Methods of Actuating Directional Valves

Methods of Actuating Valves

The are various ways of switching between the different switch settings of a directional valve, e.g. manually, pneumatically or electrically. The respective method chosen is also normally depicted on the circuit symbol of the directional valve by adding the corresponding symbol to the left-hand or right-hand side of the valve (emerging from the side in the bottom half of the symbol or in the middle). The following graphic shows an example of a 3/2-way valve with manual actuation (symbol on the left-hand edge of the box), whereby the return action into the rest position is carried out via a return spring (symbol on the right-edge of the box).

If the valve is actuated, the symbol changes in such a way that if the channel designations remain where they are, the box representing the valve appears to shift rightwards beneath them. The following graphic shows the corresponding symbol in the position for "forward bias" (air flowing from 1 to 2). The actuation is depicted by the double arrow.

In the following animation, actuate the switch by clicking on its surface to switch the valve from the rest position (venting of the cylinder from terminal 2 to 3, light blue) to the "forward bias" position (compressed air passes from 1 to 2, dark blue)

The following table lists several additional actuation methods that are commonly seen 

Electromagnetic Directional Valves

In electro-pneumatic controls electrically activated directional valves are normally used. These are operated with the aid of electromagnets and can be divided into two groups:

Spring-operated return valves maintain the actuated switch setting only as long as current flows through the solenoid coil. When current stops flowing through the solenoid, the spring restores the valve to its rest position. These valves are also referred to as monostable.

Magnetic pulse valves remain in the switch position last assumed, even if the solenoid coils are no longer carrying current. For that reason they are also called bistable. Naturally this type of solenoid valve has no unequivocal rest state because it has no reset spring.

Electromagnetic Directional Valves with Pneumatic Servo Control

Nowadays, the electromagnetic directional valves in use almost exclusively operate with pneumatic servo control because such valves are far superior to directly controlled directional valves in terms of power consumption, size of electromagnet and heat dissipation. The principle of pneumatic servo control is based on a system which has the armature of the electromagnets open an "auxiliary air channel" through which controlling air is fed to switch the actual main air channel (pneumatic servo), rather than having the armature switch the air flow directly to the load (whereby large flow rates would require a correspondingly large armature). The subsequent graphic shows the corresponding actuation symbol for this kind of electromagnetic directional valve with pneumatic servo control.