Presented by Adam Galloy¹, Kayt Frisch PhD¹, and Dylan Schmitz²

¹Dordt College, ²University of Wisconsin-Madison

Contact: kayt.frisch@dordt.edu

In order to perform an accurate dynamic simulation, external force data must be correctly prepared. One of the most common sources of external force data is a force plate. This page provides a guide to preparing external force data for use with OpenSim models. For more information on community developed tools for preparing lab data for OpenSim models see the Tools for Preparing Motion Data page.

Preparing for Application to the Model

Knowing where on a model the force is applied is just as important as knowing the quantity of that force. However, many force plates export only the force and moment components (Fx, Fy, Fz, Mx, My, Mz). Using these force and moment components the center of pressure of the net force may be calculated. The center of pressure (COP) gives the location of a force vector that will have the same net effect as the total force when coupled with an additional moment about the y-axis called the free moment. This vector will have the same force components as the total force, but will also be coupled with a free moment. The components of the center of pressure and the free moment for the force plate above can be found with the following relationships where x is the COP’s x-component, z is the COP’s z-component, and y is the distance from the surface of the force plate to the force transducers:

Once the center of pressure of the force and the free moment are calculated, care must be taken to make sure these quantities are then transformed to the model's coordinate system correctly. The equations above give the center of pressure from the force plate's coordinate system and not necessarily the same coordinate system as the model or marker data. The free moment equation above gives the free moment about the y-axis of the force plate's coordinate system.

Preparing the File Format

OpenSim reads external force data saved in the .mov file format. However, motion capture systems or force plates may not export data as .mot files. Other more commonly exported file formats for force data include .csv and .c3d. For more on how .mot files are formatted see the Motion (.mot) Files page.

Preparing the Coordinate System

OpenSim models may have a different coordinate system than the exported force data. For example, many force plates align the z axis of their coordinate systems with vertical reaction forces; however, most OpenSim models align gravity along the y-axis meaning the vertical direction in the model must be the y-axis. These differences must be reconciled before the force data can be used in OpenSim. In addition to reconciling the force coordinates with the rotation of the model coordinates, the force coordinates must agree with the translation of the marker coordinates. Often when COP data is read or calculated from the force plate, the coordinates are given relative to the center or a corner of the force plate. This might not agree with the coordinate positions markers have.

By taking advantage of the preview experimental data feature, you can check whether or not your force locations agree with your marker locations. For example, in the first of the two images below the arrow representing the force vector is far away from the subject’s markers. This indicates that the force and marker data sets’ coordinate systems are not reconciled. In the second image, the foot markers and the ground reaction force arrow are in the same spot. This indicates that the coordinate systems are more accurately reconciled.

To use this feature, locate Preview Experimental Data… in the File drop-down menu. Then load both the .trc with the motion data, and the .mot with the force data

For more on reconciling coordinate systems see the Coordinate Systems in OpenSim page.

Next: Coordinate Systems in OpenSim

Previous: Preparing Marker Coordinate Data

Home: ASB 2018: Pipeline Shared by Dordt College and UW Madison

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