Detection and alarm system

Abstract

 An intruder alarm system in which intrusion into an enclosed space is detected as a change in the air pressure in the enclosed space uses a microcomputer to monitor the enclosed space by way of a sensor such that changes in air pressure are compared with a reference set at spaced intervals in dependence upon the ambient air pressure, whereby false alarms caused by changes in the weather are avoided.

Description

[0001] The invention relates to an alarm system, and in particular to an alarm system designed to protect an enclosed space and give warning that the space has been penetrated by an intruder. The space may be a domestic or commercial building, a room in such a building, a safe, a vault or the interior of a vehicle.

[0002] In US-A-3947838 there is described an alarm system comprising a sensor responsive to air pressure within an enclosed space, the sensor providing electrical signals related to the sensed air pressure, and a signal processor to which the electrical signals are supplied and operative to initiate an alarm indication when the signal supplied by the sensor is indicative of an intrusion into the enclosed space.

[0003] In this known system the signal processor is operative to distinguish between signals supplied by the sensor indicative of an intrusion into the enclosed space and such signals deriving from changes in the air pressure in the enclosed space caused by changes in the ambient atmospheric conditions in and around the enclosed space. The signal processor effects such distinction between the different signals it receives in dependence upon the rate of change of the value of the signal, on the basis that intruder entry into the enclosed space cannot be accomplished in less than a minimum predetermined time and will be completed in less than a maximum predetermined time.

[0004] A disadvantage of this known system is that it is not completely compensated for changes in ambient atmospheric conditions and may therefore give a false alarm if such conditions change in an unexpected manner and give a rate of change of air pressure in the enclosed space similar to that produced by an intruder.

[0005] According to this invention an alarm system comprises a sensor responsive to air pressure within an enclosed space, the sensor providing electrical signals related to the sensed air pressure, and a signal processor to which the electrical signals are supplied and operative to initiate an alarm indication when the signal supplied by the sensor is indicative of an intrusion in the enclosed space is characterised in that the signal processor is operative to cause the sensor to measure the air pressure in the enclosed space at intervals and to compare the measured value with a plurality of predetermined discrete narrow reference ranges of values and select the reference range within which the measured value lies, and also to cause the sensor to measure the air pressure in the enclosed space at other more frequent intervals and compare the measured value with the reference range selected at that time, an alarm indication being given if a value measured at one of the more frequent intervals lies outside the reference range selected at that time.

[0006] Preferably said intervals are separated by about two minutes and said more frequent intervals occur about fifty times per second.

[0007] With the system of the present invention the measurements made by the sensor at the relatively widely spaced intervals serve to determine the ambient atmospheric air pressure in the enclosed space and set the signal processor in dependence thereon, while the measurements made by the sensor at the more frequent intervals serve to indicate any change in the air pressure in the enclosed space from the ambient value caused by an intrusion into the enclosed space. Thus, the reference range of values selected at any time is characteristic of the ambient weather conditions and changes in weather conditions do not initiate a false alarm indication.

[0008] The invention further includes in another aspect a sensor which comprises a housing having a diaphragm mounted therein, the diaphragm forming one plate of each of a pair of capacitors the other plates of which are coupled to a comparator, movement of the diaphragm relative to the other plate caused by changes in air pressure applied to the sensor causing changes in the relative values of the capacitors, which changes cause the comparator to provide an output signal indicative of the air pressure applied to the sensor.

[0009] This invention will now be described by way of example with reference to the drawings, in which:-

Figure 1 is a block diagram of a system according to the invention;

Figure 2 is a diagrammatic representation of a sensor for use in the system of Figure 1; and

Figure 3 is a flow chart illustrating the operation of the signal processor of the system of Figure 1.

[0010] The system shown in Figure 1 comprises a sensor S which is arranged in an enclosed space to be monitored and responsive to the air pressure in that space to provide electrical signals indicative of the air pressure at any time. The sensor S is connected to a signal processor in the form of a microcomputer MC which is also connected to an alarm-giving device A, for example a siren. Operation of the system is controlled by a switch selector SS by which a unique four digit number has to be entered to render the system operative as required. Operation of the processor MC is controlled by a timer T, e.g. a counter timer circuit.

[0011] The processor MC is programmed to operate as shown in Figure 3 from which it can be seen that after being activated the system is quiescent for sixty seconds to enable the enclosed space being monitored to be evacuated by the person activating the system. After this initial delay the processor MC compares the signal received from the sensor S with a plurality of stored signals indicative of a discrete narrow reference range of ambient air pressures and selects the range in which the measured value lies. The operation is repeated once every two minutes, the reference range selected thus being indicative of the ambient air pressure present in the enclosed space.

[0012] The signal from the sensor S is also compared fifty times a second with the signal indicative of the selected reference range at any instant, and if the comparison shows that a measured value lies outside the reference range then selected, an alarm by the device A is initiated after a sixty second delay necessary to enable an authorised person to enter the enclosed space to deactivate the system.

[0013] Thus, any sudden change in the air pressure in the enclosed space caused by entry of an intruder will cause an alarm indication to be given while any gradual change in the air pressure caused by changes in the weather conditions will result in recalibration of the system to compensate for such change.

[0014] The sensor S shown in Figure 2 comprises a housing 10 having an opening 11 in one end wall, and a diaphragm D comprising an aluminium foil 12 sandwiched between two metal plates 13, 14 which each constitute one plate of a respective capacitor, mounted across it at its centre. Arranged between the diaphragm D and the end walls of the housing 10 are printed circuit boards 15 and 16, respectively, which constitute the other plates of the capacitors. The printed circuit boards 15 and 16 have holes 17 and 18 respectively at their centres, aligned with the opening 11 in the housing 10, while the metal plates 13 and 14 of the diaphragm D have relatively large holes 20 and 19 at their centres.

[0015] The boards 15 and 16 are connected by way of coupling circuits 22 and 21 to a comparator 23 which is in turn connected to the processor MC.

[0016] Any sudden change in air pressure applied to the sensor causes movement of the diaphragm D relative to the boards 15 and 16 and thus a change in the capacitance of the capacitors formed between the metal plates 14 and 13 of the diaphragm D and the boards 15 and 16. Such change is detected by the comparator 23 which provides a signal indicative thereof, as a measure of the air pressure then prevailing in the enclosed space to the processor MC which in response thereto operates as above described.

Claims

1. An alarm system comprising a sensor responsive to air pressure within an enclosed space, the sensor providing electrical signals related to the sensed air pressure, and a signal processor to which the electrical signals are supplied and operative to initiate an alarm indication when the signal supplied by the sensor is indicative of an intrusion into the enclosed space, characterised in that the signal processor is operative to cause the sensor to measure the air pressure in the enclosed space at intervals and to compare the measured value with a plurality of predetermined discrete narrow reference ranges of values and select the reference range within which the measured value lies, and also to cause the sensor to measure the air pressure in the enclosed space at other more frequent intervals and compare the measured value with the reference range selected at that time, an alarm indication being given if a value measured at one of the more frequent intervals lies outside the reference range selected at that time.

2. A system as claimed in Claim 1, characterised in that said intervals are separated by about two minutes.

3. A system as claimed in Claim 1 or Claim 2 characterised in that the said more frequent intervals occur about fifty times per second.

4. A system as claimed in any preceding Claim characterised in that the sensor is a capacitive effect device.

5. A system as claimed in any preceding Claim, characterised in that the sensor comprises a housing having a diaphragm mounted therein, the diaphragm forming one plate of each of a pair of capacitors the other plates of which are coupled to a comparator, movement of the diaphragm relative to the other plate caused by changes in air pressure applied to the sensor causing changes in the relative values of the capacitors, which changes cause the comparator to provide an output signal indicative of the air pressure applied to the sensor.

6. A sensor for use in a system according to any preceding Claim, characterised in that the sensor comprises a housing having a diaphragm mounted therein, the diaphragm forming one plate of each of a pair of capacitors the other plates of which are coupled to a comparator, movement of the diaphragm relative to the other plate caused by changes in air pressure applied to the sensor causing changes in the relative values of the capacitors, which changes cause the comparator to provide an output signal indicative of the air pressure applied to the sensor.

Drawing