Prove Two/Three Euler Angles Needed for Orientation Change

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Discussion Overview

The discussion revolves around the mathematical proof required to establish that at least two or three Euler angles are necessary to transition between two orientations of a body in space. The scope includes theoretical aspects of rotations in three-dimensional space and their implications for mechanical design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how to mathematically prove that at least two or three Euler angles are needed for orientation change, suggesting a design context involving hinges.
  • Another participant asserts that only one rotation is necessary to transition between any two orientations, referencing the mathematical group SO(3) which describes rotations in three-dimensional space.
  • A participant expresses uncertainty about the implications of orthogonal groups and seeks clarification on whether a single axis of rotation suffices for all orientation changes.
  • It is noted that Euler's rotation theorem supports the existence of a single axis of rotation for any two orientations, but this does not imply that a physical device can rotate about that axis due to mechanical constraints.

Areas of Agreement / Disagreement

Participants present competing views on the necessity of multiple Euler angles for orientation changes, with some asserting that only one rotation is needed while others suggest that practical mechanical limitations may require multiple angles.

Contextual Notes

The discussion highlights the distinction between mathematical existence of a rotation axis and practical mechanical implementation, indicating potential limitations in applying theoretical concepts to physical devices.

TheFerruccio
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Hi, I have a simple question.

Given two orientations of a body in space, how can it be proven that at least two (or three?) Euler angles are needed to get from one orientation to the other? I am designing a mechanism and someone suggested moving it in a way that would require two hinges. I know that it is intuitively true, but I would like to prove it with math.
 
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You only need *one* rotation to get from any orientation to another.

Rotations in three space form a mathematical group, a rather important one. It's called SO(3), short for the three dimensional special orthogonal group.
 
So you can get from any orientation to any other orientation using a single axis of rotation?

I am not sure how orthogonal groups work. That is beyond what I know. Is that what I need to know in order to understand this?

*edit* I found this: http://en.wikipedia.org/wiki/Rotation_group_SO(3)#Axis_of_rotation

I am guessing that is what you mean. So, basically, every rotation between any two orientations has an axis of rotation? That's hard to wrap my head around. I am imagining specific examples where that doesn't seem to be the case, but, I guess it must be the case.
 
It's Euler's rotation theorem. (Wiki article: http://en.wikipedia.org/wiki/Euler's_rotation_theorem) Rotate an object two, three, or more different ways, and there's always some axis about which you could have rotated the object and get from the original orientation to the final orientation in one single rotation.

That said, just because a single axis rotation must exist does not necessarily mean that a physical device can rotate about that axis. For example, robotic arms need multiple joints because each joint has a limited number of degrees of freedom. Getting the arm into some desired orientation can sometimes be tricky.
 

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