Robust Path Following Control



Design a robust nonlinear path following control law for a fixed-wing UAV to track a reference path under the wind disturbance


Disturbance Observer

System model for the wind estimator

$$ \begin{bmatrix} \dot{x} \\ \dot{y} \end{bmatrix} = \begin{bmatrix} V_a \cos \psi + W_N \\ V_a \sin \psi + W_E \end{bmatrix}$$

Update law of the disturbance observer

$$ \dot{\hat{\bf{w}}} = -T_w \hat{\bf{w}} + T_w ( \dot{\bf{x}} – \bf{u}) $$
where, \( \hat{\bf{w}} = \begin{bmatrix} \hat W_n & \hat W_e \end{bmatrix}^T \) is the wind velocity estimate in the north and east direction, \( T_w \) is the diagonal observer gain matrix, and \( \bf{u} = \begin{bmatrix} V_a \cos \psi & V_a \sin \psi \end{bmatrix}^T \)


System Model


Error representation on Serret-Frenet frame

The error kinematics model becomes,

$$ \begin{align} \dot{e}_t &= V_g \cos (\chi – \chi_r ) – ( 1 – \kappa (s) e_n ) \dot{s} \\ \dot{e}_n &= V_g \sin (\chi – \chi_r ) – \kappa (s) e_t \dot{s} \end{align} $$


Path Following Control Law

Command Filtered Backstepping


Block diagram of the path following control law implementation


Hardware-in-the-loop Tests

Bspline Path Following


Reference path and actual trajectory of the UAV


Error states: Cross-track, along-track, and course angle error


Flight Tests

Circular trajectory following


Reference path and actual trajectory of the UAV


Wind velocity estimate from the flight test data (Moderate wind condition)



The experimental results show that the improved tracking performance as well as the enhanced robustness, proving the applicability of the proposed algorithm in the various mission of the fixed-wing UAV in wind disturbance environment.




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김수현(Kim Su Hyeon)
한국항공대학교 스마트항공모빌리티학과 무인시스템제어연구실(KAU USCL)
10540 경기도 고양시 덕양구 항공대학로 76 전자관 322호


Korea Aerospace University