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机器人技术在造船工业上的应用_图文

Robotics and Computer-Integrated Manufacturing 30 (2014) 442 –450

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

Review

Robots in the shipbuilding industry
Donghun Lee
Soongsil University, Korea

art ic l e i nf o
Article history: Received 17 January 2013 Received in revised form 13 February 2014 Accepted 26 February 2014 Keywords: Robot automation Shipbuilding industry Self-traveling mechanism Environment recognition Launch and recovery system

a b s t r a c t
In this paper, details of the uses of various robots in the shipbuilding process are provided, with an emphasis on newer developments and applications. The current state of robot applications will be discussed according to the priority of the shipbuilding process. First, various robots for open structures, such as several types of welding carriages and 6-axis articulated robot manipulators, will be reviewed in terms of their mechanisms and applications. Second, several attempts to design autonomous mobile robotic systems for closed blocks of the double-hulled structure of a ship will be discussed in terms of the performance characteristics of their proposed self-traveling mechanisms. Lastly, all corresponding technologies for overcoming structural complexities in closed blocks as well as future directions of robot automation in the shipbuilding industry are also discussed. & 2014 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Robots in open structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Robots in closed structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Intelligent carriages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Mobile robots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Modularized airtight controller. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Environment recognition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Multiple robot control, launch and recovery system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442 444 446 446 446 447 448 448 449 449 449

1. Introduction Over the past few decades, research on robotics has made considerable impact on many industrial ?elds [38,39]. Brie?y, these successful achievements of robotics research in industrial applications can be attributed to rising labor costs, aging skilled workers, and the inclination to avoid 3D (dirty, dangerous, and dif?cult) jobs in many industries. The shipbuilding industry, which is the major concern of this paper, is still one of the labor-intensive industries that demand numerous skilled workers. Owing to the nature of the shipbuilding industry, shortening the shipbuilding process should directly lead to

E-mail address: dhlee04@gmail.com http://dx.doi.org/10.1016/j.rcim.2014.02.002 0736-5845 & 2014 Elsevier Ltd. All rights reserved.

additional ?nancial rewards from ship owners and their increased loyalty to a business. Thus, shipbuilding companies have naturally concentrated on improving their production ef?ciencies within their quality assurance requirements through intensive investments in robot automation as well as developments in shipbuilding processes [40–42]. In this section, the necessity of robot automation technologies in the shipbuilding industry will be discussed in terms of industrial accident prevention and employment of workers, as well as production ef?ciency and quality. Recently, the amount of received orders in the shipbuilding dockyards of both Korea and China has fallen sharply. Oversupply of vessel tonnage had a strong impact on the fall of new orders. The aggressive promotion of the marine plant business by shipbuilders has not led to any remarkable achievements. Moreover, the

D. Lee / Robotics and Computer-Integrated Manufacturing 30 (2014) 442 –450

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Welding

Blasting

Painting

Double-hulled block
Transverse direction Transverse web floors Longitudinal stiffeners 5,100 mm 10,920 mm 3,000 mm

Longitudinal direction

Longitudinal girders

Fig. 1. Overall view of double-hulled structure with some overhead operations needed during the shipbuilding process. Table 1 Industrial accident rate in the Korean shipbuilding industry [1]. Year Shipbuilding industry Accident rate (%) Industrial accident victims (person) Death toll (person) Average accident rate for all industries (%) ‘06 ‘07 ‘08 ‘09 ‘10

1.89 2240 48 0.77

1.55 2065 46 0.72

1.76 2375 45 0.71

1.41 2413 53 0.70

1.20 2122 47 0.69

rate of industrial accidents in the shipbuilding industry is quite a high compared with other industries because remarkable progress has been made on transitions to ships with double-hulled structures (Fig. 1). Such structures were incorporated into ship hull design because they could prevent the out?ow of cargo when a sudden impact occurred on the outer hull. However, this has de?nitely led to an increase in the rate of working processes inside enclosed structures, which represent quite dif?cult and hazardous environments to workers. In fact, most major shipbuilding companies have readily adopted robot automation in various shipbuilding processes such as welding, one of the core working process in the ?eld. As shown in statistics from the ‘Korea Occupational Safety and Health Agency‘ in Table 1, applications of robot automation and improvement of shipbuilding processes have played major roles in reducing industrial accident rates by preventing the exposure of workers to injurious worksites. However, the industrial accident rate remains higher than the average for all industries. The working process in a double-hulled structure is likely to be one of the main causes of this phenomenon. A further issue that needs to be addressed in applying robot automation to the shipbuilding industry is that there are tacit

Table 2 Effect of increased production in the Korean shipbuilding industry on employment [1]. Amount of increase of Coef?cient of employment production (billion) induction (person/thousand million) 1006.7 6.0 Employment induction effect (person) 6040

voices of concern for long-term decline in employment and additional responsibilities of maintenance tasks due to the increasing role of robot automation. Such prejudices de?nitely lead robot designers to dif?cult situations in successful ?eld applications of developed robots. Statistics in the ‘Inter-industry relation table 2010’ from the Bank of Korea suggest that improvement in production quality through robot automation should directly lead to improved credibility of a business, and that a 10% increase in order quantity through improved credibility should enhance production with 6040 additional jobs in associated industries (Table 2). For instance, successful application of a robotic painting

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system will improve the shipbuilding process in terms of fundamental human rights by reducing harsh working environment for workers. Undoubtedly, an enhanced image or perception of improved working conditions in shipbuilding will enable more ef?cient manpower management by preventing the out?ow of skilled workers. Thus, to achieve such advantages, production and R&D departments should actively adopt cooperative attitudes toward one another for achieving mutually supportive relationships. As mentioned earlier, the working conditions in double-hulled structures as well as the labor-intensive nature of shipbuilding are likely to be one of the main causes of the high rate of industrial accidents relative to the average rate for all industries. Fig. 2 shows the manufacturing process used to obtain the closed block, which is a sub-module of the double-hulled wall of a ship. A bottom shell and an open block are assembled separately using welding processes [2]. The bottom shell is composed of a wide steel plate with several reinforcing longitudinal stiffeners welded to it in parallel. 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. The resulting closed block must then be welded. Finally, welding must be performed inside the closed block along the contact boundary of the open block and bottom shell [2]. Currently, human workers execute this welding process, as well as painting and blasting processes, by working inside the space enclosed by the top shell, the bottom shell, and a pair of transverse web ?oors and girders. These manual operations inside the closed block remain the most dif?cult and hazardous jobs performed by human workers in the shipbuilding industry (Fig. 3). As a result, shipbuilding companies are placed in a dif?cult situation in which skilled workers for double-hulled structures are lacking. Therefore, the need for automated solutions based on robotic systems has been high, and several remarkable attempts have been made.

Herein, the current state of robot applications will be discussed according to the priority of the shipbuilding process. First, various robots for open structures, such as typical articulated robot manipulators, will be reviewed in the next section in terms of their applications. In addition, several attempts to design autonomous mobile robotic systems for double-hulled structures will also be discussed in terms of mechanisms, technologies, and management, which need to be newly considered for structural complexity and their environmental hazards. Lastly, future directions of robotic research on double-hulled structures will be brie?y mentioned.

2. Robots in open structures The hull of a ship is made of welded steel plates. As shown in Figs. 1 and 3, numerous welding operations are required to successfully build ship hull structures by using many pieces of steel plate and stiffening members. First, to build an open block, various types of welding equipments, such as 1- or 2-axis welding carriages as well as 6-axis articulated welding robots installed on gantries and overhead cranes, have readily played major roles in various welding operations in shipbuilding. Before discussing robot applications and technologies in double-hulled structures, various types of robots for building an open block will be concisely reviewed here in terms of their mechanisms, operational strategies, and drawbacks in the shipbuilding process. To build solid open blocks, all boundaries among longitudinal and transverse girders, longitudinal stiffeners, and bottom plates are welded through combinations of manual welders and autonomous welding robots (Fig. 4). In this situation, welding carriages have readily played major roles in ef?cient and robust welding operations for long ranges of multiple-pass horizontal–vertical ?llet welding as well as butt welding. Here, a welding carriage is de?ned as a mechanical device having 1 axis or 2 axes for the speci?c purpose of welding. As shown in Table 3(a), a 1-axis horizontal ?llet welding carriage can weld the contact boundaries of the stiffeners and the bottom plate without any motion of the welding torch along horizontal trajectories. The vertical ?llet welding robot, shown in the left side of Table 3(a), can weld the contact boundaries in the vertical direction, with a certain rotating motion of the welding torch (i.e., weaving motion). In particular, for both carriages, guidance wheels are used to ensure straight driving by holding them against the stiffeners. Table 3(b) also shows a V-ROD, a ?xed type of commercial welding carriage for performing vertical weave-welding in speci?ed ranges. One point to note is that these carriages require careful installation for good alignment with desired welding trajectories, which is dif?cult to achieve under real-world conditions. In contrast to

Turn Over

Open block

Bottom Shell

Match slits with stiffeners

Closed Block (Needs to be welded)

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

Fig. 3. Manual welding operation in a real-world closed block.

Fig. 4. Welding carriages in shipbuilding industry.

D. Lee / Robotics and Computer-Integrated Manufacturing 30 (2014) 442 –450 Table 3 Commercialized welding carriages [19, 20]. (a) 1-axis carriage (left) and 2-axis welding carriage (right) (b) Vertical weaving carriage for ?llet welding, V-ROD

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Controller cables from the ceiling
Control and power cables from the overhead crane

Control panel Wire spool 6-axis welding robots U-shaped parts 6-axis welding robots Moving direction Wire spool

robots ?xed to factory ?oors, these types of robots require frequent installation and re-installation for different work pieces. Even for the same work piece, the relative position between the work piece and the installed robot will vary depending on how the robot is installed. Moreover, even for the same given task, the detailed shapes and conditions of the work piece vary within a certain range. Thus, these circumstances require a ?exible and tailored set of sensory systems, including an effective seam-tracking algorithm that enables the robots to plan their paths along the actual welding trajectories and to work without any complicated calibration procedures. Therefore, numerous studies have been conducted on automatic seam-tracking by using sensors based on touch, probe, vision [3–12], laser [13,14], arc [15], electromagnetic [16,17], and ultrasonic sensors [18]. Although these carriages have the excellent properties of compact size, lightness of weight, and modularized controller design, they are not suitable for more complicated tasks, such as the welding of U-shaped trajectories as well as the setting of boundary lines of the transverse web ?oor, two longitudinal stiffeners, and bottom plate. The reasons for this are the de?ciency of the degrees of freedom in the motions of the welding torch, and the unidirectional welding property [20]. Thus, most shipbuilding companies have readily employed 6-axis robotic manipulators combined with additional facilities such as gantry and overhead cranes with consideration for accessibilities in such complex circumstances for open blocks. Before the open blocks are turned over, they are required to move along the line of bay until they reach the desired position among several bays according to a scheduled block-assembly process. Then, a number of 6-axis welding robots spaced apart at regular intervals can be simultaneously installed and re-installed at the welding locations in the open block pending on that bay by lifting and lowering the overhead cranes (Fig. 5). After completing the ?rst welding, a welding robot can repetitively move to subsequent welding locations by using overhead cranes with guidance from the workers. All welding equipment pieces, such as wire spools, feeders, and teaching pendants, are fully installed on the steel frame attached to the robot. However, because the controllers for these welding robots are located at top of the overhead crane, a number of cables (heavily loaded with steel ropes) from the controllers and power sources are connected to every robot (Fig. 6). Given that most shipbuilding companies have followed this practice for a long time, it is likely to be one of the most ef?cient ways when considering the shipyard assembly process. Although this method ensures stable welding quality, operation, and maintenance through successful harmonization with the shipbuilding process, several points need to be improved

Fig. 5. 6R articulated welding robots connected to overhead crane [21,22,36,37].

RRX welding manipulator

Welding wire spool and feeder

RRX mobile platform

Modularized airtight controller

Wireless teaching pendant

Fig. 6. Field test of welding robot RRX working inside double-hulled structure of ship.

in terms of production ef?ciency. The robots cannot achieve perfect welding for all boundary lines in U-shaped parts (e.g., reinforcement brackets between the transverse web ?oor and longitudinal stiffeners) because this method does not support movement in every direction after installation of the robots in front of the welding locations. Thus, after all welding operations are performed by the robots, additional manual welding operations are performed by skilled workers. Moreover, repetitive installation and re-installation when a robot tries to move to the next welding location require continual assistance from workers. Therefore, improvements in the method in terms of production ef?ciency are needed. If any mobile platform capable of moving freely in the longitudinal and transverse directions were available in such complex circumstances, then there would be a greater chance of achieving

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Table 4 Several types of intelligent welding systems. (a) 4-axis carriage [20, 25] (b) 5-axis carriage [26] (c) 6-axis carriage [20]

both welding of high quality without any further manual welding and enhanced ef?ciency in the shipbuilding process by covering insuf?cient manipulator workspace. Moreover, because current welding robot systems cannot be used in closed blocks owing to limited accessibility of the overhead crane, any acceptable mobile platform that could freely travel inside the closed blocks would be greatly helpful in the automation of all required operations (e.g., welding, blasting, and painting). In fact, given its enclosed structure, the temperature increases and reaches 40–50 1C during the summer, air circulation is poor, and it is often too dark to freely carry out tasks, even during the daytime. However, workers still frequently perform manual welding, blasting, and painting inside the space enclosed by the top and bottom shells and a pair of transverse web ?oors and girders [20,23]. This is certainly one of the main reasons for the high rate of industrial accidents in shipbuilding industry. Thus, many shipbuilding companies have attempted to focus on developing a mobile platform capable of freely traveling inside the closed blocks and successfully performing welding, blasting, and painting.

3. Robots in closed structures We have already described the research motivations of the selftraveling mechanism for robot automation in open and closed blocks. Therefore, some previous research achievements will be discussed here in terms of the performances of the proposed selftraveling mechanisms and their corresponding technologies for overcoming structural complexities. However, before reviewing the self-traveling mechanism, it would be meaningful to review some portable intelligent carriages as an alternative of welding automation for U-shaped part welding in closed blocks. We will also discuss the recent successful achievement of the rail runner mechanism [20,24] and commercial Inrotech solution [35]. 3.1. Intelligent carriages Alongside numerous efforts to design self-traveling mechanisms combined with various types of manipulators for their own purposes, there have been other approaches to achieve autonomous welding in closed blocks through the design of portable welding carriages with only 4 to 6 axes for manipulation to perform the welding of U- shaped parts [20,25,26]. However, to cover their insuf?cient workspace, the carriages shown in Table 4 (a) and (b) have a driving mechanism, which is genuinely unacceptable because the bottom ?oor is usually quite unclean. Thus, if a certain robust algorithm for motion control does not hold, then the manner of differential driving cannot mechanically guarantee the straightness of repetitive multi-pass welding because of the high likelihood of slippages. In addition, the carriage shown in Table 4(b) uses an external controller and

driving wheels. As a result, there are a number of cables from outside to the welding locations, which in turn give rise to dif?culties in handling these cables in complex structures and negative in?uences of electric noises from neighboring welding sites. Furthermore, although these intelligent carriages have excellent properties of relatively compact size and lightness of weight compared to autonomous welding robots with their own self-traveling mechanism, several points need to be improved in terms of the weight, which can be still considered to be heavy for the workers to easily handle. In most industrial ?elds, the maximum weight of objects handled by workers and the number of times they are handled per day is strictly limited by laws on occupational safety and health acts; in the case of workers in Korea, these should not exceed 25 kg and lifting up and down 10 times per day. Thus, to successfully apply these carriages to closed blocks, auxiliary transportation devices for workers to easily launch, transport and recover a number of carriages inside the closed block are needed. However, there are no published studies on these issues, even though they are worthwhile to investigate for successful application of autonomous welding systems in closed blocks. Recently, RRXC, a new type of welding robotic system shown in Table 4(c), has been developed to perform the welding of U-shaped parts. Points to note are that it has a modularized controller, a fold-up rack system instead of driving wheels, and 6-axis joints comprising three prismatic and three revolute joints. Unlike with previous carriages, the fold-up rack in the RRXC guarantees straightness in bi-directional multi-pass welding by ?xing the fold-up rack onto the bottom plate with two on/off magnets. Moreover, it fully conducts the welding of U-shaped parts without any further manual welding by workers, because it has enhanced reachability and dexterity in its manipulator workspace compared with previous carriages [20,24]. To easily handle the RRXC, Lee et al. designed portable auxiliary transportation devices through which they controlled the mobile function within the closed blocks. An RRXC consists of electric winches, handclamps, 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 weight requirement for each device, which should be less than 10 kg to achieve the handheld mode in the real ?eld. As a result, this then provides a means of transporting the RRXC along the connected steel wire and lifting it up and down [20,24]. 3.2. Mobile robots Next, several research achievements in the development of mobile robots that autonomously move and perform welding, blasting, and painting in closed blocks will be discussed in terms of the performances of their self-traveling mechanism and feasibility of successful applications in the real world.

D. Lee / Robotics and Computer-Integrated Manufacturing 30 (2014) 442 –450 Table 5 Several types of the mobile robot system.[2, 20] (a) Hitachi's painting robot, Japan [27] (b) IAI Rower-1, Spain [28]

447

(c) SNU RRX, Korea [24,30,31]

Table 5(a) shows the painting robot, which has been developed by the Hitachi-Zosen shipyard in Japan [27]. A 6-axis painting robot, plus a self-driving carriage, is placed inside the closed block by using an expandable placer. However, this robotic system requires a large access hole 800 mm ? 1600 mm in size. Any enlargement of the hole requires the permission of the ship's owner and is almost impossible to achieve because the size of the access hole is related to ship-design safety regulations. Another serious problem of this robotic painting system is that it cannot move freely in the transverse direction inside an enclosed block. The Industrial Automation Institute (IAI) in Spain has developed a robotic system called ‘ROWER 1’, which is shown in Table 5(b) [28]. This robot moves like a spider, and has four legs that extend and contract; it moves autonomously and thus overcomes many of the welding obstacles encountered in a closed block. However, 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 [29]. Therefore, a new innovative robotic system is needed to satisfy speci?c requirements such as, compactness to go through an access hole of 500 mm ? 700 mm, possessing autonomous transverse traveling capability inside the closed block at relatively high speed, and having the required operational performance characteristics. Finally, the RRX and Inrotech welding robotic system, which overcomes all of the disadvantages of the previous robots, have recently been established. The commercial Inrotech welding robotic system is composed of a Fanuc manipulator for welding the U-shaped parts, a rail system in transverse direction for mobile functions inside of the blocks, and feeder rail for deploying the welding manipulator and the rail system from outside of the closed blocks. This impressive feeder rail system of Inrotech solution can transport all welding equipments such as welding robot, rail system, cables, controller, welding machine, etc. from outside of the blocks through the access hole as shown in Fig. 7. Moreover, this commercial welding robotic system has been successfully applied and contributed to the Odense Steel Shipyard in 2009. [35] And the RRX is composed of a 6-axis welding manipulator and a mobile platform in its newest version, where the main function of the 6-axis welding robot is to weld the U-shape boundaries of the closed block, and that of the mobile platform is to enable the transverse and longitudinal movements of the entire robotic system required to move it to subsequent welding locations. The RRX platform also displays performance characteristics that differ from those of the previous mobile robots designed for closed blocks. First, all electronic components, such as servo drivers and

Feeder rail from the outside of block

Access hole

Controller & modular welding system

Welding robot (Fanuc) Rail system

Fig. 7. Inrotech's commercial autonomous welding robotic system possible to be placed into the closed blocks by the feeder rail system [35].

power supply, are fully embedded in the airtight lower sliding body with tailored set of heat pipes for cooling; this is a reasonable design considering the hazardous environment. In addition, removing all cables from outside can lead to successful application of RRX by preventing all negative effects from various noises, hazardous air conditions, and dif?cult handling issues. One other point is that the aim of the RRX platform design is to achieve multiple functions. This platform is applicable to various tasks such as welding, blasting, and painting by changing its manipulators [30,31]. Consequently, this multifunctional mobile platform fully covers all shipbuilding processes. Lastly, RRX shows welding of high quality without further manual welding operations because it can move in the longitudinal direction after welding U-shaped parts to cover its insuf?cient manipulator workspace and successfully weld both sides of the bracket. Moreover, its welding performance and mobile functions have been veri?ed through ?eld testing over a 1-year period. Considering the circumstances as a whole, the mobile robot systems designed in the year ahead for enclosed structures must consider the following issues: 1) modularized airtight controller; 2) environment recognition for overcoming structural complexity; 3) multiple robot control; and 4) launch and recovery system. 3.3. Modularized airtight controller As discussed brie?y, the controllers in the Hitachi painting robot and IAI Rower-1 are located outside of the closed block.

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Thus, there are a number of cables from outside. This causes the following negative effects on the entire system. First, dragging a number of cables in such complex structures will certainly increase the payload of the self-traveling platforms. Second, increasing the number of robots simultaneously launched in the same closed block will give rise to dif?culties in handling these cables due to their becoming intricately tangled with one another. Lastly, there is a negative in?uence of electric noise from neighboring welding sites. However, a considerable portion of these problems can be solved by embedding the modularized airtight controller in the mobile robots. The most important issues in the design of a modular and hermetic controller are to secure robust cooling performance and dustproof quality because the temperature is 40–50 1C during the summer, and operations such as welding, blasting, and painting produce considerable amounts of metallic dust (e.g., fumes). This naturally represents a very hazardous environment for the robot controller, whose body should be kept fully airtight to prevent the in?ow of metallic dust and for which the temperature should be maintained at its rated level. Thus, to satisfy these design constraints, heat pipes can be incorporated to dissipate the amount of heat from the servomotor drivers and power units without any air?ow into the controller for cooling. For instance, Lee et al. recently proposed a new cooling system for the mobile welding robot RRX. This proposed cooling system is composed of heat pipes, cooling ?ns, fans, and L-shaped brackets for transferring the produced heat from the heating resources to the heat pipes. Validation through ?eld tests fully supports the idea that this design approach is appropriate for the controller to maintain stable performance in such a harsh environment. [34] 3.4. Environment recognition To successfully move in a closed block composed of many stiffeners and obstacles (e.g., brackets, pipes, ladders), careful consideration of environment recognition should be conducted. In the case of the longitudinal stiffeners, structural errors such as differences in height, interval distance, and straightness naturally occur in the assembly process among neighboring stiffeners. Thus, mobile robot platforms that travel on or use the longitudinal stiffeners will inevitably display uncertainty in their mobile performance. For instance, the RRX may have inclined robot postures on two neighboring stiffeners during repetitive movements in the transverse direction when their heights differ. It may also exhibit errors in travel distances in the longitudinal and transverse directions when the stiffeners have different interval distances. Such inclined postures and differences in the initial positions of the robot at every welding location will negatively affect the welding quality because of inaccurate recognition of the target structures. Namely, as the RRX uses a laser displacement sensor for ?nding every start or end point for all welding trajectories of a U-shaped part, having its own recognition algorithm is essential for successfully performing given operations even though the surrounding structures have the described errors (Fig. 8). In fact, we note that some components installed in the closed blocks (e.g., pipes and ladders) are not necessarily fully represented in the ship hull CAD data. This means that for autonomous wheeled mobile robots to successfully determine their actual locations within such environments based on given CAD data for task-path planning is highly dif?cult. Commonly, the accurate position of the robot can be estimated through maps and various sensor systems by recording information obtained from one form of perception and comparing it to a current set of perceptions. However, unreliability of even one of the components will lead to poor positioning accuracy and a high likelihood of collision with

Raw data Mobile robot

Scanning sensor Raw data Obstacle recognition Mobile robot

Hose Feeder Scanning sensor Table

Fig. 8. Result of environment recognition inside closed block for RRX blasting application.

obstacles. In addition, because slippages on the bottom ?oor and longitudinal stiffeners as well as dif?culties in installation from structural complexities are highly likely, dead reckoning or indoor GPS techniques are unhelpful in this circumstance. For welding and blasting applications of RRX, to overcome inaccurate map data based on a given set of ship hull CAD data, a spatial scanning process by rotating Hokuyo's scanning laser range?nder, UHG08LXm, was performed to detect all obstacles through postprocessing techniques such as characteristic edge detection by using the obtained scattered point cloud [32,33]. Because all welding locations are arranged along the transverse web ?oor on both sides, the RRX moves in the transverse direction with spacing apart from the web ?oor to avoid collisions with brackets on the longitudinal stiffeners. After moving to the next welding location in the transverse direction, the RRX gradually moves in the longitudinal direction along the longitudinal stiffeners to take a de?ned position in front of the welding location. Then, it measures the distance from the web ?oor by using the laser displacement sensor, and detects obstacles in any direction by using ultrasonic sensors. To realize robot automation in such complex structures, ef?cient combinations of sensor systems should be considered to overcome such structural uncertainties. 3.5. Multiple robot control, launch and recovery system Lastly, two important issues in terms of management of these mobile robot systems in closed blocks are brie?y discussed. Because the closed block is commonly composed of 4 to 6 sections (Fig. 1), a number of robots should be simultaneously launched into and recovered from the sections. Thus, to manage and control these robots in such circumstances, the following issues should be considered: 1) a wireless teaching pendant to realize simultaneous control of the robots with minimum workers, and 2) a launch and recovery system (LARS) to ef?ciently manage and transport a number of robots through the 500–700 mm2 access holes. First, the teaching pendant is a hand-held robot control terminal that provides a convenient means to run robot programs. Until the RRX had been developed, most teaching pendants were connected to a robot controller by cables. However, the wired teaching pendant is no longer acceptable because the necessity for a worker to follow a number of robots to every location in a hazardous environment is highly inef?cient. Thus, there is a great need for wireless teaching pendants to enable workers to

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control the number of working robots without any physical connections. For instance, Lee et al. have successfully applied the wireless teaching pendant in RRX and RRXC [2,20,24]. Based on previous analyses of mobile robotic systems, the mechanism for fully autonomous traveling on the structures has resulted in the enlargement of the robotic system owing to the increasing number of joints. This has led to the request for a LARS for easy handling and transport through only small access holes with openings of 500–700 mm2. Although all of the proposed mobile robotic systems have their own mobile functions, realizing functions for passing through the access holes by themselves is highly dif?cult. This is why a LARS is greatly needed for robot automation in closed blocks. Useful research achievements of LARS have been proposed for mobile robotic systems [20] and carriagetype robotic systems [24].

4. Conclusion This paper tried to deliver all detailed discussions of the recent impressive research results in terms of the robotic mechanisms for welding the ship hull structures to the present. For a fresh possibility of appearances of innovative robotic systems in shipbuilding industry, one of the important topics would be successful combinations of the proposed technologies and considerations to the present with well-designed mobile mechanisms such as RRX and Inrotech solution. To summarize, methods for convenient handling, operation, and maintenance in the real world and for ef?cient robot performance on all given tasks without any further manual operations were discussed. Lastly, as stated earlier about ‘employment induction effect’ in Korean shipbuilding industry through providing statistics from the Bank of Korea, an enhanced image or perception of improved working conditions by successful application of a robotic system in harsh environments might enable more ef?cient manpower management by preventing the out?ow of skilled workers. Though the employment stabilization is a matter to be carefully dealt with in many ways, the statistics clearly show one of the contrary evidences that the application of robot automation to the shipbuilding process may raise the long-term decline in employments. That is, it can be carefully interpreted as successful applications of robotic system that is probably one of the best ways to gradually improve the chances of such advantages in terms of fundamental human rights by reducing harsh working environment for workers. Also, it would be greatly helpful that the production and R&D departments should actively adopt cooperative attitudes toward one another for achieving mutually supportive relationships.

Acknowledgment This work was supported by the Soongsil University Research Fund in 2013. References
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