Ladder logic for 2-hand ATD

clone93 · Jun 11, 2002

I'm writing my first "real" PLC program, and am trying to incorporate a 2-hand anti-tie down control on the machine. I am starting the machine cycle w/ 2 N.O. palm buttons. To make it an OSHA-legal anti-tie down, I need the buttons to be pressed almost simultaneously. I'm thinking there has to be a way to do that w/ a timer and/or a comparison function, where only 0.xx seconds can elapse between the 2, but haven't come up with the logistics of it yet.

Thanks in advance for your help!

Terry Woods · Jun 11, 2002

Many situations can be easily handled if you draw a Timing Diagram.

Rick Densing · Jun 11, 2002

I have usually seen .5 sec for the timer.

rsdoran · Jun 11, 2002

I fought with this concept on a hot sealing device. Just wire the 2 buttons in series and make it one input...if either device fails to make contact then it wont activate. The idea is MAKE the operator use 2 hands to activate and not be in the sealer/press etc...doesnt mean the 2 pushbuttons have to use 2 inputs to the plc (if one is used).

iant · Jun 12, 2002

Hey listen to what you are saying.
OSHA ....
Two buttons in series (n.o.) one PLC input.
How is that safe??????
tie down one .... Prove it is not shorted out.

The two input idea is better, but what happens if the wires connecting to the PLC shorts the inputs??????

CANT HAPPEN??????

some one drills a hole on the machine. a small piece of Drill swarf sits accross the terminals.

with Relay logic control...
I have had a 1200t mechanical press cycle continuous with no efective guarding due to this fault. No injury Luckilly.. VERY CLOSE THOUGH.

Keep thinking of the CCT .... you are close.

The better answer is in the two input PLC. but different hard wiring of the buttons... Keep thinking

the Pilz safety button relay will give you a better wiring idea.
http://www.pilz.com/

Rick Densing · Jun 12, 2002

We may be trying to have him think for himself, but it is kind of a safety thing. You are better off with a safety relay.

Rick Densing · Jun 12, 2002

try again with the image

Alan Case · Jun 12, 2002

A good idea to practice programming on BUT if it is a real life application the legalities are horrendous. Use a Pilz type relay and do it to the applicable codes otherwise you could be calling a 20 stone tattoo-ed man mother for a long time.

RDay · Jun 12, 2002

I have a two-hand anti-tie down program written in DirectSoft that I use on the Punch Presses that we have. If you have a way to open it I will E-mail it to you.

iant · Jun 12, 2002

I had no plan to give an answer to the problem.
The idea of using a plc for start ccts is interesting.
Using a Pilz relay or similar may be better.
But doing the logic for a safety operation is good practice.
With PLC's it really doesn't matter what you use at the input stage.
The Output stage is where the Motion is...
That is where the major concentration of safety should be.
That dreaded triple redundancy....

john paley · Jun 12, 2002

Ati tie down really means anti tie down/anti repeat. This means the operator must operate both buttons simultaneously to cycle the machine. The machine should cycle once and once only. The only way the machine will cycle again is if both buttons are completely released and then both buttons are simultaneously actuated again. I don't really see a need for a timer, as the operator could hold the first button indefinitely, the cycle when the second is pressed. He could only do this once, though, as he must release both buttons to cycle again.(he couldn't tie one down) The key is, he must release both buttons in order to cycle again.

93lt1 · Jun 12, 2002

I would absolutly go with a safety relay, but if you really cant, I would incorporate some n.c. contacts as well, thereby using 4 inputs. Within the anti-tie down logic, use a retentative timer and both n.c. contacts reset it.

gbradley · Jun 12, 2002

Anti-Tie Downs

We've had this discussion before.
I subscribe to the Egg theory... it's great to have them as long as they are in place of more fatty foods.
I am of the opinion that ATD's in ladder logic are fine, especially if there was no previous ATD.
Sorry Ron, two Switches wired in series doesn't cut it.
(if that's what you suggested)

Makers of special Safety relays will quote all kinds of code as to why you have to buy their $350.00 relay and put it into your Anti tie down circuit.
It's hard to argue with them.
Good luck!
Whatever you do do make sure that it is safe.

Jim Dungar · Jun 12, 2002

I think the key word is OSHA.
If the circuit needs to be OHSA (or any lawsuit) compliant use a dedicated safety relay.
If lawsuits and fines are of no concern use the PLC. I have done this many times using John Paley's philosophy of anti-repeat.

Rick Densing · Jun 12, 2002

If you are using them as just cycle-starts and have other safety methods, logic-based ATD is probably fine. I have seen hundres of machines like this. We usually have a light curtain between the machine and the operator.

If you are requiring hands to be on the buttons for safety reasons (then they can't stick their hand in the machine), I should be hardware based. That is 100%- don't just wire the relay to a PLC input, protect your ouputs.

If you just run the relay into the PLC, you might as well save the money and implement a logic solution.

Ladder logic for 2-hand ATD

clone93

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Terry Woods

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Rick Densing

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rsdoran

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iant

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Rick Densing

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Rick Densing

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Alan Case

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RDay

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iant

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john paley

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93lt1

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gbradley

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Jim Dungar

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Rick Densing

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