Kauai Labs navX-MXP User Manual page 68

Guidance
Terminology
Magnetometers: measure earth's magnetic field, where the origin represents the center of the
navX-MXP circuit board.
Important Note: Because the navX-MXP Gyroscopes, Accelerometers and Magnetometers are
all aligned to this 3-D Coordinate System, navX-MiXP's motion processor can also use Sensor
Fusion to provide additional information and processing including Tilt Correction, "Fused
Heading", a Gravity Vector, World Reference Frame-based Linear Acceleration and
Quaternions, as discussed in the Motion Processing section below.
Motion Processing
Users should also have a working knowledge of the terms defined in the Motion Processing
Terminology.
Tilt Correction
Without correction, the compass heading calculated by a 3-axis magnetometer will only be
accurate if the magnetometers are held "flat" with respect to the earth. To ensure the compass
heading is valid even in cases when the sensor is "pitched" (Pitch angle != 0) or "rolled" (Roll
angle != 0), navX-MXP performs "tilt correction" fusing the reading from the magnetometers
with the pitch and roll angles from the accelerometers. Once corrected, the compass heading is
aligned with the navX-MXP Z axis, which ensures that the Yaw angle and the Compass
Heading measure rotation similarly.
"Fused" Heading
Given the gravity-referenced orientation provided by the Yaw angle, as well as the absolute
compass heading angle which has been aligned to the navX-MXP 3-D coordinate system, both
angles can be fused together. As shown in the table below, over a period of several minutes this
can minimize the drift inherent in the Yaw angle, as well as provide an absolute reference for
the Yaw angle – as long as the magnetometer is calibrated and a valid magnetometer reading is
available every minute or so.
Value Accuracy
.01 degrees
Yaw
2 degrees
Compass
Heading
Fused 2 degrees (as long as a valid
Heading magnetometer reading is
received in the last minute or
so)
Like the Compass Heading, the Fused Heading has a range from 0-360 degrees.
Important Note: If the Compass Heading is not valid, the Fused Heading origin is the same as
Update Rate
Up to 200 Hz
1 Hz (if not magnetically
disturbed)
Up to 200Hz
Drift
~1 degree/minute
None
None (~1 degree/minute,
during periods of magnetic
disturbance)
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