Theta angle of arm flexion/extension

Hi i have a question about Euler angles and quternions. If you have any suggestion please let me know.

Goal: I have an IMU sensor attached to my wrist that is streaming unit quaternions and Euler angles. I want to measure flexion/extension angle regardless any other motion of my arm (for example abduction/circumduction).

Ideas: First of all i’m using quaternions for all of my operations to avoid gimbal lock, and then i convert to euler angles to display Θ angle(of flexion/extension) to the user. I also set an initial quaternion (that can be my inertial coordinate frame) and compare all quaternions with initial.

result_q = Conj(initial) * q

last i get yaw, pitch, roll from result_q.

Problem: If i don’t make any other move except of flexion/extension i get the correct Θ. The problem occurs when i also rotate my wrist(circumduction). Then if the sensor is not in the initial position i get wrong angles.

submitted by /u/rombins
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