Abstract:
A mobile device for traversing a ferromagnetic surface. The device includes a frame and at least one surface contacting device attached to the frame. The device also includes a Halbach magnet array attached to the frame, wherein the Halbach magnet array provides a magnetic force to maintain the surface contacting device substantially into contact with the ferromagnetic surface.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is a continuation of Ser. No. 10/153,942 filed May 23, 2002, now U.S. Pat. No. 6,792,335, which claims priority under 35 U.S.C. 119 to U.S. Provisional Patent Application No. 60/292,948 filed May 23, 2001. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   This invention was partially funded by the U.S. Government pursuant to NASA Grant No. NCC5-223. The U.S. Government may have certain rights in this invention. 

   BACKGROUND 
   Robotic devices have become increasingly prevalent in industrial settings where automation of hazardous, time-consuming, and precise operations is desirable. For example, robots have been employed to inspect and repair storage tanks, pipelines, and nuclear facilities, and strip paint and to apply finishes. 
   In paint stripping operations, for example, the process of manually stripping paint and other finishes off of large structures such as storage tanks, ships, and bridges is a labor-intensive process that is often performed by humans using grit blasting or ultra high pressure (UHP) water jetting techniques and devices. Such techniques and devices, in addition to being labor-intensive, may also create waste disposal problems because, for example, in the case of grit blasting, the grit is intermixed with paint and coating particles (e.g. fungicides) and thus must be disposed of in an environmentally-friendly manner. 
   Various robotic devices have been developed for use in stripping paint from large structures. For example, the Flow Hydrocat™ manufactured by Flow International Corporation, uses a vacuum to attach to the surface being stripped. The Hydro-Crawler™, manufactured by JetEdge®, uses rigid magnetic tracks that attach to the surface being stripped and propel the robot on the surface. 
   SUMMARY 
   In one embodiment, the present invention is directed to a mobile device for traversing a ferromagnetic surface. The device includes a frame and at least one surface contacting device attached to the frame. The device also includes a Halbach magnet array attached to the frame, wherein the Halbach magnet array provides a magnetic force to maintain the surface contacting device substantially into contact with the ferromagnetic surface. 
   In one embodiment, the present invention is directed to a system. The system includes a generator and a mobile device in communication with the generator, the mobile device for traversing a ferromagnetic surface. The mobile device includes a frame, at least one surface contacting device attached to the frame, and a Halbach magnet array attached to the frame, wherein the Halbach magnet array provides a magnetic force to maintain the surface contacting device substantially into contact with the ferromagnetic surface. 
   In one embodiment, the present invention is directed to an apparatus for traversing a ferromagnetic surface. The apparatus includes a frame, surface contacting means, and magnetic means attached to the frame, wherein the magnetic means provides a magnetic force to maintain the surface contacting means substantially into contact with the ferromagnetic surface, and wherein the magnetic means is configured in use to be spaced from the ferromagnetic surface. 
   In one embodiment, the present invention is directed to a robotic device for operating on a ferromagnetic surface. The device includes a frame, at least one wheel attached to the frame, wherein the wheel has a polymeric coating on a surface that is configured to contact the ferromagnetic surface, and a Halbach magnet array attached to the frame, wherein the magnet array holds the wheel in substantially constant contact with the ferromagnetic surface and wherein the Halbach array is configured in use to be spaced from the ferromagnetic surface. 
   In one embodiment, the present invention is directed to a mobile device for traversing a ferromagnetic surface. The device includes a frame and at least one surface contacting device attached to the frame. The device also includes a magnet array attached to the frame, wherein the magnet array includes a plurality of magnet bars oriented such that the magnet array provides a magnetic force to maintain the surface contacting device substantially into contact with the ferromagnetic surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the present invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a diagram illustrating a robotic device according to one embodiment of the present invention; 
       FIG. 2  is a diagram illustrating a top view of the robotic device of  FIG. 1  according to one embodiment of the present invention; 
       FIG. 3  is a diagram illustrating a side view of the robotic device of  FIG. 1  according to one embodiment of the present invention; 
       FIG. 4  is a diagram illustrating the jet/vacuum system of the robotic device of  FIG. 1  according to one embodiment of the present invention; 
       FIG. 5  is a simplified schematic diagram of an electrical control device located on the robotic device of  FIG. 1  or located remote from the device according to one embodiment of the present invention; 
       FIG. 6  is a simplified diagram illustrating a control panel of a wireless control device for controlling the robotic device of  FIG. 1  according to one embodiment of the present invention; 
       FIG. 7  is a diagram illustrating a strain relief connector that can be used in conjunction with the robotic device of  FIG. 1  according to one embodiment of the present invention; 
       FIG. 8  is a diagram illustrating a system in which the robotic device of  FIG. 1  may be used according to one embodiment of the present invention; 
       FIG. 9  is a diagram illustrating a Halbach magnet array according to one embodiment of the present invention; 
       FIG. 10  is a diagram illustrating the magnetic fields of the Halbach magnet array of  FIG. 9  according to one embodiment of the present invention; 
       FIG. 11  is a diagram illustrating a side view of a magnet used in the Halbach magnet array of  FIG. 9 ; 
       FIG. 12  is a diagram illustrating detection of lift off according to one embodiment of the present invention; and 
       FIG. 13  is a graph which illustrates the difference between the holding power of a Halbach array and the holding power of a conventional, multi-pole magnet array with iron pole pieces which has identical mass. 
   

   DESCRIPTION 
   It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements. Those of ordinary skill in the art will recognize, however, that these and other elements may be desirable. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. 
   Although the present invention is illustrated herein as being embodied as a robotic device that has paint stripping and removal capabilities, it can be understood that the principles of the present invention may be employed with devices that may perform a variety of tasks such as, for example, spraying finishes, machining, welding, and inspecting surfaces or structures. 
     FIG. 1  is a diagram illustrating a robotic device  10  according to one embodiment of the present invention. The device  10  includes a jet/vacuum assembly  12  that can be used for jetting fluids and vacuuming the fluids and removed particles after jetting. The assembly  12  includes a seal  14 , a shroud  16 , and ports  18 . The seal  14  may be, for example, spring-loaded such that an adequate seal is maintained when the device  10  traverses an uneven or obstructed surface. Although the device  10  is illustrated in  FIG. 1  as having one jet/vacuum system  12 , it can be understood that multiple jet/vacuum systems may be included on the device  10 . 
   The device  10  includes surface contacting devices, such as wheels  20  that contact the surface that is to be stripped of paints or coatings. The wheels may be constructed of, for example, a metal such as aluminum with a polymeric (e.g. urethane or polyurethane) coating of, for example, ¼″ thickness. Such a coated wheel provides traction for the device  10  but does not mar the surface on which the device  10  is operating. It can be understood that any suitable type of surface contacting device may be used such as, for example, tracks or skids. Actuation devices, such as motors  22 , provide power to the wheels  20  to provide locomotion for the device  10 . The motors  22  may be, for example, sealed electric motors compliant with the National Electrical Manufacturers Association (NEMA) 17 However, it can be understood that the actuation devices may include, in addition to or instead of electric motors, a hydraulic or pneumatic drive system. 
   The motors  22  are connected via chain drives  24  to differentials  26  and the differentials  26  are connected via chain drives  28  to the wheels  20 . The differentials  26  may be, for example, limited-slip differentials. The chain drives  24  may provide, for example, a 1:1 to 2:1 reduction and the chain drives  28  may provide a 2:1 reduction. The differentials  26  may provide example, a 3.14:1 reduction. However, it can be understood that the drive system may include, in addition to or instead of chain drives, any of a variety of other devices for power transmission such as a hydraulic transmission, belt drive or gear drive. 
   The device  10  includes a steering system for providing, for example, four-wheel steering capability to the device  10 . A steering actuator  30  controls a steering linkage  32  that provides directional movement of the wheels  20 . The steering actuator  30  may provide, for example, 1200 lbs. of thrust. The linkage  32  may include, for example, pinned connections and the bushings for the steering system may be, for example, oil-impregnated bushings. 
   The device  10  includes an ultra high pressure (UHP) fluid connection  34  that accepts the fluid to be used for stripping, for example, water. The device  10  also includes air connections  36  that accept compressed air that can be used to provide downward force to hold the jet/vacuum assembly  12  against the surface on which the device  10  is operating and which can be used for a variety of other functions such as to raise and lower the jet/vacuum assembly  12 . 
   The device  10  includes lifting/safety rings  38  that can be used to lift the device  10  in place using, for example, a crane or other lift device. One or more safety lines may be attached to the rings  38  to ensure that the device  10  does not fall to the ground if the device  10  loses contact with the surface on which it is operating. 
   In one embodiment, the device  10  is designed to operate on surfaces that are ferromagnetic, such as storage tanks and ship hulls. The device  10  is thus provided with magnets  40  to adhere the device  10  to such surfaces. The magnets  40  may be any type of suitable fixed magnet or electromagnet. In one embodiment, the magnets  40  are Halbach arrays constructed of, for example, neodymium-iron-boron (NdFeB), that provide, for example, 1400 lbs. to 2400 lbs. of pull, as described further hereinbelow. The presence of the magnets  40  allows for the device  10  to operate on structures that have inclined or vertical surfaces and allows for the device  10  to operate in an upside-down position on, for example, the bottom of the hull of a ship and provides so much surplus holding force that the device  10  can pull heavy loads (such as hoses full of water) vertically up the side of a smooth ferromagnetic structure even in the presence of water and oil on the surface. The magnets  40  may be designed and constructed, as described hereinbelow, such that the magnets  40  do not wear from contact with the surface on which the device  10  is operating and so that the magnets  40  do not mar the surface on which the device  10  is operating. 
   The various components of the device  10 , including a frame  42 , may be constructed of any suitable material such as, for example, plastic, stainless steel, titanium, aluminum, or coated steel. 
     FIG. 2  is a diagram illustrating a top view of the robotic device  10  of  FIG. 1  according to one embodiment of the present invention. In addition to the elements shown in  FIG. 1 , the device  10  is illustrated in  FIG. 2  having vacuum hoses  50 , electrical cables  52 , safety tether  54 , and water supply hose  56  attached thereto. 
     FIG. 3  is a diagram illustrating a side view of the robotic device  10  of  FIG. 1  according to one embodiment of the present invention. 
   Although the device  10  is illustrated in  FIGS. 1–3  as having four wheels  20 , it can be understood that any suitable number and configuration of wheels, tracks, skids, etc. may be used depending on the application for which the device  10  will be used and the desired handling characteristics of the device  10 . For example, the device  10  could be implemented with various three-wheel configurations, four-wheel cart configurations, and four-wheel articulated configurations. 
     FIG. 4  is a diagram illustrating a bottom view of the jet/vacuum system  12  of the robotic device  10  of  FIG. 1  according to one embodiment of the present invention. The system  12  includes the seal  14 . The seal  14  may be constructed from a flexible material such as, for example, polyurethane, that creates a seal with the surface on which the device  10  is operating and allows the device  10  to operate close to obstacles. A rotating spray assembly  60  includes, for example, multiple fluid outlets. The outlets may be, for example, sapphire spray jets. The spray assembly  60  may be an assembly sold by, for example, Hammelmann Corporation. The vacuum ports  18  carry away spent fluid and debris. In one embodiment, the jet/vacuum system  12  may be constructed to have a 16 inch diameter, although any suitable diameter of the system  12  may be used depending on the desired turning radius of the device  10 . 
     FIG. 5  is a simplified schematic diagram of an electrical control device  70  located on the robotic device  10  of  FIG. 1  or remote from the device  10  according to one embodiment of the present invention. The device  70  includes a pump relay kill  72  that can stop the operation of the jet/vacuum system  12 . An emergency stop loop  74  allows the operator of the device  10  to stop the device  10  in the event of an emergency. An operator interface receiver  76  receives operator commands via, for example, a wireless control device. A control microcontroller  78  provides control signals for controlling various systems of the device  10 . An automation computer  80  provides various automated functions for the device  10  as described hereinbelow. The computer  80  receives input from, for example, one or more cameras located on the device  10  and a gyro tilt sensor. 
   A front motor control circuit  82  includes a filter  84  and an amplifier  86  and a rear motor control circuit  88  includes a filter  89  and an amplifier  90 . A turn actuator circuit  91  includes a filter  92  and an amplifier  93 . A jet/vacuum system (head) spin motor circuit  94  includes a filter  95  and an amplifier  96  and a jet/vacuum system (head) raise/lower actuator circuit  97  includes a filter  98  and an amplifier  99 . The amplifiers  86 ,  90 ,  93 ,  96 , and  99  may be, for example, Emerson EN208 amplifiers with FM3. 
     FIG. 6  is a simplified diagram illustrating a control panel  100  of a wireless control device for controlling the robotic device  10  of  FIG. 1  according to one embodiment of the present invention. The control device on which the control panel  100  is located may be any type of control device such as a wireless or a wireline control device. A vehicle speed control dial  102  allows the operator of the device  10  to control the speed of the device  10 . A rotate speed dial  104  allows the operator of the device  10  to control the rotate speed of the device  10  and a head height dial  106  allows the operator of the device  10  to adjust the height of the jet/vacuum system  12 . An emergency stop button  108  allows the operator of the device  10  to stop the device  10  in the event of an emergency. 
   A joystick  110  provides for basic control of the device  10  and allows the operator of the device  10  to easily control the direction of the device  10  during operation. A cruise control button  112  enables and disables an automatic cruise control function of the device  10 . A vision/gyro button  114  enables control of the device  10  by a computer vision system. A forward/reverse button  116  allows the operator of the device  10  to change the direction of the device  10 . A water jet button  118  allows the operator of the device  10  to start and stop the flow of water to the jet/vacuum system  12 . An end of row button  120  allows the operator of the device to cause the automatic, computer-vision controlled drive to turn the device  10  around. A right angle turn button  122  allows the operator of the device  10  to efficiently cause the device  10  to make a right angle turn during operation. A home button  124  allows the operator of the device  10  to set the desired center position for the steering joystick. 
     FIG. 7  is a diagram illustrating a strain relief connector  128  that can be used in conjunction with the robotic device  10  of  FIG. 1  according to one embodiment of the present invention. The connector  128  may connect to one of the lifting/safety rings  38  via a clip  130 . The connector  128  relieves the strain on the cables and hoses  52 ,  56  during operation of the device  10 . 
     FIG. 8  is a diagram illustrating a system  200  in which the robotic device  10  of  FIG. 1  may be used according to one embodiment of the present invention.  FIG. 8  illustrates the case where the device  10  includes a jet/vacuum system  12  for stripping paint and coatings from a surface using UHP water. A controller  202 , on which the control panel  100  of  FIG. 6  may be located, may be used by an operator to control the device  10 . The controller  202  may be, for example, a wireless or radio control device. A generator  204 , such as an enclosed diesel generator, provides electrical power to the device  10  and various other components of the system  200 . A water pump  206 , such as a diesel water pump, supplies water to the jet/vacuum system  12  of the device  10 . A vacuum  208 , such as an electric vacuum, vacuums spent water and removed particles via the jet/vacuum system  12  of the device  10 . In one embodiment, the vacuum  208  is a 56 kW vacuum that pulls approximately 128 m 3  per minute through the jet/vacuum system  12  with a vacuum of approximately 38 cm Hg. 
   The output of the vacuum  208  enters a settling tank  210  in which solid waste settles for removal. The liquid portion of the settling tank  210  is directed to a filtration unit  212 , such as an enclosed ultra filtration unit, where solids are filtered. In one embodiment, the filtration unit  212  includes a centrifuge that removes the solid waste. In one embodiment, the filtration unit  212  includes a sand filter and a secondary filter that is tailored to remove dissolved chemicals that are expected to be in the water vacuumed from the jet/vacuum system  12 . The filtered water output from the filtration unit  212  may be recycled in the system  200  by the water pump  206  or may be returned to the environment. In one embodiment, the water output from the filtration unit  212  is 1 micron filtered water. 
     FIG. 9  is a diagram illustrating a Halbach magnet array  220  according to one embodiment of the present invention. A Halbach magnetic array is a series of magnets which are so arranged as to simulate a magnetic monopole. The result is a magnetic assembly which, unlike most other magnetic devices, exhibits magnetic attraction predominately on a single surface. The Halbach array uses the power of the magnet elements which comprise it in an efficient manner to produce a magnetic device of unusual strength and ability to throw magnetic flux across significant air gaps. A Halbach device might be composed of 4 or more magnetic elements with each element having a different axis of magnetic orientation. The change in orientation from one element to the next may be 90 degrees or less. Magnetic elements may be arranged in a straight line, a circular fashion or a variety of other manners to achieve the same effect. The Halbach array  220  is used for the magnets  40  of the device  10  according to one embodiment of the present invention. The Halbach array  220  includes permanent magnet bars  222  arranged and oriented in such a way that the magnetic field of the array  220 , which varies periodically in space along the array, is concentrated on one face of the array  220  and almost canceled on the opposite face (See  FIG. 10 ). The magnetic orientation (i.e. 0 degrees, 45 degrees, and 90 degrees) of each of the bars  222 , according to one embodiment of the present invention, is indicated with an arrow. According to one embodiment of the present invention, each of the bars  222  may be, for example, 45 MGOe Neodymium (uncoated). 
   The array  220  includes 3 array cycles (i.e. 13 bars  220 ). However, various embodiments may use a differing number of cycles such as, for example, 1 cycle (i.e. 5 bars  220 ) or 2 cycles (i.e. 9 bars  220 ). In one embodiment, the x dimension of the array  220  is 6.75 in., the y dimension of the array  220  is 2 in., and the z dimension of the array  220  is 8.5 in. 
     FIG. 10  is a diagram illustrating the magnetic fields of the Halbach magnet array  220  of  FIG. 9  according to one embodiment of the present invention. In  FIG. 10 , the array  220  is included as one of the magnets  40  of the device  10 . The wheels  20  of the device  10  contact a ferromagnetic surface  224  on which the device  10  is operating. The magnet  40  does not contact the surface  224  but, rather, due to the orientation of the magnetic fields emanating from the magnet  40  as denoted by the shaded areas of  FIG. 10 , the magnet  40  is separated from the surface  224  by an air gap  226 . The magnet  40  provides the necessary force required to hold the device  10  on the surface  224 , even though the magnet  40  does not contact the surface  224  and even though there may be one or more layers of paint or coatings on the side of the surface  224  on which the device  10  is operating. The magnet  40  likewise provides sufficient force to hold the device  10  when the device  10  operates in an inverted (e.g. upside-down) or vertical position. In one embodiment, the air gap  226  is a ⅝ in. air gap. As illustrated in  FIG. 10 , the magnet  40  does not ride on the surface  224  and, thus, the magnet  40  will not mar the surface  224  during operation of the device  10 . Because the magnet  40  does not contact the surface  224 , the device  10  is able to traverse concave or convex and/or inverted surfaces that contain surface irregularities, dents, etc. because the wheels  20  provide the sole contact of the device  10  with the surface  224 . 
     FIG. 11  is a diagram illustrating a side view of a magnet  222  used in the Halbach magnet array  220  of  FIG. 9 .  FIG. 11  illustrates two embodiments of the shape of the bars  222  that comprise the array  220 . The first embodiment, designated as  300 , presents a curved working face and the second embodiment, designated as  302 , presents a segmented working face. 
   The Halbach array  200  has many advantages over methods traditionally used to hold devices on ferromagnetic surfaces such as vacuum attachments, which are unreliable and impede movement of the device, magnetic wheels and tracks, which are heavy and which mar surfaces, and conventional magnetic arrays which provide one-third the holding power of a Halbach array for their weight. The high holding power of a Halbach array for its weight, and the ability of this type of magnet to throw its magnetic field farther than other types of magnetic solutions makes it possible to build a device with unprecedented performance on ferromagnetic surfaces. 
     FIG. 12  is a diagram illustrating detection of lift off according to one embodiment of the present invention. If the magnet  40  starts to lose sufficient force to adhere the device  10  to the surface  224 , the size of the air gap  226  becomes increasingly larger until the wheels  20 , and thus the device  10 , lose contact with the surface  224 . Thus, if either the size of the air gap  226  or the magnetic flux at the surface  224  can be measured using appropriate sensors located on the device  10 , the operator of the device  10  may be alerted that the device  10  is about to lose contact with the surface and the operator may take corrective action. Alternatively, the device  10  may automatically take self-correcting action such that the device  10  does not lose contact with the surface  224 . 
     FIG. 13  is a graph which illustrates the difference between the holding power of a Halbach array and the holding power of a conventional, multi-pole magnet array with iron pole pieces which has identical mass. It can be seen in  FIG. 13  that the Halbach array solution is very substantially more efficient with any reasonable air gap. This efficiency is what makes a Halbach-equipped device, as described herein, well-suited to operation on vertical and inverted ferromagnetic surfaces where high holding power and light weight are essential. 
   In various embodiments, the device  10  may be equipped with automated mobility features that enable the device  10  to be operated more efficiently. Such features may be implemented and controlled by the automation computer  80 . One such feature is termed “cut-line tracking cruise control.” This feature may be useful when the device  10  is used to strip paint or coatings from a surface. During operation, the device  10  may make various straight-line passes over an area, with each successive pass overlapping slightly with the immediately-prior pass. Although such overlap ensures complete coverage of the device  10 , it may be difficult for an operator of the device  10  to consistently operate the device  10  with an overlap that is neither too small nor too large. 
   The device  10  may thus employ, for example, a forward-looking camera that can sense, using, for example, a computer vision algorithm, a cut line that demarcates the area on which the device  10  has operated from the area on which the device  10  has not operated. Such a computer vision algorithm may be, for example, an algorithm that relies on a color histogram-base correlation to find likely cut line points, and an aggressive line fitting algorithm to fit the most likely cut line. Because the device  10  can detect the cut line, the device  10  may automatically follow the cut line with little or no operator intervention. 
   Another automated feature is termed the “paint residue cruise control.” This feature may be useful when the device  10  is used to strip paint or coatings from a surface. As the device  10  operates, a slower speed may strip more paint or coating and a faster speed may strip less paint or coating. Because paint and coating thicknesses may vary from surface to surface or from one part of a surface to another, it may be difficult to operate the device  10  at a uniform speed and effectively remove all of the paint or coating. The device  10  may thus employ a reverse-looking camera that monitors the surface that is being stripped. The camera may feed images to an algorithm that has been trained from a set of sample images to recognize the statistical color characteristics of the stripped surface (e.g. bare steel). The algorithm may compute the percentage of paint or coating left on the surface that has been stripped and thus the device  10  may be automatically slowed if all of the paint or coating has not been removed. 
   The systems, methods, and techniques discussed herein allow for an improved device that allows for the use of non-surface marring wheels, provides for better traction on surfaces, provides for better maneuverability and obstacle clearing, does not mar or scratch surfaces, and provides a light weight and low cost magnetic assembly that has a high magnetic holding power. 
   While several embodiments of the invention have been described, it should be apparent, however, that various modifications, alterations and adaptations to those embodiments may occur to persons skilled in the art with the attainment of some or all of the advantages of the present invention. It is therefore intended to cover all such modifications, alterations and adaptations without departing from the scope and spirit of the present invention as defined by the appended claims.