need help

jlchandler81 · Jul 28, 2015

You have been asked to design an emergency alarm system for your plant. Management would like three emergency switches placed in strategic locations on the plant floor. if anyone senses an emergency, they can use any one of the three switches to turn on the alarm. the only switch capable of turning off the alarm, if activated, is the switch that set the alarm. Can anyone show how to write in Micrologix plc

keshik · Jul 28, 2015

It might be a good start to try and not copy and paste the homework question if you're looking for help with it...

bernie_carlton · Jul 28, 2015

If Emergency_Switch_1 OR Emergency_Switch_2 OR Emergency_Switch_3 THEN Alarm

That's the logic, now you enter that as a ladder.

jlchandler81 · Jul 28, 2015

thanks for the help.....

boneless · Jul 28, 2015

I would not implement this in a PLC.

Emergency Switch and regular PLC do not mix IMHO. And that is beside the fact that a PLC is overkill for a application like this

Geospark · Jul 28, 2015

boneless,

I know where you're coming from but if you read it carefully it is an "emergency alarm system" and not an emergency stop system. The "switches" are not emergency stop buttons.

They will simply be used by the operators to raise a manual alarm annunciation. This could be a flashing beacon or strobe and/or a siren or horn. They are just used to draw to the attention of anybody in the area the fact that an emergency situation has arisen.

Being ordinary control devices, they could also be used to pause, home or stop a process, but not initiate a safety function.

Whether one would do this with a PLC or not is debatable, but that decision would not be safety related.

There is a tricky part to the above FDS. Can anyone else spot it?

Regards,
George

danatomega · Jul 28, 2015

@Geospark - could it be that 'the only switch capable of turning off the alarm' ...

Geospark · Jul 28, 2015

Yes. To accomplish what they ask is trickier than the straight forward parallel rung we have all probably envisaged by now.

Why trickier though?...

danatomega · Jul 28, 2015

Seems like you'd need to make them mutually exclusive and use a flip flop on each as well. (just thinking out loud (as it were - there's no-one here to hear me)).

bernie_carlton · Jul 28, 2015

the only switch capable of turning off the alarm, if activated, is THE SWITCH that set the alarm

my emphasis added.

There is no sense in the description of more than one switch being activated at one time. Therefore (since this is a theoretical exercise) I see no need to cover complications not explicitly stated.

keshik · Jul 28, 2015

bernie_carlton said:
my emphasis added.

There is no sense in the description of more than one switch being activated at one time. Therefore (since this is a theoretical exercise) I see no need to cover complications not explicitly stated.

Even if you have multiple switches activated, each one of those would have to be reset to turn off the alarm. (In case it's not clear, I'm agreeing with you)

That does make it a pretty simple problem. I imagine the instructor meant it to be more complicated than this (but didn't word things well).

Bering C Sparky · Jul 28, 2015

This could be "tricky" or very straight forward depending on what hardware is used.
None was stated so that is up for debate.

If you set up inputs from 3 Maintained, Push - Pull, Mushroom Head Switches in Parallel to a single OTE, you only need 1 single rung, to accomplish what is stated in the post.

Push Mushroom head on switch # 2 and it stays in a maintained position and output is true until you go over to switch #2 and pull the mushroom head back out deactivating the input.

That would not be as much fun as using Non-Maintained switch's and lots of rungs to get there though.

BCS

keshik · Jul 28, 2015

I agree. I'm guessing that the "switches" are meant to be momentary buttons that trigger the set/reset of the alarm on the leading edge, not the maintained state.

harryting · Jul 28, 2015

In the real world, typically, one would use a latched switch that one can push in but needs a key to reset. Thus, once triggered, one needs to find a supervisor/lead with the key to reset.

Of course, OP's homework question really doesn't provide enough information. Is there two switches at each location? Is it latched or momentary, etc.. etc..

Bering C Sparky · Jul 28, 2015

harryting said:
In the real world, typically, one would use a latched switch that one can push in but needs a key to reset. Thus, once triggered, one needs to find a supervisor/lead with the key to reset.

Of course, OP's homework question really doesn't provide enough information. Is there two switches at each location? Is it latched or momentary, etc.. etc..

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