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That's NOT "anti tie-down"
"Anti tie-down" is ensuring that both hand controls must be released before the machine can restart. This is to ensure that one of the two hand controls isn't "tied down."
Russell Williams [2007-02-25]
Do a RISK Assessment
It depends on your machine. Do a risk assessment of your machine. If your machine can cut a body limb off, then I would never use logic in a PLC for anti-tie down. If your machine is a leak tester or a vision machine test machine or something like that that won't hurt anyone, then in my opinion, there should be no problems. It depends on YOUR machine application and how severe the injury can be to the person. Use common sense, do a risk assessment to cover your based. Also look at if you have other safety devices installed. For example, if you are using light curtain to guard a machine with hard wiring, and then two opto start buttons that run through logic in a PLC, this should not be a problem either, because your light curtain is guarding the machine with hard wiring. Do not use only software anti-tie down without another safety device, such as a light curtain.
Chris Elston [2004-12-22]
A.T.D. in ladder logic OK to OSHA?
Does anyone know OSHA's official position on whether or not a 2-hand anti-tiedown circuit can be implemented in a PLC's ladder logic? I've heard that it is NOT acceptable to use a programmable device for this. With that said, I've done A.T.D. in logic on many machines and have never had a problem. What do you guys do?
Kevin Spears [2004-12-21]
Redunant Inputs
Redundant Inputs are not good enough "such that a single failure or single operating error will not cause injury to personnel"
Redundant Inputs on redundant Input cards with redundant Outputs wired in series on redundant Output cards have been satisfactory with everyone I have ever dealt with.

Example
I:1.0 Left Hand N.O.
I:1.1 Left Hand N.C.
I:2.0 Right Hand N.O.
I:2.1 Right Hand N.C.

O:1.0 Press Down
O:2.0 Press Down

Also I believe the logic is Faulty in the fact that it does not take inconsideration logic transitions of the inputs If the inputs were shorted on PLC first Scan the logic Would fire.



Edward Dustman [2003-09-25]
Hardware suggestion
I agree with the comments from the other two gentlemen.
It has been a while since I have programmed a machine requiring anti-tiedown. However, when I did, I used a third-party piece of hardward that did the controlling itself. Specifically I used the Time-Mark Model 850 two-hand control module.
To view the item the web site is www.time-mark.com
For no more than it cost, it is better to have the liability in the lap of people who specialize in it.
Dan Herston [2003-09-25]
Redunant Inputs...Aren't they good enough?
What you need to do is evaluate your machine situation. There are some designs that anti tie down logic is fine, other cases, anti tie down logic is not ok in your design, it depends on your machine design. Two input signals are better than one, but that doesn't always make it right. If one input fails, the machine will seeze. It takes TWO inputs, a redunant start to make an action. The OSHA code you quoted states:

...constructed and arranged to operate together as a unit, such that a single failure or single operating error will not cause injury to personnel due to point of operation hazards

I don't pretend to know all the answers, however how many small machines do you see that have anti tie down logic? It makes for a good self evaluation if you are choosing to design your machine safetly or not.
Chris Elston [2003-09-20]
Anti Tie Down Logic
I don't think that implementing anti-tiedown in PLC logic complies with OSHA 29CFR 1910.211(d)(62) unless you use redundant PLCs.
WARREN SMITH [2003-09-19]


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