Abstract
This thesis is concerned with robust control theory and its application to the autonomous flight control system of small-scale unmanned autonomous helicopters.
The goal of this thesis is to control a small-scale unmanned helicopter for different flight modes in the presence of external disturbances. The control approach
is based on the backstepping control theory. Backstepping is a nonlinear control
design method that provides an alternative to feedback linearization. It is based on
Lyapunov theory, the control laws and a Lyapunov function are derived simultaneously. This Lyapunov function is used to guarantee the stability of the closed-loop
system.
Firstly, a robust backstepping control technique for the heave motion of a small-scale unmanned helicopter to automatic height control is proposed. In general, the
control of a small-scale helicopter is a difficult task, because its system is highly
nonlinear, coupled and sensitive to the control inputs and external disturbances.
For simplicity, only vertical flight motion is focused on in this study. The influences
of external wind gusts and the ground effect are taken into account within the heave
motion model. When the performance of the proposed control method is examined
in a computer simulation model, it is shown that it can achieve the desired height
and compensate the effect of external wind gusts and the ground effect.
Secondly, the horizontal position control for the linearized version of the non-linear model representing a small unmanned helicopter with external wind gusts is
considered. Based on the Lyapunov stability theory, a stable robust backstepping
controller is derived. For comparison purposes, another controller is also designed
based on the linear quadratic regulator (LQR) criteria. To test their performances
a computer simulation is conducted, which shows that the proposed controller has
rapid horizontal position tracking and a good gust attenuation capability.
Finally, a robust controller for the nonlinear system of a small-scale unmanned
autonomous helicopter with unknown external wind gusts is considered. The robust backstepping control law is derived from the Lyapunov stability analysis which
guarantees that the unknown external parameter estimation error is bounded. The
proposed controller has the ability to stabilise the helicopter system and successfully
achieves our control objective.