Abstract:
A wind velocity calibration system and method for providing highly accurate measurements of the three-dimensional wind velocity vector at high altitudes. The system includes a launcher, a projectile, an artificial aerosol cloud, at least two optical cameras, and an image processor.

Description:
[0001]    This patent application claims priority from U.S. provisional patent application 61729383, entitled “Wind Velocity Calibration Instrument”, which was filed on Nov. 22, 2012. 
     
    
     REFERENCE CITED  
     U.S. Patent Documents 
       [0002]    U.S. Pat. No. 5,174,581, Deborah A. Goodson, “Biodegradable clay pigeon”, Dec. 29, 1992.
 
U.S. Pat. No. 3,840,232, Allen C. Ludwig, “Frangible flying target”, Oct. 8, 1974.
 
U.S. Pat. No. 3,554,552, Thomas E. Nixon, “Frangible article composed of polystyrene and polyethylene waxes”, Jan. 12, 1971.
 
       OTHER PUBLICATIONS 
       [0003]    [1] Xiaoying Cao, “Modelling the Concentration Distribution of Non-Buoyant Aerosols Released from Transient Point Sources into the Atmosphere,” thesis submitted to the Dept. of Chemical Engineering, Queen&#39;s University, Kingston, Ontario, Canada, October 2007. 
         [0004]    [2] Andreas Wedel et al, “Stereoscopic Scene Flow Computation for 3D Motion Understanding, ” International Journal of Computer Vision, volume 95, 2011, pp. 29-51. 
         [0005]    [3] W. Zhao and N. Nandhakumar, “Effects of Camera Alignment Errors on Stereoscopic Depth Estimates,” Pattern Recognition, volume 29, no. 12, December 1996, pp. 2115-2126. 
         [0006]    [4] Z. J. Rohrbach, T. R. Buresh, and M. J. Madsen, “Modeling the exit velocity of a compressed air cannon,” American Journal of Physics, vol. 80, no. 1, January 2012, pp. 24-26. 
         [0007]    A wind velocity calibration system and method for providing highly accurate measurements of the three-dimensional wind velocity vector at high altitudes. The system includes a launcher, a projectile, an artificial aerosol cloud, at least two optical cameras, and an image processor. 
       FIELD OF THE INVENTION 
       [0008]    The present invention relates generally to wind velocity measurements by means of a remote optical system. More specifically, the invention discloses a calibration system and method for providing highly accurate measurements of the three-dimensional wind velocity vector at high altitudes. 
       BACKGROUND OF THE INVENTION 
       [0009]    Many applications require knowledge of the wind velocity vector at altitudes extending from the earth&#39;s surface to heights of about two kilometers. Such applications include wind-turbine energy production, dispersion of pollutants from industrial plants (especially following accidents), airport traffic control, micro and meso-scale modeling of the atmospheric boundary layer, and many others. To answer these needs, a variety of instruments have been developed, ranging from standard cup anemometers mounted on tall meterological towers to complex remote sensing systems based on radar, lidar, or sodar. These systems provide continuous measurements over an extended period of time (e.g. months), but with only moderate accuracy and at considerable cost. Typical accuracies achieved after averaging over a measurement time of one minute or more are only one to two percent, in each of the wind velocity components. 
         [0010]    For short-time wind velocity measurements, anemometers have been attached to radiosondes, balloons, dirigibles, kites, etc. Such approaches invariably yield poor measurement accuracy because they perturb the local wind conditions and because of difficulties in maintaining the sensor at a desired position in space. 
         [0011]    The present invention provides measurements of the three-dimensional wind velocity vector at a precise location in space and at discrete time intervals separated by a few seconds. Furthermore, the system is readily transportable and easily set up in a matter of minutes. Insofar as the invention significantly improves upon the accuracy of existing wind velocity sensors, it may also be used as a calibration tool for other, less accurate wind velocity sensors. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention is a wind velocity calibration system and method. The system comprises a launcher, a projectile, an artificial aerosol cloud, at least two optical cameras, and an image processor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1 : Layout of present invention 
           [0014]      FIG. 2 : Artificial aerosol cloud 
           [0015]      FIG. 3 : Optical camera 
           [0016]      FIG. 4 : Launcher 
           [0017]      FIG. 5 : Serrated projectile surface 
           [0018]      FIG. 6 : Projectile shape 
           [0019]      FIG. 7 : Graph of apogee height (H) versus pressure (P) 
           [0020]      FIG. 8 : Image processor block diagram 
       
    
    
     DETAILED DESCRIPTION  
       [0021]      FIG. 1  shows the layout of the present invention. A launcher  400  send a projectile to a desired height H, where it disintegrates, forming an artificial aerosol cloud  200 . The cloud  200  is borne by the wind, which cause it to both translate and expand. The translation to new aerosol positions  240  and  280  is caused by the local average wind velocity vector W. The expansion is caused by small-scale atmospheric turbulence. 
         [0022]    Cameras  300  and  500  track the aerosol cloud as long as it is within the field of view of both cameras. Preferably, cameras  300  and  500  have wide-angle lenses and frame rates of at least 3 image frames per second. For a maximum horizontal wind velocity of 25 meters per second and a tracking period of 2 seconds, the aerosol cloud will have moved horizontally by 50 meters and each camera will have recorded at least 6 image frames. 
         [0023]    The two cameras are separated horizontally by baseline distance L, which may be 80 centimeters or more. The length L is sufficiently long to enable parallax determination of the aerosol cloud height with an accuracy of 0.2%. This is absolutely necessary in order to enable the wind velocity components to be determined with an accuracy of 0.5% at an altitude of 100 meters. The relative positions of the cameras are fixed by stereoscopic mount  600 , which includes shock absorbing means to damp the vibrations caused by the launcher. Each of the cameras has its own set of reference axes, denoted by x 1 -y 1 -z 1  for camera  300  and by x 2 -y 2 -z 2  for camera  500 . The two sets of reference axes have been transfer-aligned prior to launch. This includes the elimination of errors caused by roll, pitch, and yaw angles between the two sets of reference axes, as well as the correction of fixed camera assembly errors, such as a tilt angle between the plane of the image sensor and the principal plane of the lens, within each camera. The alignment techniques are known to those skilled in the art of stereoscopy, and are well described in publication [3] by Zhao and Nandhakumar, which is included herein by reference. 
         [0024]    Image processor  700  is a computer whose main function is to estimate the wind velocity vector W, by means of optical flow analysis of successive image frames, as provided by cameras  300  and  500 . Optical flow algorithms are known to those skilled in the art of image processing, and are described in publication [ 2 ] by Wedel et al, which is included herein by reference. Data bus  750  is used to transfer timing, status, data, and control signals between the image processor  700 , cameras  300  and  500 , and launcher  400 . 
         [0025]    Enclosure  800  protects the image processor and cameras from severe weather conditions, such as snow, rain, and temperatures as low as −40 degrees Celsius. The enclosure has a retractable roof which is opened during measurement periods, and closed otherwise. 
         [0026]      FIG. 2  shows a two-dimensional projection of a typical artificial aerosol cloud  205 , which corresponds to any one of clouds  200 ,  240  or  280  shown in  FIG. 1 . The cloud is viewed along axis z, which is approximately parallel to the line of sight to cameras  300  and  500  of  FIG. 1 . Cloud  205  is comprised of aerosol particles  210 , which may or may not be spherical in shape. Particles  210  are non-toxic, and typically have diameters of 5 to 50 microns. The lower limit of 5 microns is considered to be safe, with regard to inhalation in the human respiratory system. The upper limit of 50 microns is still small enough for the particles to be accelerated rapidly to the wind velocity by means of Stokes drag forces. For example, particles  210  may be microspheres of polyvinyl chloride (PVC), having a diameter of 30 microns and a density of 0.2 grams per cubic centimeter. Additionally, particles  210  may be colored to be easily visible to the cameras during daytime. For nighttime visibility, a pyrotechnic powder may be used. Further information regarding aerosol materials is found in publication [1] by Xiaoying Cao, which is included herein by reference. 
         [0027]    Dashed line  220  represents an imaginary bounding surface of the artificial aerosol cloud. For example, the bounding surface may be characterized by an ellipsoid centered at the center of mass, CM, with semi-axes denoted in the figure by a, b, and c. Let N denote the total number of aerosol particles and n(x,y,z) denote the average number of particles per unit volume at a point (x,y,z). For example, n(x,y,z) may be approximated by the Gaussian distribution: 
         [0000]        n ( x,y,z )=[ N /( abc )](2π) −3/2  exp [−½ ( x   2   /a   2   +y   2   /b   2   +z   2   /c   2 )]   (equation 1)
 
         [0000]    The pixel intensities in the camera images are proportional to Radon integral transforms of the function n(x,y,z) projected along lines joining CM to the cameras. 
         [0028]      FIG. 3  shows an optical camera  305 , which may correspond to either camera  300  or camera  500  in  FIG. 1 . Camera body  310  contains an electronic image sensor  330 , based on present-day CMOS or CCD technology, and digital electronics enabling video photography at frame rates of at least 3 frames per second. For example, camera  305  may be a Canon EOS-550d digital camera, having an image sensor with 18 million pixels. Lens  320  may be a wide-angle lens for low-altitude measurements (e.g. heights of 30 to 300 meters) or a telephoto lens for high-altitude measurements (e.g. 300 to 2000 meters). For example, for low-altitude measurements, the Canon EF-S 10-22 mm lens enables the angular field of view, denoted by FOV, to be as large as 74 degrees, with negligible optical aberrations. This corresponds to a linear field of view of 150 meters at an altitude of 100 meters. For high-altitude measurements, an exemplary lens  320  may be the Canon EF-S 18-200 mm lens. Cable  340  is a high definition multimedia interface (HDMI) for transferring digital images directly from the camera to the image processor. 
         [0029]    Depending upon the color of the artificial aerosol cloud, it may be advantageous to fit the camera with optical filters which selectively enhance the image contrast between the artificial aerosol cloud and the surrounding sky. Such filters may be in the ultraviolet, visible or near-infrared region of the optical spectrum. 
         [0030]      FIG. 4  shows an exemplary launcher, in accordance with this invention, of a type known as a compressed air cannon. This type of launcher is particularly suitable for low-altitude measurements; that is, for altitudes up to about 300 meters. Launcher  400  receives compressed air  410  from an external source (not shown), such as a diesel or electrically operated compressor, a pump, or a compressed air tank. Other gases may also be used, such as propane, nitrogen, or carbon dioxide. The compressed air flows through intake valve  420  into high pressure tank  430 , until reaching a desired gauge pressure of typically 2 to 14 atmospheres. The gauge pressure is adjusted for the desired measurement height, by means of pressure sensor  440 . The cannon is fired by opening quick release valve  450 , upon receipt of an activation signal from image processor  700 . Valve  450  may be, for example, an electrically controlled, solenoid-actuated diaphragm valve or poppet valve. The pressurized gas in tank  430  expands into barrel  460 , applying a force to projectile  470  and ejecting it from barrel  460 . The inside of barrel  460  may be smooth or rifled. For low-altitude measurements, the muzzle velocity of the projectile is typically between 50 and 150 meters/sec. Further details may be found in publication [4] by Rohrbach et al, which is included herein by reference. The launcher may optionally include a means for automatic loading of projectiles from a magazine. 
         [0031]    For high-altitude measurements, the preferred launcher is a fin-stabilized missile or rocket, fueled by liquid or solid propellants. 
         [0032]    Projectile  470  contains aerosol material and a small explosive charge for both dispersing the aerosol material and for destroying the outer surface and all internal components of the projectile. The diameter of the aerosol cloud formed by the explosive charge ranges from 50 centimeters for low-altitude measurements to about two meters for high-altitude measurements. The outer surface of the projectile, as well as all components inside the projectile, are made of frangible material which disintegrates into very small pieces, on the order of 2 millimeters in size, or smaller, when the explosive charge is detonated. This is very important for both safety and environmental considerations. Suitable frangible materials are described in patents U.S. Pat. No. 5,174,581, U.S. Pat. No. 3,840,232, and U.S. Pat. No. 3,554,552, whose bibliographic information is found in the section entitled “References Cited”. These patents are included herein by reference, in their entirety. 
         [0033]    In order to guarantee total disintegration of the projectile, it is advantageous to make serrated indentations on projectile surface  471  shown in  FIG. 5 . The indentations may be on the exterior or interior side of the projectile surface, depending on aerodynamic drag considerations. The surface thickness, denoted by “t”, is typically between 0.5 and 2 mm. The depth of the indentations is about 30 to 50% of the surface thickness. The dimensions denoted by a 1  and a 2  in  FIG. 5  are, for example, 2 mm. and 0.5 mm. respectively. 
         [0034]    The apogee height reached by projectile  470  is limited by gravity and aerodynamic drag. The aerodynamic drag depends upon both the geometric shape and smoothness of the projectile. For example, it is well-known in external ballistics that the aerodynamic drag coefficient of a sphere is approximately 0.5, whereas that of a blunt cylinder is approximately 0.8. 
         [0035]      FIG. 6  shows an exemplary projectile shape. Axis  472  is an axis of rotational symmetry. The projectile is comprised of cylinder  478  and spherical caps  474  and  476 . Cylinder  478  has radius C and height A. Spherical caps  474  and  476  have a common radius R, which is equal to the square root of [C 2 +(A/2) 2 ]. The diameter 2C of cylinder  478  is slightly smaller than the inside diameter B of barrel  460 . The difference (B−2C), is known as the “windage”. Exemplary values for A, B, and C are 10, 20.4, and 10 millimeters, respectively. When inserted into the barrel, the projectile is aligned parallel to axis  472 , and chemical fuse  479  is in the proper position to be struck and activated at the time of launch. 
         [0036]      FIG. 6  is intended merely as an illustration of one possible projectile shape. Many other projectile shapes are possible. For example, spherical cap  474  may be removed or replaced by an ogive, and spherical cap  476  may be removed altogether. 
         [0037]    The small explosive charge in projectile  470  may be detonated after a specific time of flight, by means of a time-delay mechanism such as a chemical time-delay fuse or an electronic long period delay detonator (LPD). The allowed tolerance in the initial height of the aerosol cloud is about ±5 meters, at an altitude of 100 meters. Assuming a projectile velocity of less than 10 meters/sec at the time of detonation, a detonator timing error of ±0.1 seconds will add an error of only ±1.0 meter to the initial height of the aerosol cloud, which is quite acceptable. 
         [0038]    Alternatively, the small explosive charge in projectile  470  may be detonated at the maximum height reached by the projectile by means of an apogee detector. The apogee height in meters, denoted by H, depends upon the pressure of the gas in the launcher, in units of psig, denoted by P.  FIG. 7  shows an exemplary graph of H versus P, for a spherical projectile having a diameter of 2.5 cm and a mass of 20.4 grams, which is fired vertically upwards. The points represent measured values and the solid line is an empirical fit of the form: 
         [0000]        H=a  log (1 +b P )   (equation 2)
 
         [0000]    where “log” is the natural logarithm, a=60.1 (meters), and b=0.17 (1/psig). Evaluating the derivative dH/dP, from equation (1), we find that dH/dP&lt;2.33 meters/psig over the range of pressures shown in  FIG. 7 . This means that, to achieve an accuracy of ±5 meters in the apogee height, the gas pressure in the launcher must be controlled with an accuracy of about ±2 psig. This accuracy is easily achievable with inexpensive pressure sensors and controllers. 
         [0039]      FIG. 8  shows a block diagram of image processor  700 . The image processor is a digital computer which is optimized for making rapid calculations on the images provided by the cameras. PS and OS denote the power supply and operating system, respectively. The timer, which may be the internal computer clock, is necessary for synchronizing the operation of the entire system. In addition there are various control blocks which communicate with data bus  750  for controlling the launcher, the cameras, and the input-output (I/O) ports. The external communication block, which is connected to antenna  760 , enables remote operation of the system and data transfer by means of WiFi or general packet radio service (GPRS). The image processing algorithms include software routines for (a) transfer alignment of the camera reference frames, (b) locating the aerosol cloud in successive image frames and finding its center-of-mass (CM), (c) calculating the height of the CM based on the disparity between left and right camera images, and (d) determining the wind velocity vector by means of optical flow and Kalman filtering techniques, which are well-known to practitioners in the field of image processing 
       EXTENSIONS OF THE INVENTION 
       [0040]    It is evident that there are many possible extensions and generalizations to the embodiments presented above. For example, in some applications, it may be advantageous to attach stereoscopic mount  600  to a mechanical scanning mechanism so that the cameras can follow the aerosol cloud over angles that exceed the optical field of view. It also may be desirable to use more than two cameras, provided the image processor can handle the added communication and processing loads. Furthermore, the image processor may include algorithms for analyzing the spread of the aerosol cloud over time, in order to estimate atmospheric turbulence parameters, in addition to the wind velocity vector. Atmospheric turbulence parameters are of special interest in airport traffic control systems and wind energy farms, because of the effects of strong turbulence on landing aircraft and on the rotors of wind turbines. 
         [0041]    Thus, while the invention has been described with respect to certain embodiments by way of example, it will be appreciated that the present invention is not limited to what has been particularly shown and described. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described above, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.