Kauai Labs navX-MXP User Manual page 77

Guidance
Yaw Drift
understand, making it typically beyond the capabilities of many robotics engineers. The navX-
MXP circuit board uses the Invensense MPU-9250 IC, and this IC implements a proprietary
algorithm which is widely believed to be an EKF (it exhibits similar accuracy to documented EKF
implementations on MEMS acceleromter/gyroscope sensors).
With this processing, navX-MXP exhibits yaw drift on the order of ~1 degree per minute; yaw
drift is typically much lower when navX-MXP is still.
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
calculation:
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-MXP RoboRIO libraries is to
periodically "re-zero" navX-MXP by applying an offset to the navX-MXP 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-MXP 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-MXP provides a 9-axis "fused
heading" which is combined with the ~1 degree per minute of 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 that to
be effective this requires the magnetometer to be calibrated. Once calibrated, an initial
magnetometer reading undisturbed by magnetic disturbances can be acquired at the beginning
of a match, before the motors are energized. If the sensor is placed far enough away from
motors, it may be possible to also get an undisturbed magnetometer during a match.
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