Classifications

• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B13/00—Burglar, theft or intruder alarms
  • G08B13/02—Mechanical actuation
  • G08B13/04—Mechanical actuation by breaking of glass

Definitions

• 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.
• 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.
• 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.
• Fig. 1 shows a block diagram of an alarm detector device 5 according to the invention.
• 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
• amplifier 2 for amplifying the sensor signal
• processing unit 3 for amplifying the sensor signal
• an alarm circuit 4 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.
• 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
• 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.
• 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.
• the ultrasonic sensor 1 or the signal processing unit 3 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.
• 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.
• an adjustment circuit 2a may be connected to the amplifier 2 for setting the neces- sary amplification.
• 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.
• FFT frequency distri- bution
• 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 ⁇ bomb of the type B & K 2120 and was sampled with a sample period of 2 ⁇ s.
• 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 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.

Landscapes

• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)
• Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
• Length Measuring Devices Characterised By Use Of Acoustic Means (AREA)
• Re-Forming, After-Treatment, Cutting And Transporting Of Glass Products (AREA)
• Investigating Strength Of Materials By Application Of Mechanical Stress (AREA)
• Burglar Alarm Systems (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=20384342

Family Applications (1)

Country Status (2)

Cited By (3)

Citations (9)

• 1991
  • 1991-11-15 SE SE9103386A patent/SE9103386L/sv not_active Application Discontinuation
• 1992
  • 1992-11-13 WO PCT/SE1992/000786 patent/WO1993010513A1/en active Application Filing

Patent Citations (10)

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