Load cell and seat occupant weight sensing system

Abstract

A load cell particularly useful for a seat occupant weight sensing system includes a liquid filled chamber and a pressure sensor providing an electric signal indicating the pressure in the liquid. Four load cells supporting a seat provide four signals that are added to determine the weight of the seat occupant. The load cell comprises two flanged conical springs stressed to provide preload. One of the springs also forms part of the surface of the liquid filled chamber. The two springs operate in concert to resist side forces and moments. The load cell is responsive to both compressive and tensile forces while being substantially unaffected by lateral forces and moments. A seat belt tension sensor may be included to measure seat belt tension. Preferred manufacturing methods provide low cost,

1 . A load cell having an axis, and comprising: 
 two force input members for receiving force applied to said load cell,    a first spring and a second spring, and    force sensing means responsive to said force applied to said load cell by producing a force signal, and wherein:    each said spring comprises: (1) a first part linked with one of said force input members for movement therewith, (2) a second part linked with the other of said force input members for movement therewith, and (3) a uniting part unitary with said first and second parts, and    said uniting parts are offset from each other in the direction of said axis, whereby    said force signal indicates the axial component of said force applied to said load cell and is minimally affected by components of said force applied to said load cell having directions perpendicular to said axis.    
   
   
       2 . The invention as defined by  claim 1 , and wherein: 
 said force sensing means comprises a force sensor adapted for receiving force from said force input members, and    said force sensor is adapted to produce a force sensor signal responsive to said received force, whereby    said force sensor signal is said force signal.    
   
   
       3 . The invention as defined by  claim 1 , and wherein said force sensing means comprises: 
 liquid pressurized by said force applied to said load cell, and    a pressure sensor responsive to said pressure in said liquid by producing a pressure signal, whereby    said pressure signal is said force signal.    
   
   
       4 . The invention as defined by  claim 3 , and wherein said liquid has a larger thermal expansion coefficient than the thermal expansion coefficients of the materials of which said force input members and said springs are made, and 
 a said part of a said spring linked with a said force input member is linked by a link comprising a material having a larger thermal expansion coefficient than the thermal expansion coefficient of at least one of the materials of which said force input members and said springs are made, whereby    variation of said force signal with temperature is controlled.    
   
   
       5 . The invention as defined by  claim 4 , and wherein said link comprising a material having a larger thermal expansion coefficient comprises a sleeve.  
   
   
       6 . The invention as defined by  claim 1 , and wherein 
 a said spring when no force is applied thereto has the shape of a slightly conical washer unitary with two cylindrical flanges.    
   
   
       7 . The invention as defined by  claim 6 , and wherein 
 said spring that has the shape of a slightly conical washer unitary with two cylindrical flanges when no force is applied thereto is adapted to assume the shape of a flat washer unitary with two cylindrical flanges after it is preloaded during load cell manufacture.    
   
   
       8 . The invention as defined by  claim 1 , and wherein: 
 said first spring comprises: (1) a first inner cylindrical flange, (2) a first outer cylindrical flange, and (3) a first uniting part; and    one of said first flanges is linked with one of said force input members for movement therewith and the other of said first flanges is linked with the other of said force input members for movement therewith, whereby    said one of said first flanges is a said first part and said other of said first flanges is a said second part.    
   
   
       9 . The invention as defined by  claim 8 , and wherein: 
 said second spring comprises: (1) a second inner cylindrical flange, (2) a second outer cylindrical flange, and (3) a second uniting part; and    one of said second flanges is linked with one of said force input members for movement therewith and the other of said second flanges is linked with the other of said force input members for movement therewith, whereby    said one of said second flanges is a said first part and said other of said second flanges is a said second part.    
   
   
       10 . The invention as defined by  claim 1 , and wherein 
 said springs exert a spring force between said force input members in a first direction that causes said force signal to indicate a greater force than if said spring force were zero, whereby    force applied between said force input members in the direction opposite said first direction causes said force signal to indicate a lesser force than said force signal indicates when said force applied to said load cell is zero.    
   
   
       11 . A seat occupant weight sensing system comprising: (1) a seat, (2) a load cell. and (3) a processing unit for reckoning weight applied to said seat; and wherein: 
 said seat is adapted for receiving force from an occupant of said seat and applying force derived from said received force to said load cell,    said load cell comprises: (1) two force input members for receiving force applied to said load cell, (2) a spring, and (3) force sensing means for producing a force signal;    said spring comprises: (1) an outer cylindrical flange linked with one of said force input members for movement therewith, (2) an inner cylindrical flange linked with the other of said force input members for movement therewith, and (3) a uniting component unitary with said flanges; and wherein    said force signal is responsive to said force applied to said load cell,    and said processing unit is adapted to consider said force signal when said reckoning weight applied to said seat.    
   
   
       12 . The invention as defined by  claim 11 , and wherein said processing unit is adapted to categorize the occupant of said seat in accordance with said force signal.  
   
   
       13 . The invention as defined by  claim 11 , and wherein: 
 said force sensing means comprises: liquid pressurized by said force applied to said load cell and an absolute pressure sensor responsive to said pressure in said liquid by producing said force signal, and    said processing unit comprises: an atmospheric pressure sensor for measuring atmospheric pressure and means for correcting said force signal in accordance with said atmospheric pressure to obtain a corrected force signal,    whereby said corrected force signal indicates the component of the force applied to said load cell having the direction of an axis.    
   
   
       14 . The invention as defined by  claim 11 , and including: 
 occupant protection means for protecting an occupant of said seat, said occupant protection means including a microprocessor, and    a seat belt and a seat belt tension sensor responsive to tension in said seat belt by providing a tension signal to said microprocessor, and wherein    said occupant protection means is adapted to be responsive to said tension signal when said protecting said occupant.    
   
   
       15 . The invention as defined by  claim 11 , and wherein said load cell is tested after it is manufactured and numbers derived from the results of said tests are stored in said load cell.  
   
   
       16 . The invention as defined by  claim 15 , and wherein said numbers are stored as markings on the outside of said load cell.  
   
   
       17 . The invention as defined by  claim 11 , and including 
 attachment means for attaching said load cell to a structural member of said seat, and wherein    said load cell has an axis, and    said attachment means is adapted to prevent movement of said structural member relative to said load cell in a direction parallel to said axis and to permit limited movement of said structural member relative to said load cell in a direction perpendicular to said axis.    
   
   
       18 . The invention as defined by  claim 17 , and wherein 
 said limited movement is achieved by providing an opening in said structural member sized to allow said limited movement therewithin by an element fixed with respect to said load cell.    
   
   
       19 . The invention as defined by  claim 11 , and wherein a said link linking a said flange with a said force input member comprises a material having a larger coefficient of thermal expansion than at least one of the materials of which said force input members and said springs are made; whereby 
 variation of said force signal with temperature is controlled.    
   
   
       20 . The invention as defined by  claim 11 , and wherein: 
 a said force input member comprises threads for engaging a threaded fastening component, and including    a cover isolating material near said threads from said liquid; whereby    said force signal is less affected by the degree of tightness of said threaded fastening component.    
   
   
       21 . A method of making a load cell comprising a spring and a force input member, the method comprising: 
 (1) pressing a surface of said spring against a surface of a said force input member, while (2) performing a weld fixing the position of said spring relative to said force input member, (3) stressing said spring to form a gap between said surfaces, (4) injecting said liquid through an opening into said gap, and (5) sealing said opening; whereby    the effects of temperature on the performance of said load cell are accurately controlled.    
   
   
       22 . A load cell having an axis, and comprising: 
 force input members for receiving force applied to said load cell, a spring, a link linking said spring to a said force input member, and force sensing means responsive to said force applied to said load cell by producing a force signal; and wherein:    said spring comprises: a first part linked by said link with one of said force input members for movement therewith, a second part linked with another of said force input members for movement therewith, and a uniting part unitary with said first and second parts; and wherein    said link is made of material having a larger thermal expansion coefficient than the thermal expansion coefficient of the material of which said spring is made, whereby    the variation of said force signal with temperature is controlled.