Title:
Control system for hydrostatic pump
Kind Code:
A1


Abstract:
A control system that can be used on a hydrostatic pump has a feedback sensor to detect the angle of a swashplate. From the information gathered by the feedback sensor about the swashplate and another set command, this information is sent to a microprocessor. The microprocessor then uses an algorithm to create an output signal and also sends out a superimposed dither signal. This dithered output signal is then received by a pressure control that causes a dither servo pressure in a servo system. Because of the dither pressure the servo system adjusts the position of the swashplate.



Inventors:
Klocke, Craig C. (Ames, IA, US)
Application Number:
10/686369
Publication Date:
04/21/2005
Filing Date:
10/15/2003
Assignee:
Sauer-Danfoss Inc. (Ames, IA, US)
Primary Class:
Other Classes:
417/218, 417/269
International Classes:
F04B1/26; F04B1/32; F04B49/06; (IPC1-7): F04B49/00; F04B1/06
View Patent Images:
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Primary Examiner:
WEINSTEIN, LEONARD J
Attorney, Agent or Firm:
Zarley, Law Firm P. L. C. (CAPITAL SQUARE, 400 LOCUST, SUITE 200, DES MOINES, IA, 50309-2350, US)
Claims:
1. A control system for a hydrostatic unit having a swashplate comprising: an electronic means for producing a dithered output signal; a pressure control adapted to receive the dithered output signal and position the swashplate.

2. The control system for a hydrostatic unit of claim 1 wherein the electronic means is a microprocessor.

3. The control system of claim 2 wherein the microprocessor receives information from a set point command signal.

4. The control system of claim 2 wherein the microprocessor receives information from a feedback sensor.

5. The control system of claim 1 wherein the pressure control is a flapper nozzle style pilot valve with two boost spools.

6. The control system of claim 1 further comprising: a servo system operably connected to the pressure control and swashplate.

7. A method of controlling the angle of a swashplate of a hydrostatic unit having a swashplate comprising steps of: generating an electric signal based on a set point signal; receiving the electric signal in a microprocessor; interpolating the information from the electric signal using an algorithm contained in the microprocessor; sending an output signal from the microprocessor to a pressure control; dithering the output signal; and generating a dithered pressure from the pressure control that displaces the swashplate.

8. The method of claim 7 wherein the set point signal is generated by measuring an operational parameter.

9. The method of claim 8 wherein the operational parameter is the angle of the swashplate.

10. The method of claim 7 wherein the algorithm is a PID type algorithm.

11. The method of claim 7 wherein the algorithm is a PID+feed forward algorithm.

12. The method of claim 7 wherein the algorithm is a KIDT1 algorithm.

13. The method of claim 7 wherein the pressure control is a flapper nozzle style pilot valve with two boost spools.

14. The method of claim 7 wherein the pressure control is a flapper nozzle style pilot valve with one boost spool.

15. The method of claim 7 wherein the pressure control is a flow control.

16. The method of claim 7 wherein the pressure control is comprised of two pressure controls.

17. The method of claim 7 wherein the output signal is dithered by the pressure control.

18. The method of claim 7 wherein the output signal is dithered by the microprocessor.

19. A control system for a hydrostatic pump having a swashplate comprising: a feedback sensor adapted to sense the angle of the swashplate; a microprocessor adapted to receive information from the feedback sensor and produce a dithered output signal; a pressure control adapted to receive the dithered output signal and position the swashplate.

20. The control system of claim 19 wherein the microprocessor is also adapted to receive information from a set point command signal.

Description:

BACKGROUND OF THE INVENTION

This invention relates to a control system for a hydrostatic unit. More specifically, and without limitation, this invention relates to a control system that utilizes a dither signal incorporated with its normal input signal in order to adjust the angle of a swashplate.

In the art of hydraulics, oil is pumped by mechanical hydraulic pumps for the purpose of causing a hydraulic motor to revolve, a hydraulic cylinder extend, or for other useful purposes. A common aspect of many tractors, earthmoving machines, and the like is a hydrostatic transmission. In its most basic form a hydrostatic transmission consists of a hydrostatic pump which is normally driven by an internal combustion engine, and provides a source of pressurized oil flow which causes one or more hydrostatic motors to rotate. The rotation of these one or more hydrostatic motors will cause the machine to travel forward or reverse as commanded by the drive of the machine.

The swashplate is a mechanism in a hydrostatic transmission that controls the fluid flow that a hydraulic pump may deliver. Usually the angle of the swashplate is determined by a hydraulic cylinder or servo system based on information that a control system or microprocessor receives. A typical microprocessor uses an algorithm to determine an output signal that will adjust the swashplate to a position. The pump and servo system usually do not match the resolution and accuracy of the input signal to the hydrostatic unit. This can cause the incorrect positioning of the swashplate.

Thus, it is a primary object of the present invention to provide a control system for a hydrostatic pump that improves upon the state of the art.

Another object of the present invention is to use a dither signal to improve the resolution and accuracy of fluid flow (e.g. swash angle position) of the pump and/or motor in a hydrostatic control system.

Yet another object of the present invention is to provide a hydrostatic control system that is able to adjust a swashplate angle based on system parameters using a dithered signal.

These and other objects, features, or advantages of the present invention will become apparent from the specification and claims.

BRIEF SUMMARY OF THE INVENTION

The present invention is a control system that controls the angle of a swashplate. The control system can be a microprocessor that receives information from a feedback sensor and setpoint sensor. The microprocessor determines not only the angle of the swashplate but also a set point command. From the information received from the sensor(s), the microprocessor uses an algorithm to process the information and send out an output command signal. The microprocessor can simultaneously send out a superimposed dither signal that produces a resulting signal that drives a pressure control. The pressure control in turn causes a servo system to alter the angle of the swashplate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of the hydrostatic pump of this system;

FIG. 2 is a block representation of the control system of the present invention;

FIG. 3 is an enlarged scale portion 3-3 of FIG. 1;

FIG. 4. is an enlarged scale portion 4-4 of FIG. 1; and

FIG. 5 is an enlarged scale portion 5-5 of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The control system of the present invention can be connected to the hydrostatic pump seen in FIG. 1. The hydrostatic pump 10 has a central housing 12 of a standard servo hydrostatic pump. A servo piston 14 is disposed through the central housing 12. The servo piston 14 incorporates a servo screw 16, spring guide 18, servo spring 20, and spring seat 22.

The hydrostatic pump 10 also has a swashplate 24. Swashplate 24 is connected to a shaft feedback assembly 26 that works with the angle sensor assembly 28 to determine the angle of the swashplate 24. The swashplate 24 connects to the central housing 12 by use of a cone bearing 30 that is connected to cup bearing 32 which is then connected to a first O-ring 34. The first O-ring 34 rests against the bottom of plate adapter 36. A second O-ring 38 meshes between the plate adapter 36 and the bottom of the central housing 12. On the top of the housing is a bi-directional pressure control PCP assembly 39 having two coils. As one can appreciate from FIG. 1 this is a standard servo hydrostatic pump.

FIG. 2 shows in block form a typical closed loop control system can used to drive hydrostatic pump 10. The control system includes feedback sensor 40 and an operational parameter setpoint sensor 42. The setpoint sensor 42 sends a setpoint signal 44 and the feedback sensor 40 detects the angle of the swashplate 24 and sends a feedback signal 46. Both the setpoint signal 44 and the feedback signal 46 are received by a device having the capability to produce a dithered electrical control signal. This device in a preferred embodiment is a microprocessor 48 as pictured; however, in alternative embodiments the device may be an electric joystick, an electric foot pedal control, or any other device that can produce a dithered output signal. The microprocessor 48 then sends out a control signal 50 comprised of an average signal and a dither signal. A pressure control 52 then receives the control signal 50 and creates a pressure signal 54 or 56 that is sent to a servo system 58. The servo system 58 then produces a force 60 that alters the position of swashplate 24. It should be appreciated that the feedback sensor 40 is optional as the system can improve performance in a closed loop as well as open loop mode.

In the microprocessor 48 an error signal is generated which is processed through a typical closed loop algorithm. These algorithms include, but are not limited to, PID, PID plus feed forward, and KIDT1. From the output of the algorithm(s) the microprocessor 48 creates an output signal that is superimposed with a dither signal generating the resulting signal 50 that is received by pressure control 52. It should be appreciated that the pressure control 52 can be of any type, including but not limited to a flapper nozzle style pilot valve with two boost spools, a flapper nozzle style pilot valve with one boost spool, a flow control (a device that converts an electrical signal into an hydraulic signal to position the swashplate), or a plurality of pressure controls.

The pressure control 52 responds to the output signal and the dither to generate a dithering servo pressure. Based on this dithering servo pressure the max slew rate of the swashplate 24 is determined. The pump swashplate 24 position is therefore determined by the typical force balance of the servo springs, servo pressure, pressure moments, speed moments, and other system factors.

It should be appreciated that the dither signal created by the microprocessor 48 can be independent or dependent of the swash angle value related by feedback sensor 40. Furthermore, the feedback sensor 40 and the closed loop algorithm used by the microprocessor can be any standard feedback sensor or algorithm. In the current embodiment the microprocessor 48 generates the dither signal; however, the dither signal could also be generated internally in the pressure control 52 or externally in another device.

It will be appreciated by those skilled in the art that other various modifications could be made to the device without the parting from the spirit in scope of this invention. All such modifications and changes fall within the scope of the claims and are intended to be covered thereby.