Can light curtains be wired into a PLC?

[mutabi]

Thanks for the links. Makes some interesting reading but also raises a
question hehe.

I have seen people mention how their emergency stop circuit kills motors, and they also kill their PLC output bus power. Why keep your inputs hot? What if someone got hung up on a damaged limit switch? Also if you just kill your outputs, and then reset your emergency stop, arent you risking the machine, or part of the machine starting up by itself? And from my reading, that is a violation. Of course that could be put down to poor programming.

Our emergency stop circuits at present kill everything, including the plc itself. The only thing hot after it it pressed, is the power to the estop circuit. Thoughts anyone?

 

[jim dungar]

Our emergency stop circuits at present kill everything, including the plc itself. The only thing hot after it it pressed, is the power to the estop circuit. Thoughts anyone?

How do you trouble shoot your system if there is no power?

One reason to leave the inputs and PLC hot is the PLC and HMI can be used to locate and record (data logging) which safety device shut down the system, monitor the state of the unaffected portions of the system and make sure everything is in order prior to a restart.

NFPA 79 allows both controlled an uncontrolled shut downs of equipment for a reason. Uncontrolled shut down can cause more injury than it prevents; for example do you want a saw blade coasting to a stop for several seconds or would some type of controlled braking be preferred?

 

[mutabi]

How do you trouble shoot your system if there is no power?

Well we dont need to. Either the emergency stop is pushed or its not. We dont have terribly complicated machines. Now we do have safety guard limit switches wired into the PLC....dont look at me Should these also be hardwired? We use them to kill specific things on a machine.

 

[jim dungar]

Well we dont need to. Either the emergency stop is pushed or its not. We dont have terribly complicated machines. Now we do have safety guard limit switches wired into the PLC....dont look at me Should these also be hardwired? We use them to kill specific things on a machine.

If the devices are part of a type 0 or 1 emergency stop (I like to refer to NFPA 79) then they should not rely on the PLC to perform their function, although a PLC can be used for monitoring and ancillary stopping. If the devices are used for non-emergency stopping then you can do whatever you want with them.

 

[mutabi]

If the devices are used for non-emergency stopping then you can do whatever you want with them.

Could the same be said for a light curtain? I dont necessarily view these light curtains as an emergency stop. More of a way to stop certain machine functions just in case someone is dumb enough to stick their hand in front of something that is moving. You coud break the light curtain and still be perfectly safe(assuming the light curtain didnt kill everything) just like you could raise a guard and watch the machine run and still be perfectly safe.

 

[jim dungar]

Could the same be said for a light curtain? I dont necessarily view these light curtains as an emergency stop. More of a way to stop certain machine functions just in case someone is dumb enough to stick their hand in front of something that is moving. You coud break the light curtain and still be perfectly safe(assuming the light curtain didnt kill everything) just like you could raise a guard and watch the machine run and still be perfectly safe.

A light curtain may be part of the emergency circuit or it not. The fact that it is a light curtain is not important, it is what you are doing with it.

 

[tomP]

mutabi
Could the same be said for a light curtain? I dont necessarily view these light curtains as an emergency stop. More of a way to stop certain machine functions just in case someone is dumb enough to stick their hand in front of something that is moving. You coud break the light curtain and still be perfectly safe(assuming the light curtain didnt kill everything) just like you could raise a guard and watch the machine run and still be perfectly safe

A light curtain may be part of the emergency circuit or it not. The fact that it is a light curtain is not important, it is what you are doing with it.

I think the key here is that a hazard assessment needs to be done to determine what actions need to be taken.
Also a calculation needs to be done to determine the distance the light curtain needs to be placed from the hazard.

 

[petersonra]

Normally, you would not wire a light curtain into the e-stop circuit unless you are using it as a substitute for a fixed guard. Most often you are using them to guard a space that needs to have periodic access, such as a machine loading portal where it is safe during some part of the cycle to access that part of the machine.

A typical example is to allow access to the loading portal only when machine motion has halted. The light curtain output is then used to prevent motion during the access, but does not trip the estop.

There are some fairly simple calculations you need to make to determine where the light curtain is to be located. It has to be far enough away from the hazardous motion that the machine motion can be stopped fast enough that no injury can happen. That is not always that easy.

There are often much simpler and safer solutions that using a light curtain.

 

[cschmid]

see I knew the input would be good..I also agree an assessment needs to be done..

Yet if I remember correctly the use was to prevent operators from getting hurt..if that is the reason then I see it as a hazard prevention and it needs an e-stop function..which then needs to to secure all hazards associated with the e-stop area..

Now if it is to prevent machine malfunctions from occurring because of unintentional interruptions and no physical harm can come to humans then it is a different issue..

If this is to prevent an accident then I do not see there is a difference in accident levels..an accident is an accident in OSHA eyes..

http://www.nfpa.org/freecodes/free_access_document.asp

This is an access link to nfpa 79 you can not copy it but you can read it..

 

[petersonra]

Yet if I remember correctly the use was to prevent operators from getting hurt..if that is the reason then I see it as a hazard prevention and it needs an e-stop function..which then needs to to secure all hazards associated with the e-stop area.

You cannot use an estop pb to mitigate a known hazard. If you have a hazard, you have to deal with it.

I can't make any specific suggestions about your case(s) since I know very little about them, but in general I can tell you there are all kinds of ways to deal with hazards that do not involve the estop circuit at all.

 

[petersonra]

I have seen people mention how their emergency stop circuit kills motors, and they also kill their PLC output bus power. Why keep your inputs hot? What if someone got hung up on a damaged limit switch? Also if you just kill your outputs, and then reset your emergency stop, arent you risking the machine, or part of the machine starting up by itself? And from my reading, that is a violation. Of course that could be put down to poor programming.

Our emergency stop circuits at present kill everything, including the plc itself. The only thing hot after it it pressed, is the power to the estop circuit. Thoughts anyone?

The estop is not there to take away electrical power. It is there to stop machine motion. Just removing electrical power is not always a guarantee that motion will stop.

<added> as part of the estop function, power is removed, but the reason for pushing the estop is to stop the machine not to remove power.

 

[tomP]

The estop is not there to take away electrical power. It is there to stop machine motion. Just removing electrical power is not always a guarantee that motion will stop.

<added> as part of the estop function, power is removed, but the reason for pushing the estop is to stop the machine not to remove power.

Totally agree. Part of our description is:

An Emergency Stop control shall override all other machine controls and immediately deactivate and stop the machine.
Components of the fixture shall not drift or continue in motion after depression of the emergency stop pushbuttons.

 

[petersonra]

Totally agree. Part of our description is:

An Emergency Stop control shall override all other machine controls and immediately deactivate and stop the machine.
Components of the fixture shall not drift or continue in motion after depression of the emergency stop pushbuttons.

I don't like the use of the word "immediate". That brings to mind an uncontrolled crash.

It is perfectly permissible to shut it down in an orderly fashion.

For instance, it is normal to do what is called tool relief where a drill head slide is partially retracted before the slide motion is stopped.

It is also pretty normal to engage some kind of electronic brake on a spindle motor to get it to stop before power off. Sometimes you have to leave the MCR on for what seems like a very long time to accomplish this. In some cases I have seen some parts of the machine de-powered instantly, while other parts come to a stop and are de-powered 10-20 seconds later when motion is stopped.

 

[tomP]

I don't like the use of the word "immediate". That brings to mind an uncontrolled crash.

It is perfectly permissible to shut it down in an orderly fashion.

For instance, it is normal to do what is called tool relief where a drill head slide is partially retracted before the slide motion is stopped.

It is also pretty normal to engage some kind of electronic brake on a spindle motor to get it to stop before power off. Sometimes you have to leave the MCR on for what seems like a very long time to accomplish this. In some cases I have seen some parts of the machine de-powered instantly, while other parts come to a stop and are de-powered 10-20 seconds later when motion is stopped.

I guess it is up to the individual, the company policy and the type of equipment it's used on to determine what the correct definition is.
We also include a NOTE: A controlled e-stop is not considered drifting. Once the machine has completed the e-stop process, it should not drift.
We use many light curtains in operator load stations with an active robot in the station so we do want a reasonablely fast STOP.
I know if there is a robot comming at me, I'd prefer that uncontrolled crash instead of a ramp down in speed before stopping.

 

[cschmid]

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.

Okay so lets start over this is what he wants the light curtain for..I bolded the first sentence..I read it as the light curtain is for elimination of a known hazard..so if that is the case then it needs to be in the E-stop function..The known hazard arises from the known inserting of an operators hand in a moving piece of equipment..You can correct me if I have misinterpreted this..

 

[tomP]

Not trying to be wise guy but an assessment needs to be done by people knowledgable of the equipment.
Also depends whether the Estop is immediate or controlled as petersonra has indicated.
It so depends on what you are attempting to guard and and the distance to the hazard.
There are too many variables to give an absolute answer.

I think the key here is that a hazard assessment needs to be done to determine what actions need to be taken.
Also a calculation needs to be done to determine the distance the light curtain needs to be placed from the hazard.

 

[petersonra]

Okay so lets start over this is what he wants the light curtain for..I bolded the first sentence..I read it as the light curtain is for elimination of a known hazard..so if that is the case then it needs to be in the E-stop function..The known hazard arises from the known inserting of an operators hand in a moving piece of equipment..You can correct me if I have misinterpreted this..

It still does not need to trip the estop. It only needs to mitigate the hazard.

For instance, if you had a dial index machine and you used the light curtain to prevent the operators hands from going into the machine while the dial was indexing, you might wire the output of the light curtain to stop the index motor and release the spring loaded brake to bring it to a stop. Someone sticks a hand through the light curtain while the table is moving, the light curtain stops the table from moving, preventing his hand from being crushed. There is no reason the motion can't continue once the hand is removed. Its certainly logical for the PLC to record this event, but no reason to trip the machine estop. You might want to make it an alarm, have the PLC shut off the motor until the hand is removed from the curtain and the alarm reset.

Its not quite as simple as wiring the light curtain into the indexing motor contactor. you still have to show that the motion will stop fast enough to prevent injury. There are some simple calculations you use to do that. And more than likely you will want to use a safety relay system to power the motor on and off. There are some other things to consider, but you get the basic drift.

 

[mutabi]

First I want to thank everyone for their input. It makes for some very interesting reading, and I am sure it will lead me to making the correct decision.

Let me explain a bit about the hazard. The problem we have, happens when the machine is in Manual. We have had an operator, "smart" enough to stick his hand in front of a pnumatic cylinder(into a confined space), and then reach for the manual control with his other hand, and "bump" this cylinder forward, running the rod through his hand. That is the main thing we want to stop.

Running in automatic, there is no real reason to be sticking your hand in that area, and we do have safety guards in place,trying to protect the area as much as is practically possible.

Someone mentioned about the placement of the light curtain and if it would be able kill the machine quickly enough. In Automatic, with the machine running full blast, the main areas of motion come to a dead stop very very quickly, with no use of brakes.And by that I believe you could shoot your hand through the light curtain into harms way, and by the time is reaches harms way,everything would be stopped.

Our main concern, is what the operator does manually. We dont want him to be able to stick his hand in there,and then operate the machine manually.

As everyone keeps telling me, you cant fix stupid. I know it. Get this, I went and actually talked to the operator in question (he has recovered with no permanent damage) last night, to get his opinion on the light curtains in their current implementation. Well according to him, he is happy enough with them and says they work well, but thinks they need to be about 6 inches higher, so the operators can reach under them and into the machine to do what they need to do without killing everything. I just walked away shaking my head.

 

[jim dungar]

If the problem is with preventing the equipment from being started in manual, then you could put the light curtain into the "run enable" circuit.
But based on your operation description, I would have no problem with it in the e-stop circuit.

 

[petersonra]

Let me explain a bit about the hazard. The problem we have, happens when the machine is in Manual. We have had an operator, "smart" enough to stick his hand in front of a pnumatic cylinder(into a confined space), and then reach for the manual control with his other hand, and "bump" this cylinder forward, running the rod through his hand. That is the main thing we want to stop.

Running in automatic, there is no real reason to be sticking your hand in that area, and we do have safety guards in place,trying to protect the area as much as is practically possible.

Someone mentioned about the placement of the light curtain and if it would be able kill the machine quickly enough. In Automatic, with the machine running full blast, the main areas of motion come to a dead stop very very quickly, with no use of brakes.And by that I believe you could shoot your hand through the light curtain into harms way, and by the time is reaches harms way,everything would be stopped.

Our main concern, is what the operator does manually. We dont want him to be able to stick his hand in there,and then operate the machine manually.

The problem you have is now you have identified a hazard - that is someone can put their hand into the machine and get it mangled. it does not matter much if the machine is in manual or auto, he can stick his hand in there and get it chopped off.

I am not convinced a solution that only solves the issue when it is in manual is a good idea.

I am also pretty much convinced that there is some reason he is sticking his hand into the machine that also needs to be addressed, for example, a need to remove a stuck part.

It sounds to me like you might be able to add physical guarding to accomplish what you want, but I would not object to a light curtain wired into the estop in this case either.