I don't think your PLC interface to the drive adds any particular danger by itself, and in fact I would prefer that the HMI/PLC control should be the only source of VFD run signals, whether run/stop or speed control (E-stop excluded). I'm trying to envision your process. I don't mean to digress outside the area of your question, but what kind of guarding is there, to keep personnel out of harm's way, if I may ask?

Brad Waybright

It's all okay as long as it's okay.

In aerospace & defense, the preference is more KISS for mission critical systems; hardwired > FPGA > firmware > software. Anything that can be overwritten or interrupted or suborned has to be deprecated in favor of something that can't change. Stuff that doesn't run in memory that's subject to OS idiosyncracies or other software munching on memory are favored.

TTFN (ta ta for now)
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That should be OK as long as it doesn't allow dangerous operating speeds during the setup process.

Hello thebard3. Thanks for your input.

Coaxing the paper web onto the new cardboard spool is a moderately hand accomplished task, poking and squeezing and guiding and tucking the (approximately 4 foot wide) paper onto the empty cardboard spool. This occurs with the Jog function and the take-up shaft turning about 60rpm driven by the operator's foot pedal. e.g. the operator is in the paper-space which is a shaft in front of him/her. Once the paper is sufficiently started a two-handed set of buttons need to get pushed within 300ms of each other, demonstrating the operator's hands are out of the way. Directly above and slightly beyond this take-up roll is a cable E-Stop spanning the machine. Push or pull on the cable and the machine will brake to a stop. I've also added an E-Stop to the operator's panel to the operator's immediate left where the power switch and the HMI are located.

Hi IR; This is a relatively simple PLC program that will ramp the analog out to the VFD to do the major spool wrap and I'm contemplating the same system ramp only to the slow jog speed too.


Lionel; I'm really just contemplating the issues of, say, noise on the speed signal causing an unexpected speed-up during the jog phase. I suppose using a fixed pot on the front panel (the original speed setting) could have the same issue too, so no real increase in the hazard.

Keith Cress
kcress - http://www.flaminsystems.com

Sounds like you have appropriate guarding and safeties. Given that, I think neither method you described in the beginning is more or less safe than the other, so go with what you're most comfortable with.

Brad Waybright

It's all okay as long as it's okay.

So is the foot pedal just a switch to command the pre-set Jog speed in the VFD and you want to create a variable jog speed by virtue of the analog input to the VFD? If so, I can see a potential problem in ensuring the limits to that Jog speed. That's what the VFD's "Jog Speed" setting is really doing for you; taking away the ability for an error in the analog circuit that would cause a lurch when it should be a slow roll.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

Jeff... That's what I was concerned about. The setting in the VFD is a 'dumb' programmed value that just gets yes or no'd with a digital input (digital OUT from the PLC) limited to 3Hz or whatever. Whereas the full authority analog in for the VFD is only limited by whatever MAX freq-out was set to. Maybe an employee wrapper.

It's an old drive that will likely have to be replaced soonish (Reliance GP2000) and I was thinking a single analog in would carry over the easiest but thinking about it they all have digital jogs with set jog speeds so any replacement will likely work with little change.

Thanks all!

Keith Cress
kcress - http://www.flaminsystems.com

If I am following, I propose that you complete an assessment according to: ISO 13849, or equivalent.

I'm not sure I'm completely following what you mean, but if there's a Pot involved then I'd want a short in the Pot or the circuitry to only be able to generate the 60 RPM max that you're talking about. I wouldn't be comfortable with a pot failure causing a high speed event.

Thanks PersonalP. Not happening.

John, I am not using a pot this time around. I want all mechanical failure stuff minimized. Instead the PLC will be generating the 0-10V via a dedicated analog-out module set by a touch-screen setting. I've decided I'm comfortable with this, after all, because I'll be limiting the actual output by:
1) The HMI has the ability to limit the allowed speed setting during entry.
2) I can limit the output in the PLC with a standard "higher than X? : Set to X".
3) This is going out to a VFD where I will instruct the VFD to limit the output frequency to the maximum safe speed value. This is a fundamental limit that doesn't care what speed value is received.

Keith Cress
kcress - http://www.flaminsystems.com