Abstract:
In a pyrotechnical drive ( 110 ) for a vehicle-occupant restraint system, has a cylinder ( 112 ), a piston ( 114 ) that is movably arranged in the cylinder, a pyrotechnical propellant charge ( 152 ) for generating pressurized gas, and an ignition charge for igniting the propellant charge. The piston has a thrust surface area ( 142 ) which can be acted upon with the pressurized gas in order to drive the piston ( 114 ). Between the ignition charge and the thrust surface area ( 142 ), a flow connection (F) is provided. In addition, a belt retractor ( 100 ) is described having a pyrotechnical drive of this type and a method for operating a pyrotechnical drive ( 110 ) of this type. In the method, the piston ( 114 ) is propelled over a first driving distance (Z 1 ) by the gases generated by the combustion of the ignition charge ( 154 ) and is then propelled over a second driving distance by the gases generated during the combustion of the propellant charge ( 152 ).

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to a pyrotechnical drive for a vehicle-occupant restraining system. The invention also relates to a belt retractor having a drive of this type and to a method for operating a pyrotechnical drive of this type.  
       BACKGROUND OF THE INVENTION  
       [0002]     A pyrotechnical drive for a vehicle-occupant restraining system has a cylinder, a piston that is movably arranged in the cylinder, a pyrotechnical propellant charge for generating pressurized gas, and an ignition charge for igniting the propellant charge, the piston having a thrust surface area which can be acted upon by the pressurized gas in order to drive the piston.  
         [0003]     In  FIG. 1 , an example of a pyrotechnical drive of this type is illustrated on the basis of a belt tensioner drive from the related art. Belt tensioner drive  10  in  FIG. 1  has a cylinder  12 , in which a piston  14  is movably guided, and a gas generator  22  for generating pressurized gas to drive piston  14 . Gas generator  22  is provided with a propellant charge  52 , which is surrounded by a generator housing  70 . For igniting propellant charge  52 , an ignition charge is provided in the form of a squib  68 , which in turn is surrounded by a capsule  58  for protection from humidity and damage. Gas generator  22  is accommodated in the interior space of a recess  40  in piston  14 , a wall of the recess that is disposed perpendicular to the direction of movement of the piston constituting a thrust surface area  42 . To trigger belt tensioner drive  10 , squib  68  is ignited. After capsule  58  bursts, the combustion of propellant charge  52  begins and pressurized gas is generated. When a minimum pressure is reached, generator housing  70  bursts, and the gas pressure can act on thrust surface area  42  of piston  14 , setting the latter in motion. Piston  14  is provided with teeth  30 , which via a pinion  34  drive the belt reel of a belt tensioner. In  FIG. 5 , in graph i, the temporal pressure curve on thrust surface area  42  is schematically illustrated. Usually up to 3 ms pass before generator housing  70  bursts. Only then can piston  14  start to move. Because by this time point t 2  high pressure has already built up in generator housing  70 , which then abruptly acts upon piston  14 , as can be seen from the steep pressure rise in  FIG. 5 , piston  14  and, after the coupling to the belt tensioner, also teeth  30  and pinion  34  are subjected to high stresses.  
         [0004]     The objective of the present invention is to provide a pyrotechnical drive that attains great tensioning power in a short time with reduced component stress.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     For this purpose, a pyrotechnical drive for a vehicle-occupant restraint system, has a cylinder, a piston that is movably arranged in the cylinder, a pyrotechnical propellant charge for generating pressurized gas, and an ignition charge for igniting the propellant charge. The piston has a thrust surface area which can be acted upon with the pressurized gas in order to drive the piston. Between the ignition charge and the thrust surface area, a flow connection is provided. In addition, a belt retractor is described having a pyrotechnical drive of this type and a method for operating a pyrotechnical drive of this type. Thus the pressure generated by the ignition charge can be used for accelerating the piston, so that the piston starts to move already a short time after the ignition. The drive power of the piston is therefore available very early.  
         [0006]     In accordance with a second aspect, the present invention provides a belt tensioner that has a pyrotechnical drive of this type. Short time after the ignition, a thrust generated by the ignition charge is made available which is significantly inferior to the thrust generated by the propellant charge. Thus the piston is accelerated initially with smaller force, but in return at a early point in time, and therefore the components in the acceleration phase are only subject to minor stresses. This is especially advantageous when the tensioning drive is coupled to the belt retractor only in the acceleration phase, for example using a teething that engages in a pinion. Then the coupling can be accomplished with smaller stress, whereas the entire power of the propellant charge is made available for the tensioning process.  
         [0007]     According to a further aspect, the present invention also provides a method for operating a drive of this type, wherein the piston is propelled over a first driving distance by the gases generated by the combustion of the ignition charge and then is propelled over a second driving distance by the gases generated during the combustion of the propellant charge. This method reduces the stresses on the components and provides a rapid availability of propulsive power. In addition, optimal energy exploitation of both the ignition charge as well as the propellant charge is achieved.  
         [0008]     Advantageous embodiments will become apparent in the subclaims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  depicts a longitudinal section of a belt retractor having a belt tensioner drive in accordance with the related art;  
         [0010]      FIG. 2  depicts a longitudinal section of a belt retractor according to the present invention in a first condition;  
         [0011]      FIG. 3 — FIG. 3  depicts an enlarged representation of section III from  FIG. 2 ;  
         [0012]      FIG. 4 — FIG. 4  depicts a longitudinal section of a belt retractor from  FIG. 2  in a second condition; and  
         [0013]      FIG. 5  depicts a schematic comparison of the pressure curve of a pyrotechnical drive according to the related art and a pyrotechnical drive in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     Belt retractor  100  shown in  FIG. 2  is equipped with a pyrotechnical drive  110  for tensioning the safety belt. Belt tensioner drive  110  has a cylinder  112  and a piston  114  that can be moved therein along a longitudinal axis A.  
         [0015]     Cylinder  112  has a front, open end  116  and a rear end having an end wall  118 , in which a receptacle  120  for a gas generator  122  is configured. Cylinder  112  is divided by a separating wall  124  into a piston chamber  126  and a toothed rack chamber  128 , separating wall  124  extending from end wall  118  of cylinder  112  parallel to longitudinal axis A roughly to the center of cylinder  112 .  
         [0016]     Piston  114  is provided with a toothed rack section  130 , which is arranged in toothed rack chamber  128  of cylinder  112 . Toothed rack section  130  at its front end is connected to piston  114  by a bridge  132 , so that when piston  114  is moved, the toothed rack section is carried along with it and can engage in a pinion  134  in order to drive a belt reel of belt retractor  100  in a familiar manner.  
         [0017]     Piston  114  is also provided with a generator chamber  140 , a cavity which extends into piston  114  from the rear end of piston  114  parallel to longitudinal axis A. At the front end of generator chamber  140 , a wall perpendicular to longitudinal axis A forms a thrust surface area  142 . In the resting condition of belt tensioner drive  110  depicted in  FIG. 2 , piston  114  rests on end wall  118  and surrounds gas generator  122 , i.e., gas generator  122  is located in generator chamber  140 . In addition, on the shell of piston  114  in its rear section there is a seal  146 , which contacts the interior shell of piston chamber  126 .  
         [0018]     Gas generator  122  has a base  150 , via which it sits in receptacle  120  in cylinder  112 , a propellant charge  152 , and an ignition charge  154 . Base  150  is provided with connecting pins  156  for connecting the gas generator, for example, to a control unit. On base  150  sits an ignition charge sleeve  158 , which contains the ignition charge  154 . Ignition charge sleeve  158  is made up of a pot-like plastic housing that has a casing  160  that is closed by an end wall  162 . On the other end, casing  160  is pressed onto base  150 , the diameter of base  150  being greater than the diameter of ignition charge sleeve  158 . In addition, as can be seen in  FIGS. 2 and 3 , the cross section of generator chamber  140  is correspondingly configured in steps at the rear end of cylinder  112 . At this end, base  150  is also provided with a sealing ring  164 , which seals the generator chamber off from piston chamber  126 .  
         [0019]     Ignition charge  154  is made up of a booster charge  166  and a squib  168 , which is located in base  150  and is connected to connecting pins  156 .  
         [0020]     Axially adjoining the ignition charge sleeve  158 , there is a propellant charge sleeve  170 , which also has a plastic housing. The housing has a shell, whose rear end adjoins end wall  162  of ignition charge sleeve  158 . Propellant charge sleeve  170  contains propellant charge  152 , for example, in the form of propellant charge particles. Ignition charge sleeve  158  and propellant charge sleeve  170  are advantageously manufactured in one piece. It is possible, in order to save on material, to reduce the wall thickness of propellant charge sleeve  170 , because propellant charge  152  is far less sensitive to mechanical stresses than ignition charge  154 .  
         [0021]     Ignition charge sleeve  158  and propellant charge sleeve  170 , for ignition charge  164  and propellant charge  152 , respectively, form a protective sleeve that protects from mechanical damage. Alternatively, they can also be made of other materials, for example, a metal foil. Ignition charge sleeve  158  is permeable to gas at least in the area of end wall  162 , so that between ignition charge  154  and thrust surface area  142  there is a flow connection, which is illustrated in  FIG. 3  by arrow F.  
         [0022]     The ignition charge or the propellant charge, in order to be protected from humidity, can also be enclosed in a foil, for example, one that is made of plastic or metal. In addition, the ignition charge sleeve at its free front side is also closed by a transport safety device  172 , also in the form of a foil. When ignition charge  154  is ignited, foil layers located between ignition charge  154  and thrust surface area  142  are destroyed by even a slight pressure, in order to assure flow connection F.  
         [0023]     In  FIG. 5 , the temporal pressure curve of the drive according to the present invention, without booster charge (graph ii) and with booster charge  166  (graph iii), is compared to the pressure curve in a conventional pyrotechnical drive (graph i), as it was described in the introduction.  
         [0024]     For triggering belt tensioner drive  110 , squib  168  is first ignited. As graph ii of  FIG. 5  indicates, as a result of the combustion of squib  168 , already at time point t 1 , roughly 0.5 ms after the ignition of gas generator  122 , a pressure builds up which can exert a thrust on piston  114  on the basis of the flow connection between ignition charge  154  and thrust surface area  142 . Therefore, piston  114  starts to move already a short time after the ignition of the ignition charge. After a brief time, booster charge  166 , which is ignited by the combustion of squib  168 , makes further pressure available, which leads to an increase in the thrust.  
         [0025]     Booster charge  166  therefore assures that the inertia of piston  114  and the friction are reliably overcome, and piston  114  is propelled in any case over a first drive path segment Z 1  ( FIG. 5 ), until the beginning of toothed rack section  130  engages in pinion  134 .  
         [0026]     The combustion of ignition charge  154  finally leads to the ignition of the substantially larger propellant charge  152 . The latter generates a pressurized gas volume which propels piston  114  with maximum thrust, the motion being transferred to the reel of belt retractor  100  via pinion  134  in order to retract the belt as rapidly as possible in the customary manner.  
         [0027]     Since, when piston  114  moves further, base  150  comes out of the expanded section of generator chamber  140 , the cross section of piston  114  that is effective for the pressure of the combustion gases expands. If previously, for the reasons cited above, only thrust surface area  142  was available as an effective cross-section for the pressurized gas, then the pressurized gas generated by propellant charge  152  can now act additionally upon a further thrust surface area  176  at the rear end of piston  114 , so that the entire effective thrust surface area corresponds to the cross-section of piston  114  perpendicular to its direction of motion along longitudinal axis A. Thus an increased thrust is made available for the further motion of piston  114  and therefore ultimately for tensioning a safety belt.  
         [0028]     As long as toothed rack section  130  has not yet engaged in pinion  134 , only a small thrust is required, because the reel of belt retractor  100  does not yet have to be driven. Therefore, thrust surface area  142  is advantageously selected so as to be so large that it occupies a maximum of 30% of the entire piston cross-sectional surface. This has the advantage that piston  114  can be propelled over a sufficient distance even using the slight pressurized gas volume that is generated by ignition charge  154 .  
         [0029]     By selecting an appropriate shape of the particles of propellant charge  154  and by minimizing the clearance volume  178  between propellant charge sleeve  170  and thrust surface area  142 , the dead volume can be minimized so as to be able to optimally exploit the gas pressure from ignition charge  154 . If the thrust of squib  168  is sufficient, it is even possible to do without booster charge  166 .  
         [0030]     A further advantage of the drive according to the present invention lies in the fact that as a result of the initially small thrust that is produced by the ignition charge, the components, especially the first teeth of toothed rack section  130  and of pinion  134 , are only exposed to slight stresses. To further improve the meshing of toothed rack section  130  into pinion  134 , the first tooth of toothed rack section  130  is shortened by roughly 50%. Therefore, piston  114  in its resting position can also be arranged so as to be closer to pinion  134 , resulting in a smaller overall length of drive  110 .  
         [0031]     Furthermore, as a result of the propulsion of piston  114  over first drive path segment Z 1 , the combustion chamber for propellant charge  152  is expanded, because clearance volume  178  is expanded (see  FIG. 5 ). On the other hand, as was described above, the cross-section of thrust surface area  142  amounts to only a fraction of the overall cross-section of piston  114 , which assures that the combustion chamber in the first propulsion phase of piston  114  does not expand excessively. An optimized expansion is advantageous for the complete combustion of propellant charge  152  and thereby aids the optimal exploitation of the energy contained in propellant charge  152 .  
         [0032]     According to an alternative embodiment, the propellant charge, in order to save material and weight, can also be placed directly in the generator chamber, so that only the ignition charge is surrounded by a protective sleeve.  
         [0033]     The tensioner drive according to the present invention and the method are illustrated by way of example on the basis of a belt retractor. However, they are suitable for all applications in which high retraction power must be achieved, such as a buckle and fitting retractor or hood collision protection system.