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Conductive Rubber Cord: frustrating
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Conductive Rubber Cord: frustrating

by didier on Mon Jul 30, 2012 10:04 am

May be I was too much to full of hope when I ordered two pieces of conductive rubber cord in order to build a stretch sensor.
I knew the principle from a long time ago: while I was a student, I attended a lecture from a guy who presented the polymerized elastomers, concluding his speech by "Well, to be honest, we do not know yet the applications!" (Sounds like the invention of the "Post It")
I went straight to a "scientific" approach of measuring the performances of this rubber band. We used a 12 cm piece of rubber and placed contacts spaced by 9 cm. Then we stretched the rubber band by means of 5mm steps up to a +3cm elongation. The plotted results were really odd! We re-executed the test with no more success. Executing the test in a back and forth manner showed us that some "memory" effect was involved (covered in a post from this forum). I also tried to use various types of contacts between the rubber band and the measuring instrument, based on the thought that the pressure from the alligator clips could interfere in the resistance. No way. I will not even show the plots which are useless.
As I needed a quite instant response from the stretch sensor, I decided that the game was over.
Remains that the band can act as a special type of potentiometer, as long as the wiper does not presses the rubber band hard.
Any more thoughts or lessons learned from your side?

didier
 
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Re: Conductive Rubber Cord: frustrating

by adafruit_support_bill on Mon Jul 30, 2012 11:55 am

My experience with it was similar. Non-linear, slow recovery & not very repeatable. Not suitable for precision sensing, but the kids had fun with it.

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Re: Conductive Rubber Cord: frustrating

by didier on Mon Jul 30, 2012 12:23 pm

I guess that you can play funny sounds out it with simple circuitry and basic NE55! :)
I suggest that you place a comment in the shop section in order to prevent my kind of frustration :( :? .
Any alternate idea for monitoring linear displacement (other that linear pots obviously :wink: ) ?

didier
 
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Re: Conductive Rubber Cord: frustrating

by adafruit_support_bill on Mon Jul 30, 2012 12:48 pm

Cord & pulley w/rotary encoder?

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Re: Conductive Rubber Cord: frustrating

by pburgess on Mon Jul 30, 2012 1:53 pm

Or perhaps a linear optical encoder, with a limit switch at one end. You can gut an old inkjet printer for one if necessary.

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Re: Conductive Rubber Cord: frustrating

by didier on Sat Sep 22, 2012 3:58 am

Yep, both last answers are top of my mind. Long time ago I fixed a guitar pedal for a friend of mine. The potentiometer was in fact an optical sensor facing a lamp (not even a led at this time!) and in between there was a moving part having a variable aperture slit. This was the ultimate dust proof, moisture proof sensor! Easy, cheap and efficient!

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Please be positive and constructive with your questions and comments.