Aircraft Pitch Control via Second-Order Sliding Technique

Control of high-performa nce low-cost unmanned air vehicles involves the problems of incomplete measurements, external disturbances and modeling uncertainties. Sliding mode control combines high precision with robustness to the aforementioned factors. The idea behind this approach is the choice of a particular constraint which, when maintained, will provide the process with the required features and remove, therefore, the plant's uncertainty. However, standard sliding modes are characterized by a high-frequency switching of control, which causes problems in practical applications (so-called chattering effect). A second order sliding controller implemented in the present paper features bounded continuously time-dependent control and provides higher accuracy than the standard sliding mode, while preserving precise constraint fulfillment within a finite time. It possesses, also, significant adaptive properties. The general approach is demonstrated by solving a real-life pitch control problem. Results of a computer simulation and flight tests are presented. I. Introduction Aircraft and missile systems are equipped with control systems whose tasks are to provide stability, disturbance attenuation and reference signal tracking, while their aerodynamic coefficients vary over a wide dynamic range due to large Mach-altitude fluctuations and due to aerodynamic coefficient uncertainties resulting from inaccurate wind tunnel measurements. It is common practice, when designing a control system for an unmanned air vehicle (UAV), to represent the flight envelope by a grid of Mach-altitude operating