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

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Robotics and Computer-Integrated Manufacturing journal homepage: www.elsevier.com/locate/rcim

Development and application of an intelligent welding robot system 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 in fo

abstract

Article history: Received 2 November 2009 Received in revised form 14 August 2010 Accepted 17 August 2010

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 doublehulled 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.

Keywords: Rail-runner mechanism Intelligent welding robot Double-hulled block Shipbuilding

1. Introduction Commercial ships carrying liquid cargo, such as liquefied natural gas (LNG), liquefied petroleum gas (LPG), and crude oil, can cause serious environmental pollution from the risk of spillage. 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) incorporate double-hulled ship walls, as shown in Fig. 1. These consist of outer and inner walls, spaced 2- to 3-m apart; in this way, if the outer wall is holed as a result of a collision or stranding, the inner wall can still prevent the outflow of the liquid cargo [1]. However, the manufacture of double-hulled ships is more time consuming and expensive than that of single-hulled vessels. Fig. 2 also shows the manufacturing process used to obtain the closed block that is a sub-module of the double-hulled ship wall. A bottom shell and an open block are assembled separately using welding processes where the bottom shell is composed of a wide steel plate with several reinforcing longitudinal stiffeners welded to it in parallel. Forming the closed block is more complicated;

n

Corresponding author. Tel.: + 82 2 880 7144; fax: + 82 2 875 4848. E-mail address: [email protected] (D. Lee).

0736-5845/$ - see front matter & 2010 Published by Elsevier Ltd. doi:10.1016/j.rcim.2010.08.006

first, a top shell, that is the same shape as the bottom shell, must be manufactured. Then, a number of transverse web floors and girders are welded on to the top shell; second, the open block is turned over and placed alongside the bottom shell and each longitudinal stiffener in the bottom shell is aligned with the corresponding slit in the open block; and finally, third, the open block is inserted laterally along the longitudinal stiffeners of the bottom shell so that each stiffener slides into its corresponding slit to assemble the closed block, as shown in Fig. 2. The resulting closed block must then be welded. That is, the welding has to be done from inside the closed block, along the contacting boundaries of the top shell and the bottom shell. Since it is an enclosed structure, the temperature gets hot and is in the range 40–50 degrees during the summer, and it is often too 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 the closed block represents one of the most difficult and hazardous tasks 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 the double-hulled block as the overhead gantry crane cannot

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A Commercial Ships carrying Liquid cargo

Transverse web floor

300,000ton class VLCC Transverse direction Longitudinal stiffeners 5,100-mm 10,920-mm

Sectional Picture of the hull structure in assembly

3,000-mm

Access hole

Double-hulled block Longitudinal direction

Longitudinal girders

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

Table 1 Commercialized welding carriages.

Turn Over

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

Inner Bottom Block (Open block)

Slit

Bottom Shell

Put to the slit

Double Hull Structure (Need to be welded)

Fig. 2. Manufacturing of a closed block, which is part of the ship wall of doublehull structure.

Wall-guide roller

Welding torch

Torch cable Driving wheel Horizontal fillet welding

Controller Vertical fillet welding

(b) Vertical weaving carriage, V-ROD [3]

Mechanical stopper

Handle torch holder Magnets

Limit sensor

Guide rail/rack

Double-hulled structure = Closed block Top plate

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

Transverse web floor

Longitudinal stiffener Bottom plate

Girder

2. Previous works Top plate

Longitudinal stiffeners

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

approach the inside of the double-hulled block. This is because the overhead gantry cranes are installed on the ceiling of the shipyard. Therefore, it becomes clear from the current status of the welding process in the double-hulled structures, that there is a

Previously designed welding carriages and welding robots have typically played major roles in the automation of various processes in shipbuilding areas. Here, the welding carriage is defined as a mechanical device having 1- or 2-axis for the specific purpose of welding. As shown in Table 1(a), a 1-axis horizontal fillet welding carriage can weld the contact boundaries of the stiffeners and the bottom plate without any motions of the welding torch along the horizontal trajectories. On the left side of Table 1(a), it can weld the contact boundaries in the vertical direction, with a certain rotating motion of the welding torch for the so-called weaving motions. In particular, both of these use the guidance wheels to guarantee straightness in driving, by holding it against the stiffeners. And, Table 1(b) also shows a fixed type of commercial welding carriage, V-ROD, for performing the vertical weave-welding in the specified ranges. Even though these present excellent properties, such as having compact size, being lightweight, and taking

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a modularized controller design approach, they are not acceptable in more complicated tasks, such as welding of U-shaped trajectories (see Fig. 4(a)). The reasons for this are clarified as follows: (1) the deficiency of the degrees of freedom in the motions of the welding torch, and (2) the unidirectional welding property, excluding the V-ROD. Table 2A shows the intelligent mobile welding carriages. The carriage of Table 2A-(a) is composed of two prismatic and two revolute joints, and it also has an embedded controller on it. However, this carriage also uses driving wheels, which is not acceptable to the scope of this development, since the bottom floor represents quite unclean conditions. Thus, if a certain robust algorithm for motion control does not hold, then the manner of the differential driving cannot mechanically guarantee the straightness in repetitive multi-pass welding because of the high likelihood of slippages. The carriage shown in Table 2A-(b) is composed of three prismatic and two revolute joints, for U-shaped trajectory welding. However, this is not also acceptable to the scope of this development since it uses an external controller and driving wheels. In the case of the open block, as shown in Figs. 1 and 2, a commercial multi-axis articulated robotic system can be placed into the open block using an overhead gantry crane, built into the shipyard. A typical example of this is the DANDY system, as shown in Table 2B, which has been developed and successfully used in the shipyard of Daewoo Shipbuilding and Marine Engineering Co. Ltd., Korea. This system is operated by workers to weld a part of the boundaries, then moves to the next welding locations using the overhead gantry crane installed on the ceiling of the shipyard. However, as mentioned earlier, this system

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cannot be used within a closed block, as the overhead crane cannot approach the inside of the closed block. Moreover, controllers are located at the outside of the open blocks, thus there exists a number of cables from the outside. This gives rise to difficulties in handling the several cables in the enclosed structures. There are also more examples of the currently available systems, which are a combination of a multi-axis robotic system with an overhead crane, as described in [7,8]. Table 2C(a) shows the NC painting robot, which has been developed by the Hitachi-Zosen shipyard in Japan [9]. A 6-axis painting robot, plus a self-driving carriage, is placed inside the closed block using an expandable placer. However, this robotic system requires a large access hole of size 800  1600 mm2. Since the size of the access hole is related to ship-design safety regulations, any enlargement requires the permission of the ship’s owner, and is almost impossible to achieve. Another serious problem of this robotic painting system is that it cannot move freely in the transverse direction inside an enclosed block. And, the Industrial Automation Institute (IAI) in Spain has developed a robotic system called ‘‘ROWER 1’’ that can be used in a closed block [10]; the robot moves like a spider, and has four legs capable of extending and contracting. It can move autonomously and can thus overcome many of the welding obstacles encountered in a closed block but it has to be disassembled into seven modules before it can be placed into a closed block, and then re-assembled in situ. Re-assembly takes approximately 15 min, which is long enough to seriously affect the productivity of the system [11]. Finally, the RRX, which overcomes all the disadvantages of the previous robots, has recently been established, and its performances of welding and mobile functions have been verified

Transverse web floor Weaving motion welding torch Collar plate Scallop welding Scallop

z x

Bottom plate y

{ B}

~ 35-mm

Start & end-point Via-point Bracket

~ 30-mm

~ 225-mm 250 ~ 1000-mm

Radius: 50,75,100-mm

630 ~ 1050-mm

Handles

: Welding path 150 ~ 250-mm

~ 300-mm

T- Bar: 250 ~ 500-mm Angle: ~ 500-mm

Fig. 4. (a) Movement of the welding torch along the U-shaped welding trajectories (b) required dimensional ranges of each welding trajectories, and (c) dimensional ranges of each kind of bracket/stiffeners.

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Table 2 Several types of autonomous welding system. A. Intelligent welding carriages (a) 4-axis carriage [4]

B. Articulated welding manipulator (b) 5-axis carriage [5]

6-axis manipulator, DANDY [6]

Controller cables from the ceiling

Welding torch

Torch cable Wall-guide roller

Control panel Dandy, DSME C. Self-traveling welding robots (a) NC painting robot [9]

(b) Rower 1 of IAI, Spain [10]

Wire spool

(c) RRX developed by SNU [12]

Welding wire spool and feeder

Height : 2130-mm Manipulator

800× 1600-mm Access hole

RRX mobile platform

Welding manipulator

Leg Welding torch

Width : 1760-mm

NC painting robot

through field testing over a period of one year. However, points of note are that this system is still relatively hard to handle for placing in, and withdrawing out from the 500  700 mm2 accesshole, even though it satisfies the dimensional constraints. The main reason for this is its relatively large size compared to the size of human workers. Based on previous analysis of several welding systems, the mechanism for fully autonomous traveling, on the structures, has resulted in the robotic system being enlarged due to its number of joints. This has lead to the request for a compact robotic system, small enough to be easily handled with auxiliary devices for transportation instead of the fully autonomous traveling mechanism. In addition, some requests from operators in the field are as follows: (a) removal of the mobile function from the robotic system to decrease its size and weight, (b) design of auxiliary transportation devices as an alternative to the mobile mechanism, and (c) a modularized controller to the robotic system for eliminating cables connecting between robotic systems and controllers at the outside of the enclosed structures. During welding, an electric current is used to strike an arc between the base material and the consumable electrode rod. At that moment, it is known that the random movement of the electrons carrying the current, as they are welded, occurs, and affects the signal cables connecting the robots and controllers. Moreover, there are a number of robots welding simultaneously in the same block. Thus, it can be thought that modularized controllers help to prevent negative influences on the entire system, from the various noises. Based on these facts that have

been clarified so far, the design of an integrated system of a portable welding-only robot, having a modularized controller and auxiliary transportation devices, as a final alternative of a mobile robot working inside of the enclosed structures, has been encouraged.

3. Analysis on the welding task Fig. 4 represents the task structure and welding tasks, to be performed with the movement of the welding torch along the predefined welding trajectories. As shown in Fig. 4, the set of welding trajectories look like U shapes; hence this is called the U-shaped trajectory welding. The overall process of such U-shaped trajectory welding is divided into the initial positioning and the actual welding. Before initiating the welding process, the start and end points of each welding trajectory can be obtained using laser, or touch sensors with certain sensing algorithms. (i.e. the RRXC can use laser and touch sensors together). This is considered to be the initial positioning, with respect to the inertial coordinate frame. After finishing this initial positioning, the welding tasks are performed in the order of left-vertical weaving, horizontal multi-pass, and right-vertical weaving welding, respectively. The welding has to be carried out along the contacting boundaries; a vertical path of zig–zag motion on the left side is the contacting boundary between the longitudinal stiffener and the transverse web floor. That is, it is logically divided into several segments to support the job of programming and motion planning

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of the robots. In addition, there are two reasons for the zig–zag motion in the weaving, which are, (1) it is needed in order to reduce the number of times of multi-pass horizontal welding, if it requires a wide range of gap, and (2) it is also needed to prevent running down of a weldment in the vertical welding. As shown in Fig. 4, it is supposed that many kinds of U-shaped welding trajectories may currently exist with respect to the combination of the positions and dimensions of the bracket, collar plate, and scallop. The required dimensional ranges of each segment of the U-shaped trajectories are typically defined as follows: (1) the height of the longitudinal stiffeners is in the range 250–650 mm, (2) the width between two longitudinal stiffeners is in the range 630–1050 mm, (3) the thickness of the collar plate is up to 35 mm, (4) the radius of the scallop is in the range 50–100 mm, and (5) the length of the bracket is up to 500 mm. Additionally, energy sources of 3-phase electricity supplies of 220 V, and pneumatic power in the range 5 bar to 7 bar can be used in factories.

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Connectors for the motor cables

Handles

585.2-mm

356-mm 625-mm 4th axis 6th axis

A modularized controller

5th axis 3rd axis

4. Design: an intelligent welding robot RRXC The carriages, proposed in previous works, use the driving wheels on the bottom plate and passive wheels for guidance on the transverse web floor. This can guarantee its straightness in driving along the predefined welding trajectories by holding the guidance wheels against the web floor but the guidance wheel is not acceptable for use in the RRXC, whose degrees of freedom in the end-effector are represented as six-dof comprising three prismatic and three revolute joints. The main reasons for this are as follows: (1) it may lead to interference between the manipulator and the guidance assemblies in the U-shaped trajectory welding and initial positioning tasks, and (2) it may not guarantee straightness in bi-directional multi-pass welding, and only proper unidirectional welding can be ensured because of the inclined manner of driving against the wall. Naturally, this has been regarded as the most challenging subject in the mechanical design of RRXC, since it may lead to negative influences in the welding quality if it does not work well. A point to note, as shown in Figs. 5 and 6, is the design of a fold-up rack system, which consists of three foldable parts, and can be fixed onto the bottom plate with two on/off magnets, in order to settle the critical issue of guaranteeing straightness. In other words, the fold-up rack system can provide a sure method in bi-directional translations along the horizontal welding trajectories by solidly fixing two racks using the on/off magnets after folding down onto the bottom plate. The length of the two racks can be replaced with respect to the width of the U-shaped Passive guide rollers

Horizontal welding Bracket welding

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.

2nd axis

Fold-up and down 1st axis Distance setting tool

On/off magnet

Fig. 6. View of the developed RRXC, virtual mock-up with representations of dimensions, names, and number of axis.

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 a 6-axis modularized controller. And, the total weight of the system is 60 kg with the 6-axis manipulator weighing 45 kg and the modularized 6-axis controller weighing 15 kg. (1) 6-axis welding manipulator and positioning devices: the 6-axis welding manipulator is composed of three prismatic axes and three revolute axes, which are driven by AC servo motors. The first axis is driven by a rack and pinion mechanism in a parallel direction to the transverse web floor. In order words, it can make the entire body of the RRXC move on the rack, which is composed of three parts, and is connected by hinges with each other. Moreover, the total length of the racks can be changed by folding up from 760 to 356 mm. The second axis is also driven by another rack and pinion mechanism, in a perpendicular direction to the transverse web floor. In particular, the third axis is driven by combinations of a pulley, a timing belt, and a telescopic mechanism, in the vertical direction. It has three overlapping sliders, namely a multi-slider system, for elevating the welding torch from its rest state, as shown in Fig. 6. As a result, it has a stroke of 750 mm in the vertical direction, with respect to the bottom plate. The fourth and sixth axes are the yawing and rolling axes which are directly driven by servo motors through harmonic drive systems. The fifth axis is the pitching axis driven by a pulley and timing belt combination. Fig. 6 also shows the design of the end-effector, which consists of a laser displacement sensor and welding torch. This is connected to the main sixth axis of rolling through a shock sensor. It should be noted that, the reason the multi-slider system and the fold-up racks are used is that the system must be compact enough to fit through a 500  700 mm2 access hole. Thus, if the status of keeping the rest state of the third axis and folding up two

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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 CPU board, a motion controller, six AC servo motor drivers of absolute encoder type, a flash disk, some relays, power modules, and a power distributor. The welding machine controller, which is mounted on the welding machine located on the outside of the double hull structure, controls the welding machine. The communication between the two controllers is made via the RS485. The motion controller, and servo driver, applied to the RRXC are commercially available from Yaskawa, and are used because of their reliability in such hazardous environments. Three servo drivers are symmetrically arranged on both sides of the controller, and the rest are arranged in the middle-rear section of the controller to minimize the interference with the end-effector. Another challenging issue of the main controller is that it needs to be modularized for providing the portable function to enable it to carry out the tasks and to be maintained within the enclosed structures. Thus, in order to make the mechanical separation between a 6-axis robotic platform and a modularized controller possible, all the connectors are embedded on the top plate of the controller, and the two parts are bolted together. Hence the mechanical separation becomes quite simple by disjoining of five-connectors and four-bolts. The developed RRXC has to carry out welding tasks to relatively high accuracy; therefore, the positional error of the end-effector in the Cartesian coordinate frame should be less than 0.5 mm for ensuring a good welding quality. In order to execute these good movements of the end-effector, the control software which makes the RRXC perform the given welding tasks through the defined ‘actions’ and its successive combinations. Thus, the control software is defined into four layers here and the definitions of each layer are as follows

(1) Task manager: this helps to manage the task lists provided by the users, and to communicate with the teaching pendant (TP). (2) Task planner: this takes charge of receiving tasks from the task 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 trajectory of the robot through definition of the environmental data and the robot status from the task executer. (4) Task executer: this takes charge of controlling the motion controller and the actuator. It helps the RRXC to execute the

Main Controller CPU Board

RS232

USB

S

Driver #1 Motion Controller

Laser Sensor

RS485

Driver #2

AC Servo Motor #1 AC Servo M Motor #2 M

AC Servo Motor #6

Driver #6

M

On/Off Actuator Controller

Laser M Sensor CAP

Welding Machine Controller Touch Sensor Unit Arc Sensor Unit

S Shock Sensor

Welding Machine

Fig. 7. Configuration of the embedded controller for the mobile welding robot ‘RRXC’.

Teaching Pendant(TP) Task List

Task Manager

Robot status

Task Management Module

TP Module

Task Planner

Task

Move in the transverse direction Move in the longitudinal direction

Bracket welding

Straight welding

Weaving welding

Laser sensing Touch sensing

Actions for Task Action Module Linear motion of torch Transverse movement

Motion Command

Longitudinal movement Start/stop welding

Motion Generator

Activated sensor Welding Command Welding Machine Module

Task Executer ON/OFF Command Robot status Environmental status

Servo module ON/OFF module Sensor module

RS485 Voltage, Current Welding machine Welding start/stop controller AC Servo motor RC motor Shock sensor

RS232

Laser sensor Motion Controller

USB

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

received tasks and then it deciphers the environmental data and the robot status data obtained from the sensory system. Thus, it is noteworthy that the tasks can be performed through the successive combinations of ‘actions’ in the layers of actions for performing the required task. Furthermore, in order to make additions of new hardware easy, the unit functions of the task executer are modularized. Figs. 7 and 8 show a diagram of the four layered architecture and its modules, which makes the control software run.

5. Utilities 5.1. Wireless teaching pendant using a PDA The teaching pendant is a hand-held robot control terminal that provides a convenient means to run the robot programs. Nowadays, most teaching pendants are connected to a robot controller by cables. The connecting cables and the size of the teach pendant are not of concern here but a large, wired teaching pendant is not suitable for a portable mobile welding robot which has a controller inside, since a worker should follow the robot to every location in the into the double-hulled structures. Thus, there is a great need for wireless teaching pendants to enable workers to control the number of welding robots without any physical connections. Fig. 9 shows the existing and the developed wireless teach pendant and the functions and performance of the wireless teach pendant have been verified from the field testing trials carried out throughout this project [14] (Table 3).

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180-mm 85-mm

350-mm

Straight typed welding torch

180-mm

3-DOF Shock sensor Laser distance sensor with on/off sealing cap Fig. 11. Sensory systems of laser and shock sensors on the end-effector.

5.3. Portable auxiliary transportation device

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

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

Existing TP

PDA typed TP 2

Size Weight Connection

180  350 mm 1.3 kg RS232C

85  180 mm2 0.4 kg Wireless LAN(IEEE 802.3)

#1

#2

#3 PDA TP Wireless LAN

#4

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

Fig. 10 shows the hardware structure of the PDA TP and the main controller of the RRXC. The CPU board and the motion controller are connected to the wireless access point with LAN cables. Fig. 10 also shows the multiple connections for simultaneous control of a number of RRXCs, with just one teach pendant, will lead to improvements in the operational efficiency, and reduce the cost of labor. 5.2. Sensory systems The 3-dof shock sensor and laser distance sensor are installed on the end-effector of the 3P3R manipulator, through harmonic drive systems. The straight welding torch is also installed on the top side of the laser distance sensor assembly. In the case of the shock sensor, since the RRXC works in hazardous environments, malfunctions such as a wrong motion of the end-effector may suddenly occur and this can lead to critical damage to the RRXC. Thus, the shock sensor, which can detect a sudden impact, should be installed on to the industrial robot. The laser distance sensor for the initial sensing of the U-shaped trajectories, rather than the touch sensor, should help reduce the required time, thus, leading to a rise in efficiency and productivity.

Portable auxiliary transportation devices are important in the field applications of the RRXC, since they can control the mobile functions within the double-hulled structures. This consists of electric 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 requirement on the weight of each device, which should be less than 10 kg for achieving the hand-held mode in the field. In order to rigidly connect both ends of the upper longitudinal stiffeners using steel wire, the hand-clamps for fixing it at one side of the upper longitudinal stiffeners and the hand-winch for withstanding tensile forces of the steel wire at the other side are used. This can then provide a means of transporting the RRXC along the connected steel wire using the electric winches; with a roller for interfacing with the steel wire. In addition, the RRXC has an eyebolt of M12 size on the top of the vertical arm, for interfacing with the electric winch, which has a hook. Thus, it can also provide a means of lifting the RRXC up and down. Fig. 11 shows the successive process of fitting the RRXC through a 500  700 mm2 access hole in the double-hulled block. It also shows that the sliding plate provides a means of transporting three RRXCs in the longitudinal direction simultaneously, and the bridge plate provides a means of fitting through a 500  700 mm2 access hole by supporting the weight of the RRXC robot. In these cases, to fit the system through the access hole, two electric winches and two workers are needed to carry it across to the other side. Fig. 12 shows the overall process of installing the auxiliary transportation devices, commitment directions of the number of RRXC, and working directions in the double-hulled block. The retrieval of the robot is carried out by performing the process in reverse. It is noted that the proposed transportation system may lead to the useful applications in other operations, 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 DenavitHartenberg parameters used to solve the kinematics of the 3P3R manipulator 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, and joint angle, respectively, with respect to joint i. 6.1. Workspace analysis Over several years, various studies have been published on workspace analysis by Gosselin [15], Merlet [16], Waldron and Kumar [17], Tsai and Soni [18], Gupta and Roth [19], Sugimoto

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Customized Winch system

Hand clamp

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

Transport in transverse direction 6Φ steel-wire Lift up/down

500-mm Hand-held Bridge plate

700-mm

Sliding plate in longitudinal direction

Access hole Working direction Transportation direction Steel wire Winch

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

d3

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

Z3

L1 Z4 X4

L2

X3

Z1

Z5 X5 X6

α

d1

X1 X0

Y0

X2 L3

Z6

Z0

d2 Z2

Joint i

ai  1

ai  1

di

yi

1 2 3 4 5 6 7

p/2 p/2 p/2

0 0 0 L1  L2 0 0

d1 d2 d3 0 0 L3 0

p/2 p/2 p/2

0

p/2 p/2 a ¼ p/6

y4 + p y5 + 1.5p y6 + p/2 0

Note: L1 ¼201 mm, L2 ¼120 mm and L3 ¼383.3 mm.

Z7 1

Y{T}

 = 90˚  = 60˚

Y{T0}

 = 30˚

Z{T0} X{B}

 = 35˚

X-axis

et al. [20], Gupta [21], Davidson and Hunt [22], and Stan et al. [23]. However, most previous works in this area proposed using the Jacobian approaches together with the conditioning number limit for finding the manipulator workspaces [24,25]. It should note that it do not directly consider the required rotary capabilities in relation to the end-effector’s space. For the above reasons, a new concept of task-oriented workspace that considers only the predefined orientations of an end-effector required in given welding tasks is introduced here for the aim of design verification. In order to illustrate the required orientation axes, the geometric ‘‘orientation cone’’ is proposed and represented in Fig. 14. It shows the movement of the welding torch along the U-shaped welding line. Frame {B} denotes the base frame, and

Initial welding torch direction, Z{T0}

Z{B}

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

0

Y{B}=X{T0}

35˚

90˚

-1 -1

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

0 Y-axis

1

Projection result

Fig. 14. Measured yaw-pitch angles from U-shaped trajectory welding and their representation by length on the XY plane by projection.

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clear, it should note that the value of represented yaw-pitch angle could be thought as the aperture angle of the orientation cone in Fig. 14. Here, the task-oriented workspace can be defined as the set of points that the welding torch tip can approach with satisfying predefined rotational capability of 351 with respect to the initial tool frame. For the given 3P3R welding manipulator, the results of the workspace analysis are shown in Fig. 15. The conventional ‘‘reachable workspace’’ represents the larger area enclosing the

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

Task-oriented workspace on YX plane for theta = 35degree 1200

Task workspace

1000 X: -525 Y: 760

X-axis

800

600

X: 525 Y: 725

X: -680 Y: 640

Task-oriented workspace

400

200

Rack

Base coordinate frame

0

-200 -600

-400

-200

0 Y-axis

200

400

600

Task-oriented workspace on XZ plane for theta = 35degree 800 700 X: 880 Y: 560

Task workspace

600 500

X: 760 Y: 550

Z-axis

400

Task-oriented workspace

300 200 100

Base coordinate frame

0

X: 725 Y: -100

Rack

-100 -200 0

100

200

300

400 500 X-axis

600

700

Fig. 15. Results of the workspace analysis in the y–x and z–x planes.

800

900

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

other two workspaces of the ‘‘task-oriented workspace’’ of manipulator and ‘‘task workspace’’ of U-shaped trajectories. And the results also note that the task-oriented workspace also encloses the task workspace. Since the task workspace is much smaller than the task-oriented workspace, the size of the current RRX welding robot could be significantly decreased even based solely on the kinematic analysis.

Longitudinal stiffeners (Upper)

Clamp

Steel wire

6.2. Simulations for obstacle avoidance

Electric winch

Height 3-m Since many U-shaped trajectories exist, there is a great need for robot-simulations to generate the welding paths of the endeffector, and to check for interference with existing structures, such as the web faces of the longitudinal stiffeners. Simulation studies can also help to determine the strokes of each axis and the length of each link of the manipulator, in terms of satisfactory performances of the required tasks in the actual workspaces. Thus, it leads the RRXC to successfully perform numerous welding tasks in the field without any collisions. Fig. 16 shows the simulation results for avoiding interference with the web face of the longitudinal stiffeners, using the ROBCAD program through predefined CAD data of the target U-shaped part.

Clamp

Roller

RRXC

Steel wire Access hole

Hand-winch

Welding machine

Longitudinal stiffeners (Lower)

Left-vertical weaving welding

7. Field tests The field tests for the application of the developed RRXC have been carried out in double-hulled blocks over a period of several months. As mentioned earlier, the RRXC can fit through the 500  700 mm2 access hole with the help of the set of auxiliary transportation devices, and the RRXC can be located at a U-shaped trajectory by transportations in the longitudinal and transversal directions. Then, after finishing a welding job, it can be also moved to the next U-shaped trajectory by the help of an installed electric winch on a steel wire. Through these repetitive executions of welding tasks and transportations in the double-hulled structure, all performances of the RRXC have been successfully demonstrated in terms of the welding 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-hulled structures located within the ship-building factory. The 220 V supplies of single phase are only used in driving the electric winch, not the other devices. Fig. 17(b) also shows the RRXC performing a welding task, with representation of the several cables from the outside of the double-hulled structure. It consists of the welding cable from the welding machine, the power cable of 220 V and CO2 gas cable. However, the set of welding wire spool and feeder, which is connected with the RRXC by a torch cable, is typically located just behind the RRXC. Fig. 17(c), of a

Wire spool/feeder

Right-scallop welding

RRXC

Fig. 17. Field tests for validating the (a) installations of auxiliary transportation devices (b) connecting cables from the welding machine, and (c) actual arcwelding experiments in the U-shaped trajectory.

series of four Figures, shows the actual arc-welding experiments using the RRXC. It consists of vertical weaving, horizontal multipass, and scallop welding procedures. The welding voltage and the arc current are 26 V and 250 A, respectively. The welding speeds are 0.54 and 0.24 m/min, respectively, for the vertical and horizontal welding processes.

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

7.1. Experimental results In field tests, the RRXC together with the set of auxiliary transportation devices successfully demonstrated that the welding quality and operational convenience are acceptable to industrial engineers. These conclusions are made based on the following experimental results: (1) The required time in one successful sensing of the U-shaped trajectory is reduced from 3.2 to 2.0 min compared to the previous 6-axis articulated welding arm using the touch sensor. In other words, the laser sensor system shows significant improvements in efficiency and productivity, with a large reduction in the time required to accomplish the Ushaped trajectory. (2) The welding quality, from repetitive performances of the welding tasks in various types of U-shaped trajectories, has been evaluated by the industrial engineers, and is deemed satisfactory, as shown in Fig. 18. The welding conditions of Fig. 18 are also represented. (3) Some difficulty exists in operating the electric winch and the hand-winch, even though it provides convenient conditions in operating the RRXC. The operators feel unrest from the steel wires, since it seems to be quite weak. There are some requests for the improvement of the slack status of the motor cables of the RRXC. Since the third axis of the vertical arm has a large stroke of 750 mm, using a ‘‘multi-stage mechanism’’, the motor cables should be slack in the minimum stroke of the third axis. This leads to interference between the RRXC and the structures, such as the web face of the longitudinal stiffeners, in the end stroke of the first axis. Thus, this problem has been solved by changing the socket direction of the wiring and adjusting the length of the cables.

8. Conclusion Some difficulties in the applications of the previous fullautonomous welding systems have been clarified through numerous experiments, which have lead to requests for a robotic system that is easy to handle in the narrow and confined structures. For this purpose, a new type of welding robotic system, having a modularized controller, has been developed to perform the welding of U-shaped trajectories in the enclosed structures, with auxiliary transportation devices. As represented above, the welding functions and mobility using auxiliary transportation devices have been successfully carried out and verified through the ROBCAD simulations and field testing in real double-hulled blocks, during a 6 month period. The modular

controller helps the entire systems to decrease the number of controller cables, and to prevent negative influences of electric noise. Significant results have indicated that the separation of the welding and the mobility part is a great help in operating robotic systems, compared with a fully autonomous one of comparatively huge size. The wireless teaching pedant makes it possible for one operator to manage a number of RRXC systems; increasing the efficiency of the production. Although research and field tests are continuing, conclusions can be made that this system will be definitely a great practical help, and will significantly improve the productivity of welding inside of the double-hulled structures.

Acknowledgements This research was supported in part by the Brain Korea 21 Program of the Korean Ministry of Education, and Daewoo Shipbuilding and Marine Engineering (DSME) of Republic of Korea. One of the authors, namely, Donghun Lee, would like to express his thanks to Prof. Jongwon Kim, Tea-Wan Kim, and Kyu-Yeul Lee for their continuing assistance and guidance. The authors would also like to acknowledge the fact that Namkuk Ku has played major roles in programming the jobs and the field testing of the RRXC throughout the project. References [1] Lee Donghun, Kim Jongwon, et al. Development of a mobile robotic system for working in the double-hulled structure of a ship. Robotics and ComputerIntegrated Manufacturing 2010;26(1):13–23. [2] Kam BO, Kang CJ, Jeon YB, Kim SB. Development of mobile robot for welding of lattice type. Preprints of the National Meeting of Autumn, The Korean Welding Society 2000;36:34–6. [3] Thesis, MS, Lee Jung Woo. Development of weaving weld path planning algorithm for T-bar butt welding by using 1D laser sensor, 2009: p. 5–7. [4] Kam Byoung-Oh, Jeon Yang-Bae, Kim Sang-Bong. Motion control of twowheeled welding mobile robot with seam tracking sensor. In: Proceedings of the XXX ISIE, 2001: p. 851–6. [5] Lee Ji-hyoung, Kim Jong-jun, Kim Jae-kwon, Park Jong-ryon. Development of carriage-type welding Robot for double hull assembly line in shipbuilding. In: Proceedings of the 17th world congress the international federation of automatic control, Seoul, Korea, July 6–11, 2008: p. 4310–1. [6] Lee JH, Hwang HS, et al. Development of robot welding system for panel block assemblies of ship hull. Okpo Ship Technologies 1998;46(2):32–40. [7] Jacobsen NJ. Three generation of robot welding at Odense Steel Shipyard. In: Proceeding of ICCAS 2005, Pusan, Korea, vol. 1; 2005: p. 289–300. [8] Bostelman R, Jacoff A, Bunch R. Delivery of an advanced double-hull ship welding system using Robo-crane, In: Proceedings of the third international ICSC symposia on intelligent industrial automation and soft computing, Genova, Italy, vol. 1; 1999. [9] Miyazaki T, et al. NC painting robot for shipbuilding. In: Proceedings of ICCAS’99, Boston. vol. 2; 1999: p. 1–14.v. [10] Armada M, Gonzalez de Santos P. Climbing and walking robots for the maritime industries. Brest, France: European Naval Ship Design Short Course; 2002 p. 8–12. [11] Aranda J, Armada MA, de la Cruz JM. Automation for the maritime industries. 2004; ISBN:84-609-3315-6: 143.

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