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Let’s start off with a simple example. Suppose you are making a
controller for an overhead crane. The operator has a simple control box with
four push buttons: "left", "right", "up", and
"down". When the operator presses the "left" button (note
that the "left" button is an input to the PLC), then the PLC turns on
the appropriate output to the motor that makes the crane move left. The other
three buttons would operate similarly. Sounds pretty simple – right?
Suppose it takes ten minutes for the crane to reach the full left
position. Soon the operator’s fingers start to hurt (holding that button down
for ten minutes at a time hurts), and they are going to beg / bribe / threaten
you, the programmer, to latch that output on and add a stop button. Instead of
having to press the "left" button for ten minutes, the operator wants
to momentarily press the "left" button and the crane keeps moving left
till the operator presses the "stop" button. So you reprogram the
crane and now the operator picks up a 10 ton container, presses the
"left" button, realizes he forgot to get a drink (of water), and
knowing that the crane will be moving for ten minutes, goes off to get a drink.
Or suppose the crane hits the operator and knocks them out. Who is going to stop
the crane? There are some major safety considerations since you have a 10-ton
container moving around with no one to stop it.
So you start adding safety light curtains and mats around the crane’s
operational area, so that if anything comes into the crane’s operational area
the crane automatically stops. You would also add Emergency Stop (E-Stop)
buttons around the area so that anyone can press one of these buttons to stop
the crane. You would want to add end-of-travel limit switches so that when the
crane moved as far as it can go then the PLC would automatically stop the motor.
You would also want to add some more inputs (feedback) to the PLC so that when a
motor fault occurred the PLC would detect the fault, turn off the motor, and
sound alarms. There are many other safety and diagnostic inputs you should add.
Do you see how a very simple application can grow in inputs and outputs
very quickly? The good news is that by using a PLC for this application the PLC
is very quickly and easily reprogrammed for the new inputs. Other wise you have
to go get more relays and do a bunch of wiring for each new input and output.
Even More Complexity
We can extrapolate this simple crane into more complex systems:
- A "crane" that automatically loads or unloads 55 gallon drums
onto pallets, containers on or off a ship, or adds a finite amount of
reagent to a matrix of test tubes.
- Multiple cranes that have overlapping work envelopes and require
collision avoidance and cooperative handling
- "Cranes" that work in three-dimensional space to store and
retrieve items. Applications from electronics to pharmaceuticals show that
automated storage and retrieval systems reduce errors significantly.
- Two axis controllers that move a video camera around for inspecting
parts
The control systems engineer sees a lot of similarities in these different
applications. All of these applications can use a PLC but these applications are
just a tiny subset of all the control schemes that employ PLCs.
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