Can light curtains be wired into a PLC?

[mutabi]

Hey guys, We have a situation in work which we are trying to resolve. We have had some operators getting hurt by sticking their hands in places they shouldnt. So we were told to start putting in light curtains to stop people from getting hurt.

Well we installed a few, and wired them into the hard wired emergency stop circuit so when the beam is broken, it kills all power. Well that in itself raises some safety issues where someone could get hurt (having valves deenergise closing clamps etc etc). It is also a major inconvenience to the operators now, who are having to find other ways to do things, to get their jobs done, since they can no longer put their hands where they used to.

We could replace the valves with detent valves, and rework the program and that may fix the other safety issues. Maybe. And may also raise other safety issues.

Well I believe this can all be solved, by wiring the light curtain into the PLC instead of the estop circuit. That way I can kill only what I want to kill. e.g. I can stop a motor beside the light curtain, and not close a clamp 30 feet away, which is what the estop does.
My question, is it legal to wire something like a light curtain into a PLC and not the estop circuit (bear in mind we still have regular estops). I have heard that all safety devices (e.g. guard limit switches) have to be hard wired. True or False?

Thanks for any help provided.

 

[mdshunk]

The light curtain normally does trigger the estop or emergency relay. If your machine creates another hazard anytime the estop is triggered, you've got a poor design on your hands to begin with. It really has nothing to do with the light curtain. It could just as easily be a person pressing the actual estop or bumping into the cable pull or whatever.

 

[mutabi]

Oh i most certainly agree that it is a poor design. But these are old old machines. And redesigning them is out of the question(financially). Is it legal however, going by the code, to wire something like that, into a PLC instead of hard wiring it?

 

[mutabi]

I should mention, that tripping the light curtain is fairly easy to do. People walk by it regularly. Not a whole lot of room so accidental trips do happen, probably every day. Whereas accidental pushing of the emergency stop buttons, rarely if ever happens.
Again poor design, but if I had my way I would rip all the light curtains out and hire smarter operators.

 

[mdshunk]

OSHA's Directorate of Compliance Programs gave a decision on June 5, 2001, in an "interpretive letter." It stated that:

• Light curtains must comply with IEC-61496 parts 1 & 2 adopted as ANSI/UL 61496 parts 1 & 2.
• Light curtains' "TYPE" (i.e., their Safety Performance Level) must be appropriately selected.
• Light curtains' optional functions, as listed in Annex "A" of IEC-61496 part 1, must comply with Annex "A" requirements.
• Light curtains must be properly integrated into the machine.
• Light curtains must be certified by independent third party.

The ANSI standard says the light curtain should be "connected to the machine such that a stop signal from the light curtain results in an immediate halt at any point in the machine cycle or stroke." I underlined immediate halt for emphasis.

 

[ELA]

I used to design the electrical for large semiconductor processing equipment.
We used Light curtains all the time that did not EMO the entire machine.
It is probably not legal or recommended to wire the light curtain as an input to a non-safety rated PLC. You can however use the light curtain to remove power from the piece of automation that causes the operator a hazard.

In our designs light curtains removed the input AC to the servo drives that presented an immediate danger to the operator. The rest of the machine continued to be powered up.

 

[mutabi]

The ANSI standard says the light curtain should be "connected to the machine such that a stop signal from the light curtain results in an immediate halt at any point in the machine cycle or stroke." I underlined immediate halt for emphasis.

So does this specifically forbid the use of a PLC?

 

[mdshunk]

So does this specifically forbid the use of a PLC?

I doubt it, but I didn't read it all. You're welcome to, however.

 

[mutabi]

I doubt it, but I didn't read it all. You're welcome to, however.

lol thanks. appreciate it. some nice bed time reading huh

 

[nc5p]

The problem with PLC's is they scan their inputs and are dependent on programmed code to react to it. In safety critical systems listing agencies and in some cases governments will not accept this. If the PLC got hung in a section of code it wouldn't be able to respond. A hard wired circuit or programmable logic circuit (CPLD) can be used. The latter came up as an issue when I designed a control for a high level laser system. We had to satisfy CDRH (Federal Agency) and they did accept this. It effectively amounted to a hard wire system as it wasn't scanning and immediately responded to the safety condition, regardless of whatever else was going on.

 

[realolman]

I am no expert, but we have some machines that have multiple safety relays. Door switches kill some things. estops kill others.

You don't always want to kill everything... for example, in one machine there is a two axis robotic sort of thing that has 24 vdc brakes that you have to energize in order to be able to move the thing. You wouldn't want it to fall on a power loss, so the brakes release when they are energized. So you normally are on the outside of the doors when it's running, but if you have to get right in amongst it, you can move it. So there has to be some power there.

My own gut feeling, for whatever that's worth, is that you should not wire safeties into PLCs for many of the same reasons nc5p stated. I have no experience with them, but there are programmable safety relays out there. I know there are some experts on them in this forum.

 

[pfalcon]

Do Not Wire To The Plc!

Do Not Wire To The Plc!

You have several major errors as you have described your machine. Please review NFPA79-2007 before you proceed.

1) When tripping an Estop circuit you must stop all hazardous motion. This means that clamps should not change state when the power is killed due to an Estop, ram heads must not drop, etc.

2) You are reaching into a hazard area therefore the light curtain must guarantee safety - not an Estop. This is typically done with "muting". The light curtain must be clear OR the machine motion must be secure. The light curtain must be "hardwired" with the "muting" to guarantee safety against the protected motion. Not necessarily the Estop.

3) All safety issues must be protected by safety rated equipment. Until the NFPA79-2007 this meant that you could not use a PLC at all as a safety device. Now you can use safety controllers as long as access to their programming is properly restricted. A standard PLC cannot be used with the light curtain. The safety controllers are the only legitimate "softwire" permitted.

 

[cornbread]

The way we wired light curtains was to drop the voltage off the output bus. Typically we used a robust relay that was controlled by the light curtain to remove the output voltage.

 

[weressl]

You have several major errors as you have described your machine. Please review NFPA79-2007 before you proceed.

1) When tripping an Estop circuit you must stop all hazardous motion. This means that clamps should not change state when the power is killed due to an Estop, ram heads must not drop, etc.

2) You are reaching into a hazard area therefore the light curtain must guarantee safety - not an Estop. This is typically done with "muting". The light curtain must be clear OR the machine motion must be secure. The light curtain must be "hardwired" with the "muting" to guarantee safety against the protected motion. Not necessarily the Estop.

3) All safety issues must be protected by safety rated equipment. Until the NFPA79-2007 this meant that you could not use a PLC at all as a safety device. Now you can use safety controllers as long as access to their programming is properly restricted. A standard PLC cannot be used with the light curtain. The safety controllers are the only legitimate "softwire" permitted.

HIMA Himatrix offers certified safety PLC's of small size(I/O Count) that can be an economical solution when your needs are beyond the AB Safety Relay system. They list their solutions as :

Solutions for machine safety:
? Stamping and pressing
? Painting plants
? Assembly plants
? Robot cells
? Conveyer systems
? Amusement rides
? Lifts
? Locks and polders
? Cable cars
? Cranes
? Packaging machines
? Elevators and lifting tables
To assess the safety hazard you may refer to ISA/ANSI S84.01 ? 1996, "Application of Safety Instrumented Systems for the Process Industries" .

http://www.hima.com/Products/HIMatrix_default.php#solutions

They also have individual safety relays.

 

[pfalcon]

The way we wired light curtains was to drop the voltage off the output bus. Typically we used a robust relay that was controlled by the light curtain to remove the output voltage.

Actual safety-rated relays are now required not just robust.

Popping the Estop or removing output power are both acceptable. The Estop requires that the system be reset before you can resume operation. Removing output buss power may also require dropping out of automatic mode or resetting the equipment.

 

[cschmid]

I know we had this discussion before here..I remember the research but it will have to wait until later for me to find it..but I will see if a quick search can find what I am thinking about..realoman were we not talking about a project you had going on..I hate it when I can think of it but cant remember the specifics..be patient as I am a patient..:grin:

 

[cschmid]

okay will need another lesson here..if you search for E-stop it will pull up a list in the list an emegency stop plc is talked about it is under http://forums.mikeholt.com/showthread.php?t=69536&highlight=e-stop..I do not know how to post the search results as a link so you can just click on it..here is another one http://forums.mikeholt.com/showthread.php?t=89692&highlight=e-stop.. there are more but I am not for sure how to link them..I have more at home so I will post later on if this has not been cleared up by then..

edited to add sure just when I say I can not do it it works..

 

[mutabi]

I know we had this discussion before here..I remember the research but it will have to wait until later for me to find it.

I would really appreciate it Thanks to everyone for their replies. I am making note of everything!

 

[cschmid]

hope them links help gotta go..

 

[Jraef]

You have several major errors as you have described your machine. Please review NFPA79-2007 before you proceed.
...
3) ... A standard PLC cannot be used with the light curtain. The safety controllers are the only legitimate "softwire" permitted.

There are "Safety" PLCs available from several of the major manufacturers as well as some smaller companies who specialize in straight safety systems. Of them, only the Siemens S7-300 can be used both as a regular PLC AND a rated Safety PLC. The I/O modules for safety devices are different and labeled / color coded as such, but it makes integrating the functions very simple. The CPU runs two separate programs, one normal and one safety. It scans all I/O but the safety program scans the safe I/O first, using faster interrupts to enact safety protocols. But the normal operating program can also use the safe I/O information as well, so duplicate wiring is unnecessary.