FMEA help


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  1. #1
    Citius
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    Default FMEA help

    I've recently completed the design of a system for a customer...prototypes have been thoroughly tested and shown to meet customer specs, and they are 100% satisfied. They have started production on the first 1,000 units to get them ready for integration into their more complete system & ready for sale starting sometime in early June...with the plan to ramp up considerably later this year...but to date, we have exactly 10 units (of the final version at least). They have taken the prototypes to get them UL certified...all went very well electrically, and unit performed exactly as planned. None of the 10 have shown a single failure of any kind.

    BUT...the
    preliminary design review test agency that is helping them to get UL certification on their product is requesting a software FMEA (Failure Modes and Effects Analysis). The evaluator classified the software as Class C, Single Channel with Functional Test. Of course this has been dropped back into my lap (today) and has a deadline on Monday morning...

    I did receive a 42-page faxed copy (probably 20-generation copy, as I can't read a good number of items at all!), from a 2002 UL spec #
    Ul 60730-1A Annex H Requirements for Software Controls, that is nothing short of gibberish!

    with constant quotes such as:
    "H.11.12.6.1 For controls with functions declared as software class C, the manufactures shall have used systematic test H.2.17.8 and inspection H.2.17.5 or walk through H.2.17.9 or static analysis H.2.17.7.2 during software development - items of interest: H.2.16.5, H.2.20.2, H.11.12.2, H.11.12.6.1 and, of course, the table H.11.12.7."

    Has anyone ever done one of these software FMEA's? In my quick research, It appears that first, for a FMEA, failures need to be prioritized according to how serious their consequences are, how frequently they occur and how easily they can be detected...this is very difficult and somewhat subjective I think....especially with my limited test data (10-units - no known failures of any kind - and pretty thorough job on the code if I do say so myself). I've used an 18f45k20 and have it ~80% full...with probably half of the routines doing fault checks.

    I am considering a simple Excel table...where I will focus on my software solutions to potential problems & the built-in fault warnings, methods of obtaining good data for the controller such as always taking median values of multiple readings, using multiple sensors, along with some specific software settings I used when setting the micro up (Fail-safe Clock, Watchdog Timer, Brown-out setting, Stack-overflow protection, Power-up timer monitoring, etc)...

    But with the time-crunch and my prior lack of ANY experience with this task, any suggestions would be GREATLY appreciated!

    Damn UL tests - there goes my weekend!

    Thanks in advance for ANY input!!

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  3. #2
    Citius
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    Default Re: FMEA help

    Well, after about 20-hours worth of work this weekend, I was able to complete a System Level Software FMEA (Failure Modes & Effects Analysis) with I/O Fault Analysis to meet my customers UL certification needs.

    The key learning from this for anyone ever searching our forum for help with FMEA...if starting work on a project that needs UL Certification, START THE FMEA BEFORE YOU WRITE A SINGLE LINE OF CODE!

    It is a painstaking and time consuming process...but infinitely harder when you've already completed your coding! The design process is also much more thorough if FMEA is completed first, rather than patched together later.

    Essentially what you need is a through thinking out of every possible failure mode for each piece of your code, the local effects of that potential error (such as A/D module failing to read your sensor, or perhaps reading it incorrectly), all the effects that failure would have on the system as a whole, a ranking system of severity of failure, your recommendations on possible solutions to that potential failure, any current code solutions, and finally a system wide ranking of severity for that failure.

    In case anyone needs to do this in the future, I'll give you a small example of a few single software elements and how to complete the chart for this one piece. The excel chart and table are mine, so there are certainly other ways to do this...but this was how I solved this problem.

    The attached is a sanitized version of the top 6 elements of my software FMEA...(I had over 100 to write up)...but this will give you the idea.

    Carl

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  5. #3
    Fanatical Contributor Tim's Avatar
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    Default Re: FMEA help

    Many thanks for that.

    After a while doing coding you find the process is simple its all the safe guards that you have to write in that take the time.

    This is why I get so hot under the collar with interrupts, if you follow simple rules they work great, step out side and anything can happen. Following your advice on how it works is the way to go.
    Tim

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