Design of automatic landing systems using mixed H₂/H_∞ control

Mixed H₂/H_∞ control technique is employed to develop controllers for autolanding systems for a commercial airplane. A linear model of the aircraft in longitudinal motion is established using the appropriate aerodynamic coefficients. With the control actuator, tracking errors, and altitude motion, the aircraft is shown to be governed by an augmentation system along with its filter model. Two kinds of optimal and robust control requirements, which need to be satisfied simultaneously, are designed. One of the requirements is with respect to an optimal trajectory selection for landing routes. The H₂ method is used to minimize a cost function such that the optimal gain for trajectory optimization can be obtained. The other requirement is with respect to the disturbance attenuation. The H_∞ technique is employed to obtain the necessary formulation for the robust control gain to minimize the effect of the disturbance on the performance output. An algorithm is developed based on the convex theory for the mixed H₂/H_∞ control and filter gains, which provides a suboptimal solution. A large commercial aircraft (Boeing 747-200) is employed to illustrate the potential of the proposed method. It is shown that the glide slope capture motion and flare maneuver of the aircraft are accomplished quite well, and the amplitudes of all maneuvers are within Federal Aviation Administration requirements.