[Greg McKaskle]

I was looking for a decent app on the iPhone that would work to experiment with. The closest I can come to is one called Vibration. I was using it to measure how the cell phone buzzed and compare that to an external accelerometer reading.

Anyway, the app will show the three axes and it calibrates to subtract out gravity at the initial orientation. If you leave the phone sitting still and run a five second recording, you should get relatively flat lines and that's expected. The integrated area should be zero.

If you run the app and move the phone to the left and right, you'll see similar cancellation. But it probably won't quite zero. Next, during a sample recording, walk from your chair to the front door. Each step looks like a heartbeat, on each axis. And yeah, they sort of cancel out, but where is my predictor of my acceleration that tells me how far I walked. It is a tiny bump at the beginning of that heartbeat signal.

Then run a sample and simply tilt the device a bit. You'll see that a five or ten degree tilt offsets the line quite a bit. And worse, it stays there for the entire sample. The integration of the tilt is huge.

Anyway, if you can find the app, or something similar, it is helpful in understanding why IMUs are hard. After all, if it was easy, the phone or Garmin would do this instead of or in addition to GPS.

Greg McKaskle