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I am currently making software to interact with a conductive rubber cord. I am very interested in figuring out the percentage of a stretch of the cord, to determine how the resistance changes.

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Example:

Stretch the cord 1 cm, see the difference in the resistance:

Stretch the cord 3 cm, see the difference in resistance:

Programmatically, I can tell this change. When I use my hands, I can see the resistance flow change as my pressure increases on stretching the cord. However, as my hands are probably not the best instrument for accurate stretching, I am in need of an alternate machine to do this for me.

Does a such device exist, which can hold one end of this material for me, and pull the other end 1 cm? 3 cm? etc. Since I am a software engineer, and certainly lack in the field of mechanical engineering, is a good approach to take here to use motors and a raspberry pi to possibly create my own stretch machine?

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  • $\begingroup$ What precision is needed? There are any number of ways to stretch and measure. Simple and inexpensive? Try a set of digital or vernier calipers with the elastomer stretched between the fingers - accurate distance is easily read from the caliper. Some ingenuity is required. $\endgroup$ – Donald Gibson Mar 29 '17 at 15:47
  • $\begingroup$ i'd like centimeter precision. $\endgroup$ – angryip Mar 29 '17 at 17:00
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Yup,

You could use something as simple as a bar clamp. Most hardware stores will carry some version. The type you'd need is the type that can be reversed to a spreader (95% of them can do this just double check).

With one of these you'd need something to connect your cord to it. But still fairly straight forward (more clamps for example.

This wont be super accurate but it'll work.

For something more accurate you're looking at something more like a load frame. Most engineering universities will have a few of these (mechanical or civil departments). Very easy to use, but they take some set up time (a half hour ish). Every university I've worked with is usually pretty open to doing external testing, though you'll be limited to their schedule. There are other material testing places that will have them but they charge oodles usually.

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If you are at a university, you can consider using a universal tensile testing machine to stretch the rubber cord at specific stretch rates.

Otherwise, you may develop a simple setup using stepper motor to also stretch it at fixed stretch rates.

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