Abstract
This paper describes developing and simulating a Nonlinear Model Predictive Controller (NMPC) for a tiltrotor urban air mobility (UAM) aircraft. The aircraft's free flight is governed by a set of nonlinear rigid-body dynamic equations, considering multiple tiltrotors and their gyroscopic and inertial effects. The control variables include two push rotors' spin rates and the deflections of traditional control surfaces, including the elevator, aileron, and rudder. The performance of the NMPC is compared with the Linear Quadratic Regulator (LQR) and Model Predictive Controller (MPC) for vibration suppression during the level flight. The NMPC and LQR can fully remove pitch angle oscillation and stabilize altitude in approximately 15 s. The MPC, while still able to reduce the rigid body vibration cannot fully remove the oscillation in 80 s. The NMPC and LQR are compared for lateral and longitudinal trajectory path tracking, with the NMPC showing better performance in both cases due to its ability to take into account the nonlinear nature of the aircraft flight dynamics and predict the vehicle's future response when determining the best control inputs. Different from the vibration control case, the nonlinear nature of the aircraft flight dynamics should be accounted for by the controller design to properly track the ever-changing path reference, which is not the case for the LQR. While the NMPC has a higher computational cost, it demonstrates much better control performance than the MPC and LQR.
