Citation: Wahballa, H.; Duan, J.;
Wang, W.; Dai, Z. Experimental Study
of Robotic Polishing Process for
Complex Violin Surface. Machines
2023, 11, 147. https://doi.org/
10.3390/machines11020147
Academic Editors: Yuansong Qiao
and Seamus Gordon
Received: 15 December 2022
Revised: 12 January 2023
Accepted: 17 January 2023
Published: 21 January 2023
Copyright: © 2023 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
Experimental Study of Robotic Polishing Process for Complex
Violin Surface
Hosham Wahballa
1,2
, Jinjun Duan
1,
*, Wenlong Wang
1
and Zhendong Dai
1
1
Institute of Bio-Inspired Surface Engineering, School of Mechanical and Electrical Engineering,
Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
2
Faculty of Engineering, Karary University, Khartoum 12304, Sudan
* Correspondence: duan-jinjun@nuaa.edu.cn; Tel.: +86-152-5186-8386
Abstract:
This paper presents a robotic polishing process for complex violin surfaces to increase
efficiency and minimize the cost and consumed time caused by using labor and traditional polishing
machines. The polishing process is implemented based on modeling a smooth path, controlled contact
force embedded with gravity compensation and material removal depth. A cubic Non-Uniform
Rational Bases-Spline (NURBS) interpolation curve combined with an S-curve trajectory model is
used to generate a smooth polishing path on a complex violin surface to achieve stable motion
during the polishing process. An online admittance controller added to the fast gravity compensation
algorithm maintains an accurate polishing force for equal removal depth on all polished surface areas.
Then, based on Pythagorean theory, the removal depth model is calculated for the violin’s complex
surface before and after polishing to estimate the accuracy of the polishing process. Experimental
studies were conducted by polishing a wooden surface using the 6DOF robot manipulator to validate
this methodology. The experimental results demonstrated that the robot had accurate polishing
force based on the online admittance controller with gravity compensation. It also showed a precise
proportional uniformity of removal depths at the different normal forces of 10, 15, and 20 N. The
final results indicated that the proposed experimental polishing approach is accurate and polishes
complex surfaces effectively.
Keywords: robotic polishing; S-curve trajectory; controlled force; gravity compensation; violin surface
1. Introduction
The polishing process is commonly used in the manufacturing field to handle simple
surface damage, reduce roughness [
1
,
2
], and achieve shine [
3
–
5
]. Polishing is essential in
many industries requiring accuracy for surfaces, such as manufacturing molds, aircraft
airfoils, ship propellers, and many other complex surfaces [
6
,
7
]. At present, the polishing
of these surfaces is performed manually, which is time-consuming and costly, requiring ex-
perienced laborers; plus, a steady polishing process is difficult to maintain
manually [8–10]
.
In addition, manual polishing can cause many diseases related to the respiratory and
musculoskeletal systems [
11
], negatively affecting worker health. Therefore, an automatic
polishing system is required to eliminate these issues and produce desired properties, such
as improved quality, higher production yield [
12
,
13
], reduced
time consumption [14,15]
,
higher accuracy, and better reliability [
16
–
19
]. An alternative solution to manual polishing
is an industrial robot [
20
]. Generally, robotic polishing processes have attracted researchers
and companies in recent years [
21
]. In addition, a robotic polishing system can finish
different workpiece shapes, such as simple, curved, or complex surfaces. This ability
makes the industrial robot an effective and economical solution for polishing, especially
for surfaces with complex geometries [
22
]. The robotic polishing system combines various
related robotic sciences to automate the process and achieve high-quality polishing. These
sciences include robot motion planning as a polishing path, force control, and material
Machines 2023, 11, 147. https://doi.org/10.3390/machines11020147 https://www.mdpi.com/journal/machines
