Hardware to measure velocity via string/wire attatchment? beginner

Question

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 atheletes.

I'm a software engineer by trade but know nothing about hardware. I will obviously need to start with simple projects to learn, but I would like to know which hardware I should lean towards buying, so I don't have to re-buy anything?

Attach a optical rotary encoder. Similar to the one that can be found in mouse scroll-wheels. (Since direction isn't needed for you problem only a single photo-interrupter is needed.) — Gerben, Sep 25 '15 at 14:20
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? — James Wierzba, Sep 25 '15 at 14:35
Some small spring would retract the wire without exerting any noticeable force for the athlete. Or have the wire loop around the "wheel" and attach a counterweight (just like a lot of fitness devices do). — Gerben, Sep 25 '15 at 19:12

Igor Stoppa · Answer 1 · 2015-09-27T18:48:04.007

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This is an example of the optical encoder. You want to use interrupt detection, rather than polling. Or maybe even an HW counter. That's all you really need.

I am using 4 of them for a car.

I am in the process of refactoring the code, but you could already have a look at the driver & example I wrote for ChibiOS, here.

edited Sep 27 '15 at 18:48

answered Sep 25 '15 at 19:44

Igor Stoppa

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Second link appears empty – James Wierzba Sep 26 '15 at 05:30
Ooops, sorry. Fixed. – Igor Stoppa Sep 26 '15 at 07:20

score 1 · Accepted Answer · edited Jan 11 '16 at 13:16

You can use a dog lead (or replace it with a fishing line). I made one my self. I used a rotary encoder and a dog lead, an Arduino board Paralax, and Excel.

It will use interrupts. Once the wire is triggered, the Arduino will send to Excel time and distance traveled. Once the weight goes down distance and time will be printed too, however distance will run in a minus direction. Later just press one button in Excel and each set of lifts will appear on the desktop. I can see my lift speed, power generated, distance travelled and so on.

Hardware to measure velocity via string/wire attatchment? beginner

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