Passivity analysis for nonlinear, nonstationary entry capsules

Abstract

SUMMARYApplication of simple adaptive control theory to the design of guidance and control systems for winged re‐entry vehicles has been proven successful, despite the fact that the sufficient conditions of passivity are not fulfilled throughout the trajectory. This has been traced back to the hybrid control, that is, a combination of reaction and aerodynamic control for a single axis of motion, as well as a zero bank angle in the initial phase of the mission. The latter condition makes the vehicle nonminimum phase (positive transmission zeros of the open‐loop system). To further analyze the passivity of nonlinear, time‐varying systems, we study an entry capsule that enters the atmosphere in a lift‐down configuration to avoid skipping flight. The capsule is controlled by a reaction control system only. The transient effect (i.e., time‐varying aspects) is an important factor in the stability properties of the guidance system, and an analysis methodology has been set up to take both the nonlinearities and the time‐varying aspects due to the strongly changing flight regimes into account. Deriving the passivity conditions for translational motion, as well as evaluating these conditions along the trajectory, shows that the entry capsule is not almost strictly passive and the sufficient conditions are not met. The guidance‐system design should be augmented with parallel feedforward filters. For rotational motion, on the other hand, it is found the entry capsule is almost strictly passive, so the sufficient conditions to apply simple adaptive control are met.Copyright © 2013 John Wiley & Sons, Ltd.