Abstract:
Described herein is a seat belt tension sensor assembly that includes an anchor plate adapted to be secured to an object, a housing associated with and movable with respect to the anchor plate, wherein the housing defines a cavity therein, and a sense element disposed in the cavity. The sense element is adapted to produce an output in response to a force placed thereon. The output provided by the sense element is a function of the force placed on the housing by a seat belt up to a predetermined maximum value. The housing does not move until the maximum value is reached, and, after the maximum value is reached, the output of the sense element does not substantially change from the maximum value.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a seat belt tension sensor, and more particularly to a simple and accurate seat belt tension sensor assembly that includes overload protection. 
       BACKGROUND OF THE INVENTION 
       [0002]    Seat belt tension sensors are often complex, costly and less accurate than desirable. Accordingly, a need exists for a seat belt tension sensor that overcomes these issues. 
       SUMMARY OF THE INVENTION 
       [0003]    In accordance with a first aspect of the present invention there is provided a seat belt tension sensor assembly for use with a seat belt that includes an anchor plate that is adapted to be secured to an object. The anchor plate includes a sensor opening defined therethrough. The assembly further includes a housing that has first and second housing halves that define a cavity therebetween. A portion of each of the housings halves are received in the sensor opening of the anchor plate and the housing is movable with respect to the anchor plate. The housing and the anchor plate cooperate to define a seatbelt opening. The assembly also includes a sense element disposed in the cavity that is adapted to produce an electrical signal in response to a force placed thereon, and a preloaded spring disposed in the cavity between the sense element and the anchor plate. The preloaded spring is adapted to compress when the force placed on the housing by the seat belt is greater than the spring force. In one embodiment, the spring is preloaded by a clip that holds the spring in a partially compressed position, thereby forming a spring assembly. The spring assembly is disposed between the sense element and the anchor plate. In an embodiment, the housing is movable between a first position and a second position, and in the second position, the force exerted on the sense element by the spring is no greater than the spring force. 
         [0004]    In accordance with another aspect of the present invention there is provided a seat belt tension sensor assembly that includes an anchor plate adapted to be secured to an object, a housing associated with and movable with respect to the anchor plate, wherein the housing defines a cavity therein, and a sense element disposed in the cavity. The sense element is adapted to produce an output in response to a force placed thereon. The output provided by the sense element is a function of the force placed on the housing by a seat belt up to a predetermined maximum value. The housing does not move until the maximum value is reached, and, after the maximum value is reached, the output of the sense element does not substantially change from the maximum value. 
         [0005]    In accordance with yet another aspect of the present invention there is provided a method that includes providing a seat belt tension sensor assembly that comprises an anchor plate secured to an object, a housing that defines a cavity therein and is associated with and movable with respect to the anchor plate, and a sense element disposed in the cavity. The sense element is adapted to produce an output in response to a force placed thereon. The output provided by the sense element is a function of the force placed on the housing by a seat belt up to a predetermined maximum value. The method further includes placing a first force on the housing that is less than the maximum value, thereby providing a first output. The first output is directly proportional to the first force and the housing does not move as a result of the first force. The method further includes placing a second force on the housing that is greater than the maximum value, thereby providing a second output. The second output is not directly proportional to the second force and the placement of the second force on the housing causes the housing to move. In an embodiment, the method further includes placing a third force on the housing that is greater than the maximum value, thereby providing a third output that is substantially the same as the second output. The placement of the third force on the housing causes the housing to move. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a perspective view of a seat belt tension sensor assembly in accordance with a preferred embodiment of the present invention; 
           [0007]      FIG. 2  is an exploded perspective view of the seat belt tension sensor assembly of  FIG. 1 ; 
           [0008]      FIG. 3  is a perspective view of the seat belt tension sensor assembly of  FIG. 1  with the cover removed; 
           [0009]      FIG. 4  is a cross-sectional side elevational view of the seat belt tension sensor assembly of  FIG. 1 ; 
           [0010]      FIG. 5  is a perspective view of the sense element of the seat belt tension sensor assembly of  FIG. 1 ; 
           [0011]      FIG. 6  is a perspective view of the seat belt tension sensor assembly of  FIG. 1  with the cover exploded therefrom to show the wiring connection therein; 
           [0012]      FIG. 7  is a front elevational view of the seat belt tension sensor assembly of  FIG. 1  with the cover removed to show the assembly in both a non-actuated and an actuated state; and 
           [0013]      FIG. 8  is a front elevational view of the seat belt tension sensor assembly of  FIG. 1  with the cover removed to show the assembly in a protective state. 
       
    
    
       [0014]    Like numerals refer to like parts throughout the several views of the drawings. 
       DESCRIPTION OF THE INVENTION 
       [0015]    As shown in the drawings, for purposes of illustration, the invention is embodied in a seat belt tension sensor assembly  10 . In a preferred embodiment, the seat belt tension sensor assembly  10  is used in an automobile, however, this is not a limitation on the present invention. 
         [0016]    It will be appreciated that terms such as “front,” “back,” “top,” “bottom,” “left,” “right,” “above,” and “side” used herein are merely for ease of description and refer to the orientation of the components as shown in the figures. It should be understood that any orientation of the seat belt tension sensor assembly, and the components thereof described herein, is within the scope of the present invention. 
         [0017]    As shown in  FIGS. 1-4 , generally, the seat belt tension sensor assembly  10  includes a base plate  12 , housing  14  (comprising housing halves  14   a  and  14   b , sometimes referred to herein simply as housings  14   a  and  14   b ), spring assembly  16  and sense element  18 . 
         [0018]    In a preferred embodiment, the base plate  12  has front and back faces  12   a  and  12   b  and is attached to the vehicle structure through an attachment opening  22  defined therethrough, as is known in the art. The base plate  12  also has a sensor opening  24  defined therethrough that receives a portion of housings  14   a  and  14   b  and through which a seat belt  100  extends. It will be understood by those skilled in the art that the base plate  12  is an anchor point that connects the seat belt to the vehicle structure, such as the floor (by a bolt, screw, rivet or the like). Accordingly, the base plate  12  is advantageously made of a rigid metal, such as steel or the like. 
         [0019]    As is best shown in  FIGS. 2-4 , housings  14   a  and  14   b  each include a surface  26  that is adjacent to the front or back face  12   a ,  12   b  of the base plate  12  (they are not connected to the base plate  12  because they need to be able to move relative to it) and a portion that is disposed in sensor opening  24 . The housings halves  14   a  and  14   b  are held together by a press fit arrangement of the protrusion/plug  40  on one housing  14   a  or  14   b  being received in the associated groove/slot  42  in the other housing (described more fully below and shown in  FIG. 4 ). In another embodiment, the housing halves  14   a  and  14   b  can be held together by screws, rivets or the like. Housings  14   a  and  14   b  are shaped to cooperate with plate  12  to define a belt opening  26 , through which the seat belt  100  extends. Housing halves  14   a  and  14   b  also each include a recess  28  (collectively referred to as a cavity) defined therein for receiving sense element  18  and spring assembly  16 . 
         [0020]    As shown in  FIGS. 4 and 6 , in a preferred embodiment, each of the housings  14   a  and  14   b  include a ledge  29  that prevents movement of the sense element  18  in the direction toward the seat belt loop and away from the spring assembly  16 . 
         [0021]    As is best shown in  FIG. 5 , in a preferred embodiment, the sense element  18  contains a diaphragm  30  and a center pedestal  32 . In one embodiment, the sense element can be made of steel. A portion of the structure is thinned ( 30 ) to form a diaphragm so that when force is applied to the center pedestal  32  a significant stress and accompanying strain is created at the outer portion of the diaphragm  30  and opposite strain at the inner portion at the interface with the center pedestal  32 . On the back side (not shown) a continuous layer of glass is deposited, followed by patterned layers of conductor and resistor material. The resistor material is strain sensitive and if appropriately patterned (as known by those skilled in the art) creates a Wheatstone bridge with a differential voltage output that changes in proportion to the strain on the surface of the diaphragm  30 . This output is sensed by an electronic circuit, which, in an advantageous embodiment is implemented in an Application-Specific Integrated Circuit (ASIC) that is also soldered to the surface of the sense element. This circuit can then be compensated through a calibration process so that the output voltage is a precise function of the applied force. Typically, the ASIC is connected to the vehicle controller by three wires  36 —power in, ground and signal out. The strain-sensitive elements and their associated circuitry are collectively indicated by reference numeral  34  in  FIG. 6 . In a preferred embodiment, the electrical signals produced by the sense element are electrically communicated to a desired electrical component, such as a control unit for the air bags, by wires  36 . For exemplary purposes only,  FIG. 6  shows a connector  102  that might be used to make the connection. 
         [0022]    Wires  36  (that may be contained in a wire harness  36   a ) are housed in a slot  38  in one of the housings  14   a  or  14   b . In the figures, the slot  38  is defined in housing  14   b . In a preferred embodiment, for strain relief during actuation, each housing  14   a  and  14   b  includes a plug  40  that is received in a groove  42  in the opposite housing. As is shown in  FIG. 6 , in housing  14   b , groove  42  and slot  38  cooperate to provide a path for wires  36 . In a preferred embodiment, the plug  40  has a plurality of bumps  43   a  that will push the cable between them and a plurality of similar bumps  43   b  in the groove  42  in the opposite housing. When pressed together the bumps  43   a  and  43   b  provide an “S-curve” labyrinth that locks the wire harness  36   a  in place. In a preferred embodiment the wires  36  are soldered to the solder pads and initially extend in a direction opposite the exit path (groove  42  and slot  38 ). The wires  36  then make an approximate 180 degree turn, thus creating a strain-relieving loop. It will be understood that this configuration is not a limitation on the present invention, but that the wires can exit the housing  14  at any point. 
         [0023]    As is shown in  FIGS. 2-4  and  6 , spring assembly  16  is received in a portion of recess  28 . In a preferred embodiment, the spring assembly  16  includes a leaf spring  44  that is retained by a clip  46 . It will be understood that the spring  44  is preloaded in this position and has a spring force (the force necessary to begin compressing the spring) that is equal to or higher than the maximum force desired to be measured. It will be understood that the clip  46  captures the spring  44  in the desired preloaded configuration. Therefore, any way of capturing the spring (no matter what type of spring it is) and limiting its movement is within the scope of the present invention. In a preferred embodiment, the clip  46  includes a knob  48  thereon that is in mechanical communication with the pedestal  32  of the sense element  18 . 
         [0024]    Referring to  FIGS. 7-8 , the seat belt tension sensor assembly  10  basically has three states, a non-actuated state, where no tension is applied to the housing  14  by the seat belt  100 . An actuated state, where tension is applied to the housing  14  by the seat belt  100 , but the maximum force desired to be measured has not been exceeded. And, a protective state, where the tension applied to the housing  14  by the seat belt  100  exceeds the maximum force desired to be measured and the spring assembly  16  begins to compress. It will be understood that  FIG. 7  shows both the non-actuated and actuated state and  FIG. 8  shows the protective state. 
         [0025]    In the non-actuated state, the spring assembly  16  spring rests “loose” within the assembly clearances designed. At this point, there is little or no force being exerted on the sense element, as is shown in  FIG. 7 . 
         [0026]    In operation, when a tension creating incident creates tension in the seat belt  100  and places force on the housing  14 , the sense element  18  is forced against the spring assembly  16  and the resulting contact creates a proportional electric signal that is processed by the electronic circuitry  34  and transmitted to the appropriate external electric circuitry via wires  36 . This is the actuated state. 
         [0027]    In other words, when there is no tension on the belt  100  the sense element  18  has no force exerted on it because the spring  44  is captive, or trapped. With normal belt tension the spring assembly  16  remains in this state and is essentially a rigid block, transferring force without any displacement, as is shown in  FIG. 7 . 
         [0028]    The actuated state is for the purpose of measuring the seat belt tension during normal usage, not during an accident. Two exemplary tension creating incidents are as follows: The first is that the weight of the occupant as measured by an associated seat weight sensor includes the force of the seat belt, so, to get an accurate measure of the occupant weight, the seat belt tension (as is measured in the actuated state) needs to be subtracted. The second is that when using a child safety seat the seat belt should be tensioned to a high level. And this level is constant whether the vehicle is occupied or not. This constant high force can indicate the presence of an infant in the seat, disabling the air bag. 
         [0029]    It should be understood that during the actuated state the housing  14  does not move relative to the plate  12  and therefore the effect of friction that might accompany such movement is eliminated. This enables the sense element  18  to provide an accurate signal in the actuated state. 
         [0030]    However, as shown in  FIG. 8 , when the force increases to a value higher than the maximum force desired to be measured (above full scale) the spring assembly  16  begins to compress or collapse, allowing the housing halves  14   a  and  14   b  to move. When the housing halves  14   a  and  14   b  move enough to “bottom out” (as shown in  FIG. 8 ) by contacting the plate  12  through the wings  45  (discussed below) of the spring  44 , which act as pads or stops, the housing  14  ceases movement. In an exemplary embodiment, each of the housing halves  14   a  and  14   b  include shoulders  47 , and the shoulders are the portion of the housing that contact the plate  12 . After this, regardless of how much tension or force is applied, the sense element  18  is exposed only to the force of the spring assembly  16 , thereby protecting the sense element  18  and preventing it from being damaged. This is the protective state. 
         [0031]    In a preferred embodiment, the leaf spring  44  includes wings  45  at the ends. As shown in  FIG. 2 , the wings  45  are wider than the majority of the remainder of the leaf spring. The wings  45  help distribute the load from the plate  12  to the housing  14  when the load is high. This allows the housing  14  to be made of a lower strength and lower cost material than would otherwise be required if the wings  45  were not present. However, it will be understood that the wings  45  can be omitted. The wings can also include notches that cooperate with the plate  12 , and help maintain the spring assembly  16  in place. 
         [0032]    It should be understood, that while a stacked leaf spring  44  is shown other types of springs can be used as long as the spring is “captured,” in other words, preloaded. The fundamental concept is that the sense element pushes on the preloaded spring, which acts like a rigid body. In another embodiment, a projection that extends from or is integral with the plate  12  can contain the spring, and the sense element may be in direct contact with the spring. 
         [0033]    When the force exceeds the rated load, the spring  44  starts to compress, limiting the load imparted on the sense element  18 . Then the parts bottom out and the load is transferred directly from the belt  100  to the plate  12 . This force could be, for example, up to about 1,500 pounds. In this example, the assembly  10  could accurately measure a load up to about 30 pounds, but is not damaged if the load goes to about 1500 pounds. 
         [0034]    An exemplary use of the sensor assembly  100  will now be described. Normal belt tension while being worn is typically less than 30 pounds, and is usually much less. If a baby seat is installed the tension will likely be higher than 30 pounds but will be substantially constant. In the case of a baby seat, the sensor will read a fixed force. The control unit will interpret this fixed high force as an indication that a baby seat is in place and will not allow the air bag to deploy. Regardless, if the vehicle is in a crash the belt tension will be much higher than 30 pounds (possibly up to 1,500 pounds or more). In a situation where this much force is realized, it is desirable to not cause damage to the tension sensor, which will save repair costs. 
         [0035]    In a preferred embodiment, in a situation where the seat belt  100  is pulled out of plane, because the housings  14   a  and  14   b  are made of a plastic or the like, the housing  14  absorbs the force, also helping prevent and minimize damage to the assembly  10  and sense element  18 , in particular. 
         [0036]    Finally, in the case of extreme crash forces that are strong enough to break the housing  14  the seat belt  100  is still retained by the plate  12 , which is a single, strong structure. 
         [0037]    The foregoing embodiments are merely examples of the present invention. Those skilled in the art may make numerous uses of, and departures from, such embodiments without departing from the spirit and the scope of the present invention. Accordingly, the scope of the present invention is not to be limited to or defined by such embodiments in any way, but rather, is defined solely by the following claims.