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This paper investigates the output feedback control problem of underactuated spacecraft hovering in circular orbits subject to unmatched disturbances and input saturation. Two underactuated cases without either the radial or in-track thrust are considered. Besides the unavailability of velocity information, the simultaneous loss of velocity measurements and thrust also gives rise to unknown system parameters, a problem not observed in fully-actuated hovering control. To address the lack of velocity measurements, a filter is designed to generate velocity signals. An augmented system is then proposed to deal with the input saturation. By using the inherent coupling of system states, the adaptive backstepping technique is then adopted to estimate the resulting unknown system parameter and design the velocity-free underactuated controller for either case. A Lyapunov-based approach is utilized to prove the stability of the overall closed-loop system, which indicates that all state errors, parameter and velocity estimation errors are asymptotically convergent in the absence of disturbances; and are uniformly ultimately bounded in the presence of disturbances. Finally, numerical simulations are presented to illustrate the performance of the proposed controllers. [ABSTRACT FROM AUTHOR]

Underactuated collision-free controllers are proposed in this paper for multiple spacecraft formation reconfiguration in circular orbits with the loss of either the radial or in-track thrust. A nonlinear dynamical model of underactuated formation flying is introduced, which is then linearized about circular orbits for controllability and feasibility analyses on underactuated formation reconfiguration. By using the inherent dynamics coupling of system states, reduced-order sliding mode controllers are then designed for either case to indirectly stabilize the system trajectories to the desired formations. In consideration of the collision-avoidance requirement, the artificial potential function method is then employed to design novel underactuated collision-avoidance maneuvers. Rigorous proof substantiates the capabilities of such maneuvers in preventing collisions even in the absence of radial or in-track thrust. Furthermore, a Lyapunov-based analysis ensures the asymptotic stability of the overall closed-loop system. Numerical simulations are performed in a J 2 -perturbed environment to verify the validity of the proposed underactuated control schemes for collision-free reconfiguration. [ABSTRACT FROM AUTHOR]

This paper proposes the output feedback control schemes for underactuated spacecraft hovering without either the radial or the in-track thrust. In contrast to the conventional fully actuated output feedback hovering control that only lacks velocity signals, a new problem is observed in the underactuated case that the simultaneous loss of velocity measurements and thrust does not only result in the loss of velocity information but also gives rise to unknown system parameters’ effects. Furthermore, due to the loss of thrust, the disturbances turn into unmatched ones. To achieve velocity-free hovering control for the underactuated cases, a novel adaptive reduced-order observer is first proposed for the purpose of velocity and parameter estimation. With the estimates provided by the observer, an output feedback controller is then designed by using the inherent coupling of system states. The asymptotic stability of the overall closed-loop system for either case is guaranteed by a Lyapunov-based method. Finally, numerical simulations are presented to demonstrate the feasibility and validity of the proposed controllers for hovering perturbed by the unmatched disturbances with the simultaneous loss of velocity measurements and thrust. [ABSTRACT FROM PUBLISHER]

The Lorentz force acting on an electrostatically charged spacecraft as it moves through the planetary magnetic field could be utilized as propellantless electromagnetic propulsion for orbital maneuvering, such as spacecraft formation establishment and formation reconfiguration. By assuming that the Earth’s magnetic field could be modeled as a tilted dipole located at the center of Earth that corotates with Earth, a dynamical model that describes the relative orbital motion of Lorentz spacecraft is developed. Based on the proposed dynamical model, the energy-optimal open-loop trajectories of control inputs, namely, the required specific charges of Lorentz spacecraft, for Lorentz-propelled spacecraft formation establishment or reconfiguration problems with both fixed and free final conditions constraints are derived via Gauss pseudospectral method. The effect of the magnetic dipole tilt angle on the optimal control inputs and the relative transfer trajectories for formation establishment or reconfiguration is also investigated by comparisons with the results derived from a nontilted dipole model. Furthermore, a closed-loop integral sliding mode controller is designed to guarantee the trajectory tracking in the presence of external disturbances and modeling errors. The stability of the closed-loop system is proved by a Lyapunov-based approach. Numerical simulations are presented to verify the validity of the proposed open-loop control methods and demonstrate the performance of the closed-loop controller. Also, the results indicate the dipole tilt angle should be considered when designing control strategies for Lorentz-propelled spacecraft formation establishment or reconfiguration. [ABSTRACT FROM AUTHOR]