Design of Localizer Capture and Track using Classical Control Techniques

This paper discusses a typical Localizer Mode of the Automatic Landing System control laws provided in all the new generation of Boeing Commercial Airplanes. The linear control law diagram of this mode consists of three separate modules: Yaw Damper, Roll Inner-Loop and Localizer Outer-Loop. The Yaw Damper and Roll Inner-Loop are used in conjunction with other modes of the roll axis control laws of the autopilot and are, therefore, predetermined for the Localizer Mode. The design begins with a simple system to which more intricate features can be added as required and when justified. The features of the basic design are: (1) Feedforward for predicting compensation in combination with feedback control for disturbance rejection (instead of excessive gains to compromise stability); (2) Command shaping for attenuating responses to commands (instead of using reduced gains); (3) Complementary filters with inertial data for attenuating responses to gusts, beam noise and terrain irregularities, in addition to protecting against noise, granularity, and excessive transport delays in air data and ILS signals; and (4) Sensor location compensation for reducing stability and noise impact (instead of employing reduced gains). The gain schedules have been determined from aerodynamic properties, with analytic gain schedules being used for gain schedule parameters having a continuous effect on the state models.