Abstract:
A sensor package which is reliable, low cost, simple, robust, and usable to input additional seat occupant information to an airbag controller to control airbag deployment, and which is insensitive to cross axis loading of a seat belt. A second housing member is internally interfaced with the first housing member, wherein a suspension system frictionlessly suspends the first housing member springably with respect to the second housing member. A pressure sensor is mounted to one of the first and second housings, and a biasing spring is mounted to the other of the first and second housings in axial abutment with the pressure sensor.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an automobile sensor package for detecting the magnitude of a tensile force in a seat belt used in a car seat, and in particular to a sensor package that can detect the magnitude of tension in a seat belt and provide an electrical signal that is representative of the magnitude of the tensile force.  
         [0003]     2. Description of the Related Art  
         [0004]     Various devices are well known for their ability to measure force, pressure, acceleration, temperature, position, etc. by using a sensing structure combined with signal processing electronics. One general type of sensor or transducer for such applications is a resistive strain gauge sensor in which force or pressure is sensed or measured based on strain placed on the resistors. Resistive strain gauges function by exhibiting changes in resistance proportional to strain which causes dimensional changes of the resistor.  
         [0005]     Many types of strain gauge sensors have been designed and made commercially available. Various strain gauge sensors have proven to be generally satisfactory; however, these have tended to be rather expensive and not suitable in certain applications such as sensing the presence of an occupant in an automobile seat. A sensor suitable for such an application must be compact, robust, impervious to shock and vibration and yet inexpensive. In this regard, a sensor which has promise is described in U.S. Pat. No. 5,661,245 to Svoboda et al, issued Aug. 26, 1997, hereby herein incorporated by reference.  
         [0006]     Automobile seats can use sensors to activate air bags, which would be deployed during an accident. Injury to infants or small children from air bag deployment with excessive force is a current industry problem. A weight sensor in the seat can be used to control the deployment force during air bag activation. Unfortunately, however, there are several problems with detecting seat occupant weight. For example, when a seated occupant puts on a seat belt, the force of cinching down the seat belt on the occupant can cause a seat weight sensor to have false and erroneous readings. For another example, if a child&#39;s car seat is cinched down tightly in the car seat, it can appear to the weight sensor that a heavy person is in the seat, which is the wrong reading.  
         [0007]     An example of a child seat sensing system is schematically depicted at  FIG. 1 , wherein a child seat  10  is placed upon a front passenger seat  12  and held thereto by a tightened seat belt  14 . In this regard by way of example, the seat belt has an outboard portion  14   o  and an inboard portion  14   i  which are mutually coupled by a buckle  14   b.  The inboard portion  14   i  has a fixed length and is connected via an inboard anchor  16  to a vehicle component, such as for example a floor frame member. The outboard portion  14   o  is associated with, for example, an outboard anchor  18  which is also connected with a vehicle component. A shoulder belt  26  is associated with the outboard portion  14   o,  and is, for example, connected to a retractor assembly  22 , which is, in turn, connected to a vehicle component. A weight sensor  20  provides a signal to the controller. When a crash is sensed by the crash sensor, the controller manages inflation of the air bag  24  via an air bag actuation circuit. The foregoing sensing scheme is described in detail in U.S. Pat. No. 5,454,591 to Mazur et al, issued Oct. 3, 1995, hereby herein incorporated by reference.  
         [0008]     As represented schematically by  FIG. 2 , a seat belt tension sensor (BTS), which in general is used to measure the seat belt webbing tension, can be packaged in a number of locations. For example, a BTS could be packaged adjacent the outboard anchor  18 , adjacent the inboard anchor  16 , or somewhere at the buckle  14   b.  Each location has advantages and disadvantages. The BTS is required to compensate the weight sensing system such that federal government regulation FMVSS 208 may be met. This new regulation requires auto manufacturers to provide an automatic shut off of the passenger side air bag. The weight sensor may make vacover judgments under normal seating conditions. However, when a child seat is placed onto the vehicle seat and the seat belt webbing is used to cinch the child seat in place, a weight error is introduced into the sensing system. By gauging the webbing tension, the weight sensor can correct for the induced error due to the belt webbing so as to ensure the controller correctly determines whether to actuate, or whether to actuate and regulate the inflation force of, an air bag.  
         [0009]     It can be seen from  FIG. 2  that the seat belt  14  forms a load loop, the origin of which can be considered to be located at the buckle  14   b  where the latch thereof engages a tongue  14   t  connected to the end of the outboard portion  14   o  of the seat belt  14 . This area in or near the seat belt buckle is a first possible BTS location. However, a BTS could be placed adjacent the outboard anchor  18 . In this case, the rather long length of the outboard portion  14   o  of the seat belt  14  presents the possibility for a large amount of friction to be present between the buckle and the outboard anchor.  
         [0010]     With the foregoing having been said, the aforementioned advantages and disadvantages of BTS location are as follows. With regard to BTS placement adjacent the outboard retractor, advantages include limited cross axis loading variation (discussed hereinbelow), greater amount of room for packaging, and ability to be covered so as to eliminate surface requirements and avoidance of splash and debris contamination; while disadvantages include greater amount of friction from D ring and occupant body friction sources, long distance from critical contact force location (tongue to latch contact location), and specific mounting requirements due to retractor mounting considerations. With regard to BTS placement inboard adjacent the buckle or in the buckle, advantages include the sensor being located close to the contact force of the tongue to latch with a consequent lowest possible system friction therebetween, possibility for integration into the same wiring harness as the buckle switch (one dual sensor assembly), the BTS could replace buckle switch if properly designed, and a low deflection is required due to close contact force proximity (which is a key consideration for reducing hysteresis and repeatability errors); while disadvantages include a high cross axis loading being required due to buckle head flexibility, packaging considerations must include prevention of possible contamination due to socover particles and liquid spills, and packaging may be more difficult due to small size requirement for the buckle area (requiring miniaturization).  
         [0011]     Another consideration with respect to BTS placement is cross axis loading. In this regard, it should be appreciated that due to the fixed mounting in an outboard anchor based BTS, there would be limited cross axis loading, but that a buckle based BTS would have a worst case operating cross axis loading. This can be understood from  FIGS. 3A and 3B .  
         [0012]      FIG. 3A  shows the details of the potential for cross axis loading at the buckle, wherein it is assumed that the buckle  14   b  is located within a cross axis load motion cone  30 . The cone  30  is used to define a potential buckle position within or on the cone surface. In the example of  FIG. 30A , the cone  30  begins with a cross-section of 24 mm and increases to a cross-section of 100 mm. It should be appreciated that the geometry of the cone  30  should be specifically defined by the seat belt supplier in combination with the seat supplier.  
         [0013]      FIG. 3B  depicts schematically the nature of the forces involved in cross axis loading. The actual load required to achieve the motion depicted in  FIG. 3A  may be quite large. The cross axis load motion cone  30  is defined by a forward direction loading force X, an inboard direction loading force Y (which is perpendicular to force X) and a twist moment ±M XY  in the X-Y plane. Table 1 defines the range of cross axis loading motion including buckle twist.  
                               TABLE 1                       Degrees of                       Freedom   X direction   Y direction   Z direction   Twist                   Allowed Motion   +/−75 mm   +/−75 mm   Not applicable   +/−90°       Allowed Error   +/−3%   +/−3%   Not applicable   +/−3%                    
         [0014]     Accordingly a need in the art continues to exist for a reliable, low cost, simple and robust seat belt tension sensor that can be used to input additional seat occupant information to an airbag controller to control airbag deployment, and which is insensitive to cross axis loading.  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention is a seat belt tension sensor package which is reliable, low cost, simple, robust, and usable to input additional seat occupant information to an airbag controller to control airbag deployment, and which is insensitive to cross axis loading.  
         [0016]     The seat belt tension sensor package according to the present invention includes a first housing member, a second housing member internally interfaced with the first housing member, a suspension system for frictionlessly suspending the first housing member with respect to the second housing member, a pressure sensor mounted to one of the first and second housings, and a biasing spring mounted to the other of the first and second housings in axial abutment with the pressure sensor.  
         [0017]     A seat belt is connected to the first and second housings, wherein tensile force of the seat belt is registered at the pressure sensor. The first and second housings are permitted a predetermined small axial movement, the axial movement being defined between a first relative position and a second relative position. The first relative position is defined by a zero tensile force axially applied to the first and second housings, and the second relative position is defined by a predetermined tensile force axially applied to the first and second housings. The axial movement occurs without scovering friction via the suspension system. Between the first and second relative positions, the axial tensile force applied to the first and second housings is taken by the suspension system and the biasing spring. However, at the second position, all increases in axial tensile force are taken by a mechanical abutment between the first and second housings.  
         [0018]     The preferred suspension system utilizes a plurality of leaf springs which are freely flexible in the axial direction but quite inflexible in directions perpendicular thereto. As a result, the suspension system is very resistant to cross axis loading.  
         [0019]     Accordingly, it is an object of the present invention to provide a seat belt tension sensor package which is reliable, low cost, simple and robust, and which is usable, for example, to input additional seat occupant information to an airbag controller to control airbag deployment.  
         [0020]     It is an additional object of the present invention to provide a seat belt tension sensor package as aforedescribed which is insensitive to cross axis loading.  
         [0021]     These, and additional and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a schematic view of an environment of use of the present invention.  
         [0023]      FIG. 2  is a schematic depiction of an automotive seat belt system.  
         [0024]      FIGS. 3A and 3B  schematically depict cross axis loading associated with an automotive seat belt.  
         [0025]      FIG. 4  is a perspective elevational view of a seat belt tension sensor package according to the present invention.  
         [0026]      FIG. 5  is a side view of the seat belt tension sensor package of  FIG. 4 .  
         [0027]      FIG. 6  is a perspective view of the seat belt tension sensor package of  FIG. 4 , wherein a cover thereof has been removed to show internal components.  
         [0028]      FIG. 7  is an exploded, perspective view of the seat belt tension sensor package of  FIG. 4 .  
         [0029]      FIG. 8A  is a top view of the seat belt tension sensor package of  FIG. 4 , wherein the cover is removed, and the first and second housings are at a first relative position.  
         [0030]      FIG. 8B  is a top view of the seat belt tension sensor package of  FIG. 4 , wherein the cover is removed, and the first and second housings are at a second relative position.  
         [0031]      FIG. 9  is a graph of an example of sensor voltage output relative to seat belt tension. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]     Referring now to the Drawings,  FIGS. 4 through 9  depict various aspects of the seat belt tension sensor package  100  according to the present invention.  FIGS. 4 and 5  elevationally depict the seat belt tension sensor package  100 , which includes a first housing member  102  and a second housing member  104 . Each of the first and second housings  102 ,  104  have respective seat belt attachment features, as for example a fitting  106  connected to the first housing member, and a base  108  connected to the second housing member. The seat belt tension sensor package  100  is placeable anywhere in the seat belt system because of a unique suspension system between the first and second housing members (discussed hereinbelow) which negates the adverse effects of cross axis loading (described hereinabove). It is to be noted that the seat belt tension sensor package has the utility to be located at either an inboard or outboard (inclusive of the buckle) location, wherein a buckle location is preferred.  
         [0033]     Referring next to  FIGS. 6 and 7 , component details of the seat belt tension sensor package  100  will be described, wherein the first and second housing components  102 ,  104  are preferably composed of high strength steel. Further to this discussion, a coordinate convention (see  FIG. 6 ) with respect to the seat belt tension sensor package  100  will be adopted, wherein an axial axis Z coincides with axial tension forces of the first and second housing members  102 ,  104 , and wherein components of tension force along the X and Y axes constitute off axis loads.  
         [0034]     Axially opposite the seat belt interface (for example base  106 ) of the first housing member  102  a receptacle cavity  110  is formed therein (see  FIG. 7 ). The receptacle cavity  110  includes a mouth  112 , and is defined by a floor  114  and a selectively removable cover  116 , as for example via threaded fasteners  115  holding the cover to the first housing member. Adjacent a blind rear end  118  of the receptacle cavity  110 , are a pair of opposed rearward leaf spring slots  120   a,    120   b.  Adjacent the mouth are first and second forward leaf spring slots  122 ,  124 . The floor  114  and the cover  116  are each provided with a sensor mount cavities  126 ,  128 .  
         [0035]     The second housing member  104  has a nose  130  axially opposite its seat belt interface (for example the belt tongue  108 ) which is configured to be seatably received by the receptacle cavity  110 . The nose  130  has a sensor aperture  132  formed therein, wherein a forward end  134  thereof includes a biasing spring mounting feature  136 . The forward end  138  of the nose  130  includes a pin mounting feature  140  having a hole through which is affixed a pin  145 . A pair of first and second nose leaf spring slots  142 ,  144  are formed in the nose  130  at a generally medial location rearward of the forward end  138 .  
         [0036]     A rearward leaf spring  146 , having a generally elongated rectangular shape and composed of a spring material, such as a stainless spring steel, fits at its ends  146   a,    146   b  respectively into the rear leaf spring slots  120   a,    120   b  of the first housing member  102 . An aperture  146 ′ is formed medially in the rearward leaf spring  146  which is dimensioned to receive therethrough the pin mounting feature  140 .  
         [0037]     A first leaf spring  148 , also composed of a spring material (ie., a stainless spring steel), is received into the first forward leaf spring slot  122  and the first nose leaf spring slot  142 . A second leaf spring  150 , also composed of a spring material (ie., a stainless spring steel), is received into the second forward leaf spring slot  124  and the second nose leaf spring slot  144 . For mechanical anchorage purposes, it is preferred for the first and second forward leaf spring slots  122 ,  124  and for the first and second leaf springs  148 ,  150  to be generally L-shaped, wherein the base B of the “L” serves as anchorage. Further, in that the first and second housing members  102 ,  104  are relatively movable, the first and second forward leaf spring slots  122 ,  124  have a rearward wall  152  which has a finite acute angle with respect to the X axis predetermined to allow for free flexing of the first and second nose leaf springs as the first and second housing members move between the first and second relative positions.  
         [0038]     A biasing spring  154  has a connection feature  156  which interfaces with the biasing spring mounting feature  136  to attach the biasing spring to the forward end  134  of the sensor aperture  132 . A pressure sensor  158 , as for example a sensor described in aforementioned and herein incorporated U.S. Pat. No. 5,661,245, available through SenSym, Inc. of Milpitas, Calif., is mounted to a sensor base  160 . The upper and lower edges of the sensor base  160  interfit with the sensor mount cavities  126 ,  128  to thereby solidly affix the sensor  158  to the first housing member  102 .  
         [0039]     Finally, it will be noted that the contour of the axial cavity sidewalls  162  of the receptacle cavity  110  are generally complementary to the contour of the axial nose sidewalls  164  of the nose  130 . In this regard, the axial nose sidewalls and axial cavity sidewalls cooperate to allow for axial movement with a small clearance of the first housing member  102  relative to the second housing member  104  only between the first relative position and the second relative position, as will be discussed hereinbelow with respect to  FIGS. 8A and 8B .  
         [0040]      FIG. 8A  depicts the seat belt tension sensor package according to the present invention wherein the first and second housing members  102 ,  104  are at the first relative position, characterized by a relaxed state of operation in which tension between the first and second housing members  102 ,  104  is substantially zero. It will be noted that the rearward leaf spring  146  and the first and second leaf springs  148 ,  150  are in relaxed spring states. The biasing spring  154  applies a predetermined initial spring load onto the sensor  158 , the counterbalance of which is taken up by the nose to the blind rear end  118  (the rearward leaf spring  146  being sandwiched therebetween). A movement stop in the form of axially facing abutments  166 ,  168 , respectively, of the axial cavity sidewalls  162  and the axial nose sidewalls  164  are separated at the first relative position by a small predetermined distance which defines the allowed axial movement of the first housing member  102  relative to the second housing member  104  (ie., the distance between the first relative position and the second relative position), as for example 0.04 inches.  
         [0041]      FIG. 8B  depicts the seat belt tension sensor package according to the present invention wherein the first and second housing members  102 ,  104  are at the second relative position, characterized by a flexed state of operation in which tension between the first and second housing members  102 ,  104  is of a predetermined magnitude. This predetermined magnitude is the highest reasonable range of loading for the sensor under operative conditions, as for example the predetermined biasing plus the applied axial tension force, less the axial flex force of the rearward leaf spring and the first and second leaf springs. It will be noted that the rearward leaf spring  146  and the first and second leaf springs  148 ,  150  are in flexed spring states. The biasing spring  154  applies a second predetermined spring load onto the sensor  158 . Axially facing abutments  166 ,  168 , respectively, of the axial cavity sidewalls  162  and the axial nose sidewalls  164  are now in axially abutting contact, whereupon any increase in tension force applied to the first and second housing members is taken up entirely by the axially facing abutments  166 ,  168 .  
         [0042]     Because the suspension system  170  constitutes leaf springs  146 ,  148 ,  150 , frictionless, free flexibility is provided in the axial direction along the Z axis, but there is very high resistance to any flexing along non-axial directions having components along the X or Y axes (that is, the suspension system freely flexes parallel to the Z axis and is very stiff normal to the Z axis in response to cross axis loads). Accordingly, the movement of the first and second housing members  102 ,  104  is substantially immune to cross axis loading  
         [0043]     In operation, as axial tension force is applied to the first and second housing members  102 ,  104 , the first and second housing members move relative to each other in a frictionless manner, via a mutual suspension system  170  characterized by the rearward leaf spring  146  and the first and second leaf springs  148 ,  150 . As the first and second housing members relatively move, the biasing spring  154  increasingly compresses against the pressure sensor  158 , thereby causing the sensor signal output to change with the compression, and thereby, with proper pre-ascertained signal processing, provides a signal indicative of the axial tension force between the first and second housing members.  
         [0044]      FIG. 9  depicts a graph of signal output of the pressure  158  with respect to axial tension force applied to the first and second housing members  102 ,  104 . The signal can be a voltage, a current, or if needed, a digital signal using a specified protocol. The electrical parameter chosen will be dependent upon environmental considerations. For example, the signal output may be an analog voltage, ratiometric to the power supply voltage and range from 0.5 volts DC through 4.5 volts DC for full scale. The fundamentals of this transfer function consists of a zero signal (0.5 volts), a full scale signal (4.5 volts), as well as the span (4.0 volts). If the electrical measurement is to be made using a current output signal, then a typical 4 milliamp would be zero, full scale would be 20 milliamp, and have a span of 16 milliamp.  
         [0045]     In the event of an untoward incident (as for example a crash), the axial tension force would exceed the mechanical limits of the affixment of the sensor base; however, this situation can never happen by virtue of abutment of the axial facing abutments  166 ,  168  prior to approaching this mechanical limit. Table II gives an exemplar range of operational expectancies.  
                               TABLE II                           BTS error including:                       Linearity, Hysteresis, Repeatability           Sensor       Belt Load Range   Zero, and Span errors including   Number of Mechanical Cycles       Output       (lb.)   Temperature Effects. (RMS)   Typical/Design Intent   Comments   (Volts)                   0-5   BTS error shall be &lt;10% FS    50,000/200,000   Normal Use   0.5-1.1        5-10   BTS error shall be &lt;10% FS     7000/25,000   Normal Use   1.1-1.8       10-30   BTS error shall be &lt;10% FS   2500/8000   Child Seat or Infant   1.8-4.5                   Carrier       30-60   BTS error shall be &lt;25% FS   1000/8000   Child Seat or Infant   4.5-4.5                   Carrier, Occupant                   Jounce Loads        60-250   BTS shall not malfunction after    100/2500   Large occupant   4.5-4.5           exposure       Jounce Loads, Light                   Impact Loads        250-1250   BTS shall not malfunction after    3/25   Large occupant   4.5-4.5           exposure       Jounce Loads, Light                   Impact Loads       &gt;1250   BTS shall not malfunction after   1/2   Light to Moderate   4.5           exposure       Crash/Impact                   Loads                  
 
         [0046]     General considerations regarding the sensor  158  will now be detailed. The preferred pressure sensor  158  consists of a silicon micro-machined pressure transducer chip with a nominal pressure rating of 2500 psi. This pressure rating relates to the allowable micro-strain that the sensor chip can withstand under normal linear conditions. The pressure sensor mechanism is designed to collect the force exerted onto its surface axially by the biasing spring  154  in response to application of an axial tension force to the first and second housing members  102 ,  104 . This causes localized strain within the pressure sensing chip. Implanted into the silicon pressure sensor are a series of piezoresistors. These resistors change resistance under strain, and the circuit arrangement is a classical wheatestone bridge. The change in resistance causes a change in bridge differential output voltage. A detailed explanation of a suitable pressure sensor is described in aforementioned and herein incorporated U.S. Pat. No. 5,661,245.  
         [0047]     To those skilled in the art to which this invention appertains, the above described preferred embodiment may be subject to change or modification. For example, while the present invention has been described in an automotive seat belt environment of operation, the sensor package according to the present invention is not so operationally limited, in that can be used in any other operational environment, the foregoing automotive environment being merely presented herein as exemplary. Such change or modification can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.