Abstract:
A hitch on a vehicle is raised and lowered by a hydraulic actuator controlled by an electrically operated valve. A control system receives a command that indicates a designated velocity and uses the command to operate the valve. Based on a reference external force exerted on the hitch, the control system is configured with relationships for converting a plurality of command values to corresponding electric current levels for operating the valve. The control system compensates for effects due to differences between the actual force acting on the hitch and the reference external force. Velocity feedback adjusts the electric current level applied to the valve. The passive load force control provides a predictor of the hitch load force to eliminate overshoot/undershoot of hitch motion. During hitch motion, the velocity feedback also compensates for effects due to load and hitch geometry changes that occur.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to earth-working equipment, such as an agricultural implement pulled by a tractor; and more particularly to a method for controlling a hydraulic system that operates a hitch that couples the implement to the tractor. 
         [0005]    2. Description of the Related Art 
         [0006]    A variety of agricultural implements are available to be pulled by a tractor for working earth in a farm field in which crops will be or have been planted. The implement is connected to a standard three-point hitch with right and left drag links on the rear of the tractor and the hitch can be operated to raise and lower the implement. The hitch is raised and lowered hydraulically by piston-cylinder assembly that is operated by a valve that controls the flow of fluid to and from the piston-cylinder assembly. 
         [0007]    A conventional tractor has a control panel by which the operator sets desired upper and lower positions for the hitch and a desired velocity at which the hitch should travel up and down. The operator then activates an input device to select raising or lowering the hitch. An electronic control system responds to that switch by operating an electrohydraulic valve to drive the piston-cylinder assembly so that the hitch moves in the designated direction and speed until the hitch reaches the selected position at which time the valve is closed. Specifically the electronic control system applies a given level of electric current to the electrohydraulic valve which opens the valve a corresponding degree thereby providing a related amount of fluid flow through the valve. 
         [0008]    Electrohydraulic hitch valves typically have been designed with a mechanical flow compensator on the raise function. The flow compensator provides a constant flow rate (raise rate) at a given valve current regardless of the load on the hitch arms and regardless of other pressure demands of the hydraulic system. Flow compensation usually is not implemented on lower function of the hitch valve. This results in variable lowering rates for a given valve current. The lower rate varies due to different loads being placed on the hitch, as well as due to changes in hitch geometry as the hitch arms change position. The use of a mechanical flow compensation technique similar to that used during raise could provide a constant lowering rate for a given valve current, but doing so would add cost and complexity to the valve assembly. 
         [0009]    As a consequence, there is a need for a hydraulic control system that provides flow compensation during both raise and lower operations. 
       SUMMARY OF THE INVENTION 
       [0010]    A vehicle, such as a farm tractor, for example, has a hitch for towing an implement that can be raised and lowered by movement of the hitch. The hitch is moved by operating a valve to control the flow of fluid to and from a hydraulic actuator which is mechanically coupled to the hitch. 
         [0011]    To operate the hitch, a hitch command is received from a device manipulated by the tractor operator, wherein the hitch command indicates a designated velocity for the hitch. A first error value is produced that denotes deviation of the force acting on the hitch from a reference force level. For example, one or more force sensors can be attached to the hitch to detect the force acting thereon. The hitch command is altered in response to the first error value, thereby producing a first adjusted command. The hitch is moved in response to the first adjusted command. 
         [0012]    A second value is produced that relates to an actual velocity at which the hitch is moving. For example, a sensor can be attached to the hitch to provide a signal from which the actual velocity can be determined. A second error value denoting deviation of the actual velocity of the hitch from the commanded hitch velocity is derived. The first adjusted command is altered in response to the second error value, thereby producing a second adjusted command. The valve then is operated in response to the second adjusted command. 
         [0013]    One embodiment of a vehicle, that incorporates the present hitch control method, has an electrically operated valve. A control system on the vehicle converts the operator command, providing the designated velocity, into an electric current level for operating the valve. The control system is configured with relationships between a hitch motion command values and electric current levels based on a reference force level is acting on the hitch. The present method for controlling the hitch adjusts for effects that deviation of the actual exerted external force from the reference force level has on operation of the valve and the hydraulic actuator. 
         [0014]    The use of passive load force control provides a predictor of the hitch load to eliminate the overshoot/undershoot from active control. Active velocity correction further compensates during hitch motion travel to account for any load or hitch geometry changes that occur. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  illustrates a tractor pulling an agricultural implement; 
           [0016]      FIG. 2  shows a typical three-point hitch on the tractor for attaching the implement; 
           [0017]      FIG. 3  is a block diagram of an electrohydraulic system for operating the three-point hitch; and 
           [0018]      FIG. 4  is a flowchart depicting the flow compensation technique according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    With initial reference to  FIGS. 1 and 2 , an implement  10 , such as a multiple blade agricultural plow, is connected by a three-point hitch  12  to the rear of a tractor  14 . The hitch  12  comprises right and left drag links  16  and  18 , the proximal ends of which are pivotally attached to the tractor frame  17  by pins  15 . A pair of lift arms  20  and  22 , connected to the drag links  16  and  18  by lift links  24  and  25 , control the elevation of the drag links. Two hydraulic actuators  27  and  28 , in this case single acting lift hydraulic cylinders, are connected between the lift arms  20  and  22  and the tractor frame  17  to pivot the lift arms up and down with respect to that frame. 
         [0020]    The distal ends of the drag links  16  and  18  are respectively attached to vertically extending legs  29  and  30  of a coupler  26  that has a cross bar  32  connected between the upper ends of the legs. A link hydraulic cylinder  34  is attached at one end to the cross bar  32  and at the other end to the tractor frame  17  by a pin  35 . A pair of lower lift hooks  36  and  38  project rearward from the bottom ends of legs  29  and  30  and an upper lift hook  40  is positioned in the middle of a cross bar  32 . The lift arms  20  and  22  move the coupler  26  bi-directionally along a principal axis “A” of coupling motion, which in this case is vertical. 
         [0021]    The lower and upper lift hooks  36 ,  38  and  40  cooperate with mating parts on a hitch structure of the implement  10 . Specifically the lower lift hooks  36  and  38  engage the lower hitch pins that extend laterally with respect to the frame of the implement. The implement also has a laterally extending upper hitch pin that is received in the upper lift hook  40  when the implement  10  is coupled to the tractor  14 . The trio of lift hooks  36 ,  38  and  40  form the three points of the hitch  12 . 
         [0022]    With reference to  FIG. 3 , the electrohydraulic control system  50  for operating the three point hitch  12  comprises a hydraulic section  52  and an electronic section  53 . The hydraulic section  52  includes a tank  54 , which holds hydraulic fluid, and a pump  56 , that when driven by the engine of the tractor  14  sends pressurized hydraulic fluid from the tank through a supply line  58 . A supply line  58  is connected to a three-position, three-way electrohydraulic valve  60  and a tank return line  62  couples the valve to the tank  54 . The valve  60  has a workport  63  connected to two lift hydraulic actuators  27  and  28 , such as a pair of single-acting piston-cylinder assemblies having head chambers to which the workport is connected. It should be appreciated that the present flow compensation technique can be used with hydraulic sections having other configurations, such as where the valve workport  63  is connected to the cylinder rod chambers, those having only one hydraulic actuator, and those with one or two double-acting hydraulic actuators. 
         [0023]    The valve  60  is operated by a solenoid  64  that is energized by an electric current from a controller  65  within the electronic section  53  of the control system  50 . The controller  65  may be a microcomputer-based device that includes processor  66  which executes instructions of a software control program, to be described, and a memory  67  for storing the instructions and data for the control program. A valve driver  69  responds to commands from the processor  65  by producing a variable electric current level for driving the solenoid  64  to proportionally operate the electrohydraulic valve  60 . The controller  65  further comprises an input/output (I/O) circuit  68  that has analog and digital ports to receive input signals from sensors and to interface with other devices on the tractor. 
         [0024]    The input/output circuit  68  receives a signal from a position sensor  70  that indicates the vertical position of the coupler  26  of the three point hitch  12 . Any of various types of sensing mechanisms located at any of several locations on the hitch can be employed. For example, the position sensor  70  may be a linear device connected to one of the lift hydraulic actuators  27  or  28  to produce a signal as the piston rod extends and contracts from the cylinder body. Alternatively, a rotational type position sensor can be connected to one of the lift arms  20  or  22  to provide a signal indicating the rotational position of that arm with respect to the tractor frame  17 . With both of these sensing techniques, the signal from the position sensor  70  indicates a position that is geometrically related to the vertical position of the hitch coupler  26  with respect to the tractor frame  17 . 
         [0025]    The input/output circuit  68  also receives signals from right and left force sensors  71  and  72 . For example, these sensors are standard clevis pin type sensors commonly incorporated into the pins  15  that couple the left and right drag links  16  and  18  to the tractor frame  17 . The force sensors  71  and  72  detect the load force that is exerted by an implement attached to the hitch. Because that the load force results the weight of the implement due to gravity, the load force is directed downward and tends to lower the hitch  12 . The present control system  50  is being described in the context of left and right sensors which have the advantage of measuring the different forces exerted on the lateral sides of the three-point hitch  12  by the implement of other apparatus attached to the hitch. Alternatively, a single clevis pin sensor can be used in the pin  35  that connects the link hydraulic cylinder  34  to the tractor frame  17 . Other types of sensors, sensing locations and sensing mechanisms can be employed to produce electrical signals indicating the magnitude of the external force acting on the three point hitch  12 . For example, the force acting on the hitch can be detected by sensing the hydraulic pressure produced in the hydraulic actuators  27  and  28  by that force. 
         [0026]    A human interface  74  exchanges signals with the input/output circuit  68  of the controller  65 . This enables the operator of the tractor  14  to input configuration settings and send commands to the controller, thereby defining operation of the hydraulic section  52 . In particular as will be described, input switches  75  are used to select desired ultimate raised and lowered positions for the implement attached to the hitch  12 . A center-off, three-position, momentary contact toggle switch  76  enables the tractor operator to indicate that the hitch  12  should be raised and lowered. Other types of switches and input devices can be employed. The human interface  74  also has a display screen  77  by which information is presented to the tractor operator. 
         [0027]    A speed input device  78  enables the tractor operator to designate a velocity at which the hitch is to be raised and lowered. The manufacturer of the tractor  14  has determined a maximum speed for lowering the hitch  12  and different positions of the speed input device  78  indicate hitch lowering speeds within a range between a defined minimum hitch speed and that maximum speed. The maximum speed also is stored in the memory  67 , as a constant reference velocity value. The signal from the speed input device  78  indicates a percentage of the maximum speed. The control system  50  is calibrated by the tractor manufacturer so that when a constant reference force level is exerted on the hitch, each speed indicated within that range causes the valve driver  69  to send an electric current level to the valve  60  so that the hydraulic actuators are driven to achieve the desired hitch velocity. Specifically, that electric current level when applied to the solenoid  64 , opens the hydraulic valve  60  to produce a fluid flow there through that suitably operates the hydraulic actuators  27  and  28  to move the hitch at the designated velocity. The result of the calibration process is a set of relationships between hitch motion command values and electric current levels for properly moving the hitch when the reference force level is acting on the hitch. The memory  67  of the controller  65  stores a look-up table containing the set of relationships that will be use to convert hitch motion commands into electric current levels for operating the hydraulic valve. It should be understood that when a force other than the reference force level is exerted on the hitch, the velocity to electric current conversion is slightly inaccurate and the hitch may not move at the designated velocity. 
         [0028]    When the operator of the tractor  14  desires to raise or lower the hitch  12 , the operator moves the toggle switch  76  in one direction or the other from the center off position to indicate whether the hitch is to be raised or lowered. Assume for example that the hitch is to be lowered. Activation of the toggle switch  76  sends a hitch command that denotes the direction for hitch motion and a value indicating a desired speed as a percentage of the reference, or maximum speed. Thus that hitch command denotes a desired velocity for the hitch  12 . 
         [0029]    The controller  65  responds to the hitch command by executing a hitch control program  80  depicted by the flowchart in  FIG. 4 . The hitch control program  80  commences at step  82  by receiving the hitch command from the control panel  74 . Next at step  83 , the inputs from the left and right force sensors  71  and  72  are read by the controller  65  and processed to derive a net force referred to as the load force acting on the hitch  12 . One previous technique for deriving a load force simply averaged the right and left forces. Another technique calculated the load force according to the expression: 
         [0000]      Load Force=Maximum(Right Force,Left Force)+GAIN*abs(Right Force−Left Force)
 
         [0000]    where GAIN is a predefined factor that specifies the sensitivity of the force difference. 
         [0030]    Then at step  84 , a determination is made whether the hitch command designates that the hitch should be lowered or raised and in response the program execution branches to either step  85  or  86 , respectively. When the hitch  12  is lowering, the force of gravity acting on the implement  10  adds to the force from the hydraulic actuators to assist in lowering the hitch. Therefore, if the actual load force acting on the hitch is greater than a reference force level, that the manufacturer used to configure the hydraulic control system  50  and define the relationships between hitch motion command values and electric current levels, that additional force causes the hitch to lower at a faster rate than is designated by the hitch command from the operator. Similarly if the actual load force is less than that reference force level, the hitch will move downward at a slower rate than designated by the hitch command. Therefore, the present method compensates for the effects of those force differences by deriving a load error value E L  based on the reference force level. In the lowering mode, the hitch control program  80  branches to step  85  where the load error value E L  is the square root of the ratio of the reference force level over the actual load force derived from the force sensors  71  and  72  at step  83 . 
         [0031]    In the hitch raising mode, the effects due to the actual load force differing from the reference force level are inverted and the force of gravity acting on the implement counteracts the force from the hydraulic actuators and thus the hitch motion. Therefore, a greater actual load force than the reference force level causes the control system  50  to raise the hitch at a slower rate than desired, and a lesser actual load force than the reference force level causes the control system  50  to raise the hitch faster than the designated rate. Thus in the hitch raising mode, step  86  is executed instead of step  85  and the force ratio used to derive the load error value E L  is the square root of the actual load force over the reference force level. 
         [0032]    The load error value E L  then is employed at step  87  to adjust the operator&#39;s hitch command. This is accomplished by multiplying the hitch command from the human interface  74  by the load error value E L  to produce a first adjusted command. The first adjusted command has a value that is compensated for the effect that deviation of the actual load force from the reference force level has on hitch motion. The first adjusted command has a smaller value than the original hitch command when either the actual load force is greater than the reference force level in the hitch lowering mode, or the actual load force is less than the reference force level in the raising mode. In those situations, the actual load force assists the desired hitch motion and less than the calibration hydraulic force is needed to move the hitch at the designated velocity. Inversely, the first adjusted command has a larger value than the original hitch command when the actual load force either is smaller than the reference force in the hitch lowering mode, or is greater than the reference force in the raising mode. In those latter situations the actual load force counteracts the desired hitch motion and more than the calibration hydraulic force is needed to move the hitch at the designated velocity from the operator command. 
         [0033]    At step  88  the first adjusted command is converted into an electric current level using the motion command value to electric current level relationships established during control system calibration for the given reference load force acting on the hitch. That electric current level is applied by the valve driver to the solenoid  64  of the hydraulic valve  60  at step  89 . This results in a fluid flow through the hydraulic valve that drives the actuators  27  and  28  causing hitch  12  to begin moving. 
         [0034]    Producing the first adjusted command in the manners described above results in production of an electric current level that operates the valve  60  to compensate for actual load forces that are different than the given reference load force used during calibration. Thus, when the actual load force is such that the hitch does not require as much hydraulic force to move at the desired speed, a smaller electric current level is derived using the calibrated command to electric current relationship than would be produced for the given reference load force. In response to that smaller electric current level, the valve  60  opens less to apply a lower flow rate to the hydraulic actuators  27  and  28 . When the actual load force is such that the hitch requires more hydraulic force to move at the desired speed, resultant electric current level is larger and the valve  60  opens more to apply a higher flow rate to the hydraulic actuators. 
         [0035]    The control program  80  then advances to a section that provides velocity feedback control which determines any error between the actual velocity at which the hitch  12  is moving and the desired velocity as indicated by the original hitch command. Such an error then is used to alter the first adjusted command so that the electric current applied to the hydraulic valve  60  will result in the hitch moving at the desired velocity. 
         [0036]    This section of the control program  80  commences at step  90  at which the controller  65  reads the signal from the position sensor  70  to obtain an indication of the position of the hitch. At step  92 , the derivative of the position signal is calculated to determine the actual velocity of the hitch. Other sensors and sensing techniques can be employed to detect the actual velocity of the hitch  12 . Next at step  94 , any difference between the actual velocity and the velocity indicated by the original hitch command is determined, thereby producing a velocity error value E V . Note that the original hitch command is expressed as a percentage of the reference velocity, e.g. that maximum velocity defined by the tractor manufacturer for system configuration. Therefore, the arithmetic expression at step  94  uses a ratio of the actual velocity to that reference velocity to determine the velocity error value from the hitch command. 
         [0037]    At step  95 , the velocity error value is summed with the first adjusted command to produce a second adjusted command that indicates a command value that is necessary for the control system to produce an electric current to properly drive the valve  60  in a manner that achieves the hitch velocity desired by the tractor operator. In other words, if the actual velocity determined at step  92  is less than the desired velocity, the velocity error value E V  will be positive and produces a second adjusted command that is greater than the first adjusted command. In this case, the valve will open slightly more to drive the hydraulic actuators  27  and  28  a greater amount. In the opposite case, in which the actual velocity is greater than that desired by the tractor operator, the velocity error value E V  will be negative. That negative velocity error value produces a second adjusted command that is less than the first adjusted command so that the valve is closed slightly to drive the hydraulic actuators  27  and  28  less vigorously to achieve the desired hitch velocity. 
         [0038]    The second adjusted command then is converted at step  96  into a corresponding electric current level using the calibrated hitch motion command value to electric current level relationships defined during control system configuration. The so derived electric current level then is applied to the valve driver  69  at step  98  to properly operate the valve  60  and drive the hydraulic actuators accordingly. 
         [0039]    The control program  80  then advances to step  99  where the actual position of the hitch  12 , that was sensed at step  90 , is compared to the desired ultimate position as set by the tractor operator via input switches  75 . If the hitch  12  has not reached the desired ultimate position, the program execution returns to step  90  for another pass through the velocity feedback section. Eventually the hitch  12  will reach the desired ultimate position at step  99  causing the control program to terminate. 
         [0040]    The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.