Abstract:
An improved weight estimation apparatus in which an elastomeric bladder is supported on a semi-rigid backer board which is installed in a vehicle seat between the seat cushion and an underlying spring suspension. The bladder is secured onto the backer board, and the backer board is attached to the seat, enabling proper and consistent placement of the bladder relative to the seat surface. The backer board provides the reaction surface for the bladder, and masks non-uniformities of the seat structure. Additionally, the backer board will reduce or eliminate the effect of objects lying on the floor under the seat, which might otherwise pinch the bladder and produce weight estimation errors. Further, the board tends to protect the bladder from damage due to sharp objects, pinching by the suspension elements, and the effects of wires or cables routed through the seat. The backer board may be smooth to provide a uniform reaction surface, or contoured to increase or decrease sensitivity to weight applied on certain areas of the seating surface.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. Ser. No. 08/923,020, entitled Vehicle Occupant Weight Estimation Apparatus Having Fluid-Filled Seat Bladder, filed Sep. 3, 1997 and assigned to the assignee of the present invention, now U.S. Pat. No. 5,987,370. 
    
    
     TECHNICAL FIELD 
     This invention relates to detection of occupant weight in a motor vehicle seat based on the pressure in a fluid filled seat bladder. 
     BACKGROUND OF THE INVENTION 
     Vehicle occupant detection systems are useful in connection with air bags and other pyrotechnically deployed restraints as a means of judging whether, and how forcefully, to deploy the restraint. One fundamental parameter in this regard is the weight of the occupant, as weight may be used as a criteria to distinguish between an adult and an infant or small child. 
     One prior weight estimation technique is to install an array of variable resistance pressure sensitive elements in the seat, and to sum the individual pressures to determine occupant weight. A restraint system based on this technique is shown and described in the U.S. Pat. No. 5,474,327, issued on Dec. 12, 1995, and assigned to the assignee of the present invention. In practice, however, such systems tend to be relatively costly to implement, and require a significant amount of signal processing for proper calibration and weight estimation. 
     It has also been suggested to locate one or more fluid or air filled bladders in a vehicle seat, and to estimate the weight of the occupant based on a measure of the pressure in the bladder(s). However, it has been found that such systems can deliver inconsistent weight estimations because both the seat cushion and underlying suspension, which provide a reaction surface for the bladder, tend to be non-uniform in stiffness. Additionally, it is frequently difficult to properly and consistently install the bladders, and to reliably prevent shifting during use. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved weight estimation apparatus in which an elastomeric bladder is supported on a semi-rigid backer board which is installed in a vehicle seat between the seat cushion and an underlying spring suspension. The bladder is secured to the backer board, and the backer board is attached to the seat, enabling proper and consistent placement of the bladder relative to the seat surface. The backer board provides the reaction surface for the bladder, and masks non-uniformities of the seat structure. Additionally, the backer board will reduce or eliminate the effect of objects lying on the floor under the seat, which might otherwise pinch the bladder and produce weight estimation errors. Further, the board tends to protect the bladder from damage due to sharp objects, pinching by the suspension elements, and the effects of wires or cables routed through the seat. The backer board may be smooth to provide a uniform reaction surface, or contoured to increase or decrease sensitivity to weight applied on certain areas of the seating surface. A bladder port passes through an opening in the center of the backer board and is routed to a pressure sensor. A temperature sensor disposed in proximity to the bladder provides a temperature signal, and a controller estimates the weight of the occupant based on the temperature and pressure signals. Preferably, the pressure sensor is configured to sense the gage pressure—that is, the differential pressure between the fluid and atmospheric pressure—at the center of gravity of the bladder fluid in order to provide a stable output independent of changing atmospheric pressure and changing orientation of the bladder due to vehicle pitch and roll. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1A is a system diagram illustrating a passenger seat of a vehicle equipped with a fluid-filled bladder assembly and a controller for estimating the weight of a seat occupant in accordance with this invention. 
     FIG. 1B is an enlarged view of a portion of FIG. 1A, illustrating the fluid-filled bladder assembly in cross-section. 
     FIG. 2 is an exploded view of the fluid-filled bladder assembly of FIGS. 1A-1B, including a pressure sensor, a felt pad, a backer board and a fluid-filled bladder. 
     FIGS. 3A and 3B depict contoured backer boards in accordance with an aspect of this invention. 
     FIGS. 4A and 4B depict an exit port of the bladder of FIGS. 1A-1B. 
     FIG. 5 is a block diagram of the controller of FIG. 1A, in the context of an air bag deployment system. 
     FIG. 6 is a graph depicting pressure sensor output voltage as a function of occupant weight, as used in the system of FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, and particularly to FIG. 1A, 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 rigid frame and spring suspension  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 pair of tracks  20  (only one of which is shown in FIG.  1 A), which in turn, are secured to the vehicle floor  22 . A bladder assembly  24 , shown in more detail in FIG. 1B, is sandwiched between the bottom cushion  16  and the suspension elements of frame  14  for the purpose of detecting the presence and weight of an occupant of seat  12 , as explained below. Finally, a fabric cover  26  envelopes the cushion  16 , bladder assembly  24  and frame  14 . 
     Referring to FIG. 1B, the bladder assembly  24  comprises a polymeric bladder  30  in contact with the bottom cushion  16 , and a semi-rigid backer board  32  in contact with the frame  14 . Additionally, a felt pad may be inserted between the backer board  32  and frame  14 , as described below in reference to FIGS.  2  and  3 A- 3 B. The bladder  30  has a continuous weld  34  about its periphery and contains a fluid such as silicone which is non-corrosive, and not subject to freezing or break-down at extreme ambient temperatures. A bladder exit port  35  located approximately at the center of weight of the fluid is coupled to a tube  38 , which passes through an aperture  36  in the backer board  32  (and felt pad, if used). The tube  38  is coupled to a pressure sensor  40 , which may be attached to the lower portion of frame  14  as shown in FIG. 1A, the sensor  40  developing an electrical signal on line  42  that is representative of the fluid pressure in the bladder  30 . A temperature sensor (not shown), which may be integrated with the pressure sensor  40 , provides an electrical output signal on line  44  indicative of the bladder and foam temperature. The pressure and temperature signals on lines  42  and  44  are applied as inputs to an electronic controller  46  which determines if the seat  12  is occupied and estimates the weight of the occupant based on the pressure and temperature signals, possibly in combination with other inputs, such as an atmospheric pressure signal provided by pressure sensor  48 . If the controller  46  is attached to the bottom of frame  14 , the pressure and temperature sensors may be conveniently packaged within the controller housing. In any event, controller  46  produces an electrical output signal indicative of the weight estimation on line  50 , which may be used, for example, as a factor in determining whether and how forcefully to deploy air bags or other pyrotechnically deployed restraints in a crash event. The controller  46  may be mechanized with a suitably programmed microprocessor, as described below in reference to FIGS. 5-6. 
     In general, the fluid in bladder  30  has a nominal or unloaded pressure which provides a baseline pressure measurement, and the pressure increases monotonically with occupant weight applied to the seat cushion  16 . The temperature measurement is used to compensate the weight measurement for temperature dependent changes in foam and bladder stiffness to provide a weight measurement that is insensitive to temperature variations. Preferably, the pressure sensor  40  is configured to sense the differential or gage pressure of the fluid—that is, the pressure difference between atmospheric pressure and bladder fluid pressure—in order to provide a pressure measurement that is insensitive to atmospheric pressure variations due to changing weather patterns or altitude. In this case, the pressure sensor  40  includes a second inlet (not shown) open to atmospheric pressure. Alternately, the sensor  40  may provide an absolute pressure measurement of the bladder fluid, and the controller  46  may compensate the measurement for atmospheric pressure variations by reducing the fluid pressure measurement by the atmospheric pressure measurement provided by sensor  48 . 
     In general, the bladder  30  conforms in size to the seat cushion  16 . Additionally, the upper and lower surfaces of the bladder  30  may be welded together in various patterns, as indicated by the reference numeral  52  in FIG. 1B, so as to divide the bladder  30  into a series of contiguous cells  54 . The welds  52  are not continuous so that fluid can freely flow between adjacent cells. The array of cells  54  minimizes the amount of fluid required to detect occupant weight, thereby minimizing the weight of the fluid-filled bladder  30 , and prevents the fluid from pooling in proximity to an unloaded area of the seat cushion  16 . As shown in the exploded view of FIG. 2, the cells may be defined by a pattern of spot welds  52  extending over the entire surface of the bladder  30 , with the exit port  35  positioned in the center of one of the cells substantially at the center of gravity of the fluid. Preferably, the spot welds  52  are patterned so that the cells  54  assume a generally circular or domed shape, as illustrated in FIGS.  1 B and  3 A- 3 B. The domed shape of the cells  54  essentially eliminates vertical wall surfaces within the bladder  30 , thereby maximizing the measured pressure response to the weight of an occupant. 
     The sizes of the individual spot welds  52  and cells  54  are preferably chosen to minimize the weight of the fluid filled bladder  30  while maximizing the sensitivity and accuracy of the weight measurement. That is, the cell pattern can be designed to influence the sensitivity of the pressure signal to weight applied in certain areas of the seat cushion  16 . However, the geometry of the seat cushion  16 , the suspension  14  and other elements (such as wires and connectors) routed through the seat materials can also influence the pressure signal. Although these affects could possibly be compensated for through a careful design of the cell patterns, such an approach is impractical due to seat-to-seat variations in stiffness, variations in the location of wires and connectors, etc. Additionally, it can be difficult to precisely position the bladder  30  when the seat  12  is initially assembled, and once assembled, to prevent it from shifting, as shifting would defeat the compensation provided by the cell patterns. The present invention addresses these concerns through the incorporation of the-semi rigid backer board  32  between the bladder  30  and the frame  14 . The backer board  32  provides a firm and predetermined reaction surface for the bladder  30 , and also facilitates accurate and repeatable positioning of the bladder  30  in the seat  12 . The backer board  32  is positioned relative to the frame, an the bladder  30 , in turn, is firmly secured to the backer board  32 , thereby ensuring precise and secure positioning of the bladder  30  as well. 
     FIG. 2 depicts an exploded, bottom-side-up, view of the bladder assembly  24 , in a preferred implementation comprising the bladder  30 , the backer board  32  and a felt pad  58 . The tube  38  passes through openings  60 ,  36  in the pad  58  and backer board  32 , and attaches to the exit port  35  of bladder  30 , as explained below in reference to FIGS. 4A-4B. The other end of tube  38  is coupled to the pressure sensor  40 , which provides the pressure output signal on line  42 , as described above in reference to FIG.  1 A. The felt pad  58  is included primarily for noise suppression, and to provide cushioning between the backer board  32  and any wires (not shown) passing between the frame-suspension  14  and the backer board  32 . 
     In the embodiment of FIG. 2, a set of elastic straps  62   a - 62   c  are bonded or sewn to the periphery of the bladder  30  for the purpose of mutually securing the bladder  30 , the backer board  32  and the felt pad  58 . That is, the respective corners of backer board  32  and pad  58  are inserted under the straps  62   a - 62   c  so that the elements shown in FIG. 2 can be conveniently handled as one assembly. Alternatively or additionally, the backer board  32  and pad  58  may be attached with a suitable adhesive applied to the respective elements, or the pad  58  and backer board  32  may be sewn to peripheral portions of the bladder  30 . In any event, the assembly comprising the bladder  30 , backer board  32  and pad  58  is then attached to the seat frame  14 , for example by the push-pins  64  which pass through suitable openings  65 ,  66 ,  67  therein, and attachment openings (not shown) provided in the frame  14 . 
     FIGS. 3A and 3B illustrate that the backer board  32  may be contoured to influence the sensitivity—that is, th change of the pressure signal per unit change in the weight applied to the seat cushion  16 . In FIG. 1B, the backer board  32  is flat, and provides a uniform reaction surface for the bladder  30 . In FIGS. 3A and 3B, however, the backer board is contoured in axial alignment with the pattern of cells  54 . In FIG. 3A, the backer board  32 ′ is contoured to provide a concave surface feature under the center of the cells  54 ; this causes a decrease in the sensitivity, compared to the flat backer board  32 . In FIG. 3B, the backer board  32 ″ is contoured to provide a convex surface feature under the center of the cells  54 ; this causes an increase in the sensitivity, compared to the flat backer board  32 . Of course, the degree of convexity or concavity can be varied depending on the application, as can the pattern of surface features. For example, it may be desired to use concave surface features in proximity to the center of the bladder  30  since the sensitivity is typically highest for occupant weight applied to the center of the seat cushion  16 ; in this case, the degree of concavity may be reduced with increasing distance from the center position. On the other hand, it may be desired to use convex surface features near the periphery of the bladder  30  to increase sensitivity to weight applied to the edges of the seat cushion  16 . The most useful pattern for a given seat and/or desired sensitivity may be determined either experimentally or by conventional data mapping techniques. In any event, it will be appreciated that having the ability to control the contour of the backer board  32  significantly improves the ability of the designer to achieve the desired pressure vs. weight transfer function of the system. Once designed, the contoured backer board may be easily and economically manufactured by vacuum forming. 
     FIGS. 4A-4B depict the exit port  35  of bladder  30 . As viewed from inside the bladder  30  in FIG. 4A, and in cross-section in FIG. 4B, the port  35  includes a generally cylindrical plastic inlet  70  having a shoulder  72  that seats on an inner surface of the  74  of the bladder  30 , a stem  75  that extends through the opening  36  in backer board  32 , and a number of radial extending bosses  76  that protrude into the bladder  30 . The inlet  70  has a central bore  78  that receives the pressure sensor tube  38 , as shown in FIG.  4 B. The bosses  76  ensure open fluid communication between the bladder fluid and the tube  38  at all times. 
     FIG. 5 depicts the occupant weight estimation apparatus  10  in the context of an air bag deployment system in which the controller  46  provides an output signal on line  50  indicating whether deployment should be inhibited or allowed based on sensed occupant weight. Vehicle ignition voltage Vign, which may be +12 VDC, is supplied to controller  46  via line  80 , and an internal voltage regulator  82  provides a regulated system voltage of +5 VDC on lines  84  and  86 . The system and ground voltages are supplied to source voltage terminals of the pressure sensor  40 , the temperature sensor  41  (which may be a thermistor), and a microprocessor  88 . The microprocessor  88  has four analog-to-digital input channels A/D 1 -A/D 4  which receive the ignition voltage Vign, the system voltage of voltage regulator  82 , the temperature sensor output voltage on line  44 , and the pressure sensor output voltage on line  42 . 
     Based on the above-described inputs, the microprocessor  88  determines the occupant weight, and based on predetermined criterion, whether air bag deployment should be inhibited or allowed. In the illustrated control strategy, air bag deployment is to be inhibited for occupants below a predetermined weight, referred to herein as the threshold weight. In general, once the occupant weight estimation system  10  is installed in a vehicle, the relationship between occupant weight and sensed pressure (compensated for system voltage variations) is empirically determined at a variety of temperatures. In a system as suggested in FIG. 1A, the data can be used to construct either a mathematical model or a multi-dimensional look-up table of occupant weight as a function of temperature and pressure sensor output voltage, with the model or table programmed into the microprocessor  88  and used to determine and output the occupant weight on line  50 . In the system of FIG. 5, however, the output merely indicates whether the occupant is above or below the threshold weight, within the system tolerance constraints. In this case, the empirically determined data may be plotted as shown in FIG. 6, and used to develop a pressure sensor threshold voltage for determining whether deployment should be inhibited or allowed. For the data represented in FIG. 6, for example, the lines designated as upper limit UL and lower limit LL bound the pressure sensor output voltage variability for any given occupant weight. In the illustrated example, the lower limit LL defines a threshold voltage which is the minimum voltage, approximately 1.82 volts, one would expect to see for an occupant having a standing weight of 105 lbs. It should be understood that the simple control described above is exemplary in nature, and could be modified to include adaptive updating of the thresholds, or to provide a more precise output as described above in reference to FIG. 1, for example. 
     In summary, the present invention provides an improved occupant weight estimation apparatus including a fluid filled bladder  30  and a semi-rigid backer board  32 . The backer board  32  enables proper and consistent placement of the bladder  30  relative to the seat cushion  16 , and provides a known reaction surface for the bladder  30  that shapes the sensitivity and masks non-uniformities of the seat structure. Additionally, the backer board  32  reduces or eliminates the effect of objects lying on the floor under the seat  12 , which might otherwise pinch the bladder  30  and produce weight estimation errors. 
     While this invention has been described in reference to the illustrated embodiment, it should be understood that various modifications will occur to persons skilled in the art. For example, in certain applications, it may be desirable to include a semi-rigid board between the bladder  30  and seat cushion  16  to mask non-uniformities of the seat cushion  16 ; in this case, the board can also be used to control sensitivity as described above, and to protect the bladder  30  from puncture due to sharp objects penetrating the seat cushion  16 . In other applications, the upper surface of the frame  14  may be smooth and rigid, and the bottom backer board  32  may be omitted. Thus, many variations are possible depending on the application, and it will be understood that any of a number of systems may fall within the scope of this invention, which is defined by the appended claims.