# quat2rod

Convert quaternion to Euler-Rodrigues vector

## Description

example

rod=quat2rod(quat) function calculates the Euler-Rodrigues vector, rod, for a given quaternion quat. The quaternion input and resulting Euler-Rodrigues vector represent a right-hand passive transformation from frame A to frame B.

Aerospace Toolbox uses quaternions that are defined using the scalar-first convention. This function normalizes all quaternion inputs.

## Examples

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Determine the Euler-Rodrigues vector from the quaternion.

q = [-0.7071 0 0.7071 0]
r = quat2rod( q )
q =

-0.7071         0    0.7071         0
r =

0   -1.0000         0

## Input Arguments

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M-by-4 array of quaternions. quat has its scalar number as the first column.

Data Types: double

## Output Arguments

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M-by-3 matrix containing M Euler-Rodrigues vectors.

## Algorithms

An Euler-Rodrigues vector $\stackrel{⇀}{b}$ represents a rotation by integrating a direction cosine of a rotation axis with the tangent of half the rotation angle as follows:

$\stackrel{\to }{b}=\left[\begin{array}{ccc}{b}_{x}& {b}_{y}& {b}_{z}\end{array}\right]$

where:

$\begin{array}{l}{b}_{x}=\mathrm{tan}\left(\frac{1}{2}\theta \right){s}_{x},\\ {b}_{y}=\mathrm{tan}\left(\frac{1}{2}\theta \right){s}_{y},\\ {b}_{z}=\mathrm{tan}\left(\frac{1}{2}\theta \right){s}_{z}\end{array}$

are the Rodrigues parameters. Vector $\stackrel{⇀}{s}$ represents a unit vector around which the rotation is performed. Due to the tangent, the rotation vector is indeterminate when the rotation angle equals ±pi radians or ±180 deg. Values can be negative or positive.

## References

[1] Dai, J.S. "Euler-Rodrigues formula variations, quaternion conjugation and intrinsic connections." Mechanism and Machine Theory, 92, 144-152. Elsevier, 2015.

## Version History

Introduced in R2017a