Abstract:
A system and method for radiographic inspection of an aircraft fuselage includes a radiation source located on one side of the fuselage and a plurality of radiation detectors located on another side of the fuselage. The radiation detectors are located in known positions relative to the radiation source so as to receive radiation from the radiation source at different angles. The system further includes manipulators for moving the radiation source and the radiation detectors in a coordinated fashion. The system processes the radiation detected by the radiation detectors so as to display stereoscopic images of areas of interest of the fuselage. The stereoscopic images are obtained by first irradiating the fuselage and the radiation detectors with the radiation source to detect a first set of images of the fuselage from multiple angles, repositioning the radiation source and the radiation detectors with respect to the fuselage, and then irradiating the fuselage and the radiation detectors with the radiation source to detect a second set of images of the fuselage. The multiple sets of images are used to produce the stereoscopic images.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to radiographic inspection of aircraft fuselages and more particularly to methods and systems for inspecting aircraft fuselages without a-priori knowledge of interfering structures.  
           [0002]    An aircraft fuselage typically comprises a grid of circumferential frame members and longitudinal stringers covered by a skin of lightweight sheet metal. The skin is ordinarily attached to the frame members and stringers by means of rivets or the like. To ensure passenger comfort at high altitudes, aircraft are provided with cabin pressurization systems that produce near sea-level air pressure breathing environments in the aircraft cabin. The application of cabin pressure causes the skin, frame members and stringers to expand slightly. When the pressure is removed, the skin, frame members and stringers return to their normal shape. Although the pressure differentials involved are relatively small, the repeated cycles of stress imposed on the fuselage structure by the pressurization and depressurization sequence that occurs during each flight can lead to fatigue and crack formation. This fatigue damage is often assisted by corrosion of the fuselage structures.  
           [0003]    Fatigue cracks by nature can be extremely small in size and difficult to detect. The cracks are normally so small that routine pressurization of the aircraft cabin will not result in detection because the tiny cracks will not cause a detectable pressure loss in the aircraft. The combined effect of corrosion and cyclic stress can also cause looseness around the rivets and/or rivet cracking. If not detected, this condition could result in skin separation from the frame members and stringers.  
           [0004]    Traditionally, aircraft fuselage inspection relies largely on visual inspection techniques. These techniques rely heavily on human ability and are limited by ambient lighting conditions, environmental effects, and the inspector&#39;s physical and mental limitations such as eye vision corrections, time constraints, mental attitude, concentration and judgment. Furthermore, visual inspection techniques require extensive disassembly of the aircraft. This approach is thus time consuming, labor intensive and expensive.  
           [0005]    Radiography is another approach to aircraft fuselage inspection that has been proposed. While this approach can reduce the amount of aircraft disassembly required with traditional visual inspections, internal cabin objects can significantly complicate x-ray images, thereby masking defects and making their identification and quantification more difficult. These objects include overhead bins, bulkheads, air masks, oxygen plumbing, lights, electrical wiring, fasteners, lavatory and galley fixtures and so on. If the precise location of such interfering objects is known, viewing angles can usually be determined to allow the areas of interest to be imaged without interference. Some of these interfering objects are in known fixed positions. Other objects vary significantly in location from one aircraft to another. For example, electrical wiring and oxygen plumbing are flexible in nature and do not assume a fixed location. Thus, without sufficient a-priori knowledge of interfering structure location, it is difficult to plan or predict viewing angles that will avoid interference. In which case, the initial inspection will provide images where the field of view has been obstructed. This requires the affected areas to be re-inspected from another angle and perspective, which leads to additional inspection expense and time.  
           [0006]    Accordingly, there is a need for a method and apparatus for radiographic inspection of aircraft fuselages that permits all or most of a fuselage to be accurately inspected without a-priori knowledge of interfering structure locations.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    The above-mentioned need is met by the present invention, which provides a system and method for radiographic inspection of an aircraft fuselage. The system includes a radiation source located on one side of the fuselage and a plurality of radiation detectors located on another side of the fuselage. The radiation detectors are located in known positions relative to the radiation source so as to receive radiation from the radiation source at different angles. The system further includes manipulators for moving the radiation source and the radiation detectors in a coordinated fashion. The system processes the radiation detected by the radiation detectors so as to display stereoscopic images of areas of interest of the fuselage. The stereoscopic images are obtained by first irradiating the fuselage and the radiation detectors with the radiation source to detect a first set of images of the fuselage from multiple angles, repositioning the radiation source and the radiation detectors with respect to the fuselage, and then irradiating the fuselage and the radiation detectors with the radiation source to detect a second set of images of the fuselage. The multiple sets of images are used to produce the stereoscopic images.  
           [0008]    The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:  
         [0010]    [0010]FIG. 1 is a schematic view of a radiographic inspection system for inspecting aircraft fuselages.  
         [0011]    [0011]FIG. 2 is a more detailed schematic view of a radiographic inspection system for inspecting aircraft fuselages.  
         [0012]    [0012]FIG. 3 is a sectional end view of a portion of the radiographic inspection system of FIG. 2.  
         [0013]    [0013]FIG. 4 is a perspective view of an aircraft equipped with the inspection system of FIG. 2 and having a portion of the fuselage shown in partial cutaway to reveal internal fuselage structure.  
         [0014]    [0014]FIG. 5 is a partial schematic view of the radiographic inspection system with the radiation source and detectors in a first position.  
         [0015]    [0015]FIG. 6 is a partial schematic view of the radiographic inspection system with the radiation source and detectors in a second position.  
         [0016]    [0016]FIG. 7 is a partial schematic view of the radiographic inspection system with the radiation source and detectors in a third position.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 schematically shows a radiographic inspection system  10  for inspecting an aircraft fuselage  12 . The fuselage  12  generally comprises a cylindrical wall  14  made up of a grid of circumferential frame members and longitudinal stringers covered by a skin of lightweight sheet metal. The system  10  includes a radiation source  16  located on a first side of the fuselage wall  14  and a plurality of radiation detectors  18  located on a second, opposite side of the fuselage wall  14 . Although two such radiation detectors are shown in FIG. 1, the present invention encompasses more than two detectors, as will become apparent. The radiation source  16  and radiation detectors  18  are relatively situated on opposite sides of the wall  14  so that radiation emitted by the radiation source  16  irradiates the fuselage wall  14  and then impinges on each of the radiation detectors  18 . The radiation detectors  18  are positioned relative to the radiation source  16  such that the radiation impinges on each one at a different angle. As shown in FIG. 1, the radiation source  16  is located inside of the fuselage  12 , and the radiation detectors  18  are located outside of the fuselage  12 . However, it should be noted that this arrangement could alternatively be reversed so that the radiation source  16  is outside and the radiation detectors  18  are inside the fuselage  12 .  
         [0018]    The radiation source  16  is preferably, but not necessarily, a standard industrial x-ray tube powered by a high voltage power supply (not shown). Alternative radiation sources, such as an isotopic radiation source producing gamma rays, could be used as well. The radiation source  16  provides flux to a large cone-shaped or panoramic volume, but is collimated to limit this to a specific area of interest. Specifically, this zone is made large enough to expose at least two inspection areas (i.e., an inspection area for each detector) at different angles with respect to the source flux axis. The radiation detectors  18  can be any means that is capable of processing radiation emitted by the radiation source  16  into a viewable image. Although x-ray film could be used, it is generally, but not necessarily, preferred that the radiation detector  18  be of the type that converts impinging radiation into an electrical output signal. Many suitable x-ray detectors are commercially available. As is known in the art, such x-ray detectors generally have an x-ray sensitive area and means for producing an output signal that is indicative of the x-rays impinging on the sensitive area.  
         [0019]    The image data signals output by the radiation detector  18  are fed to a controller  20 , which can be a conventional computer unit. The controller  20  processes these signals and causes corresponding stereoscopic images to be displayed on a viewing apparatus  22 , as will be described in more detail below. An operator is then able to view the displayed images to inspect for defects in the fuselage  12 . The data image signals are also stored in a memory in the controller  20 . The controller  20  also controls the operation of the radiation source  16 , turning it on and off and regulating the voltage applied.  
         [0020]    A first precise manipulator  24  is provided for moving the radiation source  16  with respect to the fuselage  12 , and a second precise manipulator  26  is provided for moving the radiation detector  18  with respect to the fuselage  12 . The precise manipulators  24 ,  26  can be any type of device capable of producing the desired motion. This would include robotic devices, guide rail systems and the like. One suitable manipulator arrangement is shown in FIGS.  2 - 4  in which the fuselage wall  14  is made up of a grid of circumferential frame members  28  and longitudinal stringers  30  (shown in cutaway in FIG. 4) covered by a skin  32  of lightweight sheet metal. As seen in FIG. 3, a passenger deck  34  is disposed horizontally in the fuselage  12  so as to define the floor of an interior cabin. The cabin can be provided with conventional overhead bins  36 , ventilation panels  38  and side panels  40 . Although not shown in the Figures, the fuselage  12  typically includes other conventional structure such as lights, wiring, insulation and the like.  
         [0021]    The first manipulator  24  includes a first carrier  44  to which the radiation source  16  is mounted. The first carrier  44  is slidingly mounted on two linear guide rails  46  that are disposed on the passenger deck  34  and extend parallel to the center longitudinal line of the fuselage  12 . The first carrier  44  is moved back and forth along the guide rails  46  under the control of the controller  20 . The motion is produced by any conventional motive means such as an electric motor (not shown) in a manner known in the art. Thus, the radiation source  16  can be selectively positioned along the length of the fuselage  12 . With this arrangement, the radiation source  16  is collimated to produce a panoramic radiation beam in the circumferential direction of the fuselage  12 , but limited in the forward and aft directions to the specific area of interest. The radiation source  16  thus illuminates the fuselage  12  from floor line to floor line above the passenger deck  34  along a relatively short longitudinal section of the fuselage  12 .  
         [0022]    The first manipulator  24  is configured to move the radiation source  16  through the desired range of motion without interference with any objects located inside the fuselage  12 . Accordingly, such objects (which may include overhead bins, bulkheads, air masks, oxygen plumbing, lights, electrical wiring, fasteners, lavatory and galley fixtures, etc.) need not be removed to perform an inspection.  
         [0023]    The second manipulator  26  utilizes a rail system that includes a plurality of curved guide rails  48  mounted to the outer surface of the fuselage  12 . Mounting can be accomplished by any means such as suction cups fixed to the rails  48  and engaging the fuselage  12 . The guide rails  48  are oriented circumferentially with respect to the fuselage  12  and are spaced out along the length of fuselage  12 . Each guide rail  48  is configured to match the fuselage curvature and extends from a point adjacent to the passenger deck  34  on one side of the fuselage  12 , over the fuselage crown, and to a point adjacent to the passenger deck  34  on other side of the fuselage  12 . The guide rails  48  are thus arranged to track the path of the panoramic radiation beam emitted by the radiation source  16 . The curved guide rails  48  are situated on the fuselage  12  so as to position the radiation detectors  18  over the areas of interest of the fuselage  12 . Each radiation detector  18  is mounted between a respective pair of adjacent guide rails  48 , and each pair of adjacent guides rails  48  defines a inspection area of interest. The guide rails  48  are accordingly located on opposing sides of the fuselage structure to be inspected. For example, FIG. 4 shows the guide rails  48  straddling respective ones of the frame members  28  so that they can be inspected for defects. However, it should be noted that the system  10  could also be used for inspecting other fuselage structure such as stringers, lap joints and the like. The guide rails  48  would simply be positioned accordingly.  
         [0024]    The second manipulator  26  includes a second carrier  50  for each radiation detector  18  and a support beam  52  that supports each of the second carriers  50 . Two radiation detectors  18  are shown in FIGS.  2 - 4 , but as previously mentioned, more than two detectors can be employed. Each radiation detector  18  is mounted to the underside of the second carrier  50  so as to face the fuselage  12 . The support beam  52  is slidingly mounted on the adjacent guide rails  48  defining the selected inspection areas so as to locate the radiation detectors  18  at the desired locations with respect to the fuselage  12 . The support beam  52  is moved along the selected guide rails  48  under the control of the controller  20  by any conventional motive means in a manner known in the art. Thus, the radiation detectors  18  are capable of traveling over the outer surface of the fuselage  12  above the passenger deck  34 . The controller  20  moves the carriers  44  and  50 , and thus the radiation source  16  and radiation detectors  18 , in a coordinated fashion such that the radiation detectors  18  are precisely located relative to the radiation source  16 .  
         [0025]    The operation of the inspection system  10  is now described with reference to FIGS.  5 - 7 , which, by way of example, depict the inspection of a portion of the fuselage wall  14  that encompasses a series of adjacent frame members denoted by reference numerals  28   a - 28   e . In the illustrated example, three radiation detectors  18   a - 18   c  are mounted on the curved guide rails  48  of three selected inspection areas, although it should be noted that the present invention is not limited to this particular number of detectors. Furthermore, the present invention is not limited to inspecting frame members and can be used for inspecting other fuselage structure such as stringers, lap joints and the like. As shown in FIG. 5, the detectors  18   a - 18   c  are arranged so that detector  18   a  is aligned with frame member  28   a , detector  18   b  is aligned with frame member  28   b , and detector  18   c  is aligned with frame member  28   c . The first manipulator  24  is controlled to move the radiation source  16  into longitudinal alignment with the center detector  18   b  so that each of the three detectors  18   a - 18   c  will be exposed to radiation from the radiation source  16 , albeit at different angles.  
         [0026]    The radiation source  16  is then turned on so that the adjoining region of the fuselage  12  above the passenger deck  34  is illuminated with radiation. While the radiation source  16  is emitting radiation, the second manipulator  26  is activated to cause the radiation detectors  18   a - 18   c  to travel over the outer surface of the fuselage  12 . Radiation emitted by the radiation source  16  passes through the frame members  28   a - 28   c  and impinges on the corresponding one of the radiation detectors  18   a - 18   c . The radiation is converted into electrical signals that are fed to the controller  20 . Thus, detector  18   a  detects an image of frame member  28   a  at a first angle, detector  18   b  detects an image of frame member  28   b  at a second angle (perpendicular to the longitudinal axis of the fuselage  12 ), and detector  18   c  detects an image of frame member  28   c  at a third angle.  
         [0027]    Once the inspection of the fuselage  12  at the first position is completed, the radiation detectors  18   a - 18   c  are repositioned on the fuselage  12  so that detector  18   a  is aligned with frame member  28   b , detector  18   b  is aligned with frame member  28   c , and detector  18   c  is aligned with frame member  28   d , as shown in FIG. 6. The first manipulator  24  again moves the radiation source  16  into longitudinal alignment with the repositioned center detector  18   b  and frame member  28   c . The inspection at this position is then carried out in the same manner with the radiation detectors  18   a - 18   c  being moved over the outer surface of the fuselage  12  while the radiation source  16  is turned on. In this position, detector  18   a  detects an image of frame member  28   b  at the first angle, detector  18   b  detects an image of frame member  28   c  at the second angle, and detector  18   c  detects an image of frame member  28   d  at the third angle.  
         [0028]    Next, the radiation detectors  18   a - 18   c  are again repositioned on the fuselage  12 , as shown in FIG. 7, so that detector  18   a  is aligned with frame member  28   c , detector  18   b  is aligned with frame member  28   d , and detector  18   c  is aligned with frame member  28   e . The first manipulator  24  again moves the radiation source  16  into longitudinal alignment with the repositioned center detector  18   b . Inspection at this position is then carried out in the same manner with the radiation detectors  18   a - 18   c  being moved over the outer surface of the fuselage  12  while the radiation source  16  is turned on. In this position, detector  18   a  detects an image of frame member  28   c  at the first angle, detector  18   b  detects an image of frame member  28   d  at the second angle, and detector  18   c  detects an image of frame member  28   e  at the third angle. This process is repeated sequentially down the length of the fuselage  12  until each frame member has been imaged from each of the three angles.  
         [0029]    The controller  20  processes the various signals from the detectors  18   a - 18   c  for display on the viewing apparatus  22 . Since the images are taken at a precise and known geometry, the viewing apparatus  22  will permit an operator to view the images in a stereoscopic manner. A wide variety of electro-optical viewing apparatuses for presenting stereoscopic images are commercially available. In the event that film is used instead of electronic detectors, numerous mechanical stereoscopic viewing devices are also available. By providing multiple viewing angles of each frame member, the inspection system  10  allows for depth perception in the images. That is, an operator will be able to distinguish the different geometrical depths of the frame members and overlapping structures such as overhead bins, bulkheads, air masks, oxygen plumbing, lights, electrical wiring, and the like. The operator will thus be able to discern defects in the frame members from image artifacts caused by interfering structure located between the radiation source and the frame members. This will also enable determination of the depth location of defects within the frame members. Furthermore, known digital image techniques can be used to enhance the images.  
         [0030]    The foregoing has described a method and apparatus for radiographic inspection of aircraft fuselages that permits all or most of a fuselage to be accurately inspected without a-priori knowledge of interfering structure locations. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.