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Attaining microradian control accuracy within seconds is essential for the coarse pointing assembly (CPA) to establish and maintain the intersatellite laser link under multiple disturbances. In response to the control requirements and challenges, this article proposed a fixed-time refined disturbance observer-based composite control (FTRDOBC) scheme. First, a periscope-type CPA model is established, incorporating gimbal coupling torque, friction, and satellite-transmitted vibration to characterize the operational environment. Second, by utilizing the partially known information of the multiple disturbances, a fixed-time refined disturbance observer (FTRDO) is presented, featuring a state-dependent variable exponent coefficient auxiliary input. Combining the FTRDO with the presented time-varying fixed-time controller, an FTRDOBC scheme is proposed, aiming for high precision control in predefined time while circumventing the singularity problem. Experimental results are provided to demonstrate the effectiveness of the proposed method.

Depth estimation problem for the perspective vision systems has been extensively studied in the literature and the depth estimation can be achieved under the assumption that the exact camera motion parameters are available. However, in practice, it is hard to obtain the camera motion parameters exactly. For instance, the exact camera velocity is always unavailable and instead only the roughly estimated one can be obtained. This introduces the significant difficulties to achieve the depth estimation with partially uncertain camera motion parameters. In this article, we consider the depth estimation problem for the perspective vision system in the case that the camera angular velocities are accurate, while partial camera linear velocities are contaminated by some disturbances. This problem is theoretically formulated and solved for the first time in this article by proposing a new depth observer in the presence of the partially uncertain camera linear velocities. Local exponential convergence of the depth and disturbance estimates is achieved in presence of the partially uncertain camera linear velocities such that the estimation errors of states and disturbances converge to zero for constant disturbances or to a small error bound for bound time-varying disturbances. Simulations and experiments are carried out to verify the performance of the proposed observer.

The high-precision alignment of spacecrafts subject to multiple composite disturbances [e.g., axle frictions and back electromotive force (EMF)] and mandatory pointing constraints is an extremely challenging issue for both theoretically and practically. To address this challenge, a refined deep coupled alignment control scheme is proposed in this article. First, a deep coupled alignment model of spacecraft based on the dynamics of the reaction wheel actuator is established, which reveals the influence of the composite disturbances, such as the axle frictions and the back EMF on the system, and separates the composite disturbances into mismatched and matched disturbance according to the affected channels. Second, in view of the characteristics of the composite disturbances, heterogeneous sliding mode disturbance observers (SMDOs) are designed to estimate them accordingly. Then, in order to cope with the mandatory pointing constrains, barrier functions are introduced to convert the alignment errors to limit their variation range. Finally, combined with heterogeneous SMDOs and conversion errors, a composite refined controller is constructed to finely compensate the composite disturbances in the alignment system. High-precision control performance is achieved and the mandatory pointing requirement is satisfied. Simulations and experiments verify the effectiveness of the proposed scheme.

The disturbances of flexible spacecrafts exhibit composite property, which means that they are heterogenous and inherently coupled with the attitude variables, control inputs, and even the other uncertainties. The separation and thereby compensation of the composite disturbances are crucial to the improvement of attitude control performances. By incorporating two popular disturbance rejection methods, the disturbance observer-based control and the equivalent input disturbance (EID)-based control, into one unified framework, a refined disturbance separation-based enhanced antidisturbance control law is proposed for flexible spacecrafts. In the presence of composite disturbances including recessive flexible vibration, additive actuator error, and multiplicative inertia uncertainty, a deep-coupled attitude control model is established. On this basis, a disturbance separability condition and a refined disturbance separation method are proposed. More specifically, the recessive flexible vibration is estimated by a refined disturbance observer (RDO), while the estimation error of RDO and the other disturbances (e.g., additive actuator error) are estimated by the EID estimator. The multiplicative inertia uncertainty is attenuated by the feedback control. Finally, numerical simulations and experimental results are, respectively, given to demonstrate the effectiveness of the proposed method.

High-precision and safety control in face of disturbances and uncertainties is a challenging issue of both theoretical and practical importance. In this article, new adaptive anti-disturbance control schemes are proposed for a class of uncertain nonlinear systems with composite disturbances, including additive disturbances, multiplicative actuator faults, and implicit disturbances deeply coupled with system states. Both the cases with known and unknown control/fault directions are investigated. By properly fusing the techniques of disturbance observers and adaptive compensation, it is shown that all closed-loop signals are globally uniformly bounded and the tracking error converges to zero asymptotically, no matter the control/fault directions are known or not. In the case of known directions, the proposed control scheme, for the first time, guarantees asymptotic tracking and ${L}_{\infty }$ tracking performance simultaneously in face of disturbances and actuator faults. Moreover, novel Nussbaum functions and a contradiction argument are introduced, which allow the system to have multiple unknown nonidentical control directions and unknown time-varying fault direction. Simulation results illustrate the effectiveness of the proposed control schemes.

The actuator of attitude control system (ACS) plays an important role in providing the necessary control torque for spacecraft missions, such as orbital maneuvering and high-precision attitude stabilization. In a complex space environment and under long-term working loads, the actuator is more likely to degrade and fail compared to other components. This makes it significant to assess its reliability state and predict its remaining useful life (RUL). To overcome the drawback that traditional RUL prediction methods for the actuator are based on indirect external data, this article introduces a multiplicative fault from the control field as an intrinsic performance indicator to estimate the RUL. The multiplicative fault indicates the inherent degradation characteristic of actuation effectiveness, and its estimates can be obtained by designing a fault observer. Accordingly, a multiplicative fault observer is designed in this article, and its estimates are used to achieve the parameter estimation in the proposed generalized degradation model through maximum likelihood estimation (MLE). Furthermore, the probability density function (PDF) of the RUL is obtained. Illustrative examples of the multiplicative fault estimation and the corresponding RUL prediction are presented to demonstrate the effectiveness of the proposed method.

The high-precision and fast-response safety proximity control for servicing spacecrafts is affected by dynamic obstacle constraints and multiple disturbances, such as actuator deviations and flexible vibration. To address this challenge, an adaptive fixed-time disturbance observer (AFTDO)-based composite proximity control scheme is proposed for the servicing spacecrafts to achieve effective obstacle avoidance and high-precision position tracking. Because the multiple disturbances are coupled with the system physical variables (e.g., control input and relative position) and exhibit complicated dynamics, an AFTDO is proposed to estimate the disturbances in fixed time by adaptively learning the bound information of disturbance derivative. For the dynamic obstacle constraints, an artificial potential function-based terminal sliding mode manifold is designed to convert the obstacle constraints to a path planning problem, which effectively provides the repulsion effect of the servicing spacecraft on obstacles and the attractive effect on the target position. Through the designed finite-time composite proximity controller, the antidisturbance ability of the servicing spacecraft can be effectively improved, and the coordinated performance optimization under the obstacle constraints can be also ensured. Simulation results verify the effectiveness of the proposed scheme.