Research On Small-Scale Unmanned Helicopter Nonlinear Modeling And Control

Small-scale unmanned helicopter (SUH) has unique flight features of fixed position hovering, vertically take-off and landing, sideward flight, rearward flight, low altitude flight and urban collision avoidance maneuver, and so on. These advantages including small scale, light weight and stealthiness make SUH an exellent vehicle of vast application prospect, both in civil and military fields. SUH is a strong coupled, inherently instable, underactuated, high order, multivariable, time-varying nonlinear system. Therefore, it is a great challenge to research the aerodynamics and flight control technology of SUH. As a sophisticated national defense technique, the unmanned helicopter flight control is blockaded by the West. Nowadays, the mid-high speed autonomous flight control problem has been solved by most foreign researchers. Some of them even have implemented super-maneuver autonomous control like somersault. But the domestic research level of this field, which mostly being limited to the low speed autonomous flight, still has a large gap from the level offoreign researchers. The reasons of the backwardness of study level are not only the delays in flight control research, but also the lack of a reliable nonlinear model of unmanned helicoper which complicated enough to describe the mid-high speed motion and even super-maneuver flight dynamics. Thus, it is of important theoretical significance and application value to research the nonlinear dynamical modeling and mid-high speed flight control of SUH.By referring to the formers, integrating theoretical analysis and flight experiments, using a test platform of certain SUH, the theories and methods of modeling and flight control technique of the conventional configuration helicopters are researched in this dissertation. As a result, a set of theory system and procedures of nonlinear modeling, model parameters determination and cruise flight control of a speed range including mid-high are proposed, which have basically solved the difficult problem of the SUH mid-high speed flight control. The main study contents and innovations of this dissertation are as follows:1. The survey and summary of the research status in China and abroad of the unmanned helicopter research, especially the detailed discussion and analysis of the research status, existing problems and development trend of the two key technologies, that modeling and flight control.2. By refering to the theory of full-scale manned helicopters and analysizing the special structure of SUH, a full state nonlinear mathematical model of SUH is developed from the first priciple analysis. The main dynamic details involving the modeling precision such as blade root tip lift loss, high frequency flapping dynamics, the wake flow disturbance on other parts and so on are integrated into this model, resultantly whose modeling complexity can meet the need of applications such as flight simulation, controller design, etc.3. Refering to the methods of several literatures, and considering the current experiment conditions, the technique easy to operate is developed to determine the values of the physical parameters of the nonlinear mathematical model, such as the lift curve slope of main rotor, moment of inertia of helicopter, etc. A comparation of the parameter determined model and the experimental flight data was carried out in both time domain response and frequency domain response, which demostrated this model of high precision.4. Based on the developed high precision model, near hover flight controller was designed to control autonomous hovering and auto landing. The nonlinear model was implemented in Simulink, where small perturbation linearization technique was applied to get a 14th-order model at hover working point. To simplify the control design work, the hover linear model was order-reduced and axial decoupled by proposed approximation method, and then a cascade hover controller was designed by frequency theory, which implemented hover flight successfully in experiments. Further more, based on this hover controller as the essential control law, and considering the features of mutiple attitude dynamics sensors, a multi-sensor assisted auto landing control strategy was proposed, by which auto landing flight was successfully implemented.5. Phase lead compensation dynamic inversion control method (PLDI) with a mixed performance indices robust controller is proposed to nonlinearly control mid-high speed cruise of SUH, with a improvement of maneuver tracking performance. Because of the strong nonlinearity, the near hover controller can hardly control mid-high speed cruise flight. The linear parameter varying (LPV) dynamic inversion control method, which derived from the nonlinear model, is employed to realize the feedback linearization of cruise dyanimics of hover, low, medium and high speed. During mid-high speed cruise, rapid change of attitude due to the acceleration and deceleration maneuver should be tracked by the controller. Nevertheless, the dynamic inversion controllers of the formers were rather weak in tracking high frequency attitude, since the nonmeasurable rotor flapping dynamics was often ignored to simplify the dynamic inversion design. To solve the problem, a phase lead compensator was designed by analysizing the flapping model. The simulations demonstrated the tracking capability of closed system to high frequency attitude with the compensator.A D-stable H2/H-infinity mixed robust controller was designed for the genelized plant from feedback linearization to solve the problems of external disturbances and model perturbance due to the parameter estimation errors. This controller was compared with a PID one in simulation, and both of them were demonstrated capable of the cruise nonlinear control involving mid-high speed. However, not only the feedback matrix determination procedure of this controller is definite rather than the empirically adjustment of PID controller but also the robustness to the model uncertainty of this controller is better than PID.6. A dynamic inversion based time-delay H-infinity robust control method (IDHC) by regarding the phase lag due to the rotor flapping as pure delay was proposed as another control idea to realize the high frequency attitude tracking during mid-high speed cruise flight. The genelized plant from the dynamic inversion feedback linearization was changed to be a system that the same pure delay exist in both states and inputs due to the pure delay approximation. The Delay-dependent H-infinity controller synthesis method of the special generalized plant was deduced, and the cone complementarity linearization (CCL) method was employed to obtain feedback matrix. The simulation demonstrated the nonlinear performance of mid-high speed cruise control and the robustness to the model uncertainty. And the control method seems as a good tradeoff that, the control quality of high frequency attitude tracking decreased due to the pure delay approximation, and the actuator oscillation due to the noise amplification of phase lead compensator was avoided. The helicopter of test platform successfullly cruised in mid-high forward speed in experiments applying the IDHC, and the validity of this control method was further verified in practice.