How to check if the PLC & its program are working fine?

Hi guys,

Recently one of our client & auditors visit our plant. There are asking us a method that we can use to double check if the PLC & its prpgram are working & executing correctly. This check must be done periodically. This is a machine which test the pressure of an object.

Has anyone come across such question?

Hope to hear from you guys. Thank you.

Question #1 - is the plant online and making money? It's working.

Since there's a pressure test involved, I would assume some kind of standard is available to test or calibrate your machine... but, it's not the PLC that wears out, it's the associated equipment, such as your pressure tester.

If you are just looking for a quick visual, toggle a pilot light on/off. If the light is blinking constantly, then the PLC program is running. Beyond that, to know that every I/O module, every point on each I/O module and every part of the program is working is to know that your machine is working and making parts.

test the "tester" machine? ...

... client & auditors ...

Click to expand...

I’m just guessing , but I think that flyers is looking for something like a set of “must pass” and “must fail” tests for his PLC-controlled testing machinery ...



if the machinery performs a “pressure test” for some type of part, then he’ll probably have to provide a “KNOWN GOOD” sample part that can then be run through a test procedure - and must be marked “passed” by the testing machine ... even more important, he’ll probably have to provide another part which has a small but definite defect (maybe a small leak?) ... this “KNOWN BAD” sample part will then be run through a test procedure - and must be marked “failed” by the testing machine ...



if I’m right about what flyers is asking for, this is a very common procedure ... basic idea: the “client and his auditors” don’t want the PLC-controlled test machine to simply crank out one defective part after another simply because: (1) the test has been poorly designed ... or (2) the test machine is out of calibration ... or (3) something else along those lines ...



going further, if I’m right about the subject of the original post, then we need to know a lot more about the test machinery being used ... we can’t really tell how to design something until we know more about the specifications involved ...



plan of attack: consult the client’s specifications for the parts he’s buying ... once we’ve nailed down the precise range of tolerances which the customer considers to be “acceptable” - and which he considers to be “unacceptable” - then we can proceed from there ...


so ... did I come close, flyers? ...

Could be, but I would have expected the auditors to ask how they know the pressure test is correct. The bean counters usually have no idea what a PLC is, and so, would not have asked specifically about a PLC test for correct operatability, but instead asked in general terms about the entire process (QC).

If the question was indeed how do you know the PLC is working correctly, then I can only assume that either Flyers relayed the question incorrectly in his post or that some wandering bean counter subordinate trying to impress his boss saw or heard the term PLC and now wants to know how do you know IT (the PLC) is working.

I would expect that there should already be a working QC check on the process, especially something that is probably as critical as pressure testing and that by default would include the PLC and its program.

So, after all that blowhardedness, I guess I haven't got a clue as to what's needed

Establish a written, benchmark, functional test protocol with acceptance criteria. Perform the test periodically, note exceptions/remedial actions required, sign off on it, and maintain a file. In your case, maybe you can test a pressure standard periodically and verify expected and actual results are within prescribed tolerances.

You may also ask politely if they (clients / auditors) can give you some examples of what they want.

jamesau said:

You may also ask politely if they (clients / auditors) can give you some examples of what they want.

Click to expand...

... and if they really know what are they talking about.

This can be quite a tricky subject. I know certain auditors such as the FDA and MHRA require some sort compliance testing regarding all software, particularly in the medical field. This can lead to some embarrasing situations during an audit.

"How do you know if the software is working?"

"Well, it either does or it doesn't..."

This answer never seems to satisfy though.

Probably the best way to satisfy them would be to test the system through to the display with a DPI (with it's own calibration certificate), and perform annual and logged backups of the software in question.

They are generally happy provided they see some sort of control of the associated software being implemented, whether it's necessary or not.