Performance-Constrained Adaptive Sliding Mode Control for Guaranteed Soft Landing Using Optic Flow

This article proposes a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">scale-independent</i> adaptive control strategy that allows a quadrotor to rely solely on optic flow output to achieve a soft vertical landing on stationary surfaces. Most prior schemes that rely on a fixed-gain approach to regulate optic flow suffer from instabilities induced by nonlinear dynamics of the vertical landing process. To overcome this drawback, this scheme proposes an adaptive sliding mode control strategy with theoretical stability guarantees that incorporates performance and input constraints to achieve desired transient and steady-state performance in the presence of uncertainty arising from wind gusts, ground effects, and possible actuator saturation. In particular, detailed Lyapunov stability analysis is used to demonstrate that the optic flow error converges exponentially to a small prescribed bound in steady-state, which is shown to be the key to achieving a guaranteed soft touchdown. Extensive experimental results substantiate the efficacy of the proposed strategy using a micro-quadrotor platform in a controlled environment. Moreover, a laterally stabilized soft landing with tailored descent behavior is also shown using a mini-quadrotor platform in real-world (unstructured) settings in the presence of wind gusts. Performance comparison studies with an optic flow-based fixed-gain proportional-integral control strategy and an altitude-based vertical velocity control strategy are also included, demonstrating the superior performance of the proposed strategy in comparison with these alternative designs.