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RISYS: An Advanced Reactor Vessel Inspection System with Underwater Mobile RobotsJae Hee Kim,Jae Cheol Lee
Korea Atomic Energy Research Institute, Duckjindong 150, Yousung, Taejon, Korea 82-42-868-2085
E-mail: firstname.lastname@example.org, http://www.kaeri.re.kr/aina/
Jae Yeol Kim
Chosun University, SeoSeokdong 375, Donggu, Kwangju, Korea 82-62-230-703.
An underwater mobile robotic system for ultrasonic examination of reactor vessel welds has been developed to reduce inspection time and schedule during mandatory code inspections. Instead of a conventional inspection machine with a large structure, the underwater mobile robot with a long reach manipulator examines the whole welds of the reactor vessel. The system mainly consists of an underwater mobile robot, a laser positioning unit and a main control station. The underwater mobile robot is guided by a laser positioning unit with precise resolution of 0.01 degree. The mobile robot moves on the reactor vessel wall with four magnetic wheels. The system was integrated with the main control station, and a series of experiments were performed on the reactor vessel mockup. The system was also tested in reactor vessel of Ulchine unit 4 of Korea in January 1998. After many improvements in design and engineering of the system, RISYS's first reactor vessel examination is scheduled for November 2001. When the system is practically used, it will dramatically reduce the critical path process. This paper describes our newly developed robotic inspection system including its design and experimental results.
Recently, mobile robotic technology has been applied to industries especially in the area of plant inspections. In nuclear power plants, the reactor pressure vessel is one of the most important pieces of equipment in view of its function and safety. The vessels are usually constructed by welding large rolled plates, forged sections or nozzle pipes together.
In order to assure the integrity of the vessel, these welds should be periodically inspected using sensors such as ultrasonic transducers or visual cameras. Such inspections are usually conducted underwater to minimize exposure to the radioactively contaminated vessel walls, and have been performed using a conventional inspection machine with a big structural sturdy column. This machine, however, is so huge and heavy that its maintenance and handling are extremely difficult, requiring much effort to transport the system to a site and also, the continuous use of the utility's polar crane to move the manipulator into the building and then onto the vessel. Setup of this machine requires a large volume of area for work preparation and several shifts to complete.
In order to resolve these problems, we have developed an underwater mobile robot which is guided by a laser pointer, and performed a series of experiments in the mockup of the reactor pressure vessel. The system will reduce dramatically the critical path process for pre-service inspection of pressurized water reactors. By adopting this compact system, overall inspection time can be greatly reduced by deploying two robots simultaneously in the vessel.
This paper describes the outline of robotic inspection system developed in our laboratory, and summarizes the experimental investigation and its results.
|Fig 1: Configuration of the reactor inspection system (RISYS).|
As shown in Fig. 1, the reactor pressure vessel in a pressurized water reactor has a cylindrical shape. It has inlet and outlet nozzles around the upper shell. It has many welds such as a circumferential seam, a weld of nozzle to the upper shell, a weld of flange to the upper shell and so on. When inspecting the welds of a vessel wall, the reactor head and the reactor internals are moved to the next canal so that the inspection can be performed efficiently. The reactor vessel is filled with water up to the top of the canal to reduce the radiation exposure during inspection. Thus, the inspection machine must be operated under water.
Our reactor inspection system (RISYS) consists of a reactor inspection robot (RIROB), laser pointer (LASPO), main control computer (MCS), sonic data acquisition system (SODAS), evaluation system of sonic data (EVASOD) and so on.
Reactor Inspection Robot (RIROB)
RIROB is a submarine type mobile robot whose weight is approximately 40 kg in air and becomes zero in water by the aid of floats. Most of the reactor pressure vessel in a PWR is composed of carbon steel and clothed inside with austenitic stainless steel. In order to climb the vertical wall of the vessel, RIROB has four magnetic wheels. The magnetic material of the wheel is neodymium with 12.9 Kgauss of residual induction and 318 KJ/m3 of maximum energy product. The ring shaped magnet has N and S poles on each side of the magnet. The circular pure steel plates are attached on each side of the magnet to maximize the attraction force to the vertical wall. Smooth rubber is clothed around the magnet to prevent slippage on the vertical wall.
|Fig 3: Main display of the main control computer.|
RIROB has four magnetic wheels: two are caster wheels and the other two are driven by DC servo motors so that the robot can move in any direction on the vertical inner wall of the reactor vessel. The robot can control the linear velocity and angular velocity by the sum and difference of the velocities of the left and right driving wheels. Both the front and rear caster wheel are mounted on the parallelogram links with the robot body plate, as shown in Fig. 3. It always makes the robot body parallel to the wall, even though the wall is cylindrical.
The robot has also a light and long manipulator, and the ultrasonic probes are attached to its end effector. The manipulator has three degrees of freedom which are translation, rotation and 4 consecutive translations, as shown in Fig. 2. The manipulator can reach up to 100 cm using 4 consecutive translation links. It is not so easy to design a long reach manipulator kinematically, because it has the constraints to be light and not bulky in order to be mounted on a small mobile robot. The camera and lamp are mounted on the robot and the visual image from the camera is transmitted to the main control station.
|Fig 2: Reactor Inspection Robot (RIROB).|
The robot has an inclinometer to measure the inclination of the mobile robot and to control the robot posture. The depth sensor is also mounted on the robot body to measure the water pressure and to calculate the current vertical depth of the robot. The robot has a position sensitive detector on its back and the laser pointer induces the robot to the next position by pointing to the position using the laser beam.
Laser Pointer (LAPOS)
The robot is induced by the laser pointer (LAPOS) which is fixed in the middle of the crossbeam across the reactor upper flange. The laser pointer emits the laser beam to the next position for the robot to move. The robot, with the position sensitive detector on its back, detects the deviation of the laser beam spot from the center of the position sensitive detector, and moves in the appropriate direction to make this deviation zero. The laser pointer is a kind of pan-tilt device on which the diode laser is mounted. The device is accurately driven by the micro stepping motors of which the resolution is less than 0.01 deg/step. The laser pointer induces the robot to the next position by emitting the laser beam. The laser pointer is covered by a hemispherical shaped plastic cap to prevent the deflection of the laser beam and water penetration.
Main Control Computer
The main control computer's function is to control RIROB, the laser pointer and the sonic data acquisition subsystem. It is PC based control station with operating software and interfaces. It has the geometric information of all reactor vessels operating in Korea, so that inspections can be planned and simulated on a 3D graphic display.
During inspection, the main control system generates the scan path for RIROB to move. Simultaneously, the current posture of the robot is displayed graphically and the image captured by the camera on the robot is also displayed. After inspection, examination reports are generated using the stored data. The system can also be operated in manual mode during computer control malfunction.
The computer software includes other convenient modules: input module of reactor specification, inspection procedures module, selection of inspection item, automatic finding of the inspection robot, previous simulation of robot movements, display of inspection status, communication with RIROB, LASPO and SODAS, fully automatic inspection and manual inspection, and so on.
Sonic Data Processing System
Other systems, such as the sonic data acquisition subsystem and the data evaluation subsystem, are also under development. The data acquisition subsystem drives the ultrasonic sensor, and collects, displays and stores the reflected signal data.
Conventional inspection machine with huge manipulator can easily place its end effector equipped with ultrasonic probe to the desired weld position. It is because the machine is based on the cylindrical coordinate system, and the reactor vessel has also a cylindrical shape. However, the inspection system using an underwater mobile robot guided by a laser pointer, it needs much calculations and geometric analysis.
Laser Guidance Control
In order to inspect the welds accurately, the robot should move exactly to the given position, Xd,Yd. However, controlling a robot's exact movements is not a simple process, because the robot's position and direction are determined by the sum and difference of the velocities of the left and right wheel, which are driven by two DC servo motors, respectively. Input current of the servo motor is generated by the law of usual PD control and the PSD output is fed to the PSD controller, which is specially designed for our study.
|Fig 4: Position sensitive detector (PSD) and robot motion control.|
As shown in Fig. 4, the position sensitive detector (PSD) is mounted on the RIROB body plate. When the laser beam points to a position P on the PSD surface, the sensor generates currents corresponding to the deviation (ex,ey) of the laser spot with respect to the center of the PSD. The control objective is to drive the RIROB in such a way that
Considering the fact that the linear velocity of the robot center, vc, has a relationship with the y-directional deviation, ey, and the angular velocity of the robot center,, is strongly related to the x-directional deviation, ex, we propose the following control law:
where Kij is the corresponding control gain and L is the length between the robot center and each wheel.
More complex calculations are needed to inspect the welds near a nozzle using a mobile robot than the other welds in a reactor vessel.
When inspecting the welds around a nozzle, the robot moves around this nozzle. The equation of the intersection line of both the reactor shell and the nozzle must obtained first. Since the reactor vessel is cylindrical and the nozzle is conical, the intersection line is made by a cylinder and a cone in a 3 dimensional space. It can therefore be modeled as:
where R is the radius of the reactor vessel in the shape of the cylinder, and k is the cone angle of the nozzle.
By developing these equations, finally we can obtain the equation of the 4th degree. Using a numerical method such as the Newton - Raphson algorithm, we can obtain a solution. After these processes, some mathematical manipulations are additionally needed for each application of the robot movements.
In order to determine whether the weld has defects or not, we have conducted the well known ultrasonic testing. After emitting ultrasonic wave to the suspected welds, we monitor its reflected signal. Usually reactor pressure vessel is manufactured by welding several parts together. The welds to be inspected in the vessel is largely classified as
|Fig 5: Posture of inspection robot.|
When inspecting each weld, we have to use various incident angles of ultrasonic wave for more accurate and strict inspection. For example, in case of reactor shell welds inspection, we use incident angles of 0, 45, 60, 50/70 degrees, respectively. In addition, for each incident angle, we have to scan the welds in four directions: upward, downward, clockwise and counter clockwise direction by using a ultrasonic probes with specified incident angle. Thus we have to inspect the welds seventy seven times in total.
Korean standard reactor vessel has six nozzles, thus the number of nozzle inspection becomes sixty times, and the number of circumferential weld inspection become sixteen times. The probe assembly should cleverly designed to contact the probe plate to the weld surfaces with suitable compliance. Figure 6 shows an example of probe assembly we use for reactor shell inspection.
|Fig 6: Example of probe assembly .|
In order to confirm the integrity of our developed inspection system, we have conformed a series of experiments in the reactor vessel mockup as well as in the real reactor vessel at Ulchine nuclear power plant in Korea. As shown in Fig. 7, the reactor vessel mockup is in the shape of a cylinder, whose dimensions are 5 meters high, and 4 meters in diameter. Prior to the underwater experiments, we had performed the experiment in air to confirm our laser guidance control method.
|Fig 7: Experimental facilities .||Fig 8: launching of inspection robot .|
In this section, we describe underwater experiments of laser guidance around the nozzles, because here the guidance of the robot is most complex. Positioning accuracy is also examined, including the rotation and inclination accuracy of the underwater mobile robot, scan path accuracy of the manipulator, angle measurement accuracy of the pan-tilt unit of the laser pointer, and overall positioning accuracy of this system. The above results of functional tests indicate that the prototype inspection system satisfied most of the acceptance criteria.
The underwater experiments were usually performed using the following procedures:
|Fig 9: Inspection of nozzle welds.||Fig 10: Scanning of nozzle inner wall.|
Experimental Results and Improvements
The objective of these experiments was to check if the system satisfied the given inspection criteria concerning position accuracy and repeatability of the robot movements. The criteria is that the ultrasonic transducer attached to the end gripper of the robot manipulator can be located to the desired position within an accuracy of 3 mm. This accuracy is determined by the accumulation of position errors from the laser pointer, robot and manipulator. Each position error is measured and then the total error is also measured.
In addition, we experimented the extraction method of RIROB from the reactor vessel when malfunctioning and prepared a special procedure to deal with emergency situations. Through the functional tests, we confirmed that our robotic system met the given conditions, yet still requires many improvements. Such improvements include:
As a result, we designed a new system in consideration of the above necessary improvements, and are now carrying out the final tests for commercializing.
|Fig 11: Experiments in real reactor vessel.|
In order to improve the reactor vessel inspection system, we have developed a new robotic inspection system. We completed the laser induced control of the mobile robot, and the method is thought to be applicable to other industries. When our system is used practically for reactor vessel inspection instead of conventional machines, a lot of benefits are expected to result, such as in critical path process reduction and handling safety improvement, examination reliability and positioning accuracy, and so on.
This paper describes a part of the study on "Development of an automatic robotic inspection system of reactor pressure vessels" under the long-term nuclear research and development program in Korea. The author wishes to acknowledge the financial support of this work by the Ministry of Science and Technology, Republic of Korea.
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