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A two, three or four-way valve includes two stages each with working valve components linked by a feedback spring so that a first stage servo unit is auto-nulling and the stroke of the valve member in a second stage valve unit is proportional to the input current to the first stage. An inherently ba

A two, three or four-way valve includes two stages each with working valve components linked by a feedback spring so that a first stage servo unit is auto-nulling and the stroke of the valve member in a second stage valve unit is proportional to the input current to the first stage. An inherently balanced clevis member in the first stage controls flow of pressurized control media, which in combination with direct pressure flow, controls the position of a spool member in the second stage. The spool in turn controls pressure flow to a either one or two metering or input/output flow ports. Transient movement of the clevis due to change in input current to a magnetic servo drive assembly is opposed by bending of the feedback spring arising from the associated movement of the spool, which returns the clevis to its centered null position and stops the spool.

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1. An electrohydraulic servo valve, comprising:a first stage servo unit having a drive assembly adapted to move a forked clevis member from a null position to alternatively open and close first and second nozzle orifices such that when the first nozzle orifice is open flow is permitted between a pre

1. An electrohydraulic servo valve, comprising:a first stage servo unit having a drive assembly adapted to move a forked clevis member from a null position to alternatively open and close first and second nozzle orifices such that when the first nozzle orifice is open flow is permitted between a pressure port and a control port and when the second nozzle orifice is open flow is permitted between the control port and a return port; anda second stage valve unit having a sliding valve member and having an inlet port in communication with the first stage control port, a flow port and a pressure port, wherein flow from the second stage pressure port to the flow port is controlled by the sliding valve member;whereby the clevis member and the sliding valve member are linked by a feedback spring such that movement of the sliding valve member imparts a feedback force to the feedback spring to return the clevis member to the null position such that movement of the sliding valve member is proportional to an input signal to the drive assembly. 2. The valve of claim 1, wherein the sliding valve member is a spool cooperating with a stationary piston. 3. The valve of claim 2, wherein the second stage valve unit includes a fixed guide sleeve in which the spool is slidable. 4. The valve of claim 2, wherein the piston has a cross-sectional area essentially one half that of a cup end of the spool. 5. The valve of claim 2, wherein the piston is held stationary by flow from the second stage pressure port. 6. The valve of claim 2, wherein the sliding valve member moves under the force of flow from the first stage control port. 7. The valve of claim 2, wherein the second stage valve unit further includes a return port. 8. The valve of claim 2, wherein the second stage valve unit further includes a second flow port and wherein the two flow ports are input/output ports and the spool alternatively closes off flow from the second stage pressure port to either of the input/output ports. 9. The valve of claim 2, wherein the feedback spring is pivotally engaged with the spool. 10. The valve of claim 9, wherein the feedback spring has a ball end and the spool as a groove receiving the ball end. 11. The valve of claim 1, wherein the drive assembly is a permanent magnet motor having a wire coil and a movable actuator member connected to the clevis member and disposed along a main axis. 12. The valve of claim 11, wherein the first stage valve unit further includes a flexure pivot allowing the clevis member to pivot with respect to the main axis to control flow through the nozzle orifices. 13. The valve of claim 12, wherein the flexure pivot has a movable part and a non-moving part in a plane spaced from the movable part and joined thereto by a flexible spoke. 14. The valve of claim 1, wherein the nozzle orifices are slots formed in a nozzle body. 15. The valve of claim 14, wherein there are two pairs of nozzles orifices on opposite flat sides of the nozzle body. 16. The valve of claim 15, wherein said nozzle body is partitioned and has a bore through the partition through which the feedback spring extends. 17. The valve of claim 16, wherein the partition is disposed between pairs of the nozzle orifices. 18. The valve of claim 17, wherein the forked end of the clevis member has two prongs disposed on opposite sides of the nozzle body. 19. The valve of claim 18, wherein each prong has tapered leading edges at opposite lateral sides. 20. The valve of claim 19, wherein the feedback spring is disposed through the first stage control port and the second stage inlet port. 21. A four-way electrohydraulic servo valve, comprising:a servo first stage unit including:a valve housing having pressure, control and return ports;a magnetic drive assembly disposed in the valve housing selectively pulling an armature toward opposite sides of a main axisa flexure pivot disposed about the main axis and having a movable part mounted to the armature and a non-moving part fixed to th e housing to allow pivoting of the actuator member with respect to the main axis;a clevis member mounted to the armature and the movable part of the flexure pivot at one end and having a forked end defining two prongs symmetrical about the main axis, the clevis being pivotal about the flexure pivot in response to movement of the armature such that the clevis member moves from a null position to alternatively open and close first and second nozzle orifices;a feedback spring mounted to the clevis member; anda spool valve second stage unit having a spool disposed in a valve housing having an inlet port in communication with the first stage control port, a pressure port, a return port and two input/output ports, wherein the spool valve is linked to the clevis member by the feedback spring disposed through the first stage control port and the second stage inlet port;whereby movement of the spool valve imparts a feedback force to the feedback spring to return the clevis member to the null position such that movement of the spool is proportional to an input signal to the drive assembly. 22. An electrohydraulic servo valve, comprising:a first stage servo unit having a drive assembly adapted to move a clevis member from a null position to alternatively open and close first and second nozzle orifices such that when the first nozzle orifice is open flow is permitted between a pressure port and a control port and when the second nozzle orifice is open flow is permitted between the control port and a return port; anda second stage valve unit having a sliding valve member and having an inlet port in communication with the first stage control port, a flow port and a pressure port, wherein flow from the second stage pressure port to the flow port is controlled by the sliding valve member;whereby the clevis member and the sliding valve member are linked by a feedback member such that movement of the sliding valve member imparts a feedback force to the feedback member to return the clevis member to the null position such that movement of the sliding valve member is proportional to an input signal to the drive assembly.