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Module 3
We request you to please complete the Module 0, Module 1 and Module 2 before taking this up.
Objective of this module
1. You should be able to understand the technology of PLC.
2. You should be able to understand the hardware structure of any PLC.
3. You should be able wire Inputs and outputs for PLC.
4. You should be able to program PLC in ladder language.
5. You should be able to troubleshoot PLC operations.
6. You should be able to select / choose the PLC for any application.
PLC : Programmable Logic Controller, this is something which every aspiring Automation Engineer wants to learn. It is very very simple if the concepts are understood. Once you understand the concept then it does not matter if the PLC is of Siemens, AB, ABB, Delta, Schneider, B&R, Mitsubhishi....any make and you should be able to program or troubleshoot, if you have certain resources like programming software etc.
The idea of this module is to make you understand all the simple and complex aspects of a PLC...Lets start..
[Why do we need a PLC when we can use Hardwired Interlock System as discussed in Module 1 ?]
Can you do mathematical calculations with hardwired logic ?
Is it easy to wire if there are many many interlocks ?
How can you interface Analog devices like thermocouple / pressure transducer in hardwired circuit ?
No, so hardwired interlock circuits have their own limitations and PLC can take care of this ? PLC can do caluclations, it can read analog devices, it can take care of as many number of interlocks as applications require easily, changes in PLC are very very simple, troubleshooting is also very easy. But then
What is a PLC ?
Let us take an example, when we drive a car or a scooter...our eyes see the road, our ears listens to the horns, our eyes also see the speedometer......Where does all this information goes........To our BRAIN. Then our brain controls our hands for direction and controls our feet for brake or race. So we can say that the eyes, ears act as input devices, hands and feet act as output device, our BRAIN acts as CPU and of course it has memory and it is programmed and the process is driving the car/scooter....Isn't it ?
So a PLC is device, an intelligent device, which takes inputs from the process by Input devices/sensors and analyses these inputs continuously and based on the programming it gives the output to process actuators. It has memory to store the program and data, it has communication ability for programming by user.
BRAIN = CPU with memory
Eye / Ears = Input Devices
Hands / Feet = Output Devices.
Our Learning about Traffic Rules / Driving = Programming
Scooter / Car Driving = Process
Example 1
Process : We have to control the temperature of a room at 23 Degree C.
What all we need :
Temperature Sensor as input : To tell us what is the current Temperature.
Heater for heating if Temperature is less than 23 = Process Actuator.
Some Device to switch on and switch off the heater.
Cooler/Blower for cooling the if Temperature is more than 23 = Process Actuator.
So you refer to Module 0 and see how a heater can be switched on and how a blower can be switched on. Now the PLC is programmed in such a way that it reads the process input through some temperature sensor (Thermo couple or RTD) if it is > 23, it starts the blower, and if it is < 23 it switches off the cooler and starts the heater. At any given time either blower will be on or heater will be on.
Example 2
Process : We have to control the temperature of a room between 21 to 24 Deg C.
This is a more realistic requirement, we switch on the heater if temperature is <21 and we switch on the blower if the temperature > 24. We keep both of them off if temperature is between 21 to 24 Deg c.
Example 3
Process : We have to keep the humidity of a chamber below 85 ppm.
Humidity is kind of presence of moisture in the air, so basically we have to dry it, this can be simply done by heating, no cooling is required. So for this we need a humidity sensor which can give us value of humidity as readable by PLC, once the PLC knows the humidity value then it sees as per the program that if > 85 ppm start the heater otherwise switches off the heater.
Example 4
Process : In a pipeline steam is flowing, we have to ensure that the pressure in the pipe line is less than 4 bar..
What will happen if the pressure in the pipe line is more, it may damage the pipe line, so we have to keep the pressure in safe limit. How do we do it. rather how do we reduce the pressure ? There has to be a valve which we can open and close through PLC, isn't it. This valve will be switched on and off by the output device of a PLC. So we take the pressure input from a pressure transmitter, PLC compares and sees that this pressure is below 4 bar, if it is > 4 bar then PLC switches on the relief valve and steam gets out from a side in open (like what happens in the pressure cooker). Once the pressure again falls below 4 Bar, we close the steam valve.
Example 5
Process : In a pipe line steam is flowing as per heating in a boiler, we have to ensure that the pressure remains below 4 bar by switching on a relief valve, if even after switching on a relief valve pressure does not come below 4 bar and tries to go above 5 bar then we switch off the boiler fuel.
This is somewhat difficult. As soon as pressure > 4 bar we switch on the relief valve, now pressure is expected to fall, but still if pressure increases due to more heating at the boiler side, then if pressure > 5 the PLC closes the fuel supply to boiler (like switching off the Gas Stove in our kitchen) and pressure should come in control.
So, by now you would have understood that PLC has to continuously monitor the inputs, it continuously executes/analyses the program written and continuously should update the outputs. You never know when the pressure becomes high, this continuously loop type operation has a name, we will see later.
Example 6
Process : Crane Control, a big crane has 6 motions..UP & Down, Left and Right, Forward and Reverse. So there will be 3 motors, one for UP/DN, second for Left/Right and third for Forward/Reverse. There will be 6 Push Button with the crane operator, one for each operation/motion.
So when UP push button is pressed, PLC will read this and as per written program gives output to UP motor, and similarly when DN push button is pressed PLC will read this and as per written program gives output to DN motor and so on.....
What are the Inputs for PLC ?
Any information which PLC requires from the process side to control the PLC.
Now to start with, the input information to PLC can be given by two ways....one by changing the state (On or Off) and secondly by giving some analog signal (0 to 10 Volts, -10 to +10 volts or 0-20 mA or 4- 20 mA). Changing the state, that is on or off...is through discrete or digital inputs. Giving some values to PLC as 0-10 Volts, 4-20 mA...is through Analog Inputs.
[So the next question is how to give digital inputs to PLC]
Very simple, there will be terminals assigned for digital inputs. Read the PLC specifications for connections and voltages. A digital input can be 24 VDC type, 230 VAC Type, 110 VAC type etc....So when the specified terminal is connected with the required volateg, that digital input will change the state from 0 to 1 and vice versa. Each digital input will have a corresponding address inside the PLC, which can be used in the program.
[Now the next question is how to give Analog inputs to PLC]
Again there will be terminals for Analog Inputs connections. Refer to the PLC specifications and connect the required analog input, which will come from the sensors like pressure transmitters etc.
