Abstract:
There is disclosed an electropyrotechnic igniter which contains two ignition heads each having a body containing a resisitive heating element and a pyrotechnic ignition composition. The two ignition heads also include a means of dialogue and of triggering coded information and a means of storing electrical energy. The igniter also includes external connection means intended to be connected electrically to an electronic control unit. The igniter is intended to be used in motor vehicle safety, particular to constitute the ignition device of a gas generator associated with an airbag.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of motor vehicle safety and deals, on the one hand, with an electropyrotechnic igniter and, on the other hand, with a pyrotechnic gas generator containing such an igniter, for inflating an airbag. 
     New generators known as “adaptive generators” have appeared recently with a view to limiting the risks of bodily injury caused by the deployment of the airbag. To do that, these various generators, coupled with appropriate detection systems, make it possible to alter the gas flow rate and/or the gas volume as a function of certain parameters, such as, for example, the morphology and the position of the passenger, the ambient temperature, the nature and the intensity of the impact. These generators are connected to an electronic control unit and generally have two combustion chambers, separated from one another by a sealed partition, each of which has its own pyrotechnic charge and its own ignition system. Each of the two ignition systems is then positioned at one of the opposite ends of the generator. Patent Application EP 0 382 552 describes a generator of this type. 
     In order to reduce the number of electrical connections between the electronic control unit and the generator, it has therefore been proposed, in Patent Application WO 97/16695 and in French Patent Application FR 2 763 548, for the two separate ignition systems to be replaced by an electropyrotechnic initiator with two ignition heads which is connected to the said electronic control unit via a single electrical supply. According to a first embodiment of this initiator, the latter contains two separate resistive heating elements incorporated in a switching bridge comprising four switches which can switch in pairs and which are combined with a routing circuit produced by a diode circuit. According to a second embodiment of this initiator, the latter comprises two different resistors mounted in parallel in an electric circuit. However, motor vehicle manufacturers wish, for safety reasons, to be able at any instant to check the value of each resistive element included in the initiator by successively passing electrical test currents generated using the electronic control unit. However, because, in the first embodiment of the initiator, the junction voltage of the diodes used is too high and depends on the temperature, it is very difficult to measure the value of the said resistive elements reliably. In the second embodiment of the initiator, in order to be able to test each of the two resistors, it is therefore necessary, because these resistors have different resistances, to vary the strength of the electrical current delivered by the electronic control unit. This is relatively complicated to do and, in addition, the use of two different resistors increases the number of steps needed in assembling the initiator and therefore the manufacturing costs. 
     An initiator has therefore been proposed, as described in French Patent Application FR 2 772 909, with two ignition heads comprising two resistive heating elements incorporated into an electric circuit which comprises three electrical connections, one being common to the two resistive elements and the other two each being connected to just one of the two resistive heating elements. 
     Nonetheless, given the great increase in the number of “airbag modules” incorporated into a motor vehicle, an “airbag module” consisting of a particular entity containing a gas generator associated with an airbag, the person skilled in the art, aside from being faced with a proliferation in the number of electrical connections, has noted that in the event of a collision, the electronic control unit was unable to deliver enough electrical energy to bring about the desired triggering of the various ignition systems included in the airbag modules. 
     In addition, depending on the nature and intensity of the impact, for example, it is desirable to be able to trigger only the appropriate airbag modules that will afford appropriate protection and therefore, in each airbag module finally adopted for protection, to be able to give totally different commands to each of the two ignition heads. 
     The person skilled in the art is still looking for an electropyrotechnic igniter with two ignition heads that makes it possible to overcome the aforementioned problems. Added to these are also the constraints associated with cost and bulk. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is precisely to propose such an igniter and relates therefore to an electropyrotechnic igniter with two ignition heads, each of the said ignition heads comprising a body containing a resistive heating element and a pyrotechnic ignition composition, characterized in that 
     i) each of the two ignition heads comprises a means of dialogue and of triggering coded information and a means of storing electrical energy, 
     ii) the said igniter comprises external connection means intended to be connected electrically to an electronic control unit. 
     It therefore follows that the use of such an igniter in each of the various airbag modules incorporated into the vehicle and connected by a wired network of the bus type with an electronic control unit makes it possible, on the one hand, to leave the choice to the electronic control unit and to trigger only one or both of the two ignition heads included in the airbag module or modules likely to afford the occupant effective protection depending on the impact, and, on the other hand, to no longer ask the electronic control unit to deliver the amount of electrical energy needed to initiate such an igniter. This is achieved by virtue of the presence, in each of the two ignition heads of the igniter, on the one hand, of a means of storing electrical energy which is periodically supplied with low-strength electric current emitted by the electronic control unit and, on the other hand, of a dialogue and triggering means which is capable of detecting coded information from the electronic control unit and of giving the command to the said means of storing electrical energy to make it deliver the amount of electrical energy hitherto stored. This energy makes it possible to heat the resistive heating element through the Joule effect, so as to initiate the pyrotechnic ignition composition. 
     As a preference, the dialogue and triggering means will be a specific integrated circuit and the coded information which flows between the electronic control unit and the said dialogue and triggering means will consist of strings of specific electrical pulses. In the current application, the coded information will comprise both information constituting the command to trigger the two ignition heads of the igniter and information allowing the electronic control unit to satisfy itself of the reliability of the electronic components contained in each of the two ignition heads of the igniter. 
     Advantageously, in each of the two ignition heads, a transverse dividing wall splits the interior of the body into, on the one hand, a downstream chamber containing the resistive heating element and the pyrotechnic ignition composition, and, on the other hand, an upstream chamber containing an electronic board onto which the dialogue and triggering means and the means of storing electrical energy are integrated. 
     Advantageously also, in each of the two ignition heads, electrical connection means pass through the transverse dividing wall and connect the resistive heating element to conducting tracks which are formed on the electronic board. As a preference, the transverse dividing wall is made using a metal part having several perforations, each of these perforations having a glass side wall. Advantageously, the metal part has two perforations and the electrical connection means consist of two metal connecting pins, each inserted into one of the two perforations borne by the metal part and having a first end attached to the resistive heating elements and a second end soldered to the conducting tracks. This transverse dividing wall therefore provides sealing between the upstream chamber and the downstream chamber before and after the ignition head is operated and also electrically isolates the metal connecting pins from one another. 
     As a preference, the two electronic boards are arranged at right angles to their respective transverse dividing wall. To improve the mechanical strength of the said electronic boards and of the electronic components fixed on them, it is desirable for the assembly to be covered with overmoulding or a coating. 
     Advantageously, in each of the two ignition heads, the dialogue and triggering means is placed on one of the two flat faces of the electronic board, and the means of storing electrical energy is placed on the other flat face. 
     Advantageously, the external connection means consist of external metal pins, each of which has, on the one hand, a first part which is arranged in the continuation of the two electronic boards and which is parallel to these electronic boards, and, on the other hand, a second part which is at right angles to the first part. 
     As a preference, the resistive heating elements each consist of a thin-film resistive bridge, the latter consisting of tantalum nitride, the thickness of which is between 0.01 μm and 1 μm. The resistive bridge could just as easily consist, for example, of a layer of a nickel/chromium alloy, the thickness of which is between 0.01 μm and 1 μm. 
     The invention also relates to the use of such an electropyrotechnic igniter in a gas generator which is intended to inflate an airbag and which has an elongate tubular body. This igniter is then preferably fixed into the central part of the generator body, that is to say between the two combustion chambers thereof, and itself provide thermal and mechanical isolation between these two combustion chambers. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     There will be described hereinafter in FIGS. 1 to  4  two preferred embodiments of the igniter and, in FIG. 5, a gas generator containing an igniter according to the invention. 
     FIG. 1 is a part view in longitudinal section of an electropyrotechnic igniter equipped with three external metal pins, with the electronic components and part of the overmoulding omitted. 
     FIG. 2 is a perspective view with cutaway of the igniter depicted partly in FIG. 1, with the overmoulding omitted. 
     FIG. 3 is a part view in longitudinal section of an electropyrotechnic igniter equipped with two external metal pins, with the electronic components and part of the overmoulding omitted. 
     FIG. 4 is a perspective view with cutaway of the igniter depicted partly in FIG. 3, with the overmoulding omitted. 
     FIG. 5 is a view in longitudinal section of a gas generator containing, in its central part, the igniter that is the subject of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, it can be seen that an igniter  1  according to the invention comprises two ignition heads  2 ,  3  each of which is produced identically, independently of one another, and which are then connected to each other. These two ignition heads  2 ,  3  consist of a cap  4  which has a side wall  6  ending, on the one hand, in an end wall  8  and, on the other hand, in a free end  10 , and into which is first of all inserted a cylindrical liner  12 . The latter has a first end resting against the end wall  8  of the cap  4  and a side wall  14 , the exterior surface of which is in contact with part of the interior surface of the side wall  6  of the cap  4 . A pyrotechnic ignition composition  16  in the form of a pulverulent substance is then introduced into the cap  4 . An assembly, described hereinbelow, is finally slipped into the said cap  4 . 
     This assembly is made from a glass penetration which, on the one hand, comprises a discoid metal part  18  having an upstream face and a downstream face to which a resistive heating element  20  is attached and, on the other hand, two metal connecting pins  22 ,  23 . More specifically, the discoid metal part  18  has two perforations each having a glass side wall  26 ,  27 , the two metal connecting pins  22 ,  23  are each introduced into one of the two perforations and the said pins  22 ,  23  each have a first end soldered to the resistive heating element  20 . The latter is advantageously made using a thin-film resistive bridge consisting of tantalum nitride, the thickness of which is about 0.5 μm. A printed circuit support, in the form of a rectangular electronic board  30  having an upper flat face  32  and a lower flat face  34  on which faces conducting tracks  36 ,  37  are formed, is attached to the connecting pins  22 ,  23 . More specifically, the second end  40 ,  41  of each of the two connecting pins  22 ,  23  is soldered to the conducting tracks  36 ,  37  on the flat upper face  32  so that the edge face  44  of the rectangular electronic board  30  is in contact with the upstream face of the discoid metal part  18 . A means  46  of storing electrical energy, which consists of a capacitor, and means  48  of protection against electrostatic discharge and against electromagnetic interference are connected to the conducting tracks  36 ,  37  on the flat upper face  32  and a means  49  of dialogue and of triggering a string of specific electronic pulses is connected to the conducting tracks  36 ,  37  on the flat lower face  34 . This dialogue and triggering means  49  is advantageously produced using a specific integrated circuit. A pyrotechnic priming composition  52  in the form of a lacquer based on lead tri-resorcinate is deposited on the resistive heating element  20  and the assembly as described above is introduced into the cap  4 , the discoid metal part  18  being placed resting against the second end of the cylindrical liner  12  and the free end  10  of the cap  4  finally being fixed by laser-welding to the said discoid metal part  18 . 
     The two ignition heads  2 ,  3  are then connected to one another by connecting pins  54  to  57 . More specifically, the external pin  54  has, on the one hand, a first end which is connected to the conducting tracks  36 ,  37  of the electronic board  30  of the ignition head  2  and which lies in the continuation of the said electronic board  30  and, on the other hand, a second end  80  which lies in a plane at right angles to the plane in which the first end is contained. The same is true of the external pin  55  which has a first end connected to the conducting tracks  36 ,  37  of the electronic board  30  of the ignition head  3  and a second end  81  at right angles to the first. The external pin  56  comprises, on the one hand, a first part  82  ending at a first end attached to the electronic board  30  of the ignition head  2  and at a second end attached to the electronic board  30  of the ignition head  3  and, on the other hand, a second part  83  which is arranged in a plane at right angles to the plane in which the first part  82  lies. This second part  83  of the pin  56  and the second end  80 ,  81  of the two pins  54 ,  55  are intended to be connected, by means of a connector, to an electronic control unit. The pin  56  is intended to provide earthing and the internal pin  57  allows the conducting tracks  36 ,  37  of the electronic board  30  of the ignition head  2  to be connected electrically to the conducting tracks  36 ,  37  of the electronic board  30  of the ignition head  3 . 
     A first overmoulding  58  is produced and coats, on the one hand, the two electronic boards  30  and the various electronic components fixed on them and, on the other hand, the connecting pins  54  to  57 . 
     Thereafter, as can be seen in FIG. 5, a second overmoulding  59  is produced which has, on the one hand, a central bore leaving space for the connector coming from the electronic control unit and, on the other hand, two opposite ends in each of which is formed a groove to hold a discoid glass wall  60 . 
     It is also possible for the space left empty between the cap  4  and the discoid glass wall  60  to be filled with a strengthening powder. 
     An electropyrotechnic igniter  1  as described above operates as follows. 
     Under normal operating conditions, that is to say when the motor vehicle into which the said igniter  1  is incorporated is not experiencing any particular accident requiring the deployment of an airbag in order to protect the occupant, the two means  46  of storing electrical energy, which are positioned one on the ignition head  2  and the other on the ignition head  3  and which here consist of capacitors, are regularly powered with low-intensity current emitted by the electronic control unit and transmitted to the said capacitors via the external pins  54  and  55 . 
     When, for example depending on the nature and intensity of the impact, activation of just one of the two ignition heads  2 ,  3  will suffice, the electronic control unit delivers a triggering command in the form of a train of specific electrical pulses which can be detected only by the dialogue and triggering means  49  of the ignition head that has been chosen. When it is necessary to activate both ignition heads  2 ,  3 , the electronic control unit then delivers the specific electrical pulse trains to both dialogue and triggering means  49 . 
     In any event, each dialogue and triggering means  49  finally adopted calls upon the capacitor associated with it and which is therefore caused to release, into the two connecting pins  22 ,  23  and therefore into the resistive heating element  20 , the amount of electrical energy which was stored. This resistive heating element  20  then, through the Joule effect, causes the initiation of the pyrotechnic priming composition  52  and thereafter the initiation and combustion of the pyrotechnic ignition composition  16 , which has the effect of fracturing the end wall  8  of the cap  4  then the glass discoid wall  60 . 
     Furthermore, in each of the two ignition heads  2  or  3 , given the mechanical strength of the discoid metal part  18 , an important advantage lies in the fact that upon activation of one of these two ignition heads  2  or  3 , the various electronic components associated with them are not damaged by the pressure wave that results from the initiation of the pyrotechnic priming composition  52 , and the dialogue and triggering means  49  is then still able to exchange information with the electronic control unit in the next few milliseconds, particularly to let it know, for example, that this ignition head  2  or  3  has been correctly triggered. 
     Referring to FIGS. 3 and 4, it can be seen that an igniter  101  according to the second preferred embodiment comprises two ignition heads  102 ,  103  and largely resembles the igniter  1  described previously. This is why, on the one hand, we have kept the same reference numbers for the technical elements which do not change and, on the other hand, only those which have changed will be described hereinbelow. 
     The only change lies in the connecting of the two ignition heads  102 ,  103  to one another. This connection is achieved using two external metal connecting pins  154 ,  155 . More specifically, each of these two pins  154 ,  155  has a first part  170 ,  171  which lies in the continuation of the two electronic boards  30 , and this part  170 ,  171  ends at a first end connected to the conducting tracks  36 ,  37  of the electronic board  30  of the ignition head  102  and at a second end connected to the conducting tracks  36 ,  37  of the electronic board  30  of the ignition head  103 . Furthermore, each of these two pins  154 ,  155  has a second part  172 ,  173  arranged in a plane perpendicular to the plane in which the first part  170 ,  171  is contained and which is intended to be connected, by means of a connector, to an electronic control unit. 
     The way in which the igniter  101  works is entirely identical to the way described before on the subject of the igniter  1 . 
     Referring to FIG. 5, it can be seen that an igniter  1  according to the first preferred embodiment is incorporated into a gas generator  200  which has a body  201  of elongate tubular shape equipped with a side wall  230 . More specifically, two metal rings  202 ,  203  sit around the overmoulding  59  of the igniter  1  and this assembly thus created is fixed into the side wall  230  of the body  201  by crimping at these two rings  202 ,  203 . The igniter  1  thus positioned makes it possible to separate, in a sealed way, an upstream compartment  204  from a downstream compartment  205 . Furthermore, the side wall  230  has a central bore at the external pins  54  to  56  of the igniter  1  so as to take a connector coming from the electronic control unit. The upstream compartment  204  and the downstream compartment  205  each contain a combustion chamber  206 ,  207  and a pressure-reducing chamber  208 ,  209 . The combustion chamber  206  contains a pyrotechnic charge  210  which, when initiated by activation of the ignition head  2 , produces gases of combustion which, first of all, pass through a nozzle  211  borne by a partition  212  fixed by crimping in the side wall  230  then, next, escape through orifices  213  borne by the said side wall  230  into the pressure-reducing chamber  208 . Likewise, the combustion chamber  207  contains a pyrotechnic charge  214  which, when initiated by activation of the ignition head  3 , produces gases of combustion which, first of all, pass through a nozzle  215  borne by a partition  216  fixed by crimping in the side wall  230  then, next, escape through orifices  217  borne by the said side wall  230  into the pressure-reducing chamber  209 . In FIG. 5, the combustion chamber  206  contains a greater quantity of pyrotechnic charge than the combustion chamber  207 , but this could be reversed or, alternatively, the amount contained in each of the two chambers could be the same.