My problem is that when i hold my sensors (MPU9150) so that +y axis is downward, and y axis is on the horizontal plane, i expect that pitch = 90 degree, and roll = 0 degree, but actually pitch = 90 degree, and roll = 160 degree. However, when roll = 90 degree and pitch = 0 degree (That is what i expect). Do you know what cause my problem? Thanks
For the most common definition of roll, pitch, yaw used in UAVs, when pitch=90deg you encounter what's called gimbal Lock. Basically, when pitch=90deg you have infinite combinations of roll and yaw that will give you the same physical result.