WO1993010513A1

WO1993010513A1 - Glass break detection

Info

Publication number: WO1993010513A1
Application number: PCT/SE1992/000786
Authority: WO
Inventors: Lennart Hedlund
Original assignee: Lennart Hedlund
Priority date: 1991-11-15
Filing date: 1992-11-13
Publication date: 1993-05-27
Also published as: SE9103386L; SE9103386D0

Abstract

There are disclosed a method and a device for detection of the breaking of a pane of glass for delivery of an alarm signal. By means of a broad band ultrasonic sensor ultrasonic waves are detected, said waves being generated when a pane of glass is broken, an alarm signal being delivered upon receipt of ultrasonic waves within the frequency range of 50 - 110 kHz having amplitudes exceeding a predetermined threshold level.

Description

GLASS BREAK DETECTION

The invention concerns a method and a device for the detection of breaking glass for the delivery of an alarm signal.

A known and widely used detector for the detection of breaking glass is constituted by a vibration sensor, which is applied to a pane of glass and which will generate an alarm signal upon sensing of vibrational signals being detected when the pane of glass is broken.

A disadvantage with such detectors is that at least one vibra¬ tional sensor must be applied to each pane of glass, which is costly in premises with many panes of glass and which is also a draw-back from an aesthetic point of view.

Furthermore, a method for detecting the breaking of glass, employing frequency-selective monotoring means for detecting ultrasonic pressure waves, is known from the British patent specification 1 323 451. It is stated that the most advanta¬ geous frequency selection is in the range of 40 kHz. However, it has turned out that signals dereived from such ultrasonic frequencies may be caused by the impact of metal objects onto a pane of glass (without breaking the glass) , and, therefore, the known method does not provide an effective and secure detection of the breaking of glass.

The above-mentioned British patent specification also dis¬ closes a resonant transducer as an ultrasonic sensor for monitoring a nearby single or multiple window.

A disadvantage with using a resonant transducer is its narrow bandwidth. The breaking of glass does not generally produce ultrasound at one typical frequency, nor within a narrow frequency range. Therefore, a resonant transducer will not always reliably detect the ultrasonic spectrum arising when a particular sheet of glass is broken. The object of the invention is to provide a method and a device enabling the detection of breaking glass at a distance from the respective pane of glass, irrespective of the parti¬ cular type of glass or way of breakage, so that one or more panes of glass can be monitored by means of one detector only.

This object is achieved by a method and a device, respec¬ tively, according to the invention wherein one senses ultra¬ sonic waves, being generated when a pane of glass is broken, by means of an ultrasonic sensor, which method and device are characterized in that an alarm signal is delivered upon the receipt of ultrasonic waves having amplitudes exceeding a predetermined threshold level in the frequency interval 50 - 110 kHz or a part thereof, the detector device comprising an ultrasonic sensor, which is sensitive to ultrasonic waves within a broad part of the frequency interval well above 110 kHz, as well as a signal processing unit with means for filte¬ ring the sensor signal into the frequency interval 50 - 110 kHz or a part thereof.

It is known that a pane of glass, which is broken, will cause characteristic sound waves, not only in the form of the well known "tinkle" sound in the audible region, but also at much higher frequences in the non-audible ultrasound region. Through recent improved measurements it has been found that these frequencies occur particularly in the range around approximately 100 kHz. These ultrasonic frequencies are gene¬ rated exclusively by the fracture or crack propagation in the glass while the audible frequencies arise from vibrations and mechanical resonance of smaller pices of glass produced when a pane of glass is broken.

Since the sound generation occurs at such high frequencies, detection can be carried out without any risk of false alarm delivery because of possible sounds not originating from brea¬ king panes of glass. The invention will be explained more fully below with refe¬ rence to the appended drawings.

If

Fig. 1 shows a block diagram of an alarm detector device 5 according to the invention; and

Fig. 2a and 2b show topographical representations of sound spectra originating from broken panes of glass, a pane of glass being struck by a hammer and being broken by pressing 10 without striking, respectively.

The device shown in Fig. 1, for the detection and alarm deli¬ very is very simple and comprises basically an ultrasonic sensor or microphone 1 and associated circuitry, viz. an

15 amplifier 2 for amplifying the sensor signal, a processing unit 3 and an alarm circuit 4. These circuits may be assembled in an apparatus casing or may be separated in e.g. a detector unit, 1 - 3 or 1 - 2, and an alarm unit, 4 or 3 - 4, respec¬ tively.

20

The ultrasonic sensor 1 can be placed at a desired location in the room or the space to be monitored with respect to breaking of one or more panes of glass, in particular in window or door portions. Consequently, no detectors have to be applied to the

25 respective panes of glass, but a single ultrasonic sensor can be disposed at some distance into the premises for monitoring a plurality of panes of glass being located within sensing distance for ultrasonic waves.

30 The ultrasonic sensor is adapted to sense ultrasonic waves within a broad range of the frequency interval up to 200 kHz. Suitable filters are arranged in the signal processing unit 3 for filtering the sensor signal into a smaller frequency interval.

35

In order to avoid false alarms caused by weak noice or inter¬ ference signals in said frequency interval, the ultrasonic sensor 1 or the signal processing unit 3 must be adapted to discriminate such weak signals and to pass on to the alarm unit 4 only signals for which peak amplitudes or mean energy during a certain integration time period exceed a predeter- mined threshold level. When signals are passed on by the signal processing unit 3 an alarm will be generated by the alarm unit 4.

If desired, adjusting means, e.g. in the form of an adjustment circuit 3a connected to the signal processing unit 3, may be disposed for setting the sensitivity range (in the frequency domain) , the integration time period, and/or a desired thres¬ hold level. In a corresponding manner, an adjustment circuit 2a may be connected to the amplifier 2 for setting the neces- sary amplification.

In Figs. 2a and 2b, the measurement result is shown upon brea¬ king panes of glass having a thickness of 3 mm. The Figs, show topographical diagrams which illustrate the frequency distri- bution (after Fourier transform, FFT) as a function of time upon breaking the glass by means of a hammer, (Fig. 2a) and by means of pressing without striking (Fig. 2b) , respectively.

The measurements were performed with a microphone of the type B & K 4138 at a distance of about 1 m from the pane of glass. The measurement signal was amplified in a measurement ampli¬ fier of the type B & K 2120 and was sampled with a sample period of 2 μs.

The diagrams show clearly that strong ultrasonic waves were generated in the frequency interval 20 - 200 kHz, in parti¬ cular in the region 50 - 110 kHz having an especially high amplitude at about 90 kHz, upon breaking of a pane of glass, irrespective of the cause thereof. This knowledge is basic to the present invention. Therefore the ultrasonic sensor 1 combined with the signalprocessing device should produce a total sensitivity range corresponding to the frequency inter¬ val 50 - 110 kHz or a part thereof.

The above referred measurements, as well as other experiments, show that in the particular case, the exact frequencies within this interval may depend to a certain degree on the quality of the glass, the thickness of the glass, possible tension in the glass, and other parameters. A resonant tranducer is thus, due to its narrow bandwidth, uncapable of providing sufficient ultrasonic sensitivity in said required interval. As opposed to a resonant transducer, a broad band sensor or microphone having a sufficient sensitivity domain within the interval 50 - 110 kHz is useful for monitoring most kinds of existing panes of glass in buildings, vehicles and other places where glass break detection is desireable.

It is also evident from Figs. 2a and 2b that the detection of ultrasound may be combined with the detection of audible sound with a certain intermediate time delay to further increase the detection reliability.

The method and the device according to the invention may of course be combined with other kinds of burglar and fire alarm sensors.

Claims

1. A method of detecting the breaking of a pane of glass for the delivery of an alarm signal, wherein ultrasonic waves, being generated when a pane of glass is broken, are detected by means of an ultrasonic sensor, c h a r a c t e r i z e d in that said alarm signal is delivered upon the receipt of ultra¬ sonic waves having amplitudes exceeding a predetermined thres¬ hold level in the frequency interval 50 - 110 kHz or a broad part thereof.

2. A method as defined in claim 1, c h a r a c t e r i z e d in that said alarm signal is delivered when said received ultrasonic wawes have at least one superimposed amplitude peak exceeding a predetermined amplitude level in said frequency interval or a broad part thereof.

3. A method as defined in claim 1, c h a r a c t e r i z e d in that said alarm signal is delivered when said ultrasonic waves have a mean energy during at least one of equal-lengthed continously regarded integration time periods, exceeding a predetermined energy level in said frequency interval or a broad part thereof.

4. A method as defined in claim 3, c h a r a c t e r i z e d in that the length of said regarded integration time periods are chosen in the range of the duration of sound generation by breaking glass.

5. A method as defined in claim 1 - 4, c h a r a c t e ¬ r i z e d in that one uses a broad band ultrasonic sensor, which may also sense audible sound, and filters the received signal into said frequency interval or a broad part thereof.

6. A method as defined in claims 1 - 5, c h a r a c t e ¬ r i z e d in that one requires both the detection of said ultrasonic waves and the detection of audible sound separated by a certain intermediate time delay in order to generate said alarm signal.

7. A detector alarm device for the detection of the breaking of a pane of glass, c h a r a c t e r i z e d in that the detector device comprises an ultrasonic sensor (1) , which is sensitive to ultrasonic waves within a broad range of the frequency interval up to 200 kHz, as well as a signal proces¬ sing unit (3) with means for filtering the sensor signal into the frequency interval 50 - 110 kHz or a broad part thereof.

8. A detector alarm device as defined in claim 7, c h a r a c t e r i z e d in that the ultrasonic sensor (1) is connected to the signal processing unit (3) , if required via an amplifying unit (2) .

9. A detector alarm device as defined in claim 7 or 8, c h a r a c t e r i z e d in that the signal processing unit (3) is provided with a threshold circuit adapted to only pass on signals from the ultrasonic sensor having amplitudes excee¬ ding a predetermined threshold level.

10. A detector alarm device as defined in anyone of claims 7 - 9, c h a r a c t e r i z e d by adjustment means (2a, 3a) for setting the sensitivity range of the detector device by adapting parameters of frequency interval, signal amplifi¬ cation, integration time period and/or threshold level.