GB2124763A

GB2124763A - Alarm system

Info

Publication number: GB2124763A
Application number: GB08315824A
Authority: GB
Inventors: Werner Kormann; Heinz Pieren; Walter Enkerli
Original assignee: Securiton AG
Current assignee: Securiton AG
Priority date: 1982-06-11
Filing date: 1983-06-09
Publication date: 1984-02-22
Also published as: ATA200583A; IT8309447A0; CH662689A5; CH661135A5; DE3221997C2; IT8309447A1; DE3221997A1; AT385856B; IT1198619B; GB8315824D0; FR2528487A1; GB2124763B; FR2528487B1

Abstract

Apparatus for monitoring armoured rooms, particularly vaults, safes, and strongrooms, and for generating an alarm signal when an attempt is made to break in, comprises an acousto-electric transducer (1) connected to two signal paths (A, B) and to a monitoring circuit (C). The first signal path (A) responds to briefly occurring signals of high amplitudes, the second signal path (B) to longer-continuing signals with significantly lower amplitudes than the first signal path, and the monitoring circuit (C) is intended for thermally monitoring the apparatus in the case of heating or freezing and for monitoring the electrical connection between the transducer (1) and the evaluating circuit of the apparatus. The signal path (B) is provided with analog (10, 11) or digital (counter) means for evaluating signal voltage peaks produced during mechanical or thermal attack on vaults or the like, particularly by means of an oxygen lance. For the purpose of improving the sensitivity of the piezoelectric element of the transducer (1), there is disposed thereon a brass disc which is pressed against the piezoelectric element by means of a spring. The apparatus makes it possible to avoid false alarms. <IMAGE>

Description

SPECIFICATION Alarm system This invention relates to alarm systems, and in particular to apparatus for monitoring armoured rooms, especially vaults, safes, and strongrooms, and for generating an alarm signal when there is an attempt to break in, of the type having an acoustoelectric transducer, to the output of which there is connected a first signal path responding to briefly occurring signals of high amplitudes and comprising an amplifier stage, a first threshold-value detector, an integrator, and a second threshold-value detector, and to the output of which transducer there is connected a second signal path responding to significantly lower amplitudes than the first signal path and comprising an amplifier stage and a threshold-value detector, both signal paths being connected to a common output stage for generating the alarm signal.

Apparatus of the foregoing type has already been proposed, particularly for detecting solid-borne or mechanical vibrations of the protected object produced during burglary attempts by sawing, filing, drilling, or torch-cutting. However, the prior art apparatus is capable of detecting solid-borne vibration occurring during mechanical or thermal attacks on armoured rooms only when the adjusted sensitivity of the apparatus is sufficiently great, so that the risk exists of setting off false alarms, e.g., through environmental influences. Furthermore, with the prior art apparatus, no alarm is set off by attempts at sabotage through heating or freezing the apparatus.

Such apparatus should also be insensitive to all sorts of environmental influences such as magnetic and electric fields, disturbing electric voltages and ground balancing currents, ambient mechanical noise, airborne sound, and unintentional mechanical blows.

Thus, there is a need for improved alarm apparatus which does not have the aforementioned shortcomings and which is unaffected by the mentioned environmental influences.

According to the present invention, there is provided apparatus of the type initially defined, wherein in the second signal path, analog or digital means are connected after the threshold-value detector, which means serve the purpose of evaluating signal voltage peaks which are supplied to the thresholdvalue detector and exceed a certain value, and wherein the acousto-electric transducer comprises a piezoelectric element upon which there is disposed, for the purpose of improving the sensitivity of the transducer, a brass disc which is pressed against the piezoelectric element by means of a spring.

The apparatus according to the invention is particularly suitable for evaluating signals produced by thermal attack with an oxygen lance.

A preferred embodiment of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Figure lisa block diagram of the apparatus according to the invention, Figure 2 is a circuit diagram of the apparatus, and Figure 3 is a sectional view of an acousto-electric transducer.

As shown in Figure 1, a signal generated by an acousto-electric transducer 1 is supplied to an amplifier stage 2 of a first circuit path A and to an amplifier stage 8 of a second circuit path B. The amplifier stage 2 is followed by a threshold-value detector 3, whereas a threshold-value detector 9 is connected after the amplifier stage 8. The amplifier stages 2 and 8 and the threshold-value detectors 3 and 9 are so dimensioned that the detector 3 responds only to a transducer signal which is at least a hundred times as great as that to which the detector 9 responds. When there is a signal at the output of the detector 3, the output voltage of an integrator 4 connected thereafter quickly rises until it reaches the reference voltage of a following threshold-value detector 5 connected to a gate circuit 6 which triggers an alarm relay 7.When there is a signal at the output of the detector 9, it is supplied to a following pulse shaper 10 connected to an integrator 11, the output voltage of which rises slowly and triggers the output stage 6, 7.

The signal path A thus serves the purpose of recognizing briefly occurring signals of high amplitude such as are produced by an explosion, whereas the signal path B responds to significantly lower signal amplitudes and longer-continuing signals such as are caused by sawing, filing, or drilling, for example.

A monitoring circuit C comprises a temperature indicator 12 connected to an input of a gate circuit 13. The other input of the gate circuit 13 is connected to the output of the acouto-electric transducer 1. The output of the gate circuit 13 is connected to the output stage 6, 7. The purpose of the monitoring circuit C is the thermal monitoring of the apparatus in case of heating or freezing with an aim to sabotaging it, as well as monitoring the electrical connection between the acousto-electric transducer 1 and the evaluating circuit of the apparatus.

Figure 2 is an example of a circuit diagram for producing the apparatus according to the present invention with very few circuit components. The signal path A comprises essentially two RC sections 14, 15 and 16, 17, a threshold-value detector 18, an integrator 20, 21, and a threshold-value detector 22.

Asignal occurring at the time of an explosion, for example, is conducted over a frequency-selective stage composed of the two RC sections 14, 15 and 16, 17. This stage forms a band-pass filter for the frequencies resulting from an explosion. The filtered signal is compared in the detector 18 with a referece voltage El. If the signal voltage is greater than the reference voltage El, a rectangular signal appears at the output of the detector 18. This signal is rectified by means of a diode 19 connected at the output of the detector 18 and supplied to the integrator composed of a resistor 20 and a capacitor 21. The integration time constant is a few milliseconds.

When the output voltage of the integrator 20, 21 is greater than the reference voltage E2 of the threshold-value detector 22, the latter emits a signal, which via a diode 23, triggers the output stage consisting of a threshold-value detector 24 with a reference voltage E3, a transistor 26, and a relay 25, and sets off an alarm.

The purpose of the signal path B is the evaluation of longer-continuing signals having significantly lower amplitudes than the signal path A, such as are produced by mechanical or thermal attack on a vault.

The electric signal generated by the acousto-electric transducer 1 is supplied to a frequency-selective amplifier stage consisting of an amplifier 27 and a parallel-T filter 28-33. The amplified signal triggers a threshold-value detector 35 across a resistor 34.

When the signal voltage is greater than a reference voltage E5, rectangular signals appear at the output of the detector 35. The following pulse shaper comprises a transistor 36 and a threshold-value detector 39. The collector of the transistor 36 is earthed across a capacitor 37 and a resistor 38 connected in parallel therewith. The collector of the transistor 36 is further connected to an input of the detector 39, to the other input of which a reference voltage E6 is applied. The output of the detector 39 is connected via a diode 40 to an integrator 41-44, which is in turn connected across a resistor 45 to the output stage 24-26.

The function of the pulse shaper 36-39 will now be described by way of example in connection with a thermal attack on a vault by means of an oxygen lance, during which noises are produced that cause at the output of the acousto-electric transducer a signal having a frequency of about 8 kHz and 50 ijV amplitude and a voltage peak of 1 mV approximately every 100 ms. The repetition rate of the voltage peaks thus amounts to several Hertz. In order to be able to integrate the voltage peaks, they must be converted into rectangular pulses having a sufficiently long pulse duration, which are integrated by the integrator 41-44 connected thereafter. The signal is supplied to the threshold-value detector 35, at the output of which a group of rectangular pulses appears periodically, the duration of each pulse being 0.5 ms.These pulses are supplied to the pulse shaper 36-39, the RC section 37, 38 of which has a time constant of 60 ms, and rectangular pulses appear at the output of the detector 39, the duration of each pulse being at least 50 ms. These pulses are supplied via the diode 40 to the integrator, which comprises an amplifier 44 and an RC section 41,43.

The integration time is about 10 sec. and is determined by the RC section 41,43. This integration time corresponds to an acceptable alarm delay and to optimum sensitivity as regards false alarms. The RC section 42,43 has a time constant of about 1 min.

When the output pulses of the detector 39 appear for a period of less than 10 sec., the RC section 42, 43 causes the integration voltage to become zero and prevents a false alarm. The output of the integrator 41-44 is connected across the resistor 45 to the output stage 24-26; the integration voltage triggers this output stage and sets off an alarm.

Owing to the fact that the voltage peaks of the signal are evaluated instead of the average value of the signal voltage, the sensitivity of the apparatus can be made so low that false alarms set off by environmental influences can be avoided. The pulse shaper 36-39 and the integrator 41 -44 may be replaced by a counter, which must be so designed as to transmit a signal when it receives a certain number of pulses from the threshold-value detector 35 within a certain period of time. This signal is supplied to the output stage 24-26 and sets off the alarm.

The monitoring circuit C comprises circuit elements 46-53. Two NTC resistors 47 and 48, together with resistors 46 and 49, form a temperature sensor.

This sensor delivers a temperature-dependent d.c.

voltage which is applied via a diode 51 to an input of a threshold-value detector 52 with reference voltage E4. When the maximum or minimum temperature of the apparatus is reached, there appears at the output of the detector 52 a signal which is supplied over a diode 53 to the output stage 24-26 and sets off an alarm.

Furthermore, the acousto-electrictransducer 1 and the electrical connection between it and the evaluating circuit 52, 24-26 are monitored. The d.c. voltage applied to the output of the transducer 1 is supplied across a resistor 50 to the threshold-value detector 52 and compared with the reference voltage E4. In the event of a defect in the transducer 1, or short-circuiting or interruption of the aforementioned connection, there appears at the output of the detector 52 a signal which is supplied to the output stage 24-26 via a diode 53 and sets off an alarm.

In the output stage 24-26 the output signal of the threshold-value detector 24 triggers the transistor 26, which actuates the alarm relay 25. This relay is connected in a circuit-opening connection so that in case of an alarm, the relay drops out and sets off an alarm, by means of this circuit-opening connection, an alarm is also set off in the event of operatingvoltage failure of the apparatus. A change-over contact operating without potential is provided for transmitting the alarm.

The acousto-electric transducer is intended to receive mechanical vibrations (solid-borne vibration) occurring during attempts to break into armoured rooms and to convert these vibrations into electric signals. This transducer comprises a steel soundreceiving jacket 60 provided with a threaded bolt 61 for securing it to a steel base plate (not shown) forming part of a two-part housing of the apparatus.

Cemented to the base plate is an insulating plate of epoxy glass resting against the structure to be monitored, the plates being attached to this structure by means of two bolts.

Disposed within the jacket 60 are an insulating tube 62 and a piezoelectric element 63, e.g. of ceramics. A brass disc 64 is mounted on the element 63, and a spring 65 is disposed between the disc 64 and a small connection plate 66. The spring 65 and the brass disc 64 act to reinforce the piezoelectric effect. This reinforcement brings about a substantial improvement in the sensitivity of the acoustoelectric transducer. Hence there can be less signal amplification in the evaluating circuit of the apparatus, resulting in a greater S/N ratio.

The following steps have been taken against outside influencing of the functions of the apparatus: in orderto effect good electric and magnetic screening, the two-part housing of the apparatus is made of steel, so that it additionally provides effective mechanical protection ofthewholeappar- atus; and in order to prevent magnetic and electrical pickup in the input of the evaluating circuit of the apparatus, the housing is directly connected to the zero potential of that circuit.

Claims

1. Apparatus for monitoring armoured rooms, particularly vaults, safes, and strongrooms, and for generating an alarm signal when there is an attempt to break in, having an acousto-electric transducer, to the output of which two signal paths are connected, the first signal path responding to briefly occurring signals of high amplitudes and comprising an amplifier stage, a first threshold-value detector, an integrator, and a second threshold-value detector, and the second signal path responding to longer-continuing signals having significantly lower amplitudes than the first signal path, and comprising an amplifier stage as well as a threshold-value detector, both signal paths being connected to a common stage for generating the alarm signal, wherein in the second signal path, analog or digital means are connected after the threshold-value detector, which means serve the purpose of evaluating signal voltage peaks which are supplied to the threshold-value detector and exceed a certain value, and wherein the acoustoelectric transducer comprises a piezoelectric element upon which there is disposed, for the purpose of improving the sensitivity of the transducer, a brass disc which is pressed against the piezoelectric element by means of a spring.

2. Apparatus according to claim 1, wherein the analog means take the form of a pulse shaper and an integrator.

3. Apparatus according to claim 2, wherein the pulse shaper is designed to convert several periodically appearing pulses having a duration of approximately 0.5 ms into non-periodically appearing pulses having a duration of at least 50 ms, which are integrated by the integrator connected thereafter.

4. Apparatus according to claim 3, wherein the pulse shaper comprises a transistor and a thresholdvalue detector, the collector of the transistor being connected to an RC section having a time constant of 60 ms and to an input of the threshold-value detector of the pulse shaper.

5. Apparatus according to claim 1, wherein the digital means take the form of a counter designed to transmit an alarm signal when a certain number of pulses are supplied to the counter from the threshold-value detector within a certain time.

6. Apparatus according to claim 1,furthercom- prising a monitoring circuit connected to the acousto-electric transducer and to the output stage for thermally monitoring the apparatus upon heating or freezing and for monitoring the electrical connection between the acousto-electric transducer and the evaluating circuit of the apparatus, the monitoring circuit comprising a temperature indicator connected to one input of a gate circuit, the other input of the gate circuit being connected to the output of the acousto-electric transducer and the output of the gate circuit being connected to the output stage.

7. Apparatus according to claim 6, wherein the temperature indicator is formed of a first NTC resistor connected in series with an ohmic resistor and of a second NTC resistor connected in parallel with an ohmic resistor.

8. Apparatus according to claim 1, wherein the first signal path comprises a frequency-selective stage formed of two RC sections.

9. Apparatus according to claim 2, wherein the integrator in the second signal path comprises an amplifier and a first RC section, the integration time being about 10 sec., and wherein the integrator comprises a second RC section having a time constant of about 1 min., the whole being so designed that when the output pulses of the pulse shaper appear during a period of less than 10 sec., the integration voltage becomes zero.

10. Apparatus according to claim 1, comprising a two-part housing of steel, wherein one part of the housing takes the form of a base plate, the acoustoelectric transducer being mounted on the base plate.

11. Apparatus according to claim 10, wherein a piezoelectric element of the acousto-electrictrans- ducer is disposed in a sound-receiving jacket of steel which is provided with a threaded bolt, the sound receiving jacket being secured to the steel base plate by means of the threaded bolt, an insulating plate of epoxy glass being cemented to the base plate for electrically insulating the base plate from the structure to be monitored, and the housing being directly connected to the zero potential of the electric evaluating circuit of the apparatus.

12. Apparatus for monitoring armoured rooms substantially as hereinbefore described with reference to the accompanying drawings.