A PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.

Almost any production line, machine function, or process can be greatly enhanced using this type of control system. 

Another advantage of a PLC system is that it is modular. That is, you can mix and match the types of Input and Output devices to best suit your application.






WORKING OF PLC

SCAN Cycle: 

The scan is the cycle in which the plc

input runs,your Plc program, and then updates the output.
THis will takes some amount of time often measured in milliseconds (ms).
This amount of time it takes for the PLC to make one scan cycle is called the scan is called the scan time of the PLc.

 process of a scan cycle:

 A PLC works in a program repeatedly .The simplest scan cycle consists of 3 steps:

  • Read input,
  •  Execute the program,
  •  Write output.


The program follows the sequence of instructions. It typically takes a time span of tens of milliceond for the processor to evaluate all the instructions and update the status of all output .

What is inside in PLC 

The Central The Central Processing Unit, the CPU, contains an internal program that tells the PLC how to perform the following functions:



  • Execute the Control Instructions contained in the User's Programs. This program is stored in "non volatile" memory, meaning that the program will not be lost if power is removed
  • Communicate with other devices, which can include I/O Devices, Programming Devices, Networks, and even other PLCs.
  • Perform Housekeeping activities such as Communications, Internal Diagnostics, etc.
  • The Central Processing Unit, the CPU, contains an internal program that tells the PLC how to perform the following functions:
  • Execute the Control Instructions contained in the User's Programs. This program is stored in "non volatile" memory, meaning that the program will not be lost if power is removed
  • Communicate with other devices, which can include I/O Devices, Programming Devices, Networks, and even other PLCs.
  • Perform Housekeeping activities such as Communications, Internal Diagnostics, etc.

HOW DOES A PLC OPERATES?

There are four basic steps in the operation of all PLCs; Input Scan, Program Scan, Output Scan, and Housekeeping. These steps continually take place in a repeating loop.

 1.) Input Scan

•    Detects the state of all input devices that are connected to the PLC

2.) Program Scan

•    Executes the user created program logic

3.) Output Scan

•    Energizes or de-energize all output devices that are connected to the PLC.

4.) Housekeeping

•    This step includes communications with programming terminals, internal diagnostics, etc...





Block diagram of programmable logic controller (PLC):



Processor section (CPU)

The processor section is brain of PLC which consists of RAM, ROM, other logic circuitry. CPU controls, monitors and supervises all operation within PLC. The CPU makes decision and executes control instructions based on the program instruction in memory.  

Input and output module

The input module is a mediator between input devices and central processing unit (CPU) which is used to convert analog signal into digital signal.

The output module is a mediator between output devices and central processing unit (CPU) which is convert digital signal into analog signal.

 Power supply

Power supply is provided to the processor unit, input and output module unit. Power supply may be integral or separately mounted unit. Most of the PLC operates on 0 volts DC-24 volts.

   Memory section

The memory section is the area of the CPU in which data and information is stored and retrieved.  Data Memory is used to store numerical data required in math calculation, bar code data etc. User memory contains user’s application program.

   Programming device

Programming devices are dedicated devices used for loading the user program into the program memory or edit it and to monitor the execution of the program of the PLC. It is also used to troubleshoot the PLC ladder logic program.

# Limitations or Demerits of Relays:

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts.

1.    Relay operation can be affected due to ageing of the components and dust.
2.    Operation speed for an electromagnetic relays is limited.
3.    Requires periodic maintenance and testing.
4.    Difficulty with replacements.
5.    Suitable for simple processes.
6.    Large amount of work is required in connecting wires.
7.    Takes large time to actuate
8.    Not reliable as they suffers from contact wear
9.    Difficulty in finding errors

# Advantages of PLC over electromagnetic relays

1.    Simple and less wiring required.
2.    Fast response and designed for the rugged industrial environment.
3.    Another advantage of the PLC system is easy to troubleshoot.
4.    One of the best feature of plc system is versatility with programming and expandability                 compare      to   relay system.
5.    The program can run or simulated without actual input connection or Hardware
6.    PLC has a low power consumption.
7.    PLC has High processing speed.
8.    Smaller physical size than relay.
9.    Flexible and can be reapplied to control other systems very easily.
10.    PLCs have very few hardware failures when compared to electro-mechanical relays
11.    Special functions such as time-delay actions and counters, can be easily performed using PLCs.
12.     Easy to install.
13.     Connections can be easily changed.
14.     I costs less when compared to relayed network.
15.     Programming method is easy


PLC Input and Output Modules:

 The module which interacts with the input signal is called as Input Module. It is required to connect input devices like different types of switches.

 
The module which interacts with the output signal is called as Output Module. The output module is required to connect output devices like electric motor.


There are two types of PLCs- Compact PLC and Modular PLC.

👉    In Compact PLC, the capability of the I/O module is fixed.
👉   In Modular PLC, the capability of the I/O module is not fixed.

Classification of PLC Input and Output Modules:

The classification of input and output (I/O) modules of PLC is based on the types of signals. Basically, there are two types of signals- Discrete signals and Continuous signals. Based on the signals, I/O modules are classified into two main parts.


👉Digital I/O Module

The digital module is also called Discrete Module. In this module, the I/O signal work on the binary system. Based on Input and Output, the digital module is of two types.

•    Digital Input Module
•    Digital Output Module

Examples of digital input:

 Push switch, Toggle switch, Selector switch, Proximity switch, Limit switch and etc are the example of the Digital Input Signal.

Examples of digital output:

 Lamp, Coil, Buzzer, Relay, Motor, Fan, Heater, Actuator, Solenoid Valve and etc are the example of the Digital Output Signal.


👉Analog I/O Module

The analog module is called a Continous Module. Usually, the voltage or current is given to the input module in the form of an analog signal.
Again, analog I/O modules are also of two types.

•    Analog Input Module

•    Analog Output Module

Examples: 

Temperature detection switch, Pressure detection switch, Flow detection switch, Level detection switch, Limit detection switch, Position detection switch, PH Level detection switch are the best example of the Analog Input Signal.

Examples:

  Temperature Transmitter, Thermocouples, Pressure Transmitter, Flow Transmitter, Level Transmitter, etc., are the example of the Analog Output Signal.

Programming Terminal:  Programming terminal in the diagram is not a part of PLC, but it is essential to have a terminal for programming or monitoring PLC. It is used for programming the PLC & monitoring the PLC’s operation.


Various types of programming devices are used to enter, modify and troubleshoot a PLC program.
These programming terminal devices include handheld and PC based devices. In the handheld programming device method, a proprietary device is connected to PLC through a connecting cable.
Most popularly a Personal Computer (PC) is used for programming the PLC in conjunction with the software given by the manufacturer.
By using this PC we can run the program in either online or offline mode and can also edit, monitor, diagnose and troubleshoot the program of the PLC.

👉Power Supply:

A PLC power supply is the workhorse of the PLC system. It converts your line voltage, 120 or 240 volts AC, to a lower DC voltage, commonly 24 volts DC.
This DC voltage is then sent into the rack to power the rest of the PLC components.

 

Following is the list of PLC Manufacturers

 1.Siemens
 2. Rockwell Automation / Allen Bradley
 3. Mitsubishi Electric
 4. Schneider Electric
 5. Omron
 6. Emerson (General Electric)
 7. Keyence
 8. IDEC
 9. B & R Industrial Automation
 10. ABB
 11. Bosch Rexroth
 12. Hitachi
 13. Phoenix Contact
 14. Panasonic
 15. LS Electric
 16. Eaton
 17. Delta Electronic
 18. Yokogawa Electric
 19. Toshiba
 20. Fanuc






Different Programming Languages of PLC


The 5 most popular types of PLC Programming Languages are:

1. Ladder Diagram (LD)
2. Sequential Function Charts (SFC)
3. Function Block Diagram (FBD)
4. Structured Text (ST)
5. Instruction List (IL)

👉Ladder diagram is the universal programming language of  PLC. It has a short abbreviation as LD and also known as Ladder Logic. It is used with programmable logic controllers. And it is one of the oldest programming languages for PLC.


👉 Instruction List (IL)

Instruction List (IL) is another type of PLC programming language. It uses the mnemonic code. So the syntax of this programming language is easy to remember.


Advantages of IL:

•    It has a high execution speed.
•    It takes less memory as compared to other programming languages.

👉 Structured Text (ST)

Structured Text PLC language is shortly denoted by the ‘ST’ and ‘STX‘. It uses high-level programming language syntax.

The syntax of ST is similar to the syntax of a high-level programming language with loops, variables, conditions, and operators.

Advantages of ST:

•    ST is very easier to understand for both novice and experienced programmers.

•    Because of its standard coding format, it is easy to edit and modify program written in ST language.

👉 Function Block Diagram (FBD)

Function block diagram (FBD) is a popular and easy way to write a program like a Ladder Diagram.

FBD is represented as a box that consists of a number of lines of code for putting different programming functions.

It is a graphical language for programming logic controller. So, it makes your job easy to describe a system.



👉Sequential Function Charts (SFC)

Sequential function charts (SFC) is also a graphical programming language. It is not a textbase. It has become a popular method of accurately specifying sequential control requirements.
The main function of SFC is only the active parts of the code are executed. Due to this, it makes easier to troubleshoot and to change the code if problems occur.
These languages are accepted internationally. Among all of them, mostly the Ladder diagram (LD) programming language is used in the industry.

 

 

 How to Read Ladder Logic? 

 Ladder logic is basically read from the left hand rail to the right hand rail and from the first rung to the last rung (left to right and top to bottom). The rungs contain input symbols that either pass or block the logic flow. The last element of each rung is an output symbol which is the result of the logic expressions contained in that rung.


Contacts & Coils:

Contacts stand between power and Coils, asking questions.  If the answer to the question is true, then power is allowed to flow through the contact, if not the path is blocked.


Contacts placed side by side to form AND statement



Contacts placed one on top of each other to form OR statements.




AND and OR statements may be combined to form complex expressions.






Coils:

They represent relays that are energized when power flows to them. When a coil is energized it causes
 a corresponding output to turn ON by changing the state of the status bit controlling that output



 

 

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