Light curtain addition

We have several small machines that are manually loaded but semi-automatic thru 1 cycle. The machine(s) package paper products in cello, the procedure is the operator inserts the product then pushes the start button. The jaws of the sealer open, the product drops then jaws close and cut, jaws remain closed. The jaws are pneumatic and the cut is done with hot wire. The jaws have spring loaded limits to determine if their is an obstruction which prevents the heat wire from being activated if jaws do not fully close and should open the jaws immediately.

NOW the safety people have decided that a light curtain and some guards with safety switches should be added. The guards I can deal with simple enough but there are questions about the light curtain usage, both mine and from engineers.
1. Should the light curtain disable all outputs (this has a GE Series ONE PLC) or is there a more appropriate scheme?
2. Is it possible or can it be appropriate to use the Light Curtain to initiate the next cycle when hand is entered then removed?
3. Doing #2 would require making modifications to PLC and adding a maintenance SWITCH (or something) for those times when you didnt want to activate it when hands are removed from area..is this correct?
4. Should I NOT think of using the Light Curtain to initialize a cycle but instead keep the Start Cycle button?
5. Should the Start Cycle button be an Anti Tie Down type?

Please let me know if you can think of anything I have missed.

I have no choice on whether or not to add the Light Curtains, its already decided but the regulations in this case are fuzzy (to me) so looking for suggestions on what seems to be the most appropriate and safest way to utilize it.

Light Curtain

I would never use a light curtain to start and stop equipment automaticly. In my opinion, a light curtain should be installed the same as an e-stop circuit would be wired. I would have a hard time after installing these and walking away with that "what if" in my mind. I have had to install light curtains on diffrent machines that were wired as above, and when they do fault out, the operator has to reset this/that etc. It really causes a headache but you have to give them what they want.

We use light curtains all the time in that kind of setup.

You only need to disable the ouputs that are appropriate. That is pose a hazard. When I design from scratch, it set aside an output module or two just for LC functions. The V+ to the card runs through the LC. You monitor the light curtain with two inputs on the plc.
You definitely want to have a separate start button. Often, operator has pulled their hands out before realizing a misload or something. This gives them a chance to inspect before starting.

As far as one or two buttons. Some of our customers like to keep the two buttons, some go with one. You are covered as far as safety with the LC (properly applied). You monitor the LC because if it is broken, you want do not want to automatically re-start. Make the operators check the machine before re-starting.

Rick thanks, thats pretty much the way I expected to implement these devices. The machine and situation isnt conducive to using as a full ESTOP device because the operators must insert their hand each cycle. To require a complete system restart would make this system slower than it is and that wouldnt be good. The Engineer(s) were asking about using it to make it cycle (I think they call that PSDI Presence Sensing Device Inititate)when hand is removed. I wasnt sure about doing this but technically not sure it would hurt either. The machine at default/start position has its jaws closed so no hand can be placed inside that opening, the hand is just inserted into the cello that product is wrapped in. Even if product were misfed if hand was removed cycle is fast enough they couldnt reach back into opening, and if I understand the STI LC specs correctly their response to disable is very fast. Plus this thing has redundant safety (I implemented the redundancy aspect) features as is.

Anyway got STI coming soon but just wanted to check with those that may use this kind of device more than I do/have.

Appreciate the input and looking to hear more if anyone thinks of anything.

rs,
We just switched over to using STI products about two years ago.
I don't know about you, but I've been very happy with their products.
I built a robot cell consisting of two stations. Had to put two sets of STI light beams, four mechanical limit switches on the #1 axis of the robot,two force guided relays and a STI RM3 muting module. A lot of wiring, but after completing the project, I let that machine go to production feeling very safe about it.

I'm not going to touch the "machine start using the light curtains" issue. The only thing I can tell you is, I would find another way to do this and use the safety device as it was intended for.

Good Luck!
Tim

rsdoran,

The way you are wanting to use this light curtain is know as a double brake mode. You may want to take a look at the new IEC standards before you purchase a light curtain. OSHA has adopted these standards as there own, the only ones that I know of that meets these new standards are the Omron F3S3N, SICK Optics, and the STI 4600 & 4700 series.

I am not sure if any of the manufactures have a double brake mode as standard any more, but I am in the process of doing a project in which I am using a muteing module along with the Omron units. I know that Sick also has this but I am not sure about STI.

When you do this all componets which the light curtains are protecting must be run through a safety relay to meet OSHA requirements. You must also have a safety study done to determind the distance the light curtains must be mounted away from the pinch point, along with a study to determine the class of the hazard, class 1, 2, 3 or 4 with 4 needing the most protection. All of this information must be keep on file if OSHA ever asks for it. I found that the Omron safety catalog has all of the information you will need to do to make these calculations.

Good Luck,

Mike

If your installing 2 or more STI systems in a row, tx/rx placement is important. If the transmitter is in line with the receiver for an adjacent system you could run into crosstalk which will cause them to 'lock up'. With 2 screened areas I place the transmitters in the center and facing out. It's all in the manual but who reads those anyway.

One more thing Ron... You may already know this, but remember that there are 2 different types of LC systems:

LATCH - Once the beam is broken, the safety outputs latch OFF (lockout condition), and require a reset to turn back on (I think STI refers to this as Guard Mode)

TRIP - Safety outputs automatically turn back on when LC is clear

Sounds like your application will use a TRIP output (or an LC with a muting function)

beerchug

-Eric

Trip will probably be the way I will go..thank you

Its amazing that those that have so much to say do not say much about real world applications when presented.

I guess stating theory overrides stating what is done in the real world.

rsdoran,

I am sorry that you feel that no one offered you any real world application information. I did not offer any information on a real world application because each safety application must be looked at on it's own merits. No one should offer you any specific information about a safety application, nor would any light curtain manufacture offer you any specific information about your application even if they looked at it, doing so they would accept responsibility for your application. As I explained to you in my previous reply the Omron catalog offers a great section which explains everything you need to know about how to chose a safety system and how to document your safety evaluation. If you are looking for real world examples I am sorry but I for one will never give you specific information as to how I did a safety application, doing so would be foolish on my part. However I will be glad to give you general information and make suggestions as I did in my earlier reply. I wish you the best of luck with your application.

Mike

We implemented a light curtain on a pick and place operation a few years ago. The true failsafe way to do it was in the e-stop string. But the operation could not tolerate that, as the curtain is required to be broken between cycles, and an e-stop requires a restart and a re-homing of the drives--that stops the upstream process and it's hard to get going again.

We implemented sort of a two level solution.

level one--if the machine is not in motion, between cycles, and the curtain is broken, do not allow motion to begin. This does not require a restart of the drives and does not stop the process upstream.

level two--if the machine is in motion and the curtain is broken, turn off a relay with it's contacts wired into the e-stop string--this requires a drive restart, but who cares, nobody will get hurt.

The PLC handles the decision making as far as e-stop or not, but the contact is hard wired in the e-stop string.

We felt good about doing it that way, even though it's not really the safest way. Our thinking was that when the machine was delivered, it was deemed safe, and we only made it safer, while not crippling the process.

Since that time, we have implemented safety mat systems with the same philosophy, if it ain't movin', don't let it start--if it is movin', stop it now--and with the blessing, in writing, of our corporate engineering staff. So I guess it's alright.

rs,

Here is a web site of a manufacturer of light curtains - http://www.ktitechnologies.com The gentleman you will be speaking with has been in that specific industry for many years and would be able to assist you. He has done many installations and start ups.

Hope this helps,

God Bless,

Stephen

Ron,

It sounds to me like every situation involving a light curtain and the amount of safety you want/need varies according to the application. My suggestion is document what you do and have the company sign off on it in order to cover your arse. Ultimately whoever installed the thing is going to be held responsible.

Exactly what are you looking for, Ron? I thought that I gave a pretty real world example of what we have done. If you want more detail, just say so. All I can do is tell you what I have done or seen done. You will have to make your own judgements.

I didn't read all of the posts, but did you ever think about trying to remove the operator from the production process.

10 years ago when I was working as an electrician for Heinz, we had a similar machine for heat shrinking frozen pizzas. I even remember a similar machine at Stouffer's.

The product was fed in to the machine automatically via a conveyor belt. The conveyor belt had pegs in it that seperated the product. The belt speed was timed with shrink wrap machine to automatically insert the product. Yeah, of course, everyone once in awhile a blank cellophane bag was created from a blank slot on the conveyor (we tried to minimize this of course) and the bags would clog up the oven over time, etc.

Is this an option?