Abstract:
An airbag module for a vehicle occupant restraint system includes an airbag ( 22 ) and a gas generator ( 10 ) that is connected via at least one first outflow opening ( 18 ) to an airbag chamber ( 28 ), the airbag chamber ( 28 ) being at least partially formed by the interior of the airbag ( 22 ), an actuator unit ( 36 ) being provided on the gas generator ( 10 ) and, when the actuator unit ( 36 ) is activated, it releases a traction element ( 50 ) that causes a pressure reduction in the airbag ( 22 ), an activation of the actuator unit ( 36 ) also leading to an opening of a second outflow opening ( 20 ) in the gas generator ( 10 ), which vents generator gas to an environment without this vented gas flowing into the airbag chamber ( 28 ). 
     Moreover, the invention relates to a method of restraining a vehicle occupant with such an airbag module.

Description:
TECHNICAL FIELD 
   The present invention relates to an airbag module for a vehicle occupant restraint system, comprising an airbag and a gas generator that is connected via at least one first outflow opening to an airbag chamber that is at least partially formed by the interior of the airbag. 
   Moreover, the invention relates to a method of restraining a vehicle occupant with such an airbag module. 
   BACKGROUND OF THE INVENTION 
   In the early days of the development of vehicle occupant restraint systems using airbags, the main focus was initially to recognize a restraint situation reliably and quickly as well as to cause the airbag to be filled rapidly in order to protect a vehicle occupant. Starting from those basic requirements, the demands made of modern vehicle occupant restraint systems have increased dramatically since that time. Additional requirements that have come to the fore are, for example, that the response of the restraint system be adapted to the restraint position of the vehicle occupant and to the anticipated impact momentum of the vehicle occupant. 
   The state of the art describes numerous attempts to meet these ever-higher demands. Thus, for example, U.S. patent application 2004/0012180 A1 describes a vehicle occupant restraint system that can release an additional airbag volume as a function of the situation and, at the same time, can close an opening in the module housing. The basic idea here is to be able to use an inexpensive, one-stage gas generator that is configured for the optimal filling of the maximum airbag volume. In order to achieve approximately the same airbag hardness for the smaller airbag volume, excess gas is vented through an opening in the module housing. Depending on the embodiment, the opening or the closing of airbag openings can be coupled to the release of the larger airbag volume. 
   The object of the present invention is to create an airbag module with simple means that responds as specifically as possible to changes in individual parameters of a restraint situation such as, for example, the vehicle occupant position or the anticipated impact momentum of the vehicle occupant. 
   BRIEF SUMMARY OF THE INVENTION 
   This is achieved in an airbag module for a vehicle occupant restraint system including an airbag and a gas generator that is connected via at least one first outflow opening to an airbag chamber, the airbag chamber being at least partially formed by the interior of the airbag, an actuator unit being provided on the gas generator and, when the actuator unit is activated, it releases a traction element that causes a pressure reduction in the airbag, an activation of the actuator unit also leading to an opening of a second outflow opening in the gas generator, which vents generator gas to an environment without this vented gas flowing into the airbag chamber. 
   The term airbag chamber is used within the scope of this invention to refer to the space that essentially reaches airbag internal pressure when the airbag is deployed. As a rule, this space comprises the airbag interior and, depending on the attachment site of an airbag orifice, possibly also comprises sections of the housing of a module. 
   The second outflow opening is provided directly in the gas generator and, in general, has a relatively small outflow cross section. Therefore, a gas mass flow with a decisive effect on the airbag deployment and airbag hardness can only be vented at high pressure via the second outflow opening. Corresponding pressures of up to 150 bar (or even more, depending on the design of the gas generator) prevail in the gas generator directly after its activation. In contrast, the traction element that causes a pressure reduction in the airbag is only especially effective once a certain internal pressure has built up in the entire airbag chamber, especially in the interior of the airbag, that is to say, precisely not at the beginning of the deployment phase of the airbag. Consequently, in an inexpensive and advantageous manner, only one actuator unit has to be provided that releases the traction element as well as the second outflow opening although, depending on the point in time of the release, the effect of the traction element compared with the effect of the second outflow opening is negligible and vice versa. Consequently, a single actuator unit can respond to two independent cases such as, for example, the restraint position of the vehicle occupant and the weight of the vehicle occupant, largely independently of each other. 
   In one embodiment, when the actuator unit is activated, the traction element opens at least one airbag opening and/or releases an enlarged airbag volume. Both of these measures are very simple and effective ways to reduce the pressure in the airbag. 
   Preferably, the actuator unit has a pyrotechnical device. Pyrotechnically driven actuators are relatively inexpensive and have a fast response or activation time. 
   Together with the actuator unit, the gas generator can form a cylinder/piston unit, the piston being moved by the activation of the actuator unit, thus opening the second outflow opening. This cylinder/piston unit is a very reliable device and merely has to be slid in order to open the outflow openings. No closure devices have to be destroyed, as a result of which no free membrane fragments or the like are created that could enter the airbag and damage it. 
   In this embodiment, the piston preferably has an opening, the gas vented through the second outflow opening flowing through that opening. The cross section of this opening can serve to define the ratio of the pressures present on both sides of the piston above which ratio the piston moves. Moreover, the opening in the piston, together with the second outflow opening, defines the outflow cross section and thus the mass flow of gas that can be vented through the second outflow opening. 
   In a preferred embodiment, the vented gas exits the airbag module when it flows through the second outflow opening. Due to this direct outflow of the excess gas into the environment, an especially effective and fast pressure relief is achieved inside the airbag chamber. This significant reduction of the internal pressure in the airbag is especially necessary for vehicle occupants who are positioned relatively close to the airbag module. 
   In another embodiment, the gas generator has a separate base section and a distribution section that are securely and preferably directly connected to each other, the actuator unit being attached to the distribution section. This offers the advantage that decisive gas generator components such as the base section, can remain unchanged and only secondary components such as the distribution section have to be modified in order to receive the actuator unit. 
   Moreover, at least one airbag opening can be provided, the ratio of the outflow cross section of all of the second outflow openings to the outflow cross section of all of the airbag openings being between 1:2 and 1:8, preferably between 1:3 and 1:5. At these ratios, the timing of the effects caused by the second outflow opening and by the airbag opening are uncoupled from each other especially well. 
   The invention also provides a method of restraining a vehicle occupant, the method having the following steps: 
   a) activation of a gas generator of a vehicle occupant restraint system in case of a restraint event, whereupon the gas generator feeds gas into an airbag chamber via at least one first outflow opening; 
   b) checking the vehicle occupant position on the basis of sensor data at a prescribed first point in time; 
   c) activation of an actuator unit if the vehicle occupant is in a position that is unsuitable for a restraint, as a result of which a second outflow opening as well as a traction element are released and a pressure reduction takes place decisively as a result of venting gas into the environment through the second outflow opening; 
   d) assessment of an impact momentum of the vehicle occupant against the airbag based on sensor data at a prescribed second point in time; 
   e) activation of the actuator unit if the actuator unit was not yet activated in Step c) and if the anticipated impact momentum of the vehicle occupant lies below a predefined limit value, as a result of which the second outflow opening and the traction element are released and a pressure reduction in the airbag takes place decisively as a result of releasing the traction element. This method offers the advantage that only one actuator unit is needed to be able to respond to two different restraint parameters largely independently of each other. A significant aspect here is the point in time of the activation of the actuator unit, the configuration of the traction element and of the second outflow opening as well as the way in which they are coordinated with each other. 
   It is especially advantageous for the restraint of the vehicle occupant for the first point in time to be between 0 ms and 15 ms and for the second point in time to be between 25 ms and 40 ms after the restraint case has been recognized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a longitudinal section through the gas generator of an airbag module according to the invention, the actuator unit not having been activated; 
       FIG. 2  shows a longitudinal section through the gas generator of  FIG. 1 , the actuator unit having been activated; 
       FIG. 3  shows the schematic representation of an airbag module according to the invention in a first embodiment; 
       FIG. 4  shows the schematic representation of an airbag module according to the invention in a second embodiment; and 
       FIG. 5  shows a flowchart relating to a preferred method variant according to the invention for restraining a vehicle occupant. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows an airbag module having a gas generator  10  consisting essentially of a base section  12  and a distribution section  14 , the base section  12  being a pressure chamber section and/or a combustion chamber section. In the embodiment depicted, which is a hybrid gas generator, the base section  12  is closed by a membrane  15 . It is immaterial, however, whether the generator gas is already present in the form of compressed gas, whether it is generated as combustion gas or whether it exits from the base section  12  as mixed gas. The only important aspect is that the base section  12  has to have an activation means  16  and has to be able to establish a flow connection with the distribution section  14  in order to feed generator gas into the distribution section  14 . In the example shown, this is done by destroying the membrane  15  in response to the activation of the activation means  16 . It is especially preferred for all of the generator gas to be fed into the distribution section  14  and to be distributed there. 
   In the embodiment according to  FIG. 1 , the distribution section  14  is placed as a separate part onto the base section  12 . The two sections  12 ,  14 , however, are securely and permanently connected directly to each other, for example, by means of welding, screwing or press forming so that they form a preassembled unit. In other embodiments, the distribution section  14  is formed integrally with the base section  12 . In the present example, the gas generator  10  is configured as a tubular gas generator, the base section  12  and the distribution section  14  having a shared axis A. 
   A circumferential wall  17  of the distribution section  14  has first outflow openings  18  and second outflow openings  20  in a radial direction, the first outflow openings  18  being situated axially closer to the base section  12  than the second outflow openings  20 . The first and second outflow openings  18 ,  20  are preferably distributed along the circumference of the distribution section  14  in such a way that the generator gas is dissipated in a shear-neutral manner when it flows through the first and/or second outflow openings  18 ,  20 . 
   An airbag  22  is attached by its airbag orifice  24  to the circumferential wall  17  of the distribution section  14  in the axial direction between the first outflow openings  18  and the second outflow openings  20  so that generator gas that is flowing through the first outflow openings  18  is released into an airbag chamber  28  that starts outside of the gas generator  10 , and generator gas that is flowing through the second outflow openings  20  is dissipated into the environment outside of the airbag chamber  28 . 
   At one axial end of the gas generator  10 , the distribution section  14  has a face wall  30  with an axial projection  34  facing outwards, a centered opening  32  being provided in the face wall  30  and in the projection  34 . An actuator unit  36 , including a piston  38  running inside the distribution section  14  and a pyrotechnical device  40 , extends through the opening  32 . The pyrotechnical device  40  is, for example, an igniter or a detonator. The pyrotechnical device  40  extends from outside of the gas generator  10  into the opening  32  and is securely and tightly connected to the axial projection  34 , for example, welded. The axially movable piston  38  has a circumferential piston wall  42  that makes a transition into a base plate  44  having an axial piston projection  46 . The piston projection  46  likewise extends into the opening  32  so that it is adjacent to the pyrotechnical device  40 , forming a virtually gas-tight pressure chamber  47  with the pyrotechnical device  40 . Moreover, a hook-shaped holder  48  is formed integrally with the piston projection  46 , the holder  48  extending outwards through the opening  32  and through the pyrotechnical device  40 . 
   In an initial position according to  FIG. 1 , the second outflow openings  20  are closed by the piston  38 , or to put it more precisely, by the piston wall  42 . The base plate  44  of the piston  38  has openings  52  and, in the initial position, the base plate  44  lies against the face wall  30  of the distribution section  14  so that the openings  52  are likewise closed. 
   Outside of the gas generator  10 , the hook-shaped holder  48  engages with the pyrotechnical device  40 , thereby affixing a traction element  50 . The traction element  50  is preferably a cable or a fabric strip so that it can easily be affixed to the holder  48  by means of a loop or a recess. 
     FIG. 2  shows the section according to  FIG. 1 , but now after an activation of the actuator unit  36 . As a result of this activation, such a high pressure builds up in the pressure chamber  47  that the piston  38  is moved in the direction of the base section  12 . Due to this movement, the openings  52  in the base plate  44  move away from the face wall  30 . Furthermore, the piston wall  42  slides along the circumferential wall  26  of the distribution section  14  and releases the second outflow openings  20 . In this activation position, generator gas can flow through the first outflow openings  18  into the airbag chamber  28  as well as through the openings  52  and through the second outflow openings  20  to outside of the airbag chamber  28 . 
   As a rule, the actuator unit  36  is activated after the activation of the gas generator  10  so that a certain pressure already prevails in the distribution section  14 . The actuator unit  36  has to be configured in such a way that it can move the piston  38  against this pressure. Here, the requisite force can be influenced by the size of the openings  52 . Before the piston wall  42  reaches the first outflow openings  18 , the circumferential wall  26  of the distribution section  14  tapers slightly so that the movement of the piston  38  is stopped. Before the piston  38  reaches the tapered section, the axial piston projection  46  emerges from the opening  32  of the face wall  30  so that an equalization takes place between the pressure in the distribution section  14  and the pressure in the pressure chamber  47 . In order to prevent the piston  38  from being forced back in the direction of the pyrotechnical device  40  due to the outflowing generator gas after the piston  38  has moved in the direction of the base section  12 , a stop has to be provided so that the second outflow openings  20  continuously remain open. 
   For example, the piston projection  46  can be slightly pre-tensioned outwards relative to the axial projection  34  of the face wall  30  in the radial direction so that it widens slightly and latches outwards after emerging from the opening  32 . Due to this widening, the piston projection  46  can no longer move back into the opening  32  of the face wall  30  but rather strikes one edge of the opening. As an alternative, a spring-loaded pin  53  (indicated by means of a broken line) can also be provided in the face wall  30 . When the piston  38  moves, this pin  53  slides on the piston projection  46  until the latter emerges from the opening  32  and the pin  53  then snaps in the direction toward the axis A. The pin  53  then constitutes a stop for the piston  38  and prevents the second outflow openings  20  from closing again. 
   The hook-shaped holder  48  also moves when the piston  38  moves from the initial position according to  FIG. 1  into the activation position according to  FIG. 2 . The pyrotechnical device  40  and the holder  48  are no longer engaged, as a result of which the traction element  50  is released (see  FIG. 2 ). 
     FIGS. 3 and 4  are schematic depictions of examples of possible variants of the traction element. 
     FIG. 3  shows the airbag module in its initial position, the traction element  50  preferably being a wide fabric strip that covers an airbag opening  54 , that is to say, closes it. One end of the traction element  50  is permanently attached, preferably sewn, to the airbag  22  on the outside. 
   When the actuator unit  36  is activated and the piston  38  subsequently moves, an opposite end of the traction element  50  and thus the airbag opening  54  are released in order to reduce the pressure in the airbag  22 . The airbag opening  54  is provided in the movable part of the airbag  22 , that is to say, outside of a module housing (not shown here). Consequently, it only achieves its full effect once the airbag  22  is already in an advanced stage of its deployment. 
   A second variant of the pressure reduction in the airbag  22  is shown in  FIG. 4 . Once again, the airbag module is shown in its initial position, in this case, the airbag  22  being prevented from deploying completely by the traction element  50 . Here, the traction element  50  consists of two traction cables or traction strips, one respective end of which is attached to an airbag wall facing the vehicle occupant. The respective opposite ends of the two traction cables or traction strips are affixed onto the gas generator  10  by means of the holder  48 . 
   After the activation of the actuator unit  36  and the resultant release of the traction element  50 , the airbag  22  can occupy a larger volume, as a result of which the internal pressure in the airbag chamber  28  is reduced, which makes the airbag  22  softer. 
   In other embodiments, the traction means variants according to  FIGS. 3 and 4  are combined. 
     FIG. 5  shows the sequence of a preferred method variant for the restraint of a vehicle occupant. 
   First of all, at a point in time  0 , a restraint case is detected and the gas generator  10  is activated. As a rule, one or more suitable sensors are provided on or in the vehicle in order to detect the restraint case. At this point in time, the second outflow openings  20  are closed and the traction element  50  has not been released. This corresponds to the situation shown in  FIG. 1 . 
   After 0 to 15 ms, a first sensor detection determines the position of the vehicle occupant. If the sensor system ascertains an unsatisfactory restraint position of the vehicle occupant or if such a position is stored (e.g. if the vehicle occupant is monitored before the collision), then the actuator unit  36  is activated. This means that the second outflow openings  20  as well as the traction element  50  (secondary in terms of its effect) are released. At such an early point in time, the airbag  22  is hardly or not at all unfolded, although a high pressure is already present in the gas generator  10 . This is why the venting of the gas through the second outflow openings  20  in the gas generator  10  is decisive for the inflation behavior. Even at relatively small cross sections (diameter &lt;5 mm), a gas mass flow of 30% to 50% of the total generator gas that is present can be branched off through the second outflow openings  20 . Before this backdrop, the further pressure reduction that occurs after a certain unfolding due to the release of the traction element  50  is negligible and possibly even desirable. 
   If the vehicle occupant is in a good restraint position, then the actuator unit  36  does not respond at first and a second sensor detection is carried out after 25 to 40 ms. During this sensor detection, an anticipated impact momentum of the vehicle occupant onto the airbag is compared to a predefined, empirically determined limit value. The anticipated impact momentum is determined on the basis of the decisive sensor data such as the weight of the vehicle occupant, sitting position and/or deceleration values (as indicators of the severity of the collision). Here, of course, it is also possible that the data for determining the impact momentum or even the impact momentum itself is already present or was determined ahead of time. 
   If the anticipated impact momentum lies above the predefined limit value, which is often the case especially with excessively heavy vehicle occupants, then the actuator unit  36  does not respond and the airbag  22  reaches its maximum restraint capability. This is also the case in the embodiments in which the airbag  22  then does not reach its maximum restraint volume ( FIG. 4 ), since the airbag is very hard. 
   If the anticipated impact momentum lies below the predefined limit value, then the actuator unit  36  is activated. This means that the traction element  50  as well as the second outflow openings  20  (secondary in terms of their effect) are released. At this relatively late point in time, the airbag  22  is already largely unfolded. The pressure in the gas generator  10  and in the airbag chamber  28  has already equalized and is relatively low (approximately 0.5 bar above atmospheric pressure). Therefore, in this case, no appreciable pressure reduction due to the small second outflow openings  20  in the gas generator  10  is to be expected. At this point in time, the traction element is decisive, either releasing an airbag opening and/or an enlarged airbag volume. 
   In the embodiment with an airbag opening  54 , the ratio of the outflow cross section of all of the second outflow openings  20  to the outflow cross section of all of the airbag openings  54  lies between 1:2 and 1:8, preferably between 1:3 and 1:5. Hence, up to the time of a vehicle occupant impact, a gas mass flow in the order of magnitude of about 10% of the total generator gas can be dissipated. This increases especially the restraint comfort for lightweight vehicle occupants or at low vehicle speeds. An equivalent effect can be provided by the variant in which an additional airbag volume is made available by releasing the traction element  50 . This additional airbag volume likewise corresponds to about 10% of the original airbag volume.