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View Full Version : Re: Drop test synchronisation



Arnim Henze
08-21-2000, 08:52 PM
To all,

a couple of years ago I did some experiments with the Motion Analysis
System (four cameras FALCON HR240 (240 Hz), Software EVa 4.0, National
Instruments AD-converter AT-MIO-64F-5, Kistler force plate 9287A) and I
also had syncronisation problems.

I tried to check syncronisation dropping a stick (with a reflective
marker attached to its top end) vertically on the force plate, so that
it bounced off the plate a couple times before it fell over.

As I remember, I tried different video frequencies (at least 240Hz and
120Hz) with force platform data at 960Hz but did not find the
dis-syncronisation dependend on video frequency.
What I found was, that the offset between video and force data increased
with recording time which means that either the video or the force
sampling rate (or both) did not exactly have the value I set them to.

I did least square fits of the video data during the first free fall
phases of the stick for every trial. Trying to reproduce gravitational
acceleration as close as possible I found out that the video rate error
was ~1.e-3, so I assumed that the sampling rate of the force data
significantly incorrect.

Arnim Henze
Institut f\"ur Astronomie und Astrophysik - Abteilung Biomechanik -
Auf der Morgenstelle 10, D-72076 T\"ubingen, Germany
email: henze@tat.physik.uni-tuebingen.de
Tel.: ++49 7071 29 78654
Fax : ++49 7071 29 5889

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