Shadowing Practice: What is a PLC and how does it work? - Learn English Speaking with Video

You may never have thought about it, because it's one of those easy things to take for granted, but something told that traffic light on your drive to work today when to change from red to green.

Similarly, every time you jump in an elevator, something is monitoring those buttons to see what floor you select,

and then controlling a motor's ramp and acceleration to get you safely to your destination.

The machines responsible for all of these calculations

and controls going on in the background

and making everyday life possible are called PLCs

or programmable logic controllers like this one

and in today's video I'm gonna explain to you what they are

and how they work basically the entire world of controls

and automation and robotics and hobby electronics boil down to inputs and outputs.

Inputs are things about your environment like the state of an actuator being extended or retracted or volume, temperature, pressure, flow.

Outputs are things that you can control to affect your environment like a motor or a pump or a valve.

There's a big need in industry to monitor inputs, make decisions or calculations based on that as well as track things over time like counters,

variables, and statuses and then use those decisions to control those outputs.

If you've ever tinkered with an Arduino or a Raspberry Pi, you're probably pretty familiar with the basic concepts we just talked about.

But in industry you need something that is going to scan those inputs, make decisions about them in the exact same time, in a defined set amount of time,

every time, and control outputs very repeatedly and reliably.

This is where the PLC or Programmable Logic Controller comes in.

If you take one thing away from this video, it's keep this word in mind: determinism.

The biggest difference between an industrial PLC or industrial PC

and a standard Windows laptop or MacBook is the repeatability and determinism of the code that's running on those machines.

Your Windows PC could stop to do something like garbage collection that could create inconsistencies in how it's processing logic.

Your PLC will never do that.

The way the code is written and compiled

and the way the hardware is built is basically a promise that the inputs will be taken

of at the very beginning it will process

and execute the code in a known and repeatable way

and then it will make decisions about the states to control outputs to repeatable in a way

that is exactly defined with this comes a bit of syntactic nuance

and different programming languages even than conventional computer programming is done in

because the entire field

and the need for plc's started as a need to replace these complicated intricate relay circuits the most common way you

diagram which actually looks more like an electrical circuit than a common programming language.

And these PLCs are used absolutely everywhere.

Oil and gas, pharmaceutical, food and beverage, paper and wood products.

It literally does not matter the industry.

You pick one, PLCs are driving it.

Theme parks and roller coasters, PLCs are driving them.

Elevators and stoplights, PLCs are driving them.

The need for a highly repeatable deterministic machine is why the PLC industry manufacturers in this space are Siemens,

Allen Bradley, ABB, Modicon, Schneider Electric, as well as some other manufacturers that make them for more focused,

specialized purposes, often like Keyence, Mitsubishi, Panasonic, and Bekoff.

But that's enough of a high level.

Let's go ahead and take a look at some practical examples of how these PLCs work.

Okay, for the practical side of today's video, we're going to use a Siemens setup.

So I have a basic affordable Siemens PLC and their programming software open.

That doesn't matter.

What we're talking about is how PLCs work.

So regardless of what platform your job or your industry or your career takes you to, the stuff we're about to talk about is platform agnostic.

Okay, so I have a brand new PLC program open, there's no code running on my PLC.

The only thing I've done in advance is I have two push buttons and an output light.

So I have two digital inputs on off inputs, and one on off output we can make reference to while we talk through some of these concepts.

And then this main organization block is generated by Siemens and most PLC providers when you create a new PLC program.

You can think of this as your main loop.

So by default, every PLC has some master thing that it references while it's doing code and logic execution,

and that's your main function.

So typically, especially in larger PLC programs, you don't put any code inside main.

All you do is you want to have execute code.

And I'm going to add a new block, call it a function, and I'll just say example function.

So this will be very simple.

The way main typically works is you would have it execute the function example by calling it here.

And then inside example, we can put that code back in there, which is start push button, light turns on, and we'll actually say when the start button is pressed to turn on the light.

And when the stop button is pressed, you got it turn off the light.

And I'm not going to spend a bunch of time teaching with the ladder logic that I'm writing here, because I have tutorials on ladder programming on the channel already,

I really want to focus So to speak briefly about scan time, which is the most important concept in PLC programming,

the way a PLC is going to execute is it's going to go to its main organization block

and it's going to execute all code from top to bottom, left to right, just like you're reading a book.

does anything in example 2 and it processes code in a deterministic way like we said in the intro.

It starts by taking a snapshot view of what all the inputs are currently showing

and then it executes all of the logic that lives on the PLC

and then it makes a decision about whether or not to change the state of outputs based on that.

It's actually doing these steps in that defined order

It's just doing it at a cycle time that's typically one to two milliseconds for a well -structured small program.

So it doesn't even feel like it takes any time at all to the human.

And so to show this, I'm going to go ahead and activate this program.

We're going to load this onto the PLC really quick.

And then I'll show you that the PLC actually tells you how long it's taking to cycle the code.

So if I go to example and I go live monitor, I can press the start push button and the light turns on

and I can push the stop push button and the light turns off.

And to me, it looks instantaneous.

It looks like I pushed the button, the light turned on immediately.

But that's because if we go to online and diagnostics and go to cycle time, the PLC is telling me, hey, I am running at one millisecond.

Okay.

I'm doing an entire cycle processing everything I have on my PLC in one second.

The longest cycle time it's This is very,

very fast for larger projects and complicated projects doing advanced math.

It is normal to see these get up there to eight, nine, 10 milliseconds.

You actually want to try to keep this number down by writing efficient code.

Okay.

So every major PLC manufacturer gives you some way like this to see your cycle time.

And that's a good indicator of how your code is running, how burdened, how overloaded your PLC is.

do all your evaluation logic, and then make decisions about what to do with your outputs,

makes sense as an overall structure how PLCs execute code to you.

So let's take a little bit more of a look at what the reading top to bottom

and left to right actually means practically.

Let's go ahead back into example and let's put a few rungs of code in here that sort of conflict

light to turn on.

And if either push button is not pressed, then I want the light to turn off.

And you can maybe see already where the conflict could come in.

100

What about the case where one button is pressed and one button is released?

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Okay.

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And so again, the way this code reads, even if you're not familiar with ladder is if start push button, or if stop push button,

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so my two buttons, if either of those are pressed, turn on the light, but the next rung of code is If not start push button or not stop push button,

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turn off the light.

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I'm going to go ahead and put this on my PLC here

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and we're going to take a look at what that means.

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Okay, and you can see just when I go online to monitor it, neither of the push buttons are being pressed right now, so the light is off.

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Now I'm going to push one of the push buttons.

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What do you think is going to happen to the light?

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It doesn't turn on, okay?

111

I can verify the light is off in the field, even though the coil right here is lit up green saying, hey, I'm trying to turn the light on.

112

What's also happening is the other button's still not pressed,

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and so the light is still not turning on because this rung of code is getting executed after this piece of code, which means it's saying, alright,

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check network 1, oh, I should turn the light on.

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and not stop and not start. So...

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If I release both of them, the light goes back off.

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What's super interesting about this and a really good explanation of what's going on here is

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if I put this to not and not down here and I put this to on or on up here, flip these around.

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Now what I've done is I've changed the order of the two rungs exact same code.

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But now I've said if not start start push button or not stop push button, the light should be off.

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But then I've said if start push button or stop push button, the light should be off.

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So now when I hold just one light down and it doesn't matter which push button, excuse me, push button, not light.

123

It doesn't matter which button I hold down, the light turns on in the field.

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I don't have to hold both of them down.

125

What happened here is we changed the order that those rungs execute in.

126

So we're saying even though these rungs are often in conflict with each other where I have only one button pressed in,

127

then whichever one comes later in my program is going to win.

128

You can use this structure of left to right

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and top to bottom to your advantage as a programmer if you know what you're doing,

130

but it can also create a lot of headaches where you'll have overwritten outputs if you don't know what you're doing. So frankly,

131

a much cleaner way of structuring your code is find a way to only control outputs from one rung of code.

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If you want to make it to where start or stop turns the light on, don't have a different rung of code that turns the light off.

133

Find a way to put all of the code of how this code processes top to bottom and left to right.

134

Let's take a look at one more concept in the PLC and that's the idea behind a cyclic interrupt.

135

What if you want something that does not execute as fast as humanly possible that the PLC is continuously evaluating for.

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So it's usually generally considered pretty cool and useful that the PLC can process something every millisecond.

137

Let's go ahead and delete that function and say we actually want something to be evaluated no faster.

138

than every half second.

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I can make a new organization block and call it cyclic interrupt.

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And this is slightly different in Alan Bradley.

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They do timed function blocks as well, but a little bit different to set up again, not a syntax lesson.

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And we're going to call this half second.

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Okay.

144

Thank you.

145

I'm going to go ahead and make this

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and now I have a new organization block that's only going to be evaluated every 500 milliseconds.

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So now I can say, let's just simplify and say when the start button is pressed, turn my light on.

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Alrighty.

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And so what's pretty cool about this one we're going to see

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when I download it is I can now click the start push button and the light won't turn on.

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I'm able to click it fast enough that the light won't always turn on.

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I let go of the button and there's a time delay before the light turns back off.

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So I go ahead and click the button.

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The light does not turn on because I'm clicking it in much less than 500 milliseconds.

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And it's only evaluating this rung once every 500 milliseconds.

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But if I hold it down for a half second and the light does turn on, and if I release it,

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there's a little period of time where the light stays on because it only detects at its next 500 millisecond cliff

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One important note about these is it doesn't mean

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that there will always be a half second delay between me pressing the button

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and the light turning on or always be a half second delay between me releasing the button and the light turning off.

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Because I could be triggering that button anywhere in that 500 millisecond window.

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Likewise, I could be releasing the button anywhere in the 500 millisecond window.

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still look pretty much simultaneous when it turns on, but it could take up to a half a second.

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Again, there are actual PLC programming tutorials on my channel.

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You can check those out if you're interested in learning more.

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The most important thing to retain from this video, so I'm repeating it at the end, is PLCs are deterministic.

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They give you an exact way to know how your logic is going to behave and behave every time, repeatedly and reliably.

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They're deterministic, they're They take a snapshot of the inputs, they move it into an image table,

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they then compute all of their internal logic based on those inputs and variables,

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and then they make decisions about what outputs to change accordingly based on those inputs in a scan.

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Hopefully this didn't get too in the weeds and you found it useful

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and you found it interesting and you want to go to LeMaster Tech, like the video, subscribe to the channel, and learn more about the world of controls and automation.

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if you have any questions in the comments below, or what you want to see next on the channel.

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Thanks for watching, and see you next time.

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Bye.

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