Fault tolerant control of a small helicopter with tail rotor failures

The use of autonomous/Unmanned Air Vehicles (UAV) in variety of applications has been increased over the last decade. The UAVs are usually classified into two types: fixed wing and rotary wing. The autonomous helicopters are more versatile than the autonomous aeroplanes. They can perform wide range of manoeuvres compare to the aeroplanes, specifically they are capable of Vertically Taking Off and Landing (VTOL). Due to its high manoeuvrability, it has been used in various applications: agricultural crop dusting, search and rescue missions, inspection of bridges and power lines and surveillance of areas. Though they exhibit higher degree of applicability, they are vulnerable to faults and failures. An autonomous helicopter is naturally highly unstable vehicle and hard to control, specifically during faults/failures. So it is essential to ensure they are fault tolerant to fly safely during failures. The tail rotor failures are the failures that occur most frequently causing accidents in conventional single main rotor and tail rotor helicopters. In general, the tail rotor failures are classified into two types: Tail Rotor Control Failure (TRCF) and Tail Rotor Drive Failure (TRDF). During TRCF, the control over the tail rotor thrust is lost so it cannot be varied by the pilot or the operator and usually described as frozen control state. Where in TRDF condition the tail rotor stops spinning completely due to broken drive shaft ending up with no anti torque to counteract the main rotor torque. In this thesis, the possible TRCFs in small helicopters are investigated and fault tolerant controllers are developed based on various control schemes: PID, fuzzy-PID, LQR, H∞, and μ-synthesis to fly a small unmanned helicopter with TRCF in three different flight modes: hover, descent and forward flight. The developed controllers are tested with linear and nonlinear models and the simulation results are presented. The control of the helicopter with failure in forward flight is comparatively easier than at hover and in descent. The designed controllers are able to perform hover with minimal yaw variation in the event of failure. The proposed novel control scheme based on lateral/yaw augmented structure to utilize the lateral velocity induced yaw control during failure at hover and in forward flight is successfully implemented. A propulsion based FTC control scheme is developed using variable rotor speed as an additional control input to perform descent with failure. The fuzzy logic based controller is designed incorporating the general responses of a manned helicopter to throttle inputs given by a pilot in emergency situations. The developed controller has successfully performed the vertical descent with minimal variations in angular rates and translational velocities. The vehicle is successfully landed with very low impact energy as if it is a fault-free fully controlled descent.