Robustness Improvement for a Three-Loop Missile Autopilot Using Discontinuous State Feedback

For several decades, the classical three-loop autopilot topologies have been successfully employed as missile longitudinal autopilots. To achieve robustness against matched and unmatched parametric uncertainties, this paper proposes two possible designs from a sliding mode control perspective. This paper first introduces what is termed a variable structure three-loop controller (VS3LC), which requires the same commanded and sensed quantities as the classical three-loop autopilot, but uses discontinuous feedback gains and an additional term for robustness improvement. It is found that the solution of this topology results in continuous equivalent control having the same expression as the classical three-loop autopilot when in sliding mode. Next, another variable structure pitch rate controller (VSPRC) which is insensitive to unmatched model parameter uncertainties as well as matched uncertainties is presented. The VSPRC combined with an additional outer-loop control of acceleration results in another three-loop topology, which is named as a variable structure-combined three-loop controller (VSC3LC). The robust performance and stability of the proposed variable structure controllers are illustrated through numerical simulations.