Abstract:
A gas bag module for a vehicle occupant restraint device has a gas bag and a discharge control arrangement to expose and/or change a discharge opening through which gas can escape from the gas bag. The discharge control arrangement has at least one element made from a memory metal.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to a gas bag module for a vehicle occupant restraint device.  
       BACKGROUND OF THE INVENTION  
       [0002]     A gas bag module having a gas bag and a discharge control arrangement to open a discharge opening through which gas can escape from the gas bag is known for example from WO-A-2004/045919. The discharge region can be opened in the gas bag wall when a reduction in the gas bag internal pressure is required. A pyrotechnic charge in the form of a fuse is arranged directly on the discharge region such that the discharge region burns through or is torn open mechanically after the fuse has been ignited.  
         [0003]     In the gas bag module shown in WO-A-03/097407 a pyrotechnic pin is provided in order to expose a tube-shaped discharge region of the gas bag.  
         [0004]     In EP-A-1 279 574 a gas bag module is shown in which a slider is moved to open discharge openings in a holding piece, such that bores formed therein come in alignment with the discharge openings. The hot gas which is flowing into the gas bag melts the region of the gas bag which is situated between the bores of the slider and the discharge openings in the holding piece such that a portion of the gas emerges from the gas bag during filling.  
         [0005]     From U.S. Pat. No. 6,547,274 B a gas bag module is known in which the opening cross-section of a discharge opening in a carrier plate can be opened by piezoelectrically controlled flaps. The current supply of the piezoelectric elements is controlled, for example, depending on the posture or the physique of the vehicle occupant or on the speed of the vehicle.  
         [0006]     The invention provides a gas bag module which makes it possible to control the discharge behaviour in a manner which is not dangerous for the vehicle occupant, without enlarging the structural space.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     According to the invention, a gas bag module for a vehicle occupant restraint device has a gas bag and a discharge control arrangement to open and/or change a discharge opening through which gas can escape from the gas bag. The discharge control arrangement has at least one element made from a memory metal. Special alloys having shape memory characteristics are designated as memory metals, which include, amongst others, nickel-titanium, copper-zinc-aluminium, gold-cadmium or iron-manganese-silicon alloys. An element formed from such an alloy can be bent into any desired shape at low temperatures, but returns again into its previously impressed initial shape on heating above a so-called transformation temperature. Relatively great forces are produced thereby, i.e. upon the return deformation, an appreciable resistance can also be overcome. The heating which is necessary for the return deformation can also be brought about by an electric current flowing through the element. The invention makes use of the memory effect in order to systematically influence the discharge behaviour of a gas bag module. More precisely, the element made from memory metal according to the invention is used to control the effective cross-section of the discharge opening, i.e. the element provides for the production of a discharge opening and/or provides for a change to the discharge cross-section. Large and costly opening mechanisms can be dispensed with due to the use of such an element. The discharge control arrangement according to the invention in addition has the advantage that neither explosive substances or the like nor a melting of gas bag fabric are necessary for opening a discharge opening, i.e. a separation of particles is ruled out.  
         [0008]     Basically, the discharge opening, which is controlled by means of the discharge control arrangement according to the invention, can be provided in a fixed component of the gas bag module or in the gas bag.  
         [0009]     In the latter case, provision is made in a preferred embodiment that the gas bag comprises at least one wire made from the memory metal.  
         [0010]     Preferably, the wire is integrated into the gas bag wall, in particular woven into the fabric of the gas bag wall. Upon heating of the wire, a deformation of the fabric of the gas bag wall then takes place, whereby a discharge region can be produced, enlarged or reduced in the gas bag wall.  
         [0011]     Alternatively, the wire may also be part of a covering which is arranged over a discharge opening formed in the gas bag wall.  
         [0012]     A further preferred embodiment makes provision that the element made from the memory metal is arranged on a tube-shaped discharge region of the gas bag.  
         [0013]     A constriction of the tube-shaped discharge region to inhibit the emergence of gas or an expansion of a constriction to assist the emergence of gas can be achieved in that the element made from the memory metal is designed such that it is transformed, by heating, either from a less markedly curved shape into a more markedly curved shape, or, vice versa, from a more markedly curved shape into a less markedly curved shape.  
         [0014]     For the systematic deformation of a discharge region, the element made from the memory metal can also be designed such that it is transformed, by heating, from an elongated shape into a bent shape, or, vice versa, from a bent shape into an elongated shape.  
         [0015]     When the element made from the memory metal is arranged on a swivellable flap, a hinge mechanism can be produced in this way to control the flap.  
         [0016]     It is basically also possible to form a discharge region of the gas bag substantially entirely from a structure made from the memory metal.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIGS. 1   a,    1   b  diagrammatically show a first embodiment of the invention;  
         [0018]      FIGS. 2   a,    2   b  diagrammatically show a second embodiment of the invention;  
         [0019]      FIGS. 3   a,    3   b  diagrammatically show a third embodiment of the invention; and  
         [0020]      FIGS. 4   a,    4   b  diagrammatically show a fourth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]      FIG. 1 a  illustrates a spread-out piece of fabric  10  of a gas bag for a gas bag module of a vehicle occupant restraint device according to a first embodiment of the invention. Metal wires  12  made from a memory metal, which have the shape memory characteristics described above, are woven into the fabric piece  10 . In the state below the transformation temperature specific to the material, shown in  FIG. 1 , the fabric piece  10  is slightly gas-permeable or gas-tight and, owing to the deformability of the metal wires  12 , it is only slightly more rigid than a comparable fabric without metal wires  12  woven in.  
         [0022]     After heating the metal wires  12  above the transformation temperature, which can be brought about, for example, by a systematic flow of current through the metal wires  12 , the metal wires  12  assume their previously impressed initial shape, shown in  FIG. 1   b.  This deformation of the metal wires  12  in the fabric piece  10  leads to a region in the fabric piece  10  being created or enlarged, through which (more) gas can flow off.  
         [0023]     The fabric piece  10  may either be woven directly into the gas bag wall, or may be a covering which is arranged over a discharge opening which is formed in the gas bag wall. It is basically also possible to produce the entire discharge region from the memory metal wire  12 .  
         [0024]     In the second embodiment of the invention, shown in  FIGS. 2   a  and  2   b,  a tube-shaped discharge region  14  is provided in the gas bag. In the tube  14 , in accordance with  FIG. 2   a,  several wires  12  made from a memory metal are sewn or woven, encircling radially, into the casing  16  of the tube  14 , with the curvature of the metal wires  12  matching the adjoining regions of the tube  14 .  
         [0025]     After the metal wires  12  are heated, they assume a shape which is more markedly curved than the adjoining regions of the tube  14 , as shown in  FIG. 2   b.  Thereby, a constricted region  18  is formed, which delimits the quantity of gas being discharged per unit of time with respect to the state shown in  FIG. 2   a.    
         [0026]     A third embodiment of the invention is shown in  FIGS. 3   a  and  3   b,  in which likewise a tube-shaped discharge region  14  is provided. Here, however, the wires  12  which are made from the memory metal are sewn in or woven in in axial direction (see  FIG. 3   a ) and assume a bent shape after heating (see  FIG. 3   b ). The bending of the wires  12  brings about the formation of a fold in the tube  14 , which inhibits the exit of gas.  
         [0027]     The principle of forming a bend is also utilized in the embodiment shown in  FIGS. 4   a  and  4   b.  The metal wires  12 , which are elongated in the non-heated state, extend from a marginal region  20  to a swivellably arranged flap  22 , which closes a discharge opening  24 . After heating, the wires  12  bend in the transition region and swivel the flap  22  into an open position in which the discharge opening  24  is opened. The wires  12  made from the memory metal therefore serve as a hinge which automatically carries out a swivelling of the flap  22  through the supply of heat. The flap  22  can be provided in a suitable region of the gas bag; it is, however, also possible to construct the flap  22  from solid materal and to use it to open a discharge opening  24  which is formed in a fixed component of the gas bag module.  
         [0028]     In all the embodiments, a “reverse” construction is also conceivable, i.e. the state shown in  FIGS. 1   a,    2   a,    3   a  or  4   a  is achieved after heating from the state shown in  FIGS. 1   b,    2   b,    3   b  or  4   b,  through corresponding deformation of the metal wires  12 .  
         [0029]     A utilization of the so-called two-way effect, in which the change in shape of an element made from memory metal, brought about by heating, forms back again through cooling, additionally permits further possibilities of control for the discharge control arrangement according to the invention. Thus, for example, the associated discharge opening can be (partially) opened or closed several times by a chronologically controlled flow of current through the wires  12  which are made from the memory metal.