View Full Version : Light guards and PLC/Robot

June 5th, 2006, 01:50 AM
Hi Guys,

I've got a little problem that's puzzling me a little.

I'm installing an automated production cell we obtained from another company and by the looks of things i've got to completely rewire the cell and re-do the PLC program as we have no drawings available for any of it. The problem is this

We are using a roller conveyor to transport full pallets out of the cell, the roller conveyor entrance to the cell is to be protected by a light barrier. Obviously the light barrier will need to be muted when a pallet is leaving the cell but generate an E-stop condition at any other time. How do i stand with doing this from a safety perspective? I was thinking something along the lines of using a Pilz E-stop relay driven from the output controlling the outfeed conveyor and using that to loop out the light guard monitor temporarily the only downside is that the robot will still be working within the cell while the pallet conveys and I cant get around this as it would be detrimental to the cycle time and product quality.

What would you guys recommend?

June 5th, 2006, 04:24 AM
I've come across systems that have photo beams that detect the pallet entering the light guard zone (but haven't broken it yet), the light guard is muted and then re-enabled when the pallet leaves the other side of the light guard zone (detected with more photo beams).

June 5th, 2006, 06:14 AM

The muting is normally done either within the lightguard itself or by a muting module supplied by the lightguard manufacturer. I would be very wary of using the PLC to mute the lightguard as there are obvious safety implications. I would suggest that you look at the lightguard manual as you will probably find that it tells you exactly what you need to do. Your application seems to be very typical for this type of equipment.

June 5th, 2006, 06:43 PM
For any complex safety function I would recommend a Safety PLC.
Since you are already using Pilz, try either a Pulz Safety PLC or Programmable realy.
I would not recommend using a standard PLC for any safety function other than checking the status of your safety system.
As for your function, you should take a step back and have a good look at exactly what you want to achive and the risks involved.
1, Identify all risks
2, Analyse all risks for probability and severity
3, put in place methods of reducing these risks
4, Identify all residual risks and new risks created by your modifications
5, repeat until the risks are low enough
For example, when you have muted and the pallet is moving through the curtain, can anyone jump on it and hitch a ride? What about through the side? If no, then an ordinary mute is probably OK.
If you think this is a real risk, then you could use multiple muted light curtains, or even a curtain and some single beams. The curtain is muted and protects the entire area. The beams have no muting and protect only the space through which the pallet does not pass.

June 5th, 2006, 09:09 PM
Thanks for your suggestions guys, much appreciated.

I've been looking a bit closer at the light guards, they are Smartscan 8000 type. Just looked at some info for them on the smartscan website

In most machine applications the integrated system is the ideal solution as it eliminates the need for aligning and installing additional mute sensors with all the problems such devices introduce. The mute initiating system suppresses the shut down signal from the light curtain during a pallet transfer without interruption to the machine cycle. However, should a person attempt to pass through the entry / exit area, the light curtain will instantly detect them and shut down the operation of all associated machinery.

So it looks like they have the muting feature built-in already and are specifically for this purpose.

One other question though that springs to mind, as i have the oppertunity to re-design all this equipment before and during installing it i'm quite tempted to do away with the one large control cabinet and complex PLC program in favour of fitting smaller PLC's to each piece of equipment (the cell incorporates a PLC driven label applicator, conveying system and an assembly fixture with several cylinders) I was thinking along the lines that this would dramatically cut down on wiring as for example the labeller would only need a single input from the robot to trigger it and an output to give consent for the robot to continue, same for the assembly fixture and conveyor meaning i'd end up with three small PLCs rather than one large one and a mess of wiring, the system currently has one larger mitsubishi plc in a cabinet with the wiring just hacked off the cabinet when it was transported so i've got to do a lot of rewiring regardless.

Are there any major disadvantages i've overlooked in that or does that sound like the most sensible way to do this

June 5th, 2006, 10:01 PM
Fitting of three smaller PLCs will require some form of communications and interlocking between them. I tend to prefer to use one large PLC.
To cut down on wiring, you could use a bus system. My preferrances are Profibus and ASi, although Device net is popular and more common on Japanese PLCs. This would eliminate much of the wiring while allowing you to keep the program in one PLC.

June 5th, 2006, 10:30 PM
Good point, i overlooked the fact of safety implications there too i think. Looks like im just in for a massive rewiring job then which is going to be a little tricky as i have no drawings for any of the equipment :)

June 5th, 2006, 10:39 PM
Control Net could do your remoteIO if you've got the budget.............

June 6th, 2006, 03:04 AM
I can do the rewire ok it's just a case of having to determine how most of the equipment is supposed to operate as no-one at our place had the forethought of going to see it all working and noting down the operating sequence of anything (the automation company that originally built all this are were trying to distance themselves from it at it's last company) and chances are the PLC program that's alreay there will be password protected so I doubt i'd be able to work it out from that :)

June 6th, 2006, 07:31 AM
If you are re-wiring this machine you will need to ensure that you comply with the supply of machinery (safety) regulations and the Provision and Use of Work Equipment Regulations



Note that second hand machinery has to comply as well. If you do not substantially alter the machine and it carries a CE mark then you may not need to CE mark it again but it must still meet PUWER. This is a very tricky area and it has caused me a lot of problems over the years. The best approach is to treat it as if you are building a new machine, carry out risk assessments determine the necessary control measures, such as the light curtains and muting (in fact for pallets it is probably sector blanking you need) if the safety circuit is different from the original CE marked machine then it must be CE marked again. If the machine dates before 1993 (i.e not CE marked) and you change it then it has to be CE marked. In practice CE marking in itself isn't all that important if you are not selling machinery but it MUST comply with PUWER.

June 6th, 2006, 01:08 PM
Thanks for the info John, had a length read of the PUWER information there and was quite surprised. I've identified as many of the risks as i feel i possibly can (most if not all were already done by the automation company). I would really appreciate your input over this matter as a second opinion is always good :)

The entire production cell is guarded with all moving parts enclosed within the perimeter fence which is approximately 8ft tall and is flush to the floor. There are only two possible entrances into the guarding, one of which is through the pallet conveyor (which is linked into the E-stop circuit) and the other is via a door protected with a fortress interlock key. The injection moulding machine is flush to the side of the cell with the rear guard switches overriden but the fortess key interlock is hard-wired into the moulding machine emergency stop circuit, in addition a switch is to be installed to inform the robot that the rear guard of the moulding machine is fully open to prevent robot collision but i do not believe that this is relevant to the safe operation of the equipment. In addition i am planning to use a Pilz Estop relay to remove 24V control voltage and directly interrupt supply to all PLC outputs effectively removing power from all contactors and valves with the addition of a contactor that is latched in when the reset is pressed in addition to resetting the Pilz that drops out with the Estop relay to remove 3 phase supply to the motor contactors for the conveyors hence protecting in the event that one of the motor contactors sticks in. The E-stop relay will also interrupt the robot emergency stop circuit with the exception of the second key (normally locked in the door) being used in the teach key position on the robot which will E-stop the entire cell with the exception of the robot which can still operate at safety speed provided a deadman switch is being operated on the robot pendant. The only devices that will maintain power during an E-stop or guard open condition are the vacuum circuits on the robot which are required to prevent the moulding falling and i believe is normal practise used for this type of robot application.

I think that this should comply with the relevant regulations but do you think there is anything i may have overlooked or fallen short with? As this is probably my biggest project to date i really just dont want anything to go hideously wrong.

EDIT - forgot to add that I am planning to train the equipment operators on preforming daily equipment safety checks including verification that all the E-Stops are working as well as light barriers and fortress key switch in addition to our existing moulding machine daily safety checks which are already performed. Also forgot to mention that all pneumatic equipment (including moving parts of the robot tooling) are supplied via cut-off valves controlled by the Estop circuit with a separate permanent air supply to the vacuum circuits

June 6th, 2006, 03:48 PM
It sounds as if you are on the right lines, I would say that this type of machine will need a control category 3 e-stop and interlock system. The light curtains will probably be cat 3 as will the pilz relay, cat 3 requires that a single fault will not result in a hazardous situation and that as far as possible that fault is detected. This in practice means using dual redundant circuits, i.e. e-stops and interlock switches with 2 seperate channels to the pilz relay. The most common design failure here is not to use redundant outputs. If you are switching a 3 phase supply then you need 2 contactors in series with the NC auxilliaries in the pilz relay feed back loop. If you go to the Pilz web site and download the Pilz Safety Book it will show you how this is done.
I am going to do a risk assessment on a large assembly line for a multi-national company with similar arrangements as yours, so it is a common system.

June 7th, 2006, 03:47 AM
Thanks John, I've had a look at the Pilz website and decided to fit a PNOZ X2.1 as this seems quite well suited to the task :)