Research On Dynamics And Control Of Space Electromagnetic Docking And Separation

On-Orbit Servicing (OOS) technology, based on spacecraft docking and separationoperation, represents the frontier and important development trend of aerospace scienceresearch. The increase of on-orbit spacecraft number and spaceflight missioncomplexity highlights the requirement for regular OOS, for which the spaceelectromagnetic docking and separation technology would provide a novel docking andseparation style. This novel technology could resolve the inherent problems of thruster,such as propellant consumption and plume contamination, etc. and has non-contacting,continuous, reversible and synchronous control capability. Concentrating on dynamicsand control problems of space electromagnetic docking and separation, dynamics andself-docking characteristics, trajectory and attitude control, angular momentummanagement,6-DOF simulation and ground experiment system design are investigatedin this paper. The main research points are as follows.(1) Dynamics and self-docking characteristics of space electromagneticdocking and separation. Hypothesizing the spacecraft electromagnets as magneticdipoles, the far-field electromagnetic force/torque models, with formats of Cartesiancoordinates and spherical coordinates, are derived by the electromagnetic theory.Referenced to the orbit coordinate frame at center of mass of the spacecraft pair, therelative motion dynamic models for the two spacecraft are derived based on Hill’sequations and far-field electromagnetic force model, and then the dynamic model ofspace electromagnetic docking and separation is got by directly subtracting the twodynamic models. Based on the dynamic model, its dynamics properties, includingstrong nonlinearity, coupling, model uncertainty and controllability, are analyzed. And,the concept of electromagnetic self-docking is put forward and proved from the ground(including one-dimensional and two-dimensional) and space cases and thecorresponding sufficient conditions of self-docking are given.(2) Robust and adaptive control for soft docking trajectory motion.Considering spacecraft soft docking requirements, the desired docking trajectory isdesigned by applying the glideslope approach, and the feedforwad controller is derivedbased on the dynamic model. To resolve the control problems of space electromagneticdocking, such as strong nonlinearity, coupling and dynamic model uncertainty, a twoloop control frame with the inner loop of feedback linearization and outer loop of H∞method, is put forward and a soft trajectory tracking error feedback robust controller isfirstly designed, ensuring better robust capability to model uncertainty. Then,considering the conservative property of the H∞method, another two approaches,including inner loop feedback linearization with outer ESO(Extended State Observer) and LQR(Linear Quadratic Regulator), and the adaptive control method based onLyapunov stability theory, is explored to design the soft trajectory error feedbackcontroller, and verified by simulation cases. In addition, the estimation error of ESO andthe asymptotical stability of closed system with adaptive controller are theoreticallyanalyzed, and the tuning approach of parameters for ESO and adaptive controller arefollowed.(3) Guidance control for safe separation trajectory motion. Reversible controlcapability of electromagnetic force makes it possible to utilize one electromagneticmechanism to accomplish docking and separation operation. Different from the softelectromagnetic docking, the electromagnetic separation emphasizes safety. With thebackground of on-orbit release partner satellite, several basic hypotheses for spaceelectromagnetic separation are given. Based on constant magnetic moment vectors, thecharacteristics of H-bar, V-bar and R-Bar electromagnetic separation are analyzed andput forward by numerical simulation, and the algebraic relationship between themagnetic moment and the separation relative motion of partner spacecraft is formulated.At last, concentrating on two normal elliptical close proximity fly-around mission aimedto the master or other spacecraft, the space electromagnetic separation guidancecontroller is designed based on the cyclic pursuit theorem and the differentiablehomeomorphic mapping, and the parameters tuning of which is analyzed.(4) Decentralized and coordinated robust control for attitude motion.Considering the effects of inter-satellite electromagnetic torque and the Earth’s magnetictorque to the attitude motions of the spacecraft pair, the requirements of attitude controlare studied and the necessity of decentralized coordinate active attitude controlapproach is pointed out. Defining and formulating the absolute and relative attitudedeviation, the dynamic and kinematics models of absolute and relative attitude deviationmotion with the unit quaternion expression are derived. The attitude controller isdesigned by utilizing behavior-based decentralized control approach, adaptive controland ESO method, and then the global asymptotical stability of the closed system istheoretically analyzed. At last, the quick coordination of relative attitude, asymptoticalconvergence of absolute attitude, and robust capability to model uncertainty andexternal disturbances, are verified by simulation cases for the closed attitude controlsystem with decentralized coordinate attitude controllers.(5) Angular momentum management for space electromagnetic docking andseparation. Treating the Earth’s magnetic field as magnetic dipoles, the projectionmodels of the Earth’s magnetic torque and the inter-satellite electromagnetic torque onthe inertial coordinate frame centered at the center of mass of the spacecraft pair isderived. Then, the angular momentum management approach and correspondingsolution algorithms for magnetic dipoles are studied, including the following two. First, considering the requirements of trajectory motion control and one spacecraft’sinter-satellite electromagnetic torque counteracting with the Earth’s magnetic torque,taking V-bar, R-bar and H-bar docking for example, the "normal angular momentummanagement" approaches are explored. Second, considering the angular momentumsaturation case of attitude control actuation mechanism, the "angular momentumreduction management" approach is studied by applying the Earth’s magnetic torque.Because the designed angular momentum management approaches are negative, anAttitude Control/Momentum Management (ACMM) approach is studied, whichincludes the "sequence magnetic dipoles solution" and the "ACMM control".(6)6-DOF simulation and ground experiment system design. With thebackground of spacecraft close proximity fly-around observation based on spaceelectromagnetic docking and separation technology, a6-DOF numerical simulation caseof space electromagnetic docking and separation is implemented. A simulation frame isconstructed, and the feasibility and performance of previous dynamic models andcontrol algorithms are verified. Additionally, a ground experiment system is studied anddesigned, and the general system scheme, design results of hardware and software arepresented.In conclusion, the research of dynamics and control for the space electromagneticdocking and separation technology, on one hand, verifies the feasibility of this noveltechnology from the dynamics and control aspect; on the other hand, provides dynamicmodels, characteristics analysis, control algorithms and ground experiment base forfuture theoretical research, ground demonstration and verification experiments, orfurther on-orbit experiments.