Abstract:
A first borescope for viewing an interior surface of a cylindrical article has an image conducting tube with a beamsplitter cube adjacent a distal end of the image conducting tube. When the article allows light to pass through it, the borescope has a light source effective to provide light illuminating the inner surface from an opposing second side of the beamsplitter cube. A second borescope, useful when the article does not permit light to pass through has an image conducting tube with a reflector. A plurality of optical fibers form a light conduit mounted to optics effective to transmit light from a proximal end of the image conducting tube to the distal end, whereby the light exits through an annulus at the distal end.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This patent application claims a benefit to the filing date of U.S. Provisional Patent Application Ser. No. 62/113,709 that was filed on Feb. 9, 2015 and is titled, “Automated Stent Inspection System.” The disclosure of U.S. 62/113,709 is incorporated by reference herein in its entirely. 
       BACKGROUND 
       [0002]    (1) Field of the Disclosure 
         [0003]    Disclosed herein is an optical inspection system utilizing a borescope effective to image the inner bore (or inside diameter) of a part under inspection. More particularly, various embodiments disclose systems to provide uniform lighting and fixed magnification to facilitate use of a computer-based vision system. 
         [0004]    (2) Description of Related Art 
         [0005]    A borescope is an optical device having a rigid or flexible tube with an eyepiece or video screen at one end and objective lens at the other end. An optical relay, that may be a series of lenses for a rigid tube and optical fibers for a flexible tube, conducts an image viewed at the objective lens to the eyepiece. Representative borescopes are disclosed in U.S. Pat. No. 6,333,812, “Borescope” by Rose et al. and in U.S. Pat. No. 9,074,868, “Automated Borescope Measurement Tip Accuracy Test,” by Bendall et al. Both U.S. Pat. No. 6,333,812 and U.S. Pat. No. 9,074,868 are incorporated by reference herein in their entirties. 
         [0006]    Borescopes are commonly used to assess the quality of inner surfaces of a wide variety of industrial components. Such an inner surface may be the inside diameter of a through-hole structure, such as a pipe or a stent, or a blind bore structure, such as a cartridge case. Whether an eyepiece or a video screen is used to view the image, a person is typically required to perform an analysis and determine the surface quality of a component under inspection. One particularly important class of parts that require such inspections are small precision cylindrical components. Medical stents and rifle barrels are two exemplary members of this class. When the cylindrical component has a relatively large inner diameter, it is easier and more practical to insert a traditional camera and lens fully within the cylinder. When the cylindrical component has a relatively small inside diameter, nominally 12 millimeters or less, a borescope is preferred. 
         [0007]    Rather than rely on an inspector&#39;s judgment, manufacturers of dimension critical components prefer to rely on the more consistent and reliable performance of a computer-based vision system to assure quality. However, current borescope inspection systems generally lack a means to automatically acquire and analyze a set of borescope generated images. Further, the lighting available with current borescopes generally creates too much glare and uneven illumination for machine vision algorithms to make measurements and find defects robustly. 
         [0008]    Disclosed herein are borescopes and inspection systems useful with computer-based vision systems that do not suffer the shortcomings of previous devices and systems. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0009]    In accordance with a first embodiment, there is provided a borescope having an image conducting tube with a beamsplitter cube adjacent a distal end of the image conducting tube. This borescope is configured to view an inner surface of an object disposed adjacent a first side of the beamsplitter cube. The borescope has a light source effective to provide light illuminating the inner surface from an opposing second side of the beamsplitter cube. 
         [0010]    In accordance with a second embodiment, there is provided a borescope configured to view an inner surface of an object under inspection. This borescope includes an image conducting tube with a reflector adjacent a distal end thereof and an outer tube circumscribing the image conducting tube. This outer tube is capable of independent rotation around the image conducting tube. The borescope further has a plurality of optical fibers forming a light conduit mounted to optics effective to transmit light from a proximal end of the image conducting tube to the distal end thereof, whereby the light exits through an annulus at the distal end. An input window of the light conduit is responsive in shape to collect light from the optical fibers and a motor is effective to rotate the outer tube, reflector and light conduit so as to acquire image data anywhere along 360 degrees of the inner diameter of the object. 
         [0011]    The boroscopes may be used in an inspection system for imaging an inner surface of an object where at least a portion of the object has general rotational symmetry. The inspection system includes a source of illumination, a fixture configured to support the object, a rotary stage configured to support the fixture such that rotation of the rotary stage rotates the object about a central cylindrical axis of that portion of the object that is generally rotationally symmetric. A first digital camera and lens are capable of imaging an exterior surface of the object. A borescope has a reflector at its distal end. This reflector redirects a field of view of the borescope to capture a view of the inner surface of the object by a second digital camera located at a proximal end of the borescope. A motion controller collects encoder signals from the rotary stage and using those encoder signals calculates a set of rotary positions at which to trigger the first and second digital cameras to acquire image data. A computer is programmed to receive and process the image data and is also capable of one or more of displaying and performing quality analysis of the processed image data. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view of a borescope for use with the inspection system described herein. 
           [0013]      FIG. 2  is a flow chart illustrating computer and motion control of the inspection system described herein. 
           [0014]      FIG. 3  is a perspective view of a first system to illuminate an inner surface of a work piece being inspected with the inspection system described herein. 
           [0015]      FIG. 4  is a perspective view of the inspection system described herein. 
           [0016]      FIG. 5  is a perspective view of a second system to illuminate an inner surface of a work piece being inspected with the inspection system described herein. 
           [0017]      FIG. 6  is a flow chart illustrating steps of operation for inspection by the systems disclosed herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    With reference to  FIG. 4 , the inspection system is particularly suitable for generally cylindrical objects  19  having an inner surface  102  and an outer surface  104 . By generally cylindrical, it is meant that at least a portion of the object  100  has rotational symmetry about central cylindrical axis  106 . The object may allow light to pass through it, for example by being transparent or translucent, or being a mesh type structure, such as a medical stent. The object may not allow light to pass through it, for example by being a solid metal cartridge case or fluid pipe. Whether or not light may pass through the object impacts the illumination system as discussed below. 
         [0019]    The object under inspection is held in a fixture and rotated about its central cylindrical axis by a motorized rotary stage. A borescope is inserted into the object and images are captured sequentially by a digital camera as the object is rotated. A ninety degree (“right angle”) turning prism is affixed to an end of the borescope so that it images the inner wall surface of the generally cylindrical object. An encoder on the rotary stage may be used to trigger the digital camera at appropriate intervals. Each 360 degrees of rotation will create a strip of “unrolled” image. To then capture additional images along the length of the cylinder, a linear motion stage can be used to move the rotary stage holding the fixture and object under inspection iteratively with respect to the borescope until it is fully imaged. 
         [0020]    In one preferred embodiment, the digital camera is a line scan camera and is aligned with the right angle prism enabling the camera to build up a line-by-line image of the inner surface. By choosing a line scan camera that acquires a thin line of part image parallel to the central axis of the cylinder, problems with imaging a curved object with a flat area camera sensor can be avoided. If a telecentric stop is placed between the set of relay lenses that comprise the main body of the borescope, the magnification of the taken image will be fixed. A fixed magnification supports better image-to-image strip alignment; especially important when the individual images taken at iterative steps along the x-axis need to be joined together to form a larger whole image that represents a full scan of the inner surface across 360 degrees. Slight rotational mechanical eccentricities of the holding fixture and the inherent lack of perfect cylindricality of typical real-world parts under inspection results in variable working distances of the part to the borescope. The telecentric stop avoids distortion artifacts that might otherwise be caused by changes in magnification. Furthermore machine vision analysis is most effective if the pixels being analyzed are all based on the same magnification. 
         [0021]    A uniform illumination approach is achieved by using a beamsplitter cube in place of a simple mirror arrangement and bringing light to the object under inspection from the opposite side of the beamsplitter cube. In embodiments where the cylindrical component under inspection is not fully opaque, such as a medical stent, placing the light source outside the part under inspection and shining light towards the surface being imaged through the beamsplitter cube can create a uniformly illuminated image. 
         [0022]    For a more common inspection requirement, where the object being inspected is a generally opaque cylindrical component, bright field illumination may be obtained by bringing light through fiber optics to the beamsplitter cube and driving that light into a light guide placed behind the beamsplitter cube. If the backside of the beamsplitter cube is rounded to conform to the shape of the borescope, a wider angle of bright field illumination coverage can be introduced. A configuration that brings light up and around the rounded beamsplitter cube as well as through the beamsplitter cube using either fiber optics or a clear silvered specially shaped optical manifold can achieve both bright field and dark field illumination in the same borescope. If a color camera is used and different colors of illumination are used for the bright field opposed to the dark field, then both types of image can be obtained and analyzed separately and simultaneously. 
         [0023]    For situations where it is preferable to maintain the part being inspected stationary and instead rotate the borescope&#39;s field-of-view to create the image strips, the fiber optics can be cleaved right before the prism or beamsplitter cube and light can be transmitted across a precision annular slip ring. If the prism is mounted also on the slip ring it can rotate. A tubular member that transmits torque can be slidably positioned over the entire borescope and used to rotate the reflecting optics and the remaining end of the fiber optics on the other side of the slip ring as a unit. This tubular member that rotates can be rigid or flexible depending on the type of borescope it surrounds. 
         [0024]      FIG. 1  illustrates a borescope  10  for use with the inspection system disclosed herein. The borescope  10  has an image conducting tube  70  populated with internal relay lenses (not visible) terminating at a distal end  72  and an opposing proximal end  74 . A beamsplitter cube  5  at the distal end  72  of the image conducting tube  70  redirects the view of the borescope  10  by ninety degrees. Brightfield illumination is provided by a light guide  9  that accepts light from optical fibers  7  and redirects that light  90  degrees up and through the beamsplitter cube  5 . The optical fibers  7  are channeled back away from beamsplitter cube  5  through a gap between an outer tube  1  and an inner tube  2  and exit the proximal end  74  of the borescope  70  where they are illuminated by a light source  13 . A digital camera ( 11 ) captures the image from the borescope  70 . The borescope  70  may be extended to any desired length with an addition of more internal relay lenses. 
         [0025]      FIG. 2  illustrates in flowchart representation interaction between a computer  80  with a user interface  97  and a motion controller  82  of the inspection system. The computer  80  controls the motion controller  82  to direct the motions required by an inspection protoccol. During operation, the motion controller  82  drives a linear stage  41  by wired control  83  to position an object under inspection in the field of view of a borescope. An encoder signal  84  from the linear stage  41  assures correct positioning. Once the linear stage  41  is correctly positioned, the motion controller  82  drives rotary stage  39  by wired control  86  to rotate the object under inspection about the borescope. The motion controller  82  monitors an encoder signal  88  from the rotary stage  39  and at appropriate intervals sends a trigger  90  to the borescope camera  11  to acquire a section of image. The borescope camera  11  provides digital image data  92  to the computer  80  to display to an operator or conduct a quality assessment of the object being imaged. If the borescope camera  11  is an area camera than there will be a set of passed individual image data sets  92  passed to the computer  80 . If the borescope camera  11  is a line camera, then the trigger signal  90  is sent to acquire each needed line to build a line-by-line digital image  92 , which is then sent to the computer  80 . If the inspection protocol calls for an image to be captured from an outer surface of the object under inspection this same process is repeated, except this time using an outer diameter camera  31 . 
         [0026]    Application software running on the computer  80  allows a user to interact with the inspection system via a user interface  97  and specify, axially and rotationally, what areas of the object to image. The software is further configured to stitch together multiple image data of an inner surface or an outer surface enabling the computer to display a single unrolled view of the inner bore of the object. 
         [0027]      FIG. 3  shows a first embodiment of the inspection system. This embodiment is useful to inspect the inner diameters of generally cylindrical parts  19  that allow light to pass through, such as a stent or transparent or translucent glass tube. The borescope  10  has an outer tube  1  that contains a train of internal lenses  21  that utilize the beamsplitter cube  5  to pass along an image of the inner diameter of the part  19  to the digital camera  11 . In this embodiment, the digital camera  11  is a line-scan type with a sensor  15  having a linear array of pixels. The beamsplitter cube  5  is placed at the distal end  72  of the image conducting tube  70  of the borescope  10  to align the linear sensor  15  with a linear field of view  17  such that as the part  19  is rotated around the borescope  10  a line-by-line image can be captured. To provide highly uniform diffuse illumination, a filter  27  diffuses the light provided by a light source  25  which then passes through the beamsplitter cube  5  and then onto the part  19 . A telecentric aperture stop  23  is placed in the optical train to provide a constant magnification of the part  19 . 
         [0028]      FIG. 4  shows the part  19  held by a fixture  37  and rotated around the borescope  10  by a rotary stage  39  mounted on a linear stage  41  that can reposition the part  19  axially. The image from the borescope  10  is captured by digital camera  11  mounted on a precision alignment stage  29 , that is mounted to a common base  43  for precision alignment of the borescope  10  in response to the travel of the linear stage  41 . A second digital camera  31  mounted on a focusing stage  35  can be used to view the outer diameter of the part  19  through a lens  33 . A precision Y-Z alignment stage  29  along with a tip-tilt adjustment  28  can align the borescope  10  with the X-Axis stage holding the rotary stage  39  to enable the borescope  10  to focus on and accommodate parts  19  of varying diameter and shape. 
         [0029]      FIG. 5  shows a second embodiment of the inspection system. A borescope  63  with internal optical fibers  59  that truncate at the distal end  72  of the image conducting tube  70  expelling light in the form of a circular annulus  51  at the distal end  72 . An acrylic plastic manifold  45  is generally silver coated except for an input annulus that is of similar size and held in close proximity to accept light from the fiber optic annulus  51 . Also not silvered are exit windows  47 , to direct light on to a part under inspection (not shown). A beamsplitter cube  5  provides a field of view of the inner diameter of the inspected part to the digital camera  11  and is sized in response to the manifold  45 . An outer tube  49  extends the length of and is sized to slip fit around the image conducting tube  70 . The beamsplitter cube  5  and the manifold  45  are together affixed to the outer tube  49  and rotated by a motor  57  with a hollow shaft  53 . A light emitting diode LED light source  13  provides light to the optical fibers  59 . 
         [0030]    Although the disclosed subject matter has been described and illustrated with respect to embodiments thereof, it should be understood by those skilled in the art that features of the disclosed embodiments can be combined, rearranged, etc., to produce additional embodiments within the scope of the invention, and that various other changes, omissions, and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.