Pile of relays.

[itsmoked]

Hey folks.
I'm doing the controls on a 450 ton press.
Three motors.
Three or four valves.
Estop.
Dual cycle run buttons to prevent unsightly mushed limbs.
Couple of different settings.
Two limit switches.

Anyway a 4 or 5 relays a timer or two. Blah blah.

This is not PLC land really but it's crying for more than jungle logic relays.

So what do you guys like to use for this type stuff?
Got any suggestions?
Favorites?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

The generic name for these things seems to be 'programmable relay' or 'smart relay'.

One of the Siemens LOGO! modules sounds like a good candidate. It's a while since I used one - we are an almost exclusively A-B site for PLCs. They were fairly straightforward to program once you get to grips with the interface and had good hardware reliability. The few problems I recall were down to programming errors or oversights.

Other brands:

Moeller's 'Easy' series of programmable relays appear to be remarkably similar to A-B's 'Pico'. I expect there could be differences in the firmware but visually they are almost identical. Guess which is more expensive?

Schneider have a similar product marketed under their Telemecanique brand called 'Zelio'. I've no experience on these.


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Sometimes I only open my mouth to swap feet...

 

[DickDV]

A responsible control rebuild on a press involves using safety-rated components for the safety circuits.

I don't believe the programmable relays, etc. qualify.

 

[Skogsgurra]

Agree. There are certified "Press control relays" that seem to be mandatory at least over here. I had to do an investigation on a press control where a young girl had lost most of her right hand once back in the eighties. Never forget that. I hope things have developed since. I also tend to think Siemens LOGO!, but more is needed to have complete safety.

http://www.sea.siemens.com/contrlbu/upld/files/Sec4.pdf

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Wow lots of good stuff here. Just what I wanted!

I am surprised to hear about "Press rated safety components." I don't see a press as being as dangerous as most machines in factories.

I used to work in a tannery a lot. There were machines that were 12 feet long with three rollers that people fed leather into and it sucked it in at leather flapping speeds to make it bigger by smashing it flatter. Others imprinted patterns and grains. Some sucked in the leather passing a high speed razor thru the leather to split it in plane! Any of those would make quick work of us meat sacks. The 'splitters' were modern but had 45 ice cube relays packed into a box. They would rapidly wear out leaving a nightmare of faulty logic.

They all had safety feature stuff but I never discovered any specific stuff. Presses as big as this one generally move pretty slowly to boot.

They use it for drawing 12 inch cans out of flat steel so it actually can go too fast, as I believe the metal has to have time to flow like water.

Thanks guys. Now you've given me more homework. LOL

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Keith,
Sent you an email regarding this. Although the LOGO is not in and of itself a "Safety Relay", there is a way to integrate it into an overall safety and control system with a Safety Relay.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[richs]

Hiya-

Somewhat off the topic (philosophical rather than practical) but I wonder *IF* there could be a "safety" intelligent relay, PLC or whatever.

I would get kind of queasy if I were a manufacturer supplying a "safety intelligent relay".

By making it programmable, it would certianly lend itself to unsafe operation. The best intentions of programmers can make for deadly code. I wouldn't wonder if in normal practice, that safety relays are connected on the load side of a PLC rather than having a "safe PLC".

I can easily envision that parallel interlocks would go to both the controller (whatever) and the safety relay with appropriately different interactions; PLC trips safety relays, interlock trips safety relay, PLC sounds audio alarm for the operator.

I would suggest that you NEVER make assumptions on the condition of the operator. A tired (drunk, loaded) operator doing a repetative task can offen have what we (I) call "brain farts" and do something stupid. It's not only your client that will get hauled into court, you very well might wind up there too!

There was a book;
"Getting Sued and Other Tales of the Engineering Life"

http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=6389

A fun read. Might not be worth buying, but if you can find it in the library......

IMHO, even if you pick a PLC with a master relay, if it were me, I'd throw a safety relay in series with the master relay to deenergize everything at the first hint of a problem.
Just my 2 cents worth.

Cheers,

Rich S.

 

[ScottyUK]

Hi Rich,

There are a number of safety controllers out there. A-B have GuardLogix, there's the Pilz one that I can't remember the name of, and the Triconex range from Invensys just as a starter. They are becoming big business. Programming them must be interesting as you say. I've not had the pleasure.


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Sometimes I only open my mouth to swap feet...

 

[djs]

Hello;
When designing safety related systems there are two seperate things to look at: hardware and software. The safety related products take care of the hardware aspect for you. What this means is that the hardware has internal functional checking of itself. If the internal check fails, the hardware goes into a predetermined known safe state.
An example of this is the two pushbuttons to start the press cycle. If and discrete input connected to one of thos buttons failed in the closed condition, then the hardware would "failsafe" into a known safe condition.
The other side is the software or ladder logic. This is much more custom, however for most saftey systems that actual code is very simple.

 

[richs]

Hiya-

ScottyUK posted a reference to the Guardlogix manuals which I perused. They are are very interesting and bring up some good points for any design.

1. First and formost, safety comes from the design rigor and/or designing for safety. Well, yep. In the computer biz, the old saying of 90% of the code runs 10% of the time. A corollary is that the majority of the code takes care of "non normal" process flow.......

2. The implementation of a watchdog timer. This can be easily incorporated into almost any plc design that has timers. Although the Rockwell includes a lower level watchdog timer, one can easily implement his/her own in ladder logic. Something to think about for the design cycle of ANY plc type design. However, see note 4 below.

3. Similar to this are the concepts of remote hardware verification of I/O functionality. On input it seems to be open circuit type faults and on the output side, moitoring of the output to actually verify that the hardware output operation has occurred. Along with this the Rockwell seems to place a lot of emphasis on "pulse testing" of the system.
I'm not too sure as to the validity of this. Still things like having an extra set of contacts on the relays to monitor the state of the switch might be in order. I would feel that DC instead of AC ought to be passed as the detection of AC might lead to "interesting" delays, but OTOH, this too could be a rung in the ladder program. Something like:

If the hardware output of interest is asserted AND the input representing the voltage to the load is NOT asserted, THEN set the FAULT relay.

I found it interesting that one of the Rockwell figures showed that primarily faults occur in the sensors and actuators with only 10% of the PFS being in the controller. Hummmm, I have a feeling that this is HARDWARE only. If through system analysis is done, including the software, the figure might be a bit higher......

4. For most of the PLCs life, there have been "master relays". This one gave me pause for thought. When I designed the little PLC that I have, the master relay was/is a bit in a "special flags register" along with the state of the overflow bit, the zero bit, etc. The state of the master relay is denergized during the output of the PLC cycle. There is naturally a delay for this synchronous process to occur. Indeed one might argue to have an "asynchronous" rule set for the master relay. Hummm, going to have to think about that one. Might have to turn on the
WDT on the PIC for my code generator.......

5. Redundant systems. I have delt with redundant systems and this, IMHO, is the one of the hardest parts of systems architecture. It is also the hardest to analyze. I might suggest that as an addendum to the proof testing of the Rockwell documentation, if one were rigorous with the testing, one would include deliberately faulting the primary controller and verifying proper switch over rather than just testing both sides individually as suggested. And indeed, fault insertion of the secondary controller and watching a report of the secondary fault on the primary controller. From what I skimmed through the manual, it seems that they use a "heartbeat" type control. With carefull design, these too can be implemented with ladder logic. However, one could easily get oneself going around in circles.

As ScottyUK point programming would be interesting. From what I could see, and NOT what I had hoped for was that it's still pretty much the same old ladder logic programming. I was hoping for some sort of automated analysis of the resulting code to determine the safety feasability of the designed code. Remember the old "lint" program in unix for C code? Something a little more system integrated than just that. Sigh, maybe in the future.

So, with that in mind the old adage:
"Safety is designed in" is still the watchword.

Cheers,

Rich S.

 

[sslobodan]

Well you can always consider micro PLC if you really want PLC on your site. Any way... there is nothing wrong having relays do all the job, Its simple..You set up a schematic of what you need dig up a internet for what you need and try to make it work. Anyway i would suggest that you use

http://ecatalog.squared.com/

Because they have nice catalouge of even competitors products so you can run for actual names in specifications and get PDFs bu name of product. Trust me its pain in the a** to sit whole day(s) and search for something that would fit boss idea unless you don't know how it is named

 

[ScumPunk]

Hi,
This is a very interesting thread, as it covers 2 topics I've been looking at recently - "mini-PLC's" and Machine safety. As is correct, I'll put a few comments about safety first...
First of all, Itsmoked, according to several studies, around 60% of safety control system failures are designed in, either due to incorrect specification or poor design... a sobering thought.
(Incidently, hydraulic presses seem to be the prefferred machine for scaring students on machine safety courses. According to EN954, a hydraulic press is ALWAYS cat3 minimum.)
Probably the quickest and legally afest way to design your application would be to take a standard circuit from a book published for the purpose. These are available to the various standards that apply, and will cover your back if something goes wrong...
Secondly, Siemens LOGO modules. These are not so easy to program from the front panel (IMO), but if you use the LOGOsoft program it's really simple (similar to drawing a schematic). It also has a simulation mode so you can test it on screen before loading it into the module.
Although the LOGO isn't a "Safety" device per se, a cat 3 design can be achieved with the addition of a safety relay quite easily.
Best regards,
Martin
(Apologies if this is all old news. Next post I'll be covering the sucking of eggs by old ladies )

 

[ScumPunk]

By the way, this is a great site, I've been reading it for a while now, and finally felt I might have some sort of contribution to make, so joined. I look forward to exchanging ideas with you all.
Best regards,
Martin

Apologies for repeating old news. My next post will be on the subject of old ladies sucking eggs..

 

[itsmoked]

Thanks Scummy. I purchased the IDEC versions of the LOGO! smart relays. So far I'm quite pleased with them.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

D'oh! that cost you a lunch Keith!

 

[itsmoked]

LOL! Guilty as charged.
It shall be so.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Rodmcm]

I like the Zellio from Schneider, especially as their programming tool has a simple simulation interface. I have used this programming software to test logic for othe applications as it is so easy to use.

 

[itsmoked]

Hi Rodmcm.

I believe they are all made by the same company and just re-branded or slightly personalized and re-branded.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com