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Re: Drop test synchronisation

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  • Re: Drop test synchronisation

    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
    Tel.: ++49 7071 29 78654
    Fax : ++49 7071 29 5889

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