A Generic Approach to Missile Autopilot Design Using State-Dependent Nonlinear Control

The fundamental objective of autopilot design for missile systems is to provide stability with satisfactory performance and robustness over the whole range of flight conditions throughout the entire flight envelope that missiles are required to operate in, during all probable engagements. Depending on the control mode (skid-to-turn or bank-to-turn), intercept scenario (such as surface to surface, surface to air, air to air) and mission phase (launch, midcourse, terminal), missile autopilots can command accelerations, body rates, incidence angles, or flight path angles. To this end, classical and modern multivariable techniques from linear control theory combined with gain scheduling have dominated missile autopilot design over the past several decades. In this paper, the concept of extended linearization (also known as state-dependent coefficient parameterization) is examined for state-dependent nonlinear formulation of the vehicle dynamics in a novel and very general form for the development of a generic and practical autopilot design approach for missile flight control systems. Any extended linearization control method, such as the currently popular State-Dependent Riccati Equation (SDRE) methods, can then be applied to this state-dependent formulation for missile flight control system design. The unique contribution of this paper is the novel use of a very general and realistic nonlinear aerodynamic model that captures all major aerodynamic nonlinearities attributed to missiles, together with the fully nonlinear and coupled 6-DOF equations of motion of rigid-body missile dynamics for full-envelope, 3-axes nonlinear autopilot design, without invoking any of the usual simplifying assumptions of the traditional linear design philosophy, and independent of any flight or trim conditions. Moreover, in the development of the generic approach, all the autopilot command structures mentioned above are incorporated in one compact topology. Practical considerations such as actuator dynamics and actuator position and rate saturation are also included in the development of the nonlinear autopilot. The proposed approach has been implemented and its performance and robustness validated in detailed 6-DOF simulations in three dimensional environments, using various missile configurations with stable, unstable and nonminimum-phase characteristics.