GB2269478A - Disturbance sensor - Google Patents

Abstract

A stud 1 has a screw-threaded end portion 2 onto which a nut 3 is screwed and which extends into a coil spring 4 to support the spring. An insulative collar 14 is fixed to the nut concentrically with the spring, nut and stud and a cup-shaped metal housing 16 is located on the collar. The stud 1 and a wire 15 soldered to the housing 16 provide electrical connections which are bridged by the spring if it touches the housing when disturbed. <IMAGE>

Description

DISTURBANCE SENSOR The present invention relates to a disturbance sensor.

According to the invention, there is provided a disturbance sensor comprising: a housing at least an inner wall portion of which is electrically conductive; an electrically conductive coil spring electrically isolated from the housing but positioned therein to contact the said wall portion when suitably disturbed; a first electrical connection to the coil spring; and a second electrical connection to the said inner wall portion.

In a preferred embodiment of the sensor, the first electrical connection comprises a support post, an end portion of which extends into an end portion of the spring to support the spring.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made by way of example, to the accompanying drawings, in which Figures 1 to 3 are cross-sectional views through a disturbance sensor in different stages of its assembly, and Figure 4 shows a spring.

In the stage of assembly shown in Figure 1, the sensor comprises a support post in the form of a stud 1 of electrically conductive material one end 2 of which is screw threaded, a metal nut 3, and a coil spring 4 of electrically conductive material.

The nut 3 is cylindrical having a screw threaded bore 5 for engaging with the end 2 of the stud. The bore 5 leads into an enlarged bore 6 for accommodating a portion of the coil spring between the wall of the enlarged bore 6 and the end 2 of the stud.

The bore 6 in turn leads into a cylindrical recess 7 surounded by a circumferential flange 8.

The spring 4 is also cylindrical, one end portion 9 of it being enlarged to fit over the end 2 of the stud in the bore 6 of the nut 3. The other end portion 10 of the spring extends radially outwardly. This shape of spring allows a high inertia with minimum mass and insensitivity to gravitational deflection.

In order to assemble the stud 1, spring h and nut 3, as shown in Figure 1, the nut 3 and the stud 1 are tinned with solder over the screw threads. The stud is then supported in a suItable jig and the nut screwed onto the portion 2 until the stud 2 extends into recess 6 with the top of the stud flush with the base of recess 8. The enlarged end 9 of the spring is then placed in the recess 6, and the nut 3 is heated to fuse the nut, spring and stud together with the solder.

In order to allow free oscillation of the spring, the uppermost coil of the enlarged end 9 lies just above the base of the recess 8, and the two uppermost coils of end 9 are free of solder.

In stage 2 of the assembly as shown in Figure 2, an electrically insulative cylindrical collar 11 of plastics material (eg polycarbonate) is fixed in the recess 7 with adhesive. The collar has an enlarged base 13 which fits snugly in the recess 7 and a flange 14, of smaller radial extent than the base, upstanding from the base. A bore 12 extends through the base and flange to accommodate the spring 4.

In stage 3 of the assembly as shown in Figure 3, a copper wire 15 is welded (by a microwelding process for example) to a cup-shaped housing 16, such as a T018 transistor can, of electrically conductive material. Adhesive is applied to the flange 14 and then the housing 16 is fitted to it. The housing is secured by filling the rim between it and-the nut 3 with a cold setting adhesive 17.

The bore 12 of the collar 11 is wider than the broadest part of the spring 14, its height and breadth being such as to allow the out-turned portion 10 of the spring to touch the housing 16 when the spring 4 oscillates due to a disturbance.

Suitable circuitry (not shown) detects the completion of a circuit through the sensor as the spring touches the housing to produce a signal indicative of a disturbance.

The construction described above by way of example provides a small disturbance sensor using a coil spring 4 which is located on a stud 2 using the enlarged end portion 9 of the spring 4. The housing 16, eg a T018 can, is located concentrically with the spring by being supported on the collar 14 which Is supported concentrically with the nut 3 screw-threaded onto the stud. In this way, it is ensured that the spring 4 and housing 16 are accurately located.

The shape of the spring 4 is chosen in order to achieve the required combination of mass and inertia to ensure sensitive response to a disturbance whilst permitting the sensor to be mounted in any plane without being rendered inoperable by static deflections due to gravity.

Thus the sensor is capable of detecting disturbances in any plane and can be mounted in any plane.

Various modifications to the illustrated example may be made. For instance, the example has a metal nut 3 and a separate insulative collar 11 providing a form of construction convenient for production in low numbers. However, for production in high numbers, the nut 3 and collar 11 may be replaced by an integral plastics moulding which would be fixed to stud 2 by adhesive.

The shape of the spring may also be varied. Instead of the shape shown in Figures 1 to 3, the shape shown in Figure 4 may be used. In Figure 4, the outwardly extended portion 10 is cylindrical, extending also parallel to the axis of the spring. Variation of the shape of the spring allows its resonant frequency to be varied.

Claims (8)

What we claim is:

1. - A disturbance sensor comprising: a housing at least an inner wall portion of which is electrically conductive; an electrically conductive coil spring electrically isolated from the housing but positioned therein to contact the said wall portion when suitably disturbed; a first electrical connection to the coil spring; and a second electrical connection to the said inner wall portion.

2. A sensor according to claim 1, wherein the first electrical connection comprises a support post, an end portion of which extends into an end portion of the spring to support the spring.

3. A sensor according to claim 2, further comprising means, supported on the support post, for supporting the housing in a predetermined position relative to the post and thus to the spring.

4. A-sensor according to claim 3, wherein the supporting means -comprises an electrically insulative collar supported concentrically with the spring, and the housing comprises a cup into the mouth of which the collar extends.

5. A sensor according to claim 4 wherein the said end portion of the post is screw-threaded and the supporting means comprises a nut screw-threaded to said end portion, the collar being fixed to the nut concentrically therewith.

6. A sensor according to claim 4 or 5, wherein the cup is of electrically conductive material.

7. A sensor according to any one of claims 2 to 6, wherein the spring comprises a cylindrical central portion, a radially enlarged cylindrical end portion into which the support past extends, and a further radially enlarged end portion.

8. A disturbance sensor substantially as hereinbefore described with reference to Figures 1 to 3 of the drawings.

8. A sensor according to claim 7, wherein the further end portion is cylindrical.

9. A disturbance sensor substantially as hereinbefore described with reference to Figures 1 to 3 of the drawings.

Amendments to the claims have been filed as follows 1. A disturbance sensor comprising: a housing at least an inner wall portion of which is electrically conductive; an electrically conductive coil spring electrically isolated from the housing but positioned therein to contact the said wall portion when suitably disturbed; a first electrical connection to the coil spring; and a second electrical connection to the said inner wall portion) wherein the first electrical connection comprises a support post, an end portion of which extends into an end portion of the spring to support the spring.

2. A sensor according to claim 1, further comprising means, supported on the support post, for supporting the housing in a predetermined position relative to the post and thus to the spring.

3. A sensor according to claim 2, wherein the supporting means comprises an electrically insulative collar supported concentrically with the spring, and the housing comprises a cup into the mouth of which the collar extends.

4. A sensor according to claim 3 wherein the said end portion of the post is screw-threaded and the supporting means comprises a nut screw-threaded to said end portion, the c-ollar being fixed to the nut concentrically therewith.

5. A sensor according to claim 3 or 4, wherein the cup is of electrically conductive material.

6. A sensor according to any one of claims 1 to 5, wherein the spring comprises a cylindrical central portion, a radially enlarged cylindrical end portion into which the support past extends, and a further radially enlarged end portion.

7. A sensor according to claim 6, wherein the further end portion is cylindrical.