3 replies to this topic

[Emax]

I have posted this in the "members only" section - but realizing that the ski lift industry is far from the only user of PLCs in s safety-intensive application, I throw it out to all of you. I hope that representatives of other industries will offer their thoughts.

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The current standard (not code, please), requires safety interrupts developed by electronic means (currently, read PLC) be duplicated - or redundant.

Discounting input or output modules damaged by either poor system design or sloppy transient voltage suppression, has anyone out there experienced a real PLC fault? For the moment, let's not consider an Austrian manufacturer's programming slip-up - which was both understandable and unforgivable. I consider it a design mishap and not an equipment failure. People screw up now and then.

My own experience has been that -given an even chance - PLCs are by far the most reliable pieces of equipment in the entire control system. Yet we are currently required (perhaps pressured) to either duplicate the entire PLC hardware or (like ABB and Pilz) use a so-called "safety PLC" - which is essentially the same thing, only more expensive an unrealistically complicated to work with.

When you think about it from a control standpoint, a ski lift is a damned simple machine. I have to wonder (once again) if we're hunting jacalopes with atomic weapons. I say this while also admitting that I am involved in this open-ended (never-ending) technical pursuit. Maybe it's just the fascination with the "latest-and-greatest". If that's what it is, then maybe it's forgivable - but it's still unfortunate. But if it is in all technical seriousness, then sounder heads might say that the boys in power are overly engrossed with their private parts.

This post has been edited by Emax: 20 June 2012 - 06:45 PM

[Razvan]

At my first job, the plant had many PLCs - OK, maybe not very many (a few dozens), but of many makers: Allen Bradley, Mitsubishi, Honeywell, Telemecanique, Siemens (Simatic, Sinumerik), even some very early Toshibas (mounted on some Bulgarian-made vertical machining centers that Romania was tricked into while being a Comecon (CMEA) member state; really weird combination!)

Besides the Romanian-made discrete component "so-called PLCs" (aka "the bug cabinets"), in my 7 years there, NO OTHER PLC FAILURES. We've had some faulty motor drives, some of the older and some of the newer types, but never a PLC.

[liftmech]

I don't recall one either. I've experienced a failure of the EPROM card (old A/B unit), but that in itself is not the PLC.

[Emax]

EPROM - the UV erasable variety?

In 1978, Lift Engineering had a similar problem with the world's first microprocessor lift system: the Base 2. At load test, photographers used flash(bulbs) ... close up. The EPROMs were exposed and were randomly erased. Bad load test day. I'm thinking that EPROMs are not the best choice for PLC memory. Frankly, I don't see why standard flash drives or USB thumb drives aren't used.

Thanks for the comments up to this point. I"m hoping for more.