  Quadrotors are widely employed across various domains, yet the conventional
type faces limitations due to underactuation, where attitude control is closely
tied to positional adjustments. In contrast, quadrotors equipped with tiltable
rotors offer overactuation, empowering them to track both position and attitude
trajectories. However, the nonlinear dynamics of the drone body and the
sluggish response of tilting servos pose challenges for conventional cascade
controllers. In this study, we propose a control methodology for tilting-rotor
quadrotors based on nonlinear model predictive control (NMPC). Unlike
conventional approaches, our method preserves the full dynamics without
simplification and utilizes actuator commands directly as control inputs.
Notably, we incorporate a first-order servo model within the NMPC framework.
Through simulation, we observe that integrating the servo dynamics not only
enhances control performance but also accelerates convergence. To assess the
efficacy of our approach, we fabricate a tiltable-quadrotor and deploy the
algorithm onboard at a frequency of 100Hz. Extensive real-world experiments
demonstrate rapid, robust, and smooth pose tracking performance.
