State Consensus And Autonomous Assembly For A Team Of Flexible Spacecraft

Significant attention has been attracted to the distributed cooperative control for a team of spacecraft due to the advantages of the higher robustness,less communication requirement,shorter development cycle and more flexibility than a single spacecraft.It is noteworthy that each spacecraft in modern space mission trends towards lighter and larger,and usually carries some long appendages.However,it is a great challenge to control a team of flexible spacecraft due to the coupling between the motion of the rigid hub and the vibration of the flexible appendage.The main objective of this dissertation is to study the state consensus and on-orbit autonomous assembly for a team of flexible spacecraft by dynamic modeling,theoretical analysis,numerical simulation and experimental verification.The contents and contributions of this dissertation are as follows.1.For the spacecraft with flexible appendage,the dissertation presents three different dynamic equations,i.e.,the floating frame of reference formulation with assumed modes,the floating frame of reference formulation based on partial differential equation and the absolute coordinate based formulation.Numerical studies show that the floating frame of reference formulation only with the first cantilever beam mode is accurate enough for the spacecraft with flexible appendage in the case of low rotation speed and small deformation.It is shown by two lemmas that the stability conditions of the spacecraft with one or two appendages are that the translation and rotation speeds of rigid hub are equal to zero,and the control input variables are constant with the assumption that only the states of the hub are measured and controlled directly.2.The state consensus of multiple flexible spacecraft is more complicated than the attitude synchronization of multiple rigid or flexible spacecraft due to the coupling among the translation,the rotation and the flexible deformation.To this end,the dissertation presents three distributed controllers,i.e.,distributed PD controller,distributed finite-time controller and distributed optimal controller.Distributed PD controller is simple,easy to implement and asymptotic convergence.Distributed finite-time controller implies shorter transient stage and higher robustness,and ensures stability of the closed-loop system within finite time.Distributed optimal controller could drive the team to the target states synchronously such that a certain optimality criterion is achieved.3.A compound controller which combines an output consensus controller and a collision avoidance controller is proposed to complete the assembly at the free tip of the flexible appendage of a team of flexible spacecraft.The assembly mission is decomposed into four steps.Firstly,the individuals in the team are numbered.Secondly,the attitudes of the flexible spacecraft are regulated to the desired values synchronously.Thirdly,the team is driven to the pre-assembly configuration.Finally,the team is assembled.The collision avoidance controller is needed both in the second and the third step to actuate the team to the target configuration without inter-member collision.The effectiveness of the proposed controller algorithm is verified by two case studies.4.A novel distributed controller is proposed based on artificial potential field for the on-orbit autonomous assembly of four flexible spacecraft without inter-member collision.The potential function has no local minima,and mainly depends on the attitude errors of adjacent flexible spacecraft,the distance between the center of the hub of flexible spacecraft and the center of the virtual leader,and the radial Euclidian distance between the flexible spacecraft and the virtual leader.The global minimum of the potential function is zero.All the states that make the potential function be equal to zero are corresponding to the desired configurations.Moreover,a collision avoidance function is also included to avoid the inter-member collision.Two case studies are presented to demonstrate the successful application of the proposed controller to the assembly mission.5.Two new versions of satellite simulators are developed on the basis of the air-bearing test-bed in State Key Laboratory of Mechanics and Control of Mechanical Structures.The reliability of the test-bed and the repeatability of the experimental study are improved significantly by optimizing the hardwares and algorithms in the satellite simulator.Each satellite simulator can control its own position and attitude by its normal and tangential thrusters.To simulate the dynamics of the spacecraft with flexible appendage,each satellite simulator is equipped with an aluminium plate.With the aid of the new air-bearing test-bed,the dissertation verifies the effectiveness of the proposed distributed PD controller for state consensus,the output consensus controller and the controller based on artificial potential field for the on-orbit autonomous assembly of a team of flexible spacecraft,respectively.