Missile autopilot controller design using state transformations and optimal LQR techniques

Abstract

The design of a missile autopilot controller which provides a fast and stable response to guidance commands, and which maintains this performance throughout the entire flight envelope, provides a challenging problem to flight control engineers. This paper considers the design of a lateral acceleration autopilot controller for a generic cruciform missile, and shows how full state feedback can be applied following the transformation of a physical state space model of the missile to a form where all the states are measurable and the introduction of lateral acceleration and pitch rate error states. This facilitates the successful application of the oft-disparaged optimal Linear Quadratic Regulator (LQR) control technique. In addition to the error states, the particular optimal control formulation makes use of fin position feedback. Integral action on the acceleration error state is introduced to ensure zero steady-state error in the presence of uncertainties and unmodelled dynamics. Autopilot controller design is performed directly in the discrete domain. Lateral acceleration time histories and open-loop frequency response functions for two flight speeds of a range of incidences are used to illustrate performance. System robustness is assessed using the open-loop gain and phase margins. Prior to the autopilot control law design study, a detailed discussion of the role, function and performance requirements of a missile lateral autopilot is given.