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Dr. Guy Simoneau
11-30-1997, 05:24 AM
Here we go again with the filtering issue.

In the process of analyzing some wrist kinematic data that we collected
with the use of electrogoniometers, an interesting question was raised
by one of our graduate students.

Background: Our interest is to look at position, velocity and acceleration
of the wrist during various tasks. Since the electrogoniometers provide
position data, we differentiate the signal once for the velocity
and twice for the acceleration data. In order to eliminate noise
in the signal we use a fourth order, zero phase shift, Butterworth filter
with a cut-off frequency of 6 Hz. This filter is applied to the raw position
data.

His question was: should we apply the filter to the data again after each step
where the signal has been differentiated. His logic was that the process
of differentiating the signal would create a higher frequency component
in the signal. So, in essence we would filter the raw position data,
differentiate the signal to get velocity, filter the velocity data, then
differentiate the signal to get acceleration.

After looking through several reference books, notes, literature, etc.
I could not find anywhere that anyone does that. I also could not find
arguments either supporting or refuting this idea.

I would appreciate any comments either rejecting or supporting
the use of filtering the velocity data before calculating the acceleration.

What would be the net effect of filtering a second time?

Thank you in advance.

I will be glad to post the comments.

Guy



************************************************** ******************
Guy G. Simoneau, Ph.D., P.T., A.T.C.
Assistant Professor, Physical Therapy Department
Marquette University
Walter Schroeder Complex, Room 346
P.O. Box 1881
Milwaukee, WI 53201-1881

e-mail: simoneaug@vms.csd.mu.edu
Phone: 414-288-3380
Fax: 414-288-5987
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