FIGURE 5. This manually operated pneumatic valve is analogous
to an electrical SPDT spring-return pushbutton switch.
left one. Moving the legends to the right box positions
results in EB being blocked, P feeding I/O port B, and I/O
port A exhausting through the EA port. Figure 4 tells the
right box story.While we were up on our donkey, we also
assumed that the control valve depicted in Figure 1 had a
suitable means of actuation to transition between the three
possible control valve positions. However, there is nothing
in the 5/3 symbol we drew up in Figures 2, 3, and 4 to
support our assumption. In reality, the most common
pneumatic control valves use springs and/or solenoids to
position their internal flow control mechanisms. Figure 5
is an at-rest cutaway view of a spring loaded, manually
operated directional control valve. Note that this manually
operated control valve uses a common return port, R.
The spring has control in Figure 5 and the flow entering
P is being directed out of the B port while the A port is
exhausting to R. Let’s draw it up. The directional control
valve in Figure 5 has two positions and four ports.
Thus, we will need two boxes for our 4/2 pneumatic
valve symbol. When the 4/2 valve in Figure 5 is not
actuated, the spring forces the air flowing into P out of
port B while port A is exhausted to R. To identify that this
FIGURE 6. The spring controls the flow in the left box which
is the valve’s unactuated state. When the pushbutton
is depressed, the right box actuated logic comes into play
and the direction of flow is shifted from P to B to P to A.
position is under control of the spring, we attach the spring
symbol to this box in our 4/2 valve symbol. Since there are
only two positions, the pushbutton symbol must be attached
to the remaining box. The port symbols are usually drawn
on the normal position (unactuated) box. The 4/2 valve you
see in Figure 5 is drawn up in Figure 6.
Let’s press the button. As you can see in Figure 7, the
spool has moved to the left and the spring is compressed.
Port B is now exhausting to R, and flow into P is exiting at
port A. This is the condition that exists in the right box of
Figure 6 now that the pushbutton controls the valve.
To automate the air flow of a pneumatic system, we
must replace the manually operated directional control
valves. Being a microcontroller head, I do this by letting a
microcontroller “depress” the pushbuttons. In the case of a
microcontroller, the pushbutton becomes a solenoid that is
under control of one of the microcontroller’s I/O pins. I’ve
replaced the pushbutton with a solenoid in Figure 8. The
slash (/) winding symbol is coupled with a triangular internal
pilot symbol. This combination of pneumatic symbols tells
us that the 4/2 valve is operated by a solenoid and that all
of the necessary flow switching mechanics are contained
within the valve itself. Thus, the valve in Figure 8 is said to
have solenoid drive with an internal pilot. Think of a pilot
FIGURE 7. This is the 4/2 valve’s activated position.
The redirection of the flow is obvious.
44 SERVO 11.2009
FIGURE 8. The pushbutton has been replaced by an
internal solenoid/pilot mechanism. Now an electrical
signal under the control of a PIC microcontroller
can control the direction of flow.