Quad-rotor unmanned aerial vehicle (UAV) is a typical multiple-input-multiple-output underactuated system with couplings and nonlinearity. Usually, the flying environment is very complex, so that it is impossible for the UAV to avoid effects derived from disturbances and uncertainties. In order to improve the reliability of flight control, we established the dynamic model of quad-rotor UAV by Newton-Euler equation in unbalanced load conditions. Considering external disturbances in the attitude, a second-order sliding mode controller was designed with PID sliding mode surface and Extended State Observer (ESO). The simulation experiments have got good control performance, illustrating the effectiveness of our controller. Meanwhile, the controller was implemented in a quad-rotor UAV, which carried a pan-tilt camera for aerial photography. The actual flight experiments proved that this paper dealt with the high stabilization flight control problem for the quad-rotor UAV, which laid a good foundation for autonomous flight of the UAV.

四旋翼无人机（UAV）是具有耦合和非线性的典型多输入多输出欠驱动系统。通常，飞行环境非常复杂，因此无人机无法避免由于干扰和不确定性而产生的影响。为了提高飞行控制的可靠性，我们在不平衡载荷条件下，通过牛顿-欧拉方程建立了四旋翼无人机的动力学模型。考虑到姿态的外部干扰，设计了具有PID滑模表面和扩展状态观测器（ESO）的二阶滑模控制器。仿真实验具有良好的控制性能，说明了控制器的有效性。同时，该控制器在四旋翼无人机中实现，该无人机搭载了用于航空摄影的云台相机。