Abstract:
A system for positioning a device such as a valve with a mechanical input using a fluid operated actuator, a mechanical position feedback member coupled to a feedback element of the fluid operated actuator and an activation fluid valve. The fluid operated actuator has an output coupled to the mechanical input of the valve, a feedback element for mechanically indicating a position of the valve, and inputs for actuating fluid, such that fluid at the inputs causes the fluid operated actuator to move in opposing directions. The activation fluid valve has outputs coupled to the inputs of the fluid operated actuator, a first opposing force input coupled to the mechanical position feedback member and a second opposing force input coupled to a control input force. The position of the activation fluid valve is controlled by a balance between the force from the mechanical feedback member and the control input force.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims one or more inventions which were disclosed in Provisional Application No. 61/186,473, filed Jun. 12, 2009, entitled “PROPORTIONAL POSITION FEEDBACK HYDRAULIC SERVO SYSTEM”. The benefit under 35 USC §119(e) of the United States provisional application is hereby claimed, and the aforementioned application is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    The invention pertains to the field of servo systems. More particularly, the invention pertains to a proportional position feedback hydraulic servo system. 
       SUMMARY OF THE INVENTION 
       [0004]    An actuator system for positioning a valve or other device with a mechanical input using a fluid operated actuator, a mechanical position feedback member coupled to a feedback element of the fluid operated actuator and a pilot valve. The fluid operated actuator has an output coupled to the mechanical input of the valve or other device, a feedback element for mechanically indicating a position of the valve or other device, and inputs for actuating fluid, such that fluid at the inputs causes the fluid operated actuator to move bi-directionally. The pilot valve has outputs coupled to the inputs of the fluid operated actuator, a first opposing force input coupled to the mechanical position feedback member and a second opposing force input coupled to a control input force, the first opposing force input and the second opposing force input being reciprocal to each other such that the position of the activation fluid valve is controlled by a balance between the force from the mechanical feedback member and the control input force. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0005]      FIG. 1  shows a block diagram of a fluid servo system. 
           [0006]      FIG. 2   a  shows a schematic of a fluid servo system of a first embodiment in an equilibrium position. 
           [0007]      FIG. 2   b  shows a schematic of a fluid servo system of a first embodiment moving towards a first position. 
           [0008]      FIG. 2   c  shows a schematic of a fluid servo system in a first embodiment moving towards a second position. 
           [0009]      FIG. 3   a  shows a schematic of a fluid servo system of a second embodiment in an equilibrium position. 
           [0010]      FIG. 3   b  shows a schematic of a fluid servo system of a second embodiment moving towards a first position. 
           [0011]      FIG. 3   c  shows a schematic of a fluid servo system in a second embodiment moving towards a second position. 
           [0012]      FIG. 4   a  shows a schematic of a fluid servo system of a third embodiment in an equilibrium position. 
           [0013]      FIG. 4   b  shows a schematic of a fluid servo system of a third embodiment moving towards a first position. 
           [0014]      FIG. 4   c  shows a schematic of a fluid servo system in a third embodiment moving towards a second position. 
           [0015]      FIG. 5   a  shows a schematic of a fluid servo system of fourth embodiment in an equilibrium position. 
           [0016]      FIG. 5   b  shows a schematic of a fluid servo system of a fourth embodiment moving towards a first position. 
           [0017]      FIG. 5   c  shows a schematic of a fluid servo system of a fourth embodiment moving towards a second position. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]      FIG. 1  shows a block diagram of a fluid servo system of the present invention. A valve or other device  100  has a mechanical input connected to the output of a fluid operated actuator  110 . The fluid operated actuator  110  may be a rotary actuator, a linear actuator, or any other type of fluid operated actuator. The fluid can be oil or air or other fluids known to the art. A pilot valve  150  is connected to the fluid powered actuator  110  to operate the actuator  110  receiving mechanical position feedback through member  130  from the actuator  110 . The mechanical position feedback member is coupled to a feedback element  180  of the fluid operated actuator. The feedback element  180  may be a cam or wedge in the case of a rotary actuator or directly off an element of a linear actuator. The mechanical position feedback member  130  applies a force relative to the actuator  110  position by a follower  130  on a cam or wedge  180  connected to the mechanical position feedback member  130 , coupled to a resilient element  134  with known force versus deflection characteristics such as a spring on a first side  140  of the activation fluid valve  150 . On a second opposing side  160  of the activation fluid valve  150  is a control input force  170 . The control input force  170  may be provided by a fluid actuator, a mechanical actuator, or an electrical actuator. The embodiments discussed below exemplify the block diagram of  FIG. 1 , although other combinations are within the scope of the invention. 
         [0019]      FIGS. 2   a - 2   c  show schematics of a first embodiment of a hydraulic servo system as shown in  FIG. 1 , with proportional position feedback.  FIG. 2   a  shows a schematic of a hydraulic servo system of a first embodiment in an equilibrium position.  FIG. 2   b  shows a schematic of a hydraulic servo system of a first embodiment moving towards a first position.  FIG. 2   c  shows a schematic of a hydraulic servo system in a first embodiment moving towards a second position. The fluid circuits of  FIGS. 2   a - 2   c  are controlled by a meter in pilot. 
         [0020]    In this embodiment, the fluid operated actuator  110  is a double acting hydraulic actuator  2  and is in fluid communication with the pilot valve  150 , which is a pilot operated control valve  6 . The double acting hydraulic actuator  2  operates a valve  100  or other device that is to be positioned (not shown) through mechanical input and a feedback element  180 , for example, a rod  2   c  with a piston  2   b  that is received within the housing  2   a  of the hydraulic actuator  2 . A first fluid chamber  3   a  is formed between the housing  2   a  and one side of the piston  2   b  and a second fluid chamber  3   b  is formed between the housing  2   a  and the other side of the piston  2   b.  Mechanical position feedback  130  from the actuator is applied by the end  2   d  of the rod  2   c  opposite the valve  100  which is preferably tapered and contacts a spring  7  of a pilot operated control valve  6  through a means  8  which compresses the spring  7  in proportion to the double acting hydraulic actuator motion. The means  8  may be a tab, a rotary device that feeds back via cam/spring or feedback may be via a spring that contacts the end of the rod  2   d.    
         [0021]    The pilot operated control valve  6  preferably includes a spool with a plurality of lands. The pilot operate control valve  6  has at least three distinct positions and an infinite number of intermediate positions. In a first position  9   a  and a second position  9   c,  fluid may flow between the central pressurized oil supply  22  and the pilot operated control valve  6  and between the pilot operated control valve  6  and the chambers  3   a,    3   b  of the double acting hydraulic actuator  2 . In a neutral or third position,  9   b,  fluid is restricted from flowing to or from the double acting hydraulic actuator  2 . The pilot operated control valve  6  is moved between the positions by forces on the first side  140  and second side  160  of the valve  6 . The pilot operated control valve  6  is calibrated by adjusting a spring  10  and actuated by a piloted pressure from a pilot port  12  on a second side  160  and a spring  7  on a first side  140  of the pilot operated control valve  6  that is in contact with the double acting hydraulic actuator  2  through means  8 . 
         [0022]    The piloted pressure on the second side  160  of the pilot operated control valve  6  is provided to the pilot port  12  by a control input force  170 , which in this embodiment is a meter in pilot valve circuit. The meter in pilot valve circuit includes: a meter in analog or digital proportional flow control valve  30  that modulates the pilot pressure to the pilot port  12  of the pilot operated control valve  6 , a pressure line  40  in fluid communication with a central pressurized oil supply  22 , a hydraulic line  24  introducing fluid to chambers  3   a,    3   b  in the hydraulic actuator  2  through the pilot operated control valve  6 , a hydraulic line  26  receiving fluid from the pilot operated control valve  6  from which fluid is exiting the hydraulic actuator  2  to sump  20  and a hydraulic line  36  with a restriction  38  in fluid communication with line  26  leading to the pilot port  12  on the pilot operated control valve  6 . 
         [0023]    The proportional flow control valve  30  has at least three positions. The proportional flow control valve  30  is moved between the positions by a spring  33  one side of the valve and an analog proportional electric actuator such as a solenoid  32  on the opposite side of the valve. The proportional valve can also be a digital type that has a flow rate controlled by the duty cycle of a pulse width modulated (PWM) electrical signal In a first position  34   a,  fluid from the central pressurized oil supply  22  and line  40  are blocked and fluid to or from the pilot port  12  on the pilot operated control valve  6  is blocked from exiting through the valve  30 . In a second position  34   c,  fluid from the central pressurized oil supply  22  and line  40  flows to the pilot port  12  on a second side of the pilot operated control valve  6  unrestricted. In a neutral or third position  34   b,  fluid from the central pressurized oil supply and line  40  flows to the pilot port  12  on a second side of the pilot operated control valve  6  through a restricted orifice of the analog or digital proportional flow control valve  30 . 
         [0024]    Referring to  FIG. 2   a , the pilot operated control valve  6  and the analog or digital proportional flow control valve  30  are in equilibrium positions  9   b,    34   b.  In the equilibrium positions, the spring force  7  on the first side of the pilot operated control valve  6  and the force of the spring  10  and pilot force from the pilot port  12  on the second side of the pilot operated control valve  6  are equal. With the pilot operated control valve  6  in this position, fluid is restricted from flowing to or from the chambers  3   a,    3   b  of the double acting hydraulic actuator  2 . The force of the spring  33  on one side of the analog proportional flow control valve  30  is equal to the force of the proportional solenoid  32  on the opposite side of the proportional flow control valve  30 . If a digital proportional flow control is used, the pressure applied to the actuator on the valve  6  is dependent upon the duty cycle of the PWM signal applied to the digital pilot valve solenoid  32  rather than being dependent of the current level. In other words, if the current to the analog proportional solenoid  32  is steady or if the duty cycle to the digital pilot valve is steady, position  9   b  will be maintained. With the proportional flow control valve  30  in the equilibrium position  34   b,  fluid from line  26  flows to line  36  and through a restriction  38  to the pilot port  12  on the second side of the pilot operated control valve  6  and fluid from line  40  in fluid communication with the central pressurized oil supply  22  flows through a restricted orifice of the proportional flow control valve  30  to the pilot port  12  on the second side of the pilot operated control valve  6 . 
         [0025]    Referring to  FIG. 2   b , the current to the proportional solenoid  32  on the one side of the proportional flow control valve  30  is increased and is greater than the force of the spring  33  on the other side of the proportional flow control valve  30 , moving the valve to the left in the figure or towards the spring  33 . In moving the proportional flow control valve  30  to position  34   c,  fluid from the central pressurized oil supply  22  and line  40  flows unrestricted to the pilot port  12  on the pilot operated control valve  6  and fluid from line  26  and line  36  flow through the restriction  38  to the pilot port  12 . The same relationship exists if a digital flow control is used and if the duty cycle of the PWM signal to the digital flow control is increased. The force of spring  10  and pilot pressure from the pilot port  12  is greater than the spring force  7  on the opposite side of the pilot operated control valve  6 , moving the pilot operated control valve  6  towards the spring  7  to a position  9   a.  With the pilot operated control valve  6  in this position, fluid from the central pressurized oil supply  22  flows through line  24 , through the pilot operated control valve  6  to line  14  and the first chamber  3   a  of the double acting hydraulic actuator  2 . The fluid in the first chamber  3   a  moves the piston  2   b  mounted to the rod  2   c  in the direction of the arrow shown in the figure, moving the tapered end  2   d  of the rod and the valve  100  (not shown) to a first position. Movement of the rod  2   c  of the double acting hydraulic actuator  2  compresses the tab  8  and the spring  7 , providing position feedback of the double acting hydraulic actuator  2  to the pilot operated control valve  6 . Fluid from the second chamber  3   b  exits the double acting hydraulic actuator  2  through line  16  to the pilot operated valve  6  to line  26  leading to sump  20  or to line  36  with the restriction  38  leading to the pilot port  12  on the pilot operated control valve  6 . 
         [0026]    Referring to  FIG. 2   c , the current to the proportional solenoid  32  on the one side of the proportional flow control valve  30  is decreased and the force of the spring  33  on the other side of the proportional flow control valve  30  is greater than the force of the proportional solenoid  32 , moving the valve  30  to the right in the figure or away from the spring  33 . In moving the proportional flow control valve  30  to position  34   a,  fluid from the central pressurized oil supply  22  through line  40  is blocked from flowing to the pilot port  12  on the pilot operated control valve  6 . A small amount of fluid from line  26  and line  36  flows through the restriction  38  to the pilot port  12 , but the pressure of this fluid is just enough to maintain equilibrium with the force of the spring  7 . When the force of spring  7  is greater than the spring force  10  and the pilot port  12  on the opposite side of the pilot operated control valve  6 , it moves the pilot operated control valve  6  away, decompressing spring  7  to attain position  9   c.  With the pilot operated control valve  6  in this position, fluid from the central pressurized oil supply  22  flows through line  24 , through the pilot operated control valve  6  exhausted through line  16  from and the second chamber  3   b  of the double acting hydraulic actuator  2 . The fluid in the first chamber  3   b  moves the piston  2   b  mounted to the rod  2   c  in the direction of the arrow shown in the figure, moving the tapered end  2   d  of the rod  2   c  and the valve  100  (not shown) to a second position. Movement of the rod  2   c  of the double acting hydraulic actuator  2  decompresses the tab  8  and the spring  7 , providing position feedback of the double acting hydraulic actuator  2  to the pilot operated control valve  6 . Fluid from the first chamber  3   a  exits the double acting hydraulic actuator  2  through line  14  to the pilot operated valve  6  to line  26 , leading to sump  20  or to line  36  with the restriction  38 . The same relationship exists if a digital flow control is used and the duty cycle of the PWM signal to the digital flow control is decreased. 
         [0027]      FIGS. 3   a - 3   c  show schematics of a second embodiment hydraulic servo system as shown in  FIG. 1  which includes proportional position feedback.  FIG. 3   a  shows a schematic of a hydraulic servo system of a second embodiment in an equilibrium position.  FIG. 3   b  shows a schematic of a hydraulic servo system of a second embodiment moving towards a first position.  FIG. 3   c  shows a schematic of a hydraulic servo system in a second embodiment moving towards a second position. 
         [0028]    One of the differences between the hydraulic servo system shown in  FIGS. 2   a - 2   c  and the hydraulic servo system shown in  FIGS. 3   a - 3   c  is the replacement of line  36  with a restriction  38  in fluid communication with line  26  and that the pilot port  12  on one of the pilot operated control valve  6  is in fluid communication with line  24 , the central pressurized oil supply  22  and line  44  with a restriction  46 . Another difference is that the analog or digital proportional flow control valve  60  of the second embodiment is in a meter out pilot valve circuit instead of a meter in pilot valve circuit as in the first embodiment and is controlled by an analog or digital proportional flow control valve  60 . 
         [0029]    In this embodiment, the fluid operated actuator  110  is a double acting hydraulic actuator  2  and is in fluid communication with the activation fluid valve  150 , which is a pilot operated control valve  6 . The double acting hydraulic actuator  2  operates a valve  100  (not shown) through mechanical input and a feedback element  180 , for example, a rod  2   c  with a piston  2   b  that is received within the housing  2   a  of the hydraulic actuator  2 . A first fluid chamber  3   a  is formed between the housing  2   a  and one side of the piston  2   b  and a second fluid chamber  3   b  is formed between the housing  2   a  and the other side of the piston  2   b.  Mechanical position feedback  130  from the actuator is preferably applied by the end  2   d  of the rod  2   c  opposite the valve  100  which is preferably tapered and contacts a spring  7  of a pilot operated control valve  6  through a means  8  which compresses the spring  7  in proportion to the double acting hydraulic actuator motion. The means  8  may be a tab, a rotary device that feeds back via cam/spring or feedback may be via a spring that contacts the end of the rod  2   d.    
         [0030]    The pilot operated control valve  6  preferably includes a spool with a plurality of lands. The pilot operate control valve  6  has at least three positions. In a first position  9   a  and a second position  9   c,  fluid may flow between the central pressurized oil supply  22  and the pilot operated control valve  6  and between the pilot operated control valve  6  and the chambers  3   a,    3   b  of the double acting hydraulic actuator  2 . In an equilibrium position or third position,  9   b,  fluid is prevented from flowing to or from the double acting hydraulic actuator  2 . The pilot operated control valve  6  is moved between the positions by forces on the first side  140  and second side  160  of the pilot operated control valve  6 . The pilot operated control valve  6  is actuated by a spring  10  and piloted pressure from a pilot port  12  on a second side  160  and a spring  7  on a first side  140  of the pilot operated control valve  6  that is in contact with the double acting hydraulic actuator  2  through means  8 . 
         [0031]    The piloted pressure on the second side  160  of the pilot operated valve  6  is provided by a control input force  170 , which in this embodiment is a meter out pilot valve circuit. The meter out pilot valve circuit includes a meter out analog or digital proportional flow control valve  60  that modulates the pilot pressure of the pilot port  12  of the pilot operated control valve  6 , a pressure line  44  with a restriction  46  in fluid communication with a central pressurized oil supply  22 , line  24 ; a hydraulic line  24  introducing fluid to chambers  3   a,    3   b  in the hydraulic actuator  2  through the pilot operated control valve  6 , and a hydraulic line  26  receiving fluid from the pilot operated control valve  6  from which fluid is exiting the hydraulic actuator to sump  20 . The analog or digital proportional flow control valve  60  has three distinct positions and an infinite number of intermediate positions. The analog or digital proportional flow control valve  60  is moved by a spring  33  on one side of the valve and a proportional solenoid  32  on the opposite side of the valve. In a first position  64   a,  fluid from the pilot port  12  on the pilot operated control valve  6  flows to sump  48 . In a second position  64   c,  fluid is blocked from flowing to or from the pilot port  12  to sump  48 . In an equilibrium position or third position  64   b,  fluid from the pilot port  12  flows to the sump  48  through a variable orifice. 
         [0032]    Referring to  FIG. 3   a , the pilot operated control valve  6  and the analog proportional flow control valve  60  are in the equilibrium positions  9   b,    64   b.  In the equilibrium position, the spring force  7  on the first side  140  of the pilot operated control valve  6  and the force of the spring  10  and pilot force from the pilot port  12  on the second side  160  of the pilot operated control valve  6  are equal. With the pilot operated control valve  6  in this position, fluid is restricted from flowing to or from the chambers  3   a,    3   b  of the double acting hydraulic actuator  2 . The force of the spring  33  on one side of the proportional flow control valve  60  is equal to the force of the proportional solenoid  32 . In other words the current to the proportional solenoid  32  is steady. With the proportional flow control valve  60  in the equilibrium position  64   b,  fluid from the pilot port  12  on the pilot operated control valve  6  flows to sump  48  through a variable orifice of the proportional flow control valve  60 . Fluid also flows from central pressurized oil supply  22  into line  44 , through the restriction  46  to the pilot port  12  on the pilot operated control valve  6 . The force of the fluid from line  44  that flows into the pilot port  12  and the flow through the variable orifice of the proportional flow control valve  60  to sump  48  in addition with the force provided by spring  10  is equal to the force of the spring  7  on the opposite side of the pilot operated control valve  6 . If a digital proportional flow control is used, the pressure applied to the an actuator on the valve  6  is dependent upon the duty cycle of the PWM signal applied to the digital pilot valve solenoid rather than being dependent of the current level. 
         [0033]    Referring to  FIG. 3   b , the current to the proportional solenoid  32  on the one side of the analog proportional flow control valve  60  is increased and is greater than the force of the spring  33  on the other side of the analog proportional flow control valve  60 , moving the valve  60  to the left in the figure or towards the spring  33 . In moving the analog proportional flow control valve  60  to position  64   c,  fluid from the pilot port  12  on the pilot operated control valve  6  is blocked from flowing to sump  48 . Fluid from the central pressurized oil supply  44  flows through restriction  46  to the pilot port  12  on the pilot operated control valve  6 . The force of spring  10  and pilot pressure from the pilot port  12  is greater than the spring force  7  on the opposite side of the pilot operated control valve  6 , moving the pilot operated control valve  6  to the towards the spring  7  to a position  9   a.  With the pilot operated control valve  6  in this position, fluid from the central pressurized oil supply  22  flows through line  24 , through the pilot operated control valve  6  to line  14  and the first chamber  3   a  of the double acting hydraulic actuator  2 . The fluid in the first chamber  3   a  moves the piston  2   b  mounted to the rod  2   c  in the direction of the arrow shown in the figure, moving the tapered end  2   d  of the rod  2   c  and the valve  100  (not shown) to a first position. Movement of the rod  2   c  of the double acting hydraulic actuator  2  compresses the tab  8  and the spring  7 , providing position feedback of the double acting hydraulic actuator  2  to the pilot operated control valve  6 . Fluid from the second chamber  3   b  exits the double acting hydraulic actuator  2  through line  16  to the pilot operated valve  6  to line  26  leading to sump  20 . 
         [0034]    Referring to  FIG. 3   c , the current to the proportional solenoid  32  on the one side of the analog proportional flow control valve  60  is decreased and the force of the spring  33  on the other side of the proportional flow control valve  60  is greater than the force of the proportional solenoid  32 , moving the valve  60  to the right in the figure or away from the spring  33 . In moving the proportional flow control valve  60  to position  64   a,  fluid from the pilot port  12  on the pilot operated control valve  6  exits through the proportional flow control valve  60  to sump  48 . While fluid from the central pressurized oil supply  22  is still supplied to the pilot port  12  through line  44  and the restriction  46 , this fluid also drains through the proportional flow control valve  60  to sump  48 . Any pressure or force of the fluid flowing to the pilot port  12  is not significant enough to over power the force of the spring  7 . The force of spring  7  is greater than the spring force  10  and the pilot port  12  on the opposite side of the pilot operated control valve  6 , moving the pilot operated control valve  6  away the spring  7  to a position  9   c.  With the pilot operated control valve  6  in this position, fluid from the central pressurized oil supply  22  flows through line  24 , through the pilot operated control valve  6  to line  16  and the second chamber  3   b  of the double acting hydraulic actuator  2 . The fluid in the first chamber  3   b  moves the piston  2   b  mounted to the rod  2   c  in the direction of the arrow shown in the figure, moving the tapered end  2   d  of the rod  2   c  and the valve  100  (not shown) to a second position. Movement of the rod  2   c  of the double acting hydraulic actuator  2  decompresses the tab  8  and the spring  7 , providing position feedback of the double acting hydraulic actuator  2  to the pilot operated control valve  6 . Fluid from the first chamber  3   a  exits the double acting hydraulic actuator  2  through line  14  to the pilot operated valve  6  to line  26  leading to sump  20 . If digital proportional flow control is used, the pressure applied to the actuator on valve  6  is dependent upon the duty cycle of the PWM signal applied to the digital pilot valve solenoid rather than being dependent of the current level. 
         [0035]      FIGS. 4   a - 4   c  show schematics of a third embodiment of a hydraulic servo system as shown in  FIG. 1 , with proportional position feedback.  FIG. 4   a  shows a schematic of a hydraulic servo system of a third embodiment in an equilibrium position.  FIG. 4   b  shows a schematic of a hydraulic servo system of a third embodiment moving towards a first position.  FIG. 4   c  shows a schematic of a hydraulic servo system in a third embodiment moving towards a second position. The fluid circuits of  FIGS. 4   a - 4   c  are controlled by a meter out pilot. 
         [0036]    One of the differences between the hydraulic servo system of shown in  FIGS. 2   a - 2   c  and the hydraulic servo system shown in  FIGS. 4   a - 4   c  is the replacement of line  36  with a restriction  38  in fluid communication with line  26  and the pilot port  12  on the pilot operated control valve  6 . Line  44  contains a restriction  46  and is in fluid communication with line  24  and the central pressurized oil supply  22  and is also in fluid communication with the pilot port  12  on one side of the pilot operated control valve  6 . Another difference is that the proportional flow control valve  60  of the second embodiment is in a meter out pilot valve circuit instead of a meter in pilot valve circuit as in the first embodiment and is controlled by a proportional relief control valve instead of a proportional flow control valve as in the second embodiment. 
         [0037]    In this embodiment, the fluid operated actuator  110  is a double acting hydraulic actuator  2  and is in fluid communication with the activation fluid valve  150 , which is a pilot operated control valve  6 . The double acting hydraulic actuator  2  operates a valve  100  or other device (not shown) through mechanical input and a feedback element  180 , for example, a rod  2   c  with a piston  2   b  that is received within the housing  2   a  of the hydraulic actuator  2 . A first fluid chamber  3   a  is formed between the housing  2   a  and one side of the piston  2   b  and a second fluid chamber  3   b  is formed between the housing  2   a  and the other side of the piston  2   b.  Mechanical position feedback  130  from the actuator is preferably applied by the end  2   d  of the rod  2   c  opposite the valve  100  which is preferably tapered and contacts a spring  7  of a pilot operated control valve  6  through a means  8  which compresses the spring  7  in proportion to the double acting hydraulic actuator motion. The means  8  may be a tab, a rotary device that feeds back via cam/spring or feedback may be via a spring that contacts the end of the rod  2   d.    
         [0038]    The pilot operated control valve  6  includes a spool with a plurality of lands. The pilot operate control valve  6  has at least three positions. In a first position  9   a  and a second position  9   c,  fluid may flow between the central pressurized oil supply  22  and the pilot operated control valve  6  and between the pilot operated control valve  6  and the chambers  3   a,    3   b  of the double acting hydraulic actuator  2 . In a neutral or third position,  9   b,  fluid is prevented from flowing to or from the double acting hydraulic actuator  2 . The pilot operated control valve  6  is moved between the positions by forces on the first side  140  and second side  160  of the pilot operated control valve  6 . The pilot operated control valve  6  is actuated by a spring  10  and piloted pressure from a pilot port  12  on a second side  160  and a spring  7  on a first side  140  of the pilot operated control valve  6  that is in contact with the double acting hydraulic actuator  2 . 
         [0039]    The piloted pressure on the second side  160  of the pilot operated control valve  6  is provided by a control input force  170 , which in this embodiment is a meter out pilot valve circuit. The meter out pilot valve circuit includes a meter out proportional relief control valve  80  that modulates the pilot pressure from the pilot port  12  of the pilot operated control valve  6 , a pressure line  44  with a restriction  46  in fluid communication with a central pressurized oil supply  22 , line  24 , the pilot port  12  on the pilot operated control valve  6 , and the pilot port  52  on one side of the proportional relief control valve  80 ; a hydraulic line  24  introducing fluid to a chamber  3   a,    3   b  in the hydraulic actuator  2  through the pilot operated control valve  6 , and a hydraulic line  26  receiving fluid from the pilot operated control valve  6  from which fluid is exiting the hydraulic actuator  2  to sump  20 . The proportional relief control valve  80  has at least three positions. The proportional relief control valve  80  is moved between the positions by pressure from the pilot port  52  one side of the valve and a proportional solenoid  32  on the opposite side of the valve. In a first position  84   a,  fluid from the pilot port  12  on the pilot operated control valve  6  flows to sump  48 . In a second position  84   c,  fluid is blocked from flowing to or from the pilot port  12  to sump  48 . In an equilibrium position or third position  84   b,  fluid from the pilot port  12  flows to the sump  48  through a variable orifice of the proportional relief control valve  80 . 
         [0040]    Referring to  FIG. 4   a , the pilot operated control valve  6  and the proportional relief control valve  80  are in the equilibrium positions  9   b,    84   b.  In the equilibrium position, the spring force  7  on the first side of the pilot operated control valve  6  and the force of the spring  10  and pilot force from the pilot port  12  on the second side of the pilot operated control valve  6  are equal. With the pilot operated control valve  6  in this position, fluid is restricted from flowing to or from the chambers  3   a,    3   b  of the double acting hydraulic actuator  2 . Fluid flows from central pressurized oil supply  22  into line  44 , through the restriction  46  to the pilot port  52  on one side of the proportional relief control valve  80 . The pilot force from the pilot port  52  on one side of the proportional relief control valve  80  is equal to the force of the proportional solenoid  32  on the opposite side of the proportional relief control valve  80 . In other words the current to the proportional solenoid  32  is steady. With the proportional relief control valve  80  in the equilibrium position  84   b , fluid from the pilot port  12  on the pilot operated control valve  6  flows to sump  48  through a variable orifice of the proportional relief control valve  80 . Fluid also flows from central pressurized oil supply  22  into line  44 , through the restriction  46  to the pilot port  12  on the pilot operated control valve  6 . The force of the fluid from line  44  that flows into the pilot port  12  and the flow through the variable orifice of the proportional relief control valve  80  to sump  48  in addition the force provided by spring  10  is equal to the force of the spring  7  on the opposite side of the pilot operated control valve  6  of the pilot operated control valve  6 . 
         [0041]    Referring to  FIG. 4   b , the current to the proportional solenoid  32  on the one side of the proportional relief control valve  80  is increased and is greater than the pilot force from the pilot port  52  on the other side of the proportional relief control valve  80 , moving the valve to the left in the figure or towards the pilot port  52 . In moving the proportional relief control valve  80  to position  84   c,  fluid from the pilot port  12  on the pilot operated control valve  6  is blocked from flowing to sump  48 . Fluid from the central pressurized oil supply  44  flows through restriction  46  to the pilot port  12  on the pilot operated control valve  6 . The force of spring  10  and pilot pressure from the pilot port  12  is greater than the spring force  7  on the opposite side of the pilot operated control valve  6 , moving the pilot operated control valve  6  to the towards the spring  7  to a position  9   a.  With the pilot operated control valve  6  in this position, fluid from the central pressurized oil supply  22  flows through line  24 , through the pilot operated control valve  6  to line  14  and the first chamber  3   a  of the double acting hydraulic actuator  2 . The fluid in the first chamber  3   a  moves the piston  2   b  mounted to the rod  2   c  in the direction of the arrow shown in the figure, moving the tapered end  2   d  of the rod  2   c  and the valve  100  or other device (not shown) to a first position. Movement of the rod  2   c  of the double acting hydraulic actuator  2  compresses the tab  8  and the spring  7 , providing position feedback of the double acting hydraulic actuator  2  to the pilot operated control valve  6 . Fluid from the second chamber  3   b  exits the double acting hydraulic actuator  2  through line  16  to the pilot operated valve  6  to line  26  leading to sump  20 . 
         [0042]    Referring to  FIG. 4   c , the current to the proportional solenoid  32  on the one side of the proportional relief control valve  80  is decreased and the pilot force of pilot port  52  on the other side of the proportional relief control valve  80  is greater than the force of the proportional solenoid  32 , moving the valve to the right in the figure or away from the pilot port  52 . In moving the proportional relief control valve  80  to position  84   a,  fluid from the pilot port  12  on the pilot operated control valve  6  exits through the proportional relief control valve  80  to sump  48 . While fluid from the central pressurized oil supply  22  is still supplied to the pilot port  12  through line  44  and the restriction  46 , this fluid also drains through the proportional relief control valve  80  to sump  48 . Any pressure or force of the fluid flowing to the pilot port  12  is not significant enough to over power the force of the spring  7 . The force of spring  7  is greater than the spring force  10  and the pilot port  12  on the opposite side of the pilot operated control valve  6 , moving the pilot operated control valve  6  to decompress spring  7  to attain position  9   c.  With the pilot operated control valve  6  in this position, fluid from the central pressurized oil supply  22  flows through line  24 , through the pilot operated control valve  6  to line  16  and the second chamber  3   b  of the double acting hydraulic actuator  2 . The fluid in the first chamber  3   b  moves the piston  2   b  mounted to the rod  2   c  in the direction of the arrow shown in the figure, moving the tapered end  2   d  of the rod  2   c  and the valve  100  (not shown). Movement of the rod  2   c  of the double acting hydraulic actuator  2  decompresses the tab  8  and the spring  7 , providing position feedback of the double acting hydraulic actuator  2  to the pilot operated control valve  6 . Fluid from the first chamber  3   a  exits the double acting hydraulic actuator  2  through line  14  to the pilot operated valve  6  to line  26  leading to sump  20 . 
         [0043]      FIGS. 5   a - 5   c  show schematics of fourth embodiment of a hydraulic servo system as shown in  FIG. 1 , with proportional position feedback.  FIG. 5   a  shows a schematic of a hydraulic servo system of a fourth embodiment in an equilibrium position.  FIG. 5   b  shows a schematic of a hydraulic servo system of a fourth embodiment moving towards a first position.  FIG. 5   c  shows a schematic of a hydraulic servo system in a fourth embodiment moving towards a second position. 
         [0044]    In this embodiment, the fluid operated actuator  110  is a double acting hydraulic actuator  2  and is in fluid communication with the activation fluid valve  150 , which is a pilot operated control valve  6 . The double acting hydraulic actuator  2  operates a valve  100  or other device (not shown) through mechanical input and a feedback element  180 , for example, a rod  2   c  with a piston  2   b  that is received within the housing  2   a  of the hydraulic actuator  2 . A first fluid chamber  3   a  is formed between the housing  2   a  and one side of the piston  2   b  and a second fluid chamber  3   b  is formed between the housing  2   a  and the other side of the piston  2   b.  Mechanical position feedback  130  from the actuator is preferably applied by the end  2   d  of the rod  2   c  opposite the valve  100  which is preferably tapered and contacts a spring  7  of a pilot operated control valve  6  through a means  8  which compresses the spring  7  in proportion to the double acting hydraulic actuator motion. The means  8  may be a tab, a rotary device that feeds back via cam/spring or feedback may be via a spring that contacts the end of the rod  2   d.    
         [0045]    The pilot operated control valve  6  includes a spool with a plurality of lands. The pilot operate control valve  6  has at least three distinct positions and an infinite number of intermediate positions. In a first position  9   a  and a second position  9   c,  fluid may flow between the central pressurized oil supply  22  and the pilot operated control valve  6  and the pilot operated control valve  6  and the chambers  3   a,    3   b  of the double acting hydraulic actuator  2 . In a neutral or third position,  9   b,  fluid is prevented from flowing to or from the double acting hydraulic actuator  2 . The pilot operated control valve  6  is moved between the positions by forces on the first side  140  and second side  160  of the pilot operated control valve  6 . The pilot operated control valve  6  is actuated by a spring  10  and piloted pressure from a pilot port  12  on a second side  160  and a spring  7  on a first side  140  of the pilot operated control valve  6  that is in contact with the double acting hydraulic actuator  2 . 
         [0046]    The piloted pressure on the second side  160  of the pilot operated control valve  6  is provided to the pilot port  12  by a control input force  170 , which in this embodiment is a pressure control valve meter in pilot valve circuit. The pressure control valve meter in pilot valve circuit includes a meter in proportional pressure control valve  70  that modulates the pilot pressure to the pilot port  12  of the pilot operated control valve  6 , a pressure line  40  in fluid communication with a central pressurized oil supply  22  and in fluid communication with the proportional pressure control valve  70  leading to the pilot port  12  on the pilot operated control valve  6 , a hydraulic line  24  introducing fluid to chambers  3   a,    3   b  in the hydraulic actuator  2  through the pilot operated control valve  6 , and a hydraulic line  26  receiving fluid from the pilot operated control valve  6  from which fluid is exiting the hydraulic actuator  2  to sump  20 . 
         [0047]    The proportional pressure control valve  70  has at least three positions. The proportional pressure control valve  70  is moved between the positions by a spring  72  and pilot port  52  one side of the valve and a proportional solenoid  32  on the opposite side of the valve. In a first position  74   a,  fluid from the central pressurized oil supply  22  and line  44  are blocked and fluid to or from the pilot port  12  on the pilot operated control valve  6  exits to sump  48  through a variable orifice of the proportional pressure control valve  70 . In a second position  74   c,  fluid from the central pressurized oil supply  22  and line  44  flows to the pilot port  12  on the pilot operated control valve  6  through a variable orifice of the valve  70 . In a neutral or third position  74   b,  fluid from the central pressurized oil supply  22  and line  44  flows to the pilot port  12  on the pilot operated control valve  6  through a variable orifice of the proportional pressure control valve  70  and another variable orifice leads to sump  48 . 
         [0048]    Referring to  FIG. 5   a , the pilot operated control valve  6  and the proportional pressure control valve  70  are in the equilibrium positions  9   b,    74   b.  In the equilibrium positions, the spring force  7  on the first side of the pilot operated control valve  6  and the force of the spring  10  and pilot force on the second side of the pilot operated control valve  6  are equal. With the pilot operated control valve  6  in this position, fluid is blocked from flowing to or from the chambers  3   a,    3   b  of the double acting hydraulic actuator  2 . The force of the spring  72  and the pilot port  52  on one side of the proportional pressure control valve  70  is equal to the force of the proportional solenoid  32  on the opposite side of the proportional pressure control valve  70 . In other words the current to the proportional solenoid  32  is steady. With the proportional pressure control valve  70  in the equilibrium position  74   b,  fluid from the central pressurized oil supply  22  flows to line  44  and through a variable orifice of the proportional flow control valve  70  to the pilot port  12  on the second side of the pilot operated control valve  6 . Fluid flowing to the pilot port  12  on the second side of the pilot operated control valve  6  supplies fluid to line  73  leading to the pilot port  52  on one side of the proportional pressure control valve  70 . 
         [0049]    Referring to  FIG. 5   b , the current to the proportional solenoid  32  on the one side of the proportional pressure control valve  70  is increased and is greater than the force of the spring  72  and the pilot port  52  on the other side of the proportional pressure control valve  70 , moving the valve to the left in the figure or towards the spring  72  and pilot port  52 . In moving the proportional pressure control valve  70  to position  74   c,  fluid from the central pressurized oil supply  22  and line  44  flows through a variable orifice of the proportional pressure control valve  70  to the pilot port  12  on the pilot operated control valve  6 . The force of spring  10  and pilot pressure from the pilot port  12  is greater than the spring force  7  on the opposite side of the pilot operated control valve  6 , moving the pilot operated control valve  6  towards the spring  7  to a position  9   a.  With the pilot operated control valve  6  in this position, fluid from the central pressurized oil supply  22  flows through line  24 , through the pilot operated control valve  6  to line  14  and the first chamber  3   a  of the double acting hydraulic actuator  2 . The fluid in the first chamber  3   a  moves the piston  2   b  mounted to the rod  2   c  in the direction of the arrow shown in the figure, moving the tapered end  2   d  of the rod  2   c  and the valve  100  (not shown) to a first position. Movement of the rod  2   c  of the double acting hydraulic actuator  2  compresses the tab  8  and the spring  7 , providing position feedback of the double acting hydraulic actuator  2  to the pilot operated control valve  6 . Fluid from the second chamber  3   b  exits the double acting hydraulic actuator  2  through line  16  to the pilot operated valve  6  to line  26  leading to sump  20 . 
         [0050]    Referring to  FIG. 5   c , the current to the proportional solenoid  32  on the one side of the proportional pressure control valve  70  is decreased and the force of the spring  72  and the pilot port  52  on the other side of the proportional pressure control valve  70  is greater than the force of the proportional solenoid  32 , moving the valve  70  to the right in the figure or away from the spring  72  and pilot port  52 . In moving the proportional pressure control valve  70  to position  74   a,  fluid from the central pressurized oil supply  22  through line  44  is blocked from flowing through the proportional pressure control valve  70  to the pilot port  12  on the pilot operated control valve  6 . Any fluid in the pilot port  12  flows out through a variable orifice of the proportional pressure control valve  70  to sump  48  and to line  73  to pilot port  52 , aiding in moving the proportional pressure control valve  70  with the aid of the spring  72  to the right in the figure. With the remainder of the fluid flowing to sump  48 , the force of spring  7  is greater than the spring force  10  and the pilot port  12  on the opposite side of the pilot operated control valve  6 , moving the pilot operated control valve  6  away the spring  7  to a position  9   c.  With the pilot operated control valve  6  in this position, fluid from the central pressurized oil supply  22  flows through line  24 , through the pilot operated control valve  6  to line  16  and the second chamber  3   b  of the double acting hydraulic actuator  2 . The fluid in the first chamber  3   b  moves the piston  2   b  mounted to the rod  2   c  in the direction of the arrow shown in the figure, moving the tapered end  2   d  of the rod  2   c  and the valve  100  (not shown) to a second position. Movement of the rod  2   c  of the double acting hydraulic actuator  2  decompresses the tab  8  and the spring  7 , providing position feedback of the double acting hydraulic actuator  2  to the pilot operated control valve  6 . Fluid from the first chamber  3   a  exits the double acting hydraulic actuator  2  through line  14  to the pilot operated valve  6  to line  26  leading to sump  20 . 
         [0051]      FIGS. 5   a - 5   c  are examples of fluid circuits that are controlled by a proportional relieving pressure reducing pilot valve. 
         [0052]    The valve  100  may be a gas operated valve, a waste gate valve, an EGR valve, a turbocharger, or a bypass valve, or any other device that needs to be positioned. 
         [0053]    The pilot operated control valve and the proportional flow control valve and the proportional relieving pressure reducing pilot valve each have at least three distinct positions and an infinite number of intermediate positions. 
         [0054]    Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.