Research On Flight Control Methods For Unmanned Aerial Vehicle

Recently, with considerably expanding applications of unmanned aerial vehicles(UAVs), research on related theories and techniques has attracted increasing attention.As flight control is the fundamental requirement and important guarantee for UAVs to complete a variety of missions, automatic flight control methods have been extensively studied in recent years. Flight control problems are investigated in this thesis for fixed-wing UAVs and quadrotor UAVs, since these two types of aircrafts are widely used in practical applications.As dynamic systems, the above two aircrafts are of complicated aerodynamic characteristics, which have strong nonlinearity and serious coupling problem. Undoubtedly, there additionally exists uncertainty in the aerodynamic characteristics, which are complex and vary with flight conditions. Furthermore, both of the two mentioned aircrafts are under-actuated systems with four control inputs and six controlled outputs. Though some useful results have been reported on the aforementioned issues,there are plenty of new challenges and problems that need to be further explored and addressed. On one hand, each control method in the control context has its own advantages and limitations for each problem in practical applications, and consequently further research on control method must be problem-oriented for a specific issue. On the other hand, due to the complex aerodynamic characteristics of aircrafts, there are a variety of problems in flight control tasks. However, most existing results have been given with only one or two problems under consideration. For instance, numerous longitudinal control methods and linear control methods have been proposed, which are only effective for longitudinal motion and motion in locally linear operating range,respectively. This thesis investigates flight control for six-degree-of-freedom motion in flight envelope, which incorporates both longitudinal and lateral–directional control in large operating scope.Considering aerodynamic characteristics of fixed-wing UAVs and quadrotor UAVs,flight control problems have been analyzed and summarized, which include underactuation, nonlinearity, coupling problem and uncertainty. The degrees of a same problem will be dissimilar between different controlled systems. For example, uncertainties in aerodynamics of fixed-wing UAVs and quadrotor UAVs are slowly time-varying and fast time-varying, respectively. Based on the above analyses of control problems,study, proposal and validation of flight control method are implemented in this the-sis for fixed-wing UAVs and quadrotor UAVs. To be specific, the main work and significant contributions of this thesis are highlighted as follows:1, As far as control problems of six-degree-of-freedom motion are concerned, research on flight control is conducted in this thesis for both fixed-wing UAVs and quadrotor UAVs, which is not confined to attitude regulation or longitudinal control.It is well-known that both of the above two types of aircrafts are underactuated systems, and flight control systems for them are designed to be of hierarchical structures according to respective characteristics. In detail, applying concept of time scale here,the dynamical model and kinematical model of UAVs are defined as fast inner loop and slow outer loop, respectively, and sequentially flight control system will be hierarchically designed as well. With the hierarchical control structures having been presented,simulation models of the two mentioned UAVs and real-time flight simulation platform based on x PC Target are established, and then real-time simulations and flight test experiments are respectively conducted to verify feasibility and effectiveness of the proposed control structures for the above two kinds of UAVs.2, Concerning coupling problem, uncertainty and nonlinearity in quadrotor UAVs,attitude control method and flight control method are developed in this thesis. This thesis develops linear decoupling control for coupling problem, which guarantees both decoupling and stability. In addition to coupling effect, matched uncertainty and time-varying disturbances are concerned as well, for which L1 adaptive control is studied. With the proposed decoupling control incorporated into L1 adaptive control, an improved L1 adaptive control law is proposed. The developed control law is then utilized to address attitude control problem for quadrotor UAVs. Meanwhile,as far as nonlinearity is concerned, this thesis applies the classical gain-scheduling strategy, and a gain-scheduling decoupling control method is developed with the proposed linear decoupling control. Moreover, under the designed hierarchical structure,flight control system based on gain-scheduling decoupling control is presented for the quadrotor UAV. In summary, both the proposed improved L1 adaptive control and gain-scheduling decoupling control are considered as adaptive control. Results of flight simulations indicate feasibility and efficiency of the proposed control structure and control laws.3, This thesis is also concerned with flight control problems in flight envelope for the fixed-wing UAV. In other words, the control objective is to guarantee stability andperformance during a large scale rather than a locally linear scope.Firstly, aerodynamic characteristics of a fixed-wing UAV vary with flight height and speed, and it is difficult to establish an accurate dynamical model due to complexity and uncertainty. Additionally, the kinematical model is nonlinear. In view of the abovementioned problems, the thesis studies switching control and nonlinear dynamic inversion to realize flight control for six-degree-of-freedom motion during large scale.(i) An identification algorithm with multiple models is derived to cope with uncertainty of kinematical model,(ii) Regarding problem of poor switching transient,a smooth switching control method is proposed on basis of model reference control and neural network classifier. Feasibility and efficiency of the proposed identification algorithm and switching control scheme are demonstrated by both theoretical analyses and numerical simulations.(iii) Both the presented identification algorithm and smoothing switching control are applied to longitudinal dynamical model and lateraldirectional dynamical model, respectively. In this way, inner loop controller is put forward for dynamical model of fixed-wing UAVs.(iv) The thesis utilizes nonlinear dynamic inversion to deal with nonlinear kinematical model, which can be established accurately since only kinematic relations are involved.(v) Integrate inner loop controller and outer loop controller under the proposed hierarchical structure, and flight control system will be ultimately formed for the fixed-wing UAV.Secondly, regarding the coupling between longitudinal and lateral-directional motion during turning flight, coordinated turn is proposed and realized to avoid undesirable coupling effect such as loss of height and speed. Moreover, in order to realize desirable autonomous flight and efficient scheduling of desired heading angle, a guidance strategy is developed in this thesis. Flight simulation results are presented to illustrate effectiveness of both the proposed guidance strategy and flight control method. Moreover, results also demonstrate that flight control performance is satisfactory under coordinated turn.To point out here, reference signals with step change will be transformed into differentiable signals. Advantages of the above proposed signal transformation are validated via simulations, and then it is utilized throughout this thesis. Finally, main results in this thesis are summarized, and prospects for future research are presented.