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During the on-orbit capture task of the free-floating space robot, its operating space degenerates. The dynamic coupling and dynamic singularity also affect the task operation. In this paper, a general collision-free trajectory planning method to minimize base disturbance is proposed for a free-floating dual-arm space robot. This method can realize smooth joint trajectory planning while avoiding singularity and realizing self-collision avoidance. The singularity can be effectively avoided by transforming the trajectory planning problem into a nonlinear optimization problem. The optimization function is composed of base disturbance penalty function, end effector error function and collision repulsion potential field function, which can be used to solve the optimal joint trajectory parameters. Futhermore, self-collision avoidance is achieved by combining the bacterial foraging optimization algorithm(BFOA) with the geometric vector collision-avoidance method based on artificial potential field to simplify the calculation complexity. Finally, simulations verify the effectiveness of the proposed planning framework. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Fuzhou University is the property of Journal of Fuzhou University, Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Motion planning is one of the fundamental technologies for robots to achieve autonomy. Free-floating space robots composed manipulators and base satellite that do not actively control its position and attitude has nonholonomic characteristics, and there is a first-order differential relationship between its joint angle and the base attitude. In addition, the planning framework which first converts the goal end-effector pose to its corresponding target configuration, and then plan the trajectory from the initial configuration to the goal configuration still has the following problems: the goal configuration and the initial configuration may not be in the same connected domain. Based on the RRT framework, the motion planning of a free-floating space robot from the initial configuration to the goal end-effector pose is studied. In the algorithm design, in order to deal with the differential constraints of the free-floating space robot, and the requirement that the attitude disturbance of its base cannot exceed its limit, a control-based local planner for random configuration guiding growth of the tree and a control-based local planner for goal end-effector pose guiding growth of the tree that can adjust the attitude of the base when necessary are proposed. The former can ensure the effective exploration of the configuration space, and the latter can avoid the occurrence of singularity while ensuring that the algorithm converges quickly and the base attitude disturbance meets the constraints. The present algorithm does not need to solve the inverse kinematics, can successfully complete the planning task, and ensure that the base attitude disturbance meets the requirements. The simulation verifies the effectiveness of the algorithm.

The use of free-floating space robots for contact tasks is very promising in space exploration. However, severe damage or obvious disturbance may occur if inappropriate operation is implemented. In this paper, a novel risk assessment method is first proposed to give a clear description of the risk state before contact happens and provide guidance for configuration optimization to reduce risk on contact tasks. Firstly, the dynamics model of a free-floating space robot is given. On this basis, two important risk assessment indicators, the maximum contact force and the base attitude disturbance caused by contact, are derived. By integrating the risk assessment indicators, a novel risk assessment model is proposed for a free-floating space robot performing a contact task. It is a multidimensional and extensible risk assessment space which could give a clear description of the risk state before contact happens. Thereafter, considering the results given by the risk assessment model, the configuration optimization for a free-floating space robot is implemented based on the design of optimization factor in null space. Finally, numerical simulation for a 7-degree-of-freedom free-floating space robot performing a contact task is carried out, and the simulation results verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

We address the motion planning problem for a robotic system whose configuration manifold contains a group of rotations. Our approach is applied to a free-floating space robot composed of a three-dimensional base (a spacecraft) and an anthropomorphic onboard manipulator. The robot is actuated by the torques exerted at the joints of the onboard manipulator. A coordinate-free representation of rotations is utilized. The Lagrangian formalism is employed in order to derive a dynamics model of the robot that takes the form of a control system defined on the group of rotations and the joint space of the onboard manipulator. Using the conservation of angular momentum of the robot, a Jacobian motion planning algorithm is designed relying on the Endogenous Configuration Space Approach. The performance of the algorithm is verified by computer simulations. [ABSTRACT FROM AUTHOR]

Aiming at the force position control problem of the on-orbit insertion and extraction operation of the free-floating space robot, the system dynamics model is established. According to the interaction between the end of manipulator and the environment, the second-order impedance model is established. In order to improves the calculation efficiency, the above models are reconstructed to avoid the use of acceleration signal by introducing filtering operation. This is also conducive to the application of robot actual control. Then, an estimator requiring only the system inertia matrix is designed to compensate the modeling uncertainty, external bounded disturbance and impact effect in the process of inserting and extracting. Its structure is simple and reliable. Only one control parameter needs to be adjusted, which greatly reduces the amount of calculation. Considering that the on-orbit operation of insertion and extraction is a kind of precision operation, its control system needs to have a high-quality control performance. By introducing the prescribed performance method, the tracking error is constrained within the given range and to ensure the transient performance and steady-state performance of the control system is ensured. Finally, three simulation conditions are designed, and the results are presented to verify that the proposed algorithm has a faster convergence speed compared with traditional sliding mode controller. It can achieve vertically inserting and accurate force tracking of the manipulator end. [ABSTRACT FROM AUTHOR]

When free-floating space robots perform space tasks, the satellite base attitude is disturbed by the dynamic coupling. The disturbance of the base orientation may affect the communication between the space robot and the control center on earth. In this paper, the enhanced bidirectional approach is proposed to plan the manipulator trajectory and eliminate the final base attitude variation. A novel acceleration level state equation for the nonholonomic problem is proposed, and a new intermediate variable-based Lyapunov function is derived and solved for smooth joint trajectory and restorable base trajectories. In the method, the state equation is first proposed for dual-arm robots with and without end constraints, and the system stability is analyzed to obtain the system input. The input modification further increases the system stability and simplifies the calculation complexity. Simulations are carried out in the end, and the proposed method is validated in minimizing final base attitude change and trajectory smoothness. Moreover, the minute internal force during the coordinated operation and the considerable computing efficiency increases the feasibility of the method during space tasks. [ABSTRACT FROM AUTHOR]

The use of free-floating space robots for contact tasks is very promising in space exploration. However, severe damage or obvious disturbance may occur if inappropriate operation is implemented. In this paper, a novel risk assessment method is first proposed to give a clear description of the risk state before contact happens and provide guidance for configuration optimization to reduce risk on contact tasks. Firstly, the dynamics model of a free-floating space robot is given. On this basis, two important risk assessment indicators, the maximum contact force and the base attitude disturbance caused by contact, are derived. By integrating the risk assessment indicators, a novel risk assessment model is proposed for a free-floating space robot performing a contact task. It is a multidimensional and extensible risk assessment space which could give a clear description of the risk state before contact happens. Thereafter, considering the results given by the risk assessment model, the configuration optimization for a free-floating space robot is implemented based on the design of optimization factor in null space. Finally, numerical simulation for a 7-degree-of-freedom free-floating space robot performing a contact task is carried out, and the simulation results verify the effectiveness of the proposed method.

Aiming at the force position control problem of the on-orbit insertion and extraction operation of the free-floating space robot, the system dynamics model is established. According to the interaction between the end of manipulator and the environment, the second-order impedance model is established. In order to improves the calculation efficiency, the above models are reconstructed to avoid the use of acceleration signal by introducing filtering operation. This is also conducive to the application of robot actual control. Then, an estimator requiring only the system inertia matrix is designed to compensate the modeling uncertainty, external bounded disturbance and impact effect in the process of inserting and extracting. Its structure is simple and reliable. Only one control parameter needs to be adjusted, which greatly reduces the amount of calculation. Considering that the on-orbit operation of insertion and extraction is a kind of precision operation, its control system needs to have a high-quality control performance. By introducing the prescribed performance method, the tracking error is constrained within the given range and to ensure the transient performance and steady-state performance of the control system is ensured. Finally, three simulation conditions are designed, and the results are presented to verify that the proposed algorithm has a faster convergence speed compared with traditional sliding mode controller. It can achieve vertically inserting and accurate force tracking of the manipulator end.

The maximum contact force is one of the most important indicators for contact problems. In this paper, the configuration optimization is conducted to reduce the maximum contact force for a free-floating space robot capturing tumbling target. First, the dynamics model of a free-floating space robot is given, with which the inertial properties perceived at the end-effector can be derived. Combing the inertial properties of the contact bodies, a novel concept of integrated effective mass is proposed. It tries to transform the complex contact process into the energy change of a virtual single body with integrated effective mass. On this basis, a more general continuous contact model is established, which is also suitable for non-central collisions between space robot and the tumbling target. Thereafter, the maximum contact force is derived as an important indicator for the null-space optimization method to reduce the maximum contact force. Finally, numerical simulations with a 3-degree-of-freedom free-floating space robot and a 7-degree-of-freedom free-floating space robot, as the research objects, are carried out respectively and the results show the effectiveness of the method proposed.

Motion planning is one of the most important technologies for free-floating space robots (FFSRs) to increase operation safety and autonomy in orbit. As a nonholonomic system, a first-order differential relationship exists between the joint angle and the base attitude of the space robot, which makes it pretty challenging to implement the relevant motion planning. Meanwhile, the existing planning framework must solve inverse kinematics for goal configuration and has the limitation that the goal configuration and the initial configuration may not be in the same connected domain. Thus, faced with these questions, this paper investigates a novel motion planning algorithm based on rapidly-exploring random trees (RRTs) for an FFSR from an initial configuration to a goal end-effector (EE) pose. In a motion planning algorithm designed to deal with differential constraints and restrict base attitude disturbance, two control-based local planners are proposed, respectively, for random configuration guiding growth and goal EE pose-guiding growth of the tree. The former can ensure the effective exploration of the configuration space, and the latter can reduce the possibility of occurrence of singularity while ensuring the fast convergence of the algorithm and no violation of the attitude constraints. Compared with the existing works, it does not require the inverse kinematics to be solved while the planning task is completed and the attitude constraint is preserved. The simulation results verify the effectiveness of the algorithm. [ABSTRACT FROM AUTHOR]

Copyright of Chinese Journal of Computational Mechanics / Jisuan Lixue Xuebao is the property of Chinese Journal of Computational Mechanics Editorial Office, Dalian University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

SUMMARY: During visual servoing space activities, the attitude of free-floating space robot may be disturbed due to dynamics coupling between the satellite base and the manipulator. And the disturbance may cause communication interruption between space robot and control center on earth. However, it often happens that the redundancy of manipulator is not enough to fully eliminate this disturbance. In this paper, a method named off-line optimizing visual servoing algorithm is innovatively proposed to minimize the base disturbance during the visual servoing process where the degrees-of-freedom of the manipulator is not enough for a zero-reaction control. Based on the characteristic of visual servoing process and the robot system modeling, the optimal control method is applied to achieve the optimization, and a pose planning method is presented to achieve a second-order continuity of quaternion getting rid of the interruption caused by ambiguity. Then simulations are carried out to verify the method, and the results show that the robot is controlled with optimized results during visual servoing process and the joint trajectories are smooth. [ABSTRACT FROM AUTHOR]

Copyright of Chinese Journal of Computational Mechanics / Jisuan Lixue Xuebao is the property of Chinese Journal of Computational Mechanics Editorial Office, Dalian University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In most cases, the tracking accuracy is an elementary requirement when designing the tracking controller of free-floating space robot. However, the resource consumption of the system input is also restricted in orbit, which should be also cared about in designing tracking controller. Thus, in this paper, an optimal sample-data control method for free-floating space robot is proposed by using the discrete state-dependent Riccati equation. And, in order to overcome the effect of uncertain inertial parameters, a two-stage controller designing strategy is adopted in this method, which includes the designs of an optimal reference controller and an optimal compensation controller. In addition, the supremum of the sampling interval is investigated via uniform input-to-state stability theories. Some numerical examples are provided to illustrate the performances of the proposed control method. [ABSTRACT FROM AUTHOR]

Motion planning is one of the most important technologies for free-floating space robots (FFSRs) to increase operation safety and autonomy in orbit. As a nonholonomic system, a first-order differential relationship exists between the joint angle and the base attitude of the space robot, which makes it pretty challenging to implement the relevant motion planning. Meanwhile, the existing planning framework must solve inverse kinematics for goal configuration and has the limitation that the goal configuration and the initial configuration may not be in the same connected domain. Thus, faced with these questions, this paper investigates a novel motion planning algorithm based on rapidly-exploring random trees (RRTs) for an FFSR from an initial configuration to a goal end-effector (EE) pose. In a motion planning algorithm designed to deal with differential constraints and restrict base attitude disturbance, two control-based local planners are proposed, respectively, for random configuration guiding growth and goal EE pose-guiding growth of the tree. The former can ensure the effective exploration of the configuration space, and the latter can reduce the possibility of occurrence of singularity while ensuring the fast convergence of the algorithm and no violation of the attitude constraints. Compared with the existing works, it does not require the inverse kinematics to be solved while the planning task is completed and the attitude constraint is preserved. The simulation results verify the effectiveness of the algorithm.

In order to eliminate plume impingement on the target satellite, a free-floating space robot (also known as a chaser), which has the advantage of executing on-orbit service, is always used. This paper develops a path planning method for a safe rendezvous of chaser with a noncooperative target satellite in orbital coordinates. Safety principles for rendezvous in terminal approaching phase are proposed. Grasp points on the target satellite are analyzed and classified into two categories, and a moving ellipse trajectory is adopted to approach a rotating and uncontrolled target satellite. This method guarantees that the chaser can successfully escape if unexpected error occurs or capture fails. The simulation results show that, with this novel autonomous rendezvous method, the chaser can approach the noncooperative target satellite along the designated trajectory in any quadrant of the orbital plane. [ABSTRACT FROM AUTHOR]

Satellite capturing with free-floating space robots is still a challenging task due to the non-fixed base and unknown mass property issues. In this paper gyro and eye-in-hand camera data are adopted as an alternative choice for solving this problem. For this improved system, a new modeling approach that reduces the complexity of system control and identification is proposed. With the newly developed model, the space robot is equivalent to a ground-fixed manipulator system. Accordingly, a self-tuning control scheme is applied to handle such a control problem including unknown parameters. To determine the controller parameters, an estimator is designed based on the least-squares technique for identifying the unknown mass properties in real time. The proposed method is tested with a credible 3-dimensional ground verification experimental system, and the experimental results confirm the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

This paper presents a novel, robust, adaptive trajectorytracking control scheme for the free-floating space robot system in Cartesian space. The dynamic equation of the free-floating space robot system in Cartesian space is derived from the augmented variable method. The proposed basic robust adaptive controller is able to deal with parametric and non-parametric uncertainties simultaneously. Another advantage of the control scheme is that the known and unknown external disturbance bounds can be considered using a modification of the parameter-estimation law. In addition, three cases are certified to achieve robustness for both parametric uncertainties and external disturbances. The simulation results show that the control scheme can ensure stable tracking of the desired trajectory of the end-effector. [ABSTRACT FROM AUTHOR]

At the same time, considering the uncertain factors such as load variation, external interference and joint flexibility in engineering practice, a robust control method based on adaptive neural network is proposed. The dynamic model of free-floating space robot is established, and the error model caused by uncertain factors is deduced. Different from the traditional compensation algorithm that ignores the error model, a compensation controller based on Radial basis function neural network (RBFNN) is designed to approximate the error model. The approximation error is eliminated by robust controller to improve the control accuracy. In order to make full use of the nonlinear approximation ability of neural network, the error model is decomposed into four parts according to the input characteristics, and the neural network compensator is designed for separate and overall compensation, which further improves the control accuracy and robustness. The adaptive learning rates of network weights are designed to ensure online real-time adjustment without offline learning stage. A flexible compensator based on torque and a controller based on moment difference feedback controller (MDFC) are designed to suppress elastic vibration. Simulation and experimental studies show that the proposed strategy can have good compensation performance and robustness, and can better suppress elastic vibration, which proves the effectiveness and superiority of the proposed scheme. [ABSTRACT FROM AUTHOR]

The maximum contact force is one of the most important indicators for contact problems. In this paper, the configuration optimization is conducted to reduce the maximum contact force for a free-floating space robot capturing tumbling target. First, the dynamics model of a free-floating space robot is given, with which the inertial properties perceived at the end-effector can be derived. Combing the inertial properties of the contact bodies, a novel concept of integrated effective mass is proposed. It tries to transform the complex contact process into the energy change of a virtual single body with integrated effective mass. On this basis, a more general continuous contact model is established, which is also suitable for non-central collisions between space robot and the tumbling target. Thereafter, the maximum contact force is derived as an important indicator for the null-space optimization method to reduce the maximum contact force. Finally, numerical simulations with a 3-degree-of-freedom free-floating space robot and a 7-degree-of-freedom free-floating space robot, as the research objects, are carried out respectively and the results show the effectiveness of the method proposed. [ABSTRACT FROM AUTHOR]

This paper investigates the problem of the dynamic balance control of multi-arm free-floating space robot during capturing an active object in close proximity. The position and orientation of space base will be affected during the operation of space manipulator because of the dynamics coupling between the manipulator and space base. This dynamics coupling is unique characteristics of space robot system. Such a disturbance will produce a serious impact between the manipulator hand and the object. To ensure reliable and precise operation, we propose to develop a space robot system consisting of two arms, with one arm (mission arm) for accomplishing the capture mission, and the other one (balance arm) compensating for the disturbance of the base. We present the coordinated control concept for balance of the attitude of the base using the balance arm. The mission arm can move along the given trajectory to approach and capture the target with no considering the disturbance from the coupling of the base. We establish a relationship between the motion of two arm that can realize the zeros reaction to the base. The simulation studies verified the validity and efficiency of the proposed control method.

This paper proposes adaptive control methods based on reaction dynamics for different types of space robotic systems. Reaction dynamics feature the dynamic coupling between an actively operated part and a passively moving part in a multibody robotic system. The reaction dynamics have been used to develop trajectory tracking control of a free-floating space robot or vibration suppression control of a flexible-structure-based manipulator system. However, the presence of dynamic parameter uncertainties degrades the control performance of the above-mentioned methods, since the methods require accurate values of both kinematic and dynamic parameters. To resolve such parameter uncertainties, practical adaptive control methods are proposed in this study. The proposed methods overcome two inherent difficulties in the adaptive control design of space robotic systems, such as nonlinear parameterization of the dynamic equation and uncertainties in coordinate mapping from Cartesian space to joint space. To confirm the validity of the proposed methods, numerical simulations are carried out using three-dimensional realistic models of two types of space robotic systems. [ABSTRACT FROM AUTHOR]

This paper investigates the problem of the dynamic balance control of multi-arm free-floating space robot during capturing an active object in close proximity. The position and orientation of space base will be affected during the operation of space manipulator because of the dynamics coupling between the manipulator and space base. This dynamics coupling is unique characteristics of space robot system. Such a disturbance will produce a serious impact between the manipulator hand and the object. To ensure reliable and precise operation, we propose to develop a space robot system consisting of two arms, with one arm (mission arm) for accomplishing the capture mission, and the other one (balance arm) compensating for the disturbance of the base. We present the coordinated control concept for balance of the attitude of the base using the balance arm. The mission arm can move along the given trajectory to approach and capture the target with no considering the disturbance from the coupling of the base. We establish a relationship between the motion of two arm that can realize the zeros reaction to the base. The simulation studies verified the validity and efficiency of the proposed control method. [ABSTRACT FROM AUTHOR]

This article presents impact dynamic analysis of a free-floating space robot, subject to a force impulse at the hand. We study the joint and the base reactions in terms of finite velocity changes and clarify their role for the post-impact motion behavior of the robot. The analysis makes use of a joint-space orthogonal decomposition procedure involving the so called reaction null space. The article focuses on the specific case of a nonredundant arm and a reaction null space in terms of base angular motion. We further show that with proper post-impact control it is possible to transfer the whole angular momentum from the base toward the manipulator, and in the same time to reduce the joint velocity. [ABSTRACT FROM PUBLISHER]

A free-floating space robot exhibits strong dynamic coupling between the arm and the base, and the resulting position of the end of the arm depends not only on the joint angles but also on the state of the base. Dynamic modeling is complicated for multiple degree of freedom (DOF) manipulators, especially for a space robot with two arms. Therefore, the trajectories are typically planned offline and tracked online. However, this approach is not suitable if the target has relative motion with respect to the servicing space robot. To handle this issue, a model-free reinforcement learning strategy is proposed for training a policy for online trajectory planning without establishing the dynamic and kinematic models of the space robot. The model-free learning algorithm learns a policy that maps states to actions via trial and error in a simulation environment. With the learned policy, which is represented by a feedforward neural network with 2 hidden layers, the space robot can schedule and perform actions quickly and can be implemented for real-time applications. The feasibility of the trained policy is demonstrated for both fixed and moving targets. [ABSTRACT FROM AUTHOR]

With the increase of on-orbit maintenance and support requirements, the application of a space manipulator is becoming more promising. In actual operation, the strong coupling of the free-floating space robot itself and the unknown disturbance of the contact target caused a major challenge to the robot base posture control. Traditional Reaction Null Space (RNS) motion planning and control methods require the construction of precise dynamic models, which is impossible in reality. In order to solve this problem, this paper proposes a new Adaptive Reaction Null Space (ARNS) path planning and control strategy for the contact of free-floating space robots with unknown targets. The ARNS path planning strategy is constructed by the Variable Forgetting Factor Recursive Least Squares (VFF–RLS) algorithm. At the same time, a robust adaptive control strategy based on the Strategy Self-Adaption Differential Evolution–Extreme Learning Machine (SSADE–ELM) algorithm is proposed to track the dynamic changes of the planned path. The algorithm enables us to intelligently learn and compensate for the unknown disturbance. Then, this paper constructs a robust controller to compensate model uncertainty. A striking feature of the proposed strategy is that it does not require an accurate system model or any information about unknown attributes. This design can dynamically implement RNS path tracking performance. Finally, through simulation and experiment, the proposed algorithm is compared with the existing methods to prove its effectiveness and superiority. [ABSTRACT FROM AUTHOR]