An apparatus (10) for helping to protect an occupant of a vehicle comprises a knee bolster (20) for engaging at least one leg of the occupant to restrain the occupant. The knee bolster (20) comprises a metal tube (50) having a collapsed condition and an expanded condition. The metal tube (50) when in the expanded condition positions the knee bolster (20) to help protect the vehicle occupant. The apparatus (10) also comprises a pyrotechnic transfer line (60, 62, 64) extending within the collapsed metal tube (50). The pyrotechnic transfer line (60, 62, 64) when ignited generates fluid under pressure in the metal tube to expand the metal tube from the collapsed condition to the expanded condition. The apparatus (10) further comprises an igniter (70) for igniting the pyrotechnic transfer line (60, 62, 64).

BACKGROUND OF THE INVENTION
 1. Technical Field
 The present invention relates to an apparatus for helping to protect an
 occupant of a vehicle, and relates particularly to an apparatus including
 a knee bolster movable from a stored position to a blocking position.
 2. Description of the Prior Art
 Actuatable vehicle occupant knee bolsters are known in the art. Such a knee
 bolster is located at a lower portion of a vehicle instrument panel and is
 movable from a stored position to a blocking position in response to
 detection of a vehicle crash condition. The knee bolster can help to
 prevent the vehicle occupant from "submarining" underneath an inflated air
 bag. Knee bolster systems have been developed that use an inflatable
 device, such as an air bag, for moving the knee bolster.
 Also known in the art is a device known as a pyrotechnic transfer line, or
 gas generating linear pyrotechnic element. This device is an elongate
 assembly comprising three strands of glass fiber that are impregnated with
 an explosive material and enclosed in a plastic sleeve. It is known to use
 a pyrotechnic transfer line as an ignition device, or fuse, for an
 actuatable device such as an air bag inflator. It is also known to use a
 pyrotechnic transfer line as a gas generating device for a pretensioner or
 a side curtain. Such uses are shown in U.S. Pat. Nos. 5,145,209 and
 5,755,457.
 SUMMARY OF THE INVENTION
 The present invention is an apparatus for helping to protect an occupant of
 a vehicle. The apparatus comprises a knee bolster for engaging at least
 one leg of a vehicle occupant to restrain the occupant. The knee bolster
 comprises a metal tube having a collapsed condition and an expanded
 condition. The metal tube when in the expanded condition positions the
 knee bolster to help protect the vehicle occupant. The apparatus also
 comprises a pyrotechnic transfer line extending within the collapsed metal
 tube. The pyrotechnic transfer line when ignited generates fluid under
 pressure in the metal tube to expand the metal tube from the collapsed
 condition to the expanded condition. The apparatus further comprises an
 igniter for igniting the pyrotechnic transfer line.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION
 The present invention relates to an apparatus for helping to protect an
 occupant of a vehicle. In particular, the present invention relates to a
 vehicle occupant protection apparatus including a knee bolster movable
 from a stored position to a blocking position. As representative of the
 present invention, FIG. 1 illustrates an apparatus 10.
 The apparatus 10 includes a knee bolster 20. The knee bolster 20 is formed
 as a lower portion 22 of a vehicle instrument panel 24. The lower portion
 22 of the instrument panel 24 is the portion of the instrument panel that
 a vehicle occupant's knees would contact were the occupant to slide
 forward from a seated position to contact the instrument panel.
 The knee bolster 20 includes part of the vehicle instrument panel 24 such
 as a trim pad. The knee bolster 20 is made from a relatively soft material
 such as a resilient foam material 26 covered by a skin 28. The knee
 bolster 20 has an outer side surface 30 that is presented rearward in the
 vehicle, that is, in a direction toward a vehicle occupant. The knee
 bolster 20 also has a lower side surface 32.
 A support member illustrated schematically at 40 is located in the
 instrument panel 24. The support member 40 is illustrated as being made
 from metal but could, alternatively, be made from a stiff plastic or other
 material. An opening 42 is formed in the knee bolster 20 between the
 support member 40 and the outer side surface 30. The opening 42 extends in
 a direction generally parallel to the outer side surface 30 of the bolster
 20. The opening 42 is located about one to two inches away from (inward
 of) the outer side surface 30. The portion of the bolster 20 located
 between the opening 42 and the outer side surface 30 is a movable portion
 44 of the bolster. A fixed portion 46 of the bolster 20 is located inward
 of the opening 42.
 The knee bolster 20 includes a metal tube 50. The metal tube 50 is located
 in the opening 42 in the bolster 20. The tube 50 is preferably made from
 low carbon steel having good yield properties. The tube 50 as illustrated
 in FIG. 1 is in a collapsed condition in the bolster 20. When first made,
 the tube 50 is in an expanded condition and has a circular cross-sectional
 configuration as shown in dashed lines in FIG. 3. The tub e 50 h as first
 and second opposite end portions 52 and 54 and a central chamber 56.
 The apparatus 10 includes at least one pyrotechnic transfer line, or gas
 generating linear pyrotechnic element. In the illustrated embodiment,
 three pyrotechnic transfer lines 60, 62 and 64 are provided. Each one of
 the three pyrotechnic transfer lines 60, 62 and 64 is a known device
 comprising strands of glass fiber impregnated with an explosive material,
 as described in the above-mentioned U.S. Pat. No. 5,145,209. The strands
 are enclosed in a plastic sleeve. The pyrotechnic transfer line material
 is available commercially from Explosive Technology, Inc., Fairfield,
 Calif., under the brand name ITLX. Pyrotechnic transfer lines suitable for
 use in the apparatus 10 are also available from other vendors.
 The pyrotechnic transfer lines 60, 62 and 64 are inserted into the central
 chamber 56 in the metal tube 50 when the metal tube is in the expanded
 condition shown in dashed lines in FIG. 3. The metal tube 50 is then
 deformed into the collapsed condition shown in FIG. 1 and in solid lines
 in FIG. 3.
 After the tube 50 is collapsed with the pyrotechnic transfer lines 60-64 in
 it, the first end portion 52 of the metal tube is welded shut. An igniter
 70 is attached to the second end portion 54 of the metal tube 50. The
 igniter 70 is a known device that generates heat and pressure for igniting
 the pyrotechnic transfer lines 60-64. The igniter 70 is sealed to the
 second end portion 54 of the collapsed metal tube 50.
 The collapsed metal tube 50 is located in the opening 42 in the bolster 20.
 The metal tube 50 may be insert molded in the bolster 20, with the opening
 42 being formed in situ. Alternatively, the metal tube 50 may be inserted
 into an opening 42 that is previously formed in the bolster 20. The
 bolster 20 may alternatively comprise a separate movable portion 44
 secured to a fixed portion 46, with the metal tube 50 being secured
 between them and the opening 42 comprising the space between them.
 The igniter 70 is electrically connected to an actuation circuit in the
 vehicle shown schematically at 80. The actuation circuit 80 includes a
 crash sensor, such as an inertia switch or an accelerometer, and a
 controller. Upon detection of a crash condition requiring occupant
 protection, as sensed by the crash sensor, the controller controls the
 actuation circuit 80 so that the actuation circuit sends an electric
 signal to the igniter 70 to actuate the igniter.
 When the igniter 70 is actuated, it emits combustion products including
 heat and a shock wave. The pyrotechnic transfer lines 60-64 are ignited by
 the combustion products of the igniter 70. The pyrotechnic transfer lines
 60-64 generate gas under pressure in the chamber 56 in the collapsed metal
 tube 50. The force of the pressurized gas causes the collapsed metal tube
 50 to expand and regain its original circular cross-sectional
 configuration.
 The expansion of the metal tube 50 occurs rapidly, generally within about 5
 milliseconds. As the metal tube 50 expands, the support member 40 resists
 expansion of the metal tube inward into the instrument panel 24.
 Therefore, the metal tube 50 expands outwardly, that is, in a direction
 toward the outer side surface 30 of the knee bolster 20. The expansion of
 the metal tube 50 forces the movable portion 44 of the bolster 20 away
 from the fixed portion 46 of the bolster, in a direction toward the
 vehicle occupant compartment.
 The knee bolster 20 moves from the stored condition shown in solid lines in
 FIG. 1 to a blocking position as shown in dashed lines in FIG. 1. The knee
 bolster 20, when in the blocking position, is positioned to contact the
 knees of a vehicle occupant and block further movement of the vehicle
 occupant in a forward direction in the vehicle. This blocking action can
 help to protect the vehicle occupant by preventing contact between the
 vehicle occupant and the instrument panel 24, and by preventing the
 vehicle occupant from "submarining" underneath an inflated air bag.
 The diameter of the metal tube 50 determines how far the movable portion 44
 of the bolster 20 can move. The diameter of the tube 50 is generally equal
 to the amount of bolster movement provided. For example, expansion of a
 four inch diameter metal tube 50 will cause the movable part 44 of the
 bolster 20 to move about four inches away from the fixed part 46 of the
 bolster. Metal tubes having a diameter of from one-half inch to four
 inches or more can be used.
 The wall thickness of the material of the metal tube 50 depends on the
 amount of pressure to be generated within the tube. The tube 50 can have a
 wall thickness in the range of from one half millimeter thick to four
 millimeters thick.
 The bolster 20 shown in FIG. 1 includes a hinge line indicated
 schematically at 82, and a tear seam indicated schematically at 84. The
 hinge line 82 and tear seam 84 can enable additional movement of the
 movable part 44 of the bolster 20.
 From the above description of the invention, those skilled in the art will
 perceive improvements, changes and modifications. For example, the support
 member 40 can be eliminated to reduce cost, although possibly providing
 less movement of the movable part 44 of the bolster 20. Also, the number
 of pyrotechnic transfer lines used can vary, depending on how much
 pressure is to be generated within the tube. Such improvements, changes
 and modifications within the skill of the art are intended to be covered
 by the appended claims.