anit tie down

I have two pb ---pb1 and pb2 they must use the anit tie down...if one is realeased then the other must be released before the process starts again.

three parts of the machine must be back in plase before this process starts again

pb1 no
pb2 no

ls1 nohc
ls3 nohc
ls5 noch


how can I get this to work with what I have?

can this be done without timers

Hope you can help, i've spent hours trying to figure this out...new student

[email protected]

Hardware

See http://www.nolatron.com for a company that makes the
hardware you're looking for.

I'd be very surprised if any underwriting authority would
allow a PLC to perform this function in a real world
application.

Now, if you're a student, this is a very interesting
application to program.... View Nolatron's literature,
and see what THEY do with this problem.

You're gonna have to learn to be more precise in your descriptions. Your description is actually a bit backwards.

I wonder what a "nohc" and "noch" is.
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As far as the "parts in place"...

Are you looking to see if a part is in place? Or, are you looking to see that a part has been set in place? Are you looking for a "state" or an "action"?

Now, regarding the Push Buttons...

You need to know that the proper operation has occurred. And again, the question is, "Are you looking for a 'state' or an 'action'?"

You need to “ARM” the PB’s.

If PB-1 is OFF for X-Sec then it is “ARMED”

If PB-2 is OFF for X-Sec then it is “ARMED”

If PB-1 goes ON while it is “ARMED” then “PB-1 is Valid”, but only for X-seconds.
If PB-2 goes ON while it is “ARMED” then “PB-2 is Valid”, but only for X-Seconds.

If "Parts are in Place" AND “PB-1 is Valid” AND “PB-2 is Valid” then the cycle occurs.
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Now, you figure out how to do this and in what order these need to occur.

Let us see what you come up with.

Sorry for the typo...nohc...normally open held closed are the three ls switchs, that must be true before the anit tie down will work. I want to do this without any timers can it be done

thanks again

Jeff

I fail to see why you're placing artificial constraints on yourself. An inherent part of any two-pushbutton control with anti tiedown is that the two pushbuttons must both be actuated within a small time period. A timer is the obvious choice to establish that time period. Why would you want to try to write the routine without using a timer?

Sometimes "If it aint broke, don't fix it" is the best rule of thumb. Sometimes, it's "If it aint broke, break it". Engineering is the art of deciding which rule applies to your situation.

this was placed on me by the teacher. Trust me I hate restraints....exept

I think the reasoning behind this is understanding how to place the ladders in correct order to use the output image table.

YES... there is a way to do it without Timers.

That method involves a Counter instead of a Timer. However, you should be aware that a Timer and Counter are essentially the same thing.

Before I explain the Goofy Counter method...

Take a hint from what I said about "state" and "actions".

Tracking a "State" is "timeless".

Tracking "Relative Actions" is not timeless (i.e., it is time sensitive!).

Think very carefully in terms of the actions that need to occur in order to "KNOW" that a proper "SEQUENCE" has occurred.

(A then B) OR (B then A)

Now, "A" could have just gone ON, yesterday! And "B" just gone ON just now.

Does that serve the purpose?

It's not enough to see a PB go ON while the other is ON. That would defeat the purpose of the "Anti-Tie-Down", don't cha think?

When a PB is released, you could latch the fact that it is released.
You could do that for the other PB as well.

Now, knowing that both PB's are OFF, if BOTH PB's are pressed at EXACTLY the same time then you can get a cycle. What do you think the odds are for that to happen?

Realistically, both PB operations need to be VALID AND they need to happen within X-Seconds of each other. Thus the need for the Timer.
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The Counter Method... This is a goofy way to do it.

When a Valid PB operation occurs, if the other PB is NOT ON, then begin counting PLC scans. If the other Valid PB operation then occurs before the counter reaches the preset value then the cycle occurs.

If you made a bet with someone that you could do an Anti-Tie-Down without a Timer, this will suffice, technically.

But it can be argued that X-number of scans is actually a Timer. The time value being equal to "X-scans" times "Scan Time".




Aw, Damn!

I just posted and now see that it was a challenge put on you.

Well, there's a freebie!

WITHOUT Timers?...

Sure, you can do it without timers (or their evil twins, the counter boys), but don't you want a HUMAN to operate the buttons?...

Doing it without timers will work, but the HUMAN must press both buttons at E-X-A-C-T-L-Y the same time (taking into account input debouce time, scan time, etc.)

My thesaurus lists the words "Human" and "Exact" as antonyms!...

beerchug

-Eric

I can see the timers are the best way to go but unfortunately I have to create this with 2 push buttons. Would you know how to lay it out or the boolen equation to accomplish such a feat assuming?

Your not paying attention.

Terry laid it out for you. Use a counter to count the number of scans between the the time the first button goes high and the second. Also, please explain to your teacher for me that this is a perfect example of what not to do (EVER) in the real world of programming.

yes I see what Terry is trying to explain but I can not use a counter or a timer....

Jeff

OOOHHHH

Now I see the issue. I did not realize you could not use a counter in this exercise. That still doesn't change my opinion of using dangerous examples of fantasy situations to teach students, but that's another rant (sorry). Perhaps someone else could shed some light on this subject.

This is a very unrealistic assignment if its for a plc course, the whole concept behind an anti-tie down is to make sure someone pushes both buttons within a certain "TIME" span.

You didnt mention if a specific brand of plc was being used. A simple solution is to use an anti-tie down PB setup but that probably isnt an alternative either.

I wonder if your brand of plc being used for training has an RTC (real time clock) and the Compare instruction, sitting here thinking about this I thought that might be an option. You wouldnt be using timers but you could use "time" then use the clock to determine HOW much time has elapsed then COMPARE that to a preset value.

I know this was a student question and unrealistic in many ways but I will have to play around with this on the L sealer thing I am doing now (that has a form of an anti-tie PB arrangement).

Its just a thought and you will have to determine if its possible. The more I think about it the more I like the thought of using the RTC for the timer aspect.

I am using allen bradley plc 515 and the programming software package is RSLOGIX5.....this anti tie down is just part of the program and it only gives me 2 push buttons to use..I have spent 6 hours trying to figure this out as I thought it could be done, but everytime I change my scenario one portion would be false. Thanks to everyone who has been trying to figure this out with me....if this can't be done what kind of questions should I be asking my teacher?

- Jeff

I'm confused as to where you stated that you need a time requirement. As I read your original problem, it appeared that you have a series of input conditions that need to be met in series with an output. Just select the proper examine on/examine off symbols and addresses for your hardware. As soon as you break the rung, the output will go off.

Seems like everyone is making this out to be more difficult than it is. One rung - unless I'm not truely understanding the entire problem.