VTOL UAV Altitude Flight Control Using Fractional Order Controllers ⋆

In this paper, a fractional order flight controller design scheme for the altitude control of vertical take-off and landing (VTOL) UAV is presented. The altitude control system can be decoupled and identified as a simple first order plus time delay model under some constraints. Based on this identified model, the traditional modified Ziegler-Nichols PI controller can be designed. Meanwhile, the integer order PID controller and the fractional order controllers can be designed following the same three specifications. Two specifications on the phase margin and gain crossover frequency are used to design the controllers for the desired performance. One more specification on flat phase of the Bode plot is applied for the controller robustness to the plant gain variations. The designed integer order PID controller, two fractional order controllers and the modified Ziegler-Nichols PI controller are all implemented and compared on the OS4 VTOL simulation platform. The simulation results illustrate that the designed fractional order controllers work effectively, and outperform the modified Ziegler-Nichols PI and designed integer order PID controllers.