Abstract:
A capacitive load cell includes upper and lower capacitor plates and an intermediate dielectric comprising two or more layers of compressible material with diverse compressibility characteristics. The most easily compressed dielectric layer provides sensitivity to low occupant weight, while the less easily compressed dielectric layer(s) provide sensitivity to medium and high occupant weight.

Description:
TECHNICAL FIELD 
   The present invention relates to a capacitive load cell for estimating occupant weight applied to a vehicle seat, and more particularly to a load cell having a dielectric material that enhance the range of weight estimation. 
   BACKGROUND OF THE INVENTION 
   Various sensing technologies have been utilized to classify the occupant of a vehicle seat for purposes of determining whether to enable or disable air bag deployment, and/or for purposes of determining how forcefully an air bag should be deployed. The present invention is directed to an approach in which at least one capacitive load cell is installed in a vehicle seat, and the capacitance of the load cell is measured to provide an indication of the weight applied to the seat and/or the distribution of the applied weight. In general, a capacitive load cell includes at least first and second conductive plates separated by a compressible dielectric such as a sheet of rubber, plastic or polymeric foam. For example, representative capacitive load cells are disclosed in the U.S. Pat. No. 4,266,263 to Haberl et al., issued on May 5, 1981. Additionally, the U.S. Pat. No. 4,836,033 to Seitz; U.S. Pat. No. 5,878,620 to Gilbert et al.; U.S. Pat. No. 6,448,789 to Kraetzl; and U.S. Pat. No. 6,499,359 to Washeleski et al. show capacitive load cells as applied to vehicle seats for sensing occupant weight or weight distribution. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to an improved capacitive load cell apparatus for estimating occupant weight applied to a vehicle seat, where the load cell includes upper and lower capacitor plates and an intermediate dielectric comprising two or more layers of compressible material with diverse compressibility characteristics. The most easily compressed dielectric layer provides sensitivity to low occupant weight, while the less easily compressed dielectric layer(s) provide sensitivity to medium and high occupant weight. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded diagram of a vehicle seat and a sensing apparatus including a capacitive load cell having a multi-layer compressible dielectric in accordance with the present invention; 
       FIG. 2  is a graph depicting the compressibility of the individual dielectric layers of the load cell of  FIG. 1 ; 
       FIG. 3  is graph depicting the overall compressibility of the multi-layer dielectric of the load cell of  FIG. 1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   While the capacitive load cell apparatus of the present invention may be used in various applications, it is disclosed herein in the context of an apparatus for detecting the weight and/or distribution of weight applied to a vehicle seat. In general, a capacitive load cell comprises upper and lower conductor plates separated by a compressible non-conductive dielectric, such that mechanical loading of the cell compresses the dielectric, increasing the electrical capacitance between the upper and lower conductor plates. As applied to a vehicle seat, the capacitive load cell is preferably disposed between the frame and bottom cushion of the seat as depicted herein, but it will be understood that the load cell may be installed in a different location such as in the bottom cushion, in or behind a back cushion, and so on. 
   Referring to  FIG. 1 , the reference numeral  10  generally designates a seat bottom and sensor apparatus according to this invention. The sensor apparatus includes a capacitive load cell  12  and an electronic control unit (ECU)  14 . The load cell  12  is disposed between the seat frame  16  and a foam cushion  18 , and includes an upper substrate  20 , a dielectric  22 , and a lower substrate  24 . A reference plane conductor  28  is formed on lower substrate  24  adjacent the lower surface of dielectric  22 , and a number of charge plate conductors  30  are formed on upper substrate  20  adjacent the upper surface of dielectric  22 . The upper and lower substrates  20 ,  24  are non-conductive, and may be formed of a material such as polyurethane with a thickness of about 0.5 mm, for example. The conductors  28 ,  30  may be metal foil pads laminated to the respective substrates  24 ,  20 . The reference plane conductor  28  and each of the charge plate conductors  30  are separately coupled to ECU  14 , which periodically measures capacitance values between the reference plane conductor  28  and each of the charge plate conductors  30 . The measured capacitances provide an indication of the weight applied to seat cushion  18 , as well as the distribution of the weight, for purposes of detecting the presence of an occupant and classifying the occupant as a child, an adult, a child seat, or some other classification. 
   According to the present invention, the dielectric  22  includes multiple layers of diverse compressibility so that load cell  12  will exhibit continuous variation in capacitance over a wide range of applied occupant weight. In the illustrated embodiment, the dielectric  22  includes three juxtaposed layers  22   a ,  22   b ,  22   c  of successively lower compressibility. The top layer  22   a  is relatively soft (i.e., easily compressible), and becomes fully compressed in response to an applied force of approximately 35 pounds, as illustrated by trace  40  in  FIG. 2 . The middle layer  22   b  is firmer than the top layer  22   a , and becomes fully compressed in response to an applied force of approximately 155 pounds, as illustrated by trace  42  in  FIG. 2 . The bottom layer  22   c  is firmer than the middle layer  22   b , and becomes fully compressed in response to an applied force of approximately 240 pounds, as illustrated by trace  44  in  FIG. 2 .  FIG. 3  depicts the combined compressibility characteristic of the three layers  22   a ,  22   b  and  22   c . The overall compression of the dielectric  22  produces a continuous and high-gain relationship between applied occupant weight and separation distance between the ground plane conductor  28  and respective charge plate conductors  30  (and therefore, the measured capacitances) over a wide range of applied occupant weight. This enables ECU  14  to reliably characterize occupant weight over a much wider range than could be achieved with any dielectric of uniform compressibility. 
   Preferably, the dielectric layers are each formed of open-cell or closed-cell urethane foam, although other materials such as rubber may alternatively be used. In any event, the dielectric material must be producible with different degrees of compressibility. The compressibility or load bearing capacity of foam is commonly defined in terms of the Indentation Load Deflection (ILD) and is generally given as the force in pounds required to compress the foam to a specified percentage of its initial height. By way of example, the top layer  22   a  of the illustrated embodiment may have an ILD rating of 10 pounds, the middle layer  22   b  may have an ILD rating of 30 pounds, and the bottom layer  22   c  may have an ILD rating of 50 pounds. Also, the dielectric  22  may be manufactured as a multilayer assembly, or individually combined during assembly of load cell  12 ; in the later case, the various dielectric layers may be color coded by compressibility to ensure proper and consistent assembly. 
   While the present invention has been described with respect to the illustrated embodiment, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. For example, the load cell dielectric may comprise more or fewer layers of compressible material than in the illustrated embodiment, the load cell may be used in non-automotive environments, the charge plate conductors  30  may be single conductor instead of multiple individual conductors, and so on. Also, the dielectric layers need not be arranged in any particular order of compressibility; for example, the least compressible layer may be on top, and so forth. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.