PID Control Design of Sideslip Angle for a Fixed-Wing Mini-UAV

Abstract

Sideslip angle is one of the state variables in the lateral state-space dynamics of an Unmanned Aerial Vehicle (UAV). Large asymmetric aerodynamic loads can be induced on a UAV’s fuselage, even at zero sideslip. For steady and level flight, this angle must be controlled. To control the sideslip angle, a mathematical model for the UAV is required using static and dynamic aerodynamic as well as cross-term coefficients. Aircraft Digital Datcom was used to estimate the UAV’s aerodynamic coefficients, aerodynamic stability and control derivatives from its physical geometry. The challenge for a control engineer is the choice of transfer function (plant) to be used for sideslip angle controller design in an autopilot system. Since both the lateral model and it reduced form; Dutch Roll (DR) approximated model have sideslip angle as a state variable. In this study, the pitch plane is ignored as well as cross-terms in the moments of inertia. Dutch Roll is focused upon firstly; we investigated the dynamics characteristics of both models. Secondly, sideslip angle transfer functions obtained from both the lateral and DR approximated models were compared. The eigenvalues, natural frequencies, damping ratio, period and number of cycle to damp to half amplitude of the Dutch roll mode in the lateral dynamics are about the same with those of the Dutch roll approximated model. Steady state values and open-loop step responses computed in MATLAB & Maple differ in magnitude and direction on the Cartesian plane. Despite the limitations of this initial effort, Proportional-Integral-Derivative (PID) controllers were designed in MATLAB/Simulink for all transfer functions. Design results analysed gives intuitive and informed design choice for autopilot gains for the sideslip angle control.