Determining IMU placer rotation values

[Radhakrishnan Vignesh]

Hello,

I am trying to determine how to put the rotation values into IMU placer. I am following the webinar on Opensense and have questions regarding the same.

1) When using preview sensor data option on placement_orientation.sto, the axes are very random. How was it calculated to rotate about -90 degree on X axis? How do you use the preview sensor data option to calculate the angle of rotation?

2) Are we trying to rotate the IMU space to Opensim space similar to marker based systems or how do we calculate the rotations? How is the the placement orientation related to IMU space and Opensim space?

Thanks

[Carmichael Ong]

Responses for each of the questions below:

1) The tool cannot calculate this for you. If you have a standing trial where you know how the IMUs should be aligned, you can use this tool to give you some idea of what rotations will be needed. For our lab's setup, we saw the the data was off by 90 degrees from the OpenSim space.

2) Yes, this is similar to the marker based systems. Since every lab setup is different, the rotations between IMU and OpenSim space can be different for each lab setup, so we cannot give a general answer.

[Radhakrishnan Vignesh]

Hello Carmichael,

Thanks for your reply. Could you please explain in greater detail which coordinate system are we trying to match? Are we trying to match the imu sensor coordinates to that of opensim or are we just stating what the heading direction of the base imu is ? How does the east-north-up coordinate system affect these calculations ?

For example, in your lab setup, was the X axis facing forward ( in the direction of walk) with z upwards and y towards the left ? Or was the z facing forward ?
In the example files you shared, the preview of sensor data doesnt match the setup files, so I am confused.
Really sorry