Zhengxiong Liu

Impact Dynamic Modeling and Adaptive Target Capturing Control for Tethered Space Robots With Uncertainties

Target capturing is an essential and key mission for tethered space robot (TSR) in future on-orbit servicing, and it is quite meaningful to investigate the stabilization method for TSR during capture impact with target. In this paper, the stabilization control of TSR during target capturing is studied. The space tether is described by the lumped mass model, and the impact dynamic model for target capturing is derived using the Lagrange method with the consideration of space tether length, in/out-plane angles, and gripper attitude. Given the structure of the TSRs gripper, a position-based impedance control method is proposed for target capturing operation. The neural network is used to estimate and compensate the uncertainties in the dynamic model of TSR, and an adaptive robust controller is designed to overcome the influence of the space tether and track the desired position generated by impedance controller. Numerical simulations suggest that the proposed controller can realize the stabilization of TSR during target capturing; besides, the uncertainties of the TSR can effectively be compensated via adaptive law and the influence of the space tether can be suppressed via the robust control strategy, which lead to smaller overshoot, less convergence time, and higher control accuracy during capturing operation.

Reconfigurable spacecraft attitude takeover control in post-capture of target by space manipulators

Abstract Most current research on reconfigurable control system puts emphasis on reconfiguration for adapting to actuator failures. However, the reconfigurable control system is necessitated for spacecraft attitude takeover control in the application of capturing target spacecraft whose fuel is exhausted to extend its operational lifetime by supplying them propulsion, navigation and guidance services. In this scenario, the capture of target spacecraft by space manipulators will cause a large shift in the dynamics of the service spacecraft. Not only do the mass properties change, but also does the thruster configuration. The changes in the mass, center of mass and inertia of the combined spacecraft will cause changes in the equivalent force exerted by each thruster. In this paper, considering the changes of thruster configuration and the control reallocation, a reconfigurable control system is designed for spacecraft attitude takeover control in post-capture of target by space manipulators in order to adapt to changes in the mass properties. The unknown inertia properties of target spacecraft in the system constitute a formidable technical challenge for controller design. Therefore, a modified adaptive dynamic inverse controller is proposed to provide global asymptotic stability in the presence of model uncertainties and nonlinearities. Moreover, by the null-space intersections control reallocation method, the thrust forces of service spacecraft can be redistributed and satisfy some constraints. Numerical simulations validate the feasibility of reconfigurable spacecraft attitude takeover control with large center of mass shifts and unknown inertia properties.

Post-capture attitude control for a tethered space robot–target combination system

This paper presents a novel scheme for achieving attitude control of a tumbling combination system in the post-capture phase of a tethered space robot (TSR). Given the combination rotation characteristics, tether force is applied to provide greater control torques for stabilising the attitude. The proposed control scheme involves two attitude controllers, which coordinate the controller of the tether force and thruster force and the controller of single thruster force. The numerical simulations include a comparison between this coordinated control and the traditional thruster control and a sensitivity analysis on initial values of parameters. Simulation results validate the feasibility of the attitude control scheme for a tumbling combination system, and fuel consumption of the attitude control is efficiently reduced using the coordinated control strategies.

Dynamics Analysis and Controller Design for Maneuverable Tethered Space Net Robot

Space robots are considered as a promising solution to active space-debris capture and removal. In this paper, a brand new space robot system called the maneuverable tethered space net robot is proposed. In addition to the advantages inherited from the tethered space net, extra maneuverability in the tethered space net robot allows for wider possibilities for debris capture. The motion equations of the system are derived, and both symmetrical and asymmetrical configurations are analyzed. According to the specific vibration analysis, a modified adaptive supertwisting sliding-mode control scheme is proposed. The proposed adaption law is a function of the disturbance, which is considered as the sum of all the adjacent forces working on the controller plant: that is, the maneuverable unit. Both symmetrical and asymmetrical cases are simulated to verify that the tethered space net robot can fly toward active space debris steadily under the proposed control scheme.

Predictive Approach for Sensorless Bimanual Teleoperation Under Random Time Delays With Adaptive Fuzzy Control

Robot teleoperation techniques are applied in industrial automation, space, and surgery. Considering the high cost of force–torque (F/T) sensors, an “indirect estimation” of the external force is preferable to “direct sensing” of the force. This study provides a predictive control method for a bimanual teleoperation system without F/T and acceleration sensors. Two types of mirror predictors are built into the control architecture. One of them is a position predictor estimating the position states of the remote side under stochastic network-induced delays. The force predictor estimates the contact forces and dynamic uncertainties utilizing the local time-delayed information. By accurately estimating the local force, the position predictors/observers are used to estimate the internal dynamics of the master and slave simultaneously, and thereby, avoid using the delayed transmitted information. An adaptive fuzzy control strategy based on linear matrix inequalities (LMIs) is proposed to evaluate and suppress the uncertainties, thereby, ensuring that the synchronization errors of the system positions converge to zero and the estimated force approaches real values. The system stability of the closed-loop system is proven using LMIs based on a Lyapunov–Krasovskii functional synthesis. An experimental test involving a dual-arm robot, YuMi, holding and moving a yoga ball is performed based on a semiphysical platform.

Dynamics modeling and model selection of space debris removal via the Tethered Space Robot

This work proposes a scheme to select a proper dynamics model for space debris removal which is captured by a Tethered Space Robot. A proper dynamics model is crucial for the parameters estimation and controller design in a Tethered Space Robot mission, in particular, for the retrieval or de-orbiting of uncooperative target. A new dynamics model of the system is derived by treating the base satellite and the space debris as rigid bodies in the presence of offsets, and with the effect of the tether’s flexibility and elasticity. Then the equations of motion are simplified based on the attitude analysis and numerical simulations in different cases. It is concluded that in the Tethered Space Robot’s mission, the strong coupled attitude motions among the base, target satellites, and the tether cannot be ignored during the retrieval, which is totally different from the traditional tethered satellite system. The attitude motions of the system in different conditions are discussed respectively, and a method of the model selection of the system during post-capture and retrieval/de-orbiting phase is proposed, which is a balance of the accuracy and facility. Finally, a control scheme is used to prove this conclusion.

Autonomous Rendezvous and Docking with Nonfull Field of View for Tethered Space Robot

In the ultra-close approaching phase of tethered space robot, a highly stable self-attitude control is essential. However, due to the field of view limitation of cameras, typical point features are difficult to extract, where commonly adopted position-based visual servoing cannot be valid anymore. To provide robot’s relative position and attitude with the target, we propose a monocular visual servoing control method using only the edge lines of satellite brackets. Firstly, real time detection of edge lines is achieved based on image gradient and region growing. Then, we build an edge line based model to estimate the relative position and attitude between the robot and the target. Finally, we design a visual servoing controller combined with PD controller. Experimental results demonstrate that our algorithm can extract edge lines stably and adjust the robot’s attitude to satisfy the grasping requirements.

Recursive Differential Evolution Algorithm for Inertia Parameter Identification of Space Manipulator

This paper proposes a recursive differential evolution (RDE) algorithm to identify the inertial parameters of an unknown target and simultaneously revise the friction parameters of space manipulator joints. The inertia parameters of a space manipulator, which govern the dynamic behaviours of the entire system to a significant extent, can change for many reasons during the process of on-orbit operations; consequently, it is essential to trace these changes within the control system to ensure the stability and accuracy of the entire system. RDE is inspired by a recursive least squares algorithm, using approximate gradient information to guide the mutation operation in the standard DE. A series of contrast simulations are employed to confirm the feasibility of the RDE algorithm. The simulation results show that the identification of the RDE algorithm is more precise than for a GA (genetic algorithm) and LS (least square) algorithm, and has an appropriate convergence rate. The RDE identification method is suitable for linear, nonlinear and combined systems, and can follow system dynamics exactly.

Augmented reality for enhancing tele-robotic system with force feedback

Abstract In the teleoperation, the force feedback is indispensable, which can enhance the sense of presence of the operator and help the operator accomplish tasks comfortably. The time delay is one of the main challenges that influence the stability of the teleoperation systems, which leads to the discontinuous operation. Thus building a local virtual model in the master side is an effective way to solve this problem. In this paper, a new method is presented to reconstruct the virtual model of the remote object. The virtual model can estimate the real-time force feedback to the operator and eliminate the effects of the time delay. Then the tele-robotic system based on augmented reality technology is set up in our laboratory. In the tele-robotic system, the dynamic parameters including damping and stiffness of the virtual model are constantly updated by utilizing the positions and forces information from sensors of the remote robot site. Finally, the effectiveness of the proposed method and the correctness of the visual model parameters are verified by two experiments.

Cellular space robot and its interactive model identification for spacecraft takeover control

Facing the new challenges of the spacecraft developing, the concept of cellular space robot (CSR) for both space-craft system construction and on-orbit operation is presented in this paper. The system description and design principles are introduced to ensure the flexibility of the system. And dynamics model for takeover control is developed. After that, the regression models for the parameter identification are deduced based on the dynamics model. An interaction model identification algorithm is presented to solve the parameter identification problem for the distributed cells. Besides, the interactive model identification is validated by simulations. The simulations show that the interactive model identification method can achieve the consensus and convergence.