TRANSDUCERS AND ALARM SYSTEMS INCORPORATING THEM

Abstract: A transducer comprises a piezo electric plate member (4) which has on its face an insulating layer (6, 9), and a support (1) including means (3) for securing the support to a structure of which internal pressure waves are to be sensed, such that the plate is urged towards the structure and into close facial proximity therewith.

Description: TRANSDUCERS AND ALARM SYSTEMS INCORPORATING THEM Background to the Invention The present invention relates to transducers and in particular to a transducer which is adapted for the sensing of mechanical shock waves in a structure, such as the body of a vehicle. The invention also relates to an alarm system incorporating at least one, and preferably more than one, such transducer. Many attempts have been made to provide an alarm system, particularly though not exclusively for a motor vehicle, which provides a reliable indication of unauthorised entry or damage to a structure such as a motor car but which is immune or capable of being rendered immune to harmless movement or vibration, such as the rocking of the vehicle or the impact thereon of small objects. The present invention is based upon a transducer which can reliably sense mechanical pressure waves within the structure and an alarm system which has substantially no moving parts and can provide a reliable alarm yet be substantially insensitive to predetermined harmless conditions. Summary of the Invention The present invention is based upon a transducer which in its broadest aspect comprises a piezo electric, preferably ceramic, member normally in the form of a thin plate, which may be, though in general need not be circular; and means for securing the plate to the structure such that the plate is pressed into close facial proximity with the structure. There may be a thin pressure pad which is disposed on the face of the. plate and is composed of rubber or other suitable resilient material. In a particular embodiment, the said support may comprise a bar member having at least one and preferably two mounting pads for supporting the bar relative to a structure that the transducer, comprising the plate and the pressure pad, are urged against the said structure. Alternatively the plate may be secured to the structure by means of an adhesive pad. A transducer of the kind described is found to provide a very reliable indication of shock within the structure, yet being substantially immune to general translation of the structure. For example, when mounted in a motor vehicle against some suitable internal surface of the body, the transducer provides a reliable indication of knocking or penetration of the body or, for example, the closing of a car door yet is substantially immune to the rocking of the vehicle or the effect of rain or other precipitation on it. A detecting or alarm circuit may comprise at least one such transducer and preferably more than one such transducer in parallel, and a circuit by means of which a voltage induced in the piezo electric transducer is compared with at least one threshold. Means may provided in order to vary the sensitivity of detection. A comparator, or plurality of comparators which may be connected to an output by a multi-way switch, may trigger a resettable timer which operates an alarm device. Brief Description of the drawings Figure 1 is a schematic illustration of a transducer according to the invention; Figure 2 is a schematic illustration of the general layout of an alarm system as applied to a motor vehicle; Figure 3 is a schematic illustration of one form of alarm circui . Figure 4 is a schematic illustration of one arrangement for t_ ___c__ming the sensitivity of individual transducers; and Figures 5A and 5B are schematic illustrations of a transducer according to a second embodiment of the invention. Description of a preferred embodiment In the transducer according to the first embodiment as shown in Figure 1, a support 1 is constituted by an aluminium or other suitable metal bar 2 which at each end in this embodiment is provided with a self-adhesive mounting pad 3. At the centre of the bar is mounted, parallel thereto, a piezo electric sensing element in the form of a thin plate 4 of a suitable ceramic material. The plate 4 may be secured to the bar by an adhesive pad 5 or by a suitable clip. The mounting pads 3 are used to fasten the transducer to the structure of which internal pressure waves are to be sensed. Alternatively the support may be screwed directly on to the structure, or fastened in any other suitable way. The ceramic plate has on its face a thin pad 6 which may be of rubber or other suitable resilient and electrically insulating material. Electrical leads 7 are electrically connected to suitable locations on the rear of the plate. The arrangement of the support is such that the plate is urged facially into close proximity with the structure, such as a car body, in which shock waves are to be detected. The self-adhesive mounting pads 3 are preferably arranged so that the resilient pad is compressed between the structure and the face of the plate 4. The self-adhesive pads may be adapted for attaching the sensor to the chassis of a vehicle, for example. An alternative embodiment of the sensor is shown in Figures 5A and 5B. It comprises a transducer in which a thin piezo electric sheet or plate 8 is mounted on an adhesive pad 10 having a much larger surface area than the piezo electric sheet. In addition, the face of the plate 8 is covered with an insulating membrane 9. The adhesive pad provides a region or regions of adhesion at least partially surrounding the piezo electric plate 8, so that the plate is pressed against the vehicle body. Thus in the specific example shown, the plate is disposed centrally of the pad on the adhesive side thereof. The pad 10 has a marginal region substantially surrounding the plate and its membrane 9. Two narrow, spaced apart extensions 8a and 8b extend from the plate to the edge of the pad 10 and are connected to the leads 7. Figure 2 illustrates the general layout, by way of example, of an alarm system incorporating three transducers of the kind shown in Figure 1 or in Figures 5A and 5B. The numeral 20 illustrates a vehicle having an engine compartment 21. Two transducers 22 and 23 of the kind shown in Figure 1 or in Figures 5A and 5B are secured to a part of the bodywork defining the engine compartment, but at spaced apart locations. A third transducer 24 is secured to the car body at some other suitable place, for example inside the luggage boot of the vehicle. It is preferable for the transducers to be at widely separated locations within the vehicle, preferably to minimise the maximum distance between any point on the vehicle and one or other of the transducers. Each of the transducers is connected to a control circuit 27 which in turn can be enabled or disabled in any suitable manner by remote operation, for example by a hand-held infra-red signalling device. An incident memory indicator 47 is .mounted in the driving compartment together with an arm/disarm indicator in such a position as to be seen clearly from outside the vehicle. Figure 3 illustrates a typical circuit for use with transducers such as described above. In particular, the circuit of Figure 3 comprises a transducer 22 in parallel with further transducers 23, 24, each of the transducers 22, 23 and 24 being in parallel with an end of line resistor 30 which is in series with a thermistor 25 having a positive temperature coefficient of resistance. The transducers are significantly temperature dependent and the thermistor provides transducer temperature compensation such that the sensitivity of the transducers to internal pressure waves in the structure does not vary significantly with temperature. Each transducer is connected by way of a common line 31 to a reference such as the vehicle body and by way of another common line 32 to a respective input of each of four voltage comparators 33-36. A potentiometer 37 is in series with a resistive voltage divider to various points of which are connected the other terminals of the comparators. Each of the comparators is connected through a respective hold up resistor 39 to the tap of the potentiometer 37, and to a terminal of a three-way switch 40 of which the rotary arm 41 is connected to one input of a monostable timer circuit 42 which drives through an inhibit CCT 43, connected to the arm/disarm system, via an appropriate amplifying arrangement to an alarm klaxon 44, a hazard flasher relay 45, and an incident memory bistable 46 controlling the pulsating incident indicator 47. The thresholds set for the comparators by divider 38 and any required adjustment of potentiometer 37 are preferably such that there are four distinct levels of sensitivity, three of these levels of sensitivity are available for selection by the switch 40. The fourth level, which is of suitably low sensitivity, outputs via the major incident bistable 48 and forms the automatic hazard system. One method of trimming the sensitivity of individual transducers is illustrated in Figure 4 which shows two transducers 23, 24 in parallel with thermistor 25 and end of line resistor 30, similar to the circuit shown in Figure 3. In addition, in series with each transducer is a trimmer 50, 51 which consists of a variable resistor. This is particularly useful in high security applications such as car bomb (magnetic mine) detection, in which floor pan sensors can be installed and can be made more sensitive than the general body sensors, in order to increase the ability of the system to discriminate between general knocks to the body of the vehicle and actual bomb placement.

Claims: CLAIMS 1. A transducer comprising a piezo electric plate, a support including means for securing the support to a structure of which internal pressure waves are to be sensed, such that the plate is urged facially in close proximity with the structure, and electrical connections to the piezo electric plate for the provision of an output signal. 2. A transducer according to claim 1 wherein said securing means comprises a self adhesive pad. 3. A transducer according to claim 2 wherein said piezo electric member is attached to said self adhesive pad, and said pad has an extent greater than the piezo electric member. 4. A transducer according to claim 1 or claim 2 in which the support comprises a bar having at least two securing means for mounting the bar adjacent the said structure. 5. A transducer according to any foregoing claim and including a thin membrane or pad on the face of the plate. 6. An alarm system for sensing internal pressure waves of a structure comprising at least one piezoelectric plate, at least one comparator for comparing a signal developed by the plate with a threshold, circuit means for signalling when the said signal exceeds the threshold, and means for urging the piezoelectric plate in close proximity with the structure. 7. A system according to claim 6 and including a thin pad or membrane disposed between the face of the plate and the structure. 8. A system according to claim 6 or claim 7 in which there is at least one further comparator arranged to provide a different theshold against which a signal produced by the transducer is compared. 9. A system according to claim 8 in which means are provided for selecting the output of one of the comparators. 10. An alarm system for a vehicle, including at least two transducers mounted at substantially different locations in the vehicle and positioned to sense stress in the body thereof, at least one comparator by means of which a signal developed by each transducer may be compared with a threshold, and circuit means for signalling when said signal exceeds the threshold; wherein each transducer comprises a piezo electric plate which has on its face a thin insulating layer, and a support adapted to urge the plate into close facial proximity with the said body.

Inventors: ELLIS ANTHONY MARK, ELLIS, ANTHONY, MARK
Applicants: HCS DESIGN INNOVATIONS LTD, HCS DESIGN INNOVATIONS LIMITED
Application year: 1990
Main classification: G08B13
IPC codes: B60R25, G01V1, G08B13