What is a Programmable Logic Controller?

A Programmable Logic Controller, also known as PLC, is a robust computer designed to perform control functions in industrial applications. PLC’s were initially built to replace electromechanical relay systems that were very large and complex control systems.

When it comes down to modifying the operation of a control system there is no need to rewire a large bank of relays, instead, a quick download from PC or programming device enables control logic changes in a matter of seconds. With a very large scale of applications, PLCs today are modular meaning users can add a wide assortment of functionality including position control, analog control, motor control, etc.

Main components and Functions

            Programmable Logic Controllers consist of 2 sections, Central Processing Unit (CPU) and Inputs/Outputs. The CPU stores and process program data, while I/O modules are connected to the rest of the machine providing information to the CPU. CPU reads input data from connected field devices through the use of its input interfaces and then executes or performs the control program that has been stored in its memory system. PLC programs are created through ladder logic which closely resembles relay-based wiring schematics, some programs allow to write functions in C or program in C as well, but ladder logic is most common for plc Programming. Based on the program, the PLC writes or updates output devices via the output interfaces. This process, also known as scanning, typically continues in the same sequence without interruption and changes only when a change is made to the control program. Some applications may depend on some type of communication to export application data recorded. PLCs come equipped with numerous ports and communication protocols that allow the PLC to communicate with multiple systems. In order to communicate with PLC in real-time, an HMI or Human Machine Interface will be needed. HMI unit is usually a small or large touchscreen panel that will allow a user to input data in real time to the PLC.

PLC Applications

Today many Industries are implementing PLC’s because the return from this technology is high. Some industry applications include Manufacturing industry, Chemical industry, Assembly industry, Glass industry, Cement industry, Paper industry and many more. PLC use in automation processes increases reliability, flexibility, accuracy, efficiency, and reduction in production cost. In large process plants, PLCs are being increasingly used for automatic startup and shutdown of critical equipment. A PLC ensures that equipment cannot be started unless all the conditions for a safe start have been established. It also monitors the conditions necessary for the safe running of the equipment and trips the equipment whenever any abnormality in the system is detected. PLC use is growing between many industries, and with its modular design, these controllers can be used in numerous types of applications.