Abstract:
There is described an online inspection method and system having an illumination system that provides bright-field and dark-field illumination concurrently or sequentially, at varying intensities, in order to acquire images that may be read by an image processing device. The image processing device may obtain measurements of features in the images and evaluate acceptability of the features.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 62/086,970, filed on Dec. 3, 2014 and entitled “Online Inspection for Composite Structures”, the contents of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the field of machine vision inspection and more particularly, to an online inspection/measurement system that is able to obtain measurements of features of a surface of a composite component in order to evaluate the acceptability of the features. 
       BACKGROUND OF THE ART 
       [0003]    Composite structures (or components) are generally made from two or more constituent materials with significantly different physical or chemical properties. When combined, they produce a structure with characteristics different from the individual materials, with the aim of using the benefit of both. 
         [0004]    When manufacturing composite structures using a manufacturing process such as Automated Fiber Placement (AFP) or Automated Tape Layering (ATL), inspecting the dimensional requirements of the manufactured structures is an important part of the manufacturing process. At least part of the inspection process is done on freshly laid fiber. It is very difficult to obtain high quality images from such surfaces due to the different directional reflectivity of the fibers. In addition, the features to be inspected are black details on black background, thus producing poor contrast. 
         [0005]    For these reasons, known methods for performing dimensional inspection involve gathering data via manual inspection using a hand-held laser tracker, and having an operator compare the measured data with theoretical data from a Computer-Aided Design (CAD) file. In the case of a composite structure having many plies, manual inspection of the fibers of each ply of the structure is extremely time consuming. Another shortcoming of manual inspection is that it is dependent on the hand and eye skills of the operator, which makes it harder to validate the inspection at a later time. 
         [0006]    There is therefore a need to improve the inspection phase of the manufacturing process for certain structures. 
       SUMMARY 
       [0007]    There is described an online inspection method and system having an illumination system that provides bright-field and dark-field illumination concurrently or sequentially, at varying intensities, in order to acquire images that may be read by an image processing device. The image processing device may obtain measurements of features in the images and evaluate acceptability of the features. 
         [0008]    In accordance with a first broad aspect, there is provided a system for online inspection of a composite structure manufactured by an automated tool. The system comprises an image acquisition device in proximity to a head of the tool above a surface of the composite structure and defining a field of view on the surface of the composite structure, the image acquisition device and the surface of the composite structure defining a bright-field illumination zone and a dark-field illumination zone; and an illumination system positioned above the surface of the composite structure. The illumination system comprises a top illumination light source inside the bright-field illumination zone, for projecting light at a first intensity onto the surface of the composite structure within the field of view that is reflected by the surface towards the image acquisition device; and a side illumination light source inside the dark-field illumination zone, for projecting light at a second intensity different from the first intensity onto the surface of the composite structure within the field of view that is reflected by the surface away from the image acquisition device. 
         [0009]    In some embodiments, the system further comprises an optical adapter positioned between the image acquisition device and the surface of the composite structure for spacing the image acquisition device from the surface and/or directing a vision axis of the image acquisition device substantially perpendicularly to the surface. In some embodiments, an enclosure houses the optical adapter and the illumination system and is coupled to the image acquisition device. 
         [0010]    In some embodiments, the image acquisition device comprises a lens having a long focal length F and a corresponding viewing distance D. In some embodiments, the long focal length F is at least five times longer than a diagonal of one of a film plane and a digital sensor of the image acquisition device. In some embodiments, the viewing distance D is at least twenty times longer than the diagonal of one of the film plane and the digital sensor of the image acquisition device. 
         [0011]    In some embodiments, at least one of the top illumination light source and the side illumination light source comprises an array of Light Emitting Diodes (LEDs). In some embodiments, the array of LEDs is at least one of curved and composed of LEDs of varying intensities. In some embodiments, the LEDS comprise flash LEDs. In some embodiments, at least one of the top illumination light source and the side illumination light source comprises a backlighting plate. 
         [0012]    In some embodiments, at least one of the top illumination light source and the side illumination light source projects colored light onto the surface. In some embodiments, the colored light is one of red-orange and blue. 
         [0013]    In some embodiments, the image acquisition device comprises a shutter time synchronized with a response time of the illumination system. 
         [0014]    In accordance with another broad aspect, there is provided a method for online inspection of a composite structure manufactured by an automated tool. The method comprises illuminating a surface of the composite structure by projecting light at a first intensity onto the surface of the composite structure from a top illumination light source and projecting light at a second intensity different from the first intensity onto the surface of the composite structure from a side illumination light source. The method also comprises acquiring an image of the illuminated surface of the composite structure using an image acquisition device in proximity to a head of the automated tool, the image acquisition device and the surface of the composite structure defining a bright-field illumination zone comprising the top illumination light source and a dark-field illumination zone comprising the side illumination light source. 
         [0015]    In some embodiments, the method further comprises directing a vision axis of the image acquisition device substantially perpendicularly to the surface with an optical adapter. The optical adapter may also be used to space the image acquisition device from the surface. In some embodiments, the optical adapter and the illumination system are comprised in an enclosure coupled to the image acquisition device. 
         [0016]    In some embodiments, acquiring an image comprises acquiring the image with a long focal length F and a corresponding viewing distance D. In some embodiments, the long focal length F is at least five times longer than a diagonal of one of a film plane and a digital sensor of the image acquisition device. In some embodiments, the viewing distance D is at least twenty times longer than the diagonal of one of the film plane and the digital sensor of the image acquisition device. 
         [0017]    In some embodiments, at least one of the top illumination light source and the side illumination light source comprises an array of Light Emitting Diodes (LEDs). In some embodiments, the array of LEDs is at least one of curved and composed of LEDs of varying intensities. In some embodiments, the LEDS comprise flash LEDs. In some embodiments, at least one of the top illumination light source and the side illumination light source comprises a backlighting plate. 
         [0018]    In some embodiments, illuminating the surface of the composite component comprises projecting colored light onto the surface. In some embodiments, the colored light is one of red-orange and blue. 
         [0019]    In some embodiments, acquiring at least one image comprises acquiring a sequence of multiple images, each one with a different and customized illumination strategy applied in accordance with a specific local morphology of the surface of the composite structure. 
         [0020]    In some embodiments, illuminating the surface of the composite structure comprises projecting light at the first intensity and projecting light at the second intensity in a sequential manner. 
         [0021]    In this specification, a lens said to have a “long focal length” is intended to mean that it magnifies the image of the subject, such as a telephoto lens or a super telephoto lens. As the focal length of the lens increases, the depth of field gets shallower (for a same viewing distance) and the angle of view is narrower. As such, the term long focal length refers to the relationship between the absolute focal length of the lens and the diagonal of the film image. In some embodiments, the long focal length is provided from about 70 mm to about 300 mm. In some embodiments, the long focal length is provided from about 135 mm to about 300 mm. In some embodiments, the long focal length comprises a focal length greater than or equal to about 70 mm. In some embodiments, the long focal length is at least five times longer than the diagonal of a sensor provided in the image acquisition device. In some embodiments, a viewing distance for a long focal length is at least twenty times longer than the diagonal of the sensor provided in the image acquisition device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
           [0023]      FIG. 1  is a schematic diagram of an exemplary system for online inspection of a composite structure manufactured by an automated tool; 
           [0024]      FIG. 2 a    is an exemplary image of the surface of a composite component captured while the surface is illuminated with only left side dark-field illumination; 
           [0025]      FIG. 2 b    is an exemplary image the surface of the composite component captured while the surface is illuminated with left side dark-field illumination and top side bright-field illumination; 
           [0026]      FIG. 2 c    is an exemplary image of the surface of the composite structure with a red-orange light illumination from the left side; 
           [0027]      FIG. 2 d    is an exemplary image of the surface of the composite structure with a red-orange light illumination from the top side; 
           [0028]      FIG. 3  is an exemplary embodiment of a light guide used as an illumination source; 
           [0029]      FIG. 4  is an exemplary embodiment of a system for online inspection having an optical adapter; 
           [0030]      FIG. 5 a    is an exemplary image of the surface of a composite component without any pre-processing; 
           [0031]      FIG. 5 b    is the image of  FIG. 5 a    with standard pre-processing; 
           [0032]      FIG. 5 c    is the image of  FIG. 5 a    with color inversion pre-processing; and 
           [0033]      FIG. 6  is a flowchart of an exemplary method for the image acquisition phase of the online inspection of a composite structure manufactured by an automated tool. 
       
    
    
       [0034]    It will be noted that throughout the appended drawings; like features are identified by like reference numerals. 
       DETAILED DESCRIPTION 
       [0035]    A method and system for online inspection of a composite structure manufactured by an automated tool will be described. For illustrative purposes, the tool described is an Automated Fiber Placement (AFP) machine but other automated manufacturing tools, such as Automated Tape Layering (ATL) machines, may be used. In order to manufacture a composite structure using AFP, fiber strips (tows) are laid along a mold in multiple layers in order to create a composite structure having the shape of the mold. The fiber strips are placed along the mold in accordance with fiber laying trajectories that are input into the AFP machine to create a given structure in accordance with a set of design parameters. Some of the features that may be inspected include, but are not limited to, fiber gaps, fiber overlap, angle deviations, debris, and tow positions. 
         [0036]    The composite structure may comprise various materials, such as but not limited to cements, concrete, reinforced plastics, metal composites and ceramic composites. For example, the composite structure may be composed of composite fiber-reinforced plastics. The composite structure may be used for various applications, including but not limited to buildings, bridges, space crafts, aircrafts, watercrafts, land vehicles including railway vehicles, and structures such as wind turbine blades, swimming pool panels, bathtubs, storage tanks, and counter tops. 
         [0037]    The system for online inspection of a composite structure manufactured by an automated tool generally comprises an image acquisition device  100  and an illumination system  101 , as illustrated in  FIG. 1 . The image acquisition device  100  may be any instrument that records images that can be stored directly, transmitted to another location, or both. For example, the image acquisition device  100  may be a video camera, a still camera, a digital camera, an image sensor, a CCD sensor, a CMOS sensor, and an active pixel sensor, among other possibilities. 
         [0038]    Images are processed by an image processor (not shown) to perform inspection in real time or substantially real time, i.e. as each layer is being laid upon the mold to form the composite structure. The image acquisition device  100  is mounted to a head of the automated tool  103  and defines a field of view (FOV) on a surface of the composite structure  106 . Also defined by the image acquisition device  100  and the surface of the composite structure  106  are a bright-field illumination zone  105  and a dark-field illumination zone  107 . The bright-field illumination zone  105  is the area above the composite structure surface  106  where light emitted from a light source and projected within the FOV will be reflected into the image acquisition device  100 . The dark-field illumination zone  107  is the area above the composite structure surface  106  where light emitted from a light source and projected within the FOV will be reflected away from the image acquisition device  100 . One or more top illumination light sources  102  is provided inside the bright-field illumination zone  105 . One or more side illumination light source  104  is provided inside the dark-field illumination zone  107 . In some embodiments, four side illumination lights sources  104  are provided, namely a front side source, a back side source, a left side source, and a right side source. 
         [0039]    The illumination system  101  is thus a combination of dark-field light and bright-field light and is provided in order to reveal sufficient details available on the surface  106  of the composite structure to ensure that an image captured by the image acquisition device  100  may contain enough information to be successfully processed by an image processor. The side illumination light source  104  is provided at a first intensity and is used to create high contrast images from shadows, i.e. highlight the surface details. However, too much shadow prevents accurate measurement from the image processor. While a human operator may be able to distinguish more easily the defects in a high contrast image, excess shadow may be problematic for an image processor. The top illumination light source  102  is thus provided at a second intensity different from the first intensity to fill the edges of the shadows created by the side illumination light source  104  so that these edges are not mistaken by the image processor as real features. 
         [0040]      FIGS. 2 a  and 2 b    illustrate the difference between having only dark-field illumination and having a combination of bright-field and dark-field illumination.  FIG. 2 a    shows an image of a surface illuminated only with a left side illumination light source  104  and no top illumination light.  FIG. 2 b    shows an image of the same surface illuminated with a left side illumination light source  104  and a top illumination light source  102 . Using the right proportion of dark field and bright field illumination, the shadow edges seen in  FIG. 2 a    are reduced or substantially eliminated while preserving the high contrast of the image. The illumination system  101  is thus conceived specifically for an online system whereby images are processed in real-time and automatically. 
         [0041]    The illumination system  101  may comprise a diffuser in order to create non-coherent light and thus avoid noise introduced by sparkling reflections over the fiber details (speckles) and strong casted shadows. The diffuser may be provided for the top illumination light source  102  and/or the side illumination light source  104 . In order to increase the depth of field; the image acquisition device  100  may be provided with an aperture that closes a large portion of the optical field of the lens. A fast shutter speed (with a short opening time) may be used to ensure a short exposure time, so that the image is not blurred, as images are taken while the surface is moving. A light source with a very fast response time may be used to allow precise synchronization with the shutter speed of the image acquisition device  100 . In addition, the settings for the image acquisition device  100 , such as the shutter speed and the aperture size; may be different from image to image and may change in real time. The automatic adjustment of settings allows the quality of the images captured by the image acquisition device to be consistent while capturing images of different surface particularities, such that they may be inspected in an automated mode in real time while the surface is moving. 
         [0042]    In some embodiments, the top illumination source  102  and/or the side illumination source  104  comprises a light guide  300  as illustrated in  FIG. 3 . The light guide  300  may be composed of a prism and multiple mirrors. An LED (Light Emitting Diode) light source  302  is provided on at least one side edge of the light guide  300 . A primary scattering mirror  304  is providing along a top edge of the light guide  300  while a secondary scattering mirror  306  is provided along a bottom edge thereof. The light guide can be a prism as illustrated or a long bundle made of fiber optics. An exit scattering window  308  allows scattered (i.e. very diffuse light) light to exit from the bottom edge of the light guide  300 . Alternatively or in combination with the light guide  300 , the top illumination source  102  and/or the side illumination source  104  comprises an LED array illumination plate. The plate may be straight or curved to widen the field of view. The LEDs in the array may also be modulated in intensity in order to provide an even illumination field. In other words, each LED in the array may have its intensity set independently to obtain the desired illumination field. Alternatively or in combination with the light guide  300 , and/or the LED array illumination plate, the top illumination source  102  and/or the side illumination source  104  comprises a liquid crystal display (LCD) back illumination system, referred to herein as a backlighting plate. A backlighting plate may thus provide a very even illumination field with a high level of diffusibility, and include the side illumination source  104  and the light guide in one compact feature. The backlighting plate may also be curved and/or have individual nano-imprinted micro lenses regulated to a desired intensity level. 
         [0043]    In some embodiments, the intensity of the light sources (side and/or top) may be too high to provide continuous illumination as this may cause the surface  106  to burn or be cured. Flash LED illumination may be used. Such light sources have a very short response time and also emit “cold” light. The flash LED illumination intensity is usually at least one order of magnitude higher than of a continuous mode LED illumination, for the short time provided by the shutter, thus allowing for a brighter or more intense illumination system  101 . In addition, the small dimension of flash LEDs allows installation in a dense array, thereby achieving very even light emissivity from the light source. 
         [0044]    In some embodiments, the top illumination source  102  and/or the side illumination source  104  may be configured to project colored light onto the surface  106 . Colors may be used to distinguish between layers of the composite structure. For example, when illuminating the surface  106  with a light frequency in the red-orange portion of the visible light spectrum, light absorption is very high and strongly dependent on fiber orientation. Different plies of the structure may thus be revealed using red-orange light using fiber orientation. The last ply of the ply lay-up may be visually “detached” from the previous ply using red-orange light.  FIGS. 2 c  and 2 d    are exemplary embodiments of the composite structure imaged with a red-orange light. In  FIG. 2 c   , the red-orange light is provided only from the side illumination source  104  while in  FIG. 2 d   , the red-orange illumination is provided only from the top illumination source  102 . As shown, in both cases, the last ply is clearly distinguished from the previous ply. 
         [0045]    The red-orange light may be provided at a wavelength of about 625 nm to about 775 nm. In some embodiments, orange light at a wavelength of about 590 nm may be used. In some embodiments, red light at a wavelength of about 650 nm may be used. In some embodiments, red-orange light at a wavelength of about 621 nm may be used. In another example, blue light can be used to create high contrast images due to the high reflectivity of the AFP surface. Blue light may be provided at a wavelength of about 425 nm to about 490 nm. In some embodiments, blue light may be provided at a wavelength of about 475 nm. Other colors may also be used, as a function of a desired reflectivity and/or absorption of the light on the material. Color may be chosen based on the material of the composite structure, and/or the color of the material, and/or based on the desired purpose of the lighting and image acquisition, i.e. to distinguish between plies or to highlight certain details on the surface of the structure. Colored light may be used in combination with white light in order to create a desired effect. 
         [0046]    In some embodiments, the top illumination source  102  and/or the side illumination source  104  may be configured to project infrared light onto the surface  106 . When the surface is pre-heated by a heat lamp to improve tackiness, the last ply of carbon is hot from the compaction roller. A camera can be set to acquire images in the infrared domain in order to distinguish the hot layer from the background, similarly to the embodiment with red-orange illumination. 
         [0047]    As shown in  FIG. 1 , the surface  106  of the composite structure may be curved, thus introducing parallax errors into the images. Parallax errors may be minimized by having a small field of vision, a vision direction that is perpendicular to the surface, a small angle of vision (or long vision distance), and/or inspection performed very close to the surface. When imaging the surface of a composite structure manufactured by AFP, the vision field cannot be reduced beyond a certain size imposed by the ply lay-up geometry. The free space close to the compaction roller is limited, thus complicating installation of the camera near the surface of the composite structure and with the vision direction perpendicular to the surface. In addition, a long vision distance also makes installing the camera in proximity to the compaction roller a challenge. As a result, in some embodiments, the online inspection system is provided with an optical adapter positioned between the image acquisition device and the surface of the composite structure for spacing the image acquisition device from the surface and directing a vision axis of the image acquisition device substantially perpendicularly to the surface. The image acquisition device may thus be mounted to the head of the tool and a long focal length is used for the image acquisition device. 
         [0048]      FIG. 4  is a side view of an exemplary embodiment of an online inspection system  400  having a long focal length and an optical adapter. A camera  408  provided with a lens having a long focal length is mounted to the AFP lay-up head  402 . The optical adapter  412  spaces the camera  408  from the surface  406  and directs a vision axis of the camera  408  substantially perpendicularly to the surface  406 . The spacing of the camera  408  from the surface  406  also allows room for the illumination system  410  below the camera  408  and above the surface  406 . The illumination system  410  and the optical adapter  412  may be provided in a single housing  409 . In some embodiments, the housing may be a darkroom enclosure, in order to block out all ambient light and allow for better control of light projected onto the surface  406  of the composite structure. Alternatively, the housing  309  may enclose only the illumination system  410 , and the optical adapter  412  may be provided separately therefrom. 
         [0049]    The optical adapter  412  may be composed of one or more optical elements for directing the vision axis of the image acquisition device perpendicularly to the surface being imaged. For example, the optical adapter  412  may be composed of a half-penta prism or a pair of mirrors. Other optical elements may also be used. 
         [0050]    In some embodiments, the online inspection system  400  is provided with a long vision distance, a short shutter time, a closed aperture, high intensity illumination, very diffuse illumination, no light coherency or very low light coherency of the light beam, a shallow angle illumination very close to the surface, a top side illumination in conjunction with a shallow angle illumination, a flash illumination light source having a very fast response, and light emitted at a specific wavelength to detach the last ply from the background. While it may seem that diffuse light sources are readily available, the level of diffusibility required by the composite surface is much higher than what is currently available. This is due to the very small dimension of the fibers, which can easily create speckle if the illumination light is even only a little bit coherent. 
         [0051]    In some embodiments, color inversion is used to enhance a contrast between the dark features of the composite structure and the dark background. Color inversion facilitates filtering of the background of the image after feature extraction without losing useful information. Color inversion may be particularly useful when the features themselves are black and the background is black, thus having very low contrast.  FIG. 5 a    is an example of an original image as captured by an image acquisition device of an online inspection system. In this example, the objective is to detect gaps in the surface of the composite structure.  FIG. 5 b    is an example of the original image after standard preprocessing typically applied to the image. As shown, many features can be seen on the image, not only the gaps.  FIG. 5 c    is an example of the original image with color inversion. The gaps  502  are clearly shown without additional features in the image. The level of filtering may be automated since the feature to background ratio can be optimized in a feedback loop of the processor. 
         [0052]      FIG. 6  is a flowchart of an exemplary embodiment of a method for online inspection of a composite structure manufactured by an automated tool. The method comprises a first step  600  of illuminating the surface of the composite structure followed by a step  606  of acquiring at least one image of the illuminated surface. The illumination step  600  may be composed of a step  602  of projecting bright-field illumination onto the surface of the composite component and a step  604  of projecting dark-field illumination onto the surface of the composite component. The bright-field illumination is provided by a top illumination light source at a first intensity and the dark-field illumination is provided by a side illumination light source at a second intensity different from the first intensity. The two intensities are adjusted to provide a suitably contrasted image without too much shadow, in order to allow an image processor to detect features in the image and/or perform measurements in the image. The bright field illumination and the dark field illumination steps can be applied simultaneously or in sequence one after the other (but in the same shutter opening). The sequence application may be used if different colors are used for dark field and bright field illumination, to avoid unpredictable subtractive or additive color formation. 
         [0053]    In some embodiments, acquiring at least one image comprises acquiring a sequence of multiple images. Each one of the images in the sequence may be acquired with a different and customized illumination strategy. The illumination strategy may be selected/applied in accordance with a specific local morphology of the surface of the composite structure. The method may thus be adapted in real time to the surface of the composite component, and inspection may be performed in an automated manner. 
         [0054]    As stated above, the image acquisition device may be mounted to the head of the automated tool. Alternatively, the image acquisition device may be built into the head of the tool. The image processor may be provided remotely from the image acquisition device, thus allowing the portion of the system attached to the head of the automated tool to remain lightweight and small in volume. Color inversion image preprocessing may be used for the detection of certain types of features. Color inversion may be applied directly by the image processor or by an intermediate device receiving the acquired image from the image acquisition device, such as a Programmable Logic Controller (PLC), an upper level controller, etc. 
         [0055]    The methods and systems described herein allow for real-time inspection of freshly laid fiber in a timely manner. The high volume of features and locations on the composite structure requires speed so as to account for changes in surface reflectivity as the surface of the composite structure dries. The means described herein used to enhance the contrast of the features under inspection, such as light frequencies, illumination type, and angles of illumination, allow high quality images to be acquired for a quick and automated inspection process. 
         [0056]    The above description is meant to be exemplary only, and one skilled in the relevant arts will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, the blocks and/or operations in the flowcharts and drawings described herein are for purposes of example only. There may be many variations to these blocks and/or operations without departing from the teachings of the present disclosure. For instance, the blocks may be performed in a differing order, or blocks may be added, deleted, or modified. The structure illustrated is thus provided for efficiency of teaching the present embodiment. The present disclosure may be embodied in other specific forms without departing from the subject matter of the claims. Also, one skilled in the relevant arts will appreciate that while the systems and methods disclosed and shown herein may comprise a specific number of elements/components, the systems and methods may be modified to include additional or fewer of such elements/components. The present disclosure is also intended to cover and embrace all suitable changes in technology. Modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.