Adaptive Control Techniques for Multirotor Aircrafts

Multirotor unmanned aerial vehicles (UAVs) have shown evident advantages over traditional fixed wing UAVs in remote sensing. They are also believed to hold enormous potential in logistics, infrastructure maintenance and agriculture management. Such emerging applications bring the multirotor drones into new working environments with more uncertainties and higher levels of safety requirements. This dissertation research is to solve some adaptive control problems of multirotor drones under various uncertainties and faults. Study has shown that the desired performance of multirotors, such as fast maneuver and fault resilience, has not been rigorously achieved by current applied control approaches. The control goal of this research is to guarantee closed-loop system stability and enhance tracking performance under abnormal and uncertain system conditions. This research develops advanced adaptive control techniques for multirotors to accommodate parameter uncertainties, compensate actuator failures, and reject uncertain disturbances. The results of the research have formed some desired foundations for advanced unmanned multirotor systems and intelligent aerial systems.

The dissertation research studies both the traditional quadrotor-like under-actuated multirotors and the novel tilted-rotor fully-actuated omni-directional multirotors. Some fundamental system characteristics such as nonlinear and linearized model parameterization, relative degree, high-frequency gain matrix variation, and control allocation scheme are investigated for different multirotor systems at various operating conditions. Such studies are crucial for developing adaptive control designs which enable the multirotors working at non-hover conditions under uncertain system parameters, such as mass, inertial momentum and drag coefficients. Different rotor arrangements of hexarotor (NPNPNP and NNPPNP) and octorotor (NPNPNPNP and NNPPNNPP) aircrafts are surveyed to specify the compensable actuator failure patterns.

An input compensator is developed for multirotor systems to assure a uniform interactor matrix and a consistent pattern of the gain matrix signs over different typical operating conditions. An adaptive control scheme with input compensator is designed for quadrotors with nonlinear offsets at the non-equilibriums and uncertain parameters. Adaptive failure compensation schemes are designed to deal with unknown loss-of-control actuator failures whose pattern, time and value are all uncertain. A control signal distribution technique is developed to ensure the invariant gain matrix sign pattern under all the compensable actuator failure patterns, which enables the integrated adaptive control scheme for hexarotors subject to uncertain parameters and unknown failures simultaneously. Adaptive disturbance rejection schemes are developed to reject wind uncertainties for a multirotor-based atmospheric measurement platform. Nonlinear adaptive controllers are constructed to achieve more sophisticated maneuvers. Both analytical and simulation results are presented to verify the desired properties of the developed adaptive multirotor control systems.