Adaptive Sliding Mode Control For Proximity Relative Orbit Motion Of Spacecraft

With the complexity of outer space missions,many on-orbit spacecrafts have con-tinued a trend towards increasingly massive,exquisite,longer-lasting and high-cost satel-lites,which place increasing strain on the guarantee of safety.In addition,the increasing space debris has exacerbated the tension of orbital resources and seriously threatened the safety of on-orbit satellites.In order to support the current and future spacecrafts,on-orbit servicing(OOS)infrastructures have been proposed by many space agencies in the world.The OOS missions mainly include the maintenance of on-orbit satellites,active removal of space debris and space situational awareness,which have the important scien-tific research significance and enormous potential for economic and military fields.The precise motion control of service spacecraft is a fundamental and core problem,which is a prerequisite for the completeness of OSS missions.In OSS missions,the target spacecrafts involved are mainly divided into the coop-erative target and non-cooperative target.Different from the cooperative targets,which enable radar reflector allowing the chaser to estimate the relative position state and em-ploy the orbit control system to take cooperative operation,the targets such as space debris and enemy satellite are non-cooperative,their non-cooperative characteristics can be sum-marized into three points:1)In terms of spacecraft structure,the structural characteristics of the target is unknown,and there is lack of the matching connector for capture device,which bring great difficulties to the design of capture strategy;2)In terms of spacecraft navigation and measurement,there is no cross-link communication between the chaser and target,and the target is not equipped with the markers that facilitate the measure-ment of relative states,which may lead to large errors and uncertainties in the feedback navigation information,and even bad situations where only partial states are known;3)In terms of spacecraft motion,the target may have no ability to achieve the three-axis attitude stabilization,and even has the rolling or orbital maneuvering,which require that the attitude and orbit maneuver of the chaser must take into account the maneuver char-acteristics of the target.In conclusion,the study of relative motion control technology for the non-cooperative target is a challenging problem.Motivated by the spacecraft OSS missions,the present dissertation focuses on the adaptive sliding mode control(ASMC)of spacecraft relative orbit motion in space proximity operations.The main contents are listed as follows.First,two kinds of spacecraft relative orbit dynamics are formulated.For the coop-erative target in near-circular orbit,the relative motion model is established in the target's local-vertical-local-horizontal(LVLH)reference frame,which is a linear time-invariant system.For the non-cooperative target,the relative motion model is developed in the chaser's line-of-sight(LOS)reference frame,which is a strongly coupled nonlinear sys-tem and described by relative distance and LOS angles.Then,considering the cooperative target in near-circular orbit,the relative motion model is classified as a second-order mechanical system with the matching disturbance,and the adaptive fast finite-time control(FFTC)problem for a class of second-order me-chanical system with matched disturbance is investigated.By employing the finite-time backstepping design approach and nonsingular fast terminal sliding mode(NFTSM)con-cept,a new form of FFTC law is proposed and the sufficient conditions of the controller parameters are given,which can guarantee the fast finite-time stability(FFTS).Further,without prior knowledge of the upper bound of matched disturbance,an adaptive con-trol law is proposed,which guarantees the asymptotic stability.The simulation results of the application to spacecraft rendezvous and flying-by missions have demonstrated the effectiveness of the proposed approach.In the sequel,considering that the relative velocity is inaccessible,in the presence of external disturbance and target angular velocity uncertainty,the dynamic output feed-back(DOF)control problem of spacecraft relative orbit motion in near-circular orbit is studied.Based on the ASMC method,a reduce-order DOF sliding mode control(DOF-SMC)method for spacecraft hovering is designed,which can guarantee that the tracking error asymptotically converges to the neighborhood of zero.Besides,a linear full-order DOF controller for spacecraft autonomous rendezvous is proposed.As a stepping-stone,the H_?performance requirement,poles and input constraint are analysed separately via linear matrix inequality(LMI),and with the obtained results,the controller design prob-lem is cast into a feasibility problem subject to a set of LMI constraints.The following simulation results of the application to spacecraft rendezvous and hovering missions have demonstrated the effectiveness of the proposed approach,it can be seen that in comparison with the full-order DOF controller,the reduce-order DOFSMC method can also guarantee the completeness of spacecraft missions and has better system performance.After that,for the non-cooperative targets in any Keplerian orbit such as space de-bris and malfunctioning satellite,combining with the LOS-based relative motion model,the ASMC problem of relative orbit hovering in close proximity is discussed.First,com-bined with linear sliding mode surface and high-order sliding mode observer,a sliding mode controller is proposed to compensate the unmodeled dynamics.It can be guaran-teed that the system state reaches the sliding mode surface in finite-time and the tracking error asymptotically converges to zero along the linear sliding mode surface.However,the linear sliding surface can only guarantee that the tracking error converges to zero when the time approaches to infinity.Therefore,the linear sliding mode surface is replaced by a NFTSM surface,the unmodeled dynamics and external disturbance are categorized as a class of uncertain term,and an adaptive NFTSM(ANFTSM)control law is proposed.Under the condition that the upper bound of the uncertain term is known,the FFTS of the tracking error system can be guaranteed.Without prior knowledge of the upper bound of uncertain term,it can be guaranteed that the tracking error asymptotically converges to zero.To limit the adaptive gain and reduce the chattering amplitude,an improved adap-tive control law is introduced and can guarantee that the tracking error converges to the neighborhood of zero.Numerical simulation of spacecraft hovering to non-cooperative target is given to demonstrate the effectiveness of the proposed approach,which shows that the ANFTSM control law has faster convergence speed and higher control precision.Finally,in the presence of the external disturbance and unknown mass property of the chaser,the adaptive SMC(ASMC)problem of spacecraft relative orbit hovering to maneuvering target is studied.Integrated with the LOS-based relative motion model,the problem is formulated in the general mechanical second-order form with unknown mass parameter and matching disturbance,which makes it convenient to use some useful physical properties in the control law design.Incorporated with the continuous projec-tion algorithm and equivalent-control-dependent gain method,the modified control law is proposed,which can force the mass estimate to remain in a desired domain and efficiently overcome the drawback of the overestimation of the disturbance in the traditional ASMC law.Within the Lyapunov frame,the bounded stability is presented in the real case that the sign function is replaced by the hyperbolic tangent function.Numerical simulation of spacecraft hovering are conducted,the simulation results show that the proposed ASM-C law can ensure the completeness of relative orbit hovering mission,and estimate the unknown terms effectively.