Pretensioner with a link earthing the igniter housing

Abstract: A pretensioner for a vehicle occupant safety restraint comprising an outer casing (1); gas generant (5) contained within said outer case; an igniter comprising igniter material (2) within an igniter housing (8); a pair of electrodes (3,3a) connected to said igniter material; a resilient connector link electrically connecting the igniter housing to an earth path. The connector link may be formed by springs (12) connecting the outer case (1) to the earth electrode (3) and an interference fit connecting the igniter housing (8) to the outer case (1). The link forms a low resistance path to short circuit undesirable static charges in the unit which would otherwise tend to cause spurious detonation of the unit.

Description: The present invention relates to a pretensioner for a vehicle occupant safety restraint, particularly for a seat belt. Pretensioners are used to rapidly pull in slack in a safety restraint seat belt at the onset of a crash condition to more securely restrain the occupant against forward movement and potential injury by collision with internal features of the vehicle. In addition, the pretensioning operation aims to pull the occupant into, or at least towards, the correct seating position so as to maximise the effect of a second restraint such as an airbag. A modern seat belt is known as a 3-point restraint because it is secured to the vehicle at three points arranged about the occupant to provide a diagonal torso section and a horizontal lap portion to hold the occupant in the seat. The belt is attached to the vehicle by a spring loaded retractor tending to tighten in the belt, and by a buckle for quick release of the belt. Pretensioners are typically sited at the retractor or at the buckle end of the restraining seat belt. They are designed to store energy which when released effects the pretensioning operation. This energy may be mechanical energy in the form of a stressed spring, but more modern pretensioners are pyrotechnically operated; they comprise a sealed tube containing a gas generant which expands rapidly to release the energy to effect the pretensioning operation. The gas generant is ignited electrically via pin terminals protruding from the pretensioning tube and connected to an intermediate igniter material. Prior art document US5763814 discloses an invention relating to an electrical pretensioner initiator, whereby two electrically conductive pins form a seal upon insertion into a header. An electrical signal through a bridgewire connection between the pins causes the device to fire by generating heat, which ignites a primer that in turn ignites a solid gas generant. One problem with pyrotechnically operated pretensioners is that spurious electric charges can produce inadvertent firings of the pretensioner. For example static charge may build up within the sub-assembly of the pretensioner. When the unit passes over or through any static field a low voltage will flow across the sensitive igniter material. This low voltage will in time break down the protective coating on the igniter material and once exposed it will react with oxygen and ignite thus causing the unit to fire. In addition, inadvertent firing will occur with single discharges of static electricity, especially across the igniter material which provides a short path between the firing pins and the igniter casing. Known solutions to the inadvertent firing problem are put forward by US5140906, GB2238672 and EP0779492. The document US5140906 describes an airbag igniter having an initiator charge in an inner case and an output charge in surrounding outer case, whereby a spaced relationship is maintained by a glass seal. An inner metal header is electrically isolated from a second header and the outer case by the glass seal for protection against accidental ignition. The shunting device for electrical connectors of GB 2238672 aims to avoid the occurrence of unintended explosions upon connection of a plug-in connector to a mating connector. Contact means are straight wire ends, shorted by a coiled wire that is bent to provide camming means, such that each contact means is resiliently liased towards a respective contact member, thereby ensuring adequate contact force is provided. An inflator initiator for an airbag system with zener diode electrostatic discharge protector is described by EP0779492 such that a cup-shaped conducting metal housing containing pyrotechnic material is closed by a sealing material. Two electrodes extend through the sealing material and are interconnected by a resistive bridge element with a surface mount type zener diode electrically connected between the housing and one electrode. The present invention aims to provide an improved pretensioner which can be used at either end of the seat belt. According to one aspect of the present invention there is provided a pretensioner for a vehicle occupant safety restraint comprising: an outer case; gas generant contained within said outer case; an igniter comprising igniter material within an igniter housing wherein the igniter housing is electrically connected through an interference fit with the outer casing; a pair of electrodes connected to said igniter material; a connector link comprising a resilient member electrically connecting the igniter housing to an earth path. The connector link preferably comprises a spring pressed over one of the pins. The igniter housing is again formed as an interference fit with the outer case and the spring provide the low resistance link from the case to the pin. The resilience of the spring maintains the electrical connections and reduces the risk of the contact being broken. This entails very little or no changes to a conventional igniter housing design. Preferably the connector link electrically connects the igniter housing to one of the electrodes. Advantageously the link is a very low or even almost zero resistance link and is connected to one of the pins only with a shorting connection between the two electrodes being provided. Thus a low resistance link is provided for the charge to take through the unit so as to prevent the build up of charge and thus inhibit spurious firing of the pretensioner. According to a first embodiment of the present invention the pretensioner is constructed so that the igniter housing is electrically connected to the outer case of the pretensioner, for example by making it an interference fit in the case, and one of the electrod4e pins (preferably the earthed one) forms an electrical connection to the outer case, for example it may comprise a bent section protruding through the igniter housing and fitting, interference wise, with the outer case. Thus a short circuit is provided between the igniter housing and the earthed electrode pin via the outer case to discharge spurious static currents. This embodiment has the advantage of requiring relatively few parts and no substantial modification of a conventional igniter housing. Also it provides for only two points which need maintain electrical connection. According to a second embodiment the igniter housing is made from non-conductive material and the igniter housing has a cap which is crimped with the outer case directly into a conductive sleeve, and one of the pins is modified so that a flat anti-rotation feature is extended outwards to contact with the conductive sleeve. This embodiment has the advantage that a relatively low number of electrical contacts are required and little modification to manufacturing tools is needed. According to a third embodiment the igniter housing is moulded or cast from conductive material and one of the pins is coated in an insulating material such as plastic. The igniter housing cap and the outer case are crimped directly onto a conductive sleeve thus creating a low resistance link between the conductive igniter housing and the pin. The second pin is insulated from the igniter housing. This has the advantage of reducing the number of electrical contacts, and requiring no additional components. According to a fourth embodiment the igniter housing cap is designed as an interference fit with the outer case, so as to electrically connect the two components, and an electrical connection is formed between the outer case and one of the pins, for example by an extended leg jutting from the outer case to contact the pin. This embodiment requires no additional parts and no modification to existing manufacturing tools. Of course, modifications and combinations of the features of each of the embodiments are envisaged and are intended to be within the scope of the invention. For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made to the accompanying drawings, in which: Figure 1 is a cross-sectional view of part of a known pretensioner; Figure 2 is a cross-sectional view of part of a pretensioner according to one embodiment of the present invention; Figure 3 is a cross-sectional view of part of a pretensioner according to a second embodiment of the present invention; Figure 4 is a cross-sectional view of part of a pretensioner according to a third embodiment of the present invention; Figure 5 is a cross-sectional view of part of a pretensioner according to a fourth embodiment of the present invention; Figure 6 is a cross-sectional view of part of a pretensioner according to a fifth embodiment of the present invention; Figure 7 is a cross-sectional view of part of a pretensioner according to a sixth embodiment of the present invention. In the Figures like parts are denoted by like reference numbers. The Figures show part of a pretensioner but the general construction of a pretensioner and the manner in which it is fitted into a vehicle to control a safety restraint seat belt is well known to persons skilled in the art. One such example is shown and described in EP-A-0 827 883. The pretensioner comprises an outer case 1 in the form of a cylindrical tube of metallic material. This is filled with a gas generant 5 (of known composition). Within the outer case 1 is situated a small quantity of sensitive igniter material 2 sandwiched behind an igniter header cap 4. The igniter material 2 has a thickness indicated by 7. Two electrode pins 3 and 3a pass through a seal 6 forming the igniter housing 8 at the end of the outer case 1. They are connected by means of a thin wire (not shown) welded to each pin and lying adjacent to or embedded in the igniter material 2. The pin 3 is connected to earth and thus provides an earth path. When electricity is applied across the electrode pins 3 and 3a, the thin wire heats and ignites the igniter material 2. This activates the main gas generant 5. Spurious electric charges, such as those which result from electrostatic build up, can cause an undesirable discharge of current from the outer case 1 to the header cap 4 and thus through the sensitive igniter material 2 to the earth pin 3. When such a current passes through the igniter material 2 it often causes ignition leading to spurious detonation of the pretensioner. Figures 2 to 6 show several embodiments of the invention in which spurious discharges are reduced. A first embodiment of the invention is shown in Figure 2. The electrode 3a has a U-bend 9 formed to connect with the inside wall of the outer case 1 and form an electrical connection therewith. In addition the header cap 4 is modified to provide an electrical connection with the inside wall of the outer case 1 by an outwardly bent portion in the form of projecting tangs 10. Thus an electrical connection is provided between the header cap 4 and the earth pin 3, via the outer case 1, of relatively low resistance so that spurious charges are discharged to earth before they build up sufficiently to activate the sensitive igniter material 2. The second embodiment shown in Figure 3 utilises a pressed metal clip 11 to provide a link from the generator case 1 to the earth pin 3. As in the embodiment of Figure 1 the header cap 4 is formed as an interference fit with the outer case 1 by means of tangs 10 contacting the inside surface of the outer case 1. Again this provide a short circuit to earth for spurious charges. The tangs 10 may be formed on assembly of the pretensioner by the case 1 being inserted over the header cap 4. Figure 4 is similar to Figure 3 but shows the use of a stainless steel tension spring 12 wound around the earth pin 3 with its free end connected to the generator outer case 1, to provide the required short circuit link between the case 1 and earth. This is instead of the pressed spring 11 shown in Figure 3. The link between the header cap 4 and the case 1 is again provided by bent tags 10. An annular groove 13 of between 0.3 and 0.5mm depth is formed in the case 1 to clamp the case 1 to the fuse seal 6 and retain the two to a pull off load of up to 250 N. Stakes 18, typically eight, equispaced around the circumference of the groove serve to make the connection stronger. In Figure 5 the igniter housing 8 comprises two parts: a conductive outer sleeve 13, formed for example from aluminium, and a non-conductive insert 14 which may be moulded of standard plastics material such as that known as PBT. The outer sleeve 13 provides a direct electrical connection to the outer case 1. The sleeve 13 is connected to the earth pin 3 by a stainless steel pin 15. This pin 15 is formed by a relatively small change to the anti-rotation feature. Figure 6 illustrates an embodiment in which the earth pin 3 is directly connected to both the outer case 1 and to the header cap 4 by using a conductive plastics material for the igniter housing 8. This will provide the low resistance short circuit path required from the header cap 4 to earth. However, in this embodiment it is necessary to insulate the other electrode pin 3a and this is done by dipping it into insulating plastics material 16 prior to moulding the housing 8 around the pins 3 and 3a. Another embodiment is shown in Figure 7 where the outer case 1 is formed with an extending leg 17 which contacts the earth pin 3 directly. The header cap 4 is formed as an interference fit with tangs 10 to provide the connection from cap 4 to the case 1, as in the embodiments of Figures 2, 3 and 4. A circumferential groove 13 with spaced stakes is also provided to ensure a strong connection between the case 1 and the igniter housing 8 as in the embodiment of Figure 4.

Claims: An initiator for a vehicle occupant safety restraint pretentioner comprising: an outer case (1); gas generant contained within said outer case (1); an igniter comprising igniter material (2) within an igniter housing (8); a pair of electrodes (3, 3a) connected to said igniter material (2); a connector link (9, 10; 11, 10; 12, 10; 13, 15; 8; 10, 17) comprising a resilient member electrically connecting the igniter housing (8) to an earth path (3); characterised by the igniter housing (8) being electrically connected through an interference fit with the outer case (1). An initiator according to claim 1 wherein the resilient member comprises a spring (11; 12) pressed over one of the electrodes (3, 3a). An initiator according to claim 1 or 2 arranged so that the connector link (9, 10; 11, 10; 12, 10; 13, 15; 8; 10, 17) electrically connects the igniter housing (8) to one of the electrodes (3, 3a). An initiator according to claim 1, 2 or claim 3 wherein the link (9, 10; 11, 10; 12, 10; 13, 15; 8; 10, 17) is a very low resistance link. An initiator according to and one of the preceding claims wherein one of the electrodes (3, 3a) forms an electrical connection to the outer case (1). An initiatior according to claim 5, wherein said one of the electrodes (3, 3a) comprises a bent section (9) protruding through the igniter housing (8) and fitting, interference wise, with the outer case (1). An initiator according to any one of claims 1 to 4 wherein the igniter housing (8) is made from non-conductive material and the igniter housing (8) has a cap (4) which is crimped with the outer case (1) directly into a conductive sleeve (13). An initiator according to claim 7 wherein one of the electrodes is modified so that a flat anti-rotation feature is extended outwards (15) to contact with the conductive sleeve (13).

Inventors: BLAND STUARD, DOWNIE ANDREW, PATRICKSON JAMES, THIRLWELL BRYAN, BLAND, STUARD, DOWNIE, ANDREW, PATRICKSON, JAMES, THIRLWELL, BRYAN
Applicants: KEY SAFETY SYSTEMS INC, KEY SAFETY SYSTEMS, INC.
Application year: 2000
Main classification: F42B3
IPC codes: B60R22, B60R21, F42B3