Studies in robust control: analysis of sampled-data systems and aircraft pitch control

Abstract

This research consists of two parts: (1) robust steady-state tracking of sampled-data systems and (2) robust aircraft pitch control. In Part I, robust steady-state tracking of linear shift-invariant and periodic discretetime systems in the presence of structured norm-bounded discrete-time uncertainty is discussed first. Using the results for discrete-time systems, robust steady-state track­ ing of sampled-data systems, which are considered as continuous-time systems, in the presence of structured norm-bounded continuous-time uncertainty is addressed. E.xact conditions are derived for robust steady-state tracking of known inputs for sampled-data systems by using the lifting technique. Sampled-data systems are approximated by fast sampling of the input and output. The resulting systems are in discrete time. Based on the analysis of the resulting approximate discrete-time •'ystems, an approximate con­ verging computation algorithm is given. The same results also apply to general periodic linear time-varying continuous-time systems. In Part II. robust aircraft pitch control is presented. The discussion focuses on the longitudinal attitude control problem when aircraft weight and center of gravity are unavailable as control inputs. Due to the variation of weight and center of gravity in aircraft models, multiplicative uncertainty models for different flight conditions (three different altitudes/airspeeds) are derived for robust synthesis. Longitudinal attitude robust controllers are designed to provide consistent performance under varying weight and varying center of gravity locations.