Thursday, 11 August 2016

Industrial Automation in India | PLC SCADA DCS Training in Chennai

What is a Programmable Logic Controller ? 

A Programmable Logic Controller, also called a PLC or programmable controller, is a computer­type device used to control equipment in an industrial facility. The kinds of equipment that PLCs can control are as varied as industrial facilities themselves. Utilities Plants, Batch Control Application, Chemical Processing, Conveyor systems, food processing machinery, auto assembly lines etc…you name it and there’s probably a PLC out there controlling it

In a traditional industrial control system, all control devices are wired directly to each other according to how the system is supposed to operate. In a PLC system, however, the PLC replaces the wiring between the devices. Thus, instead of being wired directly to each other, all equipment is wired to the PLC. Then, the control program inside the PLC provides the “wiring” connection between the devices. The control program is the computer program stored in the PLC’s memory that tells the PLC what’s supposed to be going on in the system. The use of a PLC to provide the wiring connections between system devices is called softwiring.


 A PLC basically consists of two elements:

Because different input and output devices send different kinds of signals, they sometimes have a hard time communicating with the PLC. While PLCs are powerful devices, they can’t always speak the “language” of every device connected to them. That’s where the I/O modules we talked about earlier come in. The modules act as “translators” between the field devices and the PLC. They ensure that the PLC and the field devices all get the information they need in a language that they can understand. CONTROL PROGRAM We discussed a little bit earlier about the control program. The control program is a software program in the PLC’s memory. It’s what puts the control in a programmable controller The user or the system designer is usually the one who develops the control program. The control program is made up of things called instructions. Instructions are, in essence, little computer codes that make the inputs and outputs do what you want in order to get the result you need. There are all different kinds of instructions and they can make a PLC do just about anything (add and subtract data, time and count events, compare information, etc.). All you have to do is program the instructions in the proper order and make sure that they are telling the right devices what to do and voila!…you have a PLC­controlled system. And remember, changing the system is a snap. If you want the system to act differently, just change the instructions in the control program. Different PLCs offer different kinds of instructions. That’s part of what makes each type of PLC unique. However, all PLCs use two basic types of instructions: contacts coils are instructions that refer to the input conditions to the control program—that is, to the information supplied by the input field devices. Each contact in the control program monitors a certain field device. The contact waits for the input to do something in particular (e.g., turn on, turn off, etc.—this all depends on what type of contact it is). Then, the contact tells the PLC’s control program, “The input device just did what it’s supposed to do. You’d better check to see if this is supposed to affect any of the output devices.” 

A PLC’s memory system is very complex, allowing it to store information not only about the control program but about the status of all the inputs and outputs as well. To keep track of all this information, it uses a system called addressing. An address is a label or number that indicates where a certain piece of information is located in a PLC’s memory. Just like your home address tells where you live in your city, a device or piece of data’s address tells where information about it resides in the PLC’s memory. That way, if a PLC wants to find out information about a field device, it knows to look in its corresponding address location. Some addresses contain information about the status of particular field devices. Other addresses store data that’s the result of control program computations. Still others contain reference data entered by the system programmer. Nonetheless, no matter what type of data it is, a PLC uses its addressing scheme to keep track of it all. That way, it’ll have the right data when it needs it

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