After reading this:http://www.euclideanspace.com/physics/kinematics/angularvelocity/quatDiff1stAttempt.htmhttp://www.euclideanspace.com/physics/kinematics/angularvelocity/
I did something like:
btQuaternion diffQuater = gyroQuater - boxQuater;
//W(t) = 2 * dq(t) /dt * conj(q(t))
velQuater = ((diffQuater * 2) / d_time) * diffConjQuater;
But it is not working as I expect, I mean, there is written, vector part of the result quaternion should be vector of angular velocity, and scalar part should be 0, but my result is not like that.
angular velocity vector represented as quaternion with zero scalar part, i.e
W (t ) = (0, W x (t ), W y (t ), W z (t ))