Modeling and control techniques of autonomous helicopters for landing on moving platforms

Abstract

Research on Unmanned Aircraft Systems (UAS) has seen a huge growth during the last years. Currently, there is growing interest in using UAS in several applications such as aerial photography, inspection and monitoring, accurate measurement, search and rescue or disaster management, to name a few. Amongst the different available platforms, helicopters and other rotor-based aircraft with hover and vertical take-off and landing capabilities are extensively used in aerial robotics. In the framework of the EC-SAFEMOBIL project, this thesis proposes the use of a tether as a guiding element for safe landing of unmanned helicopters on moving platforms such as a ship deck. The possibility to include a Stewart platform in the tethered system in order to provide a horizontally stable landing surface by compensating ship’s roll and pitch is also analyzed. To this end, several mechanical models corresponding to each part of the tethered system (helicopter, tether, controlled winch for tether tension control and Stewart platform) are derived using Kane’s methodology, which allows to obtain uncoupled first order differential equations. The resulting models offer a trade-off between simple and manageable equations for ease of the analysis for control design, and expressions that can accurately reproduce the main behavior of the real system. Upon analysis of the model, it is proved that the tether provides a stabilizing action against external disturbances, such as wind gusts, and also provides a new way to estimate the helicopter position relative to the platform, whose reliability would not be affected by the lack of GPS accuracy. Taking into account previous conclusions on the system operation, control laws for each part of the tethered system are proposed. On the one hand, the application of linear control strategies for the helicopter (e.g. linear quadratic integral control, loop-shaping, gain scheduling, etc.) in the thesis scenario is addressed. On the other, an elaborate model-based non-linear control strategy is proposed. The design of these controllers depends on the relative position and attitude between the aircraft and the landing platform. Therefore, it is critical to develop algorithms to accurately estimate the system state with sensor data fusion. To that end, the use of a numerically efficient square-root Unscented Kalman filter is proposed to reliably close the control loop. Finally, experimental simulations with the models used for control design as well as field experiments are presented for validation. These field experiments carried out by CATEC in the framework of the EC-SAFEMOBIL project, constitute the first worldwide experiments with a tethered unmanned helicopter.