On-orbit Servicing Missions Research Articles

Dynamic analysis of a hyper-redundant space manipulator with a complex rope network

This paper studies the dynamics of a space manipulator. The space manipulator is designed for precise on-orbit servicing missions in a highly constrained environment. The concerned manipulator has hyper-redundant degrees of freedom and moves with a piecewise constant curvature, which enhances the flexibility and controllability. Such manipulator consists of a large number of links and a complex rope network. When the manipulator is driven, the interacting forces between the links and ropes introduce complexity into the dynamic behavior. In terms of dynamic modeling, the manipulator is a very complex system. This paper proposes a dynamic model of the manipulator based on methods of multibody dynamics. The ropes are assumed to be massless and linear elastic. The equations of motion are derived using space operator algebra. The vibration of the manipulator is investigated. The governing equations of the vibration are derived by applying the perturbation method to the proposed dynamic model. The values of the natural frequencies are investigated for the elasticities of the ropes. The proposed dynamic model is also applied in numerical simulation. The explicit fourth-order Runge-Kutta method is utilized to solve the equations of motion numerically. In numerical simulation, an upper bound of the time step is encountered. The value of upper bound is found to be related to the elasticities of the ropes. Such phenomena are studied by analyzing the stability region of the Runge-Kutta methods. Besides, the computational efficiency of the numerical simulation is also limited by the value of upper bound. Two modifications of the dynamic model are introduced to relax the upper bound of the time step. The effects of the modifications are demonstrated by numerical results.

A Deep Learning Strategy For On-Orbit Servicing Via Space Robotic Manipulator

Autonomous robotic systems are currently being addressed as a critical element in the development of present and future on-orbit operations. Modern missions are calling for systems capable of reproducing human’s decision-making process, thus enhancing their performance. Generally, space manipulators are mounted on a floating spacecraft in a microgravity environment, consequently leading to a mutual influence between the robotic arms and the platform dynamics, thus making the motion planning and control design more challenging than those of terrestrial robots. Another aspect to be considered is that space robots are designed as lightweight systems, resulting in a significant dynamic coupling between their rigid motion and structural elasticity. These effects involve critical issues in modelling their dynamics and designing a suitable controller. In this context, Deep Neural Network (DNN) architectures and the related Deep Learning (DL) techniques have widely proved to have powerful capability in solving data-driven nonlinear modelling problems and they can hence represent a viable solution for space activities. The present paper deals with the design of a DNN controller for a space manipulator system, which has to follow a specific path for a typical on-orbit servicing mission. The goal is to provide proper control inputs autonomously adapting to the given desired trajectory. Structural flexibility and joint friction features are implemented in the dynamic model as well.

Robotic hand-arm system for on-orbit servicing missions in Tiangong-2 Space Laboratory

PurposeThe purpose of this study is to develop a robotic hand–arm system for on-orbit servicing missions at the Tiangong-2 (TG-2) Space Laboratory.Design/methodology/approachThe hand–arm system is mainly composed of a lightweight arm, a dexterous hand, an electrical cabinet, a global camera, a hand–eye camera and some human–machine interfaces. The 6-DOF lightweight arm and the 15-DOF dexterous hand adopt the modular design philosophy that greatly reduces the design cycle and cost. To reduce the computational burden on the central controller and simplify system maintenance, an electrical system which has a hierarchical structure is introduced.FindingsThe prototypical operating experiments completed in TG-2 space laboratory demonstrate the performance of the hand–arm system and lay foundations for the future applications of space manipulators.Originality/valueThe main contributions of this paper are as follows a robotic hand–arm system which can perform on-orbit servicing missions such as grasping the electric drill, screwing the bolt, unscrewing J599 electrical connector has been developed; a variable time step motion plan method is proposed to adjust the trajectories of the lightweight arm to reduce or eliminate the collision force; and a dexterous hand uses the coordinated grasp control based on the object Cartesian stiffness to realize stable grasp. To solve the kinematic mapping from the cyber glove commands to the dexterous hand, a fingertip-position-based method is proposed to acquire precise solutions.

Point-to-Point Motion Planning of a Free-Floating Space Manipulator Using the Rapidly-Exploring Random Trees (RRT) Method

SUMMARYIt is usually proposed to use a robotic manipulator for performing on-orbit capture of a target satellite in the planned active debris removal and on-orbit servicing missions. Control of the satellite-manipulator system is challenging because motion of the manipulator influences position and orientation of the chaser satellite. Moreover, the trajectory selected for the capture manoeuvre must be collision-free. In this article, we consider the case of a nonredundant manipulator mounted on a free-floating satellite.We propose to use the bi-directional rapidly-exploring random trees (RRT) algorithm to achieve two purposes: to plan a collision-free manipulator trajectory that, at the same time, will result in a desired change of the chaser satellite orientation. Several improvements are introduced in comparison to the previous applications of the RRT method for manipulator mounted on a free-floating satellite. Feasibility of the proposed approach is demonstrated in numerical simulations performed for the planar case in which the chaser satellite is equipped with a 2-DoF (Degree of Freedom) manipulator. The obtained results are analysed and compared with the results obtained from collision-free trajectory planning methods that do not allow to set the desired final orientation of the chaser satellite.

Two-stage filter for inertia characteristics estimation of high-speed tumbling targets

The estimation of the inertia characteristics of a tumbling target before capturing is one of the greatest challenges in on-orbit servicing missions, especially when the time interval between consecutive measurements is large or the rotational speed of the target is very high. In this paper, a two-stage constant state filter (TCSF) is proposed to predict the attitude and estimate the inertia characteristics, including the principal inertia ratios and the principal inertia axes' directions, of a high-speed tumbling target. The man innovation of this paper is to design an Affiliate Filter to estimate the inertia tensor of the target, based on the estimates of angular velocity from an Adaptive Constant State Filter. Compared with the conventional estimation methods like the unscented Kalman filter method, the proposed TCSF shows better stability and accuracy in strongly nonlinear situations, such as that where the time interval between consecutive measurements is 10 s and the angular velocity is more than 14 deg/s. The Monte Carlo simulations have been proposed to demonstrate the performances of the proposed TCSF method.

Parameter estimations of uncooperative space targets using novel mixed artificial neural network

Estimating the parameters of an uncooperative space target is vital important in the on-orbit servicing missions (OOS). Considering the measurement failures caused by the complexed space environment during the measuring procedure, this paper proposes a novel dual vector quaternions based mixed artificial neural network estimating algorithm (DVQ-MANN) to estimate the parameters of the uncooperative space target. Firstly, the dual vector quaternions (DVQ) are utilized to set up the relative kinematics and dynamics model. When the measurements are available, an Extended Kalman Filter (EKF) of the DVQ-MANN will operate to accomplish parameter estimations. When the measurements failures occur, the artificial neural networks(ANN) of the DVQ-MANN will work. In the designed DVQ-MANN, the first ANN is a three-layer feedforward neural network to estimate the states of the uncooperative space target. Besides, a novel deep convolution neural network is designed to estimate the covariance matrix of the states for its advantages in processing high dimensional inputs. By training well off-board and updating onboard, the proposed ANNs of the proposed DVQ-MANN can make reliable estimations under measurements failures. Finally, the proposed DVQ-MANN is validated by mathematical simulations to show its robust performances.

On-orbit servicing of geosynchronous satellites based on low-thrust transfers considering perturbations

The goal of this paper is to investigate the on-orbit servicing (OOS) of geosynchronous satellites based on low-thrust transfers while considering the different demands of the satellites and the effects of perturbations. One or more servicing spacecraft start from a parking orbit and service the satellites in sequence via low-thrust orbit transfers, and return to the parking orbit in the end. The different demands lead to different service requirements of time and mass, while the perturbations affect the control solution of servicing spacecraft during each transfer, and induce orbit drifts of uncontrolled target satellites during the mission. An analytical method that can efficiently calculate the fuel consumption of low-thrust transfer is proposed, and methods for evaluating the orbit drifts of uncontrolled satellites are presented. The impact of perturbations on OOS mission is investigated in detail, showing that non-negligible factors include lunisolar attraction and tessral harmonic. Based on the dynamic models, a string-based simulated annealing algorithm is adopted to schedule the OOS mission. The efficiency of the proposed dynamic models and optimization method are verified through ample simulations. Results show that it is much more efficient and reasonable to take orbit drift calculations as corrections on the scheduling result assuming no orbit drifts. The proposed models and methods make it possible to schedule the OOS mission with low-thrust transfers, while achieving high accuracy under the effects of J2 perturbation, lunisolar attracion, tesseral harmonic and solar radiation pressure.

Pose Estimation of Non-Cooperative Target Coated With MLI

The relative pose estimation between the chaser and the non-cooperative target is a significant prerequisite for performing on-orbit servicing (OOS) missions. The chaser could design the close-range rendezvous trajectory to approach the non-cooperative target only when the relative pose parameters are obtained effectively and efficiently. In this paper, a pose estimation scheme is designed to obtain the relative pose parameters between the chaser and the non-cooperative target coated with multilayer insulation material (MLI). The scheme utilizes a time-of-flight (TOF) camera to acquire 3D point clouds of the non-cooperative target, and uses the iterative closest point (ICP) method to achieve the point cloud registration between every two frames. Aiming at decreasing the non-systematic errors caused by MLI in the data acquisition of the TOF camera, the corresponding point median filtering approach is adopted to filter out the bad corresponding point pairs generated in the ICP method. A semi-physical experiment is carried out to evaluate the performance of the proposed scheme, the result of which shows that the proposed scheme could notably improve the accuracy of pose estimation of the non-cooperative target, meanwhile the computational efficiency is also ensured.

Coordinated Control of a Dual-Arm Space Robot: Novel Models and Simulations for Robotic Control Methods

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].

Virtual-base modeling and coordinated control of a dual-arm space robot for target capturing and manipulation

A dual-arm space robotic system has large potential in on-orbit servicing missions, such as satellite repairing, large space structure construction and space debris removal. However, there exist multiple dynamic couplings between the base, the manipulators and the target, bringing large challenge to the motion planning and control. In this paper, a virtual-base modeling method (VBM) and coordinated control strategy are proposed for a dual-arm space robot. One of the arms’ end-effector is considered as the virtual base, and the other bodies, including each link of the arms and the center body of the spacecraft, form a combined manipulator. Correspondingly, we derive the kinematic and dynamic equations of the combined manipulator with the virtual base, taking it as a hyper-redundant space manipulator. Furthermore, the coordinated trajectory planning and control methods are presented for capturing and manipulating a space target. For the pre-contact stage, the virtual base and end-effector are controlled simultaneously to track the desired trajectory in the inertial space. During contact, the operational force is regulated using admittance control law, by simplifying the operational forces of the two arms as the equivalent operational force of the combined manipulator. After the target is captured and fixed to the two arms, its pose is adjusted by simultaneously controlling the trajectory and the operational force of the virtual base and the combined manipulator. Finally, a co-simulation system is created based on MATLAB/Simulink and MSC.ADAMS. Simulation results verify the effectiveness and robustness of the proposed methods.

Type synthesis of non-holonomic spherical constraint underactuated parallel robotics

Space robotics is taken as one of the most promising approaches for on-orbit servicing missions. The underactuated non-holonomic parallel mechanism is a new direction of parallel space robotics, with small volume, lightweight, low cost, low energy consumption, and high flexibility. The existing methods on type synthesis could not generate underactuated non-holonomic parallel mechanisms with the introduction of the non-holonomic spherical pairs in parallel mechanisms. This paper is devoted to type synthesis of underactuated parallel mechanisms with spherical motion pattern accompanied by the generation of the non-holonomic spherical pairs in parallel mechanisms with spherical motion pattern. After the introduction of the traditional roller-sphere contact, this paper proposes a novel roller-sphere contact. After this paper provides the type synthesis of parallel mechanisms with spherical motion pattern, it puts forward the generation method of non-holonomic spherical pairs in parallel underactuated mechanisms with spherical motion pattern. The non-holonomic underactuated joint mechanism of a space robot for on-orbit servicing operations is taken as an example to demonstrate the proposed method.

Novel dual vector quaternions based adaptive extended two-step filter for pose and inertial parameters estimation of a free-floating tumbling space target

Estimating the parameters of an uncooperative space target is essential to the on-orbit service missions. A good parameter estimation can provide sufficient prior knowledge for the further operations. This paper proposes a novel dual vector quaternions based adaptive extended two-step filter (DVQ-AETSF) to estimate the pose and inertial parameters of a free-floating tumbling space target. Firstly, both of the rotational and translational motions are modeled by the dual vector quaternions (DVQ). Then, by using the DVQ-based system model, the DVQ-AETSF is designed. The proposed DVQ-AETSF mainly consists of a traditional Kalman filter prediction procedure in the first step and an adaptive regularized Newton iteration technique in the second step. The new proposed two-step filter aims to deal with the high nonlinearities in the measurements equations. By using the proposed DVQ-AETSF, both of the pose and initial parameters of a free floating tumbling space target under large errors of initial guesses and high measurement noise can be well estimated. Finally, the proposed DVQ-AETSF is validated by mathematical simulations to show its performance.

Spacecraft Fast Fly-Around Formations Design Using the Bi-Teardrop Configuration

This paper investigates the feasibility, description, design, and control of fast fly-around formations around a chief spacecraft for rapid on-orbit servicing missions. The forming mechanics of two kinds of fast fly-around orbits is analyzed through an explicit geometrical demonstration in both inertial and relative coordinates. Then, relative motion equations based on relative orbit elements are presented to design the bielliptic and bi-teardrop formations, which provides intuitive geometric insight of relative motion in contrast to Cartesian position and velocity variables. The degree of urgency, fly-around distance, and relative view angle are chosen to describe the configuration of the in-plane and the out-of-plane formations. A detailed design process for the fast fly-around formations is proposed. Impulsive control strategies for the fast fly-around formations establishment and reconfiguration are derived. The design method and control strategies are validated through several numerical simulations. The effect of fly-around parameters on fuel consumption and payload performance are analyzed.

End-to-end simulation and verification of GNC and robotic systems considering both space segment and ground segment

In the framework of a project called on-orbit servicing end-to-end simulation, the final approach and capture of a tumbling client satellite in an on-orbit servicing mission are simulated. The necessary components are developed and the entire end-to-end chain is tested and verified. This involves both on-board and on-ground systems. The space segment comprises a passive client satellite, and an active service satellite with its rendezvous and berthing payload. The space segment is simulated using a software satellite simulator and two robotic, hardware-in-the-loop test beds, the European Proximity Operations Simulator (EPOS) 2.0 and the OOS-Sim. The ground segment is established as for a real servicing mission, such that realistic operations can be performed from the different consoles in the control room. During the simulation of the telerobotic operation, it is important to provide a realistic communication environment with different parameters like they occur in the real world (realistic delay and jitter, for example).

Fault-tolerant pose and inertial parameters estimation of an uncooperative spacecraft based on dual vector quaternions

Estimating the parameters of an unknown free-floating tumbling spacecraft is an essential task for the on-orbit servicing missions. This paper proposes a dual vector quaternion based fault-tolerant pose and inertial parameters estimation algorithm of an uncooperative space target using two formation flying small satellites. Firstly, by utilizing the dual vector quaternions to model the kinematics and dynamics of the system, not only the representation of the model is concise and compacted, but also the translational and rotational coupled effects are considered. By using this modeling technique along with the measurements from the on-board vision-based sensors, a dual vector quaternion based extended Kalman filter for each of the two small satellites is designed. Secondly, both of the estimations from each small satellite will be used as inputs of the fault-tolerant algorithm. This algorithm is based on the fault-tolerant federal extended Kalman filter strategy to overcome the estimation errors caused by the faulty measurements, the unknown space environment and the computing errors by setting the appropriate ratios of the two estimations from the first step dual vector quaternions extended Kalman filter. Together with the first and second steps, a novel fault-tolerant dual vector quaternions federal extended Kalman filter using two formation flying small satellites is proposed by this paper to estimate the pose and inertial parameters of a free-floating tumbling space target. By utilizing the estimation algorithm, a good prior knowledge of the unknown space target can be achieved. Finally, the proposed dual vector quaternion federal extended Kalman filter is validated by mathematical simulations to show its robust performances.

Post-capture vibration suppression of spacecraft via a bio-inspired isolation system

Abstract Inspired by the smooth motions of a running kangaroo, a bio-inspired quadrilateral shape (BIQS) structure is proposed to suppress the vibrations of a free-floating spacecraft subject to periodic or impulsive forces, which may be encountered during on-orbit servicing missions. In particular, the BIQS structure is installed between the satellite platform and the capture mechanism. The dynamical model of the BIQS isolation system, i.e. a BIQS structure connecting the platform and the capture mechanism at each side, is established by Lagrange’s equations to simulate the post-capture dynamical responses. The BIQS system suffering an impulsive force is dealt with by means of a modified version of Lagrange’s equations. Furthermore, the classical harmonic balance method is used to solve the nonlinear dynamical system subject to periodic forces, while for the case under impulsive forces the numerical integration method is adopted. Due to the weightless environment in space, the present BIQS system is essentially an under-constrained dynamical system with one of its natural frequencies being identical to zero. The effects of system parameters, such as the number of layers in BIQS, stiffness, assembly angle, rod length, damping coefficient, masses of satellite platform and capture mechanism, on the isolation performance of the present system are thoroughly investigated. In addition, comparisons between the isolation performances of the presently proposed BIQS isolator and the conventional spring-mass-damper (SMD) isolator are conducted to demonstrate the advantages of the present isolator. Numerical simulations show that the BIQS system has a much better performance than the SMD system under either periodic or impulsive forces. Overall, the present BIQS isolator offers a highly efficient passive way for vibration suppressions of free-floating spacecraft.

The effects of parking orbit elements on designing of on-orbit servicing missions

Nowadays on-orbit servicing operations such as satellite refuelling, debris removal and visual inspection are considered as the most important issues in the space missions. Mission planning has a key role on the designation of such missions and it is strongly dependent on the required fuel. In this study, a new approach is proposed for the designing of the on-orbit operations with considering the parking orbit elements and location of the servicing satellites. The proposed method improves the previous mission planning process of the multiple servicing satellites in the terms of the reduction in the mission fuel consumption. Furthermore, a special rendezvous maneuver is considered for meeting the servicing satellites and the targets. Also, the transfer orbits are obtained through Lambert targeting. The optimisation of the problem is carried out by particle swarm optimisation algorithm and Taguchi technique is employed for the robust design of the control parameters of the optimisation algorithm. The results reveal that the proposed approach is an efficient way in the reduction of the fuel consumption in the on-orbit servicing missions rather than the conventional methods.

Multibody dynamical modeling for spacecraft docking process with spring-damper buffering device: A new validation approach

In the current manuscript, the process of spacecraft docking, as one of the main risky operations in an on-orbit servicing mission, is modeled based on unconstrained multibody dynamics. The spring-damper buffering device is utilized here in the docking probe-cone system for micro-satellites. Owing to the impact occurs inevitably during docking process and the motion characteristics of multibody systems are remarkably affected by this phenomenon, a continuous contact force model needs to be considered. Spring-damper buffering device, keeping the spacecraft stable in an orbit when impact occurs, connects a base (cylinder) inserted in the chaser satellite and the end of docking probe. Furthermore, by considering a revolute joint equipped with torsional shock absorber, between base and chaser satellite, the docking probe can experience both translational and rotational motions simultaneously. Although spacecraft docking process accompanied by the buffering mechanisms may be modeled by constrained multibody dynamics, this paper deals with a simple and efficient formulation to eliminate the surplus generalized coordinates and solve the impact docking problem based on unconstrained Lagrangian mechanics. By an example problem, first, model verification is accomplished by comparing the computed results with those recently reported in the literature. Second, according to a new alternative validation approach, which is based on constrained multibody problem, the accuracy of presented model can be also evaluated. This proposed verification approach can be applied to indirectly solve the constrained multibody problems by minimum required effort. The time history of impact force, the influence of system flexibility and physical interaction between shock absorber and penetration depth caused by impact are the issues followed in this paper. Third, the MATLAB/SIMULINK multibody dynamic analysis software will be applied to build impact docking model to validate computed results and then, investigate the trajectories of both satellites to take place the successful capture process.

Adaptive pose and inertial parameters estimation of free-floating tumbling space objects using dual vector quaternions

The robust parameter estimation of unknown space objects is essential to the on-orbit servicing missions. Based on the adaptive filtering techniques along with the dual quaternions modeling methods...

Estimation of inertial characteristics of tumbling spacecraft using constant state filter

Reconstruction of dynamical parameters is one of the main challenges faced in the on-orbit servicing missions for defunct spacecraft. And the quaternion plays a major role in parameterizations of the dynamical model. In this paper, the analytical solution of the quaternion differential equations of a tumbling symmetrical object is derived. Given this solution, a constant state filter is proposed for inertial characteristics estimation and attitude prediction of tumbling spacecraft. The key idea of the present filter is to replace dynamic variables by the undetermined constant parameters of the analytical solution. These parameters are estimated during the filtering process and then used to calculate the dynamic variables of the spacecraft. Furthermore, they are also utilized to determine the inertial characteristics and predict the future attitude motions. Compared with traditional EKFs, the constant state filter shows good performance when the measurement sampling interval is large or a priori estimation of the state is unavailable, because the dynamic model and observation model are transformed into approximate linear forms by utilizing the constant state vector. Numerical simulations verify the convergence and precision of the proposed filter.