Ladder logic for 2-hand ATD

[93lt1]

Clone93-

Dont say OSHA legal!!!

What type of machine is this?
Am I no longer the only person from Iowa here?
And whats with the username? ......kinda like mine

 

[jonjon]

Question!!

Wht if the operator put some things on in one of the PB and it make held closed. He she can able to operate using one hand?

--------][-------][----------------( T1 )
pb1 pb2

--------][-------------------------( T2 )
pb1 !
--------][---
pb2


--------][------][-----------------( OUT )
T1 T2

 

[Rick Densing]

If you notice, my logic will not allow that. If you release one of the buttons you must release the other to re-actuate. Hence the term anti tie down. You can't "tie down" one of the buttons.

 

[rsdoran]

There are enumerous ways to do this. Safety is a major issue...but nothing is foolproof. After 3 safety relays ( 2 brands ) failing me in a one year period I acquiesced ( theres that word again ) that nothing is infallible.

My logic that I stated above uses 2 N.C. pushbuttons to lead into the plc input...the odds are if it fails it will fail open...if it shorts or there is a short then it wont activate. The same kind of logic I use to apply to a hardwired stop circuit. I hope someone can define a difference between using N.C. PB's in an E Stop ckt to using the same device in a safety circuit. NOTE: my circuit also has a safety circuit to prevent cutting heat if an obstruction occurs before a pulse heat signal.

To each their own, my needs arent life or limb threatening overall but someone could be burned/hurt. The machines in question I added the safety devices too in hopes to avoid problems, the devices failed and now management isnt happy about it. In one case the device failed to a point where it couldnt be stopped without powering now the system.

Overall a device can FAIL...it can cause a problem...even when a timer is involved. I dont believe my faith in normally closed (mechanical) pushbuttons is different than using an electronic or mechanical timer device, its still a device in the circuit and can fail. Just think about the requirements for using a hardwired STOP EMERGENCY circuit (in US as required by NEC code) with a plc. The more things involved the more to deal with and the more likelihood of a problem.

Each situation is different and what you do is controlled by the situation and demand. If your need requries a plc code for an ATD then Rick's suggestion is good, you may need to look at using a safety relay/start system, or my way suffice. I will state this tho..if the machine is/has life/limb threating capabilities at the point of start then you CAN NOT go wrong using redundant safety features to protect the operators and yourself. Overall these redundacies are for legal issues but I use them too.

 

[93lt1]

I have never seen details of "hard wired emergency stop circuit" in the NEC.

Do you mean NFPA79?

 

[iant]

The two button operation does not need 4 inputs .
Its the hard wiring that is needed.

I did not want to give too many answers yet.

Try this idea for Input Hardwire

      N/O             N/0

     -----           -----

0----o   o-----------o   o---------( i/p 1 )

|      |               |

|     N/C             N/C

0----o___o-----------o___o---------( i/p 2 )

 

This should assist the logic of your program now.

And if you choose to install a safety relay the wiring will already be there.

Remember this is only the PLC input.
Be aware of how you wire the outputs and their associated safety CCTS.

 

[93lt1]

iant-
I didnt say it was "required" I just think taht it is a good idea....

 

[gbradley]

ATD

My logic that I stated above uses 2 N.C. pushbuttons to lead into the plc input...the odds are if it fails it will fail open...if it shorts or there is a short then it wont activate. The same kind of logic I use to apply to a hardwired stop circuit. I hope someone can define a difference between using N.C. PB's in an E Stop ckt to using the same device in a safety circuit.

I don't understand how your ATD works.

Two button Anti-Tie-down circuit is a completely different animal than N.C. P.B. in a Stop circuit.
On a stop circuit, the N.C. push button when pressed, opens the circuit which cuts power to a relay, causing all motors etc. to stop. (chainsaw principle)

I don't think you would ever want to wire anything that way to actually start a process.

Sorry to keep harping on this but I just don't get it.
Please enlighten me.

 

[rsdoran]

OK lets look at the principle reason PLC's are used vs RLL (relay ladder logic). The PLC offers more control with less wiring and components.

I may has stated that improperly...its on the same principle as a Stop circuit as far as the wiring goes. IF power is present at all times thru N.C. pushbuttons then a short can not affect the machine.
I used N.C. pushbuttons and wrote the ladder to make the rung go true when they are pushed. Its one of those unigue situations where you Document the P.B with NOT NOT.

I spent alot of time working with this machine doing the upgrades for it (BTW the original OEM schematic shows it using the 2 Starts wired in series but I opted to use saftey relays when upgrading but the safety relays on this machine have failed 3 times with 2 different brands).

I opted to do the power NOT present to cycle the machine to eliminate an issue that may happen if something shorts.

This is an ongoing project so I may change the logic again and use timers or some better form of safety features ...this thread has shown me other ways and gave me other ideas. I also talked to the Engineer today that deals with EPA, OSHA etc to lookup 1910.217 for its specific requirements..I assume it involves not being able to lock one P.B. down while using the other to cycle the machine.

This may not be a proper way to do it but some things we learn as we go, its hard to know all the rules. IT was a quick fix that gave me some of the safety features I thought needed, the machine also has maintained contacts in the arm (that I added) that if broken by an obstruction (such as a persons arm/hand) will break the cycle. In alot of applications this may not be feasible (a 100 ton press comes to mind).

BTW 93lt1 you are correct, I misstated when saying NEC it should have been NFPA 79. I often class them together because the NEC code is also provided by the NFPA.

NOTE to original poster:
My prior statements may have been inappropriate to offer true safety for your application...follow the guidelines and look at the options offered here. I am sure you can not go wrong with their advice.

 

[Mike]

You may want to look at RK Electronics, they make a two hand anti-tie down relay which has everything in it. All you have to do is wire up your push buttons and your coil. It cost some where around $100 the best I can remember.

 

[iant]

The ATD hardwire cct I suggested is what I normally preffer.
At the end of the day the level of safety is your prefferance.

My involvement with our local OSHA tightens my reigns.
if someone gets injured I could go to jail.

with 16 years of hardwire safety CCTS on Mechanical Presses.
Our local authorities were very stringent.
I had to learn their ways early.
To date we haven't made Car Body panels out of people, we are still triing.

A N/C ATD CCT sound really good I'm Glad it works for you "rsdoran".
I only have one question -
If through normal use, 5 years down the track, a wire breaks just as the operator reaches in to the machine.....
Do we hire another operator?????

 

[daveckey]

If the controls are true anti tie down, the limit time is typically 500ms.

If the installation was in canada(my locale) there are specific rules about applying 2 hand control especially with presses.

 

[daveckey]

In Canada, specifically Ontario, special safety rules apply to machinery implementation.

The Government requires Engineering sign off and approval of any new or modified system.

If your application is in Ontario, using a normal PLC for a 2 hand control in safeguarding method, would not really be acceptable.

 

[waggs]

2hATD the way I do it

Here is a short program of 2hATD that I've used alot. It isn't fancy but it does take into account two hand actuation and timed interlock(?) between the two PBs'. Depending on the use, we were making small component assembly machines, anti-repeat was added by looking at both PBs' having to be released, i.e., 'cycle complete'. Like I was taught, there are 1000 ways to skin a cat, you just have to find the one that makes the least mess. Hope this helps...I'm open to suggestions and ideas.

Bob

 

[flexdmc]

Waggs,
Would it be possible to export the file to txt?
Some of us don't have RSLogix and would like to see what your logic looks like
Thanks