Fuzzy Altitude Autopilot Design for Unmanned Aerial Vehicles

In this paper, it is tried to design an altitude hold mode autopilot of unmanned aerial vehicles (UAVs). This autopilot has practical importance for UAV's due to flying in the vicinity of terrain to implement missions by terrain following maneuvers. This case presents an interesting challenge due to the non-minimal phase characteristics, non-linearities, and uncertainties of altitude to elevator relation. Unlike the conventional methods (in which the altitude autopilot is designed with three loops, so that altitude, pitch rate, and pitch angle variables are measured), a single loop scheme is proposed so that only altitude is measured. This single loop scheme is simple, but it is not robust with respect to the uncertainties. The fuzzy logic control is proposed to overcome parametric uncertainties and unmodeled dynamics (due to the complexity and expenses in aerospace industries, these uncertainties are usually available in modeling). Our 6DOF non-linear simulation shows that it is possible to achieve a simple and robust autopilot by fuzzy logic control.