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Space robotics has been used extensively in complex space missions. Rigid-manipulator space robots may suffer from rigid-body collisions with targets. This collision is likely to cause damage to the space robot and the target. To overcome such a problem, a novel Continuum-Manipulator Space Robot (CMSR) for performing on-orbit servicing missions is proposed in this paper. Compared with rigid-manipulator space robots, CMSRs are able to perform compliant operations and avoid rigid-body collisions with a target. The CMSR consists of two kinds of flexible components, including solar arrays and continuum manipulators. The elastic vibrations of these flexible components disturb the position and attitude of CMSRs. The beating phenomenon introduced by the energy transfer among these flexible components can cause damage to solar arrays. The complicated dynamic coupling poses enormous challenges in dynamic modeling and vibration analysis. The dynamic model for CMSRs is derived and the mechanism of the beating phenomenon is analyzed in this paper. Simulation results show that an obvious beating phenomenon occurs and the amplitude of the solar arrays increases significantly when the natural frequencies of two kinds of flexible components are close. A method is provided to avoid the beating phenomenon.

This work mainly studies the position and attitude tracking control of a free-floating space robot. With the attitude represented in modified Rodrigues parameters (MRPs), a task-space controller with predefined-time stability is developed considering the external disturbance. The tuning parameters of a predefined-time controller can be formulated as functions of the prescribed upper bound of the stabilization time. Based on the backstepping technique and a novel predefined-time stabilizing function, a predefined-time control scheme is designed for the space robot system. Moreover, to avoid ’explosion of terms’, an auxiliary variable is introduced such that the controller is independent of the derivative of the virtual control law. Numerical simulations are presented to demonstrate the effectiveness of the proposed method.

The rapid development of space technology has made it increasingly important to use space robots for on-orbit services. Space crawling robots can perform extravehicular condition monitoring, spacecraft fault location and diagnosis, repair and maintenance by carrying different payloads and traversing the spacecraft surface, which has received wide attention from researchers. A prototype of space crawling robot has been designed in this work, which is a quadrupedal insect-like configuration with integrated bionic adhesion material on the bottom of the four feet, and can walk on the satellite surface in a gravity-free space environment. The design and implementation of the crawling robot control system is carried out based on Robot Operating System, including hardware system construction and software architecture design. The tripod gait was also designed to enable the robot to crawl more stably in space. At last, the basic capabilities of the designed space crawling robot are tested in the ground environment, and the results demonstrate the robot’s body control capability, omnidirectional walking capability, and obstacle crossing and avoidance capability.

With the rapid development of space technology and the increasing demand for space missions, the traditional spacecraft manufacturing, deployment and launch methods have been unable to meet existing needs. In-space assembly (ISA) technologies can effectively adapt to the assembly of large space structures, improve spacecraft performance, and reduce operating costs. In this paper, the development and technologies for ISA are reviewed. ISA is classified from multiple angles, and the research status of ISA is shown clearly through the visual mapping knowledge domain. Then the development status of autonomous robot assembly in the United States, Europe, Japan, Canada and China is reviewed. Furthermore, the key technologies of ISA are analyzed from three aspects: assembly structure design, robot technologies and integrated management technologies. ISA technologies are still facing major challenges and need to be further explored to promote future development. Finally, future development trends and potential applications of ISA are given, which show that ISA will play a vital role in human space exploration in the future.

Space robots are playing an increasingly important role in on-orbit services. Affected by the coupling of dynamics, the base attitude of a space robot will deflect with the motion of its manipulators, which leads to unfavorable effect to the success of the mission. The trajectory planning problem for base attitude disturbance minimization of a dual-arm space robot will be studied in this article. Along the planned trajectory, the attitude of the base will keep unchanged when the end effectors of the manipulators arrive at the desired position. First, the dynamic equation of the dual-arm space robot is given, and the direct-inverse mixed dynamic equation is also derived. Then, the rotation trajectory of the manipulator joints is parameterized, and the attitude deflection of the base with respect to the given parameters is calculated based on the direct-inverse mixed dynamic equation. The particle swarm optimization algorithm is utilized to obtain the optimal trajectory. Finally, numerical simulations are carried out to verify the effectiveness of the method proposed in this article. Four cases are included, considering symmetrical or asymmetrical initial configuration and synchronous or asynchronous start of the dual arms. The simulation results show that, in each case, the method proposed can effectively plan the trajectory along which the base attitude will keep nearly unchanged before and after the motion of the dual arms.

We examine applicability of normal forms of non-holonomic robotic systems to the problem of motion planning. A case study is analyzed of a planar, free-floating space robot consisting of a mobile base equipped with an on-board manipulator. It is assumed that during the robot’s motion its conserved angular momentum is zero. The motion planning problem is first solved at velocity level, and then torques at the joints are found as a solution of an inverse dynamics problem. A novelty of this paper lies in using the chained normal form of the robot’s dynamics and corresponding feedback transformations for motion planning at the velocity level. Two basic cases are studied, depending on the position of mounting point of the on-board manipulator. Comprehensive computational results are presented, and compared with the results provided by the Endogenous Configuration Space Approach. Advantages and limitations of applying normal forms for robot motion planning are discussed.

In the first part, this article presents an overview of Guidance, Navigation and Control (GNC) methodologies developed for space manipulators to perform in-orbit robotic missions, including but not limited to, on-orbit servicing, satellite/station assembly, probing extra-terrestrial objects and space debris mitigation. Some space mission concepts are briefly mentioned, for which space robotics is discussed to be among the most practical and universal solutions. Common phases of an in-orbit robotic mission are identified as: close-range rendezvous, attitude synchronization, target identification, manipulator deployment, capture, and if needed, post-capture maneuvers. Prominent GNC methodologies that are either proposed for or applicable to each phase are extensively reviewed. In the current article, the emphasis is placed on the study of GNC methodologies utilized in attitude synchronization, manipulator deployment, and capture phases, specially the ones reported for use in the two free-floating and free-flying operating regimes of space manipulators. Kinematics and dynamics of space manipulator systems are formulated to help unifying the presentation of the main ideas behind different GNC methodologies. Using a unified notation, comparison tables and discussions provided in this paper, researchers can compare various GNC approaches and contribute to the next-generation GNC systems for space robots. In addition, this survey aids technology users to learn about in-orbit robotic missions and choose appropriate GNC technologies for specific applications. In the second part of this paper, two families of emerging control schemes based upon reinforcement learning and geometric mechanics are introduced as promising research directions in the GNC of space robotic systems. The benefits of implementing these techniques to the GNC of in-orbit robotic missions are discussed. An exclusive study of environmental disturbances affecting space manipulators and their threat to long-term autonomy concludes this article.

Purpose of Review The purpose of this review is to highlight space autonomy advances across mission phases, capture the anticipated need for autonomy and associated rationale, assess state of the practice, and share thoughts for future advancements that could lead to a new frontier in space exploration. Recent Findings Over the past two decades, several autonomous functions and system-level capabilities have been demonstrated and used in spacecraft operations. In spite of that, spacecraft today remain largely reliant on ground in the loop to assess situations and plan next actions, using pre-scripted command sequences. Advances have been made across mission phases including spacecraft navigation; proximity operations; entry, descent, and landing; surface mobility and manipulation; and data handling. But past successful practices may not be sustainable for future exploration. The ability of ground operators to predict the outcome of their plans seriously diminishes when platforms physically interact with planetary bodies, as has been experienced in two decades of Mars surface operations. This results from uncertainties that arise due to limited knowledge, complex physical interaction with the environment, and limitations of associated models. Summary Robotics and autonomy are synergistic, wherein robotics provides flexibility, autonomy exercises it to more effectively and robustly explore unknown worlds. Such capabilities can be substantially advanced by leveraging the rapid growth in SmallSats, the relative accessibility of near-Earth objects, and the recent increase in launch opportunities.

For spacecraft swarms, the multi-agent localization algorithm must scale well with the number of spacecraft and adapt to time-varying communication and relative sensing networks. In this paper, we present a decentralized, scalable algorithm for swarm localization, called the Decentralized Pose Estimation (DPE) algorithm. The DPE considers both communication and relative sensing graphs and defines an observable local formation. Each spacecraft jointly localizes its local subset of spacecraft using direct and communicated measurements. Since the algorithm is local, the algorithm complexity does not grow with the number of spacecraft in the swarm. As part of the DPE, we present the Swarm Reference Frame Estimation (SRFE) algorithm, a distributed consensus algorithm to co-estimate a common Local-Vertical, Local-Horizontal (LVLH) frame. The DPE combined with the SRFE provides a scalable, fully-decentralized navigation solution that can be used for swarm control and motion planning. Numerical simulations and experiments using Caltech’s robotic spacecraft simulators are presented to validate the effectiveness and scalability of the DPE algorithm.

Using a space robot to capture a non-cooperative spacecraft with high-speed rotation is significantly challenging since any collision generated during capturing will have great impact on both. To reduce the risk in capturing operations, it is crucial to slow down the rotation velocity of the target before capturing. Hence, this paper studies a collision control strategy for using a robotic arm to detumble a non-cooperative spacecraft. The goal of this strategy is to maintain contact between the robot and the target and to apply continuous detumbling force on the target to slow down its rotational motion. To achieve that, first the mechanism analysis of the two balls for a central collision scenario is performed. Then an overdamping control method is proposed to avoid the separation of the balls after the central collision based on the overdamping property of the mass-spring-damper system, the effectiveness of which is validated theoretically. Finally, to use this overdamping control method in the detumbling missions of space robots, a position-based overdamping control strategy is introduced. In this strategy, the relative dynamic behavior between the robotic arm tip and the contact surface of the target during detumbling is approximated to that of a mass-spring-damper system. Owing to the proposed overdamping control method, the contact between the robot and the target can be maintained, and the space robot can apply continuous control torque to slow down the rotational velocity of the target. Numerical simulations are performed to demonstrate the validity of the proposed control strategy.

Autonomous on-orbit servicing, such as capture, refuel, repair and refurbishment of on-orbit satellites using a robotic arm mounted on servicing satellite is one of the important components of future’s space missions. Space robots increase reliability, safety, and ease of execution of space operations, but pose a novel challenge due to micro-gravity and space environments. While capturing high speed orbiting objects, robotic arms undergo impact and require appropriate modeling of the system. In this paper, a unified framework is provided for modeling impact dynamics, post-capture stabilization and target maneuvering of a multi-arm robotic system mounted on a servicing satellite while capturing orbiting objects. The dynamic model of multi-arm space robot is obtained using the Decoupled Natural Orthogonal Complement (DeNOC) based formulation and closed-loop constraint equations. All three phases of the capturing operation, namely, approach, impact, and post-impact are modeled using Impulse-momentum approach and conservation of momentum. In the approach phase, robot arms are planned to move from its initial configuration to the desired capture configuration. In the impact phase, a framework is developed to estimate the impulse forces and changes in the generalized velocities caused by the impact. In post-impact phase, these velocities are used as initial conditions for the post-impact dynamics simulations. The uncontrolled dynamics during post-impact will result in an undesirable motion, thus post-impact reactionless control (minimum base disturbance) strategy is used to maneuver the space robot’s arms and target object. As such, the robotic arms can be used to maneuver an astronaut for repair of satellite. Most of the times the parameters of target object are not known. Hence, an adaptive reactionless control strategy has been devised for capturing object with unknown parameters. The effectiveness of the framework is shown using a dual-arm robot mounted on a servicing satellite performing capturing operation for multiple objects. The effects of relative velocity and angle of approach on the impact forces are also investigated.

Dual-arm space robots are capable of load transporting and coordinated manipulation for on-orbit servicing. However, achieving the accurate trajectory tracking performance is a big challenge for dual-arm robots, especially when mechanical system uncertainties exist. This paper proposes an adaptive control scheme for the dual-arm space robots with grasped targets to accurately follow trajectories while stabilizing base’s attitude in the presence of dynamic uncertainties, kinematic uncertainties and deadzone nonlinearities. An approximate Jacobian matrix is utilized to compensate the kinematic uncertainties, while a radial basis function neural network (RBFNN) with feature decomposition technique is employed to approximate the unknown dynamics. Besides, a smooth deadzone inverse is introduced to reduce the effects from deadzone nonlinearities. The adaption laws for the parameters of the approximate Jacobian matrix, RBFNN and the deadzone inverse are designed with the consideration of the finite-time convergence of trajectory tracking errors as well as the parameters estimation. The stability of the control scheme is validated by a defined Lyapunov function. Several simulations were conducted, and the simulation results verified the effectiveness of the proposed control scheme.

This paper presents a robust detumbling strategy for a spacecraft equipped with a robotic manipulator to detumble a non-cooperative target with unknown inertial parameters. Reported detumbling strategies in the literature have been formulated with the assumption of accurately known target inertial parameters (mass, inertia tensor, center of mass location) and without consideration of magnitude limits on the robotic spacecraft’s control inputs. In reality, it is unrealistic to assume accurate knowledge of the target’s inertial parameters and the robotic spacecraft will be subjected to limits on its control inputs. In this paper, a novel detumbling strategy is presented to detumble targets with unknown parameters utilizing only bounds on their inertial parameters. A desired detumbling trajectory is delineated using only bounds on the target’s inertial parameters and subjected to end-effector force/torque limits. Magnitude constraints on the robotic spacecraft are taken into account by modeling them with a hyperbolic tangent function, which results in a system that is non-affine in its control inputs. To facilitate the development of the detumbling controller, an augmented model of the robotic spacecraft is formulated. Using bounds on the target’s inertial parameters, robust adaptive control approach is utilized to design the detumbling controller with the backstepping technique to ensure successful detumbling of the unknown target attached to the end-effector and rejection of its momentum. Numerical simulations have demonstrated the effectiveness of the proposed method.

Future on-orbit servicing and assembly missions will require space robots capable of manoeuvring safely around their target. Several challenges arise when modelling, controlling and planning the motion of such systems, therefore, new methodologies are required. A safe approach towards the grasping point implies that the space robot must be able to use the additional degrees of freedom offered by the spacecraft base to aid the arm attain the target and avoid collisions and singularities. The controlled-floating space robot possesses this particularity of motion and will be utilised in this paper to design an optimal path generator. The path generator, based on a Genetic Algorithm, takes advantage of the dynamic coupling effect and the controlled motion of the spacecraft base to safely attain the target. It aims to minimise several objectives whilst satisfying multiple constraints. The key feature of this new path generator is that it requires only the Cartesian position of the point to grasp as an input, without prior knowledge a desired path. The results presented originate from the trajectory tracking using a nonlinear adaptive H ∞ controller for the motion of the arm and its base simultaneously.

The information mini-satellites (for communication, geodesy, radio- and optoelectronic Earth-observing et al.) and the free-flying space robots (used for servicing both the mini-satellites and manned orbital space stations as well as assembling the large space structures) have some principal problems with respect to their spatial attitude guidance and control. In the paper these problems are considered and obtained results are presented.

The safe realization of on-orbit capture remains a challenging task because of uncontrolled service objects, big impact forces, post-impact control, and so on. In this study, based on the relationships between the kinematics and forces established by impedance control, a force constraint function is developed to reduce the contact force during the collision process of capture. Additionally, to realize flexible contact and improve motion performance during the capture process, a control method combining impedance and a sliding mode is designed to make the end-effector exhibit mass-spring-damping behavior after contact with the target, decreasing movement overshoot and stabilization time. Finally, numerical simulations are carried out in a scenario where a planar two-link space robot is utilized to capture a non-cooperative target, and the effectiveness of the proposed method is demonstrated.

Recently, with the rapid development of aerospace technology, an increasing number of spacecraft is being launched into space. Additionally, the demands for on-orbit servicing (OOS) missions are rapidly increasing. Space robotics is one of the most promising approaches for various OOS missions; thus, research on space robotics technologies for OOS has attracted increased attention from space agencies and universities worldwide. In this paper, we review the structures, ground verification, and on-orbit kinematics calibration technologies of space robotic systems for OOS. First, we systematically summarize the development of space robotic systems and OOS programs based on space robotics. Then, according to the structures and applications, these systems are divided into three categories: large space manipulators, humanoid space robots, and small space manipulators. According to the capture mechanisms adopted, the end-effectors are systematically analyzed. Furthermore, the ground verification facilities used to simulate a microgravity environment are summarized and compared. Additionally, the on-orbit kinematics calibration technologies are discussed and analyzed compared with the kinematics calibration technologies of industrial manipulators with regard to four aspects. Finally, the development trends of the structures, verification, and calibration technologies are discussed to extend this review work.

Target capture is a fundamental and essential task of space robots for on-orbit servicing in the future. Accordingly, it is important to study the method for space robot stabilization after impact during target capture. In this paper, the stabilization control problem of a combined spacecraft after multiple impacts during target capture is studied. A dual-integral sliding mode (DISM) control method based on the reconfiguration of the reaction wheels is proposed. The stability control of the combined spacecraft is achieved by controlling the reaction wheels of the space robot to absorb the angular momentum arising from the impact. First, given the structure of the gripper, the contact detection algorithm and contact dynamics model are established, and then multiple impacts are simulated during the target capturing operation. Second, an improved sliding mode control method is proposed to overcome the influence of multiple impacts, and the stability of the controller is confirmed by the Lyapunov method. Third, through an optimization method, the configuration of the reaction wheels of the space robot is reconstructed to avoid control torque saturation. The validity and availability are eventually verified by simulations and experiments. The results show that the proposed method can achieve the stability of the combined spacecraft after the target capture, and the algorithm is robust and has engineering applicability.

Using space robots to perform capture missions are challenging problems due to potential contacts. Most state-of-the-art techniques sidestep contacts by separating a capture mission into pre- and post-capture phases, and assuming the end-effector fixes to the target instantaneously. In this paper, frictional contacts have been taken into consideration, and a corresponding trajectory optimization method is developed. The frictional contact is formulated as two sets of complementarity constraints for the compression phase and the decompression phase. The direct method for trajectory optimization with complementarity constraints is established. The combination mechanism based on the sequential quadratic programming is proposed to solve resultant highly complex optimization problems, which can provide local optimal trajectories for space robots. The method is verified via numerical simulations of a planar dual-arm space robot capturing a spinning target, and results illustrate its great performance and potential usability on other scenarios.

With the number of large-scale facilities, malfunctioning spacecraft, and debris expected to increase in space, on-orbit servicing, including assembly, monitoring, maintenance, refueling and deorbiting, has become a major concern in space missions. Space robots can efficiently cope with a large number of on-orbit servicing missions while avoiding the high risks and reducing costs of extravehicular activity. The challenges of space robots with multiple arms include the complex dynamics and the coupling between the floating base and the end-effector. To handle these problems, a dynamics model with topological structure is firstly established for a four-arm space robot based on the kinematic chain symbolic calculus system and Axis-Invariants, which have the benefits of iterative structure and can avoid complex derivation of traditional dynamics modeling methods. Then, an attitude stabilization strategy consisting of improved nonlinear model predictive control and a hybrid actuator is implemented to handle the disturbance generated by the manipulation of multiple robot arms. The hybrid actuator can provide accurate control torque while simultaneously avoiding the singularity and saturation of the control moment gyro and reaction wheel, respectively. Finally, a scenario of space debris recycling is used to demonstrate the effectiveness of the proposed modeling and control strategy for on-orbit servicing missions with multiarmed space robots.

This article elaborates on the tension control design of tethered space robots for debris removal. The novel approach is to change the tether natural length via a reel. With only the tension sensing, we leverage a Kalman filter and a recursive least-squares algorithm to estimate the actual length and the debris mass, which are fed back to a model-predictive controller to generate the optimal tangential velocity of the reel. Simulations show that the stiff tether exhibits better tension-tracking performance which is robust to the tumbling debris.

On-orbit servicing, active debris removal or assembling large structures on orbit are only some of the tasks that could be accomplished by space robots. In all these cases, a contact between a space robot and the satellite being serviced, deorbited, or assembled will occur. This contact results in a contact force exerted on the space robot, and therefore momenta of the space robot system are no longer conserved. Most of the papers that are concerned with motion planning problems of a space robot manipulator either consider that no external forces or moments are acting on the space robot system or use additional controllers when the space robot is subjected to external forces and moments. Such a controller minimizes end-effector position and orientation errors caused by the changes in system momenta due to external forces and moments acting on this system. The novelty of this work is that it proposes a new method for planning the motion of dual-arm space robot manipulators when linear and angular momenta of the space robot system are not conserved due to external forces and moments acting on the space robot base or/and manipulators’ end-effectors. In the proposed method the changes in system momenta are considered, but no additional controllers are needed. In this paper, we derive the motion planning equations for dual-arm space robot manipulators, where external forces and moments are acting on both satellite and manipulator end-effectors. The proposed method has been verified by numerical simulations, and the results are presented and discussed.

Multi-DOF flexible space robots play a significant role in the on-orbit service. The flexibility of the robots is mainly caused by the flexible links and some flexible drive elements in joints. The vibration generated by the component flexibility can affect the accuracy of control. In this paper, the dynamics and control issues of flexible-link flexible-joint (FLFJ) space robots considering joint friction are studied. Firstly, the Spong’s model is employed to depict flexible joints, and the Coulomb friction model is adopted to describe joint friction. Then based on the single direction recursive construction method and velocity variation principle, dynamic equations of the system are derived. Secondly, a trajectory of joint motion is given and a trajectory tracking controller with friction compensation is designed with the computed torque method. Finally, several numerical simulations are carried out to verify the accuracy of the dynamic model and illustrate the effect of joint friction and joint flexibility on the dynamic characteristics of space robots. Besides, simulation results suggest that the controller designed in this paper could effectively achieve the trajectory tracking control for the 6-DOF FLFJ space robot with joint friction.

Capture and removal of space debris are challenging in robotic on-orbit servicing activities. A large portion of space debris does not possess any graspable features, which makes the conventional grippers inapplicable. To handle such nongraspable objects, a space robotic capture system is presented. A dual-arm space robot simulator that has the advantages of miniaturization and scalability is designed for ground tests. Inspired by the robotic caging, we propose a novel capture method that uses a series of hollow-shaped end-effector pairs to cage the antipodal pairs of the nongraspable objects. To apply the caging-pair method steadily, space robots need exerting a squeezing action on objects, which can be characterized by the motion and force manipulation of two robotic arms in the assigned directions. Based on the velocity and force manipulability transmission ratios, a caging compatibility index is proposed to describe the capturing ability in this manner. Via the optimization of the desired caging compatibility index, an effective algorithm is proposed to plan the near-optimal joint configurations for the pregrasping cages. Finally, both simulation studies and experimental tests are conducted to evaluate the performance of the proposed capture method.

Dual-arm space robot is a promising autonomous system to remove the increasing space debris actively. Due to its limited on-orbit energy, dual-arm space robot should approach and capture the target with minimum time and base disturbance. In this paper, the multi-objective configuration optimization is proposed for maximizing manipulability and minimizing base disturbance of dual-arm space robot during the pre-contact phase. Based on virtual base modelling and coordinated planning, the free-floating dual-arm space robots can be controlled to track and approach the target satellite. In addition, in the null-space of tracking motion, the configuration optimization of dual-arm space robot can be realized by arm-angle adjustment of 7 DOF redundant manipulator. Therefore, the expressions of arm angle and arm-angle Jacobian are first derived. Then the arm-angle trajectory is parameterized by adopting 5th order polynomial interpolation. The optimal arm-angle trajectory can be obtained by Multiple Objective Particle Swarm Optimization (MOPSO), in which the objective function of configuration optimization is set to the vector consisted of base disturbance and manipulability. Therefore, in the null space of dual-arm space robot approaching the tumbling target satellite, the configuration optimization is realized through following the optimal arm-angle trajectory. Finally, several single-objective and multi-objective optimization simulations are carried out to verify the proposed method.

SUMMARYA dual-arm space robot has large potentials in on-orbit servicing. However, there exist multiple dynamic coupling effects between the two arms, each arm, and the base, bringing great challenges to the trajectory planning and dynamic control of the dual-arm space robotic system. In this paper, we propose a dynamic coupling modeling and analysis method for a dual-arm space robot. Firstly, according to the conservation principle of the linear and angular momentum, the dynamic coupling between the base and each manipulator is deduced. The dynamic coupling factor is then defined to evaluate the dynamic coupling degree. Secondly, the dynamic coupling equations between the two arms, each arm, and the base are deduced, respectively. The dynamic coupling factor is suitable not only for single-arm space robots but also for multi-arm space robot systems. Finally, the multiple coupling effects of the dual-arm space robotic system are analyzed in detail through typical cases. Simulation results verified the proposed method.

Space robots are in free-flying or free-floating mode, motions especially attitude motion of the base and motion of the manipulator are strongly coupled. Regarding to the uncertainties of the target’s inertial parameters and variation of the base’s inertial parameters, this paper presents a novel online one-step parameter identification method to estimate all the inertial parameters of the target and the base simultaneously. Momentum- and force-based identification equations are derived from the linear and angular momentum equations of the system and their derivation, and the modified recursive least square method is used for solving the equations efficiently. Compared with the traditional methods, the momentum-based equation can estimate all the inertial parameters of the base and the target simultaneously at each steps, while the force-based equation does not require torque of the joints. To verify the validity and feasibility of the proposed methods, 2D and 3D models with different targets and initial velocities are simulated and analyzed. The results show that all the estimated values show convergence to their ideal values and the method can be easily achieved online via recursive techniques.

By maximizing manipulability, the coordination of multi-arm can be enhanced. In this paper, a method to optimize the manipulability index of cooperative manipulation for a free-floating multi-arm space robot is proposed. Firstly, the manipulability optimization is formulated as a nonlinear optimize problem at position level which is hard to solve online. By redefining constraint equation and manipulability index, it is transformed to a constrained quadratic program problem at velocity level incorporating joint velocity physical limits, which generates joint velocity commands to control the multi-arm to complete predefined tasks. Owing to dynamic coupling effects and closed chain constraints formed by cooperative manipulation, the manipulability index is more complex than that of fixed-base or mobile-base manipulators. Hence, the gradient of the index is approximated by numerical algorithms. Simulations based on a dual-arm space robot model are conducted and the results prove that the proposed method is efficient to optimize the manipulability index.

This paper presents a trajectory planning method for a dual-arm space robot that is free-floating and receives no actuation from the base spacecraft. The task is to capture a target by a mission arm and accomplish base manoeuvring with the coupling effects by a manoeuvring arm. Three manoeuvring cases for the base are discussed concerning attitude regulation, attitude tracking and position tracking. In all cases, an optimization method is applied to solve for the joint velocity of the arms, which ensures that the joint states of the mission arm obtain the desired states for capturing and control the motion of the base. Moreover, the collision avoidance of the manoeuvring arm with the base and mission arm is considered an inequality constraint. In particular, the joint velocity of the mission arm is represented by a new variable to remove the disturbance caused by the mission arm on the attitude of the base which makes it easier to complete the attitude manoeuvring case. Finally, numerical simulations are carried out by the planar and spatial dual-arm robots to validate the efficiency of the control strategy during both capturing and base manoeuvring.

Simulation results of the six-link spatial industrial and space robots, namely PUMA robot, Space Shuttle Remote Manipulator System (SSRMS), and Stanford arm, with only rigid and rigid–flexible links, are presented in this chapter. A typical six-link robot has a three-link wrist attached at the end of a three-link spatial arm. Examples of PUMA robot, Space Shuttle Remote Manipulator System (SSRMS), which have revolute joints only, and Stanford arm with five revolute joints and one prismatic joint are considered here.

Game-based competitive or cooperative robotics activities constitute an effective approach to exploit the child-robot interaction perspective. However, in most game-based robotics activities robots act autonomously to achieve the goal. In this work, we aim to promote the child-robot interaction aspect through a multiplayer game where one team of robots and humans collaborates to compete with another team of humans and cobots. We describe the design of an open space that will allow children to gain access, locally and remotely, and program robotic agents to play the traditional “Capture the Flag” game in a physical stadium-arena. Through this space, we intend to teach robotics, while programming human-robot interfaces, within a computer-supported game-based learning environment. We give insights on the initial design of such an open space and the educational benefits of its use in the comprehension of abstract computational and STEM concepts.

Designing A highly-reliable modular joint system for low speed and large inertia space manipulators has been one of the grand challenges in robotics research. It is too difficult for the conventional joints for satisfy the requirements of aerospace applications. To address this challenge, this paper details the hardware and control development for a highly-reliable modular joints system, intended for use in highly space manipulators.

This paper studies the Cartesian point-to-point optimal trajectory planning for space robots oriented to space maintenance operations. Aiming at the problems of poor stability, large base disturbance, and large joint variation in the motion planning of point-to-point maintenance in space, a planning method is proposed to minimize the base disturbance and the total joint angle variation under the jerk constraint on the premise of ensuring the accuracy of the end pose. First, the attitude of the space robot is described by the unit quaternion, and the velocity relationship between the joint angle, the end effector, and the base posture is introduced. Then, the joint trajectories were parameterized by a fifth degree polynomial, and a trajectory planning model with the minimum perturbation of the base and the minimum variation of the joint of the manipulator was established under the condition that the end effector satisfied the pose and the jerk constraint. Finally, a multi-objective optimization algorithm is proposed to deal with the trajectory optimization problem under nonlinear constraints. The simulation results show that the proposed trajectory planning method can optimize the base attitude and joint angle of the space manipulator under the premise of the optimal trajectory and stability of the terminal execution tool, which ensures the stability of the space robot’s on-orbit service and reduces the energy consumption.

This paper presents intelligent approaches for swinging failure control and fuel saving of space robot by switching over the actuators. The space manipulators mainly work in extremely harsh and remote locations; hence, there may be a possibility of catastrophic failure in the manipulator. The primary concern of this work is to identify the faulty joints and subsequently switching over the actuators intelligently to control these joints and then finally optimize the trajectory based on the energy conservation criterion implementing a genetic algorithm approach. The purpose is to develop a capable robot for tackling joint failure problem in the multi-links robotic manipulators, by using redundant motorized joints. By anticipating joint failure in the form of free swinging, a power-saving six-axis space robot has been modeled. To increase the flexibility of space robots, redundancy is provided. The bond graph technique and MATLAB are used in tandem to validate and verify the proposed systems. The simulation results show that the proposed redundancy by switching over the actuator works well for failure control and optimizes 37.85% onboard fuel, which has a significant impact on satellite life. As a result, the proposed controlling method can achieve the operation of the floating body in an unknown environment with failure in the manipulator's joints. Furthermore, due to redundancy, flexibility will be economically rendered by the proposed control strategy.

The traditional trajectory planning methods of free-floating space robots have the dynamic singular problem and the difficulty of accurate modeling. Although learning-based approaches have achieved remarkable performance on such task, they mainly focus on single modular design such as the perception, planning, or control part. Optimization-based end-to-end method can well combine perception, planning and control, which not rely on parameters of the dynamic model and reduce the difficulty of manually adjusting modular controllers’ parameters. Therefore, we developed an end-to-end trajectory planning strategy based on optimization with multiple constraints. The whole strategy consists of several multi-layer neural networks and is optimized by a deep reinforcement learning algorithm based on maximum entropy. The results of visualization show that our strategy can capture the information of robotic arm from vision directly. Moreover, we evaluate the kinematic and dynamic features of the system and testify our strategy in anti-interference experiments. The performance of our strategy demonstrates the availability and robustness of the system.

Some problems on in-flight verification of the navigation, guidance and control systems for the information satellites and space robots are considered. Digital algorithms for reliable control of these spacecraft and obtained results are presented.

Space missions that focus on On-Orbit-Servicing often involve the capture of a target satellite or space debris as a key event in the operational sequence. Space robots are among the most promising technologies to perform this task and mission architectures involving manipulators proved successful in orbit. Such mission scenarios are complex and imply a considerable risk of collision between the chaser and the target vehicles. For the sake of safety, it is of key importance to accurately position the capture interface w.r.t. the gripping point on the target, especially when relative distance is very low. During these phases, the guidance and control algorithms rely on pose measurements provided by navigation sensors that are generally able to reconstruct the target position and orientation only up to a certain distance between the camera and the target. To the purpose of having a complete control of the capture interface until contact, a new sensor based on a LED and a matrix of phototransistors has been developed. This paper describes the sensor and provides its preliminary characterization. A simple theoretical description of the sensor behavior is obtained from geometrical considerations, while experiments have been performed to measure the performances of the actual hardware implementation of the system.

Visual terrain-relative navigation (VTRN) is a localization method based on registering a source image taken from a robotic vehicle against a georeferenced target image. With high-resolution imagery databases of Earth and other planets now available, VTRN offers accurate, drift-free navigation for air and space robots even in the absence of external positioning signals. Despite its potential for high accuracy, however, VTRN remains extremely fragile to common and predictable seasonal effects, such as lighting, vegetation changes, and snow cover. Engineered registration algorithms are mature and have provable geometric advantages but cannot accommodate the content changes caused by seasonal effects and have poor matching skill. Approaches based on deep learning can accommodate image content changes but produce opaque position estimates that either lack an interpretable uncertainty or require tedious human annotation. In this work, we address these issues with targeted use of deep learning within an image transform architecture, which converts seasonal imagery to a stable, invariant domain that can be used by conventional algorithms without modification. Our transform preserves the geometric structure and uncertainty estimates of legacy approaches and demonstrates superior performance under extreme seasonal changes while also being easy to train and highly generalizable. We show that classical registration methods perform exceptionally well for robotic visual navigation when stabilized with the proposed architecture and are able to consistently anticipate reliable imagery. Gross mismatches were nearly eliminated in challenging and realistic visual navigation tasks that also included topographic and perspective effects.

Aiming at addressing the problem that the joints are easily destroyed by the impact torque during the process of space robot on-orbit capturing a non-cooperative spacecraft, a reinforcement learning control algorithm combined with a compliant mechanism is proposed to achieve buffer compliance control. The compliant mechanism can not only absorb the impact energy through the deformation of its internal spring, but also limit the impact torque to a safe range by combining with the compliance control strategy. First of all, the dynamic models of the space robot and the target spacecraft before capture are obtained by using the Lagrange approach and Newton-Euler method. After that, based on the law of conservation of momentum, the constraints of kinematics and velocity, the integrated dynamic model of the post-capture hybrid system is derived. Considering the unstable hybrid system, a buffer compliance control based on reinforcement learning is proposed for the stable control. The associative search network is employed to approximate unknown nonlinear functions, an adaptive critic network is utilized to construct reinforcement signal to tune the associative search network. The numerical simulation shows that the proposed control scheme can reduce the impact torque acting on joints by 76.6% at the maximum and 58.7% at the minimum in the capturing operation phase. And in the stable control phase, the impact torque acting on the joints were limited within the safety threshold, which can avoid overload and damage of the joint actuators.

This chapter presents a unified approach to robust control of a variety of flexible mechanical systems, which are not only systems having flexible structure themselves such as a robotic manipulator with a flexible structure and a crane system, but also systems not having flexible structure but handling flexible objects such as a liquid container system and a fishery robot. So far, a lot of research efforts have been devoted to solve control problems of such flexible systems, one of the most typical problems among which is the problem of flexible robotic manipulators, e.g., [Sharon & Hardt (1984); Spong (1987); Wang & Vidyasagar (1990); Torres et al., (1994); Magee & Book (1995); Nenchev et al., (1996); Nenchev et al., (1997)]. As other types of applications, the problems of a crane system [Kang et al. (1999)] and of a liquid container system [Yano & Terashima (2001); Yano et al., (2001)] have been investigated. The common control problem for flexible systems can be stated as “how to achieve required motion control with suppressing undesirable oscillation due to its flexibility”. From the control methodology point of view, let us review those previous works. For socalled micro-macro manipulators associated with large flexible space robots, [Torres et al (1994)] and [Nenchev et al., (1996); Nenchev et al., (1997)] have proposed path-planning based control methods using a coupling map and a reaction null-space respectively, which utilize the geometric redundancy. The control methods in [Sharon & Hardt (1984)] for a micro-macro manipulator and in [Kang et al., (1999)] for a crane system rely on the endpoint direct feedback, which require sensors to measure the endpoint. In [Wang & Vidyasagar (1990)], a passivity-based control method has been proposed for a single flexible link, and in [Spong (1987)] an exact-linearization method and an integral manifold method have been presented for a flexible-joint manipulator. The method in [Magee & Book (1995)] is based on input signal filtering where the underlying concept is pole-zero cancellation. [Ueda & Yoshikawa (2004)] has applied a mode-shape compensator based on acceleration feedback to a flexible-base manipulator. For a liquid container system, H∞ control in [Yano & Terashima (2001)] and a notch-type filter based control, that is, equivalent to pole-zero cancellation, in [Yano et al., (2001)] are utilized respectively. In general, most other works have focused on individual systems and hence their control methods are not directly available for various flexible systems. For example, the path-planning methods in [Torres et

The research on underwater systems has gained an immense interest during the last decades with applications taken place in many fields such as exploration, investigation, repair, construction, etc. Hereby, control of underwater systems has emerged as a growing field of research. Underwater vehicles, in fact, accounted for 21% of the total number of service robots by the end of 2004, and are the most expensive class of service robots (UNECE/IFR, 2005). Typically, underwater vehicles can be divided into three underwater systems, namely, the manned submersibles, remotely operated vehicles (ROV) and autonomous underwater vehicles (AUV). ROVs and AUVs are mostly utilized in the oil and gas industries, and for scientific and military applications. AUVs, especially are of great importance due to their ability to navigate in abyssal zones without necessitating a tether that limits the range and maneuverability of the vehicle. However, their autonomy property directly affects the design of the control system. This requires advanced controllers and specific control schemes for given tasks. Almost all AUVs are six degrees-of-freedom (DOF) systems, and various types of actuator configurations are available in the industry for the vehicles ranging from fully-actuated vehicles to underactuated ones. The vehicle of interest here falls into the class of underactuated AUVs. Any mechanical system having fewer actuators than its degrees of freedom is defined as an underactuated system. Some examples of underactuated systems include manipulators; (Arai et al., 1998), (Oriolo & Nakamura, 1991), (Yabuno et al., 2003), marine vehicles; (Reyhanoglu, 1997), (Pettersen & Egeland, 1996), space robots; (Tsiotras & Luo, 1997), and the examples given in (Fantoni & Lozano, 2002). Controlling all of the DOF of underactuated mechanical systems is an arduous task compared to the fully actuated systems since the mathematical analysis of the system renders it difficult. Determining whether an underactuated system is controllable is one of these difficulties encountered. Control synthesis is also another challenge in this field and is still accepted as an open problem. The techniques used for fully actuated systems cannot be used directly for underactuated systems. However, there are some potential benefits over fully actuated systems depending on the efficiency of control and the task. In case of actuator failures, a fully actuated mechanical system falls into the class of underactuated systems and might still be controlled if a successful control scheme can be designed. Besides that, reduction of the weight and cost, and the increase of reliability can be considered as O pe n A cc es s D at ab as e w w w .in te ch w eb .o rg

Over the past few decades a considerable number of studies have been made on impact dynamics. Zheng and Hemami discussed a mathematical model of a robot that collides with an environment (Zheng & Hemami, 1985). When a robot arm fixed on the ground collides with a hard environment, the transition from the free space to constrained space may bring instability in the control system. Therefore, the impact between robots and environments has been the subject of controversy. Asada and Ogawa analyzed the dynamics of a robot arm interacting with an environment using the inverse inertia matrices (Asada & Ogawa, 1987). In the early 90’s, the optimum approach velocity for force-controlled contact has been enthusiastically studied (Nagata et al., 1990, Kitagaki & Uchiyama, 1992). Volpe and Khosla proposed an impact control scheme for stable hard-on-hard contact of a robot arm with an environment (Volpe & Khosla, 1993). Mills and Lokhorst proposed a discontinuous control approach for the tasks that require robot arms to make a transition from non-contact motion to contact motion, and from contact motion to non-contact motion (Mills & Lokhorst, 1993). Walker proposed measures named the dynamic impact measure and the generalized impact measure to evaluate the effects of impact on robot arms (Walker, 1994). Mandal and Payandeh discussed a unified control strategy capable of achieving a stable contact against both hard and soft environment (Mandal & Payandeh, 1995). Tarn et al. proposed a sensor-referenced control method using positive acceleration feedback and switching control strategy for robot impact control (Tarn et al., 1996). Space robots does not have fixed bases, therefore, an impact with other free-floating objects may bring the space robots a catastrophe. In order to minimize the impulsive reaction force or attitude disturbance at the base of a space robot, strategies for colliding using reaction null-space have been proposed (Yoshida & Nenchev, 1995, Nenchev & Yoshida, 1998). Most of the researches have been made to overcome the problems introduced by impacts between robots and environments. Some researchers have tried to use the advantages of impacts. When a robot applies a force statically on an environment, the magnitude of force is limited by the maximum torque of the actuators. In order to exert a large force on the environment beyond the limitation, applying impulsive force has been studied by a few researchers. Uchiyama performed a nail task by a 3-DOF robotic manipulator (Uchiyama, 1975). Takase et al. developed a two-arm robotic manipulator named Robot Carpenter, and performed sawing a wooden plate and nailing (Takase, 1990). Izumi and Hitaka proposed to use a flexible link manipulator for nailing task, because the flexible link has an advantage in absorbing an impact (Izumi & Kitaka, 1993).

Communications and navigation are fundamental to robotic spacecraft exploration. NASA spacecraft communicates via radio links with antennas in the deep space network (DSN) located in California, Australia, and Spain. Measurements of range between the DSN and spacecraft are an important and sometimes critical contribution to navigation. For a given pair of uplink and downlink carriers, ranging must generally be accompanied by telemetering, in order to allow for efficient use of spacecraft and ground resources. Regenerative ranging, which is compatible with the simultaneous presence of telemetry, is more efficient in the use of link power than is the current standard (nonregenerative) ranging. This improved efficiency arises because regenerative ranging replaces the wideband filter found in a nonregenerative transponder's ranging channel with a narrowband loop that tracks the range code, so the signal-to-noise ratio in the transponder's ranging channel is large and there is essentially no noise modulated onto the downlink carrier. This paper offers a comparison of pseudonoise (PN) regenerative ranging versus the nonregenerative method of ranging. This paper also describes the implementation of PN regenerative ranging on the Iris transponder, which is a low-power, 0.5U size, DSN-compatible, software-defined radio based on various JPL flight transponder designs. Iris is baselined to be used on 7 out of 13 of the CubeSats flying as secondary payloads on the Space Launch System Exploration Mission-1 launch of the Orion spacecraft.

Space debris or uncontrolled satellites bring new and critical challenges for space robots to capture and remove them, especially when the targets are tumbling. In the face of these challenges, most existing research only focused on capturing a fixed point on the target. Distinguished from previous research, this paper attempts to investigate an autonomous tumbling target capturing method by approaching the “Dynamic Closest Point” (DCP), that is, the most suitable point on the target to be grasped and always closest to the end effector. First, an uncontrolled space object and a space robotic system are modeled, and the existence of the DCP is proved. For a practical malfunction spacecraft, the motion characteristics of the DCP and a fixed point are theoretically analyzed and compared as regards different motion types and rotation vectors. Furthermore, the desired trajectory is generated for the end effector of the space robot to approach the DCP autonomously, based on the position and velocity of the DCP estimated in real-time by hand-eye cameras. The joint velocities and angles are then resolved by combining the differential kinematics and movement constraints on the end effector. Finally, an Adams-Simulink co-simulation system is developed. Based on this, the proposed method is verified by single-arm and dual-arm capturing cases. Simulation results demonstrate that the capture time and movement ranges of the end effector are both greatly reduced with the DCP method. In contrast with traditional capturing methods, the efficiency of this proposed method is largely improved.

In the paper a method of remote bilateral control of space robots operating in a non-deterministic environment with a large delay in control signals transmission is presented. The method provides adaptation of space robot behavior to possible changes in its external environment. Compared to the known approaches this method reduces influence of external environment variation on the control process.

Currently, space robots such as planetary robots and flexible-link manipulators (FLMs) are finding specific applications to reduce the cost of launching. However, the structural flexible nature of their arms and joints leads to errors in tip positioning owing to tip deflection. The internal model uncertainties and disturbance are the key challenges in the development of control strategies for tip-tracking of FLMs. To deal with these challenges, we design a tip-tracking controller for a two-link flexible manipulator (TLFM) by developing a sampled-data extended state observer (SD-ESO). It is designed to reconstruct uncertain parameters for accurate tip-tracking control of a TLFM. Finally, a backstepping (BS) controller is designed to attenuate the estimation error and other bounded disturbances. Convergence and stability of the proposed control system are investigated by using Lyapunov theory. The benefits (control performance and robustness) of the proposed SD-ESO-based BS controller are compared with other similar approaches by pursuing both simulation and experimental studies. It is observed from the results obtained that SD-ESO-based BS Controller effectively compensates the deviation in tip-tracking performance of TLFM due to non-minimum phase behavior and model uncertainties with an improved transient response.