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Old October 9th, 2017, 08:36 AM   #1
DLMUK
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What methods to ensure PLC program is error free?

Hi,

I am interested to know what methods people use to ensure their PLC code is error/bug free and working as per the specification.

All of the systems I do are used in the Marine/Offshore industry but are not safety critical ones. Someone has suggested GAMP to me but that seems more suited to pharma/process control etc.

Cheers!
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Old October 9th, 2017, 09:39 AM   #2
JohnCalderwood
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Depends how good/detailed your specification is....

Some can be very woolly, with just a basic functionality, and the code-writer or SI has to fill in the blanks using years of experience to come up with the full package, then resubmit to the client..

I have seen tables with lists of functions/alarms with tickboxes then signature box at the end.

Some I have seen, just require the basic safety functions to immediately stop the plant get tested, then can it be run in the expected sequence, does is shutdown sequentially.? Anything flashing red on the screen - no? then fine.... and it gets fine tuned over a number of operations....

Much prefer going through an I/O list, making sure they all function as expected. Fix the missings, then check emergency stops and interlocks, then run motors in manual/local (direction and/or speed range), open and close valves, then start up tentatively, following what is hopefully a written-down logical sequence.
Check the actual running out at the plant, confirm the SCADA shows what is expected. Then go for it.....
Any faults/alarms that should shut the plant, activate them where possible - if not possible, then simulate them.

We do not always get time to try all of these due to pressures, but cover as many as practical in the time given.
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Old October 9th, 2017, 10:01 AM   #3
tragically1969
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Quote:
Originally Posted by DLMUK View Post
Hi,

I am interested to know what methods people use to ensure their PLC code is error/bug free and working as per the specification.

All of the systems I do are used in the Marine/Offshore industry but are not safety critical ones. Someone has suggested GAMP to me but that seems more suited to pharma/process control etc.

Cheers!
You wont find one unified approach to this but following the Internal Test, Factory Witness Test (against test rig of field devices if required) then Site Acceptance Test and finally Integrated System Test, those 4 in combination should be enough to show any bugs and ensure programmed code is working correctly to expectations.

These loosely follow the 4 levels of software testing: http://softwaretestingfundamentals.c...esting-levels/

Last edited by tragically1969; October 9th, 2017 at 10:09 AM.
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Old October 9th, 2017, 10:22 AM   #4
gclshortt
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Test, test and more testing...

Even programs that have run for years without fault can then have an error when certain conditions are met.

http://accautomation.ca/plc-programm...ick-and-place/
Try to test the program under every possible condition. Input sensors are shorted out, or opened all of the time, Outputs are shorted out or open all of the time, etc.

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Old October 9th, 2017, 11:57 AM   #5
ndzied1
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When we are doing controls, we often try to grab someone not related to the project and have them run the machine and try to "break" it. They throw strange button press combinations at it. E-Stop in all phases of the system, try to put in unexpected data to operator fields in HMIs. You get the picture.

This isn't fool proof and definitely doesn't cover everything but it does get at a lot of issues that could be bad on the customer floor.

Of course you have to keep safety in mind at all times and not really break anything mechanically if you can help it ;-)
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Old October 9th, 2017, 05:49 PM   #6
ASF
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Quote:
Originally Posted by ndzied1 View Post
When we are doing controls, we often try to grab someone not related to the project and have them run the machine and try to "break" it. They throw strange button press combinations at it. E-Stop in all phases of the system, try to put in unexpected data to operator fields in HMIs. You get the picture.

This isn't fool proof and definitely doesn't cover everything but it does get at a lot of issues that could be bad on the customer floor.

Of course you have to keep safety in mind at all times and not really break anything mechanically if you can help it ;-)
Absolutely. Try as many different ways as you can think of to f*** up the system, then go find the worst operator you can and tell them to do the same thing
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Old October 10th, 2017, 02:48 AM   #7
cardosocea
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Usually, the functionality is fairly easy to test as it will have a sequence and a desirable outcome.

The main thing for me when it comes to testing is pretty much failing one thing after another and confirm the system reacts safely. What I find interesting is that some people in the automation world (noticeably more on the SCADA side) can't comprehend that what bothers me is that someone can get hurt or that the system won't handle a single failure.

I remember commissioning a winch and it was about 4 pages to test performance and about 50 to test failures and failure responses... but many people only think of the core functionality requested in the original scope.
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