FP Type 4
Similar to a FP_Type_2 platform except that a CalMatrix is specified.
C3D Parameters
The following parameters should exist:
- FP_USED= number of force platforms used
- FP_TYPE= 4
- FP_ZERO= the range of frames for which a background noise level is calculated and then subtracted from each of the analog channels
- FP_CHANNELS= Fx, Fy, Fz, Mx, My, Mz
- FP_ORIGIN= The origin of the force platform in the force platform coordinate system (see the force platform manufacturer specifications.
- FP_CORNERS= The (x,y,z) coordinates of the force platform corners in the laboratory coordinate system. Note that there is a specific ordering of the four corners.
- FP_CALMATRIX= 6 x 6 Calibration Matrix.
Compute Force, COP, and FreeMoment
C3D Channels
- [Original Analog]= [Fx_original, Fy_original, Fz_original, Mx_original, My_original, Mz_original]
- as specified by the ANALOG channel numbers
Compute Baselines
- Average the Analog signals for the specified ZERO frames
Subtract Baselines
- [Tared Analog]= [Original Analog]-[Baseline]
Pre-Multiply by the CalMatrix
- [Analog]= [Fx, Fy, Fz, Mx, My, Mz]= [CalMatrix][Original Analog]
- NOTE: For type 2 the CalMatrix is the identify matrix
Specify Force Vector
- Force[X]= Fx
- Force[Y]= Fy
- Force[Z]= Fz
For Type 2 LengthConversion = Point units conversion to meters;
- e.g. for POINT units=mm LengthConversion=0.001
For Type 4 LengthConversion = 1 because Length Conversion is part of the CalMatrix
- M[X]= Mx*LengthConversion
- M[Y]= My*LengthConversion
- M[Z]= Mz*LengthConversion
Compute Center of Pressure
- COP[X]= (ORIGIN[Z]*Force[X] - My) / Force[Z]
- COP[Y]= (Mx + ORIGIN[Z]*Force[Y]) / Force[Z]
- COP[Z]= ORIGIN[Z}
Compute Free Moment
- FreeMoment[X]= 0
- FreeMoment[Y]= 0
- FreeMoment[Z]= Mz - ( COP[X]*Force[Y] - COP[Y]*Force[X] )
These resulting signals are computed in the force platform coordinate system, and must be transformed into the laboratory coordinate system for use in Visual3D.
Applying Threshold
Under the Visual3D Force Menu is an option to set a threshold for force platform/structure data.
Any signal value less than this threshold is set to 0 (is assumed to be noise)
Transforming data to the Laboratory Coordinate System
At this point the signals have been computed in the Force Platform Coordinate system and must be transformed into the Laboratory Coordinate System to give them meaning relative to the Motion Capture Data.
Example
The following example creates the force platform parameters for a c3d file that contains the ANALOG signals from the force platform but does not contain any parameters.
Download and Open the [example c3d file].
Select Modify Force Platform Parameters from under the File Menu. The following dialog will appear
Select the Get Current C3D Parameters. There should be no change to the dialog because there aren't any parameters in the file. You can check this by looking in the Parameters section of the Data Tree; there should be only one parameter used, which is equal to zero indicating no force platforms.
For a type 4 force platform the edit boxes shown must be filled in.
The CalMatrix and ORIGIN parameters are provided in the AMTI manual. Each force platform has different values.
The channel numbers should correspond with the force platform signals. Visual3D stores the ANALOG signals alphabetically in the data tree, which is not necessarily consistent with the C3D file order. To see the C3D order you must look in the ANALOG parameters.
The force platform corners are the locations of the corners of the force platform in the laboratory coordinate system.
Download a [pipeline command] to modify the force platform parameters for the example file.
From the Visual3D pipeline, load the pipeline file that you just downloaded. Execute the pipeline and all force platform parameters will be updated.
