Another silly question about proper style

I am working on upgrading another sealer type machine, this one is automatic now but uses all relay logic. Recently a woman got her hand caught between the jaws of the sealer and the cut block, it didnt complete its cycle so it didnt heat/cut but it also had mechanical issues which made it stay in position holding her hand which in turn did cause a minor burn from the residual heat.

Anyway as the machine was designed (had to be half a century ago) it didnt use any form of ESTOP and the STOP circuit isnt what I call normal. I will see about scanning the schematics and if any would like to see them I will email.

What I plan is to build a new cabinet (the existing is on the floor and an oven has to be moved to access it). The main voltage is 240v single phase so I originally planned on using a Master Control Relay and the ESTOP plus safety limits completing that circuit that but have now considered using a Safety Relay..would that make more sense?
OR just let the ESTOP control a Master Relay which kills all power and let the safety limit switches on the cutting arm just cause the system to HALT its cycle and return to start position? With the safety relay scenario the cutting arm should also return to its start/home position. Another issue is that they have to pull cello thru the cutting space at times (new roll etc) so need to be able to feed the cello, I thought it would make more sense to add a jog/feed button to feed the cello for this purpose instead of cycling the machine as it is done now.

I also plan to use either an ATD system or create one with the plc but my primary concern is make sure that if any obstruction stops the cutting head from coming down for more than X secs(or 10ths of a sec) that the cycle halts and goes to home. I dont have a way to make it stay in place and do not want it to stay down, down is the only scenario I can see that may cause any harm.

The rest of the system is fairly straightforward..product comes down conveyor is detected then pusher places it under cutter then cutter head comes down heats/seals then conveyor moves it away to oven.

My company is on a safety thing (which I have no problem with) and my main concern is doing this properly. I want the machine to be as safe as possible. I am working now on some drawings for this but I keep changing them because of the above issues.

I have a GE Fanuc 90-30 (with the firmware upgraded recently) 5 slot with enough I/O options to do anything I need with this small system.

I also plan to modify the cutting head to lower its home position and change the rams involved to use MRS (magnetic reed switches) to show position. The head has none at this time, just limit switches to show when down and pusher also uses limit switches. Would it be more practical to use MRS over mechanical limit switches? I am hoping to increase the machines auto cycle time from 20 pcs a minute to 60 pcs a minute....I know probably biting off more than I can chew.

BTW this thing now uses Electromechanical timers (4) for dwell, heat etc and I hope to replace them with the PLC and use some form of HMI to allow them to set times if needed. I think with the PLC and if I create the right timing features with pulses I can maintain the arm at a consistent point so that it would be very rare if timing needed changed much...there will always be situations like heavier cello..thinner etc. Your recommendations on a simple HMI that works well with the GE 90-30 would be appreciated.

Again I appreciate any answers that are given.

First rule is never to have the PLC handle the safety issues... BUT you can consider the machine's safety to be handled by the PLC.

To prevent any obstructions from causing wear and eventualy braking the machine, we have done in the past a system where we installed an encoder on a serie of cylinders/pushers and we program the PLC to react on speed vs position. When the pusher plate was in a position where it was supposed to be traveling from point A to point B, it got to go at a certain speed. If not ... safety kicks in. Pulling back the pusher and waiting for service or reset from the operator.

This was not the safety feature for the human but it did preserv a few broken bones

Second: if you know the position (with a limit switch) you can heat the cutting/sealing wire with more power, faster, but for a shorter time. So if somebody has there hands under this pinching cutter/sealer... but it's not at the sealing position... No heat

also... the E-Stop is not to be messed around with. No control can be operated when the E-Stop is pressed. So if you want your movements to go back into some positions, do it mechanicaly... spring returns or such.

For the rest, it looks to me this is a fun project. You are wright to aim at 60 ppm... if you get 50 then you'still better than today... the weakness will always be mechanical... you cannot make a Chevy out of a Lada even with the best PLC in the world.

Ron, Is this machine pneumatic? From your description I'm guessing it is, the magnetic reed switch would generally be to read cylinder position. If thats the case, you might want to consider having the MCR hold in a pneumatic dump valve. We have some air operated equipment utilizing rams that work this way. Also the rams are guided through an offset collar that is held in the open or operate position by an air cylinder. This way when the e-stop is hit, the rams are trapped in the up position as the cylinder releases the locking mechanism. Works well & certainly removes all the hazards when the e-stop is hit.

Dave I am not sure I understand you, as is the pneumatic ram is spring loaded(via air valve) return to the open position....my hopes are to make it NOT close and return to home/open under any adverse conditions.

Can you explain in more detail please? Or maybe thats simpler terms, I jes a country boy dont fergit.

Ron, an analogy to Dave's air cylinder latch would be a spring set motor brake that is energized (released) all the time. If you lose power, the brake sets, preventing the motor from turning. In his system, when you lose air, the latch prevents the cylinder from dropping. BUT... only when it's fully up.

I think an air dump valve would be the best approach for your system. We put 'em on every pneumatic system we build. The one to use would be a soft-start valve. When you energize them, air pressure builds up slowly (through a flow control), then switch to full flow only after full pressure has built up. This prevents cylinders from slamming at startup.

If you need to prevent the cylinder from dropping (closing) when air pressure is lost, there are a few ways to accomplish that. Some cylinder manufacturers (Festo comes to mind) make a shaft clamp that grips the cylinder shaft at whatever position it's in when air pressure drops. What I usually do is use a pilot operated check valve at the cylinder port. These simply prevent air flow out of the cylinder unless there is pressure at the pilot. I just tee the pilot port into the air line that goes to the other cylinder port. Cylinder will only move down when your solenoid valve tells it to

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Any questions?

beerchug

-Eric

rs,

If safety is an issue and you don't want any liability issues, you might want to consider light curtains for where the operator may have to put their hands. If you need information about light curtains, I have a customer that sells them. Their web site is http://www.ktitechnologies.com/

As mentioned earlier, I would agree that you keep the estop separate from the PLC. We always state to our customers that if they require an estop, keep it separate from the controller.

God Bless,

Stephen

Not sure what the rules are un the USA, but here in the UK anyone who MODIFIES an existing machine becomes responsible for ensuring that it meets CURRENT safety standards.

So if the same applies there, you will have a major task completely upgrading the whole machine and it's control system.

You could also use a spring return cylinder to retract the cutting arm.
Or Amlok http://www.ame.com/amlok/rcn.stm makes a stand alone air-off rod clamp. Works good for holding a cylinder from dropping. We use several styles of them.

Not sure I will address the questions in sequence they were asked but I think I can clarify some things.
1. This machine is old using relay technology and older heat/wire transformer equipment. I know I can make the hot wire cutter more efficient using newer style controllers etc.
2. This machine uses electromechanical timers that were good 30 or 40 years ago but they dont offer the resolution I know we can obtain with newer devices
3. This machines company is no longer in business and we/I am aware that modifications will be our responsibility, thats why I am asking questions. There are rules and regulations that machines have to conform to but arent always explicit in details. My objective is twofold, make the machine safer than it is now (adding real ESTOP's and/or safety relays, whatever is proper or believed to be proper by those that build machines on a daily basis). I am not asking HOW TO do this but what many deem the proper way.
4. LIGHT CURTAINS This machine isnt conducive because of its physical design to add light curtains. At least I haven't figured out a way to use them but have not eliminated them from consideration.

NOW for another simplistic overview of the machine
The system is L shaped...a conveyor carries the product in front of an air ram that pushes the product and cellophane wrapping under and just past the cutting head. The head ram is activated when pusher ram is fully extended (there are timers in this sequence to delay the down to allow pusher to retract). When the head comes down it triggers first an override for the safety limits on the cut bar (these are spring maintained N.O. limit switches) then a dwell timer to maintain head down for X time then heat limit which turns on the cut wire heat which in turn cuts and seals the cellophane.

I have already got the specifications for a pusher ram and cutting head ram that can easily cylce 60 times a minute

The existing head is designed to move in an arc that has approximately 9 to 10 inches of travel, I dont need but 5 inches max. I really want to make this movement straight up and down but I may just have to live with modifying the arc length.

Another factor that I disliked was the cutting wires, the existing design uses a two wire two part transformer. I have redesigned the cutting bar to use 2 wires but act as one wire, this allows me to use 1 transformer to control the heat...easier to create a constant factor by using pulses to maintain a pre-heat.

MY primary concern was the head doing as it did with the lady on 3rd shift, at times they have to pull the cellophane thru the cutter head section to make a first cut (when adding new cellophane roll etc). The system had a short in one of the timers that triggered the head ram to come down, the limits on the cutting bar werent done correctly so they didnt break the circuit so the head came down and caught her hand but since it didnt complete the circuit it didnt heat the wire but she got a minor burn from residual heat and held her hand in the pinch point. The action of the ram isnt that great so the force wasnt enough to break bones but that is another area I want to be sure of. That the speed is there with low force...ie that the air flow/pressure cant be changed to make it come down with more force.

My thoughts and actions at this time is to redesign the cutting bar's safety system to hopefully make it more responsive when a problem occurs but also by use of a plc to act in an alarm fashion if the head doesnt complete its cycle in X time...say the head is triggered to go down but there is an obstruction that doesnt allow it to go all the way to trigger the limits and the safety switches aren't triggered either then after X time it would alarm and return to up position. My concern now is not to hold the cylinder in place or stop it but get it back UP to home position in the event of an obstruction (someones hand/arm).

I am having a problem with designing the circuit, I keep changing my mind on HOW. I want to use an MCR with ESTOP to control the Main Voltage which will feed the conveyor motors and a vacuum for the trim but the control voltage will be 24 vdc and the plc will be 120vac. This is the area that concerns me the most. I can make the head air cylinder return to home/up position mechanically with a power failure but what about that one in a million shot where air is lost or power and air? IS it best in an ESTOP condition to kill power to all circuits but the plc with it monitoring that an ESTOP has occured and reset it to start conditions and post an alarm to the HMI?

One of my questions here is when an ESTOP occurs should the plc power and inputs be left live but outputs disabled along with main power to active devices like motors?

The safety limit switches I thought about putting in the ESTOP ckt but then I lose some control, my thoughts is use them as inputs to the plc to trigger alarms and actions. Would it be more appropriate to make them part of the ESTOP ckt? OR maybe both?

I am not looking for the actual HOW to do this but I know alot of you design/build machines on a regular basis so are more familiar with the NOW and SAFETY aspects of wiring panels with the use of a plc. I am looking for (lack of better word) basic/standard procedures/practices used NOW in this area. As stated there are rules, regulations etc that have to be taken into consideration, I just want to make sure that I have looked at all aspects that may be involved in doing this.

In the long run I dont think I will do it wrong but I want to make it as RIGHT(safe) as can be done. I know alot will be thinking there is only 1 right way but I am not too sure about that. This isnt that big of a project really so I am using it to understand more of the safety regulations/issues that have to be dealt with these days. IF it was just a rewire job I could have ripped the old out and had it redone in a day or two depending on demands from other issues.

Anyway I appreciate the advice, comments etc.

BTW I think I understand about the dump valve etc now.

At my old job, we used a lot of Sergeant L-Sealers, both manual and fully automatic. They were garbage, but most other L-sealers I've seen were of similar design. Good for you that you want to improve them!

I wouldn't bother changing the arc motion design... That will probably just create a new bunch of headaches. Perhaps you can counterbalance the sealing arm and spring load it in the open position. This way with an air or power loss, the arm returns to the open position.

I'd wire the light curtain to the E-stop circuit, but only enable it while a cycle is in progress. Any other time it's blocked just drops out of automatic cycle (like pressing "STOP AUTO CYCLE"). Give the operators an indicator light (or lights) to indicate when the LC is active, so they know what will happen when they break the LC. This way they don't E-stop the machine accidentally. Even though the machine is supposed to be fully automatic, I realize that the operators need to get at the machine occasionally (often? ) to clear jams when the seal doesn't cut completely. Sounds like you're trying to fix this problem too! Can't hurt to throw a photoeye at the exit to confirm that product has left (or is at least leaving) the sealing area before allowing the next product in.

To create the initial seal (roll change, etc.), stick a two-hand control on the machine that bypasses the LC to allow manual operation. Operator has to stay on both buttons until the cycle is complete.

beerchug

-Eric

P.S. We once had a combination L-sealer/shrink tunnel (made in England I think). It would seal AND shrink the package in one operation! Pretty neat, but quite expensive...

Estop Osha rules

I'm not totally sure what all of OSHA's laws are, but I do know that for an operator to be able to reach any moving part that could cause bodily harm, those parts must be effectively guarded. I do know that you can no longer use a regular contactor as an ESTOP relay. You must use a UL listed Safety relay. They are not hard to find. We have something similar to what you are saying, except this machine is merely a cutter. The safety relay is wired in series with all outputs that could cause harm. When the machine cycle is complete, the operators can break the light curtain without tripping the estop and having to reset it again. We did this by taking the wire from the output of the PLC that runs the device in question, and run that wire thru the safety relay. This way, even if someone were to manually try to fire that device, and the PLC told it to go, it would not move until the safety relay pulled back in, which happens the second the operators are no longer blocking the light curtain. This way the outputs are locked out without even dropping power to the specific output card. Hope this gives you another idea.

Russ

Eric you got the machine dead right but not a Sargent, thank goodness the shrinktunnel/oven isnt combined,as is I have to move the shrinktunnel to get to its panel which is under the output conveyor.

I like the idea of the counterbalance/spring return. I was already thinking along the lines of a jam detect to verify product has moved before allowing next in.

Russ the rules etc get a little fuzzy when not talking about presses/punches so I am not positive what exactly applies...yet. I plan to use safety rated devices (like safety relays) but the details of wiring and implementation hasn't penetrated that fog in my head yet.

I am going to try to make a 1st drawing of the system this weekend, maybe I can post it and if I get time the associated code to see what people think. Maybe that will give people something to see what I am doing and the ability to criticize, advise etc.

Thanks again.

When you think about saftey, regulations, OSHA, and all those things and then apply them to machine design, the first thing that comes to my mind is failsafe, foolproof, and training. You seem to be thinking along the right lines but no one has mentioned operator training. What is the documented procedure for clearing machine jams? We've all heard horror stories about companies being sued for things we see as ridiculous, like someone spraying paint in their face and claiming the instructions didn't indicate which way the arrow was supposed to point. In saftey issues the first choice is engineering controls, which you are pursuing, the second choice is personal protective equipment and the last choice is administrative controls (work procedures, etc.). There are usually a combination of these three employed in most work places. I would really consider as an interim measure to have the operator place a block or some other object under the ram while they are clearing jams, at least until adequate engineering controls are fully implemented and tested.

I would never rely on a PLC for E-stopping the machine but there is no reason why you shouldn't use the e-stop circuit as an input for some logic control and at the very least to indicate an e-stop has occured. E-stop circuits should always be hard-wired. I like the idea of a spring return valve or counter weight to return the cylinder in the event of a power or air supply failure. You could also consider using an accumulator tank to ensure enough air is available to return the cylinder but what if the cylinder leaks badly or fails? Well I know I don't have a lot of specifics here but I felt I had to put my two cents in.

What about a safety mat? Nelsa make a 'monitored' unit and you can lonk mats to cover large area. Not cheap though.