## 1 Introduction
[26] considered identifying the mass properties and eliminating the unknown angular momentum of space robotic systems after capturing a non-cooperative target, designing an integrated control framework which includes a detumbling strategy, coordination control and parameter identification, and proposed a coordination control scheme for stabilizing both the base and end-effector based on impedance control implemented considering the target's parameter uncertainty. [PERSON] et al. [27] proposed a modified adaptive sliding mode control algorithm to reduce the momentum, which can reduce the unknown angular momentum of a target, and uses a new signum function and time-delay estimation to assure fast convergence and achieve good performance with a small chattering effect. [PERSON] et al. [28] developed a generic frictional contact model which can represent the contact forces between the robot's end-effector and the target object and designed a resolved motion admittance control method based on the frictional contact model. [PERSON] [29] developed a planning and control methodology for manipulating passive objects by cooperating orbital free-flying servicers in zero gravity. Although the above control algorithms focus on the dynamics and control of space robots capturing a spacecraft, the protection of the joint actuators of the space robot under the impact torque is not considered. Since a space robot's joints are easily destroyed by the impact torque during the process of space robot on-orbit capturing a non-cooperative spacecraft, therefore, the studies on compliance control of space robots during the capturing process need to be improved. For the series elastic actuator (SEA) in the ground robot, [PERSON] et al. [30] presented a modularized series elastic actuator aimed to improve the compliance of the robotic arm. [PERSON] and [PERSON] [31] refined and improved the stability analysis of the environment-adaptive force control of SEAs. [PERSON] et al. [32] presented a practical control approach for series elastic actuators which can work well even in the presence of unknown payload parameters and external disturbances. Considering that SEA devices play a key role in protecting the robot's joints from impact damage when the ground robot collides with the outside environment, therefore, this paper designs a rotary series elastic actuator (RSEA) device suitable for space robots, and at the same time, designs an active controller strategy which can timely control the opening and closing of joint actuators to achieve buffer compliance control. The RSEA also leads to joint flexibility due to the presence of a buffer spring inside the system. Since the system meets the law of conservation of linear momentum and law of conservation of angular momentum, its orbital dynamics and base attitude are coupled, which make its links' locomotion leads to the base's reactions, and consequently a variation of the end-effector position. At the same time, momentum, momentum and energy transfer change also exist in the pre-contact and post-capture phase of systems consisting of a space robot and spacecraft. In addition, due to the high velocity and rotation characteristics of the non-cooperative target spacecraft, the dynamic parameters of the post-capture hybrid system are difficult to obtain accurately. The above multiple complex situations make research on the dynamic modeling and control of the on-orbit capturing process of space robots equipped with RSEA devices very complicated. In an effort to address the various aforementioned drawbacks, this work investigates the dynamic modeling, buffer compliance control and vibration suppression of a space robot capturing a non-cooperative spacecraft. First of all, dynamic models of the space robot and the target spacecraft before capture are obtained by using the Lagrange approach and Newton-Euler method. Second, based on singular perturbation theory, the post-capture hybrid system was transformed into two subsystems, a slow rigid motion subsystem, and a fast flexible-joint subsystem.