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What representation is expected for gyro measurements? #9

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jameysharp opened this issue Aug 2, 2014 · 1 comment

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commented Aug 2, 2014

AttPosEKF::UpdateStrapdownEquationsNED constructs deltaQuat by assuming that correctedDelAng is in an axis-angle representation, where the magnitude of the Vector3f is the angle, and the direction of the vector is the axis of rotation.

I expected that any gyro measurement would instead be represented as Euler angles, that is, three orthogonal measurements of rotation rate. That's not algebraically equivalent to the axis-angle representation, as far as I can tell, except when any two of the components are 0.

What's the justification for this approximation? Is it expected that UAVs will usually only rotate around one body-frame axis at a time? Or perhaps you determined experimentally that the difference is small in practice?

The computation of summedDelAng has a related issue: summing rotation rates component-wise is only valid as long as all rotation is around the same gyro axis. Again, what's the intuition behind this?

@priseborough

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commented Aug 3, 2014

This is a standard representation for tactical grade strapdown systems. Also Euler angles are not orthogonal.
The change in direction of the rotation vector across the sample time is taken into account of by application of coning corrections.
See standard texts on strapdown nav for further info about application of coning corrections and use of delta quaternions based on delta angles.

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