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This is a broad question but I want advice on what type of hardware I should look for to make a device that could measure the velocity of an object attached via a string/wire to some sort of rotary wheel.

The purpose of this is to attach it to a barbell and measure mean-velocity for weightlifting athletes.

I'm a software engineer by trade but know nothing about hardware. I'm hoping I can find something that would interface with arduino, as that would ease the learning curve.

As with such a broad question, I'm not looking for a solution, rather I would like to be pointed in the right direction.

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    \$\begingroup\$ If it's for weight lifters I'd be tempted to consider a strain gauge on the lifting shaft and a small RF transmitter. The strain gauge signal could be used to derive the time to lift from the floor to the "final height". I guess without knowing the final height, velocity still remains an unknown but "height" can be programmed in for each athlete? \$\endgroup\$
    – Andy aka
    Sep 25 '15 at 13:48
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You're best "out of the box" solution is probably a yo-yo potentiometer (also called a string pot). For example, http://www.firstmarkcontrols.com/, but there are both cheaper and more expensive versions.

You can certainly use an off the shelf encoder, but you'd have to rig up the mechanics on your own.

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    \$\begingroup\$ +1 There are so called 'string pots' that are actually quadrature encoders with the typical cable and negator spring. I guess the OP would also need something that would unsnap or otherwise protect against a 500kg weight being dropped. \$\endgroup\$ Sep 25 '15 at 17:13
  • \$\begingroup\$ Some low-test thread for the final connection point would do that job, or a string pot with a string long enough to accommodate whatever the OP could throw at it would also (if dynamic range isn't that big of a concern). \$\endgroup\$ Sep 25 '15 at 18:31
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You're looking for a rotary encoder. https://en.wikipedia.org/wiki/Rotary_encoder

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  • \$\begingroup\$ I wonder though how I would get the encoder to "rewind" back into position (between subsequent weight lifting repetitions) without affecting the velocity measurement, or causing undue asymmetrical stress on the athlete? \$\endgroup\$ Sep 25 '15 at 14:37
  • \$\begingroup\$ I'm thinking something like a key fob amazon.com/Cosmos-Silver-Color-retractable-Fastening/dp/… Obviously you'd want something heavier duty and something that will attach to the rotary encoder. But it wouldn't need much tension at all since the string is so light. \$\endgroup\$ Sep 25 '15 at 16:02
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    \$\begingroup\$ Heavier duty versions of the key fob are called draw wire position sensors or draw wire displacement sensors, but they're somewhat expensive. At least you have a term to google for. \$\endgroup\$ Sep 25 '15 at 16:09
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Check out our open source project OpenBarbell. We designed a 3D printed enclosure around a tape measure spring and use a simple optical quadrature encoder setup. You can check it out here.

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Conceptually it will help to split the problem up into sections:

Section 1) What are you trying to measure?

"measure the velocity of an object"

Section 2) How are you going to measure it?

Option 1 - "use a rotary encoder to measure rotations of a wheel to infer velocity/position of object"

Option 2 - "use an accelerometer to measure movement of object to infer velocity/position of object"

Option 3 - "use weight sensors under the feet of the athlete to measure his change in weight and infer velocity/position of object"

Whatever you go for - you need to decide how you are going to use the data from the sensors to estimate/infer what you are trying to measure.. this will almost certainly involve calibration, trial and error.

I strongly recommend Adafruit stuff for Arduino compatibility and ease of use. This 10DOF chip comes prebuilt and with the libraries written for you!

http://www.adafruit.com/products/1604

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How about a slotted pulley, like this but cheaper. Or maybe drill holes through the supporting web. Then shine a LED through the slots, with a photo diode on the other side. Count the pulse frequency and you know the number of revolutions per second... do the math to figure out the speed. You don't get direction information unless you have a more complicated slot arrangement.

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