Fault Tolerant Control of Octoplane UAVs Using Sliding Mode Techniques

Abstract

This thesis presents an approach to designing and implementing fault-tolerant control (FTC) systems for octoplane unmanned aerial vehicles (UAVs) employing sliding mode control (SMC) techniques and control allocation (CA). The octoplane, a hybrid dual-system UAV (a fixed-wing aircraft with VTOL capabilities), presents unique challenges and opportunities due to its actuator redundancy. This research delves into the modelling, simulation, and design of FTC systems for octoplane UAVs in different flight modes: vertical, transition, and cruise. It addresses specific challenges and fault/failure scenarios associated with these flight modes, using SMC techniques to maintain flight stability and performance. The control schemes incorporate additional redundancy to manage actuator faults and failures, highlighting the scheme's efficacy under various actuator faults, failures, and uncertain conditions. SMC is a robust control method which can handle actuator faults directly. However, to deal with total actuator failures, CA is used to redistribute control signals to the remaining available actuators without reconfiguring the controller. The thesis begins with a comprehensive review of hybrid UAV configurations, focusing on quadplanes and octoplanes, and emphasizes the importance of FTC in enhancing UAV resilience and the advantages of incorporating additional actuator redundancy in the FTC design. It then introduces the SMC design process and explains its implementation of FTC, including an introduction to LPV-based SMC. Subsequently, the thesis provides an analysis of the equations of motion for an octoplane, setting the foundation for examining the FTC of different flight modes in the following chapters. In cruise mode, the proposed SMC-CA scheme shows no performance degradation under fault/failure scenarios compared to fault-free cases. An LPV-based SMC FTC approach for vertical flight mode also demonstrates robustness against actuator faults and failures. For transition mode, an LPV-based SMC FTC scheme maintains the same level of performance compared to the fault-free case within a wide speed range during climb, cruise, and descent stages. In the final chapter, a tilt-rotor mechanism is investigated, and the results show that good tracking performance has been maintained for various failure conditions during transition mode in the presence of wind and gusts. The thesis highlights the successful integration of CA and SMC techniques to an octoplane UAV across all flight modes, thus addressing FTC challenges and enhancing reliability.