Data processing: generic control systems or specific application – Specific application – apparatus or process – Robot control
Reexamination Certificate
1999-07-09
2001-08-28
Cuchlinski, Jr., William A. (Department: 3661)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Robot control
C382S106000, C382S190000, C382S291000
Reexamination Certificate
active
06282461
ABSTRACT:
BACKGROUND ART
The present invention relates to the optical determination of the physical position of tube ends in a steam generator of the type typically used in nuclear power stations.
The steam generators used in nuclear power stations typically have a tube sheet into which the ends of thousands of heat exchanger tubes are secured in a fluid tight relationship. Heated primary fluid from the nuclear reactor is passed through the tubes to exchange heat with the secondary working fluid which, in turn, drives the turbomachinery used to generate electricity. Since the primary fluid can be radioactive, the tube sheet is subject to inspection during those times that the power station is off-line for repairs and maintenance. Historically, the inspection of the tube sheet and the tube ends involved the use of technicians who would enter the steam generator and effect a visual inspection of the tube ends and conduct manual tests using various types of inspection devices to detect cracked, leaking, or otherwise damaged tubes. The defective tubes were typically “plugged” to seal the end of the tube to remove the affected tube from service and prevent leakage.
More recently, computer controlled robotic arms with specialized end-effectors have been used to effect the visual inspection of the tube ends and to effect the repairs. Contemporary robotic arms include several articulated joints with different length links between the various joints. Each joint includes, for example, a command-driven stepping motor or a rotary actuator and a cooperating angle position sensor (such as an optical encoder) that cooperate with a controller to control the angular movement of the links connected to the joint. The robotic arm is secured in place within the steam generator and beneath the tube sheet during the system shut-down. An end effector assembly is mounted to the end of the robotic arm and typically carries a television camera, one or more illumination sources, inspection tools (such as an eddy current device to detect cracked tubes), and/or tooling to effect plugging of a tube end. In general, the end effector is positioned in and moves in a plane spaced a selected distance (i.e., 2-5 cm or more) from the surface of the tube sheet.
The computer that controls the robotic arm is under the control of an operator who can move the end effector across the tube sheet using a simple joystick or by specifying a target x,y destination tube. Once the robotic arm is positionally calibrated or initialized at a known start position and knowing the tube diameter and the center-to-center pitch distance, the computer can calculate the best path from the start or initial position to the target x,y position and, in most cases, re-position the end-effector over the desired target tube. However, it is not uncommon for the robot arm to be subject to bending forces, especially when the arm is positioned at its maximum extent from its supports. Thus, an unintended deflection of one degree or so at an extension of two meters or so can cause the end effector to be positioned over a tube that is different from the target tube.
The mis-positioning of the end effector over a tube other than the intended target tube poses substantial safety implications. More specifically, the end effector can be unintentionally positioned over a known good tube and be commanded to plug that known good tube while inadvertently leaving an adjacent defective “target” tube in-service when the steam generator re-enters service.
Governmental agencies that regulate the nuclear industry require that the tube position be independently verified to maximize the probability that an end effector will be positioned over an intended target tube end. In general, tube ends can be independently counted by the system operator who counts tube ends as they move across a video monitor that displays the output of the TV camera mounted on the end effector. However, operator fatigue and inattention can cause a mis-count and the ‘loss’ of the independent count.
Representative prior art disclosures of robotic arm end-effectors used in the servicing of steam generators in nuclear power plants include U.S. Pat. Nos. 5,751,610, 5,838,882, and 5,878,151, the disclosures of which are incorporated herein by reference.
DISCLOSURE OF THE INVENTION
The present invention provides an independent tube position verification system by which successive frames of the video output of a camera or other imaging device on the end effector are processed to recognize the passage of tube ends across the visual field of the camera and identify the velocity, acceleration, and direction of the tube ends as the images thereof move across the image frame. A prediction is made as which of several possible destinations are available. When one of the several possible destinations is achieved, an independent x,y counter is appropriately incremented or decremented to track the tube position independently of the robotic arm.
When the computer controlling the robotic arm indicates the end effector has arrived at the commanded target tube end, the position coordinates of the independent counter are compared with that of the commanded position. If the positions match, the independent verification requirement is met; conversely, if the comparison does not match, an indication that the robotic arm has lost track is provided and the robotic arm is subject to a re-calibration or re-initialization step prior to re-attempting to,acquire the target tube.
The imaging device typically takes the form a television camera that is preferentially mounted on the end effector at an oblique angle relative to the tube sheet (i.e., 45-55 degrees) with the illumination source similarly mounted so that the camera perceives a perspective field of view with the illumination source creating a shadow effect that can be discerned by the image processing software. In general, it is desirable that about 40% of the image of each tube end and its shadow be a shadow cast by the tube end or cast by a part of the tube end.
As the end effector is moved over the tube sheet at speeds of up to 30 cm/sec, the imaging device takes successive image frames at a known frame rate (i.e., 32 frames/sec or 0.03135 seconds between frames). A comparison between successive frames of discernable artifacts within each frame yields a knowledge of the speed and direction of transit of the end effector across the tube sheet and yields information allowing the system to increment and/or decrement x,y counters that count relative to an initial position and thus keep track of the movement of the end effector independently of the command driven robotic arm.
The mounting of the camera at an oblique angle relative to the tube sheet causes the camera to perceive a field of view in which the tube ends appear in a perspective frame with near field tube ends appearing larger than far field tube ends and with the tube ends appearing to converge toward a distant vanishing point. Measurement of the apparent angle of convergence between successive image frame yields information as to the spacing between the end effector and the plane of the tube sheet and variations thereof. The angle of convergence information is used to control the z-axis of the robotic arm to closely and optimally control the spacing between of the end effector and the plane of the tube sheet to maximize the position tracking functionality of the system.
REFERENCES:
patent: 4873651 (1989-10-01), Raviv
patent: 5579444 (1996-11-01), Dalziel et al.
patent: 5602967 (1997-02-01), Pryor
patent: 5751610 (1998-05-01), Gan et al.
patent: 5838882 (1998-11-01), Gan et al.
patent: 5878151 (1999-03-01), Tang et al.
patent: 5887041 (1999-03-01), Zachar et al.
patent: 5943164 (1999-08-01), Rao
patent: 5963303 (1999-10-01), Allen
patent: 5974348 (1999-10-01), Rocks
International Preliminary Examination Report (IPER) from International Preliminary Examination Authority, dated Apr. 4, 2000.
Fitzgibbons Lance T.
Gan Zhongxue
Katz Jeffrey S.
Cuchlinski Jr. William A.
Kananen Robert P.
Nguyen Thu
Rader Fishman & Grauer
Westinghouse Electric Company LLC
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