Demand compensated hydraulic system with pilot line dither

Abstract

A demand compensated hydraulic system includes a pump responsive to a pressurized fluid in a demand feedback pilot line. The pilot line is connected to sense fluid demand between a hydraulic motor and an associated control valve which is closed by servo feedback from the motor. A dither is connected to the pilot line to continuously oscillate the pressurized fluid in the pilot line to prevent the occurrence of a situation wherein the flow across the control valve is sufficient to prevent the pump from going to a stand-by condition while being insufficient to operate the motor to close the valve to thereby cause the pump to go into the standby condition.

I claim: 
     
       1. An improved hydraulic system of the type having a variable displacement pump with output control means biased to increase pump output and responsive to an input of pressurized fluid to decrease pump output, a fluid motor, an output line connected between the pump and the motor, a motor control valve interposed in the output line and having servo feedback from the motor for selectively opening and closing the output line, a control line connected between the pump and the output control means, demand valve means interposed in the control line for selectively opening and closing the control line to allow and block the input of pressurized fluid to the output control means in response to pressurized fluid in a pilot line connected to the demand valve means, said pilot line .[.further connected to the output line between.]. .Iadd.operatively associated with .Iaddend.the motor control valve and .Iadd.pressurized by the pressurized fluid to .Iaddend.the motor .Iadd.when the motor control valve is moved from a neutral position, .Iaddend.wherein the improvement comprises: means operatively associated with the pilot line responsive to pressurized fluid therein to cause fluctuations in the pressure of the pressurized fluid. 
     
     
       2. An improved hydraulic system of the type having a variable displacement pump with output control means biased to increase pump output and responsive to an input of pressurized fluid to decrease pump output, a fluid motor, an output line connected between the pump and the motor, a motor control valve interposed in the output line and having servo feedback from the motor for selectively opening and closing the output line, a control line connected between the pump and the output control means, demand valve means interposed in the control line for selectively opening and closing the control line to allow and block the input of pressurized fluid to the output control means in response to pressurized fluid in a pilot line connected to the demand valve means, said pilot line .[.further connected to the output line between.]. .Iadd.operatively associated with .Iaddend.the motor control valve and .Iadd.pressurized by the pressurized fluid to .Iaddend.the motor .Iadd.when the motor control valve is moved from a neutral position, .Iaddend.and a restricted reservoir line connecting the pilot line to a fluid reservoir, wherein the the improvement comprises: dither means interposed between the restricted reservoir line and the fluid reservoir responsive to pressurized fluid in the restricted reservoir line for cyclically connecting and blocking the restricted reservoir line to and from the reservoir whereby pulsations are caused in the pressurized fluid in the pilot line. 
     
     
       3. An improved hydraulic system of the type having a variable displacement pump with output control means biased to increase output and responsive to an input of pressurized fluid to decrease pump output, a fluid motor, an output line connected between the pump and the motor, a motor control valve interposed in the output line and having servo feedback from the motor for selectively opening and closing the output line, a control line connected between the pump and the output control means, demand valve means interposed in the control line for selectively opening and closing the control line to allow and block the input of pressurized fluid to the output control means in response to pressurized fluid in a pilot line connected to the demand .Iadd.valve .Iaddend.means, said pilot line .[.further connected to the output line between.]. .Iadd.operatively associated with .Iaddend.the motor control valve and .Iadd.pressurized by the pressurized fluid to .Iaddend.the motor .Iadd.when the motor control valve is moved from a neutral position, .Iaddend.connecting the pilot line to a fluid reservoir, and a restrictor interposed in the reservoir line, wherein the improvement comprises: dither means interposed between the reservoir line and the reservoir; said dither means having a chamber therein in fluid communication through the reservoir line with the pilot line at one end and with the reservoir at the other end; said dither means including piston means positioned in the chamber and biased towards the pilot line end of the chamber and slidable under a predetermined fluid pressure to the reservoir end of the chamber, said piston means including valve means responsive to positioning at the pilot line end and at the reservoir end of the chamber to respectively block and allow fluid communication between the pilot line and reservoir ends of the chamber. 
     
     
       4. The hydraulic system as claimed in claim 3 wherein the dither means includes detent means operatively associated with the valve means for maintaining fluid communication between the pilot line end and the reservoir end of the chamber during movement of the piston means from the reservoir end and the pilot line end of the chamber and for maintaining blocking of fluid communication between the pilot line end and reservoir end of the chamber during movement of the piston means from the pilot line end to the reservoir end of the chamber. 
     
     
       5. An improved hydraulic system of the type having a variable displacement pump with output control means biased to increase pump output and responsive to an input of pressurized fluid to decrease pump output, a fluid motor, an output line connected between the pump and the motor, a motor control valve interposed in the output line and having servo feedback from the motor for selectively opening and closing the output line, a control line connected between the pump and the output control means, demand valve means interposed in the control line for selectively opening and closing the control line to allow and block the input of pressurized fluid to the output control means in response to pressurized fluid in a pilot line connected to the demand valve means, said pilot line further connected to the output line between the motor control valve and the motor, a reservoir line connecting the pilot line to a fluid reservoir, and a restrictor interposed in the reservoir line, wherein the improvement comprises: dither means interposed between the reservoir line and the reservoir; said dither means having a chamber therein closed at the ends with the pilot line through the reservoir line in fluid communication with one end of the chamber and the reservoir in fluid communication with the other end of the chamber; said dither means including a piston positioned in the chamber and dividing the chamber into a pilot line connected chamber and a reservoir connected chamber, said piston slidable towards the reservoir connected chamber in response to pressurized fluid in the pilot line; said dither means having biasing means therein urging the piston towards the pilot line connected chamber; said piston having an opening therethrough and including valve means protruding through said opening, said valve means slidable in the piston upon abutting the pilot line end of the chamber to an open position wherein the pilot line and reservoir connected chambers are in fluid communication whereby the pressure to the demand valve means is decreased and slidable in the piston upon abutting the reservoir end of the chamber to a closed position wherein the pilot line and reservoir connected chambers are blocked from fluid communication to increase fluid pressure in the pilot line. 
     
     
       6. The hydraulic system as claimed in claim 5 wherein the piston includes detent means operatively associated with the valve means to hold the valve means in the open position after the valve means abuts the pilot line end of the chamber and to hold the valve means in the closed position after the valve means abuts the reservoir end of the chamber.