GB2219171A - "Vibration sensor" - Google Patents

Abstract

A vibration sensor (20) is secured at one end to a support (10) and comprises an elongate substrate of a spring material bearing along its length a continuous layer (24) of a polymeric piezoelectric material. The substrate is non planar in shape, for example, helical or semi-helical, or L shaped, which moves about its fixed end in response to mechanical shock being imparted to the device. An electronic circuit detects a voltage output from the sensor and can be used to operate an alarm. <IMAGE>

Description

"Vibration Sensor" c = R This invention relates to sensors for detecting shock and vibration, for example for use in anti-pilfer and antiintruder alarms.

It is known to provide such alarms in which a vibration sensor initiates an alarm indication in response to unauthorised movement of the alarm itself. In one known form, the vibration sensor is in the form of a piezoelectric crystal mounted on a strip of spring steel or the like cantilevered from a base. This has the disadvantage of being sensitive primarily to movement transverse to the longitudinal axis of the cantilever spring, whereas it is desirable for the alarm device to be of approximately equal sensitivity to movement in any direction. This problem can be overcome by providing two, or even three cantilever and spring assemblies disposed at angles to one another, but at the expense of considerably increasing complexity and manufacturing cost, especially as the plurality of piezoelectric crystals must be wired to a common electronic circuit.

It is accordingly an object of the present invention to provide a vibration sensor which gives multi-axis sensitivity while being of simple construction and low cost.

The invention is defined in the appended Claims.

Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: Fig. 1 is a perspective view of an alarm device embodying the inventon; Fig. 2 is a scrap cross-sectional view, to a greatly enlarged scale, illustrating the construction of a laminar sensor element used in Fig. 1; Fig. 3 is a circuit diagram of the circuitry used in the embodiment of Fig. 1; and Figs. 4 to 6 illustrate alternative sensor elements in side view (a) and plan (b).

Referring to Fig. 1, an alarm device comprises a printed circuit board 10 mounting circuit components 12, 14, a battery 16, a siren 18, and a vibration sensor generally designated 20. The vibration sensor 20 may be mounted to the pcb 10 and simultaneously connected to its conductor tracks (not shown) by the means described in our copending application filed concurrently herewith.

The vibration sensor 20 is a laminar assembly comprising, as seen in Fig. 2, a substrate 22 formed by a strip of spring material such as spring steel or (preferably) phosphor bronze, and a film 24 of a polymeric piezoelectric material secured to the strip 22 by double-sided pressure sensitive adhesive tape 26 or any other suitable means. The film 24 may suitably be of polyvinylidene fluoride (PVDF) which has been stretch oriented and electrically polarized to obtain enhanced piezoelectric activity, such materials being well known per se. One suitable material is obtainable as KYNAR (trade mark) film from Pennwalt Corporation. The film 24 is provided on its opposed surfaces with metallised electrodes indicated at 27 and 28 in Fig. 2, as is also well known in the art.

Reverting to Fig. 1, the sensor 20 is cantilevered from the pcb 10 so as to move about its fixed end in response to mechanical shock being imparted to the device. The sensor 20 is bent before securement to the pcb 10 to form one half of a helical turn.

Fig. 3 shows one circuit arrangement for use with the sensor 20. An operational amplifier 30 is used in effect as a monostable circuit. One input of op amp 30 is connected to a d.c. voltage determined by switch SW1 and a resistor chain R1 - R4, and the sensor 20 is connected between this voltage and ground. Switch SWI acts to set the sensitivity of the device. The other input of op amp 30 is connected to a feedback capacitor 32 and to network 34 which act to determine the output period.

When a shock is applied to the alarm device, strip 22 vibrates causing a voltage output from sensor 20 which in turn produces an output from op amp 30 until transistor Q1 conducts, a period determined by the time constant of feedback capacitor 32 and the resistance of R6 - R8 set by switch SW2. The output from op amp 30 may be used to drive any suitable audible alarm via a driving circuit.

The use of a half-helix form for the sensor 20 makes the device equally sensitive to shock accelerations in all directions without the use of multiple vibration sensors.

Other forms of vibrations sensor may be used within the scope of the invention. Figs. 4 to 6 show other geometries for spring/PVDF laminates. Fig. 4 shows a simple L-shape, which would have good sensitivity in the directions shown as X and Z, but be relatively insensitive in the Y direction.

In Fig. 5, the distal arm of the L is skewed in the vertical plane, which improves sensitivity in the Y direction at the expense of reduction in the X direction. In Fig. 6, the distal arem is skewed both horizontally and vertically, which would give sensitivity predominantly in the Z direction. Thus the form shown in Fig. 1 is preferred, but other shapes may be of utility for particular purposes.

Further modifications may be made to the sensor. For example, a complete helical turn, or more than one, may be used. A mass may be secured to the end of the spring strip to give a desired mechanical resonance. The PVDF film may be replaced by a piezoceramic polymer. Instead of a laminar assembly, a spring wire coated with a polymeric piezo material could be used. The spring strip or wire may be used as one electrode for the piezo polymer.

Claims (10)

1. A vibration sensor for securement at one end to a support and comprising an elongate substrate of a spring material bearing along its length a continuous layer of a polymeric piezoelectric material, the substrate being formed into a non-planar shape.

2. The sensor of Claim 1, in which the substrate is a strip having a polymeric piezoelectric material secured to one face thereof.

3. The sensor of Claim 2, in which the strip is bent into an L-shape.

4. The sensor of Claim 3, in which the distal arm of the L is skewed.

5. The sensor of Claim 2, in which the strip is of helical formation.

6. The sensor of Claim 5, in which the strip forms one half of a helix.

7. The sensor of Claim 1, in which the substrate is a wire and the polymeric piezoelectric substance is coated thereon.

8. The sensor of Claim 7, in which the wire is formed into a helix.

9. An alarm device comprising a sensor according to any preceding Claim, said support being a printed circuit board carrying circuit components.

10. A vibration sensor substantially as hereinbefore descended with reference to and as shown in the accompanying drawings.