Part 2: Adding Electronic and Computer Control to a Pneumatic Circuit
by David A. Ward
Nis operating manually, let's add some PHOTO 1
electronics to control it. If we add an
ow that the basic pneumatic system
electromechanical relay and two limit
switches, we can make the piston travel
back and forth continuously; check out the
electrical schematic in Figure 1. The limit
switches being used are momentary SPDT
miniature switches (Photo 1) and the
electromechanical relay is a DPDT 24 VDC
coil relay with a socket (see Photo 2). By
adding electrical limit switches at the fully
retracted position, the circuit will turn the
control valve and relay on in this position and keep it on
with the electromechanical relay contacts until the cylinder
rod fully extends.
Photo 3 shows a normally open contact. Photo 4 shows
it in the fully extended position. When the extended limit
switch is hit — which de-energizes the valve solenoid, as
well as the electromechanical relay coil — the piston and rod
will go back to the retracted position and the cycle will start
PHOTO 2
over again. The speed of the cylinder travel can be
controlled with the flow control valves.
As a final control demonstration, we'll show how the
pneumatic system can be controlled by a PLC (programmable
logic controller) or industrial computer. I realize that most
people's access to PLCs outside of industrial settings is
pretty limited, however, there is a two-part series of articles
entitled Getting Started With PLCs, published in the
PHOTO 3
PHOTO 4
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