SOLVED! IMU's not correctly placed after IMUPlacer in Python/MATLAB

[Ton Leenen]

Hello,

I'm using OpenSim 4.1 via the Python API to run the IMU OpenSense IK, but unfortunately the orientation of the IMU's on the segments of the upper extremity model are not correctly placed after using the IMUPlacer, regardless of using the 'sensor_to_opensim_rotations' and 'heading_correction' function. The orientations of the Xsens DOT sensors were used as input to run the IMU OpenSense IK.

I have added an screenshot of the upper extremity model after placement of the four IMU's on the segments without using the 'sensor_to_opensim_rotations' and 'heading_correction' functions.

What could be reasons for this misalignment? I

Screenshot 2021-11-18 at 14.49.10.png (609.89 KiB) Viewed 616 times

The placement of the Xsens DOT sensors on the segments are presented below.

Screenshot 2021-11-18 at 14.53.25.png (67.59 KiB) Viewed 616 times

Screenshot 2021-11-18 at 14.53.35.png (65.33 KiB) Viewed 616 times

[Ton Leenen]

All the Xsens DOT sensors we used are defined in the earth-fixed reference system, but when using the heading reset option in the Xsens DOT application the axes in the horizontal plane are set to zero regardless of the orientation at that moment in time (in my case the calibration pose of the participants). All the sensors are logically differently positioned and oriented on the segments of the participants, but the heading reset introduces because of that a different offset to the orientation of each of the sensors. The consequence of the heading reset is that each sensor is now defined differently with respect to the earth-fixed reference system leading to unrealistic motion reconstructions.

The take home message is do NOT use the heading reset if you are using the Xsens DOT sensors and OpenSim!

[Olivier Desclaux]

Hi Ton,

Thanks for your question and answer, I'm running into the same problem! (Except that I'm using the MTw Awinda.)

I had the bad idea to call the reset Heading function multiple times, and now my IMUs all have different baseline orientations. I was wondering how you managed to "undo" the heading reset?

Thanks a lot for your help,

Olivier