Nonlinear Control of an Air-breathing Engine: A Practical Approach Using Dynamic Inversion and EKF

A practical nonlinear control design approach is presented in this paper for an airbreathing ramjet engine. The primary objective of this challenging problem is to ensure that the engine dynamically produces the thrust that tracks a commanded value of thrust as closely as possible by regulating the fuel flow to the combustion chamber. However, since the engine operates in the supersonic range, an important secondary objective is to manage the shock wave configuration in the intake section of the engine by varying the throat area of the nozzle. The nonlinear dynamic inversion technique has been used to achieve both of these objectives. To filter out the sensor and process noises and to estimate the states for making the control design operate based on output feedback, an Extended Kalman Filter based state estimation design has also been carried out. Moreover, since the process is faster than the actuators, to assure satisfactory performance of the engine, independent control designs have also been carried out for the actuators as well. After obtaining satisfactory tracking performance for standard reference signals, to test the performance of the engine for a typical realistic flight trajectory, a guidance loop has been designed for the vehicle through a trajectory optimization process, which has been augmented with a SDRE based neighboring optimal control philosophy (to cater for initial condition perturbations). Simulation results clearly indicate that the proposed EKF augmented dynamic inversion based control design approach is quite successful in obtaining robust performance of the air-breathing engine throughout the flight trajectory, i.e. both during climb as well as in cruise conditions. It is observed that a fixed set of control gains are adequate and there is no need of extensive gain scheduling.