Abstract:
Disclosed are a method and an improvement to the existing conventional magnetic flux leakage inspection device that employ some fixed magnets that are fixed inside the magnet yoke and some movably adjustable magnets, allowing their dipole orientation to be adjusted between 0° and 180° relative to that of the fixed magnets. A lever and gear set connected to the adjustable magnets can be operated to achieve desired level of magnetic strength of the device, including turning off the whole magnetic field, by causing the fields of the fixed and adjustable magnets to cancel each other. The disclosed adjustable yoke can also be used in other NDT/NDI applications where providing an adjustable magnetic strength is desirable.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to non-destructive testing and inspection (NDT/NDI) devices that employ magnetic fields induced by permanent magnets during the inspection, more particularly to an improved magnetic yoke which facilitates adjustment of the strength of the magnetic field in the devices. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is known in the art, such as in U.S. Pat. No. 4,814,705 that discontinuities such as cracks or pits below the surface of a test object of magnetizable material can be detected by magnetizing the material and sensing variations in leakage field near the surface of the test object. The device disclosed in U.S. Pat. No. 4,814,705 utilizes one or more magnets and an array of magnetic flux leakage sensors which are moved over the surface of the test object in close proximity. When a local region of the test object under the magnet is free of defects, it produces an induced magnetic flux of a known form that is highly regular. Localized defects from corrosion, pitting and the like produce irregularities in the highly regular form of the flux pattern that “leaks out” of the test object. The irregularities in the otherwise regular flux pattern may be detected by sensors in the inspection device positioned just above the test object surface. This also applies to some applications wherein the defect producing the magnetic anomaly is on the inaccessible side of the test objects. 
         [0003]    Typical magnetic flux leakage inspection devices include a carriage mounted on wheels that carries the magnet for inducing the magnetic field, the sensors for detecting the flux leakage, a motor for driving the wheels, and various other subassemblies needed for the device to function. To perform an inspection, the device is wheeled slowly across the test object surface while on-board sensors search a strip, typically about twelve inches wide, for magnetic leakage flux. 
         [0004]    The typical “magnets” used by these inspection devices are of two kinds: permanent magnets or electromagnets, which both induce a magnetic field within the magnetizable test object. To achieve good inspection sensitivity and accuracy with respect to corrosive type defects, it is desirable to apply the largest possible magnetizing force to the test object. There is a practical limitation, however, to the magnitude of the largest magnetizing force that can be applied. That is the attractive magnetic force between the magnets in the device and the material being magnetized can become unmanageably large. If the inspection device is to be operable, it must strike a balance between the magnitude of the magnetization induced in the test object under inspection (a larger induced magnetization provides greater sensitivity to magnetic anomalies) and the strength of the magnetic attraction between the test object and the device (the larger magnetic attraction, the more it hinders maneuverability of the device). 
         [0005]    Currently available magnetic inspection devices are generally difficult to operate. The devices are heavy, typically weighing 100 to 300 pounds (44 to 130 kilograms). The needed strength of the magnetic inspection power creates attraction between the on-board magnet(s) and the steel plate (test object), such as an oil tank bottom, making it very difficult to freely move the device over the inspection surface. Even with the on-board driving motor for the wheels, manipulating the device in the course of inspecting a full tank bottom can be a laborious operation. A storage tank having an 80 foot (25 m) diameter, for example, may take up to eight hours to inspect. 
         [0006]    Maneuvering the device is laborious in part because the operator must first “break” the attractive magnetic force whenever it is desired to re-position the device for inspecting a new region of the test object, for example when a sidewall is reached, or to navigate the device around or over obstacles such as plate welds. Steel plate in the order of ¼ to ½-inch thick (6 to 12 millimeters) is commonly welded to the tank bottom to patch previously discovered damage. When the edge of such patchwork is encountered, the operator must manually urge the device over or around the welded edge to continue the inspection. Operators commonly find it burdensome to manipulate the device back and forth over the tank bottom when the total attractive force exceeds about 200 pounds (about 90 kilograms) and extremely difficult if not prohibitively exhausting when the attractive force exceeds about 700 pounds (about 300 kilograms). 
         [0007]    Some eddy current inspection techniques make use of strong magnets to magnetically saturate ferromagnetic test objects. The existing magnetic yokes used for these devices are afflicted by the same inconveniences as the magnetic flux leakage inspection devices mentioned above, notably poor maneuverability, difficult cleaning and restricted air carrier transportation. 
         [0008]    Prior efforts to alievate the maneuvuerbility problem include attempts to provide a foot pedal linked to the magnet assembly so that the operator may first displace the magnet away from the inspection surface by depressing the foot pedal to break the magnetic attraction. In practice, however, the foot pedal still leads to operator fatigue over the course of several hours of inspection. Consequently, operator fatigue places a practical limitation on the maximum magnetization that may be utilized, which in turn limits the sensitivity, accuracy and overall utility of the inspection device. Furthermore, the maximum magnetization limits the plate thickness that can be inspected and the size of the gap between the inspection device and the test object. The ability to inspect with a gap between the magnets of the device and the test object is important because tank floors are sometimes covered with fiberglass coating or paint coating. 
         [0009]    Another effort attempted to overcome the maneuvrability problem due to strong permanent magnets by employing actuators to increase the distance between the permanent magnetic yoke and the inspection surface thereby decreasing the magnetic force to facilitate repositioning of the device. However, these attempts do not overcome the additional problems associated to the use of strong permanent magnets which include not being able to use ferromagnetic tools or other accessories in close proximity to the device. 
         [0010]    Additional limitations of the known efforts related to the use of strong permanent magnets that caused metallic debris to tend to adhere to the magnets and the difficulties of removing this debris. 
         [0011]    Furthermore, aviation transportation laws place limitation on the strength of the magnetic field of the equipment that can be shipped by air carriers. For many corrosion inspection service companies, this is a major limitation as shipping the inspection device by land carriers takes a significantly longer time. 
         [0012]    Electromagnets can also be used in magnetic flux leakage apparatuses or with alternative eddy current based inspection techniques. For example, the SLOFEC (saturation low field eddy current) technique, often referred to by its trademarked name as the Kontroll Technik, uses electromagnets for applications that require a specific level of magnetization and therefore an adjustable magnetic field. These eddy current based inspection techniques that currently compete with magnetic flux leakage devices require a continuously variable magnetic strength to achieve a precise level of magnetization in the test object. 
         [0013]    Although the electromagnetic field can be adjusted or completely turned off in these applications using electromagnets, electromagnets are significantly heavier than their permanent magnet counterparts and the power requirement is significant. Particularly, the high power requirement significantly affects the portability of these devices. The high amperage power requirement for electromagnets may also pose certain safety concerns. 
         [0014]    Thus given the existing problems and tried efforts, it is highly beneficial to provide a permanent-magnet based magnetic flux leakage inspection device, in which the magnetic power can be easily adjusted, to achieve better maneuverability and inspection sensitivity while avoiding the drawbacks of electromagnets based devices. 
         [0015]    It can also be appreciated that it is beneficial to provide other types of NDT/NDI devices involving permanent magnets with the capabilities of adjusting the power of their magnetic fields. 
       SUMMARY OF THE INVENTION 
       [0016]    The invention is related to NDT/NDI devices involving permanent magnets where such existing and conventional devices present the drawbacks and problems of having poor maneuverability, of being difficult to clean and being unsuitable for air transportation. 
         [0017]    Accordingly, it is a general object of the present disclosure to provide an improvement to such devices as the conventional permanent magnet type of magnetic flux leakage inspection device by employing a mechanism to adjust the strength of the magnetic field during operations. 
         [0018]    It is further an object of the present disclosure to employ an adjustable magnetic yoke to conveniently adjust the strength of the magnetic field during operations. 
         [0019]    It is further an object of the present disclosure to make use of a combination of fixed and displaceable permanent magnets to provide a magnetic yoke which allows the magnetic field to be continuously adjusted. 
         [0020]    In one embodiment of the current invention, a row of four permanent magnets are used to provide a combined magnetic field in a yoke wherein rotation of two of the magnets allows the strength of the magnetic field to be easily adjusted in a continuous fashion. 
         [0021]    In another embodiment of the current invention, two rows of four permanent magnets (eight magnets in all) are used to provide a combined magnetic field in the yoke wherein rotation of two magnets of each row of the magnets allows the strength of the magnetic field to be easily adjusted in a continuous fashion. 
         [0022]    It can be understood that the presently disclosed improvement to the existing magnetic flux inspection device provides the advantages of being easier to manipulate, clean and transport. 
         [0023]    It can be further understood that the presently disclosed improvement to the existing magnetic flux leakage inspection device provides the advantages of improved sensitivity for the inspection device due to the capability of adjusting the device to provide magnetic strength at a desired level. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a schematic diagram showing a permanent-magnet-based magnetic flux leakage inspection device according to the present invention. 
           [0025]      FIG. 2  is a schematic diagram showing an isometric view of one embodiment (embodiment A) of the inspection device according to the present invention. 
           [0026]      FIG. 3  is a schematic diagram showing an isometric cross-sectional view of embodiment A. 
           [0027]      FIGS. 4   a  and  4   b  are top cross-sectional views showing the arrangement of the permanent magnets of the preferred embodiment yielding variable magnetic power. 
           [0028]      FIG. 5  is a schematic diagram showing an isometric view of an alternative embodiment according to the present invention. 
           [0029]      FIG. 6  is a schematic diagram showing an isometric cross-sectional view of the alternative embodiment of the inspection device according to the present invention. 
           [0030]      FIG. 7  is a schematic diagram showing an isometric cross-sectional view of the alternative embodiment according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    Referring to  FIG. 1 , one embodiment (embodiment A) of a magnetic flux leakage inspection device  100  according to the present invention is shown. The device includes an inspection carriage assembly  9  mounted on wheels  5  and a handle portion  8  by which an operator steers and manipulates the device. Preferrably mouted on handle portion  8  are a control panel  10  for controlling the operation of the device and an interface screen  12  for displaying inspection results and serving as an interface for the operator communicating with the device. The device is shown positioned on a portion of a test object  14 , which is under inspection. Test object  14  is composed of a magnetizable material and for the case of storage tank bottoms, it is generally a ferromagnetic steel plate. 
         [0032]    A novel aspect of device  100  representing a significant improvement to the existing magnetic flux leakage inspection devices is that device  100  comprises a scan bar assembly  1  containing a plurality of magnets, the orientation of which can be adjusted by a lever or knob type of actuator  30  so that the power of the magnetic field can be therefore easily adjusted. 
         [0033]    Still referring to  FIG. 1 , lever  30  is configured so that its position corresponds to the orientation of the magnetic field of the magnets and the resulting overall magnetic strength of the device. A dial marking  16  is provided on top of the housing of the assembly carriage  9  and preferably surrounds lever  30  to indicate the position of the lever and the corresponding orientation of the magnetic field of the magnets, and therefore to give the reading of the magnetic strength of the device  100 . 
         [0034]    Referring now to  FIGS. 2 to 4 , more details of embodiment A, particularly of scan bar assembly  1  is shown. Carriage assembly  9  includes scan bar assembly  1  that is comprised of a yoke  28  that includes a row of preferrably four permanent magnets  20 ,  21  and  22  for inducing magnetization of plate  14  under inspection. Magnets  20  on the ends of yoke  28  are permanently embedded into yoke  28 . Magnets  21  and  22  are linked together by gears  38  and  40  and are rotatable within yoke  28 . The individual magnets  20 ,  21  and  22  are magnetically coupled to one another through yoke  28 . Rotation lever or knob  30  are coupled to magnets  21  and  22 . A locking mechanism is provided to lever  30  prevent magnets  21  and  22  from spontaneously returning to their nominal rotational positions. 
         [0035]    Positioned beneath magnets  20 ,  21  and  22  and forming a part of scan bar assembly  1  is magnetic sensor assembly  24 , which is used to detect magnetic leakage flux indicative of underlying magnetic anomalies associated with corrosive pitting and other plate damage. 
         [0036]    When the magnetic dipole of magnets  20 ,  21  and  22  are aligned and disposed accordantly, yoke  28  is magnetically coupled to plate  14 , a continuous magnetic circuit is formed. 
         [0037]    More particularily shown in  FIGS. 2A and 2B , for good measurement reliability it advantages important that yoke faces  32  and  34  which comprise the active sensor surface be disposed in an inspection position having a fixed distance from the surface of the plate  14  under inspection. The magnitude of the magnetization induced in plate  14  under inspection, and the magnitude of any consequent flux leakage due to an anomaly, depend on the distances of the yoke faces  32  and  34  from the testing surface of plate  14 , and quantitative interpretation of measurement results depends on the positioning of magnetic probes  20 ,  21  and  22  in the leakage flux. 
         [0038]    In operation, to inspect a strip of a tank bottom, the operator directs the device in a straight line over the strip. Wheels  5  are normally mounted to rotate only around their central axes in order to maintain the movement of the device in a generally straight line. The magnetic attraction between magnetic yoke  28  and plate  14  is generally quite strong. To maintain the movement against the resistive force of this magnetic attraction, wheels  5  are driven by a motor (not shown). 
         [0039]    Rotation lever or knob  30  is provided for operator to apply a rotational force to magnets  21  and  22 . This rotational force activates gear  38  which in turn activates gear  40  which provides synchronous rotation of magnets  21  and  22  in a rotational opposite direction. To facilitate maneuvering the device over and around obstacles and re-positioning in new directions, using rotation lever or knob  30  to apply a 180 degree rotation of the dipole of magnets  21  and  22  with respect to the dipole direction of magnets  20  produces a magnetic field cancelling effect that significantly reduces the strength of the magnetic force between yoke  28  and test object  14 . 
         [0040]    Referring specifically to  FIG. 4A , the dipoles of magnets  20 ,  21  and  22  are aligned and disposed accordantly thereby producing a strong magnetic circuit. Referring to  FIG. 4B , the dipoles of magnets  21  and  22  are aligned but disposed in opposite (180 degrees) direction of those of fixed magnets  20  thereby producing a very weak or null magnetic circuit. Rotation of the dipoles of magnets  21  and  22  by an angle between 0 and 180 degrees compared to the dipoles of magnets  20  produces a continuously variable magnetic field strength. Gears  38  and  40  produce an inverse rotational direction for magnets  21  and  22  which is an advantageous aspect of the invention, thereby contributing to a more uniform magnetic field in yoke  28 . 
         [0041]    It is worth noting that embodiment A uses cubic magnets. As shown in  FIGS. 4A and 4B , field couplers  42  composed of ferromagnetic material such as carbon steel are employed by the embodiment. In order to properly produce a magnetic circuit in yoke  28 , the dipole ends of cubic magnets  21  and  22  are fixed to field couplers  42 . Field couplers  42  are solidly fixed to magnets  21  and  22  and rotate with magnets  21  and  22  within yoke  28 . Note that the fixed magnets  20  are in direct contact with yoke  28  and directly produce magnetic circuit in yoke  28 . In order to prevent short circuiting of the magnetic field with the rotation of magnets  21  and  22 , these magnets are embedded in non-ferromagnetic magnet support  44 . Fillers  48  are of non-ferromagnetic material and built in between magnets to block debris from entering yoke  28 . 
         [0042]    Reference now is turned to  FIGS. 5 to 7 , an alternative embodiment B of magnetic inspection device  100  with improved magnetic yoke is shown. It should be noted that the design variations from embodiment A should be recognized by those skilled in the art to be within the scope of the present disclosure. The detailed description of embodiment B focuses on the portion of the embodiments varied from embodiment A, and should be construed to complement embodiment A. 
         [0043]    In embodiment B, as shown in  FIGS. 5-7 , an alternative yoke  61  is employed to replace yoke  28  of embodiment A. Yoke  61  embodies eight permanent magnets which are disposed in two rows, each row having four magnets. Magnets  76 ,  77 ,  78  and  79  are permanently fixed within yoke  61 . Magnets  64 ,  66 ,  72  and  74  are disposed in between the permanently fixed magnets and can be rotated by activating a rotation lever or knob  60 . Lever or knob  60 , affixed to gear  70  provides direct rotation of magnets  72  and  74  and indirect rotation via gear  68  to magnets  64  and  66 . Magnets  64  and  66  are embedded in non-ferromagnetic support  65 . Non-ferromagnetic support  65  acts as an axle between magnets  64  and  66 . As particularly shown in  FIG. 7 , a similar non-ferromagnetic support  75  is used for rotational magnets  72  and  74 . Note that the embodiment employs the use of magnetic couplers  86  to provide a stronger magnetic connection between magnets  72 ,  74  and yoke  61 . Note that the fixed magnets  76 ,  77 ,  78  and  79  are in direct contact with yoke  61  and directly produce a magnetic circuit in yoke  61 . 
         [0044]    The above descriptions and drawings disclose illustrative embodiments of the invention. Given the benefit of this disclosure, those skilled in the art will appreciate that various modifications, alternate constructions, and equivalents may also be employed to achieve the advantages of the invention. 
         [0045]    For example, other configurations or other types and shapes of permanent magnets may be used. The invention is not limited to using four or eight magnets nor is the invention limited to using cubic magnets. Cylindrical magnets, annular magnets and other shapes can also be used. 
         [0046]    In addition, the device may be configured with alternate means for rotating the rotatable magnets such as motorized means which may be controlled electronically. Other rotational means are possible beyond using gears such as using belts and chains. 
         [0047]    Furthermore, it can be recognized that magnetic flux monitoring sensors such as Hall Effect sensors can be integrated into the magnetic yoke to monitor the actual magnetic field transmitted to the test object. These sensors can be linked to a user interface device by which the user can select the magnetic force required for a given inspection. Additionally, continuous adjustment of the strength of magnetic power according to the Hall Effect sensors helps provide constant magnetic flux during an inspection. 
         [0048]    It is known in the art to use magnets to magnetize ferromagnetic test objects when performing eddy current inspections. It must be recognized that the adjustable magnetic yoke as disclosed herein would be beneficial for these inspections. In the case of eddy current inspections requiring complete saturation of the test object, the variable magnetic yoke provides the advantages of being easier to manipulate, clean and transport by air carriers. In the case of eddy current inspection techniques requiring partial magnetic saturation of the test object, such as SLOFEC, the novel adjustable magnetic yoke disclosed herein provides the advantage of being significantly lighter and more portable than electromagnets. 
         [0049]    Although the most common application for magnetic flux leakage devices is the inspection of tank floors, it must be recognized that the invention herein can also be applicable to the magnetic flux leakage or eddy current inspection of tank walls, pipes and pressure vessels. 
         [0050]    Although the novel aspect of adjustable magnetic yoke herein disclosed can also be advantageously applied to magnetic flux leakage or eddy current type inspection devices, it must be recognized that other uses are possible in the field of non-destructive testing. For example, inspection scanners for ferromagnetic test specimens employing ultrasound, phased-array, eddy current and other technologies would benefit from the use a permanent magnet arrangement for which the magnetic force can be adjusted. One can modify the existing use of magnetic wheels by adopting the herein disclosed adjustable magnetic yoke into such scanners. In this case, the suction force of the adjustable magnetic yoke is employed to maintain the scanner in contact with the inspection surface. 
         [0051]    Although the present invention has been described in relation to particular exemplary embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention not be limited by the specific disclosure.