Abstract:
An air bag cover assembly including a cover element of generally profiled configuration and including one or more mounting posts extending downwardly from the interior surface of the cover element. A reaction plate of generally curved surface contour substantially matching the contour of the inner surface of the cover element is held in place at a position below the cover element. A membrane horn switch is disposed in sandwiched relation between the reaction plate and the cover element. The matching contour of the inner surface of the cover element and the reaction plate establishes a substantially uniform operative separation between the cover element and the membrane horn switch.

Description:
TECHNICAL FIELD 
     The present invention relates to a cover assembly for a vehicle air bag, and more particularly to a cover assembly for use at the interior of a steering wheel on the driver&#39;s side of a vehicle which incorporates a contoured cover element, an underlying membrane horn switch and a contoured reaction plate beneath the membrane horn switch. The reaction plate includes an upper surface contoured to substantially match the interior contour of the cover element such that a substantially uniform distance is maintained between the cover element and the horn switch between the upper and lower edges of the cover element. 
     BACKGROUND OF THE INVENTION 
     It is well known in motor vehicles to provide an air bag module for protecting a vehicle occupant. It is also generally known to provide an air bag module including an inflatable restraint cushion and an inflator for discharging inflation gas to inflate the cushion upon sensing predetermined vehicle conditions such as deceleration exceeding a certain level. It is also known to provide air bag modules including pressure activated membrane horn switches disposed in sandwiching relationship between a molded depressable cover element and a substantially rigid reaction plate. Such air bag modules are illustrated and described in U.S. Pat. No. 6,236,309 to Haag and U.S. Pat. No. 5,762,365 to Worrell et al. the contents of which are incorporated by reference as if fully set forth herein. 
     The membrane horn switches conventionally comprise two very thin sheets having conductive coatings which are normally separated by thin spaces. Pressure on the switch forces the conductive surfaces together so as to complete a circuit and thereby actuate the horn. Typically, membrane horn switches are less than about one millimeter in thickness. To actuate the horn, the driver must sufficiently compress the cover member so as to apply the requisite pressure to the membrane horn switch to bring the opposing conducting surfaces together. 
     The use of a reaction plate in mounted relation to the cover element generally below the membrane horn switch provides a substantially solid surface against which the membrane horn switch can be compressed to actuate the horn. In practice, the reaction plate is typically held in place below the cover element and the membrane horn switch by a plurality of mounting posts which extend away from the interior surface of the cover element through aligned apertures in both the membrane horn switch and the reaction plate. The ends of the posts are melted together to heat stake the reaction plate to the cover. The interior surface of the cover element may also incorporate horn actuating projections which normally extend partially across the space between the interior surface of the cover element and the membrane horn switch. The use of such horn actuating projections reduces the distance the cover element must be compressed in order to actuate the membrane horn switch. 
     In the past, the reaction plates utilized have been of a generally planar configuration thereby forming a generally flat surface in operative opposing relationship to the interior surface of the cover element. While such a planar surface provides substantially uniform support to the membrane horn switch, the distance between the membrane horn switch and the interior surface of the cover element may vary across the module if the interior surface of the cover element is not also planar. 
     Recently, cover elements have become more stylized leading to a greater degree of curvature in the outer surface of the cover element facing the occupant. In addition, the use of so called “single shot” injection molding procedures to form the cover elements from a single layer of material has gained popularity. In such “single shot” constructions it is generally desired for the cover element to have a substantially uniform thickness in the regions overlying the, membrane horn switch so as to promote substantially uniform compressibility in those regions. However, in a contoured construction the incorporation of a substantially uniform thickness gives rise to a corresponding contour across the inner surface of the cover element. Such an inner surface contour gives rise to a variation in the distance between the interior surface of the cover element and the membrane horn switch. This differential distance may require the cover element to be depressed to a greater extent in some locations in order to contact and actuate the membrane horn switch. 
     SUMMARY OF THE INVENTION 
     The present invention provides advantages and alternatives over the prior art by providing an air bag cover assembly including a cover element of generally profiled configuration and including one or more mounting posts extending downwardly from the interior surface of the cover element. A reaction plate of generally contoured construction substantially matching the contour of the cover element is held in place at a position below the inner surface of the cover element. A membrane horn switch is disposed in sandwiched relation between the reaction plate and the inner surface of the cover element. 
     According to one aspect of the present invention, an air bag cover assembly is provided including a cover element having a curved exterior show surface and a curved interior surface and wherein a plurality of mounting posts extend downwardly away from the interior surface. A reaction plate of substantially curved contour substantially matching the contour of the inner surface of the cover element is held in place at a position below the inner surface of the cover element by heat staking of the mounting posts. A membrane horn switch is captured between the reaction plate and the interior surface of the cover element such that the distance between the interior surface of the cover element and the membrane horn switch is substantially uniform across the cover assembly. 
     According to another aspect of the present invention, the reaction plate includes a plurality of apertures for acceptance of mounting posts therethrough. The mounting posts preferably include a shoulder portion which engages a substantially planar shoulder stop in surrounding relation to the post acceptance apertures at the upper surface of the reaction plate. A plurality of protuberant boss elements are disposed across the underside of the reaction plate substantially at the post accepting apertures to facilitate heat staking at those locations. Preferably, the reaction plate is curved in an arched configuration substantially between the forward and aft edges adjacent the upper and lower edges of the air bag module. 
     The construction according to the present invention is believed to promote an enhanced degree of uniformity in the force required to actuate the membrane horn switch at various locations beneath a curved cover while at the same time facilitating proper alignment and attachment of the component parts thereby improving the manufacturing process. Accordingly, the present intention is believed to provide a useful advancement over the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings which are incorporated in and which constitute a part of this specification illustrate one embodiment of the present invention and, together with the general description of the invention given above, and the detail descriptions set forth below, serve to explain the principles of the invention wherein: 
     FIG. 1 is a plan view of a steering wheel and air bag cover assembly according to the present invention; 
     FIG. 2 is a sectional view of an air bag cover assembly according to the prior art; 
     FIG. 3 is an elevated perspective view of a contoured reaction plate for use in the air bag cover assembly according to the present invention; 
     FIG. 4 is an exploded sectional view of an air bag cover assembly according to the present invention taken generally along line  4 — 4  in FIG. 1; 
     FIG. 5 is a view taken generally along line  5 — 5  in FIG.  4  and illustrating a mounting post construction for use in the air bag cover assembly according to the present invention; 
     FIG. 6 is a plan view of a membrane horn switch for use in the air bag cover assembly according to the present invention; and 
     FIG. 7 is a sectional view similar to FIG. 2 illustrating an assembled air bag cover assembly according to the present invention. 
    
    
     While the invention has been illustrated and will hereinafter be described in relation to certain potentially preferred embodiments, practices and procedures, it is to be understood that the invention is in no way to be construed as limited to such embodiments, practices or procedures. To the contrary it is anticipated that modifications may be made and that other embodiments of the principles of the invention may occur to those of skill in the art. Therefore, it is the intent of the applicants to cover all such modifications and embodiments as may incorporate such principles broadly within the true spirit and scope of the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a vehicle steering wheel  10  includes a central hub portion  12 , a generally circular outer rim portion  14 , and a plurality of spokes  16  extending between the hub portion  12  and the rim portion  14 . An air bag module  18  is mounted within the hub portion  12  of the steering wheel  10 . The air bag module  18  includes an air bag cushion (not shown) and an inflator (not shown) for generating gas to inflate the air bag cushion. The air bag module  18  also includes an air bag cover assembly generally designated as  20 . 
     As best illustrated in FIGS. 1,  3 ,  4  and  6 , the air bag cover assembly  20  includes a cover element  22  including an upper edge  21  and a lower edge  23 , a membrane horn switch  24 , and a reaction plate  26 . It is contemplated that the reaction plate  26  may be of a multi-piece construction if desired so as to facilitate outward deployment of the underlying air bag cushion. Due to the very thin nature of the membrane horn switch  24 , the membrane horn switch may be of either a single piece or a multi-piece construction. However, a single piece construction may be preferred to minimize complexity during the assembly process. 
     In accordance with one aspect of the present invention, the cover element  22  includes a contoured outer show surface  30  (FIG. 4) and a corresponding contoured inner surface  32 . A plurality of mounting posts  34  arranged in pairs extend away from locations at the inner surface  32  for insertion through complimentary aligned openings within the membrane horn switch  24  and the reaction plate  26 . According to a potentially preferred configuration, both mounting posts  34  in each pair pass through a single opening  60  within the membrane horn switch  24  (FIG. 6) while each of the mounting posts  34  thereafter passes through an aligned discrete reaction plate post acceptance aperture  36  within the reaction plate  26 . As illustrated in FIG.  3 , the reaction plate post acceptance apertures  36  are preferably arranged in complementary pairs predominantly around the periphery of the reaction plate  26 . Following insertion of the mounting posts  34  through the aligned openings within the membrane horn switch  24  and reaction plate  26 , the distal ends of the mounting posts  34  may be fused together by heat so as to maintain their assembled positional relationship. 
     Referring to FIG. 2, a typical prior art air bag cover assembly  20 ′ is illustrated wherein the cover element  22 ′ is of a substantially contoured geometry curved along an surface between the upper edge  21 ′ and a lower edge  23 ′. In such prior art air bag cover assembly  20 ′, the reaction plate  26 ′ typically has been substantially planar in configuration thereby causing the membrane horn switch  24 ′ to adopt a similar planar orientation. As illustrated, the use of such a substantially flat reaction plate  26 ′ in combination with a curved cover element  22 ′ gives rise to a varying distance between the membrane horn switch  24 ′ and the inner surface  32 ′ of the cover element  22 ′. This variation in distance between the inner surface  32 ′ and the membrane horn switch  24 ′ may be particularly pronounced in so called “single shot” cover elements which in corporate only a single layer of material. As will be appreciated, a variation in the distance between the membrane horn switch  24 ′ and the cover element  22 ′ may give rise to varying levels of force being required to effect activating contact between the inner surface  32 ′ and the membrane horn switch. 
     As illustrated in FIGS. 3,  4 , and  7 , the present invention addresses the deficiencies of the prior art designs through incorporation of an improved reaction plate  26  of a generally contoured curved construction substantially corresponding to the curvature of the inner surface  32  of the cover element  22  across the operative region supporting the membrane horn switch  24 . The improved reaction plate  26  further provides enhanced features to facilitate the efficient and effective attachment of the curved construction to the cover element  22 . 
     As best illustrated in FIG. 3, the reaction plate  26  is preferably of a two piece construction having a general plan geometry substantially corresponding to the desired activation area for the membrane horn switch  24 . Accordingly, the reaction plate  26  preferably includes an upper plate edge  41  and a lower plate edge  43  for disposition adjacent the upper edge  21  and the lower edge  23  of the cover element  22 . 
     As illustrated, the reaction plate  26  is preferably of a two piece construction including a first segment  50  and a second segment  52  matable to one another at an interior location beneath the cover element  22  such as beneath a central tear seam  54  (FIG. 1) disposed across the cover element  22 . Such a segmented construction may reduce the force required to effect outward deployment of an underlined air bag cushion (not shown) while at the same time providing a substantially uniform support surface for the membrane horn switch. Of course, a single piece reaction plate may also be utilized if desired. 
     In accordance with the present invention, the reaction plate is preferably curved so as to form a generally convex surface between the upper plate edge  41  and the lower plate edge  43 . As illustrated, this curvature preferably corresponds substantially to the degree of curvature across the inner surface  32  of the cover element  22  such that upon assembly a substantially uniform distance is maintained between the membrane horn switch  24  and the cover element  22 . If desired, the reaction plate  26  may also be slightly curved in the dimension extending between lateral sides  44 ,  46 . However it is believed that such curvature should not be excessive so as to avoid placing undue stress upon the membrane horn switch  24 . 
     According to the potentially preferred embodiment, the membrane horn switch  24  is held in place atop the reaction plate  26  by a plurality of clip elements  56  disposed at the periphery of the reaction plate  26 . In assembly, the horn switch may be slid between the clip elements  56  and standoff elements  58  disposed across the surface of the reaction plate  26  in surrounding relation to the clip elements  56 . The clip elements  56  and standoff elements  58  thus serve to hold the membrane horn switch  24  at a desired height relative to the surface of the reaction plate. 
     In addition to the relative height control provided by the clip elements  56  and standoff elements  58 , the membrane horn switch  24  is secured against slipping out of position by passing one or both mounting posts  34  through aligned apertures  60  (FIG. 6) at the perimeter of the membrane horn switch  24  in a manner as will be well known to those of skill in the art. 
     As illustrated in FIGS. 3,  4  and  7 , in the illustrated and potentially preferred embodiment the reaction plate  26  preferably includes a plurality of rounded force concentrating projections  45  arranged across the upper surface of the reaction plate  26  and beneath the operative portions of the membrane horn switch  24 . As will be appreciated, such projections provide for the concentration of force at substantially discrete points across the membrane horn switch upon the application of pressure. Such force concentration may facilitate the ability to maintain the requisite contact between opposing surfaces in the horn switch even if force is applied across a relatively broad surface by the operator. 
     One potentially preferred configuration for the membrane horn switch is illustrated in FIG.  6 . As shown, in this configuration the membrane horn switch  24  incorporates a plurality of horn switch post acceptance apertures  60 . The horn switch post acceptance apertures  60  preferably each accept two mounting posts  34  and are positioned for alignment substantially over the reaction plate post acceptance apertures  36  so as to permit the substantially unobstructed insertion of the mounting posts  34  through both the membrane horn switch  24  and the reaction plate  26  during assembly. 
     As shown in FIG. 5, the mounting posts  34  preferably include a proximal shoulder portion  64  of enhanced width extending away from the inner surface  32  of the cover element  22 . The mounting posts  34  thereafter narrow to a reduced operative width distal portion  65  corresponding substantially to the dimensions of the reaction plate post acceptance apertures  36 . Thus, the mounting posts  34  may be inserted through the reaction plate post acceptance apertures  36  until reaching the shoulder portion  64  at which point further insertion is prevented. Conversely, the horn switch post acceptance apertures  60  are of sufficient dimension to permit the complete insertion of all portions of the mounting posts  34  therethrough. 
     As illustrated, according to the potentially preferred embodiment the reaction plate post acceptance apertures  36  are surrounded by a substantially flat surface zone  66  forming a shoulder stop surface. This substantially flat surface zone  66  is preferably formed as a depression of variable depth across the upper surface of the reaction plate  26  with depth of the depression increasing towards the interior of the reaction plate  26 . Such a variable depth depression has been found to be an effective and efficient mechanism to provide a substantially uniform support surface for the shoulder portions  64  of the mounting posts  34 . This, in turn, promotes the ability of the shoulder portions  64  to be held in stable supported contact with the substantially flat surface zone  66 . The stable interface between the shoulder portions and the substantially flat surface zones  66  promotes the establishment and maintenance of proper relative positioning between the cover element  22  and the reaction plate  26 . In addition, the(walls of the depression which surround the shoulder portions  64  assist in establishing proper positioning of the cover element  22  relative to the reaction plate  26  and provide further support against undesired lateral slippage once insertion of the mounting posts  34  has taken place. 
     As illustrated in FIGS. 4 and 7, the reaction plate  26  preferably also includes a plurality of protuberant boss elements  68  arranged across the lower surface of the reaction plate  26 . As shown, these boss elements  68  preferably surround the reaction plate post acceptance apertures  36 . The boss elements are preferably substantially flat on their underside and provide a substantially uniform surface for the fusion of the distal ends of the mounting posts  34  thereby locking the elements of the air bag cover assembly  20  in place. In addition, the boss elements  68  also serve to provide an enhanced mass heat sink to absorb excess energy during the fusion of the mounting posts  34  to one another. 
     It is believed that the present invention provides a new and useful air bag cover assembly useful in applications requiring the use of contoured air bag covers. While specific embodiments of the present invention have been illustrated and described, it is to be understood that the invention is in no way to be limited to such illustrated and described embodiments since modifications may be made and other embodiments of the principles of this invention may occur to those of skill in the art to which this invention pertains. Therefore, it is intended by the appended claims to cover all such modifications and other embodiments as may incorporate the broad principles of this invention within the true spirit and scope thereof.