###### Abstract  
This study investigates the visual servo control of the space station macro/micro manipulator system. The proposed approach is based on the position-based eye-in-hand visual servo (PBVS) and takes advantage of the hardware sensors to overcome the macro manipulator's base flexibility and joint backlash. First, a vibration suppression approach based on the reaction force feedback control is proposed, the deflection forces are measured by the six-axis force/torque sensor at the base of the micro-manipulator, and damping is injected into the flexible base in the closed-loop control to suppress the base vibration. Second, the small changes of joint backlash are compensated based on the macro manipulator joint angles sensor and converted to the desired motion of the payloads. Finally, PBVS with the lag correction is proposed, which is adequate for the precise positioning of large payloads with significant low-frequency oscillations. Ground micro-gravity experiment implementation is discussed, simulations and experiments are carried out based on the equivalent 3-DOF flexible base manipulator system and the macro/micro manipulator ground facilities, and results demonstrate the effectiveness of the proposed control algorithm. macro-micro manipulator; flexible base manipulator; visual servo; force/torque sensor; base feedback; backlash +
Footnote †: journal: Medical Imaging and Vision  
The tip position sensing cannot be obtained from the forward kinematics due to the imperceptible nature of flexural link displacements. One feasible solution to improve the positioning accuracy is the usage of a camera mounted at the EEF of the manipulator, which is a kind of eye-in-hand (end-point closed loop) servo system [7]. Visual servo (VS) is classified into a position-based visual servo (PBVS) and image-based visual servo (IBVS) [8]. In IBVS, the visual information is direct feedback for control to minimize the image plane error by extracting image features; in PBVS, the relative pos between the end-effector and the target is estimated, which can be controlled in the unified architecture in Cartesian space. PBVS is designed in our system, and markers are configured on the grapple fixtures outside the space station [9], the obtained full pose information (6D) helps to assist the maneuverring of the operators. The basic PBVS control structures have been discussed by [PERSON] in [10; 11]. For better load capacities and high precision of the EEF, the flexible states sensing and vibration control are introduced, making the control law much more complex. Various strategies have been developed to cope with a flexible base. The trajectory planning based on the input shaping [12; 13] is feedforward control, which is insufficient for the system with an initial excitation. To overcome this problem, the feedback controller combined with input shaping is proposed by [PERSON] in [14]. However, it depends on the joint space tracking and is not suitable for eye-in-hand VS. The passive analysis with large payload [15; 16] has been addressed and PD+ control is applied with the desired rigid system feedforward and tracking errors feedback. Similar PID control with a feedforward term is applied in the precise positioning of payloads by flexible space robot systems [17], however, it is an eye-to-hand IBVS. Most commonly, the vibration damping term introduced by the micro manipulator is a feedback. [PERSON] studied many different ways to damp the vibration, such as active damping control in [18] and inertial vibration damping control in [19], which is usually combined with the two-time scale control in [20]. It is assumed that the tracking control and base vibrations are separated in the frequency domain, and the dynamic model is decoupled into two-time scale based on the singular perturbation theory. This concept has been widely applied in the IBVS control [21; 22; 23; 24; 25; 26], where the slow control torque vanishes the image feature error and fast control torque sufficiently dampen the vibrations of the elastic mechanical structure.