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Programming Application - Anti Tie Down Logic

Article ID: 13
Last updated: 13 Oct, 2010

Anti-tie down logic is normally a good design practice to include in your PLC logic when you have an operator standing in front of a semi-automatic machine. Operators have a lot of things on their minds when they are working on your machine. Using anti-tie down insures safety by making sure the operator's hands are in a safe position during a potentially hazardous motion.

You should always evaluate your machine safety requirements and make sure you are allowed to use two-handed inputs directly before implementing PLC logic for two-hand controls. If your machine safety evaluation finds that you are not allowed or your conditions are too hazardous, you should always use a safety rated two-hand control module instead of PLC logic or use two-hand control PLC logic in conjunction with a light curtain to protect the front of the machine.

The idea behind anti-tie down is to make sure the operator places both hands on a set of input feedback such as a set of Opto Touch buttons. If your application will not allow for PLC logic as the anti-tie down method, then there are safety modules available such as a DUO-TOUCH module that will accomplish the same things as this PLC logic.


Anti-tie down logic requires the operator to place their hands on the input device at the same time and not release until the condition is safe to remove the hands. In the case of a rotary index table, the operator hands must remain on the Opto Touch buttons until the index table has cycled one complete index and stopped motion. If this operator releases their hands before the motion is complete, the index table should stop.

Photo Courtesy of Banner Engineering Corp


Operators' placing their hands on the Opto Touch buttons at the same time is a safety issue. If a false input on one of the buttons were sensed, then it would only take the other input to start the machine cycle. Some past causes of a false input conditions could be a person laying a rag on top of one of the opto buttons. Granted, opto technology has become far more advanced and this situation is not likely. However these types of false input considerations must be taking into account so your logic can be written to prevent false cycle starts and possibility prevent bodily harm.

If you are in doubt about your machine's safety requirements, chances are best if you choose to install a safety module instead of PLC logic or use a light curtain with your Opto Touch buttons.


Download the Anti-Tie Down RS LOGIX example, PDF included. Winzip Required.


Article ID: 13
Last updated: 13 Oct, 2010
Revision: 1
Views: 26440
Comments: 7
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WARREN SMITH | 19 Sep, 2003 12:00 AM
I don't think that implementing anti-tiedown in PLC logic complies with OSHA 29CFR 1910.211(d)(62) unless you use redundant PLCs.
Posted: 10 years 10 months ago   | Permalink
Chris Elston | 20 Sep, 2003 12:00 AM
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.
Posted: 10 years 10 months ago   | Permalink
Edward Dustman | 25 Sep, 2003 12:00 AM
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.


Posted: 10 years 10 months ago   | Permalink
Dan Herston | 25 Sep, 2003 12:00 AM
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.
Posted: 10 years 10 months ago   | Permalink
Kevin Spears | 21 Dec, 2004 12:00 AM
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?
Posted: 9 years 7 months ago   | Permalink
Chris Elston | 22 Dec, 2004 12:00 AM
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.
Posted: 9 years 7 months ago   | Permalink
Russell Williams | 25 Feb, 2007 12:00 AM
"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."
Posted: 7 years 5 months ago   | Permalink

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