What is a servo valve?
A servo valve is an electrohydraulic valve that turns a low-power electrical signal into precise control of hydraulic flow or pressure. It is used in closed-loop systems that need fast, precise motion.
What does a servo valve do?
It sets the direction and amount of oil going to a hydraulic actuator according to an electrical command. A cylinder’s position, speed or force can then be controlled in a closed loop, together with feedback from a sensor.
Working principle
In a common two-stage servo valve, operation goes like this:
- Current through the coil creates a torque in the torque motor, proportional to the current.
- This torque upsets the balance in the pilot stage (for example a nozzle-flapper) and changes the pressure difference across the spool.
- The pressure difference moves the main-stage spool, and the spool sets the opening the oil flows through.
- A feedback mechanism reports the spool position to the torque motor or the electronics, and the spool settles at the position that matches the command.
Why is it driven by current?
Torque is proportional to coil current. Because coil resistance changes with temperature, the current also changes under voltage drive. Servo valve drivers therefore control the coil current in a closed loop.
Where is it used?
- Hydraulic actuators in aerospace and defence systems
- Presses and machines that need precise position control
- Material test systems and test benches
- Industrial hydraulic applications that need high dynamics
What is checked in servo valve testing?
Servo valve tests usually look at the flow curve against command, hysteresis, null shift, internal leakage and frequency response. Some of these measurements need a hydraulic test setup that measures flow and pressure.
Frequently asked questions
What is the difference between a servo valve and a proportional valve?
Servo valves are generally designed for higher dynamics and precision and are used in closed loops. Proportional valves are simpler and generally less sensitive to oil contamination, but their dynamics are usually lower.
Why is a servo valve sensitive to oil contamination?
The narrow passages in the pilot stage and the fine clearances of the spool are easily affected by particles in the oil. Oil cleanliness therefore matters in servo valve systems.
To drive and test servo valves
MotorTech HSV drives servo valves under current control and reads LVDTs on the board.