Control limitation of a three-loop acceleration autopilot

Abstract

The essential characteristics for a three-loop acceleration autopilot of tactical missile were presented from a frequency analysis perspective. The basic limitation of right-half-phase pole and zero from airframe static instability was analyzed using an open-loop crossover frequency inequality, which showed a consistent conclusion with the final overall stability condition in nature. To analyze the three-loop autopilot control capacity, the rate feedback, rate plus attitude feedback, and three loop feedback, were calculated and simplified based on engineering requirement, respectively. Then the equivalent actuator dynamics were introduced to compute the stability condition at high frequency. Such that the overall stability condition was obtained in explicit formulation. The results showed that the autopilot control capacity is dominated by actuator bandwidth, and a compromise should be determined between the flight performance and the actuator requirement for a statically instable missile.