hschmiedmayer88

10-17-2005, 05:36 PM

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

[15new]

^^^^^^^^^^^^^^^^^^^^^^^

Y = [ b*( F1z + F2z - F3z - F4z ) - c*Fy + T1x + T2x + T3x + T4x ] / Fz

[16new]

^^^^^^^^^^^^^^^^^^^^^^^

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

of:

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

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

[15new]

^^^^^^^^^^^^^^^^^^^^^^^

Y = [ b*( F1z + F2z - F3z - F4z ) - c*Fy + T1x + T2x + T3x + T4x ] / Fz

[16new]

^^^^^^^^^^^^^^^^^^^^^^^

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

of:

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