Abstract:
A method of inspecting an object including locating an object on a machine vision apparatus, attaching a light panel to the object to backlight a region of the object, obtaining an image of the region when backlit by the light panel and identifying a geometric property of the object from the image.

Description:
FIELD OF INVENTION 
       [0001]    This invention concerns a method and apparatus for use in machine vision. The invention has particular, but not exclusive, application to detecting an edge profile of an object through imaging of the object. 
       BACKGROUND 
       [0002]    US2011/0181715 A1 describes a back-lit vision machine comprising a planar backlight located in a base of a coordinate measuring machine (CMM) for illuminating objects placed on the base. Backlit images are acquired by a CCD camera, these images processed by a computer to measure the location of edges of the object. Backlighting of non-planar parts facilitates the identification of the edges of an object. 
         [0003]    For simple objects such apparatus detects edges of the object with the required accuracy. However, it has been found that for complex objects, such as objects comprising a series of overlapping walls or non-planar parts that cannot be uniformly illuminated by the backlight located in the base, edges of the object are not detected with the required accuracy. 
         [0004]    WO94/01716 discloses a light emitting panel assembly that may be used as backlighting in a camera system used in inspection. 
         [0005]    Stemmer Imaging Ltd provides Phlox High Bright LED backlight panels suitable for use in machine vision situations. 
       SUMMARY OF INVENTION 
       [0006]    According to a first aspect of the invention there is provided a method of inspecting an object comprising locating an object on a machine vision apparatus, attaching a light panel to the object to backlight a region of the object, obtaining an image of the region when backlit by the light panel and identifying a geometric property of the object from the image. 
         [0007]    In this way, the region of the object may be illuminated sufficiently in order that geometric properties, such as edges of the object, in this region can be accurately detected from the image. In particular, the light panel can be attached to the object to uniformly illuminate regions of the object which cannot be uniformly illuminated from below or by using lights, such as light boxes, mounted remote from the object. For example, it may not be possible to illuminate an object having features that face in significantly different directions using a single light box. The use of multiple light boxes generates gaps that may hinder the identification of the geometric properties, such as the edges of the object. The light panel of the invention when attached to the object may have a shape that substantially corresponds to a shape of the region backlit by the light panel. In this way, substantially uniform, and possibly continuous, backlighting may be achieved across the region of the object to be imaged. 
         [0008]    The light panel may comprise a flexible light panel that can be bent into a desired shape, such as a shape that substantially corresponds to a shape of the region backlit by the light panel. Attaching of the light panel to the object may bend the light panel into the desired shape. The light panel may be electroluminescent sheeting. It has been found that electroluminescent sheeting provides a uniformity of light that aids detection of geometric properties of the region. 
         [0009]    The light panel may be attached to the object using a fixture. The fixture may also attach the object in place in the machine vision apparatus. The fixture may hold the light in the desired shape. The fixture may be arranged such that attachment of the light panel to the object using the fixture results in the light panel being shaped to follow contours of the object, thus providing a suitable shaped light panel to provide uniform illumination over the region to be illuminated. 
         [0010]    The fixture may comprise a mounting to which the light panel can be releasably attached. For example, the mounting may be integrated into the fixture in such a manner that the mounting is intended to remain part of the fixture during normal use. This may allow the light panel to be replaced, when required. 
         [0011]    The fixture may comprise a means for supplying power to the light panel attached thereto. The means may be a power supply. Alternatively, the means may be electrical connections for connecting to a power supply. 
         [0012]    The fixture may comprise mounting formations for repeatably mounting the fixture to the object at a location defined by the formations. The location on the object may be defined in 6 degrees of freedom. The method may comprise repeatably mounting the fixture on the same or mounting the fixture on each of a set of nominally identical objects using the mounting formations, and inspecting the or each object when the fixture bearing the light panel is mounted thereon. In this way, each image is obtained under comparable conditions. 
         [0013]    The fixture may be arranged to attach the light panel to the object such that the light panel is spaced from the object, for example, a distance that is the order of millimetres, such as about 5,6,7,8,9,10 or more millimetres. This may ensure that the light panel remains out of focus when obtaining an image of the object such that dust settled on the light panel is not mistaken as an edge of the object. 
         [0014]    The method may comprise locating the or each of a set of nominally identical objects to the machine vision apparatus using the fixture. The method may comprise attaching the lighting panel to each of a set of nominally identical objects, wherein the lighting panel is attached at approximately the same position on each object. The fixture may comprise further mounting formations for mounting the object on the machine vision apparatus in a repeatable manner at a location defined by the mounting formations. The location on the machine vision apparatus may be defined in six degrees of freedom. The method may comprise repeatably locating the same or locating each of a set of nominally identical objects to the machine vision apparatus using the mounting formations on the fixture, and inspecting the or each object when the or each object is located at the defined location on the machine vision apparatus. In this way, each image is obtained under comparable conditions. 
         [0015]    The and/or the further mounting formations may comprise a kinematic mount to define the location on the object/machine vision apparatus. 
         [0016]    The light panel may be attached to the object so as to be located within the object. In this way, the light panel may illuminate a region that would otherwise fall within a shadow cast by the object if the object was backlit by a light located externally to the object. 
         [0017]    It will be understood that the region of the object that is illuminated may be a region that would otherwise fall within the umbra or penumbra of a shadow cast by the object if the object was backlit by a light located externally to the object. 
         [0018]    According to a second aspect of the invention there is provided a lighting unit for illuminating an object in a machine vision apparatus comprising a light panel and a fixture for attaching the light panel to the object such that the light panel backlights the object from a point of view of a camera of the machine vision apparatus. 
         [0019]    The fixture may comprise a mount to which the light panel can be releasably attached, the mount arranged such that, when the light panel is attached thereto and the fixture attaches the light panel to the object, the light panel backlights the object from a point of view of a camera of the machine vision apparatus. 
         [0020]    The fixture may be arranged to locate the light panel within the object to illuminate a region of the object that would otherwise fall within a shadow cast by the object if the object was backlit by a light located externally to the object. 
         [0021]    The light panel may be a flexible panel that can be bent into a desired shape. For example, the light panel may be flexible to the extent that the light panel can be bent into a shape that follows a contour of a region of the object to be backlit. The fixture may be arranged to hold the flexible light panel bent into the desired shape when the light panel is attached to the object using the fixture. 
         [0022]    The fixture may comprise mounting formations for repeatably mounting the fixture to the object at a location defined by the formations. The location on the object may be defined in 6 degrees of freedom. 
         [0023]    Additionally or alternatively, the fixture may comprise further mounting formations for mounting the object on the machine vision apparatus. The further mounting formations may be arranged for repeatably mounting the object at a location on the machine vision apparatus defined by the further mounting formations. The location on the machine vision apparatus may be defined in six degrees of freedom. The machine vision apparatus may comprise a coordinate positioning machine and the further mounting formations may be arranged for repeatably mounting the object to a bed of the coordinate positioning machine. 
         [0024]    The and/or further mounting formations may comprise a kinematic mount to define the location on the object/machine vision apparatus. 
         [0025]    The fixture may comprise a power supply for the light panel. The fixture may comprise connections for connecting the light panel to a power supply. 
         [0026]    The light panel may be an electroluminescent sheet. Alternatively, the light panel may be a string of LEDs, such as the light string as described in U.S. Pat. No. 7,048,413, which is incorporated herein by reference, and sold as LED Neon Flex™. The string of LEDs may be contained in a scattering body for scattering the light emitted by the LEDs. A cladding may enclose the scattering body and the string of LEDs, the cladding having an arcuate top face for emission of light emitted by the LEDs. 
         [0027]    According to a third aspect of the invention there is provided a fixture for attaching a light panel to an object located in a machine vision apparatus, the fixture comprising a mount to which a light panel can be attached, the mount arranged such that when the light panel is attached thereto and the fixture attached to the object located in the machine vision apparatus, the light panel backlights the object from a point of view of a camera of the machine vision apparatus. 
         [0028]    According to a fourth aspect of the invention there is provided a machine vision apparatus in combination with an object to be scanned using the machine vision apparatus, comprising a light panel attached to the object to backlight a region of the object from a point of view of a camera of the machine vision apparatus. 
         [0029]    The region may be a region that would otherwise fall within a shadow cast by the object if the object was backlit by a light located externally to the object. 
         [0030]    The light panel may be a flexible light panel that is attached to the object in a bent shape. 
         [0031]    According to a fifth aspect of the invention there is provided a method of inspecting an object comprising locating an object on a machine vision apparatus, locating a light panel within the object to backlight a region of the object, obtaining an image of the region when backlit by the light panel and identifying a geometric property of the object from the image. 
         [0032]    According to a sixth aspect of the invention there is provided a machine vision apparatus in combination with an object to be scanned using the machine vision apparatus, comprising a light panel located within the object to backlight a region of the object. 
         [0033]    According to a further aspect of the invention there is provided a method of inspecting an object comprising locating an object in a machine vision apparatus, fixing a flexible light panel in the machine vision apparatus such that the flexible light panel is held bent in a required configuration for backlighting a region of the object, obtaining an image of the region when backlit by the flexible light panel and identifying a geometric property of the object from the image. 
         [0034]    The flexible light panel may be held in a bent shape that substantially corresponds to a shape of the region of the object to be backlit. 
         [0035]    The flexible light panel may be fixed in the bent shape through attachment to the object and/or to a bed of the machine vision apparatus. 
         [0036]    According to another aspect of the invention there is provided a lighting unit for illuminating an object in a machine vision apparatus comprising a flexible light panel and a fixture for fixing the flexible light panel in the machine vision apparatus such that the flexible light panel is held bent in a required configuration for backlighting a region of the object from a point of view of a camera of the machine vision apparatus. 
         [0037]    According to yet another aspect of the invention there is provided a machine vision apparatus in combination with an object to be scanned using the machine vision apparatus, comprising a flexible light panel fixed in the machine vision apparatus such that the flexible light panel is held bent in a required configuration for backlighting a region of the object from a point of view of a camera of the machine vision apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1  is a schematic cross-sectional view of a machine vision apparatus according to one embodiment of the invention scanning an aircraft engine combustor; 
           [0039]      FIG. 2  is a schematic plan view of the machine vision apparatus shown in  FIG. 1 ; 
           [0040]      FIG. 3  is a plan view of a connecting member for attaching a lighting panel to an object in accordance with one embodiment of the invention; 
           [0041]      FIG. 4  is a perspective view of a connecting member for attaching a lighting panel to an object in accordance with another embodiment of the invention; 
           [0042]      FIG. 5  is a plan view of the connecting member shown in  FIG. 4 ; 
           [0043]      FIG. 6  is a perspective view of a connecting member for attaching a lighting panel to an object in accordance with yet another embodiment of the invention; 
           [0044]      FIG. 7  is a cross-sectional view of the connecting member shown in  FIG. 6  connected to an object with a wall at an oblique angle to the vertical; 
           [0045]      FIG. 8  is a schematic cross-sectional view of a machine vision apparatus according to another embodiment of the invention scanning an aircraft engine combustor; and 
           [0046]      FIG. 9  is a schematic cross-sectional view of a machine vision apparatus according to another embodiment of the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0047]    Referring to the Figures, a machine vision apparatus  1  is a bridge-type (Cartesian) coordinate measuring machine comprises arms  2  for supporting a quill  3 , the arms  2  mounted on a base, such as a solid, granite base  7 . The base  7  provides a bed  11  on which object  6  can be located. Mounted to the end of the quill  3  is a rotary head  4  carrying a camera  5 , in this embodiment a CCD camera. It is also possible to use a CMOS image sensor. The quill  3  can be moved around the object  6  with the rotary head  4  adjusting the position of the camera  5  in order to capture images of the object. 
         [0048]    The object  6  is located on the base  7 . A fixture  8  attaches one or more light panels, such as one or more electroluminescent sheets  10 , to the object  6  via a plurality of connecting members, four of which are shown as  17   a ,  17   b ,  17   c  and  17   d  The fixture  8  and the light panel  10  together form a lighting unit. 
         [0049]    Referring to  FIG. 3 , each connecting member  17   a ,  17   b ,  17   c ,  17   d  comprises a first member  30  that is slidably connected with second member  31  such that the length of the connecting member  17  can be adjusted. In this embodiment, the first and second members  30 ,  31  comprise tubular members that connect in a telescopic manner. A spring or like element (not shown) biases the members  30 ,  31  apart such that the connecting member  17  can be located within the object  6  between two walls  15 ,  18  of the object and is held in place by the biasing pushing ends  32 ,  33  of the connecting member against the walls  15 ,  18 . The ends  32 ,  33  comprise a compliant material, for example rubber, to grip the walls  15 ,  18  and to prevent engagement of the connecting member  17  with the object  6  damaging the object  6 . The larger diameter tubular member  30  comprises resilient grips  34 ,  35  in the form of a pair of resilient rings that are angled towards each other to engage at their outer peripheries. Edges of light panels  10  are inserted between the grips  34 ,  35  such that the light panels can be held in place on the object by the connecting members  17 . 
         [0050]    When attached to the object  6  the light panel(s)  10  is/are bent into a shape that substantially corresponds to a shape of at least a region of the object  6  to be imaged. In this embodiment, the light panel  10  follows the curvature of an external wall  15  of the object  6  and is spaced approximately the same distance from the external wall  15  along the region to be illuminated (in this embodiment, along the entire length of the external wall  15 ). 
         [0051]    The light panel  10  comprises electrical connections (not shown) for connecting the light panel  10  to a power source, such as a battery pack  16 . 
         [0052]    In use, an image is obtained using the CCD camera  5  of a region of the object  6  by locating the object  6  on the table  11  of the CMM and attaching a light panel  10  bent into the required shape to the object  6  such that the light panel  10  is located within the object  6 . In this position, the light panel  10  can backlight the external wall  15  of the object  6 , which otherwise would fall within a shadow cast by the object  6  if the object was backlit by a light located externally to the object  6 , i.e. outside the external wall  15 . The CCD camera  5  is then moved around the outside of the object  6  whilst capturing images of the external wall  15  whilst it is backlit by the light panel  10 . These images can then be passed to appropriate software, which is known in the art, to identify edges, such as edges  15   a , of the object  6 . 
         [0053]    Use of this method may increase the accuracy in which edges of the object  6  are detected as the edges to be identified by the software are less likely to be obscured by shadows cast by the object  6 . 
         [0054]      FIGS. 4 and 5  show an alternative connecting member that may be used to attach the lighting panel  10  to an object  66  in the machine vision apparatus described above. In this embodiment, each connecting member  47  comprises three magnetic members  40 ,  41 , and  42 . A central magnetic member  40  is located between two outer magnetic members  41  and  42  with the lighting panel  10  located between members  40  and  42  and the object located between members  40  and  41  The magnetic attraction between the magnetic members  40 ,  41  and  42  hold the lighting panel  10  in place on the object  6 . The connecting member  47  is suitable for situations where there are not two adjacent walls between which a connecting member can be attached. In the embodiment shown, four connecting members  47   a ,  47   b ,  47   c  and  47   d  hold the lighting panel  10  in place on the object  66 . However, it will be understood it may not be necessary to hold the lighting panel  10  at the bottom and the lighting panel  10  may simple be allowed to hang loose. 
         [0055]    Referring now to  FIG. 6 , a further connecting member  57  is shown. This connecting member  57  comprises a hook shaped body  58  connected to resilient grip elements  54  and  55 . The lighting panel  10  is pushed into the gap between the resilient grip elements  54 ,  55 , the resilience of the elements holding the lighting panel  10  in place. It will be understood that other means for gripping the lighting panel could be used. For example, the gripping elements  54  and  55  may be relatively stiff elements that are biased towards each other by a spring or the like or the elements themselves may provide the biasing force, for example the elements could be a U-shaped metal clip, the ends of which are biased together by the resilience of the clip. 
         [0056]    To attach the lighting panel  10  in place on the object  66 , the lighting panel  10  is attached in the connecting members  57   a , and  57   b  and the hook shaped body  58  of each member  57   a ,  57   b  hooked over the top of the object  66 . In  FIG. 6 , the lighting panel  10  is shown illuminating an object  66  with a vertical wall, the lighting panel  10  hanging from the connecting members  57   a  and  57   b . However, for an object with non-vertical walls, such as that shown in  FIG. 7 , it may be necessary to provide a spacer  59  for maintaining the required spacing between the wall  75  and the lighting panel  10 . A spacer may also be used with the connecting member shown in  FIGS. 4 and 5 , if the lighting panel is allowed to hang loose at the bottom. 
         [0057]      FIG. 8  shows another embodiment of the invention wherein a fixture  8  comprises connecting members  19  attached to a support  9  that bears the lighting panel  10 . Connecting members connect the lighting panel  10  via the support  9  to the object  6 . A length of each connecting members  19  may be adjustable to clamp the connecting member  19  in place on walls  15 ,  18  of the object  6 . In one embodiment, cooperating formations on the object and formations on the connecting members allow for a kinematic mounting of the connecting members to the object  6 . The bottom of the support  9  has a mounting formation  12  that can cooperate with one or more inserts  13  received in slots  14  in the bed  11  of the machine vision apparatus for attaching the support  9  in place on the bed  11 . The mounting formation and insert  12 ,  13  may be of the type described in WO2010/038009, wherein the support member  9 , rather than a tool setter, can be kinematically mounted to the insert  13  located in the slots  14 . 
         [0058]    Referring to  FIG. 9 , a further embodiment is shown in which a flexible light panel  110  backlights a curved region of object  106 . Like reference numerals but in the series  100  have been used to identify features of this embodiment that are the same or similar to features of the previously described embodiments. 
         [0059]    The flexible light panel  110  is bent into the required shape and attached to support  109  of fixture  108  such that it is held in the bent shape. The light panel  110  is attached to the support  109  by suitable fixing devices (not shown), such as magnetic fixing devices. To locate the bent light panel in the machine vision apparatus, the fixture  108  is attached to the base  107  of the machine vision apparatus, in the manner hereinbefore described with reference to previous embodiments, and is not attached to the object  106 . The light panel  110  is located such that it backlights the object  106  in a manner suitable for measurement. Such an arrangement may be appropriate when the shape of the object makes it inappropriate to attach the light panel to the object. 
         [0060]    It will be understood that modifications and alterations can be made to the described embodiment without departing from the scope of the invention as defined herein. For example, the light panel may be integrally connected with the fixture. The invention is not limited to the light panel being located within the object but may also include attaching a light panel to the object such that the light panel is located outside of the object. Such an arrangement may have advantages when the object has a complex shape that cannot be illuminated adequately with a light source mounted remote from the machine vision apparatus.