GB2183876A - Vibration sensing unit - Google Patents

Abstract

A vibration sensing unit 10 for use, for example in a loop security system, includes a vibration sensor 11, an analyser circuit 16 for controlling a loop switch 20 and a visual indicator to indicate a tripped condition. Circuitry including a reset switch and a timer 23 is connected to the analyser and is effective to ensure that after tripping, a visual indication of tripping is retained on the unit until it is reset, but the loop switch is re-closed after a short period to maintain the effectiveness of a loop system as a whole even after tripping of one or more units of the invention in such a system. The vibration sensor (Fig. 2, not shown) includes a hollow casing mounting a pair of electrically separate contacts (11, 12) and a body of mercury (15) of a volume smaller than the interior volume of the casing. <IMAGE>

Description

SPECIFICATION Vibration sensing unit This invention relates to a vibration sensing unit which can be attached to a structure, for example a window or door to sense vibration thereof consistent with an attempt to tamper with the structure or gain access to premises containing the structure. The unit is adapted to be connected to a central unit to give a warning of excessive vibration of the structure to which it is attached. The unit can be part of a security system or the like.

A known vibration sensing unit contains a sensor and a connected analyser circuit which can discriminate between acceptable vibration due to normal factors such as movement due to wind and nearby traffic, and abnormal vibration caused by interference or an attempt to gain entry. A conducting loop can unite a plurality of such sensors in series and connect them to a central control unit. In such a known unit the sensor has heretofore been in the form of a ball switch or a piezo electric bimorph or multimorph. In the ball switch a set, e.g. three, conducting spheres rest on a concave surface and each contacts the others and a respective fixed contact beneath it. The attached circuitry senses the presence of a short circuit between each pair of spheres.

When this switch is knocked the spheres move and separate causing one or more open circuits which can be sensed. In the second type a piezo electric bimorph is fixed at one end and vibrateswhen sensing a mechanical disturbance on the surface to which the complete unit is mounted. The vibrating bimorph generates an electrical signal which can be sensed.

Both these devices are relatively expensive, the first because the spheres and contacts must be coated with gold or similar non-oxidising metal to function over a period and the second because the piezo device is intrinsically expensive.

It is an object of the present invention to provide a vibration sensing unit which is cheaper than those previously known devices.

Accordingly, the invention provides a vibration sensing unit including a vibration sensor in the form of a hollow casing mounting a pair of electrically separate contacts and a body of mercury, within the casing of volume smaller than the interior volume of the casing, vibration of the casing causing the body of mercury to move to change the electrical condition between the two contacts.

The casing can have a concave generally nonconducting base with a central contact and a surrounding cylindrical conducting wall, the mercury normally forming a generally partspherical body spaced from the side walling, vibrating of the body causing the mercury to bridge between the central contact and the wall to form a short circuit therebetween, detectable by a connected analyser circuit.

The mercury containing switch has the considerable advantage over the piezo-electric device that its natural or resonant vibrational frequencies are very much lower. Accordingly its susceptibility to false alarms due to spurrious vibrations is very much lower.

As mentioned, vibration sensing units are usually connected in series in a conducting loop extending from and back to a control unit at a central position in a security office or the like. Such a conducting loop is normally in a "closed circuit" condition, i.e. all the individual sensing units have respective loop switches which are normally closed and the control unit passes a current through the loop and all the switches in it either constantly or intermittently to test the integrity of the loop. When a sensing unit is activated its analyser circuit produces an output signal which opens its loop switch and places the whole loop on "open-circuit". This condition is sensed by the control unit and the control unit, or monitoring personnel, take appropriate security action.This will involve checking all the units in the loop to ascertain which has tripped and why. It is also necessary to reset the tripped unit to render the system operative as soon as possible. Each unit normally has an indicator, such as a light, which is illuminated upon tripping. To reset a tripped unit it is usually necessary to disconnect its power supply and re-connect it. Such resetting usually is done by disconnecting all the units in a loop by means of a button or similar switch at the control unit. It will be appreciated, however, that resetting cannot occur until after the tripped unit has been located. Even if only resetting is necessary some considerable time may elapse between the tripping of a unit and the loop becoming operative once again.During this time all the other units are, of course, useless, because the control unit cannot sense the existence of more than one break in a normally-closed loop. If a second unit is triggered during this period the act of resetting cancels the second interference unless the second unit is damaged or destroyed.

Such a system can thus have a weakness when used as a long perimeter defence system. A knowledgeable malfactor may attack at one iocation to trip a unit and effectively disable the loop, and then attack subsequently at a second location whilst the loop is disabled.

A further object of the present invention therefore is to provide a vibration sensing unit which does not have this disadvantage.

Accordingly the invention further provides a vi.bration sensing unit including a sensor, an analyser circuit connected to the sensor and controlling a loop switch, and a visual indicator to indicate a tripped condition, circuitry including a reset switch and a timer being connected to the analyser circuit and effective to ensure that upon tripping the indicator is activated and the loop switch is opened, the loop switch being subsequently closed automatically after a pre-determined time interval, but the visual indicator remaining activated until the reset switch is operated.

Thus, after a unit has tripped, the loop becomes operative a short time after tripping, but the tripped unit maintains a visual indication that tripping has occurred directing personnel to check why tripping has occurred.

Thus, any further breach of the loop shortly after an initial breach can be detected and checked.

The unit can additionally have test circuitry including a test switch and sensitivity adjustment. Thus upon installation or modification of a particular unit in a loop (for example to desensitize a unit to reduce false tripping) the test switch can be operated to maintain the loop switch closed, whilst the remainder of the circuitry stays operative. Thus the unit or its associated structure can be caused to vibrate and the indicator will give indication of "tripping", without the loop switch opening and without the control unit being alerted.

Once a desired sensitivity has been set, the test switch is reversed and the unit becomes an operative part of the loop again.

The invention will be described further, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a block diagram showing a preferred vibration sensing unit of the invention; Figure 2 is a cross-sectional view of a sensor of the unit of Figure 1; and Figure 3 is a block diagram iliustrating a system in which the unit of the invention can be used.

A preferred vibration sensing unit 10 of the invention is shown in Figure 1 and includes a sensor 11 in the form of a mercury switch.

Sensor 11 (Figure 2) includes a hermetically sealed casing, a concave base 13 of insulating material mounting two contacts 11 and 12 and a body of mercury 15 of volume less than the interior volume of the casing. The sensor 11 is shown in its inactive position where in the mercury body 15 forms an electrical connection between both contacts 11 and 12. When the sensor 11 is knocked or vibrated the body 15 moves and breaks the normally closed circuit between 11 and 12 which can be sensed by the analyser. For increased versatility the analyser can also support sensor devices whose contacts are in the normally open condition and vibration of the unit will cause the mercury to move to create a normally closed circuit. Analyser 16 functions on the principle of the input signal reaching a trigger threshold thereby causing the timer to initiate the operation of the relay.An adjustable input filter comprises the sensitivity control, integrating the input pulses from the detection sensor. When there are such as to indicate an attack rather than usual wind movement or random occasional disturbances, which two conditions can be distinguished by empirical tests and adjustments, the analyser 16 products an output signal on line 17 which passes to an indicator 18, such as an L.E.D.

or other lamp, and to relay 19 which opens a loop switch 20 in a alarm loop 21 which causes an alarm indication at a central control unit 22. A timer 23 keeps the loop switch 20 open for a period of, say, two seconds, whereafter it closes the loop switch 20, but keeps the indicator 18 in an "on" condition to indicate where the alarm has occurred, but reinstating the "armed" condition of the entire loop.

The unit 10 includes a test switch 24 which can be used upon installation or during servicing to isolate the loop switch 20 whilst leaving the remainder of the unit 10 operative.

The unit 10 can then be tested and its sensitivity adjusted, tripping being indicated by indicator 18, without the alarm loop 21 being broken during testing. Thus, the majority of the system remains operative during installation and testing of an individual unit.

Figure 3 shows a system which includes a plurality of units 10 and control unit 22 connected by four wire loops. Two of the wires form power loops; the third the alarm loop 21 and the fourth a tamper loop which interconnects tamper switches or the like one on each unit and operative to indicate removal of or damage to the unit casing.

The unit of the invention is superior to prior known units in that it is cheaper to produce and gives an improved performance to the system. However, its connections are wholly compatible with existing wiring systems and existing control units.

An additional feature of this invention involves a 'multi-shot' sampling of an alarm signal to overcome false alarm situations caused by unusual vibrational conditions initiated for example by heavy traffic, strong wind or the spontaneous fracturing of window sealants due to severe changes in environmental factors. In this situation the relay coil and latching indicator are only energised when the analyser has detected a pre-determined number of alarm signals, for example, two, within a specified period of time which may be the order of 2 seconds.

Claims (11)

1. A vibration sensing unit including a sensor, an analyser circuit connected to the sensor and controlling a loop switch, and a visual indicator to indicate a tripped condition, circuitry, including a reset switch and a timer, being connected to the analyser circuit and effective to ensure that upon tripping the indicator is activated and the loop switch is opened, the loop switch being subsequently closed automatically after a pre-determined time interval, but the visual indicator remaining activated until the reset switch is operated.

2. A unit as claimed in claim 1 wherein said circuitry includes a test switch and sensitivity adjustment, so that upon installation or modification of a particular unit in a loop the test switch can be operated to maintain the loop switch closed, whilst the remainder of the circuitry stays operative for testing.

3. A vibration sensing unit as claimed in claim 1 or 2 wherein the analyser includes an input filter which is adjustable to provide for said sensitivity adjustment.

4. A vibration sensing unit as claimed in claims 1, 2 or 3 and including a tamper switch.

5. A vibration sensing unit substantially as hereinbefore described with reference to the accompanying drawings.

6. A security system including a plurality of vibration sensing units as claimed in any preceding claim interconnected by a plurality of loops including an alarm loop.

6. A security system as claimed in claim 5 and further including a tamper loop.

7. A security system substantially as hereinbefore described with reference to the accompanying drawings.

8. A vibration sensing unit including a hollow casing mounting a pair of electrically separate contacts and a body of mercury, within the casing, of volume smaller than the interior volume of the casing, vibration of the casing causing the body of mercury to move to change the electrical condition between the two contacts.

9. A unit as claimed in claim 8 wherein the casing has a concave generally non-conducting base having a central contact and a surrounding cylindrical conducting wall, the body of mercury, in a rest condition, forming a generally part-spherical body spaced from the side walling, vibration of the body causing the mercury to bridge between the central contact and the wall to form a short circuit therebetween, detectable by a connected analyser circuit.

10. A vibration sensing unit as claim in claim 8 wherein the casing has a generally non-conducting upwardly concave base from which protrude a pair of contacts normally short-circuited by the body of mercury.

11. A vibration sensing unit substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.