Abstract:
The present invention relates to a steering device for facilitating the steering of an implement towed by a work vehicle. The towed implement, for example, a farm implement such as a seeding device, comprises a main frame having front and rear ends and at least one steerable surface engaging wheel attached to the frame. The steering device comprising a sensor means mounted on the main frame of the towed implement for sensing a misalignment of the towed implement and the work vehicle and producing a signal corresponding to the misalignment. The steering device further comprises a steering means associated with the steerable wheel for receiving the signal produced by the sensor means and for effecting movement of the steerable wheel in response to the signal until the towed implement and the work vehicle are once again realigned.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a steering device for a farm implement having at least one steerable wheel that is towed by a work vehicle such as an agricultural tractor. In particular, the present invention relates to a steering device that helps a towed implement manoeuvre around an obstruction, such as a slough, or keeps the implement in alignment with a work vehicle when the work vehicle is traversing a slope. 
   BACKGROUND OF THE INVENTION 
   Many seeding and harvesting equipment are designed to be drawn behind tractors and the like. Most of these towed devices are steered primarily by the tractor and do not have their own steering mechanisms. 
   Under certain circumstances, however, it would be advantageous to be able to steer a towed implement independently from the tractor that is pulling it. For example, with seeding implements it is desirable for the seeding implement to remain aligned with the tractor at all times during seeding in order to obtain straight and even seed rows. However, often when a tractor is traversing in a direction perpendicular to the slope of the land, there is a tendency for the towed seeding implement to start “side slipping” or moving in the direction of the slope thereby falling out of alignment with the tractor. This results in uneven crop rows. 
   Another circumstance when it would be desirable to have an independently steerable implement would be where a tractor must manoeuvre the implement around an obstruction such as a slough, telephone pole, large boulder and the like. If the towed implement does not have independent steering, there is a tendency for the towed implement to “cut corners” thereby, in the case of a seeding implement, the seeding openers are moving sideways instead of straight ahead, which is the intended use. 
   Thus, a towed implement having independent steering would be able to steer itself back into alignment with the tractor. This can be accomplished by equipping the towed implements with at least one steerable surface engaging wheel, and more preferably, with two steerable surface engaging wheels attached to the rear of the main frame of the towed implement. Steering can either be automatically controlled by means of a turning sensor or could be operator controlled. 
   French Patent No. 2034151 (“&#39;151”) teaches the use of ropes and pulleys to steer the rear wheels of a towed implement. In this instance, ropes are attached to the front hitch that connects the implement to the tractor and to two pivotally mounted wheels. When the angle of the front hitch strays from 90° relative to the front end of the main frame, the attached ropes are pulled in the direction of the hitch. The ropes, which are also attached to the rear wheels, will then pull the rear wheels to a degree corresponding to the angle of the hitch. 
   The system taught in &#39;151 is undesirable for several reasons. First, the system would be constantly re-adjusting the angle of the rear wheels, resulting in the towed implement “wobbling” down the field. Second, if the side-hills on which the towed implement is being towed are particularly steep, the excess tension on the ropes could cause the ropes to break. Finally, there is no method for the operator to be able to control the amount the implement turns. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a steering device for facilitating the independent steering of a towed implement. The present invention allows a towed implement to correct any misalignment with the vehicle towing the implement and allow the implement to stay on course once such realignment has been achieved. In particular, the present invention does not require constant re-adjustment so the towed implement does not wobble down the field when traversing a hill. 
   In its broadest embodiment, the present invention relates to a steering device for facilitating the steering of an implement towed by a work vehicle, said implement comprising a main frame having front and rear ends and at least one steerable ground engaging wheel attached to said frame, comprising:
         (a) a sensor means comprising a ground engaging sensor member having side to side movement about a vertical axis, said sensor means mounted on the frame of the towed implement for sensing a sideways displacement of the towed implement and producing a signal corresponding to said displacement; and   (b) a steering means associated with said steerable wheel for receiving said signal and for effecting movement of said steerable wheel in response to said signal.       

   In one embodiment, the present invention provides a steering device, which steers the towed implement in response to a ground engaging sensor member. The ground engaging sensor member is mounted to the frame of the implement in such a fashion that if there is sideways movement of the frame at that location, the sensor member will move in a direction opposite to that of the frame and send the appropriate signal to one or more steerable wheels to turn accordingly. Therefore, the sensor member may be either pivotally mounted to said frame to allow it to move from side to side about a vertical axis or be rigidly mounted to said frame but have a degree of flexibility allowing it to move from side to side about a vertical axis. 
   Thus, in accordance with one embodiment of the present invention, a steering device for facilitating the steering of an implement towed by a work vehicle, said implement comprising a main frame having front and rear ends and at least one steerable ground engaging wheel attached to said frame, is provided, comprising:
         (a) a ground engaging sensor member having a first and second end, said first end mounted to said frame such that said second end engages the ground, said ground engaging sensor member having side to side movement about a vertical axis and being in a neutral position when it is traveling in a plane perpendicular to a horizontal member of the frame;   (b) a sensing means responsively associated with said ground engaging sensor member for sensing the side to side movement of said ground engaging sensor member and for creating a signal corresponding to said side to side movement; and   (c) a steering actuating means associated with said steerable wheel for receiving said signal and for effecting movement of said steerable wheel in response to said signal until the sensor member is back to the neutral position.       

   In a preferred embodiment, the implement is a seeding device having a main frame, a plurality of individual seeders attached to the main frame and two steerable wheels positioned at the rear of the main frame. 
   In another preferred embodiment, the surface engaging sensor member is pivotally mounted to the frame and comprises at least one arm carrying a rotatable wheel, a rolling disc or coulter, or a skid, all of which are capable of riding on or engaging the ground. 
   In another preferred embodiment, the sensor member is kept in constant engagement with the surface by means of a hydraulic or pneumatic cylinder, a spring biasing means or simply the force of gravity upon the sensor member. 
   In another preferred embodiment, the sensor member is mounted more rigidly but the sensor member itself is flexible thus capable of side to side movement about a vertical axis. 
   In another preferred embodiment, the sensing means comprises a valve means responsive to the mechanical movement of said sensor member, said valve means having a receiving port for receiving a source of hydraulic fluid or air and a pair of supply ports for supplying hydraulic fluid or air (i.e. the signal) to the steering actuating means so that when the sensor member moves in one horizontal direction the valve means releases hydraulic fluid or air through the first supply port and when the sensor member moves in the opposite horizontal direction the valve means releases hydraulic fluid or air through the second supply port. 
   In another preferred embodiment, the sensor member further comprises a potentiometer for producing an electrical signal corresponding to the direction of the horizontal movement of said sensor member and the sensing means comprises a valve means responsive to said electrical signal produced by said potentiometer. In this embodiment, the valve means comprises a receiving port for receiving a source of hydraulic fluid or air and a pair of supply ports for supplying hydraulic fluid or air to the actuator means so that when an electrical signal is given corresponding to the sensor member moving in one horizontal direction the valve means releases hydraulic fluid or air through the first supply port and when an electrical signal is given corresponding to the sensor member moving in the opposite horizontal direction the valve means releases hydraulic fluid or air through the second supply port. 
   In another preferred embodiment, said actuator means comprises at least one hydraulic or pneumatic cylinder having first and second chambers, whereby when the sensing means provides a signal comprising hydraulic fluid or air, the hydraulic fluid or air is directed to one or the other of the chambers thereby causing the cylinder rod  62  to move horizontally and effect movement of said steerable wheel either to the left or to the right relative to the direction of travel of the tractor. 
   In a further preferred embodiment, the actuating means comprises at a hydraulic or pneumatic cylinder having first and second chambers, whereby one end of the cylinder is attached to one steerable wheel and the other end of the cylinder is attached to the other steerable wheel such that each steerable wheel moves in the same direction in response to said signal received. 
   In another embodiment of the present invention, both the towing vehicle and the towed implement are each equipped with an antenna that can receive Global Positioning System (GPS) satellite signals. A receiver/CPU is provided which can compare the relative position of the towing vehicle to the position of the towed implement. Thus, if the towed implement is off track relative to the towing vehicle, the receiver can send an appropriate signal to the steerable wheels to turn accordingly. 
   Thus, in accordance with another embodiment of the present invention, a steering device for facilitating the steering of an implement towed by a work vehicle, said implement comprising a frame having front and rear ends and at least one steerable ground engaging wheel attached to said frame, is provided, comprising:
         (a) means for determining the location of the centre of said towed implement and the location of the centre of said work vehicle and for producing a first signal when the centre of said towed implement and the centre of said work vehicle are misaligned;   (b) a sensing means for receiving said first signal and creating a second signal corresponding to the position of said towed implement in relation to said work vehicle; and   (c) a steering actuating means associated with said steerable wheel for receiving said second signal and for effecting movement of said steerable wheel in response to said second signal until the centre of said towed implement and the centre of said work vehicle are in alignment.       

   In a preferred embodiment, the means for determining the location of the centre of said towed implement and the location of the centre of said work vehicle and for producing a signal when the centre of said towed implement and the centre of said work vehicle are misaligned comprises a first global positioning antenna located on said work vehicle, a second global positioning antenna located on said towed implement and a receiver. 
   In another preferred embodiment, the sensing means comprises a steering control valve means responsive to said first signal which producing a second signal in the form of the release of hydraulic fluid or air. The steering actuating means comprises at least one hydraulic or pneumatic cylinder operative to steer the steerable ground engaging wheel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic top view of a towing vehicle and a towed implement having a steering device of the present invention. 
       FIG. 2  is a schematic top view of a towed seeding device having a steering device of the present invention. 
       FIG. 3   a  is a schematic of the hydraulic circuit for the steering device of  FIG. 2 . 
       FIG. 3   b  is a schematic of the hydraulic circuit of an alternate embodiment for the steering device of  FIG. 2 . 
       FIG. 4  is a side view of a preferred embodiment of the ground engaging sensor member of the present invention. 
       FIG. 5  is a perspective view of the ground engaging sensor member of  FIG. 4 . 
       FIG. 6  is a side view of the ground engaging sensor member of the present invention utilizing one embodiment of the steering control valve means. 
       FIG. 7  is a perspective view of the ground engaging sensor member of  FIG. 6 . 
       FIG. 8  is a side view of a preferred embodiment of the castoring device used to rotate the steerable wheels of the present invention. 
       FIG. 9  is a bottom perspective view of the castor device of  FIG. 8 . 
       FIG. 10  is a rear view of the assembled castoring device. 
       FIG. 11  is a top view of the assembly of  FIG. 10 . 
       FIG. 12  is a side view of the assembly of  FIG. 10 . 
       FIG. 13  is a schematic top view of a preferred embodiment of a towing vehicle and a towed implement each equipped with GPS antennae. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings in general, and in particular to  FIGS. 1 ,  2  and  3 , the number  10  refers to a tractor operable to pull a trailing (or) towed agricultural implement, which in this preferred embodiment is seeding device  14 . Seeding device  14  is connected to tractor  10  by means of front hitch assembly  12 . Seeding device  14  comprises a main frame  22  having a plurality of horizontal bars  60  on which a plurality of ground engaging individual seeders or openers  16  are mounted. In this embodiment, two steerable wheels  24 ,  26  are connected to axle  54  by conventional castoring means to allow the steerable wheels to pivot about a vertical axis. Axle  54  is mounted to the rear of frame  22 . 
   The steering device of the present invention comprises a ground engaging sensor member  18  which is pivotally mounted more or less in the centre of frame  22  on one of the horizontal bars  60 , i.e. so that ground engaging sensor member  18  is essentially in the centre of the gang of openers  16 . Thus, sensor member  18  is pivotally mounted at a position that results in the least amount of sideways motion of the ground engaging individual openers  16  when seeding device  14  is kept in alignment with tractor  10  by means of the present invention. 
   Sensor member  18  is attached to frame  22  by means of a pivot  20  allowing horizontal or side-to-side movement of the sensor member  18  relative to horizontal bars  60 . Sensor member  18  thus acts more or less like a pendulum. When the vertical plane of the sensor member  18  is positioned at a 90° angle relative to the horizontal bars  60  it is considered to be in the neutral position and the tractor  10  and seeding device  14  are in alignment. 
   Sensor member  18  is operably linked to a sensing means comprising steering control valve  40 . Steering control valve  40  is a 4 port-3 position valve, for example, a Walvoil SD4/1(KG3-120)/1CP18L-SAE valve (as shown in  FIG. 3   a ). In this embodiment, steering control valve  40  is a mechanical valve operably linked to sensor member  18  in such a fashion that when sensor member  18  goes off of 90°, a mechanical force is asserted on steering control valve  40  to activate a lever in one direction or the other. 
   In another preferred embodiment, sensor member  18  is operably linked to a sensing means comprising a closed centre, non-load reaction steering control valve, which is shown schematically in  FIG. 3   b  and labelled numerically as 400. An example of one such steering control valve is Eaton Steering Control Unit Series 6 212-1069. 
   In another preferred embodiment, steering control valve  40  is a 4 port-3 position solenoid valve (not shown) capable of receiving an electrical signal, which activates an electromagnet that shifts the position of the valve, altering the course of hydraulic fluid flow. In this embodiment, sensor member  18  further comprises a potentiometer (not shown), which sends an electronic signal to a solenoid steering control valve that corresponds to movement of the sensor member either to the right of centre (i.e. centre being when the sensor member is perpendicular to the frame and thus in neutral position) or to the left of centre. 
   Steering control valve  40  operates to control a steering actuating means, which in this embodiment comprises a horizontally adjustable hydraulic cylinder, commonly called a double-acting hydraulic cylinder  28 , by sending a signal (i.e. in this case, sending a volume of hydraulic fluid) instructing hydraulic cylinder  28  to move the cylinder rod  62  left or right. Hydraulic cylinder  28  is operably attached at either end to steerable wheels  24  and  26  by means of two tie rods  52 . 
   With particular reference to  FIGS. 2 ,  3   a  and  3   b , the present invention will now be described when in operation. The operator of tractor  10  will be able to control when to put the steering device of the present invention into action. Generally, however, it is desirable that the steering device be operable during the entire seeding procedure. An electronic switch, generally situated in the cab of the tractor, turns on the steering  372  device. The switch sends an electronic signal to diverter valve  34 , which in this embodiment is a 3 port-2 position directional solenoid valve having one receiving port  70  and two supply ports  72  and  74 . 
   Tractor  10  is equipped with hydraulic fluid reservoir  30  that supplies hydraulic fluid to receiving port  70  of diverter valve  34  via hydraulic line  32 . Generally, flow control needle valve  68  controls the flow of hydraulic fluid from reservoir  30  through pump  118  to diverter valve  34 . Thus, when the operator manually turns on the switch necessary to operate the steering device of the present invention, the electronic signal is sent to diverter valve  34  opening supply port  72 . 
   In the case when steering control valve  40  is used, hydraulic fluid is directed to steering control valve  40  via hydraulic line  36  through receiving port  76  and the steering device is activated and continuously operating. When sensor member  18  is in the neutral position (i.e. its vertical plane is perpendicular to the frame), for example, in instances where seeding in a straight line on a level ground surface, the centre supply port  78  of steering control valve  40  is open and hydraulic fluid from the diverter valve  34  is simply recycled back to hydraulic fluid reservoir  30 . Hence, the steering device is considered to be in neutral and the steerable wheels  24  and  26  are not activated (i.e. they are not turning). Those skilled in the art would recognize this as an open-centre system. 
   In the alternative, when using steering control valve  400 , the centre port  780  is closed when sensor member  18  is in the neutral position (i.e. its vertical plane is perpendicular to the frame). Thus, hydraulic fluid is only released by steering control valve  400  when the sensing member is either to the left or right of 90°. Therefore, hydraulic fluid does not constantly have to recycle when the work vehicle and the farm implement are in alignment. Those skilled in the art would recognize this as an open-centre system. 
   When tractor  10  and trailing seeding device  14  are traversing a slope or hill, the tendency is for the seeding device  14  to “fall” or sideslip down the slope and the desire is to correct this slippage by changing the direction of the steerable wheels. Side slipping causes the sensor member  18  pivot to one side or the other relative to the ground and the frame  22 . For example, when traversing a slope where area “A” as shown in  FIG. 1  is of higher elevation than area “B”, this causes frame  22  to slide downhill toward area “B” and sensor member  18  to pivot to the right relative to the frame (as shown in phantom in  FIG. 1  as sensor member  18 ′). The rotation of sensor member  18  to the position of sensor member  18 ′ will activate steering control valve  40  by activating a lever which mechanically opens the right supply port  82  of steering control valve  40 . Hydraulic fluid is then released through supply port  82 . 
   When steering control valve  400  is used, valve  400  is operably attached to pivot pin  120  of sensor member  18  (shown in more detail in  FIGS. 6 and 7 ). Thus, when sensor member  18  pivots to one side, the internals of steering control valve  400  rotate to open either supply port  780  or supply port  800 . The greater the rotation of sensor member  18 , the larger the opening will be of supply ports  780  or  800 . Hence, the amount of released hydraulic fluid, which will dictate the degree of rotation of the steerable wheels, is directly proportional to the degree of rotation of sensor member  18 . 
   In yet another embodiment, the sensor member  18  is linked to a potentiometer (not shown), which sends an electronic signal to activate steering control valve  40 . 
   Hydraulic line  44 , which is linked to right supply port  82  or  820 , then transports the released hydraulic fluid to a steering actuating means. In the present embodiment, steering actuating means is a double-acting hydraulic cylinder  28 . Double-acting hydraulic cylinder  28  comprises first and second chambers,  84  and  86 , respectively. Thus, when the steering control valve  40  (or  400 ) releases hydraulic fluid through the right supply port  82  (or  820 ) said hydraulic fluid is directed via hydraulic line  44  to the first chamber  84  of the hydraulic cylinder  28  causing the hydraulic cylinder rod  62  to move position (i.e. either to the left or to the right). Because the hydraulic cylinder is operably connected at each end by means of tie rods  52  to steerable wheels  24  and  26 , when hydraulic cylinder rod  62  moves steerable wheels will rotate or turn into the slope, i.e. towards area “A” (shown in phantom in  FIG. 1  as steerable wheels  24 ′ and  26 ′). 
   Now assuming that area “B” is of higher elevation than area “A”, the sensor member  18  will pivot to the left relative to the frame. The rotation of sensor member  18  will activate steering control valve  40  (or  400 ) to open the left supply port  80  (or  800 ) and hydraulic fluid is then released through said port via hydraulic line  42  to double-acting hydraulic cylinder  28 . Hydraulic fluid is released into second chamber  86  of hydraulic cylinder  28 , which causes the hydraulic cylinder rod  62  to move and in turn causes the steerable wheels to rotate or turn into the slope, i.e. towards area “B”. 
   It is understood that other hydraulic or pneumatic cylinders could be used in the present invention and are within the scope of the present invention. For example, a retractable and extensible hydraulic cylinder is particularly useful when the towed farm implement has only one steerable wheel, but could also be adaptable to two wheels, having two tie rods extending from the same end. It is also within the scope of the present invention to use more than one hydraulic cylinder, for example, each steerable wheel could be operated by its own hydraulic cylinder. 
   Once the steerable wheels are rotated or turned into the slope the sensor member  18  goes back to its neutral position and the centre supply port  78  of steering control valve  40  is open and hydraulic fluid from the diverter valve  34  is simply recycled back to hydraulic fluid reservoir  30 . Thus, the steerable wheels  24  and  26  will be “held” in this corrected position during the course of the tractor traversing the slope. However, once tractor  10  and seeding device  16  encounter level ground again, the rotated steerable wheels will cause sensor member  18  to rotate again, this time in the same direction as the steerable wheels  24  and  26 . This will cause hydraulic fluid to be supplied to hydraulic cylinder  28  thus rotating the steerable wheels back into alignment with the direction of travel of tractor  10 . 
   Sensor member  18  also acts in a similar fashion as above when the operator wishes to seed while changing direction. In particular, the present steering device is useful when tractor  10  must be manoeuvred around an obstruction such as a slough or telephone pole or the like. When seeding while changing direction, the tendency is for tractor  10  to pull the trailing, towed implement, seeding device  14 , off line and hence it will cut across the corner. This will cause sensor member  18  to pivot relative to horizontal bars  60 . 
   By way of example, when tractor  10  is turning around an obstruction on the left side of the obstruction, sensor member  18  pivots to the right, causing hydraulic cylinder rod  62  to shift position, thus turning or rotating the steerable wheels  24  and  26  to the left. This will ensure that the seeding device  14  manoeuvres around the obstruction and doesn&#39;t simply “cut the corner” of the turn. 
   In another preferred embodiment, the trailed agricultural implement can be further equipped with a third valve means, in particular, a centering valve  48 . Centering valve  48  is a 4 port-3 position valve, for example, a Walvoil SD4/1(KG3-120)/1CP18L-SAE valve. Centering valve  48  is operably connected to hydraulic cylinder  28  and is operable when seeding device  14  is not in seeding operation. 
   Two circumstances where centering valve  48  is useful are (1) when tractor is transporting or towing the seeding device to and from the field, and (2) when the seeding device has finished a pass of seeding (i.e. reached the end of the field) and now the tractor must turn the seeding device around for the next seeding pass. During both of these circumstances, the seeding device is not in operational (seeding) mode, i.e. the steering device of the present invention has been deactivated. 
   In the towing or transport situation, it is desirable to have the steerable wheels in a straight-ahead position. This is accomplished by the operator turning off the steering device of the present invention causing the hydraulic fluid to flow through supply port  74  of diverter valve  70  instead of supply port  72 . The hydraulic fluid is transported to receiving port  88  of centering valve  48  via hydraulic line  46 . In other words, in this embodiment, when the steering is turned off, the diverter valve  70  automatically turns “on” the centering valve. An alternate embodiment would be to have a diverter valve operable to engage either the centering device, the steering device or none at all. 
   Operably connected to centering valve  48  is activating lever  50 , which is attached at one end to either of tie rods  52  such that when the steerable wheels  24  and  26  are straight ahead, activating lever would be at 90° relative to the tie rods  52 . This is considered the centred position, whereby centre supply port  90  would be opened and hydraulic fluid would simply be recycling back to the hydraulic fluid reservoir  30 . 
   If the steerable wheels are not at 90° when the operator wishes to transport the farming implement, the operator would first have to activate the centering valve. Because the steerable wheels are not at 90°, activating lever  50  will also not be at 90° relative to the tie rods  52 . Thus, depending upon which direction the steerable wheels are turned, either supply port  92  or supply port  94  will open to provide hydraulic fluid to either chamber  86  or chamber  84  of double-acting hydraulic cylinder  86  via hydraulic lines  66  and  64 , respectively. Cylinder rod  62  will in turn shift, which will cause steerable wheels to rotate until they eventually are at 90° to the frame  22 . Once this occurs, activating lever  50  will go back to being perpendicular to the tie rods  52 , thereby causing the centering valve  48  and hydraulic fluid to bypass back to the tractor reservoir  30 . 
   With reference now to  FIGS. 4 and 5 , the preferred embodiment of the ground engaging sensor member  18  is shown. Ground engaging sensor member  18  comprises top plate  96  and bottom plate  104 . Permanently mounted (welded) to top plate  96  is mounting plate  98 , which attaches sensor member to one of the horizontal bars  60  of main frame  22 . Side plates  100  are provided for support and make the sensor member  18  more rigid. Rotational stops  102  are also provided to limit the rotation of sensor member  18 . 
   Sandwiched between top plate  96  and bottom plate  104  is a wear plate  106 , which is preferably made from ultra-high molecular weight nylon (UHMW) that has a low coefficient of friction. Permanently mounted (welded) to bottom plate  104  are arm lugs  110  and, in one embodiment, a hydraulic cylinder mount  108 . 
   Preferably, top plate  96 , wear plate  106  and bottom plate  104  are held together by a bolted connection. A bearing is shown in  FIG. 5  as a preferred embodiment. A threaded connection would tighten against the bearing to ensure smooth rotation of the sensor member  18 . 
   Sensor member  18  further comprises a pair of downwardly extending sensor arms  112 , which are attached to the arm lugs  110  by means of a pinned connection. Sensor wheel  116  is attached to sensor arms  112  by means of a hub and spindle assembly to ensure free rolling of sensor wheel  116  on the ground. It is understood that instead of a sensor wheel, rolling coulters or discs, or a stationery skid could be used. 
   To ensure that sensor member  18  is engaging the ground at all times, in a preferred embodiment a hydraulic cylinder  114  is attached to cylinder mount  108  at the cap end of the cylinder by means of a pinned connection, and at the rod end of the cylinder to the sensor arms  112 , also by means of a pinned connection. Those commonly skilled in the art would understand that a hydraulic pressure would need to be supplied to hydraulic cylinder  114 , as well as including in the system a pressure relief valve to relieve excessive pressure from the hydraulic cylinder  114 . Excessive pressure may develop if sensor member  18  were to encounter an obstacle, where it is necessary for sensor member  18  to “trip” over such obstacle. Although only one biasing element is shown, it is implied that two biasing elements could also be used. It is understood that other means for providing a force to keep sensor member in constant engagement with the ground would also work, for example, a spring or other type of biasing member. In addition, it would be possible to add mass to sensor member  18  by filling tire  116  with a fluid (i.e. calcium solution), adding sufficient mass that gravity would work as a method of ensuring constant engagement with the ground surface. 
   With reference now to  FIG. 6  and  FIG. 7 , illustrated here is the means by which steering control valve  400  is linked to sensor member  18  for operation. Steering control valve  400  comprises mounting plate  122 , which is welded to mounting plate  98 . Steering control valve  400  is operably connected to pivot pin  120  such that rotation of pivot pin  120  will cause either the left supply port or the right supply port of steering control valve  400  to be open. When either supply port is open, hydraulic fluid will be released through hydraulic lines  42  or  44  to hydraulic cylinder. 
   With reference now to  FIGS. 8 ,  9 ,  10 ,  11  and  12 , one embodiment of the ground engaging steerable wheels of the present invention will be described. Essentially, steerable wheels  24  and  26  are attached to axle  54  by conventional castoring means. With reference first to  FIGS. 8 and 9 , bottom castoring assembly  124  comprises castor pivot shaft  126 , which is permanently attached (welded) to bottom plate  128 . Also permanently attached to bottom plate  128  are two spindle mounts  130 . Additionally, securing hardware is also permanently attached to bottom plate  128 , which would allow the spindle  138  to be removed should it become damaged. Alternatively, spindle  138  could be permanently attached (welded) to the spindle mounts  130 . 
     FIGS. 10 ,  11  and  12  show one of the steerable wheels  24  attached to spindle  138  in conventional way. Axle  54  comprises at each end gusset  136  and castor top plate  134 . Sandwiched between top plate  134  and bottom plate  128  is wear plate  142 , which is preferably made from ultra-high molecular weight nylon (UHMW) that has a low coefficient of friction. Pivot shaft  126  passes through wear plate  142  and top plate  134  and is secured by collar  140  and a pin (not shown), pivotally connecting bottom castoring assembly  124  and top castor plate  134 . 
   Tie rod  52  attaches to bottom plate  128  as best shown in  FIG. 11 . Bottom castoring assembly  124  is free to pivot about pivot shaft  126 , which results in the direction of travel of wheel  24  to be altered with respect to the main frame&#39;s direction of travel. 
   With reference now to  FIG. 13 , another embodiment of the present invention is shown. The work vehicle, tractor  1000 , and the towed implement, the seeding device  1400 , are equipped with global positioning antennae  150  and  152 , respectively. In a preferred embodiment, global positioning antenna  150  is placed in the centre of tractor  1000  and global positioning antenna  152  is placed in the centre of the seeding device  1400 . It is understood, however, that any location would work if the appropriate software were provided. GPS receiver/CPU  154  can track the precise location of the centre of the tractor  1000  and the same receiver/CPU  154  can also identify the precise location of the centre of the seeding device  1400 , thus will be able to determine if both the tractor  1000  and the seeding device  1400  are in alignment and following the same path. 
   If the seeding device  1400  starts to sideslip when the tractor  1000  is traversing a slope, the path of the seeding device will now be different from the path of the tractor. Depending upon the direction of the slope, the path of the seeding device  1400  will be to the right or to the left of the tractor  1000 . The receiver/CPU will be able to determine exactly to what degree the seeding device  1400  is off course from the tractor  1000  and will send an electrical signal that will activate an electromagnetic hydraulic steering control valve  4000 , which would operate in much the same fashion as steering control valve  40 , described above. The steering control valve would activate double-ended hydraulic cylinder  2800 , causing steerable wheels  2400  and  2600  to rotate accordingly. 
   While various embodiments in accordance with the present invention have been shown and described, it is understood that the same is not limited thereto, but is susceptible to numerous changes and modifications as known to those skilled in the art, and therefore the present invention is not to be limited to the details shown and described herein, but is intended to cover all such changes and modifications as are encompassed by the scope of the appended claims.