Ammunition and explosives – Igniting devices and systems – Accidental fuse ignition prevention means
Reexamination Certificate
1999-10-06
2001-04-24
Carone, Michael J. (Department: 3641)
Ammunition and explosives
Igniting devices and systems
Accidental fuse ignition prevention means
C102S202200, C102S202400, C102S202500, C102S200000
Reexamination Certificate
active
06220163
ABSTRACT:
The present invention relates to the field of electro-pyrotechnic initiators intended for motor-vehicle safety and especially to the field of initiators intended for initiating seat belt retractors or pyrotechnic gas generators for air bags. The invention relates more particularly to initiators whose heating system consists of a resistive metal element connected to two thin layers of conductive metal.
Conventionally, electro-pyrotechnic initiators intended for motor-vehicle safety consist of an insulating body extended by a fragmentable metal cap, through which insulating body two electrodes pass. The electrodes are connected together by a resistive heating filament surrounded by an explosive ignition composition, for example a composition based on lead triresorcinate. Such initiators, which are for example described in U.S. Pat. No. 3,572,247; U.S. Pat. No. 4,517,895; U.S. Pat. No. 4,959,011; or U.S. Pat. No. 5,099,762, have the drawback, however, of being sensitive to the vibrations of the motor vehicle at the soldered joints between the resistive filaments and the electrodes. These soldered joints, when they are repeatedly stressed by the vibrations of the vehicle, can break and make the initiator inoperable.
In order to remedy this drawback, initiators have therefore been developed in which the electrodes are in contact with two separate conductive metal areas extended over the surface of the insulating body which is inside the metal cap. These two areas are connected together by a narrow flat resistive strip deposited on the surface of the insulating body. The conductive areas and the resistive strip are covered with an explosive ignition composition. Such initiators, which are for example described in U.S. Pat. No. 5,544,585 or U.S. Pat. No. 4,690,056, are no longer sensitive to the vibrations of the motor vehicle, but they have a new drawback. Because of the very small thickness of the conductive areas, often less than 50 micrometres, these initiators are sensitive to electrostatic discharges which may cause an undesirable triggering of the initiator or, at the very least, a serious deterioration of the state of these conductive areas. Attempts have been made to protect these initiators from this type of discharge, as described for example in U.S. Pat. No. 5,616,881 or EP 0,802,092, but these protective means are not always effective in the case of electrostatic discharges of very high voltage.
Those skilled in the art are therefore still confronted with the problem of having a reliable electro-pyrotechnic initiator which, when it is mounted in a gas generator or in a seat belt retractor with which a motor vehicle is fitted, is both insensitive to mechanical vibrations of the vehicle and insensitive to high-voltage electrostatic discharges which may be induced, for example, by a high-voltage electrical line.
The object of the present invention is specifically to propose such an initiator.
The invention therefore relates to an electro-pyrotechnic initiator comprising especially a resistive heating element which is covered by a pyrotechnic ignition composition and which is connected to two electrodes which can themselves be connected to an electric current source by means of an electrical connector, the said resistive heating element having a resistance of value Rc allowing ignition of the said ignition composition when the potential difference between the two electrodes reaches an operating value Uf, the said initiator being characterized in that the two electrodes are also connected together by a conductive filter device which is connected in parallel with the said resistive heating element, the equivalent resistance of the said conductive filter device having a value at least equal to 100Rc when the electric potential difference between the two electrodes is less than or equal to the operating voltage Uf, this equivalent resistance having a value of less than or equal to 0.25Rc when the potential difference between the two electrodes is greater than a predetermined value Uo greater than Uf.
Thus, by virtue of the presence in the initiator of a conductive filter device that can be modulated as a function of the voltage and is connected in parallel with the resistive heating element, when the voltage applied to the electrodes is the operating voltage the current will pass via the resistive heating element and will cause the initiator to operate. On the other hand, should the voltage be greater than the predetermined threshold value Uo and be due to an electrostatic discharge, the current will pass through the conductive filter device, the equivalent resistance of which will have dropped and will provide the resistive heating element. Finally, it should be noted that in the event of electrostatic discharge with a voltage less than the predetermined threshold value Uo, there is no risk of degradation of the initiator since, in this situation, the energy which is delivered to it is too low.
According to a first embodiment of the invention, the conductive filter device consists of a capacitor and a varistor which are connected in parallel with each other. In this arrangement, the capacitor reinforces the effectiveness of the varistor in the event of very high overvoltage.
Preferably, the resistive heating element will consist of a thin flat resistive element deposited on an insulating printed-circuit support and connected to the two electrodes via two separate conductive metal areas extended over the said support, each area being in contact with one of the two electrodes.
The resistive heating element may be based on a nickel-chromium alloy, the conductive areas will generally be based on copper and the support will be made of ceramic, glass or based on a glass/resin mixture, the resin being, for example, an epoxy resin.
Advantageously, the varistor will consist of assemblies of thin semiconductor layers, for example layers based on tantalum or on germanium, which will be attached to the said conductive areas.
Also advantageously, the capacitor will consist of assemblies of thin layers that are alternately conductive and insulating and are attached to the said conductive areas.
According to a second embodiment of the invention, the said conductive filter device consists of two Zener diodes, the threshold voltage of which is equal to the predetermined value Uo, which Zener diodes are connected in parallel with each other and with opposed conducting directions, in other words they are connected in parallel “head to tail”.
Preferably, the resistive heating element will consist of a thin flat resistive element deposited on an insulating printed-circuit support and connected to the two electrodes via two separate conductive metal areas extended over the said support, each area being in contact with one of the two electrodes.
The resistive heating element may be based on a nickel-chromium alloy, the conductive areas will generally be based on copper and the support will be made of ceramic, glass or based on a glass/resin mixture, the resin being, for example, an epoxy resin.
Also advantageously, the said Zener diodes will consist of assemblies of thin semiconductor layers, for example layers based on tantalum or on germanium, these layers being attached to the said conductive areas.
In order to further increase the operating reliability of the initiators according to the invention, a filter coil will advantageously be connected in series with one of the electrodes so as to clip the overvoltage peak.
This coil may be mounted in the initiator itself or else in the connector connected to the electrodes of the initiator.
The invention therefore also relates to the use of an initiator as described above, by connecting the electrodes of this initiator to a connector having two electrical conductors, characterized in that one of the conductors, for example a tubular socket, is connected in series with a filter coil.
The invention thus makes it possible, in its preferred embodiments, to have particularly reliable electro-pyrotechnic initiators which are insensitive to mechanical vibratio
Duguet Jean-Rene
Vedel Jean-Pierre
Carone Michael J.
Livbag SNC
Pillsbury & Winthrop LLP
Semunegus Lulit
LandOfFree
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