Automatic Flight Control Of Unmanned Helicopter Based On Nonlinear Model

Owing to its unique characteristics, unmanned helicopter (UMH) has become increasingly important in military and civilian areas. After the installation of automatic flight control system (AFCS), UMH can fly over the horizon and accomplish many tasks which can't be fulfilled by conventional aircrafts. Research and development of AFCS for UMH has a very important practical significance, however, UMH itself is a typical unstable, strong coupling, time-varying nonlinear systems, so that it has been a very challenging task to design AFCS.In order to deal with problems existing in the study of UMH's control, this paper selected a small UMH with a single main rotor and a tail rotor ("Thunder Tiger 90"), developed a control algorithm combined state-dependent Riccati equation (SDRE) with nonlinear feedforward compensation technique. Flight tests were conducted by actual flight validation. The main work done is shown as follows:1. SDRE control theorySDRE approach was elaborated in theory. For the present three key directions in SDRE study, namely, full state feedback, nonlinear observer and numerical solution of SDRE, the paper gives results of the study.2. Mathematical modeling and characterization of UMHIn accordance with actual different response rate of each component in UMH (main rotor's flapping, attitude dynamics, trajectory dynamics), the overall nonlinear model was divided into fast mode,middle mode and slow mode, and then, a"three-timescale"mathematics model was established. On the basis of multi-timescale affine nonlinear model, the paper quantificationally analyzed major and minor factors affecting state variables'movement and proved the rationality of model.3. UMH attitude controller designThe paper improved and expanded state dependent riccati equation (SDRE) control approach, deriving analytical conditions for achieving global asymptotic stability with lyapunov stability theory. Proof was given. By combining improved SDRE control with nonlinear feed-forward compensation technique, the full envelop flight attitude control laws could be designed. In addition, the online estimation of main rotor speed (Ω) and flapping angles ( a f ,b f) were studied. Simulation results were given.4. UMH trajectory controller design By combining SDRE approach with nonlinear feed-forward compensation technique, a speed (u, v, and w) controller was developed. The block diagram of speed control system was given. As the speed control was decoupled, three-dimensional coordinates (x, y, and z) were controlled through traditional PID method. Simulation results show the effectiveness of the controller.5. Flight testActual flight data was collected and numerical analysis was carried out in order to further validate the controller's overall performance. The actual performance of attitude controller was given.