Nonlinear missile autopilot design with θ-D technique

In this paper, a new nonlinear control method is used to design a full-envelope, hybrid bank-to-turn (BTT)/skid-to-turn (STT) autopilot for an airbreathing air-to-air missile. Through this new approach, called the theta - D method, we find approximate solutions to the Hamilton-Jacobi Bellman (HJB) equation. As a result, the resulting nonlinear feedback law can be expressed in a closed form. In this paper, a theta - D outer-loop and inner-loop controller structure is used in an autopilot design. A hybrid BTT/STT autopilot command logic is used to convert the commanded accelerations from the guidance laws to reference angle commands for the autopilot. The outer-loop theta - D controller converts the angle-of-attack, sideslip, and bank-angle commands to body-rate commands for the inner loop. An inner-loop theta - D controller converts the body-rate commands to fin commands. This design is evaluated using a detailed six-degrees-of-freedom simulation. Numerical results show that the new controllers achieve excellent tracking performance and exhibit insensitivity to parameter variations over a wide flight envelope.