Article
Robotic Assembly for Irregular Shaped Peg-in-Hole with
Partial Constraints
Wei Li
1,
*, Hongtai Cheng
2,
*, Chengpeng Li
3
and Xiaohua Zhang
1
Citation: Li, W.; Cheng, H.; Li, C.;
Zhang, X. Robotic Assembly for
Irregular Shaped Peg-in-Hole with
Partial Constraints. Appl. Sci. 2021, 11,
7394. https://doi.org/10.3390/
app11167394
Academic Editor: Giovanni Boschetti
Received: 29 June 2021
Accepted: 9 August 2021
Published: 11 August 2021
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1
Faculty of Electronic Information & Electrical Engineering, Dalian University of Technology,
Dalian 116024, China; xh_zhang@dlut.edu.cn
2
School of Mechanical Engineering and Automation, Northeastern University, Shenyang 110819, China
3
SIASUN Robot & Automation Co., Ltd., Shenyang 110179, China; lichengpeng@siasun.com
* Correspondence: davidlee83@126.com (W.L.); chenght@me.neu.edu.cn (H.C.)
Abstract:
The tight tolerance peg-in-hole process brings great challenges for robotic assembly. Force
control-based methods have been proposed to generate complex compliant behavior to deal with the
shape and clearance variances. However, existing solutions are based on the assumption that the peg
and hole parts are fixed during the assembly process and can absorb the contact force completely.
For this purpose, customized fixtures have to be designed and utilized, which greatly affect the
system’s deployment cost, time, and flexibility. Considering the fact that in an assembly, the parts
are naturally related to each other, this paper studies the irregular-shaped peg-in-hole assembly
problem with partial constraints. Firstly, geometric and force model are developed for the natural
constraints between the parts; by analyzing the behavior of the partial constraint, a control policy is
proposed to compensate the position errors and drive the parts to a stable equilibrium point; For the
irregular-shaped parts, a multiple-stage searching method is developed to efficiently search for the
real hole location; Finally, a switching force/position hybrid controller is designed to coordinate the
alignment, searching and insertion processes. The method is implemented in a real platform. The
experiment results verify the effectiveness of the proposed methods.
Keywords: peg-in-hole assembly; partial constraint; robotic assembly
1. Introduction
Automatic assembly is an important part of an industrial automation system. The
robot, due to its high flexibility, plays a key role in the fully automatic assembly system.
However, in a typical assembly task, the workpieces have to contact each other in order
to accomplish the mating process. Therefore, different from material handling, welding
or spraying tasks, the robotic assembly not only requires precisely controlling the robot
position but also needs to regulate the contact force compliantly and safely. Therefore,
complex hybrid control policies have to be designed according to different configurations.
Peg-in-hole is one of the most common operations in assembly applications. Because
of the limitations on the robot repeatability, fixturing and grabbing errors, the tight tolerant
peg-in-hole task used to be a difficult task [
1
]. Without compliant control policy, the initial
errors may generate huge reaction force, which may damage the robot or workpieces.
Without a sophisticated control policy, the success rate will be low for large variation
scenarios.
Passive compliance and active compliance are two ways to implement the compliance
peg-in-hole operation. The passive compliance uses a flexible device like a rubber spring to
link the end effector and gripper. The error will be compensated by the device’s deforma-
tion [
1
–
3
]. However, the passive compliance device is not generally applicable and has to
be redesigned according to different task configurations. With the development of force
sensors, the active force control is more and more used in many applications. The force
sensor is installed at the end effector to detect the contact force. This force information is
Appl. Sci. 2021, 11, 7394. https://doi.org/10.3390/app11167394 https://www.mdpi.com/journal/applsci
