Fault tolerant adaptive control of an unmanned aerial vehicle

[摘要] ENGLISH ABSTRACT: This thesis presents the development of an adaptive longitudinal control system for an unmannedaerial vehicle (UAV). The project forms part of a research effort at Stellenbosch Universityinto different fault-tolerant control techniques for UAVs.In order to demonstrate the usefulness of fault-tolerant adaptive control, the control systemwas designed to handle damage-induced longitudinal shifts in the centre of gravity (CG)of the aircraft, which are known to have a dramatic effect on the stability of a fixed-wing aircraft.Using a simplified force and moment model, equations were derived which model theeffect of longitudinal CG shifts on the behaviour of the aircraft. A linear analysis of thelongitudinal dynamics using these equations showed that the short period mode can becomeunstable for backward CG shifts.An adaptive pitch rate controller with the model reference adaptive control structure wasdesigned to re-stabilise the short period mode when the CG shifts backwards. The adaptivelaw was designed using Lyapunov stability theory. Airspeed, climb rate and altitude controllerswere designed around the pitch rate controller to allow full autonomous control ofthe longitudinal dynamics of the UAV. These outer loops were designed with constant parameters,since they would be unaffected by CG shifts if the adaptive pitch rate controllerperformed as desired.Pure software simulations as well as hardware-in-the-loop simulations showed that theadaptive control system is able to handle instantaneous shifts in the centre of gravity whichwould destabilise a fixed-gain control system. These simulation results were validated inflight tests, where the aircraft was destabilised using positive feedback and re-stabilised bythe adaptive control system.Thus the simulation and flight test results showed that an adaptive control can re-stabilisean unstable aircraft without explicit knowledge of the change in the aircraft dynamics, andtherefore could be effective as part of an integrated fault-tolerant control system.

[发布机构] Stellenbosch University