Implementing Electro-Pneumatic Controls

Relay and programmable logic controls


Relay controls

The signal control component of an electro-pneumatic control system consists of three function blocks, as shown in the graphic below:

Signal input unit (left), containing all sensors (two reed switches in this case), actuators and status switches.
Signal processing unit (centre, with yellow background), that receives input signals (I1 and I2) and generates actuator signals (O1 and O2). This component is implemented using relays connected together via conducting tracks.
Signal output unit (right) that passes signals to the actuators (in this case, the two electromagnetically operated directional control valves).


The entire signal processiing aspect of an electro-pneumatic control system can be implemented using relays. Relays were manufactured in large numbers in the past and are thus still used today. The prime advantage is in the simplicity of their design and their easily understood functionality. Nowadays, however, the trend is increasingly towards the use of programmable logic circuits as described below.


Programmable Logic Controls

With programmable logic controls the signal processing component is handled by a programmable microprocessor system, usually a PLC. The following graphic shows the same electro-pneumatic control system as above but this time it is designed with PLC.


Programming the PLC is accomplished with the aid of programming languages such as function charts, ladder diagrams or statement lists that are mostly graphically based. The screenshot below shows the user interface for a typical PLC software environment (STEP7), here shown in "function chart" mode. Use of PLC control is particularly beneficial for complex control systems with a large number of interconnected inputs and outputs.


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