TECHNICAL FIELD  
       [0001]     The present invention is directed to a fluid-filled seat bladder for vehicle occupant weight estimation, and more particularly to a seat bladder having internal tethering.  
       BACKGROUND OF THE INVENTION  
       [0002]     Occupant weight estimation systems are useful in connection with air bags and other pyrotechnically deployed restraints for purposes of discriminating the occupant category and judging whether, and how forcefully, to deploy the restraints. A well known weight estimation technique is to locate one or more fluid-filled bladders in or under a vehicle seat cushion, and to estimate the weight of the occupant based on a measure of the pressure in the bladder(s). See for example, the U.S. Pat. Nos. 5,987,370 and 6,246,936 to Murphy et al., and the U.S. Pat. Nos. 6,101,436 and 6,490,936 to Fortune et al., all of which are assigned to Delphi Technologies, Inc., as well as the U.S. Pat. No. 5,957,491 to Cech et al. and the U.S. Pat. No. 5,975,568 to Speckhart et al.  
         [0003]     Typically, seat bladders are formed by peripherally joining two sheets of elastomeric material, by radio frequency welding, for example. Additional welds may also be formed at various locations within the periphery of the bladder in order to reduce fluid pooling and distention or bulging of the bladder due to localized loading, as well as to reduce the fluid volume, and therefore its weight. These inter-peripheral welds may be spot welds or seam welds, and can be used to form flow-through cells or fluid-free regions. See particularly the aforementioned U.S. Pat. Nos. 5,957,491; 5,975,568; 5,987,370; and 6,101,436.  
         [0004]     A drawback of inter-peripheral welding in weight estimation seat bladders is that the bladder material in the vicinity of the weld provides a vertical support that effectively shunts a portion of the occupant weight through the bladder without significantly influencing the fluid pressure within the bladder. As a result, a given weight distributed over a relatively large area of the seat will produce a different pressure reading than the same weight when distributed over a relatively small area of the seat. Accordingly, what is needed is a bladder structure that will achieve the benefits of inter-peripheral welding without providing significant vertical support of occupant weight.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention is directed to an improved occupant weight estimation apparatus for a vehicle seat in which an elastomeric seat bladder defined by peripherally welded upper and lower sheets of elastomeric material includes a plurality of elastomeric tethers defined by bands or sheets of elastomeric material coupling the upper and lower sheets within the peripheral weld in order to reduce fluid pooling and distention or bulging of the bladder due to localized loading. The elastomeric tethers are spot or seam welded to upper and lower sheets of the bladder; they extend to limit distension where required, and otherwise partially or fully collapse to minimize vertical shunting of occupant weight through the bladder. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-  
         [0007]      FIG. 1  is a diagram of a vehicle seat including an elastomeric bladder according to this invention for occupant weight estimation.  
         [0008]      FIGS. 2A, 2B  and  2 C depict the elastomeric bladder of  FIG. 1 .  FIG. 2A  shows the bladder in a fully distended condition,  FIG. 2B  shows the bladder in a condition of nominal distension, and  FIG. 2C  shows the bladder in a flattened condition.  
         [0009]      FIGS. 3A, 3B  and  3 C depict different tether configurations for the bladder of  FIG. 1 .  FIG. 3A  depicts a triangular pattern,  FIG. 3B  depicts a rectangular pattern, and  FIG. 3A  depicts a hexagonal pattern. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]     Referring to the drawings, and particularly to  FIG. 1 , the reference numeral  10  generally designates a vehicle occupant weight estimation apparatus according to this invention. The vehicle seat, generally designated by the reference numeral  12 , includes a frame  14 , a bottom foam cushion  16  and a back foam cushion  18 . A set of posts  19  integral to the frame  14  attach the seat  12  to a pair of tracks  20  (only one of which is shown in  FIG. 1 ), which in turn, are secured to the vehicle floor  22 . A bladder assembly  24  is sandwiched between the bottom cushion  16  and the frame  14  for the purpose of detecting the presence and weight of an occupant of seat  12 . Finally, a fabric cover  26  envelopes the cushion  16 , bladder assembly  24  and frame  14 .  
         [0011]     The bladder assembly  24  includes an elastomeric bladder  30  defined by upper and lower sheets  30   a ,  30   b  of elastomeric material such as polyurethane or pellethane that are peripherally seam welded as indicated by the reference numeral  36  to form a closed volume that is filled with silicone or another suitable fluid, and a pressure sensor  40  for measuring the fluid pressure in bladder  30 . In general, the weight of a vehicle occupant seated on the cushion  16  is applied to the bladder  30 , and an electronic controller  42  coupled to the pressure sensor  40  develops an estimation of the occupant weight based on the measured pressure and its variation with respect to time; see, for example, the aforementioned U.S. Pat. Nos. 5,987,370 and 6,246,936. The assembly  24  typically also includes an upper interface panel  32  disposed between the upper bladder sheet  30   a  and the seat cushion  16 , and a lower interface panel  34  disposed between the lower bladder sheet  30   b  and the seat frame  14 . In general, the interface panels  32  and  34  protect bladder  30  from puncture damage, and limit the system sensitivity to variations caused by manufacturing irregularities.  
         [0012]     In conventional practice, the upper and lower bladder sheets  30   a ,  30   b  are joined by spot or seam welding within the peripheral weld  36  to form a plurality of flow-through cells as disclosed in the aforementioned U.S. Pat. Nos. 5,957,491; 5,975,568; 5,987,370; and 6,101,436. The cells advantageously serve to reduce the fluid volume (and therefore weight) and to reduce fluid pooling and distention or bulging of the bladder due to localized loading. Additionally, the cells can be sized and positioned to tailor the bladder sensitivity to the occupant weight, as described for example in the aforementioned U.S. Pat. No. 6,101,436. As explained above, however, the bladder material in the vicinity of the inter-peripheral welds tends to shunt a portion of the occupant weight through the bladder without significantly influencing the fluid pressure within the bladder. As a result, a given weight distributed over a relatively large area of cushion  16  will produce a different pressure reading than the same weight when distributed over a relatively small area of cushion  16 .  
         [0013]     According to the present invention, the upper and lower layers  30   a ,  30   b  of the bladder  30  are internally coupled within the peripheral weld  36  by a number of elastomeric tethers  38  defined by bands or sheets of elastomeric material such as polyurethane or pellethane that are selectively welded to the upper and lower layers  30   a ,  30   b . Referring to  FIGS. 2A-2C , spot welds attaching the tethers  38  to the upper layer  30   a  are designated by the reference numeral  44 , and spot welds attaching the tethers  38  to the lower layer  30   b  are designated by the reference numeral  46 . The tethers  38  effectively create an array of flow-through cells within the bladder  30  as shown in  FIGS. 1 and 2 A- 2 C to achieve the advantages of conventional inter-peripheral spot welding of the layers  30   a ,  30   b , but without establishing a vertical support structure through which occupant weight can be shunted through the bladder  30 . Since occupant weight shunting is minimized, the bladder fluid pressure due to occupant seat weight is less sensitive to the area over which the weight is applied, leading to more accurate and consistent weight estimation. Of course, the length and spacing of the tethers  38  can be varied to control the degree of permitted separation of the layers  30   a ,  30   b . As illustrated by  FIG. 2A , the tethers  38  restrict separation of the layers  30   a ,  30   b  when occupant weight applied to one portion of bladder  30  tends to pool the bladder fluid in other portions of the bladder. As illustrated in  FIG. 2B , the tethers  38  normally are in a state of partial collapse, and offer little or no vertical support structure through which occupant weight can be shunted through the bladder  30 . Finally as illustrated in  FIG. 2C , the tethers  38  fully collapse when the bladder  30  is flattened by an applied load; in this case, nearly all of the bladder fluid is displaced to other portions of the bladder  30  and the layers  30   a ,  30   b  are essentially pressed into engagement. In the region of the tethers  38 , there will be a slight bulge in the bladder  30 , but the bulges are very small and will not affect occupant comfort.  
         [0014]      FIGS. 3A, 3B  and  3 C illustrate three different possible arrangements or repeat patterns of tethers  38 . In each of the tethers  38 , the upper triangular end  38   a  signifies a spot weld to the upper layer  30   a  of bladder  30 , and the lower triangular end  38   b  signifies a spot weld to the lower layer  30   b  of bladder  30 .  FIG. 3A  depicts a triangular or diamond-shaped repeat pattern,  FIG. 3B  depicts a rectangular repeat pattern, and  FIG. 3A  depicts a hexagonal repeat pattern. Obviously, other basic shapes may be used as well, and the pattern of tethers may be non-uniform if desired. As with spot welds, the repeat pattern of tethers  38  defines the cell shape; and in general, a spherical cell shape is preferred as the lowest energy configuration.  
         [0015]     In summary, the present invention provides a seat bladder for occupant weight estimation that provides more accurate and consistent weight estimation without sacrificing the advantages achieved by inter-peripheral spot welding. While described in reference to the illustrated embodiments, it should be understood that various modifications will occur to persons skilled in the art. For example, the number and shape of the tethers  38  may be different than shown, the two or more individual tethers  38  may be joined into one or more sheets that are perforated for fluid flow, the tethers  38  may be joined to the upper and lower layers  30   a ,  30   b  of bladder  30  by sewing or adhesives, and so on. Thus, many variations are possible, and it will be understood that any of a number of configurations may fall within the scope of this invention, which is defined by the appended claims.
An elastomeric seat bladder for a vehicle occupant weight estimation system includes a plurality of elastomeric tethers defined by bands or sheets of elastomeric material coupling upper and lower layers of the bladder within a peripheral weld in order to reduce fluid pooling and distention or bulging of the bladder due to localized loading. The elastomeric tethers are spot or seam welded to upper and lower sheets of the bladder; they extend to limit distension where required, and otherwise partially or fully collapse to minimize vertical shunting of occupant weight through the bladder.