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Guidance
Terminology
Important Note: Because navX-sensor Gyroscopes, Accelerometers and Magnetometers are all aligned to
this 3-D Coordinate System, a navX-sensor'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), a navX-sensor
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-sensor 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-sensor 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
Compass
2 degrees
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 the Yaw
angle. When valid (magnetically undisturbed) compass readings are received, the Fused Heading's
origin shifts to magnetic north (0 degrees on the Compass).
Gravity Vector
Accelerometers measure both acceleration due to gravity, as well as acceleration due to linear
Update Rate
Up to 200 Hz
1 Hz (if not magnetically
disturbed)
Up to 200Hz
Drift
See
Technical Specifications
None
None (however during periods
of magnetic disturbance, the
heading is subject to yaw
drift)
65
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