Draft control method for operating an earth moving machine
View Patent ↗The hitch on a vehicle is moved by a hydraulic actuator. A method for controlling the hydraulic actuator defines a draft force setpoint in response to separately averaging two forces acting on lateral sides of the hitch, while operating in a configuration mode. During regular operation, a draft load is calculated from sensing those two forces and deriving a draft force error from difference between the draft load and the draft force setpoint. The draft force error is used to control fluid flow to and from the hydraulic actuator. One aspect of the control method derates the draft force error as the hitch moves beyond a predefined threshold position.
1. A method for controlling a hitch on a vehicle, wherein the hitch is raised and lowered by a hydraulic actuator, said method comprising:
defining a draft setpoint that specifies an amount draft force desired to be exerted on the hitch;
sensing a first draft force acting on one side of the hitch;
sensing a second draft force acting on another side of the hitch;
deriving a draft load as a function of a greater one of the first draft force and the second draft force and of a difference between the first and second draft forces;
producing a draft force error in response to the draft load and the draft setpoint; and
operating the hydraulic actuator in response to the draft force error to selectively raise and lower the hitch.
2. The method as recited in claim 1 wherein defining a draft setpoint comprises operating the vehicle while:
sensing a first plurality of samples of the first draft force;
sensing a second plurality of samples the second draft force;
calculating a first average of the first plurality of samples;
calculating a second average of the second plurality of samples;
deriving the draft setpoint as a function of a greater one of the first average and the second average and of a difference between the first and second averages.
3. The method as recited in claim 2 wherein deriving the draft setpoint adds the greater one of the first average and the second average to a term based on an absolute value of the difference between the first and second averages.
4. The method as recited in claim 2 wherein deriving the draft setpoint employs the following equation:
Draft Setpoint=Maximum(first average,second average)+Gain*abs(first average−second average)
where the “Maximum” term selects the greater of the first and second averages, the “abs” term selects the absolute value of the difference between the first and second averages, and the Gain is a factor that defines a sensitivity for the difference.
5. The method as recited in claim 1 wherein deriving a draft load adds the greater one of the first draft force and the second draft force to a term based on an absolute value of the difference between the first and second draft forces.
6. The method as recited in claim 1 wherein deriving a draft load employs the following equation:
Draft Load=Maximum(first draft force,second draft force)+Gain1*abs(first draft force−second draft force)
where the “Maximum” term selects the greater of the first and second draft forces, the “abs” term selects the absolute value of the difference between the first and second draft forces, and the Gain1 is a factor that defines a sensitivity for the difference.
7. The method as recited in claim 1 further comprising defining a threshold position for the hitch; and derating the draft load in response to an amount that an actual position of the hitch exceeds the threshold position.
8. The method as recited in claim 1 wherein there is a position limit for motion of the hitch, and further comprising:
defining a threshold position for the hitch;
producing a hitch draft load value in response to the draft load, a first relationship between an actual position of the hitch and the position limit, and in response to a second relationship between the threshold position and the position limit; and
producing the draft force error in response to the hitch draft load value.
9. The method as recited in claim 8 wherein producing a hitch draft load value comprises deriving a first difference between the actual position of the hitch and the position limit, and deriving a second difference between the threshold position and the position limit.
10. The method as recited in claim 8 wherein producing a hitch draft load value employs the following equation:
Hitch
Draft
Load
=
(
Position
Limit
-
Actual
Position
Position
Limit
-
Upper
Position
Threshold
)
*
Draft
Load
.
11. A method for controlling a hitch on a vehicle, wherein the hitch can be moved between a first position limit and second position limit by a hydraulic actuator, said method comprising:
defining a draft setpoint that specifies an amount of draft force desired to be exerted on the hitch;
defining a threshold position between the first and second position limits;
sensing a first draft force acting on one side of the hitch;
sensing a second draft force acting on another side of the hitch;
deriving a draft load as a function of the first draft force and the second draft force;
when an actual position of the hitch is less than the threshold position, setting a hitch draft load value equal to the draft load;
when the actual position of the hitch is greater than the threshold position, setting the hitch draft load value in response to the draft load, a first relationship between an actual position of the hitch and the first position limit, and a second relationship between the threshold position and the first position limit; and
producing the draft force error in response to the hitch draft load value and the draft setpoint; and
operating the hydraulic actuator in response to the draft force error to selectively raise and lower the hitch.
12. The method as recited in claim 11 wherein, when the actual position of the hitch is greater than the threshold position, setting the hitch draft load value comprises deriving a first difference between the actual position of the hitch and the first position limit, and deriving a second difference between the threshold position and the first position limit.
13. The method as recited in claim 11 wherein, when the actual position of the hitch is greater than the threshold position, setting the hitch draft load value employs the following equation:
Hitch
Draft
Load
=
(
First
Position
Limit
-
Actual
Position
First
Position
Limit
-
Upper
Position
Threshold
)
*
Draft
Load
.
14. The method as recited in claim 11 wherein defining a draft setpoint comprises operating the vehicle while:
sensing a first plurality of samples of the first draft force;
sensing a second plurality of samples the second draft force;
calculating a first average of the first plurality of samples;
calculating a second average of the second plurality of samples;
deriving the draft setpoint as a function of a greater one of the first average and the second average and of a difference between the first and second averages.
15. The method as recited in claim 14 wherein deriving the draft setpoint adds the greater one of the first average and the second average to a term based on an absolute value of the difference between the first and second averages.
16. The method as recited in claim 14 wherein deriving the draft setpoint employs the following equation:
Draft Setpoint=Maximum(first average,second average)+Gain*abs(first average−second average)
where the “Maximum” term selects the greater of the first and second averages, the “abs” term selects the absolute value of the difference between the first and second averages, and the Gain is a factor that defines a sensitivity for the difference.
17. The method as recited in claim 11 wherein the draft load is derived as a function of a greater one of the first draft force and the second draft force and of a difference between the first and second draft forces.
18. The method as recited in claim 11 wherein deriving a draft load adds a greater one of the first draft force and the second draft force to a term based on an absolute value of a difference between the first and second draft forces.
19. The method as recited in claim 11 wherein deriving a draft load employs the following equation:
Draft Load=Maximum(first draft force,second draft force)+Gain1*abs(first draft force−second draft force)
where the “Maximum” term selects the greater of the first and second draft forces, the “abs” term selects the absolute value of the difference between the first and second draft forces, and the Gain1 is a factor that defines a sensitivity for the difference.
20. A method for controlling a hitch on a vehicle, wherein the hitch can be moved between a first position limit and second position limit by a hydraulic actuator, said method comprising:
defining a draft setpoint that specifies an amount of draft force desired to be exerted on the hitch;
defining a threshold position between the first and second position limits;
sensing a first draft force acting on one side of the hitch;
sensing a second draft force acting on another side of the hitch;
deriving a draft load as a function of the first draft force and the second draft force;
when an actual position of the hitch is less than the threshold position, setting a hitch draft load value equal to the draft load;
when the actual position of the hitch is greater than the threshold position, setting the hitch draft load value derived as a function of the draft load and an amount that the actual position of the hitch exceeds the threshold position; and
producing the draft force error in response to the hitch draft load value and the draft setpoint; and
operating the hydraulic actuator in response to the draft force error to selectively raise and lower the hitch.