Gyro/Accelerometer Calibration - Kauai Labs VMX-pi User Manual

Guidance
Terminology
integrated velocity and displacement values calculated in real-time by the VMX-pi.
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 VMX-pi circuit board is in it's original orientation. However as the VMX-pi 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 World
Reference Frame solves this issue by providing a reference upon which to measure "leftness".
To account for this, VMX-pi's motion processing adjusts each linear acceleration value by
rotating it in the opposition direction of the current yaw angle. The result is an acceleration value
that represents acceleration with respect to the area in which VMX-pi operates, which is known
as "World Reference Frame". This world-frame linear acceleration value is much simpler to use
for tracking motion of an object, like a robot, which might rotate while it moves.
Important Note: VMX-pi Linear Acceleration values are in World Reference Frame.
Advanced
Advanced users may require knowledge of the following terminology.
Quaternions
A quaternion is a four-element vector that can be used to encode any rotation in a 3D
coordinate system. This single 4-element vector value can describe not only rotation about a
reference frame's origin (Pitch, Roll and Yaw) but also the rotation of that entire reference
frame with respect to another. Furthermore, when Pitch, Roll and Yaw measures to perform
certain calculations, it is not possible to clearly ascertain orientation when two axes are aligned
with each other; this condition is referred to as "Gimbal Lock". For robotics applications, Pitch,
Roll and Yaw are sufficient, however for certain aerospace applications, Quaternions may be
required to handle all possible orientations.
VMX-pi uses Quaternions internally, and also provides the 4 quaternion values for use by those
who might need them.
VMX-pi onboard orientation sensors require calibration in order to yield optimal results. We
highly recommend taking the time to understand this calibration process – successful calibration
is vital to ensure optimal performance.

Gyro/Accelerometer Calibration

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