Free-floating Space Robot Research Articles

Dynamic coupling analysis of multi-arm space robot

The base pose of a free-floating space robot can be disturbed by the motion of its manipulators. When a multi-arm space robot captures a space target, a closed-loop system will be formed, and the dynamic coupling between the base and manipulators or the target are more complex. This paper analyzes the dynamic coupling effect of the open-loop multi-arm space robot and the closed-loop system. According to the momentum conservation equation, the velocity correlation matrices between the base and the joints or end-effectors in the open-loop free-floating space robot, and those between the base and the joints or target in the closed-loop system are derived. On this basis, the dynamic coupling factors and coupling ellipsoids are proposed as two tools to measure the coupling degree. The coupling factors proposed in this paper are based on the singular values of the correlation matrices to expand the scope of application. The two tools are applied to analyze the coupling of a dual-arm space robot and its composite body with the captured target, where the influences of dynamic parameters, link lengths and joint variables are mainly studied.

基于控制变量参数化方法的自由漂浮空间机器人路径规划

基于控制变量参数化方法的自由漂浮空间机器人路径规划

PI Sliding Mode Control and Vibration Suppression of a Space Robot with Elastic Base and Two Flexible Arms

The trajectory tracking and vibration suppression of elastic base and two flexible arms for a free-floating space robot system with flexible arms and elastic base is discussed. Using singular perturbation theory of two kinds of time scale, a slow-subsystem describing the rigid motion and a fast-subsystem corresponding to vibration of base elastic and two flexible arms are obtained. For the slow-subsystem, neural network is used to approximate the two uncertainties in the dynamic model of the space robot. The integral term of proportional integral (PI) sliding mode control makes initial state of the system falls on the sliding surface, which can offset the unknown disturbance at the beginning of control and eliminate the chattering of controller output, so the robustness of the control law is enhanced. The PI sliding mode neural network controller is designed to improve the tracking performance of the rigid point-to-point movement. For the fast-subsystem, an optimal linear quadratic regulator(LQR) controller is adopted to damps out the vibration of the two flexible links and base elastic, which guarantees the stability and tracking accuracy of the system. Finally, computer simulation results show the effectiveness of the compound control method.

Motion planning of a space robot

The plane problem of motion control of a free-floating space robot is investigated. The robot is supposed to be in a state of passive flight. It is assumed that the vectors of momentum and kinetic moment of the robot equal to zero. The movement of the arm relative to the robot body changes the position of the center of masses of the body and leads to its rotation around the center of masses. There are constraints on the range of variation of the manipulator length and the angle of its rotation relative to the body. It is shown that while carrying out special movements of the manipulator, it is possible to ensure the movement of robot gripper from arbitrary initial to an arbitrary final position, if they are located inside the workspace, which is a ring with a centre in the centre of masses of the robot. Moreover, it is possible to get the desired value of the angle between the manipulator and the body in the final position.

Adaptive robust control for free-floating space robot with unknown uncertainty based on neural network

It is difficult to obtain a precise mathematical model of free-floating space robot for the uncertain factors, such as current measurement technology and external disturbance. Hence, a suitable solution would be an adaptive robust control method based on neural network is proposed for free-floating space robot. The dynamic model of free-floating space robot is established; a computed torque controller based on exact model is designed, and the controller can guarantee the stability of the system. However, in practice, the mathematical model of the system cannot be accurately obtained. Therefore, a neural network controller is proposed to approximate the unknown model in the system, so that the controller avoids dependence on mathematical models. The adaptive learning laws of weights are designed to realize online real-time adjustment. The adaptive robust controller is designed to suppress the external disturbance and compensate the approximation error and improve the robustness and control precision of the system. The stability of closed-loop system is proved based on Lyapunov theory. Simulations tests verify the effectiveness of the proposed control method and are of great significance to free-floating space robot.

Coupling minimization with obstacles avoidance of free-floating space robots based on hybrid map in configuration space

To avoid the saturation of momentum wheels and the harm due to the thruster plume during the on-orbital manipulation, space robots usually stay in a free-floating state which follows the linear and angular momentum conservation leading to a kinematic coupling effect of the satellite base and the space manipulator. Emphasizing the stability of satellite base and execution safety, it is significant to minimize the kinematic coupling effect as well as avoid obstacles in the environment. Nevertheless, coupling minimization and obstacles avoidance are considered separately in previous work. By applying a hybrid map in the Configuration space, this article proposes a unified method dealing with the above two problems together. First, coupling factors are defined to evaluate the kinematic coupled effect which can be described by a coupling map; second, an obstruction map is generated by transforming obstacles in the Cartesian space to the Configuration space; the proposed hybrid map is finally generated from an overlay of a coupling map and an obstruction map. Numerical simulations verify the effectiveness of the method on a two degree-of-freedom planar space robot.

Real-time trajectory planning for free-floating space robot in close range using continuous thrust

This paper investigates the trajectory planning problem of free-floating space robot in proximity with continuous thrust. Sampling-based motion planning algorithm is viewed as promising method in such high-dimensional, complex scenario. A novel, fast trajectory planning algorithm AGB-RRT* is proposed using linearized orbital dynamics, which is capable of preventing collisions efficiently. Several constraints are considered, such as obstacle avoidance, plume impingement and control feasibility. Two extensions of AGB-RRT* algorithm are made: adaptive sampling strategy and LQR steering method. Finally, the performance of proposed algorithm are tested in 2D planar case and 3D case. Simulation results indicate the real-time feasibility of the proposed AGB-RRT* algorithm and the collision-free trajectories are displayed.

Vibration suppression control of free-floating space robots with flexible appendages for autonomous target capturing

Some flexible appendages, such as solar panels, communication antenna and other large structures are mounted on the base of the space robot. The structure flexibilities will cause vibration during the operation of manipulators. Due to complicated dynamic coupling among manipulators, the rigid base and flexible appendages, it is very challenging to control the end-effector to track inertial trajectories, especially when the target states are constantly changing. This paper proposes a vibration suppression method for autonomous target capturing during the preimpact phase without controlling the base. Firstly, we derive the rigid-flexible coupling dynamics of a space robot system with flexible appendages. Then, the relationship among joint rates, elastic motion and the end-effector velocities is established by using the linear momentum and angular momentum conservation equations. Secondly, a closed-loop control system is designed based on the dynamic coupling model. And the control system is composed of target motion prediction, autonomous trajectory planning, energy-based joint controllers and so on. Thirdly, the energy-based joint control is detailed, which is proved to be stable by Lyapunov direct method. Finally, simulations of a planar space robot with two flexible appendages and a 3D space robot with single flexible appendage are provided to verify the effectiveness of the presented approach. The effectiveness of energy-based joint control for vibration suppression is verified by a single-degree-of-freedom space robot experimental system. The simulation and experimental results show that the space manipulator can successfully capture the moving target while suppressing the structure vibration.

Minimum Base Attitude Disturbance Planning for a Space Robot During Target Capture

This paper presents a method to minimize the base attitude disturbance of a space robot during target capture. First, a general dynamic model of a free-floating space robot capturing a target is established using spatial operator Algebra, and a simple analytical formula for the base angular velocity change during the impact phase is obtained. Compared with the former models proposed in the literature, this model has a simpler form, a wider range of applications, and O(n) computation complexity. Second, based on the orthogonal projection matrix lemma, we propose the generalized mass Jacobian matrix (GMJM) and find that the base angular velocity change is a constant multiple of the component which the impact impulse projects to the column space of the GMJM. Third, a new concept, the base attitude disturbance ellipsoid (BADE), is proposed to express the relationship between the base attitude disturbance and the impact direction. The impact direction satisfying the minimum base attitude disturbance can be straightforwardly obtained from the BADE. In particular, for a planar space robot, we draw the useful conclusion that the impact direction unchanged base attitude must exist. Furthermore, the average axial length of the BADE is used as a measurement to illustrate the average base attitude disturbance under impact impulses from different directions. With this measurement, the desired pre-impact configuration with minimum average base attitude disturbance can be easily determined. The validity and the efficiency of this method are verified using a three-link planar space robot and a 7DOF space robot.

GA Based Adaptive Singularity-Robust Path Planning of Space Robot for On-Orbit Detection

As a new on-orbit detection platform, the space robot could ensure stable and reliable operation of spacecraft in complex space environments. The tracking accuracy of the space manipulator end-effector is crucial to the detection precision. In this paper, the Cartesian path planning method of velocity level inverse kinematics based on generalized Jacobian matrix (GJM) is proposed. The GJM will come across singularity issue in path planning, which leads to the infinite or incalculable joint velocity. To solve this issue, firstly, the singular value decomposition (SVD) is used for exposition of the singularity avoidance principle of the damped least squares (DLS) method. After that, the DLS method is improved by introducing an adaptive damping factor which changes with the singularity. Finally, in order to improve the tracking accuracy of the singularity-robust algorithm, the objective function is established, and two adaptive parameters are optimized by genetic algorithm (GA). The simulation of a 6-DOF free-floating space robot is carried out, and the results show that, compared with DLS method, the proposed method could improve the tracking accuracy of space manipulator end-effector.

Optimal sample-data control for free-floating space robot with uncertain inertial parameters

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

Optimal reorientation of a free-floating space robot subject to initial state uncertainties

This paper focuses on the dynamics and optimal reorientation of a free-floating space robot system in the presence of initial state uncertainties. A control strategy combining optimal motion planning and feedback control is presented based on the dynamic model of the system. In the design of the optimal motion planning, Legendre pseudospectral method (LPM) is used to transform the optimal reorientation problem into a nonlinear programming problem. Then, sequential quadratic programming algorithm is employed to solve the nonlinear programming problem and off-line generate the optimal reference trajectory of the system. In the design of feedback control, the state equation is linearized around the reference trajectory obtained by LPM. The tracking control problem is converted into a two-point boundary value problem based on Pontryagin’s maximum principle. Then LPM is used to discretize the two-point boundary value problem and transform it into a set of linear algebraic equations. This process does not require any integration calculations and has good performance in real time. Numerical simulations indicate that the control strategy is effective with good robustness.

Optimal trajectory planning of free-floating space manipulator using differential evolution algorithm

The existence of the path dependent dynamic singularities limits the volume of available workspace of free-floating space robot and induces enormous joint velocities when such singularities are met. In order to overcome this demerit, this paper presents an optimal joint trajectory planning method using forward kinematics equations of free-floating space robot, while joint motion laws are delineated with application of the concept of reaction null-space. Bézier curve, in conjunction with the null-space column vectors, are applied to describe the joint trajectories. Considering the forward kinematics equations of the free-floating space robot, the trajectory planning issue is consequently transferred to an optimization issue while the control points to construct the Bézier curve are the design variables. A constrained differential evolution (DE) scheme with premature handling strategy is implemented to find the optimal solution of the design variables while specific objectives and imposed constraints are satisfied. Differ from traditional methods, we synthesize null-space and specialized curve to provide a novel viewpoint for trajectory planning of free-floating space robot. Simulation results are presented for trajectory planning of 7 degree-of-freedom (DOF) kinematically redundant manipulator mounted on a free-floating spacecraft and demonstrate the feasibility and effectiveness of the proposed method.

Bionic Attitude Transformation Combined with Closed Motion for a Free Floating Space Robot

In order to realize the small error attitude transformation of a free floating space robot, a new method of three degrees of freedom(DOF) attitude transformation was proposed for the space robot using a bionic joint. A general kinematic model of the space robot was established based on the law of linear and angular momentum conservation. A combinational joint model was established combined with bionic joint and closed motion. The attitude transformation of planar, two DOF and three DOF is analyzed and simulated by the model, and it is verified that the feasibility of attitude transformation in three DOF space. Finally, the specific scheme of disturbance elimination in attitude transformation is presented and simulation results are obtained. Therefore, the range of application field of the bionic joint model has been expanded.

On the Workspace of a Free-Floating Space Robot

The planar problem of controlling the motion of a free-floating space robot is investigated. The robot consists of a body and a telescopic manipulator arm. The motion of the manipulator arm changes the body’s center of mass and causes the body to rotate. It is assumed that the length of the manipulator arm and the angle of its turn relative to the body are restricted. It is shown that the workspace of such a robot is significantly larger than the workspace of a robot with a fixed body. Due to the special motions of the manipulator arm, the robot’s body can be turned and the gripper can be moved from an arbitrary position to another arbitrary position if they are within the workspace, which is a ring centered at the robot’s center of mass. In addition, the prescribed angle between the manipulator arm and the body in the terminal position can be ensured.

Design of a Tracking Controller for Object Interception in Space

The paper presents a model-based tracking controller design for a freefloating space robot for a mission scenario of intercepting an object. Such missions are of interest due to a growing number of objects needed to be removed from space. The free-floating mode requires spacecraft thrusters to be off and linear and angular momentum are conserved then. Momentum conservation generates holonomic and nonholonomic constraint equations, respectively. The free-floating mode implicates underactuation, so the robot becomes multi-constrained. Many control algorithms are designed for underactuated robots but they are specific mission and robot dependent. Motivations for the presented research come from the growing space exploration, which results in more space debris and requires sophisticated removal services. Service tasks and debris removal need to be performed by specialized robots. The debris interception scenario presented in the paper consists of estimation of target properties, a controller design to track and intercept the debris, and move it to the graveyard orbit. Simulation results of the theoretical control development for the robot intercepting a non-tumbling object are provided.

Adaptive reactionless control strategy via the PSO-ELM algorithm for free-floating space robots during manipulation of unknown objects

The operation of free-floating space robots causes significant challenges to the attitude control of a spacecraft body, resulting from the strong dynamic coupling existing in the system as well as the appending unknown property from a captured object. Conventional reactionless motion planning and control methods require the precise knowledge of system dynamics with ideal path tracking performance, which are impossible to achieve in reality. To solve this problem, an adaptive reactionless path planning and control integrated strategy is proposed for free-floating space robots while manipulating unknown objects. For details, a SW-RLS (Slide-windowed Recursive Least Square) algorithm is employed to construct the adaptive reactionless path planning by identifying and compensating the unknown property in the form of a time-variable matrix for the momentum equation of the system. Simultaneously, a robust adaptive control strategy via the PSO-ELM (Particle Swarm Optimization-Extreme Learning Machine) algorithm is proposed to track the dynamic change in the planned path, based on the foundation of a designed PD controller for reducing the error. With computational efficiency, an adaptive controller via PSO-ELM aims at learning and compensating for the unknown property intelligently; then, a robust controller is used to reject the model uncertainties. The notable feature of the proposed strategy is that it requires neither an accurate system model nor any information about the unknown property. Most importantly, the design can dynamically realize reactionless path tracking performance. Finally, the simulations of a free-floating space robot capturing an unknown object and comparisons with existing methods are presented to demonstrate the effectiveness of the proposed scheme.

Hybrid task priority-based motion control of a redundant free-floating space robot

This paper presents a novel hybrid task priority-based motion planning algorithm of a space robot. The satellite attitude control task is defined as the primary task, while the least-squares-based non-strict task priority solution of the end-effector plus the multi-constraint task is viewed as the secondary task. Furthermore, a null-space task compensation strategy in the joint space is proposed to derive the combination of non-strict and strict task-priority motion planning, and this novel combination is termed hybrid task priority control. Thus, the secondary task is implemented in the primary task’s null-space. Besides, the transition of the state of multiple constraints between activeness and inactiveness will only influence the end-effector task without any effect on the primary task. A set of numerical experiments made in a real-time simulation system under Linux/RTAI shows the validity and feasibility of the proposed methodology.

Coordinated motion control of a dual-arm space robot with joint-limit avoidance and uncertain inertial parameters

In this article, a new adaptive coordinated motion control approach is introduced for a dual-arm free-floating space robot. This adaptive algorithm is used for the post-capture of a large noncooperative target with joint-limit avoidance and uncertain dynamic parameters. To overcome the problem of dynamics coupling between the space base, its manipulators, and the target, we develop a dual-arm space robotic system. One arm is used to complete the capture task and the other is used to counteract the disturbance to the space base. In this case, a new coordinated motion control law is derived based on reaction null space control. An improved joint-limit avoidance algorithm is implemented for large noncooperative target capture; otherwise, a significant base disturbance may result if the joint-limit constraints are not explicitly considered. Based on momentum conservation, the linear regression form of the estimation problem is obtained, and we further identify the unknown inertial parameters of the target. Finally, the simulation results demonstrate the effectiveness of the proposed algorithm.

Rapid satellite capture by a space robot based on delay compensation

PurposeRapid satellite capture by a free-floating space robot is a challenge problem because of no-fixed base and time-delay issues. This paper aims to present a modified target capturing control scheme for improving the control performance.Design/methodology/approachFor handling such control problem including time delay, the modified scheme is achieved by adding a delay calibration algorithm into the visual servoing loop. To identify end-effector motions in real time, a motion predictor is developed by partly linearizing the space robot kinematics equation. By this approach, only ground-fixed robot kinematics are involved in the predicting computation excluding the complex space robot kinematics calculations. With the newly developed predictor, a delay compensator is designed to take error control into account. For determining the compensation parameters, the asymptotic stability condition of the proposed compensation algorithm is also presented.FindingsThe proposed method is conducted by a credible three-dimensional ground experimental system, and the experimental results illustrate the effectiveness of the proposed method.Practical implicationsBecause the delayed camera signals are compensated with only ground-fixed robot kinematics, this proposed satellite capturing scheme is particularly suitable for commercial on-orbit services with cheaper on-board computers.Originality/valueThis paper is original as an attempt trying to compensate the time delay by taking both space robot motion predictions and compensation error control into consideration and is valuable for rapid and accurate satellite capture tasks.