Preliminary Design of an Aircraft Automatic Painting andPaint Removal System

Umberto Morelli

Thesis to obtain the Master of Science Degree in

Aerospace Engineering

Supervisors: Prof. Filipe Szolnoky Ramos Pinto CunhaProf. Alexandra Bento Moutinho

Examination Committee

Chairperson: Prof. Fernando José Parracho LauSupervisor: Prof. Filipe Szolnoky Ramos Pinto Cunha

Member of the Committee: Prof. Full Name 3

October 2016

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Alla mia famiglia

unita nella tempesta

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Resumo

Com o crescimento acelerado da industria aeroespacial decorrente dos ultimos anos e previsto para o

futuro, novas tecnologias e metodologias de producao tornam-se cada vez mais fundamentais. Uma

das areas mais carentes de inovacao e a manutencao do acabamento das aeronaves, incluindo os

processos de pintura e despintura. Atualmente. a manutencao e realizada manualmente, o que requer

muitas horas de mao de obra bracal num ambiente perigoso. Muitas solucoes para este problema

tem sido desenvolvidas, no entanto, um sistema eficaz seria pela automatizacao do processo, o qual

ainda nao esta disponıvel. Esta solucao podera acelerar drasticamente o processo, consequentemente

dimunuir o envolvimento direto da mao de obra, custos e riscos ambientais. Este trabalho tem como

objetivo aprofundar o tema apresentado e realizar uma proposta preliminar de projeto de uma solucao

automatizada respondendo a complexidade da questao a partir de uma solucao de baixo custo.

Palavras-chave: Pintura, Remocao de Tinta, Robo para Pintura, Sistema Automatico, Aeron-

aves.

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Abstract

The maintenance of the aircraft finish system is executed completely manually at present, involving

a big amount of manual labor for a long time and in a hazardous environment. The automation of

the process would be able to dramatically speed it up and to decrease manpower involved, with a

consequent contraction in costs and environmental risks. It is at the moment an important challenge

within the aerospace industry also because of the expectations of airplanes fleet growth over the coming

years. Several solutions are being developed, nevertheless, a system able to achieve the maintenance

process automatically is not yet available. Along this thesis, a preliminary design of an automatic system

for aircraft painting and paint removal has been carried out. The work points out that a low cost solution

for this complex problem is possible. As a preliminary study, this is intended to be a starting point for

further development on this subject.

Keywords: Aircraft finish system, Paint Removal, Spray Painting Robot, Automatic System,

Aircraft.

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Contents

Resumo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii

1 Introduction 1

1.1 Finish System Maintenance Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Paint Removal Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.1 Chemical Removers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.2 Mechanical Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.3 Optical Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3 Painting Methods and Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3.1 High Volume Low Pressure Spray Method . . . . . . . . . . . . . . . . . . . . . . . 6

1.3.2 Airless Spray Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3.3 Hot Spray Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3.4 Air-Assisted Airless Spray Method . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3.5 Electrostatic Spray Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3.6 Spray Painting Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4 Aircraft Painting Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4.1 Priming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.4.2 Topcoating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.5 Thesis Motivation and Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.6 Existing Automated Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.7 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2 Finish System Automatic Maintenance Solutions 17

2.1 Specifications and Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.1.1 Aircraft impact on the design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.1.2 Painting requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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2.1.3 Coating removal requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.2 Possible Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2.1 Multirotor UAV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2.2 Rail Mounted Robot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.2.3 Mast Mounted Robot on AGV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.2.4 Multi DoF Structure on AGV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.2.5 Lifting Structure on AGV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.3 Design Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3 System Design 29

3.1 Robotic Arm Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2 Structure Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.2.1 Horizontal Beam Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.2.2 Beam Support Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.2.3 Lifting System Structure Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.3 Lifting System Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.3.1 Linear Guides Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.3.2 Lifting Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.4 Subsystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.5 AGV Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.5.1 AGV Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.5.2 Subsystems Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

3.6 Cost Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4 Conclusions 55

4.1 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Bibliography 59

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List of Tables

2.1 Geometrical features of the C-130 H. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2 Trade-off between different possible solutions. . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.1 Different robotic arms specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2 Aluminium alloy Al 6061-T6 properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.3 Sample of the bean cross section properties table. . . . . . . . . . . . . . . . . . . . . . . 32

3.4 Beam cross section properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.5 SSETWNO M16-CR linear bearing specifications. . . . . . . . . . . . . . . . . . . . . . . 43

3.6 Vertical load on the lift actuator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.7 Angular velocity, lead and Vertical velocity for different screws. . . . . . . . . . . . . . . . 45

3.8 KGS-4040-023-RH ball screw specifications. . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.9 FANUC R-30iATMMate Controller features. . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.10 Paints specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.11 Specifications of different paint tanks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.12 AGV payload. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

3.13 Cost estimation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

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List of Figures

1.1 Finish system scheme. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Cleaning of the tail os a F16 aircraft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Aircraft masking before painting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Chemical stripping of helicopter coating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.5 Scuff sanding a KC-10 aircraft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.6 Paint removal by PMB method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.7 Portable handheld laser stripping device. . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.8 Primer application on aircraft fuselage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.9 Topcoat application on aircraft fuselage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.10 Aircraft painter at work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.11 Hyundai Alabama robotic painting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.12 Rotor blades for wind power systems coating achieved by ABB’s painting robots IRB 5400. 12

1.13 ABB’s robotic mining truck-washing system in Brazil. . . . . . . . . . . . . . . . . . . . . . 13

1.14 Robotic system cleaning up the Sydney Harbor Bridge. . . . . . . . . . . . . . . . . . . . 13

1.15 UltraStrip Systems, Inc.’s M-2000 removing paint from the hull of a ship. . . . . . . . . . . 14

1.16 Advanced Robotic Laser Coating Removal System. . . . . . . . . . . . . . . . . . . . . . 14

1.17 Laser Coating removal Robot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.18 Robotic Aircraft Finishing System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.19 Robotic system coating the B-777 wing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.1 A C-130E Hercules from the 43rd Airlift Wing, Pope Air Force Base, N.C. . . . . . . . . . 18

2.2 C-130H side and front views with dimensions. . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.3 C-130H top view with dimensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.4 Multirotor UAV system sketch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.5 Rail mounted robotic system sketch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.6 Mast mounted robotic system on AGV sketch. . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.7 Multi DoF robotic system on AGV sketch. . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.8 Arm and lifting robotic system on AGV sketch. . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.1 Grafic representation of a human arm workspace. . . . . . . . . . . . . . . . . . . . . . . 30

3.2 FANUC PaintMate 200iA/5L and its workspace. . . . . . . . . . . . . . . . . . . . . . . . . 31

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3.3 Forces acting on the horizontal beam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.4 Scheme of the I-beam cross section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.5 Beam weight parametric study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.6 Beam forces at the root section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.7 Stresses in the beam root section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.8 Horizontal beam support structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.9 Horizontal beam and support structure assembly. . . . . . . . . . . . . . . . . . . . . . . . 36

3.10 Beam support free body diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.11 Beam support parametric study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.12 Beam support forces diagram and section. . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.13 Lifting system exploded top view. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.14 Lift structure plate cross section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.15 Lift structure detail. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.16 Lift structure base. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.17 Lift base approximate structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.18 Lift structure parametric studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.19 Lift structure forces diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.20 Lift structure truss scheme. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.21 SSETWNO M16-CR linear bearing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.22 Linear guides: round rails. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.23 Lift system view with beam support. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.24 40MMx40MM ball screw with nut. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.25 PK599BE-N7.2 stepper motor torque vs. speed graph. . . . . . . . . . . . . . . . . . . . . 47

3.26 FANUC R-30iATMMate Controller. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.27 Automatic system overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.28 Top view of the system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.29 System sideview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4.1 Complete system drawing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

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Nomenclature

Greek symbols

ρ Material density

σ Normal stress

τ Shear stress

Roman symbols

A Area

db Horizontal beam tip deflection

E Modulus of elasticity

F Force

H Lift structure height

I Moment of inertia

lb Horizontal beam length

M Bending moment

nresonance Ball screw angular velocity at which resonance occurs

Sp Ultimate tensile strength

Sy Tensile yield strength

t Thickness

V Vertical shear force

W Weight

w Width

Subscripts

arm Robotic arm

xv

beam Horizontal beam

bear Linear bearings

cr Critical

house Beam support

leg Base column of the lift structure

lift Lifting system structure

s Ball screw

x, y, z Cartesian components

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Acronyms

AGV Automatic Guided Vehicle

ARLCRS Advanced Robotic Laser Coating Removal

System

ASM Automated Spray Method

ATEX ATmosphere EXplosibles

CG Center of Gravity

CTC Concurrent Technologies Corporation

DoF Degree of Freedom

HVLP High Volume Low Pressure

JPL NASA’s Jet Propulsion Laboratory

LARPS Large Aircraft Robotic Paint Stripping

LCR Laser Coating removal Robot

MPW Medium Pressure Water

NREC National Robotics Engineering Consortium

PMB Plastic Media Blasting

RAFS Robotic Aircraft Finishing System

SLAM Simultaneous Localization and Mapping

UAV Unmanned Aerial Vehicle

USAF United States Air Force

iGPS indoor Ground Positioning System

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Chapter 1

Introduction

The aircraft external structure is covered by a number of paint layers generally referred as finish sys-

tem. It has a large number of functions that differ between military and civil aviation. The primary

purposes of coating military aircraft are reducing radar and infrared signature, protecting the structure

(from corrosion, abrasion, chemicals . . . ) and appearance. Civilian aircraft coatings are used for struc-

ture protection, company identification and aesthetics appearance [1].

In both military and civilian cases, all along the aircraft life, the coating system is applied and removed

several times for a variety of reasons, a replacement of frayed coatings or changing in the livery are the

most common. Presently, the finish system is mainly removed by chemical stripping. This method

removes the paint through toxic chemicals and workers labor. If some paint resists to the chemicals, the

laborers strip it with wire brush or sandpaper. This task exposes them to health hazard because of the

highly toxic paint dust and residual chemicals. There is also a risk of damaging the aircraft structure in

case of error [2].

Beside the worker health hazard, both painting and paint removal processes present a critical envi-

ronmental pollution threat due to the chemicals used, especially in the paint stripper. Consequently, the

application, storage and waste disposal of these chemicals is highly regulated [3].

Other paint removal methods are mechanical and optical. The latter is still under development and

is being implemented just recently [4]. These methods are generally less polluting, due to the absence

of chemical products. They also tend to be more efficient but require higher skilled workers as well as

higher initial investment for training and equipment [2].

After the removal of the finish system, an aircraft structure inspection follows to detect cracks or

corrosion damages, and finally the aircraft is repainted [5].

Usually, the finish system consists of three layers: a pretreatment, a primer and a topcoat (see

Figure 1.1). The finish system is generally deposited with spray guns by highly skilled and experienced

workers. During both painting and paint removal, the workers use protective garments for the body,

hoods with visor and respirators [6].

1

Figure 1.1: Finish system scheme.

1.1 Finish System Maintenance Process

The maintenance of the finish system is a long-lasting process that follows a specific routine indicated

by the regulations. According to the United States Air Force (USAF) technical manual [5] the process

is divided into the following stages: (i) preparation for paint removal, (ii) paint removal, (iii) surface

preparation for painting and surface treatment, (iv) painting and (v) coating inspection.

First of all the aircraft is de-energized. Then, before the paint is removed, all the surfaces to be

worked are cleaned from grease, oil and dirt. These materials would act as a barrier protecting the

coating to be removed [5]. After washing the surfaces (Figure 1.2), all special areas, equipment and

material are protected by masking. The masking is essential to protect delicate areas of the aircraft, like

electronic equipment and windows, during the paint removal.

Figure 1.2: Cleaning of an F16 aircraft [7].

The finish system removal is a complex and critical process. To accomplish this task it is possible

to use different methods: chemical, mechanical and optical [2]. Each one has its advantages and

disadvantages (see Section 1.2).

After the finish system is removed the surface of the aircraft is prepared for the application of new

coatings. This is the most important stage for ensuring proper adherence and performance of the new

finish system. The life of a coating system, its effectiveness and appearance depend more on the

condition of surfaces receiving it than any other factor [5].

The surface to be painted is carefully cleaned and then inspected for corrosion and damages. Before

the painting operations begin, the aircraft is masked again (Figure 1.3). The mask is changed because

2

during the painting the parts to mask are different from the ones masked during the paint removal. The

masking material used is also different for the decoating and painting processes. The masking operation

generally consumes more man-hours than the actual painting [5].

Figure 1.3: Aircraft masking before painting1.

When the aircraft is entirely cleaned, treated and masked, it is ready to be painted. Painting can

involve many skilled workers at the same time depending on the aircraft size. Finally the coating system

is inspected to ensure its effectiveness all over the aircraft.

1.2 Paint Removal Methods

The objective of the paint removal process is the complete removal of the coating system of the aircraft

without damaging the surfaces on which it is applied. In order to do it, a variety of methods are used.

In this section they are individually described, pointing out advantages and disadvantages of each one

according to the USAF technical manual [5].

The removal methods can be divided into chemical, mechanical and optical. While the chemical and

mechanical methods are widespread, optical coating stripping is a young technology applied just since

few years.

1.2.1 Chemical Removers

The paint strippers are a mixture of five chemical components: organic solvents, thickeners, corrosion

inhibitors, surfactants and evaporation retardants. The components have to be mixed immediately before

use as chemicals tend to separate on standing.

The chemical remover is selected relatively to the finish system to be removed. If the component

to be stripped is small, it is possible to immerse it in a pool of hot removal (approximately 85 oC). This

method is obviously not applicable to the whole aircraft but is generally applied to the landing gear and

other small components.

1URL http://www.european-coatings.com/Homepage-news/New-coatings-to-enhance-range-for-aircraft [Ac-cessed:3 September 2016]

3

Alternatively, the mixture can be applied directly on the surface using sprayer, brush or roller. The

thickness of the stripper layer has to be light to medium as thick coats of it slow down the removal

rate and increases the operational cost. When the finish system is stripped as in Figure 1.4, or the

chemicals dwell time is exceeded, the whole area is agitated with a brush Immediately after all loosened

finish system is scraped from the surface. Chemical removers are reapplied in spots where the finish

system has not been removed. The complete process is repeated a maximum of three times. If after

the third application the finish system is not completely removed, the process has to be completed with

the mechanical removal of the coatings. Finally, the workers rinse the area thoroughly with hot water

between 37 oC and 49 oC at a pressure of 1-1.7 MPa.

Figure 1.4: Chemical stripping of helicopter coating2.

This is an old but still widespread method because of its effectiveness and economy. It requires few

equipment investments and low skilled workers.

Chemical paint stripping was developed for metallic surfaces and it is not possible to use it on com-

posite material surfaces because the chemicals would react with the composite structure [2]. This prob-

lem has to be taken into account because of the trend of aerospace industry to use more and more

composite materials. Moreover, it requires a long time and it is heavily influenced by the environment

temperature and humidity.

1.2.2 Mechanical Removal

Mechanical removal methods include the use of motor- or hand-driven wire brushes, abrasive paper and

mats, as well as abrasive blasting. These methods are recommended if chemical stripping is impractical

due to structural complexity, environmental restriction and working difficulties. Mechanical removal is

generally very effective. Nevertheless, it can cause severe damage to the structure if improperly used.

Mechanical removal consists of a simple mechanical abrasion of the finish system (see Figure 1.5).

It produces highly toxic dust, requiring workers and environment protections. The mechanical removal

is generally made by abrasive blasting of grit or sand (effective on iron and steel alloys), or hand or

motor-driven abrasive equipment.

2URL http://www.aviationpros.com/product/10472836/solvent-kleene-inc-aircraft-paint-stripper [Accessed:14September 2016]

3U.S. Air Force photo/Margo Wright - URL http://www.af.mil/shared/media/photodb/photos/081002-F-4094W-807.jpg

[Accessed:14 September 2016]

4

Figure 1.5: Scuff sanding a KC-10 aircraft3.

Other mechanical methods are Plastic Media Blasting (PMB) and Medium Pressure Water (MPW)

methods. PMB consists of blasting polyester plastic particles at a pressure in the range of 0.28 to

0.41 MPa with the nozzle tip at a distance within the range of 30 to 60 cm, while the angle of incidence

should be within the range of 30 to 60 degrees (see Figure 1.6). In the end, the plastic media can be

collected and reused. This method can not be used on metal structures having a thickness less than

0.4 mm. It is an efficient and rapid method both for metallic and composite structures, but a proper

waste management must be ensured for economic and environmental reasons. This method requires

specialized workers and high initial investment It is more environmental friendly than chemical stripping

and has a high removal rate (about 7 cm2/min).

Figure 1.6: Paint removal by PMB method4.

MPW method is the blasting of water and sodium bicarbonate. The injection system is a positive

feed control system (computer controlled). The water pressure is 100 MPa with a flow rate of 11 liters

per minute. The nozzle distance from the surface is within the range of 5 to 10 centimeter and the angle

of incidence in the 40 - 60 degrees range [5]. The speed on the nozzle across the surface should be 10

4URL http://www.fus.de/uploads/images/Gallery/FUS-DS/5D.jpg [Accessed:14 September 2016]

5

cm/s. As the PMB, this technique requires specialized workers and high initial investment. Nonetheless,

it is relatively environmental friendly and has a high removal rate.

1.2.3 Optical Removal

The optical removal of the finish system is made by a laser wave. The possibility to remove the coating

on a substrate by laser wave is being investigated since the early 90’s [8]. The results obtained show

that it is possible to remove the coating from inorganic as well as organic substrate. Many handheld

laser removal systems have been designed (see Figure 1.7) but they are heavy, dangerous and the final

result is really affected by the human-factor. With the finish system removal automation trend, in the last

years, laser removal is the subject of many new projects [9, 10].

Figure 1.7: Portable handheld laser stripping device [11].

The interest on laser removal is motivated by advantages it has when compared to the other tech-

niques. It does not involve the use of raw materials like chemicals or water and because of that the

waste produced is just the coating removed that can be easily vacuumed This leads to a cleaner envi-

ronment and a easier disposal of waste. This method can be used on aluminum as well as composite

substrate without damage of the structure. Furthermore, it is possible not to operate on delimited areas

and therefore the masking of the aircraft can be partly avoided [9].

Beside the stated advantages, a laser system involves a really high initial investment, and being a

relatively new technique, in many aircraft maintenance manuals it is not contemplated or allowed.

1.3 Painting Methods and Techniques

This section describes the different painting methods and techniques according to the USAF technical

manual [5].

Spray application is the standard for painting aircraft. It is a fast procedure and produces films of

good uniformity and quality. Other methods are brush or roller applications. These are useful in special

cases, particularly in non-aeronautical or less critical applications, but are not described here.

1.3.1 High Volume Low Pressure Spray Method

The standard spray method in the aerospace industry is the High Volume Low Pressure (HVLP) method

Using a high volume of low pressure air, the coating material is atomized through the spray gun nozzle.

6

The spray equipment generally utilizes low pressure gun cups to assist in the delivery of the coating

material to the gun nozzle, while low pressure air is used to atomize the coating material at the spray

head. A high volume of air pushes the coating material, forming a very soft, low velocity pattern. The

soft spray generally provides more consistent coverage and a better overall finish.

With this spray method, the gun is held closer to the surface (15 to 25 cm) than with other methods

because of the lower speed of the paint particles. Moreover, this method applies, in a single coat, a

thicker film than any other spray method here described.

1.3.2 Airless Spray Method

In the airless spray system no air pressure is used. Instead, hydraulic pressure is used to deliver the

coating material to the gun head. The paint is atomized by ejection from special spray nozzles that

increase the pressure by a factor of about 100. The paint droplets, moving toward the surface by their

momentum, are appreciably slowed down by air resistance. This method produces less bounce of the

coating material on arrival at the work surface and, therefore, less over-spray.

The paint is not cooled by the expansion of the air as in the conventional spray method, so the only

heat loss is through solvent evaporation.

1.3.3 Hot Spray Method

Hot spraying is the application of coatings with HVLP or airless spraying system using heat as a sub-

stitute for all or a portion of the thinner, generally used to reduce viscosity of the coating material. It is

most frequent and efficiently used with airless spray systems.

The hot paint is cooled rapidly when atomized but retains sufficient heat to still be close to the

ambient air temperature when it reaches the work surface. So the possibility of blushing, that is due to

condensation of moisture, is reduced and it is possible to spray under high humidity conditions. However,

heating the paint reduces its pot life (period in which the chemicals remain usable when mixed).

1.3.4 Air-Assisted Airless Spray Method

The coating material is atomized by hydraulic pressure as in the airless spray system but at a much

lower pressure. Low pressure air is added at the gun head through jets at the nozzle and directed at the

paint mist to control and form the spray pattern. This allows the operator to control the atomized coating

pattern which cannot be done with standard airless systems.

This method offers the same advantages of the airless method while being safer and requiring lower

maintenance on pumps, due to the lower hydraulic pressure. Moreover, the appearance of the coating

applied is better as the tendency to obtain a bumpy surface (usually called orange peel) is lessened.

7

1.3.5 Electrostatic Spray Method

This method is a modification of the spray methods previously described. It adds the feature of electro-

static charging of the paint material which is attracted by the grounded work piece. The coating material

can be charged either inside the gun or at a fine metal probe at the gun nozzle exit, where the latter is

the one generally used. This method achieves the best result with airless spray because the low velocity

of the paint particles and the electrostatic attraction produces a high transfer efficiency rate.

This method reduces over-spray and allows a better painting of hard-to-coat areas such as edges.

The Faraday, however, effect limits the effectiveness of this method in coating interior corners, crevices

and cavities.

1.3.6 Spray Painting Techniques

Different techniques are important when handling a spray gun. The coating quality is highly dependent

on how well these techniques are applied. This section describes the most important spray techniques

and theirs effects on the final result.

When spraying a surface, the distance of the gun to the aircraft surface depends on the width of the

spray pattern desired and on the type of gun used. If the spray gun is too far from the surface, it will

result in a dry spray, called dusting, and an excessive over-spray. Contrarily, if the spray gun is kept too

close to the surface, the coat will be too heavy, developing a tendency for sags or runs.

Stroking is the movement of the spray gun along the surface to paint. It is essential to maintain the

spray gun at the same distance to the surface, move it at the same speed, and hold it at the same time

perpendicular to the surface throughout the pass. Generally, the painters, when in an uncomfortable

position or fatigued, tend to arc the stroke, causing an uneven application with a thicker coat in the

middle of the stroke than at the ends. The rate of stroke should be uniform to produce an even wet coat.

Each stroke should be parallel to the other with a 50% overlap.

The technique that ensures the best coating integrity and coverage is the cross coating. It consists of

applying two layers of coat, one with a stroke perpendicular to the other. This technique should always

be used when applying multiple coats, except for high solid primers and topcoats with HVLP, airless, or

air-assisted airless equipment.

Another important technique is the triggering. It is the pressure and release of the spray gun trigger

during the stroking. This is a difficult technique that requires a long experience to be acquired. Generally,

the painter should press the trigger after the beginning of the stroke and release it before the end.

1.4 Aircraft Painting Process

This section presents a description of the aircraft painting process according to the technical manuals [5,

12].

The painting of an aircraft is accomplished by at least two painters supported by helpers. For large

aircraft, it may be necessary to increase the number of painters. Small aircraft are generally positioned

8

with the tail towards the exhaust filter bank of the painting hangar and vice-versa for larger aircraft. The

process will be described for small aircraft. For large aircraft the sequence of events presented has to

be inverted

1.4.1 Priming

The first coat to apply is the primer (see Figure 1.8). The priming starts at the end of the aircraft near

the exhaust filter bank and moves towards the air supply.

Figure 1.8: Primer application on aircraft fuselage5.

Starting from the tail, the priming begins from the higher surface (horizontal stabilizer for ”T” tail

aircraft, vertical stabilizer(s) for different configurations). The horizontal stabilizer is primed starting from

the upper surface at the center moving outboard to the tip stroking perpendicularly to the leading edge.

Then, the lower surface is primed, and finally the primer is applied to the edge from the outboard to the

junction with the vertical stabilizer. The priming of the vertical stabilizer(s) starts at the top and leading

edge of each side moving down and aft with vertical strokes. Finally the leading edge is primed from the

top down.

After the tail, a full wet coat of primer is applied at the aft section of the fuselage starting from the

aft end and the top moving forward and down with vertical strokes to the junction with the wing trailing

edge.

The wings are primed starting from the lower surface at the tip moving inboard with a stroke perpen-

dicular to the leading edge. Then the lower fuselage between the wings is primed. A full wet coat of

primer is applied to the upper surface of the wings and the upper fuselage with the same technique as

of the lower surface.

Finally the primer is applied to the forward section of the fuselage starting at the wing leading edge

and the top moving down and forward to the nose with a vertical stroke.

5URL http://www.flyingcolorspaintandinterior.com/paint/ [Accessed: 03 September 2016]

9

1.4.2 Topcoating

Topcoats can be applied in one coat as well as two coat system. The one coat system is applied with a

stroke in one direction followed by a stroke in the perpendicular direction working small areas at a time

(see Figure 1.9). The two coat system is applied stroking the two coat in perpendicular directions.

The topcoating follows the same process sequence of the priming but backwards. It starts from the

nose of the fuselage and ends on the upper part of the tail.

After topcoating, it is important to allow the paint to cure in a dust-free atmosphere.

Figure 1.9: Topcoat application on aircraft fuselage6.

1.5 Thesis Motivation and Objective

In many industries the painting process is a completely automatic procedure for over 50 years. This

is especially true for the automotive industry [13]. The aerospace industry has an automation delay

compared to other industries for many reasons (complexity, regulatory, materials, etc.). Dan Friz, director

of business development, KUKA Systems (Shelby Township, MI), said ‘The major automation challenge

within the aerospace industry is simply the aircraft was never designed for an automated process.’

The fleet of commercial airplanes is growing. Boeing, for example, forecasts that by 2032 more than

35,000 new airplanes will be built [14]. To manufacture and maintain all these airplanes, the whole

process has to get faster and cheaper, reason for which many aerospace manufacturers, especially in

commercial aviation, are looking for solutions to automate the aircraft finish system maintenance.

The automation of the aircraft painting and paint removal procedures leads to many benefits for both

workers and companies. The robotic application of paint on the aircraft can lead to material savings

of between 30 to 50%, which means a 30-50% savings in airplane weight as well [14]. This weight

saving interests the airplane manufacturers because it leads to a fuel saving for airline customers, with

economic and environmental benefits.

6URL http://www.csiro.au/en/Research/MF/Areas/Chemicals-and-fibres/Materials-for-industry-and-environment/

Coatings-and-surfaces/TopCoat [Accessed: 26 September 2016]

10

On the other hand, both painting and paint removal processes require a number of highly skilled

workers operating for a long time on each aircraft, in a polluted environment (see Figure 1.10) and often

without the possibility to pause the ongoing process. The introduction of an automatic system reduces

the number of workers involved and their exposition to health hazards. It also ensures the possibility to

process an aircraft as soon as it is possible reducing the time waste due to the employees working time.

OGMA Industria Aeronautica de Portugal, S.A founded in 1918 in Alverca, is an international player

in aerospace maintenance, repair and overhaul, and manufacturing business. The company is nowa-

days accomplishing the whole finish system maintenance of both military and civil aircraft completely

manually. The subject of this study is to make the preliminary design of an automatic painting and paint

removal system on behalf of this company.

Figure 1.10: Aircraft painter at work7.

1.6 Existing Automated Solutions

The first painting robot was presented in 1967 by the Norwegian company Trallfa that was producing

wheelbarrows. It was an electro-hydraulic robot which could perform continuous movements and that

was meant for their internal use only. It was developed into a commercial success as in 1985 ASEA

(later ABB) took over Trallfa [15].

From the 1960s on, robots have been taking the place of human workers in carrying out many

manufacturing tasks increasing the productivity in many industries. This transition is still going on, the

market research company Forrester says in a report that robots will eliminate 6% of all US jobs by

2021 [16].

Many projects of interest for this study have been developed. The majority accomplish only the

painting of surfaces, a process involved in many manufacturing processes, while only few concern the

paint removal and none was found about the masking and demasking.

All systems developed to paint surfaces always involve the use of a robotic arm spraying on the

surface and, except for small parts, any other technique (brush, roller, etc.) has not been experimented

so far.

The automotive industry completely automated the painting of almost all car parts not only because of

the production volume but also because of the quality and consistency aspects [17]. Chad Henry, North7URL http://excelaviation.ie/jobs/aircraft-spray-painter/ [Accessed: 26 September 2016]

11

American Sales Manager at Staubli Corporation in Duncan, South Carolina, speaking about robotic

painting said ‘You simply cannot get repetitive performance with manual operation.’

Generally, in car manufacturing, the robot base stays steady while the part to be painted moves along

the assembly line as in Figure 1.11. An exception is the system developed by Aerobotix, that using two

rail mounted FANUC P-250iA/15 robots, is able to paint camouflage pattern on military vehicles [18].

Figure 1.11: Hyundai Alabama robotic painting8.

The robotic leader industry ABB automated the coating of 80 meters long wind turbine blades using

two rail mounted IRB 5400 robotic arms (Figure 1.12), achieving a reduction of energy consumption by

up to 60 percent and of paint consumption of 25 percent compared to the standard paint application [19].

Figure 1.12: Rotor blades for wind power systems coating achieved by ABB’s painting robots IRB 540010.

In another field, Figure 1.13 shown the mining trucks cleaning system designed by ABB. It uses high

pressure waterjet from a IRB 6650S robot mounted on movable rails, saving 60 percent of the time

compared to the manual process [20].

Another interesting project is the system developed by the University of Technology of Sydney for

steel bridge maintenance. It is able to remove the paint and rust from steel bridges with a grit-blasting

technique. This system avoids that workers stay in partially closed spaces with highly polluted air or in

dangerous freefall conditions [21]. The system is composed of a robotic arm able to sense the workable

8URL http://www.hotrod.com/articles/14-steps-great-paint/ [Accessed: 26 September 2016]10URL http://www.abb.com/cawp/seitp202/90c5c8ab0a1fd46ac125759a003ec090.aspx [Accessed: 27 September 2016]11URL http://www.pngindustrynews.net/commonlib/ImageEnlarge.asp?strImageFileName=ABB_robots.jpg [Accessed:

27 September 2016]

12

Figure 1.13: ABB’s robotic mining truck-washing system in Brazil11.

surface inside its workspace and complete the paint removal task autonomously. However, it is not able

to move its base by itself.

Figure 1.14: Robotic system cleaning up the Sydney Harbor Bridge12.

In the naval industry there are many projects for the robotic removal of paint from the ships hulls

while few studies have investigated the paint application [22]. Differently from the previews projects, all

the ships paint removal robots found are able to attach their structure to the surface and move along it.

The most popular naval paint remover is the M-2000 in Figure 1.15. It is a semi-autonomous robotic

paint removal system, built out of a partnership between NASA’s Jet Propulsion Laboratory (JPL), the

National Robotics Engineering Consortium (NREC) at Carnegie Mellon University, and UltraStrip, that

strips paint from ships hulls [23]. It attaches itself magnetically to the hull of a ship, with a vacuum hose

running from it. A controller helps navigate the robot along the surface of the ship with 360 degrees

of movement. The M-2000’s high-pressure water jet generates 270 MPa of pressure to blast away the

paint right down to the ship’s steel substrate. The water and the stripped paint are then captured by the

vacuum system.

Research papers about automatic painting and/or paint removal systems in the aerospace industry

go back to the early 90’s [24] but just during the last years the aerospace companies began implementing12URL http://www.pulse-pr.co.uk/service-robots-revolutionize-clean-up-of-the-sydney-harbor-bridge-85.asp

[Accessed: 27 September 2016]13URL https://spinoff.nasa.gov/spinoff2000/er1.htm [Accessed: 7 October 2016]

13

Figure 1.15: UltraStrip Systems, Inc.’s M-2000 removing paint from the hull of a ship13.

these systems in their manufacturing process [14]. Most of the projects are still under development and,

with few exceptions, concern only the paint application or the paint removal.

During the 90’s the NASA and the USBI Company developed a robotic high pressure waterjet system

for the paint stripping of the Space Shuttle solid rocket boosters. Years later, this project leaded to

the Large Aircraft Robotic Paint Stripping (LARPS) designed by the Pratt & Whitney Waterjet System,

a commercial robot able to remove the finish system from civil and military aircraft by high pressure

waterjet [25], but the project never came to an end.

The Carnegie Mellon University’s NREC and Concurrent Technologies Corporation (CTC) of John-

stown developed, and are now testing, the Advanced Robotic Laser Coating Removal System (ARLCRS)

in Figure 1.16. It uses a continuous wave laser mounted on a state-of-the-art mobile robot to remove the

coating system from medium to small size military aircraft. It is also able to avoid selected areas limiting

the masking needed [10].

Figure 1.16: Advanced Robotic Laser Coating Removal System14.

Another similar project under development by STRATAGEM is the Laser Coating removal Robot

(LCR). This system uses a 20 kW CO2 laser to evaporate and combust the paint that is immediately

14URL http://www.nrec.ri.cmu.edu/projects/ctc/ [Accessed: 27 September 2016]

14

vacuumed from the surface and passed through a filtration system. The laser is mounted on a eight

Degree of Freedom (DoF) robotic arm and four DoF mobile platform (see Figure 1.17). On the end-

effector is also mounted a forward scanner to check the aircraft geometry in real-time [9]. STRATAGEM

expects 50% reduction in processing time and 90% labor reduction. Moreover, STRATAGEM is planning

to implement on the same structure paint spraying tools to build also an aircraft painting system.

Figure 1.17: Laser Coating removal Robot15.

Regarding the coating of aircraft there are two military projects both for stealth coating of fighting

aircraft: the Sandia F117 robot for the coating of the F117 Nighthawk and the Robotic Aircraft Finishing

System (RAFS) developed by Lockheed Martin for the F-35 coating (see Figure 1.18). The first system

has three robotic arms: two rail-mounted and one floor-mounted, all with seven DoF. In 1999 it has

successfully painted the first F-117 Nighthawk fighter [26]. The latter coated its first aircraft in 2008, it

comprises three FANUC R2000iA 125L robots, each with six DoF, mounted on auxiliary axis rails [27].

Figure 1.18: Robotic Aircraft Finishing System [27].

Another painting system, shown in Figure 1.19, was developed by Boeing to automatically coat the

B-777 wings. It has two robotic arms rail mounted and is called Automated Spray Method (ASM). The

system is replacing 35 to 40 painters and is able to apply two different paints at two different thickness

at the same time. The company is now planning to extent the process to other parts of the aircraft [28].

15URL http://www.lcrsystem.com/ [Accessed: 27 September 2016]16URL http://mashable.com/2013/06/04/boeing-777-robots [Accessed: 27 September 2016]

15

Figure 1.19: Robotic system coating the B-777 wing16.

1.7 Thesis Outline

Along the present thesis the design of an aircraft automatic painting and paint removal system is ex-

plained in detail.

Specifically, in Chapter 2 the system requirements are expounded. Then, different possible solutions

are illustrated and evaluated. Between these, a trade-off process is carried out to select the solution to

be developed.

In Chapter 3, the system components are designed and a cost estimation is performed. Finally, in

Chapter 4, a general overview of the present work is carried out giving a path for the future work to be

executed for the completion of the project.

16

Chapter 2

Finish System Automatic Maintenance

Solutions

Along this chapter the problem is analyzed in detail and a set of possible solutions is presented.

The subject of this study is the automation of aircraft painting and paint removal. The automation

of the other phases of the finish system maintenance (i.e. masking, cleaning etc.) has not been taken

under consideration for two specific reasons: there is a lack in the technology development (masking)

and the advantages for a further development into automation compared to the development time and

investments required have been considered neglectable.

To satisfy the client requirements neither a painting nor a paint removal method is selected. The

present study has been confined to describe the possible solutions available and leave the painting and

paint removal method selection to the client.

2.1 Specifications and Requirements

The automation of painting and paint removal is influenced primarily by three factors: the aircraft size

and shape, the paint application requirements and the paint removal requirements. In this section these

factors are analyzed in detail.

2.1.1 Aircraft impact on the design

The automatic system has to process any aircraft the company is presently maintaining and possibly the

ones it will maintain in the future, in order to provide a real economic benefit. Generally the airplanes

are different in size and shape. While the dimension of the aircraft influences the size of the system

workspace, its shape determines the dexterity the system needs to process all the required surfaces.

The system will be sized on the Lockheed C-130 Hercules (Figure 2.1). This is the bigger aircraft

OGMA is maintaining as well as the one with the lower distance between the fuselage and the ground.

The geometrical features of this aircraft are set out in Table 2.1 where the heights are measured with the

17

retracted landing gear. To ensure the possibility to process differently shaped aircraft, the system will be

oversized.

Figure 2.1: A C-130E Hercules from the 43rd Airlift Wing, Pope Air Force Base, N.C.1

Table 2.1: C-130 H geometrical features [29, 30].

[m]

Length 29.3Height 11.4Wingspan 39.7Wing root chord 4.9Fuselage height 4.6Fuselage width 4.3Landing gear height 0.52

The four dimensions in Figure 2.2 are used to size the system workspace. Moreover, to size a system

composed of a moving vertical structure another dimension is required. This is computed as the lenght

of the hypotenuse of a right triangle having as legs half the wing root chord and half the fuselage width

(see Figure 2.3). Then, a system composed of a vertical structure to reach any point of the surface has

to locate its end-effector 3.25 m away from it. Applying a conservative approach, the above length is

multiplied by a factor 1.25, obtaining 4 m.

Figure 2.2: C-130H side and front views with dimensions.

The dimensions of the system workspace are here defined in accordance with the previews results.

The minimum height the system has to reach is 13 m while the width and length are 45 m and 35 m1URL http://www.af.mil/shared/media/photodb/photos/990101-F-5502B-002.jpg [Accessed: 28 September 2016]

18

Figure 2.3: C-130H top view with dimensions.

respectively. Furthermore, it also has to process the bottom of a fuselage located 0.5 m over the ground.

This initial sizing of the automatic system is useful in order to carry out a preliminary analysis on different

designs.

2.1.2 Painting requirements

To paint an aircraft the system has to handle one of the methods described in Section 1.3. To do it the

required equipment is: an air compressor, a paint tank and a spray gun. The system has to handle

this equipment and to apply the paint with the required tickness following the technical prescriptions [5].

Moreover, the system has to clean the hoses and the spray gun, and to be able to change paint.

To paint an aircraft, at least one painter at each side of it is needed. This is because the overlap

of the coating has to happen while it is still wet. With only one painter it would not be possible to do it

because the paint would dry on the middle line of the fuselage causing a thickness discontinuity. For the

same reason, stopping and restarting the painting process reduces the quality of the final result.

Because of the solvents inside the paint spread in the air, in the painting area the antiexplosion

regulation ATmosphere EXplosibles (ATEX) is applied [31]. The ATEX directory divides the working

area into different zones depending on the explosion risk. According to it all equipment has to be ATEX

certified.

2.1.3 Coating removal requirements

The requirements for the automation of the paint removal depend upon the removal method selected

by the client among those described in Section 1.2. In this section, each method requirements are

described.

To automatize the chemical paint stripping it is possible to use the same equipment used to paint

changing the spray gun, because to atomize the paint stripper is not allowed. The thickness control and

precision required is lower compared to the paint application [5]. The system has also to agitate the

whole surface and scrape all loosened coatings with a squeegee. At the end it has to clean the surface

19

with hot water.

This removal method needs a long time because of the chemical reactions involved, thus the main

advantage of a robotic execution is not to have workers doing an hazardous task. Moreover, to make the

chemical stripping completely automatic, the system must be able to handle many different tools (spray

gun, squeegee and water gun) increasing the complexity of the system as well as its cost.

The paint removal by mechanical methods requires a higher system precision and accuracy because

there is a risk to damage the aircraft structure and equipment. Moreover, the system has to know when

the paint is removed from the area it is working.

If an electric sander is used, the system has to hold the sander, keep it in contact with the surface

and react to the friction forces acting on it. On the other hand, the PMB, MPW and abrasive blasting

methods require the system to handle a blasting equipment and a media or water tanks. Moreover, it

has to react to the forces generated by the blasting as well as keep the end effector at the right distance

and orientation with respect to the surface.

Finally, the laser removal requires the system to supply and handle with high precision and accuracy

a laser equipment. It also has to sense when the paint is removed from the area it is working.

2.2 Possible Solutions

Along this section various automatic solutions for both painting and paint removal are analyzed and

evaluated.

Taking into account the systems described in Section 1.6 as well as the above specifications and re-

quirements, two different general solutions are evaluated: a robotic arm mounted on a movable structure

and a multirotor Unmanned Aerial Vehicle (UAV).

Relatively to the robotic arm solution, there are many possible structures able to position the arm

in the space. In the present study four different designs are presented and evaluated. Assuming the

system is able to work in any painting hangar, the proposed designs do not produce any load on the

hangar structure besides the system weight on the ground.

2.2.1 Multirotor UAV

This design is made up of a multirotor UAV able to fly in the painting hangar. The UAV holds a three

rotational DoF robotic wrist that controls the orientation of its end-effector. In Figure 2.4 the system is

illustrated with a sketch.

The flying platform can position the end-effector in any point of the workspace while the robotic wrist

can control its orientation with respect to the work surface. In this way it is possible to process the whole

surface of the aircraft.

During the painting, the UAV carries the electric power storage, the paint tank, the air compres-

sor, while the end-effector is a spray gun. The paint tank, the spray gun, and the compressor can be

substituted for the paint removal with the dedicated equipment.

20

Figure 2.4: Multirotor UAV system sketch.

The implementation of most of the removal methods of Section 1.2 on this system is an issue. To

install a motor-driven abrasive remover can be difficult due to the torque the propellers have to counter-

act. Also the MPW, PMB and grit blasting methods are hard to implement because of the big amount of

media the vehicle has to lift and the force due to the blasting. The laser removal is difficult to implement

too because it needs a big amount of electrical power, thus heavy batteries, and very high precision in

positioning with respect to the work surface.

This solution presents some benefits mainly due to its fly ability. The UAV can reach any point of

the hangar, thus it can process aircraft of any size. Moreover, it is able to paint with stroke as long as

needed achieving a better final result. Furthermore, the multirotor structure allows an easy maintenance

of the system because the maintainer has clear access to all the component of the system.

The use of UAVs gives advantages as well as many disadvantages. Due to their design, they produce

a lot of noise while flying. They have low flying autonomy especially in heavy payload applications as this

one [32]. Moreover, they have poor positioning and accuracy skills as well as vibration problems [33].

A common problem with the multirotor platform is the failure of one of the engine causing the failure

of the whole system. In this application the problem is even bigger because a failure of the system can

cause serious damages to the aircraft structure if it falls on breakable parts. Not least, the air flow due

to the propellers can affect the painting final result.

The precision control of multirotor UAVs is an hard task, and in this application the weight change due

to the paint (paint remover) consumption increases the complexity of the problem. The more accuracy

and precision required, the more work has to be done on the control of the UAV, thus a long development

time is expected.

21

2.2.2 Rail Mounted Robot

A robotic arm is mounted on a three linear DoF rail able to move it everywhere in the workspace defined

in Section 2.1.1 (see Figure 2.5). Because the rail structure is big and heavy, its motion is supposed to

be slow. Therefore, it locates the robotic arm close to the surface to process and the arm positions and

directs the end-effector, ensuring a proper final result. Thus, the robotic arm shall have at least 6 DoF

for both the position and orientation of the end-effector along the aircraft surfaces.

Figure 2.5: Rail mounted robotic system sketch.

This design allows an accurate positioning of the end-effector because it is possible to determine the

position of the arm along the rails with precision.

The rails ensure a energy supply line to the robot as well as a supply of compressed air, therefore

there is no need of placing batteries and air compressor close to the robotic arm. Thanks to this, the

laser paint removal method, that requires a big amount of electric energy, is easy to implement. In this

way, all the paint removal methods of section 1.2 can be implemented on this design.

Due to the big workspace, the rail structure has to be really long, heavy and expensive. A limit of this

solution is that it is impossible to use the present system to process aircraft bigger than the workspace.

For this system, positioning the aircraft with precision inside the workspace is essential. Furthermore,

it has a problem processing some part of the aircraft especially if complex shapes are involved. For

example, to reach the nose of the aircraft the rail gets as close as possible to the fuselage but then a big

robotic arm is needed to go from the rail to the nose increasing the cost of the whole system.

Because of the paint requirement of subsection 2.1.2 at least two robots at opposite sides of the

aircraft are needed. In this architecture, the motion of the two robots is not completely independent one

from the other but the elevation of the rail must be the same to share part of the structure.

22

2.2.3 Mast Mounted Robot on AGV

A robotic arm is mounted on a mast that is located on an omnidirectional Automatic Guided Vehicle

(AGV) able to move in any direction as well as to perform zero radius turns [34]. The mast rises and

lowers the robotic arm while the AGV positions and rotates it [34]. The robotic arm is then required for

the motion and orientation of the end-effector along the surface, requiring at least 6 DoF. A sketch of this

solution is shown in Figure 2.6.

Figure 2.6: Mast mounted robotic system on AGV sketch.

From Section 2.1.1 considerations, the length of the robotic arm workspace has to be of 4 m in a

ground parallel plane. Painting robotic arms of this size are not available on the market, so it has to be

made ad hoc for this system, increasing the cost of the system and the development time. A bigger arm

for equal DoF has generally a lower dexterity, which can be a problem with complexly shaped aircraft.

Some features of this design are due to the use of an omnidirectional AVG and are common to all the

following solutions. It adds to the system 3 DoF, i.e. the system can move everywhere on the ground as

well as rotate around a vertical axis. Therefore, a high precision in positioning the aircraft in the hangar

is not required. However, complex systems to localize and determine the attitude of the system are

required [35, 36].

23

All the heavy and cumbersome parts of the system, i.e. tanks, batteries, compressor, etc., are

located on the AGV platform. Nevertheless, because each wheel has a maximum load the positioning

of the center of gravity on the AGV is important.

This design can accommodate any of the finish system removal methods described in section 1.2

but, depending of the material of the AGV wheels, the chemical stripping can be unsuitable, reacting with

the wheels material. Furthermore, the paint removal can be accomplished by just one robot because it

is able to move around the whole aircraft while for the coating always two robots are needed.

2.2.4 Multi DoF Structure on AGV

A robotic arm is located at the tip of a beam which is supported and moved up and down by a lifting

system mounted on an AGV. The beam is able to move back and forward along its axis as well as to

rotate around the joint with the lifting structure (see Figure 2.7).

Figure 2.7: Multi DoF robotic system on AGV sketch.

In accordance with Section 2.1.1, the length of the beam plus the length of the extended arm has to

be at least four meters. While the height of the lifting system depends on the height of the AGV, on the

maximum height reached by the robotic arm and on the beam length and maximum rotation angle.

24

The idea behind this solution is to use a small and light robotic arm with a structure able to position

it at any point of the work surface. However, the robotic arm has to have 6 DoF at least to control the

end-effector orientation with respect to the surface as well as to control the stroke of it because the other

motions of the structure are supposed to be too slow to control it adequately.

With a six DoF robotic arm, this architecture has 12 DoF. It allows the robot to work on complexly

shaped aircraft. Nevertheless, it needs a sophisticated control software increasing the development

time and system cost. Moreover, it is possible to have precision and accuracy problems due to the many

movable joints involved. For the same reasons, the reliability of this system is supposed to be low.

Mainly not to overload the beam, most of the equipment of the system should be located on the

AGV. So does the paint (paint remover) tank. The paint (paint remover), the electric power and the

compressed air have to be supplied to the robotic arm. Because the lifting structure and the beam can

move and rotate, the design of the supply lines is expected to be difficult.

2.2.5 Lifting Structure on AGV

A robotic arm is located at one end of a beam which has its longitudinal axis in a ground parallel plane.

The beam is supported on the other end and moved up and down by a lifting system positioned on a

omnidirectional AGV (see Figure 2.8).

The AGV and the lift, providing the system with 4 DoF, position the robotic arm with respect to

the work surface. The arm has to position and direct its end-effector, thus it needs at least 6 DoF.

Accordingly, the system has 10 DoF.

Compared to the previous solution, the lifting structure is higher and therefore heavier. However, the

lack of the two movable joints between the beam and the lifting structure makes the control of this robot

and the design of the supply lines to the robotic arm easier. It also increases the reliability of the system

as well as its precision and accuracy.

2.3 Design Selection

Along this section a trade-off between the designs expounded in Section 2.2 is carried out. According

to the K. Otto and K. Antonsson trade-off strategy [37], the most important features for the final design

are also selected. A weight from 1 to 5 is assigned to each feature according to the importance it has in

the project. Then, a mark per feature is assigned to each design of Section 2.2. The marks, from -5 to

5, follow the analysis made in the previous Section 2.2.

To accomplish a selection between the five solutions, each mark is multiplied by its weight and then

summed to the other marks of the same solution. The chosen design is the one with the higher final

score. This trade-off process is resumed in Table 2.2. The selection criteria are rewarding the design

with the lowest cost and complexity from both the mechanical and control point of view. A higher weight

has been given to features like cost and control but also development time and reliability. From the

performance point of view, accuracy and repeat ability have been considered important as well as the

25

Figure 2.8: Arm and lifting robotic system on AGV sketch.

adaptability of the system to different aircraft shapes and sizes.

Table 2.2: Possible solutions trade-off.Weight Multirotor UAV Rail Mounted Mast on AGV Multi DoF on AGV Lift on AGV

Control 5 -3 3 3 0 3Cost 5 3 -2 -4 2 3Simplicity 4 1 3 3 -1 4Accuracy 4 -4 4 3 1 2Repeatability 4 -3 4 2 2 3Development time 4 -1 3 -4 2 3Adaptability 4 5 -4 3 4 3Multiple tasks 3 0 2 4 2 2Reliability 3 -1 4 2 1 3Maintenance 3 5 2 2 2 3Rapidity 2 3 2 2 0 2Payload 1 -4 4 2 2 3

Result 6 77 59 53 120

The solution selected by the trade-off analysis is the lifting structure on AGV. It has also been selected

26

because between the AGV mounted systems it is the cheapest and the simplest solution. The UAV

solution has big control problems and this impairs its selection. The rail mounted system, on the other

hand, has costs and adaptability problems.

27

28

Chapter 3

System Design

In this section the preliminary design of the automatic system is presented.

From the trade-off analysis of Chapter 2 the solution selected is composed of a small robotic arm

located at the tip of a cantilever beam moved along a vertical axis by lifting system mounted on an AGV.

This system has to reach any point of the workspace in accordance with Section 2.1.1. Moreover, it has

to ensure a proper paint application and removal.

As already mentioned, the system is designed in order to allow the client to choose the painting and

paint removal methods to implement.

To start designing the components of the structure it is necessary, first of all, to select the robotic arm.

This is essential to know the load the structure has to support as well as the dimensions of the other

parts of the system. Once the weight and the workspace of the robotic arm are known, the horizontal

beam and the lifting system (lift) are designed. With the selection of the other subsystems required, the

weight the AGV has to support is known, then it is selected. Finally, the location of the parts on the AGV

is decided to avoid an overload of its wheels.

3.1 Robotic Arm Selection

In this project an off-the-shelf robotic arm is used for the following reasons: the design and manufacture

of a robotic arm would increase the development time and the system costs (especially because of the

safety certification involved), secondly the arm control system is already developed and implemented.

Many robotic arms are available on the market. To select one of these, the following criteria are

applied:

• Lightness

• Workspace equal to or bigger than a human painter

• ATEX certification

• Production company able to ensure spare parts supply in the next decades

29

• Different end-effector tools

The arm has to be light to reduce the weight of the whole structure on the AGV. To be light the arm

has to be small, but to ensure a proper length of the strokes it needs a workspace equal to or bigger than

an human arm. Moreover, by regulation the equipment handled by the workers has to be ligher than 5 kg,

thus, to install these tools on the arm, its maximum payload can not be less that 5 kg [38]. The human

arm workspace, shown in Figure 3.1, has a maximum reach in the horizontal plane of 60 cm as well

as on the vertical plane. Its shape is approximately three quarter of a sphere. Following the preceding

criteria different robotic arms are selected from the catalog of the most solid robotic industries.

Figure 3.1: Human arm workspace [39].

The specifications of different robotic arms chosen following the above criteria are shown in Table 3.1.

To have an approximate indication of the workspace dimensions, in Table 3.1 reach means the maximum

extension in a horizontal plane.

Table 3.1: Different robotic arms specifications.

Robotic arm Reach [mm] Weight [kg] Payload [kg]

FANUC Paint Mate 200iA [40] 704 35 5FANUC Paint Mate 200iA/5L [41] 892 37 5Motoman EPX 1250 [42] 1250 110 5ABB IRB 52 [43] 1200 250 7

The two robots by FANUC are the lightest thanks to their reduced dimensions and their aluminum

structure. Between these two is selected the FANUC Paint Mate 200iA/5L (Figure 3.2) because of its

higher reach compared to the FANUC Paint Mate 200iA that weights 2 kg less but has a reach lower by

188 mm. The bigger reach allows a reduction of the supporting beam length as well as the possibility to

work larger surface without moving the whole system.

Important features of this robotic arm are its avarage power consumption of 0.5 kW, the dimention

of its footprint (260 x 265 mm), and the possibility of mounting it in any position with respect to the

ground [41].

30

(a) FANUC PaintMate 200iA/5L. (b) FANUC PaintMate 200iA/5L workspace in mm.

Figure 3.2: FANUC PaintMate 200iA/5L and its workspace [41].

3.2 Structure Design

According to Section 2.1.1, the system has to reach a height of 13 m and be able to locate its end-effector

at least 4 m away from the lift in a horizontal plane.

The robotic arm can extend up to 1267 mm from its base in the vertical plane (see Figure 3.2(b)).

To make a conservative design, the lift structure height is computed without take into account the AGV

height. So the height of the lifting system is Hlift = 13− 1.3 = 11.7 m. Following the same process, the

length of the beam should be 3.1 m but is chosen to use a 3.5 m long beam to oversize the system.

3.2.1 Horizontal Beam Design

The horizontal cantilevered structure supporting the robotic arm can be approximated as a uni-dimensional

structure, then easy computations about its deformation are done using the Euler-Bernoulli beam theory.

The weight of the robotic arm and the weight of the beam itself cause a bending moment on the

cantilever beam. It is important to decide how much the tip of the beam is allowed to move downward

from its ideal position taking into account that the final position of the robotic arm depends also from the

deformation of the lifting structure.

The forces acting upon the beam are the weight of the robotic arm and the weight of the beam itself

as shown in Figure 3.3. The weight of the robotic arm Warm is known (37 kg) and is applied at the

tip of the beam, this is an approximation as the arm center of gravity changes position with its motion.

However, the robotic arms are made with a structure getting lighter from the base to the tip [13], thus

this is a good approximation. On the other side, the weight of the beam is applied in the beam center of

gravity and depends by its material and shape.

31

Figure 3.3: Forces acting on the horizontal beam.

The materials generally used for structural beams are steel and aluminum alloys. In this project an

aluminum alloy beam is used to have a lighter structure. Between the aluminum alloys the Al 6061-T6

is selected. It is the most commonly used aluminum alloy for structural applications, has above average

corrosion resistance, good machinability, and is excellent for welding [44]. Its properties are shown in

Table 3.2.

Table 3.2: Aluminium alloy Al 6061-T6 properties [44].

Density ρ 2.7·10-6 kg/mm3

Ultimate Tensile Strength Sp 310 MPaTensile Yield Strength Sy 276 MPaModulus of Elasticity E 68.9 GPa

It is decided to use a constant section beam to have easy structure computations and a cheap off-

the-shelf part. The cross section of the beam determines how the beam deforms under a load. To have

a pure bending, a symmetrical cross section is selected and specifically a I-section beam that has high

strength to weight ratio. I-sections beams are generally rolled or extruded, thus are produced in large

quantities at economic prices [45].

To select the dimensions of the beam section an iterative computation is used. The American Society

for Testing and Materials developed a standard for the section of metal beams, the ASTM A6 [46],

where standard dimensions and properties of I-section beams are listed. It is possible to make a table

of different I-sections sorted according to the cross section moment of inertia Ix about the x axis of

Figure 3.4. A sample of such table is shown in Table 3.3 where h, s, and w are the dimensions in

Figure 3.4.

Table 3.3: Part of the bean cross section properties table.

Metric Depth Width Web Thickness Sectional Area Weight Ix Iymm x mm x kg/m h [mm] w [mm] s [mm] [cm2] [kg/m] [cm4] [cm4]

W 310 x 310 x 44.2 318.0 308.0 13.1 165.0 44.22 30770.0 10040.0W 310 x 310 x 48.8 323.0 309.0 14.0 182.0 48.78 34760.0 11270.0W 310 x 310 x 53.9 327.0 310.0 15.5 201.0 53.87 38630.0 12470.0W 310 x 310 x 61.1 333.0 313.0 18.0 228.0 61.10 44530.0 14380.0

The bending moment Mx acting about the x axis of the beam is given by Equation 3.1 where Wbeam

32

Figure 3.4: Beam cross section.

is the beam weight, Warm the robotic arm weight and lb the length of the beam. As a result of the Euler-

Bernoulli theory the tip deflection dbeam of a cantilever beam is given by Equation 3.2, where E is the

Modulus of Elasticity of the beam material and Ix is the moment of inertia about the x axis.

Mx =Warm · lb +Wbeam · lb/2 . (3.1)

dbeam =Mx · l2b2 · E · Ix

=⇒ Ix =Mx · l2b

2 · E · dbeam. (3.2)

The iterative computation calculates the bending moment starting from a zero weight beam, then

computes the moment of inertia needed to have the imposed tip deflection from Equation 3.2. Known

the moment of inertia, it looks into the ASTM A6 sorted table for the first section with a moment of inertia

bigger than the required one. From the weight per meter column of the table it computes the weight of

the beam, then the process restarts. It stops when it selects twice the same section.

A parametric study of the beam weight as a function of the maximum tip deflection has been per-

formed. The result of the study is set out in Figure 3.5. The graph illustrates that the weight drops to

53.2 kg for a displacement of 2.75 mm, then it remains stable. Because the weight decreases with an

average gradient of 170 kg/mm, to have a light structure the beam section of Table 3.4 is selected. Its

weight is 53.2 kg corresponding to 2.75 mm of tip displacement. This displacement is constant during

the whole operative life of the system, is then possible take it into account during the system control

design restricting the error introduction.

To have the total weight of the beam is necessary to add 500 mm, i.e. 7.6 kg, that is the part of the

beam inside its support, designed in Section 3.2.2. Finally, the total beam weight is 60.8 kg.

Table 3.4: Beam cross section properties.

Metric Depth Width Web Thickness Sectional Area Weight Ix Iymm x mm x kg/m h [mm] w [mm] s [mm] [cm2] [kg/m] [cm4] [cm4]

W 310 x 165 x 15.2 313,00 166,00 6,60 56,70 15,20 9934,00 854,70

A stress analysis is now carried out to ensure that the stress on the beam is lower then the material

33

Figure 3.5: Beam weight vs. tip deflection.

tensile yield strength. The beam section with the higher stresses is the cantilever section, the shear

force and moment in this section are computed by Equations 3.3 according to Figure 3.6.

Vy =Wbeam +Warm = 884.9 N , (3.3a)

Mx = 3500 ·Warm + 1750 ·Wbeam = 2.184 · 106 N ·mm. (3.3b)

Figure 3.6: Beam forces at the root section.

The normal stress in the section is given by Equation 3.4a. For I-sections, the shear load is given by

Equation 3.4b and is assumed to be evenly applied only to the section web [47]. The total stress σ is

computed using the Von Mises yield criterion in Equation 3.4c [48]. To find the maximum stress in the

section, it is computed for points A and B of Figure 3.7. The maximum total stress is 3.44 N/mm2 at

point A. It ensures safety working condition to the structure being 80 times lower than the tensile yield

strength of the material (276 MPa).

σz =Mxy

Ix, (3.4a)

τzy =Vy

Areaweb, (3.4b)

σ =√σ2z + 3τ2zy . (3.4c)

34

Figure 3.7: Stresses in the beam root section.

3.2.2 Beam Support Design

The support of the horizontal beam consists of four aluminum alloy plates welded together to form the

structure in Figure 3.8, the aluminum alloy of Table 3.2 is used. The beam is held and bolted to the

support as in Figure 3.9, where also the bearings and the screw nut that connect the support to the

lifting structure are drawn. The bearings are able to transmit to the lifting structure only forces normal to

their axis, while the vertical load is transmitted to the ball screw through the screw nut (their functions

are explained in details in Section 3.3).

Figure 3.8: Horizontal beam support structure.

The forces and moment applied to the beam support are shown in Figure 3.10. The forces are

assumed symmetrical with reference to the z-y plane where the z axis coincides with the beam axis of

Figure 3.3. In Figure 3.10, Vy and Mx are the force and moment due to the weight of the robotic arm

and the portion of the beam outside the support computed by Equations 3.3. Whouse is the weight of the

portion of the beam inside the support. Fs is the lifting screw force, while Fbear1,Fbear2,Fbear3 and Fbear4

are the forces on the bearings. All the computations are made in a steady situation but a conservative

approach is adopted to take into account dynamic loads.

According to Figure 3.10, the equilibrium Equations 3.5 are written. In these equations the distance

between the bearing axis and the support edge is neglected (wbear ' whouse).

There are three equilibrium equations and seven unknowns, then in Equations 3.6 the following

35

Figure 3.9: Horizontal beam and support structure assembly.

Figure 3.10: Beam support free body diagram.

assumptions are made: Fbear4 = 0 and Fbear3 = Fbear2. Finally, the parametric analysis of Figure 3.11

is executed to choose wbear and hbear.

To select wbear and hbear has been taken into account the load on the bearings and the general

dimensions of the beam support. The load on the bearing influences their rails dimensions and weight,

on the other hand the beam support can not be too cumbersome. Thus, wbear is chosen equal to 500

mm and hbear equal to 700 mm, it implies a design force of 3436 N on the bearings and of 959.5 N on the

screw. It is noted that, throughout these computations, the beam support weight has been neglected.

Fs =Whouse + Vy = 0.149 N/mm · wbear + 884.9 N , (3.5a)

Fbear4 + Fbear3 − Fbear1 − Fbear2 = 0 , (3.5b)

(2Fbear1 + 2Fbear2 + 2Fbear3 + 2Fbear4)hbear2

=Mx + Vywbear

2. (3.5c)

36

Fs =Whouse + Vy = 0.149 N/mm · wbear + 884.9 N , (3.6a)

Fbear1 = 0 N , (3.6b)

Fbear2hbear =Mx + Vywbear

2. (3.6c)

Figure 3.11: Beam support parametric study.

To compute the plates thickness, it is assumed that all the load acts only on one of the two horizontal

plates. The stress on this plate is computed studying it as the cantilever beam of Figure 3.12.

The maximum stress is located in the cantilever section, the shear force Vr and moment Mr in this

section are computed by Equations 3.7. The normal stress due to the bending is σz = MryIx

where

Ix = 166t3

12 is the section moment of inertia about the x axis of Figure 3.12 where t is the plate thickness.

The shear stress is τyz = ( 6Vr

166t3 )(t2

4 − y2). Then, applying the Von Mises yield criterion of Equation 3.4c,

the maximum stress is σ(y = t/2) = Mr

27.7t2 . Equaling this value with the material tensile yield strength,

a minimum thickness of 18.6 mm is computed. A 19 mm plate is then selected for the beam support

structure which weights 44 kg.

Vr =Whouse + Vy = 959.5 N , (3.7a)

Mr =Mx + 500 · Vy + 250 ·Whouse = 2.645 · 106 N ·mm. (3.7b)

3.2.3 Lifting System Structure Design

Along the present section the lifting system structure design is described. The lift is 11.7 m tall and its

functions are: holding the horizontal beam support and moving it in a vertical direction. The system is

mounted on an AGV platform, then it is important to limit its weight.

The first lift concept, shown in Figure 3.13, was composed of two continuous metal plates, with two

37

Figure 3.12: Beam support forces diagram and section.

linear rails and a ball screw mounted on each side of the beam support. The function of the linear

bearings that connect the beam support with the lift is to transmit the moment due to the robotic arm

and the beam weight to the lift structure (see Figure 3.19), while the screw carries the vertical loads and

transmits the motion to the horizontal beam through the screw nut. Then, in first approximation, the only

loads on the lift structure are: the moment transmitted by the bearings and the weight of the structure

itself.

Figure 3.13: Lifting system exploded top view.

Being a tall column, the lift structure is designed to avoid buckling. According to the Euler bucking

theory, maximum height for a free-standing, vertical column, loaded by its own weight, is given by Equa-

tion 3.8 [48]. Where E is the Young’s modulus, I is the minimum moment of inertia of the beam cross

section, g is the acceleration due to gravity, A the cross section area and ρ the material density.

Hlift = 7.84EI

ρgA. (3.8)

tmin =

√12Hliftρg

7.84E= 31 mm. (3.9)

The lift plate has the rectangular cross section in Figure 3.14. The minimum moment of inertia is

Iz = t3 w12 , while the area is A = t w. Where t is the plate thickness and w its width. To minimize the

lift weight, the plates width w is selected equal to 600 mm, this is the minimum width for the lift due to

the bearings rails spacing selected in Section 3.2.2. Substituting the moment of inertia and the area

38

equations into Equation 3.8, the minimum thickness to avoid buckling is given by Equation 3.9, where

the material is the aluminum alloy which properties are in Table 3.2. It corresponds to a weight of 588 kg

per plate. To reduce the lift weight, a different lift structure has been designed.

Figure 3.14: Lift structure plate cross section.

To make the lift light, a truss structure is used. It is manufactured from only one aluminum alloy plate

bent and cut as in Figure 3.15. Again, the material selected is the aluminum alloy Al 6061-T6 whose

properties are summarized in Table 3.2. To allow preliminary computations, all its structural elements

have the same thickness and width.

Figure 3.15: Lift structure detail.

To size the structure, it is studied as composed of jointed beams. The idea is to analyze the buckling

of the column at the lift base, pointed out in Figure 3.16. As shown in Figure 3.17(a), the concentric

axial load due to the structure weight Wleg is applied to the column. It is approximated by Equation 3.10,

where Hlift is the lift height, ρ the material density, g the gravitational acceleration and Aleg the column

area.

Wleg = Hlift · ρ · g ·Aleg . (3.10)

The column has the boundaries conditions shown in Figure 3.17(a), i.e. one end fixed and the other

supported. To size the structure the following dimensions have to be selected: the structure section

thickness tleg and width Lleg (shown in Figure 3.17(b)), and the column height hleg.

According to the Euler buckling theory [48], the critical load for the described column is given by

Equation 3.11. Where E is the material Young’s modulus, hlegcr is the column height at which buck-

39

Figure 3.16: Lift structure base.

(a) Base support forces and constraints. (b) Base support section.

Figure 3.17: Lift base approximate structure.

ling occurs and Ileg is the column section smallest moment of inertia about its principal axes given by

Equation 3.12 (for equation clarity, tleg and Lleg are abbreviated to t and L respectively).

Pcr =π2EIleg

(1.2 hlegcr )2. (3.11)

Ileg =t(2L4 − 4L3t+ 8L2t2 − 6Lt3 + t4

12(2L− t). (3.12)

Substitution of Equation 3.10 into Equation 3.11 gives Equation 3.13, where a safety factor f = 3

is added. The parametric studies in Figure 3.18 have been performed to size the column section. In

Figure 3.18(a), the behavior of the column critical height hlegcr as a function of tleg and Lleg is shown,

while Figure 3.18(b) shows the behavior of the lift structure weight (approximated as ρ Aleg Hlift) as a

function of the same parameters.

hlegcr =

√π2 E Ileg

1.44 f Hlift ρ g Aleg. (3.13)

From Figure 3.18, it is clear that there is not bucking problem under this load for the column and that,

to have the lightest structure, the section has to be as small as possible. Then, to have enough material

to bolt the bearings rails, the dimensions selected are hleg = 780 mm, Lleg = 60 mm and tleg = 5 mm.

40

(a) Column critical height. (b) Lift structure weight.

Figure 3.18: Lift structure parametric studies.

Consequently, the lift structure weight is 103.2 kg.

On the lift structure, beside its own weight, are acting the forces transmitted by the linear bearings.

To make a structural analysis on the structure, the load acting on it is supposed to be the moment Mlift

computed by Equation 3.14, according to Figure 3.19. Where Warm is the robotic arm weight located at

the beam tip and Wbeam is the horizontal beam weight acting in the beam Center of Gravity (CG).

Figure 3.19: Lift structure forces diagram.

Mlift = 3750mm·Warm+2000mm·Wbeam = 3750mm·521.9N+2000mm·363.0N = 2.684·106 N ·mm.

(3.14)

To compute the stresses along the structure, Ftool is used [49]. It is a simple two-dimensional frame

analysis tool that solves forces equations of truss structures. The lift structure has been analyzed as the

trass pillar in Figure 3.20 loaded by its weight and half of the moment computed by Equation 3.14.

The software computed that the maximum load acts on the root pillar (red in Figure 3.20) and it is

a compression load of 2.7 kN. Then, because the pillar critical load, computed by Equation 3.11, is

41

Figure 3.20: Lift structure truss scheme.

62 kN, it is stated that there is not buckling in the structure. The compression stress related to the load

is 4.696 N/mm2. It is 59 times lower than the material compressive yield strength (assumed equal to the

material tensile yield strength [50]) ensuring enough structure stiffness.

As a simulation result, the lift tip deflection is 17 mm along the z axis of Figure 3.19. The structure

deformation decreases almost linearly to zero at the root of the column. This deformation has to be taken

into account during the control design because it affects the position of the robotic arm end-effector.

3.3 Lifting System Design

The lifting system moves the horizontal beam along the lift, carries its vertical load, and transmits the

moment, due to the beam and robot weight, to the lift structure. The first two tasks are demanded to a

screw-nut system, while the latter is demanded to two sets of linear guides. The system is symmetrical

about the vertical plane in order to split equally the load between two systems and to avoid torsion in the

lift structure. Along this section the linear bearings, the rails and the lifting mechanism are selected.

3.3.1 Linear Guides Selection

Each linear guide is composed of a rail and two linear bearings. In Section 3.2.2 a bearings design load

equal to 3436 N has been computed. Along this section the linear bearings and the rails are selected.

To ensure a spare parts supply along the entire life of the system, all the linear motion systems in this

project are selected from the catalog of Thomson Industries, it is a linear systems leader manufacturer .

The linear rails can be round or square, and end or continuously supported. In this application, the

rails have to transmit a moment to the lift structure, then, continuously supported rails are selected. They

ensure a reduced bending on the rail itself and do not present bucking problems. Between round and

square rails, round rails are selected because they present self-alignment, i.e. the friction increases

much less than for square rails when the lift structure, and then the rails, deforms [51].

For the bearings selection, the following criteria is applied: between the bearings able to support

the load required (3436 N) the one with the smaller shaft diameter is selected. Moreover, it has to be

corrosion resistant to ensure a long working life in a polluted environment. The rail shaft diameter is

42

important because the rail weight increases approximately with the squared diameter of its shaft.

The SSETWNO M16-CR bearing in Figure 3.21 has beam selected, its specifications are summa-

rized in Table 3.5.

Figure 3.21: SSETWNO M16-CR linear bearing1.

Table 3.5: SSETWNO M16-CR linear bearing specifications [51].

Shaft Nominal Diameter 16 mmLoad Capacity 4400 NWeight 0.37 kgBearing Type Ball Bushing BearingMax. Operation Temperature 85o C

Two different continuously supported rails can be coupled to the selected bearings. The rail in Fig-

ure 3.22(a) is an aluminum alloy rail, the height from the base to the mean shaft center is 30 mm, has

a weight of 4.7 kg/m and its attachment bolts are from above. On the other hand the LSRM16 rail in

Figure 3.22(b) is a steel rail, the height from the base to the mean shaft center is 18 mm, has a weight

of 2 kg/m and its attachment bolts are from underneath. The latter is selected to have a lighter and more

compact system. Moreover, the attachment of the rails to the lift structure is easier because the access

to the bolts is from outside the structure. According to the previews selections, the weight of the linear

guiding system is 96.56 kg.

Because the rail material is steel while the lift structure is made of aluminum alloy, a different rail

choice can be done if the system is installed in an environment with considerable temperature gradients.

3.3.2 Lifting Mechanism

The lifting mechanism moves the horizontal beam and the robotic arm as well as supports their weight.

Many linear actuators are able to perform this task, they can be divided into hydraulic, pneumatic and

electromechanical systems.

Hydraulic actuators, using a pressurized fluid to generate thrust, are generally heavier than others

actuators and require continuous electric power to hold the load. Furthermore, their positioning accuracy

1URL http://www.thomsonlinear.com/en/product/SSETWNOM16DD [Accessed: 06 October 2016]

43

(a) SRM16 rail. (b) LSRM16 rail.

Figure 3.22: Linear guides: round rails [51].

is low and require more maintenance than both pneumatic and electromechanical actuators. Pneumatic

actuators are the cheapest and the most powerful actuators but, as the hydraulic actuators, they have

poor positioning accuracy and high maintenance costs. Electromechanical systems are the most ex-

pensive nevertheless they have low maintenance costs, high accuracy and easy control. Moreover, they

hold the load without consuming power [52].

In view of the above, an electromechanical linear actuator is installed. Specifically, it is composed

of screws driven by electric motors. The screws have an end mounted on the AGV, where the motor is

located, while the other is supported by the lift structure as in Figure 3.23. A screw nut is bolted to the

beam support structure, when the screw rotates the horizontal beam moves along the screw axis. In the

present section, screws, screw nuts and electric motors are selected.

Figure 3.23: Lift system view with beam support.

Two type of screws are available: lead screws or ball screws. For this application, ball screws

are used because of their higher precision and efficiency, lower vibrations and longer operative life;

nevertheless they are more expensive [48].

44

According to Table 3.6, the load on the screws is 144.8 kg. The load is supposed to be equally

divided between two screws, one on each side of the beam.

Table 3.6: Vertical load on the lift actuator.Part Weight [kg]

Robotic arm 37Horizontal beam 60.8Bearings 3Beam support 44

Total 144.8

Each screw has approximately the same length of the lift structure and is loaded by concentric axial

load. To find the screw diameter dscrew, a buckling analysis has been carried on.

The minimum axial load causing the bucking of the screw Fbuckling is given by the Euler’s column

buckling formula in Equation 3.15, where n is a factor accounting for the end conditions, E [MPa] the

modulus of elasticity, I [mm4] the screw section moment of inertia and L [mm] the screw length [48, 53].

Fbuckling[N ] = n · π2E · IL2

= 4 · 9.687 · 104 d4screw

L2⇒ dscrewmin

= 26.6 mm. (3.15)

Equation 3.16 gives the screw angular velocity at which resonance occurs [53]. Finally, the screw

lead is computed by Equation 3.17. In this application, the screw is mounted with both the ends fixed to

minimize bucking problems.

nresonance[RPM ] = 1.2 · 108C dscrewL2

= 2.23 · 1.2 · 108 dscrewL2

. (3.16)

Screw Lead =V ertical V elocity

Angular V elocity. (3.17)

From Equation 3.15 the minimum screw diameter is 26.6 mm. On the Thomson catalog are available

ball screws with 32 mm and 40 mm diameters [54]. For these screws the maximum angular velocity

is computed multiplying for a 0.8 safety factor the natural frequency. Each screw has a maximum lead

available, thus, using Equation 3.17, the maximum vertical velocity for each one is computed. The

results are shown in Table 3.7.

Table 3.7: Angular velocity, lead and Vertical velocity for different screws.

Diameter [mm] 32 40

Max.angular velocity [RPM] 50.0 62.6Max. screw lead [mm] 40 40Vertical velocity [m/min] 2.0 2.5

Finally, the 40 mm diameter screw in Figure 3.24 is selected because, in spite of being heavier, it

allows a vertical velocity of 2.5 m/min that is comparable with the vertical velocity of commonly used

human lifts [55, 56]. Screw specifications are summarized in Table 3.8.

45

Figure 3.24: 40MMx40MM ball screw with nut2.

Table 3.8: Ball screw specifications [54].

Model KGS-4040-023-RHDiameter 40 mmLead 40 mmStandard lead accuracy ±23 µm/300 mmMax. backlash 0.041 mmMax. dynamic load 35 kNMax static load 101.9 kNWeight per meter 9.0 kg/mWeight 105 kg

The screw buckling load, computed by Equation 3.15 is 7250 N. It is five times bigger than the total

vertical load on the system (1420 N). Then, to limit weight and costs, only one screw is used. This

design induce an torsion on the beam support structure, in this preliminary study it is neglected because

the arm of the screw thrust with respect to the beam support axis is small.

Between the possible nuts to couple with the ball screw, the flanged nut in Figure 3.24 is selected

because it can be easily bolted to the beam support while is less cumbersome than a round flanged nut.

To drive the screw an electric motor is required. Its minimum torque and power are computed re-

spectively by Equation 3.18 and Equation 3.19, where ε is the screw efficiency equal to 0.9 and the Load

and Angular velocity are the same of the screw.

Torque = Load · lead2πε

= 9.7 Nm. (3.18)

Power = Torque ·Angular velocity = 63.6W . (3.19)

To drive screw linear actuators, generally three type of electric motor are used: Direct Current (DC),

stepper and servo motors. DC motors are continuous rotation motors, they generally run at high speed

2URL http://www.thomsonlinear.com/en/product/7115-448-076 [Accessed: 06 October 2016]

46

and, due to their poor accuracy, are rarely used for accurate positioning. Between servo and step-

per motors, the latter are selected because they are cheaper, can work in an open loop, have higher

performance at low speeds, and require less maintenance (stepper motors are brushless) [57].

Knowing speed and torque ranges, the motor is selected on order to connect it directly with the

screw without adding a gear, this design reduces the system weight and transmission looses. The

stepper motor is then selected from the Oriental Motor catalog that provides the torque-speed graph for

each motor. The motor used is, finally, the PK599BE-N7.2 with the torque-speed graph in Figure 3.25.

Figure 3.25: Stepper motor torque vs. speed graph3.

3.4 Subsystems

In this section are listed, described and selected the required subsystems not covered in the previews

sections.

To control the robotic arm, it has to be linked to its controller, the R-30iATMMate Controller in Fig-

ure 3.26 whose specifications are summarized in Table 3.9.

To paint the aircraft, paint and compressed air are supplied to the robotic arm. The air pressure and

flow rate depend from the technique used. In the manual process the spray guns are fed by long hoses

linked to one or two common air compressor.

To limit the weight and the cost of the system, the air is supplied by hoses linked to an external

compressor. This solution also avoids the air compressor to introduce vibrations into the system. To

get out of heavy and expensive batteries on-board, also the electric power is supplied to the system by

cable linked to an external power source. Then, the system does not have to stop to recharge or change

batteries.

3URL http://catalog.orientalmotor.com/?plpver=11 [Accessed: 06 October 2016]

47

Figure 3.26: FANUC R-30iATMMate Controller.

Table 3.9: Robotic arm controller specifications [40].

Model FANUC R-30iATMMate ControllerWidth 470 mmDepth 320 mmHeight 950 mmAmbient temperature 0 - 45◦ CPower supply 200 - 230 VAC Single PhasePower consumption 0.5 KW (average), 1.2 KW (maximum)Controlled axis 24Built-in I/O Up to 28 digital inputs / 24 digital outputs 24VDCWeight 56 kg

It has to be ensured that the AGV platform does not run over the supply line. Thus, the electric cable

and the air hose are mounted on a retractable reel. Two type of retractable reel are possible: spring or

motor driven. For this system, electric motor driven retractable reel are used in order to control actively

the tension force on the cables and hoses.

48

The paint tank is located on the AGV platform. To size it, the paint usage is estimated. The Lockheed

C-130 Hercules has a wetted area of 2323 m2 [58]. Assuming two robots painting it, each robot paints

1163 m2, the half of it. In Table 3.10 the specifications for different coatings used are shown, they may

vary in accordance with color, manufacturer and environment conditions.

Table 3.10: Paints specifications.Paint Theoretical Coverage [m2/l] Volume [l] Density [kg/l] Weight [kg]

Epoxy Primer [59] 24 48.5 1.32 63.7Elastomeric Polyurethane Primer [60] 11.9 97.7 1.32 128.8High Solid Polyurethan Coating [61] 20 58.1 1.28 74.5Topcoat [62] 82 14.2 1.28 18.2

Multiplying the maximum paint volume needed by a 1.2 factor to take into account coatings with

lower coverage, a 120 liter paint tank is needed. A tank of this size is not required when painting smaller

aircraft or with higher coverage paints, it is difficult to clean and handle, heavy, expensive and increase

the paint waste due to the paint left-over.

In Table 3.11 different paint tanks specifications are shown. To reduce the weight, the cost and the

paint waste a 60 liter tank is selected. This selection implies that painting larger aircraft a refill of the

tank may be required. The total tank weight is then 128 kg (assuming 80 kg of paint).

Table 3.11: Paint tanks spacifications [63].

Volume [l] Weight [kg] Max. pressure [bar] Cost [e]

22 31 6 278045 44 6 310060 48 3 - 6 3600 - 3700

120 72 1.5 - 6 6700 - 8000

The subsystems required for the paint removal depend from the method selected. Chemical stripping

does not requires any additional subsystem, while to optical remove the paint a laser equipment is

necessary. The mechanical removal by water or media blasting requires a dedicated tank and blasting

equipment on the robotic arm tip. On the other hand, if the paint is removed by a motor driven abrasive

equipment, it is only required to replace the arm manifold. Therefore, if the customer requires laser,

MPW or PMB paint removal ad hoc studies are are required to select and locate the subsystem.

3.5 AGV Platform

In the present section an omnidirectional AGV is selected. Then the location of the subsystems on the

vehicle is analyzed.

3.5.1 AGV Selection

To select an AGV it is necessary to know the load it supports. According to Table 3.12, the payload on

the vehicle is 635.6 kg. The weight of the parts not yet designed (electric power, paint and air supply

49

lines, bolts etc.) is unknown. Thus, to select conservatively the vehicle, a minimum payload of 1000 kg

is selected.

Table 3.12: AGV payload.

Part Weight [kg]

Robotic arm 37Arm controller 56Horizontal beam 60.8Beam support 44Ball screw 105Linear bearings 3Rails 93.6Stepper motor 5Lift structure 103.2Paint tank 48Paint 80

Total 635.6

For the present project, it is required a zero turning radius vehicle and the possibility to guide it with

high accuracy. Moreover, it has to be possible to locate the robotic arm at an height of 0.5 m above the

ground. The majority of AGVs are used in warehouses or in assembly lines and are designed for much

higher payload [64].

Only one AGV that satisfies all the requirements has finally been found. It is the RoboMate 17

by Vetex, the vehicle is equipped with four omnidirectional wheels that support up to 1000 kg each.

Unloaded, its maximum speed is 67 m/min. Moreover, it is a modular system and the vehicle structure

is designed meating the client requests [65].

The company only sells the vehicle, the navigation control has to be developed ad hoc for the ap-

plication and implemented. Many different navigation systems have been developed, they include ultra

wide band indoor Ground Positioning System (iGPS), laser, and vision based Simultaneous Localization

and Mapping (SLAM). Typical accuracy of an omnidirectional vehicle under autonomous global naviga-

tion can be from +/- 20 mm to +/- 10 mm, depending on the type of system used. Often, when greater

accuracies are required, a combination of sensing systems can be used, such as switching from ultra

wide band iGPS indoors, to vision based localization at a micro level [66, 67, 68]. The development of

the navigation systems is demanded to future works.

With the selection of the AGV, the main components of the system have been designed or selected

off-the-shelf. In Figure 3.27, an overview of the whole system is shown.

3.5.2 Subsystems Location

Locating the subsystem on the vehicle, an even distribution of load has to be ensured. There are two

main problem to take into account: paint consumption and positioning of the lift axis in the center of the

vehicle. The paint consumption causes a system center of gravity shift, thus the load on each wheel

50

Figure 3.27: Automatic system overview.

changes during the painting. This problem could be solved positioning the paint tank in the wheels

centroid (from here on called only centroid), but this is not possible because the lift axis has to be

located coincident with the centroid. It ensures that, when the vehicle is performing a zero radius turn,

the result is only a rotation about the lift axis without any translation.

The CG positioning problem is divided into lateral and longitudinal positioning. Along this section,

the AGV is supposed to have a longitudinal and a lateral plane of symmetry.

The lateral positioning of the subsystems is easier because the CG of most of the parts is located

in the vehicle plane of symmetry (z-y plane), as shown in Figure 3.28. The only two parts with the CG

away from the z-y plane are the screw and the robotic arm controller. The latter is used to balance the

screw moment and locate the system CG in the z-y plane. The controller position is then computed

by Equation 3.20, where Wscrew and Wcontroller are the ball screw and the robotic arm controller weights

respectively.

Figure 3.28: Top view of the system.

51

xcontroller = −150Wscrew

Wcontroller= −216 mm. (3.20)

The longitudinal positioning of the subsystem is shown in Figure 3.29. From the system drawings,

it is measured that the paint tank CG can be located at the minimum distance of 540 mm from the y

axis. Neglecting the paint weight, to balance the system only with the arm controller it shall be located

4 m away from the y axis. Considering an AGV length of 2.5 m , the controller CG is located at the

distance of 1000 mm from the y axis. The position of system CG is then, 318 mm along the z axis with

the empty tank and 210 mm with the tank fulled with 80 kg of paint. It corresponds to a load per wheel

of 211 kg on the front wheels and of 109 kg on the real wheels. The weight of the AGV itself and of

the yet-to-design parts have to be added. Locating them in the rear part of the vehicle, it is possible to

reduce the unbalance between front and rear wheels load.

Figure 3.29: System sideview.

3.6 Cost Estimation

In the present section a system cost estimation is carried out. The estimated cost is related to the parts

of the system already designed or selected. The labor cost for the development of the missing parts and

for the assembly has not been considered. Thus, the estimated cost is simply the sum of the costs of

the parts described along the present chapter.

In Table 3.13 the cost for each part is shown. Most of this data comes from a direct contact with

the manufacturers. The total cost for the system is estimated to be 86.6 ke. To this value the price

of the parts not yet designed, as the system sensors, has to be added as well as the control system

development costs and assembly costs.

52

Table 3.13: Cost estimation.Part Cost [e]

Robotic arm w/ controller 20000Horizontal beam 900Beam support 100Lifting system 4000Lift structure 2000Stepper motor 1000Paint tank 3600AGV 55000

Total 86600

53

54

Chapter 4

Conclusions

The fleet of commercial aircraft is growing, and in the future years more aircraft will require maintenance.

A phase of the aircraft maintenance is the removal and application of the finish system to check the

substrate integrity and protect it from corrosion.

The whole process is, nowadays, achieved completely manually. It requires a big amount of time

and labor. Furthermore, it has to be accomplished inside a dedicated hangar for environmental safety

reasons, thus at the same time only one aircraft per painting hangar is processed. To tackle the aircraft

number growth, there are two ways: either to increase the painting hangars number and, consequently,

the number of workers, or to increase the finish system maintenance rate.

The automation of the aircraft finish system maintenance is nowadays a big challenge in the aerospace

industry. The implementation of an automatic system induces great advantages under many aspects.

The maintenance rate increases as well as the quality of the final result. Meanwhile, the whole process

cost drops because a big amount of highly skilled labor is no more required and the material waste is

reduced. Moreover, the introduction of an automatic system drastically reduces the workers exposition

to a toxic environment during the painting and paint removal of airplanes. Not least, the environmental

impact of the process is reduced thanks to the waste optimization.

Consequently, as an aerospace maintenance leader company, OGMA required the preliminary de-

sign of an automatic finish system maintenance procedure. This has been the subject of the present

work.

Presently, there is no robotic system able to achieve the whole coatings system maintenance. Al-

though several projects are under development especially for the paint removal automation, only a few

involve the painting of aircraft. Beside painting and paint removal, the automation of the aircraft masking

would give big benefits but the technology to do it is not yet available and/or the system would get too

complex and expensive. Therefore, along the present thesis a solution for the automation of aircraft

painting and paint removal has been designed.

There are many different methods and technologies for both painting and paint removal. The design

criteria of this thesis is to study a system able to implement as many of them as possible, in order to give

the client the opportunity to choose methods and technologies taking into account the advantages and

55

disadvantages of each one.

The system has been sized in order to process airplanes with a maximum height of 13 m and a

minimum height of 0.5 m. The airplanes OGMA maintains are within these dimensions as most of the

medium to small commercial and military aircraft.

Five possible systems have been evaluated and, through a trade-off process, the one in Figure 4.1

has been selected. The selection criteria were rewarding the design with the lowest cost and complexity.

Specifically, the system consists of a robotic arm mounted on the tip of a cantilever beam which is held

by a structure able to move it along a vertical axis. This structure is mounted on an omnidirectional AGV

where all the required subsystems are located.

Figure 4.1: Complete system drawing.

To compete with the state-of-the-art robots under development, the system has to ensure high per-

formance and rapid development at the lowest investment and operating costs. Thus, the driving design

criteria has been simplicity.

To ensure high performance, high accuracy selection criteria were adopted during the design. More-

over, the system has been designed to process as many aircraft as possible taking into account different

shapes and sizes. Finally, to reduce the costs, it is composed mostly by off-the-shelf components. They

do not require to be designed and manufactured but are mass-produced by specialized companies at

a lower cost. Furthermore, it is possible to have a spare parts supply during the operative life of the

system.

Because this is a complex problem, the development time for a project of this kind is generally

long. Namely, the developing and validation of the control system require a long time. To reduce the

development time, the system has been designed as simple as possible both from the structural and the

control point of view.

In detail, the system is composed of a FANUC Paint Mate 200iA/5L robotic arm installed at the tip of

an aluminum alloy I-beam 4 m long. The beam is held by a support that connects it to a lifting system

56

composed by an aluminum truss structure, four linear guides and a ball screw driven by a stepper motor.

The whole structure is then mounted on a Vetex RoboMate onmidirectional AGV on which the paint tank

and the robotic arm controller are installed. The AGV receives electric power and compressed air from

external sources through cables and hoses.

Except for the lift truss structure and the beam support, all the components listed above are off-the-

shelf.

4.1 Future Work

Being a preliminary design study, this is a stepping stone for future works. First of all, some components

of the system are yet do be designed, i.e. the electric power and air supply lines, AGV chassis and ball

screw support.

To lead preliminary evaluations, simplified theories and big safety coefficients have been used. This

led to a conservative design from the structural and mechanical point of view. An interesting future work

would be to carry on more precise analysis to reduce the structure weight and increase its stiffness.

From the structural point of view also a dynamic analysis has to be done. Especially vibration a analysis

is required to avoid resonance in the structure.

Based on the present work, an active collaboration with the client is necessary to define in detail the

system features. Specifically, painting and paint removal methods have to be selected to advance on

the design.

Once the system equipment, structure and mechanics are completely defined, the design of the

control system begins. First of all, the accuracy and precision required have to be computed. To do that,

tests and simulations for both painting and paint removal processes have to be done. Then, the general

approach to the problem has to be selected.

Generally, there are two possibilities: open-loop and closed-loop control. The first one does not use

feedback and would process the aircraft based on the drawings and process software without checking

its real position with respect to the surface and the result accomplished. The second control technique

uses the feedback loop, based on sensors, to check the system error with respect to the required position

and nullify it [13]. Beside the control of the system itself, a navigation system has to be designed as well

as a collision avoidance system to ensure the aircraft integrity.

Not last, the system cost estimation of Section 3.6 has to be improved including the control develop-

ment and implementation costs as well as the operative costs.

As already said, this work is a stepping stone, this complex problem is far from being solved. But, as

Mattie J.T. Stepanek said, ’even though the future seems far away, it is actually beginning right now’.

57

58

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