Robust control of systems subject to input non-linearities with application to high performance helicopters
This thesis addresses some of the issues associated with linear systems subject to nonlinearities at their input, and considers robust control of high performance helicopters. In particular, the problems of substitution and saturation nonlinearities are discussed. In the case of the former, a new procedure based on ideas from optimal control is developed to help facilitate dynamic transfer between two linear controllers. For the latter, a generalisation of an existing scheme is made which allows asymptotic set-point tracking for all states belonging to a given subset of the state space. The closely related problem of limited authority control is considered and promising results are demonstrated on the Westland Lynx helicopter in a pilotted simulation. The thesis culminates with a discussion of the design and implementation of an Hinfinity controller for the Bell 205 helicopter. The results show that in flight, the helicopter attained good handling qualities ratings, even though the fidelity of some of the mathematical models was relatively low.