Force measuring device // Flexiforce or Piezo Vibra Tab??
steve_b
Posts: 1,563
Hey guys,
Some of you might recall that I work on Weather radars.
Well, some of the smallest things on these things can really screw it up.
We have a multiplatter slipring assembly (this is a way to get electrical signals to the top of a turning turret -- wires would get tangled)....
Well, we have issues with the wear on our slipring brushes.
It's a bit of an art on aligning them. There's about 40 'leaf spring' style brushes that have to be set up in blocks...which means that one brush will have more pressure than another, which means some wear more than others.
The weight difference is so slight that it's hard to "feel" with the human hands (ok, I'm like a bull and have notion of 'nice and easy'!)
So, the boss asked me if we had a way to measure the pressure the brushes are putting on the platters (in hopes that we could get them all at a uniform force).
I looked at the Flexiforce and the Piezo off of Parallax's website and like both.
The flexiforce is thinner and offers an easy RCTime interface.
The Piezo Vibra tab has a better package (in taht the flexi has an 8" ribbon type wire--hard to move about) but I'm concerned about the potential high voltages that they could output (up to 70V at a 90deg bend) and would also require an A/D (probably)....or could be used as the power source in an RCtime cct.
Blah blah blah...haha I make short story's long!
Does anyone have experience with either sensor?
Would be interested in resolution/force!
thanks
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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
Some of you might recall that I work on Weather radars.
Well, some of the smallest things on these things can really screw it up.
We have a multiplatter slipring assembly (this is a way to get electrical signals to the top of a turning turret -- wires would get tangled)....
Well, we have issues with the wear on our slipring brushes.
It's a bit of an art on aligning them. There's about 40 'leaf spring' style brushes that have to be set up in blocks...which means that one brush will have more pressure than another, which means some wear more than others.
The weight difference is so slight that it's hard to "feel" with the human hands (ok, I'm like a bull and have notion of 'nice and easy'!)
So, the boss asked me if we had a way to measure the pressure the brushes are putting on the platters (in hopes that we could get them all at a uniform force).
I looked at the Flexiforce and the Piezo off of Parallax's website and like both.
The flexiforce is thinner and offers an easy RCTime interface.
The Piezo Vibra tab has a better package (in taht the flexi has an 8" ribbon type wire--hard to move about) but I'm concerned about the potential high voltages that they could output (up to 70V at a 90deg bend) and would also require an A/D (probably)....or could be used as the power source in an RCtime cct.
Blah blah blah...haha I make short story's long!
Does anyone have experience with either sensor?
Would be interested in resolution/force!
thanks
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
·
Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
Comments
This seems really rather tricky, without attaching a genuine strain gauge to the back of each brush. How big are these brushes?
(I can't quite imagine where you're planning to install the flexiforce sender, and I worry about its long-term stability under a fixed load)
Would it be possible to throw a beam of light onto the back of the leaf spring, and the more it's bent under pressure, the more the light's deflected? I know it's not measuring the pressure itself, but it may be less invasive, if you can get away with it.
(It's possible I'm imagining something completely other than the real problem, though - any chance of a photo / sketch?)
Steve
the slipring assembly looks like a bunch of large hard drive platters!
Here's a quick pic...hopefully it's viewable!·
We couldn't leave the force sensor in place while operating.· The platters spin and would just wear the sensor right off, plus, it interferes with an electrical connection, so our radar would go in to alarm and wouldn't run anyhow!
What we'd do is, upon inspection or replacement of the brushes, we'd insert the sensor to get a reading on the amount of pressure each brush was putting on the platter.· Any brushes that were out of tolerance (too light or too heavy a force) would be adjusted.
We've used dynanometers (hope I remembered that word right) to measure the gram force before....but didn't really understand what we were doing...if you measure the force at the end of the leaf spring, the pressure would be different than if you measured it closer up to it's holding point....
So this would seem like a way to measure the force that's actually being applied to the platter....this is all relative of course.
The image I attached is our Mark2 design.· We've since removed all the brushes that you see on the left handside....as they were just redundant and were causing excessive wear of the slipring.· But the large block at the top is our 'Power block' (larger voltages/currents for the elevation motor)....most of the other contact points are just for signal lines.
So, with our Mark 4 design (half the brushes you see, exluding the power section) we'd have the room to get a small sensor in there for checking....
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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
Aah, that's the bit I was missing. Couldn't see how to measure the contact pressure without putting the sensor in the gap [noparse]:)[/noparse]
Last time I fancied measuring force without resorting to strain gauges (which, despite being a pain to use, do work rather nicely), I got some pressure-sensitive paint. Strange pink stuff, sort of worked, but really wasn't very stable, sensitive or reliable. Gave up in the end... Looks like both of the Parallax devices are much better candidates, but I've not tried them - best of luck!
Steve
I used to work at a place that manufactured carbon (and metal) brushes and brush holders. Depending on who made your actual slip ring assembly, it is quite possible that we made the brushes.
We tried looking at the flex sensor technology for measuring brush contact forces, and found them not very accurate in the application, and that was with larger brushes and higher forces. As a general or realitive measurement they were OK, but the inconsistantcy in measurement was greater than the difference in forces we were trying to measure.
We had the best luck using the good old fashoined spring scales. (For the larger size brushes, we used an electronic "fish scale".)
If the slip rings (and specifically the brushes) are large enough, you might also be able to retrofit or redesign them with a constant force brush holder.
I hope you find the information useful. If you would like more details on any of this, please let me know.
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John R.
8 + 8 = 10
What kind of currents are we talking about here? Are you transferring just data or are there some current or RF circuits involved?
I've worked on a couple of projects where it made sense to only transfer the power circuits by brushes and all the data came off via a short range data radio. Still leaves you with brushes, but fewer of them.
As for brush wear, we found brush wear was influenced quite a bit by the current transferred by the brushes. At BOTH low and High currents, brush wear was much greater than at the optimum amps/sq. cm.
On the big generators I worked on (before the days of brushless excitors) the solution to brush wear was a "constant tension spring" which kept the pressure constant throughout the life of the brush.
Cheers
The "leaf spring" method of loading brushes, while popular with this type of application is also one of the more inconsistant ways (in terms of force).· There are too many variables involved, and current going through the leaf spring (and heating it up) can affect things even after the installation is "tweaked".
I was involved on the manufacturing side, and latter IT.· For a while I actually managed a small division that did the "leaf spring" type assemblies with contacts on the end, usually a high silver content metal alloy.· Our main brush divsion also made brushes used on slip ring assemblies, and some small holders for constant force springs in this type of application.
Using torsion springs is also an option.· The effects of spring rate can be minimized by having severa slots or pins to move the "non-brush" end of the spring as the brushes wear.· This was done, and still is done, on some types of larger brush holders.
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John R.
8 + 8 = 10
We can't trust a wireless 'signal' path as some of our interlocks travel on the sliprings too....most of them are failsafes (if signal is lost/dead, then everything stops)...but it would also mean that there would be operational outages which is a bad thing to have during a storm!
FWIW, we cover Toronto, Ontario....big political issues if an ice storm drops onthe city and the radar wasn't working....
We've been in contact with some engineers about our slipring brushes and some of our other motor brushes....
For us, we rely on a layer of silver oxide dust (from the brushes) that gives us some friction relief and keeps us in contact. A perfectly clean surface will actually cause severe wear! inversely, a severely dirty (lots of dust) might build up and cause shorts with the next platter.
So it's a bit of a game....you don't want human fingers getting in there and "tuning" things...as this usually increases wear and faults....so the balancing act of "just enough" is what we're after. And a starting place is determining how much pressure is the right amount of pressure!
Another method for passing signals in rotating platforms is, instead of using a brush, using a ballbearing type holder.
A small gold (I think) ball sits in a cup that is tensioned and pushing against the ring contact....the ball rolls and it's rather nice. $$ though!
Anyhow....we'll see what happens!
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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
www.tekscan.com/industrial/iscan_handheld.html
kelvin
Sorry didnt read all the the post
Looking at the picture of the multiplatter slipring assembly.
Does the shaft go to the bottom of the housing
or does it stop halfway down the housing were your hand are ?
I cant tell by looking at the picture?
japer
There are 2 crossbars along the bottom of the slipring assembly that tie in to the pedestal (big yellow steel structure).
The pedestal has an upper and lower bearing that allows the inside portion of it to spin (so the platters spin and the brushes stay still).
You can see the small gap at the top of the assembly....this is where the upper portion rotates.
The only thing going down the center of the slipring assembly (as it's like a donut--hollow in the middle) is the waveguide.
The waveguide has to have a rotary joint so it too can spin freely.
Not sure if that answers your questions....
cheers
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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
the reason i asked is because the lower part of the shaft dosent seem
to be supported
i wonder if the shaft spins at a constant speed or are there variations in
acceleration and deceleration, which could cause symetric wear on the
brushes, if thats the case
then a vibration type of sensor on the main shaft would work...
to see if their is any harmonic vibration.
also if their is no coating on the platters then maybe you could wipe some prusion blue
dye on each platter to make sure the wear is even ,with the old brushes installed.
the brushes are proably resting on springs and can absorb a certain amount of flex
but there will be a price, wear.
this is probably the best design for this application a compromise because of the high voltage judging by the wire size.
it could be a slightly warped shaft or platter causing premature brush wear.
japer