Prox Speed Problems

micky

Lifetime Supporting Member
Join Date
Aug 2003
Location
DeRidder, La
Posts
83
I have a prox looking at two steel targets to monitor speed on a gearbox. The prox goes to a frequency converter/intrinsic barrier which converts it to a 4-20 then sent to the PLC. Everything works fine and dandy until you get to speeds up above 2500 rpms, which then it starts to swing and settle out about 200-300 rpms short of actual speed. In other words 2800rpms reads at about 2500, with a swing every now and again closer to 2800. Speed range I am looking at it is 0-2800rpms looking at two pulse per revolution. The prox in question is the one on the yellow gearbox.

Prox P/N Pepperl+Fuchs NCB1.5-8GM25-N0-V1
Barrier P/N Pepperl+Fuchs KFD2-UFC-Ex1.D

Any ideas as to why at higher speeds I start losing accuracy?

P5210268.jpg
 
Is the signal on long enough for the prox to change state or for the frequency converter to recognize it? You might need a target that provides a longer on time.

Or you could look for a faster prox. What's you max switching speed and min ON time for the prox you're using? Making the target larger (more ON time) may help.

Along those same lines, make sure your Hz-->ma converter isn't running out of headroom either. What's it's max input frequency?



-rpoet
 
I agree that it is likely a matter of a too-small target.

The datasheet for that proximity switch suggests that it can handle 2000 Hz. Two thousand pulses per second gives a pulse duration of 1/4 millisecond (0.00025 seconds).

Let's say for the sake of approximation that it's a 16 inch diameter gearbox, with a 1/2 inch wide flag.

At 2800 revolutions per minute, the outer rim of the gearbox is moving at (2800 / 60) x (3.14159 x 16) = 2346 inches per second.

At that speed, the 1/2 inch target is in front of the proximity sensor for 1/4690 of a second = 0.00021 seconds.

That's just a little shorter than the 0.00025 seconds we theoretically need to let the proximity sensor reliably read the flag.

I could be mis-assuming a number of factors, here, but just with bar-napkin math it makes sense.
 
Micky,

Had a similar problem a couple of years ago on a centrifuge. If I recall correctly the prox was replaced and setup to close to the target. Moving it back slightly gave the prox a larger target area hence a longer on time.

Regards,
Barry.
 
It also looks like you have four targets per revolution. Two targets covering 25% of the circumference would work better. Try taking out the pins you have and milling slots in the collar so that it looks sort of like a cross.

When I've had this kind of problem, I went to a photo prox and used reflective tape on the shaft. No balance problems, so I could use one pulse per rev at 50% of the circumference. It was also a lot more forgiving on alignment and gap adjustment.
 
He mentioned it was the prox switch on the bigger gearbox that was the problem, though the hex screws used as flags on the smaller ones are pretty small targets, too.

Any screw extending from a rotating hub or collar gives me the willies; I've seen what they an do if they grab a glove.
 
One of two of Mickey's targets.

Remove each attachment screw, cut off the targets, weld on new 1" x 1" x 1/8" curved sections out of a 4" diameter steel pipe, then replace in the old positions.

Mickey's Target.jpg
 
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They didn't get the new targets put on yet, hopefully tomorrow.

Took me a week to convince them to do the same on the coupling setup. Originally on the coupling the flags were Allen screws and the prox was mounted off center horizontally and vertically so I was actually only seeing half the head. Readings were all over the place. Told them I needed something better and that is what I got. Readings are better now but still not as stable as I would like.

Back to the topic at hand,

Ken, could I use some of your bar napkins. The gearbox is actually 16" in diameter, the flags are a little smaller than 1/2" closer to 3/8-1/4. There's a guard that covers the entire gearbox and coupling area.

Barryoc, this is actually a centrifuge also.

Tom, I thought about going with photo/reflective tape but figured eventually one or the other would get covered in grease or drilling mud.

Lancie1, when can you start on the new targets?
 
Double Ken's calculation for a safety margin that should work, then double the flag width if possible. The actual on time of the flag at high speed can't be guesstimated very well by rolling it past it nice and slow making pencil marks. Distance is killer with inductive proxes. Get it as close to the target as possible for as long as possible and make it as sudden a transition as possible to fully blocked. I have used 1mm feeler gauges to set the gap on a solid mount to improve this type of arrangement where the sensing range is supposed to be 5mm. Closer is better. A bigger flag with a consistent gap is always better too.

If you can put a scope on the signal you can prove that it is in or out of the signal conditioner spec. We used Wilkerson Instruments for this sort of application with excellent accuracy, if you want to try a different signal converter.

You will likely find very short on duration periods. Consider, if the off pulses were just as short, find that frequency and compare with the converter rating. Note the off and on switching speeds of the device if specified, sometimes the sensor is your high freq. bottleneck by having a slightly longer off switching speed.

If the minimum pulse width is too short or sensor speed too slow, you will have issues as you describe (missing pulses at high speed).

If the sensor is fast enough but the on time is just too quick, the problem can sometimes be overcome with a "pulse stretcher". (A Pulse Stretcher amounts to a hardwired solid state off delay timer.)

If you use one, you may get away with a single flag until you get way slow...But it can even out the on to off ratio for a specific speed and keep the on pulses above the signal converter rating, extending the upper end speed range of the system.

Sometimes, weight is an issue for the flag size, and hall effect sensor works better with a small flag.

Sometimes switching flag materials is the ticket. Higher ferrous content materials are easier to pick up with standard inductive proximity switches.

In a pinch, I like to have a nice programmable laser PNP photocell and some $100/ft stickers laying around. Have it mounted, plug it in, set the off delay timer, apply reflective sticker to about 40% of the flag frequency, in just a few minutes.

Prepare the surface well to set the adhesive well if you use peel and stick polarized retro-reflective material. Polarized is important near shiny metals or volatile ambient lighting. I clean the surface with sandpaper if necessary, crosshatch if possible, alcohol, (primer if called for).

Do this for high speed and/or semi-permanent installation. It weighs practically nothing and overcomes signal pulse width issues and distance to sensor problems.

EDIT: Keep two targets, looking at pic., photocell and million dollar tape on the orange part (throw away the flags) would be sweet. Sensor must be fast though will co$t extra for 100 microsecond speed or anything faster than 250 in my experience.

http://www.bannerengineering.com/en...oelectric-Sensors/67/PicoDot-PD-Series/#specs
 
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Lancie1, when can you start on the new targets?
I could have made them for you - 5 years ago. Now I sold my welding equipment and most other tools, sold the farm, moved to town, retired to a life of ease.
 

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