IMU quarternion output interpretation?

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If you check the IMU message type defined by ROS, you can see that it has three main elements. Linear velocity, angular velocity and orientation. The angular velocity is the one expressed in rad/sec containing three values for three axes. Only the orientation is expressed in Quaterions and when you transform it to Euler angles, it represents the rotation around the three Cartesian axes, which means it is a pose information and not velocity (where the z axis is probably the opposite gravity direction and the x and y axes are initialized at the beginning). It is not a good idea to try imagining or visualizing quaternions as it is impossible (At least for the average human being). Here is a nice video about them. The best thing to do is to get Rviz to display your IMU message.