iphone - core motion (relative rotation)

Question

Is there a way to obtain a relative rotation from core motion?

What I need is: how much it rotated in one axis and which direction (+ sign = anti-clockwise, - = clockwise, according to the right-hand rule).

I have found the property rotationRate, but I am now sure how I would extract the angle out of it, as this is giving me radians per second.

I have done all kind of stuff on the last days but nothing is giving me stable values. I have tried to do a timed sample of core motion data, using a NSTimer and calculate the difference between two samples, so I would have how much it rotated since the last sample, but from times to times it gives me crazy numbers like 13600 degrees even when the iPhone is resting on the table.

Any thoughts on how this can be accomplished?

thanks

Answer 1

There is indeed. You can get what you're looking for by drilling down into the properties of CMMotionManager, through CMDeviceMotion and finally to CMAttitude. The attitude of the device is defined as:

the orientation of a body relative to a given frame of reference.

In the case of DeviceMotion's CMAttitude, that frame of reference is established by the framework when starting device motion updates. From that point in time on, the attitude of the device is reported relative to that reference frame (not relative to the previous frame).

The CMAttitude class provides some handy built in functionality to convert a CMAttitude to a form that is actually useful for something, like Euler Angles, a rotation matrix, or a quaternion. You sound like you're looking for the Euler Angle representation (Pitch, Yaw, Roll).