ARTICLE

International Journal of Advanced Robotic Systems

A Robot for the Unsupervised Grit-Blasting of Ship Hulls

Regular Paper

Daniel Souto, Andres Faiña, Alvaro Deibe, Fernando Lopez-Peña and Richard J. Duro*

Integrated Group for Engineering Research, Universidade da Coruña, Spain * Corresponding author E-mail: [email protected]

Received 3 May 2012; Accepted 20 Jun 2012

DOI: 10.5772/50847

© 2012 Souto et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract This paper describes the design and the control of the robot has been implemented and tested in realistic architecture of an unsupervised robot developed for grit environments, ascertaining that the design and the blasting ship hulls in shipyards. Grit blasting is a very control system are perfectly suited to the functions which common and environmentally unfriendly operation, the robot must carry out. required for preparing metallic surfaces for painting operations. It also implies very unhealthy and hazardous Keywords Autonomous robots, layered robot control, working conditions for the operators that must carry it Climbing robots, safe gritblasting, Shipyard automation out. The robot presented here has been designed to reduce the environmental impact of these operations and completely eliminate the health associated risks for the 1. Introduction operators. It is based on a double frame main body with magnetic legs that are able to avoid the accumulation of Shipyards are a paradigmatic example of a dynamic and ferromagnetic dust during its operation. The control unstructured environment where the real application of system presents a layered structure with four layers that robots presents a very significant challenge. These types are physically distributed into two separate components of environments are very extensive, requiring moving in order to facilitate different operational modes as well robotic platforms, and in constant change, making them as to increase the safety requirements of the system. A very difficult for traditional robotic approaches. The low‐level control component has been implemented on different operators and tools that are required are usually the robotic unit itself, and a mission planning and control carried to the product itself (ship or marine structure) and component has been developed on a base station that is move over it, as opposed to the way things are done in also used for interaction with the operator, when the more traditional assembly lines. Thus, any robot that has monitoring of the robot’s operation is required. This base to work in these settings will be faced with a constantly station component contains three layers of the control changing product that grows as construction proceeds. At system that permit the manual, semiautonomous and the same time, the robot needs to move and operate in an autonomous operation of the whole system. A prototype environment that is filled with all kinds of changing

www.intechopen.com Daniel Souto, Andres Faiña, Alvaro Deibe,Int Fernando J Adv Robotic Lopez-Peña Sy, 2012, and RichardVol. 9, 82:2012 J. Duro: 1 A Robot for the Unsupervised Grit-Blasting of Ship Hulls obstacles: other moving robots or humans, scaffolding, texture level for the surface. However, it presents many machinery, etc. As a consequence, shipyards are among drawbacks when considering the environmental those industries with a low degree of robot penetration. implications of the process and the hazards for the human operators performing the task. It obviously Notwithstanding the previous comments, some groups generates a lot of toxic waste, which includes the particles and companies have addressed the introduction of robots themselves as well as any of the materials that have been in these environments since the end of the Eighties [1]. stripped off the surface (paint, metal, oxides, etc.). It also However, these systems have not really been adopted by involves very unhealthy working conditions for the the industry until much more recently, and only for operators carrying it out, as they have to work in an specific and very controlled and simplified operations, environment full of small metal or sand particles that such as inspection [2], welding [3],[4], and hull cleaning bounce off the surface at very high speeds, paint and [5]. The aspect which the authors of the different other toxic particles that are stripped off the surface, as proposals pay the most attention to is how the robots well as being exposed to very high acoustic levels move over the areas they are supposed to work in. In this [12],[13] and [14].s A a consequence, over the last few line, different approximations are possible, going from years new environmental regulations have led to an wheeled robots to different types of walking machines. increasing use of ultra‐high pressure water jetting as an General introductory reviews of climbing and walking alternative to abrasive blasting in certain operations. In robots in the marine industry, as well as of cooperative this case, no particles are used ‐ it is just the water itself, robots, may be found in [6],[7] and [8]. Just to cite some of projected at extremely high pressures, which performs the most recent applications in this line, we should the stripping of the surface. Unfortunately, these mention those of Fei, Zhao and Wan [9] who developed a techniques, although much more environmentally climbing inspection robot with four magnetic wheels, and friendly, are generally slower and more costly than Lee et al. [10] whose proposal is very specialized for a abrasive blasting and, more importantly, cannot really certain type of environment, namely that of double hulls. prepare steel surfaces for optimal paint adherence, like in They have designed a rail runner mechanism that carries the case of sand or grit blasting. They basically remove a robotized arm for welding in double hull structures. only the coating that is present. It is for this reason that the most commonly used systems in shipyards are still Surface preparation is a very important operation in based on manually operated hoses that project grit at shipyards. It involves removing any surface coating high speeds by injecting pressurized air at a pressure of (paint or oxides) as well as providing a surface with a around 8 kg/cm2. given texture that is adequate for the subsequent painting operations [11]. This is a process that is carried out just With the aim of reducing the environmental and health before painting in order to prepare the metal surfaces. impact of these operations, over the last decade some When this operation is performed during the construction inroads have been made in their automation [2] and in of the ship, the objective is for the surfaces to conform to the development of robots for hull stripping and surface a given texture standard before applying the paint preparation. Most of the work carried out in this line, coatings in order to improve durability and coverture. On however, has considered water jetting to be the main the other hand, when this operation is carried out during stripping technique [15],[16] and [17]. A clear example of ship maintenance, the objective is usually to first remove this is the Ultrastrip series of Robots developed at the the old paint coats and prepare the surfaces for new ones Carnegie Mellon University Robotics Institute [15] and when it is repainted. In both cases, it is an later commercialized by Ultrastrip Systems. These 200+ environmentally problematic process, with all kinds of kg remote controlled robots are based on air gap magnets health‐related risks, which is usually carried out by for fixation and wheels for motion, and they carry around human operators. a water jet head within an enclosure for vacuuming the residue in order to reduce the environmental impact of Different technologies exist for cleaning and stripping the system. The results obtained by this system were metal surfaces, the two most relevant being abrasive quite successful but, as indicated above, water jetting blasting and ultrahigh pressure water‐jetting [2]. does not achieve the same level or steel surface Abrasive blasting is a traditional technique and has been preparation as grit blasting systems and, thus, is not around for more than a century. It consists in blasting the appropriate for all cases. It is for this reason that some hull with small particles of sand (sand blasting) or metals groups have devoted great efforts to the development of (grit blasting) using a high pressure jet of fluid, generally robotic systems capable of safely performing grit blasting air or water, to project these particles onto the surface at operations on ship hulls. One of the main problems here very high speeds. It is a very effective technique in terms lies in the adherence of the robots to the metallic surfaces, of the final surface results as, in addition to removing as blasting produces ferromagnetic particles or dust that whatever coating is present, it is able to provide a given tends to collect on the fixation magnets, rendering them

2 Int J Adv Robotic Sy, 2012, Vol. 9, 82:2012 www.intechopen.com useless. The authors of [18] tried to solve this problem by cleaning an 80 cm wide path as it moves. It is attached to using vacuum‐based adhesion and moving the robot the surface of the hull by means of a NdFeB permanent using four wheels. Others, like Ortiz and his collaborators magnet‐based fixation system whose individual elements [19], have addressed the problem through a remote can be demagnetized, as needed by means of internal controlled robot arm with a blasting head positioned on demagnetizing coils. A double sliding structure provides an elevation platform that moved over the ground enough degrees of freedom for the system to turn as alongside the ship. The externally controlled platform is desired, avoiding obstacles and comply with the curved displaced along the length of the ship and raises the arm shape of the hull. The robot is quite modular and can to the operational positions. Very high‐levels of surface accommodate an image‐based quality verification quality can be achieved using this approach. In fact, the subsystem that modulates its operation in order to quality levels meet the most stringent requirements in the provide a homogeneous quality level throughout the sector. However, on the down side, this system is very surface. cumbersome and costly to run, and its operation, like in the case of Ultrastrip, requires human operators to The paper is organized as follows: Section 2 is devoted to remotely control the system. This same group proposed the description of the architecture and the kinematics of another robot [20] based on magnetic treads. Recently, the robot. A description of the control system is provided [21] have developed a robot for grit blasting the inside of in nsection 3. I section 4 we present some examples of the double hulls. The robot moves by supporting itself on the real operation of the robot and, finally, section 5 provides inside beams of the hull and controls the blasting head some conclusions of this work. through a seven axis manipulator. 2. Robot architecture Thus, it is clear that there is a need for compact and easy to handle robotic systems that can carry out sand or grit The design of the robot architecture has been carried out blasting tasks in an autonomous or semiautonomous taking into account that its primary mission is grit mode while preserving the environmentally and operator blasting. As a consequence, it should be able to navigate friendly character required for their use in modern on the hull surface, transporting the blasting head and shipyards. In other words, a grit blasting system is moving it according to certain rules while fulfilling a needed that can compete in terms of environmental series of actuation and safety requirements. One of these friendliness with current water blasting systems and requirements imposes that, in the case of a total loss of which is not hazardous for human operators. At the same the power supplied to the robot, it should remain time, this system should be able to attach itself and move stationary in its current location and entirely attached to securely over the surface as well as receive protection the hull. For this reason, we have resorted to the use of from the effects of the grit and particles generated during permanent magnets as a means of attachment. However, the blasting operation. since some of the blasting materials are ferromagnetic, the magnets need to be placed away from the blasting head To fulfil these requirements, which have not been covered and demagnetized at some points during the robot’s by any system yet, in this work we present a novel small, operation in order to avoid the attachment of large light and autonomous robotic grit blasting system that amounts of grit. Thus, we have opted for a legged robot can achieve the same or even better surface quality results with magnets ‐ which are demagnetized when moving than those provided by a human operator while away from the hull surface ‐ as its feet. Starting from this minimizing the amount of waste the environment has to proposition and taking into account the robot’s primary support. Two main blocks make up the grit blasting purpose, its architecture has been finally based on a system: a closed circuit grit blasting head that recovers double frame main body. We found that this design the grit and the stripping residues through a vacuum facilitates the sideways oscillating motion of the blasting system ‐ which is being developed jointly with the head while the robot is advancing in a straight or mildly research department of the Navantia Shipyards in Spain ‐ curved path, which are the basic actuations needed for and a semi‐autonomous climbing robotic system whichs i the robot to be able to accomplish its task. The robot able to perform the whole surface preparation operation architecture and different aspects of its design and with very little human assistance. motion are described in the following subsections.

The objective of this paper is to present a semi‐ 2.1 Mechanical Design autonomous climbing robotic system that carries the blasting head as well as the elements that participate in The robot architecture (Fig. 1) is based on two four‐ its control. It is based on an 80 kg sliding type legged frames or modules linked through two complex pneumatically actuated structure similar to [22] that can joints allowing them to move relative to each other while walk on the hull carrying the blasting head while allowing the whole robot to walk. Some details of the

www.intechopen.com Daniel Souto, Andres Faiña, Alvaro Deibe, Fernando Lopez-Peña and Richard J. Duro: 3 A Robot for the Unsupervised Grit-Blasting of Ship Hulls robot kinematics are shown in Fig. 2. We have resorted to to magnets at their feet through ball and socket joints pneumatic actuators for all of the moving parts. The main (BS5‐BS8). This way of coupling allows the magnets to reason for this choice is that they are light and that ‐ due adapt to small angle deviations of the hull surface, to the compressibility of air ‐ movement driven by increasing their grip. This module has a double action pneumatic actuators is passively compliant, facilitating linear actuator that drives slide S3, which is mounted the absorption of vibrations or small irregularities and aligned with respect to S4. It is on these two elements (S3 thus leading to safer operation and S4) where the two bearings (R1 and R2) are mounted. These R1 and R2 joints are linked through their top ends The first complex joint is made up of the S1 and S3 slides to the S1 and S2 slides mounted on the upper frame, thus and the passive R1 joint, which is just a bearing. The linking the two modules together. This way of coupling second one is made up of the S2 and S4 slides and the both modules permits relative linear displacements passive R2 joint, which is similar to R1. All four slides, between them along two axes, as well as the rotation of except for S4, are driven by pneumatic linear actuators, the modules relative to each other. Therefore, this being S4 passive. The lower module has four legs that are arrangement of the modules allows the robot to advance, driven by linear actuators (T5‐T8) and which are coupled to change its direction, and to move horizontally.

Figure 1. The complete robot (left) and the lower and top modules (right).

The grit blasting head and a vision and quality control subsystem are mounted on the lower module. The quality control subsystem consists of two intelligent cameras provided with illumination elements. One is placed before and the other behind the blasting head, allowing vision‐related alignment and blast strip length control tasks. In fact, one of the cameras sees the surface just before being blasted and the other just after blasting. The former is located before the advancing blasting head and discriminates between the blasted and non‐blasted regions, allowing the correcting of the robot trajectory. The latter is located behind the head and allows the performance of surface quality verification. Obviously, the cameras switch their functions depending on the direction of the motion of the head. Both the cameras and the blasting head are moved together by means of a linear actuator (S5). Figure 2. Robot kinematics diagram.

4 Int J Adv Robotic Sy, 2012, Vol. 9, 82:2012 www.intechopen.com The top module is a frame formed by a rigid rectangular robot in order to blast a working area is shown in Fig. 3. structure that has four actuated legs located at the This figure also displays the trace of one of these blasting vertices of the rectangle. These are identical to the legs of operations. the lower module. It also presents two parallel linear actuators driving the S1 and S2 slides that move along two parallel sides of the rectangular frame. These actuators are in charge of the vertical motion of the robot on the hull when actuated together and in charge of the performance of relative rotations between the modules when actuated independently. In these actuators, a pneumatic breaking system in the slider prevents the vertical displacement of one module with respect to the other.

When one module moves, the other remains attached to the hull by its four legs. In case of power failure, the air in the leg actuators is purged and all eight feet are placed on the surface. This attaches the robot to the hull. Before moving, each module demagnetizes its magnets and retracts its legs. In order to prevent uncontrolled moments from appearing at the couplings of the modules, there is a mechanical support element beside each magnet consisting of a leg with a spherical wheel.

2.2 fMotion o the Robot

The operation of grit blasting the hull is carried out as follows. The robot is placed on the hull. The blasting compressor is turned on and the head starts blasting away. The compressor is not turned off until the end of the operation. The robot starts by moving both the top module and the blasting head horizontally, allowing blasting a strip of around 80 cm, until the head reaches the end of the working area. The bottom module slides Figure 3. Blasting sequence (left to right, top to bottom). up appropriately and the blasting head and the top module begin to move horizontally in the opposite Since the hull is a three dimensional surface, in order to direction, until they reach the end of the working area on blast it properly in the way described and without the other side. During this sequence, the magnets and the leaving non‐blasted spots ‐ or blasting some areas twice ‐ actuators in the legs of the modules must be activated it will be necessary to correct the robot trajectory along its appropriately so as to attach or move the module at the path. This can easily be achieved by introducing corresponding instant of time. differences in the commands to the actuators driving slides S1 and S2. In this manner, small turns can be Consequently, the head will blast horizontal strips introduced in the robot path without harming its stacked on top of each other until the bottom module actuation. The normal blasting operation does not require reaches the upper end of the top module. At this point, large turns to be made; however, there could be some the bottom module is fixed to the hull surface and the top situations where the robot will need to make a sharp turn. module is displaced up until the bottom module is These kinds of turns are represented in Fig. 4 and can be located at its bottom, whereupon the operation resumes achieved as follows: with the bottom module fixed and as before. When the top of the area which must be grit the top one released, slides S1 and S2 are moved all the blasted on the ship hull is reached, the bottom module is way and in opposite directions to one another; then the moved completely to one side of the top module; it is top module is fixed to the surface and the bottom one is then fixed to the hull and the top module is displaced released; after that, the S1 and S2 slides move all the way over it until it reaches a position where a new vertical back (or however far is necessary, which in the case of the strip can be blasted. Then, the blasting operation starts figure is only halfway). At this point the robot has turned again, this time descending down the hull. The sequence by around 37º. The operation is repeated as many times of motions performed by the different elements of the as is necessary to complete the required turning angle.

www.intechopen.com Daniel Souto, Andres Faiña, Alvaro Deibe, Fernando Lopez-Peña and Richard J. Duro: 5 A Robot for the Unsupervised Grit-Blasting of Ship Hulls system unusable. This characteristic was important in choosing a legged robot over a wheeled one as ‐ in this last case ‐ it would be very difficult to demagnetize the wheels at any time without risking the robot becoming detached from the surface. A further advantage of this implementation is that the electromagnets within the permanent magnet shell can also be polarized to produce a magnetic field that is aligned with that of the permanent magnets, thus increasing the attraction and grip forces when necessary.

3. Electronics And Control

For any robot or automated structure to perform a task, a control system implemented over a certain type of electronics determines the sequences of actuations and the responses to the different signals provided by the sensors or the commands of users. This is no different in the case of a grit blasting robot. That is, to carry out the task of blasting, the robot needs to monitor its sensors, which provide information on its own kinematic and dynamic state, its location on the shipʹs hull and the surface quality achieved. Using this information, it needs to calculate in real‐time the correct configuration of its actuators in order to perform the task assigned to it. In other words, the robot must achieve the appropriate Figure 4. Actuation sequence for turning the robot kinematic configurations to allow it to move following a specific path and, in particular, a specific blasting head 2.3 Fixation System path. In fact, as the speed at which the head moves determines the quality of the stripping process, it needs According to the considerations made earlier, and given to be able to follow a specific blasting head path at a that the robot has to move over a ferromagnetic surface, particular speed, which must be adapted depending on we have chosen a permanent magnet‐based fixation the characteristics of the surface to be stripped. system. As pointed out earlier, these types of fixation elements are safer in case of system malfunctions than The size and weight of the compressors needed for grit other options ‐ such as electromagnets or vacuum‐based blasting, and the grit itself, are too heavy to carry fixation elements ‐ since none of these other methods can onboard the robot without ending up with it becoming guarantee that the robot will not fall when a total loss of overburdened. Therefore, the most sensible thing to do energy supply occurs. would be to leave the infrastructure needed to generate the blast shot on the ground. A special hose is used to The selected magnetic fixation system consists of NdFeB bring the grit jet to the blasting nozzle, which is onboard magnet shells with electromagnetic coils inside them the robot, and moves with it. Likewise, from the point of which permit the system to cancel the magnetic field and, view of the robot’s energy requirements and actuation, an consequently, unfix the feet from the surface by autonomous and independent robot solution should be generating magnetic fields opposed to those of the found. This solution would carry on board the robot the permanent magnets. This results in a very simple and equipment needed for supplying compressed air to the effective means of achieving the fix‐unfix actuation. In pneumatic actuators, with enough batteries to power the addition, there is another reason to choose this type of necessary electrical and electronic equipment. Again, the strategy based on the robotʹs operation: as mentioned weight involved in this solution would be high and above, grit blasting can generate a lot of ferromagnetic would make the robot more cumbersome and harder to metallic dust and particles that will have a tendency to handle. Here, we have chosen to design a system based accumulate and remain attached to the magnets. The on two hardware blocks: a robotic structure that moves demagnetizing action, used as part of thet robo along the hull transporting the blasting head and a base movement operation, permits the elimination of stray station that comprises all of the hardware on the ground particles from the magnets in a cyclic way. Otherwise, required for providing energy, i.e., compressed air for the they would accumulate and render the magnets and the pneumatic actuators and compressors for producing the

6 Int J Adv Robotic Sy, 2012, Vol. 9, 82:2012 www.intechopen.com

Figure 5. Hardware involved in the control and operation of the robot. The left side of the image shows the base station devices while the right side shows the different actuators and sensors on the robot.

pressurized blasting shot. The base station is linked to the robot through two hoses (one with a blasting shot and the other with compressed air), an electrical power line and a control line.

Thus, taking into account that there will be a physical connection between the robot and the ground, a decision was made to design the robot control system as a layered structure that is divided into two main parts: a robot control system, onboard the robot, which monitors the sensors and makes real‐time decisions regarding the status of the actuators; and a mission control system, located at a base station on the ground, whose function is to permit the interaction with the operators, perform the necessary path planning required to cover the area to be blasted, and monitor the surface quality achieved using the information received from the cameras. As indicated above, these two control subsystems are joined together through a communications line. Fig. 5 displays a schematic representation of the different hardware elements involved in the control system and the operation of the robot. From another point of view, a schematic of the layered control system employed is presented in Fig. 6. As noted before, this control system can be split into a mission control subsystem and a robot control subsystem. These two different control substructures are explained in detail in the following subsections.

3.1 Robot Control Subsystem

In terms of electronics, the robot control subsystem is Figure 6. Schematic representation of the control system, which based on an electronic unit designed and developed can be split into the robot control subsystem and the mission control specifically for this task. The core of this circuit is a subsystem. PIC24HJ256GP206 microcontroller designed to work in

www.intechopen.com Daniel Souto, Andres Faiña, Alvaro Deibe, Fernando Lopez-Peña and Richard J. Duro: 7 A Robot for the Unsupervised Grit-Blasting of Ship Hulls

Figure 7. Pictures of the robot control subsystem hardware. The left image shows the box containing the electronic board and on its cover can be seen the heat sinks of the H bridges; to the right hand side of the image, we can see the electrovalves for controlling the pneumatic actuators. The right image shows the electronic board of the robot.

industrial environments. It communicates with the directly by the microcontroller’s 10‐bit A/D converter. mission control subsystem through an RS485 bus to The information from the cameras as well as the receive operating instructions and to send information positioning information provided by the cricket mots is from the sensors. Among its responsibilities are the not processed on board, instead being sent as‐is to the supervision of the hardware circuitry for the control of base station. the actuators and the conditioning of the signals and information from the sensors. The robot control subsystem is a simple finite state machine which acquires data from the sensors cyclically. Two kinds of actuators may be found in the robot: For instance, the linear sensors are sampled at 100Hz pneumatic actuators that change the kinematic with a minimal use of the microcontroller because these configuration of the robot and coils in the magnets of samples are obtained using the analogue to digital each leg. The pneumatic actuators are driven by solenoid converter hardware of the microcontroller. This hardware valves, which are essentially low power inductive loads. produces an interrupt when the sample is obtained and Low power Darlington transistor nets based on ULN2003 the software only has to save the values to a variable. IC’s and free‐wheeling diodes are used to drive these Moreover, if the robot is moving its actuators, the valves. In addition, there is a proportional valve for the interrupt checks whether any actuator is reaching its control of the blasting head motion speed, which is target position in order to stop them. The state machine controlled by an analogue voltage proportional to the receives commands through the RS485 line and, after desired output. This voltage is generated by an checking them, generates the low‐level actions. An AD724CMOS monolithic amplified digital‐analogue example of this is when the robot receives a command to converter. Finally, the coils of the magnets are also move the lower module to a new position. In this case, essentially inductive loads, but of higher power. They are the microcontroller unfixes the magnets of the lower used to cancel or strengthen the magnetic field of the module, employing the H‐bridge. Afterwards, the robot magnets by forcing currents through these coils in the waits for at least half second so as to guarantee that the right direction. For this purpose, full H‐bridges of current is flowing into the coil of the magnets and, thus, TLE5205 transistors are used. Some images of the that there is no adhesion force left so that the legs of the hardware involved are presented in Fig. 7. module can be retracted smoothly. Next, the microcontroller activates the valves in order to retract the The system comprises four types of sensors: linear legs and move the lower module in the desired direction. sensors, which measure the position of the pneumatic When the actuators achieve the desired position, the actuators and allow the real‐time acquisition of the microcontroller stops the motion, activates the brake and robot’s kinematic configuration; a triaxial accelerometer start the sequence to fix the lower module to the hull. that measures the local gravity and is used to estimate the This sequence basically implies activating the magnets of spatial orientation of the robot and, thus, improve the the lower module and, at the same time, extending the leg estimate of the robotʹs position on the shipʹs hull actuators. Again, after a few moments to guarantee that provided by a cricket‐based positioning system; and there is no current in the coil of the magnets, the legs are finally, video cameras for controlling the quality of the contracted so as to reduce the distance to the hull without blasted surface and helping to position the robot. any risk of unfixing the magnets from the surface.

The linear sensors and accelerometers provide analogue The robot control is basically an on/off control of the voltage signals that are conditioned and then read actuators because we employ on/off pneumatic valves,

8 Int J Adv Robotic Sy, 2012, Vol. 9, 82:2012 www.intechopen.com which do not allow for any PWM modulation. The on the hull using information from the time of flight of program of the microcontroller calculates the velocity of simultaneously emitted radiofrequency and ultrasound each actuator to switch off the valve at the exact moment signals. This system comprises a series of modules ‐ or needed so as to make the actuator stop with precision at mots ‐ one of which (a listener mot) is placed on the robot the desired position. Only the grit blasting head actuator blasting head. The rest of the mots (emitter mots) are is controlled by a proportional pneumatic valve, which placed on the ground along the length of the ship. In this can be used to control the grit blasting head speed system, the higher the density of mots, the greater the through a simple PID controller. precision of the position information provided. The position estimation obtained from the cricket system is In addition, the robot control subsystem checks the status augmented by information coming from odometric of the robot and sends alarms to inform the base station. measurements of the motion of the robot, the robotʹs In the particular case where the robot detects a strong orientation in space obtained by the triaxial accelerometer, vibration, an emergency stop is activated. This implies and the hull’s geometry contained in the digital hull model stopping any movements, activating all the magnets to used by the mission control application. obtain the maximum adhesion force and starting the routine to fix the unattached modules. This emergency The mission control is based on three hierarchical layers stop can also be activated by an operator by pressing an which correspond to the three different operating modes, emergency switch. The signal thus generated produces a as shown in Fig. 6. The lower layers are employed by the high priority interrupt in the microcontroller which upper layers and each layer is accessible to the operator activates the emergency stop routine. through a graphical user interface (Fig. 8).

3.2 Mission Control Subsystem In this interface, the user can choose between the three operating modes: manual, semi‐autonomous and The mission control subsystem, which is physically located autonomous. In the first one, the manual mode, the user can in the base station, interacts with the operator thereby set the state of each robot actuator manually in real‐time allowing him to control the robot, calculates the paths for and thus control the robotʹs kinematic state and its the blasting head, and monitors and controls the blasted movement over the shipʹs hull with incremental surface quality achieved. The hardware used for this movements. This layer provides functionality to the purpose is a standard PC with an RS485 interface for layers on top of it and, when managed directly by the communication with the robot control subsystem as well as operator through its graphical user interface, it is mostly an appropriate input for the information sent from the used for testing and maintenance operations or else to quality control video cameras. In addition, out of the remotely control the blasting head for very particular different possible alternatives that can be found in the applications. In addition, this layer is responsible for literature for positioning [23] and [24], we have chosen a handling communications with the robot and periodically cricket system [25] that provides the position of the robot checking that there are no active alarm conditions.

Figure 8. Graphical user interface of the manual, semi‐autonomous and autonomous modes.

www.intechopen.com Daniel Souto, Andres Faiña, Alvaro Deibe, Fernando Lopez-Peña and Richard J. Duro: 9 A Robot for the Unsupervised Grit-Blasting of Ship Hulls

Figure 9. Graphical user interface of the autonomous mode. The work area is selected over the 3D model of the ship hull and this layer generates a path to completely grit‐blast this area.

In the second mode of operation, the semi‐autonomous in the hull of the vessel and forbidden blasting zones, and mode ‐ which corresponds to the second control layer of excludes them from the automatic path generation the mission control subsystem ‐ the user chooses the process, adapting the trajectories as needed. This layer, direction of travel of the robot over the hull, and the after automatically generating the path, sends the high‐ mission control application automatically generates the level commands to the lower layer. Next, it waits for the required sequence of commands to the actuators that acknowledgment that informst i that each high‐level perform the requested movement. This layer basically command has been carried out. decomposes these high‐level commands into low‐level commands, which are sent to the lower layer. The The blasting speed is directly related to the initial state of blasting action is user‐selectable and the robot continues the surface to be treated and inversely related to the to move in the selected direction until the user provides a surface finish quality achieved. When the blasting speed stop command increases, the surface quality decreases, and vice versa. The mission control application tries to find an optimum In the third mode, the autonomous mode, the user where the blasting speed is maximized while ensuring interactively selects the area to blast in a graphical the desired finish standards. This minimizes the total interface that shows the ship’s hull. This is achieved by time required for the mission and its related costs. providing four or more points that determine a polygon that corresponds to the borders of the area to be blasted A detailed description of the operation of the mission (see Fig. 9). The mission control subsystem automatically control subsystem is presented in the second example of generates the appropriate paths to accomplish the the next section. blasting of the selected area, calculates the corresponding commands for the robot, and monitors it using the 4. Operation examples location system while following those paths in order to correct possible deviations. This subsystem contains a In this section, we will present some examples of the quality control module that uses the information from the operation of the robot. The objective of these runs is to cameras to determine the surface finish achieved. It show the operation of the robot and to tune its different controls the speed of the blasting head to adapt to the subsystems, as well as the whole robot. To this end, this operational conditions and, if required, commands the section is divided into three parts, each devoted to a robot to repeat the blasting in areas that do not reach the different aspect. First of all, some examples are presented desired surface finish. where the robot moves over a horizontal surface. Their purpose is to present the system as a whole and test the It is important to note here that the selection made by the different motion strategies of the robot without having to user of the area of the ship hull to be blasted results in a worry about the attachment or the location problem. hull surface portion that is usually warped. The These tests also helped to tune the local robot control automatic trajectory planning algorithm takes into parameters and strategies. account the warped nature of the selected surface and inserts the required rotation commands between linear After this, the next subsection addresses the location translations of the robot to cover the entire surface problem, i.e., determining in real‐time the location of the portion. Furthermore, it also takes into account openings robot on the surface on which it must operate so that the

10 Int J Adv Robotic Sy, 2012, Vol. 9, 82:2012 www.intechopen.com mission control module can make it work autonomously. These experiments were carried out in a more realistic environment that suffered most of the electromagnetic shielding and interference problems typical of shipyards and ships where most surfaces are made of steel. The mission control module was tuned during these tests.

Finally, we have addressed the attachment problem in an experiment in which the robot must operate while moving over a vertical surface, which is the worst condition for magnetic attachment strategies. It was during these tests that the optimum attachment/release strategy for the different operations of the robot has been determined. This last experiment involves the whole robotic system and, as such, can also be considered a validation experiment for the robot as a whole.

4.1 Low‐level module tuning

As indicated above, the first example involves moving over a horizontal, flat and non‐ferromagnetic surface. These tests have been carried out in order to check the different subsystems of the robot and to tune the parameters of the local or low‐level control system, except for the magnet control parameters. In this first example, we wanted to ignore the attachment problem and, therefore, the magnets were changed for rubber tipped legs in order to increase the friction coefficient. This also shows the versatility of the semi‐modular design of the robot, which allows the magnets to be changed for rubber tips or suction cups in order to allow the system to deal with hull surfaces other than ferromagnetic ones. Obviously, since magnetic fixation is not considered, the routine to fix or unfix of one module to the ground is different from that employed on ferromagnetic surfaces. When the modules extend their legs to contact the ground, it is the weight of the module on the rubber legs and the friction between the ground and the rubber tips that fix the module. Conversely, a module becomes unfixed from the surface just by contracting its legs. Figs. 10 and 11 display the behaviour of the robot when performing translation and rotation operations. The sequence of motions needed to perform these operations and, consequently, the actuations that are necessary, have been described in section 2. Suffice it to say that after several trials in which the parameters of the low‐level controller were adapted, the robot performed both the linear movements that are necessary to move up and down the hull and sideways and the rotational movements necessary for turning and/or adapting to the curvature of the hull flawlessly for Figure 10. Sequence of images of the robot performing all the different motion speeds and sequences. movements required for grit blasting several bands.

www.intechopen.com Daniel Souto, Andres Faiña, Alvaro Deibe, Fernando Lopez-Peña and Richard J. Duro: 11 A Robot for the Unsupervised Grit-Blasting of Ship Hulls operation of the robot when working for long periods of time without human intervention. In order to achieve this, it is obviously necessary for this module to be aware of the exact location and orientation of the robot, as well as of its current state. This state is obtained from the sensors in the different components (see the diagram in Fig. 5) that allow the mission control subsystem to know in which position each of the actuators is at any time, as well as other variables such as its speed or, in the case of magnetic coils, whether they are active or not and whether their activity complements the magnets or cancels them. The orientation of the robot with respect to the hull is obtained, as indicated in the electronics section, by means of a triaxial accelerometer. Finally, the position on the hull is obtained using a cricket‐based [25] system. These positioning systems triangulate the distances from a listener module, placed on the grit blasting head of the robot, to some beacon modules placed in the environment. The idea is to place several beacons near the ship all along its length at the shipyard where the ship is grounded for grit blasting. In the experiment presented here, the beacon modules were placed on tripods near the operating surface and facing the robot, in order to approximate reality. This scenario is displayed in Fig. 12, where two beacon modules as well as the robot placed on the vertical surface and the base computer can be seen. This setup has been used to test the position and orientation determination capabilities of the system in order to ascertain its validity for the niche it is supposed to cover, as well as to validate the mission control subsystem in terms of being able to generate the appropriate trajectories and guide the robot along them without mistakes.

The procedure followed prior to blasting a given area of the surface is for the operator to mark four or more points on the graphical user interface (GUI) of the mission control subsystem, which displays the 3D model of the ship hull as displayed in Fig. 9. These points determine the working area of the robot. To reliably obtain the position of the robot on the ship hull during operation, and before starting the blasting process, the listener module must be placed on four or more points of the hull and these points must be selected by an operator on one of the screens of the user interface that shows a real‐time video of the robot and the surface to be blasted. Using this information, the mission control software and, in particular, the cricket system, can calibrate itself and better adapt to the particular characteristics of the environment. Once the working area has been determined, the mission control software ‐ which contains a 3D representation of the surface to be blasted ‐ Figure 11. The robot rotates on a horizontal surface. calculates the trajectory of the robot and the movements of the blasting head needed to fully blast the area. This 4.2 Positioning system and mission control trajectory is then translated into command sequences for the low‐level control module that take into account the In order to make the system autonomous, a mission control turns the robot must make in order to compensate for the module has been developed, as explained in section 3. The curvature of the hull or to avoid forbidden areas. The objective of this module is to be able to supervise the operation of the robot is constantly monitored and any

12 Int J Adv Robotic Sy, 2012, Vol. 9, 82:2012 www.intechopen.com deviations in the position of the robot and/or blasting head that the robot will experience in terms of attachment, as the which are detected are corrected appropriately. In order to only way to prevent the robot from sliding is through avoid any errors induced by electromagnetic noise ‐ which friction between the magnets and the steel plate. The is always present in industrial environments ‐ the position achievement of a high enough frictional force using of the blasting head is filtered, taking into consideration elements such as the magnets and steel surfaces ‐ which past positions and the constraints on its movement. present quite low friction coefficients ‐ requires a very strong normal force from the magnets that can compensate For the purpose of illustrating the results obtained, an for a robot weight of around 80 kg, as well as for vibrations image of the mission control GUI in a given instant of the and other undesired effects using just half of the magnets robot’s operation is displayed in Fig. 13. It is split into (as those are the only ones available when the robot is two sections: the world of the robot and the real world. In moving). On the other hand, the size of the magnets must both sections we can see the four points that determine be contained in order to make the robot easy to handle. the working area of the robot in this case and the path Consequently, we chose to use a magnet that provides a followed by the grit blasting head (painted yellow). normal attraction forcef o around 1000N, which is enough to fix the robot to the vertical plate when one module is detached. Nonetheless, when there are a lot of vibrations, the robot ‐ even with the four magnets that provide a joint normal force of about five times the robot’s weight ‐ may slide down the surface. Therefore, for safety reasons, the coil inside the magnets is used to increase the attraction force in the magnets placed in the fixed module before unfixing the other module and until this module is again attached. We found that in this way, the robot never detaches or slides down the surface, even under high vibrations. To further improve on this situation and reduce the moments acting on the magnets, a few seconds after the magnets of one of the modules of the robot are attached to the hull their legs are contracted so as to reduce the Figure 12. Experimental environment for testing the robot. We distance between the hull and the robot’s centre of mass. can see the steel plate to which the robot is attached, the This also ensures that all of the magnets are really in computer running the mission control subsystem and two contact with the hull, as it may present imperfections that tripods holding two cricket beacons. would leave one leg in the air if all the other legs are extended. In the images showing the foperation o the robot on the steel plate ‐ and again, for safety reasons ‐ the robot always operates while linked to a crane by two ropes. However, these ropes are not in tension and do not provide any support to the robot.

A sequence of pictures showing the typical grit blasting operation is displayed in Fig. 14. Basically, and as explained in section 2, the robot moves the grit blasting head to strip a horizontal band and then the lower module descends the width of the strip to permit the stripping of a new band (each pair of pictures that are Figure 13. Graphical user interface showing the work area of the positioned side by side show the beginning and end of a robot and the path of the grit blasting head at a given instant of strip. We have omitted all the strips between the first one time during the operation of the robot. in the top two pictures and the last one within the same position of the top module shown in the middle). When 4.2 Robot attachment and operation no a vertical surface the actuators of the top module reach their end stroke, the lower module cannot continue its descent, which is the The last set of experiments was carried out on the case for the pictures in the middle. At this point, the environment presented in the previous subsection bottom module is attached to the surface to allow the top containing a ferromagnetic surface placed vertically with module to move down all the way, reattach itself and respect to the ground (see Fig. 13). This ferromagnetic thus permit the lower module to continue descending surface is a 10mm steel ship hull plate (2.0 x 2.8 m) with a and stripping new bands with the grit blasting head (as shop primer and, therefore, it is a realistic environment. In shown in the bottom two pictures). The whole sequence addition, this environment represents the worst condition is carried out in less than 40 seconds. In the images of Fig. www.intechopen.com Daniel Souto, Andres Faiña, Alvaro Deibe, Fernando Lopez-Peña and Richard J. Duro: 13 A Robot for the Unsupervised Grit-Blasting of Ship Hulls 14, we have intentionally included the stairs beside the Fig. 15 shows the robot performing a horizontal steel plate so as to provide a reference of size and displacement. In this case, the first picture on the left shows distance. the initial state of the robot, which had been working on the right side of the steel plate. Thus, the robot fixes the lower module to the plate and displaces the top module towards the left. Next, it fixes the top module to the plate and displaces the bottom one leftwards. It will continue doing this until it reaches the new vertical area to be blasted. The limited size of the plate does not allow for greater displacements to be shown.

Figure 15. Robot performing a horizontal displacement.

Finally, Fig. 16 shows a more complicated manoeuvre in this test environment ‐ a 90º rotation, as explained in section 2. The robot takes 30 seconds to carry out all the necessary movements. In this case, starting from picture a, the robot first moves the sliders of the top module opposite to each other in order to rotate the bottom module. After this, it fixes the bottom module and rotates the top one by moving its sliders opposite to the previous case and then fixing the bottom module. It continues to perform this sequence until the final rotation angle for the blasting head strip is achieved.

In summary, the robot operates successfully in realistic environments and conditions even when complex movements are required. It is able to move forwards, backwards or sideways and it is able to turn appropriately. The mission control subsystem provides the sequences of actions that the robot must carry out to this end based on Figure 14. Sequence of pictures showing the typical procedure the location information provided by the cricket system, for grit blasting a surface when moving vertically. the state information provided by the different sensors on the robot and the area defined by the operator as the area to be blasted on the 3D model of the ship hull.

14 Int J Adv Robotic Sy, 2012, Vol. 9, 82:2012 www.intechopen.com a b performing translations and rotations over curved hulls. These two modules are attached to the hull through hollow permanent magnets with coils inside them. This setup provides for a very simple actuation mechanism for detaching the robot legs from the surface as well as for increasing the attachment force when required. An additional advantage of this approach in terms of grit blasting is that it makes it very easy to get rid of any d ferromagnetic dust that may become attached to the magnets during operation, since whenever a leg detaches the magnetic structure is demagnetized by the coils.

A layered control system has been created for this robot. There are four control layers, one of them on the robot itself and three on the base station. This control structure

allows the robot to operaten i manual, semi‐autonomous c and autonomous or unsupervised modes. In fact, in the unsupervised mode, the operator simply provides the points defining a polygon corresponding to the area that the robot must operate over and the higher level mission control system or layer calculates the trajectory the robot and its blasting head must follow in order to adequately blast the area to the specified quality. This trajectory is then converted into robot actions by the lower layers, taking into account the 3D nature of the hull and inserting the appropriate turns. These actions are finally converted into actuations by the lower level control layer and these are carried out and monitored in order to make the adjustments that are deemed necessary to successfully finalize the task.

e f The different systems that make up the robot ‐ as well as the whole robot ‐ have been implemented as a prototype and tested in a realistic environment and the results were completely satisfactory. It is now the object of current and future work to produce a final version of the robot that is able to operate for extended periods of time in real shipyard environments.

6. Acknowledgements

The authors would like to acknowledgee th support of the Navantia shipyards of Spain in this project, especially in the tests and experiments performed, as well as their permission for publishing the results. We would also like to acknowledge the support of the Xunta de Galicia Figure 16. 90º rotation manoeuvre of the robot carried out in 30 through project 09DPI012166PR seconds. 7. References 5. Conclusions [1] J. M. Kalogerakis, “Use of Robots in the Shipbuilding This paper describes the design and implementation of a Industry,” The Naval architect, 1986. robot for grit blasting ship hulls, including its architecture [2] A. McCrea, “Automated inspection and restoration and its control system. In terms of architecture, the robot of steel bridges—a critical review of methods and is based on two modules that move with respect to each enabling technologies,” Automation in Construction, other in order to endow the system with the capability of vol. 11, no. 4, pp. 351‐373, 2002.

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