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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.

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.

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.

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.

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.

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.

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).

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.

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.

Large-aperture space telescope is the key equipment for studying the origin and evolution of the universe and various celestial bodies. Developing large-aperture space telescope is challenging and has become a priority in the field of optical engineering. Due to the mass and volume constraints of launch vehicles, it has to be designed and launched in modules and then assembled on-orbit by space robots. In this paper, we present a conceptual of 10-meter diameter modular space telescope together with robotic assembly strategy. To fulfill on-orbit assembly and maintenance of the space telescope, a novel assembly robot with a ring-shaped mobile base and a redundant stretchable manipulator is proposed. The time-optimal trajectory planning based on genetic algorithm is utilized to achieve efficient assembly. The preliminary ground experiment of assembling submirror modules of space telescope using the KUKA LWR iiwa-7 is carried to verify the feasibility of the proposed method, and the results show that the assembly task of telescope modules can be accomplished successfully with appropriate forces and torques.

The methods for modeling and detection of anomalous in the automatic control systems, and also practical methods for active analysis of anomalous situations, are presented. We develop algorithms for consecutive classification of onboard equipment failures and reconfiguration of control loop. The simulation results are presented for the attitude control systems of the information satellites and space robots.

Space robots have attracted increasing attention for performing, autonomously or telerobotically, on-orbit servicing missions such as repairing, refueling, and upgrading spacecraft; reusing space assets; and on-orbit assembly. The extension of robot application to space can release astronauts from risky, time-consuming, and expensive extravehicular activities [1]. However, in the microgravity environment, the floating base of a space robot will be disturbed by the robot's arm motion when it approaches or manipulates a target. The motion of the spacecraft base resulting from this disturbance will, conversely, affect the motion of end effectors (known as coupling dynamics), making control of space robots more complicated than that of fixed-base robots. In addition, such a disturbance of spacecraft attitude may result in a communication interruption between the spacecraft and the ground station or a failure of energy accumulation caused by disorientation of solar panels [2].

With the development of robotics in recent years, space robotic manipulators have become a research focus on space technologies. In the TianGong-2 (TG-2) spacelab mission, a human-robot collaborative on-orbit servicing experiment was planned as one of the three key tasks. The central component of this task is the TG-2 robotic manipulator, which is a multisensory in-vehicle space robot with multiple control modes, including a 6-DOF robotic arm, a five-fingered humanoid hand, hand-eye and global cameras, and on-orbit human-machine interfaces(space mouse and cyber glove). The design purpose of TG-2 robotic task is to complete various prototypical experiments under micro-g environment to validate key technologies of space robots and on-orbit human-robot collaboration and gain experience and experimental data about robotic on-orbit servicing by assisting or cooperating with human astronauts. A variable time-scale motion plan method, which could adjusts the trajectories of the robot to reduce or eliminate the collision force between the robot collide with the environment, is developed for safe human-robot collaboration. The five-fingered hand grasp control is based on an approach coined as “coordinated grasp control based object Cartesian stiffness” for the adaptation of external forces and robust grasp. To solve the kinematic mapping from cyber glove commands to five-fingered robotic hand, a fingertip-position-based method is proposed to acquire precise solutions. The TG-2 robotic manipulator was launched to space in Sep. 15, 2016. From Oct. 27 to Nov. 13, 2016, all the planned experiments, including dynamic parameter identification, grasping floating objects and prototypical on-orbit servicing, were carried out successfully under the supervision and assistance of the two astronauts, laying foundations for the future applications of space robots.

Space missions have required the development of specific embedded systems for spacecraft applications, making reuse of existing software difficult. JAXA's space robot operating system (SP-ROS) initiative tackles this problem in order to realize more intelligent robotic space missions. JAXA's SP-ROS initiative proposes an open-source software framework for robots used in outer space and will facilitate the development of space robotics through the ROS and cFS system (RACS). This paper introduces RACS and describes a planned demonstration in space. RACS provides two functionalities. The first one is ROS/cFE converter, which makes it possible to directly incorporate the code of a ground robot application into an environment that can be ported as a spacecraft application. The porting of existing robotic applications will aid in developing new applications using ROS's rich development environment. The second functionality of RACS is the ROS/cFE bridge, which allows native cFS and ROS programs to interact. This is aimed mainly at making ground robotics software testing easier. Another benefit would allow ROS's development tools (Rviz and Gazebo) and libraries (OpenCV and MoveIt), as well as packages developed by the roboticists anywhere in the world, to be directly incorporated in the testing. Provided the onboard computer supports cFS, it will be possible to implement these software packages into space robots. JAXA has made the open-source RACS module available to everybody. We currently propose to test the system in a space demonstration using Int-Ball2, a space drone in the International Space Station (ISS).

The increased complexity of the tasks that on-orbit robots have to undertake has led to an increased need for manipulation dexterity. Space robots can become more dexterous by adopting grasping and manipulation methodologies and algorithms from terrestrial robots. In this paper, we present a novel methodology for evaluating the stability of a robotic grasp that captures a piece of space debris, a spent rocket stage. We calculate the Intrinsic Stiffness Matrix of a 2-fingered grasp on the surface of an Apogee Kick Motor nozzle and create a stability metric that is a function of the local contact curvature, material properties, applied force, and target mass. We evaluate the efficacy of the stability metric in a simulation and two real robot experiments. The subject of all experiments is a chasing robot that needs to capture a target AKM and pull it back towards the chaser body. In the V-REP simulator, we evaluate four grasping points on three AKM models, over three pulling profiles, using three physics engines. We also use a real robotic testbed with the capability of emulating an approaching robot and a weightless AKM target to evaluate our method over 11 grasps and three pulling profiles. Finally, we perform a sensitivity analysis to demonstrate how a variation on the grasping parameters affects grasp stability. The results of all experiments suggest that the grasp can be stable under slow pulling profiles, with successful pulling for all targets. The presented work offers an alternative way of capturing orbital targets and a novel example of how terrestrial robotic grasping methodologies could be extended to orbital activities.

Cartesian trajectory planning of a free-floating space robot is impacted by dynamic singularities due to the inverse kinematics equations. Although various methods have been proposed to avoid the singularities, very few of them are suitable for the trajectory planning of a high degree-of-freedom (DOF) space robot in the Cartesian space. In this paper, a method of combining Damped Least Squares (DLS) and feedback compensation is developed to avoid such singularities. The trajectories of the end-effector are parametrized with Bezier curves, which are simple and make it easy to limit the joint velocities. Moreover, because of certain missions, such as communication and observation, base attitude disturbance and moving time are considered to establish a cost function and the trajectory planning is transformed into a multi-objective optimization. Chaotic particle swarm optimization (CPSO) is employed to solve the optimization, which can improve the premature phenomenon of particle swarm optimization (PSO). Simulation results are presented for the trajectory planning of a 6 DOF space robot and demonstrate the effectiveness of the proposed method.

Recently, some manned moon base projects have been considered in Japan and other countries for the next mission of the International Space Station. Space robots, especially humanoid robots, are expected to play an important role in cooperating with the astronauts for lunar exploration in future space projects. This research investigates the effect of gravity on bipedal robot motion, for example, walking, running, and jumping, from the viewpoint of energy consumption using dynamic simulations. In these simulations, the central pattern generator (CPG) was used to control a space robot, because CPG can easily generate both walking and running motions by changing some parameters in the CPG models. In this research, the motion of a humanoid robot with low moving speed used in the rooms of the moon base is investigated. By considering the amplitude of joint torque during robot motions, the optimal motion form of a robot is shown. A robot can easily do precise control of about 10% of walking torque on the earth. So walking is an optimal motion because of small torque and energy saving, unlike an astronaut jumping on the surface of the moon.

A geosynchronous-orbit (GEO) satellite that is out of order due to component failure, will bring critical challenges for space robots to capture and repair it. To overcome the above challenges, a space robot system equipped with a traditional redundant manipulator (Arm-a) and a segmented hyper-redundant manipulator (Arm-b) is introduced in the paper. Firstly, the kinematics model considering the conservation of momentum is established. The coordination trajectory planning method is then proposed to make space robot detect the target point in the confined space. Based on the position, attitude, and configuration deviations, the linear and angular velocities of the two arms are planned at the same time. In order to realize the configuration planning of the system, a modified generalized Jacobian (MGJ) is derived. The joint angular velocities of two arms are resolved by the MGJ. Furthermore, joint angles of Arm-a and Arm-b are obtained according to numerical integration. Finally a co-simulation system is designed and the numerical simulation is carried out. The simulation results demonstrate the proposed method.

As the space environment is difficult to be recreated on the earth, it is essential to develop a software toolkit to validate the parameters designing for space robots. In this paper, a toolkit addressed for dynamic control rapid prototype simulation system of the space robot was presented. Based on the real-time control technology, the toolkit was consisted of real-time dynamic computing, redundant dual-arm control system, visual imaging and measurement and teleoperation. Specifically, the collision problem involved in the manipulator end effector and the target could be analyzed by contact dynamics models in the real-time dynamic computing module. Furthermore, a peg-in-hole (multi cores) assembly experiment involving the disassembly for the space electrical connectors was carried out, which was adopted to evaluate the simulation effect of toolkit. Our results showed that the connection between the space plug and socket with multi cores was well built up in the simulation environment. Moreover, the toolkit can be used to make simulations of on orbit operational missions by multiple of human-computer interaction, which is the key for the evaluation of control algorithm applied on the space robot.

Space debris removal is very important for ensuring space safety and further space exploration. However, most of the debris is tumbling in space and its corresponding large mass and inertia result in non-negligible momentum. During the detumbling manipulation of large-momentum space debris, there will be large base disturbance force which makes the control of the floating base of space robots much more challenging. In this paper, we propose a trajectory optimization framework for stabilizing a tumbling object using a dual-arm space robot. In this proposed trajectory optimization framework, only the object dynamics is used to minimize the detumbling time and base disturbance during the detumbling manipulation, while the optimal dual-arm motion is generated from the object motion by considering the closed-chain constraint. The objective function of trajectory optimization is defined as weighted detumbling time and base disturbance force to save energy of the dual-arm space robot for further manipulation. In order to verify the proposed method, a typical simulation of dual-arm space robot stabilizing a tumbling target is carried out.

This paper adds on to the on-going efforts to provide more autonomy to space robots and introduces the concept of programming by demonstration or imitation learning for trajectory planning of manipulators on free-floating spacecraft. A redundant 7-DoF robotic arm is mounted on small spacecraft dedicated for debris removal, on-orbit servicing and assembly, autonomous and rendezvous docking. The motion of robot (or manipulator) arm induces reaction forces on the spacecraft and hence its attitude changes prompting the Attitude Determination and Control System (ADCS) to take large corrective action. The method introduced here is capable of finding the trajectory that minimizes the attitudinal changes thereby reducing the load on ADCS. One of the critical elements in spacecraft trajectory planning and control is the power consumption. The approach introduced in this work carry out trajectory learning offline by collecting data from demonstrations and encoding it as a probabilistic distribution of trajectories. The learned trajectory distribution can be used for planning in previously unseen situations by conditioning the probabilistic distribution. Hence almost no power is required for computations after deployment. Sampling from a conditioned distribution provides several possible trajectories from the same start to goal state. To determine the trajectory that minimizes attitudinal changes, a cost term is defined and the trajectory which minimizes this cost is considered the optimal one.

The prerequisite for space robots on-orbit capturing and servicing is to inspect and track the attitude of the target satellite. However, the number of DOFs (degrees of freedom) of space robot is limited, which cannot satisfy the position and attitude tracking of space robot end and the zero disturbance of the satellite base attitude at the same time. The multi-task priority motion planning method of space robot is adopted, which can realize the attitude tracking of space robot end and the zero disturbance of the satellite base attitude, while the position tracking of space robot end can be optimized. The simulation experiment results show that the method is effective.

In 1981, the Shuttle Remote Manipulator System (SRMS), developed by Canada, was put into orbit by the space shuttle Columbia, becoming the world’s first on-orbit operating robot for space applications (Siciliano and Khatib in Handbook of robotics. Springer, New York (2007) [1]).

Kinematics and dynamics are the basis of analyzing the characteristics and control of space robots. In this chapter, the kinematics equations of a typical space robot system are given, including a description of topological relationships and the kinematics modeling process, which can be used for robot dynamics analysis and path planning. Based on analytical mechanics and vector mechanics, the dynamic models for rigid space robots and flexible space robots are established in this chapter, which can be used for robot dynamics analysis and control algorithm design.

For different types of space robots, motion planning has different meanings. Motion planning of space robots usually refers to the process of generating the desired motion trajectory in the robot joint space or Cartesian space according to the mission target.

Space robots involve different operating objects and operating environments while performing missions. For example, for on-orbit servicing robots, the operating objects are mostly cooperative targets with the preset visual markers and dedicated operation interfaces, and the operating environment is a spacecraft surface structured environment, while for planetary exploration robots, the operating objects are mostly noncooperative targets without visual markers or dedicated operation interfaces, and the environment features are unknown and unstructured. Space robot perception refers to the process in which the space robot obtains the information of the operating objects and the operating environments by analyzing raw data generated by various sensors onboard.

We present a numerical method to compute singularity sets in the configuration space of free-floating robots, comparing two different criteria based on formal methods. By exploiting specific properties of free-floating systems and an alternative formulation of the generalized Jacobian, the search space and computational complexity of the algorithm is reduced. It is shown that the resulting singularity maps can be applied in the context of trajectory planning to guarantee feasibility with respect to singularity avoidance. The proposed approach is validated on a space robot composed of a six degrees-of-freedom (DOF) arm mounted on a body with six DOF.

For precise attitude control of space robots intended to capture a target, the inertial parameters of the target need to be well estimated because the motions of arms and bases are coupled....

Small space robots have the potential to revolutionise space exploration by facilitating the on-orbit assembly of infrastructure, in shorter time scales, at reduced costs. Their commercial appeal will be further improved if such a system is also capable of performing on-orbit servicing missions, in line with the current drive to limit space debris and prolong the lifetime of satellites already in orbit. Whilst there have been a limited number of successful demonstrations of technologies capable of these on-orbit operations, the systems remain large and bespoke. The recent surge in small satellite technologies is changing the economics of space and in the near future, downsizing a space robot might become be a viable option with a host of bene�ts. This industry wide shift means some of the technologies for use with a downsized space robot, such as power and communication subsystems, now exist. However, there are still dynamic and control issues that need to be overcome before a downsized space robot can be capable of undertaking useful missions. This paper �rst outlines these issues, before analyzing the effect of downsizing a system on its operational capability. Therefore presenting the smallest controllable system such that the benefi�ts of a small space robot can be achieved with current technologies. The sizing of the base spacecraft and manipulator are addressed here. The design presented consists of a 3 link, 6 degrees of freedom robotic manipulator mounted on a 12U form factor satellite. The feasibility of this 12U space robot was evaluated in simulation and the in-depth results presented here support the hypothesis that a small space robot is a viable solution for in-orbit operations. Keywords: Small Satellite; Space Robot; In-orbit Assembly and Servicing; In-orbit operations; Free-Flying; Free-Floating.

Current plans for the exploration of Moon and Mars envision the use of telerobotic systems controlled from orbiting laboratories. The advantage of telerobotics is that it combines the resilience, endurance and precision of robots with the inherent flexibility, anticipation and decision making capabilities of humans. The primary disadvantage of telerobotics is the communication time delay in the human-robot control loop. The delay can lead to a loss of situation awareness, an increase in operator work load, and an overall decrease in effectiveness and efficiency of the human-robot system. Most of the effects of the delay can be mitigated by the use of predictive displays, presenting the operator with a simulated system state. This paper presents current work on such a predictive display designed to support an operator in remote flight and landing of space robots and unmanned aerial vehicles. The Adaptable Human-Machine Interface was developed for hardware-in-the-loop laboratory experiments with a Parrot A.R. Drone 2.0 quadcopter as test case. Based on live video from two on-board cameras, attitude and velocity telemetry, and control inceptor deflection, the interface calculates a predicted flight path and attitude and presents it in a “tunnel in the sky” display. The graphical display itself was developed in the Unity3D game engine. The paper describes the implementation of the interface between Unity3D and the A.R. Drone, the dynamic model of the quadcopter, and the prediction algorithm. The paper also discusses the results of flight tests involving a number of test subjects and projects the path forward in the development of this technology.