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--- visual3d:documentation:kinematics_and_kinetics:external_forces:force_platforms [2024/12/02 15:38] – wikisysop
+++ visual3d:documentation:kinematics_and_kinetics:external_forces:force_platforms [2024/12/02 15:43] (current) – wikisysop
@@ Line 76: / Line 76: @@
 | [[visual3d:documentation:pipeline:force_commands:fp_type_1|FP_TYPE 1]] | [[visual3d:documentation:pipeline:force_commands:fp_type_2|FP_TYPE 2]] | [[visual3d:documentation:pipeline:force_commands:fp_type_3|FP_TYPE 3]] | [[visual3d:documentation:pipeline:force_commands:fp_type_4|FP_TYPE 4]] | [[visual3d:documentation:pipeline:force_commands:fp_type_5|FP_TYPE 5]] | [[visual3d:documentation:pipeline:force_commands:fp_type_6|FP_TYPE 6]] | [[visual3d:documentation:pipeline:force_commands:fp_type_7|FP_TYPE 7]] |
+==== Computing Force, COP, and Free Moment ====
+The force signals of interest for our kinetic analysis are the [[visual3d:documentation:visual3d_signal_types:force_data_type|Force]], [[visual3d:documentation:visual3d_signal_types:cofp_data_type|Centre of Pressure]], and [[visual3d:documentation:visual3d_signal_types:freemoment_data_type|Free Moment]] signals. This figure depicts a Kistler force platform, but the diagrammed force vector is consistent for all force platforms manufacturers.
+{{:fp_type6.jpg}}
+Although each force platform type records different information in its different channels, the process for converting the original recorded analog signals into the desired force signals follows the same common steps:
+  - Compute baselines
+  - Subtract baselines from original signals
+  - Pre-multiply by the calibration matrix (if applicable)
+  - Calculate the Ground Reaction Force
+  - Apply a threshold
+  - Transform data to the Laboratory Coordinate System
+== Compute Baselines ==
+The [[Visual3D:Documentation:Pipeline:Force_Commands:FP_ZERO|FP_ZERO]] C3D parameter defines the range of frames used to calculate an average baseline value for each ANALOG signal.
+**Note:** most of the manufacturers arbitrarily set the range to be the first 10 frames of the trial. If the force platform is loaded during these first 10 frames, then **the ground reaction force signals will be incorrect**. If this is the case, then the user should either set the frames to 0,0 so that no baseline is subtracted, or should select frames in which the force platform is unloaded.
+== Subtract Baselines ==
+The baseline value for each ANALOG channel represents the value of that channel's signal under no load. It is subtracted from that ANALOG channel's original signal at every frame to produce a tared signal. This does not affect the original ANALOG signals as seen in the data tree since this processing takes place when the Force Platform signals are computed.
+<code>
+[Tared Analog] = [Original Analog]-[Baseline]
+</code>
+== Pre-multiply by the CalMatrix ==
+Several of the force platform types have a Calibration Matrix (Inverse Sensitivity Matrix) that is provided by the manufacturer. Visual3D pre-multiplies the values reported by the analog channels with this calibration matrix in order to convert the recorded analog signals from volts to the appropriate Force and/or Moment signal units.
+For example, for a type 4 force platform:
+<code>
+[Converted Analog] = [Fx, Fy, Fz, Mx, My, Mz]= [CalMatrix][Tared Analog]
+</code>
+**Note:** These signals are intermediate signals that are then used to compute the Ground Reaction Force.
+== Calculate the Ground Reaction Force ==
+The Ground Reaction Force is represented by 3 vectors: Force, Centre of Pressure (COFP), and Free Moment. Each is calculated from the the appropriate analog signals. Different force platform types require different calculations for these Ground Reaction Force vectors.
+For a Type 4 force platform, the force vector is equal to the intermediate force signals from the previous step.
+<code>
+Force[X] = Fx
+Force[Y] = Fy
+Force[Z] = Fz
+</code>
+For a Type 4 force platform, the centre of pressure vector is computed as follows:
+<code>
+COP[X] = (ORIGIN[Z]*Force[X] - My) / Force[Z]
+COP[Y] = (Mx + ORIGIN[Z]*Force[Y]) / Force[Z]
+COP[Z] = ORIGIN[Z]
+</code>
+**Note:** The [[Visual3D:Documentation:Pipeline:Force_Commands:FP_ORIGIN|ORIGIN]] is necessary for this calculation.
+For a Type 4 force platform, the free moment vector is computed as follows:
+<code>
+FreeMoment[X] = 0
+FreeMoment[Y] = 0
+FreeMoment[Z] = Mz - ( COP[X]*Force[Y] - COP[Y]*Force[X] )
+</code>
+== Apply the Threshold ==
+The Visual3D Force Menu contains an option to set a [[Visual3D:Documentation:Kinematics_and_Kinetics:External_Forces:Force_Platforms#Minimum_Force|threshold for force platform/structure data]]. Any computed signal value that is less than this threshold is assumed to be noise and is set to 0.
+== Transform 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.
+As long as the force platform is accurately located within the lab, for example by using the [[caltester:caltester_mode_overview|CalTester]], this is accomplished by a straightforward [[Visual3D:Documentation:Kinematics_and_Kinetics:External_Forces:Transform_from_FCS_to_LCS|translation]] from the force platform coordinate system to the lab coordinate system.
 ==== Learn More ====