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Trey Crisco
10-23-1995, 11:30 PM
Greetings,

I do not have a recommendation for a standardized coordinates
systems (CS). I do however have a concern over some recommendations of
Euler/Bryant sequences (i.e. the six sequences of [X][Y][Z], [X][Z][Y],
etc.) that appear to be finding their way into the standardization of CS.
If there is a concensus for a standard CS, fine, but I strongly disagree
with a standard for the sequence of rotations.
When rotation occurs about an axis of the defined CS, there are
negligible differences between the six sequences. Thus, there is not a
problem. However, as the axis of rotation deviates from the axes of the
CS, the differences between the sequences can easily reach factors of 2
(100% difference) or greater. Which sequence is best? NONE. When the
rotations are highly coupled (i.e. the axis of rotation has significant
components in each direction), each of the sequences gives rotations that
are non-intuitive. In these cases, I believe that the helical axis of
motion is the best choice for describing complex coupled motions.
In summary, if there is a concensus to standarized CS, fine (my
vote is one is not needed), but do not attempt to include a standard
sequence for calulating the rotations in that standard.

Sincerely,
Trey Crisco, Ph.D.
joseph_crisco_iii@brown.edu