F-18 robust control design using H2 and H-infinity methods

Abstract

The open loop F-18 longitudinal control system is stabilized using H2 and H-infinity singular value loop shaping for a multivariable feedback control system. The H2 and H-infinity control theories involve suppressing the sensitivity matrix transfer function at the lower frequencies for high gain performance and suppressing the transmissivity at higher frequencies, i.e., loop shaping. The singular value Bode plot is used for MIMO systems in analogy with the classical Bode frequency analysis for SISO (Single Input Single Output) systems. There are two control inputs with input 1 controlling the stabilator and input 2 controlling the leading edge flap and trailing edge flap in tandem. There are two output: angle of attack and pitch rate. The H-infinity design achieved a separation in that input 1 controlled angle of attack and input 2 controlled pitch rate. the first design is an optimum design which imposed no limitations on control input. A cost penalty associated with control actuator limitations is imposed to achieve a limited performance design.