Abstract:
With the rapid development of space technology, space robots are playing an important role in on-orbit services, such as refueling, debris removal, and malfunctioning satellite repair. These space robots typically consist of a base spacecraft and
n degrees_of_freedom (
n-DOF) manipulators. The manipulators could be equipped with different end-effectors to capture space objects and perform various operations. In the precapture phase, space robots are required to follow a preplanned trajectory to the designated capture points. However, the dynamic characteristics of space robots introduce challenges in path planning. First, dynamic singularity might occur when solving the inverse kinematics in task-space path planning. Unlike fixed-base manipulators, the motion of the base spacecraft could be influenced by the manipulator’s movements due to dynamic coupling. Thus, base disturbance minimization becomes a key consideration while planning the trajectory of the manipulator. In addition to dynamic factors, practical concerns such as obstacle avoidance as well as input and velocity constraints should be satisfied to ensure the success of the mission. Furthermore, for some complicated space missions involving dual-arm space robots, coordination between two arms should be considered. Once the desired trajectory is planned, a tracking control strategy is designed to drive the end-effector to the capture points. However, the high nonlinearity, multidimensionality, and strong coupling of the space robot system increase the difficulty of tracking control. During on-orbit capture, the relative motion between the space robot and the target limits the available operational time, requiring the end-effector to quickly reach the capture points along the desired trajectory. In addition, the controller design should guarantee both steady-state and transient performance, such as slight overshoot, small steady-state error and short transient time. Working in a complex space environment, space robots are inevitably subjected to unknown external disturbances and parametric uncertainties, which could negatively affect the control accuracy and require compensation. Consequently, the proposed control strategy should ensure overall system stability, fast convergence, and strong robustness against disturbances and uncertainties. Before deployment, comprehensive ground-based verifications are important to evaluate the performance of the system. The effectiveness of ground-based verifications in accurately reflecting real space motions depends on the simulations of the space environment, particularly the microgravity environment. To address these technological challenges, this paper provides an overview of recent advancements in path planning and tracking control, with a focus on the inherent dynamic characteristics of space robots and the complexities of the space environment. The application of intelligent methods in multiobjective optimization and disturbance rejection control is also introduced in light of the rapid development of artificial intelligence. Additionally, relevant ground-based verification technologies are discussed. Finally, this paper presents the existing limitations and potential future developments in the field of space robots.