plc's??

I've just started studying PLC's at college, but I'm not sure what they actually do. Do they allow control of machinery? Or is it a little more complex than that? How could a PLC be used to automate a machine?

Look at this:
http://www.plcs.net/contents.shtml

Hey, Ron...

How about this for an idea...

Any new guy (or maybe, any unregistered guy) that wants to post his first question gets redirected, automatically, to the "Learn PLC's" page... then, before he gets to post a question, he has to take, and pass, a test at the end of the tutorial before he can post!

Maybe this could continue to happen until he registers...

Wha cha think?

Works for me

Cant tell ya how hard it was not to make one of those remarks that I am guilty of blasting when I first came here.

Don't be so hard on a guy

You guys shouldn't be so hard on that guy. He is just trying to learn. Maybe you guys think you are know-it-alls, but I bet at some point in your life you were asking dumber questions than that. Lighten up!

Well if the guy is taking a course then I am sure he has a book, which from his question he obviously didn't read.

Well, I agree that we as a group should not be overly hard on newbies with basic questions. And I also think we have been (especially recently).

But don't you think there should be an expectation (requirement?) for at least a minimum of effort to find the answer to your question before you post? I mean, lets get real. At the very top of this forum is a link labeled "Learn PLCs". If you go there, the first topic available to you is "What is a PLC?".

It takes two mouse-clicks to get to w22tjw's answers (I know, I did it) and then you just read. It seems to me that this is not too much to ask.

Steve

I think its a good question.
A PLC replaces the banks of relays, timers, and counters that once were used to control complex machines. It also allows extra control by using its ablity to connect with display units and digital command interfaces often called HMI's "Human Machine Interface". Because of the costs of timers and counters once 4 timers are required in a process it is often cheaper to use a PLC. It is also easyer to change the way things work. For example on a concrete press a new product required re-bar to be inserted. If it was a Pre-PLC relay based system it would have taken a rewire of the control box and extra relays taking about 2 weeks to convert. With a PLC this was done in 4 hours.

OK does that answer the question? He did not ask how to program them.

Well, how do you program them?????

Sorry I couldn't resist asking.

Thanks Eric

Eric answers the question of what plc is very clearly. It probably took him 2 minutes to satisfy the questioner. There's no such thing as a stupid question. I was looking for a simple answer to this question as well and was glad to find Eric's among useless comments.

Eric, my next question is how PLC different from Microcontrollers such as Intel 68HC11 or a Motorola chip coded by assembly language? Is PLC more popular only in electromechanical field and not Electrical Engineering field? Hope you understand my question.

In the main the PLC has a better EMC than a PC. PC boards are used in some equipment but they are harder to get repaired. Normaly it needs manufacturer to change any programing and repair so if they go to the wall the equipment is also non up-gradeable. There are many people who can modify PLC programs. Normaly the manufacturer includes a ladder print out. And even if one has not got the software one can follow what should happen. We have a robotic arm and product code printers with PC boards in. The robot is OK but printers are for ever being messed up by the setters and so hard to work out if fault is software or hardware. With price of PC's dropping I think we will see more of them used directly. But its taken some time. Look at the old BBC computer used in most UK schools in early days and look what they could do without any extra boards fitted and it seems we have gone backwards. Many radio Hams used spectrum +3 as it would interface with radio without modem etc. As would many of the old tape loaded computers. But my PC fails quite often and needs rebooting. I have see PLC's run for years without a reboot. I note most PC's in machines run Win98 not any of the new operating systems?

The difference between a classic microcontroller and a PLC are essentially packaging and operating system. The PLC CPU is essentially a microcontroller with a specialized operating system. After ladder logic is compiled and downloaded to the PLC it is essentially operating in machine language, and the PLC operating system takes care of reading and writing I/O and executing the code. The PLC packaging genearlly makes I/O simpler, and also industrially hardens the electronics for factory floor temperatures, electrical noise, etc.