Field of the Invention

The present invention relates to an intruder deterrent system of the type which generates a screen of smoke or vapour.

Background to the Invention

Regardless of whether a country's economy is booming or is suffering from a recession, the one almost inevitable "growth industry" is crime against property. It is widely appreciated that nationwide burglary statistics show an almost unerring upward trend. Many companies and individuals have endeavoured to develop systems which will reduce the vulnerability of unguarded property. Sophisticated modern burglar alarms provide the most widespread form of protection, serving to detect the presence of intruders and generate audible and/or visible warning signals normally both in the vicinity of the property and also at the neighbourhood police station. A severe limitation of the audible/visible alarm system is that the triggering of the alarm will not automatically deter a burglar and in most circumstances there will be a delay of as much as 10 to 15 minutes before the alarm is responded to by the owner, the pohce or other concerned citizens.

Others have recognised the inherent drawback of relying upon alarm systems alone and have designed devices which will impede or harm the burglar.

Some of these systems would be regarded as inhumane and others are practically undesirable for the damage which they cause to the property or the inconvenience to the owner if accidentally triggered.

Of the more humane means for deterring or repelling intruders, smoke generating devices are amongst the most suitable for the purpose. French patent applications numbers FR-A-2 558 625 and FR-A-2 538 147 describe burglar detection and alarm systems which liberate noxious gases such as tear gas or smoke to deter and/or hinder an intruder. Use of non-toxic smokes in anti-theft devices for protecting vehicles is disclosed in United Kingdom patent appUcation number GB-A-2 247 094 and Netherlands patent application number NL-A-8 402 896.

Known types of smoke-generating intruder deterrent system are, however, invariably subject to significant problems. The existing systems have difficulty in generating a high density smoke screen. Secondly, they have difficulty in maintaining that smoke screen, especially should the intruder take action to disperse the smoke by, for example, opening or breaking doors or windows to the smoke-filled room.

It is a general objective of the present invention to overcome these significant drawbacks of existing smoke-generating intruder deterrent systems.

Summary of the Invention

According to a first aspect of the present invention there is provided an intruder deterrent system of the type which generates a smoke screen and which comprises: smoke screen generating means; activating means to activate the smoke screen generating means when an intruder detection means, operatively linked to the activating means, is triggered; and smoke screen density regulating means having a sensor to sense the density of the smoke screen and control means responsive to the sensed density to adjust the output of the smoke screen generating means to maintain the screen density at a desired level.

The smoke screen generating means suitably comprises a pump the rate and/or duration of operation of which is controllable by said control means to regulate the density of the smoke screen.

Advantageously the smoke screen generating means comprises a storage tank for an aqueous liquid composition and delivery pipes extending from the tank to deliver the liquid composition to an outlet, there being further provided heating means to heat the liquid composition which has left the tank to thereby convert the liquid composition into a dense vapour.

Suitably the duration of operation of the pump is controlled to regulate the density of the smoke screen and the rate of operation of the pump is controllable to regulate the efficiency of use of the liquid composition.

Suitably the liquid comprises a mixture of Glycol and water. Heating of this mixture as it passes through the delivery pipes to the outlet will vaporise the mixture to form a fog.

Preferably the smoke density sensor is housed independently of the smoke screen generating means.

The activating means is advantageously operatively linked to a fire detection system so as to be inactivated if fire detection system detects a fire. This will prevent hindrances to firemen breaking and entering a blazing building in which the intruder deterrent system is installed and armed.

According to a second aspect of the present invention there is provided a smoke screen generating means which comprises a storage tank for an aqueous liquid composition and one or more delivery pipes extending from the tank to deliver the liquid composition to an outlet, the system further comprising heating means adjacent the delivery pipe (s) to heat the liquid composition in the delivery pipe (s) to thereby convert the liquid composition into a dense vapour, control means being provided to control the rate of operation of the pump in dependence upon the temperature of the heating means.

Brief Description of the Drawings

A preferred embodiment of the present invention will now be more particularly described, by way of example, and with reference to the accompanying drawings Figure 1.

Description of the Preferred Embodiments

Referring to Figure 1, there is shown a smoke-generating intruder deterrent unit 1, comprising a robust metal or plastics casing housing a liquid storage tank 2 which is adapted to act as a reservoir for a Glycol/water mixture or other suitable heat labile fog-generating mixture. One or more delivery pipes

3 extend from the storage tank 2 to a powerful oscillating piston pump 4 which, when activated, operates to draw the liquid mixture from the tank 2 and pump it at a selectable variable rate. Suitably two such pumps 4 are provided in parallel to each other to improve fluid flow. The fluid pumped by the or each piston pump 4 is forced through a narrow coiled tube 5 located within an insulated heating block 6, to be heated to a thermostatically controlled temperature and vaporised prior to being expelled from the unit 1 via an outlet 7. Within the unit 1 there is mounted a switch 8 which receives an input from an intruder detection system A, of any suitable known type which is suitably part of a burglar alarm system. Switch 8 which may, for example, comprise a latching relay serves to activate the pump 4 when the intruder detection system is triggered.

Switch 8 may be over ridden or switched off by a signal resulting from activation of a fire smoke detection system B. Furthermore, inhibit circuits may be provided to switch off or over ride the switch 8 in response to switching off of the burglar alarm system which the intruder detection system complements or forms part of. This will prevent accidental activation of the smoke screen generator during dayhght hours when the building is normally occupied

Although the intruder detection system is, in the illustrated embodiment shown as being external to the unit 1, it may, if desired, be formed integrally with the unit 1, making the unit 1 a wholly self-contained system enabling the unit to be positioned in a wide range of locations. In such case, the sensor is suitably of an infrared or microwave transmitting and receiving nature.

The unit 1 suitably also incorporates its own internal power supply which is suitably a 12 volt battery. This may serve as a back-up power supply in case of failure of the external mains electricity power supply.

An indicator panel is suitably provided on the unit 1, or, more preferably on the main burglar alarm system control panel to indicate the operational status of the unit 1 and display outputs from fault detection sensors provided in the unit 1 for detecting power failure or malfunction of the heater block 6. The indicator panel suitably also displays an output from a reservoir level senses in the unit 1 which sensors the level of fluid within the tank 2 to warn of depletion.

The intruder detection system may be configured to activate the smoke- generating unit 1 when the intruder breaks the perimeter of the building prior to entering the room in which the unit 1 is installed. By doing so the room will be effectively smoke-filled, preventing vision, even before the burglar enters that room.

Attainment and maintenance of a satisfactory level of smoke screen density is achieved through feed-back control of the rate of vapour production by the unit 1.

One or preferably a pluraity of smoke screen density sensors 10 sense the density of the generated smoke screen. These are similar to the sensors used in conventional fire safety smoke detectors employing infrared transmissions and utilizing the Tyndle effect of light dispersion to sense smoke. Unlike conventional smoke detector sensors, however, because the threshold level of smoke screen density for triggering the system to activate it to produce more smoke is suitably around 450 mg of Glycol vapour per cubic metre of air, the

sensitivity setting of the relay switch 1 in each sensor 10 which activates the system is suitably lower than conventional. Additionally, the rate at which the sensors 10 sample is faster than conventional, being of the order of once per second to three times per second.

A control unit 9 having comparator circuits controls the duration of operation and rate at which the piston pump 4 pumps the liquid mixture from the tank 2 and hence controls the rate of vapour generation. Use of a triac in the control unit 9 provides not only an on off control but also continuously variable adjustabhty of the rate of pumping. This variable adjustabihty enables economical use of the stored glycol mixture.

When the smoke screen generating system is first activated in response to triggering of the intruder detection system A the control unit 9 will provide power to the piston pump 4 such that the piston pump will operate.

As the level of smoke density within the room builds up through operation of the system it will eventually reach the point at which it meets the threshold level for triggering of the relay switches 11 of the sensor 10. This will inhibit supply of further power to the piston pump 4 by the triac of the control unit 9 unless or until the smoke density level drops below the sensor 10 threshold level at which point, assuming that the system is still activiated and has not been disarmed, the switches 11 of the sensors 10 will switch the triac in the control unit 9 to allow the pump 4 to operate to pump out further vapour.

The smoke sensors 10 are suitably independent of the unit 1 and operatively linked thereto by one or more electrical cables. By provision of the sensors 10 independently of the unit 1, substantial flexibility in the arrangement of the system is obtained. Amongst other factors, the independent positionability of the sensors enables adjustment by that means of the sensitivity of the system to the smoke screen density.

Because efficient use of the limited supply of glycol mixture is dependent upon the temperature of the heating block 6 being sufficient to fully vaporise the glycol mixture, temperature sensors 13 are provided to sense the

temperature of the heating block 6 and send control inputs to the triac of the control unit 9 to adjust the rate of operation of the pump 4 to correspond with the temperature of the heating block 6. Thus, at start up, the rate of pumping of the fluid to and through the heating block 6 will initially be maintained at a low level and made to rise as the temperature of the heating block 6 rises.

A number of smoke-generating units may be inter-linked and connected to one or more sensors at a range of locations to provide optimal coverage of an area to be enshrouded in the smoke screens. By trial and error a suitable configuration of the smoke-generating units 1 and the sensors 10 may be arrived at to ensure that no area of a room is vulnerable to dispersion of the smoke screen by opening of doors or windows or the like.

By using a Glycol/water mixture or similar heat labile fog-forming liquid composition, a long-lasting dense fog may be produce with little or no residue being deposited on the exposed surfaces of the room, leaving the room practically untouched by smoke once it has been dispersed and the smoke being totally innocuous to occupants of the room. A smoke screen of this nature is ideal for the purpose in question.

Variation of the composition of the mixture by, for example, changing the glycol used or altering the water content, provides further selectability of the duration of persistence of the generated smoke screen.