What is a Programmable Logic Controller?

Programmable logic controllers, otherwise known as PLCs are a type of computer which has been broken down to perform specific tasks. PLCs are used in the industrial automation industry, where they are programmed to perform a single task. Such as controlling a single robotic arm, accomplishing a single function or even managing a whole production line (if used as cascaded systems).

Different Components of PLC

Most PLCs have a fixed amount of internal components. Some of the different components of PLC are listed below:

1- Rack Assembly

As most PLC systems have multiple internal components, a rack (something like a bookshelf) is used to stack the components so that all the electrical modules are held together.

2- Power Supply

The power supply provides the internal components of the PLC with the required DC voltage. Most PLCs require 24v to power up.

3- Processor (CPU)

The central processing unit is considered as the brain of the PLC. As a PLC can only perform certain specified tasks, the size of internal memory, processing speed, size of application logic as well as the number of programmable functions is quite small as compared to a desktop computer’s CPU.

programmable logic controller

Working Principle of Programmable Logic Controller

As there are multiple components present in a PLC, the working of a PLC is comparatively complex as well. There are two types of inputs and outputs of a PLC, Analog and Digital. For the input of PLCs, external components such as pressure sensors, input switches or even operator inputs may be used. As for the output, motors, lights, relays are used. A PLC is a programmable device, which can be programmed by using special software with which the user can calibrate the equipment to perform certain tasks by altering the logic used.  The PLC receives input from the external components which have been programmed to collect information about the ongoing tasks. The PLC, using the CPU and the programmed logic, makes decisions and gives the output in terms of a digital or analog signal. A PLC also has the feature of monitoring data in real time and alter the output in a short period.

programmable logic controller

Different types of PLC

Programmable logic controllers are ruggedized computers that have flexibility in programming the logic functions, due to which they can be made to be adaptable to any circumstances. There are different types of PLC that are available in the market. Some of them are listed below:

1- Integrated PLC

A PLC which is made by linking components within a single case, where the input and output ports and the specifications are fixed according to factory settings by the manufacturer.   

2- Modular PLC

A PLC in which the internal components are plugged into a standard rack. The internal components, such as the I/O ports, CPU or the memory may be altered according to the user requirements. By simply replacing the components that are plugged into the rack, the specifications of the PLC may be changed.


3- Small/Medium/Large PLC

These PLC are used in industries according to the input and output requirements and the computation speed needed. Small PLCs may be used for a single piece of equipment, such as a robotic arm in an assembly line. The medium and large size PLCs may be used to control the whole manufacturing line or even multiple assembly lines simultaneously.