Abstract:
A pyrotechnic igniter for adaptation to a specific gas generator or pyromechanism, the igniter including at least one pyrotechnic composition located inside a cartridge composed of a case extended by at least two pins. The cartridge is located in a casing including at least an upper shell joined to a lower shell, the upper shell containing the cartridge and the lower shell having at least two protruding electrodes internally connected an internal circuit and to the pins.

Description:
FIELD OF THE INVENTION 
     The technical scope of the present invention is that of pyrotechnic igniters, that is to say components enabling a pyrotechnic effect to be transmitted in a pyrotechnic train. 
     BACKGROUND OF THE INVENTION 
     Igniters whose cartridge is at least partially made of a plastic material are known by patents EP600791 and EP711400. 
     These igniters are, for example, intended to activate a pyromechanism or else to ignite a gas generating composition in an automobile safety system. 
     Their external profile is thus of a globally cylindrical shape intended to co-operate with a matching cavity in the gas generator or in the pyromechanism. They also have a rim that acts as an abutment during assembly operations, such rim also providing sealing by acting as a bearing surface for an O-ring. 
     These igniter cartridges are generally made by injection or duplicate-moulding. 
     The disadvantage of such cartridges is that they are of a shape that is specific to a given gas generator or pyromechanism. 
     However, there are different types of gas generators and pyromechanisms are available on the market and the attachment interfaces for the igniters are not, at present, standardised. 
     It is thus essential for the igniter production line to be specially adapted for each customer&#39;s order so as to carry out duplicate-moulding of specific dimensions. 
     This results in an increase in production costs. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to propose an igniter as well as an assembly process for such an igniter that does not suffer from such drawbacks. 
     Thus the igniter according to the invention is of a structure that allows it to be inexpensively manufactured for a given external geometry. 
     The assembly process according to the invention also allows igniters that have various external dimensions to be inexpensively manufactured. 
     The process according to the invention also allows for the easy integration of electronic components providing complementary functions (electromagnetic or electrostatic protection, logic circuits, firing device . . . ). 
     Thanks to the invention, the inner part of the igniter is the same whatever the outer geometry of the whole component. It is therefore possible to mass produce the essential and a priori the most expensive part of the igniter, that is to say the cartridge enclosing the pyrotechnic composition as well as the ignition means. 
     The igniter is thereafter customised by being installed in a casing having the external dimensions required for a specific customer. 
     Thus the subject of the invention is a pyrotechnic igniter comprising at least one pyrotechnic composition placed inside a cartridge composed of a substantially cylindrical case extended by at least two pins, such igniter being characterised in that the cartridge is placed inside a casing formed by joining at least two shells, such casing delimiting an inner volume accommodating the cartridge. 
     The casing is advantageously formed by joining an upper shell, delimiting the inner volume accommodating the cartridge, and a lower shell, provided with holes to allow the cartridge pins to protrude. 
     The casing can be formed by joining an upper shell, delimiting the inner volume accommodating the cartridge, and a lower shell provided with holes allowing two electrodes to protrude, both pins and electrodes being connected to a circuit placed in the inner volume. 
     The circuit can be carried by a card arranged perpendicularly to the pins and electrodes. 
     The card can be housed in a bore arranged in the lower shell. 
     The circuit can alternatively be housed in the upper shell. 
     Advantageously, the circuit will incorporate at least one component providing protection with respect to electromagnetic and/or electrostatic discharges. 
     The circuit can incorporate at least one component to decode a firing signal. 
     According to one characteristic of the invention, the casing has an outer profile enabling it to be attached to a gas generator or pyromechanism. 
     The shells can be joined together by ultrasonic sealing. 
     A further subject of the invention is an assembly process for a pyrotechnic igniter that allows it to be adapted to a specific gas generator or pyromechanism, such process being characterised in that it comprises the following steps: 
     firstly, an igniter cartridge is manufactured incorporating a substantially cylindrical case extended by at least two pins, 
     secondly, at least one casing is manufactured that has an outer profile allowing it to be adapted to a specific gas generator or pyromechanism, such casing incorporating an upper shell and a lower shell, said shells delimiting an inner volume to accommodate the cartridge, 
     the cartridge is placed in the casing, 
     the two shells forming the casing are joined together. 
     According to one variant of the process, at least two casings can be manufactured, each having a different outer profile, and each incorporating an upper and lower shell, said shells delimiting an inner volume to accommodate the cartridge. In this case, a casing will be chosen that has an outer profile corresponding to the gas generator or pyromechanism to which the igniter is to be adapted. The cartridge will then be placed inside the casing thus selected, and finally the two casing shells will be joined together. 
     The igniter cartridge can be attached to an electronic circuit before being housed in the selected casing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood after reading the following description of the different embodiments, such description being made with reference to the appended drawings, in which: 
     FIG. 1 shows a section view of an igniter according to the invention, 
     FIGS. 2 a  and  2   b  show the lower shell on its own, FIG. 2 a  being a section of FIG. 2 b  along plane AA, 
     FIG. 3 is a section view of the upper shell on its own, 
     FIG. 4 is a schematic showing the different steps of an assembly process according to the invention, 
     FIGS. 5 and 6 show an igniter according to a second embodiment, FIG. 5 being a partial longitudinal section along plane CC in FIG.  6  and said FIG. 6 being a transversal section of FIG. 5 along plane BB, 
     FIG. 7 shows a partial longitudinal section of an igniter according to a third embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a pyrotechnic igniter  1  according to the invention comprises an inner cartridge  2  placed inside a casing  3 . 
     The cartridge  2  comprises a metallic case  4  (for example, made of stainless steel) that is substantially cylindrical and encloses a pyrotechnic igniting composition  5 , for example a composition combining Boron and Potassium Nitrate in proportions in mass of 70% Boron to 30% Potassium Nitrate. 
     Case  4  is closed by a base that is also metallic and carries two electrodes  7   a ,  7   b . Electrode  7   a  is welded to the base  6  and electrode  7   b  passes through the base  6  and is electrically insulated from it by an insulating sleeve  8  (for example, made of glass). 
     A semi-conductive card  9  is formed by a non-doped silicon-based insulating substrate  10  bonded to the base  6 . This card incorporates a semi-conductive bridge  11 , for example of doped silicon, that is partially covered by two conductive pads  12   a ,  12   b , for example of aluminium. 
     The pads are spaced at between 60 and 100 micrometers and preferably around 80 micrometers. Pad  12   b  is connected to electrode  7   b  by a connecting wire  13  attached by welding. Pad  12   a  is connected to electrode  7   a  by means of the metallic base  6  through a semi-conductive well  14  (doped silicon) that passes through the insulating substrate  10 . 
     Such as structure is described in detail in patent FR2720493. 
     A igniting pyrotechnic composition  15  covers the card  9  as well as the wire  13  and the welds. This composition is formed, for example, by combining Zirconium and Potassium Perchlorate in classical proportions of 60% in mass of Zirconium to 40% in mass of Perchlorate. It is ignited by the semi-conductive bridge and is intended to ignite the igniting composition  5 . 
     The average particle size of the composition  15  will be selected of the same magnitude as that of the dimensions of the semi-conductive bridge. Such an arrangement ensures heat transfer by convection and/or projection during the temperature build-up of the semi-conductive bridge. Reliable ignition of the pyrotechnic composition is thus ensured by the semi-conductive bridge. 
     The dimensions of the bridge are generally speaking of around 50 micrometers×150 micrometers. Therefore, a particle size will be adopted for the composition that is less than or equal to 50 micrometers. 
     The base  6  is joined to the case  4 , for example by laser welding. 
     The cartridge is loaded in the following manner: 
     First of all, the case  4  is loaded with the igniting composition  5  then with its priming composition  15 , the two compositions being lightly compressed. 
     The base  6  carrying the semi-conductive card  9  is then put into place. 
     The base  6  is then welded to the case  4 . 
     The cartridge  2  is placed inside a casing  3  that is formed by joining two shells: a lower shell  16  and an upper shell  17 . 
     The two shells are made by injection of a plastic material, for example of the polyamide or polycarbonate type. They are joined together, for example by ultrasonic sealing. 
     They delimit an inner volume accommodating the cartridge  2 . The lower shell  16  is shown on its own in FIGS. 2 a  and  2   b . It incorporates a bearing face  18  into which a circular groove  19  has been made and it is pierced by two holes  20 ,  21  connected by a notch  22 . 
     Holes  20  and  21  are intended to allows pins  7   a  and  7   b  to pass through the cartridge  2 . The notch  22  accommodates the curve of pins  7   a  and  7   b.    
     The external profile of the lower shell  16  comprises two cylindrical surfaces  23 ,  24  connected to one another by a conical surface  25 . 
     The upper shell  17  is shown on its own in FIG.  3 . It incorporates a bearing face  26  that carries circular toothing  27  intended to co-operate with the groove  19  in the lower shell  16 . 
     The upper shell  17  delimits an inner cylindrical volume  28  that is intended to accommodate the cartridge  2 . This volume is thus of the same diameter as the external diameter of the cartridge  2  and is of a height equal to that of the cartridge. 
     The external profile of the upper shell  17  comprises two cylindrical surfaces  29 ,  30  connected to one another by a conical surface  31 . 
     The thickness e of the upper shell is of around 0.5 mm. This shell is thin enough to be able to be fractured by the composition  5  igniting. 
     The igniter according to the invention is assembled as follows. 
     After the cartridge  2  has been made following the above-described steps, the cartridge is positioned on the lower shell  16  with pins  7   a  and  7   b  penetrating into holes  20  and  21 . The base  6  of the cartridge  2  is thus pressing against the bearing face  18 . The cartridge  2  is then topped by the upper shell  17 . The two shells are then joined together by ultrasonic sealing. 
     Ultrasonic sealing is carried out in a known manner by applying the sonotrode of the welding apparatus to the conical profile  31  of the upper shell. 
     The vibrations produced by the sonotrode seal the contacting surfaces. The circular toothing  27  penetrates into the groove  19  during assembly allowing the stresses caused by the ultrasonic vibrations to be concentrated during the sealing operations. Heating at the contact point between groove and toothing is thus focussed enabling sealing to take place. 
     The two shells may naturally also be joined together by other means, for example by bonding. 
     After the two shells have been joined together, the assembly of the external profiles  25 ,  24 ,  30  and  31  for a rib that enables the igniter to be fastened onto a gas generator or pyromechanism (not shown). 
     Sealing of the igniter with respect to the exterior is ensured by laser sealing the cartridge  2 . There are no particular precautions to be taken with respect to sealing during the assembly of the casing  3 . Manufacturing is thus simplified. 
     The sealing of the igniter with respect to the pyromechanism and relative to the gases generated during operation is ensured by the base  6  pressing on the bearing face  18 , such pressure having the additional effect of applying the base and the case  4  firmly onto the bearing face making the cartridge  2  penetrate slightly into the lower shell  16  thereby increasing gas tightness. 
     To further improve gas tightness, the external cylindrical surface of the case  4  will advantageously be given a tight fit in its housing in the upper shell  17 , and pins  7   a  and  7   b  will also be tightly fitted into holes  20  and  21 . 
     Additionally, one or several circular grooves (not shown) can be provided on bearing face  18 , such grooves acting as baffles for the expanding gases or each accommodating an O-ring. 
     Different variants are possible without departing from the scope of the invention. It is thus possible for the semi-conductive card  9  to be replaced by other means of ignition known to the Expert, for example a hot wire or else an exploding wire or a resistive element arranged on a support according to the printed circuit technique. 
     It is also possible for the cartridge case to be made of a non metallic material. 
     The notch  22  can also possibly be replaced by a larger housing that could accommodate a filtering electronic component, for example a capacitor welded in parallel onto pins  7   a  and  7   b . Such an arrangement ensures protection with respect to static electrical discharges (refer notably to U.S. Pat. No. 5,099,762). 
     Lastly, it is possible for igniters to be manufactured that have more than two pins. 
     One of the advantages of the igniter according to the invention lies in its simple structure allowing it to be manufactured at a reduced cost for a given external geometry. 
     The cartridges  2  enclosing the pyrotechnics can be produced in one place and the plastic casings elsewhere. It is therefore no longer necessary to carry out a duplicate moulding operation on a pyrotechnically active cartridge. 
     A further advantage of the igniter according to the invention lies in that igniters having different external diameters can be manufactured at a reduced cost. 
     FIG. 4 shows the different stages in the assembly process for a pyrotechnic igniter according to the invention. 
     Industrially speaking, it is necessary for pyrotechnic igniters to be manufactured that have different external shapes and/or dimensions according to the assembly interfaces of the safety systems to which they are to be adapted (gas generators for air bags, safety belt tighteners, embrittlement devices for example for car pedals, divers pyromechanisms . . . ). 
     By way of a non limiting example three different forms of igniters are shown here as  1   a ,  1   b  and  1   c.    
     Following the process according to the invention these three forms are easily manufactured at a reduced cost since they all incorporate the same cartridge  2  enclosing the pyrotechnic compositions as well as the igniting means (semi-conductive card or hot wire). 
     Thus, to make a pyrotechnic igniter that can be adapted to a specific gas generator, an igniter cartridge  2  is manufactured first of all that incorporates a substantially cylindrical case extended by two pins (step A). 
     The shells enabling the required casing to be defined for the final igniter are manufactured separately. 
     Thus, for step B three separate sets of shells ( 16   a / 17   a ,  16   b / 17   b ,  16   c / 17   c ) are presented. 
     The shells all have in common the fact that they delimit an identical inner volume  28   a ,  28   b ,  28   c  enabling them to accommodate the same cartridge  2 . 
     Thus, the cartridge can be indifferently adapted to any of casings  3   a ,  3   b  or  3   c.    
     According to the client&#39;s needs, a casing will be chosen that has an external profile matching the gas generator or pyromechanism onto which it must be adapted. 
     The cartridge is placed in the selected casing and the two shells of the casing are joined together to make the required igniter  1   a ,  1   b  or  1   c.    
     It is thus possible for large quantities of identical pyrotechnic cartridges  2  to be made. 
     It is only during assembly that the igniter will be customised by adapting it to a casing  3  having a specific external geometry required by a client. 
     The most expensive part of the igniter (the pyrotechnic system) can thus be manufactured in large quantities since it is only afterwards that it will be integrated into a specific casing adapted to the client&#39;s requirements. 
     Production costs for pyrotechnic igniters are thus substantially reduced. 
     FIGS. 5 and 6 show another embodiment of an igniter according to the invention. 
     This igniter  1  also comprises a cartridge  2  enclosing the pyrotechnic composition or compositions, such cartridge being placed inside a casing  3  formed by joining two shells  16  and  17  together. 
     For the sake of clarity, the cartridge  2  is not shown on the figure as a section view. 
     This igniter differs from that shown in FIG. 1 in that the pins  7   a  and  7   b  of the cartridge do not protrude from the casing  3 . These pins are welded to an electronic circuit that, in this case, is located on printed circuit card  32  of a circular shape housed in a matching bore  33  that is a countersink arranged in the lower shell  16 . 
     The lower shell  16  is provided with holes that allow two connecting electrodes  34   a ,  34   b  to pass through. These electrodes are also welded to the printed circuit  32 . 
     The printed circuit card  32  allows the pins  7   a  and  7   b  to be electrically connected to the connecting electrodes  34   a ,  34   b . It can advantageously be fitted with the electronic components that ensure various functions: protection with respect to electromagnetic waves and/or electrostatic discharges, introduction of a logic function notably the coding of the igniter and means allowing this code to be compared with a coded signal to trigger firing (decoding), electrical power storage capacitor. 
     By way of a non limiting example, FIGS. 5 and 6 show printed circuit card  32  having thereon a capacitor  35  mounted in parallel between pins  7   a  and  7   b  and ensuring protection against electrostatic discharges (a value of between 0.1 and 10 microfarads will be selected for the capacitor capacitance). 
     The circuit shown also incorporates an integrated circuit  36  that receives signals coming from the two electrodes  34   a ,  34   b  that only supply pin  7   b  if a code corresponding to that of the igniter is transmitted by the connecting electrodes. 
     Such a logic circuit is well know to the Expert and will not be described here in any further detail. 
     The active or passive circuits will preferably be surface mounted components in order to limit the space taken up by the circuit and to reduce assembly costs. 
     A radio frequency filter can also be arranged onto the circuit  32  that notably combines resistors, inductive resistors and capacitors. 
     This igniter is assembled in accordance with the previously described assembly process. 
     The cartridge  2  is made separately from the casing  3  formed of two shells  16  and  17 . The shells are defined such that they have an inner volume able to accommodate the casing  2  as well as the printed circuit  32 . The external profile of the shells will be adapted according to the specific needs of the client. 
     In the specific case of this variant, the printed circuit  32  carrying the two electrodes  34   a ,  34   b  is also made. 
     The cartridge  2  and circuit  32  carrying the electrodes  34   a ,  34   b  are welded together, then the assembly thus formed is placed into the casing  3 . 
     Lastly, the two shells  16  and  17  are joined together, for example by ultrasonic sealing. 
     As in the previous example, sealing means such as O-rings can be placed between the circuit  32  and the lower shell. 
     FIG. 7 shows another embodiment of an igniter according to the invention. 
     This embodiment also incorporates a cartridge  2  (not shown in detail) that encloses the pyrotechnic composition or compositions and is placed inside a casing  3  formed by joining together two shells  16  and  17 . 
     Pins  7   a  and  7   b  of the cartridge do not protrude from the casing  3 , but are attached to an electronic circuit  37  shown here in a cylindrical form and housed in the inner volume  28  of the upper shell  17 . 
     This circuit is not shown in detail and can be defined such as to ensure the previously described functions (electrical protection, decoding of a firing signal, electrical energy supply . . . ). 
     The lower shell  16  is provided with holes allowing the two electrodes  34   a ,  34   b , which are also connected to the electronic circuit  37 , to protrude. 
     According to the axial space taken up by the electronic circuit  37  it is possible thanks to the assembly process according to the invention for an igniter to be made at a very reduced cost. In fact, the cartridge  2  is identical to that shown in reference to FIG.  5 . It is therefore enough to manufacture an upper shell  17  having a suitable shape to accommodate both the cartridge  2  and the circuit  37 . 
     By way of a variant, such a cylindrical circuit can be replaced by a plane printed circuit arranged in parallel to pins  7   a  and  7   b.