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The menu item above lists all the pages giving basic information with regard to
PLCs. The horizontal menus, top and bottom are links to other electrical
related information.
Programmable Logic Controllers
The solution lies in the name. A controller controls
something, in this case it could be a manufacturing process for example.
Programmable means that the sequence of operation can be altered since it is not
fixed. So what is logic?
Every day each and every one of us makes desicions,
often the right one, sometimes the wrong one. It maybe a decision as the whether
to skip breakfast or which program to watch on TV. There is a therfore process
at work here that allows us to make decisions about the actions we take. I say
process since although it appears that we have just make a single decision in
reality many other much smaller decisions were needed to be made. Well to me,
logic is the process by which we make desicions.
Now that is a little unfair I know since all I have done
is to explain the name.
So what is a PLC?
At this early stage let us call it a black box that has
electrical inputs, electrical outputs, some sort of connection for a programming
device, lamps, and power connections.
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The function of the inputs is to provide feedback on the status of the world
outside the PLC
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The function of the outputs is to make something happen in response to the
status of the world outside the PLC.
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The programming connection allows logic to be written as to how the outputs
should respond to the inputs
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The lamps are there to give information to us.
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The PLC requires a power to be able to function.
What are PLCs used for and why?
The only simple answer I can give as to what they are used for is in the
automation of a process. Automation means removing people from being involved in
the process, where possible. As to why, I hope to make it clear a little later
since we are going to now go back in time and start a company making washing
machines for clothes. I have chosen a washing machine since most people know
what they are.
In the beginning we copied the concept of having two vertical drums, one for
washing and the other for spinning the clothes dry. The housewife was in almost
total control of the whole process, since all we did was to provide her with
simple controls to turn each drum motor on or off. We did have a safety feature
installed that meant she could not operate the spinner without the lid closed.
One of our rivals came up with an idea of having a horizontal drum or front
loader, also this new machine controlled or automated the process. The housewife
placed washing powder into a compartment, clothes in drum, set a dial to the
approriate wash time and left the machine, returning later to empty the damp but
rinsed clothing. Our company was losing ground to this new innovation since it
was proving very popular with the housewife. Action was urgently needed to bring
out a similar automatic model. We purchased one of our rival's machines and took
it to pieces to see if we could improve on the design and make our machine the
housewife's choice.
We found on investigation that the process was controlled by a mechanical timer
that operated a set of cams as it slowly turned. These cams when lifted operated
switches to control the individual parts of the process. The timer could be
stopped until a particualr part of the process had been completed. Our engineers
constructed a flow diagram to show the different steps and how the
machine knew that the step had been completed.
A brainstorming session was called to see what we could come up to make a far
superior machine to theirs. Someone came up with the idea of making ours future
proof in that it could be modified to deal with new washing powders or new
materials amonst other things. If we used the same controller that our rivals
had used this would mean a possible rewire of the machine to acheive the new
functional specification. We needed a controller that would allow us to
modify the logic to acheive the new functional specification without having to
resort to time consuming and thus expensive rewiring. Rather than re-invent the
wheel it was decided that a PLC should be used to control the process since only
the minimum of wiring would be needed to upgrade the system where a new input or
output needed to be added.
I hope from the little scenario that you can see that PLCs are used since it
does not require a lot of rewiring to modify the functional specification. The
functional specification is a term used to define what the process actual
does and how this needs to be acheived, more on this later.
Now some of you will no doubt be throwing down the gauntlet with me in that the
price of a PLC can be higher than the cost of an automatic machine, I have to
agree with you there. Again it is not in the best interests of the manufacturer
of the washing machine to make it last over 5 or so years. However since most
people are familar with the automatic washing machine, they will be able to
relate to what is to come. This is because the automatic washing machine can be
used in understanding a lot of aspects relating to PLCs.
In part two it is time to find out how PLCs are
programmed before we return to the automatic washing machine.