Robust Attitude Tracking for Aerobatic Helicopters: A Geometric Approach

This article highlights the significance of the rotor dynamics in control design for small-scale aerobatic helicopters and proposes two singularity-free robust attitude-tracking controllers based on the available states for feedback. The first employs the angular velocity and the flap angle states (a variable that is not easy to measure) and uses a backstepping technique to design a robust compensator (BRC) to actively suppress the disturbance induced tracking error. The second exploits the inherent damping present in the helicopter dynamics leading to a structure-preserving, passively robust controller (SPR), which is free of the flap angle feedback. The BRC controller is designed to be robust in the presence of two types of disturbance: structured and unstructured. The structured disturbance is due to the uncertainty in the rotor parameters, and the unstructured disturbance is modeled as exogenous torque acting on the fuselage. The performance of the controller is demonstrated in the presence of both types of disturbances through numerical simulations. In contrast, the SPR tracking controller is derived such that the tracking error dynamics inherits the natural damping characteristic of the helicopter. The SPR controller is shown to be almost globally asymptotically stable, and its performance is evaluated experimentally by performing aggressive flip maneuvers. Throughout this study, a nonlinear coupled rotor-fuselage helicopter model with first-order flap dynamics is used.