Abstract:
A seat weight sensor for detecting the weight of a seat occupant. The weight sensor has a case mounted between a seat pan and a seat member. One or more strain gauge resistors are mounted in the case. The resistors generate an electrical signal in response to the case being stressed by the weight of the seat occupant. The electrical signal changes as a function of the weight of the occupant. A fastener passes through the seat member, the case, and the seat pan. The fastener secures the sensor between the seat pan and the seat member. The case is adapted to transfer to the strain gage resistor the weight of the occupant up to pre-determined level. The case prevents the strain gage from receiving weight beyond that of the pre-determined level such that the sensor is not damaged by an excessive load. The case also allows the weight sensor to be insensitive to off-axis forces that might otherwise contribute to inaccurate weight readings.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an automobile weight sensor for detecting the presence of a person in a car seat, and in particular to a sensor that can detect the presence of an occupant using strain sensitive resistors and provide an electrical signal to control activation of an airbag. 
   2. Description of the Related Art 
   Automobiles are equipped with restraint systems, such as seat belts, and inflatable restraint systems, such as airbags, to improve passenger safety. In some situations, these safety devices can injure the occupants. For example, an occupant in the front passenger seat may be injured by deployment of an airbag, if the occupant is a baby or child. It is desirable to control the operation of the airbag according to the weight of a passenger for improved performance of seat belts and airbags. A device for measuring the weight of a passenger sitting on a vehicle seat is needed to prevent or modify the deployment of the airbag when the weight is less than a predetermined amount. 
   There have been a number of attempts to measure the weight of a seat occupant, all with significant disadvantages. For example load cells or strain gages have been used in a vehicle seat. One problem encountered in measuring the weight of a seat occupant is that the weight reading needs to be uniform when the vehicle is moving. When the vehicle travels around a curve or in a turn, the weight sensor cannot have a large change in its reading. In other words, the weight sensor needs to be somewhat insensitive to loads that are not in the vertical direction. The load cells of the prior art have suffered from giving false readings when subjected to side loads. 
   The seat weight sensor also needs to be manufactured at a low cost and must be able to withstand large loads. The sensor cannot be damaged by crash forces or other overloads. Prior art seat weight sensors have suffered from requiring an extensive redesign of the seat frame in order to be installed. It is desirable for a seat weight sensor to be installed in existing car seats with a minimum of changes to the existing seat design. 
   A current unmet need exists for a reliable, low cost, robust automobile seat weight sensor that is insensitive to off axis loads and that can be installed in a vehicle with a minimum of changes to the existing seat design. 
   SUMMARY 
   It is a feature of the invention to provide a reliable and cost-effective vehicle seat weight sensor for detecting the weight of a seat occupant. The sensor uses strain sensitive resistors. 
   An additional feature of the invention is to provide a weight sensor for sensing the weight of an occupant in a vehicle seat. The seat has a seat pan and a seat member. The weight sensor includes a case mounted between the seat pan and the seat member. One or more strain gauge resistors are mounted in the case. The resistors generate an electrical signal in response to the case being stressed by the weight of the seat occupant. The electrical signal changes as a function of the weight of the occupant. A fastener passes through the seat member, the case, and the seat pan. The fastener secures the sensor between the seat pan and the seat member. The case is adapted to transfer to the strain gage resistor the weight of the occupant up to a first magnitude. The case prevents the strain gage from receiving weight beyond that of the first magnitude. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a vehicle seat weight sensor mounted in an automobile seat. 
       FIG. 2  is an exploded perspective view of the vehicle seat weight sensor. 
       FIG. 3  is an assembled cross-sectional side view of a FIG.  2 . 
       FIG. 4  is an enlarged cross-sectional view of the weight sensor of FIG.  2 . 
       FIG. 5  is a top view of the sensor strain gage. 
       FIG. 6  is an exploded perspective view of an alternative embodiment of a vehicle seat weight sensor. 
       FIG. 7  is an assembled cross-sectional side view of a FIG.  6 . 
       FIG. 8  is an enlarged cross-sectional view of the weight sensor of FIG.  6 . 
       FIG. 9  is a cross-sectional view of an alternative embodiment of a vehicle seat weight sensor in accordance with the present invention. 
   

   It is noted that the drawings of the invention are not to scale. In the drawings like numbering represents like elements between the drawings. 
   DETAILED DESCRIPTION 
   The present invention provides a vehicle weight sensor for detecting the weight of a seat occupant. Referring to  FIG. 1 , there is a seat assembly  20  shown. Seat assembly  20  has a seat  22  with a seat back  23 , and a seat bottom  24 . A metal first seat member or pan  26  is located between a second seat member or rail  34  and seat bottom  24 . Seat member  34  can be a slide rail. Seat pan  26  has a seat pan bottom  28  that has four bolt holes  30  formed therein. Seat pan bottom  28  also has four wire holes  32 . A seat pan outer rim  27  runs around the outside perimeter of seat pan  26 . Member  34  is slidably attached to a carriage  36  by roller bearings (not shown). Member  34  has holes  35 . The seat member or slide rail allows seat assembly  20  to slide forward and backwards in a vehicle. The carriage  36  is attached to a vehicle floor  38  by a fastener  39  such as a bolt or rivet. 
   Four weight sensor assemblies  40  are shown mounted at the four corners of seat assembly  20  between seat pan  26  and rail  34 . A fastener  42  such as a bolt, rivet or screw passes through rail  34 , sensor  40  and pan  26 . A nut  44  attaches to fastener  42  above seat pan bottom  28 . 
   Referring to  FIGS. 2-5 , details of the weight sensor assembly  40  are shown. Sensor  40  has a metal substrate  46  with two ends, a first end  47  and a second end  48 . Substrate  46  has a top surface  49 , a bottom surface  50  and an aperture  51 . Substrate  46  is preferably formed from  430  stainless steel. An insulative dielectric layer  52  is shown disposed on top surface  49 . Four strain gauge resistors  54 A,  54 B,  54 C and  54 D are arranged on top of dielectric layer  52  around aperture  51 . 
   Resistors  54 A-D are strain sensitive and will change resistance based on the amount of strain in substrate  46 . Circuit lines  56  connect resistors  54 A-D to terminals  58 . The terminals are used to solder to individual wires  59  in a wiring harness  60 . A cover coat (not shown) would be placed over resistors  54 A-D and circuit lines  56 . The cover coat protects the resistors from damage and acts as a solder mask. Dielectric layer  52 , Resistors  54 A-D and terminals  58  can be formed from conventional thick film materials using conventional thick film screening and processing techniques that are commercially available. Dielectric layer  52 , Resistors  54 A-D and terminals  58  can also be formed from a ceramic green tape. Such methods of forming resistors on metal substrates are detailed in U.S. Pat. No. 4,556,598 titled, “A porcelain tape for producing porcelainized metal substrates”, the contents of which are specifically herein incorporated by reference. In a typical configuration, Resistors  54 A-D would be connected to form a wheatstone bridge circuit that is well known in the art. 
   Metal substrate  46  is overmolded with a slightly elastomeric plastic cover or case  70 . Case  70  surrounds substrate  46 . Wire harness  60  is attached to terminals  58  and then overmolded with case  70 . The overmolded case acts as a strain relief for the wire harness. Case  70  is preferably formed from a thermoplastic material that is slightly compliant or elastomeric. Case  70  has a pair of downwardly extending blades  72  and  73 . An upper boss  74  extends from upper surface  49 . A lower boss  76  extends from lower surface  50 . Aperture  51  passes through the bosses and substrate  46 . Case  70  has a top center portion  78  and a bottom center portion  79 . Over-molding the substrate provides environmental protection for the substrate. The over-molding operation reduces the cost of the sensor. The over-molding could also be implemented as discrete pieces that are adhesively attached to the substrate. 
   Fastener  42  is used to attach sensor  40  between seat pan  26  and seat member  34 . Fastener  42  can be a bolt and nut  44  or a bolt and a threaded hole or can be a rivet. An elastomeric washer  80  is located between seat member  34  and fastener  42 . Washer  80  reduces noise and serves as a compliant member. Fastener  42  passes through hole  35 , washer  80 , aperture  51  and hole  30 . Nut  44  is located above seat pan bottom  24  and threadedly mates with fastener  42 . Fastener  42  has a non-threaded shoulder  43 . Wire harness  60  passes through wire hole  32  and runs along seat pan bottom  24 . The four wiring harnesses can be connected together at a junction box (not shown) in the center of the seat, if desired. 
   Case  70  is able to move slightly from side to side in hole  35 , this allows side loads on assembly  20  that are not in the vertical axis to be absorbed by case  70  but not measured. In other words, case  70  decouples the strain gage resistors from side or off vertical axis loads. Sensor  40  therefore is somewhat insensitive to side loads which are undesirable to be measured. Loads in the vertical direction are representative of seat occupant weight and are measured by sensor  40 . 
   Referring to  FIG. 3 , lower boss  76  fits into hole  35 . Upper boss  74  and lower boss  76  both surround shoulder  43 . A gap  82  is formed between seat pan bottom  28  and top center portion  78 . Similarly, another gap  84  is formed between rail  34  and bottom center portion  79 . When an occupant sits on seat bottom  24 , the seat occupants weight is transferred from seat bottom  24  to seat pan  26 , through sensor  40 , to seat member  34 , then to carriage  36  and floor  38 . The entire weight of the seat occupant is supported by the four sensors  40 . This weight causes strain in the sensor and is measured by strain gage resistors  54 A-D. A voltage is applied to the resistors through wires  59 . An electrical output signal is generated that is proportional to the seat occupant&#39;s weight. The electrical signal is transmitted over one of the wires to a conventional air bag controller (not shown). The air bag controller then can control deployment of the airbag based upon the seat occupant&#39;s weight. Typically, the air bag is disengaged or turned off below a minimum weight, such as for a child. 
   The length of shoulder  43  is slightly less than the distance between the lower end of boss  74  and the upper end of boss  76 . The elastomeric bosses are compressed slightly upon the application of weight. This is not a requirement for the function of the weight sensor; however, it helps to reduce noise. Boss  76  is free to move vertically in hole  35 . The bosses  74  and  76  and bolt  42  can move a small amount in the vertical direction. The blades  72  and  73  are in contact with seat member  34  and have a minimum amount of travel in the vertical direction. Boss  76  and hole  35  resist horizontal motion while allowing for some compliance such that a large weight reading by the sensor is not generated for a small misalignment of the seat pan and seat member. 
   When weight is applied to seat pan  26 , it is transferred to boss  74 . The load is then carried through central portions  78 ,  79  and substrate  46  to blades  72  and  73 , which transfer the load to seat member  34 . The bosses and blades and elastomeric washer  80  work together to allow the weight sensor to isolate substrate  46  from torque loads and off-axis loads that are not applied in the vertical direction. In addition, the symmetrical strain resistor placement is inherently insensitive to applied torque loads. These features together allow the device to function, even though, in practice, the seat pan and seat member will not be parallel. 
   Weight sensor  40  has many advantages. Weight sensor  40  provides overload protection for the strain sensitive resistors. If an excessive force is applied to substrate  46 , it could be permanently deformed, if it is stressed beyond its elastic region. This would result in a permanent voltage shift for the strain sensitive resistors. Overload protection for down loads or positive loads is achieved when the lower gap  84  is closed and central portion  79  contacts seat member  34 . Any additional loading then passes directly to seat member  34  without passing through substrate  46 . Overload protection for upward loads or negative loads is achieved by the non-linear spring rate of elastomeric washer  80 . 
   Additional negative overload protection can be added by the addition of a spring around washer  80 . As an example a Belleville washer (not shown) could be added adjacent washer  80  surrounding fastener  42 . A negative load would compress a Belleville washer, applying a pre-load, until it reaches its solid height. At this point, any additional applied load would bypass weight sensor  40 . 
   Elastomeric washer  80  is provided to prevent metal to metal contact when the net loading tends to separate seat pan  26  and seat member  34 . It also can serve as a compliant member, which fills the gap while allowing for dimensional variation and it may also provide a pre-load on the assembly. A spring (not shown) may work in concert with or replace Washer  80  if more consistent pre-load values are desired or if the changes in the elastomer&#39;s properties with temperature affect measurement performance. Adding a defined or calibrated pre-load would allow for the measurement of negative (separation) loads. 
   A particular feature of sensor  40  is that strain sensitive resistors  54 A-D can be placed in either compression or tension but not both. The design of sensor  40  provides a mechanical structure that only allows either compression or tension forces to be transferred to strain gages  54 A-D but not both. This is a major advantage because when a strain sensitive resistor is cycled from compression to tension or from tension to compression, it causes hysteresis in the strain sensitive resistors. Previous sensors have had a bi-directional operation which causes more hysteresis. 
   1 st  Alternative Embodiment 
   Referring to  FIGS. 6-8 , details of an alternative weight sensor assembly  96  are shown. Sensor  96  is similar to sensor  40  except that a spring washer  90  and standoffs  92 ,  93  have been added. 
   Spring washer  90  is located between boss  74  and seat pan  26 . Other types of springs could also be used for washer  90  such as coil springs or leaf springs. The spring  90  allows for more motion of sensor  96  and allows for more deflection of substrate  46  when weight is applied. Shoulder  43  passes through the hole in washer  90 . 
   Standoffs  92  and  93  extend upwardly from upper surface  78  at the ends of case  70 . Standoffs  92  and  93  are opposed from blades  72  and  73 . The standoffs  92  and  93  are molded from plastic in the same manner as blades  72  and  73 . 
   The operation of weight sensor  96  is as follows: 
   The fastener  42  is free to slide up and down in aperture  51 . Spring  90  is pre-compressed to a pre-determined load designated as (L). This load is carried in tension by fastener  42  and in compression by case  70  of sensor  96 . 
   When an occupant sits on seat bottom  24 , the seat occupants weight is transferred from seat bottom  24  to seat pan  26  through spring  90 , through upper boss  74 , through substrate  46 , through blades  72  and  73  to seat member  34 , then to carriage  36  and floor  38 . The entire weight of the seat occupant is supported by the four sensors  96 . This weight causes strain in the sensor and is measured by strain gage resistors  54 A-D. A voltage is applied to the resistors through wires  59 . An electrical output signal is generated that is proportional to the seat occupant&#39;s weight. The electrical signal is transmitted over one of the wires to a conventional air bag controller (not shown). The air bag controller then can control deployment of the airbag based upon the seat occupant&#39;s weight. Typically, the air bag is disengaged or turned off below a minimum weight such as for a child. 
   From zero load up to load L, the seat pan  26  moves downward according to the spring rate of sensor  96 . Case  72  and substrate  46  together flex and act as a spring with a limited range of motion. This is the spring rate associated with sensor  96 . When the pre-compression load L is reached, fastener  42  is no longer in tension. At loads slightly greater than load L, the seat pan  28  moves downward according the series combination of the spring rates of sensor  96  and spring  90 . Spring  90  is chosen to have a spring rate much lower than that of sensor  96  so that seat pan  28  moves relatively much farther for load increments slightly above load L than it does for loads below load L. The gap  82  between standoffs  92  and  93  and pan  28  is sized such that this gap is closed at a maximum load (M) before spring  90  is fully compressed. Substantially all additional load applied above the maximum load M is transferred from pan  28  to standoffs  92  and  93  and then through blades  72  and  73  to seat member  34 . The maximum load M is chosen to be sufficiently below the maximum design load of substrate  46  such that substrate  46  is protected from loads above its design limit. Sensor  96  therefore provides overload protection for the strain sensitive resistors. The load bearing standoffs  92  and  93  are representative of alternate load paths in general and could be designed differently. The overload protection is not dependent upon the shape of the substrate  46 . 
   The overload path structures need not be a part of the sensor if the tolerances of the external structure are taken into consideration in the design of the sensor. For example, a load bearing member may be placed an appropriate distance below the head of bolt  42  or directly below seat pan  26 . Generally, these structures would be part of or somehow tied to seat member  34  in order to move in concert with seat member  34 . 
   It is noted that there may be some advantage to inverting spring  90  from the position shown in FIG.  7 . This would allow the load transferred to sensor  96  to be transferred closer to the center of sensor  96 . This would tend to increase the load levels near the center portions  78  and  79  and thus increase the maximum stress levels near the center of substrate  46 . 
   Washer  80  is designed to provide one or more of the following functions: load spreading, impact damping, noise reduction and gap filling. Washer  80  could also be a spring, similar to spring  90 . Washer  80  could also be an assembly including multiple components such as a washer and a spring. Washer  80  can perform one or more of the functions listed above. In addition, washer  80  could apply a load that is some fraction of the pre-compression load L called N. This load would appear as a constant offset in the weight reading of sensor  96 . Applying this type of load allows weight measurements in the reverse or negative (vertical) direction. The reverse direction is when the seat pan is pulled upwards. A washer  80 , pre-compressed to load N, would allow weight measurements up to a maximum weight of N in the reverse direction and in the normal load direction up to the maximum load M minus the pre-compression load N. 
   In the reverse direction, there is no problem with a sensor overload since the sensor  96  is not directly attached to seat member  34 . The pre-compression load N can be applied with equal efficacy either at the position of washer  80  or between nut  44  and seat pan  26 . 
   Similar to sensor  40 , the bosses and blades and elastomeric washer  80  work together to allow the weight sensor  96  to isolate substrate  46  from torque loads and off-axis loads that are not applied in the vertical direction. In addition, the symmetrical strain resistor placement is inherently insensitive to applied torque loads. These features together allow the device to function even though, in practice, the seat pan and seat member will not be parallel. 
   2nd Alternative Embodiment 
   Referring to  FIG. 9 , details of an alternative weight sensor assembly  100  are shown. The location of sensor  100  is different than in previous embodiments. Sensor  100  is located above seat pan  26 . In some seat configurations, it may be desirable to have the sensor located above the seat pan in order to reduce the overall height of the seat assembly. Seat pan  26  has openings  102  through which blades  72  and  73  extend to contact seat member  34 . Fastener  42  has a boss  104  that fits into hole  35 . 
   First spring washer  90  is retained between plate  108  and upper boss  74 . Spring washer  90  is compressed during assembly by nut  44  pushing plate  108  downwardly. Spring washer  90  is compressed to a load L 1  of approximately 100 kilograms of spring force. Elastomeric washer  80  is compressed between boss  104  and seat pan  26 . A second spring washer  106  is located between fastener  42  and seat member  34 . Spring washer  106  is compressed during assembly by nut  44  pulling on fastener  42 . Spring washer  106  is compressed to a load L 2  of approximately 15 kilograms of spring force. The spring washer  106  allows for more motion of sensor  96  and allows for more deflection of substrate  46  when weight is applied. Shoulder  43  passes through the hole in washer  90 . 
   The operation of weight sensor  100  is as follows: 
   The fastener  42  is free to slide up and down in aperture  51 . Spring  90  is pre-compressed to a load L 1 . Spring  106  is pre-compressed to a load L 2 . When an occupant sits on seat bottom  24 , the seat occupants weight is transferred from seat bottom  24  to seat pan  26  through fastener  42  to plate  108 , through spring  90 , through upper boss  74 , through substrate  46 , through blades  72  and  73  to seat member  34 . This weight causes strain in the sensor and is measured by strain gage resistors  54 A-D. A voltage is applied to the resistors through wires  59 . An electrical output signal is generated that is proportional to the seat occupant&#39;s weight. The electrical signal is transmitted over one of the wires to a conventional air bag controller (not shown). The air bag controller then can control deployment of the airbag based upon the seat occupant&#39;s weight. Typically, the air bag is disengaged or turned off below a minimum weight such as for a child. 
   From zero load up to load L 1  minus L 2 , the seat pan  26  moves downward according to the spring rate of sensor  100 . Case  72  and substrate  46  together flex and act as a spring with a limited range of motion. This is the spring rate associated with sensor  100 . When the load reaches L 1  minus L 2 , boss  76  separates from seat pan  26 . For loads above L 1  minus L 2 , the seat weight is still carried through sensor  100 . Spring  90  starts to be compressed allowing seat member  26  to move toward seat member  34  until at a load L 3  washer  80  contacts seat member  34 . Further loads beyond load L 3  are transferred from seat member  26  through washer  80  directly to seat member  34 . This is the overload position. The maximum load seen by sensor  100  is approximately load L 3 . 
   Spring  90  is chosen to have a spring rate much lower than that of sensor  100  so that seat pan  26  moves relatively much farther for load increments slightly above load L 1  minus L 2  than it does for loads below load L 1  minus L 2 . The gap  84  is sized such that this gap is closed at a maximum load L 3  before spring  90  is fully compressed. Substantially all additional load applied above the maximum load L 3  is transferred from pan  26  through washer  80  to seat member  34 . The maximum load L 3  is chosen to be sufficiently below the maximum design load of substrate  46  such that substrate  46  is protected from loads above its design limit. Sensor  100  therefore provides overload protection for the strain sensitive resistors. 
   It is noted that the design of sensor  100  allows the measurement of both loads that are pressing down on the seat and loads that are pulling up on the seat. It is the addition of spring  106  that allows for the measurement of upwardly directed loads while maintaining the unidirectional loading of sensor element  46 . In other words, sensor  46  is always exposed to one of either compression or tension even when the seat loading is in either direction. This feature prevents hysteresis in the sensor and provides a more accurate sensor. 
   Variations of the Invention 
   Although the illustrated embodiment shows resistors  54 A-D on the top surface  49 , more or fewer resistors could be used. If desired, the resistors could be placed on bottom surface  50 . 
   The weight sensor shown used a thick film resistor, one skilled in the art will realize that the preferred embodiment would work with other types of resistors. For example, discrete chip resistors could be attached to substrate  46  or thin film resistors could be used. Furthermore, the shape of substrate  46  could be varied to any configuration that would transfer the weight from the seat and concentrate it in the desired location on the substrate. For example a cross, round, or triangle shape could be used. 
   Another variation of the weight sensor would be to utilize other electrical connections. For example, other types of connectors or terminals could be used in place of wire harness  60 . 
   Yet, a further variation would be to place signal conditioning circuitry on substrate  46  to amplify and filter the electrical signal before it is transmitted to the airbag controller. 
   The illustrated embodiment showed the use of the weight sensor between a seat pan and a seat member. It is contemplated to utilize the weight sensor in other locations. For example, the weight sensor could be mounted between the floor  38  and carriage  36 . 
   While the invention has been taught with specific reference to these embodiments, someone skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. The invention should therefore be limited only by the scope of the human imagination. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.