flexiforce sensor resolution/specs
steve_b
Posts: 1,563
Hey guys (in particular Parallax)...
I'm interested in the Flexiforce sensor.....I need to measure the contact pressure of some slipring brushes we use in our radars.
There's always been a "feel" approach with installing these things, and some people put them on too heavy and others too light....in one case we worry about aggressive wear and scouring of the contact plate and in the other, not enough contact and loss of signal (obviously this isn't as bad as destroying a plate).
Anyhow, I took a look at the flexiforce on Parallax's website and on the webpage it states: "- 0V is no force and 4.2V is 100 lbs".
Looking at the datasheet....Parallax's calls it a 1.6lb rate device when using the indicated circuit.
What's the lowest reading this circuit could measure?
In the footnote it says that a full-scale reading could be attained by changing driving voltage and feedback resistors....they indicate the fullscale reading to be 1lb-1000lbs.· Can it be made to have a smaller reading?
Essentially, I'm looking to measure sub-1lb forces.· Not quite sure what....but using a torsion gauge a while back, I believe we were looking at around 100grams/in2 or something like that.· I've no idea how this computes to lbs/in2 but t'is small!
Just looking at the - 0V is no force and 4.2V is 100 lbs statement....if it's linear...1lb = 42mV!· this is pretty small on it's own and noise would certainly be an issue.· Now, we don't necessarily care that the readings show an accurate weight reading....but we would like to keep it repeatable so we could take relative measurements from contact to contact.
OK...just found a converter program....1lb force is equivalent to 453.5924 gram force.· I'm assuming these are the units I'm interested in....
So, for 100 gram force....I'd be looking at around 0.22 pound force....so just under a quarter pound (with cheese?).
Any suggestions would be appreciated.
**the flexiforce sensor looks to be small enough to slip in between the brush and contact....other "in situ" testers are somewhat larger and not useable.
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."
I'm interested in the Flexiforce sensor.....I need to measure the contact pressure of some slipring brushes we use in our radars.
There's always been a "feel" approach with installing these things, and some people put them on too heavy and others too light....in one case we worry about aggressive wear and scouring of the contact plate and in the other, not enough contact and loss of signal (obviously this isn't as bad as destroying a plate).
Anyhow, I took a look at the flexiforce on Parallax's website and on the webpage it states: "- 0V is no force and 4.2V is 100 lbs".
Looking at the datasheet....Parallax's calls it a 1.6lb rate device when using the indicated circuit.
What's the lowest reading this circuit could measure?
In the footnote it says that a full-scale reading could be attained by changing driving voltage and feedback resistors....they indicate the fullscale reading to be 1lb-1000lbs.· Can it be made to have a smaller reading?
Essentially, I'm looking to measure sub-1lb forces.· Not quite sure what....but using a torsion gauge a while back, I believe we were looking at around 100grams/in2 or something like that.· I've no idea how this computes to lbs/in2 but t'is small!
Just looking at the - 0V is no force and 4.2V is 100 lbs statement....if it's linear...1lb = 42mV!· this is pretty small on it's own and noise would certainly be an issue.· Now, we don't necessarily care that the readings show an accurate weight reading....but we would like to keep it repeatable so we could take relative measurements from contact to contact.
OK...just found a converter program....1lb force is equivalent to 453.5924 gram force.· I'm assuming these are the units I'm interested in....
So, for 100 gram force....I'd be looking at around 0.22 pound force....so just under a quarter pound (with cheese?).
Any suggestions would be appreciated.
**the flexiforce sensor looks to be small enough to slip in between the brush and contact....other "in situ" testers are somewhat larger and not useable.
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."
Comments
Tekscan, the makers of the Flexiforce sensor, have a 1 lb, 25 lb and a 100 lb sensor available form their websight. I used the 25 lb sensors to measure the force applied to tubing when compressed by pinch valves used in a medical product. I calibrated the sensor with a calibrated weight set and a custom milled plate that applied force to the whole pad of the sensor. The whole sensing pad needs to be utilized in order to be accurate. If only a part of the sensing pad is used then all bets are off as to accuracy. Pinching between two sharpe points can ruin the sensing pad. I have PIC basic code for a 16F819 PIC and can draw a schematic for the system I built. The code can easily be ported to basic stamp with an external A/D converter.
Jason
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He who dies with the most toys wins!
The boss has asked me to order one of their "variety" packs (with 8 different rated ones) for us to play with.
I might pick your brain down the road wrt code....but I think our original intention is to measure the force using a multimeter.
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."
First, good luck with your project, depending on what you're trying to learn about your slip rings, and depending on how mucy the sensors have progresses, you may, or may not see meaningful and repeatable results.
At my former employer (a carbon brush manufacturer), we tried using the sensors about 10 years ago or so to do exactly what you're trying. I believe National Carbon actually came out with a device that they sold (I didn’t see it on their current web site) based on the sensors.
I'm not trying to discourage you, and repeating, this was 10 years ago or so.
What we found was that the results were not repeatable. We found the force measurments from the sensors varying by over 100% on repeated readings from a single brush. We found two things that were major factors in this variance. The first (and probably most significant) was that because the sensor had to be put in place between the brush and commutator/slip ring the equipment had to be turned off. This meant that you were doing a "static" measurement, and friction of the brush in the holder, side loads of the spring (especially when you have a beveled top, hysteresis in the spring, etc. could all contribute to differences in reading. Just pulling the spring off the brush and gently setting the spring back down would result in wide variations.
The second factor was the sensor itself, especially on a curved surface. Jason’s comments correlate with our findings. The mating surfaces of the brushes and commutator/slip ring don’t provide for an even “full sensor” contact. We made a sand that put the sensor between two curved surfaces (12” Dia) and had a force stand (with strain gauge) apply pressure. We did not get repeatable results.
Of less concern because it could be calibrated out, was the variation from sensor to sensor. I suspect that this is less of an issue now, but I believe (from memory) that the variance between sensors was like +/- 20%.
As any rate, the net result is that we scrapped the project as being unfeasible. Our opinion was that you could get results, but the results were not meaningful.
I believe there was another thread on this topic and specific application where I shared these comments and there was additional discussion.
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John R.
8 + 8 = 10
I realize yours is a truly professional application, and what I'm about to propose in the second paragraph, is more in line with the hobbiest answer to your quandry. However, just for the halibut, see if these thoughts bring anything else to mind, or possibly set you on a path to a different type of product, just as an alternative to the Flexforce Sensor.
You may find it interesting that the black sponge-like foam in which many IC's are packed will show a rather linear resistance response to pressure. The more the pressure, the less the resistance (if I remember correctly, but check it). So long as the foam isn't completely deformed and crushed, it will return to its original size, or the size of its original "container", and will respond again to second and additional applications of presure. I know there are plastic-like substances which operate in a similar manner. I just don't know how linear their response is. Additionally, Plasti-Gauge (I think that's the name of the product) will give you a visual reading of presssure, but must be removed to be read.
Above and beyond the Flexforce sensor, there are other peizo materials that will also present a voltage upon deformation, and spring back to their original shape. Check for "piezo force transducers" with Google or your favorite search engine.
The last item that may be of use to you, is the strain gauge. Check those out with your favorite search engine as well. They generally work on a Wheatstone Bridge type principle.
I hope one of those thoughts is helpful.
Regards,
Bruce Bates
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One of our main hurdles is the small area to work in. The slipring platters are 5/8 inch apart with the brushes being 1/2inch in size (when new)...so there doesn't offer a lot of space to get a sensor in.
I tried to do a search for it...but couldn't quite find what I wanted....but we used a small handheld dynamometer gauge (I wonder if I'm googling the proper spelling). Essentially it has a little arm that you apply force to and a needle on a gauge deflects and indicates a certain force applied. In our case we were looking at gram forces.
We have double sets of brushes on our sliprings....this gauge can fit in and measure the force on half of each set, but can't fit in for the others.
So we need something with a smaller footprint that could get in there....
We'd looked at the piezo's (particularly the one Parallax offers) and figure it's just too big to slip in.
The flexiforce sensor looks like it can be trimmed slightly and is thin enough (300mils?) to fit in and allow us to measure a brush.
I figure some trial and error will be needed in order to get the brushes to sit right in the middle of the sensor. I've read that they really don't work well with lateral forces or sharp objects....work didn't mind forking out a few bucks to try this (really not that $$)....and if it works it'll allow us to extend the life of these brushes (which cost upwards of $2kcanadian per set...youch).
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."
we ordered the variety pack of 8 sensors (4x1lbs;2x25lbs;2x100lbs)...we only really need the 1lbs for our current needs.
Anyhow, the idea was to slip these little sensors under some conductive slipring brushes in order to ascertain what their pressure was/is.
Current methods of installing these brushes was by "feel" and being one with sausage fingers, I don't have that kind of 'feeliness' haha
So, I've gotten our sensors and have been playing with them.
The spec sheet says their conductance is linear (or near-linear).· So, if you were to increment their force on the sensor and plot the resistance change, you'd get an exponential graph (can't remember what the name of the curve actually is), but if you inverted the resistance you'd get conductance.·· So, conductance (in mho's) is 1 / resistance (in ohm's)·· [noparse][[/noparse]notice that ohm and mho are reverse spellings of each other...and also the opposite physics-wise]....
the data sheet says that you need to pre-condition the sensor by applying 120% of it's rating...so a 1lb sensor would have a 1.2lb weight on it for "some duration"...and thus repeated 4 or 5times.· I assume this is to make the resistive paste more uniform across its sensing area.
Well, I did this, then I set up a little test-station on a machined flat surface (within 2thous) and used a gramm dynamometer to apply force.· Since the end of the force meter was a dull point, I had to use an old electrical box knockout to distribute the force over the entire sensing area.
I applied a range of gramm forces from 0 to 250 using a resistance meter set to record the minimum reading....well, I tried this a number of times and plotted the resistance/conductance graphs and couldn't quite get the same readings repeated nor a truly smooth graph.
My issue, at present, are the way I apply the force...it's a hand operated force gauge, so if you have too much coffee, you may overshoot your intended reading (wish I had those high school scale weights -- to buy them today makes you look like a 'dealer').
Anyways, I mucked with this for a while and eventually received our slipring assembly.· I "pre-conditioned" the sensor and went ahead and placed the sensor under a few brushes.· Surprisingly I was able to get some repeatability (assuming the brushes are fairly consistant in their tensions)...this may be explained as the slipring brushes have a wide surface area and that the knockout I was using previously was wider than the sensor and may have been touching the test surface and relieving some weight.
Anyhow, still working on it....here's some pics of what I'm working with...excell graph included!
Look at the slipring photo and you'll see the small silver brushes....they are on small leaf springs and everyone has a different idea of how to install them for tension/wear.· Anyhow, I'm just after a way to get a proper value to shoot for!
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."