Journal of Robotics, Networking and Artificial Life, Vol. 5, No. 2 (September 2018) 105-109___________________________________________________________________________________________________________Optimizing the Motion for Robotic Snap Assembly Using FEM
Peihao Shi, Kensuke Harada, Weiwei Wan, Ixchel G. Ramirez

Department of Systems Innovation, Osaka University, 1-3 Machikaneyama-cho
Toyonaka, Osaka, Japan.
Juan Rojas

School of Mechanical and Electrical Engineering, GDUT, 100 Waihuanxi Lu, Gongxue 2, 105
Guangdong University of Technology, Guanzhou, Guandong, China
Hiromu Onda

AIST, 1-1-1Umezono
Tsukuba, Ibaraki, Japan

Abstract
In this paper, we aim to provide a better assembly method for a snap joints assembly task. We create a 3D
cellphone model and use ADAMS simulation environment to analyze the relative motion between screen andbacker part. We focus on two points in this research. 1) Two kinds of relative motion between the screen andbacker parts, i.e., the rotation-based and the translation-based methods, are compared, and 2) difference between
assembly and disassembly is analyzed. By using the maximum elastic energy in an assembly process, we showthat 1) the rotation-based assembly motion has better robustness than the translation-based assembly motion incellphone assembly tasks when we set the same initial position error. 2) The rotation-based assembly method ismore effective when we disassembly the cellphone screen part.
Keywords: Robotic Assembly, Snap joints, Strain Energy.

1. Introduction
Rojas et al. have proposed a force control method [4-6]and identification of assembly state for the snap assemblyproblem.
Rojas et al. focus on the identification of assemblystate in 2012 and prove that the rotation-based assembly
method is more robust against the error of part’s posethan the translation-based method in robotic snap
assembly [7].
In 2016, we proposed the Extend Directional
Blocking graph for snap assembly [7]. We also proposed
the Maximum Elastic Energy (MEE) used for theSnap joints connection is one of the typical examples ofplastic parts assemblage. As the widely application andmass-production of plastic parts, snap joints assembly isbecoming more and more popular in assemble method ofIndustrial products. In snap joint assembly, theconnection between two parts is built by elastic
deformation of snap joints. [1] Fig.1 shows an exampleof the snap assembly task. Because of the elastic
deformation of the snap joints, it becomes relatively hard
for an industry robot to finish snap joints assembly task
[2-3]. As for the research on robotic snap joints assembly.

Copyright © 2018, the Authors. Published by Atlantis Press.
This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
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Journal of Robotics, Networking and Artificial Life, Vol. 5, No. 2 (September 2018) 105-109___________________________________________________________________________________________________________Extended Directional Blocking graph to evaluate theeasiness or difficulty of snap assembly where smaller
MEE implies the easiness of snap joints assembly [8].Weanalyzed the motion of snap joints assembly based
dynamic simulation by using FEM. We proposed asimulation based method to compute the MEE [9-10].
In this paper we aim to provide a better assembly
method for a cellphone snap joints assembly task. We
create a 3D model of a cellphone, and use ADAMS
simulation environment to analyze the assembly processof the cellphone. Using the Maximum Elastic Energy(MEE) as an index for evaluating the quality of a snap
joints assembly tasks, we compare two kinds of assembly
methods, i.e., rotation-based and translation-based
methods. We also compare the difference of MEE inassembly and disassembly when we choose different
assembly method.
By using the maximum elastic energy in an assembly
process, we show that 1) Rotation-based assembly
motion has better robustness than the translation-based
assembly motion in cellphone assembly tasks. When we
set the same initial position error. 2) The rotation-based
assembly method is more effective when we disassembly
the cellphone screen part.

Fig.2 3D cellphone model.

We consider Ei(s) as the total elastic energy in theassembly process. Here, s is a scalar ranging from 0 to 1.
It indicates the states of assembly. 0 is the starting state.
1 is the goal state. A value between them is anintermediate state. The maximum elastic energy (MEE)
Ui is being defined for snap assembly/disassembly taskof Part i. [8]𝑈𝑚𝑎𝑥 𝐸(1)We use Adams physics simulation environment
analyze the assembly process. We can get the strainenergy-duration function curve in Adams. And select theMEE from the curve. By compare the MEE of different
assembly methods, we can choose the better assembly
method which the MEE is smaller.
3. Experiment and Analysis.
In this section, we set two kinds of relative motion forcellphone assembly task, the rotation based relative
motion is shown in Fig. 3, and translation based relative
motion is shown in Fig. 4. We compute and compare theMEE of rotation-based/translation-based relative motion.
And the better assembly method is be defined as the one
bring smaller MEE in assembly process.

Fig.1 Snap assembly task

2. Approach and Definitions
We create a 3D model of a cellphone used in ADAMS
physics simulation environment where the screen partcontains four snap joints as shown in Fig. 2. We simulate
the assembly process by using the redefined relative
motion between two parts. Throughout a physics
simulation of a snap joint assembly, we consider
obtaining the function curve of strain energy. And selectthe MEE from the strain energy curve.

3.1 Initial Position Errors.
In robotic assembly tasks. Influence of initial positon
errors always been used as a criterion of robustness. In
our experiment, we set the initial position errors of screenpart to 0mm, 1mm, 2mm (Fig. 3) to compare the rotationbased and translation-based assembly motion.

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Journal of Robotics, Networking and Artificial Life, Vol. 5, No. 2 (September 2018) 105-109___________________________________________________________________________________________________________Fig. 6 Strain energy of 0mm position error in rotation-based
assemblyFig.3 Rotation-based relative motion and Initial positionerrors.

Fig. 5 and Fig 6 shows the simulation results when
we set the initial position errors to 0mm. The result
shown the strain energy of translation based-motion(1.19J) is bigger than the rotation-based motion (0.94J).
Fig. 7 and Fig. 8 shows the simulation result when weset the initial position errors to 1mm. The result shown
the strain energy of translation based-motion (2.17J) isbigger than the rotation-based motion (1.12J). Fig. 9 andFig. 10 shows the simulation result when we set the initial
position errors to 1mm. The result shown the strainenergy of translation based-motion (3.51J) is bigger than
the rotation-based motion (1.13J).

Fig. 7 Strain energy of 1mm position error in translation-based
assemblyFig.4 Translation-based relative motion.

Fig. 8 Strain energy of 1mm position error in rotation-based
assembly

Fig. 5 Strain energy of 0mm position error in translation-based
assembly

Fig. 9 Strain energy of 2mm position error in translation-based
assembly

107Journal of Robotics, Networking and Artificial Life, Vol. 5, No. 2 (September 2018) 105-109___________________________________________________________________________________________________________Fig. 10 Strain energy of 2mm position error in rotation-based
assembly

Fig. 12 Strain energy of assembly/disassembly process inrotation-based assembly

The simulation result shown a conclusion: when we
set the same initial position errors, the MEE in rotationbased assembly process is smaller than translation-based
assembly process. And proved rotation-based assembly
method has better robustness than translation-based
assembly method.

4. Conclusions.
This paper analyzes the 3D cellphone snap joints
assembly task. We compare two kinds of assembly
method from the following points. First is the initial
position errors of screen part. Second is difference
between assembly and disassembly.
We use ADAMS to simulate the snap joints assembly
motion and get the strain energy curve to compute theMEE. Take the cellphone screen snap assembly task forexample.
We proposed method evaluates the MEE of rotation
based and translation based assembly, selects a better
assembly method for a cellphone snap joints assemblytask. We also compare the difference of MEE inassembly and disassembly when we choose different
assembly method.
Based on the simulation result. We can draw thefollowing conclusions: the rotation-based assembly
method has better robustness than translation-based
assembly method when we set same initial positon error.
The rotation-based assembly method is more effective
when we disassembly the cellphone screen part.

3.2 difference of assembly and disassembly.
As the second point, we analyze the difference ofassembly and disassembly in two kind of assembly
methods. The MEE is different in assembly anddisassembly when we use translation-based assembly
method. Fig. 11 shows the MEE in assembly/disassembly
process based on translation-based relative motion. The
MEE in assembly process is 1.19J, it is smaller than theMEE in disassembly process (1.52J). Nevertheless, Fig.
12 shows the MEE in assembly/disassembly process
when we use rotation-based assembly method, the MEE
is almost same in assembly and disassembly process(0.94J).

References
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Fig. 11 Strain energy of assembly/disassembly process intranslation-based assembly

The simulation result shown that the rotation based
assembly method is more effective when we disassembly
the cellphone screen part.

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Journal of Robotics, Networking and Artificial Life, Vol. 5, No. 2 (September 2018) 105-109___________________________________________________________________________________________________________

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