Dynamics And Control Of Continuous Low-Thrust Spacecraft On Libration Points

Since the beginning of the 21st century,China has achieved a brilliant success in the two fields of near-Earth satellite and manned spaceflight.The primary goal of China Aerospace in the next decade is deep-space exploration.Pioneered by the Chang'e Project of the Chinese Lunar Exploration Program,several deep-space projects,including Mars exploration,asteroid exploration,and external solar sys-tem exploration,are under consideration or development.Since the targets are much farther,deep-space exploration needs the support of new propulsion technol-ogy,and the space platform to hold probers should be selected properly according to their mission requirement.Due to its higher fuel efficiency,thrust controlla-bility,and payload capability,continuous low-thrust propulsion technology,rep-resented by electric propulsion and solar sailing,is achieving a more and more significant position in deep-space missions.As for space platforms,the collinear libration points in the sun-Earth/moon system are of great dynamic characteris-tics and an excellent view,and thus become ideal positions for solar and cosmic observation.However,collinear libration points are inherently unstable,which challenges the design and implement of missions nearby.Based on continuous low-thrust propulsion technology,this thesis studies the station-keeping control of libration point orbits and provides solutions with satisfactory performance.The result is of certain significance in theory and practice for the development of deep-space programs in China.The energy required for long-term libration point missions is directly related to the accuracy of their orbits.The model of the circular restricted three-body problem is a good approximation of celestial mechanics for the motion of space-craft near libration points.With this model,the Lindstedt-Poincare perturbation method is utilized to construct three-order analytical periodic orbits around libra-tion points and artificial libration points,and the differential correction method is utilized to obtain accurate numerical orbits with initial conditions given by ana-lytical orbits.These numerical orbits are to be used as pre-determined trajectories of libration point missions.Spacecraft running in the vicinity of libration points can maintain the po-sition for a long time with low thrust and modest fuel expenditure.However,collinear libration points are unstable and active station-keeping control strategy is required to keep spacecraft from drifting away due to external disturbances.In consideration of the shortcomings of existing methods,a novel strategy com-bining an extended state observer and a back-stepping sliding mode controller is proposed in this thesis.In the proposed method,the system dynamics and un-certainties axe treated as total disturbance and estimated by the extended state observer using only the input and output information.The back-stepping sliding mode controller is designed to compensate for the total disturbance and ensure the actual trajectory converges to the nominal orbit.A rigorous stability analysis of the synthetical controller using Lyapunov's method is presented.The active disturbance rejection control and the back-stepping sliding mode control without an extra observer are employed for comparison.The results indicate that the pro-posed controller is adequate for station-keeping of unstable libration point orbits,and performs outstandingly in the presence of orbital insertion errors,external disturbances,and random uncertainties.The implementation difficulty and cost of deep-space missions are dramati-cally higher than near-Earth missions because of their long distance.In order to prevent tasks from failure,deep-space missions put higher requirements on the reliability of the entire spacecraft system.Among all subsystems,the propul-sion system is relatively vulnerable and has witnessed several malfunctions in aerospace practice.In order to ensure the stability of spacecraft under the loss-of-effective malfunction in the propulsion system,an extended state observer-based adaptive robust active fault-tolerant control strategy is proposed.The extended state observer is utilized to estimate total disturbance and ensure the robustness of the control system.A parameter adaptation law is designed to estimate the level of thrust malfunction.The adaptive robust is designed to compensate for the total disturbance and ensure system stability.In comparison with a pas-sive fault-tolerant controller,the proposed method is proved capable to ensure high-precision orbital tracking under the loss-of-effective malfunction,and having adequate recovery capability from short-time propulsion outage and stuck.In order to expand natural libration points' solar monitoring ability,the aerospace community proposed the conception of artificial libration point mis-sions.Artificial libration points are neither stable nor balanced.The amount of fuel consumption to maintain artificial libration point orbits is beyond the ca-pability of electric propulsion.Solar radiation pressure generated by large-area solar sails is employed to maintain orbits.The model of reflectivity modulated solar sails and solar power sails axe constructed using reflectivity control devices and thin-film solar cell technology developed by JAXA's IKAROS sailcraft.In order to overcome the under-actuation issue of conventional solar sail control,electrochromic materials are used in the reflectivity modulated solar sail to make reflectivity adjustable,and an auxiliary electric propulsion system is mounted in the solar power sail.Due to the restriction of large-area structure,the attitude change of the sail film is required to be smooth and slow.A time-varying gain ex-tended state observer-based back-stepping sliding mode controller is proposed to avoid the "peaking phenomenon" introduced by time-invariant gain extended state observer.Numerical simulations indicate that high-precision and robust artificial libration point orbits tracking is achieved using the proposed solar-sail crafts.Solar sailing is still in the testing stage because of the high requirement of lightweight materials and unsolved difficulties in deployment of large-area flexible structure.Electric solar wind sailing,an emerging propulsion concept,generates thrusts by electrostatic Coulomb interaction between positively charged chains and protons in the solar wind.As a result,the deployment of a large-area flexible structure is avoided and larger thrusts can be obtained.In this thesis,electric solar wind sailing is utilized to maintain artificial libration point orbits.The time-varying gain extended state observer-based back-stepping sliding mode controller proposed before is employed.Numerical simulations indicate that high-precision and robust tracking of deeper orbits is achieved.