What is your nomenclature for 4-20mA speed signals?

defcon.klaxon · Apr 13, 2021

Hi guys,

For the longest time I've been calling the 4-20mA loops I use for controlling pumps via VFD "command" for the signal that tells the VFD what speed to run at, and "reference" for the signal that indicates current VFD speed.

However when working on an Allen Bradley VFD, their nomenclature is "reference" for what I call "command" and "feedback" for what I call "reference". The electrician and I were talking about it, and he said he usually sees "reference" used as "command". Thus, I wanted to ask what you guys typically see.

ASF · Apr 13, 2021

I would view "reference", "command", and "setpoint" as referring to the "required speed" signal from PLC to drive, and "feedback" or "process variable" for the actual speed signal from the VSD back to the PLC.

Typically I use "reference" and "feedback", exactly as AB's terminology - but that may be because I primarily use AB and have adopted their terminology.

jhenson29 · Apr 13, 2021

I generally use “reference” for the setpoint and “feedback” for the process variable (if I’m not using “setpoint” and “process variable”).

But I don’t think it matters much as long as what you use is both clear and consistent.

OkiePC · Apr 13, 2021

jhenson29 said:
I generally use “reference” for the setpoint and “feedback” for the process variable (if I’m not using “setpoint” and “process variable”).

But I don’t think it matters much as long as what you use is both clear and consistent.

Me too, but I have more and more been using the word "command" along with (or in place of) "reference" when describing the analog input to a drive/valve. "Feedback" is pretty clear, but "reference" might be misunderstood. "command" or "command reference" is usually well understood.

drbitboy · Apr 13, 2021

I don't see how "(speed) reference" can refer the "feedback" signal, but that, i.e. that I don't see how, only proves OP's point, because obviously some people do.

Steve Bailey · Apr 13, 2021

I've found that when explaining things to others, the term, "reference" is less intuitive than "command". If I use the term, reference, those who have spent time working with drives understand the meaning better than those who haven't. If they ask, "what's reference mean" I'll say "speed command" and they'll understand. When I use the term, command, I'll generally include an adjective and say "speed command" or "run command" or "forward command".

defcon.klaxon · Apr 13, 2021

drbitboy said:
I don't see how "(speed) reference" can refer the "feedback" signal, but that, i.e. that I don't see how, only proves OP's point, because obviously some people do.

I think it's just a matter of what "seems" right to various people. For me, "command" is obvious (as I'm sure it is with everyone else). "Reference" for "feedback" makes sense to me because I've always thought of it like a reference manual; it's read only, you grab a reference manual to acquire specific information if that makes any sense.

"Feedback" to me is not interchangeable with other analog signals coming from analyzers/sensors, since "feedback" is an engineering term where you've commanded something to be at a certain position and feedback is telling you where it is. So what would you all call a signal from a sensor?

jhenson29 · Apr 13, 2021

OkiePC said:
Me too, but I have more and more been using the word "command" along with (or in place of) "reference" when describing the analog input to a drive/valve. "Feedback" is pretty clear, but "reference" might be misunderstood. "command" or "command reference" is usually well understood.

Again, I think as long as it’s clear and consistent, it doesn’t matter much. I’ll cry much more over inconsistency or completely meaningless names than than I will over specific names.

But “reference” makes more sense than “command” to me in the context of a speed control loop. I would associate a command with telling the drive to perform a task (like run).

E.g. if a drive has 50% value for the speed loop control setpoint and no run command, I think it makes more sense to say the drive does have a reference but does not have a command.

If a drive has a run command with an 0% speed loop setpoint, I would say it has a command with a zero reference.

drbitboy · Apr 13, 2021

I agree that feedback is pretty obvious that is passing information in the motor=>VFD=>PLC(=>scada?) direction, but even so the feedback is from the motor to the VFD that is doing the control. The signal from the VFD back to the PLC is not what I would call real feedback, even though it is in the right direction.

For reference, the process cannot provide a reference: the control systems provide the reference that the process has to match.

Anyway, opinions are like noses (and other body parts ...)

jhenson29 · Apr 13, 2021

drbitboy said:
I agree that feedback is pretty obvious that is passing information in the motor=>VFD=>PLC(=>scada?) direction, but even so the feedback is from the motor to the VFD that is doing the control. The signal from the VFD back to the PLC is not what I would call real feedback, even though it is in the right direction.

For reference, the process cannot provide a reference: the control systems provide the reference that the process has to match.

Anyway, opinions are like noses (and other body parts ...)

I think you need to localize your context more. E.g. a control variable in one context can be a setpoint in another.

A variable doesn’t have to make sense as just one thing in the context of the system as a whole. It has to to make sense in the context of its use. In this regard, “feedback” can be used at many levels.

And the process can provide a reference. If the drive participates in a process and the process creates a signal and the signal is used as the setpoint for the speed control loop...well...there you go. In this case it’s a reference for the speed control loop, but I may also pick up the signal as feedback for another process in which case the signal itself is either a reference or a feedback depending on context.

RET · Apr 14, 2021

Command and feedback

drbitboy · Apr 14, 2021

Originally Posted by drbitboy

For reference, the process cannot provide a reference

jhenson29 said:
I think you need to localize your context more.

I should have been more explicit: the VFD, which is the process in the context of this thread, cannot provide a reference, external from itself, to itself.

I understand cascades. My context was that of the OP, which is PLC talking to VFD; reference information only flows in that direction.

Even in your response you use the word reference in the way I am understanding it.

daba · Apr 14, 2021

What's wrong with SP and PV, like we use in other control loops.

It shouldn't matter whether it's temperature, pressure, flow, level, position, altitude, azimuth, or anything else, it all boils down to setting a desired SetPoint (SP), which is measured by some sensing device or calculated feedback, to give a Process Variable (PV).

Why can't speed or velocity be included in that not-so-private club ?

jhenson29 · Apr 14, 2021

drbitboy,

I disagree. I think if I have a VFD setup with a positive feedback loop because the process feeds back into the reference to keep the process at a limit set by another device, then the reference comes from the process.

dogleg43 · Apr 14, 2021

Isn’t this kind of a point of reference thing? Kinda like using the terms upload and download?

What is your nomenclature for 4-20mA speed signals?

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