Thrust command based integrated reference model with envelope protections for tilt-rotor vtol transition UAV

This paper proposes an integrated reference model for a tilt-rotor vertical take-off and landing (VTOL) transition aircraft in a unified incremental nonlinear dynamic inversion (INDI) control strategy. In addition to the preceding work in [1], new enhancements to the functionality and structure of the reference model are presented here. Firstly, a smooth transition to thrust command is added to transform the pilot stick behavior to that of a conventional aircraft stick in wingborne mode. Additionally, thrust level control provides stall protection hence improving robustness against wind. Moreover, to prevent bleeding of velocity during climb, additional compensation thrust is also added to the pilot command. Instead of using command variable derivative of the order of relative degree as pseudo control, derivative of one order higher than the relative degree is used. Consequently, desired dynamics for the pseudo controls can be defined within the reference model and actuator dynamics are taken into account by the INDI controller framework. Aircraft flight envelope protections using command value limitation are enforced based on the specifications in the phase plane of a protected variable. Flight test results show that the reference model produces physically meaningful trajectories for different flight maneuvers in hover as well as wing-borne flight leading to good tracking of the reference commands by the aircraft.