Abstract:
A sensing system employs multiple sensors utilizing mounting structure integrated into a fifth wheel hitch and which is protected from the environment, is capable of accurately measuring forces along longitudinal and vertical axes for providing information as to roll, pitch, yaw, and drawbar load and which utilizes sensors which provide output signals for display and control. In one embodiment, a fifth wheel includes mounting boxes formed on an undersurface thereof on opposite sides of the kingpin receiving slot and a force-sensing unit mounted within each of said mounting boxes. Each force-sensing unit includes a vertical sensor positioned fore and aft of the vertical hitch axis and forward and aft horizontal force sensors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/457,900, entitled “FORCE-SENSING FIFTH WHEEL,” by Applicants Jack L. Gisinger et al., filed on Dec. 9, 1999, the entire disclosure of which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a fifth wheel hitch for receiving a kingpin of a trailer and one which has the ability to sense forces between the trailer bolster plate, kingpin and the fifth wheel and a circuit to process such sensed information.  
           [0003]    The identification of forces between a truck trailer and a truck hitch is useful for a variety of reasons. Such information can be employed, for example, in an automatic braking system whereby signals from a sensing system can be employed to prevent excessive braking on one or more wheels or otherwise control braking for safe deceleration. In addition, such systems are helpful in alerting the vehicle operator to excessive trailer movement, such as pitching, yaw and/or potentially dangerous roll conditions.  
           [0004]    As can be appreciated, the interconnection between a trailer kingpin and a fifth wheel assembly provides a relatively harsh environment for detection of the large forces involved and efforts to provide accurate sensing information with signals from a sensor which are linearly related to the detected forces has been difficult. U.S. Pat. Nos. 5,286,094 and 5,289,435 represent one sensor construction and a mounting employed for measuring push and pull forces on a hitch connection. It would be desirable, however, to provide additional information such as pitch, yaw and roll information for use by the driver in safely operating the vehicle.  
           [0005]    The sensing of multiple axis force information between the coupling of a trailer to a tractor is difficult not only due to the harsh environment to which the equipment is exposed but also the relatively large and rapidly varying forces encountered and finally the difficulty in providing a linear output signal from sensors which may be employed.  
         SUMMARY OF THE INVENTION  
         [0006]    The system of the present invention provides a sensing system which employs multiple sensors utilizing a mounting structure which can be integrated into a fifth wheel hitch and which is protected from the environment and capable of accurately measuring forces along longitudinal and vertical axes for providing information as to roll, pitch, and yaw. The system utilizes sensors which provide a relatively linear electrical output signal which can be used for displaying such forces, for generating alarms, or for controlling the vehicle operation.  
           [0007]    Systems embodying the present invention comprise a fifth wheel having mounting boxes formed on an undersurface thereof on opposite sides of the kingpin receiving slot and a force-sensing unit mounted within each of said mounting boxes. Each force-sensing unit includes a forward and aft vertical sensor and a forward and aft longitudinal sensor. With a force-sensing unit on the left-side and the right-side of the fifth wheel assembly, eight such sensors provide signal information which can be used to determine roll, pitch and yaw movements derived from the detected vertical and longitudinal forces on the hitch.  
           [0008]    In one embodiment of the invention, each of the sensors are mounted to the sensing unit utilizing elastomeric springs coupling the fifth wheel plate to the force-sensing unit and a plunger for coupling forces from said elastomeric spring to a force sensor itself. In this embodiment, the longitudinal sensors also include a pair of elastomeric springs for preloading the sensor such that it is capable of sensing forces in both directions. Also, in one embodiment of the invention, the plungers are mounted within a cylindrical aperture having a curved opening allowing the elastomeric spring to deform linearly into contact with the plunger as force is applied thereto.  
           [0009]    With such a system, a force-sensing fifth wheel assembly is provided which measures forces in vertical and horizontal axes between the kingpin coupling to the fifth wheel and provides accurate signal information to an electrical circuit which can display pitch, roll and yaw and vertical load information to the vehicle for use in controlling the safe operation of the vehicle.  
           [0010]    According to another embodiment of the present invention, a force-sensing unit for a fifth wheel assembly includes a housing, a first vertical sensor, a second vertical sensor, a first horizontal sensor and a second horizontal sensor. The housing is coupled to a fifth wheel between a tractor mountable bracket and a fifth wheel. The first vertical sensor is positioned in the housing forward of the lateral axis of the fifth wheel and the second vertical sensor is positioned in the housing rearward of the lateral axis of the fifth wheel. The first horizontal sensor is positioned in the housing forward of the lateral axis of the fifth wheel and the second horizontal sensor is positioned in the housing rearward of the lateral axis of the fifth wheel. In this manner, the force-sensing unit provides a total of four channels of force-sensing information, with each of the sensors being embedded within an elastomeric pad.  
           [0011]    These and other features, objects and advantages of the present invention will become apparent upon reading the following description thereof together with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a right-side elevational view of a fifth wheel assembly, according to a first embodiment of the present invention;  
         [0013]    [0013]FIG. 2 is an enlarged bottom plan view of the fifth wheel shown in FIG. 1;  
         [0014]    [0014]FIG. 3 is an enlarged, fragmentary cross-sectional view of the left-side force-sensing unit taken along section lines III-III of FIG. 2;  
         [0015]    [0015]FIG. 4 is an enlarged, fragmentary bottom plan view, partly broken away and in cross section, of the sensing unit shown in FIG. 3;  
         [0016]    [0016]FIG. 5 is an enlarged, partly exploded, perspective view of one of the force-sensing units, according to one embodiment of the present invention;  
         [0017]    [0017]FIG. 6 is a fragmentary perspective view, partly broken away, of the left bottom side of the fifth wheel shown in FIGS.  1 - 4 , with the force sensor removed therefrom;  
         [0018]    [0018]FIG. 7 is a fragmentary perspective view of the structure shown in FIG. 6, with the force-sensing unit installed therein;  
         [0019]    [0019]FIG. 8 is an enlarged perspective, exploded view of one of the horizontal (longitudinal) sensors for mounting to one of the force-sensing units;  
         [0020]    [0020]FIG. 8A is an enlarged exploded view of a horizontal sensor, showing its mounting relationship;  
         [0021]    [0021]FIG. 9 is an electrical circuit diagram in block form of a circuit employed to utilize the information provided by one embodiment of the force-sensing system of the present invention;  
         [0022]    [0022]FIG. 10 is a right-side elevational view of a fifth wheel assembly according to another embodiment of the present invention;  
         [0023]    [0023]FIG. 11 is an enlarged bottom plan view of the fifth wheel shown in FIG. 10;  
         [0024]    [0024]FIG. 12 is an enlarged, fragmentary cross-sectional view of the left-side force-sensing unit taken along section lines XII-XII of FIG. 11;  
         [0025]    [0025]FIG. 13 is an enlarged, fragmentary bottom plan view, partly broken away and in cross section, of the sensing unit shown in FIG. 12;  
         [0026]    [0026]FIG. 14 is an enlarged, partly exploded, perspective view of one of the force-sensing units, according to another embodiment of the present invention;  
         [0027]    [0027]FIG. 15 is a fragmentary perspective view, partly broken away, of the left bottom side of the fifth wheel shown in FIGS.  10 - 13 , with the force sensor removed therefrom;  
         [0028]    [0028]FIG. 16 is a fragmentary perspective view of the structure shown in FIG. 15, with the force-sensing unit installed therein;  
         [0029]    [0029]FIG. 17 is an enlarged perspective, exploded view of one of the horizontal (longitudinal) sensors for mounting to one of the force-sensing units; and  
         [0030]    [0030]FIG. 18 is an electrical circuit diagram in block form of a circuit employed to utilize the information provided by one embodiment of the force-sensing system of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    Referring initially to FIGS.  1 - 3 , there is shown a fifth wheel  10  embodying the present invention which includes a top surface  12  (FIGS. 1 and 3), a bottom surface  14  (FIGS. 2 and 4), and a kingpin receiving slot  16  (FIG. 2) defined by bifurcated projections  18   a  and  18   b  for the left-side and right-side of the fifth wheel, respectively. Extending downwardly from the undersurface  14  of the fifth wheel  10  is a left-side mounting box  20   a  and a right-side mounting box  20   b,  each box being substantially the same and symmetrical. Each of the mounting boxes  20   a,    20   b  receive a force-sensing unit  70   a,    70   b,  respectively, described in greater detail below. Each of the boxes  20   a  and  20   b  are substantially identical and mirror images of one another, as are the force-sensing units  70   a  and  70   b.  Accordingly, following is a detailed description of the force-sensing unit  70   a  and its mounting relationship with respect to the fifth wheel  10 , it being understood that force-sensing unit  70   b  is structurally substantially the same. The eight sensors are, however, separately identified, as are the signals provided by the fifth wheel sensing unit of the present invention.  
         [0032]    The mounting box  20   a  (FIGS.  2 - 4 ) includes a forward wall  22 , a rear wall  24 , which may extend the width of the fifth wheel  10  and thereby also function as a strengthening rib, an outside wall  26  and an inside wall  28 . Outside wall  26  includes an aperture  27  aligned with an aperture  29  in wall  28  for receiving a mounting pin  30  (FIG. 3), which couples the fifth wheel  10  to a tractor mounting bracket  32  secured to a tractor frame. The coupling of the fifth wheel  10  to the mounting bracket  32  is conventional and may employ an elastomeric interface  31  which, together with mounting pin  30 , secures the fifth wheel  10  to the tractor. It is understood, however, that the force-sensing units  70   a  and  70   b  interface between mounting brackets  32  and the fifth wheel  10  such that all of the forces transmitted between the kingpin and the fifth wheel assembly  10  are transmitted through the force-sensing units  70   a  and  70   b.  Not shown in FIGS. 1 and 2 is the kingpin locking mechanism or other mechanical details of the fifth wheel assembly, which are described in greater detail in U.S. Pat. No. 4,659,101, the entire disclosure of which is incorporated herein by reference.  
         [0033]    Before describing the force-sensing unit  70   a  (and the substantially identical forcesensing unit  70   b ) in detail, the location of the eight sensors employed in the system of the present invention with respect to the longitudinal axis “L 0 ” (FIG. 2 of the fifth wheel  10 ) and lateral axis “L a ”, which intersect at the vertical axis V, are described in connection with FIGS. 1 and 2. The left-side of the fifth wheel assembly includes a forward longitudinal (horizontal) sensor  40  and an aft longitudinal (horizontal) sensor  41  as best seen in FIGS. 2 and 3. The left sensing unit  70   a  also includes a forward vertical sensor  42  and an aft vertical sensor  43 . Similarly, the right force-sensing unit  70   b  includes a forward longitudinal (horizontal) sensor  44 , an aft longitudinal (horizontal) sensor  45 , a forward vertical sensor  46 , and an aft vertical sensor  47 . The placement of the sensors and their mounting to the boxes  20   a  and  20   b  of the fifth wheel  10  is shown in FIGS.  1 - 4 . Each of the sensors  40 - 47  may be capacitive sensors that include a pair of conductive plates spaced from one another and mounted within a compressible bellows-type assembly, which includes air and a dielectric material between the conductive plates. The capacitive sensors can be generally of the type disclosed in U.S. Pat. Nos. 5,286,094 and 5,289,435, the entire disclosures of which are incorporated herein by reference. Other sensors which can withstand the load levels encountered in this environment can also be used. The forces transmitted to the sensor, as described in greater detail below, causes the movement of the plates toward and away from one another a distance of approximately 2 mm, which results in a capacitance change which is employed to provide electrical signals for use with the electrical control circuit shown in FIG. 9 and described below.  
         [0034]    As is best seen in FIGS. 3 and 5, each of the force-sensing units  70   a  and  70   b  include a body  75  having semicylindrical downwardly formed surface  76  which mates with a similarly shaped mounting bracket  32 , as seen in FIG. 3. The top surface  74  of the body  75  is positioned in a spaced relationship to the lower surface  14  of fifth wheel  10  with four elastomeric springs, which are pads  62 ,  63 ,  66 , and  67  positioned in pockets (i.e., pockets  62 ′ and  63 ′ for pads  62  and  63 , respectively) formed in the lower surface  14  of fifth wheel plate as best seen in FIGS. 3 and 6 for the left-side illustrated. Similarly, the right-side includes pockets for receiving the elastomeric springs, e.g., polymeric pad for the right-side force-sensing unit. Thus, the elastomeric springs fore and aft of the lateral axis L a  of the fifth wheel  10  on each side of the longitudinal axis L o  provide the interface between the fifth wheel plate and the force-sensing units  70   a  and  70   b.    
         [0035]    In a preferred embodiment of the invention, the vertical elastomeric springs  62 ,  63 ,  66  and  67 , as well as the longitudinal elastomeric springs described below are made of natural rubber, having a  60  IRHD, although polyurethane or other elastomeric materials having minimal hysteresis can be employed as well. In one embodiment, the vertical pads  62 ,  63 ,  66  and  67  are approximately 90 mm by 70 mm and have a thickness or depth, as viewed in FIG. 3, of approximately 15 mm. The top surface  74  of the force-sensing unit bodies  75  likewise includes pockets (e.g., pockets  62 ″ and  63 ″ (FIG. 3)) for receiving the resilient elastomeric pads, e.g., pads  62  and  63 , leaving a gap between the lower surface  14  of the fifth wheel and the top surface  74  of a force-sensing unit sufficient to allow the elastomeric springs to deform under the compressive vertical forces provided by the kingpin coupled to the fifth wheel.  
         [0036]    The vertical pads  62 ,  63  engage spaced pairs of cylindrical plungers  52 ,  53  slidably extending in cylindrical apertures  72  of the body  75  which apertures extend downwardly from the floor of spring-receiving pockets  62 ″ and  63 ″ defining an interface having a shallow concave recess  77 , as best seen in FIG. 3, such that the lower surface  62   a,  for example, of spring  62  can deform into the aperture  72  and engage the end of plunger  52  moving in a distance linearly related to the vertical force applied to the fifth wheel. The concave recess  77  and each of the interfaces between the vertical springs and the associated plungers  52  assure such forces are linearly translated into plunger motion that is transferred to the associated sensor  42 . Sensors  42 ,  43 ,  46  and  47  are mounted in associated pockets (i.e., pockets  42 ′ and  43 ′ for sensors  42  and  43 ) formed in body  75  such that the vertical forces on plate  12  are transmitted through pads  62 ,  63 ,  66 ,  67  and their associated plunger assemblies to the sensors  42 ,  43 ,  46  and  47 . Electrical conductors  142 ,  143 ,  146  and  147  (FIGS. 2 and 9) electrically couple the sensors to the signal processing circuits  200 ,  200 ′ on the protected undersurface  14  of the fifth wheel  10 . Each of the four vertical sensors  42 ,  43 ,  46  and  47  are of substantially identical construction as are the mating cylindrical apertures  72 , recesses  77  and plungers associated therewith.  
         [0037]    With reference now to FIG. 5, the horizontal or longitudinally extending sensors  40 ,  41 ,  44  and  45  are mounted within recesses  90  formed in the body  75  of each of the force-sensing units  70   a  and  70   b.  As shown, the sensor  40  is captively held to plate  101  by opposed facing slots  40   b  (FIGS. 8 and 8A) that engage a mounting clip  103 ′ (FIG. 8A). The clip  103 ′ is welded to a top surface of plate  101  such that edges  105 ′ are spaced above the top surface. The clip  103 ′ includes a stop tab  109  for positioning the sensor  40  with respect to the plate  101  and the pressure plate  50 ′ with the outer facing of an associated plunger  50  acted upon by a pair of elastomeric springs, such as pads  100  and  102 ,  104  and  106  associated with sensors  40  and  41 , respectively. The clip  103 ′ is not shown in FIG. 8 and the plunger  50  is not shown in FIG. 8A. A pair of stacked serially coupled elastomeric springs are used in connection with each of the longitudinal or horizontal force sensors  40 ,  41 ,  44  and  45  to allow preloading of the sensors such that both acceleration and deceleration forces are detected by each of the four horizontal sensors (two on each side of the longitudinal axis L o  of the fifth wheel).  
         [0038]    The elastomeric springs, such as pads  100  and  102 , are mounted on opposite sides of a downwardly extending intermediate plate  101  with apertures  101 ′ (FIG. 8) for receiving plungers  58  and  59  of the plunger  50 . Each of the plates  101  includes a rectangular seat  103  (FIG. 8) for receiving an elastomeric spring or pad, such as pad  100  illustrated in FIGS. 5 and 8. The outer walls  110  and  112  of recesses  90  have rectangular apertures  111  therethrough to allow elastomeric pads, such as pad  102  mounted on end plate  120 , to engage pad  100  for preloading its associated sensor  40  as described below.  
         [0039]    End plates  22  and  24  extend over the outer plates  120  and  122  (FIG. 3) and include apertures  108   a  for receiving a pair of spaced, threaded fasteners  107  at the forward and aft sides of each of the two sensing units for preloading the sensors  40 ,  41 ,  44  and  45  for each of the two sensing units. Fasteners  107  are threadably received by threaded apertures  105  in plates  101 , as best seen in FIG. 4. Thus, the tightening of fasteners  107  compresses the elastomeric springs or pads  100  and  102 ,  104  and  106 , and the corresponding springs on the opposite side of the force-sensing units for each of the force sensing units to deflect plungers  50  into engagement with corresponding sensors  40 ,  41 ,  44  and  45  for preloading the sensors. The elastomeric springs  100 ,  102 ,  104 , and  106  also communicate with apertures  72  having a bell-shaped concave entrance  77 ′ (FIGS. 3 and 4) similar in shape to that of the vertical sensors discussed above such that the extrusion of the elastomeric springs into the apertures  72  translate the forces applied to the hitch from the trailer in a linear motion which is transferred to the horizontal sensors to provide a linear output signal in response thereto.  
         [0040]    In a preferred implementation of the first embodiment of the invention, each of the concave radii surrounding the corresponding cylindrical apertures  72  for each of the vertical and horizontal sensor plungers have a radius of about 2 mm such that the diameter of the concave entry recesses  77  and  77 ′ are approximately 4 mm larger than the diameter of apertures  72  for receiving the plungers associated with each of the plunger assemblies  50 . Each of the longitudinal springs  100 ,  102 ,  104 , and  106 , in a preferred embodiment, have a dimension of approximately 73 mm by 36 mm and a thickness (left to right in FIG. 3) of approximately 13 mm and are made of the same material as that of the vertical springs discussed above.  
         [0041]    The assembly of each of the force-sensing units is illustrated in FIGS. 5 and 8, with the sensor and plungers being positioned with respect to the elastomeric springs subsequently inserted into the force-sensing unit bodies  75  as illustrated in FIG. 5, which, in turn, is mounted within the rectangular housing  20   a,  as shown in FIG. 6, into which the vertical springs  62  and  63  have previously been mounted. With the force-sensing unit partially assembled, fasteners  107  are extended through aperture  108   a  in the plates  22  and  24 , apertures  108  in the plates  120  and  122  and into the apertures  105  in plates  101  of the assembly to preload the longitudinal sensors  40 ,  41 ,  44  and  45  with the elastomeric springs  100 ,  102 ,  104 , and  106  deforming into the bell-shaped recesses  77 ′ communicating with aperture  72  to move plungers  50  into engagement with the sensors  40 ,  41 ,  44  and  45  for preloading the sensors such that acceleration and deceleration forces can be sensed by each of the four longitudinal sensors.  
         [0042]    Each of the sensors  40 - 47  are coupled by conductors  140  through  147  (FIGS. 2 and 9) comprising coaxial conductors which couple each of the sensors to interface electrical circuits  200  and  200 ′ for processing the signals for each of the force-sensing units. A temperature sensor  190  may also be coupled to the fifth wheel at a convenient location, such as on wall  24 , as shown in FIG. 2, and coupled to at least electrical circuit  200 , via conductor  192 .  
         [0043]    Having described the mechanical construction of the sensors, the sensor actuating plungers, and the elastomeric springs together with their relationship to each of the sensing units and the relationship of the sensing units to the fifth wheel under frame, a description of the electrical circuit  300  shown in FIG. 9 and the signals from the sensors are now briefly described in connection with FIG. 9.  
         [0044]    The capacitive sensors  40 - 47  are coupled to circuits  200  and  200 ′, which are of a conventional design such as a voltage controlled oscillator which responds to a change in capacitance to change the frequency thereof, which frequency can be converted to a digital signal representative of the frequency and, therefore, the capacitance which is related to the force applied to the sensors from the kingpin applying pressure on the fifth wheel. Forces on the vertical sensors range from approximately 0 up to 160,000 Newtons with 80,000 Newtons on the left-side and right-side. The longitudinal forces applied to the longitudinal sensors varies from −80,000 Newtons to +80,000 Newtons. Circuits  200  and  200 ′ are coupled by suitable electrical conductor  310  mounted to the under-surface of the fifth wheel and coupled to the electrical circuit  320  mounted to the vehicle itself.  
         [0045]    Circuit  320  includes a microprocessor  330 , which is coupled to conductors  310  through suitable interface circuit  340  and to an information display unit  350  via bus  355  in a conventional manner. Microprocessor  330  is programmed to apply any corrective information for the elastomeric material as a function of the temperature detected by the temperature sensor  190  and responds to the signals from each of the eight sensors to provide left and right vertical load information which can be added and subtracted to provide roll moment information. Further, the eight signals are employed by detecting the front and rear vertical loads, which can be added and subtracted to provide pitch moment information. The four vertical sensors are added to provide total vertical load information while the longitudinal left and right signals can be added and subtracted to provide yaw information, all of which can be supplied to the information display unit  350 . These signals also can be applied to a tractor control module  360 , which may include a microprocessor and which is typically provided by the tractor manufacturer for limiting braking activity for safe deceleration of the vehicle and trailer or for providing warning signals to the driver indicating excessive pitch, yaw or tendency to roll, such that the driver can respond to audible or visual alarm signals to control the tractor trailer safely.  
         [0046]    Thus, with the system of the present invention, signals are provided for use by the vehicle operator which accurately measures the coupling forces extending between the trailer and tractor in vertical and horizontal directions to the left and right of and forward and aft of the vertical axis of the kingpin. The system provides electrical signals, which are related in a known manner to the forces applied from the trailer to the tractor by the improved force-sensing units of the present invention.  
         [0047]    FIGS.  10 - 17  depict a fifth wheel  410 , according to another embodiment of the present invention. With reference to FIG. 10, which is a right-side elevation view of the fifth wheel  410 , a forward vertical pad  467  and an aft vertical pad  466  each include a sensor embedded within the pads. In this manner, the pressure that builds up in the pads  466  and  467  when they are under a load can be measured. Such a system may generally be more reliable and also may generally provide more repeatable results than a system that implements an actuator type system, such as the system described with respect to FIGS.  1 - 9 . FIG. 11 is an underside view of the fifth wheel  410  depicting the various components associated with force-sensing units  470   a  and  470   b.  The force-sensing units  470   a  and  470   b  are located within pockets  420   a  and  420   b,  respectively, of the fifth wheel  410 . Similar to the fifth wheel  10  of FIG. 2, the fifth wheel  410  includes a throat  416  that accepts a kingpin and is defined by bifurcated projections  418   a  and  418   b  for the left-side and right-side, respectively, of the fifth wheel  410 . The mounting boxes  420   a  and  420   b  are substantially the same and symmetrical and each contain a force-sensing unit  470   a  or  470   b,  respectively, as is further described in more detail below.  
         [0048]    As the force-sensing units  470   a  and  470   b  and the boxes  420   a  and  420   b  are substantially the same, the following description is limited to a description of the force-sensing unit  470   a  and its mounting relationship with respect to the fifth wheel  410 . It should be understood that the force-sensing unit  470   b  is structurally similar to the force-sensing unit  470   a.  Each of eight sensors  440 - 447  are separately identified as each of the sensors provide an individual signal to a processing unit  720  (FIG. 18), which in response to the supplied signals determines a load applied to the fifth wheel  410  by a given trailer.  
         [0049]    The box  420   a  includes a forward wall  422 , a rear wall  424 , which may extend the width of the fifth wheel  410 , an outside wall  426  and an inside wall  428 . The outside wall  426  includes an aperture  427  aligned with an aperture  429  in the wall  428  for accepting a mounting pin  430  that couples the fifth wheel  410  to a tractor mounting bracket  432 , which is secured to a tractor frame. The coupling of the fifth wheel  410  to the mounting bracket  432  is conventional and may employ an elastomeric interface  431 , which, together with the mounting pin  430 , secures the fifth wheel  410  to a tractor. The force-sensing units  470   a  and  470   b  interface between the mounting brackets  432  and the fifth wheel  410  such that the forces transmitted between the trailer and the fifth wheel assembly  410  are transmitted through the force-sensing units  470   a  and  470   b.    
         [0050]    As with the fifth wheel  10  (see FIG. 2), the fifth wheel  410  (see FIG. 11) includes a longitudinal axis L o  and a lateral axis L a,  which intersect at a vertical axis V. The left-side sensing unit  470   a  of the fifth wheel assembly  410  includes a forward horizontal (longitudinal) sensor  440  and an aft horizontal (longitudinal) sensor  441 . The sensing unit  470   a  also includes a forward vertical sensor  442  and an aft vertical sensor  443 . Similarly, the right-side forcesensing unit  470   b  includes a forward horizontal (longitudinal) sensor  444 , an aft horizontal (longitudinal) sensor  445 , a forward vertical sensor  446  and an aft vertical sensor  447 . The placement of the sensors and their mounting within the boxes  420   a  and  420   b  of the fifth wheel  410  are further illustrated in FIGS.  10 - 13 . Each of the sensors  440 - 447  may be of various types, e.g., pressure sensitive films, ceramic load buttons or small cylindrical piezoceramic elements, which are embedded within elastomeric pads, which may be, for example, rubber or polyurethane.  
         [0051]    When polyurethane pads are used they may exhibit a typical IRHD of about 80 and when natural rubber pads are used they may exhibit an IRHD of about 60. It should be appreciated that a variety of materials can be utilized that exhibit an IRHD within the range of about 60-80 providing the materials exhibit reasonably low hysterisis properties. It should be appreciated that the embodiment shown in FIGS.  10 - 17  does not include actuators, as the sensors are embedded within resilient pads. In this manner, the embedded sensors sense the pressure condition of the pad material without the need for a plunger, piston or actuating pin.  
         [0052]    As is best shown in FIGS. 12 and 14, force-sensing unit  470   a  (and  470   b ) includes a body  475  that has a semi-cylindrical downwardly formed surface  476 , which mates with a similarly shaped mounting bracket  432 . A top surface  474  of the body  475  is positioned in a spaced relationship to the lower surface  414  of the fifth wheel  410  with four elastomeric pads  462 ,  463 ,  466 , and  467 . The pads  462  and  463 , as shown, are positioned in pockets  462 ′ and  463 ′ formed in a lower surface  414  of the fifth wheel  410 . Similarly, the right-side of the fifth wheel  410  includes pockets for receiving pads for the right-side force-sensing unit. As shown in FIG. 13, the pad  463  is received by a pocket  463 ″ within the top surface  474  of the body  475 . In one embodiment, the vertical pads  462 ,  463 ,  466  and  467  are approximately 90 mm by 70 mm and have a thickness or depth, as viewed in FIG. 12, of approximately 15 mm.  
         [0053]    The top surface  474  of the force-sensing unit body  475  likewise includes a pocket  462 ″ for receiving the pad  462 . The pads  462  and  463  provide a gap between the lower surface  414  of the fifth wheel  410  and the top surface  474  of the force-sensing unit sufficient to allow the pads  462  and  463  to deform under a compressive vertical force provided by a trailer coupled to the fifth wheel  410 . Electrical conductors  542 ,  543 ,  546  and  547  electrically couple sensors  442 ,  443 ,  446  and  447 , respectively, to the signal processing circuits  600  and  600 ′ on a protected undersurface  414  of the fifth wheel  410 .  
         [0054]    [0054]FIG. 14 depicts how the pads  500  and  504  and their associated horizontal sensors  440  and  441  are mounted within a recess  490  formed in the body  475  of the force-sensing unit  470   a.  The horizontal sensors  444  and  445 , mounted within pads, are positioned within the force-sensing unit  470   b  in a substantially similar manner. As is best shown in FIG. 17, the sensor  440  is embedded within the pad  500 . The pads  500  and  502  are mounted on opposite sides of a downwardly extending intermediate plate  501 . Each of the plates  501  includes a rectangular seat  503  for receiving a pad, such as pad  500  illustrated in FIGS. 14 and 17. The outer walls  510  and  512  of the recesses  490  have rectangular apertures  511  therethrough to receive elastomeric pads. For example, the pad  502  is mounted on end plate  520  to engage pad  500  for preloading its associated sensor  540  as is further described below. End plates  422  and  424  extend over the outer plates  520  and  522  and include apertures  508  for receiving a pair of spaced threaded fasteners  507  at the forward and aft sides of each of the two sensing units for preloading the sensors for each of the two sensing units. The fasteners  507  are threadibly received by threaded apertures  505  in the plates  501 .  
         [0055]    As is best shown in FIG. 13, the tightening of the fasteners  507  compresses the elastomeric pads  500  and  502  and the corresponding pads  504  and  506  on the opposite sides of the force-sensing unit so as to preload the sensor  440  within the pad  500  and the sensor  441  within the pad  504 . In one embodiment, the pads  500 ,  502 ,  504  and  506  have a dimension of approximately 73 mm by 36 mm and a thickness of approximately 13 mm. The assembly of the force-sensing unit  470   a  is further illustrated in FIG. 14 with the various pads being inserted into the force-sensing unit body  475 , which in turn is mounted within the rectangular housing  420   a,  shown in FIG. 15, to which the pads  462  and  463  have been previously mounted. With the force-sensing unit partially assembled, the fasteners  507  are extended through apertures  508   a,    508  and into the apertures  505  in the plates  501  of the assembly to preload the longitudinal sensors  440  and  441  contained within the elastomeric spring  500  and  504 , respectively. In this manner, acceleration and deceleration forces can be sensed by each of the longitudinal sensors  440  and  441 .  
         [0056]    Each of the sensors  440 - 447  are coupled by conductors  540 - 547 , e.g., coaxial conductors, which couple each of the sensors to interface electrical circuits  600  and  600 ′ for processing signals received from each of the force-sensing units. A temperature sensor  590  may also be coupled to the fifth wheel  410  at a convenient location, such as on the wall  424  as is shown in FIG. 11, and coupled to at least an electrical circuit  600 , via conductor  592 . As is further shown in FIG. 18, sensors  440 - 447  are coupled to the circuits  600  and  600 ′, which may be voltage controlled oscillators, which respond to changes in a signal to change the frequency thereof, which frequency can be converted to a digital signal representative of the frequency and therefore, the signal, which is related to the force applied to the sensors from the trailer applying pressure on the fifth wheel.  
         [0057]    Typical forces on the vertical sensors  442 ,  443 ,  446  and  447  range from approximately 0 up to 160,000 Newtons on the left-side and right-side. Longitudinal forces applied to the longitudinal sensors  440 ,  441 ,  444  and  445  vary from about −80,000 Newtons to about +80,000 Newtons. As shown, the circuits  600  and  600 ′ are coupled by a single electrical connector  710  mounted to the under-surface of the fifth wheel  410  and coupled to the electrical circuit  720  mounted to the vehicle itself. Circuit  720  includes a microprocessor  730  that is coupled to the conductors  710 , through a suitable interface circuit  740  and to an information display  750  via a cable  755  in a conventional manner. The microprocessor  730  may be programmed to, for example, apply correction factors for the elastomeric material as a function of temperature, among other functions.  
         [0058]    The signals provided by the sensors  440 - 447  may also be supplied to a tractor control module  760 , which may limit brake activity for safe deceleration of a vehicle and trailer or may provide warning signals to a driver indicating excessive pitch, yaw and/or roll such that the driver can respond to the audible or visual alarm signals to control the tractor in a safe manner.  
         [0059]    It should be appreciated that the outputs of the sensor may also potentially be used to detect conditions/faults (e.g., the force of a kingpin against a plate may substitute for a kinpin sensor) and in systems that implement coupling detection the display and processor may be shared. For example, the system described herein may be implemented with a coupling alignment warning system as is described in U.S. patent application Ser. No. 09/836,796, filed Apr. 17, 2001, and entitled “TRUCK REARVIEW MIRROR ASSEMBLY HAVING A DISPLAY FOR DISPLAYING TRAILER COUPLING STATUS INFORMATION,” which is a continuation-in-part of U.S. patent application Ser. Nos. 08/951,250 (now U.S. Pat. No. 6,252,497), filed Oct. 16, 1997, and entitled “COUPLING ALIGNMENT WARNING SYSTEM” and Ser. No. 09/493,534, filed Jan. 28, 2000, and entitled “ELECTRONIC SYSTEM FOR MONITORING A FIFTH WHEEL HITCH.” The entire disclosures of U.S. patent application Ser. Nos. 09/836,796; 08/951,250; and 09/493,534 are hereby incorporated herein by reference.  
         [0060]    It will become apparent to those skilled in the art that various modifications to the preferred embodiments of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.