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