Staged air bag inflator

An apparatus (10) for inflating an inflatable vehicle occupant protection device (32) comprises a container (20) defining a gas storage chamber (14), and inflation fluid (66) under pressure in the gas storage chamber. An opening (40, 42) in the container (20) enables fluid flow from the gas storage chamber (14) to the inflatable device (32). A rupturable burst disk (60, 62) extends across the opening (40, 42). An assembly (80) in the gas storage chamber (14) is provided for producing combustion products for heating and pressurizing the inflation fluid (66). The assembly (80) includes a housing (82) and a separator (100) in the housing defining first and second chambers (112, 114). A first pyrotechnic charge (122) in the first chamber (112) has a first burn rate. The housing (82) has first fluid outlets (96) for enabling flow of combustion products from the first chamber (112) to the gas storage chamber (14). A second pyrotechnic charge (124) in the second chamber (114) has a second burn rate less than the first burn rate. The housing (82) has second fluid outlets (98) for enabling flow of combustion products from the second chamber (114) to the gas storage chamber (14).

BACKGROUND OF THE INVENTION
 1. Technical Field
 The present invention relates to a vehicle safety apparatus and, in
 particular, to an inflator for an inflatable vehicle occupant protection
 device, such as an air bag.
 2. Description of the Prior Art
 It is known to inflate a vehicle occupant protection device, such as an air
 bag, to help protect a vehicle occupant. The air bag is inflated by
 inflation fluid from an inflator.
 Some air bag inflators include inflation fluid stored under pressure in a
 chamber in a container. A burst disk blocks flow of inflation fluid from
 the chamber. An initiator is actuatable to rupture the burst disk and to
 initiate flow of inflation fluid from the chamber to inflate the air bag.
 One type of inflator includes a pyrotechnic material which, when ignited,
 generates combustion products for heating and pressurizing the inflation
 fluid stored in the chamber.
 It is known to tailor the output pressure of an inflator, in order to
 inflate an air bag in a predetermined manner over a period of time. For
 example, it is known to begin inflating an air bag at first with inflation
 fluid at a relatively low pressure, then finish inflating the air bag with
 inflation fluid at a relatively high pressure. Some known inflators of
 this type have two independently actuatable pyrotechnic charges for
 heating and pressurizing inflation fluid stored in a container.
 SUMMARY OF THE INVENTION
 The present invention is an apparatus for inflating an inflatable vehicle
 occupant protection device. The apparatus comprises a container defining a
 gas storage chamber, and inflation fluid under pressure in the gas storage
 chamber. An opening in the container enables fluid flow from the gas
 storage chamber to the inflatable device. A rupturable burst disk extends
 across the opening. An actuator assembly in the gas storage chamber is
 provided for producing combustion products for heating and pressurizing
 the inflation fluid. The actuator assembly includes a housing and a
 separator in the housing defining first and second chambers. A first
 pyrotechnic charge in the first chamber has a first burn rate. The housing
 has first fluid outlets for enabling flow of combustion products from the
 first chamber to the gas storage chamber. A second pyrotechnic charge in
 the second chamber has a second burn rate less than the first burn rate.
 The housing has second fluid outlets in the housing for enabling flow of
 combustion products from the second chamber to the gas storage chamber.

DESCRIPTION OF PREFERRED EMBODIMENTS
 The present invention relates to a vehicle safety apparatus and, in
 particular, to an inflator for an inflatable vehicle occupant protection
 device, such as an air bag. The present invention is applicable to various
 inflator constructions. As representative of the present invention, FIG. 1
 illustrates an inflator 10 which forms a part of a vehicle safety
 apparatus 11.
 The inflator 10 includes a container 12 which defines a gas storage chamber
 14. The container 12 has a cylindrical main body portion 20 which includes
 an axially extending side wall 22 of the container. The side wall 22 of
 the container 12 is centered on a longitudinal central axis 24 of the
 inflator 10. First and second end walls 26 and 28 of the container 12 are
 fixed to the main body portion 20 of the container.
 A diffuser 30 is fixed to the second end wall 28 of the container 12. The
 diffuser 30 is in fluid communication with an inflatable vehicle occupant
 protection device in the form of an air bag indicated schematically at 32.
 The diffuser 30 defines a diffuser chamber 34 which is located outside of
 the container 12. The diffuser chamber 34 is at ambient air pressure.
 First and second outlet passages 40 and 42 are formed in the second end
 wall 28 of the container 12. The first outlet passage 40 terminates in an
 opening 41 in an inner end surface 31 of the second end wall 28. The
 second outlet passage 42 terminates in an opening 43 in the end surface
 31. The outlet passages 40 and 42 establish fluid communication between
 the gas storage chamber 14 and the diffuser chamber 34.
 The outlet passages 40 and 42 are spaced apart from each other on opposite
 sides of the axis 24 and are "in parallel" with each other. Specifically,
 each one of the outlet passages 40 and 42 provides a separate,
 independent, fluid flow path between the gas storage chamber 14 and the
 diffuser chamber 34.
 The outlet passages 40 and 42 are identical to each other in size and
 configuration. The first outlet passage 40 has a frustoconical main
 portion 44 adjacent the opening 41 and a stepped, cylindrical portion 46.
 The second outlet passage 42 has a frustoconical main portion 48 adjacent
 the opening 43 and a stepped, cylindrical portion 50. Because the outlet
 passages 40 and 42 are identical to each other in size and configuration,
 the effective flow area of the first outlet passage 40 is the same as the
 effective flow area of the second outlet passage 42. While the passages 40
 and 42 are shown as being identical, they may not be identical. For
 example, cylindrical portion 50 of passage 42 may have a larger diameter
 than cylindrical portion 46 of passage 40.
 A first burst disk 60 is welded to the inner end surface 31 of the second
 end wall 28. The first burst disk 60 has a domed, circular configuration
 and extends across the opening 41. The first burst disk 60 blocks fluid
 flow through the first outlet passage 40.
 A second burst disk 62 is welded to the inner end surface 31 of the second
 end wall 28. The second burst disk 62 has a domed, circular configuration
 and extends across the opening 43. The second burst disk 62 blocks fluid
 flow through the second outlet passage 42. The second burst disk 62 is the
 same size, including diameter, as the first burst disk 60.
 A quantity of inflation fluid 66 is stored under pressure in the chamber
 14. The inflation fluid 66 preferably comprises an inert gas, such as
 argon or helium, under pressure. Also, oxygen could be mixed with the
 inert gas. The inflation fluid 66 is stored at a pressure in the range of
 about 1,500 psig to about 10,000 psig. The inflation fluid 66 may
 alternatively comprise a combustible mixture of gases including a primary
 gas and a fuel gas. The primary gas comprises the majority of the
 inflation fluid that inflates the air bag 32. The fuel gas, when ignited,
 heats the primary gas to increase the pressure and temperature of the
 fluid in the gas storage chamber 14.
 The first and second burst disks 60 and 62 have inner side surfaces 70 and
 72, respectively, which are exposed to the pressure of the inflation fluid
 66 when the first and second burst disks are unruptured. The burst disks
 60 and 62 could, alternatively, be formed in one piece with the second end
 wall 28, as thin-walled sections of the second end wall, for example.
 A pressure differential exists across the first burst disk 60 when the
 inflator 10 is in the unactuated condition shown in FIG. 1, because the
 inflation fluid 66 in the gas storage chamber 14 is maintained at a
 pressure greater than the ambient air pressure in the diffuser chamber 34.
 The first burst disk 60 is rupturable when the pressure differential
 across the first burst disk exceeds a first predetermined pressure
 differential. When the inflator 10 is in the unactuated condition shown in
 FIG. 1, the pressure differential across the first burst disk 60 is less
 than the first predetermined pressure differential, and the first burst
 disk does not rupture.
 A pressure differential also exists across the second burst disk 62 when
 the inflator 10 is in the unactuated condition shown in FIG. 1, because
 the inflation fluid in the gas storage chamber 14 is maintained at a
 pressure greater than the ambient air pressure in the diffuser chamber 34.
 The second burst disk 62 is rupturable when the pressure differential
 across the second burst disk exceeds a second predetermined pressure
 differential. When the inflator 10 is in the unactuated condition shown in
 FIG. 1, the pressure differential across the second burst disk 62 is less
 than the second predetermined pressure differential, and the second burst
 disk does not rupture.
 The first burst disk 60 is designed to rupture at a different pressure than
 the second burst disk 62. Specifically, the first burst disk 60 is
 designed to rupture at a lower pressure differential than the second burst
 disk 62. Because both the first burst disk 60 and the second burst disk 62
 are exposed to ambient pressure on one side, from the diffuser chamber 34,
 the two burst disks rupture at different pressures of inflation fluid 66
 in the gas storage chamber 14.
 The two burst disks 60 and 62 can be configured in alternative manners to
 provide the different rupture pressures. For example, the first burst disk
 60 could be thinner and therefore weaker than the second burst disk 62.
 Alternatively, the first burst disk 60 could be scored with score lines
 and thereby made weaker than the second burst disk 62.
 The inflator 10 includes an actuator assembly 80 for producing combustion
 products for heating and pressurizing the inflation fluid 66. The actuator
 assembly 80 is located in the gas storage chamber 14 in the container 12.
 The actuator assembly 80 includes a propellant housing 82. The propellant
 housing 82 is a cup-shaped metal member having a generally cylindrical
 configuration. The propellant housing 82 includes a side wall 84 having a
 cylindrical inner surface 86. An end portion 88 of the side wall 84 is
 screwed into a projecting flange 90 on the first end wall 26 of the
 inflator 10. An end wall 92 at the opposite end of the side wall 84 closes
 the propellant housing 82.
 An annular, radially extending shoulder surface 94 is formed on the inner
 surface 86 of the side wall 84 of the propellant housing 82. The shoulder
 surface 94 is presented toward the first end wall 26 of the inflator 10. A
 circular array of primary nozzles 96 is formed in the side wall 84 of the
 propellant housing 82. The primary nozzles 96 are located between the
 shoulder surface 94 and the first end portion 88 of the propellant housing
 82.
 A circular array of secondary nozzles 98 is formed in the side wall 84 of
 the propellant housing 82. The secondary nozzles 98 are located between
 the shoulder surface 94 and the end wall 92 of the propellant housing 82.
 The total flow area of the secondary nozzles 98, as a group, preferably is
 larger than the total flow area of the primary nozzles 96.
 The actuator assembly 80 includes a separator 100 located in the propellant
 housing 82. The separator 100 is a metal plate having a radially extending
 central wall 102 and a cylindrical, axially extending side flange 104. A
 circular opening 106, centered on the axis 24, is formed in the central
 wall 102 of the separator 100.
 The side flange 104 of the separator 100 is in abutting engagement with the
 inner surface 86 of the side wall 84 of the propellant housing 82. The
 radially outermost portion of the central wall 102 of the separator 100 is
 in engagement with the shoulder surface 94 on the propellant housing 82.
 The engagement of the separator 100 with the shoulder surface 94 blocks
 movement of the separator in a direction toward the end wall 92 of the
 propellant housing 82.
 The separator 100 is located between and partially defines primary and
 secondary propellant chambers 112 and 114 in the propellant housing. The
 primary nozzles 96 establish fluid communication between the primary
 propellant chamber 112 and the gas storage chamber 14. The secondary
 nozzles 98 establish fluid communication between the secondary propellant
 chamber 114 and the gas storage chamber 14. The opening 106 in the central
 wall 102 of the separator establishes fluid communication between the
 primary propellant chamber 112 and the secondary propellant chamber 114.
 A primary propellant 122 is disposed in the primary propellant chamber 112.
 The primary propellant 122 is preferably provided in the form of discs or
 pellets, illustrated schematically in FIG. 1, of a known pyrotechnic
 material. A secondary propellant 124 is disposed in the secondary
 propellant chamber 114. The secondary propellant 124 is preferably
 provided in the form of discs or pellets made of the same material as the
 primary propellant 122.
 The pellets which form the secondary propellant 124 are larger than the
 pellets which form the primary propellant 122. As a result, the secondary
 propellant 124 has a relatively slow rate of generation of combustion
 products ("burn rate"), and the primary propellant 122 has a relatively
 fast rate of generation of combustion products ("burn rate").
 The inflator includes an initiator 130. The initiator 130 is a known device
 which is electrically energizable, by an electric signal over lead wires
 132, to produce hot combustion products for igniting the primary
 propellant 122. The initiator 130 is located in an initiator housing 134
 secured in the first end wall 26 of the inflator 10. The initiator 130 is
 connected in fluid communication with the primary propellant chamber 112
 by a passage 136 in the first end wall 26 of the inflator 10. A rupturable
 burst disk 138 closes the passage 136 prior to energization of the
 initiator 130.
 The vehicle safety apparatus 11 includes known means indicated
 schematically at 140 (FIG. 1) for sensing a collision involving the
 vehicle and for energizing the initiator 130 in response to the sensing of
 a collision. The means 140 may include a sensor 142 and vehicle electric
 circuitry for energizing the initiator 130 in response to sensing a
 vehicle condition having a severity greater than a predetermined threshold
 value.
 In the event of sensing such a condition, the sensing means 140 provides an
 electrical signal over the lead wires 132 to the initiator 130 in the
 inflator 10. The initiator 130 is actuated in a known manner and ruptures
 the burst disk 138. The combustion products of the initiator 130 ignite
 the primary propellant 122. The primary propellant 122 burns and produces
 hot combustion products. The relatively small primary nozzles 96 maintain
 a high pressure in the primary chamber 112 and help the primary propellant
 122 to react with a fast burn rate.
 The combustion products of the primary propellant 124 flow through the
 primary nozzles 96 in the side wall 84 of the propellant housing 82 and
 into the gas storage chamber 14. The combustion products heat and
 pressurize the inflation fluid 66 in the gas storage chamber 14. The
 pressure in the gas storage chamber 14 increases sufficiently that the
 pressure differential across the first burst disk 60 exceeds the first
 predetermined pressure differential. The first burst disk 60 ruptures and
 the inflation fluid 66 flows out of the gas storage chamber 14 into the
 diffuser chamber 34.
 The inflation fluid 66 flows from the diffuser chamber 34 to the air bag
 32, to inflate the air bag. The second predetermined pressure differential
 (for the second burst disk 62) is selected so that the second burst disk
 does not rupture at the pressure levels reached in the gas storage chamber
 14 when only the primary propellant 122 is ignited.
 The separator 100 prevents the secondary propellant 124 from being ignited
 directly by the combustion products of the initiator 130. When the primary
 propellant 122 burns, however, it produces a jet of hot combustion
 products, or flame, which travels through the opening 106 in the central
 wall 104 of the separator 100 into the secondary propellant chamber 114.
 The combustion products of the primary propellant 122 ignite the secondary
 propellant 124. Thus, the secondary propellant 124 is ignited only after
 some burning of the primary propellant 122, thus achieving a desired time
 delay.
 The relatively large secondary nozzles 98 help to reduce the pressure in
 the secondary chamber 114 and thus help to provide a lower burn rate for
 the secondary propellant 124. The combustion products of the secondary
 propellant 124 flow through the secondary nozzles 98 in the side wall 84
 of the propellant housing 82 and into the gas storage chamber 14. The
 combustion products further heat and pressurize the inflation fluid 66 in
 the gas storage chamber 14.
 The pressure in the gas storage chamber 14 increases sufficiently that the
 pressure differential across the second burst disk 62 exceeds the second
 predetermined pressure differential, thus rupturing the second burst disk.
 The inflation fluid 66 flows out of the gas storage chamber 14 into the
 diffuser chamber 34.
 The sequential opening of the burst disks 60 and 62 provides an output
 curve for the inflator 10 in which pressure rises over time, as may be
 desired. Several other features and characteristics of the inflator 10 can
 be varied and controlled to provide a desired output curve for the
 inflator. These items include the absolute and relative burn rates of the
 first and second propellants 122 and 124; the absolute and relative sizes
 of the primary and secondary nozzles 96 and 98; and the rupturing
 characteristics of the two burst disks 60 and 62.
 In addition, the size of the flame jet passing through the separator
 opening 106 into the secondary propellant chamber 114 affects the burning
 time of the secondary propellant 124. Therefore, the size and location of
 the opening 106 from the primary propellant chamber 112 into the secondary
 propellant chamber 114 can be varied, or a plurality of such openings can
 be provided.
 As noted above, the secondary propellant 124 has a lower burn rate than the
 primary propellant 122. The different burn rates for the two propellants
 122 and 124 can be achieved in a variety of ways. For example, different
 propellant materials or mixtures of different propellant materials can be
 used to provide the propellants 122 and 124 with different burn rates.
 Alternatively, the propellants 122 and 124 can have different
 fuel-oxidizer ratios, or different consolidation densities (obtained by
 subjecting the propellant materials to different pressures during
 formation of the propellants).
 FIG. 2 illustrates a vehicle safety apparatus 11a including an inflator 10a
 in accordance with a second embodiment of the invention. The inflator 10a
 is generally similar in construction and operation to the inflator 10
 (FIG. 1), and similar parts are identified by similar reference numerals
 with the suffix "a" added for clarity.
 The inflator 10a includes only one burst disk 60a for enabling flow of
 inflation fluid 66a out of the gas storage chamber 14a into the diffuser
 30a. As a result, the output characteristics of the inflator 10a are
 different from those of the inflator 10. Specifically, the single burst
 disk 60a is rupturable under the pressure of the inflation fluid 66a when
 the inflation fluid is heated and pressurized by the combustion products
 of the primary propellant 122a. Subsequent ignition of the secondary
 propellant 124a results in increased flow of inflation fluid out of the
 inflator 10a, rather than rupturing of a second burst disk.
 FIG. 3 illustrates an inflator 10b in accordance with a third embodiment of
 the invention. The inflator 10b is similar in construction and operation
 to the inflator 10 (FIG. 1), and similar parts are identified by similar
 reference numerals with the suffix "b" added for clarity.
 The inflator 10b includes an actuator assembly 80b having first and second
 propellants 122b and 124b which are separated radially rather than axially
 as in the inflator 10. Specifically, the actuator assembly 80b includes a
 two-piece propellant housing 82b. The propellant housing includes a
 primary housing 83b and a secondary housing 85b. The primary housing 83b
 has a cup-shaped configuration including a cylindrical side wall 84b. An
 end portion 88b of the side wall is screwed into a projecting flange 90b
 on the first end wall 26b of the inflator 10b.
 The side wall 84b of the primary housing 83b has an outwardly flared
 portion 87b which merges into a radially extending end wall 92b of the
 primary housing. A plurality of primary nozzles 96b extend through the end
 wall 92b of the primary housing 83b. A primary propellant chamber 112b is
 defined inside the primary housing 83b.
 The secondary housing 85b extends around the primary housing 83b. The
 secondary housing 85b has a cylindrical side wall with a plurality of
 secondary nozzles 98b. The secondary housing 85b has a radially inwardly
 projecting end wall 99b which is engaged by the flared portion 87b of the
 primary housing 83b. This engagement holds the secondary housing 85b in
 position against the first end wall 26b of the inflator 10b. An annular
 secondary propellant chamber 114b is defined inside the secondary housing
 85b, radially outward of the side wall 84b of the primary housing 83b.
 The side wall 84b of the primary housing 83b acts as a separator between
 the primary chamber 112b and the secondary chamber 114b. A plurality of
 openings 106b in the side wall 84b of the primary housing 83b establish
 fluid communication between the primary propellant chamber 112b and the
 secondary propellant chamber 114b.
 A primary propellant 122b is disposed in the primary propellant chamber
 112b. A secondary propellant 124b is disposed in the secondary propellant
 chamber 114b. The secondary propellant 124b has a relatively slow burn
 rate, and the primary propellant 122b has a relatively fast burn rate.
 The propellant assembly 80b functions in a manner similar to the propellant
 assembly 80. Upon actuation of the initiator 130b, combustion products of
 the initiator ignite the primary propellant 122b. The primary propellant
 122b burns and produces hot combustion products which flow into the gas
 storage chamber 14b. The combustion products heat and pressurize the
 inflation fluid 66b in the gas storage chamber 14b. The pressure in the
 gas storage chamber 14b increases sufficiently to rupture the first burst
 disk 60b.
 Combustion products of the primary propellant 122b also travel through the
 openings 106b into the secondary propellant chamber 114b. The combustion
 products of the primary propellant 122b ignite the secondary propellant
 124b. The combustion products of the secondary propellant 124b flow
 through the secondary nozzles 98b into the gas storage chamber 14b. The
 combustion products further heat and pressurize the inflation fluid 66b in
 the gas storage chamber 14b, and the second burst disk 62b ruptures.
 Alternatively, the inflator lob could, like the inflator 10a, include only
 one burst disk rather than two.
 From the above description of the invention, those skilled in the art will
 perceive improvements, changes and modifications in the invention. For
 example, an inflator could include three or more burst disks which rupture
 at different pressure differentials across the respective burst disks.
 Also, an inflator could include a single burst disk structure having burst
 areas of different thicknesses, for example, and which areas burst at
 different pressure differentials across the respective areas. Such
 improvements, changes and modifications within the skill of the art are
 intended to be covered by the appended claims.