Research On De-tumbling And De-orbiting Control Of Space Debris Target Using Tethers After Capture

Space debris poses a serious threat to the normal operation of spacecraft as well as the space activities of astronauts since its huge kinetic energy.According to the collision cascade effect,when the number of space debris reaches a certain point,frequent collisions would be triggered,renders the debris level be completely out of control.The situation is equally serious in GEO,where debris objects from collision would remain permanently that makes precious orbital resources no longer exist.Facts show that large-scale abandoned spacecraft is the main source of collisions,aiming at which active debris removal is considered as a promising way for space debris mitigation.With increasing number of launches in recent years,the demand for active debris removal is becoming urgent.In addition,spacecraft with initial residual angular momentum,such as spin-stabilized satellites or the spacecraft with attitude control failure and wrong spurt of thruster occurred,will finally tumbled under the action of complex environment torques.Tumbling brings great challenges to the capture as well as safety of platform especially in close range.Based on the active debris removal mission,this paper investigates the tumble mitigation and de-orbit of the target using space tethers.The system contains the target,chaser and a tether,in which a harpoon is used to capture the target.The safety of the platform could be guaranteed since the chaser would not contact the target directly.The main research work is as follows:By using Newton Euler method,the attitude dynamics model of the two spacecraft considering tether torques,as well as the relative motion model between the two mass centers are established.The former considers the influence of force moment produced by tether tension on the attitude of the two spacecraft,and the later takes into account the translational acceleration from tether tension and is similar to CW equation.Target includes rigid and flexible spacecraft are studied.For the de-orbiting scheme using electrodynamics tether system,the dumbbell model considering in-plane and out-of-plane angular motion equation is established.The deformation of the tether under the action of electric force is evaluated by the pearl point method.For satellites with different altitude,the perturbation effects such as the earth's oblateness,atmospheric drag and solar-lunar gravitational perturbations are considered by actual circumstances.Choosing a rigid spacecraft as target,a Takagi-Sugeno(T-S)fuzzy guaranteed costcontroller for the tethered space-tug(TST)system is designed.The problem of de-tumbling the target is transformed into tether tension control and guidance of the chaser.Firstly,by designing the tension control law similar to sliding mode structure,the T-S fuzzy model for guidance and control of the chaser are established.The guaranteed cost controller is used and the guidance and control law is obtain by solving a set of linear matrix inequalities(LMIs).Then the identification of target inertia parameters is studied.The simulation results show that tumbling of the spacecraft could be stabilized by the space tether system,and the target is finally rotating along the tether,which is beyond the control capability of the tether.In addition to rigid target,the de-tumbling problem of spacecraft with flexible appendages is studies.A linearization and discretization method to approximate the dynamics of the target using feedforward neural network model is proposed.Considering that modal coordinates of flexible appendages are not measurable,an Extended State Observer(ESO)is introduced to estimate the effect of appendages.The measurable state,control input and extended state of the system are taken as the input vector of the neural network,and the linearization model is obtained by weight calculation.The controller with quadratic index is obtained and its stability is proved.The simulation results show that the tethered space-tug system successfully de-tumbles the flexible target,and during the process,the vibration of flexible appendages gradually decreases due to the structural damping as well as tether tension control.The trajectory planning of chaser to de-tumble a rigid spacecraft based on tether expansion method,pseudo-spectral optimization method and sequence convex optimization method are studied respectively.The fuel consumption is chosen as the performance index,and the winding avoidance between tether and the two spacecraft as well as the control saturation of chaser are considered.For the target whose moment of inertia is much larger than chaser,the method of tether deployment is much more appropriate,which increase the system's moment of inertia,therefore reduce the rotational speed of the target along with the tether releasing.Otherwise,optimize method is more suitable.Compared to Gauss method,Radau pseudospectral optimization method could obtain the initial control and implementation is much easier,the simulation shows that the target could be perfectly de-tumbled to zero angular velocity of three axis.The sequential convex optimization method is based on discrete control,and the control quantity problem is solved iteratively through the way of rolling time domain,simulation verifies theeffectiveness of the strategy.Deorbiting strategy using chemical thrust and the LEO de-orbiting scheme using electrodynamics tether system are studied in the last section.Dual pulse transfer is used to de-orbit the geosynchronous target to the grave orbit.In order to maintain the relative position as well as target's attitude,a small thrust is used to keep the tether tight during the two maneuvers.The fuel consumption for GEO target is small,and one chaser could successively remove multiple targets.For LEO targets,when chemical thrust is used,single maneuver to enter the atmosphere costs a high demand of ?V,and the fuel consumption may exceed the capacity of chaser.Therefore,the scheme of decreasing the perigee height of the target with a small ?V,reduce orbital lifetime is studied.The disadvantage is that the falling point is uncontrollable,which might cause property or personnel loss.For LEO target.Current constraints and stability of tether attitude are considered in electrodynamics tether system.The simulation results show that the reducing rate decreases with the increase of orbit inclination.