Tek-Tips is the largest IT community on the Internet today!

Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!

Register Log in
  • Congratulations TugboatEng on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Safety question.

Status
Not open for further replies.
itsmoked

itsmoked

Electrical
Joined
Feb 18, 2005
Messages
19,114
Location
US
I'm refurbing a paper roller that pulls paper off a three ton roll of paper and spools it onto 20 pound rolls.

During the small roll setup the operator uses a foot pedal that runs the machine at a very slow speed using the VFD's jog function. Subsequently the machine switches to run mode and spools up to high speed.

I'll be using the analog out of a PLC to control the speed allowing the user to set the ultimate peak speed via the HMI.
I can obviously and simply use the same speed control to jog the system for setup.

The question is this.
Do you think this injects a hazard that wouldn't be there if I stuck with running the jog via the VFD's digital jog function.

Keith Cress
kcress -
 
Sort by date Sort by votes
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.
 
Upvote 0 Downvote
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)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
Upvote 0 Downvote
That should be OK as long as it doesn't allow dangerous operating speeds during the setup process.
 
Upvote 0 Downvote
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 -
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
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
 
Upvote 0 Downvote
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 -
 
Upvote 0 Downvote
If I am following, I propose that you complete an assessment according to: ISO 13849, or equivalent.
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
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 -
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

toothless48
  • Locked
  • Question Question
Looking for advice on a validation procedure for E-Stop buttons tied to a safety PLC.
Replies
3
Views
900
IRstuff
IRstuff
Indy
  • Locked
  • Question Question
IO list question
Replies
3
Views
467
Indy
Indy
cliff turnell
  • Locked
  • Question Question
stepper drive input question
Replies
1
Views
296
itsmoked
itsmoked
MazukFarnas
  • Locked
  • Question Question
Question about PLC education
Replies
10
Views
820
PNachtwey
PNachtwey
KevinBrown
  • Locked
  • Question Question
Safety Relays in America
Replies
5
Views
416
KevinBrown
KevinBrown

Part and Inventory Search

Sponsor

Back
Top