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Topic: Measuring Stretch (Read 2425 times) previous topic - next topic


  Hi all,

  I am looking for a way to measure how much something has stretched.  I am probably looking for something like a strain gauge, but one which will allow for motion over a relatively long variation in length (something like 50-100mm, ideally with about 1mm accuracy). 
  Does anyone have any experience with something like this?  More generally, does anyone have any advice on a good source for inexpensive "hobby" grade strain gauges?

CrossRoads

I've seen these things in hardware stores called retractable tape measures ...

Since I imagine you want the arduino to read this tho, perhaps a wheel that turns as your something moves, and you read the amount of wheel turning.
Could even have the wheel be on a potentiometer shaft, with the pot set up as a voltage divider, and read the voltage - then have a  lookup table to see what distance goes with a voltage.
Need a wheel that is 100mm over ~270 degrees of rotation, so ~135+mm total circumference.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

retrolefty

A strain gauge measures force not travel, plus strain gauges don't have any travel or give at all to speak of.
To use a strain gauge you would have to couple it with a suitable spring that can handle the physical travel distance and transfer the stress force to the strain gauge.

Lefty

tkbyd

Deep within....

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1276355492

.. an excellent discussion of various possibilites developed.

There are devices called "linear potentiometers" which would be fine, if the mechanical aspects are suitable to your needs, e.g...

http://www.celesco.com/linearpot/

(The term is also used to speak of a pot with a linear change in resistance, though, so you'll get some "false positives" out of Google.)

mowcius

Possibly some of this:
http://www.inventables.com/technologies/stretch-sensing-rubber
Can do 100% elongation and is probably relatively precise. The issue would be measuring the resistance. Using a voltage divider would work but you might want to use an external ADC (as the 10-bit one on the atmega chips isn't the greatest for precision) and an accurately measured resistor for the other side of the voltage divider.


  Thanks for the good tips.  For my use, the "Stretch Sensing Rubber" seems to be exactly the right thing.  At $13 a strip, that is a little steep, but I may be able to get by with it.  The option of using a strain gauge with some kind of elastic was the more obvious choice, maybe I should have some kind of bake off between the two.  Thanks for the tips though, I think I will order some of this rubber stuff and see how it works out.

mowcius

Well the prices on that site are a little steep but they are generally 'sample' materials rather than something you'd buy a lot of from them. If you wanted a few hundred of something you'd contact the people who make it and probably get something custom a bit cheaper (as it'd be in bulk).

They have some interesting materials and it's one of the few places you can get little bits of things to play with.

CrossRoads

So you will be measuring the stretch sensing rubber as you stretch it in parallel with the "something" you began with?
Seems plausible. Perhaps even repeatable.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

mowcius

Quote
Seems plausible. Perhaps even repeatable.

Sounds easy enough to me.
Resistance change presumably is linear so just measure the limits, pick a resistor, do some minor calibration and you're away.

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