Research On Control Issues Of Quadrotors With A Cable-suspended Payload

Thanks to simple mechanical structure and rapid development of IC,MEMS sensors,communication and automatic techniques,multi-rotor UAVs have received continuous attention and develops rapidly in recent years e.g.quadrotors.As a kind of ideal aerial platform,quadrotors are often equipped with different devices for different tasks.Generally,there are two ways for quadrotors to carry these equipments.The first one is to carry them with a specifically designed gripper,which will rigidly connect them as a whole part.Another choice is to suspend the payload with a cable,which can effectively decrease its bad impacts on the UAV's agility.Moreover,there is no need to consider the problem that the shape and size of the loads might not match the quadrotors.As a result,it is very meaningful to do the research on the topic of quadrotors with a cable-suspended payload.In order to enlarge the application domain of quadrotors,several control issues in terms of quadrotor-load system are discussed in this thesis,which are detailed as follows:(1)Build the dynamic model of single and two quadrotors with a cable suspended payload respectively.Firstly,a complete hybrid model is built for the single quadrotor case according to whether the cable's tension is zero.Similarly,a non-linear dynamic model with exogenous disturbances is built for the second situation,where the cables are always assumed to be taut.Then its linearised form is obtained by analysing its hovering equilibrium point.Finally,the corresponding simulation platform is built in MATLAB,which lays the foundation for the next chapters.(2)Identify the altitude,pitch and roll channel for a quadrotor which is equipped with an inner-loop attitude stabilization system.Firstly,a linear low order model is chosen for each channel to approximate the input and output relationship.The unknown parameters are estimated using a least square method which is based on recursive frequency spectrum of real flight data.According to identified results,a state space equation is presented and its prediction performance is also analysed.Afterwards,the identified model is utilized to design a performance index in Simulink for tuning position tracking controllers,which is finally validated by practical flight tests.(3)Control a quadrotor to carry a cable-suspended payload flying through a window under the assumption that its exact dynamic model is unknown.Firstly,the system is simplified in the two-dimensional vertical plane and its corresponding low order model is chosen.After being excited by some specifically designed manoeuvres,the unknown parameters in the model are obtained by previously proposed identification algorithm.Based on that,the task of "passing window" is then transformed into a trajectory optimization problem,considering trajectory's smoothness as its objective and identified model as its dynamical constraints.Several aggressive trajectories are numerically generated and analysed when different boundary constraints are given.Finally,a practical flight test is demonstrated which validates the proposed solution.(4)Design a mixed H2/H? controller for two quadrotors to carry a cable-suspended payload to prevent the problem of swing and collisions.Firstly,a general form of the mixed H2/H? controller for this task is introduced and it is solved by using a Linear Matrix Inequality(LMI)approach.In order to strike a balance between smoothing the tracking trajectory and resist the disturbance,the performance outputs of H2,H? and constraints are successively determined.Meanwhile,the functions and tuning strategies of parameters ? and ?H2/H?C are numerically analysed and summarized.Finally,in order to evaluate the system performance,two numerical simulation results are implemented including hovering and tracking a predefined trajectory.Specifically,the trajectory passes through a "narrow door" and is optimized in its differential flatness space.The simulation results illustrate that the proposed controller meets the requirements.(5)Design a receding Nash controller for two quadrotors to transport a cable-suspended payload in case that they conceive different interests.Firstly,basic concepts of non-cooperative dynamic game are introduced and quadrotors are considered as two decision-makers with different quadratic objective functions.Then the linearised model is considered as a difference game under open-loop information structure so that its Nash equilibrium is derived in recursive finite-time form.Combined with the idea of reced-ing horizon control,a state-feedback Nash controller is then designed to synthesize this problem.Finally,two numerical simulations are presented and the results illustrate that the proposed controller exploits their individual interests and makes the system become more flexible.