Dr. Chris Kirtley
12192002, 09:36 AM
More replies. still astonishes me that we don't have a standard method
after all these years... (begging the flames)
From:
Richard Baker
About 90% of problems can be resolved by using ASIN rather than ATAN.
This
returns a positive or negative value depending on whether the angle is
one
side or the other of vertical as opposed to the ATAN function which
returns
both as positive. It doesn't work beyond 90 degrees either way of
course
but few joints have a functional range of movement of more than 180
degrees
and thus by sensible choice of the reference line about which ASIN is
calculated for each joint/segment this problem can generally be avoided.
Richard
From:
rsrlee
Dear Chris,
For atan(y/x), you can tell the quadrant where the angle lies by
examining the
signs of y and x. For instance, if both y and x are negative, y/x will
be in
the third quadrant. If one does not examine the signs of y and x, since
y/x
will be positive, atan(y/x) will be mistaken to be in the first
quadrant.
Of course, if you do atan(value), i.e. with no knowledge of x and y,
there is
no way that you can tell the quadrant. Fortunately, in gait analysis,
since
you derive x and y from the marker positions, eg (Yknee  Yankle)/(Xknee

Xankle), you should have no problem.
In Matlab, there is actually a function atan2 which does the above
automatically. I prefer atan2 to atan so that we don't have to do manual
testing of the signs of x and y.
Does Ray Smith use the same method?
Happy Christmas to you too.
Raymond

Dr. Chris Kirtley MD PhD
Associate Professor
Dept. of Biomedical Engineering
Catholic University of America
620 Michigan Ave NE, Washington, DC 20064
Tel. 2023196247, fax 2023194287
Email: kirtley@cua.edu
http://engineering.cua.edu/biomedical/faculty/kirtley

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after all these years... (begging the flames)
From:
Richard Baker
About 90% of problems can be resolved by using ASIN rather than ATAN.
This
returns a positive or negative value depending on whether the angle is
one
side or the other of vertical as opposed to the ATAN function which
returns
both as positive. It doesn't work beyond 90 degrees either way of
course
but few joints have a functional range of movement of more than 180
degrees
and thus by sensible choice of the reference line about which ASIN is
calculated for each joint/segment this problem can generally be avoided.
Richard
From:
rsrlee
Dear Chris,
For atan(y/x), you can tell the quadrant where the angle lies by
examining the
signs of y and x. For instance, if both y and x are negative, y/x will
be in
the third quadrant. If one does not examine the signs of y and x, since
y/x
will be positive, atan(y/x) will be mistaken to be in the first
quadrant.
Of course, if you do atan(value), i.e. with no knowledge of x and y,
there is
no way that you can tell the quadrant. Fortunately, in gait analysis,
since
you derive x and y from the marker positions, eg (Yknee  Yankle)/(Xknee

Xankle), you should have no problem.
In Matlab, there is actually a function atan2 which does the above
automatically. I prefer atan2 to atan so that we don't have to do manual
testing of the signs of x and y.
Does Ray Smith use the same method?
Happy Christmas to you too.
Raymond

Dr. Chris Kirtley MD PhD
Associate Professor
Dept. of Biomedical Engineering
Catholic University of America
620 Michigan Ave NE, Washington, DC 20064
Tel. 2023196247, fax 2023194287
Email: kirtley@cua.edu
http://engineering.cua.edu/biomedical/faculty/kirtley

To unsubscribe send SIGNOFF BIOMCHL to LISTSERV@nic.surfnet.nl
For information and archives: http://isb.ri.ccf.org/biomchl
