Servo - February 2009

Part 2: Adding Electronic and Computer Control to a Pneumatic Circuit

by David A. Ward

Nis operating manually, let's add some ^PHOTO ¹electronics 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