GB2207555A

GB2207555A - Inertia sensor switch

Info

Publication number: GB2207555A
Application number: GB08814798A
Authority: GB
Inventors: Keith Ronald Polwin
Original assignee: Inertia Switch Ltd; First Inertia Switch Ltd
Current assignee: Sensata Technologies Ltd
Priority date: 1987-07-31
Filing date: 1988-06-22
Publication date: 1989-02-01
Anticipated expiration: 2008-06-22
Also published as: GB8718206D0; GB2207555B; GB8814798D0

Abstract

A inertia sensor includes an inertia body (11) having a surface of revolution and a conductive shaft (12) arranged along the axis of said surface which moves along a path in response to accelerations which exceed a threshold. Two contacts 15 are bridged by the shaft (12) to form an electrical switch which is closed as the inertia body (11) moves along said path in response to the accelerations. The contacts (14) may be flexible and may be resilient and there may be a wiping action of the shaft (12) with the contacts. The inertia body (11) may be returned to its rest position by the resilience of the contacts (14) but on the other hand may be kept in the position to which it is moved by the accelerations by a magnet (21, Figure 3, 4, not shown) acting on the inertia body (11) which in that case must be of magnetic material. <IMAGE>

Description

INERTIA SENSOR In an inertia sensor, an inertia body moves in response to an acceleration and causes an electrical switch to operate when the acceleration passes a threshold. The present invention is concerned with keeping the electrical switch operated for a predetermined minimum period irrespective of the subsequent acceleration history. In a known type of switch in which an inertia body travels down a track and engages the movable contact of an electrical switch at the end of the track, forcing it into engagement with a fixed contact, the inertia body is likely to rebound from the movable contact if the acceleration drops below the threshold again, so that the closing of the electrical switch in this arrangement is only trarntarv.

When the inertia body or part of it is conductive, the conductive part can be arranged to bridge contacts of an electrical switch when the body moves in response to applied acceleration. The path of the inertia body can be arranged so that the body moves to the position in which its conductive part bridges the contacts when the acceleration threshold is exceeded and the body remains in a position or range of positions in which its conductive part bridges the contacts for the predetermined minimum period required for operation of the switch.For example, the inertia body could be a conductive sphere which runs along a track having a first nonconductive portion and a second conductive portion and a vertical profile such that the inertia body remains on the first portion while the applied acceleration remains below the threshold but passes to the second portion when the threshold is exceeded and remains in that second portion for the predetermined minimum period irrespective of the subsequent acceleration history.

The construction of a conductive sphere is expensive, since the whole of the spherical surface has to be made conductive and when conductivity has to be held within precise limits, it is usually necessary to coat the sphere with a noble metal such as gold. It would be preferable to make only a part of the inertia body conductive, since the expensive noble metal can then be restricted to only part of the surface of the inertia body, thus saving in cost.The present invention therefore provides an inertia sensor in which an electrical switch is operated in response to an applied acceleration which exceeds a threshold, the electrical switch comprising a pair of spaced electrical contacts and an inertia body which moves in response to the applied acceleration along a path, the inertia body comprising a surface of revolution and a conductive shaft arranged along the axis of that surface, the shaft contacting said two contacts through a region of said path into which the inertia body moves in response to said applied acceleration over said threshold. Coating the ends of the shaft with a noble metal is much cheaper then coating the whole surface of the cylinder or a sphere; furthermore the coated and rolling surfaces of the inertia body are different, so the coating is not so likely to be damaged by motion of the body.

The contacts may be flexible being moved by engagement with the shaft of the inertia body as the inertia body moves along said region of the path. There may be a wiping action of the shaft on the contacts and the contacts may be resilient, tending to return the inertia body towards the position at which contact was first made between the shaft and the contacts. The wiping action keeps the engaging surfaces clean. The inertia body may be of magnetic material, and a magnet may be provided adjacent the path tending to retain the inertia body in said region of said path once it enters it.

Examples of the invention will now be described with reference to the accompanyingdrawings, in which: Figure 1 is a side elevation, partly broken away, of an inertia switch, Figure 2 is a section on line A-A of the switch of Figure 1, also partly broken away, and Figures 3 and 4 are respectively a side elevation and plan of an alternative switch.

In both illustrated embodiments, the inertia body comprises a cylinder 11 having an axial shaft 12 with connected conductive surfaces at the protruding ends. In its rest position, the cylinder 11 is supported on the walls of a recess 13 in a chamber in the switch body which extend parallel to the shaft 12, the width of the recess 13 being less than the diameter of the cylinder 11.

A pair of contacts 14 are provided, one contact on each side of the cylinder, each having a flexible upper portion 15 which in the rest position of the cylinder is spaced from the shaft 12.

The switch as seen in Figures 1 and 2 is responsive to acceleration towards the left. When the applied acceleration has a moment about the point of contact 16 of the cylinder 11 with the right hand wall of the recess greater than the moment of gravity, both accelerations acting on the centre of gravity of the inertia body, the inertia body will rotate about the line of contact 16 with the right hand wall of the recess so that the ends of the shaft 12 will rise and strike the upper portions 15 of the contact springs 14.

If the applied acceleration is sufficient, the inertia body moves further to the right, rolling up the inclined floor 18 of the chamber deflecting the contact springs 14 so that the upper portions 15 rotate about the coiled mid-portions 17 and the ends of the shaft 12 wipe over the upper portions 15. The resilience of the springs 14 and the inclination of the floor 18 tend to return the inertia body towards its rest position.

In an alternative embodiment shown in Figures 3 and 4, the device is shown reversed so that it responds to accelerations to the right. The cylinder 11 is of magnetic material. Beyond the left hand wall of-the switch body above the recess is a magnet 21, which attracts the cylinder 11 with a force sufficient to delay the return of the inertia body from its raised position above the recess 13 due to the return forces of gravity and any flexure of the contact springs 14. The forces acting on the inertia body due to gravity, the magnet and the springs can be so arranged that once the switch has been closed by the shaft 12 engaging the contact springs 14, it d does s not open again for at least 50 msecs irrespective of the subsequent history of the applied acceleration.

In this embodiment there is no inclined floor 18 and the chamber to the left of the contact line 16 is foreshortened in relation to the chamber of Figures 1 and 2.

Claims

CLAIMS:

1. An inertia sensor in which an electrical switch is operated in response to an applied acceleration which exceeds a threshold, the electrical switch comprising a pair of spaced electrical contacts and an inertia body which moves in response to the applied acceleration along a path, the inertia body comprising a surface of revolution and a conductive shaft arranged along the axis of the surface, the shaft contacting said two contacts through a region of said path into which the inertia body moves in response to said applied acceleration over said threshold.

2. A sensor as claimed in Claim 1 wherein said contacts are flexible and are located for engagement with the shaft of the inertia body as the inertia body moves along said region of the path.

3. A sensor as claimed in Claim 2, wherein said contacts are resilient and are arranged to wipe said shaft as the body moves along said region of the path, the resilience of the contacts tending to return the inertia body towards the position at which contact was first made between the shaft and the contacts.

4. A sensor as claimed in any one of Claims 1 to 3, wherein the inertia body is of magnetic material, the sensor comprising a magnet adjacent said path tending to retain the inertia body in said region of said path once it enters it.

5. An inertia sensor substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.