Abstract:
A load sensor is provided on a tractor to sense a load in the upper link of a three point hitch. The load sensor is fixed to the upper link, rather than the tractor to move with the upper link rather than the tractor. This arrangement permits the load sensor to always indicate the longitudinal load placed on the upper link.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to load sensing in work vehicles. More particularly, it relates to load sensors located between a work vehicle and a towed implement. 
     BACKGROUND OF THE INVENTION 
     Work vehicles such as tractors and backhoes, for example, tow implements using a variety of hitches to connect the vehicle and the implement. One of the most common hitches is called a “three point hitch.” It is called a three point hitch since it has three substantially parallel links or connections that extend between the vehicle (usually a tractor) and the implement. These links permit the motion between the vehicle and the implement to be constrained to relative translation, and substantially prevent relative rotation. 
     Many tractors or other work vehicles are able to electronically monitor and control their travel through the field, as well as control the height of and load placed upon the implement. This control is especially useful to regulate the speed of the tractor, the power output of the engine, the tractor&#39;s gear ratio and the height of the implement above or below the ground. Using various sensors on the vehicle, the tractor can raise the implement to reduce the implement load, increase the power output of the engine, down shift to a lower gear or reduce the speed of the tractor. 
     To sense the load placed on the tractor by the implement, a load sensor is typically provided between the implement and the tractor to sense the load placed on one or more of the links connecting the tractor to the implement. These sensors can be arranged to provide two different types of sensings: indirect and direct load sensing. Both of these methods have serious limitations. The direct method places a load sensor between the tractor and the implement that directly senses the strain applied by the implement on the tractor. As an example, a load pin is placed between one or more of the three links and the tractor or between one or more of the three links and the implement. A common direct sensing method has been to replace one or more of the pins coupling the three links of the three point hitch to the tractor or one or more of the three links connecting the three point hitch to the implement with a load sensing pin that provides an electronic signal indicative of the load applied to the pin. As the links are pulled by the tractor towing the implement, they in turn apply a force to the load pin. Load cells placed on the load pin provide an electrical signal indicative of the load on the load pin. 
     There are problems with the direct method. In the direct method, the pin is fixed with respect to the tractor. A bracket fixed to the tractor commonly has two ears or webs with holes through which the load pin extends. The bracket defines the axis of the load pin perpendicular to the direction of travel of the tractor, extending either vertically or horizontally, and prevents the pin from moving. Since the load sensed by the load pin typically indicates a load applied perpendicular to the load pin axis, it is responsive only to loads applied straight backward away from or forward toward the tractor. As the implement moves from side to side, however, it places a bending or torsional load on the load pin. This causes the load pin to give a false reading of the load in the link. 
     In the indirect method a spring member is located between the upper link and the tractor. As a load is applied to the upper link, the spring member deflects. This deflection is either sensed by a position sensor or a load sensor. While this eliminates some of the problems of the direct method, it is less accurate than the direct method. 
     What is needed, therefore, is an improved method and apparatus for sensing the load applied by an implement to a tractor. It is an object of this invention to provide such a method and apparatus. 
     SUMMARY OF THE PRESENT INVENTION 
     In accordance with the first embodiment of the invention an agricultural vehicle is disclosed including a tractor, an implement and a multi-point hitch connecting the tractor to the implement, the hitch including a plurality of links extending from the tractor to the implement, a swivel disposed between and coupling at least one of the links to the tractor, and a load sensor disposed between the swivel and the at least one link. The load sensor may be fixed with respect to the at least one link. The load sensor may have a primary sensing direction and that sensing direction may be fixed with respect to the at least one link. Load sensor may be a cylindrical load pin having a longitudinal cylindrical axis. The swivel may be constrained to pivot with respect to the tractor about a first pivotal axis. The at least one link may be constrained to pivot about the swivel about a second pivotal axis substantially orthogonal to the first pivotal axis. The first pivotal axis may be substantially horizontal. 
     In accordance with a second embodiment of the invention, a multi-point hitch for connecting a tractor to an implement is disclosed including an upper link extending from the tractor from an upper point on the tractor and couplable to an implement, two links extending from the tractor at two laterally spaced points below the upper point and couplable to an implement, a swivel coupled between the tractor and the upper link, and a load sensor coupled between the swivel and the upper link. The load sensor may have a primary sensing direction and this direction may be fixed with respect to the upper link when the upper link pivots with respect to the tractor. The load sensor may be disposed between the swivel and the upper link to transmit substantially the entire load in the upper link to the tractor. The swivel may be constrained to pivot with respect to the tractor about a first pivotal axis. The upper link may be constrained to pivot about the swivel about a second pivotal axis substantially orthogonal to the first pivotal axis. The first pivotal axis may be substantially horizontal. 
     In accordance with a third embodiment of the invention, an agricultural vehicle is disclosed including a tractor, and an elongate load sensing member having first and second ends and a longitudinal axis and coupled to the tractor at the first end, the member including, a first coupler disposed at the first end, a second coupler disposed at the second end and a load sensor fixed to the first coupler. The vehicle may further include a threaded cylinder disposed between and coupling the first coupler to the second coupler. The member may pivot with respect to the tractor at least about a horizontal and a vertical axis. The load sensor may pivot together with the member about the horizontal and vertical axes. The threaded cylinder may be rotatable with respect to the first and second ends to shorten or lengthen the elongate member. The first coupler may include an eye. The first coupler may also include a clevis. 
     Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a tractor having a three point hitch, including an upper link, and two lower links as well as a load pin disposed between the upper link and the tractor; 
     FIG. 2 is a perspective view of the hitch of FIG. 1 with the tractor removed showing the arrangement of upper and lower links, the actuators and the lifting links; 
     FIGS. 3 and 4, respectively, are assembled and exploded perspective views of the upper link mounting bracket and a portion of the upper link of the hitch shown in FIG. 2; and 
     FIG. 5 is a partial cross-sectional side view of the upper link mounting bracket and upper link of FIGS. 3 and 4. 
     Before explaining at least one embodiment of the invention in detail it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a tractor  10  having a three point hitch  12  to which implements can be coupled. Three point hitch  12  includes an upper link  14  and a pair of lower links  16  (only one shown) disposed below upper link  14  to which the implement is coupled. These links are pivotally coupled to the tractor at lower link pivot point  18  and upper link pivot point  20 . The pivotal couplings disposed between the lower link and the upper link at these pivot points permit the upper and lower links to pivot birth up and down and from side to side about the pivot points with respect to the tractor. Upper link  14  includes a coupler  22 , shown in FIG. 1 as a swivel, for coupling upper link  14  to the implement at an upper coupling point (not shown) on the implement. Lower links  16 , include a lower link coupler  24 , here shown as an eye in the ends of lower links  16  which are connected to two lower coupling points on the implement. Alternatively, lower link coupler may be a hook. 
     Hitch  12  also includes a pair of actuators  26 , here shown as hydraulic cylinders, that raise and lower the hitch with respect to the tractor. When the actuators are extended, they raise the lower link couplers  24  with respect to the tractor, causing the implement to raise with respect to the tractor. Since upper link coupler  22  is also pivotally coupled to the implement, it is similarly lifted by the implement as the implement is raised by lower link  16 . Upper link  14  and lower links  16  pivot about upper link pivot point  20  and lower pivot point  18  to cause the implement to substantially translate with respect to the tractor, and not to rotate significantly with respect to the tractor. 
     FIG. 2 shows a detailed view of multi-point hitch  12  with tractor  10  removed. Upper link  14  is in the form of a turnbuckle having an internally threaded cylinder  28  into which two devises  30 ,  32  are threaded. Clevis  30  is disposed at the outer end of the upper link  14 . The two ears or webs of clevis  30  define a vertical passage for receiving pin  34 . Clevis  30  receives a swivel  22  to which it is pivotally ceupled. Clevis  30  and swivel  22  are constrained to rotate with respect to each other about the vertical axis defined by pin  34 . Swivel  22  includes at an end distal from the tractor a second pin receiving passage  38  that is adapted to be coupled to the upper connection point of the three point connection on the implement. Passage  38  is preferably substantially orthogonal to pin  34 , and thus is horizontally disposed. 
     By coupling the implement about a horizontal pivotal axis of swivel  22  defined by pin receiving passage  38  and coupling upper link  14  to swivel  22  about a vertical pivotal axis defined by pin  34 , upper link  14  can pivot with respect to the implement with two degrees of freedom. It is to be recognized that the same two degrees of freedom could be provided by reversing the pivotal axes. Similarly, an eye could be provided instead of clevis  30  and swivel  22  could have a mating clevis to engage that eye. Several different arrangements are possible that permit two degrees of rotational freedom of upper link  14  with respect to the implement. 
     Clevis  32  is similarly coupled to the tractor. The two ears or webs of clevis  32  have holes  39  (FIG. 4) that define a vertical pin receiving passage  40 . Passage  40  receives pin  42  which passes through hole  43  in swivel  44 . The pin, pin receiving passage and swivel are configured to permit swivel  44  to rotate about a vertical axis with respect to upper link  14 . The other end of swivel  44  has a hole  45  that defines a horizontal pin receiving passage. This is disposed between two ears of mounting bracket  66  which is coupled to and extends from the tractor and define a clevis  46 . Each ear of clevis  46  has a hole  47  that, together with hole  45  of swivel  44 , defines a horizontal pin receiving passage into which pin  48  is inserted, which constrains swivel  44  to rotate with respect to the tractor about a horizontal axis defined by pin  48 . 
     As can be appreciated from the above description, both ends of upper link  14  are similarly configured to provide two degrees of rotational freedom to upper link  14  both with respect to the tractor and with respect to the implement. 
     While the pivotal axes at each end of upper link  14  are preferably orthogonal they may nonetheless be somewhat less than orthogonal and still provide relatively free pivoting with two degrees of freedom without binding. The degree to which they vary from the preferred orthogonal relationship will depend primarily on the materials, lubricants and tolerances of the parts involved. Too great a reduction in orthogonality will cause enhanced wear of the pins and premature failure. 
     In addition, the axes, while preferably shown as vertical and horizontal, need not be so arranged. It is primarily for convenience in manufacturing that horizontal and vertical axes are preferred, since this arrangement provides for more balanced loads on the components. While upper link  14  is shown as terminating in clevises, it need not do so. It could terminate in an eye, and swivels  22  and  44  have devises instead of eyes at each end. 
     To adjust the length of upper link  14 , cylinder  28  can be grasped and rotated to reduce or increase the overall length of upper link  14 . This is possible since the internal threads in cylinder  28  have opposite hands at opposing ends. One clevis is threaded into cylinder  28  with a left hand thread and the other clevis is threaded into cylinder  28  with a right hand thread. Rotation of cylinder  28 , therefore, results in retraction of both devises toward cylinder  28  or extension of both devises away from cylinder  28 . 
     Hitch  12  is lifted when actuators  26  fill with hydraulic fluid under pressure. The rod ends of actuators  26  are pivotally coupled to arms  56 . Arms  56  are also coupled to tractor  10  and pivot about rotational axis  58  when actuators  26  extend or retract. The hitch is raised when actuators  26  extend. Arms  56  rotate counterclockwise (FIG. 2) around axis  58  causing ends  60  of arms  56  to raise with respect to the tractor. Ends  60  of arms  56  are pivotally coupled to lifting links  62  which, in turn, are pivotally coupled to lower links  16 . Thus, when ends  60  raise, the free ends of lower links  16  raise with respect to the tractor as the other ends of lower links  16  pivot about pivot points  18 . When connected to an implement, this causes the implement to lift with respect to the tractor as well. Since upper link  14  is also pivotally coupled to the tractor, the outer end of upper link  14  coupled to the implement also rises pivoting with respect to the implement about a horizontally disposed pin inserted in passageway  38  in swivel  22 . The inner end of upper link  14 , however, is rotationally coupled to the tractor and cannot rise. Thus, the inner end of upper link  14  pivots about the tractor around the horizontal axis defined by pin  48 . 
     FIGS. 3 and 4 show an assembled and exploded view, respectively, of the tractor-end of upper link  14  and the structures that couple it to the tractor. Upper link  14  is coupled to mounting bracket  66  which is removably fastened to the tractor by removable fasteners here shown as bolts  68 . These fasteners extend through holes in mounting bracket  66  and screw into the gear case of the tractor. Mounting bracket  66  includes a base plate  72  through which the holes pass and two outwardly extending webs or ears  74  disposed in parallel in spaced apart relation. A bore or hole  47  passes through both ears  74  to define clevis  46  and a substantially horizontal pivotal axis for swivel  44 . Eye  78  of swivel  44  has a horizontal bore or hole  45 . Eye  78  is inserted between ears  74 . Hole  45  together with holes  47  define a horizontal passageway for receiving pin  48 . In this manner, swivel  44  is constrained to rotate about a substantially horizontal axis with respect to mounting bracket  66 . Alternatively, the mounting bracket can be disposed of and webs or ears  74  can be formed integral with and extend directly from the tractor transmission housing. 
     The inner facing surfaces of ears  74  are spaced apart to provide only a small gap between eye  78  and ears  74 . This minimizes the twisting of eye  78  and concomitant wear of pin  48 . The end of swivel  44  that is disposed away from the tractor and mounting bracket has a second eye  82  that has a hole or bore  43  substantially perpendicular to hole  45  at the other end of swivel  44 . In the preferred embodiment, when swivel  44  is extended horizontally, hole  43  extends vertically. 
     Eye  82  of swivel  44  is inserted between ears or webs  84  and  86  of clevis  32 . The inner facing surfaces of ears  84  and  86  are spaced apart to provide only a small gap between eye  82  and ears  84  and  86 . This minimizes the twisting of eye  82  and concomitant wear of pin  42 . The inner facing surfaces of ears  84  and  86  are preferably parallel and perpendicular to the axis of pin  42 . 
     Pin  42  is a load sensor. It is inserted through holes  39  and  43  in clevis  32  and swivel  44 , respectively. It carries the entire load transmitted from upper link  14  to tractor  10 . Load cells (not shown) mounted inside pin  42  sense the load applied to the pin and transmit a signal indicative of this load through cable  88  to connector  90 . Connector  90 , in turn, is coupled to an electronic controller on the tractor (not shown) which uses this signal to control hitch motion. Pin  42  has a primary sensing direction indicated by arrow  92  on the top of the pin. This direction indicates the direction of greatest sensor sensitivity. When loads are applied to the pin in the direction of the arrow, the primary sensing direction, the signal provided at connector  90  will be at its greatest. Since pin  42  is fixed with respect to clevis  32 , and hence upper link  14 , its primary sensing direction cannot change position with respect to the upper link. As a result, the signal provided by pin  42  is always indicative of the longitudinal load applied to upper link  14 , regardless of the orientation of upper link  14 . This is achieved by constraining pin  42  so it cannot rotate within pin receiving passageway  40  and therefore always rotates with upper link  14 . Mating alignment surface  94 , here shown as a flat surface on pin  42 , is configured to abut a similar mating aligning surface  96  on clevis  32 . These surfaces are disposed such that pin  42  cannot rotate when it is inserted into pin receiving passage  40 . To prevent pin  42  from falling out of passage  40 , a pin retainer, here shown as plate  98 , is fastened over the top of pin  42  once it is inserted into passage  40 . Plate  98  is fixed to clevis  32  by bolts  100  which pass through holes  102  in plate  98  and screw into holes  104  on clevis  32 . 
     Referring now to FIG. 5, the arrangement of clevis  32  with respect to threaded cylinder  28  can be seen. Threads  106  are provided on an outer surface of shaft  108  extending from clevis  32 . The longitudinal axis  110  of shaft  108  is preferably coaxial with the longitudinal axis of cylinder  28  and upper link  14 , and is preferably perpendicular to longitudinal axis  112  of pin  42 . In this manner, only a limited if any twisting moment is applied to pin  42  by swivel  44 . 
     Thus, it should be apparent that there has been provided in accordance with the present invention an improved upper link sensing that fully satisfies the objectives and advantages set forth above. Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. In particular, the swivel may be a different forms such as those commonly used in agricultural or work vehicles for coupling lnks to implements or towing vehicles. In addition, the load pin may be disposed between the link and the vehicle, or between the link and the implement. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.