Design of Lifted Dual-Rate Digital Controllers for X-38 Vehicle

Abstract

A new state-matching digital redesign method to Ž nd the lifted dual-rate pulse-amplitude-modulated (PAM) and pulse-width-modulated (PWM) digital controllers from a predesigned, state-feedback, continuous-time controller is presented. The proposed method provides close matching of the states of the continuous-time controlled analog system with those of the digitally redesigned system (including intersample behavior). The redesigned lifted dual-rate controller enables the digitally controlled X-38 vehicle to track the desired trajectory while remaining insensitive to modeling parameters, initial conditions, and exogenous disturbances for larger sample times than existing methods. The X-38 will require a PAM controller for commanding the electromechanical actuators that operate its aerodynamic surfaces for  ight in the atmosphere. PWM control will be used to command the reaction control jets during orbital operations. The proposed method provides a new alternative for indirect digital design of state-feedback multivariable continuous-time systems.