Citation: Xue, Y.; Sun, Z.; Liu, S.; Gao,
D.; Xu, Z. Stiffness-Oriented
Placement Optimization of
Machining Robots for Large
Component Flexible Manufacturing
System. Machines 2022, 10, 389.
https://doi.org/10.3390/
machines10050389
Academic Editors: Yuansong Qiao
and Seamus Gordon
Received: 12 April 2022
Accepted: 16 May 2022
Published: 18 May 2022
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Article
Stiffness-Oriented Placement Optimization of Machining
Robots for Large Component Flexible Manufacturing System
Yuan Xue
1
, Zezhong Sun
1
, Shiwei Liu
2,
* , Dong Gao
1
and Zefan Xu
1
1
School of Mechatronics Engineering, Harbin Institute of Technology, No. 92 Xidazhi Street, Harbin 150001,
China; xueyuan106@hit.edu.cn (Y.X.); sun_zezhong@stu.hit.edu.cn (Z.S.); gaodong@hit.edu.cn (D.G.);
20s108329@stu.hit.edu.cn (Z.X.)
2
State Key Laboratory of Robotics and System, Harbin Institute of Technology, No. 92 Xidazhi Street,
Harbin 150001, China
* Correspondence: swliu@hit.edu.cn
Abstract:
A large component flexible manufacturing system provides more application scenarios for
industrial robots, and, in turn, these robots exhibit competitive advantages in machining applications.
However, the structural characteristic of low stiffness is the main obstacle for the industrial robot.
Aiming at obtaining sufficient stiffness in the whole machining process, this paper focuses on robot
placement optimization in the flexible manufacturing of large components. The geometric center
of the machined feature is selected as, firstly, the base point, and the center-reachable placement
space of the robot base is obtained by establishing the kinematic model considering a variety of
motion constraints. Then, according to the reachability of the machining feature contour, the global
placement space meeting all machining boundaries is further extracted. The mapping relationship
between joint force and posture is established, and the most suitable robot placement is selected
based on the criterion of global stiffness optimization. A series of numerical and finite element
simulations verify the correctness and effectiveness of the proposed optimization strategy. The
developed stiffness-oriented placement planning algorithm can provide beneficial references for
robotic machining applications.
Keywords:
placement optimization; large component flexible manufacturing system; robotic machining;
robot stiffness; motion constraints
1. Introduction
In general, traditional computer numerical control (CNC) machine tools in the field of
mechanical manufacturing are dedicated to high-accuracy tasks, such as milling, grinding,
and drilling, while industrial robots are usually adopted for dull and dirty work scenes
to replace human workers. The robots are mainly designed to perform repetitive work
with relatively low accuracy, such as transferring, palletizing, and painting [
1
–
3
]. However,
owing to their considerable advantages such as high flexibility, large workspace, and
cost-efficiency, industrial robots have attracted increasing attention in the manufacturing
industry. With tremendous support from the academic and industrial communities, robots
have become viable and competitive candidates in machining applications, especially for
large-scale components [
4
,
5
] which usually exceeds 10 m in size, far beyond the processing
range of conventional CNC technology. Moreover, large-scale components are usually
produced in a single part or small batch. Hence, the flexible machining system based on
industrial robot has also been proposed [6–9].
Unfortunately, low stiffness is the main obstacle for industrial machining robots. The
stiffness of robots is generally less than 1
N/µm
, which is only one-fiftieth of CNC machine
tools [
10
,
11
]. Due to low stiffness, the end-effector of the machining robot suffers from static
deformation and dynamic vibration in machining applications, and large deformation can
seriously deteriorate manufacturing accuracy, which results in poor processing quality.
Machines 2022, 10, 389. https://doi.org/10.3390/machines10050389 https://www.mdpi.com/journal/machines
