Electrohydraulic servo valve

A servo valve is a variant of the continuous valve. It allows for any intermediate position of the valve opening and thus of the fluid flow. It differs from the simple proportional valve by the execution of the piston control edges, with the help of a larger bandwidth of the frequency response ( in the industry as " dynamic " called ) is reached. This makes servo valves in particular in control theory very valuable. The precision, which is necessary for the production, however, makes them expensive.


Servo valves are solenoid controlled proportional valves. You have pistons that give a possible linear dependence between control current and flow. Proportional valves have a dead zone at zero crossing and its advantage lies in other areas. The advantage of servo valves is that they have there is no dead zone. Therefore, the zero crossing is fully utilized, ie, on both sides, the positive and negative side, which means flow in both directions. Therefore servo valves are at least three-way valves or valves of higher order.

Single-stage servo valves are driven by an electromagnetic force and move a piston. Two-stage servo valves are available, distinguished by the functional principle, in flapper jet version ( flapper servo or flapper - nozzle servo) and with a movable nozzle ( jet pipe servo). Servo valves can be combined with larger medium-controlled proportional valves to multi-position valves. Medium -controlled servo valves are usually only as factory installed main stage of a two-stage servo valve, and not separately available.


Single-stage servo valve

The piston of the single-stage servo valve is moved directly from the coil. It is similar in the drive a proportional valve. It is so constructed that the coil ( with the current Is ) attracting the magnetically relevant superstructure on the piston in a substantially linear area. A spring acts on the other hand, in this case on the other side. When de-energized, the valve shown is not in the blocked state. The coil must be driven with about 50 % that the piston moves to the center.

The servo valve is driven by a linear motor or a torque motor, instead of a coil, the plunger can be actively moved in both directions. He then in the middle of an evoked by springs electroless zeroing in locked position.

Flapper servo valve

The baffle plate is part of a respective servo to the first stage of the two stages the existing valve. Furthermore, the two jets and the electromagnetic driving part of it. At the second stage is one of the piston of the servo valve. The baffle plate is placed in the middle between two narrow opposed nozzles. It is influenced by the electric control current of the servo valve and of the valve piston of the second stage, and it affects the hydraulic fluid in the nozzle. The influence of the piston of the second stage is the negative feedback of the integrated mechanical control circuit which ensures that the piston reaches the desired position, and as long as the coil current korriert until this is the case.

Still no current flows, the plate is in the middle or oscillates rapidly through the loop in the middle of one.

Now a current flows as in the second image of the anchor exerts a force on the baffle. From the two nozzles always comes a liquid jet. The plate is drawing to a nozzle, here to the right, there can not easily escape through the liquid. Thus, the pressure in the underlying line increased. Because it is connected to the piston of the second valve stage, it moves to the left.

Now the loop comes into play. The piston pushes the flapper about the spring back in such a manner, until it has restored the balance between left and right-hand nozzle and the piston thus does not move. The baffle plate thus has the desire to always be in the middle. So that the displacement of the piston is proportional to the electromagnetic force exerted by the anchor, in this case approximately half the opening of the valve. The current changes, the piston follows after it.

This is also true for the case that the current, as shown here in the second image starts. The line at the left nozzle has increased pressure and pushes the piston to the right. This continues until it levels off in normal position.

Zero point adjustment

The flapper servo valve thus has a mechanical zero point comes when no current is applied. In the state of the working line terminals A and B should be blocked. Because these valves are also reasons of precision to the pistons have a matching sleeve so that the zero point can be adjusted.

It is to the sleeve, to the left, move like this. The valve is in operation so that the flow can be controlled. If behind a cylinder is directly connected, his movement can be observed for flow measurement if another valve is connected, the movement of the piston this indirectly controlled valve can be measured.

Measurement and control

A servo valve can have an internal mechanical control when two stages are involved. In addition, the position of the piston can be monitored also in single-stage servo valves. Normally this is done with non-contact differential transformers ( LVDTs ). With this information, an electric control circuit can be built. He is, if any, superior to the mechanical loop, and gives him the calculated setpoint. The electrical control is limited to the most rapid and accurate reaching the desired piston position. You can also correct an incorrect zero point setting, but needs for a short time, and can work only with available power.

The piston position control closes another parent control is not enough. You could check the cylinder position, and correct for deviations between the setpoint of the controller and the actual value.

Zero overlap of the piston control edges


The control edges are the parts on the valve piston, the seal or release the openings to the working line ports A and B. They are ultimately responsible for the flow of the valve. The piston of the servo valve is formed so that it lies at rest just above the operating line opening, here it is on A. The piston or the notches are contained with the terminal edge of the edge -to-edge. At the slightest movement of the piston, the hydraulic fluid in the working port begins to flow, here in the outlet A.

Modular design

Servo valve with electrical feedback

Servo valve with barometric feedback

Servo valves are in practice - modular design - as shown in the pictures. This makes it possible to standardize individual components and servo valves according to the modular principle - optimized for the particular purpose - to assemble.


Due to the usually required in practice precision of a servo valve servo valves are the most expensive type of valve on the market. Therefore, servo valves are used only for special applications, such as when a proportional valve is not sufficient. In addition, they consume during operation always some hydraulic fluid, even without moving anything. Servo valves come about in the power steering in a car or in the operation of control surfaces in an aircraft for use, as well as in various positioning tasks of hydraulic drive technology.