Research On Leo Space Tether Deployment Control And Electrodynamic Deorbiting Technology

Funded by the Natural Science Foundation of China and Discovery Grant of the Natural Sciences and Engineering Research Council of Canada,this research explores the hotspots and future development trends of the tethered satellites and develops space tether deployment control,deorbiting control and navigation reliability methods.A tethered satellite system(TSS)is a space system in which two or more satellites connected by tether(s).It is a very promising technology for space science exploration due to its reconfigurable,large-scale advantages,which has become a research hotspot of the world’s space powers in recent 20 years.Space tether deployment problem is a critical issue in all tethered missions.Because of its under-actuativity and the Coriolis force during the releasing process,the tether may have a large swing.Without proper control,it will lead the tethered system to instability or even breakage.In order to improve the releasing stability of the tethered satellite,it is necessary to design a control law that satisfies the control constraints and deploys efficiently.In the past decades,with the increasing number of LEO malfunctioning satellites and space debris,space junk has become a serious threat to human space activities.The Electrodynamic tether(EDT)is a prospective technology for the removal of space debris with the advantages of propellant-less mechanism.However,on account of the inherent complex dynamics and control problems in the EDT,its application is confronted with technical issues.The main research is devoted to the following five aspects:1.The research status of the tethered satellite deployment control technology and the electrodynamic tether deorbiting technology both in domestic and abroad is investigated.The past and current the space tether missions are reviewed.Combined with the requirement of the research project and the existing research development,the scope of research is determined as the issues of space tether deployment control and its deorbiting application.Based on the Lagrange method and the analytical mechanics,the dynamic model of the TSS is derived,and the equilibrium point,controllability and underactuated characteristics of TSS are analyzed.2.The non-overshooting releasing control problem of TSS is studied.A hierarchical-dimensionality reduction analysis method is proposed,which divides the control process into two steps: reference trajectory design and tracking control.Firstly,an indirect control associated with the tether length and deployment velocity is designed to stabilize the internal dynamics of the system.Positive velocity is ensured by keeping the control law greater than zero.Then,a non-overshooting reference trajectory is generated according to the indirect control law and initial deployment condition.Secondly,partial feedback linearization combined with back-stepping method is adopted to design the tracking control law.Proper parameter range for nonovershooting tracking control is deduced and proved.In addition,a simple On-Off control law is designed to achieve no overshoot and stable deployment.3.The constraint of maximum in-plane libration angle and tension disturbance attenuation problems during the deployment of the tethered satellite are studied.A large swing angle may cause the tether slack which needs to be avoided.A modified barrier Lyapunov method is proposed to design the internal dynamic control law of the tethered satellite,which constrains the in-plane angle to a preset threshold.Then the internal dynamic control law is integrated to obtain the reference trajectory.To consider the influence of tension disturbance,a nonlinear disturbance observer is designed.The Lyapunov’s second method and the back-stepping method are employed to design the tracking control law with the disturbance observer.Then the stability of the observer with bounded disturbance is deduced,and the parameter adjustment strategy of observation error and tracking error is studied.Simulation validates the performance of the proposed control strategy under high-frequency and low-frequency disturbances.4.Considering the issue of three-dimensional stable release of the TSS,a realtime MPC based on active set and real-time iterative is proposed.Firstly,an open-loop optimization problem is discretized by multiple shooting method,which is transformed into sequential quadratic programming problem.Then,the Gauss gradient descent method is used to solve the sequential convex quadratic programming problem.Finally,a real-time iterative strategy is adopted to improve the optimization speed.The effectiveness and robustness of the proposed method are verified by simulations with different initial conditions and random tension disturbances.Besides,in order to tackle the problem of navigation reliability in TSS deployment,a fault detection and exclusion method of one defective GPS satellite is proposed.It adopts a progressive sample consensus strategy to identify the faulty GPS satellite.Then the influence of the positioning errors on TSS deployment are analyzed via simulations.5.A scheme of electrodynamic tether fast deorbiting is proposed.Aiming at the singular problem of the traditional orbital elements,a modified elements description method is adopted,and the real-time MPC control is employed to station keeping control and deorbiting of EDT.The effectiveness is verified via simulations.The main innovations of this paper are listed as followings:1.A two-stage control law for space tether deployment is proposed.Firstly,establishing the relationship between direct control variable and indirect control variable,the indirect control input is used to stabilize the internal dynamics.With the acquired internal dynamic control law and the initial state,the reference trajectory is obtained by integration.Secondly,the tracking control law is developed by partial feedback linearization,and the parameter range for non-overshooting tracking is derived.A simple On-Off control law is designed to make it particularly suitable for simplifying the release mechanism of the TSS.2.A modified barrier Lyapunov method is developed to regulate the sensitivity of the feedback control when the constrained state approaching the threshold.In addition,a robust tracking controller based on the disturbance observer is designed,and the convergence of the disturbance observer for bounded disturbance is derived and proved.Simulation demonstrates the effectiveness of the proposed scheme.It is also revealed that low-frequency disturbance is the main source of system error.3.A MPC control law using real-time iterative strategy is employed to improve the optimization speed.It is revealed that if the initial value of the out-of-plane angle is zero,two-dimensional deployment is equivalent to the two-dimensional deployment.The deployment results under different initial conditions and in the presence of random tension disturbance are analyzed.A progressive sample consensus method is proposed to improve the efficiency of the GPS satellite fault detection and exclusion comparing with the random sample consensus method.The influence of GPS positioning error on the deployment of TSS is analyzed.4.The orbital dynamics equations of EDT based on modified elements are established.The real-time MPC is employed to the electrodynamic tether station keeping and fast deorbiting.The relation between the initial libration angles and the convergent speed is analyzed.An open-loop trajectory optimization and closed-loop real-time MPC is designed for EDT deorbiting control.