I am facing some problems with the OpenSense tool in OpenSim, especially with the orientation of the IMUs in the OpenSim world.
I'm working with Inertial Measurement Units to measure upper-body kinematics of Downhill Mountainbike athletes. I'm still in the testing phase, trying to make an OpenSim Model based on the IMUs (I use Shimmer3 IMUs) placed on a participant (2x ulna, 2x humerus, sternum, and pelvis) doing basic movements. The Shimmer IMUs provide both 6- and 9-degree-of-freedom quaternions. I converted the Shimmer3 file format to an APDM file format to create a .sto file as described in the OpenSense documentation. However, the problem arises when I use the IMU Placer and IMU Inverse Kinematics tools.
I have my pelvis IMU as the base IMU, which is placed so that the IMU -z axis is facing the same direction as the participant's direction facing forward. I set the pelvis_imu -z as the heading correction in the GUI. After applying a -90 degrees rotation around the x-axis, the axes displayed in OpenSim align with how I placed the IMUs on the participant. However, when I start IMU IK, the model rotates in an unusual way (see picture - it should be in an N-pose).
- Orientation_initial.png (116.27 KiB) Viewed 83 times
- orientations_afterIK.png (161.17 KiB) Viewed 83 times
As mentioned, I use Shimmer3 IMUs, which likely have a North-West-Up orientation (though I could not confirm this in the documentation). The file I use for the IMU Placer is the same as the first row of my IMU IK file, where the participant is in an N-pose. The default pose of my model is also an N-pose.
Has anyone faced similar issues? Do you have any insights on determining the correct settings for sensor heading correction and rotation in the IMU Placer tool?
Thank you in advance for your time and expertise!