Safety PLC

[Govind Lathkar]

Dear All, I want to know guidelines for using safety plc My requirement safety input 20 DI - 500 D0 - 400 Here should I use only safty PLC or safty + normal PLC combination Regards Govind L. Lathkar

[KAZAH]

[edda]

Hi Govind Lathkar! I cant say if you shuld only use a Safety PLC or a combo. But I´n my latest project I have used a combo. I have used a saftety plc from Jobab (Jokab Pluto) and a ordinary plc (siemens S7 300) and to comunicate between them I used profibus and I think that this works fine. Becuse you wrote this post in Mitsubishi I assume that you are normaly using Mitsubishi. I know that Mitsu have a Saftey CPU called QS001 that uses the busssystem CC-Link Safety. I have never tried this so cant say if it good or not, but maby anyone else can share some experience of this? Best Regards!

[panic mode]

there could be many things to consider when selecting hardware for the project, for example: - complexity - functionality (is the PLC supposed to do any other high priority task such as motion for example) - number of I/O - required safety category - scale (distributed or local), - cost, - maintainablity etc safety plcs are great and allow things that used to be so cumbersome to do before. combining safe and nonsafe functions into one controller may be more cost effective when looking only at the hardware cost or panel space. however I would rather keep safety and non-safe functions in separate controllers (any day, at least until I see certain things changed). i see that Mitsubishi went for dedicated safety controller which imho is better. this way maintenance guy poking through PLC at 3AM has much less chace of making mistake or allowing built in design flaws stick their ugly head and byte someone. note that safety in general is not defined very clearly (this is why risk assessment process is highly influenced by personal experience, opinion or interpretation). also only small percentage of controls guys know how to make proper safety circuit - even when they are just following circuit from datasheet of the controller. it is even worse when creating safety circuit based on safety plc. most will not even know what will be expected response time before the system is build (and after...), what is monitoring and how to do it properly, what is redundancy and how to do it properly etc. chances are they will make large margin mistakes just in selecting strategy, components, circuits and program as well, specially since safety plcs also often allow mixing of safe and non-safe signals in the safety program. imho this defeats very purpose of safety controller. some don't even clearly identify non safe signals, don't allow commenting of the I/O etc. since i'm not in position to set the rules, i will just try yo help. for example: - learn what are the safety categories and what is the difference from circuit design perspective - find out what exactly is meant by redundancy and monitoring - learn what standards apply to particular job (is is general purpose machine or additioanl standards apply such as press, robotic etc.) - i would ask someone experienced to do risk analisys and suggest safety category (knowing if above mentioned standards apply can easily narrow down selection). - identify needed I/O, for example you mention 20 inputs but the actual count will depend on category and number of used outputs (!) safety categories basic concepts: B - good quality components and design practices but no specific safety circuit (no light curtain or simlar on washing machine or plasma TV) Cat1 - everything from B applies but with single channel circuit and no monitoring (often simple E-Stop circuit using seal in contact to hold the relay energized after "reset") Cat2 - everything from Cat1 but with monitoring (both inputs and outputs) Cat3 - everything from Cat2 plus redundancy Cat4 - everything from Cat3 plus prevention mechanism for fault accumulation now think about possible design (or maintenance) errors. what happens if you try to design Cat4 but don't do monitoring properly? clearly it's not Cat4 any more but what category will this circuit meet? assuming that everything was corrected but one of the redunant outputs is driving only single contactor instead of two. what category would this be? assuming that everything was restored to normal but some day someone ends up replacing relays or contactors on that redundant output for non-safety rated relays. what category is the circuit now? redundancy - if you read the standards you may find out that safety gate of the robotic cell for example may need monitoring by safety circuit and that it needs to be redundant but there is no mention of what part of the safety input is supposed to be "redundant". is it enough to just use two contacts from same sensor (gate switch) and why not? what controller can be used with particular coded magnet safety or transponder type contactless sensors? they certainly can work on just about any controller but you may be surprised that nobody will stand behind their product if the low current device such as those is to be used on controller not specifically listed in the datasheet (regardless if from competitor or not). can you wire solid state outputs from the light curtain directly to safety controller input and if answer is no what can be done? once you get through such exercises, selecting safety system is no longer a problem. for a medium size project (cell for example) i like to use Pilz Multi. it is compact, expandable, very powerfull. Cat4 sensors are small, easy to use, inexpensive etc.

[MDH]

If you go to the MEAU site there is a Power Point presentation (under Downloads/Presentations) explaining the Safety PLC and CC-Link functions. Hope it helps a little. Regards, Mitchell Production Engineering

[KAZAH]

http://www.meau.com/eprise/main/sites/publ..._Safety/default .see presentation.use no script in browser

[Crossbow]

There are many manuals in the Downloads section at www.meau.com which deal with the Safety PLC. I don't have the numbers handy, but will post them later. Try searching on part number QS001CPU. Are all of those I/O safety related? Only the safety I/O needs to be on a safety PLC. You can then use a standard PLC for the others and network the safety PLC and the normal PLC with Melsecnet/H or MelsecNet/G.