Superaugmentation and stability augmentation control system for unmanned aerial vehicle

Abstract

It is always a challenge to compromise between stability and controllability in the design of an aircraft. The challenge is becoming bigger in designing a flight control system of a small, light weight and low speed unmanned aerial vehicle (UAV). This type of UAV is facing a higher degree of difficulty because of its constraints in stability margin due to the limitation of the centre of gravity locations and experiencing more problems in control system when flying in air turbulence (severe wind gust or crosswind). This research work is focused on analysis, design and simulation of a robust flight control system (FCS) for a small UAV to make it capable of flying in severe gusty conditions. A combination of the variable stability technique along with advanced flying and handling qualities (FHQ) requirements are used to reduce the gust effect on the aircraft. A low-speed UTM-UAV is used as a testbed for this research. A mathematical model for the aircraft including gust velocity components was formulated based on a combination of experimental wind tunnel with theoretical and empirical methods to estimate the aerodynamics coefficient, thus stability and control derivatives. A linearized longitudinal and lateral-directional equations of motion of the aircraft in the state-space form were developed and validated against a non-linear model. Matlab/Simulink simulation algorithm was developed to analyse and evaluate the dynamic behaviour of the UAV at different speeds and CG locations. The simulation results show that the selection of particular stability and control derivatives has a significant influence on the FHQ level of the aircraft gust response for a small UAV. The superaugmentation FCS that consisted of stability augmentation system (SAS) and command stability augmentation system (CSAS) was developed to improve the dynamic characteristics of the longitudinal aircraft. A simulation result shows that the superaugmented aircraft is capable of operating in severe gust environments than augmented aircraft, and puts less strain on the elevator activity in both extreme and calm weather conditions. A comparison of superaugmented aircraft to augmented aircraft shows a significant reduction (70-80%) in undesirable pitch motion caused by a vertical gust in which, that level 1 flight phase Cat.C can be achieved.