What are the different types of PLC ?

PLCs focus on implementing logic and switching operations to efficiently control systems. It is used to control various types of machines or processes through digital or analog input/output.

In this article, we will discuss the Types of PLC, We will start our Article with the Definition of PLC, Then go through the Functional Components of PLC, Then we will go through its Various Types of PLC, At last, we will conclude our Article with the Application, Advantages, Disadvantages and Some FAQs.

What is a PLC?

A programmable logic controller(PLC) is a specialized microprocessor-based controller that is capable of storing instructions to implement control functions such as sequencing, timing, counting, arithmetic, communication, and data manipulation. Engineers with limited computer knowledge can operate them. They feature a user-friendly programming language.

PLCs mainly execute logic and switching tasks, such as “if P or Q happens, activate R; if P and Q occur, activate S.” Input sensors and output devices like motors and valves connect to PLCs. Then the controller monitors inputs and outputs executing programmed control rules for which it has been programmed.

PLCs are similar to computers but PLCs are optimized for control tasks whereas computers are optimized for calculation and display tasks. PLCs are rugged and can withstand vibrations, temperature, and noise. These are easily programmed and have an easily understandable programming language. The most basic PLC system contains two terminal blocks, one for the input and the other for the output.

Block Diagram and Functional Components of PLC

The basic components of a programmable logic controller are a processor, power supply , memory , input/output interface , communication interface and programming interface.

Block diagram of PLC

• Processor: The central processing unit (CPU) which contains microprocessor , that interprets input signals and executes control actions based on the program stored in its memory . It communicates decisions as action signals to the outputs . It commands and governs the activities of the entire PLC systems.
• Programming device: This serves as the interface for programming the PLC. Engineers use it to develop and input the required control program into the processor’s memory . The program is created in the programming device and then transferred to the PLCs memory unit.
• Memory unit: The memory unit stores the control program that contains logic instructions , sequencing and other control algorithms .It also stores data obtained from inputs for processing and information destined for outputs .The memory unit serves as storage for the programs instructions and any temporary data needed during execution .
• Power supply unit :It is vital for converting AC(alternating current) voltage to the low DC(direct current) voltage required by the processor and circuits within the input and output interface modules . This ensures stable power distribution throughout the system . It is used to run the PLC components .
• Input /output interface :These interfaces facilitates communication between the processor and external devices .Inputs encompass signals received from various sensors and switches , including but not limited to switches ,photoelectric cells ,temperature sensors and flow sensors .These inputs provide data for the PLC to process and act upon .It outputs transmit signals from the processor to external devices , influencing their operation based on the programmed logic .

The Two Major Types of PLC

PLCs are classified into various types based on size and based on hardware

The Two major Types of PLC are :

• Fixed / Integrated / Compact PLC
• Modular PLC

Fixed/Integrated/Compact PLC

The name compact PLC ,itself indicates that the size of the PLC is small and having integrated design . Compact PLCs integrate separate units of power supply , CPU and input/output interfaces into a single unit. Due to this complexity of the system reduces and simplifies the installation. These PLCs having inbuilt programming language which helps users to create and modify control logic for various tasks.

Now a days PLCs are equipped with powerful processors that are capable of executing complex algorithms .These PLCs are having user friendly programming interfaces that make it easy for users to develop and modify control algorithms .They offer cost effective solution for small to medium sized applications .

The input and output ports of the compact PLC are integrated into the microcontroller itself . The number of input and output ports in compact PLCs cannot be expanded and reduced.

Modular PLC

Modular PLCs consists of separate units for power supply , CPU and input/output interfaces mounted on same base. These PLCs can be divided into small , large and medium PLCs .

The input and output ports can be added or removed according to the user requirement .These PLCs consists of powerful CPU capable of dealing with complex control algorithms. The user can choose from various programming languages to develop control algorithms.

Modular PLCs can be expanded so no need to replace the existing ones .Modular PLCs having capability of storing high volume of information . As they are having individual blocks , these are easier to repair and it does not effect other blocks .

What are the advantages of using Modular PLC?

Scalability : It provides greater scalability PLC control system and to the company that uses the controller.

Number of I/O : Due to scalability feature, it has number of input devices and output control is higher than Fixed PLC.

Easier Fault Detection : As the name suggest, it is a modular PLC so each function is separated module wise. Therefore, the fault detection becomes far easier than that of the Fixed PLC.

Less Downtime : As the name suggest, it is a modular PLC so each function is separated module wise. Therefore, the repair time is less.

Difference Between Compact and Modular PLC

Compact PLC	Modular PLC
The size of compact PLC is small as it integrates all units into one unit .	The size of modular PLC is large as it contains separate units .
Due to the integrated design , wiring is simplified	Due to separate units , wiring design is complex
The initial cost of the system is low for smaller systems.	The initial cost of the system is high due to modular design.
It requires low maintenance due to integrated design	It requires high maintenance as it is having separate units .
The number of input and output ports cannot be expanded and reduced.	In modular PLC , we can add or remove input and output ports according to the requirement .
It requires less space	It requires more space
Due to its simple Design it can be Easily installed	Installation becomes more complex due to the modular design.
It supports limited programming languages	It supports wide variety of programming languages

Type of PLC According to Size

On the Basis of the Size the PLC can be classified as

• Small PLCs
• Medium PLCs
• Large PLCs

Small PLCs

Small PLCs also called as micro PLCs .Small PLCs are the smallest form of programmable logic controllers .These compact devices are designed to control smaller machines such as conveyors and motors. Due to their compact size ,they contain sufficient memory to store logic programs .These PLCs having less features and capabilities compared to larger PLCs .These are often more affordable than larger PLCs ,that makes them cost effective solution for small machines.

These PLCs support simple and easy programming languages like ladder diagram and ladder logic programming . That make them accessible to users with limited programming knowledge. These are designed to be robust and reliable in harsh environmental conditions . They are having built in communication ports for connecting to external networks. These small PLC having 32 to 128 IO points .

Medium PLCs

Medium PLCs are larger in size compared to compact PLCs but smaller than modular PLCs .These are user friendly that supports multiple programming languages. They offer expansion capabilities , that allows users to add additional i/o modules .They may not have same scalability as modular PLCs but they offer sufficient flexibility to adapt to changing needs .These are characterized by their robust performance capabilities .

They are having higher number of I/O points compared to the compact PLCs which allows more extensive control .They provide cost effective solution for medium sized automation projects .These are designed to handle control tasks of moderate complexity ,such as in manufacturing process .They can withstand harsh industrial environments as they are rugged and reliable .

They ensure continuous operation even in demanding conditions .They accommodate to future expansions allowing users to adapt the control system for the requirements without replacing the entire PLC .

Large PLCs

Large PLCs are advanced industrial control systems that are designed to handle complex tasks. They allows users to expand the system by adding various types of input /output modules ,communication modules providing flexibility to accommodate to various requirements .These modules can be added or removed from the rack without shutting down the entire system .

These are equipped with powerful CPUs capable of executing complex algorithms and handling large volumes of data processing .They support analog and digital signals for specific applications such as high speed counting .These PLCs support multiple programming languages . This allows users to choose suitable language for implementing control logic algorithms based on the complexity.

Difference Between Small , Medium and Large PLCs

SMALL PLC	MEDIUM PLC	LARGE PLC
Smaller size	larger size compared to small PLCs	Larger size compared to medium PLCs
Limited programming languages	It supports multiple programming languages	It supports multiple programming languages
It consists of few number of input/output points	It consists of moderate number of input/output points	It consists of many number of input/output points
Lower cost compared with medium and large PLCs	moderate cost	Higher initial costs
Memory capacity of small PLCs is limited	Memory capacity of medium PLCs is moderate	Memory capacity of large PLCs is high
It is suitable for smaller machines	It is suitable for medium sized automation projects	It is suitable for large scale industrial automation.

Type of PLC According To Power Supply

on the Basis of the Power Supply Plc can be Classified as

• Built-in power supply PLCs
• External power supply PLCs

Built-in Power Supply PLCs

Built-in power supply PLCs consists of power supply unit within the PLC housing. It includes transformer , rectifier to convert incoming AC or DC supply into required voltage levels. It reduces overall space and wiring requirements as it eliminates external power supply . These are used in compact machines where space is limited.

External Power Supply PLCs

External power supply PLCs requires separate power supply units connected to PLC through power input terminals. This external power supply provides necessary voltage and current levels required by the PLC components .These are used where high power is required than the built in power .These are also used in applications where backup power sources are required .

Type of PLC According To Output

On the Basis of the output, Plc can be Classified as :

Analog output PLC

Analog output PLCs are used to control the speed of continuously running machines like motors or turbines .These are capable of generating analog signals to control variable speed drives and devices where continuous control is required .These are used where precise control of temperature ,pressure ,speed is required .

Relay output PLCs

Relay output PLCs are used to control external devices such as motors and solenoid valves .These PLCs are suitable for both AC and DC output devices .These relays provide electrical isolation and can switch to high currents and voltages . These PLCs have limited switching speed and shorter life span .

Transistor output PLCs

Transistor output PLCs are uses solid state devices to control external devices . As these solid state devices does not have moving parts , faster switching is achieved. These transistor PLCs have higher reliability and longer lifespan compared to relay output PLCs. These PLCs are used in applications where high speed switching , precise control is required.

Advantages of PLCs

• PLCs are well known for their robust reliability , capable for enduring prolonged operation without breakdowns . This minimizes downtime , ensures uninterrupted operation and improves overall reliability .
• PLCs having faster response time , so that system performance can be improved.
• PLC programming languages are designed to be intuitive and user friendly . Engineers or technicians with minimal programming language knowledge can develop and modify control logic .
• PLCs are rugged and having ability to withstand vibrations , humidity and temperature .
• PLCs can be easily programmed and easy to modify changes in production process or system requirements .This allows for quick adaptation to new tasks or production lines without extensive hardware modifications .
• PLCs can be programmed to ensure safe operation of equipment.

Disadvantages of PLCs

• Once a PLC system is installed and programmed , it is difficult modify the control logic .
• It requires regular maintenance to ensure proper operation ,otherwise it increases operational costs and downtime .
• PLCs are having limited processing power and memory capacity that may restrict the complexity and performance of control algorithms .
• PLCs programs are difficult to develop as well as for debugging.
• Initial cost of PLCs is high that includes hardware , software and programming services .
• These are designed for use in industrial environments with specific temperature , humidity and vibration requirements .
• Fixed circuit operations.

Applications of PLCs

• These are widely used in manufacturing industries for automating production processes , packaging machinery and robotic arms.
• These are used in paper production industry to manufacture products such as books ,newspapers etc.
• These are utilized in glass industries for the production high standards of quality in glass production .
• These are essential for efficient and safe operation of all types of transportation systems including escalators and elevators.
• PLCs are utilized in automated drainage water pump monitoring and control systems to regulate water levels in areas like basements, reservoirs, and more.
• These are used in smart grid system to monitor and detect fault conditions .

Conclusion

Thus programmable logic controllers come in various types according to different application requirements . Small PLCs are compact units suitable for controlling smaller machines such as conveyors and motors .They are cost effective and equipped with sufficient memory for logic programs .

Compact PLCs are ideal for smaller machines and simple control tasks and limited functionality .Modular PLCs offer scalability and flexibility that allows users to expand systems by adding modules as per requirement. Medium PLCs provide moderate processing power and scalability for controlling mid sized production lines and machinery .

Large PLCs are advanced control systems designed for handling complex control algorithms. They offer high processing power and scalability for the control of extensive production lines. They play vital role in optimizing efficiency ,ensuring safety and enhancing productivity across various industries .

Types of PLC – FAQs

What are the programming languages used in PLCs ?

The five programming languages are ladder logic ,functional block diagram ,sequential function charts , structured texts and instruction list .

What are the essential parts of PLCs ?

The main components of PLCs are CPU( central processing unit) , power supply unit ,programming device and input/output modules .

What is the ladder diagram in PLC ?

It is a graphical programming language used to create control logic for Programmable logic controllers that looks like a ladder with horizontal rugs and vertical rails .