An improvement on PWPF modulation of discrete RCS and design of the blended control logic

The re-entry control of reusable launch vehicles (RLV) often involves both discrete reaction control system (RCS) and continuous aerodynamic controls, in which the avoidance of limit cycles and the blended control logic are the key issues of design. In this paper the limit cycle oscillation behavior of the discrete pulse-width and pulse-frequency (PWPF) modulation is predicted by application of the discrete describing function analysis technique, the appearance conditions are then derived, and an asymmetric dead zone is further constructed and used in the forward path to eliminate the limit cycle oscillations, without performance loss. Based on the improved PWPF modulation, a RCS and aerodynamic surfaces blended control logic is designed with concern on implementation. Simulations on the control of a typical RLV show that the improved discrete PWPF modulation and the blended logic work satisfactorily to the requirements of control.