IMU Plus Relative Orientation Initial Frame of Reference

I have been using my MMR sensors in IMU plus mode and am wondering how it sets the initial frame of reference. According the the documentation IMU plus mode calculates the orientation of the sensor relative to its initial start position.
When i have the sensor sitting flat on a table and start a sensor fusion the angles are initially 0,0,0 (which makes sense, since the orientation is relative to its initial position)
Where I am getting confused is if I tilt the sensor up on its edge (i.e. pitch 90) and then start sensor fusion I would expect to have that position as my initial reference frame and the orientation again being 0,0,0 since it is relative to my initial position but I am instead getting a pitch of 90. I seems as if the x and y axis are alway perpendicular to gravity and not actually relative to the sensors start position.

I am wondering if this is normal behaviour and specifically how the initial reference frame is set in IMU plus mode.
Thank you in advance.


  • The sensors are factory calibrated.
    This is normal behavior.

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