Robust Trajectory Tracking for a Control-Surface-Free Tilt-Rotor Convertible UAV

Abstract

Convertible UAVs unlock a new range of applications, by combining the flight features of vertical take-off and landing (VTOL) and fixed-wing UAVs. Tilt-rotor UAV (TRUAV) is a popular category, in which switching from one flight mode to another is achieved by tilting some or all the rotors. In this work, we consider a new TRUAV design that does not include any control surfaces. This design, which we named control-surface-free TRUAV (CSF-TRUAV), exploits only propellers to control the drone’s position and attitude in both VTOL and fixed-wing modes. We also consider a control scheme that, unlike existing works, uses a single controller to handle both flight modes. This makes the transition from VTOL to cruise mode no longer an issue. This control scheme is implemented using a sliding mode controller (SMC), and validated on the full nonlinear model of the CSF-TRUAV, including all coupling and aerodynamic effects. The obtained results show the incapacity of a first-order SMC in dealing with the aerodynamic forces and moments, which act as external perturbations if they are not accurately estimated and fed to the controller. To deal with this issue, a super-twisting SMC (ST-SMC) is designed. The ST-SMC was capable of accurate trajectory tracking in both VTOL and fixed-wing modes.