Search

In this paper, two kinds of sliding-mode guidance laws with impact time constraint are proposed for intercepting various target motions, including nonmaneuvering and maneuvering targets. To achieve...

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.

This paper aims at the trajectory optimization problem of an accompanying satellite in the space station under multi-obstacles by model predictive control (MPC) with successive linearization. This optimization problem is reformulated as a mixed integer second-order cone programming (MISOCP) problem by considering the obstacles avoidance, and various physical constraints. More specifically, a novel variation weight cost function is established with the distance information of the accompanying satellite and obstacles, such that a multi-objective optimization performance is achieved by incorporating fuel consumption and time expenditure further. Through the successive linearization of the MPC scheme, the satisfaction of the multiple constraints is established. These results lead to a feasible solution with harsh constraints as well as the fast convergence to the optimal solution. The obstacles avoidance constraints are first implemented through the appropriate selection of initial values, and then solved by successive approximation with the MPC framework for the convergence of solution. Numerical simulation has been conducted to demonstrate the effectiveness and applicability of the proposed method.

The attitude tracking control problem of a rigid spacecraft with actuator saturation is investigated in this paper. A finite-time attitude tracking control scheme is presented by incorporating sliding mode control (SMC) and adaptive technique. Specifically, a novel time-varying sliding mode manifold is first developed that aims at regulating the attitude tracking error to equilibrium point within a certain finite time. Moreover, it can be specified a priori by the designer according to the mission requirement. Subsequently, an adaptive controller is derived by using the SMC in conjunction with adaptive technique. The designed controller is capable of ensuring that the state trajectories reach to sliding regime within a finite time, and hence that attitude tracking error can converge to zero in a finite time with the aid of the developed sliding dynamics, despite the presence of exogenous disturbances, unknown inertia properties and saturation nonlinearities. Finally, the simulation experiments are carried out to demonstrate the effectiveness of the proposed control scheme.

In this paper, a new guidance strategy is proposed for intercepting stationary and constant-velocity moving targets with desired impact time and angle via a virtual target approach, subject to nonl...

This paper studies the rest-to-rest attitude reorientation problem of a three-axis stabilized satellite subject to inertia uncertainties, external disturbances, actuator faults and input saturation. An arc tangent function is first adopted to model the constrained control input, and meanwhile an augmented plant is constructed to facilitate the control law derivation. Then, a novel adaptive fault-tolerant control scheme is proposed by incorporating the prescribed performance control and adaptive estimation techniques into backstepping design. Exploiting the dynamic surface control method, the complexity problem residing in traditional backstepping approaches is effectively averted. It is shown that the control algorithm developed is not only robust against environmental disturbances and adaptive to unknown time-varying inertia properties caused by the mass displacement of large-scale deployable appendages, but also able to steer the attitude reorientation errors along prescribed transient and steady-state behavioral bounds, despite the presence of actuator faults and input saturation. Based on standard Lyapunov synthesis, all signals in the closed-loop system are proved to be semi-globally uniformly ultimately bounded. Finally, simulation experiments carried out on a miniature satellite testify the effectiveness of the proposed control approach.

This paper addresses the integrated attitude and position control problem for the final phase proximity operations of spacecraft autonomous rendezvous and docking, in which important motion constra...

In this paper, an observer-based fault-tolerant control scheme is proposed for the attitude stabilization of rigid spacecraft in the presence of actuator fault, configuration misalignment, input saturation and even external disturbances simultaneously. More specifically, an iterative learning observer is firstly developed to estimate the torque deviation and steer the estimation errors into some small residual sets. And also the detailed derivations of the observer are provided, along with a thorough analysis for the associated ultimate bounded stability and estimation error convergence property. Then, an integral-type sliding mode control law is designed to produce the three-axis virtual control signals with the desired performance for being distributed among the individual actuators. Under this, a robust control allocation algorithm is developed to map the virtual control demand onto individual actuator in an optimal manner, which takes into account the estimation uncertainties and ensures some fault-tolerant ability. The key feature of the proposed strategies is that the whole closed-loop fault tolerant control system can be guaranteed theoretically to be stable by the development of Lyapunov methodology. Numerical simulation results are presented to illustrate and highlight the fine performance benefits obtained using the proposed schemes.

During the aerospace missions for the on-orbiting spacecraft, actuator faults are frequently encountered such that the control system performance may be degraded, even the unstable ACS may result in the completely collapse, immeasurable losses or catastrophic consequences. Thus, the spacecraft ACS should be capable of dealing with or tolerating possible actuator faults, while at the same time guaranteeing an acceptable performance in the presence of actuator faults. There exist some main approaches to improve the stability and reliability of the spacecraft ACS, such as high reliable control system design with redundant devices, FDD based FTC system design, CA based FTC system design and so on. Recently, CA techniques have drawn significant interests in the aircraft and spacecraft control [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13], especially in the FTC system design. Gui et al. [14] introduced an adaptive SMC law to dynamically compensate the perturbations due to actuator faults and other uncertainties, in which the real time CA algorithm delivers the control command to actuators. In [15], a novel dynamic near-optimal CA scheme with combination of a saturated baseline controller for spacecraft attitude control using single-gimbal control moment gyros. This dynamic control allocation is implemented with online update law, which has a modest computational complexity. All these methods can primarily generate commands to control effectors to achieve some demand moment or acceleration. Some of them take in account the position and rate limits, while some also allow for satisfying a secondary objective. Generally speaking, all the aforementioned CA can be implemented practically to address the concerned constraints and objectives. However, the authors in [16] illustrated that different CA algorithms can lead to different closed-loop maneuver performances owing to the CA errors between the designed signal and actual input. Usually, this CA error is treated as small and neglectable in the control design, which, in fact, may lead to a degraded performance or even potential closed-loop instability. In all the aforementioned references, it is assumed that the system state trajectories are governed by the baseline controller, and CA algorithms have no/little effect on the closed-loop performance.

The science objectives of a spacecraft mission place stringent performance requirements on the spacecraft attitude control system. However, it remains an open problem how to guarantee consistent control performance necessary to meet these requirements, especially in the event of actuator faults and input saturation. Motivated by this fact, in this paper, we address the problem of attitude tracking control with prescribed performance guarantees for a rigid spacecraft subject to unknown but constant inertia parameters, unexpected disturbances, actuator faults, and input saturation. First, certain performance functions specified a priori by the designer are adopted to impose desired performance metrics on the attitude tracking errors. Then, the original attitude tracking error dynamics with performance constraints is transformed into an equivalent “state-constrained” one whose robust stabilization is shown to be sufficient to solve the stated problem via a novel error transformation. Subsequently, based on the transformed system, an adaptive fault-tolerant controller is derived by incorporating backstepping control, the barrier Lyapunov function, and Nussbaum gains. It is proved that the designed controller is able to guarantee the satisfaction of the prespecified constraints on the transformed errors, as well as the boundedness of all other closed-loop signals, without resorting to a judicious selection of the control parameters. Finally, the effectiveness of the proposed control scheme is evaluated by means of simulation experiments carried out on a microsatellite.

This paper addresses the control problem of air-breathing hypersonic vehicles subject to input nonlinearities, aerodynamic uncertainties and flexible modes. An adaptive backstepping controller and a dynamic inverse controller are developed for the altitude subsystem and the velocity subsystem, respectively, where the former eliminates the problem of “explosion of terms” inherent in backstepping control. Moreover, a modified smooth inverse of the dead-zone is proposed to compensate for the dead-zone effects and reduce the computational burden. Based on this smooth inverse, an input nonlinear pre-compensator is designed to handle input saturation and dead-zone nonlinearities, which leads to a simpler control design for the altitude subsystem subject to these two input nonlinearities. It is proved that the proposed controllers can guarantee that all closed-loop signals are bounded and the tracking errors converge to an arbitrarily small residual set. Simulation results are carried out to demonstrate the effectiveness of the proposed control scheme.

This article investigates the velocity-free attitude coordinated tracking control scheme for a group of spacecraft with the assumption that the angular velocities of the formation members are not available in control feedback. Initially, an angular velocity observer is constructed based on each individual's attitude quarternion. Then, the distributed attitude coordinated control law is designed by using the observed states, in which adaptive control method is adopted to handle the external disturbances. Stability of the overall closed-loop system is analyzed theoretically, which shows the system trajectory converges to a small set around origin with fast convergence rate. Numerical simulations are performed to demonstrate fast convergence and improved tracking performance of the proposed control strategy.

In this paper, a novel second-order sliding mode control (NSOSMC) scheme with finite time convergence is proposed for the autopilot design of skid-to-turn (STT) missiles considering coupling effects to track the desired maneuver commands. Firstly, the control system formulation is developed based on the mathematical model for nonlinear missile dynamics with system uncertainties and unknown multiple disturbances. Secondly, the NSOSMC scheme is developed by using a PID sliding mode surface with twisting control algorithm, which is to suppress the system uncertainties and unknown external disturbances, and to eliminate the chattering phenomenon. Then, the proposed NSOSMC law is proved to be finite time convergent to the origin. Finally, the effectiveness and robustness of the proposed NSOSMC scheme is verified through the numerical simulations with comparisons.

A smooth attitude-stabilizing controller is proposed for rigid spacecraft actuated by thrusters that achieves finite-time stability in the presence of external disturbance, input saturation, and in...

The main problem addressed is the quaternion-based attitude stabilization control of rigid spacecraft without angular velocity measurements in the presence of external disturbances and reaction wheel friction as well. As a stepping stone, an angular velocity observer is proposed for the attitude control of a rigid body in the absence of angular velocity measurements. The observer design ensures finite-time convergence of angular velocity state estimation errors irrespective of the control torque or the initial attitude state of the spacecraft. Then, a novel finite-time control law is employed as the controller in which the estimate of the angular velocity is used directly. It is then shown that the observer and the controlled system form a cascaded structure, which allows the application of the finite-time stability theory of cascaded systems to prove the finite-time stability of the closed-loop system. A rigorous analysis of the proposed formulation is provided and numerical simulation studies are presented to help illustrate the effectiveness of the angular-velocity observer for rigid spacecraft attitude control.

This paper investigates the flight longitudinal control problem of a hypersonic vehicle subject to input constraints and aerodynamic uncertainties. A modified control-oriented model is first derived with explicit consideration of the actuator dynamics property. Then, based on the novel dynamics model developed, adaptive backstepping controller and dynamic inverse controller are designed for the altitude subsystem and the velocity subsystem, respectively. To avoid the complexity problem residing in traditional backstepping control caused by the repeated derivations of the virtual control variables, the dynamic surface control technique is involved in conjunction with the backstepping control approach, and a novel second-order sliding-mode-based integral filter instead of the conventional first-order filter is employed. Additionally, in order to deal with actuator magnitude constraints, two auxiliary systems are constructed to generate certain compensating signals to guarantee tracking errors suitable for adaptive parameter estimation. It is proved that the proposed control scheme can guarantee that the tracking errors converge to an arbitrarily small neighborhood around zero in the presence of actuator constraints and aerodynamic uncertainties as well. Finally, numerical simulations are presented to demonstrate the effectiveness of the proposed control scheme.

A novel smooth attitude algorithm is presented for rigid spacecraft in the presence of assigned sensor and actuator saturations, as well as external disturbances. More specifically, an analog of variable structure control law is first developed by explicitly taking into account the velocity and control constraints simultaneously. In the proposed controller, behavior of the adaptive parameter is investigated to ensure the close-loop convergence and disturbance rejection. The straightforward relationship between the constraints given and parameters tuning is further analyzed. In addition, to avoid the unwinding phenomenon problem caused by quaternion attitude representation, a class of definition of attitude error vector is proposed to ensure that both two equilibria are locally asymptotically stable. Numerical simulations illustrate the performance of proposed attitude controller and the improvement of the anti-unwinding technic.

This paper addresses the translational control problem for the final phase of spacecraft rendezvous and docking. For safety concerns, during the approach process, the pursuer spacecraft is required...

In this paper, attitude coordinated tracking control algorithms for multiple spacecraft formation are investigated with consideration of parametric uncertainties, external disturbances, communication delays and actuator saturation. Initially, a sliding mode delay-dependent attitude coordinated controller is proposed under bounded external disturbances. However, neither inertia uncertainty nor actuator constraint has been taken into account. Then, a robust saturated delay-dependent attitude coordinated control law is further derived, where uncertainties and external disturbances are handled by Chebyshev neural networks (CNN). In addition, command filter technique is introduced to facilitate the backstepping design procedure, through which actuator saturation problem is solved. Thus the spacecraft in the formation are able to track the reference attitude trajectory even in the presence of time-varying communication delays. Rigorous analysis is presented by using Lyapunov-Krasovskii approach to demonstrate the stability of the closed-loop system under both control algorithms. Finally, the numerical examples are carried out to illustrate the efficiency of the theoretical results. Published version

This paper investigates a velocity-free finite-time attitude control scheme for a rigid spacecraft with actuator saturation and external disturbances. Initially, a finite-time observer is designed to compensate the unknown angular velocity information. With the estimated values, a finite-time controller is further developed under which the time-varying reference attitude trajectory can be tracked precisely. Moreover, input saturation problem is solved by adaptive method. Rigorous Lyapunov-based analysis shows that the states of the closed-loop system converge to a small neighborhood of the origin in finite time. Finally, the dynamic performance of attitude control system is presented by numerical simulation examples and the superiority of the finite-time control scheme is verified.

Summary In this paper, an output-feedback adaptive consensus tracking control scheme is proposed for a class of high-order nonlinear multi-agent systems. The agents are allowed to have unknown parameters, unknown nonlinearities, and input quantization simultaneously. The desired trajectory to be tracked is available for only a subset of agents, and only the relative outputs and the quantized inputs need to be measured or transmitted as signal exchange among neighbors regardless of the system order. By introducing a kind of high-gain K-filters and a smooth function, the effect among agents caused by the unknown nonlinearities is successfully counteracted, and all closed-loop signals are proved to be globally uniformly bounded. Moreover, it is shown that the tracking errors converge to a residual set that can be made arbitrarily small. Simulation results on robot manipulators are presented to illustrate the effectiveness of the proposed scheme. Copyright © 2017 John Wiley & Sons, Ltd.

Background Chemotherapy of nasopharyngeal carcinoma (NPC) can lead to significant side effects and complications. Exercises during chemoradiotherapy have potential to reduce complications and fatigue and improve quality of life. The aim of the randomised clinical study was to investigate the benefits of resistance exercise during chemoradiotherapy in NPC patients. Methods A total of 146 patients were randomised to perform resistance or relaxation exercises during chemoradiotherapy. Resistance exercise consisted of eight machine-based progressive resistance exercises, and relaxation control consisted of progressive muscle relaxation. Side effects and complications were analysed, and fatigue was assessed by Multidimensional Fatigue Symptom Inventory-Short Form (MFSI-SF) scores. The European Organization for Research and Treatment of Cancer Quality of Life Questionnaire Core-30 (EORTC QLQ30) scale was used to evaluate the effects of resistance exercise or relaxation control on quality of life. Per-protocol analysis was performed on the collected data. Results Resistance exercise has stronger effects than relaxation in reducing complications, including oral mucositis, mouth-opening difficulties, xerostomia, hearing loss and nasal congestion, and alleviating both physical fatigue and mental fatigue. The improvement in quality of life was also more prominent among patients performing resistance exercise. Conclusions For NPC patients undergoing chemoradiotherapy, resistance exercise has a better efficacy in reducing complications, alleviating fatigue and improving quality of life.

Based on the dual-quaternion description, a smooth six-degree-of-freedom observer is proposed to estimate the incorporating linear (translational) and angular velocity, called the dual-angular velocity, for rigid bodies. To establish the observer, some important properties of dual vectors and dual quaternions are presented and proved, additionally, the kinematics of dual-transformation matrices is deduced, and the transition relationship between dual quaternions and dual transformation matrices is subsequently analyzed. An important feature of the observer is that all estimation states are ensured to be $\mathcal {C}^{\infty }$ continuous, and estimation errors are shown to exhibit asymptotic convergence if the signals to be estimated are bounded. Furthermore, to achieve tracking control objectives, the proposed observer is combined with an independently designed proportional-derivative-like feedback control law (using full-state feedback), and a special Lyapunov “strictification” process is employed to ensure a separation property between the observer and the controller, which further guarantees almost global asymptotic stability of the closed-loop tracking error dynamics. Numerical simulation results for a prototypical spacecraft pose tracking mission application are presented to illustrate the effectiveness and robustness of the proposed method.

This paper proposes a model predictive controller for power-limited spacecraft attitude maneuver considering the actuators’ power limitation and the torque and angular momentum saturation. In the space exploration mission, the power provided by the solar panels is limited, and the performance of the reaction wheels is related to the available power. Therefore, the available power for reaction wheels is taken into account in controller design. Besides, the control torque and angular momentum constraints of reaction wheels are also described as inequalities herein. Then, the constrained discrete-time spacecraft attitude dynamics actuated by redundant reaction wheels is derived and the model predictive control is implemented to calculate the control torque while minimizing the special performance index. The validity and effectiveness of the proposed scheme are illustrated through numerical simulations.

This paper addresses the attitude synchronization problem for multiple flexible spacecraft subject to directed communication topology. Taking the time delays of communication network into account, a distributed attitude coordination control scheme is proposed only through local information exchange. It is shown that the control law can not only guarantee that all spacecraft within a formation can track a common reference attitude synchronously, but also ensure the undesirable vibration induced by flexible attachments are perfectly suppressed even when only a subset of formation members have access to the reference attitude. The result is rigorously proved by resorting to the Lyapunov stability analysis and algebraic graph theory. Finally, simulation examples for attitude synchronization among three flexible spacecraft are provided to demonstrate the effectiveness of the proposed control algorithm.

This paper investigates the translational control problem for autonomous spacecraft rendezvous with a freely tumbling target under approaching path constraints. Specifically and firstly, a dynamic model resolved in the target’s body-fixed frame is established for relative translational motion of the pursuer with respect to the target, which allows to facilitate the closed-loop control design and stability analysis significantly. To ensure that the approaching path constraint is satisfied, the constrained zone is parameterized in terms of spatial geometrical feature, and then an artificial potential function (APF) is designed accordingly. Moreovre, by mathematical analysis, it is shown that the APF is free of local minima problem inherent in traditional APF-based control. Subsequently, an adaptive controller is synthesized within the backstepping control framework using the designed APF. The designed controller enables the pursuer to reach the desired position, while complying with the approaching path constraint. Following Lyapunov’s direct method, the closed-loop stability is analyzed. Finally, simulation results are given to demonstrate key features and illustrate the effectiveness of the proposed control scheme.

An analytical guidance law considering impact time constraint and reduced filed-of-view (FOV) limit is proposed. To achieve the desired constraints, the relative range profile is designed as a quadratic polynomial in terms of an auxiliary variable defined by the seeker look angle. Specifically, the reduced FOV is ensured by guaranteeing the positive first-order derivative of the developed range profile such that the seeker’s look angle decreases monotonically from its initial value to zero at the collision point. The impact time and zero miss distance are met by integrating the range profiles with boundary constraints, and thus the analytical guidance law is derived by transforming the guidance model in terms of the auxiliary variable. Finally, numerical simulations are conducted to validate effectiveness of the guidance law.

Satellite mission planning is an important premise for earth observation. Traditional satellite mission planning is mainly aimed at fixed ground targets, which cannot meet the increasingly complex mission requirements. This paper considers the moving target observation, and puts forward a method of satellite mission planning via data driven approach. This method forecasts the future track and position information of the moving target through a modified Long Short-Term Memory (M-LSTM) networks algorithm, and proposes a Constraint Satisfaction Genetic Algorithm (CSGA) to plan the missions of moving target observation based on the results of M-LSTM algorithm. In view of the complexity of the constraint and task conflict in the moving target observation, CSGA embeds the constraints into the genetic algorithm through conditional forms, and a conflict resolution operator is designed in CSGA to resolve task conflicts. Simulation results demonstrate the efficiency of the LSTM-CSGA method to solve the mission planning and get a higher observation accuracy.

In this paper, a homing guidance law considering impact time and impact angle constraints under limited field-of-view (FOV) is proposed. To deal with these constraints, the line-of-sight (LOS) shaping approach is employed to create the reference LOS profiles that are quartic polynomials in time. The boundary conditions on the LOS angle and LOS rate lead to a single parameter in the desired LOS profiles. By investigating the relation between the look angle dynamics and the LOS profiles, a partial numerical routine is provided to determine a set of feasible LOS profiles such that the seeker’s FOV limit cannot be violated. Moreover, the non-singular terminal sliding mode technique is applied to achieve the precise tracking of the desired LOS profiles. Finally, numerical simulations are conducted to validate effectiveness and feasibility of the guidance law.

This paper addresses the problem of finite-time output feedback decentralized control of double tethered satellite system (2-TSS) attitude tracking in the presence of external disturbances in deep space. More specifically, the finite-time attitude tracking control law with the reconstructed information achieving from the pseudo-angular velocity filter is proposed in this paper. The significant feature of proposed strategy is that the whole closed-loop control system can be guaranteed theoretically to be finite-time stable by the development of adding a power integrator method, which indicates that the tracking errors for 2-TSS are confronted into a small residual region around origin within finite time. Finally, numerical simulation analysis is presented to validate the effectiveness and feasibility of proposed scheme.

A look angle shaping guidance law with impact angle and seeker's field-of-view (FOV) constraints considering the time-varying speed is proposed. A look angle profile with respect to the relative range is first constructed as a cubic polynomial. The desired impact angle and the maximum look angle are expressed as functions of a parameter in the look angle profile. The impact angle constraint under limited FOV is achieved by solving this parameter with boundary conditions. Moreover, the feasible region of the desired impact angle is derived to ensure seeker's lock-on condition. By combining the first order look angle dynamics and the reference look angle profile, the impact angle control guidance (IACG) law with FOV constraint is obtained. The proposed guidance law is derived without relying on the linearized engagement kinematics, switching logic, constant speed assumption, and the LOS rate information during the guidance process. Moreover, the proposed guidance technique is helpful for user/designers to select feasible impact angle constraints in advance without tuning parameters. Finally, numerical simulations under a realistic model are performed to validate the effectiveness of the proposed guidance law.

This paper addresses the attitude stabilization problem with the optimization objective regarding system performance and energy consumption for rigid spacecraft. Novel state-dependent coefficient (SDC) parameterizations are developed to reduce the optimization problem to solving the state-dependent Riccati equation (SDRE). To be explicit, a proper SDC parameterization is selected for the stabilization mission among infinite feasible SDC representations by utilizing an improved guideline. Subsequently, a novel analytical SDC reconstruction is evoked alternatively at the mission breakdown region in which the derived SDC parameterization fails to operate. Behind the scenes, the anti-unwinding behavior is supported by the explicit sampling SDC forms derived for the scalar part of the unit quaternion. The novelty of two developed SDC parameterizations lies in that they not only render the maximal pointwise controllable space to facilitate the control mission, but also cover the whole maneuvering domain without any breakdowns. Moreover, the computational burdens for online SDRE solvability checking and numerical SDC reconstruction are alleviated significantly. Finally, comparative numerical simulations are implemented to highlight the superior performance of the presented scheme.

This paper investigates the problem of attitude tracking control for a rigid spacecraft subject to parametric uncertainties, external disturbances, actuator faults and actuator saturation constraints. By combining the finite-time passivity technique into adaptive sliding mode control approach, a novel smooth fault-tolerant control algorithm with finite time convergence is proposed. Then, the finite-time convergence of the relative attitude errors will be achieved by implementing the proposed smooth fault-tolerant controller, even under actuator faults and magnitude constraints. In particular, a solution to the unwinding phenomenon, arising from the usage of the redundant four-parameter attitude representation, is explored in the sense of finite-time passivity. Besides detailed controller design procedures and rigorous theoretical proofs of all related closed-loop finite-time stability, numerical simulation results are exhibited to demonstrate the effectiveness and superior control performance of the proposed control scheme.

Summary This paper deals with the task-space trajectory tracking control problem of robot manipulators. An improved adaptive backstepping controller is proposed to deal with the uncertainties in kinematics, dynamics, and actuator modeling. To avoid the explosion of computation in conventional backstepping techniques, a modified dynamic surface control algorithm is proposed, which guarantees the asymptotic convergence rather than the uniformly ultimately boundedness of tracking errors in conventional dynamic surface control methods. Furthermore, the expression of the ℒ2 norm of tracking errors is explicitly derived in relation to the controller parameters, which provides instructions on tuning controller parameters to adjust the system performance. Moreover, the passivity structure of the designed adaptation law is thoroughly analyzed. Simulation of a free-floating space robot is used to verify the effectiveness of the proposed control strategy in comparison with the conventional tracking control schemes. Copyright © 2016 John Wiley & Sons, Ltd.

The single-axis maneuver planning problem is investigated for underactuated spacecraft with one unactuated axis fixed in a certain direction of the spacecraft in the presence of a single conical forbidden pointing zone. Specifically, a nonnominal Euler rotation scenario is formulated by imposing admissible constraints on rotation axes, and it is proven that the singular sensitive axis can be pointed along an arbitrary inertial direction outside the forbidden zone by at most two principal rotations. Given this fact, an exact solution is obtained by numerically solving a quartic trigonometric function equation about the first rotation angle, while enforcing the rotation axes tangent to the forbidden cone for the shorter path. Furthermore, an approximate but computationally efficient solution is analytically developed by Ferrari's formulas for the linearized quartic trigonometric function equation around the first tangent direction, where the smallest real one is selected from the four candidate roots. The solvability of the two-step planning schemes is also assured by the corresponding accessible regions. Finally, comprehensive numerical simulations are conducted to corroborate the effectiveness of both the exact and approximate planning schemes.

Stereo matching is important to many robot applications. Recent work has shown that we can use convolutional neural networks (CNNs) to compute disparity map from a stereo image pair directly. However, current methods don’t take the problem of geometric deformation caused by perspective projection into account. To address the problem, we design a new model that utilizes Gabor convolutional neural networks (Gabor CNNs) in the feature extraction part of the network architecture. In Gabor CNNs, the learned filters from ordinary CNNs are modulated by the Gabor filters, which can enhance their abilities to address the problem of geometric deformations. Finally, we test our model on KITTI and Scene Flow datasets. The results show that our model outperforms the ordinary model that uses ordinary CNNs by a large margin.

The two-photon microscope is a powerful tool in life science. Conventional two-photon microscopy can image only a plane of a particular axial position at a time. Axial scanning can get the volumetric information, but it gets signals from different axial positions serially, which means that the exposure time at every plane is limited. Here we demonstrate a novel method, to the best of our knowledge, that can simultaneously record images from two planes at different xyz positions. The demultiplexing of the signal is realized using a confocal strategy. The experimental results show that it can be used for simultaneous two-photon imaging at two focal planes with little cross talk.

This paper is concerned with a rest-to-rest attitude maneuver of spacecraft subject to angular velocity constraint and external disturbances. An auxiliary sliding vector is first introduced and its state trajectory is restrained a priori by relative prescribed performance functions. Then, an adaptive controller incorporating error transformed technique is proposed, which enables the spacecraft to orientate any specified trajectory with high precision. It is shown that the developed control scheme is not only robust against unexpected environmental disturbances, but also able to ensure the constraint on angular velocity is perfectly fulfilled. Based on the standard Lyapunov theory, all signals in the closed-loop system are proved to be uniformly ultimately bounded. Finally, numerical simulations performed on a microsatellite are presented to illustrate the effectiveness of the proposed control algorithm.

This paper investigates the problem of attitude tracking control for a nonrigid spacecraft subject to time-varying inertia matrix and external disturbances. Based on the deployable spacecraft with both fuel depletion and mass displacement, the time-varying inertia matrix is characterized with both rigid components and time-dependent components. Not only the direct influences of the fuel depletion and the mass displacement, but also the resulting change of center of mass (CM) are taken into account during the characterization. And the time-varying inertia matrix is applied to the mathematic model of the nonrigid spacecraft’s dynamics. Then, an adaptive attitude tracking control algorithm is proposed for the nonrigid spacecraft. The proposed controller directly compensates for inertia variations and achieves high control accuracy and good system robustness. Furthermore, a theoretical analysis of the system stability is also presented. Finally, numerical simulations are carried out to illustrate the effectiveness and superior control performance of the proposed control scheme.

A saturated controller is developed for the final phase of rendezvous and proximity operations in the presence of disturbances, parametric uncertainties and loss of control effectiveness. To achieve this, an integrated auxiliary filter structure is first established using hyperbolic functions that work in tandem with filtered tracking error. Then a robust saturated controller for nominal system is proposed. Furthermore, saturated fault-tolerant controller based on the nominal control law and sliding-mode-like method is derived. The bound on the proposed controller can be adjusted by changing the feedback gains. A Lyapunov-based stability analysis is provided to prove uniformly ultimate bounded (UUB) tracking despite uncertainties, disturbances and even actuator faults in the six-degree-freedom dynamics. A numerical simulation is presented to demonstrate the performance of the controller.

A novel pointing control law is presented to the problem of the spacecraft pointing with the presence of the constraints in space configuration. With the pointing constraint expressed by the reduced attitude, a potential function with global minimum located at the desired attitude is presented. Then, a PD-like control law is given with the synthesis of the proposed potential function. To prove the asymptotic stability of the closed-loop system, a Lyapunov-based analysis is performed. Simulation results are conducted to illustrate the effectiveness of the control scheme.

This paper investigates the problem of attitude tracking control with prescribed tracking performance for a spacecraft exposed to unknown inertial matrix and actuator failure. Based on the prescribed tracking performance technique, an adaptive fault-tolerant controller is adopted by taking backstepping control and neural-network into consideration. By estimating the bounds of time-varying parameters induced by fault uncertainties, the influence of actuator faults is successfully compensated for. By estimating the norm of the optimal weight vector of neural networks, there is only one parameter needs to be estimated, which significantly reduces the computational burden. It is proved that the proposed controller can guarantee that all closed-loop signals are bounded and the tracking errors are converged into an arbitrarily small residual set with prescribed performance. Simulation results are presented to demonstrate the effectiveness of the proposed control scheme.

This paper proposes a Laguerre Model Predictive Control (LMPC) method for trajectory generation of the accompanying satellite in space station to execute detection tasks. The design of ensuring the accompanying satellite reach the target position and avoid obstacles simultaneously are difficult due to the critical non-convex and nonlinear constraints, and solving the large number of decision variables as a result of model predictive control is too computationally intensive to be implemented. To counter this problem, the trajectory generation problem is reformulated as a mixed integer quadratic programming (MIQP) problem according to the quadratic objective function and obstacles constraints, and the LMPC with smaller variables method is employed in this paper. The obstacles avoidance constraints are solved by mixed integer quadratic programming with LMPC framework, with particularly advantages of the method could obtain a feasible and satisfied solution when the constraints are rigorous. Besides, this paper introduces a LMPC approach to reduce the computational burden by a set of Laguerre functions, guaranteed the long control horizon condition is solved by the small number of control variables. Application for trajectory generation of satellite in highly constrained space station is conducted to demonstrate the effectiveness of the newly proposed method.

An anti-unwinding attitude controller is presented under angular velocity constraint. As a stepping stone, a novel attitude error function is introduced to eliminate the detrimental unwinding phenomenon of the unit quaternion. Then, the angular velocity constraint is considered, and a new form of barrier Lyapunov function is used to achieve this constraint. Furthermore, a controller aiming at solving the preceding two issues simultaneously is presented, and system's asymptotical stability is guaranteed due to Lyapunov analysis. Numerical simulations are employed to demonstrate the effectiveness and merit of the provided controller.