Releasing Dynamics And Control Of Tethered Space Net Robot

Ever since the rapid development of space technology and the strong exploration in space science,space resourse and space military by human,the total number of the new launched satellites in each year by each country increases rapidly.Accordingly,the number of the space debris is growing as well.Thereinto,space junk satellites get more attention by the science researchers and institutions,because of their high value and high risk.Tethered Space Net Robot(TSNR)is a new kind of space robot for active space debris capture and removal,which is composed of a net and Maneuverable Units(MUs).A SNR has the ability of large working range,flexible maneuverability,large envelop area,and low capuring conditions,which are not required in other space debris capture devices.So the SNR will be a promising solution for the active space junk statellite capture.However,a TSNR is a flexible multi-dynamics under-actuated system,and there is rare research,to my best knowledge,dedicated to the releasing dynamics charecteristics and control problems of such a system.A stable flying of the TSNR is the first precondition for space capture mission.Therefore,releasing dynamics and control of Space Net Robot is a foundamental research of the active space junk satellite capture.The releasing dynamics and stability control after releasing are proposed in this thesis,which is based on the dynamics modeling first.The structure of the thesis is organized as follows:(1).A full mathematical dynamics modeld is derived in the very beginning by treating the knitted tethers as a serial mass points connected via massless springs.The derived motion equations can present the flexibility,elasticity and high-freedoms of the TSNR.Based on this dynamics model,a simplified dynamics is derived and a set of singularity points are solved of this simplified dyanmics.After the phase analysis,a conclusion is acquired that this set of singularity points are equilibrium points but not stable points.(2).The releasing dynamics is studied and an optimal selecting scheme for the TSNR releasing is given based on a full analysis of net folding scheme,MUs' shooting conditions.Fistly,the motion dynamics of a TSNR when it is totally deployed is studied,which can be considered as a standard free flying of the system.Then,three candidate folding schemes are prposed based on the classic origami dynamics and analysis.Criterion are presented to evaluate the free flying of the TSNR after releasing.So three candidate folding schemes are compared and analysis based on these criterion.Based on the dicided folding scheme,initial shooting conditions of the four MUs are discussed later,including the initial shooting angles and velocities.(3).A super-twisting second order sliding mode controller is designed to solve the hop from free flying stage to control flying stage of the TSNR.This proposed control scheme can suppress the oscillations casued by flexibility and elasticity,and keep stable configuration of the TSNR from free flying to control flying.Besides,the proposed controller can reduce the chattering a lot compared to the traditional sliding mode control.A brand new control strategy is presented that the motion of uncontrollable part of the TSNR,namely the underactuate net,is considered as an external disturbance working of the MUs.A classic super-twisting sliding mode control schemem is updated with a compensation term to suppress the unknown disturbance(oscillation).The scheme is proved via both mathematical verification and numerical simulations.(4).Aiming to the same problem that the hop from free flying stage to control flying stage of the TSNR,more real and complex space environment is further considered.The stability prolem of the TSNR is solved with a complex,continue and asymmetrical configuration.Based on the previous dynamics analysis,it is known that the dynamics of the TSNR in an asymmetrical configuration is chaotic and unstable,and will accurate the whole TSNR with oscillations.To solve this problem,a brand new controller named fuzzy based adaptive super twisting sliding mode control is proposed.In this controler,a fuzzy scheme is used to approximate the unknown complex disturbance,which is composed by external orbital perturbance,dynamics errors and oscillations from the net.This proposed control scheme inherits both strong robustness and easy-design from sliding mode control and general approximation from the fuzzy scheme.So this scheme can solve the over-control problem,and achieve the exact control of this flexiable underactuate system and low fuel consumption.(5).A hierarchical fuzzy based adaptive sliding mode control scheme is proposed to solve the problem of configuration maneuverability,specificly a flexible underacutate high freedoms system.The hierarchy concept is involved on the sliding variables,so that all the knit tether nodes are involved in the control scheme.A fuzzy scheme with adaptive learning law is used to estimate the dynamics equations and unknown time-varing upper bound of the disturbance.A Lyapunov function is designed to prove the closed system stability and tracking performance.The conclusion shows that the closed system is stable,and the tracking errors of the states are convergency.This proposed control scheme inherits both strong robustness and easy-design from sliding mode control and general approximation from the fuzzy scheme.So this scheme can solve the problem of configuration maneuverbility,and achieve the exact control of this flexiable underactuate system and low fuel consumption.