Research On Agile Technologies Of Three-rigid-body Spacecraft Based On DFP System

The disturbance-free payload(DFP) system is a two-rigid-body spacecraft consisting of the payload module(PM) and the support module(SM), and these two modules are linked by non-contact actuators. The DFP system has an excellent isolation performance. In order to improve the agile performance of DFP system, a link module(LM) and a spherical joint are introduced to improve the two-rigid-body DFP system into the three-rigid-body DFP system.As the foundation of the research for agile technology, the dynamic model is built, using the method of Newton-Euler. In the process of modelling, the force and torque generated by six non-contact actuators in a hexapod configuration, the dynamic equations of three modules, and the relative motion equation of PM and LM are derived. Finally, the complete dynamic model is built as a nonlinear 12 degree-of-freedom matrix equation with tight coupling.The total angular momentum of the three-rigid-body spacecraft is studied, and the reference trajectory of the internal motion of SM is designed. This is done in order to allow the angular momentum variation of PM and LM to be absorbed by SM, and reduce the limit on maneuver velocity caused by the ability of flywheel. This maneuver strategy is also suitable for the uncertainty of the moment of inertia.However, the uncertainty of the moment of inertia will cause big overshot with the use of the traditional cascade-saturation controller, leading to an increase of maneuver time. In order to improve the robustness against the uncertainty of the moment of inertia, the traditional cascade-saturation controller is improved by adding an acceleration compensation term, which can significantly decrease the overshot. Moreover, to solve the problem of multiple actuator constraints of the three-rigid-body spacecraft, the controller parameter selection method is presented.Furthermore, the collision may happen between LM and SM during the internal motion if the maneuvering angle is large. To solve this problem, the constraint condition of collision is given, a condition which is suitable for the special architecture of the three-rigid-body spacecraft, and two plans are presented for collision avoidance. The first plan is to lock the spherical joint when the two modules are about to collide, and then the spacecraft continues to maneuver to the target attitude. However, some unknown disturbance will be introduced when locking the spherical joint. So the parameter self-adjusting plan is presented to keep the spherical joint from locking. In the second plan, the internal motion reference trajectory of SM is improved into the self-adjustment reference trajectory, adjustable with the relative attitude between LM and SM. The second plan enables us to avoid the collision without locking the spherical joint.