Robust Attitude Tracking in the Presence of Parameter Uncertainty for a Variable-Pitch Quadrotor

A variable-pitch quadrotor is designed with a single power plant which drives the four rotors at constant rotational speed. Thrust generated by each rotor is varied by changing the collective pitch angle of the blades. Unlike mass of the vehicle, rotational inertia of the quadrotor cannot be estimated exactly and always carries a bounded uncertainty. Controllers like PD, PI and PID are known to inherently take care of such uncertainties. However, existing works do not give a tuning method to determine the gains of such controllers based on the uncertainties present in the system parameters. This paper presents a robust linear controller for attitude tracking of a variable-pitch quadrotor in the presence of parameter uncertainties. The attitude dynamics is linearized about the hover condition and a PD controller along with a low-pass filter is designed for tracking a desired trajectory. A systematic approach to tune gains based on the robust stability condition for handling almost 100 percent uncertainty in inertia values is presented. Numerical simulations and experimental results are presented to validate the proposed control design.