GB2305296A - Inertia device - Google Patents

Abstract

An inertia device comprises a base 15, a position sensor 21,22, having one portion 21 mounted on a member 16 pivotable with respect to the base and a second portion 22 responsive to the position of the first portion relative to the base and an inertia body 11 having a rest position resting on the base and movable relative to the rest position when the device is subject to an acceleration over a given threshold. The inertia body may be mounted on the member, the member having two pivot points A,B relative to the base, pivoting about the first pivot point A on initial movement of the inertia body from its rest position and pivoting about the second pivot point B on further movement of the inertia body from its rest position. Alternatively the member (44, Fig 6) may be caused to pivot by the movement of the inertia body (41), the inertia body being movable relative to the base (42) without direct connection to the pivoted member (44), the pivoted member being movable about its pivot axis by engagement with the inertia body when the inertia body moves from its rest position.

Description

INERTIA SWITCH An inertia device comprises an inertia body, a support for the inertia body which allows it to move from a rest position when the inertia device is subject to an acceleration over a given threshold and means responsive to the movement of the inertia body to change the state of a component, such as a lock if the device is mechanical or an electrical switch if the device is electrical. The acceleration to which the device is responsive can be a component of gravity if it is in a mounting which is tilted.

Seat belts in motor vehicles have in recent years been provided with a mechanical inertia device which allows the belt to be extended until the vehicle is subject to an acceleration over a given threshold, after which the belt is locked and cannot be further extended. When the inertia locking devices are mechanical, it is necessary as a practical matter to provide a separate mechanical inertia locking device for each belt. To operate reliably they have to be mounted on a permanently upright part of the vehicle, and so it is difficult to integrate them with reclining seats which deviate from the vertical.

An electrical inertia device is not subject to these problems and it would be possible to provide a single electrical inertia device within the vehicle, controlling all of the seat belts within the vehicle, thus achieving a considerable saving in cost. An electrical switch is not required to remain vertical, so it can be integrated into a seat which can recline.

It is desired in inertia devices that the inertia body should not move sufficiently to change the state of the component mentioned above until an acceleration above a given threshold is experienced. One method of achieving this arrangement is to make the inertia body of magnetic material and to provide a magnet biasing the inertia body to its rest position. It is then only when the acceleration experienced is sufficient to overcome the magnetic attraction to the rest position that the inertia body will move. However, when the inertia body is a freely movable device, without magnetic restraint to its rest position, it is necessary to provide an alternative arrangement to achieve the response of the device only to accelerations over the given threshold.

According to one aspect of the invention there is provided an inertia device comprising a base, a position sensor having one portion mounted on a member pivotable with respect to the base and a second portion responsive to the position of the first portion relative to the base, an inertia body mounted on the member and having a rest position relative to the base and movable relative to the rest position when the device is subject to an acceleration over a given threshold, the member having two pivot points relative to the base, pivoting about the first pivot point on initial movement of the inertia body from its rest position and pivoting about the second pivot point on further movement of the inertia body from its rest position.

The pivot points are preferably arranged so that the movement of the inertia body in response to a given increase in acceleration is smaller when it is pivoting about the first pivot point than when it is pivoting about the second pivot point. The inertia body has different moments of inertia with respect to the respective pivot points and the non-uniformity of the moment of inertia as the inertia body pivots in response to different accelerations destroys any resonances which might have built up if a uniform moment of inertia had been present.

This is particularly important in a vehicle which is subject to certain vibrations from travel over the ground, thus causing the inertia body to resonate and change the state of the component when it is not desired.

In another aspect of the invention, any resonances are damped out by the weight of the inertia body. In this aspect the invention provides an inertia device comprising a base, a position sensor having one portion mounted on a member pivotable with respect to the base and a second portion responsive to the position of the first portion relative to the base, an inertia body having a rest position resting on the base and movable relative to the rest position when the device is subject to an acceleration over a given threshold, thereby causing the member to pivot, the inertia body being movable relative to the base without direct connection to the pivoted member, the pivoted member being movable about its pivot axis by engagement with the inertia body when the inertia body moves from its rest position.

In all aspects of the invention, the output of the device is taken from the position sensor. The output can be arranged to make or break an electrical path, the change of state occurring at a definite position sensed, or the output can be an electrical signal which varies as a function of the position sensed and a further device is adapted to operate when this electrical signal reaches a desired threshold, for example.

Examples of the invention will now be described with reference to the accompanying drawings in which Figure 1 is a diagram of a first embodiment, Figure 2 shows a detail of Figure 1 in a different position, Figure 3 shows a detail of Figure 1 to a larger scale, Figure 4 shows a second embodiment, and Figure 5 shows a detail of Figure 4 in a different position.

In Figure 1 an inertia body in the form of a mass 11 is integrally mounted with a pivoted T-shaped member 12. The stem 13 of the T passes through an aperture 14 in a base member 15 with plenty of clearance laterally with the cross-piece 16 of the T extending symmetrically across the top of the base so that the mass is suspended from the base by engagement of the cross-piece with the upper surface of the base.

The underside of the cross-piece and the upper surface of the base are so shaped that on initial tilting of the stem of the T from the vertical the underside of the cross-piece and the upper surface of the base engage at a first pivot point A a small distance from the stem. On further pivoting of the stem of the T from the vertical in the same direction, the crosspiece of the T and the base come into engagement at the end of the cross-piece of the T so that the mass then pivots relative to the base about the new pivot point B.

The centre of gravity of the mass is lifted more when the second pivot point is operative, so further movement of the mass will require greater energy.

In the embodiment of Figure 1, the underside of the T is horizontal from the stem outwards to the first pivot point A and then is inclined downwardly at first angles of inclination 17. The upper surface of the base below the cross-piece of the T is horizontal outwards from the aperture to the first pivot point and is then inclined at second angles of inclination 18 greater with respect to the horizontal than the first angles.

Although this is the preferred arrangement, the general rule for the relative shapes of the underside of the cross-piece on the upper surface of the base is that they should diverge outwardly from the first pivot point A at a greater angle than on the inside of the first pivot point A.

A magnet 21 is mounted on the base of the mass for movement with the mass and a magnetic sensor 22 is mounted below the base of the mass fixed relative to the base. A stationary pole piece 23 is mounted below the sensor to provide a strong magnetic field from the magnet when in its rest position through the sensor and to attract the mass to its stable rest position. A magnetic sensor will sense the decrease of magnetic field as the mass moves from its rest position and thus provide an indication of the position of the inertia body.

The position sensor 22 should give an output which shows a marked discontinuity when the cross-piece of the T stops pivoting about the first pivot point A and starts pivoting about the second pivot point B. A preferred form of magnetic sensor is a Hall-effect device. The inertia body has however freedom of horizontal movement in any direction. If the device of Figure 1 is turned upside-down, the stem 13 will either slide through the aperture 14 so that the mass 11 and the magnet 21 will move away from the sensor 22 or it will tilt over at maximum angle; in either case the sensor output will correspond to that when a high sideways acceleration is experienced. This makes it suitable for roll-over detection in vehicles.

In Figure 4, a similar arrangement is shown in which the pivoted member 31 is not T-shaped but is generally the upper portion of a circle integral with and above the inertia body 32 which is the lower portion of a circle of greater diameter.

The two circles are coaxial and there is a coaxial aperture 33 which is generally circular within the combined member/mass body having a flat portion 34 across the top. Within the aperture is a fixed cylindrical shaft 35 having a corresponding flat portion at the top whose radius is appreciably less than the radius of the aperture. The magnet 21, the sensor 22 and the pole piece 23 are arranged below the inertia body in a manner similar to that of Figure 1. In both embodiments the discontinuity of movement due to the pivot point change is effective to destroy resonances.

As sideways acceleration is experienced, the mass 32 will tend to rotate about the shaft 35 in one direction, but will require sufficient acceleration in that direction before it will pivot about the point A at the end of the flat portion of the shaft since the acceleration will be required to cause the centre of gravity of the mass to lift. This will be clear from the dotted line 37 in Figure 5 which represents the position of the bore in the mass in its rest position. Once the mass does start to pivot about the pivot axis along the side edge of the flat portion of the shaft, it will continue to pivot about that point until the flat portion of the aperture engages the curved surface of the shaft after which the relative rotation will be about the common axis B of the shaft and the aperture, as shown by the full line 38 in Figure 5.

The position sensor illustrated as a Hall-effect device using a magnet with a magnetic sensor and a pole piece is not an essential feature of the invention. It would be possible to sense the position of the mass in any suitable way, for example by means of a beam of light passing through a notch in the mass which is cut off as soon as the mass starts to pivot about the second pivot point. Other position sensors include magneto resistive sensors and giant magneto resistive sensors to detect very low levels of magnetic field and change their resistance in response. Another example is a reed switch and this is preferably mounted with its longitudinal axis at 450 to the vertical below the magnet.

In Figures 6 and 7, there is an inertia mass 41 freely movable in relation to a base casing 42 which is shaped with a recess 43 to provide a stable support for the inertia mass in its rest position. In Figure 6, the inertia body 41 is a sphere and the recess 43 on the upper surface of the base is dish-shaped to support the sphere in its rest position. Across the top of the base there is provided a member 44 pivoted at one side of the base and provided on its underside with a part-spherical recess 45 resting on the top of the inertia body. On the free end of the pivoted member 44 is a magnet 46 which cooperates with a magnetic sensor 47 which senses the position of the magnet and hence of the pivoted member which is in turn dependent on the position of the inertia body relative to its rest position on the base.

When the sphere is subject to a sideways acceleration, it will be restrained within the part-spherical recesses of the base and the pivoted member until a threshold of acceleration is reached when it will pivot about the edge 48 of the spherical recess at the base and move towards one side of the base, lifting its centre of gravity and its upper surface lifting the pivotable member 44 as it does so. This will cause the magnet 46 to lift in relation to the sensor 47 and the sensor output will be changed accordingly.

A similar operation occurs in Figure 8, except that the mass is not spherical but is of truncated Christmas-tree shape. The bottom part of the trunk 51 of the Christmas tree fits into a corresponding recess 43 in the base and the upper surface of the mass is a part-spherical recess 52 into which a lug 53 on the pivoted member extends. As sideways acceleration is experienced of increasing magnitude, the inertia body will be retained in its approximately vertical position until a threshold of acceleration is reached when the lug 53 will move up the wall of the part-spherical recess 52 in the top of the inertia body and the inertia body will tilt on one edge of its base until its conical side 54 engages the sidewall 55 of the base.The lug 53 remains within the recess 52 at all times but as it moves to the edge of the recess it lifts the pivoted member 44, moving the magnet 46 away from the sensor 47 and causing the sensor output to change.

In Figure 7, the deflected position of the sphere is unstable, so that it will roll back to its rest position once the acceleration experienced has been removed. Similarly, in Figure 9, the lug will then drop back into the centre of the recess assisting gravity to return the inertia mass to its original rest position with the full width of the trunk resting squarely on the base recess.

The first, third and fourth embodiments have a 360" response to sideways accelerations. The second embodiment is only responsive to accelerations at right-angles to the shaft axis B.

Inertia devices according to the invention are suitable for operating air-bags in vehicles. They can replace mercury switches which have been used in automatic braking systems and for lighting when bonnet or boot lids are opened, but which are now considered unsuitable due to their toxic effect.

The electrical circuits responsive to the electrical inertia devices may include delay devices leading to locking solenoids, so that for example a seat belt retractor solenoid, which normally operates to retract belts when vehicle ignition is switched off, will release the belt for, say, 2 minutes after a vehicle door is opened, so that a passenger can enter the vehicle and put the belt on. A half second delay applies at any time to prevent the solenoids "chattering" as a vehicle passes over rough ground.

Claims (8)

1. An inertia device comprising a base, a position sensor having one portion mounted on a member pivotable with respect to the base and a second portion responsive to the position oz the first portion relative to the base, an inertia body mounted on the member and having a rest position relative to the base and movable relative to the rest position when the device is subject to an acceleration over a given threshold, the member having two pivot points relative to the base, pivoting about the first pivot point on initial movement of the inertia body from its rest position and pivoting about the second pivot point on further movement of the inertia body from its rest position.

2. A device as claimed in claim 1 wherein the pivot points are arranged so that the movement of the inertia body in response to a given increase in acceleration is smaller when it is pivoting about the first pivot point than when it is pivoting about the second pivot point.

3. An inertia device comprising a base, a position sensor having one portion mounted on a member pivotable with respect to the base and a second portion responsive to the position of the first portion relative to the base, an inertia body having a rest position resting on the base and movable relative to the rest position when the device is subject to an acceleration over a given threshold, thereby causing the member to pivot, the inertia body being movable relative to the base without direct connection to the pivoted member, the pivoted member being movable about its pivot axis by engagement with the inertia body when the inertia body moves from its rest position.

4. A device as claimed in any one of claim 1 to 3 wherein the output of the device is taken from the position sensor.

5. A device as clawed in claim 4 wherein the output is be arranged to affect the continuity of an electrical path, the change of state occurring at a definite position sensed.

6. A device as claimed in claim 4 wherein the output is an electrical signal which varies as a function of the position sensed.

7. A device as claimed in claim 6 further comprising a further device adapted to operate when said electrical signal reaches a desired threshold.

8. An inertia device substantially as herein described with reference to the accompanying drawings.