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This paper is devoted to real-time optimal attitude reorientation control of rigid spacecraft control. Particularly, two typical practical problems - actuator misalignment and forbidden pointing constraints are considered. Within the framework of adaptive dynamic programming (ADP), a novel constrained optimal attitude control scheme is proposed. In this design, a special reward function is developed to characterize the environment feedback and deal with the pointing constraints. Notably, a novel argument term is introduced to the reward function for overcoming the inevitable difficulty in actuator misalignment. By virtue of the Lyapunov stability theory, the ultimate boundedness of state error and the optimality of the proposed method can be guaranteed. Finally, the effectiveness and performance of the developed ADP-based controller are evaluated by not only numerical simulations but also experimental tests with a hardware-in-loop platform.

This paper proposes a neural network-based fault diagnosis scheme to address the problem of fault isolation and estimation for the Single-Gimbal Control Moment Gyroscopes (SGCMGs) of spacecraft in a periodic orbit. To this end, a disturbance observer based on neural network is developed for active anti-disturbance, so as to improve the accuracy of fault diagnosis. The periodic disturbance on orbit can be decoupled with fault by resorting to the fitting and memory ability of neural network. Subsequently, the fault diagnosis scheme is established based on the idea of information fusion. The data of spacecraft attitude and gimbals position are combined to implement fault isolation and estimation based on adaptive estimator and neural network. Then, an adaptive sliding mode controller incorporating the disturbance and fault estimation results is designed to achieve active fault-tolerant control. In addition, the paper gives the proof of the stability of the proposed schemes, and the simulation results show that the proposed scheme achieves better diagnosis and control results than compared algorithm.

For high-resolution imaging implementations, the spacecraft attitude tracking control accuracy is crucial to determining the imaging quality. This investigation addresses the attitude tracking issue of imaging spacecraft subject to system uncertainties (unavailable inertia tensor, unexpected disturbances, and actuator faults). An adaptive sliding mode control (SMC) strategy is proposed to guarantee practical fixed-time closed-loop stability even in the presence of system uncertainties. Unlike existing methodologies, the sliding mode surface is developed to satisfy a novel sufficient condition of fixed-time stability. The sliding manifold design also circumvents the unwinding phenomenon arising in quaternion representations. Particularly, this controller is developed to generate a smooth control profile by using a new parameter update law. Rigorous Lyapunov analyses are further employed to ensure the fixed-time closed-loop stability irrespective of the system initial states. Finally, numerical examples are performed to demonstrate the feasibility and highlight the inherent features of the derived control law.

This paper aims to handle the optimal attitude tracking control tasks for rigid bodies via a reinforcement learning-based control scheme, in which a constrained parameter estimator is designed to compensate system uncertainties accurately. This estimator guarantees the exponential convergence of estimation errors and can strictly keep all instant estimates always within pre-determined bounds. Based on it, a critic-only adaptive dynamic programming (ADP) control strategy is proposed to learn the optimal control policy with respect to a user-defined cost function. The matching condition on reference control signals, which is commonly employed in relevant ADP design, is not required in the proposed control scheme. We prove the uniform ultimate boundedness of the tracking errors and critic weight's estimation errors under finite excitation conditions by Lyapunov-based analysis. Moreover, an easy-to-implement initial control policy is designed to trigger the real-time learning process. The effectiveness and advantages of the proposed method are verified by both numerical simulations and hardware-in-loop experimental tests.

This paper proposes a coordinate-free controller for a nonholonomic vehicle to circumnavigate a fully-actuated moving target by using range-only measurements. If the range rate is available, our Proportional Derivative (PD)-like controller has a simple structure as the standard PD controller, except the design of an additive constant bias and a saturation function in the error feedback. We show that if the target is stationary, the vehicle asymptotically encloses the target with a predefined radius at an exponential convergence rate, i.e., an exact circumnavigation pattern can be completed. For a moving target, the circumnavigation error converges to a small region whose size is shown proportional to the maneuverability of the target, e.g., the maximum linear speed and acceleration. Moreover, we design a second-order sliding mode (SOSM) filter to estimate the range rate and show that the SOSM filter can recover the range rate in a finite time. Finally, the effectiveness and advantages of our controller are validated via both numerical simulations and real experiments., 13 pages,17 figures

This article details the development and evaluation of a practical solution for path-constrained proximity maneuvers of spacecraft. Whereas no velocity measurement is utilized within the feedback structure, the controller rigorously enforces actuator magnitude constraints. Specifically, the control algorithm is constructed from a potential function method repelling the spacecraft from possible collisions. The proposed controller can guarantee potential functions to be navigated to the origin and thus overcome the stubborn local minima problem. Moreover, the control capability under any given control limit can be estimated and adjusted by changing the feedback gains. The specific performance with guaranteed safety can be also explicitly calculated by designers. The results are obtained through a Lyapunov-based stability analysis to prove uniformly ultimate boundedness. Numerical simulation results illustrate the performance and features of the developed control method.

A passive fault tolerant control scheme is proposed for the full reentry trajectory tracking of a hypersonic vehicle in the presence of modeling uncertainties, external disturbances, and actuator faults. To achieve this goal, the attitude error dynamics with relative degree two is formulated first by ignoring the nonlinearities induced by the translational motions. Then, a multivariable twisting controller is developed as a benchmark to ensure the precise tracking task. Theoretical analysis with the Lyapunov method proves that the attitude tracking error and its first-order derivative can simultaneously converge to the origin exponentially. To depend less on the model knowledge and reduce the system uncertainties, an incremental twisting fault tolerant controller is derived based on the incremental nonlinear dynamic inversion control and the predesigned twisting controller. In this article, it is shown that not only the benefits of both incremental control and twisting control are inherited, but also their side effects are reduced. Notably, the proposed controller is user friendly in that only fixed gains and partial model knowledge are required. Numerical simulations in various cases and comparison studies are conducted to verify the effectiveness of the proposed method.

This article addresses the event-triggered coordinated control problem for multiple Euler-Lagrange systems subject to parameter uncertainties and external disturbances. Based on the event-triggered technique, a distributed coordinated control scheme is first proposed, where the neural network-based estimation method is incorporated to compensate for parameter uncertainties. Then, an input-based continuous event-triggered (CET) mechanism is developed to schedule the triggering instants, which ensures that the control command is activated only when some specific events occur. After that, by analyzing the possible finite-time escape behavior of the triggering function, the real-time data sampling and event monitoring requirement in the CET strategy is tactfully ruled out, and the CET policy is further transformed into a periodic event-triggered (PET) one. In doing so, each agent only needs to monitor the triggering function at the preset periodic sampling instants, and accordingly, frequent control updating is further relieved. Besides, a parameter selection criterion is provided to specify the relationship between the control performance and the sampling period. Finally, a numerical example of attitude synchronization for multiple satellites is performed to show the effectiveness and superiority of the proposed coordinated control scheme.

Blood analysis, though complete blood count (CBC), is the most basic medical test for disease diagnosis and health monitoring. However, the need for bulky and expensive laboratory facilities and skilled technicians limits the universal medical practices based on blood analysis outside of well-equipped laboratory environments. Here, we proposed a multiparameter mobile blood analyzer utilizing label-free contrast-enhanced defocusing imaging (CEDI) with the assistance of machine vision for instant and on-site blood analysis. The analyzer using CEDI obtains both the blood cell morphological characteristics and hemoglobin spectrophotometric information for simultaneous five-part white blood cell differential count, red blood cell count, and hemoglobin quantification without the need for sample staining. In addition, we have designed a miniature microscope with a sub-micron spatial resolution of (~0.98 μm), which is quite small, lightweight, and cost-effective for blood imaging. We have shown that our assay can analyze a sample within 10 minutes, using just a drop of fingertip blood (~7 μl) and measurements (30 samples) from the analyzer have strong correlations with clinical reference values (significant level: P < 0.0001). The proposed mobile blood analyzer can help provide universal access to blood analysis and has great potential for integrated surveillance of various epidemic diseases, including coronavirus infection, invermination, and anemia, especially in low-and middle-income countries.

This article addresses the formation tracking control problem for multiple spacecraft systems subject to limited communication resources and external disturbances. Considering that only a subset of the follower spacecraft can have access to the motion states of the dynamic leader, an event-based distributed observer is first developed to reconstruct the leader's information. Subsequently, to achieve the accompanying flight of the follower spacecraft around the leader with a desired formation configuration, for each follower spacecraft, a distributed event-triggered coordinated controller is proposed. In particular, by embedding simultaneously the position information and observer output into the triggering function, a novel unified event-triggered mechanism is designed to schedule information transmission in multiple spacecraft systems. The salient characters of the distributed coordinated control scheme are twofold: unnecessary occupancy of communication resources can be avoided significantly; and asymptotic stability of the whole closed-loop system is guaranteed without resorting to the separation principle. Finally, numerical simulations are carried out to illustrate the efficiency of the theoretical results.

Glioblastoma (GBM) is the most frequent tumor in the central nervous system. Long non-coding RNAs (lncRNAs) have been widely accepted as essential participators in cancer progression. Nonetheless, the specific role and mechanism of lncRNA SRY-box transcription factor 2 overlapping transcript (SOX2-OT) in GBM have not been studied. We evaluated expression levels of SOX2-OT, miR-192-5p and Ras-related protein Rab-2A (RAB2A) in GBM cells via qRT-PCR. To investigate the roles of SOX2-OT in GBM cells, CCK-8, JC-1, EdU, and western blot assays were performed. The connection among SOX2-OT, miR-192-5p and RAB2A in GBM cells was explored through pull down, luciferase reporter, and RIP assays. Western blot and qRT-PCR were employed to analyze the activity of extracellular-signal-regulated kinase (ERK) signaling pathway. SOX2-OT expression was higher in GBM cell lines than in normal cells. SOX2-OT knockdown repressed proliferation and promoted apoptosis of GBM cells. Mechanism assays revealed that SOX2-OT could sponge miR-192-5p. Moreover, RAB2A was certified to be the target gene of miR-192-5p. Overexpression of RAB2A reversed the repressive function of SOX2-OT knockdown on GBM cell growth. Furthermore, SOX2-OT activated ERK signaling pathway in GBM cells. SOX2-OT regulated miR-192-5p/RAB2A axis and ERK pathway to promote GBM cell growth.

Background: Concurrent chemo-radiotherapy (CCRT) is an ideal treatment for advanced head and neck squamous cell carcinoma (HNSCC). The performance of CCRT induces severe toxicities in HNSCC patients and decreases the quality of life (QOL). Maitake D-Fraction is proteoglycan which has anti-tumor function associated with its immunomodulatory capacity. The polysaccharides of Maitake also have anti-radiation effect in radiation therapy during cancer treatment. This research aimed to illustrate Maitake D-Fraction effects on CCRT-associated adverse events and QOL. Methods: During CCRT, Maitake capsules were taken orally 3 times a day, each time 4 capsules, one hour before meals. QOL were analyzed by EORTC QLQ-C30-Chinese version and EORTC QLQ-H&N-35-Chinese version. 141 patients were recruited and divided into an intervention group and a placebo group. Results: Frequencies of severe CCRT-associated adverse events in intervention group were less than in placebo group. Global QOL score in intervention group was higher than in placebo group 5 weeks post treatment. The proportion of patients returning to baseline global QOL score at 6-month was increased by Maitake D-Fraction administration. Conclusion: In conclusion, this randomized clinical trial demonstrated that in advanced laryngeal and pharyngeal cancer patients, the oral administration of Maitake D-Fraction alleviated CCRT-related adverse events and deterioration in QOL.

This article studies the six-degree-of-freedom pose control issue of spacecraft proximity operations subject to kinematic and dynamic constraints, as well as parameter uncertainties. A transformed relative translational dynamics and a line-of-sight frame are introduced to facilitate the pose control design. In this context, an immersion and invariance (I&I) adaptive pose control scheme based on artificial potential functions (APFs) is proposed. Specifically, a class of APFs that are free of local minima are first constructed to deal with kinematic and dynamic constraints imposed by certain safety and physical requirements. Second, using the dynamic scaling technique, an I&I adaptive pose controller is tactfully designed. This controller is shown to be able to circumvent the realizability condition that is required for most of the existing adaptive control approaches, but nonetheless may not hold under dynamic constraints. Third, through Lyapunov's direct method, the asymptotic stability of the closed-loop system is analyzed. The proposed method enables the pursuer to arrive at the desired anchoring point with a specified pointing, while satisfying both kinematic and dynamic constraints. Moreover, it depends upon the I&I adaptive design philosophy and, therefore, introduces an attracting manifold, whereby the deterministic case of closed-loop performance (no effect of parameter uncertainties) can be asymptotically recovered. Simulation results illustrate the efficiency of the proposed control scheme.

This paper addresses the relative position coordinated control problem for spacecraft formation flying under an undirected communication graph, whilst considering mass uncertainties, external disturbances, and limited communication resources. A new event-triggered information transmission mechanism is first presented, where each spacecraft only requires accessing to the states of neighbors intermittently. Subsequently, a novel event-based coordinated control scheme is proposed by combining a smooth adaptive projection rule that confines the parameter estimations to well-defined bounded convex hypercubes. Under the proposed control framework, the information exchange among spacecraft occurs only when the specified event is triggered, thereby significantly reducing the communication load and saving the onboard resources. Furthermore, a positive lower bound on interevent time intervals is guaranteed to exclude Zeno behavior. By virtue of Lyapunov stability analysis and graph theory, it is proved that the relative position tracking errors can converge to small invariant sets around the origin, and that all closed-loop signals are bounded, even in the presence of mass uncertainties and external disturbances. Finally, numerical simulations are given to evaluate the effectiveness and highlight the advantages of the developed control algorithm.

An analytical 3-D guidance law with impact angle and field-of-view (FOV) constraints considering nonlinear coupled dynamics is proposed. As a stepping stone, the guidance model is transformed to a set of nonlinear differential equations in terms of the relative range variable. To meet the desired impact angles in the pitch and yaw planes, two cubic polynomials including eight coefficients are developed with respect to the relative range for creating reference line-of-sight (LOS) profiles. The unknown coefficients are explicitly solved by initial and terminal conditions on the LOS angles and LOS rates in the mutually orthogonal planes. Then, the analytical 3-D impact angle guidance (IAG) law is derived via formulating the second-order LOS dynamics of the transformed model. Moreover, the relation between the seeker's look angle and the reference LOS profiles is developed, such that the achievable impact angle set can be obtained to handle the FOV limit. Numerical simulations with comparison study and a realistic model are conducted to verify effectiveness and robustness of the guidance law. Its feasibility is additionally validated by applying it to the guidance of unmanned aerial vehicles landing on surface moving carriers.

The mission of simultaneous attack and warhead lethality enhancement requires the constraints on impact time and angle in the guidance design. To this end, a two-stage guidance law considering constrained impact as well as the robustness against external disturbances and heading errors is proposed. The first stage is based on a circle involute geometrical rule that guarantees the desired velocity direction at the involute's end. With this goal achieved, the involute guidance is switched to the guidance of nullifying line-of-sight rate such that near-zero maneuver in the second stage is ensured. To achieve both impact time and angle constraints, a simple parameter determination approach is provided to find explicit solutions for the desired two-stage trajectory. Meanwhile, a robust two-stage guidance law is constructed to implement the geometrical rule, guarantee terminal constraints, and ensure robustness. The proposed technique is easy to implement, in that it does not involve model linearization, time-to-go estimation, and numerical optimization routine. Additionally, the achievable range of different constraints is analyzed considering practical issues such as initial launch angles, initial line-of-sight angles, and acceleration limits. Extensive simulations are carried out in various engagement scenarios under uncertainties and disturbances to validate the effectiveness and robustness of the proposed guidance law.

Hematology analysis is the most fundamental clinical test in differential diagnosis. The conventional way uses a laboratory hematology analyzer for complete blood count and requires highly trained professionals for morphology examination. The whole procedure is time- and labor-intensive, which severely affects the disease diagnosis efficiency. Meanwhile, the requirement for laboratory instruments and professionals impedes its popularization. Here, we proposed a deep-learning assisted, label-free hematology analysis technique through a regular microscope to solve these limitations. We have demonstrated that the subcellular morphology of unstained leukocytes, erythrocytes, and platelets can be revealed by simply defocusing a regular microscope. We have shown that this technique can automatically differentiate five-part leukocytes with high precision (mean average precision achieves 98.0%) and convert the label-free blood images into virtual Giemsa images with the assistance of deep-learning algorithms. The Pearson coefficients for the leukocyte counts from this technique and the manual counting method are large than 0.9083. The proposed technique can not only enhance the clinical hematology analysis efficiency but also has great potential to be quickly popularized since the regular microscope is a broadly used imaging tool.

In this paper, a novel adaptive neural network (NN) control scheme is proposed for a class of nonlinear systems with unknown control direction. By introducing some differentiable functions and high-order Lyapunov functions, the obstacle caused by unknown control direction in NN control is successfully circumvented and all closed-loop signals are shown to be uniformly bounded up to infinite time. Meanwhile, by introducing an error transformation technique, it is rigorously proved that the argument of the unknown nonlinearities remains within a compact set which can be explicitly calculated a priori , making the NN approximation always valid. Moreover, with the aid of a bound estimation approach, we effectively compress the impact of approximation errors and external disturbances and steer the tracking error into a predefined small residual set. Simulation results illustrate the effectiveness of the proposed scheme.

This paper proposes a novel dynamic near-optimal control allocation scheme with combination of a saturated baseline controller for spacecraft attitude control using single-gimbal control moment gyros (CMGs) and reaction wheels. First, a saturated controller is proposed to stabilize the nominal system in the presence of actuation mismatch. Aided by a state-dependent variable, a dynamic control allocator is then proposed that allows for smooth switching between two actuation sets. Unlike the previous static constraint optimization formulations, the control allocation augments its performance function with penalty terms in order to enforce individual input constraints and configuration singularity avoidance. Moreover, this dynamic control allocation is implemented with an online update law, which has a modest computational complexity compared to its numerical optimization counterpart. The closed-loop boundedness is guaranteed by a constructive Lyapunov-design method. Simulation results demonstrate that during the attitude maneuvers, the input saturation constraint and the active avoidance of CMG singularities are enforced with a relatively small computational cost.

This paper is devoted to attitude tracking control of fractionated spacecraft with wireless communication. We consider the practical case that the spacecraft suffers from uncertain inertia parameters, external disturbances, and even unknown and time-varying actuator faults. Within the framework of the backstepping method, a novel event-triggered adaptive fault-tolerant control scheme is proposed. In our design, an event-triggering mechanism is introduced to determine the time instants for communication, which successfully avoids continuous communication and Zeno phenomenon. Then, with the aid of a bound estimation approach and a smooth function, the impacts of the actuator faults, as well as the network-induced error, are effectively compensated for. Moreover, by employing the prescribed performance control technique, it is shown that the attitude tracking errors can converge to predefined arbitrarily small residual sets with prescribed convergence rate and maximum overshoot, no matter if there exist unknown actuator faults. Compared with conventional adaptive attitude control schemes, the proposed scheme significantly reduces the communication burden, while providing high reliability and stable, rapid, and accurate response for attitude maneuvers. Simulation results are presented to illustrate the effectiveness of the proposed scheme. This work was supported in part by the National Natural Science Foundation of China under Grant 61673036, Grant 61661136007, and Grant 61603021, in part by the Beijing Natural Science Foundation under Grant 4182036, and in part by the Program for Changjiang Scholars and Innovative Research Team under Grant IRT_16R03.

In this paper, a twisting control-based guidance law considering nonlinear/coupled dynamics and terminal angle constraints is proposed in a three-dimensional (3-D) space. The terminal angle refers to the impact angle for nonmaneuvering targets, and the approach angle for maneuvering targets, respectively. To meet the terminal angle constraints in the 3-D space, the coupled line-of-sight (LOS) error dynamics is controlled to track command LOS angles, which can be obtained from the relation between the desired terminal angle and the LOS angle. Then, a barrier function-based adaptive multivariable twisting controller is designed to obtain the guidance law for nullifying the LOS angle and rate tracking errors in the mutually orthogonal planes simultaneously. By virtue of the Lyapunov stability theory, the convergence of the LOS tracking errors to a predefined neighborhood of zero is proved in the presence of bounded disturbances with unknown bounds. In addition, several modifications are included in the proposed guidance law to enhance the guidance performance. Numerical simulations in different scenarios with a realistic missile model are conducted to validate the effectiveness and robustness of the guidance law.

In this note, a novel event-triggered adaptive control scheme is proposed for a class of nonlinear systems with unknown control direction and unknown sensor faults. The effects of the network-induced error and sensor faults are compressed by introducing some auxiliary filters and a bound estimation approach. Additionally, by introducing some differentiable auxiliary functions and high-order Lyapunov functions, we successfully circumvent the obstacle caused by unknown control direction and completely avoid Zeno phenomenon. The proposed scheme is able to ensure that all closed-loop signals are globally uniformly bounded and the tracking error converges to a residual set. Simulation results are presented to illustrate the effectiveness of the proposed scheme.

This work mainly discusses the attitude tracking issue of spacecraft which ensures fixed-time convergence property. To achieve this, the attitude tracking dynamics model is first established. Then a novel sliding manifold is constructed based on the presented sufficient condition of fixed-time stability. Moreover, a sliding mode controller is developed to ensure fixed-time convergence. Also, the closed-loop fixed-time stability of the whole system is proven to be guaranteed based on Lyapunov stability theory. Finally, the simulation results are provided to verify the superior performance of the designed controller.

This paper proposes a reinforcement learning (RL)-based six-degree-of-freedom (6-DOF) control scheme for the final phase proximity operations of spacecraft. The main novelty of the proposed method are from two aspects: 1) the closed-loop performance can be improved in real-time through the RL technique, achieving an online approximate optimal control subject to the full 6-DOF nonlinear dynamics of spacecraft; 2) Nontrivial motion constraints of proximity operations are considered and strictly obeyed during the whole control process. As a stepping stone, the dual-quaternion formalism is employed to characterize the 6-DOF dynamics model and motion constraints. Then, an RL-based control scheme is developed under the dual-quaternion algebraic framework to approximate the optimal control solution subject to a cost function and a Hamilton-Jacobi-Bellman equation. In addition, a specially designed barrier function is embedded in the reward function to avoid motion constraint violations. The Lyapunov-based stability analysis guarantees the ultimate boundedness of state errors and the weight of NN estimation errors. Besides, we also show that a PD-like controller under dual-quaternion formulation can be employed as the initial control policy to trigger the online learning process. The boundedness of it is proved by a special Lyapunov strictification method. Simulation results of prototypical spacecraft missions with proximity operations are provided to illustrate the effectiveness of the proposed method.

This paper provides an exponential stability result for the adaptive anti-unwinding attitude tracking control problem of a rigid body with uncertain but constant inertia parameters, without requiring the satisfaction of persistent excitation (PE) condition. Specifically, a composite immersion and invariance (I&I) adaptive controller is derived by integrating a prediction-error-driven learning law into the dynamically scaled I&I adaptive control framework, wherein we modify the scaling factor so that the algorithm design does not involve any dynamic gains. To avoid the unwinding problem, a barrier function is introduced as the attitude error function, along with the tactful establishment of two crucial algebra properties for exponential stability analysis. The regressor filtering method is adopted in combination with the dynamic regressor extension and mixing (DREM) procedure to acquire the prediction error using only easily obtainable signals. In particular, aiding by a constructive liner time-varying filter, the scalar regressor of DREM is extended to generate a new exciting counterpart. In this way, the derived controller is shown to permit closed-loop exponential stability without PE, in the sense that both output-tracking and parameter estimation errors exponentially converge to zero. Further, the composite learning law is augmented with a power term to achieve synchronized finite/fixed-time parameter convergence. Numerical simulations are performed to verify the theoretical findings., 15 pages

In this work, a robust incremental three-dimensional (3D) guidance law is proposed considering terminal angle constraint against maneuvering targets. As a stepping stone, the line-of-sight (LOS) tracking error dynamics is employed for the 3D guidance law design. A sliding variable is constructed such that its first-order derivative excludes the relative range in the perturbation, which avoids the unboundedness of system perturbation induced by target maneuvers near collision. A time-varying version of the sliding variable is designed to accelerate convergence of the LOS tracking errors and avoid large initial sliding variables. Then, two guidance laws are derived as a benchmark via the nonlinear dynamic inversion (NDI)-based sliding mode control (NDI-SMC) and NDI-based time-varying sliding mode control (NDI-TVSMC), respectively. To further improve guidance robustness with reduced system perturbation, the sensor-based incremental nonlinear dynamic inversion (INDI) control is used to design the INDI-SMC-based and INDI-TVSMC-based guidance laws. The sensor-based guidance laws exploit the LOS angular acceleration and guidance command output at the latest step, which result in smaller guidance gains to reject the perturbation than the NDI guidance laws. Numerical simulations in various cases and comparison studies are conducted to verify the effectiveness and robustness of the proposed method.

This work addresses the problem of observer-based closed-loop attitude stabilization in the presence of actuator faults, reaction wheel friction and external disturbances. As a stepping stone, an iterative learning disturbance observer (ILDO) is developed to estimate and compensate for the synthetic disturbances mentioned above. Specially, this ILDO does not need complex faults isolation operations or require knowledge related to these uncertainties. Furthermore, a typical proportional-derivative controller incorporating with the presented ILDO is employed to realize attitude stabilization for spacecraft subject to the actuator failures and uncertainties. The significant feature of the proposed strategy is that the uniformly ultimately bounded stability of the overall closed-loop system with observer-controller architecture could be guaranteed. The effectiveness and robustness of the developed scheme are investigated via a set of numerical simulations and analyses. Particularly, an experimental verification involving a hardware-in-loop platform is further provided to validate the engineering feasibility of the proposed strategy.

This paper addresses the difficult problem of fault-tolerant control and closed-loop control allocation for spacecraft attitude control system with actuator failures, actuator saturation, and external disturbances. As a fundamental step, a modified fault diagnosis observer is proposed utilizing the iterative learning methodology to reconstruct the actuator failures in real time. On the basis of the previously reconstructed failure information, a fault-tolerant control scheme incorporating with a parameter adjusting law is presented to enforce the spacecraft attitude control system to reach the real sliding mode surface in finite time. Meanwhile, the singularity of control command will be avoided using the saturation function, and the chattering problem restrained by the adjusting law. Furthermore, with the concept of control allocation, a novel on-line closed-loop constrained optimal fault-tolerant control allocation scheme is employed to distribute the signals synthesized by the baseline controller over the redundant actuators with failures and constraints. The stability of the closed-loop fault-tolerant control allocation process is guaranteed by the theory of the closed-loop discrete-time feedback control system. The key achievement of the proposed systematic strategy is that the whole closed-loop attitude control system can theoretically be guaranteed to be stable. Numerical simulations are carried out to verify the effectiveness and superiority of the developed approach.

This paper addresses the approaching trajectory control problem for the final phase of non-cooperative spacecraft proximity operations in the presence of disturbances and even input saturation. As a stepping stone, a novel line-of-sight tracking model is developed to describe attitude maneuver towards the target in the proximity. Then, a two-stage artificial potential function is employed to enable the pursuer spacecraft to comply the safety trajectory during approaching the target. In each stage, gradients of potential function are analyzed and the local minimum problem associated with potential function can be addressed. Furthermore, a saturated control law is proposed to ensure a reliable real-time tracking measurement of the target, and the controller rigorously enforces safe path and actuator magnitude constraints. The associated stability proof is accomplished through Lyapunov method. Simulations are carried out to evaluate the effectiveness of the proposed method.

This paper considers the fixed-time translational operation control issue for the spacecraft rendezvous task with a free-tumbling target. The translation dynamics model of the rendezvous mission under actuator misalignments is first established in the line-of-sight coordinate frame. Then, a novel time-varying sliding mode surface is constructed and a control scheme with the sliding manifold is proposed to achieve the fixed-time convergence of relative parameters’ tracking errors. It is interesting that the preceding convergence time can be altered explicitly according to the specific rendezvous requirements. Meanwhile, a modified updating strategy is developed to ensure system's fixed-time convergence property in the presence of lumped uncertainties (external disturbance and actuator misalignments). The associated stability proof is constructive and accomplished by the development of a Lyapunov function candidate. Finally, numerical simulations on rendezvous missions are performed to demonstrate the effectiveness and the superiorities of the designed fixed-time control scheme.

This paper is devoted to adaptive attitude tracking control for rigid spacecraft in the presence of parametric uncertainties, actuator faults and external disturbance. Specifically, a dynamic model is established based on one-tank spacecraft, which explicitly takes into account changing Center of Mass (CM). Then, a control scheme is proposed to achieve attitude tracking. Benefiting from explicitly considering the changing CM during the controller design process, the proposed scheme possesses good robustness to parametric uncertainties with less fuel consumption. Moreover, a fault-tolerant control algorithm is proposed to accommodate actuator faults with no need of knowing the actuators’ fault information. Lyapunov-based analysis is provided and the closed-loop system stability is rigorously proved. Finally, numerical simulations are presented to illustrate the effectiveness of the proposed controllers. Keywords: Adaptive control, Attitude tracking, Changing center of mass, Fault-tolerant, Spacecraft

Plug-and-play technology is an important direction for future development of spacecraft and how to design controllers with less communication burden and satisfactory performance is of great importance for plug-and-play spacecraft. Considering attitude tracking of such spacecraft with unknown inertial parameters and unknown disturbances, an event-triggered adaptive backstepping controller is designed in this paper. Particularly, a switching threshold strategy is employed to design the event-triggering mechanism. By introducing a new linear time-varying model, a smooth function, an integrable auxiliary signal and a bound estimation approach, the impacts of the network-induced error and the disturbances are effectively compensated for and Zeno phenomenon is successfully avoided. It is shown that all signals of the closed-loop system are globally uniformly bounded and both the attitude tracking error and the angular velocity tracking error converge to zero. Compared with conventional control schemes, the proposed scheme significantly reduces the communication burden while providing stable and accurate response for attitude maneuvers. Simulation results are presented to illustrate the effectiveness of the proposed scheme. Keywords: Adaptive control, Attitude tracking, Event-triggered control, Spacecraft, Uncertainties

This work investigates the challenging problem of fast robust fault tolerant attitude control for spacecraft to handle external disturbances, actuator failures and misalignments. More specially, a novel nonsingular terminal sliding mode based finite-time extended state observer is first designed to estimate and compensate for the lumped system faults or uncertainties. And the proposed extended state observer is analysed and proved to be stable in the sense of fast finite-time uniformly ultimately bounded stability. Then, utilizing the techniques of super-twisting and terminal sliding mode control synthetically, a novel continuous attitude control algorithm is developed. The finite-time stability of the closed-loop attitude control system is proved by using a continuously-differentiable, homogeneous and strict Lyapunov function. And also the proposed control scheme is continuous with the property of chattering restraining. Finally, some numerical simulation results are shown to verify the effectiveness and superior performances of the spacecraft attitude stabilization control system driven by the proposed fast robust fault tolerant attitude control scheme.

This work develops a novel disturbance observer and integral sliding mode technique based fault tolerant attitude control scheme for spacecraft, which is subject to external disturbance torques and actuator failures. More specifically, a simple and novel finite-time disturbance observer is first designed to reconstruct the synthetic uncertainty deriving from actuator failures and disturbances, by which the synthetic uncertainty is also compensated or restricted. Then, an integral sliding mode based finite-time fault tolerant attitude stabilisation controller incorporating with an adjusting law is investigated to ensure the closed-loop attitude control system converge to the stable region in finite time. And also the finite-time stability of the closed-loop attitude control system driven by the proposed attitude control scheme is analysed and proved utilising Lyapunov methodology. Finally, a simulation example for a rigid spacecraft model is carried out to verify the effectiveness and superiority of the proposed attitude control approach.

Label-free imaging technology is a trending way to simplify and improve conventional hematology analysis by bypassing lengthy and laborious staining procedures. However, the existing methods do not well balance system complexity, data acquisition efficiency, and data analysis accuracy, which severely impedes their clinical translation. Here, we propose defocusing phase-contrast imaging under the illumination of 415 nm light to realize label-free hematology analysis. We have verified that the subcellular morphology of blood components can be visualized without complex staining due to the factor that defocusing can convert the second-order derivative distribution of samples’ optical phase into intensity and the illumination of 415 nm light can significantly enhance the contrast. It is demonstrated that the defocusing phase-contrast images for the five leucocyte subtypes can be automatically discriminated by a trained deep-learning program with high accuracy (the mean F1 score: 0.986 and mean average precision: 0.980). Since this technique is based on a regular microscope, it simultaneously realizes low system complexity and high data acquisition efficiency with remarkable quantitative analysis ability. It supplies a label-free, reliable, easy-to-use, fast approach to simplifying and reforming the conventional way of hematology analysis.

This article considers the connectivity preservation coordinated control problem for multiple spacecraft systems subject to limited communication resources and sensing capability. By constructing a novel bump function, a distance-dependent dynamic communication network model is first presented, which characterizes the interaction strength as a nonlinear smooth function varying with the relative distance of spacecraft continuously. Subsequently, based on an edge-tension potential function, a distributed event-driven coordinated control scheme is proposed to achieve formation consensus, while ensuring that adjacent spacecraft is always within the allowable connectivity range. Meanwhile, to avoid redundant data transmissions, a hybrid dynamic event-triggered mechanism with maximum triggering interval is developed to schedule the communication frequency among spacecraft. It is proven that the onboard communication resources occupation can be reduced significantly and the Zeno phenomenon is strictly excluded. Finally, the efficiency of the proposed method for, as an example, four-spacecraft formation system is substantiated.

Large aberrations are induced by non-collimated light when the convergence or divergence of the incident beam on the back-pupil plane of the objective lens is adjusted for 3D non-inertial scanning. These aberrations significantly degrade the focus quality and decrease the peak intensity of the femtosecond laser focal spot. Here, we describe an aberration-corrected 3D non-inertial scanning method for femtosecond lasers based on a digital micromirror device (DMD) that is used for both beam scanning and aberration correction. An imaging setup is used to detect the focal spot in the 3D space, and an iterative optimization algorithm is used to optimize the focal spot. We demonstrate the application of our proposed approach in two-photon imaging. With correction for the 200-µm out-of-focal plane, the optical axial resolution improves from 7.67 to 3.25 µm, and the intensity of the fluorescence signal exhibits an almost fivefold improvement when a 40× objective lens is used. This aberration-corrected 3D non-inertial scanning method for femtosecond lasers offers a new approach for a variety of potential applications, including nonlinear optical imaging, microfabrication, and optical storage.