"The signal must have good quality"

KingPresident · Dec 18, 2015

I'm building an And function block for pre-process standby requirements. One of the customers written requirements is "Motor control must have good signal quality." I will be using a standard 4-20mA loop coming back from the VFD. I just don't how to code for that... I'm not asking to be spoon fed, just looking for a pointer. Thanks in advance.

Tom Jenkins · Dec 18, 2015

That's kind of like saying you support motherhood and the flag. It really accomplishes nothing.

KingPresident · Dec 18, 2015

That's what occurred to me. Either a signal is present, or not. The logic will work itself out. I'm fairly green and wanted some opinions.

Steve Bailey · Dec 18, 2015

There are "quality" codes associated with OPC data exchanges. The spec could be interpreted as meaning that the "quality" of any communication channel involved in motor control must be monitored. I'm not an expert in OPC, but google "OPC quality" for more information. Then go back to the customer and ask for clarification.

Saffa · Dec 18, 2015

Ask the client to clarify. This may be more of a physical wiring requirement, i.e. using screened instrument cable that is properly grounded.

Or it could just be the usual copy-paste waffle that some consultants love.

KingPresident · Dec 18, 2015

Good enough! Thanks guys.

Ken Roach · Dec 18, 2015

It makes sense to me that a pre-process standby requirement would include checking the signal quality of the feedback loop.

Since it's a 4-20 mA loop, an open circuit will give you 0 mA and you'll know your logic should not go past standby.

If they were requiring this for discrete wiring, you would probably have to use an I/O module that did some diagnostic pulse-checking.

ASF · Dec 20, 2015

I think Ken's on the money. It could also be from the perspective of talking to a VFD via comms of any type - it's one thing to say "my Drive Healthy bit is on", but you also need to know that the data from the drive is valid - or "good quality" if your spec prefers

. A "Drive Healthy" signal is only any use if the drive is online and giving you correct, up-to-date information

Tom Jenkins · Dec 21, 2015

Here's the problem with this requirement: "good quality" is not defined in measurable terms. Is it referring to voltage level for discrete signals? Is it referring to wiring practice? IS it referring to mean time between failure? For analog signals is it referring to % deviation from nominal value? Is it referring to EMI/RFI as a % of signal voltage?

A standard that is not measurable is not enforceable. A standard that is not enforceable is useless.

Paullys50 · Dec 21, 2015

tom jenkins said:
a standard that is not measurable is not enforceable. A standard that is not enforceable is useless.

+1

KingPresident · Dec 21, 2015

Update: After some conversation, the "Good Signal Quality" is simply a matter of coding for sensor error. Example: My device will only put out 4-20mA. If I see a reading out of bounds, say 3mA or 24mA, then stop process. It's just a comparator... Learning is fun. Thanks for the input.

rguimond · Dec 21, 2015

It may be difficult to detect a signal outside 4-20mA. Most sensors need the first 4mA to operate and most (probably all) 4-20mA analog cards I've worked with ready full scale @ 20mA.

MrAnonEMoose · Dec 21, 2015

*whispers from the shadows* +1 for traditional DCS.

ASF · Dec 21, 2015

rguimond said:
It may be difficult to detect a signal outside 4-20mA. Most sensors need the first 4mA to operate and most (probably all) 4-20mA analog cards I've worked with ready full scale @ 20mA.

Most analog cards I've worked with do at least 3.5mA to 20.5mA. It is important to check though, because I've seen plenty of times where someone has defined "below 3mA or above 21mA is a fault", because the sensor outputs 21.5mA on fault (or 0mA in the case of a wire break). But then their card only ranges from 3.5mA to 20.5mA, so sure, that transmitter will do it's darndest to tell you there's a problem, but you'll never trigger the fault!

"The signal must have good quality"

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