Kauai Labs navX2-MXP User Manual page 78

Guidance
Yaw Drift
older "Classic" navX-MXP sensors used the Invensense MPU-9250 IC, which internally implemented a
proprietary algorithm widely believed to be an EKF (it exhibits similar accuracy to documented EKF
implementations on MEMS acceleromter/gyroscope sensors).
With this processing, navX-sensors exhibit very low yaw drift as documented in the
Specifications.
Tips
What follows are some tips on how to deal with the yaw drift within the context of a FIRST FRC
competition.
In general, the yaw will not drift significantly during a FRC match, based upon the following
calculations:
"Generation 2" navX-sensors:
yaw drift(degrees) at end of match = yaw drift (~.5 degree/minute) x match length (2.5 minute) =
~1.25 degrees or better
"Classic" navX-sensors:
yaw drift(degrees) at end of match = yaw drift (~1 degree/minute) x match length (2.5 minute) =
~2.5 degrees
However, during long practice matches the drift may become noticeable, and can be dealt with using the
following approaches:
1) The simplest approach which is supported by the navX-sensor libraries is to periodically "re-zero"
navX-sensor by applying an offset to the navX-sensor yaw angle. To use this approach, when the robot is
in the correct orientation, a driver can press a button which causes an offset to be added so that the
reported angle at that orientation is 0.
2) Even though the navX-sensor magnetometer will likely give erroneous readings once the robot motors
are energized, a calibrated magnetometer can potentially provide a stable reading during the moments
before a FRC competition round. The navX-sensor provides a 9-axis "fused heading" which is combined
with the drift in the yaw angles. Using the "fused heading", it is possible to calculate the robots absolute
orientation and maintain it. With the "fused heading", that drift will be updated w/the absolute heading
from the compass whenever a compass reading which is free from magnetic disturbance is detected. Note
Technical
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