Drifting yaw angle after moving fast

Question

In my current project I ran into trouble regarding the quaternion provided by Core Motion\'s CMAttitude. I put the iPhone 5 (iOS 6.0.1) at a well defined start position. The

Answer 1

I've done something similar in my own code and found the same z-axis rotational drift (yaw). I've applied a balanced filter. At each motion manager time interval, I grab the current quaternion (z-component) and then save that after calculations as oldZ, for use in the next set of calculations. My filter simply balances the NEW z value with the z value immediately preceeding it, preventing it from moving too much too quickly. Depending on your hardware and the exact tolerance in your program, you can manage drift quite well in this way. You will see the gyro drift slightly, but then begin to be corrected as the filter continues to act. My filter looks something like this and prevents more than 0.5 degree of "stray":

filtZ = 0.65 * oldZ + 0.35 * z;

The 0.65 and 0.35 values were determined experimentally and I would advise you to play with those as you have time. Output will still be scaled 0-1 and can then be utilized in the same way you've been doing (or re-introduced to the quaternion if you must retain all 4 dimensions throughoutf).

Answer 2

Not the definitive solution but at least a workaround for my own question (I leave it as unanswered to invite you). It turned out that at least DeviceMotion.gravity is not affected by the bug. So I decided to redesign this pretty simple part of motion detection and use arcsin (gravity.x/||gravity||) for moving the main player character to the side when tilting the device.

This is definitely the second best solution as it destroys information about the full rotation status contained in a quaternion. I decided to it that way for strategical considerations:

1. I think most developers do tilt motion detection with the gravity vector rather than CMAttitude.quaternion because most people are not that amused about quaternion maths ;-) Thus any future bugs related to the gravity vector will probably be fixed during the beta phase because of a larger number of users.
2. If it is a software bug and not related to hardware issues, what I assume, and if the bug will be fixed ASAP, there is still a number of devices that might not get updated for what reason ever. Thus the risk that a potential future customer will run into trouble is small but > 0. So the second best solution might be sometimes the best.