Attitude and position control for a small scale quad-tiltrotor

Small scale unmanned aerial vehicles (UAVs) are getting very popular among people and research communities due to their different applications such as traffic monitoring, exploration of disasters (fire, earthquake, flood), military purposes, etc. Although numerous research has been conducted so far, conventional quad-rotors still suffer from limited mobility and maneuverability. Having four independent control inputs (four propeller spinning velocities) versus six degrees of freedom (position and orientation) in space makes this kind of UAVs not fully controllable. In most of cases, position (X,Y,Z) and yaw angle are considered as outputs, then pitch and roll angles cannot be controlled. Eventually, they cannot track an arbitrary trajectory in space, they cannot stay in a desired position and orientation, they cannot remain in a predefined altitude and attitude in presence of disturbances such as wind, they are not able to land on ramp surfaces, etc. In this thesis we overcome the aforementioned problems by designing attitude and position controllers for a novel overactuated quad-rotor which has four extra motors actuating on the propellers tilting angles. To achieve the aim of this thesis, first, thanks to Newton-Euler formulation a comprehensive and accurate mathematical model is developed and then it is linearized around the hovering state. Second, an adjustable parameter beta (0=<beta=<1) is introduced which enables us to choose a desired mode of operation between conventional quad-rotor (beta=0) and full quad-tiltrotor (beta=1) dynamically. Finally, two optimal controllers (LQ and H_infinity ) are developed to control the position and attitude of the quad-tiltrotor in a desired state. These controllers are designed not only to stabilize the system, but also to ensure that the outputs track the reference inputs in presence of disturbances. Finally, the results of several simulations are reported to illustrate the capabilities of the proposed novel UAV design.