Inflator initiator with zener diode electrostatic discharge protector

Abstract: An initiator (10) for a vehicle occupant restraint airbag system with electrostatic discharge protection to prevent inadvertent system actuation. The initiator includes a cup-shaped conducting metallic housing (12) containing pyrotechnic material closed by a sealing material (16) such as glass which either extends across an open end (14) of the housing (12) or across an annular conducting header ring (18) fitted against an inner surface of the housing (12) to encapsulate the pyrotechnic material. First and second electrodes (20, 22) extend through the sealing material (16) and are interconnected by a resistive bridge element. A surface mount type zener diode (30) has its anode electrically connected to the housing (12) and its cathode electrically connected to the second electrode (22).

Description: The present invention relates generally to improvements in initiators of the type utilized with inflators for automotive vehicle occupant restraint or airbag systems. More particularly, the invention relates to an initiator having a zener diode to provide electrostatic discharge protection. Generally speaking, an automotive vehicle occupant restraint system or airbag includes an inflatable cushion and an inflator for providing a quantity of gas for rapidly inflating the cushion at the appropriate time. Such inflators may be of the pyrotechnic type wherein a quantity of pyrotechnic material, once ignited, rapidly produces a quantity of gas for inflating the inflatable cushion. Other types include a so-called hybrid type of inflator wherein a quantity of inflating gas is stored under pressure and supplemented with a quantity of gas produced by a pyrotechnic material. Yet another type of inflator, referred to as a fluid-fueled type, utilizes a quantity of one or more fluid fuels and one or more oxidants to form a volatile mixture which, when activated or energized, will ignite and produce a quantity of gas. In this type of inflator, a quantity of additional pressurized gas may also be provided in a gas storage chamber which, upon ignition of the volatile mixture, will combine with the gas expelled thereby to inflate the inflatable cushion. The various types of inflator have in common the requirement for an initiator, sometimes called a squib, which is responsive to a pulse of electrical energy, commonly at the 12 volt DC voltage typical of an automotive electrical system, for producing a burst of energy to initiate or begin the process of gas generation and/or release to the inflatable cushion. Typically, this initiator is an electro-explosive device (EED) which contains a quantity of pyrotechnic material having a pair of spaced electrodes embedded therewithin. Typically, the ends of the electrodes embedded within the pyrotechnic material are connected by a relatively thin bridge element which has thermal characteristics selected such that it will rapidly heat to a relatively high temperature when the burst of electrical energy passes therethrough. The heat of this bridge element will ignite the pyrotechnic material within the initiator, providing a rapid burst of energy to trigger or initiate the operation of the inflator device. In initiators of this type, it is necessary to prevent electrostatic energy, which may build on the external housing or header, from discharging through the pyrotechnic to ground, causing inadvertent deployment of the initiator. Moreover, even if the energy discharged in this way is not sufficient to deploy or fire the initiator, it can cause dielectric tunneling in the pyrotechnic material, resulting in carbonizing, or an oxidizer rich zone of material to form around the electrodes and/or bridge element. This material will act generally as a heat insulator, preventing the heat of the bridge element from adequately reaching the pyrotechnic, which may compromise or even prevent adequate firing of the device when desired, thus resulting in a "dud" or reject initiator. Such electrostatic charges commonly occur on the outer surface of the initiator during the manufacture, assembly and handling of the initiator devices, prior to their assembly with an inflator device. In so-called coaxial type initiators, only a single electrode or lead enters the pyrotechnic, with a "header" acting as the other electrode. In this case, electrostatic discharge may be provided by coupling the header to ground and operating in a polarity wherein the firing current is passed from the internal electrode through the pyrotechnic to the grounded header. However, in the case of two-pin or two-electrode initiators, a number of other arrangements have been utilized to try to provide such a discharge path for electrostatic energy. One such arrangement includes a shunt element such as a bridge wire, a quantity of silver epoxy, a conductive link or a spark gap provided between one of the electrodes and an internal surface of the outer housing. Typically, this shunt element may connect to an internal surface of a sleeve which is interposed intermediate to an external housing or charge cup and a glass header or other seal which encapsulates the pyrotechnic material and the ends of the electrodes in contact therewith within the housing or header. However, such an initiator is more difficult and expensive to construct. Moreover, most of these alternatives will also allow the firing energy to flow to ground unless some additional secondary insulation is provided. In the event of insulation resistance failure of such secondary insulation, the device may fail to fire, due to the firing pulse being drawn off through this additional ground path. Or, if the polarity of the device is altered, such that the discharge path is provided to the energized or "hot" pin or electrode rather than the ground pin, an insulation resistance failure could result in inadvertent firing or deployment of the device. Yet other arrangements provide complete electrical isolation of the charge cup or housing, for example, by providing insulation for the external surfaces of the housing and insulation between the housing and the electrode(s). As an additional matter, most applications also require some minimum insulation resistance, typically in the order of 500 volts between the charge cup or housing and the electrodes. Bridge wires, conductive epoxies or other conductive links and spark gaps must be carefully specified and assembled in order to provide a specific insulation resistance requirement. This adds to the complexity and expense of such an initiator. An example of a prior art initiator is described in Patent US 4061088 (on which the preamble of claim 1 is based). The initiator includes a housing containing an explosive charge into which two electrodes extend through a silicone rubber plug, which electrodes are connected by a bridge wire extending through the charge. A non-linear zinc oxide resistor element is sandwiched between electrode plates and connected between the electrodes and the housing for preventing accidental electrostatic actuation of the initiator. The resistor element and plates are cast in the rubber plug or an insulating body of resin, thereby complicating the manufacture of the initiator. Accordingly, it is a general object of this invention to provide electrostatic discharge protection for an initiator which overcomes the above-noted problems. A further object is to provide such electrostatic discharge protection which allows electrostatic energy to flow from the outside surfaces of the initiator to ground without affecting the pyrotechnic material. A related object is to provide such electrostatic discharge protection which eliminates the need for complete electrical isolation. Another object is to provide such electrostatic discharge protection which allows energy to flow only in one direction, thereby preventing energy from flowing to ground during the firing pulse. According to the invention there is provided an initiator with electrostatic discharge protection comprising a generally cup-shaped housing having an open end; a quantity of pyrotechnic material in said housing; sealing means for closing said housing open end and encapsulating said pyrotechnic material within said housing; a pair of electrodes in contact with said pyrotechnic material and extending through said sealing means; and a zener diode coupled in electrical circuit between said housing and one of said electrodes to provide a path for electrostatic discharge and to prevent electrostatic discharge from adversely affecting said pyrotechnic material, said sealing means comprising a quantity of sealing material encapsulating said electrodes and said quantity of pyrotechnic material, said sealing material defining oppositely facing surfaces, one surface facing inwardly of said housing and one surface facing outwardly of said housing, characterised in that said zener diode is of the surface mount type and mounted to one of said surfaces of said sealing material. The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof may best be understood by reference to the following description, taken in connection with the accompanying drawings in which like reference numerals identify like elements, and in which: Fig. 1 is a longitudinal sectional view through an initiator, somewhat diagrammatic in form, illustrating electrostatic discharge protection in accordance with the invention; Fig. 2 is a view similar to Fig. 1 showing an equivalent electrical circuit superimposed upon the elements of Fig. 1; and Fig. 3 is a view similar to Fig. 2 illustrating a reverse polarity of the equivalent electrical circuit. Referring now to the drawings and initially to Figs. 1 and 2, an initiator is designated generally by the reference numeral 10. This initiator 10 is provided with a novel form of electrostatic discharge protection in accordance with the invention, as will be more fully described hereinbelow. Generally speaking, the initiator 10 includes a generally cup-shaped housing 12 which has an open end 14. A sealing means such as a glass seal 16 is provided for normally enclosing the open end 14 of the housing 12 and encapsulating a quantity of pyrotechnic material 15 which is contained within the cup-shaped housing 12. This pyrotechnic material may comprise one of a number of materials which when heated will produce a rapid burst of energy, for example, for use in an inflator device for an automotive vehicle occupant restraint system. A number of such pyrotechnic materials are well known in the art. In the illustrated embodiment, the open end 14 of the housing 12 is sealed by a quantity of electrically nonconductive glass material 16 and a metal header 18. The housing 12 is of an electrically conductive metallic material, and an additional intermediate generally cylindrical header 18 of electrically conductive material, and preferably material similar to that of the housing 12, is interposed between an inner surface of housing 12 and an outer surface of the sealing material 16. In the illustrated embodiment, the housing 12 and header 18 are constructed of, but are not limited to, stainless steel material. A pair of electrodes 20, 22 extend through the glass seal 16 and into the pyrotechnic material 15 encapsulated within the housing 12. The glass or other material forming the seal 16 may be poured or otherwise introduced following the placement of the electrodes 20 and 22 within the header cylinder 18 in the housing 12. Thus, the electrodes 20 and 22 extend back outwardly of the encapsulated pyrotechnic material through the now sealed open end of the housing 14 for electrical contact with appropriate electrical circuit elements for firing or energizing the pyrotechnic material 15 by introducing an electrical pulse through a circuit including the electrodes 20 and 22. Referring to Figs. 2 and 3, two such electrical circuits (of opposite polarity) are illustrated in simplified form. In order to energize or fire the pyrotechnic material 15 in response to an electrical pulse introduced by way of electrodes 20 and 22, a bridge element 24 is provided embedded in the pyrotechnic material 15 and electrically coupled between the ends of the electrodes 20 and 22. Preferably, this bridge element 24 has thermal resistive characteristics such that it will rapidly heat in response to an electric current or a firing pulse delivered through the electrodes 20 and 22. The heat energy of the bridge element 24 will normally deploy the pyrotechnic material 15. Thus, in Figs. 2 and 3 the bridge element 24 is represented electrically by a resistor element. In accordance with the invention, in order to provide a path for electrostatic discharge protection, a zener diode 30 is coupled in electrical circuit between the housing 12 through the header 18 and one of the electrodes 20 and 22. It will be noted that this arrangement also protects this electrostatic discharge from affecting the pyrotechnic material. The zener diode 30 is interposed in a position extending between an inner surface of the header 18 and one of the electrodes 20 and 22. The zener diode 30 is of the surface mount technology (SMT) type and thus comprises a relatively compact, flat element, which advantageously is also a relatively simple, low cost and robust device. This relatively flat SMT zener diode 30 is mounted in the illustrated embodiment between an inner surface of the header 18 and the electrode 22, which as will be seen in Figs. 2 and 3 may be either coupled with ground or coupled with the energizing potential for firing the initiator 10, here symbolically shown as a battery. As also best viewed in Figs. 2 and 3, the anode of the zener diode 30 is electrically coupled with the housing 12, by way of the header 18, while its cathode electrode is electrically coupled with the electrode 22 of the initiator 10. Also, to avoid any contact with or disturbance of the pyrotechnic material 15 and also to simplify the assembly of the initiator 10, the zener diode 30 is mounted to an outer surface of the glass seal 16. In this regard, the glass seal 16 has oppositely facing surfaces, one of which faces generally into the encapsulated portion of the housing 12 and one of which generally faces oppositely, that is, toward the open end 14 of the housing 12. The zener diode 30 may be selected or specified to have a forward breakdown voltage at least as great as the firing voltage of the initiator 10, which in most automotive applications is 12 volts. In cases where there is a required insulation resistance between the housing 12 and the electrodes 20, 22 the zener diode 30 may be selected to have a forward breakdown voltage at least as great as this insulation resistance. In many cases the insulation resistance is specified as a test voltage, typically 500 volts. Accordingly, the present invention provides a path to ground for electrostatic energy, wherein this energy runs through a zener diode 30 rather than through the pyrotechnic material 15. It will be appreciated that typical electrostatic charge voltages are on the order of from 6,000 to 25,000 volts. Thus, the zener diode 30 provides a path to ground for electrostatic energy, which protects the initiator 10 from inadvertent deployment due to electrostatic discharge through the pyrotechnic. Moreover, this arrangement prevents the electrostatic discharge from adversely affecting the pyrotechnic material. That is, with this arrangement, energy is not passed through the pyrotechnic material, which as mentioned above, can cause carbonizing of the material. Moreover, this arrangement prevents the loss of normal firing energy when it is applied. Advantageously, as noted above, the SMT zener diode 30 comprises a simple, low cost and robust device.

Claims: An initiator (10) with electrostatic discharge protection comprising: a generally cup-shaped housing (12) having an open end (14); a quantity of pyrotechnic material (15) in said housing; sealing means (16) for closing said housing open end and encapsulating said pyrotechnic material within said housing; a pair of electrodes (20, 22) in contact with said pyrotechnic material and extending through said sealing means; and a zener diode (30) coupled in electrical circuit between said housing and one of said electrodes (22) to provide a path for electrostatic discharge and to prevent electrostatic discharge from adversely affecting said pyrotechnic material, said sealing means (16) comprising a quantity of sealing material encapsulating said electrodes (20, 22) and said quantity of pyrotechnic material (15), said sealing material defining oppositely facing surfaces, one surface facing inwardly of said housing (12) and one surface facing outwardly of said housing, characterised in that said zener diode (30) is of the surface mount type and mounted to one of said surfaces of said sealing material (16). An initiator according to claim 1 wherein said zener diode (30) is mounted to the surface of the sealing material (16) at the open end (14) of said housing (12). An initiator according to any preceding claim wherein said zener diodes (30) has an anode coupled electrically with said housing (12) and a cathode coupled electrically with said one electrode (22) of said initiator. An initiator according to any preceding claim wherein said zener diode (30) has a forward breakdown voltage at least as greet as the firing voltage of the initiator. An initiator according to any preceding claim wherein said zener diode (30) has a forward breakdown voltage at least as great as any required insulation resistance between said housing (12) and said electrodes (20, 22). An initiator according to any preceding claim wherein said sealing means (16) extends across said open end (14) of said housing (12) and said zener diode (30) is mounted to said initiator in a position extending between an inner surface of said housing and one of said electrodes (22). An initiator according to claim 1 wherein said housing (12) comprises a conductive metallic cup member and further including a header (18) of conductive material interposed between said housing and said sealing means (16), wherein said sealing means comprises a quantity of sealing material extending across said header and wherein said zener diode (30) is mounted to a surface of said sealing means extending between said header and one said electrode. An initiator according to claim 7 wherein said zener diode (30) has an anode electrically coupled to said header (18) and a cathode electrically coupled to the electrode (22) with which it is coupled in electrical circuit. An initiator according to claim 7 or 8 wherein said zener diode (30) extends between said header (18) and the electrode (22) with which it is coupled in electrical circuit. An initiator according to claim 7, 8 or 9 wherein said zener diode (30) is electrically connected between said header (18) and the electrode (22) with which it is coupled in electrical circuit.

Inventors: HANSEN DAVID D, MONK DAVID B, WOODBURY MARK B, HANSEN, DAVID D., MONK, DAVID B., WOODBURY, MARK B.
Applicants: AUTOLIV ASP INC, AUTOLIV ASP, INC.
Application year: 1996
Main classification: F42B3
IPC codes: F42B3