Abstract:
An occupant classification sense element is provided. The sense element has a body having a center portion, a first attachment portion connected to the center portion by a first flexible beam, and a second attachment portion connected to the center portion by a second flexible beam. First and second strain sensing elements are attached to the first flexible beam and third and fourth strain sensing elements are attached to the second flexible beam. The first, second, third and fourth strain sensing elements cooperate to produce an electrical signal indicative of a force deflecting the first and second flexible beams.

Description:
FIELD OF INVENTION  
       [0001]     This invention relates generally to devices for determining the weight of a seated vehicle occupant. More particularly, this invention relates to such a device that operates to isolate and measure a seat occupant&#39;s weight from other forces acting on the seat.  
       BACKGROUND  
       [0002]     In modern vehicles, passive safety systems such as airbags are becoming increasingly dependent on data about vehicle occupants. The data is often used by the system to determine each occupant&#39;s seating position, mass, and center of gravity. Each occupant&#39;s weight is useful in making the determinations.  
         [0003]     One method of measuring an occupant&#39;s weight is to place a force sensor in the load path between the vehicle floor and the seat structure. The force sensor then provides a signal indicative of a seat occupant&#39;s weight. Such sensors, as disclosed in Oestereicher et al., U.S. Pat. No. 6,070,115, are undesirably tall and cause the seat structure to raise from its original position when the sensor is introduced in the load path. In order to return the seat structure to its original height, the seat base, the vehicle floor, or both, must then be redesigned to lower the seat height to its original level prior to introducing the sensor in the load path.  
       SUMMARY  
       [0004]     It is therefore an aspect of the invention to provide an occupant classification sensor having a low profile to minimize its effect on seat height when installed.  
         [0005]     In accordance with this aspect, an occupant classification sense element is provided. The sense element has a body having a center portion, a first attachment portion connected to the center portion by a first flexible beam, and a second attachment portion connected to the center portion by a second flexible beam. First and second strain sensing elements are attached to the first flexible beam and third and fourth strain sensing elements are attached to the second flexible beam. The first, second, third and fourth strain sensing elements cooperate to produce an electrical signal indicative of a force deflecting the first and second flexible beams. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0007]      FIG. 1  depicts a schematic diagram in accordance with electrical elements of the present invention;  
         [0008]      FIG. 2  depicts a side view of an occupant classification element at rest; and  
         [0009]      FIG. 3  depicts a side view of an occupant classification under load. 
     
    
     DETAILED DESCRIPTION  
       [0010]     The following description is merely exemplary in nature and is in no way intended to limit the invention, its applications, or uses. Other functions may also be implemented as indicated in this specification.  
         [0011]     Turning now to  FIG. 1 , a schematic diagram of electrical aspects of a sense element is shown. Strain sensing elements R 1 -R 4  are arranged in a Wheatstone bridge configuration and supplied with excitation voltage at node A. Node C is referenced to ground  8 . Nodes B and D provide the differential output across the bridge. Placement of the strain sensors is discussed later herein.  
         [0012]     The differential output of the bridge is connected to a signal conditioning circuit, which is preferably embodied in the form of an application specific integrated circuit (ASIC) U 1 . The ASIC U 1  conditions the signal from the differential output of the bridge. Signal conditioning may include functions such as a low-pass filter, thermal compensation, and zeroing an offset in the differential output of the Wheatstone bridge. ASIC U 1  has first 16 and second  18  bridge inputs for receiving the signal from the differential output. A signal output  14  provides a conditioned electrical signal for use by external devices, such as components of a passive safety system. In a preferred embodiment, the signal output is an analog signal, thereby avoiding quantization errors that may be introduced by a digital output. The ASIC U 1  receives power through power input  12  and is electrically referenced to ground  8 . Connector J 1  is attached to the ASIC U 1  by cable  20  and may be used to connect the circuit to the external devices. Output terminal  2  provides the conditioned electrical signal, excitation terminal  4  provides voltage to the ASIC U 1  and the Wheatstone bridge, and ground terminal  6  provides a connection to ground  8 . Both output terminal  2  and excitation terminal  4  are referenced to ground terminal  6 .  
         [0013]     Turning now to  FIG. 2 , a side view of a sense element is shown. The sense element is installed in a load path between a seat frame  32  and a floor pan  24 . A body  22  has a first attachment portion  42 , a second attachment portion  44 , and a center portion  46 . A first flexible beam  50  connects the first attachment portion  42  to the center portion  46 . Similarly, a second flexible beam  48  connects the second attachment portion  44  to the center portion  46 . Preferably first and second attachment portions are located opposite each other and coplanar with the center portion  46 , located equidistant between them. Body  22  is preferably formed from a metal having a high yield strength and low modulus of elasticity, such as titanium.  
         [0014]     The center portion  46  is attached to a structure, such as floor pan  24 . Attachment may be achieved with a bolt  26  threadably engaged to a nut  30 , or by other suitable means.  
         [0015]     The first and second attachment portions  42 ,  44 , are each connected to seat frame  32  by seat bolts  36  and nuts  38 . Each bolt  36  has a flanged head for evenly distributing the bolt-clamping load around its respective mounting portion. A washer is interposed between each mounting portion and its mating section of seat frame  32  to manage the clamping load. Each washer also cooperates with the flange of its mating bolt to provide a uniform strain field throughout their respective attachment portion.  
         [0016]     Each flexible beam  48 ,  50  has strain sensing elements positioned on either the top or bottom of its surface, but not both. Strain sensing elements R 4  and R 3  of the Wheatstone bridge are positioned on flexible beam  48 , with R 4  being positioned towards mounting portion  44  and R 3  being positioned towards center portion  46 . Similarly, strain sensing elements R 1  and R 2  are positioned on flexible beam  50 , with R 2  being positioned towards mounting portion  42  and R 1  being positioned towards center portion  46 . The strain sensing elements are preferably positioned symmetrically about center portion  46 . In addition, positioning the strain sense elements closer to their neighboring mounting portions, or center portion, increases the sensitivity of the sense element. However, doing so increases the possibility of the strain sense elements being biased due to residual strain from clamping forces by bolts  26  and  34 . Strain sense elements R 1 -R 4  are preferably a film resistor formed from ruthenium dioxide.  
         [0017]     ASIC U 1  is positioned on body  22  and preferably potted to protect it from the environment. Cable  20  and connector J 1  provide a connection to ASIC U 1 .  
         [0018]     In operation, the weight of an occupant is applied to seat base  32  in a direction towards floor pan  24 . With the weight applied, mounting points  42 ,  44  deflect downward toward the floor pan  24 . With mounting points  42 ,  44  so deflected, flexible beams  48  and  50  will assume an S-shaped curvature. In the regions of strain sense elements R 3  and R 1 , the flexible beams  48  and  50  will be in tension, thereby reducing the resistance of elements R 3  and R 1  from their nominal values. In the regions of strain sense elements R 4  and R 2 , flexible beams  48  and  50  will in compression, thereby increasing the resistance of strain sense elements R 4  and R 2  from their nominal values. These changes in resistance will upset the balance of the Wheatstone bridge. The unbalanced voltage at nodes D-B of the bridge is then amplified by ASIC U 1  to provide an indication of the weight applied to the seat base  32 . The indication appears as a voltage at pin  2  of connector J 1 .  
         [0019]     Turning to  FIG. 3 , operation of the sense element is shown with a moment force F applied to seat base  32 . In this situation, flexible beams  48  and  50  are in compression in the proximity of strain sense elements R 3  and R 2 , thereby decreasing their resistance from their nominal values. Simultaneously, flexible beams  48  and  50  are in tension in the proximity of strain sense elements R 4  and R 1 , thereby increasing their resistance from their nominal values. In this case, the Wheatstone bridge remains in balance and no, or little, net force is indicated by a voltage across nodes D-B. The effect of the moment force on any occupant weight reading is thereby minimized.  
         [0020]     The description of the invention is exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.