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Re: Summary: force plate issue

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  • Re: Summary: force plate issue

    Dear Young-Hoo and *!

    Just to add another important point: A force plate is normally constructed
    like a table standing on four posts. In each post three components of the
    forces transmitted through the posts are measured. Due to the construction
    of force plates there are generally also force couples (free torque)
    transmitted through the posts, as these posts are clamped to the ground and
    to the `table's surface'. The equations presented by Young-Hoo Kwon are
    correct under the assumption that in the `plane of measurement', which is
    located at a distance c beneath the force-plate's surface, these free
    torques sum up to zero. Due to the deformation characteristic of
    force-plates the plane, where the free torques transmitted through the posts
    sum up to zero, depends on the loading conditions. Therefore Kwons Equations
    [15] to [17] should be written

    X = [ a*( F1z - F2z - F3z + F4z ) - c*Fx + T1y + T2y + T3y + T4y ] / Fz
    Y = [ b*( F1z + F2z - F3z - F4z ) - c*Fy + T1x + T2x + T3x + T4x ] / Fz
    Tz = a*( Fy14 - Fy23 ) - b*( Fx12 - Fx34 ) - X*Fy + Y*Fx + T1z + T2z + T3z +
    T4z [18new]


    Where Tix, Tiy, and Tiz are the components of the free torques transmitted
    through each post. Generally these torques are not measured in the
    force-plates and it is assumed that in the plane of measurement these
    torques vanish. This is one of the reasons for the errors in the
    determination of the COP wit force-plates. For more details see appendix B

    H.-B. Schmiedmayer, and J. Kastner, 2000, Enhancements in the Accuracy of
    the Center of Pressure (COP) Determined With Piezoelectric Force Plates Are
    Dependent on the Load Distribution. J.Biomech.Eng., 122, 523-527, (2000).

    For a siple 2-D Model of force plates see:
    Schmiedmayer, H.-B., and Kastner, J., 1999, Parameters Influencing the
    Accuracy of the Point of Force Application Determined With Piezoelectric
    Force Plates, J.Biomech., 32, pp. 1237–1242.

    Kind regards

    Heinz-Bodo Schmiedmayer

    Vienna University of Technology
    Institute for Mechanics and Mechatronics
    Wiedner Hauptstraße 8-10/325/A1
    A-1040 Vienna – AUSTRIA – EUROPE