Help with Idec PLC FC4A-D40K3 - safety stuff

[a70duster]

I work for Keystone Resort in Colorado. We are using Idec PLCs (FC4A-D40K3) to control our ski lifts. I was curious about some safety features that are associated with PLCs. I have searched on the internet and have found some general information about watchdog timers and power failure checks. My two main concerns would be the watchdog timer and a low voltage dropout. I would like to have the ability to implement a watchdog timer fault and a low voltage fault.

Any suggestions appreciated.

 

[dpc]

If this involves life safety, you need an emergency shutdown system that does not depend on the PLC, such as an emergency stop or pull cord system that uses hard-wired contacts that will directly de-energize the motor control circuit. These safety switches should connected in a fail-safe configuration such that a component failure or loose wire will cause a safe system shutdown.

You cannot rely on the PLC internal logic or PLC inputs/outputs for life safety functions.

For better PLC reliability, you can get into parallel PLC processors, redundant I/O, voting logic, fail-safe configurations, UPS power, etc.

 

[a70duster]

dpc,

Thanks for your concern but you are assuming. We have safeties in place that are wired in the fashion you mentioned. My question was directed at the PLC level. Say the program craps out on a divide by zero or gets caught in some "loop", that is what the watchdog timer is for. Say our batteries start to crap out, then can this particular PLC throw a fault and shut down?

The PLC monitors detachable chairs in the couture of the terminal. How can one "hard wire" in a safety monitoring system? It would not be feasible or maybe even possible to do.

 

[dpc]

I was assuming because you didn't tell us. The PLC can always be programmed to shut down its outputs in response to a set of conditions. The problem is what happens if the PLC output fails? By far the most common PLC hardware failures are output modules and they can fail open or shorted, take your pick.

 

[a70duster]

dpc, so far you have not answered my questions. And instead of assuming you could ask questions.

 

[dpc]

a70duster,

That's an interesting attitude.

Best of luck.

 

[ScottyUK]

dpc, so far you have not answered my questions. And instead of assuming you could ask questions.

Of course you could always pay a consultant who will charge enough to make it worth his while putting up with your attitude problem. None of us get paid for doing this: we just have a chance to help like-minded individuals and get some help ourselves now and then. A little bit of friendly courtesy goes a long way.


As for your questions:

Low voltage alarm -

Could implement it either through an analogue input on the PLC with a compare function to set a digital flag, or use an external sensing relay and wire it in through the digital I/O. The latter will almost certainly be cheaper unless you have a spare analogue input. Use the input to drive the PLC to a safe state.

Watchdog -

Are you trying to detect that the processor is 'alive' or that the program is executing? What do you intend to do with the watchdog if it detects a fault condition?


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[a70duster]

All I posted was for dpc not to assume. Read it for what it is.

ScottyUK,

No analog inputs used with our application. And with the watchdog in the event of a timeout, I would have it shut down the lift. If the PLC can't cycle thru its "ladder", then it is not monitoring properly.


An email I got today from Idec. My questions to Idea were the same that I posted to this forum. Guess my "bad attitude" really got to him.

Yes there are low voltage and WDT circuits in the MicroSmart
Which can be read directly in your ladder program.

Here's the current user's manual which provides the most
detailed information on the FC4A-D40K3.

http://www.idec.com/Products/ENG/PDF/manuals/MiSmManual0604.pdf

You can add logic to your program to look at special data register D8005
for various error codes. Refer to pages 28-3,-4,-5, in the manual for
more information. You can also view D8005 directly in the WindLDR's
monitor mode by selecting: Online, Monitor, PLC Status, Error, Details.

Bill Ulett, IDEC-USA

 

[a70duster]

In case someone else has a similar question in the future and tries to find some info with a search...

The Idec MircoSmart does have two analog inputs. One is connected to a pot and the other is two open pins that measure 0-10 volts. It is located on second to left "column" and above the programming port.

As

 

[a70duster]

As ScottyUK mentioned, I can use this ADC port for a low voltage drop out.

 

[ScottyUK]

a70duster,

Good info - thanks. I've not met this brand of PLCs before. Sorry about earlier post - I shouldn't post when I'm in a cranky mood. Can I give myself a black star?


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!