Abstract:
A ground based wind turbine blade inspection system and method consists of a thermal imaging camera configured to detect propagating defects by acquiring thermal imaging data from a wind turbine blade when it is substantially at thermal equilibrium with respect to surrounding air and analyzing the thermal imaging data with a processor to identify thermal effects associated with latent defects caused by internal friction due to cyclic gravitational stresses and wind loads during normal turbine operation. The system permits latent defects to be identified using a ground-based in situ inspection before they become visually apparent, which allows repairs to be made economically while the blade is in place.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 15/275,889, filed Sep. 26, 2016, entitled “System and Method for Ground Based Inspection of Wind Turbine Blades,” currently pending, which is a continuation of U.S. patent application Ser. No. 14/469,292, filed Aug. 26, 2014, entitled “Method and Apparatus for Remote Feature Measurement in Distorted Images,” currently pending, which is a continuation-in-part of U.S. patent application Ser. No. 13/839,908, filed Mar. 15, 2013, entitled “System and Method for Ground Based Inspection of Wind Turbine Blades,” now U.S. Pat. No. 9,330,449, the entire contents of all of which are incorporated by reference herein. 
     
    
     INCORPORATIONS BY REFERENCE 
       [0002]    Applicant further 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, entitled “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, now U.S. Pat. No. 9,194,843, entitled “Method and Apparatus for Monitoring Wind Turbine Blades During Operation.” and U.S. patent application Ser. No. 13/840,470, filed on Mar. 15, 2013, now U.S. Pat. No. 9,395,337, entitled “Nondestructive Acoustic Doppler Testing of Wind Turbine Blades from the Ground During Operation.” 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The invention relates to methods and apparatuses for inspecting wind turbine blades on rotating wind turbine generators, from the ground. The invention also relates to methods and apparatuses for remotely measuring features in geometrically distorted images, in particular digital photographic and thermal images of wind turbine blades. The invention facilitates such inspections from the ground, on operating wind turbine generators and has utility for remotely detecting propagating latent defects, existing damage and broken adhesive bonds within the skin of a wind turbine blade. This permits subsurface 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. 
         [0005]    2. Description of the Related Technology 
         [0006]    Due to their large size, extensive surface area and complex shape, wind turbine blades are difficult to non-destructively 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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    Detecting progressive subsurface 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 trailing edge splits, cracks, lightning damage and blade erosion. In addition, major subsurface delaminations, cracks and debonded adhesive joints can easily go undetected with current technology. 
         [0010]    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 prohibitively expensive, slow to operate, require repair and maintenance themselves. Their effectiveness is also questionable. Thermal imaging of blades using solar heating during the transition from day to night has been attempted but is very limited in both the time over-which data may be taken and by being limited to blades facing the sun. Further this technique requires stopping the rotors, with consequential loss of revenue. 
         [0011]    Helicopter access is both expensive and dangerous in wind farms, and no means are included to quantitatively measure or locate indications. Thermal imaging of blades using solar heating during the transition from day to night is very limited by both the time over-which data may be taken and by being limited to blades facing the sun. Finally, it is common practice to use optical and digital photographic imaging of blades in an attempt to detect visible damage from the ground. Again these methods suffer from complex logistics, insensitivity to defects, poor repeatability and do not allow precise measurement of the defect size, area or the location. 
         [0012]    There accordingly exists a need for a fast, cost effective nondestructive inspection system and method 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. There also exists a need for a method and apparatus for precision measurement of features or anomalies and locating these on the target blade. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    Accordingly, it is an object of the invention to provide a fast, cost effective nondestructive inspection system and method 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, and to provide analysis of thermal and digital photographic images of turbine blade surfaces for such systems. 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. 
         [0014]    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, comprised of a camera sensitive to the thermal emissions from friction caused by defects subjected to cyclic strain from gravity and wind loading as well as surface or aerodynamically cooled near-surface damage that block thermoelastic emissions from the blade or defect breaches in the blade shell cooled by airflow from pressurized air from within the blade and a means to process thermal images of these anomalies to determine location, signal to noise ratio, size or other quantitative measurements such early detection may allow repair of the blade up tower instead of more costly replacement. 
         [0015]    Another embodiment includes a means to stabilized the thermal images of a wind turbine blade over at least several successive video frames to reduces the image degrading effects of blade motion. 
         [0016]    A third embodiment includes a means of stabilizing the camera when making inspections of off shore wind turbines from a boat. 
         [0017]    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 
         [0018]    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. 
           [0019]      FIG. 1  is a schematic representation of a utility scale, horizontal axis, wind turbine generator; 
           [0020]      FIG. 2  is a schematic representation of a wind turbine blade cross section; 
           [0021]      FIG. 3  is a schematic diagram showing the relative location of a thermal camera for inspecting the wind turbine blades that are oriented so as to be substantially perpendicular with respect to the ground or sea level; 
           [0022]      FIG. 4  is a diagrammatical plan view of a wind turbine site with an indication of six placement locations for the thermal camera to view various locations on the blades; 
           [0023]      FIG. 5  shows a basic thermal system including an image processing computer that is suitable for direct viewing and recording blade surfaces in or near a wind turbine site; 
           [0024]      FIG. 6  is a schematic showing the thermal camera viewing a low pressure, or suction side of a blade in the horizontal position; 
           [0025]      FIG. 7  shows how an electronic image peak store unit captures the trajectory of thermal emitting defects as the blade passes through the thermal camera field of view; 
           [0026]      FIG. 8  is a schematic diagram of a manually controlled moving mirror assembly mounted in front of the thermal camera to stabilize, or derotate, thermal images of a rotating turbine blade and suitable for use when the blade is in approximately a horizontal orientation; 
           [0027]      FIG. 9  is a schematic of computer controlled moving camera image stabilizer for derotating a sequence of thermal images of a rotating turbine blade using the presence of the blade in the video image to trigger the start of blade tracking; 
           [0028]      FIG. 10  shows the timing of the ramp voltage waveform used to drive the linear motor to drive the hinged plate supporting the thermal camera to track one blade only turning with a period of four seconds; 
           [0029]      FIG. 11  is a schematic diagram of a gimbal mounted thermal camera for testing off shore wind turbine blades from a boat or ship; 
           [0030]      FIG. 12  is a schematic diagram of a thermal camera testing a blade with a visible light camera and light source to image the serial number of the blade and showing the image of the serial number presented with the test images of the blade; 
           [0031]      FIG. 13  includes two images of a wind turbine blade exhibiting defect indications following inspection according to embodiments of the present invention; 
           [0032]      FIG. 14  is a screenshot of a display image generated by the image processing computer showing a thermal image of the high pressure side of an operating wind turbine rotator with a video frame selected with the blade positioned horizontally blade showing a dark defect during nighttime; 
           [0033]      FIG. 15  is a screenshot of a display image generated by the image processing computer showing the use of a caliber overlay function to mark on the digital image the width or diameter of the cylindrical root end of a blade; 
           [0034]      FIG. 16  is a screenshot of a display image generated by the image processing computer showing the use of the caliper overlay function to determine a distance between the blade root end and the black indication of a defect in a wind turbine blade; 
           [0035]      FIG. 17  is an infrared image showing a flaw in a wind turbine blade near the tip of the blade; 
           [0036]      FIG. 18  is an infrared image showing the caliper overlay function being used to measure the location of two areas of damage on a wind turbine blade with respect to the blade root end; and 
           [0037]      FIG. 19  is a flowchart depicting a method according to a preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    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 apparatus 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.” 
         [0039]    Certain embodiments of the present invention disclosed herein describe a system and method for the nondestructive inspection of wind turbine blades suitable for both onshore and off-shore wind power generators and capable of detecting propagating defects or damage during normal turbine operation. Inspections are made from ground or sea level without any requirements for access to the wind generator tower or any disruption of power generation. Large utility scale wind power turbines are generally of the HAWT design using composite air foil shaped blades to generate the rotational torque needed to drive the electrical generator. Current utility scale wind turbine blades may range from 9 m in length up to more than 50 m, with much larger blades being designed for offshore wind power generators. The application of this invention may achieve good results on blades of all lengths manufactured with thermoelastic composite materials such as fiber glass and carbon fibers in an epoxy matrix. 
         [0040]    Wind turbines blades, during normal operation, are subjected to continuous cyclic loading due to gravity and variable wind forces. Thermal imaging of blade anomalies requires the test be conducted after sunset or on cloudy days during normal operation. After the blades have come to thermal equilibrium with the ambient air temperature, the only remaining thermal emissions from the blade occur at the site of propagating damage anywhere in the blade. Tribological damage including plastic deformation, fretting, adhesive wear, oxidation, and phase transformations, such as melting can occur at rubbing crack faces. C. J. Pye et al. (Ref 1) where cyclic gravitational loads pass through structural anomalies. Heat is generated by three mechanisms (J. Renshaw, Ref. 2): internal frictional rubbing of contacting surfaces (asperities) on crack walls, deformation of the plastic zone surround the crack, and viscoelastic losses. 
         [0041]    While the local blade surface temperature anomaly is small and the area may be small, embodiments of the present invention provide excellent signal to noise ratio and quantitative data to evaluate the condition of the blades. Telephoto optics are necessary to resolve the small angle α subtended by for example a 6 inch long crack at 300 ft. height, where a=arc sin 1/600=0.0955° or only 343 arc seconds. Further, if the defect is located near the blade tip, the thermal indication will be moving at a high rate of speed. For example, on a typical 50 m blade operating at 20 rpm, the thermal indication will be moving at 176.8 ft./second. 
         [0042]    Depending on the characteristics of the thermal camera, which commonly uses a relatively slow micro bolometer thermal sensor, the rotation of the blade through the field of view typical thermal camera makes it difficult for an operator to see, much less analyze, the thermal indications, even if the camera has the thermal resolution performance to detect the emissions at all. Positioning the thermal camera on the right side of the tower (on the ground or at sea level for an off shore turbine with ones back to the wind) at a location approximately mid-span under the turbine rotor disk (the plane containing the blade tips) allows the camera to image the leading edges of the turbine blades as they move directly towards the camera with the angle of the blade changing relatively slowly. Good results for defects on the blade leading edges can be obtained. The trailing edges may be inspected in the same manner from the left side of the tower (with ones back to the wind) where the blades are moving directly away from the camera. Imaging a defect that is located elsewhere on the blades is more challenging since the blades are viewed up or down wind of the turbine disk and the image moves rapidly across the field of view of the camera. 
         [0043]    The thermal images of the rotating blades from positions upwind or downwind of the plane of the turbine disk can be captured with the thermal camera using the camera memory, if present, or a computer with the appropriate drivers for capturing digital images using a GigE or camera link interface or an analog to digital video frame grabber or any other appropriate camera/computer interface well known in the art. A software algorithm to display each frame by frame in a series of sequenced images allows defects to be identified as the blade rotates into view of the camera. Certain embodiments of the invention also include a peak-store image function that records the stored maximum value for each pixel in each frame. As a hot spot on the blade, caused by stress induced internal friction at the site of a propagating defect, plasticity or thermoelastic emissions, rotates into view, each pixel is locked to its maximum value. With a series of sequential frames combined into one peak stored image the motion trajectories of defect indications are recorded through each blade pass. Because wind turbine blades are painted to reflect heat from the sun, the heat from sources other than active defects in the blade may appear in the thermal images of blades as reflections. One example might be the heat from a car situated near the wind tower. Heat signal from a defect will form a sine wave trajectory track around the blade path, while reflection of heat sources on the ground or on adjacent towers appear “painted” on the image of the blade through a small number of frames or a small angle of blade rotation. 
         [0044]    In multiple embodiments of this invention a blade image de-rotation stabilization device can be added to the front aperture of the thermal camera to derotate the motion of the blade as it passes through the camera field of view. 
         [0045]    A two axis mirror motion using two actuators would allow more precise image motion compensation as the blade is viewed from a distance of from 100 feet to 1000 feet with frequently a considerable off-axis view. As such, the blade motion and the motion of any thermal emitters on the blade surface appears to move in a combined vertical and horizontal motion approximating a sine wave. Blades are best inspected when within approximately 45 degrees of the 90 and 270 degree horizontal position so that the images across the span of the blade have approximately the same scale. 
         [0046]    The output signal from a function generator producing a ramp function can be used to drive a voice-coil linear actuator at an amplitude and repetition rate required stop the appearance of motion for several video frames. As the blade starts to pass through the field of view, the mirror pivot motion starts and tracks the blade image across the field of view. The frequency and amplitude can be set with each cycle corresponding to the turbine rotational period r seconds to track one blade or τ/3 to track all three blades sequentially. Other embodiments include the use of the graphical target generated by software that is place on the video image from the thermal camera that triggers the de-rotator to initiate a mirror motion cycle. As the blade enters the thermal camera field of view and crosses the graphical target on the display, the computer starts to move the mirror to approximately track the blade as long as possible allowing the thermal camera time to capture multiple high quality images. The de-rotator motion may also be triggered by a photo detector established to detect light from a laser beam illuminating the blade from the ground. The unexpanded low power laser and detection electronics would provide an electronic trigger signal when the blade was in the correct position to start tracking. 
         [0047]    In another embodiment, a light weight thermal camera can be mounted to a hinged plate and operated with any of the methods for image de-rotation of the rotation blade as describe herein. 
         [0048]    Referring now to the drawings, wherein like reference numerals indicate corresponding structure throughout the views, and referring in particular to  FIG. 1 , a schematic diagram of a HAWT is shown that 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. 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. 
         [0049]      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 or other like composite material are bonded to the edges of the sandwich panels  18 . The blade spar web  30 , which 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. Sometimes a second or even third spar web is present forming a box beam. 
         [0050]    Defects such as adhesive disbonds or unbonds present at the spar cap  26  to spar web  30  adhesive bond  28  may lead to catastrophic failure of the blade in service. Fiber waves in the solid spar cap  26  laminate can also lead to cracking and ultimately to blade failure. Further, trailing edge  21  splits or cracks in the high pressure  16  and low  14  pressure side shell adhesive bond  24  may be signs or excessive blade flex during operation. The trailing edge  21  adhesive bond  24 , in the area of greatest blade chord width towards the root end  10  supports blade twist loads. Cracks and breaks in the adhesive bond  24  at these locations can also lead to blade failure unless detected in time and the turbine shut down and promptly repaired. When one of the reinforcing elements such as a glass or carbon fiber breaks, it makes a distinct sound, like a stick breaking. The sound propagates throughout the structure of the wind turbine blade, and throughout the enclosed space defined by the interior surface of the outer skin of the blade. In addition, a pressure gradient develops within the enclosed space as a result of centripetal acceleration. The pressure differential between the portion of the enclosed space that is proximate the wind turbine hub and the outermost portion of the enclosed space can be on the order of 2 psi. 
         [0051]      FIG. 3  is a schematic diagram showing the side view of thermal camera  222  at locations down wind of tower  6  for inspection of the low pressure side of the blade  11 . Thermal emissions  204  from defects in the blade  11  propagating due to cyclic stress load due to gravity and wind fluctuations are detected by thermal camera  222 . The side view also shows the plane  20  as a line extending to the ground or sea level  28 , containing the blade tips  12 . This approximate location is a good viewing position for blade  11  leading edge  20  inspections. 
         [0052]      FIG. 4  shows thermal camera  222 , positioned at locations both on the up wind-high pressure side of the turbine blades, at two locations on either side of the tower in the plane of the blade disk and at a location on the down wind, low pressure or suction side of the blades to provide advantageous viewing of the entire blade surfaces during rotation. For the best view angle of the low pressure side  14  of the blades  11  is achieved with the thermal camera  222  positioned at approximately location  212 , however the best results are obtained using an image stabilizer as described here in or an image peak storage unit or software due to the relatively high speed the image of the blade passes through the camera field of view. The best view of the blade leading edges  20  are at position  214 , in the plane  20  of the blade tips, since the blade  11  is rotating down directly at the camera and the angles of points on the leading edge  20  are changing relatively slowly. 
         [0053]    Moving down wind from position  214  gives good views of the forward low pressure side surfaces  14 , of the blade  11  with relatively low rate of angle changes over three to four video frames. Position  216  is excellent for the trailing edge  21  of the blades and position  220  offers a full view of the high pressure side  16  of the blade  11 , which is best viewed with the image stabilizer or peak storage embodiment described herein. Position  210  offers relatively poor views of the low pressure side except for the trailing edge  21  and the aft low pressure sandwich structure  18 , but with fast moving angle changes and with the blade twist. Wind turbine farms are often located on hills or mountain ridges with little room to move away from the tower  6 . Local conditions and land shapes often dictate where the best viewing angles can be obtained. 
         [0054]      FIG. 5  is a schematic diagram showing a thermal camera  222 , positioned to receive low level thermal radiation  228  from the blade  11  due to thermoelastic emission from the stresses acting on the blade material due to gravitational forces from the blade rotation motion  17 . 
         [0055]    Emission  226  from the mechanically stressed defect  224  appears warmer in the image produced by thermal camera  222  due to internal friction and plasticity around defect  224 . 
         [0056]    The camera  222  is positioned under the blade at position  214  to receive thermal radiation  228  from the leading edge and the forward part of the low pressure side  14 . This position reduces the angular changes due to blade rotation in the image during the frame acquisition. The streamed video images from the thermal camera  222  are recorded by Image Processing computer  230 , or in a memory device in the thermal camera  222  as video files, and processed presented using peak store or other image processing techniques and presented on monitor  232 . Various means of processing the images including video image peak store, frame by frame analysis, histogram normalization, unsharp filters and so on to obtain good image quality and quantitative measurements of size and location comparing features of known size at the range to the target. 
         [0057]      FIG. 6  shows how a thermal camera located at position  212  that can be used to test the entire blade from one location by pointing to a blade section inboard adjacent to the generator nacelle  8  and acquiring the thermal image sequences. Moving or rotating the camera in an arc  230  outboard and overlapping the next blade section and so on until the tip is reached. The use of programmed servo or other motor drives to move the camera would allow for a fast automated test. 
         [0058]      FIG. 7  shows a representation of the video peak store function. The thermal emitting defects  236 ,  238  and  240  on blade  11  as shown, rotate counter-clockwise in this view of the low pressure side  14  of blade  11 , as seen from down wind of tower  6 . The axis of rotation  7  is shown in the approximate middle of the nacelle  8 . The larger thermal emitting defect,  236 , happens to be the closest defect to the axis  7  and generates the strongest signal  248 . In a peak stored image, each pixel is locked to the highest gray level value while image data is being acquired by the post-test analysis software or in the field during the test. The result is sources of thermal emissions can be tracked through the field of view as show in defect traces  248 ,  246  and  244  and as shown in  FIG. 13 . A thermal emitting defect has a trajectory following the blade in rotation as viewed greatly off-axis. A line scan  252 , through the peak stored image  242 , maps the gray level value for each pixel in a line from one side of the image to the other and allows a graph  254  to be plotted showing the gray level signal intensity vs. position on the blade. The distance scale is calibrated in software from the size of know features on the nacelle  8  or tower  6 . These values can be corrected mathematically for the difference in velocity for different span wise locations on the blade using a look up table (LUT) or other techniques for correcting image data based on geometric parameters well know in scientific programming and photogrametry. 
         [0059]      FIG. 8  shows one embodiment of the thermal image de-rotator consisting of a mirror  241 , attached to a hinge  250  at one end and the opposite end is connected to a linear actuator or voice coil linear actuator  238  through a ball joint or universal joint  240 , whose cyclic motion in the direction of the blade motion can be adjusted to track the approximate motion of each blade  11  as it rotates through the field of view. The derotation will essentially stop the rotation motion for several frames. The blade appears to hang in space giving the thermal camera time to generate higher resolution images of the moving blade. The adjustments consist of amplitude of the ramp voltage  260 , which will compensate for the speed of blade rotation, the duration of the ramp function  262 , and the time  264 , between each start of the ramp function. The output of function generator  244  is amplified in amplifier  242  which then drives the linear motor actuator  238 . The motion of mirror  241  can de-rotate short sequences of images produced by thermal camera  222 . By selecting the repetition rate  262 , the user can select to image all the blades by starting the tracking motion every τ/3 seconds, where τ=the period of rotation of the turbine in seconds, or by selecting a repetition rate of τ, the images of the same blade  14 , will be presented and de-rotated. A spring can be used to provide a restoring force to bring the mirror back to its start position, ready for the next blade pass. Only one or more video frames need to be tracked to substantially improve blurring of the thermal camera image due to blade rotation. The frequency of motion of the de-rotating mirror  241 , can be calculated in the following example. Assume a turbine operating @ 15 rpm, a thermal camera  222  with a 30 frames per second frame rate, and it is desired to de-rotate 4 video frames. The period, τ, of the turbine is the time required to make 1 revolution, in this case 60 seconds/15 rpm=4 seconds. In this time, the thermal camera  222  (operating at 30 frames/second) captures (4 seconds×30 frames/second)=120 frames. The 4 frames we wish to de-rotate are captured over a time of (4 frames/30 frames/sec.)=0.133 seconds. To de-rotate these four frames then, we start moving the mirror to track the blade starting at 0 seconds, when the blade  14  enters the field of view of the thermal camera. The mirror  241  angle is changed (increased or decreased depending on what direction the blade is moving through the field of view) continuously over 0.133 seconds, then returned to the start position. To track only one blade, the tracking motion is started every 4 seconds (τ). To track all three blades sequentially, the tracking motion is started every τ/3 seconds, 4/3=1.33 seconds. After each tracking motion, the mirror is returned to the start position to await the next tracking cycle. 
         [0060]      FIG. 9  shows a second embodiment where the thermal camera  222  is moved in rotation to follow the blade  11  motion around axis  7 . The thermal camera  222  is mounted on a frame or plate  251  and connected to hinge  252  which is also attached to frame member  250  which supports actuator  238 . The opposite end of plate  251  is connected to the electric actuator through a flexible joint, such as a hinge or universal joint. The movement of the actuator causes the thermal camera  222  to move up and down which when aligned with a portion of the blade here the motion is substantially the same, will tend to stabilize the blade in the field of view of the thermal camera  222 . Additional actuators can operate simultaneously to move the thermal camera in multiple directions however with additional complexity. In practice, one actuator is sufficient if a clear view of the blade  11  when it reaches the horizontal position during rotation can be attained in the field. Another option is to rotate the entire actuator  238 , support plates  250  and  251  as well as the thermal camera  222  to align the motion of the actuator with the motion of the blade during data acquisition. Reducing the driven mass reduced vibration and power requirements for the electronics and electric actuator, which may be battery operated or power from the vehicle used by the operator. The motion of the camera should be aligned approximately with the direction as the motion of the turbine blade  11  in the camera field of view during its rotation on axis  7 . 
         [0061]    As the high pressure side of the blade, which viewed from the up wind side of the tower  6  at position  220  is turning clockwise, rises into the field of view of camera  222 , the leading edge of the blade eventually crosses threshold marker  242 . This motion can be identified as a change in the pixel intensity value from that of the open sky as seen by thermal camera  222 , and the software electronically triggers the start of the motion of linear actuator  238  by computer  230  the waveform generator  244  and amplifier  242 . Again, an optimal waveform is a ramp function which will move the camera  222  at a constant angular velocity to receive in lens  228  the thermal emissions  226  from the defect in the blade  224  as it moves vertically across the field of view at a constant velocity. This embodiment can also use a moving move a mirror. The tripod mount can also be fitted with a mechanical azimuth and elevation, for example, using rod  258  connected with a flexible joint  254  to the base frame for the camera  250  and locked with clamp  260  with the linear motor providing fine movement of the thermal camera or a mirror. Although the embodiment shown uses a tripod  204  to support the thermal camera and image de-rotation mechanism, any sturdy support can be used including a truck, van, car or a wheeled cart to easily move the equipment around the site of the wind turbine.  FIG. 10  shows a graph of voltage vs time for the signal used to drive the image de-rotation actuator. For the manually operated de-rotation embodiment, the function generator operates continuously. The operator adjusts the repetition rate to match the turbine rotation period τ,  268 . The ramp function shown in  FIG. 10  has a 0.133 second duration to match the time for four video frames to be displayed. The electronic or software system can be designed to allow the operator to input these values directly and have the de-rotation device, using either the moving mirror  241  or the moving thermal camera configurations. 
         [0062]      FIG. 11  is a schematic drawing showing the addition of a gimbal mount to stabilize the thermal camera  222 , and it&#39;s various motion mechanisms and embodiments described here in, during use on water aboard a ship or vessel to inspect blades  11  mounted on off shore wind power generators. In one embodiment, counter weight  280  is supported by gimbal mount comprised of inner frame  270  and outer frame  272  connected by bearing pins  274  and  276 , as well as support housing or frame members  278  which is supported on the deck  29  of the ship or vessel. This allows camera  222  to remain aligned with blade  11  during inspections. This gimbal assembly may also be actively powered and use servo actuators and accelerometers, well known in the art, to provide a stable platform with respect to the vessel rolling and pitching movement for successful operation of the blade inspection system described herein. In addition, the shipboard blade test system may use an inertia platform to keep the camera aimed at the target blade during tests to compensate for ship motion.  FIG. 12  is a schematic diagram of the wind turbine blade inspection system described herein with the addition of a video recorder, camcorder, photographic camera or video camera  242  and a light source  244 , configured to record the serial numbers  282  written on the root end  10  of each blade as it is tested. Frequently the optimal position for the thermal camera during the test is not the same position need to place the serial number camera to image the serial numbers. Any one of a number techniques may be used to synchronize the thermal images with the serial number images. First, a GPS clock can provide timing signals on the sound track of a video camcorder as well as timing signals to the thermal camera  222  or the computer  230 , if connected at the time of data acquisition. Second, if the image de-rotator is being used an audible or electrical or visual signal can be sent by electrical line  243  to the serial number camera when the actuator begins to track the blade  11 . A delay may be needed to time the serial number capture with the position of the blade to allow the blade to move from the test angle to the best angle to capture the correct serial number. 
         [0063]      FIG. 13  shows two test results on operating 1.5 MW utility scale wind power generators made with embodiments of this invention. The top image shows seven video frames of one blade from the high pressure side as it rotates through the thermal camera field of view. The images were recorded as a .MOV file and played back through software that generates a peak store image. The trajectory  246  of the thermal emissions from an active propagating defect is seen as the blade rotates from top to bottom clockwise. The dark lines in the image of the high pressure side  14  of this blade are thermal camera  222  artifacts due to the fast blade  11  motion. The bottom image shows a test image of a leading edge of blade  11  that includes thermal response from hub  18  nacelle  8  and tower  6 . The test was made from position  214  in  FIG. 4 . Two indications of defects  236  are seen. Scaling from known features in the image, the defects are located at 7.2 and 9 meters from the root end  10 . 
         [0064]    One preferred embodiment for this method of remotely inspecting a land or offshore wind turbine blade or blades uses a sensitive IR camera with a fast integration time, wherein a single image or a continuous series of images of each blade is recorded as it passes through the field of view of the thermal camera. The IR camera integration time, t, should be fast enough to reduce the degrading effects of blade rotation creating an image without apparent motion smearing at the blade tip, ranging from 0.001 to 100 milliseconds but ideally, t seconds=τ/πD, where D is the diameter of the turbine rotor and r is the rotation period of the rotor in seconds. 
         [0065]    The camera should be sensitive to mid-wave or long wave IR radiation, having a wavelength from 4 to 18 microns, corresponding to the wavelength of energy emitted by visco-elastic material undergoing cyclic stress loads. Such cameras avoid the further complexity of requiring an means to track the blades as they rotate but are more expensive than microbolometer based IR cameras. Wind turbine blade anomalies on an rotating rotor at night, after the blade has come to thermal equilibrium, present in three ways. First, heat may be caused by friction when a defect such as a crack is stressed cyclically by the various forces acting on the blade including the rotating gravity vector, varying wind speed, blade actuation torque, lift from the wind acting of the blade airfoil. This heat flows through to the surface of blade and appears as a hot spot. Defects such as fiber waves and fatigue cracks produce such higher temperature thermal indications. 
         [0066]    The cyclic forces acting on the blade also generates heat due to the visco-elastic properties of the composite materials. Other types of defects such as delaminations in the composite sandwich construction of the blade can block the flow of heat from deeper within the blade. Such defects appear dark, as the surface temperature of the blade surface at that location is cooler than the adjacent areas. 
         [0067]      FIG. 14  is a screenshot of a display image generated by the image processing computer showing a thermal image of the high pressure side of an operating wind turbine rotator with a video frame selected with the blade positioned horizontally blade showing a dark defect. This image was taken at night after the blade has come to thermal equilibrium from the effects of solar heating during the day. This delamination type defect is blocking the heat generated deeper in the blade and preventing it from reaching the surface. The camera used to record this image has a fast integration time of 0.001 seconds. Another defect type that appears dark is a perforation of the blade shells, the two bonded halves that comprise the blade. 
         [0068]    Air inside the blade is pressurized by the centripedal acceleration due to the rotator rotation. Compressed air flowing through holes cools the material adjacent to such holes. The step of imaging the wind turbine blade may thus be performed while the turbine is rotating and compressing the air within an open cavity inside a hollow turbine blade and causing an inflow of outside ambient air into the cavity. The air cools the composite material within or around defects where it escapes through breaches of blade shell. Other defects that allow material to be partially supported at the surfaces may be cooled by cool ambient air flowing over the blade. Many of these defect types are caused by lightning strikes, transportation damage with through shell cracks and fatigue cracks. 
         [0069]    The IR camera may be advantageously mounted on appropriate vehicles to maneuver through the wind farm to test each turbine. Using a remote control pan/tilt mechanism to steer the camera, mounting on the roof of a truck or car greatly increases testing throughput. A GPS enabled helicopter flying a programmed route can reach all of the best test locations for a given wind direction and rotor orientation. In practice, 20 towers or more may be tested in one night. The IR camera can also be mounted on a boat for offshore turbine inspections. 
         [0070]    The dimensions and location of this defect indication need to be determined to allow proper evaluation and maintenance decisions.  FIG. 15  shows how a known distance can be defined on a still IR image of the blade. Here the diameter of the blade root end, known to be 6.2 feet, is defined by clicking on one side of the blade root and dragging the measurement line across the image to the opposite side of the blade root, hence defining the diameter of the blade root or a reference line. 
         [0071]    The number of pixels in the image that comprise this line are counted to determine the image scale in unites of pixels per unit distance on the image of the blade. The image scale is accurate for all areas of the image of the blade at the same distance from the IR camera as the line segment defining the blade root diameter. The location of the defect with respect to a known datum such as the blade root end can then be measured as shown in  FIG. 16 . The computer mouse is used to move the cursor to the blade root, where a left click anchors a line at the datum which is then dragged across the image to the defects indications. So long as the line is moved across areas of the blade that are substantially equidistant from the IR camera as the defined reference line, the pixel count divided by the image scale will yield the distance of the defect to the datum. 
         [0072]    Using the high speed IR camera images of blade tip defects, as shown in  FIG. 17  are easily detected with sufficient resolution to enable sizing and determination of location, in spite of the faster than 250 ft. per second speed. 
         [0073]      FIG. 18  shows how two defect indications near the blade root end can be measured using a caliper overlay function. 
         [0074]    Digital thermal and 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 600 horizontal by 420 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 you move to an edge of the field of view, the angle and distance increase and the image scale decreases. 
         [0075]      FIG. 19  shows one preferred method. Objects in the field in a digital image may be measured with respect to other objects of features in the image. First, a the digital image must be calibrated by counting the number of pixels in a line segment that cross 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 features 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 dived by the image scale. 
         [0076]    For wind turbine generators we can select a calibration feature in the digital image that changes 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 attached 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. 
         [0077]    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. 
         [0078]    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. 
         [0079]    A video image of root end of the blade and blade hub may be recorded continuously with a video camera and synchronized with video frames from a thermal camera imaging the blade for anomalies using at least one of GPOS timing signals, wireless signals, or other means in order to identify blade serial numbers or the rotor lift lugs, thereby identifying the blade positions of specific blades with anomalies or features of interest. 
         [0080]    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 to 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. 
         [0081]    A sequence of digital thermal images or photographic 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. 
         [0082]    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. In other words, segments broken up by length along the longitudinal axis of the blade, are scanned incrementally from the inside out as the blades rotate. 
         [0083]    The analyst reviewing the frame by frame sequence over the full blade length may use an image scale with a distance correction factor to obtain accurate measurements. 
         [0084]    It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.