Introduction to PLC

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PLC

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Today. I am gonna give an overview about PLC. We will have a look on basics i.e. what is PLC? Why we use PLC instead of microcontroller like Arduino or PIC Microcontroller? What's its advantages and disadvantages? I will try to cover all about the basics. After reading this tutorial, you must have a look at Introduction to Ladder Logic for P L C, Ladder Logic is programming language for PLCs.

There are different types of PLCs available in the market manufactured by different companies, so its impossible to cover all of them. In this tutorial, I am gonna discuss Fatek PLC as I have worked on it during my project. The model I have used is Fatek PLC Fbs-20MA. The reason I used this model because it was cheap and has enough input/output ports sufficient for my project. That's why I preferred it as its engineers' task to optimize the cost as well. Let's get started with PLC.

What is PLC?

Its a basic question, which is normally asked by all the starters so I am gonna reply it first for the newbies.

• PLC is nothing but an advanced form of Microcontroller. It is usually used in industries because of its flexibility and ease of use.
• It can be attached quite easily with computer via serial port as well as usb port.
• PLC is used when we need to automate anything just like microcontroller. We attach our sensors and actuators etc with PLC and then insert some programming code in it and let it do its job.
• You have seen automated lifts, they all are operated with PLC.
• We can use timers, counters, registers in PLC and can get any kind of output from it.
• We can program PLC with different languages and the most commonly used language for PLC is named as Ladder Logic.

Internal Overview of PLC

What's inside PLC, which makes it so cool ? That's a good question and normally engineers wonder about it. PLC can be divided into 3 sections, which are as follows:

• Power Supply - Thissection provides power to the PLC, in my case it is operated on 220V AC, so when I provide 220V AC to my PLC, it got activated and start performing functions.
• Centeral Processing Unit (CPU) - Its the actual brain of PLC, it is further divided into several parts i.e. RAM, ROM, EEPROM, microcontroller etc. The programming code is uploaded in this CPU and according to that program, it performs its functions.
• Input / Output Section - This section is the one from where PLC communicates with the external world. We can attach sensors to the inputs of PLC and can operate our motors, actuators etc from the outputs of PLC.

Types of PLCs

• There are different types of PLCs available in the market manufactured by different companies.
• Few famous PLC companies are Siemens, Mitsubishi, Fatek etc.
• Moreover, they are also available in different sizes and functions. The one I used has 14 inputs and 8 outputs. It doesn't support analog inputs as I don't require them.
• There are PLCs available with analog inputs or you can also buy cards which are interfaced with the PLC and make them capable to work on analog inputs.
• Another function which is not available in my PLC is the Serial communication, but such models are available which supports serial communication.
• So in short, there are several models of PLC available in the market and you have to consider your project demands while buying a PLC.

Why use PLC instead of microcontrollers?

• Microcontroller is normally used in small products, where you need to control some sensors or some motors etc but when we talk about big automated plants in industries then PLC is always preferred over microcontroller.
• The reason for preferring PLC over microcontroller in big projects is because of its flexibilty and ease of use. PLC can be programmed frequently with computer, suppose you have an automated system and you find some bug in it which you wanna remove, then what you need to do is simply attach a computer with the PLC of that plant and make changes in the code, which isn't possible with the microcontroller.
• Moreover, PLC has lot of memory, you can add any size of data in it.
• PLC is also long life as compared to microcontroller.
• Last but not the least, PLC has built in cards to control heavy AC voltages, you can get any kind of voltage from PLC i.e. 220V AC etc but if you wanna get such voltages from microcontrollers then you have to add some extra circuitry.
• In short, in all industrial automated plants, PLC is used.

Getting Started With PLC

I think now you have the idea about PLC, so now I am getting started with PLC. I am gonna explain the functioning of Fatek PLC as I have used that one but if you are using another model of PC then no need to panic as all PLCs have same functionality. So, it doesn't matter which one you are using. If you check the below image then you will see I have marked three sections in it.

• Section 1 is indicating the status of input pins. If inputs are off then this section will remain as shown in above figure, but if any of the inputs get high, then the respective pin indicator will also glow into red, which indicates that this pin is ON. It helps while you are writing programming code for the PLC.
• Section 2 indicates the status of PLC. If PLC is powered up then the POW led will go red, if you have uploaded the code in PLC and start it then the RUN led will go red and if your code has some error then ERR led will go red.
• Section 3 indicates the status of output pins, it will tell you which output is currently ON.

In the below image, I have indicated Section 4 and 5, these are the input/ output section. If you have a look at it closely then you can see there are two rows of screws, where you plug your wires for inputs and outputs and above them, they are also labelled with white color. So, it goes like that, first row of labelling is for first row of screws and second row is for second row of screws.

• Section 4 is the inputs pins section, so if you check above there are inputs from X0 to X13, which makes it overall 14 inputs. Moreover, there are two pins labelled as + 24V and - 24V, which PLC is providing us, so if you wanna give any input to this PLC you have to make sure that its 24V, otherwise PLC not gonna recognize it.
• Section 5 is the output pins section. and you can see there are total 9 outputs starting from Y0 to Y8, now you are getting confused with C0 to C6. C pins are actually the voltage setter, let me explain, in projects there are different outputs are required like your motor is running at 12V DC while your solenoid valve is running at 220VAC. So, there's a need to set these voltages at the output. Here C pins are used. Suppose you need to give 12V output at Y0 and Y1 then give this 12V at C0 and when Y0 or Y1 is ON then they will give 12V at output. In short, when output gets ON it is actually connecting with their respective C pin.
• Lastly, check the Port 0 in the above image, this is the port where you plug your serial wire inPLC and connect it with your computer in order to upload the programming code.

That's all for today. I hope you got the basic idea of Programmable logic controller and now its time to have a look at Introduction to Ladder Logic for P L C, ladder logic is programming language for PLC. Your feedback are warmly welcome. In the next tutorial, I am gonna cover about ladder logic and will show you how to program a PLC. Till then Take care and have fun.