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Servo Valves Information

Servo valves (or servo valves) provide closed loop flow or pressure response to an electrical or electronic control signal. They can be infinitely positioned to control the amount, pressure and direction of fluid flow. The distinction between servo valves and proportional valves is inconsistently defined, but in general, servo valves provide a higher degree of closed-loop control. Both types of valve are used for control in pneumatics, hydraulics, gas, steam, water transport, and other specialized applications. In a conventional open-loop force control system, servo valves output pressure, which is applied to the hydraulic piston that drives the load. The controlled pressure may be the differential between the two sides of the load actuator or it may be the pressure in a single line connected to one side of the load actuator.



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The response of the load to pressure from the servo valve may be measured electrically and fed back to create closed-loop servo. Pressure control servo valves are sometimes used in position servos (with load position feedback) where severe load resonance problems exist and where servo stiffness is of secondary importance.


In servo valves, a given electrical signal produces a definite position of the main-stage spool, but it does not necessarily produce a fixed flow. Flow is a function of the square root of the difference between supply pressure and load pressure. Thus, as load pressure increases, both flow and effective pressure drop across the valve decrease.


Gain-compensated servo valves incorporate internal feedback to correct for load-fluctuation effects on output and, thus, more nearly approach the ideal steady-state curve. Amount of correction depends on the valve design. Where high accuracy is needed for either velocity or position, experts recommend an external transducer and feedback loop.



Stage Designs

Servo valves are available in one, two, or three stage designs.  A single stage is a directly operated (direct drive) valve.  Two valve stages are comprised of a pilot stage and final / main stage.  Three stage valves are similar, except that the pilot itself is a two-stage servo valve.  Three stage servo valves are used in situations where very high flow is anticipated.


The first stage of servo valves is the hydraulic or pneumatic amplifier.  This stage may be one of four main designs.  Flapper-nozzle, jet pipe, force motor, or solenoid.  Flapper-nozzles use a cylindrical curtain orifice area, formed by a flat plate moving toward a sharp edged orifice, to amplify the flow. Jet pipes convert fluid pressure into momentum via a moveable nozzle.  Force motors are direct drive pilot stages that use a permanent magnet differential motor to amplify flow.  Solenoid servo valves simply use a solenoid to control the flow. 


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