How do PLCs work?

 In Documentary

PLCs (programmable logic controllers) are essentially a computer – but smaller and built for industrial applications. They are used to automate control processes such as operation of an HPU (hydraulic power unit), ballast system on a large vessel, or motors, pumps and other functions in a manufacturing environment. PLCs were invented to streamline the first attempt at industrial automation – electromechanical relay circuits. These relay circuits required one circuit per function controlled.  PLCs replaced this with one computer using ladder logic which essentially stacks multiple relay circuits all controlled by one central machine.

PLCs have three main components: a processor (CPU), inputs and outputs. The CPU acts as the “brain” as it receives the inputs, applies the rules associated with that input and produces a corresponding output which will control the actuator – pump, motor, valve, etc. Inputs come from signals such as sensors, switches or push buttons.

The two main types of I/O (inputs or outputs) are discrete and analog.  Discrete refers to a function that is “on/off” like a switch controlling a light or a “power-on” switch.  Analog refers to signals that have a value range greater than 1 or 0 and usually refers to measurements of voltage or current. Examples of this in the industrial space are pressure sensors, weight sensors or temperature sensors. 

The final piece of the puzzle with PLCs is the language that they speak when transmitting signals. These are called industrial communications protocols and many different options exist.  The oldest option is Modbus which is still widely used – others include DNP, ControlNet, EtherNet/IP and much more.

Standing alone, a PLC is not a powerful machine. It still needs human programming expertise to build the ladder logic which will determine how the processor will receive inputs and generate outputs to successfully control the application. Elite Controls, Inc. has the engineering expertise to help you program a PLC to use for your application.