GB2088107A

GB2088107A - Arrangement for detecting the breaking of window glass

Info

Publication number: GB2088107A
Application number: GB8132407A
Authority: GB
Original assignee: Shorrock Security Systems Ltd
Current assignee: Shorrock Security Systems Ltd
Priority date: 1980-11-06
Filing date: 1981-10-28
Publication date: 1982-06-03
Also published as: GB2088107B

Abstract

Detector 10 in the form of a piezoelectric element or accelerometer is disposed on a window pane or its frame and provides a signal, on physically sensing breakage of the pane, to AND gate 12. Microphone 13 faces the pane and provides a signal on audibly detecting breakage of the pane, to the AND gate 12, which provides an alarm output only when both signals are received substantially simultaneously, thereby avoiding spurious alarms. <IMAGE>

Description

SPECIFICATION Arrangement for detecting the breaking of window glass This invention relates to an arrangement for detecting the breaking of window glass.

Breaking glass detectors of various different types are well known. One known type is shocksensitive; it is physically attached to a window and responds to the physical shock of glass breakage.

Such detectors usually operate on either the principle of an accelerometer or the stress applied to a piezo-electric element to generate a voltage.

Detectors of this kind can be effective, but frequently nuisance alarms can be generated, for example by persons tapping on the window. In particular, children often delight in deliberately actuating the detector and setting off the alarm system connected thereto.

A further type of detector is sound-sensitive. It is based on the use of a microphone located so as to look at the glass to be protected and arranged to indicate breakage in the event of receipt by the microphone of a short sharp sound characteristic of that of breaking glass.

Again, detectors of this kind can be effective, but problems of false alarm generation can arise, for example from thunderclaps, sonic and supersonic bangs and other extraneous noises.

An object of the present invention is to provide an arrangement which reduces enormously the possibility of false or nuisance alarms.

With this object in view, the present invention provides an arrangement for detecting the breaking of window glass which comprises a first shock sensitive detector attached to a pane of the glass whose breakage is to be detected or to a frame around said pane, a second sound-sensitive detector associated with said pane, sensor means adapted to receive signals from said two detectors and operative in a logical AND function so as to provide an alarm output only upon receipt of signals indicative of glass breakage substantially simultaneously from both of said detectors.

The shock-sensitive detector may comprise a piezo-electric element or accelerometer which generates a signal upon breakage of the pane, in combination with a monostable to provide a modified and extended signal to the sensor.

The sound-sensitive detector may comprise a microphone which faces the glass pane, this too being in combination with a respective modified and extended signal to the sensor.

The sensor may, of course, simply be an electronic AND gate, and the alarm output may serve to initiate an audible alarm in the vicinity of the glass being protected, or it may provide an alarm at a remote location, such as at a police station or security post.

The arrangement of the invention ensures that false or nuisance alarms are minimised. For instance, tapping on the window will not activate the sound-sensitive detector, and the likelihood of tapping on the window occurring simultaneously with a loud noise similar to glass breaking, such as a sonic boom, is low.

Further, with the arrangement of the invention the sensitivity of both of the detectors may be adjusted to levels which would be unacceptable with single element detectors, as the false alarm rate would be intolerable.

In order that the invention may be fully understood, it will be described further, by way of example, with reference to the accompanying drawing in which the single figure is a block diagram of the circuitry of a preferred practical embodiment thereof, the following description being illustrative, and not restrictive, of the scope of the invention.

The preferred embodiment of the arrangement of the invention is an alarm installation fitted to a shop window (not shown). It comprises, attached directly to the pane of glass of the window, or alternatively to the window frame, a first breakage detector 10 which is a shock-sensitive detector in the form of a piezo-electric element which generates a voltage upon being subjected to stress. This element is connected electrically to a monostable 1 1 the function of which is such that upon receipt of a signal from the piezo-electric element it modifies it so as to produce correspondingly extended signal; the monostable 11 in turn is connected to an AND gate 12.

The installation further includes a soundsensitive detector 13 which is mounted at a spacing from the pane and facing the latter, this detector 13 being for example in the form of a microphone and adapted to produce a respective signal upon sensing sound originating from the pane. The microphone is connected to a shaping amplifier 14 which serves to transmit an effective signal to a respective monostable 1 5 when the sound detected is such as would be consistent with breaking of the glass pane. This monostable 15, too, is connected to the AND gate 1 2.

The AND gate 12, in its turn, is connected to alarm apparatus (not illustrated).

The mode of operation of the installation will readily be understood from the foregoing description. If either of the detectors 10, 1 3 separately supplies a respective signal to the AND gate 12, no alarm output is supplied by the AND gate 12 to the alarm. Thus, spurious alarms and nuisance alarms are effectively minimised.

However, should both the detectors 10 and 1 3 supply signals substantially simultaneously to the AND gate 12, the latter produces its alarm output to drive the alarm, and provide an audible or other suitable warning indicative of the likelihood of the glass pane having been broken.

Naturally the invention is not confined to the precise details of the foregoing example. The forms of the two detectors may differ from what has been described, as also can the manner in which said detectors are connected to the alarm arrangement provided the latter is driven only when both detectors produce signals consistent with glass breakage.

Claims

1. An arrangement for detecting the breaking of window glass which comprises a first shock sensitive detector attached to a pane of the glass whose breakage is to be detected or to a frame around said pane a second sound-sensitive detector associated with said pane, sensor means adapted to receive signals from said two detectors and operative in a logical AND function so as to provide an alarm output only upon receipt of signals indicative of glass breakage substantially simultaneously from both of said detectors.

2. An arrangement as claimed in claim 1 wherein the shock-sensitive detector comprises a piezo-electric element or accelerometer which generates a signal upon breakage of the pane, in combination with a monostable to provide a modified and extended signal to the sensor.

3. An arrangement as claimed in claim 1 or 2 wherein the sound-sensitive detector comprises a microphone which faces the glass pane, in combination with a respective monostable to provide a respective modified and extended signal to the sensor.

4. An arrangement as claimed in claim 1, 2 or 3 wherein the sensor is an electronic AND gatO.

5. An arrangement as claimed in any preceding claim wherein the alarm output serves to initiate an audible alarm or provides an alarm at a remote location.

6. An arrangement for detecting the breaking of window glass substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.