Shadowing Practice: PLC State Machines Explained - Learn English Speaking with Video

Hello and welcome back to the channel.

In today's video, we're going to cover a super important concept in the world of PLC programming, controls programming, called the finite state machine, or sometimes just state machine.

In a very basic setting, sometimes a state machine is also just considered like a sequencer or sequential logic.

They overlap a little bit in small systems, but when you get into large,

complex systems, a state machine is really like a hierarchy or a fundamental way of structuring your code that is super scalable,

easy to debug, easy to write additional code onto.

one together.

Okay, so let's talk about a very simple example of something that we could create a state machine to control.

And let's take a look at an industrial setting.

Okay, we have a tank, right?

We have some vessel that we want to be able to fill with material.

And then we want to be able to blend it and mix it up.

And then we want to be able to drain it and discharge material.

So I have all the IO, all the inputs and outputs used in this system shown here.

We're going to have a fill valve that will gravity feed raw material in.

We'll have a high and low level sensor, just digital inputs that tell us there is material in the tank at different levels

so we can see if we're below a low level set point or above a high level.

We'll actually have a mixer, kind of like a mixer descending into the tank to blend the material, and then we'll have a discharge valve that will also be gravity fed.

So we're not going to worry about pumps or things like that to

case and let's talk about how we want this system to operate normally okay

so we want the tank sort of sitting there waiting for a command someone would hit start

On an HMI or a push button doesn't really matter someone says okay start a blend and

and the system will check make sure everything's good no faults we're ready for

that command we're going to start by opening the fill valve until high level is reached

and then starting the mixer mix for some set point desired

and then we'll stop the mixer open the discharge valve

and once low level is reached then we'll close the discharge valve and return to idle okay so

conceptually you can see how this might make sense if you've been in industry or in a chemical plant

They are very simple.

It's like raw material in sometimes an operator will put a couple cups of additives in the top

or you use little diaphragm pumps for dosing.

You'll blend it up and mix it and then you'll send it on its merry way to go get packaged out.

So this is a very realistic real world scenario.

Now to program this in a logical way, a well documented, well structured way.

We want to create a state machine for it and state machines, broadly speaking, are defined by states and transitions between states.

Okay?

So for this system that we've just described, the states could effectively be idle, it's sitting there not doing anything,

filling where the mixer is off and the discharge valve is closed

and only the fill valve is open and the tank is building up its volume of raw material.

And then mixing where it's blending, no new materials coming in, no new materials going out.

We're just mixing and blending and then draining where the mixer's off, the fill valve's off, and we're just letting the blended material leave.

And then another state to think about is if something goes wrong, like the mixer fails to start or a valve gets stuck open,

that might go into alarm state.

So here shown in kind of the blue bubbles are what you could think of as the states.

schematic I'm showing here is known as a state transition diagram

because you're defining the states and the transition events

that take you from one state to another right so

when you're sitting there in idle the only thing that's going

to take you out of idle the way I've described the

system would be someone pressing the start button you could have an automated system where just like high level

or replacing a tote or something automatically triggers a fill,

that's a transition from the idle state to the filling state.

Now if I'm in filling and something bad happens, I might need to go into alarm state.

But if a person says this was a mistake, call it off, and they hit the stop button, I might just go right back to idle.

But in normal circumstances, I've hit start, I've started a fill, and I've reached high level,

then I will transition from the filling state to the mixing state.

Here, once low level is reached, I'll go back to idle.

So a normal operation flow is idle.

Someone presses start.

I fill to high level.

I mix for a desired set point.

And I drain to low level.

But you can see just in this one tank, 5IO point, simple system, how these state transition diagrams also capture a lot of unique edge cases.

And the PLC programmer needs to be thinking about these edge cases when they write the code for it.

diagram actually does not look too different from an IEC 61131 PLC programming language called SFC or sequential function chart.

Sometimes I think mislabeled sequential flow chart,

but it actually is a programming language that uses this concept of states and transitions similar to this.

It's not common in PLC programming.

Far more common is ladder diagram and structured text.

So we're going to stick with those.

But it is worth calling out that that is basically what an SFC is, is a document of the states and transitions between the states.

And you need to write lower level code to actually automate the system.

Okay, so enough PowerPoint engineering.

Let's take a look at what this actually means in a PLC program.

And because it would probably take, you know, hour to write together,

I'm going to go ahead and talk about the state machine that I have already written and walk you through it.

So let's go to main.

The main PLC let me close this side by side real quick.

Okay, and

so what I have set up ahead of time for this

Demonstration is something to handle the basic states something to simulate as

if we had these inputs and outputs This isn't a hardware tutorial

So I'm not you know actually standing up a tank in my garage to show you guys this

and then I have that running logic that normal sequence built Okay, so that's all that's in Maine.

Let's go to state handler, right?

at my main states and the things that could transition between primary states

so active just not being true me not being in sequence

is going to say I want to be in zero I want to be an idle

running will be our main control code this is where the things are happening that'll be

when someone hits the start command we'll start it up we'll say we're in active we're in run mode

One quick just sort of syntax thing, it's not a good idea to program yourself into a corner when you're doing PLC programming.

So when you're setting up a program initially for a small system, maybe you want to take jumps of tens rather than individual integers.

When you're doing a large system, maybe you would want to go from zero to 100 to 200 or 1 ,000, 2 ,000, 3 ,000.

added, sometimes you want to add a step that's actually halfway between idle and running, right, like a startup sequence,

and maybe you want to make that five or something so that these numbers still make sense and are sort of sequential.

So that's why this is zero to 10 and not zero to one.

It's just sort of like a programmer's delight sort of feature.

But anyways, so we have running, stopping, and alarmed or faulted, okay, those are kind of what you could think of as like your

modes and then we'll go into running

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and we'll take a look at what actually happens

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when we enter running

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so someone has hit start we've entered active first thing we do is we check are there no active alarms

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if there are no active alarms let's go start a fill

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if there are active alarms let's go to that mode 30

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and let's sit there until someone clears them or acknowledges them

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Now we're in mode 11 right which is just right after starting up the running sequence.

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Now we're in filling and we're going to open the fill valve until we reach high level.

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And then once high level is reached we're going to move to step 12 and step 12 is going to be mixing.

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Here we're going to start the mixer and again

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if you've been following along with the PLC programming tutorials on the channel you already know this.

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the fill valve just once this condition is no longer true the fill valve will shut

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so these momentary coils imply if I'm in step 11

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and I don't have high level open the fill valve

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but it also says if I'm not in step 11

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or I do have high level close the fill valve so

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that you implicitly get that code for using these momentary coils hopefully that's clear go watch the

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previous tutorials if

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if you're unfamiliar with why we don't need to also command the fill valve to close okay

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but now moving on to look at the mixing step again

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we're turning on the mixer we'll wait to watch for mixer feedback

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so this is a very common convention in digital outputs for motors

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or pumps is you command a plc output to turn on a pump

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or motor but usually that chunk some big motor contactor pulls in

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and you want feedback off of that contactor that it has and pulled in.

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So other than just looking at the output and saying is the output on ideally, you'd actually be looking at a feedback on that contactor that says, Yep, it pulled in.

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And now I'm counting up in timing.

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That's a normal source of alarms and faults is like, Hey, I commanded this output to turn on, but I never got feedback that the contactor pulled in.

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And anyways, then we're going to time for some set point, I have one minute in there right now, this could be a

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I just have one minute in there and when

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that timer finishes we'll go to step 13 similar to the

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valve we won't need to tell the mixer to turn off

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it will just turn off once we're no longer in step 12.

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Step 13 will be draining it looks exactly like filling

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but now we open the drain valve until low level is met

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and then when low level is met we're going to turn off the active bit

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and we're going to go back to step 0 which is idle.

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Okay so that's the normal sequence of events and you can see how handles this little discrete chunk of logic.

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And it's really easy to see how from an HMI or a monitoring system of any sort,

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you can always know exactly what your code thinks it ought to be doing just by looking at this one tag.

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This one mode tag will tell you exactly what it should be trying to do

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and what state it should be in right now.

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This is a lot cleaner than a world of dozens of

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Find what mode currently is and you go look at what it's trying to do in that specific step.

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Now, because this is a software setup and I don't have all this physical IO, I'll walk you through the sim logic I have here really quickly.

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When fill valve is open, I have every 500 milliseconds, so twice a second, I have add one to tank level,

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just this integer value that's tracking the tank level.

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fill and tank drain.

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And then when the mixer turns on, I today tell the feedback to act as if that worked properly.

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And then when tank level gets above 75, we say, okay, we have high level when it gets below 10, we say we have low level.

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So that's the that's the simulation logic I've built so that we can test this sequencer.

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All right.

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So let's actually walk through what this looks like.

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And then we'll just close the tutorial real quick.

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So we're sitting here in idle system is just chilling.

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Everything seems okay.

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Nothing's faulted.

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A person comes along and hits a start button.

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Now I go to step 10.

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Now I move into step 11.

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I'm in running.

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And if I go in and monitor my running logic, you can see we're sitting here with the fill valve open, waiting for high level to be reached.

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So if I go over to my sim logic, you can see the tank level is rapidly counting up, it's counting up twice per second.

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So we'll hit 75 in no time.

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But if I had an HMI setup for this, and you know, we'll do some HMI tutorials on the channel soon, it would say, you know,

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a graphic of the tank that is showing the volume counting up.

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And then it just hit 75.

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So now we're in mixing and you can see our mix timer is counting up the fill valve closed.

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The level is holding at 75, which is what it filled to there.

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And in 45 seconds, we're going to finish this mix timer.

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And here you could is a good spot to say like you have the main logic of what you want to

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this code is like what's a common issue you could have

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at this step it would be like the mixer is being commanded to be on

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but i don't have mixer feedback for five seconds

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or one second like i gave the contactor a chance to pull in

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and relay that back to the plc and

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so you could have a oh i commanded the mixer to start

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and i didn't get the feedback in that case i want to go to my

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when everything's good and we get the feedback, we count up to 60 seconds, we get to our drain state.

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Now the mixer's off, the fill valve's off, and all the system is trying to do is drain until we get low level.

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We can see the level hurtling down now, kind of opposite of the filling step that we were just in.

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And in a couple of seconds, we'll be at 10, which will trigger our low level indication, and we should go back to idle, right?

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So this very simple logic actually would let us completely automate a filling and draining sequence.

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And it all goes back to there.

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There we go.

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We hit 10.

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Okay, so we should be back in idle.

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All right, we hit low level, it moved to step zero, and it reset active.

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And now everything is just chilling, waiting for commands and transitions.

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It all goes back to being able to

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writing the PLC code to actually do the thing is relatively trivial

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and there are great ways to go through step by step

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and check for edge cases

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and of course it gets more complicated as you get into dozens

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or thousands of IO points and large interconnected systems with DCSs

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and SCADAs but if you can take every system and subsystem back to a state transition diagram and say what

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move me from filling to mixing to draining, then you can create well -structured, easy to debug, easy to program PLC code.

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And a big thing in our industry in controls

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and automation is writing code for the next person who has to work on that system.

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Over decades, it's very unlikely that one person will be responsible for all of the control code for a facility.

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programmers and people working on a system what you were thinking when you created that system.

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Okay, so like I said early on in the video, in a smaller system like this one I've demonstrated here, creating a sequencer where you just say like,

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I'm in step 10, 11, 12, 13, is not super different from a state machine.

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When you get to very large systems, you often have like zones of control that

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have a mode that says I've gone from you know idle to fill

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but you'll have a new like sub step

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or sub mode that's like command all the valves to do a thing

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and they'll all be sub functions inside of the fill sort of master mode control

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so state machines can actually have state machines nested inside of them it gets very complicated at very large systems

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but if you're able to break it down doing.

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So I hope you found this useful.

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As always, if you have questions about what you saw in this video, feel free to ask them in the comments below.

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Please consider leaving a like on the video and subscribing to the channel.

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And let me know what you want to see more of on the channel next time.

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Thanks for watching and see you next time.

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Thanks.

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Bye.

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