Helicopter Robust Nonlinear Controller Synthesis Using Approximate Linearization

For nonlinear systems such as helicopters, feedback linearization offers an attractive method for the design of controllers. Most applications of this technique start with a nonlinear model of the helicopter which is assumed to exactly represent the vehicle flight dynamics. However, due to the empirical representation of the aerodynamic forces, any mathematical helicopter model, linear or nonlinear, will have a significant amount of uncertainty which must be accounted for by some means in the design and implementation of the controller. Furthermore, even when the equations of motion are known exactly, problems may occur in the inversion process due to small control sensitivities. In this paper we present a general technique for synthesizing nonlinear flight controllers which begins in the identification process with a fixed structure nonlinear model that includes a representation for the uncertainty. This is followed by an approximate feedback linearization procedure in which we assume that the control enters linearly into the nonlinear equations of motion. Results are first shown for three different maneuvers performed using a "nearly exact" linearization design which illustrate the achievable performance with these methods as well as problems which may occur. Next, preliminary results are shown for the current approximate linearization procedure. Finally we present a method of nonlinear controller synthesis in which we apply modern robust control theory to the "linear part" of the controller design.