Abstract:
A method and apparatus for inspecting utility scale wind turbine generator blades from the ground includes a digital camera and an adjustable polarizing filter on a stable platform. The camera is used to record multiple digital images with varying polarization angles using an artificial polarized or polarized solar illumination to produce images with the same or similar registration but different polarization angles, followed by image processing using subtraction or absolute difference routines to yield high resolution, high contrast images with reduced glare and excellent surface definition over the field of view.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 62/042,102, filed on Aug. 26, 2014, currently pending, entitled “Method and Apparatus Contrast Enhanced Photography of Wind Turbine Blades,” the entire contents of which are incorporated by reference herein. 
       INCORPORATIONS BY REFERENCE 
       [0002]    Applicant hereby incorporates by reference, as if set forth fully herein, the entirety of the disclosures of U.S. patent application Ser. No. 13/731,085, filed Dec. 30, 2012, now U.S. Pat. No. 8,553,233, and having the title “Method and Apparatus for the Remote Nondestructive Evaluation of an Object,” U.S. patent application Ser. No. 13/837,145, filed on Mar. 15, 2013, and having the title “Method and Apparatus for Monitoring Wind Turbine Blades During Operation,” U.S. patent application Ser. No. 13/839,908, filed on Mar. 15, 2013, and having the title “System and Method for Ground Based Inspection of Wind Turbine Blades,” U.S. patent application Ser. No. 13/840,470, filed on Mar. 15, 2013, and having the title “Nondestructive Acoustic Doppler Testing of Wind Turbine Blades from the Ground During Operation,” and U.S. patent application Ser. No. 14/469,292, filed Aug. 26, 2014, and having the title “Method and Apparatus for Remote Feature Measurement in Distorted Images.” 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    This invention relates to a method and apparatus for remotely detecting surface features and anomalies on wind turbine blades in situ, in particular utilizing digital photographic imaging technology. The invention facilitates such inspections from the ground, on stationary operating wind turbine generators and has utility for remotely detecting propagating latent defects and existing damage on the surface of a wind turbine blade. This permits surface defects to be detected before they become too large for repair in situ, which provides significant economic advantages, as the cost of repairing the wind turbine blade in situ is typically 10% of the cost of replacing a blade. 
         [0004]    Due to their large size, extensive surface area and complex shape, wind turbine blades are difficult to nondestructively inspect even within a fabrication or repair facility. Visual inspection cannot identify defects below the surface of the outer skin of the wind turbine blade, which typically is fabricated from a fiberglass material. Active thermography inspection techniques using heat are effective for near surface defects but can give false positives and false negatives due to variations in material thickness and surface emissivity. 
         [0005]    Shearography with either thermal stress or during flexure testing of the blade in the factory can be used to detect fiberwaves in spar caps and other areas of the blade, but the technique is slow, expensive and is usually performed only if known issues are suspected. Angle beam ultrasonic techniques are very slow and may not work through thick carbon fiber spar caps. 
         [0006]    As a result, blades are commonly installed on towers and put into service with a significant probability of latent manufacturing defects. Furthermore, composite blades are subject to extreme temperature variations. Entrapped water in blades can undergo freeze/thaw cycles, which can cause internal damage. Cyclic forces of gravity and varying forces from the wind acting on the blades as they rotate can cause fatigue damage or the propagation of latent defects over time while manufacturing process mistakes can lead to early blade failure. Defects can grow below the surface of a wind turbine blade to the point that by the time cracks and damage breach the surface and can be detected visually, the damage may not be repairable on tower. 
         [0007]    Detecting progressive damage and propagating defects in wind turbine blades in situ is difficult for a number of different reasons. Inspectors using sky cranes or rope access are expensive, time consuming and put personnel in a very dangerous working environment. While on tower, close access allows inspectors to visually detect blade defects such as gel coat cracking, lightning damage, and blade erosion, which may be precursors to further degradation of the blade over time. Access to a wind turbine blade in situ with portable instruments for nondestructive testing also requires rope access or sky platforms and cranes. Blade and tower crawlers with nondestructive testing sensors for in situ inspection have been developed and tested, but they can be extremely expensive, slow to operate. 
         [0008]    It is also common practice to use optical and digital photographic imaging of blades in an attempt to detect visible damage from the ground. Standard telephotography often has very low contrast, even with post image processing with current contrast enhancement software. Pixels may be saturated with glare or areas of the blade may be in shadows. The normalization of the histogram, a frequent image processing function, may not be effective in achieving the effect of uniform blade illumination, except in small areas of the blade in the images. 
         [0009]    There accordingly exists a need for a fast, cost-effective inspection method and apparatus for wind turbine blades using digital imaging to detect latent and propagating damage, which includes a method and apparatus for precision measurement of features or anomalies and locating these on the target blade. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Accordingly, it is an object of the invention to provide images and analysis of digital photographic images of turbine blade surfaces for fast, cost-effective surface inspection for wind turbine blades to detect latent and propagating damage early enough to allow on-tower repair before it becomes necessary to remove the wind turbine blade from the tower and repair it off-site or replace it with a new blade. The sizing and location of anomalies can be used to classify blade status to provide a useful definition of actions needed to maintain safe turbine operation. 
         [0011]    In accordance with the embodiment described herein, a system for inspecting utility scale wind turbine generator blades from the ground for propagating subsurface anomalies during normal operation comprises a digital camera and an adjustable polarizing filter on a stable platform that is used to record multiple digital images with varying polarization angles during daylight hours. Images with the same or similar registration but different polarization angles are subsequently image processed using subtraction or absolute difference routines to yield high resolution, high contrast images with reduced glare and excellent surface definition over the field of view. Images of blades may be calibrated to determine the location, signal to noise ratio, size, or other quantitative measurements of anomalies, and such early detection may allow repair of the blade up tower instead of more costly replacement. 
         [0012]    These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]    The following detailed description of preferred embodiments of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of illustration, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
           [0014]      FIG. 1  is a schematic representation of a utility scale, horizontal axis, wind turbine generator; 
           [0015]      FIG. 2  is a schematic representation of a wind turbine blade cross section; 
           [0016]      FIG. 3  is a schematic diagram of a first preferred embodiment of the invention using a single camera and polarizer rotated manually or automatically; 
           [0017]      FIG. 4  is a diagram of a second embodiment of the invention using multiple cameras in pairs or singling with fixed or automatic polarization rotation; 
           [0018]      FIG. 5  shows a first image with 0 degrees polarization; 
           [0019]      FIG. 6  shows a second image with 90 degrees polarization; 
           [0020]      FIG. 7  shows the final image after digital image subtraction and brightness and contrast adjustment; and 
           [0021]      FIG. 8  is photograph of a wind turbine blade after image processing. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Certain terminology is used in the following description for convenience only and is not limiting. The words “right”, “left”, “lower”, and “upper” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, the geometric center of the device and designated parts thereof. The terminology includes the above-listed words, derivatives thereof, and words of similar import. Additionally, the words “a” and “an”, as used in the claims and in the corresponding portions of the specification, mean “at least one.” 
         [0023]    Referring now to the drawings, wherein like reference numerals indicate corresponding structure throughout the views, and referring in particular to  FIG. 1 , provided is a schematic diagram of a horizontal axis wind turbine (HAWT) that is typical of both land based and off-shore turbine generators. The view  1  from behind the turbine facing the wind includes tower  6  extending up from the ground or ocean surface  2  to support the nacelle  8  which contains the generator and gear reducers, unless it is a direct drive generator. 
         [0024]    There are typically three blades on a utility scale wind turbine having root ends  10  and blade tips  21 . As seen from the side view  2 , the blade root ends attach to the rotatable hub  18 . Blade side  16  facing the wind  4  is often referred to as the high pressure side. The blade side  14 , facing away from the wind is referred to as the low pressure or suction side. As the blade speed increases the blade pitch is adjusted to the optimal angle of attack to the wind to create the maximum lift and torque required to drive the electricity generator.  FIG. 2  shows the construction cross section of a typical HAWT blade. Wind turbine blades are generally manufactured with adhesively bonded composite shells forming the high pressure side  16  and the low pressure side  14 . The trailing edge  21  is adhesively bonded, as is the leading edge  20 , with adhesive bonding in some cases between two flanges  22  formed by the inner and outer fiberglass skins that make up sandwich panels  18 . Two spar caps  26 , which may be made from fiberglass or carbon fiber laminate, are bonded to the edges of the sandwich panels  18 . The blade spar web  30  can be a solid fiberglass laminate or a sandwich construction with fiberglass or carbon fiber face sheets and a core material made with foam, balsa wood or other suitable material with high compressive strength. The spar web  30  is bonded with adhesive  28  to the spar caps  26  to form an I-beam. 
         [0025]    The preferred embodiment includes a digital camera with a polarizing filter and an image processing computer.  FIG. 3  shows a schematic diagram of a single camera  102  setup with a single rotatable polarizer  112 . The camera  102  is preferably mounted on a remotely controlled pan/tilt mechanism  122  located on an outside of a vehicle  124 , such as a car, truck, boat, or the like. While a rotatable polarizer  112  is shown, any type of adjustable polarizer filter that allows for variation of the amount of passed light can be used, such as other types of mechanical polarizer filters, adjustable liquid crystal phase retarders, or the like, or combinations thereof. 
         [0026]    With the blade stopped, two images of a section of a wind turbine blade are photographed with the polarized filter  112  set at first to reduce the amount of reflected light from the blade then second, to pass the maximum amount of reflected light from the blade. The filter rotation of 90 degrees may be manual or automatic. The resulting images must either be in perfect registration using a tripod (not shown) or a means must be available to align the images into registration with each other. The images are the subtracted or the absolute difference is calculated using the image processing computer. The image may also be further enhanced with a brightness and contrast adjustment to yield a glare-free, very high contrast image revealing even subtle surface waviness, repairs, gel coat defects, blade erosion and surface cracking. 
         [0027]    As shown in  FIG. 4 , multiple cameras  102 ,  104  with fixed or automatic polarizer  112 ,  114  rotation may be used for stationary blades. For blades in rotation, simultaneity of the exposures is critical to maintain image registration. Both cameras  102 ,  104  must record the same field of view at the same time. The slight parallax between the two cameras  102 ,  104  does not significantly degrade the final image. 
         [0028]    As with any digital image, pixel counting software may be used to measure features, locations and areas. Digital photography images are formed by focusing electromagnetic energy with a lens onto a flat sensor comprised of energy sensitive elements or pixels positioned in generally a rectangular array, for example 3000 horizontal by 2400 vertical. As with any image of an object, the image scale, measured in pixels per unit distance on the surface of the object, changes continuously over the field of view as the angle θ between the camera lens and the position of each imaged point on the target changes. For a large flat object with the camera pointed straight on, the line from the center of the camera lens to the target is the shortest distance and the image scale will be largest value. As one moves to an edge of the field of view, the angle and distance increase and the image scale decreases. 
         [0029]    Objects in the field in a digital image may be measured with respect to other objects of features in the image. First, the digital image must be calibrated by counting the number of pixels in a line segment that crosses and ends at the extremes of a feature of known dimension, located at approximately the same distance from the camera to the feature desired to be measured. The number of pixels counted divided by the feature&#39;s known dimension gives the image scale at that location in pixels/unit of measure (ft. or meters). Other features or objects can then be measured by counting pixels for either their length or wide or distance from a known datum, if in the field of view, the dimension being the pixel count divided by the image scale. 
         [0030]    For wind turbine generators, one can select a calibration feature in the digital image that changes in apparent width with changes in distance, but does not change if the viewing angle changes, giving a more reliable measurement. One example of such an object is a sphere, whose diameter changes with distance, but not with the viewing angle. A cylinder is another example. The root end of wind turbine blades are cylindrical where they attach to the pitch bearing in the hub. Regardless of the blade pitch or the viewing angle from the ground, the blade root end diameter can be used to calibrate the image scale of a wind turbine blade and then allow measurement of other features at the same approximate distance. 
         [0031]    Another area of known dimension could be the diameter of the tower at the base of the nacelle or at a visible weld joint. Due to the increased thickness of steel at the tower joint welds, these joints retain heat from the sun and remain visible with infrared cameras during most of the night, when thermal inspection of wind turbine blades is best due to washout of defects with thermal emissions. The known dimensions at the blade roots or at tower welds may be used to calibrate the image scale. 
         [0032]    The imaging of the area of known dimension could be done using digital thermography, photography or any other passive or active imaging technique. Software is then used to determine an image scale in units of pixels/ft., pixels/meter, or any other scale of pixels to length. Pixel counting is then used to determine a dimension calibrated in pixels of the size and/or area of the indication. This will permit the software to comparatively determine the dimensions of other features or objects, such as anomalies, that are located at approximately the same distance. The dimensions of such features or anomalies may then be converted back to a conventional dimensional measurement, such as feet, meters, or other units of length. The imaging software may then integrate dimensional measurements to determine the surface area of the feature or anomaly. 
         [0033]    Such imaging may be conducted with the wind turbine blade at multiple radial angles of rotation in order to generate an image scale template that corrects for image distortion over the field of view of the digital image as the wind turbine blade rotates. The imaging may be performed as the wind turbine blade rotates, so it is not necessary to immobilize the wind turbine blade or take the unit off-line during the inspection process. With the blade oriented horizontally, the angle of the arc subtended by a defect indication on the blade in the image is relatively small, so the error in sizing and locating the defect is relatively small. Images where the blade is pointing at angles other than horizontally have an image scale much more distorted by changes in the distance from the IR camera to points on the blade. If the blade is pointing down, the distance from the camera to the blade tip is approximately equal half the height of the tower. The image scale at the blade tip would be twice the value for the image scale at the root and defect measurements would be twice the actual size. 
         [0034]    A sequence of digital thermal images or photographs may be played back, frame by frame to allow the analyst the ability to select frames with optimal image quality for the definition of the boundaries of an anomaly. Image measurement tools that measure each pixel value along a line through a defect indication can be programmed to measure the signal to noise ratio, which may be defined as the pixel values for the area adjacent to the defect squared divided by the pixel values for the defect indication squared. Such a tool, among many known to those skilled in the art of image processing, may be used to quantitatively select the image with the best image quality. 
         [0035]    The field operator may image the wind turbine blade in position segments in order to capture a sequence of multiple images of all three blades at least once as they pass through the camera field of view. This may be followed by a rotation of the camera on its mount to capture the next blade segment in order to image a sequence of multiple images of all three blades at least once as they pass through the camera field and so forth until images of the entire blade are captured. 
         [0036]      FIG. 5  is a photograph of a wind turbine blade in the polarized solar light with the polarizer set to transmit the minimum light reflected from the blade. 
         [0037]      FIG. 6  is a photograph of a wind turbine blade in a polarized solar light with the polarizer set to transmit the maximum rate reflected from the blade. 
         [0038]      FIG. 7  is a photograph of a wind turbine blade made by taking the absolute difference images between the blade photographs, as shown in  FIG. 5  and  FIG. 6 , followed by adjusting the brightness. This can be shown as P3=[P1−P2]. Note that this is the absolute value for each pixel in the digital image after the subtraction of the images. 
         [0039]      FIG. 8  depicts an area on a final high contrast image selected, using a dotted line segment by clicking on one side of the indication in dragging the cursor across the indication. The signal-to-noise ratio is the pixel value of adjacent areas divided by the pixel value of the indication, as shown in  FIG. 8 . Signal-to-noise ratio is 1.4. Other arithmetic routines may be used so long as each method is defined in the context of use. Another routine to measure the signal to noise ratio is to square the pixel values, prior to the division. 
         [0040]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.