Abstract:
A method is provided for automatically determining a hitch raise rate calibration value for a hitch control system having a hydraulic actuator for moving the hitch, a valve for controlling flow of hydraulic fluid to the actuator and an electronic hitch control unit. The method includes applying a first control signal to the valve to cause the hitch to raise, determining a first hitch velocity as the hitch moves in response to the first control signal, and repeating these steps for a second control signal. The raise rate calibration value is then calculated as a function of a desired raise velocity, the first and second control signals and the first and second velocities.

Description:
BACKGROUND  
       [0001]     The present invention generally relates to an electro-hydraulic hitch control system for controlling a hitch mounted on an agricultural tractor, and more particularly, relates to a method of calibrating a control parameter of such a hitch control system.  
         [0002]     Currently available hitch control systems have various parameters which must be determined or calibrated and stored in order for the hitch control system to function as desired.  
         [0003]     For example, U.S. Pat. No. 5,012,415, issued to Boe et al., in 1991 describes a hitch control system raise rate calibration method which requires the operator to manipulate the hitch control lever and a drop rate potentiometer.  
         [0004]     U.S. Pat. No. 4,931,967, issued to Boe et al., in 1990 describes a hitch control system having a calibration and configuration algorithm which operates to establish the sensor ranges of all sensors installed and disables certain configurable features if the associated sensors are not present.  
         [0005]     U.S. Pat. No. 5,472,056, issued to Orbach in 1995 describes a hitch assembly and control system and calibration methods therefore. The calibration methods determine and store calibration parameters relating to lower hitch position, upper hitch position, hitch lower threshold and hitch raise threshold.  
         [0006]     U.S. Pat. No. 5,918,195, issued to Halgrimson et al., in 1999, describes a system for calibrating a control system command device by moving the command device into predefined positions and storing sensed calibration values.  
         [0007]     U.S. Pat. No. 5,810,095, issued to Orbach et al., in 1998, describes a hitch control system wherein valve control signal threshold values are modified in response to detection of undesired hitch movement.  
         [0008]     But, none of the systems or methods described in these patents provides any system or method for determining hitch raise rate calibration parameters.  
         [0009]     For prior production of John Deere tractor hitch control systems, the hitch raise rate calibration parameter was experimentally determined. In particular, a small group of sample tractors having different combinations of hitch cylinder sizes and raise valves was chosen. For each sample tractor, a range of different raise valve currents was tested to determine what raise valve current would cause the hitch to fully raise in 3 seconds. An average number was determined for groups of vehicles with the same cylinder size and raise valve. A lookup table of these numbers and the corresponding cylinder sizes and valves was stored in the hitch control units on the tractors. Then, during the manufacturing process of each tractor, a technician selected and entered into the hitch control unit data for the cylinder and valve for that particular tractor, and the hitch control unit then selected or enabled and used the corresponding valve raise rate current. However, it was time consuming to test multiple different experimental raise current values. Also, the experimentally determined average number may not produce the desired raise rate for a particular tractor. Also, the technician may enter incorrect data pertaining to cylinder size and valve.  
         [0010]     If the raise rate calibration value is determined during field operation, inaccuracies may be caused by varying conditions and limitations on the number of changes allowed to the available nonvolatile memory hardware. For example, the system may set the calibration value artificially high due to low engine speed or other conditions. Then, when these conditions are adjusted or removed, the hitch will raise at an excessive rate for a moment with a rapid deceleration as the control system adjusts to these new conditions. Such changes in acceleration can cause excessive jerk that can cause customer dissatisfaction.  
       SUMMARY  
       [0011]     Accordingly, an object of this invention is to provide a method for rapidly determining an accurate hitch raise rate calibration value.  
         [0012]     A further object of the invention is to provide such a method which is reliable.  
         [0013]     A further object of the invention is to provide such a method which is not subject to operator error.  
         [0014]     These and other objects are achieved by the present invention, wherein a method is provided for automatically determining a hitch raise rate calibration value for a hitch control system having a hitch mounted on a vehicle, a hydraulic actuator for moving the hitch, a hitch position sensor, a valve for controlling flow of hydraulic fluid to the actuator and an electronic hitch control unit for supplying valve control signals to the valve. The method includes applying a first estimated control signal to the valve to cause the hitch to raise, determining a first hitch velocity as the hitch moves in response to the first control signal, applying a second estimated control signal to the valve, determining a second hitch velocity as the hitch moves in response to the second control signal, and calculating the raise rate calibration value as a function of a desired raise velocity, the first and second control signals and the first and second velocities. The hitch velocities are determined by storing a hitch start position obtained from the position sensor, after a certain delay time period T storing a hitch end position, and calculating the hitch velocity by dividing the change in position by the time period. The method includes waiting for a certain time period between applying the estimated control signal and storing the hitch start position.  
         [0015]     The raise rate calibration value determined during this process is retained in nonvolatile memory in the hitch controller for use during normal hitch operations. Because raise valve flow rate is affected by variables such as hitch load, hydraulic oil temperature, engine speed, lift cylinder volumes and valve characteristics, this allows the control system to determine the raise rate calibration flow rate in a controlled environment where these variables are preset. In addition, the raise rate calibration value is determined automatically by the control unit, reducing the chances of operator error. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic diagram of a hitch control system; and  
         [0017]      FIGS. 2A-2C  show a logic flow diagram illustrating an algorithm executed by the ECU of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0018]     Referring to  FIG. 1 , a hitch system  10  includes a hitch  12 , such as a conventional 3-point hitch, to which tools and implements (not shown) can be integrally attached, as distinguished from towed implements which are coupled to a tractor drawbar (not shown). The hitch  12  is raised and lowered by conventional hitch cylinders  14 . Conventional electro-hydraulic raise and lower valves  16 ,  18  control communication between pump  20 , reservoir  22  and cylinder  14 . Valves  16  and  18  are controlled by valve command signals generated by a microprocessor-based electronic control unit  24  which includes conventional valve drivers (not shown). Control unit  24  receives signals from a conventional external raise/lower switch  26 , and parameter signals including a hitch position signal from a conventional hitch position sensor  28 , a draft force signal from a conventional draft force sensor  30 , a vehicle or wheel speed signal from a conventional vehicle or wheel speed sensor  32 . A hitch command lever unit  34  includes a lever  33  and a lever position sensor or transducer  35 . The control unit  24  also receives the lever position signal from sensor  35 .  
         [0019]     A conventional CAN bus  36  communicates the control unit  24  with a display/control unit  40 . Display/control unit  40  includes a controller  41 , display  42 , and a rotary encoder  44  which is controlled by a rotary control knob  46 . Control unit  40  also includes a set switch  50 , a lock switch  52 , a load/depth switch  54 , a height or raise limit switch  56  and a drop rate switch device  58 . Control unit  24  transmits signals from sensors and control devices  26 - 34  to control unit  40  via CAN bus  36 . Display/control unit  40  receives signals from encoder  44  and switches  50 - 58  and transmits the signals to display  42  and control unit  24 .  
         [0020]     The switches  50 - 58  are preferably implemented as touch switches on the display/control panel  40  utilizing known touch switch technology. Such a touch switch control/display panel is known and has been commercially available on production equipment, such as John Deere Model 7020 tractors. As in this known control panel, the control knob  46  is preferably also mounted on the control panel  40 .  
         [0021]     A load/depth mix signal associated with load/depth mix select switch  54  can be set by actuating switch  54  and adjusting the associated value with the rotary control knob  46 . The height limit signal associated with height limit select switch  56  can be set by actuating switch  56  and adjusting the associated value with the rotary control knob  46 . A drop rate signal associated with drop rate select switch  58  can be set by actuating switch  58  and adjusting the associated value with the rotary control knob  46 . Alternatively, load/depth, height limit and drop rate values could be set or adjusted using for each value a separate conventional rotary transducer coupled to control knob, such as used on production John Deere 7000 tractors.  
         [0022]     The control unit  24 , in response to the various inputs to it, generates the valve command signals according to an algorithm which is basically similar to the algorithm described in U.S. Pat. No. 4,979,092, which is incorporated by reference herein. For example, the control unit  24  generates, through a closed control loop, the valve command or hitch control signals as a function of signals representing the lever position, hitch position, draft force, wheel speed, load/depth mix, height limit and drop rate signals and as a function of various criteria. Most of these basic hitch control functions are known and have been available on commercially available tractors with non-spring centered command levers.  
         [0023]     When a valve control signal is applied to raise valve  16 , hydraulic fluid flows from pump  20  to cylinders  14 , extending cylinders  14  and raising the hitch  12 . The rate at which the hitch raises is a function of the rate of fluid flow from pump  20  and the volume of cylinders  14 , and the rate of fluid flow is controlled by valve  16  and as a function of the signal applied to valve  16 . It is necessary to determine and store (or calibrate) the particular valve control signal which will cause the hitch  12  to raise at a predetermined desired rate, the “raise rate”.  
         [0024]     In the present invention, the control unit  24  automatically determines and stores this raise rate calibration value by performing the algorithm  100  illustrated by the logic flow chart  FIG. 2 . The conversion of the above flow chart into a standard language for implementing the algorithm described by the flow chart in a digital computer or microprocessor, will be evident to one with ordinary skill in the art.  
         [0025]     Referring now to  FIG. 2 , in step  102  an initial current is applied to raise valve  16 , where this initial current is the current which will cause valve  16  to begin to open and allow fluid to flow (typically called the dead band current).  
         [0026]     Step  104  calculates a first new or estimated current which is equal to the initial current plus a current offset, so that the new current represents a first estimate of the current necessary to provide the desired hitch rate of movement. Step  106  applies the first estimated current to valve  16 .  
         [0027]     Step  108  causes the algorithm to wait until a timer (not shown) counts down for a time period T 1 , such as 0.10 seconds so that the hitch movement can reach a steady-state condition.  
         [0028]     Step  110  stores a first hitch start position P 1  obtained from position sensor  28 .  
         [0029]     Step  112  resets the timer.  
         [0030]     Step  114  causes the algorithm to wait until the timer (not shown) counts down again for a time period T 2 , such as 0.50 seconds, so that the hitch will have time to move in response to opening of the raise valve  16 .  
         [0031]     Step  116  stores a first hitch end position P 2  obtained from sensor  28 .  
         [0032]     Step  118  calculates a first hitch velocity V 1 =(P 2 −P 1 )/T 2 .  
         [0033]     Step  120  calculates a second estimated current which is equal to the initial current plus a second current offset, so that the second estimated current represents a second estimate of the current necessary to provide the desired hitch rate of movement.  
         [0034]     Step  122  applies the second estimated current to valve  16 .  
         [0035]     Step  124  causes the algorithm to wait until a timer (not shown) counts down for a time period T 1 , again so that the hitch movement can reach a steady state.  
         [0036]     Step  126  stores a second hitch start position P 1  obtained from position sensor  28 .  
         [0037]     Step  128  resets the timer.  
         [0038]     Step  130  causes the algorithm to wait until the timer (not shown) counts down again for a time period T 2 , again so that the hitch  12  will have time to move in response to the opening change of raise valve  16 .  
         [0039]     Step  132  stores a 2nd hitch end position P 2  obtained from sensor  28 .  
         [0040]     Step  134  calculates a 2nd hitch velocity V 2 =(P 2 −P 1 )/T 2 .  
         [0041]     Step  136  calculates a revised current offset value according to the following equation. 
 
REVOFFSET=[((Vdesired−V 1 )×2 nd  CURRENT)/(V 2 −V 1 )]−[((Vdesired−V 2 )×1st CURRENT)/(V 2 −V 1 )]
 
         [0042]     This revised OFFSET current value is then stored as the raise rate calibration value. Thereafter, when the hitch  12  is commanded to raise, a current equal to the deadband current plus the REVOFFSET current is applied to raise valve  16  so the hitch  12  will be raised at the desired velocity or rate Vdesired.  
         [0043]     While the present invention has been described in conjunction with a specific embodiment, it is understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.