Citation: Zhang, L.; Wang, S. Risk
Assessment Model-Guided
Configuration Optimization for
Free-Floating Space Robot
Performing Contact Task. Machines
2022, 10, 720. https://doi.org/
10.3390/machines10090720
Academic Editors: Xiaochun Cheng
and Daming Shi
Received: 30 June 2022
Accepted: 19 August 2022
Published: 23 August 2022
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Article
Risk Assessment Model-Guided Configuration Optimization
for Free-Floating Space Robot Performing Contact Task
Long Zhang * and Shuquan Wang
Key Laboratory of Space Utilization, Technology and Engineering Center for Space Utilization,
Chinese Academy of Sciences, Beijing 100094, China
* Correspondence: zhanglong@csu.ac.cn
Abstract:
The use of free-floating space robots for contact tasks is very promising in space exploration.
However, severe damage or obvious disturbance may occur if inappropriate operation is implemented.
In this paper, a novel risk assessment method is first proposed to give a clear description of the
risk state before contact happens and provide guidance for configuration optimization to reduce
risk on contact tasks. Firstly, the dynamics model of a free-floating space robot is given. On this
basis, two important risk assessment indicators, the maximum contact force and the base attitude
disturbance caused by contact, are derived. By integrating the risk assessment indicators, a novel
risk assessment model is proposed for a free-floating space robot performing a contact task. It is a
multidimensional and extensible risk assessment space which could give a clear description of the
risk state before contact happens. Thereafter, considering the results given by the risk assessment
model, the configuration optimization for a free-floating space robot is implemented based on the
design of optimization factor in null space. Finally, numerical simulation for a 7-degree-of-freedom
free-floating space robot performing a contact task is carried out, and the simulation results verify
the effectiveness of the proposed method.
Keywords:
free-floating space robot; risk assessment; contact task; configuration optimization;
maximum contact force; base attitude disturbance
1. Introduction
In the past decades, space robots have been playing an important role in space contact
tasks, such as on-orbit construction and assembly, target capture and asteroid sampling
[1–3]
.
During the contact process, there are two major problems, namely, the maximum contact
force and the base attitude disturbance caused by contact, that need to be focused on. If
the maximum contact force exceeds the limit that the space robot allows, the robot itself
or the target may be damaged. When the inertia of the base is not large enough, the
eccentric external force and torque will cause an obvious base attitude disturbance, which
will affect the communication with the ground and power supply for the space robot, and
compensating for the disturbance using the attitude control system will consume a large
amount of fuel, which is limited in space. Therefore, the minimization of maximum contact
force and the minimization of base attitude disturbance attract the attention of scholars.
On the basis of the assumption that the contact is an instantaneous phenomenon, and
the contact effect can be regarded as a contact—impact impulse applied on the end-effector,
the contact model is then established by the theorem of impulse [
4
–
7
]. This method is called
the discrete contact dynamics modeling method, which can realize the simple and efficient
contact dynamics analysis of space robots. However, it can only obtain the final changes of
the space robotic system caused by contact and cannot obtain the information during the
entire contact process, including the maximum contact force. Thereafter, the null-space op-
timization methods for redundant space robots or the intelligent optimization methods are
adopted to minimize the contact-impact impulse by adjusting capture configuration
[8–11]
.
Machines 2022, 10, 720. https://doi.org/10.3390/machines10090720 https://www.mdpi.com/journal/machines
