The modeling and attitude stabilization control problems of a four-rotor vertical takeoff and landing unmanned air vehicle (UAV)

known as the quadrotor are investigated.The quadrotor’s attitude is represented by the unit quaternion rather than Euler angles to

avoid singularity problem. Taking dynamical behavior of motors into consideration and ignoring aerodynamic effect, a nonlinear

controller is developed to stabilize the attitude.The control design is accomplished by using backstepping control technique.The

proposed control law is based on the compensation for the Coriolis and gyroscope torques. Applying Lyapunov stability analysis

proves that the closed-loop attitude system is asymptotic stable. Moreover, the controller can guarantee that all the states of the

system are uniformly ultimately bounded in the presence of external disturbance torque.The effectiveness of the proposed control

approach is analytically authenticated and also validated via simulation study.