I have a sensor that gives me a quaternion. I convert the quaternion to an axis-angle representation using http://www.euclideanspace.com/maths/geometry/rotations/conversions/quaternionToAngle/. When I hold the sensor on a flat surface and rotate the sensor pressed against the flat surface, I would have expected the axis to be the same and the angle to vary from 0-360 degrees - but this does not happen (the axis varies significantly). Any ideas why? Maybe I don't understand the axis-angle representation?
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$\begingroup$ How sensitive is the sensor to variations in the surface? What happens if you put pressure on a surface like a book and then rotate the book and sensor together? $\endgroup$– John DoumaApr 14, 2013 at 23:09
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$\begingroup$ In short, axis–angle representation ensures that “angle” parameter is non-negative by flipping the direction of “axis” parameter. Until you understand this purely geometrical thing, quaternions will remain a piece of unhelpful higher algebra. Also, for rotations close to the identity (0 angle) axis is poorly defined. $\endgroup$– Incnis MrsiNov 2, 2014 at 10:24
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