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Enki
Senior Member
 
Join Date: Nov 2004
 
2006-12-30, 14:42

I can imagine treadmills would provide some very difficult problems for the inertial sensors. When you run over solid ground the sensor never goes forward-backwards, just forwards-stop-forwards. Knowing the size constraints of the accelerometers they have, every direction change, forward-stop-forward or forward-backward-forward will cause a period of noise that is compensated for in the distance measuring sofware. Compensating for the noise over ground is straightforward, that's what the caliberation period is doing, measuring the number of footfalls and the getting the difference in measured distance, divide it out and apply as the noise correction factor. On the treadmill the backwards part of the foot motion would be corrected against your direction of motion because that't the direction the accelerometer says you were moving. It is correct as far as it tracks your foot, but YOU aren't moving the same direction as your foot. YOU are actually quite stationary.

Even if Nike got that part worked out there is another component of the treadmill running that is impossible for them to compensate for. The mat is always in motion even if your feet are not on it. So as your feet go forwards there is a slight acceleration disconnect between your foot motion and the decoupled constant non-accelerated motion of the mat. Since the treadmill measures the mat motion, not your motion, and is true, the sensor cannot measure the correct distance. The sensor can only measure your foot acceleration connected to a stationary body, which is lower in an absolute sense than a moving foot connected to a moving body. Multiply the small difference over every step you take and it can add up in a hurry.

Even the accelerometers in the human ear deal with the same issues, it is nearly impossible to run on a treadmill without using your eyes because your non-vision aided balance is crap, but you can run for awhile over solid ground before suffering the same problems.
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