Nonlinear autopilot control design for a 2-DOF helicopter model

The paper illustrates a robust control scheme for application to helicopters in vertical flight mode (both take-off and landing) to guarantee altitude stabilisation. A nonlinear helicopter model is used to derive the proposed control in which Lyapunov's direct method is used to establish the overall system stability. A recursive design technique is applied to design a nonlinear robust controller using the highly coupled system structure. It will be shown that the proposed robust controller provides semiglobal stabilisation of uniform ultimate boundedness for achieving the desired altitude. That is, the control design is valid for all values of the helicopter's collective pitch angle away from zero. The vertical flight application demonstrates a unique robust control mechanisation for helicopter and V/STOL autopilot augmentation systems.