Tracking speed and distance for a weight training analysis tool

g'day,

I'm pondering the idea of making a measuring tool to calculate and log wattage and work done when doing weight training with barbells - this is computationally simple - work is force x distance and power is work / time - so if I can track the movement of a barbell in the vertical, the sums are simple. All I really need to know is distance moved and time taken, but with a reasonably high sample rate, probably at worst to the tenth of the second or better if we want to analyse some of the faster lifts.

Using xbee and an ultrasonic rangefinder to the ground attached and hanging from a barbell (so it always points at the ground) might work, or some kind of retracting and low drag cable and spool arrangement with a box on the ground is another possibility - or even an accelerometer (although how it would know how far the bar had traveled I don't know?) I guess it could provide instantaneous power figures with some slighly more real-timeish calculations.

Does anyone here know of a suitable sensor to do this?  Any suggestions?

thanks!

Carl

In theory, by integrating acceleration, you can back-port velocity, and from there distance (and force, and other fun things) so if you can get that working, it'd be a nice trick. Plus, a good 3D accelerometer would give you other things in the long term, like the ability to plot the weight path, and wasted energy, etc. At the same time, we're talking about inertial navigation, essentially - accuracy is something I've got no experience with in this case. Might be a nightmare. You could also try side-facing IR leds to an IR sensing bar with the sensors set up in a thermometer configuration. There may also be a way to triangulate in three-space using a high-frequency sound source and some microphones (a la smart whiteboards). Just some ideas.

I've seen some using a roller and thin line, an example is here :

http://www.kinetic.com.au/products/gymaware/

That's got to be a pretty simple way to do it, but where do you get the rollers? I'm sure they'd be an off the shelf part somewhere? I'm not even sure of what you'd call them? Google doesn't help when you don't know the name of something!

you could do this with a wii remote and some IRLEDs but you'd need a PC for that.

Are you planning on building a custom weight set around your project, because that would have a lot of possibilities if you did. That would allow you to do weight tracking, instantaneous force, all kinds of good stuff...