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plate coordinate system origin to the working surface of the force platform.
Normally the force plate coordinate system origin is below the surface of the
platform and the coordinate system z-axis is directed downwards, so that the sign of
the distance entered in ORIGIN(3,) will be negative.
TYPE-2
For a Type-2 force platform, this parameter must hold the components of the vector
pointing from the origin of the FP coordinate system to the point at the geometric
center of the force platform working surface. This vector must be expressed in the
force platform coordinate system.
The vector locates the center of the working surface of the force plate within the
force plate coordinate system. The Z component of this vector should be negative.
Please note that the AMTI documentation has the opposite sense of this vector. The
force plate offset vector should locate the center of the working surface of the plate
relative to the force plate measurement origin and in the force plate coordinate
system. The direction of the force plate coordinate system axis (Z axis) that is
normal to the working surface of the force plate (usually the vertical axis but the
force plate could be on its side) is directed away from the working surface of the
force plate. Thus, you must travel in a negative Z direction in the force plate
coordinate system to reach the working surface.
At the time of writing, applications written by one well known motion capture
system do not store the correct ORIGIN value for TYPE-2 force plates. Instead, C3D
files are generated using the supplied FP values. This will produce significant errors
in any application that calculates center of pressure, power, and moments as these
calculations will assume that the force plate is mounted above the force surface,
based on the incorrect Z ORIGIN value.
Supplied FP values
ORIGIN parameter values
X
3.9
-3.9
Y
-4.6
+4.6
Z
40.2
-40.2
Figure 31 – Force platform ORIGIN values
The AMTI documentation locates the force plate origin relative to the middle of the
working surface and reports this vector in the force plate coordinate system.
Therefore, the C3D file and AMTI values should be equal and opposite as shown
above.
The origin of the force plate coordinate system is determined by the positions and the
gains of the transducers and may not lie exactly below the center of the force
platform surface.
TYPE-3
For a Type-3 force platform, these values record the sensor offsets. ORIGIN(1,) must
contain the distance between the transducer axes and the force platform y-axis.
ORIGIN(2,) must contain the distance between transducer axes and the force platform
x-axis. ORIGIN(3,) should contain the distance between the force plate origin and the
surface of the force platform. Since the force platform z-axis projects down, this
value will normally be negative as it records the distance within the force plate
coordinate system.
92 • Required Parameters
The C3D File Format User Guide