TECHNICAL FIELD 
       [0001]    The present invention relates to a method for testing a stored gas container. More particularly, the present invention relates to a method for pressure testing stored gas containers. 
       BACKGROUND OF THE INVENTION 
       [0002]    A stored gas container is often used in a vehicle occupant protection system to provide inflation fluid for inflating an air bag. The gas stored in the stored gas container is generally pressurized and may include a single gas alone, a mixture of gases, or a combination of a single gas or a mixture of gases and an ignitable material for heating the gas or gas mixture. The ignitable material may be solid ignitable material or a fuel gas. 
         [0003]    To assess whether the stored gas container can, without leaking over time, maintain sufficient pressure, a pressure test is performed on selected filled containers. One currently performed pressure test includes heating the stored gas container to increase the pressure of the gas. During this type of pressure test, a statistically significant number of the stored gas containers are removed from the assembly line and placed in an oven for a predetermined period of time. The oven is heated to a temperature for increasing the pressure of the stored gas to a predetermined value, generally 1.5 times the designated fill pressure of the stored gas. This pressure test is inefficient, however, due to the necessity of removing the stored gas containers from the assembly line. Also, this pressure test is performed on only a selected number of the stored gas containers. A more efficient pressure test in which all of the stored gas containers are tested is desirable. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention relates to a method for assessing whether a container for a stored gas can maintain a predetermined gas pressure. The method comprises the steps of: producing a magnetic field with an inductive heating element; placing the gas container containing a quantity of stored gas in the magnetic field; inductively heating the gas container to elevate the pressure of the stored gas to an elevated pressure greater than the predetermined gas pressure; and thereafter testing the container for an indication of potential inability of the container to maintain the stored gas at the predetermined gas pressure. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The foregoing and other features of the present invention will become apparent to one skilled in the art to which the present invention relates upon consideration of the following description of the invention with reference to the accompanying drawings, in which: 
           [0006]      FIG. 1  is a schematic view of a first testing system for performing the method of the present invention; and 
           [0007]      FIG. 2  is a schematic view of a second testing system for performing the method of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    The present invention relates to a method for testing stored gas containers. More particularly, the present invention relates to a method for testing stored gas containers that are to be used as inflation fluid sources for inflating an air bag or other inflatable device of a vehicle occupant protection system. 
         [0009]      FIG. 1  illustrates a first system  11  for pressure testing stored gas containers  12  in accordance with the method of the present invention. The system  11  includes a conveyor for transporting the containers  12 . The conveyor illustrated in  FIG. 1  is a rotary table  13 . The rotary table  13  rotates at a rate of approximately one revolution every six minutes in a counter-clockwise direction, as viewed in  FIG. 1 . Empty containers  12  are loaded successively onto the table  13 .  FIG. 1  schematically illustrates a loading area  14  adjacent the rotary table. Empty containers  12  are loaded onto the rotary table  13  in a manner such that the containers are spaced circumferentially from one another about the periphery of the rotary table.  FIG. 1  illustrates the rotary table  13  holding thirty five containers  12 . The table  13  can be designed to hold any number of containers  12 . 
         [0010]    A fill station  55  is associated with the rotary table  13 . The fill station  55  includes mechanisms for filling the empty containers  12  with gas and sealing filled containers. Shortly after being loaded onto the rotary table  13 , each empty container  12  enters the fill station  55 , is filled with gas, and is sealed. 
         [0011]    A heating station  17  is also associated with the rotary table  13 . The heating station  17  is located downstream of the fill station  55  in the direction of rotation of the rotary table  13 . The containers  12 , after being filled with gas and sealed, pass successively into the heating station  17 . 
         [0012]    The heating station  17  includes an inductive heating element  19 . The inductive heating element  19  is located on an arm  16  that is configured to move the inductive heating element vertically relative to the rotary table  13 . Prior to a container  12  entering the heating station  17 , the inductive heating element  19  is spaced away from the rotary table  13  by a distance sufficient to enable the container to enter the heating station  17 . The rotary table  13  stops momentarily while the container  12  is at the heating station  17 . While the rotary table  13  is stopped, the inductive heating element  19  is moved to a position proximate the container for heating the container. The inductive heating element  19  is again moved to a position spaced away from the rotary table  13  after the container  12  is heated so that the rotary table may rotate to move the container out of the heating station  17 . 
         [0013]    The heating station  17  is electrically coupled to a computer  21  and a power supply  25 . The computer  21  is also electrically coupled to a pyrometer  23 . The computer  21  controls operation of the heating station  17  and the pyrometer  23  and also records data associated with each container  12 . The pyrometer  23  is located immediately upstream of the heating station  17  and is operable for measuring the temperature of the containers entering the filling station. The pyrometer  23  provides the measured temperature with the computer  21 . The computer associates the measured temperature with the respective container. 
         [0014]    The power supply  25  provides power to the heating station  17 , computer  21 , and pyrometer  23 . The power supplied to the heating station  17  has an alternating current. Upon receiving the alternating current from the power supply  25 , the inductive heating element  19  produces a magnetic field. When a container  12  is located at the filling station  17 , the container  12  is located within the magnetic field and becomes inductively coupled to the inductive heating element  19 . When a respective container is inductively coupled to the inductive heating element  19 , the container  12  is heated. The heating of the container  12  heats the gas stored within the container. As a result, the pressure of the gas stored within the container  12  increases. The container is made of a conductive material, such as a ferrous metal, that allows for the heating of the gas in the container by the inductive heating element  19 . 
         [0015]    According to the method of the present invention, an empty container  12  is loaded onto the rotary table  13 . The empty container  12  is moved into the fill station  55  by rotation of the rotation table  13  and is filled with gas. In one example, the container  12  is filled with a mixture of gases comprising air, helium, and hydrogen. During filling of the container  12  at the fill station  55 , the amount or quantity of each gas that is introduced into the container  12  is determined. The determined amount of each of the gases is provided to the computer  21 . The computer  21  associates the determined amounts of the gases with the respective container being filled and stores this data in memory. After the container  12  is filled with the gases, the container  12  is sealed. 
         [0016]    After being filled and sealed, the container  12  is transported toward the heating station  17 . Prior to reaching the heating station  17 , the pyrometer  23  measures the temperature of container  12  and the measured temperature is provided to the computer  21 . The computer  21  associates the measured temperature with the respective container and stores this data in memory. 
         [0017]    After the temperature of the container is measured, the container  12  enters the heating station  17 . When the container is located at the heating station  17 , the rotation of the table  13  is stopped momentarily. The inductive heating element  19  is moved into a position proximate the container and power is supplied to the inductive heating element. In response to receiving power, the inductive heating element  19  produces a magnetic field for inductively coupling the inductive heating element and the container  12  located at the heating station  17 . While producing the magnetic field, the inductive heating element  19  is moved vertically along the height of the container  12 . The inductive coupling of the inductive heating element  19  and the container  12  heats the container. No contact occurs between the inductive heating element  19  and the container  12  during heating of the container. 
         [0018]    Using the stored gas amounts and temperature for the respective container  12 , the computer  21  controls the amount of power applied to the inductive heating element  19  and the period of time that the inductive heating element  19  produces the magnetic field so that the pressure of the gas within the container  12  is raised to a predetermined elevated pressure. The predetermined elevated pressure is generally equal to or greater than 1.5 times the designated fill pressure of the gas. In one example, the computer  21  may determine that the container  12  should be heated to approximately 150 degrees Celsius for the stored gas to reach the desired elevated pressure. The computer  21  then controls the inductive heating element  19  to heat the container  12  to this determined temperature. 
         [0019]    After the container  12  is heated and the pressure of the gas within the container is raised to the predetermined elevated pressure, the inductive heating element  19  is moved away from the container  12 . The rotary table  13  begins to rotate and the container  12  is moved away from the heating station  17 . As one container  12  moves away from the heating station  17 , the next container  12  enters the heating station  17  to be heated. This next container  12  is heated using the same steps as the previous container. The computer  21  adjusts the time period for heating the next container  12  based on the measured temperature and the stored gas amounts of the next container  12 . 
         [0020]    As a container  12  is moved away from the heating station  17 , it is allowed to cool. Cooling generally occurs in a relatively short time period. The container  12  then enters a testing station  26 . At the testing station  26 , the container  12  is tested for actual leaks or one or more other indications of potential inability of the container  12  to maintain the stored gas at a predetermined gas pressure, which may be the designated fill pressure or another pressure. 
         [0021]    For example, a ring test may be performed at the testing station  26  to determine if the hoop stress of the container  12  is adequate to prevent leakage over time. In another example, each of the containers  12  may be weighed upon arrival at the testing station  26  and then weighed again at a predetermined time after its arrival at the testing station  26 . A leak is determined when the weight of the container  12  at a predetermined time after its arrival at the testing station  26  is less than the weight of the container  12  upon arrival at the testing station  26 . After leaving the testing station  26 , the container  12  remains on the rotary table  13  until reaching an unloading station  32 . At the unloading station  32 , the container  12  is removed from the rotary table  13 . 
         [0022]      FIG. 2  illustrates a second system  27  for performing a method in accordance with the present invention. The system  27  of  FIG. 2  includes a belt conveyor  29  for transporting the containers  12  from right to left as viewed in  FIG. 2 . A fill station  57  for filling the containers  12  with gas and for sealing the containers  12  is associated with the belt conveyer  29  and is located near a loading area for the containers  12 . 
         [0023]    A heating station  31  is also associated with the belt conveyor  29 . The heating station  31  is located downstream of the filling station  57  in the direction of movement of the belt conveyor  29 . The heating station  31  includes an inductive heating element  35  that extends over the belt conveyor  29  and along part of the length of the belt conveyor  29 . In particular, the inductive heating element  35  includes a first straight elongated portion  37  that is spaced from and parallel with a second straight elongated portion  39 . The first and second elongated portions  37  and  39  extend horizontally over the belt conveyor  29 . The inductive heating element  35  also includes an inverted u-shaped portion  41  that joins the first and second elongated portions  37  and  39  at their upstream ends and a similar inverted u-shaped portion  43  that joins the first and second elongated portions  37  and  39  at their downstream ends. Each of the inverted unshaped portions  41  and  43  extends vertically upwardly from the elongated portions  37  and  39 . 
         [0024]    The heating station  31  is electrically coupled to a power supply  51  and a computer  53 . The computer  53  is also electrically coupled to a pyrometer  49 . The power supply  51 , computer  53 , and pyrometer  49  operate in a manner similar to the power supply  25 , the computer  21 , and the pyrometer  23  described with reference to  FIG. 1 . 
         [0025]    The power supplied to the heating station  31  has an alternating current. Upon receiving the alternating current from the power supply  51 , the inductive heating element  35  produces a magnetic field. Each container  12  passing through the filling station  31  passes through the magnetic field produced by the inductive heating element  35  and becomes inductively coupled to the inductive heating element. When a respective container  12  is inductively coupled to the inductive heating element  35 , the container  12  is heated. The heating of the container  12  increases the pressure of the gas stored within the container. 
         [0026]    According to the method of the present invention, each container  12  is loaded onto the belt conveyor  29 . The container  12  then enters the fill station  57 . At the fill station  57 , the container  12  is filled with gas and is subsequently sealed. During the filling process, the amount or quantity of each gas that is introduced into the container  12  is determined. The data related to the stored amount of each of the gases is supplied to the computer  53 . The computer  53  associates the stored gas amounts with the respective container  12  and stores the information in memory. After the container  12  is filled and sealed, the belt conveyor  29  transports the container  12  toward the heating station  31 . 
         [0027]    Prior to reaching the heating station  31 , the temperature of each container  12  is measured by the pyrometer  49 . The pyrometer  49  supplies the measured temperature information to the computer  53 . The computer  53  associates the temperature information with the respective container  12  and stores the information in memory. 
         [0028]    Upon entering the heating station  31 , the container  12  moves under the inductive heating element  35 . When the container  12  is located under the inductive heating element  35 , the inductive heating element  35  and the container  12  become inductively coupled via the magnetic field produced by the inductive heating element. As a result, the container  12  is heated. 
         [0029]    Using the stored gas amounts and the temperature, the computer  51  controls the amount and time duration of electrical power applied to the inductive heating element  35 . The speed at which the container  12  passes by the energized inductive heating element  35  may also be controlled. The inductive heating element  35  heats the container  12  to raise the pressure within the container to the predetermined elevated pressure, which is generally equal to or greater than 1.5 times the designated fill pressure of the gas within the container  12 . 
         [0030]    After exiting the heating station  31 , the container  12  is allowed to cool. The container  12  then enters a testing station  61  in which the container is tested for actual leaks or one or more other indications of potential inability of the container  12  to maintain the stored gas at a predetermined gas pressure, which may be the designated fill pressure or another pressure. 
         [0031]    After one container  12  is moved away from the heating station  31 , the belt conveyor  29  transports the next container  12  through the inductive element  35  of the heating station  31 . The computer  53  adjusts the amount of heating of the next container  12  based on the stored gas amounts and the temperature of the container  12  as measured by the pyrometer  49 . 
         [0032]    From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications in the invention. For example, the inductive heating element design, the alternating current frequency, the element to container proximity, and other factors may be altered for controlling the heating of the container for increasing the pressure of the stored gas. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.
A method is provided for assessing whether a container ( 12) for a stored gas can maintain a predetermined gas pressure. The method includes the steps of: producing a magnetic field with an inductive heating element ( 19, 35); placing the container ( 12) containing a quantity of the stored gas in the magnetic field; inductively heating the container ( 12) to elevate the pressure of the stored gas to an elevated pressure greater than the predetermined gas pressure; and thereafter testing the container ( 12) for an indication of potential inability of the container to maintain the stored gas at the predetermined gas pressure.