensing means comprising a transducing means for generating a signal in response to air movement within the environment, the transducing means comprising a rigid diaphragm mounted within a housing by mounting means allowing the whole diaphragm to move as a body relative to the housing with the mounting means sealing the diaphragm to the housing so as to define behind the diaphragm a closed chamber which is unaffected by movements of air outside the housing, the diaphragm thereby being operative to move in response to air movements resulting from the breaching of the closed environment, and processing means responsive to the movement of the diaphragm for generating an alarm.

In apparatus in accordance with the invention, the diaphragm may be provided by a dynamic, generally a moving coil, loudspeaker having a substantially conical diaphragm and a chamber behind the diaphragm may be sealed to extend the normal frequency response range of the loudspeaker beyond 30Hz down to about 1Hz. The mouth of the conical diaphragm may be about 3 to 4 inches (6 to 10 cm) in diameter. A base loudspeaker may be used because of its sensitivity in the relevant frequency range.

Where a closed chamber is provided behind the diaphragm, the chamber may be provided with air bleeding means, for example a small hole of about 0.5mm (or possibly even smaller) in diameter, to enable the air pressure within the closed chamber to be allowed to acclimatise to changes in the ambient pressure, for example if the aircraft is parked at an airport at high or low altitude.

Where the diaphragm is sealed to a housing, a deformable porous foam member sealed by an airimpermeable sealing means such as a grease or oil may be used to form the seal. As another possibility, an expansible bellows arrangement or any suitable mounting means which seals the coupling of the diaphragm to its housing against the entrance of air yet also allows the diaphragm to move as a whole relative to the housing may be used.

A further diaphragm may be disposed to face in a direction opposite to that of the diaphragm to enable a signal indicating whether or not the closed environment is subject to vibration to be derived. Such a signal may be used to inhibit the generation of an alarm when a signal indicating vibration above a given threshold is derived.

The use of two diaphragms enables the apparatus to determine whether the closed environment is subject to vibrations and enables the apparatus to avoid giving a false alarm in the event of such vibration. This may be of particular advantage where the apparatus is to be used on a parked aircraft because, for example, the take-off and landing nearby of a large jet such as Boeing 747 may induce severe vibrations in the parked aircraft. In addition, this should enable the apparatus to be unaffected by atmospheric conditions such as thunderstorms which may result in vibrations that, in the absence of the further diaphragm, might result in a false alarm and also prevents deliberate attempts to induce a false alarm by generating loud noises.

In an embodiment, the present invention comprises an apparatus for detecting the breaching of a closed environment, comprising sensing means for sensing movement of air resulting from the breaching of a closed environment within which the apparatus is located, the sensing means comprising two transducing means each comprising a diaphragm mounted within a housing by mounting means allowing the diaphragm to move relative to the housing, the transducing means being arranged so that front surfaces of the two diaphragms face in opposite directions, and processing means for deriving signals representing any air movement within the environment and any vibration exerted on the environment from the transducing means and means for generating an alarm when a signal indicating movement of air but no signal indicating vibration above a given threshold is derived.

Compensation can thus be provided for the effects of vibrations induced in the closed environment, for example vibrations resulting from severe weather conditions or the take-off or landing of other aircraft or other loud noises, so helping to apparatus giving a false alarm.

Where two diaphragms are provided, at least one of them may be sealed to a housing so as to define behind the diaphragm a closed chamber unaffected by movements of air.

If. two diaphragms are provided, then the two diaphragms may form part of respective identical transducing means and signals from the two transducing means summed to provide a signal representing any subsound and subtracted the signals to provide a signal indicating any vibrations induced in the closed environment. As another possibility, one of the transducing means may have air vents or apertures enabling movements in air to be communicated to both sides of the diaphragm so that the diaphragm does not respond to such air movements but only to other influences such as vibrations or audio signals. This enables separate transducing means to be used to detect air movement and vibrations.

If two diaphragms are provided, they may be axially aligned and opposed to one another so as to be spaced apart by an air gap which is sufficiently large to avoid having any affect on the measurements being made. This should enable more accurate measurements to be made because it ensures that the same air mass or volume is sensed by both diaphragms.

The giving of an alarm may be inhibited if an audio signal deriving means derives an audio indicating signal above a given threshold. This enables the apparatus to avoid the possibility of a false alarm if the enclosed environment is subjected to loud noises, for example thunderclaps or the noise of an aircraft taking off or landing.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a diagrammatic part-sectional view through one example of a sensing means for use in an apparatus in accordance with the present invention; Fig. 2 is a view similar to Fig. 1 of another example of a sensing means for use in an apparatus in accordance with the invention; Fig. 3 shows a block diagram of one example of processing means for an apparatus in accordance with the invention for processing signals supplied by the sensing means; Fig. 4 is a view similar to Fig. 1 of another example of sensing means for use in an apparatus in accordance with the invention;; Fig. 5 is a block diagram similar to Fig. 3 of one example of processing means suitable for use in an apparatus in accordance with the present invention for processing signals from the sensing means shown in Fig.

4; Fig. 6 shows a block diagram of another example of processing means suitable for use in an apparatus in accordance with the present invention for processing output signals from the sensing means shown in Fig. 4; Fig. 7 is a schematic block diagram of one example of an apparatus in accordance with the invention; Fig. 8 illustrates diagrammatically one example of an operation panel for an apparatus in accordance with the invention; Figs. 9A to 9C show a flowchart for illustrating operation of an apparatus in accordance with the present invention; Fig. 10 illustrates a very diagrammatic view of apparatus in accordance with the invention; and Fig. 11 illustrates very diagrammatically the placement of apparatus in accordance with the invention in an enclosed environment.

It should, of course, be understood that the drawings are not to scale and that like components are referred to by like reference numerals throughout.

Referring now to the drawings, Fig. 1 shows a partsectional view of one example of sensing means 1 in accordance with the present invention suitable for use in an apparatus for detecting the breaching of a closed environment.

As shown in Fig. 1, the sensing means 1 comprises a first transducing means 2 having a housing 3 within which a rigid diaphragm 4 is mounted so as to enable the diaphragm 4 to move as a body relative to the housing 3.

In the example shown in Fig. 1, the rigid diaphragm 4 comprises a rigid cone having its periphery coupled to a flange 2a of the housing 2 by means of a foam mounting member 5 which is sufficiently flexible or deformable to allow the cone 4 to move relative to the housing 3. The cone 4 has a rear extension 4a carrying a moving or voice coil 6. The voice coil 6 is surrounded by either, as shown, an electromagnet 7 or a permanent magnet. The cone 4, voice coil 6 and electromagnet 7 form a conventional dynamic base loudspeaker. Indeed, a commercially available loudspeaker such as the Audax AT080M0 manufactured by Audax a French company of Chateau-du-Loir, near le Mans, France 72500 may be used.

The transducing means 2 shown in Fig. 1 differs from the commercially available base loudspeaker in that the housing 2, conical diaphragm 4 and mounting means 5 define a chamber 8 which is completely sealed apart from a small air bleed which enables the pressure within the chamber 8 to follow or acclimatise to the ambient pressure. The small air bleed may be provided by the air-permeable foam mounting means 5. However, as the characteristics of the foam mounting means are not reproducible, it is preferred that the foam mounting means is sealed or made non air-permeable by a coating Sa of grease or the like and that a small air bleed hole 3a is formed in the housing 3. Typically, the air bleed hole should have a diameter less than about 0.5mm.Any other suitable air bleeding arrangement which allows for the pressure within the closed chamber to acclimatise to the ambient pressure may be used. The sealing of the chamber 8 extends the frequency sensitivity of the transducing means 2 beyond the normal 30Hz roll off frequency of the conventional loudspeaker to below 1.75Hz, typically to about 0.5-1Hz. The air bleed provided to the chamber 8 by either the air bleed hole 3a or the air permeability of the foam mounting means 5a enables the quasi-DC or extremely low frequency (below 0.05Hz, typically) response of the transducing means 2 to go to zero and avoids changes in the overall ambient air pressure affecting the response of the transducing means 2.

A completely sealed chamber 8 could be used if, for example, a separate sensor was provided to detect the ambient air pressure enabling the apparatus to compensate for changes in the response of the loudspeaker with the ambient air pressure.

The sensing means 1 shown in Fig. 1 also comprises a second transducing means 20 which is identical to the transducing means 2 apart from the fact that the housing 30 of the second transducing means 20 is formed with apertures 31 so that the chamber 8a is not sealed. The apertures 31 may be formed in any desired pattern but their total area should be equivalent to the open area at the front of the conical diaphragm 4 so that any changes in air pressure are transmitted equally to the front and the back of the conical diaphragm 4 so that conical diaphragm 4 of the transducing means 20 is not responsive to changes in air pressure.

The first and second transducing means 2 and 20 are arranged so that the front faces of the cones 4 face in opposite directions. As shown in Fig. 1, the cones 4 are axially aligned and the transducing means 2 and 20 are spaced apart by an air gap 9 by, for example, spacer bars 10 (two of which are shown) which couple the housings 3 and 30 together at specific points around the periphery of the housing so as to allow free movement of air into the air gap between the two transducing means 2 and 20.

The spacing or separation of the first and second transducing means 2 and 20 should be sufficiently large to avoid the gap affecting the measurements being made.

Typically, the separation of the first and second transducing means 2 and 20 should be at least 1cam. The maximum separation between the first and second transducing means 2 and 20 is determined by the desirability for the first and second transducing means to respond to the same air mass and may, of course, be constrained by the size requirements for the sensing means 1. In practice, the separation between the first and second transducing means 2 and 20 may be about 3cm.

However, the air gap may, as indicated above, be larger or smaller than 3cm.

Fig. 2 shows another example of a sensor la in accordance with the present invention. The sensor la shown in Fig. 2 differs from that shown in Fig. 1 in that the positions of the first and second transducing means 2' and 20 are reversed so that the second transducing means 20' is placed on top of the first transducing means 20. Of course, the positions of the first and second transducing means shown in Fig. 1 could also be reversed.

In addition, the conical diaphragms of the first and second transducing means 2' and 20' shown in Fig. 2 are coupled to their respective housings 3 by flexible rubber or plastics material bellows arrangements 50 rather than the foam mounting means 5 shown in Fig. 1.

As shown in Fig. 2, the first and second transducing means 2' and 20' may each be provided with a perforated front grill 11 as conventionally used for loudspeakers.

Such grills could also be provided in the sensing means 1 shown in Fig. 1. The air inlet area of the apertures 11a of the grills 11 should be equal to the area of-the apertures 31 in the housing 3 of the second transducing means 20 so as to ensure pressure equalisation on both sides of the conical diaphragm 4 in the second transducing means 20 and to match the air inlets of the first and second transducing means 2' and 20'.

Because the chamber 8 of the first transducing means 2 or 2' is sealed, the conical diaphragm 4 of the first transducing means 2 or 2' responds to movements of air or changes in air pressure caused the breaching of a closed environment within which the sensing means 1 or la, for example by a person trying to gain access to or exit from the closed environment. However, because the second transducing means 20 or 20' has a non-sealed chamber 8a, the conical diaphragm 4 of the second transducing means 20 or 20' does not respond to such air movements or changes in air pressure but only to audible sounds and vibrations occurring in the direction of the axis of the conical diaphragm.The second transducing means 20 or 20a thus enables separate detection of vibrations and can be used to avoid the sensing means 1 or la giving a false alarm if, for example, the enclosed environment within which the sensing means 1 or la is located is subject to vibrations from an external source such as, in the case where the sensing means is located on an aircraft, the take-off or landing of another aircraft nearby, or atmospheric disturbances such as thunderstorms or even a deliberate attempt to generate a false alarm by making a loud noise outside the closed environment.

In operation of the sensing means 1 or la, axial movement of the conical diaphragms 4 results in an electrical signal in the corresponding voice coil 6 with a frequency equal to that of the frequency of vibration of the conical diaphragm. Fig. 3 illustrates one example of processing means 40 for processing signals from the output leads 6a, 6b of the moving coils 6.

In the interests of simplicity, the first and second transducing means 2 and 20 are shown only schematically in Fig. 3. It will, of course, be appreciated that the processing means shown in Fig. 3 may be used with both the sensing means 1 and la.

As shown in Fig. 3, the output from the voice coil 6 of the first transducing means 1 is coupled to the input of an amplifier 41 of a first processing arrangement 40a. The amplifier 41 may be of any suitable conventional form. Generally, the amplifier 41 will be an operational amplifier. The output 41a of the amplifier 41 is supplied to a first band-pass filter 42 which is arranged to pass frequencies in the range of from 1-3Hz. Reference may be made to any suitable text book on electronics for suitable forms for the filter 42.

For example, the second edition of "The Art of Electronics" by Horowitz and Hill published by the Press Syndicate of the University of Cambridge describes various suitable forms of active filters, such as Butterworth filters. The output 42a of the first filter 42 is coupled to the input of a second amplifier 43 which may be, for example, a variable gain amplifier of known construction. The output 43a of the variable gain amplifier 43 is supplied to the input of a second filter 44. The second filter 44 is a sharp filter designed to pass the frequency which it is desired to detect, in this example 1.75Hz plus or minus 0.25-0.5Hz. The chapter entitled "Active Filters and Oscillators" in the second edition of Horowitz and Hill describes examples of suitable circuits for forming the sharp filter 44.For example, the sharp filter 44 may be a so-called "biquad" filter as shown in Fig. 5.2 and as described at pages 278 and 279 of the second edition of Horowitz and Hill.

Other suitable forms of state variable filters may be used.

The output of the sharp filter 44 is supplied to a full wave rectifying circuit 45 of conventional form.

The rectified output is then supplied to an amplitude modulator 46 which smooths the rectified output using a lower frequency signal. The amplitude modulator 46 is coupled to a peak detector 47 which again may be of conventional form. The peak detector 47 detects the maximum voltage of the amplitude modulated signal supplied by the amplitude modulator 46. The detected peak is then supplied to a threshold circuit which may be, for example, a Schmitt circuit or a conventional comparator which provides a high output signal only when the peak exceeds a given voltage. The threshold detector 48 thus provides an output signal OP1 when the amplitude of vibration of the conical diaphragm 4 at the frequency of 1.75Hz exceeds a threshold value determined by the threshold circuit 48.

The second transducing means 20 is coupled to a second processing arrangement 40b which is matched to the processing arrangement 40a of the first transducing means 2 and so comprises an amplifier 41', a first filter 42', a variable gain amplifier 43', a sharp, that is a narrow band pass, filter 44', a full wave rectifier 45', an amplitude modulator 46', a peak detector 47' and a threshold circuit 48. The second processing arrangement outputs a high output signal OP2 when the amplitude of the vibration of the conical diaphragm 4 of the second transducing means 20 at the detection frequency, in this case 1.75Hz, exceeds a threshold value.As the second transducing means 20 does not respond to changes in air movement or air pressure but only to vibrations, a high output signal OP2 indicates that the sensing means 1 is subject to vibrations above a given threshold at a frequency of 1.75Hz, for example, where the sensing means 1 is mounted on a parked aircraft, due to, for example, atmospheric effects such as thunderstorms and like or the take-off or landing of a large plane such as a Boeing 747, or other external loud noises.

The output from the voice coil 6 of one of the first and second transducing means 2 and 20 is also coupled to a third processing arrangement 40c which provides an output signal when the associated transducing means detects an audio signal. The third processing arrangement is similar to the first and second processing arrangements but differs in that only a single filter 42a is provided. The filter 42a is similar to the filters 42 and 42' but is designed to pass frequencies in the range of from 1.5-500Hz so as to enable detection of low frequency audio signals such as may be caused by atmospheric effects or passing aircraft.The third processing arrangement provides a high output signal OP3 when the amplitude of vibration of the conical diaphragm 4 of the associated transducing means 2 or 20 within the frequency pass band of the filter 42a exceeds a given amplitude as determined by the threshold circuit 48".

Preferably, the third processing arrangement is coupled to the first transducing means 2 because, as indicated above, the first transducing means 1 is more sensitive, in the frequency range concerned, than the second transducing means 20.

Fig. 4 illustrates another example of a sensing means 1b in accordance with the present invention. In the example shown in Fig. 4, the two transducing means 2" each have a sealed rear chamber 8. As shown in Fig.

4, the construction of the first and second transducing means 2" is similar to that of the first transducing means 2' shown in Fig. 2. However, of course, the construction of the transducing means 2" may be similar to that of the transducing means 2 shown in Fig. 1.

Fig. 5 illustrates one example of processing means for processing the output signals from the voice coil 6 of the sensing means 1b shown in Fig. 4.

The third or audio processing arrangement 40c shown in Fig. 5 is the same as that shown in Fig. 3. The first and second processing arrangements 40'a and 40'b however differ from those shown in Fig. 3. Thus, as shown in Fig. 5, the outputs of the sharp filters 44 and 44' of the first and second processing arrangements are supplied to respective adding and subtracting circuits 60 and 61 of conventional form. The summed and subtracted signals are then processed as described above with reference to Fig. 3 by full wave rectifying circuits 45, 45', amplitude modulators 46, 46', peak detectors 47, 47' and threshold detectors 48, 48' to provide output signals OP1 and OP2.Because the conical diaphragms 4 of the two transducing means 2' are vibrating in the opposite directions, the signal resulting from the processing following the subtracting circuit 61 provides a signal OP2 when vibration occurs at the detection frequency, 1.75Hz in this example, while the signal resulting from the further processing of the signals supplied by the summing circuit 60 provides a high output signal OP1 when movement of air or a pressure change occurs as a result of a person trying to gain entry into the enclosed space within which the sensing means is situated.

Fig. 6 illustrates an alternative arrangement for processing the output signals from the sensing means ib.

The processing means shown in Fig. 6 differs from that shown in Fiq. 5 merely by virtue of the fact that the outputs of the filters 44 and 44' are summed after amplitude modulation. This arrangement does, of course, require further full wave rectifying and amplitude modulating circuits 45 and 46 because subtraction has to occur prior to rectification.

As another possibility, the addition and subtraction may occur before the filtering and amplification or at any suitable point in the filtering and amplification chain 41 to 44.

In other respects, the processing means shown in Fig. 6 is the same as that shown in Fig. 5.

Fig. 7 is a block diagram of one example of an intruder detection apparatus 100 using the sensing means 1, la or lb described above. The apparatus 100 is powered from a rechargeable, battery 101 so enabling the apparatus to operate as a stand-alone piece of equipment.

The battery 101 is coupled to a power supply unit 102 via a battery charger 103 and a power control unit 104, all of known form. The power supply unit 102 is capable of being coupled to any suitable mains voltage MV to enable recharging of the battery 101 via the power control unit 104 and the battery charger 103. The battery charger 103 is arranged to adapt the rate of charging of the battery to the actual state of charge of the battery as is known in the art.

The power control unit 104 comprises conventional voltage regulation and level shifting circuitry and supplies three output voltages on lines 104A, 104B and 104C.

The power supplies 104A and 104B provide the positive and negative power supply rails, typically +5 volts, for the sensing means 1 and the processing arrangement. For simplicity in Fig. 7, the processing arrangement is illustrated simply as a block 400 providing three outputs OP1, OP2 and OP3 which are supplied to a microprocessor 105. The third power supply line 104C (again typically providing 5 volts) from the power control circuit 104 is used to power the microprocessor 105. This enables the power control circuit 104 to disconnect the power supply to the power supply lines 104A and 104B if the battery voltage drops below a certain desired minimum level while still allowing power supply to the microprocessor 105 so that the status of the apparatus can be indicated. The microprocessor 105 is associated with a storage device 106 which may be of any suitable conventional form, for example the storage device 106 may be a hard disc drive.

The apparatus 100 is also provided with a keypad 107 for inputting instructions to the microprocessor 105. A display 108, for example an LCD display is provided for enabling display of instructions input to the microprocessor 105 and also of data stored in the memory 106.

As will-be explained below, the microprocessor 105 also receives input signals from an entry/exit key 109 and a tilt switch 110. The apparatus may also be provided with suitable radio transmitters or transceivers 111 and 112 for supplying signals to a radio pager and a base station, respectively. Of course, suitable transceivers may be provided for supplying signals via any appropriate communication link, for example a telephone or computer communication link. The microprocessor 105 also controls operation of a number of LEDs 113-116 for providing a visual indication of a low battery voltage, entry into the enclosed space in which the apparatus 100 is situated, exit from the enclosed space and the fact that an unauthorised entry into the enclosed space has occurred. The microprocessor may also control a siren 117 or other similar device for providing an audible signal.

Fig. 8 indicates very schematically the operation panel 100a of the apparatus. As shown in Fig. 8 the display 108 may be controlled by the microprocessor 105 so as to provide an indication 101a of the current battery charging level.

As indicated above, the apparatus 100 is designed to be a portable standalone piece of equipment which operates independently of the power supply to the environment within which the apparatus is to be located.

Figure 10 shows a very diagrammatic perspective view of apparatus 100 in accordance with the invention. As shown the apparatus 100 has a case 120 which is generally formed of metal or a plastics material and has the shape of a briefcase or attache case. The case 120 has a carrying handle 120 to facilitate transport of the apparatus to and from the closed environment within which it is to be located in operation. The control panel 100a shown in Figure 8 is mounted into, as shown, a side wall of the case 120. The case 120 also has a number of air inlets, in the example shown two air inlets 120b and 120c each of which communicates with the air gap 9 between the transducing means of the sensor 1. The air inlets 120b and 120c may communicate with the air gap 9 by means of air pipes 121 as shown in phantom lines in Figure 10.

The remaining components of the apparatus 100 are mounted within the case 120, for example within a foam mounting block (not shown). Figure 10 illustrates a first block 1020 which represents a common housing for the power supply unit 102, battery charger and power control unit 104, a second block 1010 which represents the location of the rechargeable battery and a third block 1050 which represents the printed circuit board or boards carrying the processing arrangement 400, the microprocessor 105 and associated circuitry. If desired, a separate printed circuit board may be provided for the driving circuit for the display 107. The case 120 has a socket 122 for enabling connection to a mains power supply MV as discussed above.The case 120 will usually be locked or otherwise secured in a manner which allows the case only to be opened for maintenance, repair or replacement of the operational components of the apparatus. Of course, any other suitable housing arrangement for the apparatus 100 may be used.

The operation of the apparatus 100 will now be described with reference to the flowchart shown in Figs.

9A to 9C.

Figure 11 shows very schematically a closed environment 200 (which may be the passenger cabin of an aircraft) having at least one door or hatch 200a and within which the apparatus 100 is located.

The apparatus 100 is carried by its case handle 120a and placed at a suitable location within the passenger cabin 200 of an aircraft by an authorised keyholder either of the airline or of the airport security. The authorised keyholder then activates the apparatus by inserting an entry/exit key (not shown) into the entry/exit key slot 109. As another possibility, the authorised keyholder may be required to key in a code via the keypad 107. Once the microprocessor detects the entry of the key at step S2 in Fig. 9A, the microprocessor then determines at step S1 whether or not a signal indicating incorrect orientation of the sensing means 1 is being received from the tilt switch 110 which, where the sensing means 1 is in a separate housing, is, of course, mounted in the housing of the sensing means 1.If the microprocessor detects that the orientation is not correct, a message may be displayed on the LCD screen 108 at step S3 and an audible warning given by means of the siren 117.

Once the sensing means 1 has been correctly oriented, the microprocessor checks at step S4 the current battery voltage and displays it on the LCD screen as indicated at step S5.

The microprocessor 105 will also check, as shown in Fig. 9A at step S6 whether the battery voltage is low and, if so, may light the battery low warning LED and display on the LCD display 108 a message indicating that the apparatus cannot be alarmed until the battery has been recharged. Assuming that the battery voltage is satisfactory then the microprocessor may check at step S8 for any inputs via the keypad 107 from the authorised keyholder and then execute any input instructions at step S9, for example alter the date or time stored in the memory 106. To reduce even further the possibility of an unauthorised person tampering with the apparatus, the microprocessor 105 may be programmed only to allow inputs via the keypad 107 to alter the date or time setting etc.

when the apparatus is not in use and is plugged into a mains power supply, for example to recharge the battery, at the base station.

When the microprocessor subsequently detects at step S10 that arm-/disarm key has been removed, the exit LED 115 begins to flash to warn the authorised keyholder that he should exit the aircraft as the apparatus will shortly become armed. The microprocessor 105 may also display a warning message to the authorised user on the LCD screen 108.

Assuming that the authorised user does not reinsert the arming key, the microprocessor 105 then waits for the preset time period, for example thirty seconds, at step S11 to enable authorised keyholder to exit the aircraft and ensure all hatches 200a are closed. The apparatus 100 is now in a condition in which any subsequent entryinto the aircraft will be detected and an alarm given, unless the arm/disarm key is inserted to deactivate the apparatus 100. In a preferred embodiment, the apparatus 100 will be arranged to give a first signal indicating entry into the closed environment and then, if the arm/disarm key is not inserted, a second signal indicating that the entry into the closed environment is unauthorised. During its operational condition, the microprocessor 105 will monitor, as shown at step S12 in Figure 9B, the battery voltage and provide a warning at step S12a if the battery voltage is low. The warning preferably will take the form of an alert signal sent via the radio transceivers 111 and 112 to both the base station and a radio pager carried by either an employee of the airline or a member of the airport security responsible for the aircraft.

As illustrated in Fig. 9B at steps S13 and S13a, the microprocessor may optionally transmit the current status of the apparatus to the base station 111 at predetermined times, for example every hour. In addition, the microprocessor may, as shown by steps S14 and S15, periodically store data regarding the status of the apparatus in the memory 106.

Once the apparatus has been armed, the microprocessor continuously monitors, at steps S16 to S18, the output lines OP1, OP2 and OP3 from the processing arrangement to see if a high signal has been received.

In the example shown, the microprocessor 105 first checks at step S16 to see whether the signal is an audio signal, that is whether the signal OP3 is high. If the answer is yes, the microprocessor resets, at step S17, the count CT of its internal or an external counter (not shown) and continues to monitor for other signals.

If no audio signal is detected at step S16, the microprocessor then checks at step S18 whether a high output signal OP2 indicating a vibration signal has been received. If the answer is yes, the microprocessor again resets the counter and continues to monitor for further output signals OP1, OP2 and OP3.

When the microprocessor detects at step S19 that a high output signal OP1 has been received indicating the presence of sub-sound, it increments at step S20 the count CT of the counter and then checks at step S21 whether the count of the counter has exceeded a predetermined value CTc. If the answer is yes, the microprocessor activates the entry LED at step S22, provides a signal, preferably to the base station and/or radio pager indicating that the aircraft has been entered, and then checks at step S23 to see if the arming key has been inserted. If the answer at step S23 is yes, the microprocessor then returns to step S8.If, however, the arming key is not inserted, the microprocessor determines that the entry is unauthorised and supplies a second signal via the radio transceivers 111 and 112 to alert the base station and the authorised keyholder or user on his radio pager. The status of the entry LED may be altered. For example on entry the entry LED may flash and upon detection of an unauthorised entry the entry LED may be lit continuously or may flash at a different rate. As another possibility, separate LED for indicating entry and a subsequent alarm may be provided.

Where desired, and if the siren 117 does not present too big a drain on the battery, the siren 117 may also be activated.

If the count has not reached the predetermined value, then the microprocessor continues to monitor for signals until the count reaches the predetermined value when the alarm will be given indicating the occurrence of an intrusion.

As will be appreciated from the above, the counter is only incremented if a signal is received which indicates air movement or a change in air pressure resulting from breaching of the closed environment, for example entry into or exit from the aircraft. Any vibrations or audible signals due to severe weather conditions or the take-off or landing nearby of large airplanes is avoided because these signals are separately detected by the second and third processing arrangements and used to reset rather than increment the counter.

The provision of the radio transceivers to supply signals to a base station and a radio pager enables appropriate personnel to be alerted immediately to the fact that an intrusion has occurred. However, if desired for, for example, costs reasons, these could be omitted so that the apparatus simply provides an indication that the closed environment has been breached (by lighting the entry LED) when it is later inspected by authorised personnel.

In the apparatus described above, high output signals OP1 to OP3 are provided when a peak exceeds a given threshold. However, alternatively or additionally the rate of change of the outputs from the amplitude modulators 46, 46' and 46" may be determined by using a simple differentiation circuit and a high output signal OP1, OP2 or OP3 provided when the rate of change exceeds a given threshold. This may give a more accurate detection of a breach if the closed environment. As another possibility, the rate of change may be determined by using the microprocessor to control the gain of the variable gain amplifiers 43, 43' and 43a. The gain of these amplification stages may also be adjusted as required for the characteristics of a particular aircraft.

Generally, the power control unit will operate to disconnect the power supply to the sensing means 1 if the battery voltage drops below a predetermined level, for example 8 to 7 volts. The use of a separate power supply line 104C for the microprocessor 105 enables the microprocessor to detect the situation and to, for example, transmit warning signals to the base station or radio pager indicating that the battery needs recharging.

The inventors have found that the amplitude modulation envelope produced by an intrusion is very reproducible for a given aircraft and therefore, as an alternative to using peak threshold detection, a conventional correlation circuit may be provided to compare the shape of the detected amplitude modulated envelope with that of the amplitude modulated envelope expected for that aircraft.

As another possibility, frequency modulation may be used in conjunction with amplitude modulation to enable more information to be extracted from the incoming signal.

It will be appreciated that the second transducing means only detects vibration in one perpendicular plane.

If desired, of course, three mutually orthogonal sensing means could be provided to enable detection of vibration in any direction.

Various modifications and changes will be apparent to the person skilled in the art. Apparatus and sensing means in accordance with the invention may be used in any situation where it is desired to detect the breach of a closed environment. The closed environment may be, for example, a closed and empty vehicle other than an aircraft, for example a spacecraft or submarine or even a road vehicle or even a closed room or a closed suite of rooms within a building, for example a secure laboratory or a bank vault. Breaching of the closed environment may occur when, for example, a person or animal tries to gain access to or tries to exit the closed environment or when a normally secure door, window or hatch fails or a bulkhead or seal cracks or fails.

Where it is not likely that vibrations will give rise to false alarms, the second transducing means may be omitted and reliance made simply upon the signals from the first transducing means. Similarly, if the particular conditions in which the apparatus is situated mean that false alarms resulting from audio signals are unlikely, then the audio signal processing arrangement may be omitted. The apparatus may also be used in sealed pipe lines and the like to monitor for breaches in the sealed pipe line.

The fact that the apparatus is battery operated and portable is particularly advantageous where the apparatus is intended to be used in a vehicle such as an aircraft and enables the apparatus to be easily removed and kept in a secure place during operational use of the vehicle.

This may be of particular advantage in the event of a subsequent accident involving the vehicle as data stored in the apparatus may be of assistance in determining the reasons for the accident, for example the data may help eliminate the possibility of sabotage by an unauthorised entry while the aircraft (or other vehicle) was parked.

As another possibility, if it is not desired that the apparatus be completely standalone, then the apparatus may be designed to be operated by a mains power supply with, possibly, a back-up battery to ensure continued operation in the event of a mains power supply failure. A mains operated version of the apparatus may be suitable for use in buildings, for example in bank vaults and secure laboratories.

It will of course be appreciated that movable diaphragms other than conical diaphragms may be used and that the frequency of sub-sound to which the apparatus is sensitive may be adjusted to meet the particular use to which the apparatus is to be put.

Claims (20)

CLAIMS:

1. An apparatus for detecting the breaching of a closed environment, comprising sensing means for sensing movement of air resulting from the breaching of a closed environment within which the apparatus is located, the sensing means comprising a transducing means for generating a signal in response to air movement within the environment, the transducing means comprising a rigid diaphragm mounted within a housing by mounting means allowing the whole diaphragm to move as a body relative to the housing with the mounting means sealing the diaphragm to the housing so as to define behind the diaphragm a closed chamber which is unaffected by movements of air outside the housing, the diaphragm thereby being operative to move in response to air movements resulting from the breaching of the closed environment, and processing means responsive to the movement of the diaphragm for generating an alarm.

2. An apparatus according to claim 1, wherein the transducing means comprises a dynamic loudspeaker having a substantially conical diaphragm.

3. An apparatus according to claim 1 or 2, wherein the closed chamber is provided with air bleeding means for enabling the air pressure within the closed chamber to acclimatise to the ambient air pressure.

4. An apparatus according to claim 3, wherein the air bleeding means comprises a hole in the housing having a diameter of about 0.5mm.

5. An apparatus according to any one of the preceding claims, wherein the mounting means comprises a deformable porous foam member sealed by air-impermeable sealing means.

6. An apparatus according to any one of claims 1 to 4, wherein the mounting means comprises an expandable bellows arrangement.

7. An apparatus according to any one of the preceding claims, wherein the sensing means comprises a further transducing means similar to the transducing means and having its diaphragm disposed to face in a direction opposite to that of the diaphragm of the transducing means and wherein the processing means is responsive to movement of the two diaphragms to derive a signal indicating whether or not the closed environment is subject to vibration and for inhibiting the generation of an alarm when a signal indicating vibration above a given threshold is derived.

8. An apparatus for detecting the breaching of a closed environment, comprising sensing means for sensing movement of air resulting from the breaching of a closed environment within which the apparatus is located, the sensing means comprising two transducing means each comprising a diaphragm mounted within a housing by mounting means allowing the diaphragm to move relative to the housing, the transducing means being arranged so that front surfaces of the two diaphragms face in opposite directions, and processing means for deriving signals representing any air movement within the environment and any vibration and/or noise exerted on the environment from the transducing means and means for generating an alarm when a signal indicating movement of air but no signal indicating vibration and/or noise above a given threshold is derived.

9. An apparatus according to claim 8, wherein in at least one of the transducing means the mounting means seals the diaphragm to the housing so as to define behind the diaphragm a closed chamber unaffected by movements of air.

10. An apparatus according to claim 7, 8 or 9, wherein the two transducing means are identical and the processing means comprises means for summing signals from the two transducing means to provide a signal representing any air movement within the closed environment and means for subtracting signals from the two transducing means to provide a signal representing any vibrations acting on the closed environment.

11. An apparatus according to claim 7, 8 or 9, wherein the two transducing means differ in that the housing of one of the transducing means has air vents thereby causing both sides of the diaphragm to be affected by air movements within the closed environment so that the diaphragm of that transducing means is not responsive to such air movements and wherein the processing means comprises means for deriving a signal representing any air movement in the closed environment from the transducing means having the closed chamber and means for deriving a signal representing any vibration induced in the closed environment from the transducing means having the vented chamber.

12. Apparatus according to claim 7, 8, 9 or 10, wherein the two transducing means are axially aligned and opposed to one another so as to be spaced apart by an air gap.

13. Apparatus according to any one of the preceding claims, wherein the processing means comprises, for the or each transducing means, means for amplifying a signal from the transducing means, means for filtering the signal to pass only signals having a predetermined frequency or range of frequencies and means for generating an alarm when the filtered signals have predetermined characteristics.

14. Apparatus according to claim 10, wherein the processing means comprises means for amplifying and means for filtering the signals from each of the transducing means before supplying the amplified and filtered signals to the summing means and the subtracting means and means for generating an alarm when the summed and subtracted signals have predetermined characteristics.

15. Apparatus according to claim 13 or 14, wherein the generating means is arranged to generate a alarm when the amplitude of the signal representing air movement but not the amplitude of the signal representing vibration exceeds a predetermined threshold.

16. Apparatus according to any one of the preceding claims, wherein the processing means comprises means for inhibiting an alarm when audio signal deriving means derive an audio signal above a given threshold from the or one of the transducing means.

17. Apparatus according to claim 16, wherein the audio signal deriving means comprises filter means for passing signals in - the range of from 0.5-500Hz from the transducing means or one of the transducing means and means for generating an audio indicating signal when the filtered signal has a predetermined characteristic.

18. Apparatus according to claim 17,wherein the audio signal generating means generates an audio indicating signal when the filtered signal has a predetermined amplitude.

19. Use of a dynamic loudspeaker in which the substantially conical diaphragm of the loudspeaker is sealingly mounted to the loudspeaker housing so as to define a closed chamber behind the diaphragm as a sensing means for sensing movement of air resulting from a breach of a closed environment within which the sensing means is located.

20. Use of two axially aligned and opposed dynamic loudspeakers one of which has a closed chamber behind the substantially conical diaphragm and the other of which has a vented chamber behind the substantially conical diaphragm as sensing means for sensing movement of air resulting from a breach of a closed environment within which the sensing means is located.