ELECTRIC DISPLACEMENT CONTROL FOR AN OPEN CIRCUIT VARIABLE DISPLACEMENT PUMP

An electric displacement control system has a hydraulic variable displacement pump that operates in an open hydraulic circuit. A servo piston is disposed within a servo bore that is connected to the hydraulic variable displacement pump. Located in the servo bore is a control spool valve having an orifice that vents fluid pressure from the servo bore to a pump case. The flow rate of the system depends upon a fluid force between a feedback spring on a first side of the control spool valve and a solenoid actuator force on a second side of the control spool valve.

BACKGROUND OF THE INVENTION

The present invention is directed to a control system for a hydraulic variable displacement pump, and more particularly a control system for an open circuit variable displacement pump which is adjustable by means of a servo piston that shifts inside of a servo cylinder.

Control systems for an open circuit variable displacement pump are known in the art. In one example, as disclosed in U.S. Publication No. 2015/0050165 by Zavadinka, a control device is used to set various power levels for the variable displacement pump that are reliably maintained by automatically regulating the control device without requiring any external control intervention. Electric displacement controls (EDC) for a pump regulate the speed of a machine function such as speed of a conveyor belt or drilling speed on a drill rig. EDCs typically replace PVG-like valves allowing cost reductions as well as lowering the hydraulic losses across the valve for better system efficiency. While useful, problems still exist with respect to cost and space while still meeting application requirements.

A solution to these problems involves an EDC concept that includes use of a two-way, two or three position, spool type valve located in the pump servo bore and having an orifice to vent fluid from the servo bore to the pump case. The spool type valve meters fluid from system pressure to the servo bore and the flow rate across the valve depends on a force balanced between a feedback spring force on one side of the spool and a solenoid actuator force on another side of the spool.

An objective of the present invention is to provide an electric displacement system that reduces the cost of manufacturing and still meets application requirements.

Another objective of the present invention is to provide an electric displacement system that reduces the space needed and still meets application requirements.

These and other objectives will be apparent to those having skill in the art based upon the following written description, drawings, and claims.

SUMMARY OF THE INVENTION

An electric displacement control system has a hydraulic variable displacement pump that operates in an open hydraulic circuit. A servo piston is disposed within a servo bore that is connected to the hydraulic variable displacement pump. Located in the servo bore is a control spool valve having an orifice that vents fluid pressure from the servo bore to a pump case. The flow rate of the system depends upon a fluid force between a feedback spring on a first side of the control spool valve and a solenoid actuator force on a second side of the control spool valve.

The control spool valve is located in a housing and has a plug with a system pressure access to a porting hole, a moveable control spool, a feedback compression spring at a first side and a solenoid actuator at a second side. Preferably the control spool valve is a two-way, two-position spool type valve or a two-way, three-position spool type valve. The control spool valve has a groove where fluid in the groove is connected with a fluid in the servo bore and fluid volume at a solenoid actuator. An axis of the of the control spool valve is aligned with an axis of the servo bore. Positioned between the servo piston and a first side of the control spool valve is a feedback spring.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the figures, an electric displacement control system10includes a variable displacement pump12. The variable displacement pump12has an inlet14and an outlet16. The outlet16is connected to a control device18via a pressure line20.

Control device18has a control piston22mounted within a housing24and is adapted to shift or move longitudinally. A first end26of the control piston22has an actuator27that is exposed to high pressure fluid that exerts hydraulic force toward a second end28having an adjustable spring30.

The housing24has a first port32, a second port34, and a third port36. The first port32is positioned to selectively align with pressure line20. The second port34is positioned to selectively align with drain line37that extends between the control device18and a tank38. The third port36is positioned to selectively align with pressure line40. The control device18has a first position that permits flow through the control device18between drain line36and pressure line40. The control device18has a second position where fluid flows from pressure line20to pressure line40.

The pressure line40extends from the control device18and a spool type valve42. The spool type valve42is of any type and preferably is a two-way two position spool valve42A for a zero flow and minimum fail-safe control design, or a two-way three position spool valve42B for a full flow and maximum pressure fail-safe control design.

The spool valve42is located in a pump servo bore44and the axis of the spool valve42is aligned with the axis of the servo bore44. The spool valve42includes a plug46with a system pressure access to a porting hole48, a moveable control spool50, a feedback compression spring52on one end, and a solenoid actuator54on the opposite end. A groove56on the control spool50selectively permits flow from the outlet16, through the control spool50to pressure line40. The two-way three position spool valve42B has a pair of plugs46A and46B on each side of the groove56.

The servo bore44has an orifice58to maintain a pressure differential between servo pressure and case pressure. Fluid pressure from the spool valve42acts upon a servo piston60moving the piston60against the force of a return spring62. Movement of the servo piston60adjusts the variable displacement pump12as required via an activation link64. This adjustment results in a change in the deflection angle of a swashplate.

In operation, using the two-way two position spool valve42A, with no or minimum solenoid force, fluid pressure flows from outlet16, through control spool50to pressure line40. This results in sending the pump12to minimum displacement.

As solenoid force increases, the spool valve42A moves toward and against feedback spring52reducing flow to the servo. As a result, the pump bias system returns pump12displacement to maximum while compressing the feedback spring52until the spring force and the actuator force are balanced.

When the actuator force is decreased, moveable control50is pushed by feedback spring56against the solenoid actuator54and system pressure is communicated to the servo piston60. The servo piston60moves out of the servo bore44and force from the feedback spring56is lowered until spring force and solenoid actuator force are balanced again. With maximum solenoid force, system pressure is blocked at porting hole48and fluid in servo bore is all vented to case as the pump bias system returns the pump12to maximum displacement.

The two-way three-position spool valve42B operates in a similar manner to move the control member50between maximum and minimum displacement, but also provides an emergency or fail safe when electrical power to the solenoid actuator54is lost. More specifically, when solenoid force drops below a preset minimum force from the feedback spring52moves the control member50all the way against the solenoid actuator which blocks system pressure. Pump displacement would be returned to maximum as all fluid would be vented to the pump case through the orifice.