Kauai Labs navX-MXP User Manual page 69

Guidance
Terminology
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
acceleration. This fact makes using raw accelerometer data difficult. navX-MXP's automatic
accelerometer calibration determines the component of measured acceleration which
corresponds to gravity, and uses this information together with gyroscope readings to calculate
a gravity vector, which represents acceleration due to gravity. Pitch and Roll angles are derived
from this gravity vector.
Once the gravity vector is understood, this value is then subtracted from the raw accelerometer
data to yield the acceleration due to linear motion.
Velocity and Displacement
Acceleration is defined as the change in Velocity. Therefore, linear velocity can be calculated by
integrating linear acceleration over time.
Velocity is defined as the change in Position, otherwise known as Displacement. Therefore,
linear displacement can be calculated by integrating linear velocity over time.
Important Note: Using currently-available MEMS-based accelerometers to calculate linear
velocity and displacement is subject to large amounts of error primarily due to accelerometer
"noise" (a difference between the actual acceleration and the measured acceleration inherent
with MEMS sensors). This noise not only accumulates, but is also squared in the case of
velocity, and is cubed in the case of displacement. Therefore, the resulting estimated velocity
and displacement values are not typically useful for robotic navigation. The amount of error in
displacement estimation can be several feet per second. As MEMS sensors improve in the
coming years and accelerometer noise is reduced by approximately 100 times its currently
value, this technique will become more useful for robotics navigation.
If you would like to experiment with using the navX-MXP to calculate displacement and velocity,
you can use the navXUI's "Experimental" button to bring up a dialog which displays the
integrated velocity and displacement values calculated in real-time by the navX-MXP.
World Reference Frame
Raw acceleration data measures acceleration along the corresponding sensor axis. This
measurement occurs in a reference frame known as "Body Reference Frame". This works well
as long as the navX-MXP circuit board is in it's original orientation. However as the navX-MXP
circuit board rotates, the X and Y accelerometer axes no longer point "forward/back" and
"left/right" with respect to the original orientation. To understand this more clearly, consider how
the meaning of the term "left" changes once a robot has rotated 180 degrees? Introducing a
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