IEEE Robotics & Automation Magazine - December 2018 - 95

without any aliasing. A slow sampling rate would cause
AVSC to produce a gain value that is not in tune with how
the error states evolve.
Conclusions
This article proposed coordinated control methods for a
dual-arm, free-flying space robot. Both arms of the space
robot perform as mission arms to execute space tasks. The
spacecraft base attitude is actively controlled while its
translation is left free to save energy. In consideration of
external disturbances and unknown targets merged as system uncertainties, two types of robust controllers were
developed. System performance under these controllers
was investigated in three cases: path tracking, set-point
regulation, and inaccurate system model. The AVSC controller was demonstrated to achieve higher tracking accuracy in all scenarios. Furthermore, the AVSC controller
saves a large amount of energy compared to SQC2S. This
is a critical advantage because energy is limited on board
spacecraft and is quite valuable. The SQC2S controller
cannot deal with large-value system uncertainties (e.g., the
tracking error of the end-effector position becomes as
large as 6 mm when the system uncertainty is 30% of its
nominal value), whereas AVSC can restrict the position
tracking error within 1 mm. Therefore, AVSC can realize
fast and high-accuracy tracking operation of space robots
in space missions.
It is worth mentioning that we have assumed a stationary target in the article; in a realistic on-orbit servicing mission, the target is often nonstationary. However, on the
basis of the designed trajectories for approaching a nonstationary target, the proposed controllers can also be used. In
addition, the controllers can be easily extended to other
floating-base systems, e.g., an unmanned aerial vehicle or
underwater robotic system. Future work will focus on
force/torque control of a space robot capturing a target and
dynamics and control of a space robot with flexible links
and joints.
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Lingling Shi, School of Mechanical Engineering, Beijing Institute of
Technology, China. E-mail: l.shi@bit.edu.cn.
Hiranya Jayakoby, School of Mechanical and Manufacturing
Engineering, University of New South Wales Sydney, Australia.
E-mail: hiranya.jayakody@unsw.edu.au.
Jayantha Katupitiya, School of Mechanical and Manufacturing
Engineering, University of New South Wales Sydney, Australia.
E-mail: j.katupitiya@unsw.edu.au.
Xin Jin, School of Mechanical Engineering, Beijing Institute of
Technology, China. E-mail: goldking@bit.edu.cn.

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