Robust Multi-Mode Control of High Performance Aero-Engines

This thesis describes the application of H∞ design techniques to the control of high performance aero-engines. The design study presented is practical and realistic, the work being motivated by problems that arise naturally in real engineering situations. The aero-engine is multivariable and highly nonlinear: the dynamics vary considerably with the thrust being produced, and with the altitude and forward speed of the aircraft. Moreover, there are operational constraints that must never be violated for reasons of safety: certain engine variables should always be limited to safe vales. Furthermore, not all the engine parameters to be controlled are directly measurable; instead a number of related measurements are available. A methodology is presented to choose from the available measurements, those that are preferable for feedback control. Different techniques of model reduction using balanced realizations are considered. Two illustrative examples are presented, and the methods compared in detail. Explicit state-space formulae for an H∞-based two degrees-of-freedom robust controller are derived in discrete time. The controller provides robust stability with respect to coprime factor perturbations, and a degree of robust performance in the sense of making the closed-loop system match an ideal reference model. Special attention is paid to the structure of the controller. It is shown that the controller consists of a plant observer, the reference model, and a generalized state feedback law associated with the plant and model states. Multi-mode control logic is developed to ensure that safety limits are never violated. Actual engine test results are presented for sea-level static conditions. All the different modes of operation are tested. Full flight envelope evaluation of the controller is carried out using a nonlinear engine simulation. The robust performance of the controller is demonstrated and comparisons made with existing engine control systems.