The invention relates to an intruder detector according to the preamble
of Patent Claim 1 and in particular to an acoustic pressure wave glass monitor.
German Offenlegungsschrift 2,656,318, which is the starting point
of the preamble of Patent Claim 1, discloses a device for detecting the movement
of bodies emitting heat, in which two different detectors are connected such that
an alarm is triggered only given the response of both detectors. In this case,
one detector is embodied as an ultrasonic detector and one detector is embodied
as an infrared detector.
A problem with an intruder detector of this type consists in that
an infrared detector can detect only movements perpendicular to the principal axis
of its security zone and an ultrasonic detector can detect only movements in the
direction of the principal axis of its security zone. An intruder who moves only
on the principal axis of the security zone or only at a right angle to the principal
axis of the security zone of the detector can, by adopting this method of locomotion,
avoid triggering the alarm since in so doing he always triggers only one of the
It is the object of the invention to develop an intruder detector
according to the preamble of Patent Claim 1 in such a way that intrusions are reliably
detected and false alarms are avoided with the aid of simple means.
This object is achieved by means of the measures specified in Patent
It is therefore provided according to the invention to combine a
pressure fluctuation detecting device with a glass breakage detecting device in
such a way that an alarm is triggered only if both detecting devices are addressed
with frequencies which are situated in a frequency range that is characteristic
of the respective detection criterion. A high security against intrusions and
false alarms is guaranteed by the selection of the glass breakage criterion and
of the pressure fluctuation criterion, since on the one hand both criteria are
always fulfilled given the destruction of a pane in the security zone, while on
the other hand both criteria are virtually incapable of being fulfilled by a person
legitimately staying in the room to be secured.
Moreover, the security against false alarms is further enhanced owing
to the fact that triggering of the detecting devices is restricted to frequencies
the combination of which arises exclusively given the destruction of the pane
in a room.
Moreover, by using the trigger according to Patent Claim 3 it is
achieved that too slight pressure fluctuations, such as can arise upon movement
of a door, for example, cannot trigger the pressure fluctuation detecting device,
so that security against false alarms is further improved by these measures.
Since the sound waves of the noise of glass breakage generally reach
the detecting devices before the pressure waves, by using the time-delay circuit
according to Patent Claim 4 it is achieved that the two detecting devices pass
on an intrusion at the same time.
Further advantageous embodiments of the invention are specified in
the remaining subclaims.
The invention is explained in more detail below using the description
of exemplary embodiments with reference to the drawing, wherein:
- Figure 1
- shows a block diagram of a first exemplary embodiment of the intruder detector
according to the invention in the form of an acoustic pressure wave glass monitor,
- Figure 2
- shows a block diagram of a second exemplary embodiment of the intruder detector
according to the invention with a joint detector for both detecting devices.
A pressure detector 10 according to Figure 1 has a structure known
per se and is designed such that it can effectively sense fluctuations in room
pressure down to 0.3 mbar. It is arranged in the room to be secured in such a
way that it senses pressure fluctuations in the entire security zone.
The output signal of the pressure detector 10 is fed to a first bandpass
filter 15 which suppresses interfering signals not situated in the pressure fluctuation
frequency range characteristic of a breaking pane. As could be established by
trials, the frequencies of the pressure fluctuations arising given the destruction
of a pane are situated between 5 Hz and 10 Hz.
Since the amplitudes of the pressure fluctuations are relatively
low, the output signal of the bandpass filter 15 is fed to an amplifier 20 which
has a gain of 300 at a frequency of 7.5 Hz.
The filtered and amplified signal of the pressure detector 10 is
fed to a trigger 25 or to a Schmitt trigger circuit. This trigger is the junction
between the analog pressure-sensing circuit and the downstream digital circuit.
The output of the trigger is a high signal when the output signal of the amplifier
20 overshoots an adjustable threshold value, or a low signal when a smaller signal
than the threshold value is present at the input of the trigger 25. The threshold
value of the trigger is selected such that fluctuations in room pressure of less
than 0.3 mbar or in a range from -0.3 mbar to +0.3 mbar correspond to a low signal
at the output of the trigger 25. The output signal of the trigger 25 is fed to
an AND element 60 and a light-emitting diode 50 which displays the undershooting
or overshooting of the threshold value pressure of +0.3 mbar.
A glass breakage detector 30 is either mounted directly on a pane
or arranged such that it can sense any noises from the breakage of a plurality
of panes, which need not be situated in a plane window front. In the first case,
the glass breakage detector 30 has a conventional structure, and in the second
case the glass breakage detector 30 is a microphone having a frequency response
and a pickup pattern by means of which a glass breakage in the entire security
zone of the glass breakage detector can be effectively sensed.
The output signal of the glass breakage detector 30 is fed to a second
bandpass filter 35 which suppresses interfering signals which are not situated
in the frequency range of the noises of glass breakage. In order for the glass
breakage frequencies of panes such as bullet-proof glass, plastics-containing panes
or pure glass panes to be covered, the lower limiting frequency is selected at
4 kHz and the upper limiting frequency at 12 kHz.
The output signal of the second bandpass filter is fed to a rectifier
40, since the downstream digital circuit operates with positive levels. Furthermore,
the signal is smoothed or lengthened by the rectifier, so that it can be more
effectively detected by the downstream circuit.
The glass breakage signal is delayed by a time T1 = 0.1 s in a time-delay
circuit 45. Given glass breakage, there is a high signal at the output of the time-delay
circuit 45, and in the absence of glass breakage there is a low signal. This signal
is fed to the AND element 60 and a light-emitting diode 55, which displays a noise
of glass breakage.
If a high signal is fed simultaneously to the AND element 60 from
the trigger 25 and the time-delay circuit 45, a high signal likewise arises at
the output of the AND element 60.
A downstream storage device 65 passes the high signal on to a downstream
relay 70 and a light-emitting diode 75, which displays the case of an alarm. In
this regard, the storage device 65 stores the high signal of the AND element 60
until a reset signal is applied to the storage device 65 via the reset input 80,
or until the operating voltage of the storage device 65 is switched off. The storage
device 65 is designed such that it stores a low signal given application of the
Finally, the alarm report is transmitted, for example, to an alarm
indicator or a report control centre by means of a contact assembly (not shown)
of the relay 70.
The detector 85 according to Figure 2 is arranged in the room to
be secured in such a way that it can effectively sense pressure fluctuations and
noises of glass breakage in the entire security zone.
The detector 85 can detect vibrations in a range from 5 Hz to 12
kHz, and can be formed, for example, by a microphone which can sense this range.
The output signal of the detector 85 is fed to the bandpass filters
15 and 35, which transmit the corresponding frequency components thereof, as already
in the case of the first exemplary embodiment. The circuit downstream of the glass
breakage detector 30 and the pressure detector 10 or the detector 85 is identical.
The mode of functioning of the acoustic pressure wave glass monitor,
which is the same in both exemplary embodiments, is explained in more detail below.
In this connection, the sequence in the case of an actual intrusion is first described.
When a glass pane is pushed in or smashed in, or when a hole is cut
out of it, in the security zone a noise of glass breakage arises, on the one hand,
and pressure fluctuations are triggered in the room to be secured, on the other
hand. The noise of glass breakage is recorded by the glass breakage detector 30
or the detector 85, and the pressure fluctuations are recorded by the pressure
detector 10 or likewise by the detector 85, the pressure fluctuations generally
being recorded with a delay of approximately 0.1 s, since pressure waves have
a lower rate of propagation than sound waves.
The sound waves of the noise of glass breakage are detected by the
glass breakage detector 30 or the detector 85, conducted via the second bandpass
filter 35 to the rectifier 40, where they are rectified and smoothed or maintained
for a specific time T2 and transmitted with a 0.1 s delay time T1 via the time-delay
circuit 45 as a high signal to the AND element 60, a light-emitting diode 55 simultaneously
lighting up. The time T2 can be selected in the range from 0.1 s to 1 s. In the
meantime, the pressure fluctuation has also reached the AND element 60 via the
pressure detector 10 or the detector 85, the first bandpass filter 15, the amplifier
20 and the trigger 25 as a high signal, the light-emitting diode 55 lighting up
and there arising at the output of the AND element 60 a high signal which is maintained
via the storage device 65 and actuates the relay 70 as well as switching on a light-emitting
diode 75. Finally, an alarm report is transmitted to an alarm indicator or a report
control centre via a contact assembly of the relay 70.
A high security against false alarms is achieved by means of the
acoustic pressure wave glass monitor according to the invention. If the glass breakage
detector is constructed as a microphone and mounted in a room in such a way that
it monitors a plurality of windows or glass doors, the noises that are not situated
in the frequency range of the noises of glass breakage are suppressed via the
second bandpass filter 35.
Moreover, by using the first bandpass filter 15 it is achieved that
only the fluctuations typical of a glass breakage are transmitted.
In addition, it is ensured by means of the delay time T1 of the time-delay
circuit 45 that a pressure change and a noise of glass breakage trigger an alarm
only in a sequence characteristic of a glass breakage.
Finally, by means of the AND element 60 the breaking of a glass or,
for example, the crack of a supersonic aircraft breaking the sound barrier is
reliably detected as a false alarm and an alarm report is prevented.
Pressure fluctuations in the room owing to wind conditions around
the building in which the zone to be secured is situated, or pressure fluctuations
owing to the opening of a door or a window have fluctuation frequencies which
are situated below the lower limiting frequency of the first bandpass filter 15.
Moreover, pressure fluctuations owing to the impact of a bird on a pane are generally
of so low an amplitude that the signals generated by the pressure detector 10 or
the detector 85 are situated below the threshold voltage of the trigger 25.
The pressure detector and the glass breakage detector or the entire
acoustic pressure wave glass monitor can be accommodated in a single housing.
The acoustic pressure wave glass monitor according to the invention
can be used in rooms of up to 50 m². Its mode of functioning is ensured even if
individual windows or doors of the house to be secured are open.
A description is given of an intruder detector having two different
detecting devices for the same security zone, which triggers an alarm only if both
detecting devices respond in a specific way, one detecting device being a glass
breakage detecting device which comprises a glass breakage detector and a bandpass
filter and the other detecting device being a pressure fluctuation detecting device
which comprises a pressure detector and a second bandpass filter, it being possible
for the pressure detector and the glass breakage detector to be formed by a single
sensing element. An alarm is triggered only if both detecting devices are addressed
in specific frequency bands.