Reactionless Control Research Articles

Reactionless control of free‐floating space manipulators with parameter uncertainty and input disturbance

AbstractThis paper studies the control problem of free‐floating space manipulators, and a control scheme is proposed to solve reactionless control of the end‐effector pose tracking with parameter uncertainty and input disturbance. First, based on dual modeling to treat the end‐effector as a virtual base spacecraft, the dynamics with uncertainties are established which map the joints' torque to the end‐effector pose and base spacecraft attitude, while the inverse kinematics and the derivative of the generalized Jacobi matrix can be avoided in controller design. Then, the reference acceleration stabilization schemes satisfying prescribed performance constraints are carefully designed for tracking errors, and based on these schemes the steady‐state and transient performance of the tracking control can be guaranteed. Further, the radial basis function neural network is adopted to estimate modeling errors caused by parameter uncertainty and input disturbance. In addition, a concurrent learning method is introduced in the network weights matrix update law, which allows the estimation errors to converge a neighborhood of zeros without the need for satisfying the persistent excitation condition. The simulation results verify the effectiveness of the proposed control scheme.

Adaptive reactionless control of free-floating space manipulators for detumbling a target with dynamic uncertainties

Detumbling targets in the post-capturing phase is significant for an on-orbit capturing mission. However, the momenta of targets usually cause disturbances to the attitude of a free-floating base during the detumbling process. This paper proposes an adaptive controller for free-floating space manipulators detumbling targets with dynamic uncertainties in the post-capturing phase. The key feature of the controller, termed as adaptive reactionless detumbling controller (ARDC), is that it can produce null disturbance to base attitude. First, a reaction null-space controller is established based on momentum conservation, by which the null disturbance can be achieved at the displacement level. Second, considering the target’s dynamic uncertainties, an adaptive dynamic compensation is included. Furthermore, the reaction null-space vector in the controller is determined by an optimization model with the aim of detumbling target. A reference guiding vector is defined in the optimization model to ensure a steady detumbling process. Finally, the stability of the proposed controller is verified using the Lyapunov theory. The effectiveness of our proposed controller is validated through simulations, wherein the effect of the initial velocity and dynamic uncertainties of the target and the parameters of the controller are also studied.

Impact modeling and reactionless control for post-capturing and maneuvering of orbiting objects using a multi-arm space robot

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.

Reactionless Control of Free-Floating Space Manipulators

This paper presents a new dynamic formulation as well as control scheme for space manipulators in order to drive the end-effector along a desired trajectory while minimizing the base disturbances caused by the arm movements. Through the new dynamic formulation, the end-effector is viewed as a virtual base, and the end-effector variables are also considered as generalized coordinates. As a result, joint controllers can be designed without having to solve for the inverse kinematics problem and computing the derivative of the generalized Jacobian matrix. Consequently, the joint control torque can be obtained analytically through the Lagrange multipliers method. Further, the joint control torque is also obtained through a quadratic programming problem in order to take into account the joint torque constraints. Several case studies are simulated to demonstrate the new control scheme and compare its performance with that of other controllers.

Parameters concurrent learning and reactionless control in post-capture of unknown targets by space manipulators

This paper studies parameters identification and minimizing base disturbances problems after the space manipulator capturing an unknown target. A concurrent learning algorithm that concurrently uses past motion data points and instantaneous motion data of the system is proposed for the parameters identification. Given a condition for selecting the used past data points as well as a scaling technique to make the parameters have the same magnitude, the concurrent learning algorithm guarantees that parameters identification errors can globally converge to zero at an exponential rate and without the need for satisfying the persistent excitation (PE) condition. An adaptive reactionless control method is proposed based on the passivity theorem and Task-priority method, which ensures that the base attitude is stationary and joint motions satisfy the limits during the system generating excitation motions for the parameters identification. Simulation results verify the effectiveness of the proposed method.

Attitude reactionless and vibration control in space flexible robot grasping operation

When a space flexible robot captures a floating target, the manipulator vibration and the impact reacting onto the base attitude are the key issues which may cause critical safety problems. In this investigation, the attitude reactionless control and the vibration suppression are considered in the meantime to reduce the risk in grasping operation. The dynamic models of the space flexible robot and the target are established, and then the Dynamic Grasping Area is introduced to describe the contact procedure of the capture device grasping target. Even though the flexible vibration of the manipulator can be easily suppressed by using the conventional joint active damping control, the reaction impact to the base attitude is serious and unacceptable. The simulation results indicate that, by using the optimal control for the vibration suppression in the attitude reaction null space, the vibration of the manipulator could be alleviated significantly and the base attitude is also almost undisturbed in the meantime during the whole grasping procedure.

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.

Reaction torque control of redundant free-floating space robot

This paper presents the reaction torque based satellite base reactionless control or base disturbance minimization of a redundant free-floating space robot. This subject is of vital importance in the study of the free-floating space robot because the base disturbance minimization will result in less energy consumption and prolonged control application. The analytical formulation of the reaction torque is derived in this article, and the reaction torque control can achieve reactionless control and satellite base disturbance minimization. Furthermore, we derive the reaction torque based control of the space robot for base disturbance minimization from both the non-strict task priority and strict task priority control strategy. The dynamics singularity in the proposed algorithm is avoided in this paper. Besides, a real time simulation system of the space robot under Linux/real time application interface (RTAI) is developed to verify and test the feasibility and reliability of the method. The experimental results demonstrate the feasibility of online reaction torque control of the redundant free-floating space robot.

Reactionless robust finite-time control for manipulation of passive objects by free-floating space robots

On-orbit servicing requires efficient techniques for manipulating passive objects. The paper aims at developing a reactionless control method that drives the manipulator to manipulate passive objects with high precision, while inducing no disturbances to its base attitude. To this end, decomposition of the target dynamics from the base dynamics is discussed, so that they can be considered as two independent subsystems. A reactionless nonlinear controller is presented, which ensures high-precision manipulation of the targets and that the base orientation is unchanged. This is achieved by combining the robust finite-time control with the reaction null space. Finally, the performance of the proposed method is examined by comparing it with that of a reactionless PD controller and a pure finite-time controller.

Adaptive Reactionless Motion and Parameter Identification in Postcapture of Space Debris

This paper presents a new control scheme for the problem of a space manipulator after capturing an unknown target, such as space debris. The changes in the dynamics parameters of the system, as a result of capturing an unknown target, must be accommodated because they may lead to poor performance of the trajectory control and attitude stabilization system. To address this issue in the postcapture scenario, the adaptive reactionless control algorithm to produce the arm motions with minimum disturbance to the base is proposed in this study. In addition, the online momentum-based estimation method is developed for inertia-parameter identification after the space manipulator grasps an unknown tumbling target with unknown angular momentum. This control scheme is intended for use in the transition phase from the instant of capture until the unknown parameters are identified and/or the available stabilization methods can be applied properly. To verify the validity and feasibility of the proposed concept, MSC.Adams simulation platform is employed to implement a planar base–manipulator–target model and the three-dimensional model of the Engineering Test Satellite VII system. The numerical results show that the space manipulator is able to perform reactionless motion while the inertial parameters converge to their real values.