Abstract:
A seat belt tension sensor for measuring the magnitude of tension in a seat belt. The seat belt tension sensor has a housing with a cavity. An anchor plate has a portion located in the cavity. The housing moves relative to the anchor plate between a first position and a second position. A sensor is mounted to the housing. The housing presses on the sensor as the housing moves from the first to the second position. The sensor generates an electrical signal in response to the housing moving between the first and second positions. The electrical signal changes as a function of tension on the seat belt. The sensor is a strain gage. The housing engages the anchor plate in the second position to protect the sensor from damage in a vehicle crash situation.

Description:
CROSS REFERENCE TO RELATED AND CO-PENDING APPLICATIONS 
     This application is a continuation in part of U.S. patent application Ser. No. 09/923,151 filed Aug. 6, 2001 and titled, “Seat Belt Tension Sensor”. 
    
    
     This application is related to the following U.S. patent applications: 
     U.S. patent application Ser. No. 09/884,615 filed Jun. 19, 2001 and titled, “Seat Belt Tension Sensor with Overload Protection”. 
     U.S. patent application Ser. No. 09/441,350, filed Nov. 15, 1999 and titled, “Automobile Seat Having Seat Supporting Brackets with a Stepped Weight Sensor”. 
     U.S. patent application Ser. No. 09/374,874, filed Aug. 16, 1999 and titled, “Automobile Seat Weight Sensor”. 
     U.S. patent application Ser. No. 09/374,870, filed Aug. 16, 1999 and titled, “Vehicle Occupant Position Detector and Airbag Control System”. 
     U.S. patent application Ser. No. 09/422,382, filed Oct. 21, 1999 and titled, “Vehicle Seat Weight Sensor”. 
     U.S. Pat. No. 6,209,915, issued Apr. 3, 2001 and titled, “Seat Belt Tension Sensor”. 
     The foregoing patents have the same assignee as the instant application and are herein incorporated by reference in their entirety for related and supportive teachings. 
     1. Field of the Invention 
     This invention relates to an automobile sensor for detecting the magnitude of a tensile force in a seat belt used in a car seat, and in particular to a sensor that can detect the magnitude of tension in a seat belt and provide an electrical signal that is representative of the magnitude of tensile force. 
     2. Description of the Related Art 
     Air bags have been heralded for their ability to reduce injuries and save lives. However, since their incorporation into automobiles, a problem has existed with people of smaller size and small children. Air bags are designed to cushion the impact of occupants and thus reduce the injuries suffered. However, the force needed to properly cushion the occupant varies based on the size and position of the person. 
     For example, a larger person requires the bag to inflate faster and thus with more force. A smaller person may be injured by a bag inflating at this higher inflation force. A smaller person is more likely to be sitting close to the dashboard and would therefore stand a higher chance of being injured by the impact of the inflating bag, as opposed to the passenger hitting the fully inflated bag to absorb the impact of the accident. An average-sized person can also be injured by an airbag inflation if they are leaning forward, as for example, if they are adjusting the radio. 
     Because of the concern over injury to passengers in these situations, the National Highway Transportation Safety Administration (or NHTSA), an administrative agency of the United States, is instituting rules under FMVSS 208 requiring the air bag deployment system to identify the passenger size and position and inflate the air bag accordingly. 
     One way to accomplish this task is to use a seat belt tension sensor in conjunction with an occupant weight sensor. The weight sensor can provide an indication of the force placed by an occupant on the seat. However, if the seat belt is unduly tightened, it can place an additional downward force on the passenger, creating an erroneous weight reading. Similarly, it is common for infant car seats to be secured tightly to the seat. In this circumstance, it is critical for the system to recognize that the passenger does not warrant inflation of the air bag. By sensing the tension on the seat belt in addition to the weight reading from the seat, the actual weight of the occupant can be determined. This allows for the system to safely deploy the air bag. 
     SUMMARY 
     It is a feature of the present invention to provide a seat belt tension sensor for attachment between a seat belt and a vehicle. 
     Another feature of the invention is to provide a seat belt tension sensor that includes a housing that has a cavity. An anchor plate has a first portion located in the cavity. The housing moves relative to the anchor plate between a first position and a second position. A sensor is mounted to the housing. The housing presses on the sensor as the housing moves from the first to the second position. The sensor generates an electrical signal in response to the housing moving between the first and second positions. The electrical signal changes as a function of the tension on the seat belt. A spring is located between the sensor and the anchor plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective exploded view of a seat belt tension sensor. 
     FIG. 2 is a partial assembled view of FIG.  1 . 
     FIG. 3 is an assembled view of FIG. 1 with a portion of the housing removed. 
     FIG. 4 is an assembled view of FIG.  1 . 
     FIG. 5 is an assembled view including the belt and fastener. 
     FIG. 6 is a cross-sectional view of FIG.  5 . 
     FIG. 7 is a side view of the strain member. 
     FIG. 8 is a schematic diagram of the wheatstone bridge circuit. 
     It is noted that the drawings of the invention are not to scale. 
    
    
     DETAILED DESCRIPTION 
     The present invention is a seat belt tension sensor. Referring to FIGS. 1-8, a seat belt tension sensor assembly  20  shown. Assembly  20  has a limit structure  22  and a sensor  24 . Limit structure  22  is fastened between a seat belt webbing  30  and a structural part of the vehicle such as a floor (not shown). The belt webbing  30  has an end  31 , an end  32 , a belt loop  33  and stitching  34  that retains end  32 . 
     The limit structure  22  includes a housing  200 , an anchor plate  220  and a strain sensor or member  240 . Housing  200  has a bottom portion  201 , top portion  202 , flange  203 , hole  204 , spring channel  205  and posts  206 . A cavity  210  is located within housing  200 . Posts  206  have slots  207  that hold strain member  240 . Housing  200  has a notch  211 , pylons  208 , pins  209  and an upwardly extending lip  212 . Housing  200  has a narrow portion  214  on an end of the housing. 
     Anchor plate  220  is loosely fitted within housing  200  resting on pylons  208 . Anchor plate  220  includes ends  221  and  222 , a step section  223 , a cutout  224 , aperture  226  and an aperture  228 . Arm  227  extends between aperture  226  and cutout  224 . A projection  230  extends from arm  227  into cutout  224 . A fin  232  extends into cutout  224 . The anchor plate  220  is located in cavity  210 . Aperture  226  goes over and surrounds flange  203 . A gap  236  is formed between flange  203  and edge  234   
     Seat belt webbing  30  is attached through hole  204  and aperture  226 . The end  32  of webbing  30  is routed through hole  204  and aperture  226 , wrapped back onto itself forming loop  33  and sewn with stitching  34  to secure the seat belt webbing to assembly  20 . 
     A spring  260  is mounted in spring channel  205 . One end of spring  260  is mounted over projection  230 . Sensor  24  has a strain member  240  that is mounted in slots  207 . A support  262  fits into one end of spring  260 . Support  262  rests adjacent a surface of strain member  240  and serves to focus the forces from spring  260  onto strain member  240 . Fin  232  is in contact with the back surface of strain member  240 . 
     A wire harness  280  has several wires  282 . Wires  282  are pressed or soldered into circuit board holes  256  in printed circuit board  252 . 
     The top portion  202  of the housing  200  is attached to the bottom portion  201  by ultrasonic welding along lip  212 . 
     Seat belt tension sensor  20  is attached to a vehicle floor or seat or other member (not shown) by a fastener  40  such as a bolt, rivet or screw. Fastener  40  goes through aperture  228  and is attached to a vehicle structure or seat. The fastener shown is threaded; however, other types of fasteners would work such as a rivet. 
     Strain member  240 , shown in FIGS. 7 and 8, is formed of a material capable of supporting the tension applied by spring  260  and actuator  262  when the seat belt is tightened. Preferably, the strain member  240  is formed of 430 stainless steel. The strain member  240  includes strain sensitive resistors  242   a,b,c,d  formed thereon. These are formed by first depositing a dielectric layer  244  onto a substrate  241 . Substrate  241  is preferably steel. The strain member  240  is then kiln fired at 850° C. Next, electrically conductive traces  245  and connector pads  246   a,b,c,d  are similarly deposited onto the strain member  240 . The strain member  240  is again kiln fired at 850° C. The strain sensitive resistors  242   a,b,c,d  are next screened onto the strain member  240  in positions defined by the electrically conductive traces  245 . The strain member  240  is again kiln fired at 850° C. At this point, a final coating of a covercoat or epoxy (not shown) can be applied to protect the electrical components of strain member  240 . This coating is not required, but may be desirable in circumstances where high abrasion or contaminants are expected. It should be noted that the strain sensitive resistors  242   a,b,c,d  and connector pads  246   a,b,c,d  together form the Wheatstone bridge circuit of FIG.  8 . 
     Details of the construction and operation of resistors  242  are shown in U.S. patent application Ser. No. 09/441,350, filed Nov. 15, 1999 and titled, “Automobile Seat Having Seat Supporting Brackets with a Stepped Weight Sensor”. 
     Terminals  250  connect to strain member  240  and are soldered to pads  246   a, b, c, d . Terminals  250  have one end that are soldered in holes  258  of printed circuit board  252 . Electronic circuitry  254 , such as an integrated circuit is attached to printed circuit board  252  to amplify and filter the signal from the strain gage resistors  242 . Printed circuit board  252  has holes that fit over pins  209  in housing  200 . Circuit board  252  is held in position in cavity  210  by pins  209 . 
     When a tension is applied to seat belt  30 , housing  200  transfers force to posts  206  which applies pressure to the ends of strain member  240 . The spring force of spring  260  resists this force in the center of strain member  240  causing strain in member  240 . As the tension increases, the strain sensitive resistors  242  will change resistance resulting in an electrical output signal that changes in proportion to the amount of tension in seat belt  30 . This electrical signal is processed by electronic circuitry  254  and provided to an external electrical circuit by wire harness  280 . 
     In a collision situation, a large force is applied to the tension sensor. The force applied to the seat belt overcomes the spring resistance of spring  260  moving housing  200  and flange  203  into contact with anchor plate  220 . In this case, edge  234  is in contact with flange  203 . The large force from the seat belt is transferred through the anchor plate  220  to fastener  40 , which is attached to the vehicle structure or seat. Thus, in a collision, the large seat belt tension force is transferred from the seat belt to the vehicle structure. In this way, no further tension is applied to the strain member  240  and the strain member  240  is thus protected from excessive damaging forces by limit structure  22 . 
     An electrical output signal is generated by the resistors  242  that is proportional to the magnitude of the tension in the seat belt and is transmitted over a wire harness  280  to a conventional air bag controller or occupant classification module (not shown). The air bag controller can then use the seat belt tension information to compute a more accurate profile of the seat occupant and use that information to control deployment of the airbag. This is the normal operational state of the seat belt tension sensor in which all of the seat belt tension is carried through the sensor  20 . 
     In a situation where the vehicle is involved in a crash, the seat belt tension sensor operates in a different mode called a high load or crash state. In the high load state, the limit structure  22  carries the majority of tension placed on the seat belt. The amount of tension in the seat belt in a crash situation is much larger than in normal operation. If the strain member  240  was designed to carry all of this tension, it would not flex enough to properly function as a strain gage sensor. Therefore, in a crash situation, the limit structure  22  carries the tension through the much stronger limit structure  22 . 
     Remarks 
     The seat belt tension sensor has several advantages. It allows accurate sensing of seat belt tension, while at the same time providing the structural strength needed for occupant restraint in a crash situation. The seat belt tension sensor allows an airbag controller to make better decisions as to when and how to deploy and airbag based upon more accurate seat occupant information. In the case of a child&#39;s car seat being strapped into a car seat, the seat belt tension sensor in conjunction with a seat weight sensor allows the airbag controller to properly compute that the seat occupant has a low weight and to prevent deployment of the airbag. 
     The gap between the anchor plate and the housing flange is the travel range of the sensor as it is actuated. This design solves several problems. 
     1) Maintaining Sensitivity at Low Loads Without Damage at Higher Loads. 
     When the gap between the edge and the flange is closed the load applied to the strain sensor elements reaches its limit. After this, the load is transferred to the fastener. Limiting the maximum load applied to the strain sensor is necessary since the working range of the sensor is generally below 100-lbs. but the sensor must withstand large (often greater than 1000-lb.) loads without damage and must not compromise the integrity of the passenger restraint system. 
     2) Maintaining Restraint System Integrity. 
     The present design allows the use of the same or very similar mounting bolts and anchors and mounting technique as do current seatbelt attachment methods. Thus, safety engineers are very familiar with the requirements of the attachment method and installation procedures are changed as little as possible. 
     3) Integration Into Multiple Restraint Systems. 
     This present invention allows the sensor to be attached at the most common point of a wide variety of belt systems. It is useable even with very short bolt to seat belt buckle distances. 
     Variations 
     The sensor shown had several strain gage resistors, one skilled in the art will realize that the preferred embodiment would work with other types of sensors. For example, discrete chip resistors could be attached or foil type strain gages could be used. 
     Another variation of the seat belt tension sensor would be to utilize other electrical connections other than a wire harness. For example, a integral connector or terminals could be added. 
     The seat belt tension sensor shown was mounted between a seat belt and a vehicle structure. One skilled in the art will realize that the preferred embodiment could be mounted to various locations on the seat or vehicle interior. For example, the seat belt tension sensor could be attached to the upper or lower B pillar or at the seat frame. 
     The illustrated embodiment showed the use of the seat belt tension sensor in an automobile seat. It is contemplated to utilize the seat belt tension sensor in other occupant sensing applications such as chairs, sofas, scales, beds and mattresses, hospital equipment, cribs, airplane seats, train seats, boat seats, amusement rides, and theater seats. 
     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 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.