Introduction to Industrial Automation

valve (a), and a hand-actuated, 3/2 directional control valve (b).

(a)

3/2 Valve energized

Air

inlet

Solenoid

Exhaust

 

To cylinder

From cylinder

Air

inlet

Exhaust

(b)

3/2 Valve non-energized

Spring

Plunger

Orifice

seals

t

enoid

Exhaust

To cyli

yy nder

From cy

cc

Air

inlet

Spring

Plunger

seals

Figure 5.14  A 3/2 directional control valve with electrical actuation in two operation states 

(a) energized and (b) non-energized.

Figure 5.13  Symbols of various actuation mechanisms for the mechanical control of a direc-

tional control valve.

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Elements of Electro-Pneumatic Components 

◾

 

173

the piston rod in one direction, while the termination of the voltage application causes the return 

of the piston rod to its rest position. As has been mentioned, the return of the rod to its normal 

position is achieved by a return spring; however, this could also be accomplished by the utilization 

of a second solenoid that could actuate in the opposite direction. In Figure 5.15a, the symbol of 

an electrically actuated three-way, two-position valve with return spring is presented, while Figure 

5.15b shows the same valve with two solenoids and without return spring. The direction valves 

with three switching positions and electrical actuation are carrying two solenoids, one in each side 

of the valve. Due to the numerous types of valves, solenoids, and their corresponding function-

alities, a generalized form of symbolic representation has been adopted for the electro-pneumatic 

valves, which is as follows:

 

◾

In a valve with two switching positions, the “operation” that is indicated by the rectangle 

located next to the spring symbol, takes place when the valve is not energized.

 

◾

In a valve with three switching positions, the “operation” of the central rectangle takes place 

when the valve is not energized.

 

◾

When a solenoid is energized (in any kind of a valve), then the operation of the rectangle that 

is next to the solenoid takes place.

The previous rules, as well as the operation of two classical directional valves, are presented in 

Figure 5.16.

Return

spring

Solenoid

Double solenoid

actuation

(a)

(b)

Figure 5.15  Symbol of a 3/2 direction control valve with the electrical actuation of one sole-