Anti -Tie Down - Non Repeat - No Timer

Hello, I am in school learning about PLC and RLL and my professor gave us an assignment to try to figure out how to write code for a anti tie down non repeat circuit using just 2 push buttons and 1 output light but we cannot use timers just b3 internal relays.

I have sat for hours writing different codes and cant seem to figure out how he does it. I have read every thread on here about these circuits so I know people are gonna say its been discussed many many times. Sorry for the repeat... I just haven't seen too much info doing it this method.

any help would be greatly appreciated.

I've not seen this done without a timer. I think it is impractical to do so, and I sympathize with your struggle.

With that said, lets get a clear definition of the problem.

By anti tie-down that means to me:
1) if either button is pressed by itself nothing happens
2) if both buttons are pressed at the same time, an output occurs (for a oneshot or limited amount of time).


In reality, there must be some tolerance to allow one button press to occur up to 100ms (typical) later than the other. This type of control is not safe when handled by an ordinary PLC. If two hand control is for safety, do it with a safety relay or properly rated controller, not an ordinary PLC.

Also in reality, there are situations where two hand (or similar) control needs to be done in a PLC. In a PLC I would use timers. I have yet to see a PLC without a couple of free timers to use for something like this. I know your instructor is trying to make you think and show you how slick he is, but it is unrealistic. In a PLC, you would have each button run a timer set for say 1 second, and when both "timer timing" bits (or each is enabled and NOT done) then allow the output to be on, otherwise it is off.

It's that simple with two timers.

Without timers, I would expect that you will have to be very exacting in pressing the buttons in order for the program to behave as intended, so in principle, it's the same sort of logic.

If both buttons are on, seal in the output, if either button is not on, drop it out:

I don't understand the problem, what's the function of the two pushbutton?

A two-handed start, anti-repeat, anti-tie down control is required for typical mechanical presses where single-stroke capability is required.

The two-handed feature forces the operator to use both hands, simultaneously, to initiate a stroke. Typically the stroke must also make it past 180 degrees before either of the two hands can be lifted. The purpose is to assure that the operator is clear of machine. There are obviously other safety devices typically in play here, too, but this is one.

The anti-tie down is a feature that looks at each push button and requires that it cycle state between strokes. This prevents someone from laying a glove on the push button or finding some other "tie down" technique to bypass the use of one of the push buttons.

Switching back to the original OP's question, in a press, you typically have either cams or a resolver to monitor the stroke of the press. When you know where you are within the cycle of the press, you can make additional decisions that would prevent the use of a timer. Clearly you don't have that here, so we have to think about it differently than with a press.

As I think about your description of the problem, it sound like the professor wants the following:

1 - Starting with the light off, both buttons must be pressed to turn on the light.
2 - With the light on, if either one (but not both) of the buttons is released, the light will go out.
3 - With the light on, if only one button is released and then pressed again, the light WILL NOT go on.
4 - Once both buttons are released the light goes out.
5 - Pressing both buttons again will turn the light on again.

Would this be a correct description of what the professor wants?
Steve

You could program anti tie-down circuit in the PLC without timer.
Just use counter or add function and count/add scan cycles of the PLC program.
This way you will satisfy instructor request.
Tom

Thank you everyone for all the quick responses !

OkiePC: Yes your problem statement is pretty accurate and I will test you example as soon as possible when i have access to the schools PLC. Your explanation made clear sense thank you for that.

Harryting: the reason for it was explained already for me and it was for a two handed press also.

Steve: you guess correctly with what the professor wants. I included a scan of the paper he handed us so everyone can see it.

TomekPLC: I am not sure we are allowed to use counters or the add function for this lab only what he has taught so far in the class and that was not covered yet.

The paper itself does not say that you can't use timers. It must just be an extra condition given. I think the exercise is more to emphasize the anti-tiedown rather than the simultaneity of the button presses.

yes you are correct. he told us in class no timers. he used to give this assignment for a grade but to many students got frustrated and quit because they couldn't figure it out so now it is extra credit assignment.

I understand the problem now, but seriously... Does this "professor" have any real world experience to be teaching people?

No timer? Use elasped time assuming the PLC got a clock.

Serioulsy, this is a waste of time, unless, that is the point the "professor" wants to make.

...and yes, I'm old and grumpy.

Whoa! Your teacher let them quit? He should have adviced, not make one assigment optional..

My Professor is actually the dean of the entire program/building. I think the man reason for this exercise is to show the importance of writing a code that is safe for the operator because if it is wrong injury and/or death can occur. He has been working with PLC's for over 30 years teaching and for companies both in that time.

For the students that quit in college its hard to make them change their mind once its made up especially they don't need to talk to him in order to quit you just submit 1 form to admissions and you never have to go back. no contact with the teacher is needed.

Originally posted by harryting:

Serioulsy, this is a waste of time, unless, that is the point the "professor" wants to make.

Click to expand...

I strongly disagree with this statement. We don't have enough information to justify it. We don't know what the instructor is attempting to point out.

Exactly how many times in your life have you had to determine where a train heading northeast out of LA at 125MPH will meet a train heading southwest out of NYC at 145MPH assuming 3200 miles of track between them? I never have. But it is a classic freshman math problem. It forces the student to think through the information they have and how they can use it. I love that one now that my kids are in school. But I thought it was the dumbest thing in education when I had to do it.

Keith

First, you have to understand the a lot of people here know their stuff better than your "professor".

2nd, the reason the problem is a waste of effort is very well answered by Okie

In reality, there must be some tolerance to allow one button press to occur up to 100ms (typical) later than the other. This type of control is not safe when handled by an ordinary PLC. If two hand control is for safety, do it with a safety relay or properly rated controller, not an ordinary PLC.

Click to expand...

3rd, IMO. when you teach vocational program (as opposed to liberal art - philosophy). You don't waste students time by handing out trick question that takes away real-world tool.

I never claimed my professor knows more then anyone else. I am sure others have more knowledge on this matter. This isn't a required assignment no more for the class because it is a "tricky" problem. It is more of a add-on problem.

Its school assignment, it has nothing to do with real deal. Its meant for students to think how plc works, its just in nice wrapper to do it more motivating for them.

Assignment itself does not require looking for how long one button is pressed, it requires only to not be able to keep it pressed while operated by one button only. Only thing that is needed is to make sure that both buttons are depressed before pressed again.