Robust Integrated Attitude And Orbit Control For Proximity Motion Of Spacecraft

The attitude and orbit control of spacecraft is the key technology in space missions.With the development of worldwide space activities, especially for the proximity mis-sions such as on-orbit maintenance, formation flying, astroid soft landing and space in-terception, the position and the attitude of spacecraft are often required to simultaneouslyachieve the desired states with high maneuverability and control accuracy. To this extent,traditional separated position and attitude control strategy is hard to meet the increas-ing requirements of these future space missions. In contrast, integrated translation androtation control scheme fully takes into consideration the mutual couplings between thetranslation dynamics and the rotation dynamics, and can simultaneously control the posi-tion and attitude motion, which is able to essentially improve the system performance andguarantee high control accuracy and maneuverability. The present dissertation focuses onthe integrated translation and rotation modeling and control of spacecraft in space prox-imity operations and space orbit maneuver, both of which are based on spacecraft relativemotion in proximity. The main researches are listed as follows.Two kinds of coupled relative attitude and orbit dynamics of spacecraft are formulat-ed for two type proximity missions mentioned above. The mutual couplings between theorbital dynamics and the attitude dynamics are analyzed. During the modeling, accordingto the requirements of both missions, two actuator configurations are given to providecontrol force and control torque. Due to the diferent actuator layout, the coupled dy-namics for proximity operations performs a full-actuated system, while the one for orbitmaneuver missions possesses an under-actuated system.Considering space proximity operations, in order to make full use of full-actuatedsystem such that the system maneuverability and control precision can be improved, westudy the robust integrated translation and rotation finite-time control problem of a s-pacecraft. To do so, by designing a class of well-behaviored trajectory with finite-timeconvergence, the finite-time control problem is equivalently transformed into a trajectorytracking problem, and thus a robust finite-time control law is developed via adaptive back-stepping technique. In addition, an approach for computing the thrust input is addressedsince it cannot be evaluated explicitly and directly from the developed control law. Fur-thermore, the control scheme is modified to eliminate potential chattering phenomenonand the stability analysis is given as well. The following numerical simulation shows that the position and the attitude of spacecraft are able to converge to their desired values ina pre-determined time, despite of unknown mass properties, thruster misalignment andexternal disturbances, which demonstrates the efectiveness of the proposed control law.In the sequel, based on the same mission, since the actuator outputs are limited inapplications, the integrated translation and rotation control problem of a rigid spacecraftwith control saturation is considered, and further, the integrated finite-time control prob-lem with control saturation is studied. To solve the first problem, based on anti-windupphilosophy and adaptive backstepping method, an anti-windup robust integrated transla-tion and rotation control law is proposed, where an auxiliary signal is introduced into thevirtual control to compensate for the influence caused by control saturation. Then, in or-der to obtain the high control accuracy and maneuverability from the designed finite-timecontroller and simultaneously deal with control saturation, based on switched control phi-losophy, the proposed finite-time controller and the anti-windup controller are synthesizedand thus the robust integrated translation and rotation anti-windup finite-time control lawis developed, which solves the second problem. The closed-loop stability is guaranteed byusing switched system theory. It can be demonstrated from numerical simulations that,the anti-windup controller takes efect initially to deal with input constraints, and oncethe control saturation no longer happens and the given switch condition is satisfied, thefinite-time controller takes over the system such that the relative position and the attitudeof spacecraft are able to accurately converge to the desired values in a fast response.After that, we focus on the integrated translation and rotation control for relativeorbit maneuver, whose dynamics possesses an under-actuated system. In these missions,a single orbital thruster is often fixed on the spacecraft body and thus the orbital thrustvector for orbit control performs nonlinear with respect to the attitude angles, which be-comes the chief difculty of the control problem. To deal with this nonlinear cascadedrelationship, based on transformations of trigonometric functions, a vector decompositionis proposed such that the backstepping philosophy is applicable. It follows that the fil-tered backstepping technique is used to construct the robust integrated control law, wherea first-order filter is introduced in each design step to facilitate the derivation of the vir-tual controls. Then, the rigorous closed-loop stability analysis is given by using singularperturbation theory and Lyapunov theory. Numerical simulation of a lunar soft landingscenario shows the efectiveness of the proposed control law.Finally, we study the spacecraft rendezvous mission with a target in near-circularorbit, which is a special-type relative orbit maneuver mission. For this mission, by us- ing the proposed vector decomposition, the integrated translation and rotation controlissue can be equivalently transformed into the output stabilization of a class of extend-ed semi-strict feedback systems, whose structure holds the features that1) the low-orderdynamics perform linear systems,2) the high-order dynamics take the semi-strict feed-back form, and3) the whole system possesses the cascaded structure. Thus, accordingto these features, a H∞-based robust adaptive backstepping control law is proposed suchthat the robust adaptive control problem of this type nonlinear system can be solved. Theproposed control scheme is able to not only guarantee the ultimate uniform boundednessbut also attenuate the system uncertainties with a given level. Based on the proposed con-trol scheme, a robust integrated translation and rotation control law is constructed suchthat the spacecraft is able to rendezvous with target along the reference trajectory andsimultaneously attenuate various system uncertainties. The following numerical simula-tion of a space interception scenario is given to illustrate the efectiveness of the designedintegrated controller.