Dual-Loop Robust Attitude Control for an Aerodynamic System With Unknown Dynamic Model: Algorithm and Experimental Validation

This paper proposes two kinds of dual-loop nonlinear robust control strategies that are implemented on an open-loop unstable two-degree-of-freedom (2-DOF) helicopter system with unmodeled dynamics and uncertainties. The inner feedback loop is considered as a nominal controller realized by an existing "intelligent" proportional differential controller (iPD) while the outer layer feedback control is regarded as a compensation loop. We study two different forms of outer loop in this paper. One is model-free sliding mode compensator (MFSMC) and another is model-free data-driven compensator (MFDDC). The combination of the shared inner loop and either of the outer loops forms two different kinds of model-free robust control strategies, i.e., iPD-MFSMC and iPD-MFDDC. Both robust control approaches are validated experimentally on the attitude tracking control of a 2-DOF laboratory helicopter, whose control objective is to have the helicopter attitudes, i.e., pitch and yaw motions, track specified trajectories. To demonstrate the utility of the two control approaches, we compare them with linear quadratic regulator (LQR), optimal feedback linearization control (OFLC) and iPD, respectively. The extensive comparison of the simulation and experimental results shows that the dual-loop robust control approaches are quite promising in controlling the systems with unknown dynamical models.