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visual3d:tutorials:kinematics_and_kinetics:force_platforms [2024/12/02 16:20] – [Force Platforms] wikisysopvisual3d:tutorials:kinematics_and_kinetics:force_platforms [2025/01/15 19:42] (current) – [Practical Example] wikisysop
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 Once familiar with the foundational concepts outlined in the overview, return to this page to follow a step-by-step example of working with Type 4 force platforms in Visual3D. This tutorial provides practical guidance to help you set up and analyze data effectively. Once familiar with the foundational concepts outlined in the overview, return to this page to follow a step-by-step example of working with Type 4 force platforms in Visual3D. This tutorial provides practical guidance to help you set up and analyze data effectively.
 +
 +If you would prefer to watch a video walk-through of this tutorial, please follow the link here: [[https://youtu.be/K0tWmFYWpGc?feature=shared|Visual3D Tutorial Video 5: Force Platforms]]
 ==== Interpreting the C3D File ==== ==== Interpreting the C3D File ====
  
-We will begin by reviewing the information stored in the [[visual3d:documentation:c3d_signal_types:c3d_format|C3D file]].+We will begin by a brief review of the information stored in the [[visual3d:documentation:c3d_signal_types:c3d_format|C3D file]], this is also available on the Force Platforms Overview page mentioned previously.
  
 === C3D Parameters === === C3D Parameters ===
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 Type 4 platforms also have a 6x6 CalMatrix that specifies how the original analog signals should be transformed. Type 4 platforms also have a 6x6 CalMatrix that specifies how the original analog signals should be transformed.
  
-==== Computing Force, COP, and Free Moment ==== +==== Preparing for Tutorial ==== 
- +The required material for this tutorial are contained within [[https://has-motion.com/download/YouTubeTutorial/Visual3D Tutorial 5 Force Platforms.zip|Visual3D Tutorial 5 Sample Data]], containing: 
-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. +  * **Tutorial5_ForcePlatforms_Start**: Used to start the tutorial, .cmz file containing the .c3d movement trial file. 
- +  * **walkNoFpParameters**: Movement trial file containing no force platform parameters, changes made throughout the tutorial. 
-{{:fp_type6.jpg}} +  * **Modify_Force_Platform_Parameters.v3s**: Pipeline script to modify force platforms for the example file. 
- + 
-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. +
 ==== Practical Example ==== ==== Practical 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. 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 [[https://www.has-motion.com/download/examples/walkNoFpParameters.c3d|example c3d file]]. \\ \\ {{:Amti1.png}} \\ \\+  - Download the ZIP file provided above and open **Tutorial5_ForcePlatforms_Start.cmz**. \\ \\ {{:Amti1.png}} \\ \\
   - Select Modify Force Platform Parameters from under the Force Menu. The following dialog will appear \\ \\ {{:Amti2.png}}\\ \\   - Select Modify Force Platform Parameters from under the Force Menu. The following dialog will appear \\ \\ {{:Amti2.png}}\\ \\
   - 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. \\ \\ {{:Amti3.png}} \\ \\   - 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. \\ \\ {{:Amti3.png}} \\ \\
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   - 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. \\ \\ {{:Amti5.gif}} \\ \\   - 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. \\ \\ {{:Amti5.gif}} \\ \\
   - The force platform corners are the locations of the corners of the force platform in the laboratory coordinate system.   - The force platform corners are the locations of the corners of the force platform in the laboratory coordinate system.
-  - Download a [[https://www.has-motion.com/download/examples/modifyForcePlatforms.v3s|pipeline script]] to modify the force platform parameters for the example file. +  - Make sure that the **Modify_Force_Platform_Parameter.v3s** pipeline script is within the ZIP file, this will be used 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. \\ \\ {{:Amti6.gif}} \\ \\+  - From the Visual3D pipeline, load the in the pipeline file. Execute the pipeline and all force platform parameters will be updated. \\ \\ {{:Amti6.gif}} \\ \\
  
 === Modifying the Force Platform Parameters === === Modifying the Force Platform Parameters ===
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 **Troubleshooting Tip:** If the ANALOG signals for the force platform exist, but the FORCE signal is zero, examine the FZ1 graph and verify that the Fz Channel of the ANALOG signals is negative. If not, then the ANALOG scale factor in the C3D file has probably been stored incorrectly by the motion capture software. The scale factor will need to be changed in the motion capture software and/or the file. **Troubleshooting Tip:** If the ANALOG signals for the force platform exist, but the FORCE signal is zero, examine the FZ1 graph and verify that the Fz Channel of the ANALOG signals is negative. If not, then the ANALOG scale factor in the C3D file has probably been stored incorrectly by the motion capture software. The scale factor will need to be changed in the motion capture software and/or the file.
  
-Continue to the [[visual3d:tutorials:events:event_processing|next tutorial]].+Continue to the [[visual3d:tutorials:events:event_processing|Event Processing]].
  
visual3d/tutorials/kinematics_and_kinetics/force_platforms.1733156411.txt.gz · Last modified: 2024/12/02 16:20 by wikisysop