On-orbit Maintenance Dynamics And Control Of Space Manipulator For Maintainable Spacecraft

On-orbit faults may degrade the performance of spacecraft,or lead to the paralysis of the whole satellite.As most hardware failures cannot be repaired through software upgrade and system restart,the development of on-orbit maintenance has become an inevitable trend.Due to the rapid development of ground technology and the extension of spacecraft life,the longer the spacecraft life,the greater gap between the performance of spacecraft in the middle and late life and the level of ground technologies.Therefore,it has become a consensus in the industry to design the maintainability of large and medium-sized spacecraft to make it easier to accept on-orbit services such as maintenance and upgrading.However,the design method of maintainable spacecraft has not formed a complete theoretical system,and on-orbit maintenance for maintainable spacecraft is facing new technical challenges.On the one hand,it is necessary to solve the difficult problem of maintenance convenience and cost in the design of maintainable spacecraft.On the other hand,it is necessary to solve the problem of realizing highprecision and high compliance maintenance operation under the influence of the configuration and inertia changes of maintainable spacecraft,the complex obstacle constraints in confined space and the nonlinear factors of maintenance tools.Therefore,the on-orbit maintenance dynamics and control technology of space manipulator for maintainable spacecraft is studied.Based on the convenience of on-orbit maintenance for space manipulator and low cost,this paper puts forward a general design method for maintainable spacecraft from three aspects: maintainable architecture,replaceable functional module architecture and plug-and-play interface architecture.Combined with the proposed method,a modular maintainable satellite with variable main structure configuration is designed,which as the object of on-orbit maintenance operation of space manipulator in this paper.The designed serviceable satellite takes the open mechanical system with variable configuration,distributed heterogeneous network system and refillable propulsion system as the core architecture,and the two-level functional modules and three-level interfaces as the repairable carrier to support the on-orbit maintenance of space manipulator.From the three aspects of maintenance operation process,maintenance operation constraints and maintenance cost,the maintenance operation requirements of maintainable spacecraft for space manipulator and the economic value of spacecraft maintainability development are analyzed.The analysis shows that the designed maintainable spacecraft realizes the maintainability at a low cost,reduces the capability requirements for maintenance operators.A fine dynamics model of space manipulator system considering the influence of internal and external factors such as joint flexibility and end contact collision is established,which can characterize the influence of joint flexibility on the on-orbit maintenance accuracy of space manipulator.Firstly,the coordinate system of a 7-DOF space manipulator is described by D-H parameter method,and its kinematics and dynamics equations are derived.Aiming at the harmonic reducer joint commonly used in space manipulators in unmanned on-orbit maintenance missions,a nonlinear fine dynamic model of harmonic driven joint is proposed,which comprehensively considers nonlinear stiffness,hysteresis,torsional clearance,different types of friction,pure kinematic error of harmonic drive and joint damping.The coupling effects of nonlinear stiffness,hysteresis and torsional clearance are modeled by ordinary differential equation,and the algebraic solution under periodic input is given.Next,the nonlinear characteristics of joints are introduced into the system dynamic model of the space manipulator,the collision detection and collision force calculation are carried out in the operation process,thus,the coupling dynamic model of the space manipulator and the target is established.The dynamic simulation shows that joints nonlinearities cause high-frequency vibration of the manipulator and lead to significant increase of the end position error,which needs to be compensated in the manipulator control.A trajectory planning and optimization method of space manipulator under the constraints of complex obstacles in confined space and its own performance is proposed,which can meet the trajectory planning requirements of high-precision on-orbit maintenance operation of maintainable spacecraft.In order to have high-dimensional complex obstacle space search ability,efficient calculation efficiency and ensure the smoothness of planning results,an improved RRT-connect and RRT * obstacle avoidance trajectory planning method based on artificial potential field is proposed.And an obstacle free trajectory of the space manipulator in the process of module replacement of the designed maintainable spacecraft is given.Considering that the maintenance operation process is limited by many factors,such as the energy supply and thermal control capability of spacecraft are affected by spacecraft configuration transformation and docking of the servicing spacecraft and the serviced spacecraft,onorbit maintenance is strictly limited by maintenance operation time and energy consumption.Therefore,based on the NSGA-II(Non-dominated Sorting Genetic Algorithm)algorithm,a multi-objective trajectory optimization method is established,which takes maintenance time,energy consumption and deviation from obstacle free trajectory as the objectives,and the physical constraints of manipulator joints being considered.Simulation results show that the proposed method can guide the space manipulator to effectively avoid various complex obstacles such as satellite body,solar wings,and obstacles inside the maintainable spacecraft with variable configuration,and the joints motion and torque change smoothly.And the planned trajectory is optimized in terms of maintenance time,energy consumption and deviation from the obstacle free trajectory.A new force and position hybrid control method is proposed,which can overcome the influence of internal and external uncertain factors and realize high precision and high compliance of on-orbit maintenance operation.First,the dynamic model of the manipulator is reconstructed in the adaptive coordinate system.The force and position hybrid control architecture of redundant manipulator and a kind of torque optimization method based on the zero-space method are proposed,and the corresponding control law is designed.In this process,the decoupling of position control and force control is realized.On this basis,a modified force position hybrid control method based on radial basis function neural network(RBFNN)compensation is proposed,the corresponding adaptive control law is designed and its stability is proved.The comparative simulation of different algorithms and the simulation of functional module replacement task of maintainable spacecraft shows that the proposed control method can adapt to the large changes of load mass and inertia,overcome the influence of nonlinear factors of space manipulator,and realize the accurate control and compliant operation of on-orbit maintenance process.