Measuring Acceleration/Speed for Small Vehicles?

stampfli_brit asks: "I am the captain of rowing at my local club, and was looking for a way to improve training. We have an NK SpeedCoach. However, I was looking for something that could measure the speed of the shell several times a second, more like this [pdf]. Any ideas on a cheap system using IC accelerometers or GPS, that could get this kind of information, log it for an hour or two, then dump it to an Apple Mac for analysis?"

Comments

Tricky

[linuxwrangler]

This can be a lot harder than it seems at first. In the yachts I race on we have some top-of-the-line instrumentation. Sensors feed GPS info, wind speed/direction, speed-through-water, compass-heading, etc. into the instrumentation computer (black box) and on to the various displays on-deck as well as into a pc for tactics and analysis.

It's very useful info but you also learn rather quickly that it's full of errors. Consider that the wind speed and direction is usually read from a anemometer/vane at the top of the mast (often on a small pole to attempt to keep it out of the wash of the sails). Speed through the water is done ultrasonically or with a paddle wheel. All these direct measurements are converted to things like true wind speed, true wind direction, velocity made good and so on.

Damping is a huge problem. You may think that several readings a second is good but as the boat rides over waves and heels to varying degrees all of your measurements will be off. Wind due to the mast swinging (doesn't take much heel to really move things around 7 stories up - I've been there many times), boat speed due to water-flow differences at different heel angles (some boats average readings from multiple sensors to combat this issue), and even magnetic heading due to compass-dip if the compass sensor isn't gimballed or otherwise compensated. Essentially you become limited to short-term running averages of the readings.

You don't have quite as many inputs but still need to be concerned with yaw and tilt of the shell, turbulence from the oars and so on. Boat motion will be especially problematic if you are trying to directly measure acceleration.

I suspect that an appropriately designed towed speed sensor might get you the data you desire (though the data-quality will still be sketchy). Perhaps a small propellor/chopper-disk/optical sensor. You could probably build one for a few dollars. The problem then will be to convert the pulses to something you can use.

Why?

[Zebra_X]

Several times a second? What can you garner from that kind of resolution in speed? Perhaps you are looking to analyze the impact of the rudder on the boats performance?

Having been a varstiy cox for a top D1 rowing team I've never needed such information to make my boat faster.

The speed coach is a very good device. Short of rebuilding it, you will not find anything that will come close to it's level of accuracy (or durability). If you're looking for a device that can dump to a PC (presumably Mac as well) have a look here. It's not that expensive and comes with all of the hardware that you would have to build yourself.

Your only other option is to find an impeller, and get a data logger and make it waterproof. You'll also need a display and a C compiler to drive the impeller -> display conversions.

I guess the question is how much is your time worth?

Ball bearing solution

[stick_figure_of_doom]

My crew club is having another problem (which I know all crews are subjected to). The steel ballbearings in the slide wheels rust out, and when you have 4 wheels for 8 seats you have 32 wheels rusting. We end up replacing the shotty we find about every two weeks. The old slide solution didn't involve ballbearings, but it sucked. It involved rolling axles attached to the wheels, a sliding plate, and another set of rollers below the actual seat. It jams all the time, but it never rusted out. What would anyone say is a more appropriate mechanical solution? Thanks for not modding this off-topic, because you don't see crew on /. very often...

More than Accel

[BSDevil]

Correct me if I'm wrong, but you're looking for something that will do everything a SpeedCoach does, but you want a better resolution, and you want it (presumably) for less money.

My immediate thought would be to go grab a few Vernier sensors (as someone else mentioned) and a TI-92, and do it on that. However, NK has spend a whole lot of time and cash making sure their impellers work perfectly for what they're supposed to do, and produce the minimum drag possible. I know if I was rowing at that level (I rowed in HS for a while), I wouldn't want some homemade impeller giving me excess drag.

Upon further though, why would you want that much resolution in terms of speed? The SpeedCoach measues at the midpoint of each stroke, as the power phase is finishing. If you want something more specific than that (maybe to analyze for the effect of small changes in stroke shape or recovery speed), I would think that you'd be using a tank anyways, and you could do something with video for that (not needing an impeller at all).

Also, think about this: most national teams use SpeedCoaches for their training. If it's good enough for them, why do you need more prescision?

I think you need to rethink your problem. If you're measuring for the impact of rudder usage on speed, you can do that with models in a flow tank (go talk to your hydrodynamics departement). If you're doing stroke geometry, use an erg (or a tank, a videocamera, and a few Vernier probes). I can't think of a good reason for needing that much prescision in an on-water situation.