Robust control of underactuated 3-DOF helicopter based on lower order disturbance estimator

An active disturbance rejection control strategy based on Uncertainty and Disturbance Estimator (UDE) is designed, analyzed, and validated for underactuated 3-Degree-of-Freedom (3-DOF) helicopter affected by significant model uncertainties and external disturbances. In detail, an attitude trajectory tracking strategy of the elevation and travel angles is developed using feedback linearization based on the idea of full-drive theory for the helicopter experimental setup, which exhibits severe dynamic coupling and obvious underactuated characteristics, while the stabilization of the internal state (pitch angle) is guaranteed. Then, the first-order UDE is further proposed and combined with nominal control law to construct a robust control scheme for the design of the virtual control algorithm of the three channels with inner-outer loop structure, which can effectively compensate for uncertainty and external disturbance of the helicopter. The control scheme addresses the underactuated characteristics of the plant with continuous and smooth control signals, while the disturbance compensator has fewer parameters and the adjustment of system performance is concise and clear. Based on the singular perturbation theory, the convergence of tracking error, the input-state stability of the closed-loop system, and the influence of UDE parameters on the ultimate bound of tracking error are analyzed. In addition, the potential multiple time scale attributes of the system are revealed. The necessity of disturbance compensation, the effectiveness of UDE, and the convenience of control parameters adjustment are verified by both simulation and experimental results.