How to determine absolute orientation

Question

I have a xyz accelerometer and magnetometer. Now I want to determine the orientation of the device using both. The problem I see is that depending on the device orientation, I'd need to use the sensors in different order.

Let me give an example. If I have the device facing me then changes in both the roll and pitch can be determined with the accelerometer. For yaw I use the magnetometer.

But if I put the device horizontally (ie. turn it 90º, facing the ceiling) then any change in the up vector (now horizontal) isn't notice, as the accelerometer doesn't detect any change. This can now be detected with the magnetometer.

So the question is, how to determine when to use one or the other. Is this enough with both sensors or do I need something else?

Thanks

Answer 1

I think the question "how to determine when to use one or the other" is misguided. You should always use both sensors for orientation. There are cases where one of them is useless. However, these are edge cases.

Answer 2

If I understand you correctly, you'll need something to detect pitch (tilting) and orientation according to the cardinal points (North, East, South and West).

The pitch can be read from the accelerometer.

The orientation according to the cardinal points can be read from a compass.

Combining the output from these two sensors correctly with the right math in your software will most likely give you the absolute orientation.

I think it's doable that way.

Good luck.

Answer 3

The key is to use the cross product of the two vectors, gravity and magnetometer. The cross product gives a new vector perpendicular to them both. That means it is horizontal (perpendicular to down) and 90 degrees away from north. Now you have three orthogonal vectors which define orientation. It is a little ugly because they are not all perpendicular but that is easy to fix. If you then cross this new vector back with the gravity vector that gives a third vector perpendicular to the gravity vector and the magnet plane vector. Now you have three perpendicular vectors which defines your 3D orientation coordinate system. The original accelerometer (gravity) vector defines Z (up/down) and the two cross product vectors define the east/west and north/south components of the orientation.