Beneficial actuator-induced bifurcations in compressor control

There has been a concerted effort to use feedback to either stabilize or ameliorate the detrimental effects of the aerodynamic instabilities of rotating stall and surge in compression systems of jet aircraft engines. Typical model-based efforts have largely focused on eliminating a treacherous hysteresis loop in the Moore-Greitzer equations, making stall recovery possible without resorting to drastic measures. The present note is motivated by peculiar and very interesting observations of the behavior of certain control laws when the performance of the actuator deteriorates (increased time lag between the commanded and actual control input). The degradation induces an instability which surprisingly improves the performance of the controller. In the following sections, we briefly describe the phenomenon and then offer an explanation by unfolding a doubly-degenerate bifurcation