GB2207001A - Inertia sensor - Google Patents

Abstract

An inertia sensor comprises a conductive inertia body (22) movable in a chamber when accelerated or tilted beyond a certain angle, to bridge contacts (12, 13). The contacts remain bridged when the chamber is upside down, because a deflector (part of contact (13)) deflects the body (22) to the bridging position. The chamber floor (21) is depressed to define a rest position for the body (22). A second inertia body (25) bridges the same contacts, but not when the chamber is upside down (due to a domed contact portion (26)). The two bodies jointly ensure bridging of the contacts (12, 13) over a very wide angle of tilt. In an alternative arrangement, a chamber is defined between top and bottom walls comprising different parts of one contact, the second contact being annular and part-way between the walls. The bottom wall defines the rest position and the top wall the deflector. Magnetic biasing can be used. <IMAGE>

Description

INERTIA SENSOR Inertia sensors are used to operate an electrical switch when subjected to an acceleration or tilting. The switch can be used to operate safety devices which are required to be brought into operation when acceleration or tilting over a given threshold is experienced.

In GB-A-1 368 492, an inertia body in a chamber moves in response to acceleration or tilting to depress the plunger of a microswitch.

In W084/03585, an inertia sensor with a conductive inertia body within a chamber in which two contacts are arranged for bridging by the inertia body when the sensor is subject to certain accelerations and tiltings is disclosed, but if the sensor of that publication is turned upside down, the inertia body rests in a position in which it does not bridge the contacts. The sensor of that publication therefore only closes the contacts in certain nonrest positions and does not close the contacts when the sensor is upside down.

In order to provide a sensor which closes the contacts when the sensor is upside down from its normal rest position, the invention provides a sensor as claimed in claim 1.

Examples of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a central diametral section through a first inertia sensor; and Figure 2 is a central diametral section through a second inertia sensor.

The sensors of both embodiments comprise an insulating body 11 with a first electrical contact 12 and a second electrical contact 13.

The electrical contact 12 has a lower portion 23 and an upper portion 24, between which portions the second contact 13 extends.

The body 11 and the contacts 12 and 13 define a chamber which has axial symnetry.

The surfaces of the insulating body facing the interior of the chamber are of thermoplastic material with a sealing lip, so as to seal the chamber from the ingress of fluid, such as that found in an ultrasonic cleaning bath which might be used to clean the exterior electrical contacts of the sensor. In the embodiment of Figure 1, the insulating body 11 is formed with a dish-shaped base portion 21 in which a first conductive spherical inertia body 22 rests in the normal position of the sensor. The lower portion 23 of contact 12 is annular and encircles the dished portion 21 so as to engage the ball when it rolls to the side of the dished portion 21. In the rest position, the sphere 22 does not contact either contact 12 or 13 and even if it does roll to the side of the dished portion 21, it only contacts the lower portion 23 of the contact 12 and does not bridge the two contacts.

The sensor of Figure 1 contains a second inertia body 25, also a conductive sphere, which moves in an upper chamber defined by the second contact 13 and the upper portion 24 of the first contact which has a central dome 26. The upper surface of the second contact 13 is dished so that the second sphere 25 is retained at the centre of the chamber during the rest position of the sensor, and at the centre of the chamber the contacts are well separated.

They approach each other towards the sides of the chamber so that as the sensor is tilted or as it is subjected to horizontal acceleration, the upper sphere 25 moves to the side wall of the upper chamber to bridge the second contact 13 with the upper portion of the first contact, thus completing the electrical connection and operating the safety devices connected to the sensor. When this tilt or acceleration is experienced, the lower sphere simply moves to the side of the lower chamber and does not bridge the contacts.As the tilt increases beyond about 1600 from the rest position (as iluustrated) there will come a time when the ball in the upper chatter, which corresponds to the sensor of W084/03585, will move out of contact with the second contact 13 and will rest in the recess formed by the domed centre portion 26 of the upper portion 24 of the first contact. By this stage the first ball in the lower chamber has already rolled on the inner rim of contact portion 23 into contact with the underside of contact 13.

Any tendency to move to the centre of the chamber when the sensor is upside down (hence breaking contact with contact portion 23) is prevented due to the shape of the second contact which is convex with respect to the lower chamber and thus retains the first sphere in contact with the side of the lower chatter. This contact is maintained until the tilt reaches about 2000, when the second sphere moves to the other side of the upper chamber and takes over the bridging of the two contact portions 13, 23 again and maintains this bridging until the tilt reaches about 3400 when the bridging between the contacts is broken and both spheres move back to their initial central position in which the contacts are not bridged.

The sensor thus provides bridging of the contacts at all tilts except about 200 on either side of the rest position.

The embodiment of Figure 2 provides a similar arrangement, using a single inertia body and a single chatter. In this embodiment it is the second contact 13 which is annular. The lower portion 23 of contact 12 is dished downwards, and the upper portion 24 is also dished downwards. As the tilt increases past a minimum threshold, the bridging of contacts by the sphere is between the second contact and the first portion of the first contact and as the tilt increases to an angle just over 900, the bridging is taken over by the second portion of the first contact when the sphere rolls on the inner rim of the contact 13. As the tilt increases towards 180 degrees, the sphere is biased against movement away from the inner rim of the annular second contact by the convex shape of the second portion of the first contact.As the tilt passes the upside down position and continues to about 2700, the first portion of the first contact takes over the bridging of the contacts as the sphere rolls back on the inner rim of the contact 13 and the sphere returns to its initial position in the dished concave first portion of the first contact at a tilt of about 3400. The embodiment of Figure 2 thus provides bridging between the contacts for all tilts except about 200 on either side of the initial position, with a momentary break in contact as the sphere rolls on the inner rim of the annular second contact, moving between the first and second portions of the first contact.It is usually possible for the electrical circuit connected to the sensor to include a small time delay device which will make it insensitive to the small break in connection between the two contacts at this point.

In the embodiment of Figure 1, it would be possible for the first portion of the first contact to extend continuously across the underside of the first chamber provided that the separation of the two contacts was greater than the diameter of the sphere at the centre of the chamber and less than the diameter of the sphere at the sides in order to ensure that the sphere bridged the two contacts at the sides of the chamber but not at the centre. The base portion 21 may have a central dip to retain the body from rolling about when subjected to small accelerations or tilts. In either embodiment a magnet may be used to retain the body in its rest position, the strength of the magnet determining the response of the sensor. The body must in this case be of magnetic material.

For ease of manufacture, the insulating body is preferably formed as a base with a plurality of pillars on which is mounted insulating rings for separating the portions of the contacts, the tops of the pillars being swaged over to hold the components of the sensor together. Noble metal plating is preferably applied to the surface of the spheres and on the surfaces of the contacts facing the interior of the chambers.

Claims (8)

1. An inertia sensor comprising means defining a chamber1 and a conductive inertia body within the chamber, said means comprising a pair of contacts separated by less than the maximum dimensions of the body at the sides of the chamber1 said means including means defining a rest position for the body in the centre of the base of the chamber where it does not bridge the contacts when the sensor is. upright, said means including means to deflect the body to the side of the chamber when the sensor is inverted.
2. 2. A sensor as claimed in claim 1 wherein one of said contacts extends continuously across the whole width of the chamber.
3. 3. A sensor as claimed in claim 1 or claim 2 wherein the other contact of the pair conprises an annulus whose inner rim defines part of said chamber.
4. 4. A sensor as claimed in any one of the preceding claims wherein the deflecting means comprises an inverted peak extending into the chamber.
5. 5. A sensor as claimed in claim 4 wherein the peak is part of one of said contacts.
6. 6. A sensor as claimed in wherein said contacts define a second chamber within which is a second conductive inertia body, the contacts being bridged by the second body when the sensor is subjected to sideways accelerations above a predetermined minimum.
7. 7. A sensor as claimed in any one of the preceding claims wherein the chamber defining means comprises a plurality of pillars mounted on a base, and means on the top of the pillars for securing together the components for defining the chamber(s).
8. 8. An inertia sensor substantially as herein described with refeerence to and as illustrated in Figure 1 or Figure 2 of the accompanying drawings.