Electric displacement control for an open circuit variable displacement pump

Abstract

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.

What is claimed is: 
     
         1 . An electric displacement control system, comprising:
 a hydraulic variable displacement pump operated in an open hydraulic circuit;   a servo piston disposed within a servo bore connected to the hydraulic variable displacement pump;   a control spool valve located in the servo bore and having an orifice that vents fluid pressure from the servo bore to a pump case.   
     
     
         2 . The control system of  claim 1  wherein flow rate 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. 
     
     
         3 . The control system of  claim 1  wherein 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. 
     
     
         4 . The control system of  claim 1  wherein the control spool valve is a two-way two-position spool type valve. 
     
     
         5 . The control system of  claim 1  wherein the control spool valve is a two-way three-position spool type valve. 
     
     
         6 . The control system of  claim 1  wherein the control spool valve has a groove where fluid in the groove is connected with fluid in the servo bore and fluid volume at a solenoid actuator. 
     
     
         7 . The system of  claim 1  wherein an axis of the control spool valve is aligned with an axis of the servo bore. 
     
     
         8 . The system of  claim 1  wherein a feedback spring is positioned between the servo piston and a first side of the control spool valve. 
     
     
         9 . A method of controlling the displacement of a variable displacement pump, comprising the steps of:
 providing a hydraulic control system that includes a spool type valve having an axis aligned with an axis of a servo bore, wherein the spool type valve has a solenoid actuator on one end and a feedback spring on an opposite end;   communicating system pressure through the spool type valve with servo pressure when there is no solenoid force acting upon the spool type valve sending a variable displacement pump to minimum displacement.   
     
     
         10 . The method of  claim 9  further comprising the step of increasing solenoid force to move the spool type valve against feedback spring force reducing flow to a servo control to send pump displacement to maximum until the feedback spring force and solenoid force are balanced. 
     
     
         11 . The method of  claim 10  further comprising the step of decreasing solenoid force so that the spool type valve is pushed by the feedback spring force against the solenoid force. 
     
     
         12 . The method of  claim 9  wherein the spool type valve is a two-direction three position spool valve that blocks system pressure when solenoid force drops below a preset minimum.