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You must also input orientations from one or more IMU sensors. We currently support Xsens and APDM file formats. You can create your own file converter to support any other IMU system and we plan to add support for additional sensor manufacturers in the future. We currently assume that sensor fusion and syncing have been performed using a vendor’s or third-party algorithm. Several open-source sensor fusion algorithms are also available on GitHub. 

Registration and Calibration

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An inverse kinematics method is used to compute the set of joint angles at each time step of a motion that minimizes the errors between the experimental IMU orientations and the model’s IMU Frames. The angles can then be used as inputs to other OpenSim tools and analyses or you can visualize these angles in the OpenSim GUI. The OpenSense capabilities are available through the command line and through scripting (Matlab or Python). The resulting Model and Motion can be loaded, visualized, and analyzed in the OpenSim 4.0 GUI. In the future, we will also provide a direct GUI-based tool to run IMU-based kinematics. 

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Now that you've installed OpenSim, we will show you how to use the software through a hands-on example using example experimental IMU data from a study of lower extremity gait kinematics. The example data. , models, scripts, and setup files can be found in your OpenSim resources directory under /Code/Matlab/OpenSenseExample. You can also download a zip of the example files _here_.

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Once you have collected data and prepared it (i.e., compiled IMU-based orientations for calibration and movement trials), you must convert it to OpenSim's file format and associating associate it with an OpenSim model. Data from IMUs can be in various formats: a single file with numbered sensor names (e.g., APDM) or multiple files with sensor-specific numbering (e.g., Xsens). Upon import, OpenSim will create a single, time synced, Storage (.sto) file format for orientations, converting the rotation matrices into quaternions. 

In this example, we will be using data from an Xsens system that has been processed and exported to an Xsens text format. You can find this data in the IMUData folder. Each Xsens sensor is represented by a single text (.txt) file with time histories of the internal sensor data.

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