GB2217441A

GB2217441A - Alarm system for enclosures

Info

Publication number: GB2217441A
Application number: GB8729189A
Authority: GB
Inventors: John Philip Dakin
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1987-12-15
Filing date: 1987-12-15
Publication date: 1989-10-25
Also published as: GB8729189D0

Abstract

An alarm system mounted within an enclosure, such as a vehicle passenger cabin 24, detects opening of the enclosure by means of a light source 23 arranged to scatter light 27 broadly within the enclosure and a detector 29 arranged to receive reflected backscattered light 30. The light source may be pulsed in order that phase difference detection can be used in the detector arrangement. <IMAGE>

Description

AN ALARM SYSTEM The present invention relates to an alarm system and more particularly to such an alarm system using light signal disruption to cause activation of the alarm.

There is a requirement to provide alarm systems for enclosures, particularly vehicle cabin enclosures. Figure 1 illustrates a previous alarm system for a vehicle passenger cabin. A transmitter/detector element 3 provides a well collimated optical beam 5 accurately projected against walls of the vehicle cavity or cabin 1. The optical beam 5 has its angles of reflection upon the walls arranged such that a major proportion of the reflected beam 7 is eventual substantially incident upon the detector element 3 again.

The beam 5 is principally reflected by windows in the vehicle cavity 1 due to their inherent reflectivity.

It will be appreciated that when a door of the cabin 1 is opened the angles of reflection will be disrupted and thus the reflected beam 7 will not be incident upon the element 3. Consequently, a loss of signal can be detected and an alarm raised.

These previous alarm systems are limited by the requirement for accurate reflection in order that the reflected beam 7 becomes incident upon the element 3. However, various environmental factors such as ambient light or condensation on windows of the cabin 1 will alter the reflectivity of those windows. Variations in the reflectivity will consequently change the proportion of reflected light incident upon the element 3. These changes in proportions of reflected light incident upon the element 3 may cause false alarms.

The sensitivity of this prior system must thus be balanced against possible environmental changes. These prior systems are thus problematic and relatively limited in their application.

It is an objective of the present invention to provide an alarm system which substantially relieves the above problems.

According to the present invention there is provided an alarm system for an openable enclosure having at least one light source and a detector element, the light sources being arranged, in operation, to scatter light in the enclosure such that a proportion of light is reflected back by a substantial portion of the enclosure walls to be incident, either directly or following scattering from additional surfaces within the enclosure, upon the detector element, the proportion of light reflected being dependent upon whether the enclosure is open or closed, the detector element being arranged to provide an alarm signal when a change in the proportion of light reflected by the enclosure is detected.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawing in which: Figure 2 illustrates an alarm system according to the present invention; Figure 3 illustrates, in functional block diagram form, a detection arrangement for the system as shown in Figure 2; and, Figure 4 illustrates an oscillator signal, and reflected backscatter signals typically associated with the alarm system as illustrated in Figure 2.

In Figure 2, an alarm system according to the present invention is illustrated within a vehicle body 21. The interior of a passenger cabin 24 in the body 21 is broadly illuminated by light 25 scattered from sources 23. The sources 23 may emit infra-red light and be light emitting diodes (L.E.D's). The light 25 is initially reflected principally by windows 27 etc. of the passenger cabin 24. It will be appreciated that the sources may be arranged at several points within the interior of the vehicle body cabin 24 or as shown in Figure 2 in a central location. The light 25 is thus reflected by a substantial portion of the surfaces of cabin 24 and any objects therein.

A detector arrangement 29 is positioned to receive reflected backscattered light 30 from the interior surfaces of the passenger cabin 24. The effects of moisture on the surfaces of the vehicle cavity 24 are reduced as the alarm system operates over entire surfaces rather than specific small areas as in the previous system.

In Figure 3 the detector arrangement 29 (Figure 2) is illustrated in functional-block diagram form. A detector element 36 detects the backscattered light 30. Typically, the sources 23 are modulated by an oscillator 31 and the detection arrangement uses a technique of phase sensitive detection. By using such a technique the alarm system is insensitive to extraneous ambient light.

Furthermore, the phase difference between an oscillator signal provided by the oscillator 31 and the detected backscattered light 30 detected by detector element 36 may be shifted by a phase shifting element 33. This relative shifting of the reference/detected signal phase difference allows null-point and sensitivity optimisation for the alarm system. A phase-sensitive detector (P.S.D) 39 is arranged to receive an output from the detector element 36 and the oscillator signal from the oscillator element 31.

In Figure 4, the oscillator signal 41 is illustrated along with typical backscattered light signals 30 associated with the present alarm system. The oscillator signal 41 is a simple square wave sufficient to switch the sources 23 ON and OFF. With the cabin 24 (Figure 2) completely closed the backscattered light signal is represented by the pulse train 30' whilst, when a window or door is opened or damaged, there is a displacement to pulse train 30". The displacement between the pulse trains 30' and 30" is due to a variation in detected phase and/or intensity for the backscattered light 30. The displacement between pulse trains 30' and 30" is detected by the P.S.D 39 (Figure 3) and an alarm signal 43 output.

The alarm signal or detector arrangement output may be fed to an AC coupling arrangement or "D.C. - blocking buffer" in order that slow displacements in phase or the reflected light intensity may be prevented from stimulating an alarm device. A capacitor-resistor network can provide such a buffer. The transmitted signal after such "high-pass" electrical filtering, may be fed to a threshold detection or "comparator" device which may be triggered and if required, be reset after a specific time period. The triggered state of the detection circuitry may be arranged to stimulate an alarm device such as a car horn.

It will be appreciated that, although the present invention has been described with respect to the passenger cabin of a vehicle, the invention could be equally applicable, with adaption, to other enclosures such as greenhouses, boat cabins or rooms within a house.

Claims

1. An alarm system for an openable enclosure having at least one light source and a detector element, the light source being arranged1 in operation, to scatter light in the enclosure such that a proportion of light is reflected back by a substantial portion of the enclosure walls to be incident, either directly or following scattering from additional surfaces within the enclosure, upon the detector element, the proportion of light reflected being dependent upon whether the enclosure is open or closed, the detector element being arranged to provide an alarm signal when a change in the proportion of light reflected by the enclosure is detected.

2. An alarm system as claimed in claim 1 wherein the light source is an infra-red radiation emitter.

3. An alarm system as claimed in claim 1 or 2 further including an oscillator arranged, in operation, to pulse or otherwise modulate the intensity of the emitted light.

4. An alarm system as claimed in claim 3 wherein the detector element is arranged, in operation, to compare the phase between the oscillator signal and the light reflected by the enclosure whereby an ambient phase difference is established and displacements from such ambient phase difference are arranged to stimulate an alarm signal.

5. An alarm system as claimed in claim 4 wherein a phase shifter element is arranged, in operation, between the oscillator element and the detector element to shift the phase of the oscillator signal.

6. An alarm system as claimed in claim 3 wherein the detector element is arranged to detect changes in the amplitude of the received light signal and such amplitude changes are arranged to stimulate an alarm signal.

7. An alarm system as claimed in any preceding claim wherein the enclosure is a vehicle passenger cabin.

8. An alarm system as claimed in any preceding claim further including a buffer element arranged, in operation, to prevent slow changes in the light reflected by the enclosure from stimulating an alarm device.

9. An alarm system as claimed in claim 7 wherein the buffer element comprises an electrical storage device arranged, in operation, to be charged or triggered by the detector arrangement, when a change in the proportion of light reflected by the enclosure is detected, and to stimulate an alarm device when a pre-set change in detected signal has occurred and to continue to stimulate such alarm device for a preset period.

10. An alarm system substantially as hereinbefore described with reference to the accompanying drawings.