Servo Valves

Objectives

  • Explain what makes a servo valve different from normal valves
  • Determine how a servo valve works
  • List common types of servo valves

What is a Servo Valve?

Also known as an electrohydraulic servo valve, a servo valve is a control valve which tracks its own output and uses that information to make internal adjustments in order to more closely match the desired output.

This allows the servo valve to make corrections to errors caused by outside forces (ie vibrations, movement), internal system changes (oil temperature) or even imperfections in the valve itself.

Electrohydraulic servo valves are found in a wide range of industries, from aerospace to manufacturing to steering giant cargo ships.

These valves will be found in any system that requires a high level of precision or fast actuator response.

How it Works

A servo valve is controlled using two electrical signals.

Control Signal
P
T
A
B
Controller/Amplifier
Magnet
Example servo circuit. The cylinder has a magnetic displacement transducer (MDT) which tracks the rod position.

The first signal is the command signal.

This determines what the actuator should be doing and what the valve has to do to make that happen. This signal comes from a control system such as a PLC program or a joystick.

The second signal is feedback from after the output of the valve.

This is taken from a sensor that is tracking what the actuator is actually doing.

These two signals are then compared by the servo valve controller.
Any difference between the two signals is used to generate a new input signal that drives the servo to a new position.

This process continuously repeats until there is no difference between the control signal and the feedback signal.

Port
Land
No Overlap

Servo valves normally use a zero lapped (critical) center valve spool.
This means that there is virtually no overlap between the valve ports and lands.

This gives the valve the ability to respond immediately and in a linear fashion to changes in the input signal.

Flow
Displacement
Closed Center
Position
Spool Displacement vs Flow

These valves need to be manufactured to a very exact level of precision in order to work correctly.

Normal wear, seen as rounding (erosion) at the valve ports, is enough to drastically reduce the effectiveness of the servo valve.

Sleeve
Spool

To compensate for this, servo valve commonly use a hardened steel sleeve between the ports and the spool itself.

This sleeve acts as a sort of guide or funnel to compensate for the imperfections in the porting, directing fluid more precisely to the spool.

Metering Notch

The sleeve may also have metering notches to further increase the spools response accuracy.

Servo valves can be configured to adjust for a number of different feedback types.

The most common configurations are

  • Position
  • Speed
  • Force
Position Servos

This is the most common of the servo configurations.
These can be used to track either linear or rotational position.

Speed Servos

Used to control the speed of an actuator.
This can be either linear or rotational speed.

Force Servos

Controls the force acting on an actuator.
For example, the hydraulic pressure in a cylinder or the torque acting on a spinning shaft.

Try it Out

Use the slider to change the target speed for the motor.

Observe how the valve responds as the motor nears the desired speed.



The response speed has been slowed waaaaay down for dramatic effect!


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We hope you enjoyed Servo Valves

Vacuum Pressure
Atmospheric Pressure
Low Pressure
Medium Pressure
High Pressure
Ground/Common
Lowest Voltage
Medium Voltage
Highest Voltage
Magnetic Field
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