Does anyone know how non-GPS speed & distance calculators work? You know those ones you put on your shoes. I know they can figure out number of footfalls using accelerometers, but how can they calcaulate distance from that?
Has anyone used one of these things? How accurate was it?
here’s a free one … but it may be kind of hard to run with a computer
Calculating time, distance or speed is trivial when you have two of the variables. What I am asking is how do you calculate distance from footfalls? Do you have to enter your stride length?
You have to calibrate it. The Fitsense I use is has a calibration for walking and for running. I am not sure of the exact algorithm they use, but by measuring the acceleration and time of each stride, relative to the acceleration and time for a known stride, they can calculate the distance of the stride (and your pace). This is not the same as the cheapo pedometers that just count strides and use a fixed stride length.
I find they produce very “repeatable” results, it measures the same course to the same distance with an insignificant amount of error. Whether that is the “correct” distance is a factor of the calibration, which is affected by changes in shoes, and I think to some degree large changes in speed. Whether it is better than GPS or not is a matter of preferance then.
Thanks Dan. POlar have a new HRM coming out with the speed/distance pod and they’re claiming 97% accuracy. I guess I’ll just ahve to enter my stride length into the watch or something.
With the Fitsense, and probably others (Nike, etc), you calibrate it by running a fixed distance (400M for instance), telling it “start” and “stop”. You can then tweak the value if you find it is off.
The Polar sounds interesting because the Fitsense heart rate monitor strap is a battery hog, while my Polar straps have lasted years.
Dan
It’s the S625X if you want to check it out at www.polar.fi.
I’ve emailed their tech support as well, so hopefully they’ll give me an informed answer, rather than some marketing speak.
Combination of accelerometers gives you the forward acceleration by the second.
Numerical intergration of this number gives you the forward foot velocity.
Numerical integration of the forward foot velocity gives you the forward distance. Knowing that your foot stops every time it hits the gorund, you are in business.
I have the fitsense and the wife has the timex. They match to within 2 decimal places when hers is working.
We are getting rid of hers because if dropped signals. She will be getting a fitsense or nike instead.
the guy who designed the nike sdm came and talked to my highschool calculus class (they were originally from the university of alberta) and there are some pretty intense calculations that go into it.
it tracks the path the foot makes through the air, as well as the distance each stride is…it’s pretty intense stuff…
apparently if calibrated correctly, they are 99% accurate…97 or so out of the box.
My observed accuracy is probably about 99.5%. In other words, if I calibrate it on a known mile, I estimate I would have to run at least 2 miles before it is off by 100th (2.01 or 1.99). Actually, it is usually better than that. I think 97% would be too inaccurate to be satisfactory, by 5K you would be off by nearly a 10th mile. Calibration out of the box was well off for me.
They also don’t match well with treadmills (although better than the GPS would do 
My guess would be an accelerometer and a timer chip. Accelerometers are really small and cheap these days. Keep track of the time and accel and you get the distance. A coworker uses this method for robot navigation. Running on treadmills probably doesn’t create the same accel as real running so that’s probably where the error comes from. As you can see the error is cumulative with no chance for self correction unlike a GPS.
dist=1/2at^2 +vt+x0.