Design of two-degree-of-freedom robust controllers for a seeker scan loop system

A mathematical model of a seeker scan loop plant is derived. The scanning accuracy of the seeker scan loop is degraded by modelling uncertainty and disturbance. To satisfy robust stability as well as robust performance, a new two-degree-of-freedom (TDF) robust control is formulated in the framework of H optimization problem. The TDF controller consists of the feedback controller for robust stabilization and the feedforward controller for good command following. Two structures, such as the conventional and modified structures, of TDF control are introduced, and the formulations of the TDF H control are presented for both structures. The feedback and feedforward controllers are designed in a single-step procedure. The proposed formulation is able to reflect not only frequency-domain specifications but also time-domain specifications such as transient response characteristics and multivariable interaction between output channels, differently from the mixed-sensitivity problem. The seeker scan loop system with the TDF control satisfies design requirements and in particular shows good scanning performances for conical and rosette scan patterns.