Quote:
Originally Posted by efoote868
Does anyone know off hand what expected drift is over time?
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It's...complicated. Drift happens because you are taking an angular velocity measurement and integrating it over time. Small errors in velocity measurements add up to big errors in position given sufficient time. There are many sources of errors, some of which are random and others of which are systemic:
1) Bias drift. MEMS gyros are sensitive to temperature...and they self-heat when powered. Several minutes after booting your cold robot, the gyro will think it is spinning because the null voltage when it was calibrated with has changed. Leaving your robot on for several minutes prior to match start (and recalibrating the gyro soon before the match starts) helps somewhat.
2) Axial misalignment. If your gyro is not perfectly level with the field, you will accumulate small errors over time. Aligning the gyro to your frame is one thing; going over a bump or doing a wheelie on the field is another.
3) Saturation. If your gyro measures up to 250 deg/s rotation and you spin faster than that, you will underestimate your rate of turn and drift will accumulate quickly.
4) ADC discretization and conversion noise. Your analog measurements lose some precision during the conversion to a digital measurement. Carefully selecting the bandwidth to use during sampling helps somewhat, though narrower bandwidth may limit your ability to sense rapid turns.
5) Cross axis sensitivity. Unfortunately, it turns out that gyros only MOSTLY measure angular velocity...they also pick up linear accelerations (typically <1% of cross axis sensitivity, but every little bit counts when integrating).
6) Thermomechanical noise. Unfortunately, even if you perfectly compensate for all of the other factors, Brownian motion occurs within the gyro and will add up over time. There is nothing you can do about this one other than to buy a more expensive gyro.
Various specs for all of these factors are available for most gyros. Turning this into a "degrees per minutes" position drift estimate is possible using complex math; for the
KOP gyro from a few years back (which I believe is still the gyro available through FIRST Choice as of last season), about 200 degrees per hour is the quoted drift rate. Drift in position occurs exponentially, so in about a minute you would expect ~.05 degrees of drift...IF you perfectly account for the accountable factors above (which is almost never the case in FRC). In my experience, a degree or two of position drift per minute is more achievable (as long as you don't spin too fast and stay on a flat and level field).