# Quadrotor Cascade Control System Design Based on Linear Active Disturbance Rejection Control
###### Abstract
In addition to their own shortcomings, the control methods described above cannot simultaneously take into account internal parameter uncertainties by coupling between quadrotor control channels, wind disturbances, and other nonlinear disturbances, and can only deal with one or two disturbances; thus, further research is needed. If the quadrotor is disturbed, the goal is for it to continue to fly on the optimal path [25] to reach the desired position using the shortest path and to obtain a longer endurance based on the original energy. Combining the advantages of the above control schemes, this paper utilizes LADRC and SMC to design a simple and more robust quadrotor cascade control scheme for quadrotor UAVs with long endurance. The contributions of this paper are outlined as follows:  
First, when the quadrotor UAV is in flight, external disturbances, internal disturbances, and controller parameter deviations can all affect its smooth operation. LADRC can effectively estimate the internal and external disturbances of the system, but there are many adjustable parameters; therefore, we introduce adaptive control to adjust the PD controller parameters online according to the input and output of the system. This adaptive LADRC-based quadrotor UAV control system can effectively compensate for the tracking error caused by the controller parameters and make the control system more robust. Secondly, this paper proposes a cascade control system design based on linear active disturbance rejection control with sliding film control as the outer loop of the control system and adaptive control-based LADRC as the inner loop of the control system. This strategy, by introducing SMC, can effectively improve the robustness of the controller and accelerate the response speed. The position control channels of the quadrotor system are controlled by independent SMC&A-LADRC. This strategy can effectively suppress the external interference of the quadrotor, overcome the shortcomings of low LADRC tracking accuracy, and realize the quadrotor UAV flying with the optimal path to achieve long endurance flight. Third, LESO can estimate and compensate for the total disturbance of the quadrotor system in real time. We treat the chattering caused by SMC as disturbance, observe it in real time by LSEO, and then compensate for it by PD controller to further improve the robustness of SMC&A-LADRC. The control scheme combines the advantages of both, uses LESO to solve the problem of chattering in SMC, and applies it to the position control channel, so that the quadrotor still follows the desired trajectory when disturbed, and achieves the purpose of an optimal flight path. The structural framework of this paper is presented as follows: Section 2 presents a six-degrees-of-freedom quadrotor UAV dynamics model. Section 3 proposes a LADRC-based tandem control scheme for the quadrotor system and proves the stability of the system by Lyapunov theorem of stability. Section 4 simulates and analyzes the control strategy proposed in this paper and tests the trajectory tracking of the quadrotor UAV under different conditions. Finally, Section 5 provides a detailed summary of this paper.