Programmable Logic Controller (PLC) Explained

Ever heard of something called a PLC, but wonder what it is and does? 

PLC stands for "Programmable Logic Controller." It’s a small computer used to automatically control various processes and components in an industrial system. 

Like some people’s brains, a PLC can multi-task – and it's lightening fast.

Think of a PLC like a brain controlling a finger on the trigger of a paint spray gun or applicator.

• The brain sends a signal to the finger, directing it to pull the trigger so something is dispensed.
• When the brain wants the finger to stop pulling the trigger, it sends another signal, so the dispensing ends. 

While the brain controls the finger on the trigger, it’s also controlling the eyes. The eyes would be like a scanner reading a barcode in a PLC controlled system – think of the scanners you’ve seen at your local grocery store.

Like a scanner, the eyes read and signal something to the brain – in this case that the dispense is complete - and then again, instantly signaling back from the brain to the finger to stop pulling the trigger.

A PLC can control many inputs and outputs. For example, it can direct a paint line to change color, so that all the widgets being painted go from red to black. 

A Programmable Logic Controller (PLC) can both talk and listen via a communication protocol or discrete I/Os.

"I/O" stands for inputs and outputs.

Communications protocols include, for example, Ethernet IP, DeviceNet, Modbus TCP, Profinet, etc.

Think of these ways of communicating as languages or sets of rules that define these languages.

The discrete I/Os include both digital and analog signals. A discrete system uses a digital signal to communicate. 

The easiest way to think about this is the signal used in a simple light switch.

• When the switched is flipped one way, the light is on. 
• When it’s flipped the other way, the light is off.

On or off, that’s it.

Analog is more like a dial with a range values, instead of on or off; and it has to be converted to digital or digitized. 

Say there was an amplifier with a range that went from 1 to 11, hooked up to an electric guitar.

• When setting the dial at one, the guitar is quiet. 
• At 11, it’s REALLY LOUD!  

Unlike digital with its on-or-off only, the amplifier can be turned to control the volume in a range. 

In fact, that’s how a Graco electric circulation pump works – with a dial to control its speed over a range.

Building on our color change story, this example has a proportioner, pumps, gun, scanner and robot. The Programmable Logic Controller or PLC or "brain" monitors the entire system, telling the components to go on and off in the correct order and for the correct amount of time.

• The pump is pumping the material, the proportioner is measuring and mixing it, and the “finger” on the gun trigger is now on the end of the robot arm.
• The batch of widgets coming down the conveyor on hangers are all getting a nice red coating of paint. But now a barcode moves in front of the scanner.
• The scanner reads the barcode signaling the PLC to change the color to black for the next batch of widgets. 
• The PLC says “okay” and, starts the process to change the system to black by sending out the correct signals for that color. 

When we said multitasking, we really meant multitasking, because the PLC “brain” is monitoring color, flow, parts, pressure, ovens, fans, filters, alarms and more.

A PLC can control all kinds of systems and components in that system, all at the same time. Not to mention the fact that they are extremely accurate, fast and can consistently repeat a process, over and over. 

Contrast this level of control with an individual painter. While there are advantages and disadvantages to both, even the most highly skilled likely can’t keep up the level of accuracy of the PLC-controlled system.

Plus, the PLC isn’t going to make adjustments to the system on its own. For example, it won't turn up the flow on the sprayer without asking.

The kind of control we’re talking about can give you the precision you need to save material, and lessen rework.

It'll just plain save you money, cash, lettuce, moula, greenbacks.

Enough said? Enough said.