Patent Document

FIELD OF THE INVENTION  
       [0001]     The present invention relates to airbag inflators, more specifically a single stage driver side inflator.  
       BACKGROUND OF THE INVENTION  
       [0002]     The deployment of an airbag module requires a device for rapid, controlled release of a gaseous fluid. This device is commonly referred to as an inflator.  
         [0003]     The inflator uses a gas generant that when ignited rapidly generates a large but controlled release of hot gases to fill the airbag.  
         [0004]     As shown in  FIG. 1 , a typical single stage inflator has an initiator that is most typically a squib type device encased in an initiator housing filled with a booster charge. When the squib is activated it ignites an enhancer charge in the initiator housing and that charge generates hot particles and gases, which ignite the gas generant within the disk shaped housing. This prior art device is centrally and symmetrically charged. The initiator is centered in the centerline of the housing and accordingly the generant surrounds the initiator housing and therefore can be easily ignited uniformly and thus burns radially outwardly with a neutral or almost neutral thrust. One such device is found in U.S. Pat. No. 6,796,579 B1. As shown, the prior art device is commonly referred to as a single stage inflator.  
         [0005]     More commonly used, but quite a bit more complex is a dual stage inflator. The dual stage inflators require two initiators and separate enhancer and gas generant chambers. The squibs can be fired one first then the second after a delay or may only fire one squib not firing the second squib at all or both squibs can be fired simultaneously. One such device can be found in U.S. Pat. No. 6,648,370 B1.  
         [0006]     Dual stage inflators are obviously more complex and costlier than a single stage inflator. The dual stage inflator also requires typically more enhancer charge to permit the deployment sequence to perform adequately in a sequential fashion with sufficient dwell time during deployment. Single stage inflators are far simpler in design based on fewer components and a simpler firing initiation. Dual stage inflators offer a variable range of protection to passengers, which a single stage inflator cannot provide. Therefore, the use of single stage inflators is far from simple in design complexity when factored into the various requirements of passenger protection. For this reason the use of single stage inflators has been decreasing in spite of the benefits of cost, reliability and simplicity.  
         [0007]     In a related, but somewhat ignored consideration is the fact that assembly lines building dual stage inflators are not readily compatible with automated tooling and fixtures used in building single stage inflators. As a result a large investment in capital must be laid out for two separate building lines, one for a single stage inflator and a separate line for a dual stage inflator.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention described hereafter provides a single stage inflator with one initiator having a single initiating squib located in an initiator housing, the initiator housing being offset relative to the centerline of the disk shaped inflator housing.  
         [0009]     The design can be made compatible with an automated dual stage inflator building line, greatly reducing the required capital cost needed for a second redundant assembly line.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a bottom plan view of a prior art single stage disk shaped inflator.  
         [0011]      FIG. 2  is a cross sectional view of the prior art inflator of  FIG. 1  taken along lines  2 - 2 .  
         [0012]      FIG. 3  is a bottom plan view of the single stage inflator according to the present invention.  
         [0013]      FIG. 4  is a cross sectional view of the inflator of  FIG. 3  according to the present invention taken along lines  4 - 4 .  
         [0014]      FIG. 5  is a first chart of Pressure KPa versus Time in milliseconds of a first generant charge.  
         [0015]      FIG. 6  is a second chart of Pressure KPa versus Time in milliseconds of a second generant charge. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     With reference to  FIGS. 1 and 2 , a prior art single stage disk shaped inflator  100  having a single centrally located initiator housing assembly  140  is illustrated. The inflator  100  as shown has a housing  101  having a top half portion  104  and a bottom half portion  102  welded or otherwise joined together. Encircling the two portions  102 ,  104  is a mounting flange  106  having a plurality of mounting holes  107  for attaching the inflator  100 . As shown, these disk shaped inflators  100  are commonly attached to an airbag module (not Illustrated) for location in the driver side steering wheel.  
         [0017]     On the top portion  104  of the housing  101  is shown a plurality of gas vent holes  108  covered or sealed by a foil type burst tape. A cylindrically shaped annular filter  110  of wire mesh or similar material is shown extending from the top portion  104  to the bottom portion  102 . The filter  110  blocks burning particles from passing through the vent opening  108  when the inflator gas generant is ignited. A seal  124  is positioned internal to the annular filter  110  upon which gas generant pellets  120  are located along with auto ignition pellet  122 . In a central location of the bottom half portion  102  is an initiator housing assembly  140 . The initiator housing assembly  140  has a single initiating squib  142  with projecting electrical connectors  143 ,  144  adapted to connect to a wiring connector (not illustrated). The initiating squib  142  has an explosive charge encapsulated in one end surrounded by an enhancer charge  150  comprising small pellets  152 . When activated the initiating squib  142  ignites causing the enhancer charge  150  to ignite which in turn causes a pressure rise internal to the initiator housing  141  forcing hot particles and expanding gases through small openings  164  thereby igniting the generant pellets  120  and auto ignition pellet  122 . This creates a further rise in pressure causing the foil  109  to burst and gases to fill the airbag (not shown) upon deployment. The initiator housing  141  has an opening  162  sealed by an end plate  160 . Although the auto ignition pellet is not needed for a normal deployment, it is consumed during deployment. In the event that the inflator is heated by an outside source, the auto ignition pellet will begin to burn at a predetermined level, causing the inflator to deploy without structural failure.  
         [0018]     With reference to  FIGS. 3 and 4 , a prior art single stage disk shaped inflator  10  according to the present invention having a single offset located initiator is illustrated. The inflator  10  as shown has a housing  11  having a top half portion  14  and a bottom half portion  12  welded or otherwise joined together. Encircling the two portions  12 , 14  is a mounting flange  16  having a plurality of mounting holes  17  for attaching the inflator  10 . As shown, this disk shaped inflator  10  is also preferably attached to an airbag module (not illustrated) for location in the driver side steering wheel.  
         [0019]     On the top portion  14  of the housing  11  is shown a plurality of gas vent holes  18  covered or sealed by a foil type burst tape  19 . A cylindrically shaped annular filter  21  of wire mesh or similar material is shown extending from the top portion  14  to the bottom portion  12 . The filter  21  blocks burning particles from passing through the vent opening  18  when the inflator gas generant is ignited. A seal  24  is positioned internal to the annular filter  21  upon which gas generant pellets  20  are located along with auto ignition pellet  22 . In an offset location of the bottom half portion  12  is an initiator housing assembly  40 . The initiator housing assembly  40  has an initiator housing  41  with a single initiator squib  42  with projecting electrical connectors  43 ,  44  adapted to connect to a wiring connector (not illustrated). The initiator squib  42  has an explosive charge encapsulated in one end surrounded by an enhancer charge  50  comprising small pellets. When activated the initiator squib  42  ignites causing the enhancer charge  50  inside the initiator housing  41  to ignite which in turn causes a pressure rise internal to the initiator housing  41  forcing hot particles and expanding gases through the small openings  64  in the initiator housing  41  thereby igniting the generant pellets  20  and auto ignition pellet  22 . This creates a further rise in pressure causing the foil  19  to burst and gases to fill the airbag (not shown) upon deployment in a single stage fashion. The housing  41  has an end opening  62  sealed by an end plate  60 . Although the auto ignition pellet is not needed for a normal deployment, it is consumed during deployment. In the event that the inflator is heated by an outside source, the auto ignition pellet will begin to burn at a predetermined level, causing the inflator to deploy without structural failure.  
         [0020]     Referring to  FIG. 3 , an inflator  10  was constructed of steel. The generant pellets  20  are preferably made of a non-azide gas generant. Representative gas generant compositions useful in the inventive inflator housing include fuels such as aminotetrazoles, tetrazoles, bitetrazoles, triazoles, the metal salts thereof, guanidine nitrate, nitroguanidine, aminoguanidine nitrate and mixtures thereof; in combination with an oxidizer such as the alkali and alkaline earth or transition metal nitrates, chlorates, perchlorates, ammonium nitrate and mixtures thereof. A preferred gas generant comprises a mixture of nitroguanidine with strontium and potassium nitrates. Typically, the gas generant or gas producing material can comprise about 15 to about 70 weight % fuel, about 2 to about 80 weight % oxidizer and about 1 to about 30 weight % other materials, such as coolants, catalysts, binding agents and processing aids. The gas generant can be formed into various shapes using various techniques known to those skilled in the art.  
         [0021]     It is desirable to pelletize the gas generant composition. To do so, up to about 5.0 weight %, typically 0.2-5 weight % of a pressing aid or binder may be employed. These may be selected from materials known to be useful for this purpose and include molybdenum disulfide, graphite, elastomers, polyesters, boron nitride, silicon dioxide, talc, calcium stearate and clays.  
         [0022]     The gas generant composition may optionally contain a catalyst at up to about 3 weight %, typically between about 1 and about 2 weight %. Cupric oxide is a representative combustion catalyst.  
         [0023]     The initiator housing  41  has the plurality of vent holes or openings  64  oriented to direct exhaust into the gas generant pellets  20  with a strong but directional thrust. The openings  64  are located less than 270 degrees, preferably less than 180 degrees around the periphery of the initiator housing  41  in the direction of the gas generant pellets  20  as shown. To compensate for this strong directional thrust effect, the holes  18  on the inflator housing top portion  14  are radially oriented about 360° in a spaced pattern and as these gases move radially outwardly the overall thrust becomes almost thrust neutral with an almost immeasurable thrust bias opposite the initiator housing  41 .  
         [0024]     With reference to  FIG. 3 , the offset single stage disk shaped inflator  10  is shown having the initiator housing assembly  40  spaced a distance (d) from a center location (C) of the inflator. The offset displacement is quite unique in that it shifts the initiating squib  42  from a central firing position to one very close to the annular filter  21 . Accordingly the openings  64  are also offset meaning the hot particles and gases spraying through the inflator  10  are directionally oriented. The openings  64  preferably are directed on the portions of the initiator housing  41  exposed to the gas generant pellets  20  and the auto ignition pellet  22 . In this way the openings  64  of the initiator housing  41  orient the hot particles and gases to facilitate burning of the gas generant pellets  20 .  
         [0025]     In testing of the inventive inflator  10 , it was determined that the enhancer charge should be increased to about 2.0 grams, and the initiating squib charge to be in the range of 180 to 260 mg of zirconium potassium perchlorate (ZPP) for proper performance in terms of airbag deployment. Initiator booster charges in excess of 260 mg were not necessary and in fact could potentially damage the initiator housing by exceeding rated burst pressures of the crimp. Lower amounts of charge, below 180 mg, could cause a delay in the gas generant burning and lead to unsatisfactory airbag deployment pressure or fill rates.  
         [0026]     Empirical studies show that the size of the pellets of the enhancer could be beneficially altered to increase or improve surface area for rapid burn rates, accordingly, a pellet having a 4 mm diameter and a 1.2 mm length was found to be ideal from a pressure versus time analysis.  
         [0027]     In  FIG. 5 a  chart is illustrated showing an inflator  10  having a 260 mg initiator booster charge  52 , 1.80 g of enhancer pellets  50 , 33.5 g of gas generant pellets  20  of a size 8.0 mm diameter by 1.7 mm length. Each line represents a separate test of an inflator  10 .  
         [0028]     In  FIG. 6 a  chart is illustrated showing the same proportions by weight, but wherein the 33.5 g of gas generant pellets  20  are of a size 8.0 mm diameter by 2.0 mm length.  
         [0029]     As a result of these empirical charts it was projected that a more preferred gas generant pellet  20  size for the offset inflator  10  was a size of 8.0 mm diameter by 2.0 mm length.  
         [0030]     For comparison purposes, the dual stage inflator with two initiators uses 180 mg of initiator booster charge and the enhancer charge is 1.1 grams in the primary initiator housing and 1.3 grams in the secondary initiator housing. The present invention uses 180 to 260 mg of initiator booster charge and 2.0 grams of enhancer. This represents a 0.4 gram reduction in the enhancer. The enhancer pellets  50  are more expensive and burn with more toxicity than the generant pellets  20 , accordingly, the reduction in enhancer load is believed to be a valuable improvement.  
         [0031]     Another benefit of the single stage offset design is it has 20% more free volume than the dual stage inflator. This means the height of the inflator housing could be reduced by at least 10% if further miniaturization is desirable.  
         [0032]     The current invention uses an internal spacer of aluminum to occupy the free volume space and thus the housing upper and lower portions are almost identical to the dual stage housing permitting either style to be used on the same production line.  
         [0033]     Variations in the present invention are possible in light of the description of it provided herein. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. It is, therefore, to be understood that changes can be made in the particular embodiments described which would be within the full intended scope of the invention as defined by the following appended claims.

Technology Category: 7