ELECTRO-HYDRAULIC VALVES: A TECHNICAL LOOK

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.

Moog Valves
Operating principles

Nozzle Flapper Operated Valves

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.

Functional Principle of a Torque Motor

A torque motor consists of permanent magnets, an upper and a lower pole piece, an armature and a coil.

The basic principle is to create a torque that rotates the armature around its fixed point.

The charged permanent magnets polarize the pole pieces.

A DC current applied to the coils causes an increased force in diagonally opposite air gaps. The magnitude of the force is proportional to the current. The direction of the force depends on the direction of the current.

Functional Principle of a Nozzle Flapper Hydraulic Amplifier

Armature
Flapper
Flexure sleeve
Inlet orifice
  • The armature and the flapper are rigidly joined and supported by a thin-wall flexure sleeve, which also acts as a centering spring for the torque motor.
  • The fluid continuously flows from the pressure side PS, through both inlet orifices, past the two nozzles into the nozzle flapper space, and then through the drain orifice to tank.
  • A rotary motion of the armature/flapper throttles the flow through one nozzle or the other. This diverts flow to one end of the spool.

Nozzle Flapper Piloted Valve with Mechanical Feedback

  • These valves contain a closed loop control circuit to control the position of the spool. Since the feedback of the spool position is transmitted via a mechanical feedback spring, they are often referred to as mechanical feedback valves (MFB valves).
  • An electrical current in the torque motor coils creates magnetic forces on the ends of the armature. Thus, the armature and flapper assembly rotates about a fixed point.
  • The flapper closes off one nozzle and provides for an increase in pressure in that flow path. The pressure increase acts on the end of the spool causing the spool to move and opens PS to control port A. At the same time it also opens control port B to T.
  • The spool pushes the ball end of the feedback spring creating a restoring torque on the armature/flapper.
  • As the feedback torque becomes equal to the torque from the magnetic forces, the armature/flapper moves back to its centered position. At that time the spool stops at a position where the feedback spring torque equals the torque due to the input current. Therefore, the spool position is proportional to the input current. With constant pressures and loads, the flow to the actuator is proportional to the spool position.

Valve responding to change in electrical input

Valve condition following change

Moog Valve Types Explained

Moog produces many different valve types to suit the different applications of our customers. If you ever asked yourself what is the difference between a servo and a proportional valve, or the difference between an MFB or EFB valve, or what a pQ valve or ACV can do, then you should read the entire guide.

Servojet Operated Valves

Functional Principle of the ServoJet Hydraulic Amplifier

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.

Jet pipe
Receiver
Annular area
Nozzle
  • The ServoJet® pilot stage consists mainly of a torque motor, a jet pipe, and a receiver.
  • A current through the coil displaces the jet pipe from its neutral position. This displacement, combined with the special shape of the nozzle, directs a focused fluid jet on both receivers towards one receiver. This produces a pressure differential in the control chambers to the end of the spool.
  • This pressure difference causes the spool to move.
  • The pilot stage drain is through the annular area around the nozzle to tank.

ServoJet® Piloted Valve with Electrical Feedback

  • An electrical command signal corresponding to the desired spool position is applied to the integrated electronics, which drives the valve coil.
  • The current through the coil displaces the jet pipe from its neutral position.
  • The displacement of the jet directs the flow to one end of the spool thereby increasing the driving force on the end of the spool.
  • The spool moves and opens P to one control port, while the other control port is open to tank.

D661 valve in center position

D661 valve in open position P - > B

Need Help Sizing a Valve?

When selecting a servo or proportional valve for an application, it is important to not only consider the valve’s characteristics, but also take a look at the system around the valve. Our complete guide provides advice on the different valve and system parameters you should consider when selecting a valve.

LINEAR FORCE MOTOR OPERATED VALVES

Functional Principle of the Linear Force Motor

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.

  • The motor consists of a pair of high energy rare earth magnets (1), centering springs (2), an armature (3), and a coil (4).

  • Without a current being applied to the coil, the magnets and springs hold the armature at equilibrium.

  • 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.

Direct Drive Valve (DDV) with Electrical Feedback

Bushing
Position transducer
Null adjust cover plug
Integrated electronics
Spool
Valve connector
Centering spring
Linear Force Motor
  • An electrical signal corresponding to the desired spool position is applied to the integrated electronics and produces a pulse width modulated (PWM) current in the linear force motor coil.
  • The current causes the armature to move which then directly activates the spool.
  • The spool moves and opens pressure P to one control port, while the other control port is opened to tank T.
  • The position transducer (LVDT), measures the position of the spool by creating an electrical signal that is proportional to the spool position.
  • The demodulated spool position signal is compared with the command signal, and the resulting electrical error changes the current to the Linear Force Motor’s coil.
  • The spool moves to its commanded position and the spool position error is reduced to zero.
  • The resulting spool position is thus proportional to the electrical command signal.

Pilot Operated Valve with DDV Pilot

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.

  • An electrical command signal corresponding to the desired spool position is applied to the integrated electronics which drives the Linear Force Motor.
  • The current through the Linear Force Motor displaces the pilot valve’s spool from its neutral position.
  • The pilot valve’s spool moves and opens pilot pressure to one of its control ports AV or BV, and thus to one of the main spool’s control chambers.
  • The main stage spool moves and opens P to one control port, while the other control port is open to tank T.

D684 valve in center position

D684 valve in open position P - > B

The Complete Technical Look

When laying out an electro-hydraulic system, a lot of different system components and characteristics have to be considered. In our complete guide, we will give you an overview of some important guidelines you should take into account when planning your system.

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