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)/skidto- turn (STT) autopilot for an air-breathing air-to-air missile. Through this new approach, called the θ − D method, we find approximate solutions to the Hamilton- Jacobi-Bellman (HJB) equation. An interesting and important feature of this new technique is that the nonlinear feedback law can be expressed in a closed form. In this autopilot design, a θ − D outer-loop and inner-loop controller structure is adopted. 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 outerloop θ − D controller converts the angle-of-attack, the sideslip, and the bank angle commands to body rate commands for the inner loop. An inner-loop θ − D nonlinear controller converts the body rate commands to fin commands. The nonlinear design is evaluated using a detailed six-degrees-of-freedom simulation. Simulation results show that the new controllers achieve excellent tracking performance and exhibit insensitivity to parameter variations over a wide flight envelope