Nonlinear H∞ missile longitudinal autopilot design with &thetas;-D method

In this paper, a new nonlinear control synthesis technique, the θ- D method, is employed to design a missile longitudinal autopilot. The θ-D technique yields suboptimal solutions to nonlinear optimal control problems in the sense that it provides approximate solution to the Hamilton-Jacobi-Bellman (HJB) equation. Semi-global asymptotic stability can be achieved by manipulating the perturbation terms which are added to the cost function in developing a series solution. Furthermore, this method can be used to provide an approximate closed-form solution to the state dependent Riccati equation. The particular θ-D methodology adopted in this paper is referred to as θ-D H_∞ design. The θ-D H_∞ design has the same structure as that of linear H_∞, except that the two Riccati equations are state dependent. By using the θ-D technique, we would eliminate the need for online computations of Riccati equations as in the recently popular state dependent Riccati equation technique. A missile longitudinal autopilot design demonstrates the capabilities of θ-D method.