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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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