Programmable Logic Controllers (PLC): Basics, Types & Applications

In modern society, everyone depends upon automated machinery to carry out several fundamental industrial mechanisms starting from producing electricity to weaving fabric. Automation technology has enabled increased production and reached new heights of innovation. To make all the processes possible, a critical invention known as a Programmable Logic Controller (PLC) is required. 

Defining Programmable Logic Controller or PLC Controller 

A PLC is a type of computer specifically developed to function reliably under harsh industrial surroundings like dry, dusty, or wet conditions, and extreme temperatures. They are used to automate industrial procedures like wastewater treatment plants, processing plants, and production plant assembly lines. 

PLCs share several characteristics similar to PCs that any individual use at home. PLC and PC both have inputs and outputs (I/O), a CPU (Central Processing Unit), a power supply, and various operating software. The only major difference is that PLCs are better with rough industrial surroundings. Often, PLC is regarded as the “ruggedized” digital computer which handles the electromechanical procedure of any industrial unit. 

Understanding the basics of PLC 

In 1964, Dick Morley invented PLCs and since then, it has transformed the manufacturing and industrial unit. There is a broad range of functions that PLC performs such as calculating, processing, timing, counting, and comparing several analog signals. 

The most significant benefit of PLC as compared to a “hard-wired” control unit is that any individual can reform a PLC after they have programmed it (at the cost of the programmer’s time). On the other hand, in a “hard-wired” system, you need to start from scratch by ripping out the wires (it is costlier and takes longer). 

How does a PLC Controller work?

The functioning of a PLC controller can be easily understood using the cyclic scanning technique which is also known as the scan cycle. The following steps are included in a PLC Scan Process:

  • The operating unit began cycling and monitoring time.
  • The CPU begins to read the information from the input module and analyze the condition of all the inputs. 
  • The CPU began to execute the application or user program written in PLC-programming language or relay-ladder method.
  • Following that, the CPU carries out all the communication tasks and internal diagnosis. 
  • As per the programming outcome, it writes the information into the output module such that all outputs are modified. 
  • This mechanism continues if the PLC is in run mode. 

What are the main types of PLC Controllers?

There are two main types of PLCs as mentioned below:

  • Compact PLC: There are several modules within a single case. It has a pre-defined number of external I/O cards and I/O modules. Hence, it cannot extend its modules. It is the manufacturer who decides every input and output. 
  • Modular PLC: This PLC allows varying expansions with the help of “modules” and so, is known as Modular PLC. I/O elements can be expanded. Modular PLC is much easier to use as every element is independent of one another. 

PLCS is segmented into three types depending upon the output that are Transistor output, Triac output, and Relay output PLC. Transistor output PLCs are used inside microprocessors and use switching operations. Relay output PLC is most appropriate for both DC and AC output tools. A PLC is further segmented into Nano, Micro, and Mini depending upon physical size. 

PLC Applications 

PLCs are used in industrial areas such as the steel industry, automobile industry, chemical industry, electricity industry, paper industry, process automation plants, and in boilers like thermal power plants. Depending upon the growth of all these technologies; the scope, application, and functionality of the PLC have increased drastically. 

  • In the glass industry: In the glass industry, a PLC is commonly used in every mechanism and workshop for the processing of flat glasses, managing the material ratio, and so on. For manufacturing glass, companies use bus technology to build the PLC control mode using a distributed-control unit. 
  • In the cement industry: In addition to the best quality raw materials, during the mixing mechanism within the kiln, the precise data about process variables helps to ensure that the output given must be of the best possible quality. 

Benefits of having a PLC 

  • PLC controllers are moderately direct to the program. They are easier to use as compared to other industrial controlling systems and it helps industries to reduce cost and complexity. 
  • They are highly versatile, and many PLC models are appropriate for a managing broad range of systems and processes. 
  • There is a wide range of pricing for PLCs which simply means that they have affordable ranges as well that can be used by small companies and startups. 
  • They do not consume much electrical power and hence, are efficient. It supports conserving energy and can ease wiring considerations. 
  • They have a few elements that make PLC easier to troubleshoot and further support in minimizing maintenance downtime. 
  • PLCs are entirely solid-state tools which means they have no moving sections and so, they become highly reliable. This is why PLC can survive the critical circumstances that arise in industrial units. 


A PLC is a microprocessor-derived controller that utilizes a programmable memory to uphold instructions for timers, sequencing, and counting. A PLC takes in inputs from human input points like buttons or switches or from automated data capture points.  PLCs are majorly applied in industrial sectors such as safety device monitoring, event recording, and process automation.

James Morkel

Tech website author with a passion for all things technology. Expert in various tech domains, including software, gadgets, artificial intelligence, and emerging technologies. Dedicated to simplifying complex topics and providing informative and engaging content to readers. Stay updated with the latest tech trends and industry news through their insightful articles.

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