GB2110446A

GB2110446A - Device for monitoring the state of a body capable of conducting sound waves

Info

Publication number: GB2110446A
Application number: GB08230402A
Authority: GB
Inventors: Roland Marki; Kurt Schluep
Original assignee: Securiton AG
Current assignee: Securiton AG
Priority date: 1981-10-28
Filing date: 1982-10-25
Publication date: 1983-06-15
Also published as: FR2515399A1; IT1192514B; FR2515399B1; CH659335A5; DE3142705A1; GB2110446B; IT8209522A0; DE3142705C2

Abstract

The device, which is particularly suitable for detecting breakage of a pane of glass, comprises a piezoelectric element (10) connected to two signal paths. The first signal path (1, 2, 3, 4, 5) responds to signals having a long duration and low amplitude as compared with the signals to which the second signal path (1, 8) responds. A blocking element (9) serves the purpose of suppressing false alarms. The serviceability of the device can be tested by means of a sound generator disposed on the body. <IMAGE>

Description

SPECIFICATION Device for monitoring the state of a body capable of conducting sound waves This invention relates to monitoring devices, and more particularly to a device for monitoring the state of a body capable of conducting sound waves, especially for detecting breakage of a pane of glass, and for generating an alarm signal, of the type comprising a piezoelectric element which converts into electric signals the acoustic vibrations occurring in the body, and a first signal path containing a first amplifier, a second amplifier, a resonant circuit, a demodulator, an integrator, and an output stage for generating the alarm signal.

Devices of the foregoing type have been disclosed which are intended particularly to detect the breaking of a pane of glass and to produce an alarm when the glass is cut, drilled, ground, or heated, as well as shattered by blows.

These previous devices have the drawback, however, that they do not set off any alarm in the event that the glass is suddenly violently destroyed by means of explosives or firearms.

Furthermore, signal components appear at the output of the piezoelectric element not only when the glass breaks but also when sand, gravel, small bits of metal, or the like strike the pane, or when the glass is battered or abraded for some time without breaking. The result is a false alarm or supposed glass breakage.

Thus, there is a need for an improved monitoring device which remedies these drawbacks, i.e., which responds to short, highamplitude signals as well as to long, lowamplitude signals, as compared with the former, and which suppresses false alarms. Such a device should also be very sensitive to the sounds produced by breaking glass.

According to the present invention, there is provided a monitoring device of the type initially mentioned, wherein a second signal path having a pulse shaper connected to the output stage is connected to the output of the first amplifier, the whole being designed in such a way that the second signal path responds to signals produced by the piezoelectric element having a short duration and a high amplitude as compared with the duration and amplitude of the signals to which the first signal path responds.

A preferred embodiment of the invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a block diagram of the device according to the invention, Figure 2 is a circuit diagram of the device, and Figure 3 is a graph showing the relationship between the voltage magnitudes at the inputs of a switching circuit of the device.

As shown in Figure 1, the signal produced by a piezoelectric element 10 is supplied to an amplifier 1. The output of the amplifier 1 is connected to an amplifier 2 of a first signal path A and to a pulse shaper 8 of a second signal path B.

Following the amplifier 2 is a resonant circuit 3 having a resonance frequency of 1 80 kHz. When the vibration amplitude is high enough, a demodulator 4 connected thereafter causes the output voltage of an integrator 5 connected to the output of the demodulator 4 to rise until this output voltage has reached a value at which a switching circuit 6 connected to the integrator 5 is actuated, and a relay 7 drops out.

When an explosive charge is detonated for the purpose of violently destroying a pane of glass, or when the latter is fired upon, the output signal of the amplifier 1 is supplied to a pulse shaper 8 which is connected to and activates the switching circuit 6, so that the relay 7 drops out. Thus, the device according to the invention responds not only to signals of long duration and low amplitude but also to signals which are of short duration and high amplitude as compared with the former type of signals. The circuitry illustrated in Figure 1 further comprises a blocking element 9 connected to the pulse shaper 8 and the integrator 5. The blocking element 9 causes the suppression of a false alarm, e.g., when the pane of glass is worked at mechanically for some time without breaking.

Figure 2 is an example of a circuit diagram for the device according to the invention. A piezoelectric element 10, e.g., of a piezoelectric ceramic material, is attached to the pane of glass.

A coil 11, together with capacitors 14 and 15, forms a pi-type filter for lessening disturbances caused by the supply voltage. Stabilization of the supply voltage takes place by means of a transistor 17, a Zener diode 16, a resistor 13, and a capacitor 1 8. A diode 12 protects against inverted polarity.

The piezoelectric element 10 is connected to the negative pole of the supply voltage and to the base of a transistor 26 which is protected against over-voltage by means of a diode 31. A resistor 25 produces the base bias of the transistor 26.

The collector of the transistor 26 is connected to the base of a transistor 28. A resistor 29 and a potentiometer 30 are connected in series between the emitter of the transistor 26 and the negative pole. The emitter of the transistor 28 is connected to the negative pole across a resistor 34, and its collector is connected across a resistor 22 to an LC parallel resonant circuit 19,20, 21, component 20 being a damping resistor.

Transistors 26 and 28 form an amplifier stage.

The collector of the transistor 28 is further connected to the base of a switching transistor 35, the emitter of which is connected to the positive pole, and the collector of which is connected across two series-connected resistors 40, 41 to the negative pole. The junction point of the resistors 40, 41 is connected across a resistor 38 to the positive pole and also to the non inverting input of an operational amplifier 46. The inverting input of the operational amplifier 46 is connected via a capacitor 39 and a resistor 36 connected in series therewith to the collector of a transistor 27, the base of which is connected to the emitter of the transistor 26. The inverting input of the operational amplifier 46 is further connected to a voltage divider 43, 44, 45. The emitter of the transistor 27 is connected to the negative pole.A capacitor 42 is connected in series with the capacitor 39 and the resistor 36 and to the positive pole. These components determine the collector voltage of the transistor 27. The output of the operational amplifier 46 is connected via a capacitor 54 to the junction point of the resistors 40,41 and to the non-inverting input of the operational amplifier 46.

Furthermore, this output is connected via a VMOS transistor 55 to a relay 56, which is protected by means of a diode 57.

The output of the operational amplifier 46 is further connected across a resistor 48 to the noninverting input of an operational amplifier 47 and via a reed contact 50 to the negative pole.

Furthermore, the non-inverting input of the operational amplifier 47 is connected to the negative pole via a delay capacitor 49 for delaying the energizing of an LED 53. The inverting input of the operational amplifier 47 is connected to the voltage divider 43, 44, 45, viz., to the junction point of the resistors 43, 44. The output of the operational amplifier 47 is connected to the negative pole across a resistor 52 and the LED 53. A diode 51, connected between the noninverting input of the operational amplifier 47 and its output, serves the purpose of keeping the LED 53 energized.

A VMOS transistor 33 is connected to the junction point of the resistor 29 with the emitter of the transistor 26 and to the negative pole and serves the purpose of by-passing the resistor 29 and the potentiometer 30 which form the emitter resistance of the transistor 26. A capacitor 32 is connected to the negative pole and to the drain electrode of the transistor 33 and serves as a neutralising capacitor.

The circuit described in Figure 2 operates as follows: In the case of cutting, drilling, grinding, heating or striking against the glass, resulting in breakage (first situation), the signals transmitted by the piezoelectric element 10 are amplified by the transistors 26, 28 and supplied to the LC parallel resonant circuit 19, 20, 21. If the vibration amplitude exceeds a certain value, the transistor 35 becomes conductive, and the capacitor 54 is charged across the resistor 40. When the capacitor voltage becomes greater then the reference voltage at the inverting input of the operational amplifier 46, the output voltage of the latter becomes positive, and the relay 56, which is triggered by the VMOS transistor 55, drops out and remains in that state for a certain period of time (e.g., 100 ms).Simultaneously, the LED 53 is triggered via the resistor 48 and the operational amplifier 47 for indicating the state of alarm, and it remains energized until the reed contact 50 is closed by means of a magnet.

If a pane of glass is suddenly destroyed by the detonation of explosives or by the use of firearms (second situation), the signal transmitted by the piezoelectric element 10 is amplified by the transistor 26 and supplied to the base of the transistor 27. The latter becomes conductive when the amplitude of the signal is sufficiently high relative to the voltage to which the potentiometer 30 is adjusted. The negative pulse at the collector of the conductive transistor 27 is supplied via the resistor 36 and the capacitor 39 to the inverting input of the operational amplifier 46. If the voltage at the inverting input of the operational amplifier 46 is less than the reference voltage at the non-inverting input, the output voltage of the operational amplifier 46 becomes positive, and the relay 56, which is triggered by the VMOS transistor 55, drops out.

If the pane of glass is worked on (e.g., battered or abraded) for some time without breaking (third situation), the capacitor 54 is charged as in the first situation, so that the voltage at the noninverting input of the operational amplifier 46 rises and reaches a certain value. At the same time, as in the second situation, negative pulses are present at the collector of the transistor 27 and are supplied to the inverting input of the operational amplifier 46, so that a false alarm might be given. This is prevented by means of the VMOS transistor 33. When the positive voltage of the capacitor 54 reaches the threshold voltage of the transistor 33, the latter becomes conductive and bypasses the resistor 29 and the potentiometer 30, thus disabling the transistor 27.

The graph of Figure 3 shows the voltages as a function of time at the two inputs of the operational amplifier 46. In the rest state, there are 6 V at the inverting input of the operational amplifier 46 (straight line A) and 1.5 V at the noninverting input (straight line B). Curve C shows the course of the voltage as a function of time at the non-inverting input in the first situation. As soon as the voltage at the input is greater than that at the inverting input, an alarm is given. Curve D shows the course of the voltage at the inverting input of the operational amplifier 46 in the second situation. As soon as the voltage at that input is less than that at the non-inverting input, the alarm is set off.In the third situation, the voltage at the non-inverting input of the operational amplifier 46 is represented by the curve B'. Without the VMOS transistor 33 in the circuit, the voltage at the inverting input would be represented by curve E.

However, this transistor causes this voltage to remain equal to the rest potential, which, as shown, is greater than the voltage at the noninverting input, so that no alarm is set off.

In order to safeguard the device more effectively against sabotage by means of magnetic induction, the relay 56 is shielded by an additional metal housing. The piastics housing of the device is likewise provided with an electrically conductive coating. This is an effective measure against electromagnetic fields.

As already intimated above, the concepts described here are not limited to alarm systems for panes of glass but can be applied as well to bodies of other sound-conductive materials instead of glass, say, for monitoring bodies of plasticis, metal, concrete, or other materials.

The device can be tested for serviceability by means of a sound generator (not shown) having a range of about 2.5 meters, which is to be disposed on the bod3! to be monitored. This sound generator comprises an oscillator, an amplifier, and a piezoelectric element, which are accommodated in a housing similar to that of the device itself. Continuously variable adjustment of the starting sound output is achieved by means of a potentiometer. The device is to be checked for serviceability at regular intervals.

Claims

1. A device for monitoring the state of a body capable of conducting sound waves, particularly for detecting breakage of a pane of glass, and for generating an alarm signal, having a piezoelectric element (10) which converts into electric signals the acoustic vibrations occurring in the body, and a first signal path (A) containing a first amplifier (1), a second amplifier (2), a resonant circuit (3), a demodulator (4), an integrator (5), and an output stage (6, 7) for generating the alarm signal, wherein a second signal path (B) having a pulse shaper (8) connected to the said output stage (6, 7) is connected to the output of the first amplifier (1), the whole being designed in such a way that the second signal path (B) responds to signals produced by the piezoelectric element having a short duration and high amplitude as compared with the duration and amplitude of the signals to which the first signal path (A) responds.

2. A device in accordance with claim 1, wherein a blocking element (9) is connected to the pulse shaper (8) and to the integrator (5) in such a way that at a certain output voltage of the integrator, the blocking element blocks the pulse shaper.

3. A device in accordance with claim 1, wherein the blocking element is a transistor (33).

4. A device in accordance with claim 1, wherein the sensitivity of the second signal path (B) is adjustable by means of a potentiometer (30) for each body monitored.

5. A device in accordance with claim 1, wherein a switching circuit (6) forming part of the output stage comprises an operational amplifier (46) and a transistor (55).

6. A device in accordance with claim 5, wherein another operational amplifier (47) is connected to the output of the said operational amplifier (46), which other operational amplifier (47) is connected in such a way that in case of alarm it triggers an LED (53) which emits light until a reed contact is actuated by means of a magnet.

7. A device in accordance with claim 1, wherein the pulse shaper (8) comprises a transistor (27), the collector of which is connected to two resistors (24, 36) and two capacitors (39, 42).

8. A device in accordance with claim 1, wherein an alarm relay (7) of the output stage is shielded against magnetic induction by means of an additional metal housing.

9. A device in accordance with claim 1, wherein a plastics housing in which the device is disposed is provided with an electrically conductive coating against electromagnetic fields.

10. A device in accordance with claim 1, the serviceability of which can be periodically tested by means of a sound generator disposed on the body and comprising an oscillator, an amplifier, and a piezoelectric element.

11. A device for monitoring the state of a body capable of conducting sound waves and for generating an alarm signal, substantially as hereinbefore described with reference to the accompanying drawings.