A servo valve is a high-precision electro-hydraulic control valve that regulates fluid flow and pressure based on an electrical input signal. It is commonly used in systems requiring extremely accurate motion control, such as aerospace, robotics, injection molding, and industrial automation.

Understanding how a servo valve works requires breaking the process into stages: electrical control, electro-mechanical conversion, and hydraulic actuation.

1. Electrical Signal Input

The process begins when a controller (PLC, CNC, motion controller, or computer) sends an electrical command signal to the servo valve.

• The signal is usually a current (mA) or voltage (V).

• It represents the desired movement, position, speed, or force of a hydraulic actuator.

Example:
A +10 V signal may command the actuator to extend.
A –10 V signal may command it to retract.

The servo valve continuously adjusts based on these changing inputs.

2. Electro-Mechanical Conversion

Inside the servo valve, the electrical signal is converted into a small mechanical movement by:

• a torque motor

• or a piezoelectric actuator

The most common design uses a torque motor, which consists of:

• Two coils

• A magnetic armature

• A flexure tube to support movement

When the electrical signal energizes the coils, the armature rotates slightly—typically only a few microns—which creates a controlled movement inside the valve.

3. Hydraulic Modulation (Flapper or Jet Mechanism)

The mechanical movement shifts a small internal component—either a flapper or jet pipe—depending on valve type.

A. Flapper–Nozzle Servo Valve

This is the most widely used design.

1. The torque motor moves a flapper positioned between two nozzles.

2. The flapper partially blocks one nozzle more than the other.

3. This creates a pressure difference between the two nozzles.

4. The pressure imbalance shifts the main spool.

B. Jet Pipe Servo Valve

Used in contamination-resistant systems.

1. A small jet of hydraulic oil is directed toward two receiver ports.

2. The electrical input rotates the jet pipe left or right.

3. More flow enters one receiver than the other.

4. This creates a pressure difference that moves the spool.

Both designs convert tiny mechanical motions into a hydraulic pressure change that controls spool movement.

4. Spool Movement and Fluid Control

The spool is the main hydraulic control element of a servo valve.

When the pressure difference acts on the spool:

• It shifts left or right

• Opens or closes flow passages

• Precisely meters fluid to the hydraulic actuator

The spool movement is proportional to the electrical input signal.

The result:

• Higher signal → greater spool movement → higher fluid flow

• Lower signal → smaller spool movement → fine control

• Neutral signal → spool centered → no flow

The relationship is continuous and extremely precise.

5. Closed-Loop Feedback for Accuracy

Most servo valves include internal feedback mechanisms, such as:

• a mechanical feedback spring

• or a position transducer (LVDT)

The feedback system ensures:

• The spool always returns to the correct position

• The actual hydraulic output matches the input command

• Stable, accurate, repeatable control

Without feedback, the valve would drift or become unstable.

6. Controlling the Hydraulic Actuator

After the spool meters the fluid, hydraulic oil flows to:

• a cylinder

• a hydraulic motor

• or another actuator

This creates precise movement such as:

• Position control

• Velocity control

• Force / load control

• Dynamic oscillation

The servo valve adjusts flow continuously in real time—often hundreds of times per second.

Summary: How a Servo Valve Works

Servo valves provide extremely accurate, dynamic, and responsive hydraulic control—far beyond standard valves.