The FORCE_PLATFORM:TYPE parameter is an array of signed integers that define the type of force platform output expected from each force platform. The TYPE array size must be equal to or greater than the value of the FORCE_PLATFORM:USED parameter. Initially, the C3D specification supported three force platform types (1-3), with the Type-4 platform added in the early 90’s to support the inclusion of the full force plate calibration matrix. Since that time various manufacturers have added further force platform descriptions as the need arises.
The analog data from each force platform is stored in the associated analog channels defined by the FORCE_PLATFORM:CHANNEL parameter – the data stored from each force plate channel is scaled by the ANALOG:SCALE parameter. All motion capture systems except the Vicon Motion Systems Nexus system store unprocessed raw analog data from each force plate channel in the associated analog channel. These values are then scaled using the ANALOG:SCALE parameters and, if available, the FORCE_PLATFOM:CAL_MATRIX parameters.
Introduced in 2007, the Vicon Motion Systems Nexus system stores only processed force plate data – the force plate TYPE is described as TYPE-2 while the calculated force and moment data are stored in the analog channels defined by the FORCE_PLATFORM:CHANNEL parameter. The relevant ANALOG:SCALE parameters set to a value of 1.00, indicating that the data has already been scaled by the Nexus software and can be interpreted directly as three forces (Fx, Fy and Fz) and three moments (Mx, My and Mz). This scheme relieves the end-user of the problems of applying the SCALE and CAL_MATRIX parameters to the data but eliminates the ability to review the raw force plate signals in the event of a problem with the force plate.
In addition, Vicon Nexus C3D data must be stored using the floating point format and can not be converted to the integer format without rescaling the force plate data. This is because integer overflow can occur as the scaled force plate data (especially the Mx and My moments) can easily exceed the 16-bit integer storage range.
The following force platform types are described in the C3D specification:
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