Abstract:
An improved ignition system for vehicle airbags is described. This invention includes a novel means for venting igniter material gases in such a manner as to more evenly distribute the flow in a more disperse manner, thereby permitting the gas generant to burn more uniformly throughout the generant bed. This invention by venting igniter gases more efficiently produces improved airbag ballistics, namely lowered average airbag deployment delay and increased consistency of operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to automobile airbags inflators. More specifically, this invention relates to the control of the ignition train of the gas generant used in airbag inflators. 
     2. Description of Related Art 
     The safety of automobile passengers has taken important steps forward with the development of automobile airbags. Airbags are inflatable automobile safety devices that help protect a driver or front-seat passenger in a collision. Airbags are most effective when the collision is a head-on or front-angle collision and when they are used in combination with lap and shoulder belts. The typical airbag system consists of a cloth airbag, an inflator, a cover, a can or module to hold the various components, and sensors for detecting the sudden deceleration of the automobile. A computerized control unit typically monitors the system for malfunctions. The sensors, generally mounted on the front of the vehicle and in the passenger compartment, operate on power from the automobile&#39;s battery or from the computerized control unit. Typically, airbags are designed to inflate when the automobile strikes an immovable object at more than about ten miles per hour or another automobile at about twice that speed. After an impact, sensors send an electric current to an igniter system or, in some cases to the computerized control unit. This unit evaluates the situation and then sends an electrical impulse to the igniter system. Typically, electrical impulse produces a current which in turn heats a filament, which ignites a pyrotechnic material contained within a capsule. The ignition capsule supplies heat at pressure to ignite gas generating pellets. In most systems, the pellets are made of sodium azide and produce nitrogen gas when they burn. In some systems, pressurized argon gas is used instead. The gas expands quickly and inflates the airbag, which then breaks through a plastic cover in the steering wheel or, on the passenger side, in the dashboard of the automobile. Typically, the whole process takes about 0.1 second from the moment a crash is detected. Once deployed an airbag starts to deflate immediately, venting the harmless gas through openings in the bag or through the bag fabric itself. 
     A variety of igniter chambers, cavity, and ignition ports have been developed and are widely used in automobile airbags. Traditionally igniter tubes have used cylindrically shaped orifices to control the flow of the ignition material. However, such cylindrically shaped orifices have contributed to relatively poor ignition because of the lack of uniform dispersal of ignition material through the combustion chamber. Because ignition efficiency is directly related to the uniformity of the dispersal of the ignition material or gas generant, through the combustion chamber, improved orifices which provide greater uniformity of gas dispersal, resulting in reduced ignition delay and enhanced performance repeatability of ignition. 
     For general background material, the reader is directed to U.S. Pat. Nos. 4,272,102, 4,817,828, 4,846,368, 4,928,991, 5,058,921, 5,100,171, 5,222,761, 5,280,946, 5,372,380, 5,458,364, 5,483,896, 5,492,361, 5,505,488, 5,518,268, 5,520,411, 5,533,751, 5,542,702, 5,564,736, 5,623,115, 5,668,345, 5,669,631, 5,681,055, and 5,692,768, each of which is hereby incorporated by reference in its entirety for the material disclosed therein. 
     SUMMARY OF THE INVENTION 
     It is desirable to provide a method of improving the ballistics of the ignition system in airbag inflators. Since the ballistic characteristics of airbag ignition systems are directly related to the system inflation delay and operation consistency, improving the flow of the igniter material through the igniter tube to the combustion chamber which provide improvements in the uniformity of the dispersal of the igniter material enhances the operation and performance of vehicle airbags. Improved performance of airbags enhance the safety of automobiles for both drivers and passengers. Therefore, providing improved automobile airbag ignition system ballistics is important in the effort to continue to enhance the safety of automobile travel. 
     Accordingly, it is the general objective of this invention to provide ignition ports of the igniter chamber which are configured in such a manner as to distribute the flow of igniter material in a more disperse manner. 
     It is a further objective of this invention to provide improved uniformity of igniter material through the combustion chamber. 
     It is a further objective of this invention to provide improved ballistics of ignition in airbag igniter. 
     It is a still further objective of this invention to provide igniter tubes having an improved orifice design which provides improved ignition material flow. 
     It is another objective of this invention to provide igniter tubes have a slotted orifice structure to improve the repeatability of ignition. 
     Another objective of this invention is to provide igniter tubes having orifices which provide reduced ignition delay. 
     Another objective of this invention is to provide igniter tubes which provide improved pressure rise consistency within the combustion chamber. 
     These and other objectives of this invention will be readily understood by those of ordinary skill in the art upon consideration of the following detailed description, preferred embodiment, drawings and claims of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a partial cross section of an inflator containing the ignition system of this invention. 
     FIG. 2 shows a full cross section of an inflator providing additional detail of the igniter components of the invention. 
     FIG. 3 a  shows the side view of the traditional prior art cylindrically orificed igniter tube. 
     FIG. 3 b  shows the end view of the traditional prior art cylindrically orificed igniter tube. 
     FIG. 4 a  shows the side view of a first preferred embodiment of the invention using an igniter tube with slotted orifices. 
     FIG. 4 b  shows a section view of the first preferred embodiment of the invention using an igniter tube with slotted orifices. 
     FIG. 5 a  shows the side view of a second preferred embodiment of the invention using an igniter tube with slotted orifices. 
     FIG. 5 b  shows a section view of a second preferred embodiment of the invention using an igniter tube with slotted orifices. 
     FIG. 6 a  shows an experimentally measured pressure verses time chart for an igniter tube having traditional cylindrical orifices. 
     FIG. 6 b  shows an experimentally measured pressure verses time chart for an igniter tube having the slotted orifices of this invention. 
     FIG. 7 shows a table of measured performance values, comparing the performance of cylindrical orifices and that of the slotted orifices of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention is an igniter chamber, for use in automobile airbag systems, which is designed to distribute the gas flow from the igniter material in a more disperse manner, thereby permitting the ignition of the gas generant to proceed more uniformly throughout the generant bed, thereby producing improved airbag ballistics. The traditional igniter tubes in use have cylindrical orifices. The preferred igniter tube of this invention has slotted orifices which when employed in the airbag system have been shown to provide lower ignition delay with faster and more consistent increase in pressure in the combustion chamber. This results in more repeatable performance in the cylindrical orificed igniter tube design, as well as lower ignition delay and more consistent (lower standard deviation) performance. 
     Referring now to the Figures and particularly to FIG. 1, which shows a partial cross section of an automobile airbag inflator containing the ignition system of this invention. The several key components of a preferred airbag inflator assembly are shown in relative position to each other. A preferred embodiment of the igniter tube  101 , with one preferred embodiment of the igniter slots  110  of the invention, is press fit in place onto a squib adapter  102  mounted on its first end  109 . The airbag inflator base  103  is shown with the squib adapter  102  welded in place. Adjacent to the generant retainer and welded to the base  103  is a diffuser  105 . Within the airbag igniter is provided a screen pack  104 , as well as the gas generant pellets  107 . The gas generant pellets  107  are confined within the screen pack  104  by the retainer  106 . 
     FIG. 2 depicts a full cross section of an inflator providing additional detail of the igniter components and is useful for providing an overview of the use of the operation of the igniter. Once an impact is detected by sensors (not shown), an electric current is sent through conductors to the squib  203 , which is held in place by the squib adapter  102 . A filament within the squib  203  is heated by the applied electric current. Upon reaching a desired temperature, the filament within the squib  203  ignites a pyrotechnic contained within it  203 . The pyrotechnic burns, producing both gas and heat, which in turn ignites the igniter material  201 . The igniter material  201  ignites and produces gas and heat, quickly bursting the igniter cup  202  that normally holds the igniter material  201 . The gas produced by the burning igniter material  201  then passes through the slots  110  in the igniter tube  101  thereby entering and igniting the gas generant pellets  107 . The gas generant pellets  107  burn producing gas, heat and solid particulate matter. Gas produced from the burning of the gas generant pellets  107  passes through the screen pack  104  where it is cooled and filtered to remove the solid particulate matter. The resulting cooled and filtered gas then passes through the diffuser  105  to the airbag, which is thereby inflated. FIG. 3 a  shows the side view of the traditional prior art cylindrically orificed igniter tube  301 . The cylindrical orifices  303  are shown in their typical configuration. FIG. 3 b  shows a section view  302  of the traditional cylindrically orificed igniter tube  301  along a section plane  304 . As can be seen the traditional igniter tube  301  has a plurality of orifices  303  which are generally cylindrical in shape. Such traditional tubes  301  require that the gas generated by the ignition of the igniter material  201  pass through the generally constant and confined cylindrical orifices  303 , thereby causing undesirable gas generant ignition delay and variability of performance. 
     FIG. 4 a  shows the side view of a first preferred embodiment of the invention using an igniter tube  101  with slotted orifices  403 . The slotted orifices  403  of this embodiment are adapted such that each slot  403  appears to be generally rectangular in shape, having two sides  405 ,  406  that are substantially of greater dimension than the other two sides  407 ,  408 . Each slot  403  is presented with three portions  409 ,  410 ,  411 . The portions  409 ,  411  are portions which designate the expansion of the slot  403  as it extends from portion  410  which is the opening into the interior  412  of the igniter tube  101 . The reader will observe that portion  410  has generally the same relative dimensions as portions  409 ,  411 . This embodiment permits the expansion of the gas as it exits the ignition tube of approximately three to one. The increase in slot dimension permits the gas to diffuse throughout the generant bed, efficiently expanding in a relatively wide arc within the generant bed with less delay, which leads to an improvement in combustion chamber pressure. FIG. 4 b  further shows the end section view of the first preferred embodiment of the invention, having slotted orifices  403  in the igniter tube  101  being cut along the plane  404 , further demonstrating the increase in slot dimension between the inner opening  413  and the outer opening  414  of this preferred slot  403 . 
     FIG. 5 a  shows the side view of a second preferred embodiment of the invention using an igniter tube  101  with slotted orifices  503 . The slotted orifices  503  of this embodiment are adapted such that each slot  503  appears to be generally rectangular in shape, having two sides  505 ,  506  that are only somewhat of greater dimension than the other two sides  507 ,  508 . Each slot  503  is presented with three portions  509 ,  510 ,  511 . The portions  509 ,  511  are portions which designate the expansion of the slot  503  as it extends from portion  510  which is the opening into the interior  512  of the igniter tube  101 . The reader will observe that in this embodiment of the invention portion  510 , the opening into the interior  512  of the igniter tube  101 , is generally significantly larger than the cut-away portions  509 ,  511 . This embodiment permits the expansion of the gas as it exits the ignition tube of approximately 1.5 to 1. Such an increase in slot dimension permits the gas to diffuse throughout the generant bed, efficiently expanding in a relatively wide arc within the generant bed with less delay than the traditional cylindrical orifice, but somewhat more delay than the first preferred embodiment of the invention. This embodiment also leads to an improvement in combustion chamber pressure and demonstrates the tune ability of the invention. By adapting the various dimensions of the slots the ignition tube  101  itself can be tuned to various igniter delays, while maintaining a degree of consistency in tubes having the same types of slots, not found in the traditional cylindrical orifices. FIG. 5 b  further shows the end section view of the first preferred embodiment of the invention, having slotted orifices  503  in the igniter tube  101  being cut along the plane  504 , further demonstrating the increase in slot dimension between the inner opening  513  and the outer opening  514  of this preferred slot  503 . 
     FIG. 6 a  shows an experimentally measured combustion pressure verses time chart for an igniter tube design having traditional cylindrical orifices. This figure shows the wide variation in performance  601  between various identical cylindrical orificed ignition tubes, as well as the typical time from point of ignition  602  of the igniter material  201  until the combustion pressure has dropped to ambient levels  603 . 
     FIG. 6 b  shows an experimentally measured combustion pressure verses time chart for an igniter tube design having new slotted orifices. This figure shows the narrow variation in performance  604  between various identical slotted orificed ignition tubes, as well as the typical time from the point of ignition  605  of the igniter material  201  until the combustion pressure has dropped to ambient levels  606 . 
     FIG. 7 shows a table of measured performance values, comparing the performance of cylindrical orifices and that of the slotted orifices of this invention. As this chart demonstrates, the use of slotted orifices in ignition tubes result in performances which have lower average ignition delay (3.7 milliseconds verses 5.8 milliseconds), higher average tank pressure (58 kPa at 20 milliseconds and 140 kPa at 40 milliseconds verses 55 kPa at 20 milliseconds and 137 kPa at 40 milliseconds), and increased tank pressure consistency (standard deviations of 3 kPa and 2 kPa respectively verses 8 kPa and 4 kPa), than otherwise identical cylindrical orifices. Since, in the operation of vehicle airbags two of the most important constraints on total airbag system performance are ignition delay and repeatability, it can be seen that this invention provides an important improvement in the performance of vehicle airbag systems. 
     It should be understood that the above described embodiments of this invention are merely illustrative of numerous and varied other embodiments which may constitute applications of the principles of the invention. For example, the relative dimensions of the lengths of the sides of individual slots may be easily varied. The relative sizes of each portion of the slots could also be varied. Even the generally rectangular shape of the invention may be altered without departing from the intended scope of this invention. These and other embodiments of this invention may be readily devised by those skilled in the art without departing from the spirit or scope of this invention and it is the inventor&#39;s intent that they are deemed to be within the scope of this invention. Therefore, the scope of this invention is indicated by the range and equivalency of the appended claims.