At Moog, we have been producing electro-hydraulic servo valves since 1951. Our experts have had countless discussions with our customers about their applications and how to use our valves to achieve the best performance for their applications. We have written a guide to deal with topics that are essential for making the best use of our valves.
Below is an interactive portion of our guide covering the operating principles of our valves. You can download the full guide that includes this material, as well as valve sizing, valve types, practical considerations when designing EH control systems, and routine maintenance information.
Nozzle flapper operated valves are piloted by a hydraulic amplifier that is actuated by a torque motor. We will start by describing the functional principle of a torque motor. Then we will explain the nozzle flapper hydraulic amplifier, and finally we will show you the function of a nozzle flapper piloted servo valve with mechanical spool position feedback.
A torque motor consists of permanent magnets, an upper and a lower pole piece, an armature and a coil.
Valve responding to change in electrical input
Valve condition following change
ServoJet® piloted Moog Valves are exclusively built as electrical feedback valves (EFB valves). This means that the valve’s internal spool position control loop is closed by an electronic controller and a position transducer. And onboard electronics (analog or microcontroller based) control the current to the torque motor coils.
D661 valve in center position
D661 valve in open position P - > B
Moog Direct Drive Valves (DDV) use our proprietary Linear Force Motor. A Linear Force Motor is a permanent magnet differential motor. The permanent magnets provide part of the required magnetic force. The Linear Force Motor has a neutral mid-position from which it generates force and stroke in both directions. This is an advantage against a proportional solenoid drive, which can only operate in one direction. Force and stroke of the Linear Force Motor are proportional to the applied current.
When a current is applied to the coil with one polarity, the flux in one of the air gaps surrounding the magnets is increased, cancelling out the flux in the other.
This unequilibrium allows the armature to move in the direction of the stronger magnetic flux.
The armature is moved in the opposite direction by changing the polarity of the current in the coil.
DDV piloted valves are always built as electrical feedback valves. This means that the valve’s internal spool position control loop is closed by an electronic controller: The spool position is fed back by a position transducer, and an onboard electronics (analog or microcontroller based) controls the current to the Linear Force Motor coils.
D684 valve in center position
D684 valve in open position P - > B