Abstract:
A method of radiographic inspection of an object includes the steps of: providing a radiation source and a radiation detector located on opposite sides of the object; positioning the radiation detector to receive radiation transmitted through the object from the radiation source; radiographically imaging an region of interest of the object with the radiation source and the radiation detector, using an set of initial imaging parameters, to produce a test image; obtaining at least one quality measurement of the test image; comparing the quality measurement to predetermined image quality limits; and in response to the quality measurement exceeding the predetermined image quality limits, changing at least one of the initial imaging parameters to generate a new set of image parameters. The process may be repeated iteratively until a final set of imaging parameters is obtained.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to radiographic inspection and more particularly to a method of radiographic inspection of complex objects such as airframes. 
         [0002]    Aircraft, including their fuselages and nacelles, and other large structures, often require periodic inspection to determine their structural condition. This may be done visually or with non-destructive evaluation (NDE) techniques. Because of the complicated physical structure of aircraft fuselages and nacelles, radiographic inspection, such as X-ray inspection, is used to avoid having to disassemble overlapping components, insulation, wall coverings, etc. Modern digital electronic detectors are replacing X-ray film in many X-ray inspection applications. 
         [0003]    Prior art radiographic inspection requires substantial a-priori knowledge of the structure and materials. Development of inspection parameters is typically done in an iterative fashion, with informed radiographers choosing initial process parameters, then making sequential improvements. After obtaining a satisfactory image, the parameters are recorded and used in future inspections of that product. This works well when the product&#39;s materials and configuration are consistent, but sufficient variability in either makes the inspection&#39;s parameters unsatisfactory. For example, differences in aircraft airframe and interior materials, including interior panels and insulation used, provide considerable variability in the absorption of x-rays and thus require different inspection parameters to be used. Since these differences are not known in advance, one must discover them in an expensive trial and error process. 
         [0004]    In radiography, an x-ray beam is projected through an object. Depending on the density and dimensional configuration of the object, portions of these X-rays are absorbed. X-ray film or electronic detectors measure the x-rays transmitted through the object. Mapping the geometric pattern of the ratio of absorbed to transmitted x-rays reveals significant information of the object&#39;s materials and dimensional configuration. Defects in the material such as cracking, porosity, mechanical assembly errors, and a wide range of other defects can be accurately observed. Often however, objects not of interest to the inspection mask the features of interest. In airframe inspection, these include insulation and interior panels, as well as other objects. Changes in these objects materials or physical configuration can change the total absorption, moving the transmitted x-ray signal outside the useful dynamic range of the inspection system&#39;s settings, thus making it difficult to characterize the object to the required sensitivity. This requires that a radiographer review the images to make sure the parameters used are appropriate. When the objects change enough to exceed these limits, a new set of parameters must be selected and iteratively tried, greatly increasing the inspection time required. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The above-mentioned shortcomings in the prior art among others are addressed by the present invention, which provides a radiographic inspection system and method that will improve the efficiency of inspection processes, when compared to prior art methods. The system accommodates greatly increased variability in the materials and object locations within the field of view of the x-ray image. In airframe inspection, this difference will permit the use of X-ray inspection during shorter maintenance intervals, permitting greatly enhanced flexibility in airframe maintenance operations, and in the inspection of other objects with variability of density or construction. 
         [0006]    The above-mentioned need is met by the present invention, which according to one aspect provides a method of radiographic inspection of an object, including: providing a radiation source and a radiation detector located on opposite sides of the object; positioning the radiation detector to receive radiation transmitted through the object from the radiation source; radiographically imaging region of interest of the object with the radiation source and the radiation detector, using a set of initial imaging parameters to produce a test image; obtaining at least one quality measurement of the test image; comparing the quality measurement to predetermined image quality limits; and in response to the quality measurement exceeding the predetermined image quality limits, changing at least one of the initial imaging parameters to generate a new set of image parameters. 
         [0007]    According to another aspect of the invention, a system for radiographic inspection of an object includes: a radiation source carried by a first manipulator operable to position the radiation source on one side of a region of interest of the object; a radiation detector carried by a second manipulator operable to position the radiation detector on another side of the region of interest of the object such that the radiation detector can receive radiation transmitted through the region of interest from the radiation source to produce a test image using a set of initial imaging parameters; means for obtaining at least one quality measurement of the test image and comparing the quality measurement to predetermined image quality limits; and means for changing at least one of the initial imaging parameters to generate a new set of image parameters in response to the quality measurement exceeding the predetermined image quality limits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0009]      FIG. 1  is a schematic front view of an inspection system constructed according to one aspect of the present invention and positioned around an aircraft; 
           [0010]      FIG. 2  is a schematic cross-sectional view of an aircraft fuselage with a radiation detector and source positioned for inspection thereof; 
           [0011]      FIG. 3  is a schematic view of a radiation source and detector operatively connected to a controller; 
           [0012]      FIG. 4  is a schematic front view of a radiation detector; and 
           [0013]      FIG. 5  is a view taken along lines  5 - 5  of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIGS. 1 and 2  illustrate a radiographic inspection system  10  for inspecting an aircraft fuselage  12 . The fuselage  12  generally comprises a cylindrical wall  14  made up of circumferential frames  16  and longitudinal stringers  18  covered by a skin  20  of lightweight sheet metal. The inspection system  10  may be used with other types of structures as well. 
         [0015]    The inspection system  10  includes a radiation source  22  located on a first side of the fuselage wall  14  and a radiation detector  24  located on a second, opposite side of the fuselage wall  14 . The radiation source  22  and radiation detector  24  are relatively situated on opposite sides of the wall  14  so that radiation emitted by the radiation source  22  irradiates the fuselage wall  14  and then impinges on the radiation detector  24 . As depicted in  FIG. 1 , the radiation source  22  is located outside of the fuselage  12 , and the radiation detector  24  is located inside of the fuselage  12 . However, it should be noted that this arrangement could alternatively be reversed so that the radiation source  22  is inside and the radiation detector  24  is outside the fuselage  12 . 
         [0016]    As shown schematically in  FIG. 3 , A first manipulator  26  is provided for moving the radiation source  22  with respect to the fuselage  12 , and a second manipulator  28  is provided for moving the radiation detector  24  with respect to the fuselage  12 . The manipulators  26 ,  28  can be any type of device capable of producing the desired motion. This would include robotic devices, guide rail systems and the like. As shown in  FIG. 1 , the first manipulator  26  comprises an articulated boom  30  mounted to a carrier vehicle  32 . The boom  30  may be of a known type in which multiple-axis movement of the various members is provided by hydraulic actuators (not shown). In the illustrated example, the second manipulator  28  comprises a base  34  mounted on a rail  36  which extends parallel to the longitudinal axis of the fuselage  12 . The radiation detector  24  is attached to the base  34  with a manipulator arm  38 . The base  34  can be driven forward and aft along the rail  36  by an electric motor (not shown), and the manipulator arm  38  is able to move the radiation detector  24  in one or more axes to position it as required. It is noted that the present invention is equally suitable for use with any other kind of radiographic inspection system and does not require the specific arrangement of manipulators described above. 
         [0017]    The radiation source  22  may be 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  22  provides a large cone-shaped or panoramic volume radiation flux, but may be collimated to limit this to a specific region of interest. 
         [0018]    The radiation detector  24  can be any means that is capable of processing radiation emitted by the radiation source  22  into a viewable image. Although X-ray film could possibly be used, it is preferred that the radiation detector  24  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]    As shown in  FIG. 4 , the radiation detector  24  is divided into a two-dimensional array of individual detector elements  40 . It is noted that the number and size of the detector elements  40  are exaggerated for purposes of illustration. The output signal of each detector element  40  is provided within a range. An analog signal may be used, but more commonly the output signal would be digital data representing a discrete step value. For example, the output may be an integer value from 0 to 16,000 (or other suitable maximum value), with 0 representing no flux impinging on the detector element  40 , and 16,000 representing the maximum detectable flux (i.e. saturation of the detector element  40 ). To produce preferred-quality images, it is desired that the flux striking the detector element  40 , and thus its output, be in a smaller range. While the preferred range will vary with the specific application, an example of a preferred output range would be about 5000 to about 7000. 
         [0020]    As shown schematically in  FIG. 3 , the image data signals output by the radiation detector  24  are fed to a controller  42 , which can be a conventional computer unit. The controller  42  processes these signals, as described in more detail below, and may optionally cause corresponding images to be displayed on a display  44 . 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  42 . The controller  42  also controls the operation of the radiation source  22 , turning it on and off and regulating the voltage applied, and the manipulators  26  and  28 . 
         [0021]    In operation, the first and second manipulators  24  and  26  are controlled to move the radiation source  22  into alignment with a region of interest (ROI)  46 , e.g. a geometric area of selected size and shape, on the fuselage  12 , as shown in  FIG. 5 , and with the radiation detector  24  so that the detector  18  will be exposed to radiation from the radiation source  22 , under the direction of the controller  42 . 
         [0022]    Once the radiation source  22  and the radiation detector  24  are aligned with the region of interest  46 , the radiation source  22  is then turned on so that the region of interest  46  is illuminated with radiation at an initial flux level. Radiation emitted by the radiation source  22  passes through the fuselage wall  14  and impinges on the radiation detector  24 . The radiation is converted into electrical signals that are fed to the controller  42 . These signals represent a test image. 
         [0023]    The test image is evaluated, for example using software running on the controller  42 , to determine if at least one image quality measurement is within acceptable predetermined limits by comparing the test image to a pre-selected image quality standard, such as the above-noted desired detector output range. One or more statistical methods may be used to compare the test image to the standard. For example, as shown in  FIG. 5 , the region of interest  46  encloses a portion of the skin  20 , which is generally of uniform thickness, as well as portion of a stringer  18  and a frame  16 , both of which are substantially thicker (as measured in a radial direction) than the skin  20 . At any given flux level, more flux will be absorbed by the stringer  18  and the frame  16  than the skin  20 , resulting in lower flux striking the detector element  40  and thus lower image density, in the detector elements  40  that are aligned with the thicker objects. This will reduce the mean image density within the region of interest  46  by a statistically significant amount, and depending on the relative dimensions of the various components, may reduce the mean image density to below the desired range. 
         [0024]    In response to the image quality measurement exceeding the predetermined limits, the controller  42  automatically repeats the exposure with different parameters, for example, a different source flux intensity, exposure time, collimation, or source-to-detector distance, and again evaluates the image. This iteration continues until a test image meets the predetermined standard. A final set of imaging parameters, as well and the X-Y-Z location in space of the region of interest  46 , may be stored in the controller  42  or other storage device for use in subsequent evaluations. The region of interest  46  may then be imaged and the image stored for human-readable display or computerized evaluation. For example, in  FIG. 5  a defect such as a small crack  48  in the skin  20  may be observed and evaluated in the final image. It is expected that any defects in the region of interest  46  would be sufficiently small compared to the larger structures such as the stringers  18  so as not to significantly affect the image density mean or other image quality measurement. 
         [0025]    Many statistics and image processing methodologies can be applied to the test image and used to determine if the exposure is appropriate, such as the above-noted image density mean; range, standard deviation, image segmentation, or histogram. 
         [0026]    The above-described method is highly useful for evaluating objects without a-priori information of their structure. However, to the extent such information is available, it may be used in combination with the method described herein to improve inspection efficiency. In a particular model of airframe or other object with a known interior construction, the initial imaging parameters can be adjusted to accommodate those structures. For example, if the location of stringers, ribs, or other thick structural elements is known, a higher initial flux level may be used when the region of interest  46  is aligned with those elements. This would reduce the number of iterations needed to arrive at a final set of imaging parameters. 
         [0027]    Furthermore, the method described herein may be used to build a database of information about a particular structure to enhance subsequent operations. As each region of interest  46  is evaluated using the iterative process described, the final imaging parameters may be stored and then used for later reference as initial or baseline parameters. 
         [0028]    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. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.