Effects of endowing tilt-rotor mechanisms in the context of multi-copters

Abstract

Standard fixed-rotor multi-copters are classified as underactuated systems. For such cases, position and attitude control cannot be achieved independently due to the number of control inputs. To overcome this, novel multi-copter designs containing vectoring thrust mechanisms have been studied. This strategy, denominated tilt mechanism, has been widely employed on bi-copters, tri-copters, quad-copters and hexa-copters. In this context, this study concerns the dynamical modeling of a tilting rotor multi-copter aerial vehicle capable of tilting its motors laterally. Based on that, the control decoupling properties of the model are explored emphasizing the trade-off between mechanical complexity versus system maneuverability and controllability. For this, a tilt-rotor bi-copter test bench is developed for model and control validation. Then, two control design formulations are proposed and validated experimentally. First, the Linear Quadratic Tracking (LQT) is employed for trajectory tracking considering fixed and tilt-rotor configurations. In the sequel, a Model Predictive Controller (MPC) is designed for position tracking considering tilt deflection limitations, actuator dynamics and control decoupling between attitude and horizontal displacement. The results have shown that the tilt mechanism dynamics have major influence in lateral motion of the multi-copter. Moreover, the MPC dynamic decoupling control strategy presented some improvement over the LQT controller being able to properly handle the tilt-deflection constraints.