Student needs help

[BDKuhns]

Ok, Ok, Ok. I've tried everything I can think of to get this to work.
I'm a student, and am taking my first PLC class. We are using the SLC-500, RSLogic500, and a practice simulation console. One of our projects is to do the following:


Inputs: Device type Addrs.

1 Start Button Momentary I:3/6
1 Stop Button Momentary I:3/7
1 Motor#1 Auxiliary Contact Maintained I:3/0
1 Motor#2 Auxiliary Contact Maintained I:3/1
1 Motor#3 Auxiliary Contact Maintained I:3/2
1 Motor#4 Auxiliary Contact Maintained I:3/3

Output:
1 Motor Starter #1 O:4/0
1 Motor Starter #2 O:4/1
1 Motor Starter #3 O:4/2
1 Motor Starter #4 O:4/3
1 Pilot Light- “Motors Running” O:4/4
1 Pilot Light- “Motors Stopped” O:4/5

Sequence of Operations:

Actuate Start Button, Starts Motor #!.
After confirming Motor #1 has energized (Aux Contact), start Motor #2.
After confirming Motor #2 has energized (Aux Contact), start Motor #3.
After confirming Motor #3 has energized (Aux Contact), start Motor #4.
After confirming Motor #4 has energized (Aux Contact), turn on “Motors Running” Pilot Light.

Actuate Stop Button, stops Motor #4.
After confirming Motor #4 has de-energized, stop motor #3.
After confirming Motor #3 has de-energized, stop motor #2.
After confirming Motor #2 has de-energized, stop motor #1.
After confirming Motor #1 has de-energized, turn on “Motors Stopped” Pilot Light.


I have no problem with the first part of the project, but I can't seem to figure out how to reverse the process. We are allowed to use the B3: file.

Anybody have any suggestions.
P.S. I do not have the logic program here at home, it is only at school. I've been doing these project on paper and programming them at next class.
Thanks.
Barry.

 

[Steve Bailey]

When you press the 'Stop' button, latch up a B3 bit called 'Stop_Sequence'. If 'Stop_Sequence' is true and all four aux contacts are true fire a B3 bit that stops motor 4, etc. Last motor off resets the 'Stop_Sequence' bit.

Now all you have to do is figure out how to recover from an interrupted sequence. What will happen if you pull the plug on the PLC in the middle of the sequence?

 

[Terry Woods]

BE THE COMPUTER !!!

"We are allowed to use the B3: file"

Does that preclude your use of timers? It shouldn't.

From a logical point of view...
It appears that the purpose of the exercise is to point out that subsequent actions are "DEPENDENT" upon the resulting conditions of previous actions.

From a practical point of view...
It sounds like the aim of the exercise is to start a set of motors and give ol'-man power-grid half a chance to continue working without being brought to his knees.

Or, could it be that the instructor is simply looking to see if you can organize a sequence... even if it brings the grid down? In that case, someone, somewhere, will feel a "great disturbance in the Force" as the set of motors drag ol'-man power-grid to his knees.

When you start the motors, you need to use a delay for each motor. You want to verify that each AUX Contact is ON for a certain minimum amount of time before moving on and starting the next motor.

There are only a few dozen ways of accomplishing that goal.

When it is time to kill (stop) the motors... you might want to do it in a slow & easy sequence, lest someone, somewhere, feels a "great disturbance in the Force"!

So again, you use timers. If Motor-4 is ON and the stop signal is asserted then interrupt the Run Motor-4 circuit and run a timer.

If the AUX Contact is OFF for a certain minimum amount of time then move on and interrupt the next motor.

As long as there are motors running and the goal is to stop the motors then continue with the stop sequence.

THINK...

BE THE MANUAL CONTROL SYSTEM...
If YOU were a manual control system (you are.. you know), how would you accomplish the required task? Think very carefully... every detail counts... don't assume anything!

Now that you know what needs to happen (from a manual point of view)...

BE THE COMPUTER...
If YOU were a computerized control system (you are... you know), how would you accomplish the required task? Think very carefully... every detail counts... don't assume anything!

 

[Allen Nelson]

Sealing circuits.

Although I would use a timer in a system that I design (like the one I'm programming right now, in fact, with cascaeded starts and stops), a timer isn't necessary to solve the problem.

This sort of problem is hard to teach (especially over a message board), because it is both simple and fundemental to the understanding of PLCs. I don't think that it's enough to say "Be the computer". He's clear on the sequence. But he doesn't know how to translate what he knows into ladder.

What Barry needs to learn about is the concept of Sealing Circuits.

If I had to guess at his code, I'd say it looks something like this:

   START       STOP     RUN

----| |----+----|/|-----( )

           |

    RUN    |

----| |----+



    RUN                  M1

----| |-----------------( )



    RUN     M1_AUX       M2

----| |------| |--------( )



etc.

RUN is an internal bit (a "B3") that is used to "remember" that the START pushbutton has been pressed. When the STOP pushbutton is pressed, RUN drops out, and ALL the motors stop all at once. The reason that they stop, is that the RUN bit drops out in every ring.

So what's needed is to "remember" that RUN had been on, so that the motor can keep running. Well, one way (and certainly not the only) is to use the fact that if the motor is running, then RUN must have started it.

Since we want to keep the motor running when RUN drops out, we put the contact in parallel with it, like so:

    RUN     M1_AUX       M2

----| |---+--| |--------( )

          |

    M2    |

---| |----+

Great. Now, once the motor is running, it will stay running, even if RUN drops out.

But wait, that's not good. We don't want it to stay running forever. We only want it to run while M3 is still running. Once M3 stops, M2 should stop. So we add the "M3 is still running" condition to the part of our logic that's keeping the motor running:

    RUN             M1_AUX       M2

----| |-----------+--| |--------( )

                  |

    M2    M3_AUX  |

---| |----| |-----+

Lather.
Rinse.
Repeat.

Now let's step back at re-examine that last rung, this time when the system isn't running, before the START pushbutton has been pressed.
- Run isn't active so it can't make M2 come on.
- M2 isn't on, so it can't make M2 come on.
Therefore M2 doesn't come on.

When the START pushbutton is pressed,
- RUN come on making that BRANCH true.
- M2 isn't on yet, so this BRANCH is false.
- The two branches are in parallel, which is the logical OR. (TRUE) OR (FALSE) = TRUE
- But M1 isn't running (just yet), so M2 stays false.

In some future scan, all the above remains the same, and so THEN AND ONLY THEN does M2 come on.

On the next (and subsequent scans):
- RUN is on, making that branch true.
- M2 is on (and eventually M3_AUX will be on), making that branch true
-- True OR True = True
- M1_AUX is still on, so M2 will continue to be energized.

And so on.

When you get this, truly, in your heart, get this, PLC programming will be a snap.

 

[BDKuhns]

Allen, thanks! I had constructed my ladder about 50 times with the sequence similar to your first example-

START STOP RUN
----| |----+----|/|-----( )
|
RUN |
----| |----+

RUN M1
----| |-----------------( )

The used B3:00 as you "RUN" memory contact. It was at this point that I could not get past.....that is until now. I see how you third example would work. It was difficult to see that one can use a condition of a devise that has not been addressed (energized) yet as a condition for one prior to it. I see now that this would allow us to reverse the process as a stop sequence. THANKS!

Terry Woods,
I have seveal spec/command books for the SLC-550 and PLC-5 that came with the book. The instructor is just now going over the more advanced commands (sequencing, timing, counting). For this project, he instructed for us to use only the B3 file if necessary. I'm still confused about one thing though. Apparently, one of the other students in the class was able to accomplish this task without the use of B3 files. I don't know how he did it.

Thanks for everyone's help. If I need more, I'll be sure to ask.
Barry.

 

[Allen Nelson]

You're welcome. By your reply, I can see that you GET IT. That's what we're here for.

Apparently, one of the other students in the class was able to accomplish this task without the use of B3 files. I don't know how he did it.

I can tell you, in general principles how. You might want to see if you can do it on your own with the hints I'll give:

Hint #1. The "RUN" bit is used to 'remember' that START had been pressed. All of those "Sealing Circuits" are there to 'remember' that "RUN" had been active. In other words, you are "remembering" a "memory".

Hint #2. The START button only has to start Motor #1. The STOP button only has to stop Motor #5.

Hint #3. It's possible (although not desirable) that the START button and/or the STOP button needs to be held down through entire sequence to make the code work. It's possible (I think) to write the ladder without that requirement (and using no internal bits), but not easy.

Hint #4. When posting ladder logic to the Forum, even copied from a previous post, you need to include the vB tag [ladder] before the line-art, and [/ladder] after it. That way it will post exactly as you drew it. There's a handy button labelled LADDER just above the Reply window to put those code in for you (you don't have to put any code in when it prompts you to if you don't want to).

 

[Terry Woods]

Allen...

If indeed the instructor is only trying to see if the "kid" can "sequence" (which it now sounds like might be the case) then... yes, of course, the code you showed would work.

However... as a practical matter, doing as you showed would not be very desirable... ol'man-grid, don't cha know?

Your method would (could possibly) have all four motors come on as quickly as 4 scans! Poor ol'man grid.

I don't think bringing a half-dozen 100-HP Blowers on line at, what amounts to be, the same time, would bring down part of the national grid... however, it most probably will bring down part, if not all, of the in-house grid.

It doesn't hurt to teach the practical along with the theoretical. That is, I think it's just as important for "the kids" to develop a practical understanding of the stuff being controlled as well as how to control the stuff.

If KD actually had some relays connected to the outputs (with aux inputs) then, with the particular code, he might or might not be able to determine if the relays operated, as expected, in the correct sequence.

Using timers would make it REAL and REASONABLE.

Yeah, Yeah, Yeah, I know you know, Allen... this is more for the "kid" than it is for you.

BD (the "kid") said...
I'm still confused about one thing though. Apparently, one of the other students in the class was able to accomplish this task without the use of B3 files. I don't know how he did it.

I don't know how to give a "clue" for this without going into a long explanation of what BE THE COMPUTER really means... so...

An unused Input is a valid "scratch-bit" until the next scan. That is, at some point in the ladder, you can turn that bit ON to remember something, for a short time, a very short time... then Alzheimers kicks in. You have only until the end of the current ladder scan to use the bit.

An unused Output is also a valid "scratch-bit". However, it won't exhibit the Alzheimer effect. This kind of bit is as valid as a "B" bit.

 

[Pierre]

Dear BDK,

Sometimes the EE Inspector pays a visit to the EE working in the private sector.

One of the most funny tinghs he looks for is if you have nailed your credential to the wall. They say it must be apparent how proud an EE is to be an EE or a PE or whatever.

He also looks for methods. One of them being the way an EE holds a good neet archive system.

"Apparently, one of the EE in the shop was able to accomplish this task without the use of a file cabinet . I don't know how he did it"

 

[BDKuhns]

Thanks

Once again, thanks to all.

Allen- It looks like I am not only learning to program PLC’s, but learning the in’s and out’s of this forum. I’ll get it right soon enough. I was first pretty apprehensive about posting. I come from Emergency Medical (Paramedic/Firefighter) background. In that field, there is a lot of intimidation from within, so one is usually shy about asking questions. To me that is too dangerous, especially when on is dealing with the life of others. With regard to accomplishing this task without the Bit file, I’m not going to worry about it. As long as the job gets done and done right, that is my target. Besides, this project apparently is the basis of future projects (they compound on one another) and I think without the use of the Bit file, the other student has built in difficulty for the future projects.

Terry- Thanks for the input. A question thought. Instead of using the contacts of the motors to verify the actual energizing of the motors, if an RPM or AMP sensor were used to trigger this light at a “running normal” condition, would this prevent the in-rush concern with the power?

Pierre- I’ll take this to heart. I wish I had initially chosen the EE field instead of the medical. Once I get into the actual Higher EE academia, I plan to search out a couple of what I think as good practioners as mentors.

 

[Eric Nelson]

A question thought. Instead of using the contacts of the motors to verify the actual energizing of the motors, if an RPM or AMP sensor were used to trigger this light at a “running normal” condition, would this prevent the in-rush concern with the power?

Now you're thinking!...

In essence, that it what a timer would provide. You just want to delay the starting of the NEXT motor until the previous stops drawing so much current. Usually this will be somewhere near a fixed amount of time, so a timer could be used.

What you propose is actually a better method than using timers. By monitoring the speed and/or current of the motor, you can KNOW when it's safe to start the next motor, instead of assuming it SHOULD be safe. This becomes a 'smart' timer, with the perfect delay setting, because that delay setting automatically varies as needed to compensate for different starting conditions.

Using a timer vs. an RPM or current sensor probably boils down to, for the most part, cost. Timers (internal) are "free", but a 'proving' sensor will cost $$$. In more ways than just the cost of the sensor... Added installation time, wiring, more input points, etc...

Please keep thinking this way! Start by imagining a system without worrying about budget, then remove the items you can live without one by one until the costs reach your budget...

Also, your statement "As long as the job gets done and done right, that is my target" will get you very far. That's the correct attitude to have!...

beerchug

-Eric

 

[BDKuhns]

Eric,
Sorry if I seem to ponder on this too much, but if inputs from exterior (field) sensors were used as conditional devise to start a sequence of other actions, could we program the PLC to monitor the “time taken” or “amperage needed” to bring a device to a "running normal" from start state. If so we could build into a program a trigger to flag us if one of these motors were having difficulty and showed signs of impending failure.
Barry.

 

[Eric Nelson]

Pondering is a good thing...

Eric,
Sorry if I seem to ponder on this too much, but if inputs from exterior (field) sensors were used as conditional devise to start a sequence of other actions, could we program the PLC to monitor the “time taken” or “amperage needed” to bring a device to a "running normal" from start state. If so we could build into a program a trigger to flag us if one of these motors were having difficulty and showed signs of impending failure.
Barry.

All I can say is, E-X-A-C-T-L-Y!...

As cousin Allen said, you seem to GET IT... :nodi:

You catch on quick Barry... Are you sure you weren't a programmer in a previous life?...

beerchug

-Eric