Lateral track control of UAVs using the sliding mode approach: From design to flight testing

This paper develops sliding-mode-based nonlinear logic for guidance of unmanned aerial vehicles (UAVs) for curved and straight path following. UAV trajectories generally consist of straight path segments, curved arcs, circular loiters and other manoeuvres; tight ground track control is desired throughout the trajectory. This is achieved by controlling the lateral (cross-track) deviation of the vehicle in flight. The main objective of the guidance algorithm is to keep the lateral track error of the vehicle as small as possible while performing graceful and stable manoeuvres despite the presence of uncertainties and disturbing winds. Lateral track control is usually achieved by banking the vehicle, that is, by executing roll manoeuvres. The scheme must perform well without saturating the roll angle of the vehicle, which serves as the control input for the guidance algorithm. The algorithm proposed here is shown to perform well for both straight and circular path tracking while ensuring control boundedness, and hence no saturation. Crosswinds are a major source of disturbance for the guidance problem. This is incorporated into the design formulation and guidance gains are selected to provide the desired performance despite the presence of disturbing winds. A sliding-mode-based scheme is developed which includes a feedforward component related to the rate of change of the desired path heading. Stability of the algorithm is proved using an appropriate Lyapunov function. The algorithm is implemented in the flight control computer of a scaled YAK-54 research aircraft; flight test results are presented and compared with those from other guidance algorithms. Flight results demonstrate the effectiveness and performance of the proposed guidance scheme. The algorithm considers guidance in the 2D lateral plane only and minimizes deviations from the desired ground track of the vehicle.