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Topic: Convetion of quaternion functions
Replies: 2   Last Post: Aug 13, 2014 1:49 PM

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William Frane

Posts: 36
Registered: 2/14/12
Convetion of quaternion functions
Posted: May 29, 2013 9:34 AM
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The various quaternion functions in the Aerospace Toolbox (see http://www.mathworks.com/help/aerotbx/flight-parameters-1.html) all indicate that an input quaternion q should have "its scalar number as the first column" (i.e., [w x y z] ordering rather than [x y z w]). However, this convention doesn't always seem to produce the expected result.

For example, the quaternion
[cos(pi/2) + 0i + 0j + sin(pi/2)] = [1/sqrt(2) + 0i + 0j + 1/sqrt(2)k]
represents a rotation about the z axis ([0 0 1]) by 90 degrees (Wolfram Alpha confirms this: http://www.wolframalpha.com/input/?i=quaternion%3A+%281%2Fsqrt%282%29%29%2B0i%2B0j%2B%281%2Fsqrt%282%29%29k). Accordingly, if this quaternion is used to rotate the vector [1 1 1] in a right-handed coordinate system,, the result should be [-1 1 1].

If MATLAB's quatrotate function is used to perform this rotation, the result is
quatrotate([1/sqrt(2) 0 0 1/sqrt(2)], [1 1 1]) = [1 -1 1]
This result corresponds to a rotation about the z axis by -90 degrees, not 90 as expected.

Another example: rotating [0 0 1] by the quaternion [1/sqrt(2) 1/sqrt(2) 0 0] (which represents a 90 degree rotation about the x axis) yields
quatrotate([1/sqrt(2) 1/sqrt(2) 0 0],[0 0 1]) = [0 1 0]
when the expected result is [0 -1 0].

The result from quatrotate (at least for these two examples) seems to be determined using a left-handed coordinate system, yet the documentation for the Aerospace Toolbox explicitly states that "[t]he Aerospace Toolbox software uses right-handed (RH) Cartesian coordinate systems" (see http://www.mathworks.com/help/aerotbx/ug/defining-coordinate-systems.html#bqsgj4j-5). Other quaternion functions exhibit similar behaviour (e.g., using the quaternion from the first example, quat2dcm produces a matrix that performs a -90 degree rotation about the z axis).

Have I missed a part of the documentation that explains this behaviour, or am I using these functions incorrectly? Any assistance would be appreciated.

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