Development and application of an intelligent welding ... · Development and application of an...

Robotics and Computer-Integrated Manufacturing 27 (2011) 377–388

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Robotics and Computer-Integrated Manufacturing

0736-58

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/rcim

Development and application of an intelligent welding robotsystem for shipbuilding

Donghun Lee n, Namkug Ku, Tae-Wan Kim, Jongwon Kim, Kyu-Yeul Lee, Youg-Shuk Son

Robust Design Engineering Laboratory (RoDEL), School of Mechanical and Aerospace Engineering, Seoul National University, Building 301 Room 210, San 56-1, Shillim 9-dong,

Gwanak-gu, Seoul, 151–742, Republic of Korea

a r t i c l e i n f o

Article history:

Received 2 November 2009

Received in revised form

14 August 2010

Accepted 17 August 2010

Keywords:

Rail-runner mechanism

Intelligent welding robot

Double-hulled block

Shipbuilding

45/$ - see front matter & 2010 Published by

016/j.rcim.2010.08.006

esponding author. Tel.:+82 2 880 7144; fax:

ail address: [email protected] (D. Lee).

a b s t r a c t

Over the last few decades, there have been a large number of attempts to automate welding in the

shipbuilding process. However, there are still many non-automated welding operations in the double-

hulled blocks, even though it presents an extremely hazardous environment for the workers. And, the

hazards come about mainly because of the dimensional constraints of the access-hole. Thus, much

effort has been recently directed toward the research on compact design of the fully-autonomous robot

working inside of the double-hulled structures. This paper describes the design, integration,

simulations, and field testing trials of a new type of welding robotic system, the RRXC, which is

composed of a 6-axis modularized controller, a 3P3R serial manipulator, and an auxiliary transportation

device. The entire cross section of the RRXC is small enough to be placed inside the double-hulled

structures via a conventional access hole of 500�700 mm2, from the outside shipyard floor. The weight

of the manufactured RRXC is 60 kg, with a 6-axis manipulator and modularized controller, and the

weight of an auxiliary transportation device is 8 kg, with a 2.5 m steel wire of 6F. Throughout the field

tests in the enclosed structures of shipbuilding, the developed RRXC has successfully demonstrated

welding functions without the use of any additional finishing by manual welders, and has shown good

mobility using an auxiliary transportation device in double-hulled structures.

& 2010 Published by Elsevier Ltd.

1. Introduction

Commercial ships carrying liquid cargo, such as liquefiednatural gas (LNG), liquefied petroleum gas (LPG), and crude oil,can cause serious environmental pollution from the risk ofspillage. In an attempt to minimize such possibilities of spillage,vessels such as very large crude oil carriers (VLCCs), bulk carriers(B/C), and liquefied natural gas carriers (LNGCs) incorporatedouble-hulled ship walls, as shown in Fig. 1. These consist of outerand inner walls, spaced 2- to 3-m apart; in this way, if the outerwall is holed as a result of a collision or stranding, the inner wallcan still prevent the outflow of the liquid cargo [1].

However, the manufacture of double-hulled ships is more timeconsuming and expensive than that of single-hulled vessels. Fig. 2also shows the manufacturing process used to obtain the closedblock that is a sub-module of the double-hulled ship wall. Abottom shell and an open block are assembled separately usingwelding processes where the bottom shell is composed of a widesteel plate with several reinforcing longitudinal stiffeners weldedto it in parallel. Forming the closed block is more complicated;

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first, a top shell, that is the same shape as the bottom shell, mustbe manufactured. Then, a number of transverse web floors andgirders are welded on to the top shell; second, the open block isturned over and placed alongside the bottom shell and eachlongitudinal stiffener in the bottom shell is aligned with thecorresponding slit in the open block; and finally, third, the openblock is inserted laterally along the longitudinal stiffeners of thebottom shell so that each stiffener slides into its correspondingslit to assemble the closed block, as shown in Fig. 2. The resultingclosed block must then be welded. That is, the welding has to bedone from inside the closed block, along the contacting bound-aries of the top shell and the bottom shell.

Since it is an enclosed structure, the temperature gets hot andis in the range 40–50 degrees during the summer, and it is oftentoo dark to freely carry out tasks, even during the daytime.However, human workers currently execute this welding process,working inside the enclosed space surrounded by the top shell,the bottom shell, a pair of transverse web floors and the girders[2]. As shown in Fig. 3, this manual welding process inside theclosed block represents one of the most difficult and hazardoustasks to human workers in the shipbuilding industry. Moreover,the welding robot, which is currently used in the open blocks,with a 6-axis articulated manipulator, cannot be used in thedouble-hulled block as the overhead gantry crane cannot

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Longitudinal stiffeners

Longitudinal girdersLongitudinal direction

Transverse direction

A Commercial Ships carrying Liquid cargo

300,000ton class VLCC

10,920-mm

5,100-mm

Access hole

3,000-mm

Transverse web floor

Double-hulled block

Sectional Picture of the hull structure in assembly

Fig. 1. Overall view of the double-hulled structure of shipbuilding industry.

Bottom

Shell

Inner Bottom

Block

(Open block)

Turn

Over

Put to the slit Double Hull Structure

(Need to be welded)

Slit

Fig. 2. Manufacturing of a closed block, which is part of the ship wall of double-

hull structure.

Double-hulled structure = Closed block


Longitudinal stiffener

Top plate

Longitudinal stiffeners

Bottom plate Girder

Top plate

Fig. 3. Manual welding processes inside double-hulled blocks.

Table 1Commercialized welding carriages.

(a) (left) 1-axis carriage [3] and (right) 2-axis

welding

carriage [3]

(b) Vertical weaving

carriage,

V-ROD [3]

Wall-guideroller

Welding torch

Driving wheel

Torch cable

Controller

Horizontal fillet welding

Vertical fillet welding

Magnets

Mechanical stopper

Limit sensor

Guide rail/rack

torchholder

Handle

D. Lee et al. / Robotics and Computer-Integrated Manufacturing 27 (2011) 377–388378

approach the inside of the double-hulled block. This is becausethe overhead gantry cranes are installed on the ceiling of theshipyard.

Therefore, it becomes clear from the current status of thewelding process in the double-hulled structures, that there is a

great need for an acceptable solution based on a robotic systemthat can move around within the closed block, to weld thecontacting boundary of the top and the bottom shells, with mobilefunctions or other suitable alternatives. This is the basic researchmotivation and objective of the research presented in this paper.

2. Previous works

Previously designed welding carriages and welding robots havetypically played major roles in the automation of various processesin shipbuilding areas. Here, the welding carriage is defined as amechanical device having 1- or 2-axis for the specific purpose ofwelding. As shown in Table 1(a), a 1-axis horizontal fillet weldingcarriage can weld the contact boundaries of the stiffeners and thebottom plate without any motions of the welding torch alongthe horizontal trajectories. On the left side of Table 1(a), it can weldthe contact boundaries in the vertical direction, with a certainrotating motion of the welding torch for the so-called weavingmotions. In particular, both of these use the guidance wheels toguarantee straightness in driving, by holding it against the stiffen-ers. And, Table 1(b) also shows a fixed type of commercial weldingcarriage, V-ROD, for performing the vertical weave-welding in thespecified ranges. Even though these present excellent properties,such as having compact size, being lightweight, and taking

D. Lee et al. / Robotics and Computer-Integrated Manufacturing 27 (2011) 377–388 379

a modularized controller design approach, they are not acceptablein more complicated tasks, such as welding of U-shaped trajectories(see Fig. 4(a)). The reasons for this are clarified as follows: (1) thedeficiency of the degrees of freedom in the motions of the weldingtorch, and (2) the unidirectional welding property, excluding theV-ROD.

Table 2A shows the intelligent mobile welding carriages. Thecarriage of Table 2A-(a) is composed of two prismatic and tworevolute joints, and it also has an embedded controller on it.However, this carriage also uses driving wheels, which is notacceptable to the scope of this development, since the bottomfloor represents quite unclean conditions. Thus, if a certain robustalgorithm for motion control does not hold, then the manner ofthe differential driving cannot mechanically guarantee thestraightness in repetitive multi-pass welding because of thehigh likelihood of slippages. The carriage shown in Table 2A-(b)is composed of three prismatic and two revolute joints, forU-shaped trajectory welding. However, this is not also acceptableto the scope of this development since it uses an externalcontroller and driving wheels.

In the case of the open block, as shown in Figs. 1 and 2, acommercial multi-axis articulated robotic system can be placedinto the open block using an overhead gantry crane, built into theshipyard. A typical example of this is the DANDY system, asshown in Table 2B, which has been developed and successfullyused in the shipyard of Daewoo Shipbuilding and MarineEngineering Co. Ltd., Korea. This system is operated by workersto weld a part of the boundaries, then moves to the next weldinglocations using the overhead gantry crane installed on the ceilingof the shipyard. However, as mentioned earlier, this system

welding torch

Weaving motion

Scallop

Collar plate

Bottom plate


Scallop welding

Start & end-pointVia-point

Bracket~ 30-mm

250 ~ 1000-mm

630 ~ 1050-mm

Radius: 50,75,100-mm

~ 225-mm

: Welding path

Handles

150 ~

Fig. 4. (a) Movement of the welding torch along the U-shaped welding trajectories (b) re

of each kind of bracket/stiffeners.

cannot be used within a closed block, as the overhead cranecannot approach the inside of the closed block. Moreover,controllers are located at the outside of the open blocks, thusthere exists a number of cables from the outside. This gives rise todifficulties in handling the several cables in the enclosedstructures. There are also more examples of the currentlyavailable systems, which are a combination of a multi-axis roboticsystem with an overhead crane, as described in [7,8].

Table 2C(a) shows the NC painting robot, which has beendeveloped by the Hitachi-Zosen shipyard in Japan [9]. A 6-axispainting robot, plus a self-driving carriage, is placed inside theclosed block using an expandable placer. However, this roboticsystem requires a large access hole of size 800�1600 mm2. Sincethe size of the access hole is related to ship-design safetyregulations, any enlargement requires the permission of the ship’sowner, and is almost impossible to achieve. Another seriousproblem of this robotic painting system is that it cannot movefreely in the transverse direction inside an enclosed block. And, theIndustrial Automation Institute (IAI) in Spain has developed arobotic system called ‘‘ROWER 1’’ that can be used in a closed block[10]; the robot moves like a spider, and has four legs capable ofextending and contracting. It can move autonomously and can thusovercome many of the welding obstacles encountered in a closedblock but it has to be disassembled into seven modules before itcan be placed into a closed block, and then re-assembled in situ.Re-assembly takes approximately 15 min, which is long enough toseriously affect the productivity of the system [11].

Finally, the RRX, which overcomes all the disadvantages of theprevious robots, has recently been established, and its perfor-mances of welding and mobile functions have been verified

xz

y{

B}~ 35-mm

250-mm ~ 300-mm T- Bar: 250 ~ 500-mmAngle: ~ 500-mm

quired dimensional ranges of each welding trajectories, and (c) dimensional ranges

Table 2Several types of autonomous welding system.

A. Intelligent welding carriages B. Articulated welding manipulator

(a) 4-axis carriage [4] (b) 5-axis carriage [5] 6-axis manipulator, DANDY [6]

Welding torch

Wall-guide roller

Wire spool

Controller cables from the ceiling

Dandy,DSME

Torch cable

Control panel

C. Self-traveling welding robots

(a) NC painting robot [9] (b) Rower 1 of IAI, Spain [10] (c) RRX developed by SNU [12]

800×1600-mm Access hole

NC painting robot

Height : 2130-mm

Width : 1760-mm

Manipulator

Leg

Welding manipulator

RRX mobile platform

Welding wire spool and feeder

Welding torch


through field testing over a period of one year. However, points ofnote are that this system is still relatively hard to handle forplacing in, and withdrawing out from the 500�700 mm2 access-hole, even though it satisfies the dimensional constraints. Themain reason for this is its relatively large size compared to thesize of human workers.

Based on previous analysis of several welding systems, themechanism for fully autonomous traveling, on the structures, hasresulted in the robotic system being enlarged due to its number ofjoints. This has lead to the request for a compact robotic system,small enough to be easily handled with auxiliary devices fortransportation instead of the fully autonomous traveling mechan-ism. In addition, some requests from operators in the field are asfollows: (a) removal of the mobile function from the roboticsystem to decrease its size and weight, (b) design of auxiliarytransportation devices as an alternative to the mobile mechanism,and (c) a modularized controller to the robotic system foreliminating cables connecting between robotic systems andcontrollers at the outside of the enclosed structures.

During welding, an electric current is used to strike an arcbetween the base material and the consumable electrode rod. Atthat moment, it is known that the random movement of theelectrons carrying the current, as they are welded, occurs, andaffects the signal cables connecting the robots and controllers.Moreover, there are a number of robots welding simultaneouslyin the same block. Thus, it can be thought that modularizedcontrollers help to prevent negative influences on the entiresystem, from the various noises. Based on these facts that have

been clarified so far, the design of an integrated system of aportable welding-only robot, having a modularized controller andauxiliary transportation devices, as a final alternative of a mobilerobot working inside of the enclosed structures, has beenencouraged.

3. Analysis on the welding task

Fig. 4 represents the task structure and welding tasks, to beperformed with the movement of the welding torch along thepredefined welding trajectories. As shown in Fig. 4, the set ofwelding trajectories look like U shapes; hence this is calledthe U-shaped trajectory welding. The overall process of suchU-shaped trajectory welding is divided into the initial positioningand the actual welding. Before initiating the welding process, thestart and end points of each welding trajectory can be obtainedusing laser, or touch sensors with certain sensing algorithms. (i.e.the RRXC can use laser and touch sensors together). This isconsidered to be the initial positioning, with respect to the inertialcoordinate frame. After finishing this initial positioning, thewelding tasks are performed in the order of left-vertical weaving,horizontal multi-pass, and right-vertical weaving welding, re-spectively. The welding has to be carried out along the contactingboundaries; a vertical path of zig–zag motion on the left side isthe contacting boundary between the longitudinal stiffener andthe transverse web floor. That is, it is logically divided into severalsegments to support the job of programming and motion planning

A modularized controller

6th axis

5th axis

4th axis

HandlesConnectors for the motor cables

356-mm

585.2-mm

625-mm


of the robots. In addition, there are two reasons for the zig–zagmotion in the weaving, which are, (1) it is needed in order toreduce the number of times of multi-pass horizontal welding, if itrequires a wide range of gap, and (2) it is also needed to preventrunning down of a weldment in the vertical welding.

As shown in Fig. 4, it is supposed that many kinds of U-shapedwelding trajectories may currently exist with respect to thecombination of the positions and dimensions of the bracket, collarplate, and scallop. The required dimensional ranges of eachsegment of the U-shaped trajectories are typically defined asfollows: (1) the height of the longitudinal stiffeners is in the range250–650 mm, (2) the width between two longitudinal stiffeners isin the range 630–1050 mm, (3) the thickness of the collar plateis up to 35 mm, (4) the radius of the scallop is in the range50–100 mm, and (5) the length of the bracket is up to 500 mm.Additionally, energy sources of 3-phase electricity supplies of220 V, and pneumatic power in the range 5 bar to 7 bar can beused in factories.

Distance setting tool

3rd axis

1st axis

2nd axis Fold-upand down

On/off magnet

Fig. 6. View of the developed RRXC, virtual mock-up with representations of

dimensions, names, and number of axis.

4. Design: an intelligent welding robot RRXC

The carriages, proposed in previous works, use the drivingwheels on the bottom plate and passive wheels for guidance onthe transverse web floor. This can guarantee its straightness indriving along the predefined welding trajectories by holding theguidance wheels against the web floor but the guidance wheel isnot acceptable for use in the RRXC, whose degrees of freedom inthe end-effector are represented as six-dof comprising threeprismatic and three revolute joints. The main reasons for this areas follows: (1) it may lead to interference between themanipulator and the guidance assemblies in the U-shapedtrajectory welding and initial positioning tasks, and (2) it maynot guarantee straightness in bi-directional multi-pass welding,and only proper unidirectional welding can be ensured because ofthe inclined manner of driving against the wall. Naturally, this hasbeen regarded as the most challenging subject in the mechanicaldesign of RRXC, since it may lead to negative influences in thewelding quality if it does not work well.

A point to note, as shown in Figs. 5 and 6, is the design of afold-up rack system, which consists of three foldable parts, andcan be fixed onto the bottom plate with two on/off magnets, inorder to settle the critical issue of guaranteeing straightness. Inother words, the fold-up rack system can provide a sure methodin bi-directional translations along the horizontal weldingtrajectories by solidly fixing two racks using the on/off magnetsafter folding down onto the bottom plate. The length of the tworacks can be replaced with respect to the width of the U-shaped

Passive guide rollers

Bracket welding

Horizontalwelding

Driving guide roller

Driving wheels

Welding torch

Fig. 5. The conceptual principle of operation of the guidance wheel in the top view

of the U-shaped trajectory.

trajectories and the operations of the on/off magnets are simple,as shown in Fig. 6.

The RRXC is composed of a 6-axis welding manipulator and a6-axis modularized controller. And, the total weight of the systemis 60 kg with the 6-axis manipulator weighing 45 kg and themodularized 6-axis controller weighing 15 kg.

(1) 6-axis welding manipulator and positioning devices: the6-axis welding manipulator is composed of three prismatic axesand three revolute axes, which are driven by AC servo motors. Thefirst axis is driven by a rack and pinion mechanism in a paralleldirection to the transverse web floor. In order words, it can makethe entire body of the RRXC move on the rack, which is composedof three parts, and is connected by hinges with each other.Moreover, the total length of the racks can be changed by foldingup from 760 to 356 mm. The second axis is also driven by anotherrack and pinion mechanism, in a perpendicular direction to thetransverse web floor. In particular, the third axis is driven bycombinations of a pulley, a timing belt, and a telescopicmechanism, in the vertical direction. It has three overlappingsliders, namely a multi-slider system, for elevating the weldingtorch from its rest state, as shown in Fig. 6. As a result, it has astroke of 750 mm in the vertical direction, with respect to thebottom plate. The fourth and sixth axes are the yawing and rollingaxes which are directly driven by servo motors through harmonicdrive systems. The fifth axis is the pitching axis driven by a pulleyand timing belt combination. Fig. 6 also shows the design of theend-effector, which consists of a laser displacement sensor andwelding torch. This is connected to the main sixth axis of rollingthrough a shock sensor.

It should be noted that, the reason the multi-slider system andthe fold-up racks are used is that the system must be compactenough to fit through a 500�700 mm2 access hole. Thus, if thestatus of keeping the rest state of the third axis and folding up two

Task Manager

Task Planner

Task Executer

ON/OFF moduleWeldingMachineModule

TP ModuleTask Management Module

MotionGenerator

Teaching Pendant(TP)

Task List Robot status

Task

Motion Command

Laser sensor

Motion Controller

Welding machine controller

Move in the transverse direction

Move in the longitudinal direction

Straight welding

Bracket welding

Laser sensing

Touch sensing

Servo module

Sensor module

Shock sensor

RS485

USB

RS232

ON/OFFCommand

Robot statusEnvironmental status

Voltage, CurrentWelding start/stop

Welding Command

Actions for TaskAction Module

Transverse movement

Linear motion of torch

RC motor

AC Servomotor

Weaving welding

Start/stop welding

Longitudinal movement

Activated sensor

Fig. 8. The four layered architecture and the modules of the RRXC [13].


racks hold, then the dimensional constraint of 500�700 mm2

access hole is clearly satisfied.6-axis controller: The controller hardware consists of a main

controller and a welding machine controller. The main controller,which is mounted on the mobile welding robot, consists of a CPUboard, a motion controller, six AC servo motor drivers of absoluteencoder type, a flash disk, some relays, power modules, and apower distributor. The welding machine controller, which ismounted on the welding machine located on the outside of thedouble hull structure, controls the welding machine. The com-munication between the two controllers is made via the RS485.The motion controller, and servo driver, applied to the RRXC arecommercially available from Yaskawa, and are used because oftheir reliability in such hazardous environments. Three servodrivers are symmetrically arranged on both sides of the controller,and the rest are arranged in the middle-rear section of thecontroller to minimize the interference with the end-effector.

Another challenging issue of the main controller is that itneeds to be modularized for providing the portable function toenable it to carry out the tasks and to be maintained within theenclosed structures. Thus, in order to make the mechanicalseparation between a 6-axis robotic platform and a modularizedcontroller possible, all the connectors are embedded on the topplate of the controller, and the two parts are bolted together.Hence the mechanical separation becomes quite simple bydisjoining of five-connectors and four-bolts.

The developed RRXC has to carry out welding tasks torelatively high accuracy; therefore, the positional error of theend-effector in the Cartesian coordinate frame should be less than0.5 mm for ensuring a good welding quality. In order to executethese good movements of the end-effector, the control softwarewhich makes the RRXC perform the given welding tasks throughthe defined ‘actions’ and its successive combinations. Thus, thecontrol software is defined into four layers here and thedefinitions of each layer are as follows

(1)

M

Fig.‘RRX

Task manager: this helps to manage the task lists provided bythe users, and to communicate with the teaching pendant (TP).

(2)
Task planner: this takes charge of receiving tasks from thetask manager and then it helps to choose a series of required‘actions’.
(3)
Actions for the task: this takes charge of receiving the ‘actions’from the task planner. Then it helps to generate the trajectoryof the robot through definition of the environmental data andthe robot status from the task executer.
(4)
Task executer: this takes charge of controlling the motioncontroller and the actuator. It helps the RRXC to execute the
CPUBoard

ain Controller

Arc Sensor UnitTouch Sensor Unit

Motion Controller

Driver #1

On/Off Actuator

Controller

M AC ServoMotor #1

S Shock Sensor

SLaser Sensor

MLaser SensorCAP

RS485

RS232

Driver #2 MAC ServoMotor #2

Driver #6 MAC ServoMotor #6

WeldingMachine

Welding Machine Controller

USB

7. Configuration of the embedded controller for the mobile welding robot

C’.

received tasks and then it deciphers the environmental dataand the robot status data obtained from the sensory system.

Thus, it is noteworthy that the tasks can be performed throughthe successive combinations of ‘actions’ in the layers of actions forperforming the required task. Furthermore, in order to makeadditions of new hardware easy, the unit functions of the taskexecuter are modularized. Figs. 7 and 8 show a diagram of thefour layered architecture and its modules, which makes thecontrol software run.

5. Utilities

5.1. Wireless teaching pendant using a PDA

The teaching pendant is a hand-held robot control terminalthat provides a convenient means to run the robot programs.Nowadays, most teaching pendants are connected to a robotcontroller by cables. The connecting cables and the size of theteach pendant are not of concern here but a large, wired teachingpendant is not suitable for a portable mobile welding robot whichhas a controller inside, since a worker should follow the robot toevery location in the into the double-hulled structures.

Thus, there is a great need for wireless teaching pendants toenable workers to control the number of welding robots withoutany physical connections. Fig. 9 shows the existing and thedeveloped wireless teach pendant and the functions and perfor-mance of the wireless teach pendant have been verified from thefield testing trials carried out throughout this project [14](Table 3).

85-mm

180-mm

180-mm

350-mm

Fig. 9. Existing TP of fixed type robot and PDA TP.

Table 3Specifications of the fixed type welding robot TP and the PDA TP.

Items Existing TP PDA typed TP

Size 180�350 mm2 85�180 mm2

Weight 1.3 kg 0.4 kg

Connection RS232C Wireless LAN(IEEE 802.3)

#1

# 2

#3

#4PDA TP

Wireless LAN

Fig. 10. Multiple connections of the PDA TP and the RRXC through the wireless

access point.

3-DOF Shock sensor

Straight typed welding torch

Laser distance sensor with on/off sealing cap

Fig. 11. Sensory systems of laser and shock sensors on the end-effector.


Fig. 10 shows the hardware structure of the PDA TP and themain controller of the RRXC. The CPU board and the motioncontroller are connected to the wireless access point with LANcables. Fig. 10 also shows the multiple connections for simulta-neous control of a number of RRXCs, with just one teach pendant,will lead to improvements in the operational efficiency, andreduce the cost of labor.

5.2. Sensory systems

The 3-dof shock sensor and laser distance sensor are installedon the end-effector of the 3P3R manipulator, through harmonicdrive systems. The straight welding torch is also installed onthe top side of the laser distance sensor assembly. In the caseof the shock sensor, since the RRXC works in hazardousenvironments, malfunctions such as a wrong motion of theend-effector may suddenly occur and this can lead to criticaldamage to the RRXC. Thus, the shock sensor, which can detect asudden impact, should be installed on to the industrial robot.The laser distance sensor for the initial sensing of the U-shapedtrajectories, rather than the touch sensor, should help reducethe required time, thus, leading to a rise in efficiency andproductivity.

5.3. Portable auxiliary transportation device

Portable auxiliary transportation devices are important in thefield applications of the RRXC, since they can control the mobilefunctions within the double-hulled structures. This consists ofelectric winches, hand-clamps, a bridge plate, a sliding plate,hand-winches, and steel wire. The electric winch, bridge plate,and sliding plate are customized to meet a common requirementon the weight of each device, which should be less than 10 kg forachieving the hand-held mode in the field. In order to rigidlyconnect both ends of the upper longitudinal stiffeners using steelwire, the hand-clamps for fixing it at one side of the upperlongitudinal stiffeners and the hand-winch for withstandingtensile forces of the steel wire at the other side are used. Thiscan then provide a means of transporting the RRXC along theconnected steel wire using the electric winches; with a roller forinterfacing with the steel wire. In addition, the RRXC has an eye-bolt of M12 size on the top of the vertical arm, for interfacing withthe electric winch, which has a hook. Thus, it can also provide ameans of lifting the RRXC up and down.

Fig. 11 shows the successive process of fitting the RRXCthrough a 500�700 mm2 access hole in the double-hulled block.It also shows that the sliding plate provides a means oftransporting three RRXCs in the longitudinal direction simulta-neously, and the bridge plate provides a means of fitting through a500�700 mm2 access hole by supporting the weight of the RRXCrobot. In these cases, to fit the system through the access hole,two electric winches and two workers are needed to carry itacross to the other side. Fig. 12 shows the overall process ofinstalling the auxiliary transportation devices, commitmentdirections of the number of RRXC, and working directions in thedouble-hulled block. The retrieval of the robot is carried out byperforming the process in reverse. It is noted that the proposedtransportation system may lead to the useful applications in otheroperations, such as transportation of ladders.

6. Workspace analysis and simulation

Fig. 13 shows a kinematic model of the RRX welding robot,which is a 3P3R manipulator consisting of a PPPRRR serial chain,where P and R denote prismatic and revolute joints, respectively.The manipulator has six degrees of freedom with six active joints,each of which is indicated by an arrow in Fig. 6(a). The Denavit-Hartenberg parameters used to solve the kinematics of the 3P3Rmanipulator shown in Fig. 13 are listed in Table 4, where ai–1, ai–1,

di, and yi are the link twist angle, link length, joint distance, andjoint angle, respectively, with respect to joint i.

6.1. Workspace analysis

Over several years, various studies have been published onworkspace analysis by Gosselin [15], Merlet [16], Waldron andKumar [17], Tsai and Soni [18], Gupta and Roth [19], Sugimoto

Winch systemCustomized

Sliding plate in longitudinaldirection

Hand-heldBridge plate

Transport in transverse direction

Lift up/down

6Φ steel-wire

500-mm

Transport in longitudinal direction through the 500 × 700-mm access hole

700-mm

Hand clamp

Steel wire

Winch

Transportation direction

Working direction

Access hole

Fig. 12. (a) Overall process of transporting the RRXC in double-hulled structures and (b) overall process of installing the auxiliary transportation devices.

X1

X3

Y0

Z0

X0Z1

Z3

αZ7

X5

X6

Z5

Z6

L3 Z2

X2

X4

Z4

L2

L1

d1

d2

d3

Fig. 13. Kinematic model of the RRX robotic system.

Table 4Denavit-Hartenberg parameters of the 3P3R manipulator.

Joint i ai�1 ai�1 di yi

1 p/2 0 d1 p/2

2 p/2 0 d2 p/2

3 p/2 0 d3 p/2

4 0 L1 0 y4+p5 p/2 �L2 0 y5+1.5p6 p/2 0 L3 y6+p/2

7 a¼p/6 0 0 0

Note: L1¼201 mm, L2¼120 mm and L3¼383.3 mm.

90˚

Y{B}=X{T0} 35˚

Z{B}

X{B}

Z{T0}

Initial welding torch direction, Z{T0}

Y{T} Y{T0}

-10

1-1

0

1

� = 35˚

� = 60˚� = 90˚

� = 30˚

Projection of Z{T} axes onto XY{T}-plane

X-a

xis

Y-axisProjection result

Fig. 14. Measured yaw-pitch angles from U-shaped trajectory welding and their

representation by length on the XY plane by projection.


et al. [20], Gupta [21], Davidson and Hunt [22], and Stan et al.[23]. However, most previous works in this area proposed usingthe Jacobian approaches together with the conditioning numberlimit for finding the manipulator workspaces [24,25]. It shouldnote that it do not directly consider the required rotarycapabilities in relation to the end-effector’s space. For the abovereasons, a new concept of task-oriented workspace that considersonly the predefined orientations of an end-effector required ingiven welding tasks is introduced here for the aim of designverification. In order to illustrate the required orientation axes,the geometric ‘‘orientation cone’’ is proposed and representedin Fig. 14. It shows the movement of the welding torch along theU-shaped welding line. Frame {B} denotes the base frame, and


frame {T} denotes the tool frame. The initial tool frame for thewelding process {T0} is defined to be rotated (901, 01, 1141) withrespect to frame {B}, in order to have symmetric yaw-pitch angles.The measured yaw-pitch angles of the welding torch, with respectto frame {T0}, are expressed as the length from the origin by theprojection to the YZ plane, which is shown in Fig. 14. The requiredyaw-pitch angles are determined as 351 about the z{T0} during theentire welding process. To perform the welding process success-fully, the 351 of yaw-pitch rotational capability should beguaranteed. To make the concept of geometric ‘‘orientation cone’’

-600 -400 -200-200

0

200

400

600

800

1000

1200

X: -525Y: 760

X: -680Y: 640

X-ax

is

Task-oriented workspace o

Ta

Base coordinate frame

Task-oriented worksp

0 100 200 300 400-200

-100

0

100

200

300

400

500

600

700

800Task-oriented workspace o

X

Z-ax

is

Task-oriented w

Tas

Rack

Base coordinate frame

Fig. 15. Results of the workspace an

clear, it should note that the value of represented yaw-pitchangle could be thought as the aperture angle of the orientationcone in Fig. 14.

Here, the task-oriented workspace can be defined as the set ofpoints that the welding torch tip can approach with satisfyingpredefined rotational capability of 351 with respect to the initialtool frame.

For the given 3P3R welding manipulator, the results of theworkspace analysis are shown in Fig. 15. The conventional‘‘reachable workspace’’ represents the larger area enclosing the

0 200 400 600

X: 525Y: 725

Y-axis

n YX plane for theta = 35degree

sk workspace

Rack

ace

500 600 700 800 900

X: 725Y: -100

X: 760Y: 550

n XZ plane for theta = 35degree

-axis

orkspace

k workspaceX: 880Y: 560

alysis in the y–x and z–x planes.

Fig. 16. Simulation results in terms of the interferences avoidance with existing structures.

Hand-winch

Clamp

Longitudinal stiffeners (Lower)

Longitudinal stiffeners (Upper)

Height 3-m

RRXC

Steel wire

Steel wire

Roller Clamp

Electric winch

Access hole


other two workspaces of the ‘‘task-oriented workspace’’ ofmanipulator and ‘‘task workspace’’ of U-shaped trajectories. Andthe results also note that the task-oriented workspace alsoencloses the task workspace. Since the task workspace is muchsmaller than the task-oriented workspace, the size of the currentRRX welding robot could be significantly decreased even basedsolely on the kinematic analysis.

6.2. Simulations for obstacle avoidance

Since many U-shaped trajectories exist, there is a great needfor robot-simulations to generate the welding paths of the end-effector, and to check for interference with existing structures,such as the web faces of the longitudinal stiffeners. Simulationstudies can also help to determine the strokes of each axis and thelength of each link of the manipulator, in terms of satisfactoryperformances of the required tasks in the actual workspaces.Thus, it leads the RRXC to successfully perform numerous weldingtasks in the field without any collisions. Fig. 16 shows thesimulation results for avoiding interference with the web face ofthe longitudinal stiffeners, using the ROBCAD program throughpredefined CAD data of the target U-shaped part.

Welding machine

Wire spool/feeder

RRXC

Left-vertical weaving welding

Right-scallop welding

Fig. 17. Field tests for validating the (a) installations of auxiliary transportation

devices (b) connecting cables from the welding machine, and (c) actual arc-

welding experiments in the U-shaped trajectory.

7. Field tests

The field tests for the application of the developed RRXC havebeen carried out in double-hulled blocks over a period of severalmonths. As mentioned earlier, the RRXC can fit through the500�700 mm2 access hole with the help of the set of auxiliarytransportation devices, and the RRXC can be located at a U-shapedtrajectory by transportations in the longitudinal and transversaldirections. Then, after finishing a welding job, it can be alsomoved to the next U-shaped trajectory by the help of an installedelectric winch on a steel wire.

Through these repetitive executions of welding tasks andtransportations in the double-hulled structure, all performancesof the RRXC have been successfully demonstrated in terms of thewelding quality, the welding functions, the electrical reliability,and the overall operational convenience.

Fig. 17(a) shows the view of the actual installation of the RRXC,with the set of auxiliary transportation devices in double-hulledstructures located within the ship-building factory. The 220 Vsupplies of single phase are only used in driving the electricwinch, not the other devices. Fig. 17(b) also shows the RRXCperforming a welding task, with representation of the severalcables from the outside of the double-hulled structure. It consistsof the welding cable from the welding machine, the power cableof 220 V and CO2 gas cable. However, the set of welding wirespool and feeder, which is connected with the RRXC by a torchcable, is typically located just behind the RRXC. Fig. 17(c), of a

series of four Figures, shows the actual arc-welding experimentsusing the RRXC. It consists of vertical weaving, horizontal multi-pass, and scallop welding procedures. The welding voltage andthe arc current are 26 V and 250 A, respectively. The weldingspeeds are 0.54 and 0.24 m/min, respectively, for the vertical andhorizontal welding processes.

Fig. 18. Successful welding quality of U-shaped trajectories.


7.1. Experimental results

In field tests, the RRXC together with the set of auxiliarytransportation devices successfully demonstrated that the weld-ing quality and operational convenience are acceptable toindustrial engineers. These conclusions are made based on thefollowing experimental results:

(1)
The required time in one successful sensing of the U-shapedtrajectory is reduced from 3.2 to 2.0 min compared to theprevious 6-axis articulated welding arm using the touchsensor. In other words, the laser sensor system showssignificant improvements in efficiency and productivity, witha large reduction in the time required to accomplish the U-shaped trajectory.
(2)
The welding quality, from repetitive performances of thewelding tasks in various types of U-shaped trajectories, hasbeen evaluated by the industrial engineers, and is deemedsatisfactory, as shown in Fig. 18. The welding conditions ofFig. 18 are also represented.
(3)
Some difficulty exists in operating the electric winch and thehand-winch, even though it provides convenient conditions inoperating the RRXC. The operators feel unrest from the steelwires, since it seems to be quite weak.
There are some requests for the improvement of the slackstatus of the motor cables of the RRXC. Since the third axis of thevertical arm has a large stroke of 750 mm, using a ‘‘multi-stagemechanism’’, the motor cables should be slack in the minimumstroke of the third axis. This leads to interference between theRRXC and the structures, such as the web face of the longitudinalstiffeners, in the end stroke of the first axis. Thus, this problem hasbeen solved by changing the socket direction of the wiring andadjusting the length of the cables.

8. Conclusion

Some difficulties in the applications of the previous full-autonomous welding systems have been clarified throughnumerous experiments, which have lead to requests for a roboticsystem that is easy to handle in the narrow and confinedstructures. For this purpose, a new type of welding roboticsystem, having a modularized controller, has been developed toperform the welding of U-shaped trajectories in the enclosedstructures, with auxiliary transportation devices. As representedabove, the welding functions and mobility using auxiliarytransportation devices have been successfully carried out andverified through the ROBCAD simulations and field testing in realdouble-hulled blocks, during a 6 month period. The modular

controller helps the entire systems to decrease the number ofcontroller cables, and to prevent negative influences of electricnoise. Significant results have indicated that the separation of thewelding and the mobility part is a great help in operating roboticsystems, compared with a fully autonomous one of comparativelyhuge size. The wireless teaching pedant makes it possible for oneoperator to manage a number of RRXC systems; increasing theefficiency of the production. Although research and field tests arecontinuing, conclusions can be made that this system will bedefinitely a great practical help, and will significantly improve theproductivity of welding inside of the double-hulled structures.

Acknowledgements

This research was supported in part by the Brain Korea 21Program of the Korean Ministry of Education, and DaewooShipbuilding and Marine Engineering (DSME) of Republic of Korea.One of the authors, namely, Donghun Lee, would like to express histhanks to Prof. Jongwon Kim, Tea-Wan Kim, and Kyu-Yeul Lee fortheir continuing assistance and guidance. The authors would also liketo acknowledge the fact that Namkuk Ku has played major roles inprogramming the jobs and the field testing of the RRXC throughoutthe project.

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