Dynamics And Control Of Flexible Spacecraft With Time-varying Parameters

Large flexible appendages,such as solar wings and onboard deployable antennas,usually rotate relative to the mainbus of the spacecraft.And the relative motion would result a system with time-varying parameters,which is a great challenge to the high precision attitude control of flexible spacecraft.In this study,taking the flexible spacecraft with time-varying parameters as the research object,its dynamic characteristics,vibration control methods and attitude stabilization methods are systematically studied in respects of the theoretical modelling,numerical simulation and the ground experiment.The main work of this paper can be summarized as follows:1.The first order coupled dynamic model for flexible spacecraft with time-varying configuration are established.Characteristics of the appendages and effects on the spacecraft bus are analyzed based on the complex mode.(1)The structural models for a flexible beam and a flexible plate under rigid motion are established based on the Hamilton principle and by incorporating the torsional inertia.Then the electrical–structural coupling model and control electronics of the smart spinning beam with distributed piezoelectric sensor and actuator layers are established.Effects of the spinning speed,the noncoincidence of centroid and shear center,and the warping on natural frequencies and mode shapes are investigated.The model reveals the objective law that the spinning about the longitudinal axis would induce complex modes.(2)The first order coupled dynamic model of the flexible spacecraft is formulated by using Lagrange's equations in terms of quasi-coordinates.The model includes the six freedom rigid motions and rotations of the platform and elastic vibration of flexible appendages.The model considers the first time derivative of the transformation matrix,which defines the transformation between the platform's body frame and the appendage's floating frame.Based on the model,dynamic characteristics of the system are derived and responses under thermal loads are calculated.Numerical results show that the first order terms are of great importance on the attitude of the rigid body and dynamic response of the flexible appendage.2.Three active control theories,i.e.linear state feedback,variable positive position feedback,and sliding mode control methods are proposed for systems in complex mode space.Numerical simulations validate all the three control methods.(1)A novel nonlinear state feedback controller is developed for system subjected to actuator saturation in complex space.The novel controller utilizes the nested technique and integrates the nested controller with a time-varying coefficient to utilize actuators' full potential.The controller is applied to a spinning smart beam in the cases that single input and multiple-input are simulated.(2)A variable PPF controller with inconstant gain and frequencies is proposed to damp the system with complex mode based on the LQR method.Simulations of the closed-loop system under impact load,random load and initial displacement are conducted to validate the proposed control method.(3)For systems with parameter uncertainty and actuator saturation,a methodology for sliding mode controller(SMC)based on complex mode theory is proposed.And numerical simulations are conducted to demonstrate the effectiveness of the proposed controller.3.In order to solve the problem of high precision attitude control for flexible spacecraft with time-varying parameters,an optimal variable amplitudes input shaping control method,an adaptive sliding mode control method and an attitude coupling control method are proposed for slew maneuver of the flexible spacecraft.(1)For the flexible spacecraft with invariable parameters,the maneuver trajectory optimization is studied based on the input shaping method.The optimal trajectory is derived by convolving the bang-bang signal with a sequence of impulses with appropriate magnitudes and time spacing.Numerical simulations show that the method can effectively reduce the residual vibration of the flexible appendages.(2)For the slew maneuver of flexible spacecraft with time-varying parameters,a variable amplitudes Zero-Vibration(ZV)shaper is proposed for mathematic model with time-varying parameters.Then an optimal variable amplitudes input shaping control method is developed based on the shaper.To improve the robustness,the variable amplitudes Zero-Vibration and Derivative(ZVD)shaper and Zero-Vibration and Double-Derivative(ZVDD)shaper are designed.And the corresponding optimal control methods for slew maneuver are proposed.Numerical simulations show that the method is robust to the frequency perturation and damping uncertainties.(3)The closed-loop strategies based on H? robust control and an adaptive sliding mode control are proposed for attitude maneuver of flexible spacecraft.Numerical results show that the strategies are effective for systems with time-varying parameters and severe nonlinearities.(4)The analysis on an attitude coupling control method,which combines the adaptive sliding mode control with the positive position feedback control is researched.Simulation results show the method discussed herein can effectively decrease the stabilization time and improve the attitude accuracy of the flexible spacecraft.4.The flexible spacecraft testbed is established to validate the proposed controllers.Ground tests of attitude maneuver and stabilization are conducted to validate the optimal variable amplitudes input shaping control method,the adaptive sliding mode method and the coupling control method.(1)The attitude control testbed of flexible spacecraft is established.And the testbed is capable of conducting experiments for attitude control,vibration control and coupling control of flexible structures.(2)Attitude maneuver tests for flexible spacecraft are carried out and the results are compared to the responses under classical bang-bang maneuver methods.Experiments results show that the maneuver trajectory based on the input shaping method can reduce the residual vibration of flexible appendage and improve the performance of the attitude.(3)Attitude control tests for flexible spacecraft are carried out and the results show good agreement with the simulation.The proposed adaptive sliding mode control method is validated by comparing to the PID control.(4)Attitude coupling control tests for flexible spacecraft are carried out.And the attitude coupling control method is demonstrated by comparing the results of coupling control with that of adaptive control.This dissertation addresses key issues of flexible spacecraft with time-varying parameters considered.And the analyses are meaningful for flexible spacecraft to achieve high precision.