AN INERTIA SWITCH IMPACT SENSOR

Abstract: A ferromagnetic ball (1) is mounted between dished first and second contacts (2 and 5) and subjected to magnetic restraint tending to retain the ball (1) in its central, rest position by means of a rare earth type magnet (7) which is accurately positioned relative to the ball (1) by means of a polyester shim (6) clamped between the first contact (2) and the magnet (7) as a result of resilient deformation of an annular portion (9) of a cap (8) attached to a housing member (11) enclosing the first and second contacts (2 and 5). Lugs (18) extending from the first and second contacts (2 and 5) pass through openings in the housing member (11) and through apertures in a printed circuit board (16) to which the sensor is attached by dowels (19). The lugs (18) are then attached to the printed circuit board (16) by soldering.

Description: An Inertia Switch Impact Sensor Field of the Invention The invention relates to an inertia switch impact sensor, for vehicle crash detection, for the interruption or initiation of electrical currents such as those controlling fuel flow and central door locking units. Background Art Vehicle manufacturers are increasingly employing electronic control units which operate in response to signals from inertia switch impact sensors. Electronic controls are already common both in door locking and fuel management systems. This move to electronic processing of sensor signals and the use of driver stages or relays to perform power switching has prompted the development of smaller and smaller low power switches and transducers which are frequently required to be small enough to be directly mountable within electronic control modules. Known inertia switch impact sensors for this purpose comprise a ferromagnetic ball; a dished first contact having a circular portion of smaller diameter than the ball, for supporting the ball in a rest position, and an upwardly inclined outer portion extending from the circular portion; a second contact extending around a circle for engagement by the ball, on movement of the ball away from its rest position as a result of impact by the vehicle, to complete an electrical path between the two contacts; and a magnet disposed below the ball and spaced from the first contact. One way to adjust the strength of the magnet so as to ensure that the sensor operates correctly is to fit the magnet in a fully magnetised condition and then to demagnetise the magnet until the restraint it exerts on the ball is reduced to the required level. Where magnets are not demagnetised, to avoid this additional process step, in mass production in different runs where magnets of the same strength are required to hold the balls with different retaining forces, trouble is encountered unless each magnet is positioned precisely in relation to the first contact and, as a result of the manner in which magnetic field strength varies with distance, it has been found necessary to space the magnet a relatively large distance from the first contact since, if the magnet is too close to the first contact, small deviations from the correct position of the magnet cause large variations in the . force exerted by the magnet on the ball. This is important because the position of the magnet depends on dimensional imperfections in at least two parts: the member supporting the magnet and the magnet itself. Thus, in practice, it is normal to leave a relatively large air gap between the magnet and the first contact so that any imprecision in the disposition of the magnet will result in only minor variation in the force exerted by the magnet on the ball. Disclosure of the Invention It is the object of the present invention to overcome the deficiencies of known inertia switch impact sensors so as to provide smaller sensors for use with electronic control apparatus and to provide a form of construction in which the retaining force exerted by the magnet in different runs of mass produced sensors can be simply and accurately changed. This can be achieved by providing the sensor with a non-magnetic shim which is disposed between the first contact and the magnet; and a support for the magnet to cause the magnet to press the shim against the first contact to thereby precisely position the magnet relative to the first contact. Thus, according to the invention there is provided an inertia switch impact sensor, for vehicle crash detection, comprising a ferromagnetic ball; a dished first contact having a circular portion of smaller diameter than the ball, for supporting the ball in a rest position, and an upwardly inclined outer portion extending from the circular portion; a second contact extending around a circle for engagement by the ball, on movement of the ball away from its rest position as a result of impact by the vehicle, to complete an electrical path between the two contacts; a magnet disposed below the ball and spaced from the first contact; a non-magnetic shim disposed between the first contact and the magnet; and a support for the magnet to cause the magnet to press the shim against the first contact. The support preferably has a resiliently deformable portion which, when the sensor is assembled, is deformed so as to effect the necessary clamping. Where it is desired to form a sensor in which the force with which the magnet holds the ball is to be changed, using the same magnets in a different mass production run, the shim disposed between the magnet and the first contact of each sensor is of different thickness and, where the support has a resiliently deformable portion, this different thickness is accommodated within the resilient deformation of the resiliently deformable portion. In a preferred embodiment of the invention, the magnet is a rare earth type magnet. These magnets are smaller than other permanent magnets and so this permits even further reduction in the size of the sensor. Moreover, it is understood that demagnetisation of rare earth type magnets is more difficult than with other types of permanent magnet and so the use of spacer shims is particularly useful when rare earth type magnets are used. Three embodiments of the invention are hereinafter described, by way of example, with reference to the accompanying drawings. Brief Description of the Drawings Figure 1 is a sectional side elevation of an inertia switch impact sensor, according to the present invention, attached to a horizontal printed circuit board; and Figures 2 and 3 are sectional side elevations of assemblies respectively including sensors, as shown in Figure 1, and two forms of spring mounting respectively mounted in vertical and inclined printed circuit boards. Modes for Carrying Out the Invention As shown in Figure 1, a 5mm steel ball 1 is supported in a dished first contact 2 formed by pressing a substantially circular piece of brass sheet with a radially extending portion 18 which is bent upwardly to form a connecting lug. A second contact 5 is formed by a similar pressing operation on an identical piece of brass plate so that the first and second contacts 2 and 5 have identical concave portions 10. However, the radially extending portion 18 of the second contact 5 is bent in the opposite direction so that when the two concave portions 10 are arranged to face each other, both contact lugs project upwardly for connection to a printed circuit board 16. The first and second contacts 2 and 5 are enclosed in a plastic cup- shaped part 11 having two dowels 19 for connecting the sensor to the printed circuit board 16. The cup-shaped member 11 has a plastic cap 8 which is held within the cup-shaped member 11 by resilient engagement with a lip 20 on at least part of the rim of the cup- shaped member 11. The cap 8 and cup-shaped member 11 are respectively formed with internal shoulders 21 and 22 which locate "the peripheral edges of the first and second contacts 2 and 5 and a sleeve 23 disposed between the first and second contacts 2 and 5 maintains the first and second contact 2 and 5 in correct spaced relation. The first contact 2 has a central part-spherical portion of smaller diameter than the ball 1 and so the ball 1 rests on the circular portion 3 between this part-spherical portion and an upwardly inclined outer portion 4. This prevents the ball 1 from rolling freely in the concave portion 10 of the first contact 2 when the sensor subject to horizontal vibration. The inclination of the outer portion 4 of the first contact 2 (and thus the inclination of the equivalent portion of the second contact 5) is chosen so as to ensure that the ball 1 will neither rebound too rapidly from the second contact 5 nor wedge itself between the first and second contacts 2 OMPI and 5. As shown, cap 8 has a flexible annular portion 9 supporting a hub 24 formed with recesses for accommodating a rare earth type magnet 7 and a polyester shim 6 which is clamped between the part-spherical portion of the first contact 2 and the magnet 7 as a result of resilient deformation of the annular portion 9 of the cap 8. Although the space between the ball 1 and magnet 7 may be kept small, by using a shim 6 having a thickness of only 0.5mm, the accuracy of the space between the first contact 2 and the magnet 7 depends only on the tolerance on the thickness of the shim 6 and so it is possible to assume that the magnetic field strength to which the ball 1 is subjected falls within acceptable limits. If it is desired to produce sensors in which the ball is subjected to a different magnetic field strength, this can be effected simply by replacing the shim 6 with a shim of different thickness. However, it is of course necessary to ensure that the cap 8 can accommodate the replacement shim 6 and that the annular portion 9 of the cap 8 is resiliently deformed to such an extent that the replacement shim 6 is clamped between the first contact 2 and the magnet 7. Figure 2 shows an assembly in which a sensor similar to that shown in Figure 1 is fitted with a clip, shown in dashed outline, having a resilient band 25 surrounding the housing member 11 and two spring arms 12 extending above and below the sensor for engagement with the edge of a circular aperture 14 in a printed circuit board 16. The sensor can thus be rotated about a horizontal axis extending perpendicular to the printed circuit board 16 so as to ensure that its own central axis is perfectly vertical. The spring arms 14 can then be soldered in place on the printed circuit board and soldered connections can also be made between the lugs 18 of the first and second contacts 2 and 5. Quite clearly, it is possible to modify the sensor so as to ensure that the lugs 18 are shaped differently and project through more conveniently spaced openings in the housing member 11 so as to facilitate connection of these lugs 18 to the printed circuit board 16. Figure 3 also shows an assembly of a sensor similar to that shown in Figure 1 fitted with a clip having a resilient band 25 surrounding the housing member 11. However, in this case, two arcuate spring arms 13 extend from the band 25 on opposite sides of the sensor for engagement with diametrically opposite sides of a circular aperture 14 in a printed circuit board 16 so as to permit rotation of the assembly about a diameter extending between these diametrically opposite edges of the aperture 14 and also about a horizontal axis extending perpendicular to this diameter so as to ensure that the central axis of the sensor is perfectly vertical. The spring arms 13 are of arcuate form so as to permit the sensor to be mounted equally well in a circular aperture 15 formed in a printed circuit board 17 which is oppositely inclined to the printed circuit board 16. Once again, it is possible to solder the spring arms 13 to the printed circuit boards 16 or 17 and modify the lugs 18 of the first and second contacts 2 and 5 so as to facilitate their connection to the required printed circuit boards 16 or 17. It is to be understood that the sleeve 23 may be replaced by complementary arcuate projections formed respectively on the cap 8 and the cup-shaped member 11. In this case, the edges of the first and second contacts 2 and 5 must be recessed to accommodate the arcuate projections. The cap 8 and its projections therefore support the undersides of the first and second contacts 2 and 5 whereas the cup-shaped member 11 and its arcuate projections support the upper surfaces of both contacts 2 and 5. OMPI

Claims: Claims 1. An inertia switch impact sensor, for vehicle crash detection, comprising: a ferromagnetic ball (1); a dished first contact (2) having a circular portion (3) of smaller diameter than the ball (1), for supporting the ball (1) in a rest position, and an upwardly inclined outer portion (4) extending from the circular portion (3); a second contact (5) extending around a circle for engagement by the ball (1), on movement of the ball (1) away from its rest position as a result of impact by the vehicle, to complete an electrical path between the two contacts (2 arid 5); and a magnet (6) disposed below the ball (1) and spaced from the first contact (2); characterised in that: a non-magnetic shim (7) is disposed between the first contact (2) and the magnet (6); and a support (8) for the magnet (6) to cause the magnet (6) to press the shim (7) against the first contact (2) to thereby precisely position the magnet (6) relative to the first contact (2). 2. A sensor, according to Claim 1, in which the support (8) has a resiliently deformable portion (9) which, when the sensor is assembled, is deformed so as to effect the necessary clamping. A sensor, according to Claim 1 or Claim 2, in which the magnet (6) is a rare earth type magnet. 4. A sensor, according to any preceding claim, in which the first and second contacts (2 and 5) respectively include identical concave portions (10) one of which is inverted so as to face the other. 5. A sensor, according to any preceding claim, in which: a housing member (11) surrounds the first and second contacts (2 and 5) ; and two spring arms (12 or 13) extend from opposite sides of the housing member (11) for engagement with a circular aperture (14 or 15) in a supporting plate (16 or 17) so as to permit rotation of the sensor about a horizontal axis through the centre of the aperture (14 or 15). 6. A sensor, according to Claim 5, in which the arms (13) are curved and capable of adopting the same radius of curvature as a circular aperture (14 or 15) when disposed within the circular aperture (14 or 15) so that the sensor can also be rotated about an axis extending diametrically across the aperture (14 or 15). OM

Inventors: JACKMAN PETER RONALD, STRATTON PAUL PHILIP, JACKMAN, PETER, RONALD, STRATTON, PAUL, PHILIP
Applicants: INERTIA SWITCH LTD, FIRST INERTIA SWITCH LIMITED
Application year: 1984
Main classification: H01H35
IPC codes: H01H35