Abstract:
An assembly structure and method for housing a remote lens unit assembly of a laser Doppler velocimeter. The housing includes one or more transceiver telescopes and an optical window. The transceiver telescopes are aligned such that optical beams from the mounted optical transceiver telescopes cross paths at the optical window. The housing is mountable on a wind turbine.

Description:
PRIORITY CLAIM 
       [0001]    The present application is a continuation of U.S. application Ser. No. 12/723,333, filed Mar. 12, 2010, entitled REMOTE LENS UNIT ASSEMBLY FOR LASER DOPPLER VELOCIMETER, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The disclosure relates to a structure housing components of a laser Doppler velocimeter and in particular to a structure housing the telescopic elements of a laser Doppler velocimeter. 
         [0003]    A laser Doppler velocimeter (“LDV”) transmits light to a target region (e.g., into the atmosphere) and receives a portion of that light after it has scattered or reflected from the target region or scatterers in the target region. Using the received portion of scattered or reflected light, the LDV determines the velocity of the target relative to the LDV. Actual or non-relative velocity may also be determined. LDVs are extremely useful and have a wide range of applications including, but not limited to: blood-flow measurements, speed-limit enforcement, spaceship navigation, projectile tracking, and air-speed measurement. In the latter case the target consists of aerosols (resulting in Mie scattering), or the air molecules themselves (resulting in Rayleigh scattering). 
         [0004]    An air speed LDV includes a source of coherent light, a beam shaper and one or more telescopes. The telescopes each project a generated beam of light into a target region. The beams strike airborne scatterers (or air molecules) in the target region, resulting in one or more back-reflected or backscattered beams. In a mono static configuration, a portion of the backscattered beams is collected by the same telescopes which transmitted the beams. The received beams are combined with reference beams in order to detect a Doppler frequency shift from which velocity may be determined. 
         [0005]    An LDV, as disclosed in International Application Publication No. WO/20091134221 (“the &#39;221 publication”), the entirety of which is hereby incorporated by reference, may include at least three transceiver telescopes that are remotely located from the LDV coherent light source. As disclosed in an embodiment of the &#39;221 publication, the disclosed LDV includes an active lasing medium, such as e.g., an erbium-doped glass fiber amplifier for generating and amplifying a beam of coherent optical energy and a remote optical system coupled to the beam for directing the beam a predetermined distance to a scatterer of radiant energy. The remote optical system includes “n” duplicate transceivers (where n is an integer that may be, for example, three) for simultaneously measuring n components of velocity along n noncolinear axes. As disclosed in the &#39;221 application, the optical fiber is used to both generate and wave guide the to-be-transmitted laser beam. A seed laser from the source is amplified and, if desired, pulsed and frequency offset, and then split into n source beams. The n source beams are each delivered to an amplifier assembly that is located within the n transceiver modules, where each of the n transceiver modules also includes a telescope. Amplification of the n source beams occurs at the transceiver modules, just before the n beams are transmitted through the telescope lens to one or more target regions. When the n source beams are conveyed through connecting fibers from the laser source to each of the n telescopes within the respective transceiver modules, the power of each of the n source beams is low enough so as not to introduce non-linear behaviors from the optical fibers. Instead, power amplification occurs in the transceiver module, just before transmission from the telescope. Consequently, fiber non-linear effects are not introduced into the system. 
         [0006]    By using the LDV disclosed in the &#39;221 application, object or wind velocities may be measured with a high degree of accuracy. Because the source laser is split into n beams, the measurements taken along all of the n axes are simultaneous. Additionally, splitting the source beam into n beams does not necessarily require that the source laser transmit a laser with n times the necessary transmit power, because each of the n beams are subsequently power amplified before transmission. Additionally, the disclosed LDV has no moving parts, and is thus of reduced size and improved durability. The disclosed LDV may be used with a platform motion sensing device such as e.g., an inertial measurement unit (“IMU”) or global positioning satellite (“GPS”) unit so that the motion of the LDV platform may be compensated during calculation of the measured velocities. Thus, because of the light-weight and non-bulky nature of the LDV, and because of the LDV&#39;s ability to compensate for platform motion, the disclosed LDV may be mounted on any moving platform (e.g., a helicopter, a boat, etc.) and still obtain highly accurate readings. 
         [0007]    As mentioned above, the transceiver telescopes, as well as their respective amplifier assemblies, may be located remotely from the LDV light source and other components. The remotely located transceiver assemblies may be positioned in a variety of locations not necessarily suitable for mounting an entire LDV, such as, for example, on the nacelle or hub of a wind turbine. Mounting the transceiver assemblies remotely from the remainder of the LDV can subject the transceiver assemblies to harsher environmental conditions than that of the remainder of the LDV. What is needed is a durable housing that protects the components from environmental conditions including temperature fluctuations and moisture. The desired housing must be able to house one or more telescope transceivers in an environmentally protected manner. The housing may also enclose additional components of the LDV, such as one or more amplifiers. In an embodiment wherein the entire LDV is enclosed within the housing, the housing must be capable of encasing a source laser for the transceiver telescopes. 
         [0008]    What is needed, then, is an environmentally-protective structure to house the remote lens assembly of an LDV, or alternatively, the entire LDV. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates a remote lens unit assembly in accordance with embodiments of the disclosed invention; 
           [0010]      FIG. 2  illustrates a transceiver telescope assembly in accordance with embodiments of the disclosed invention; 
           [0011]      FIG. 3  illustrates a side view of the remote lens unit assembly in accordance with embodiments of the disclosed invention; 
           [0012]      FIG. 4  illustrates a front view of the remote lens unit assembly in accordance with embodiments of the disclosed invention; 
           [0013]      FIG. 5  illustrates a mounting frame for a transceiver telescope assembly in accordance with embodiments of the disclosed invention; 
           [0014]      FIG. 6  illustrates the remote lens unit assembly and mounting frame in accordance with embodiments of the disclosed invention; 
           [0015]      FIG. 7  illustrates a top view of the remote lens unit assembly in accordance with embodiments of the disclosed invention; 
           [0016]      FIG. 8  illustrates a bottom view of the remote lens unit assembly in accordance with embodiments of the disclosed invention; and 
           [0017]      FIG. 9  illustrates a remote lens unit assembly mounted on a wind turbine in accordance with embodiments of the disclosed invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    In order to solve the problems described above in connection with protecting the sensitive components of the remote transceiver assembly, the remote lens unit assembly of the LDV or, alternatively, the entire LDV disclosed in the &#39;221 application, may be housed in a structure as illustrated, for example, in  FIG. 1 .  FIG. 1  illustrates a remote lens unit (“RLU”) assembly  100  that includes, for example, an optical window  120  through which laser transmission and reception is achieved, a wiper motor assembly  140  and a housing  160  for up to three individual light detection and ranging (“LIDAR”) transceiver telescope assemblies. In an embodiment of the disclosure, the remote lens unit (“RLU”) assembly  100  is egg-shaped. However, other shapes may be used, as understood by a person of ordinary skill in the art. 
         [0019]    The RLU assembly  100  includes a single optical window  120 . The optical window  120  is used by each of the transceiver telescope assemblies within the structure. By angling the transceiver telescope assemblies so that the output lasers each cross paths at the position of the optical window  120 , the diameter of the optical window  120  is reduced to the approximate diameter of a single output laser from one of the transceiver telescope assemblies. For example,  FIG. 2  illustrates how three transceiver telescope assemblies  220   a,    220   b,    220   c  can be angled so that the respective beam paths each cross at a single plane. The plane at which the beams cross is where the optical window  120  is positioned. By minimizing the size of the optical window  120  and by limiting the number of optical windows to one, maintenance concerns for the structure are greatly reduced and simplified. A small, single optical window  120  means that only a small surface area needs to be maintained as clean for the transceiver beams. A single window  120  also reduces the potential for moisture ingress and other environmental effects on the components within the RLU assembly  100 . 
         [0020]    The single optical window  120  is protected by a visor  320  mounted above the window  120 , as illustrated in  FIG. 3 , which shows a side view of the RLU assembly  100 . The single window  120  needs only one wiper motor assembly  140 . The single wiper motor assembly  140  is mounted via mounting bracket  142  onto, or within, the RLU assembly  100  so that a wiper blade  340  can wipe moisture away from the optical window  120  as needed. As illustrated in  FIG. 4 , which shows a front view of the RLU assembly  100 , the wiper blade  340  is of sufficient size to clean at least the portion of the optical window  120  that covers an aperture  420  in the housing  160  that allows the telescope beams to pass into and out of the RLU assembly  100 . Adjacent to the wiper blade  340  is a washer fluid nozzle  440  for dispensing washer fluid to the optical window  120  when the wiper motor assembly  140  is in operation. The washer fluid is supplied to the washer fluid nozzle  440  via a washer fluid supply tube  445 . Although not illustrated here, other known moisture-removing devices may also be used to remove moisture from the window  120 . 
         [0021]    In connection with an exemplary embodiment, the optical window  120  is formed of silicon. A silicon window allows for transmission of the laser beams at the expected operational wavelengths for LIDAR operation. A silicon window also has a high thermal conductivity relative to other optical materials. This promotes the inclusion of simple heaters to heat or defrost the silicon window  120 . For example, the optical window  120  includes a rod heater  425  affixed to or near the window  120  to heat the window  120  so as to avoid ice buildup on the window  120 . The temperature of the rod heater  425  is controlled using an independent thermostat  435 . The window  120  is supported by a window mount  430  that includes a mount for the window  120 , the rod heater  425  and the window thermostat  435 . The window mount  430  is thermally isolated from the remainder of the RLU assembly  100  so that heat from the rod heater  425  is localized to the window  120  and has minimal effect on the assembly&#39;s optical components. The mount  430  is adjustable so that the window surface can be made to be flush with the external surface of the RLU assembly  100 . A seal fitting the window  120  to the window mount  430  prevents moisture and other contaminants from entering the RLU assembly  100  through the window mount  430 . 
         [0022]    Inside the RLU assembly  100  is a rigid internal mounting frame  580  to accurately position each telescope assembly  220   a,    220   b,    220   c  (collectively,  220 ) with respect to each other, as is illustrated in  FIGS. 5 and 6 . In  FIG. 5 , only two telescope assemblies  220   a,    220   b  are illustrated, while the third telescope assembly  220   c  is positioned behind one of the other visible telescope assemblies  220   a,    220   b.  As shown in  FIGS. 5 and 6 , the mounting frame  580  may be located near the rear of the housing  160  for mounting up to three telescope assemblies  220 . In one embodiment, the telescope assemblies  220  are mounted equidistant from each other. In the case of three telescope assemblies  220 , the assemblies  220  are mounted on the mounting frame  580  at 120 degree intervals. Variations in this spacing of the telescope assemblies  220  may also be used. The mounting frame  580  also establishes the orientation of the telescope assemblies  220 . In an exemplary embodiment, as shown in  FIG. 5 , the telescope assemblies  220  are each oriented along the surface of a cone whose apex is at the silicon window  120 . As an example, the cone may be a 15-degree half angle cone, meaning that the telescope assemblies  220  are each positioned at a 15 degree angle from the longitudinal axis of the RLU assembly  100 . Variations in the orientation angle may also be made, according to the size of the RLU assembly  100  and the dimensions of the space to be measured by the LDV. 
         [0023]    In addition to the telescope assemblies  220 , the mounting frame  580  is able to mount up to three power amplifier subassemblies  585   a,    585   b,    585   c  for corresponding telescope assemblies  220 . The mounting frame  580  may also be used to mount an altitude and heading reference system (“AHRS”)  590 . The AHRS  590  is used to determine the orientation of the RLU assembly  100  itself. The optical components are mounted to the mounting frame  580  using ceramic bushings  595  in order to thermally isolate the optical components from the rest of the structure. The mounting frame  580  also includes heaters  599  operating from minimal heating power in order to maintain an operating temperature for the optical system. 
         [0024]      FIGS. 7 and 8  respectively illustrate top and bottom views of the RLU assembly  100 . In  FIG. 7 , the housing  160  includes covers  760 ,  762 ,  764 . Covers  762  and  764  are formed, for example, as spun aluminum covers which form a complete  3600  shell for the front and back of the RLU assembly  100 . Cover  760  is a cylindrical cover that fits between covers  762 ,  764 . Cover  760  may also be formed of aluminum, for example. The resulting covers  760 ,  762 ,  764  are cost effective and strong with a minimal amount of joints. The main covers  760 ,  762 ,  764 , and other components of the RLU assembly  100  may be joined together via brazing or welding or using other methods known in the art. The housing  160  provides a hermetic shell that limits humidity ingress to the interior of the RLU assembly  100 . In addition, the RLU assembly  100  includes a serviceable desiccant for absorbing excess moisture within the RLU assembly  100 . Sealants are used at joints and openings to reduce the amount of moisture that enters the RLU assembly  100 . 
         [0025]    Referring to  FIG. 8 , the housing  160  also includes a center body  860  that includes cable egress and mounting features. In order to provide power and control to the components within the interior of the RLU assembly  100 , the center body  860  includes connectors for fiber optic and electrical inputs. Fiber optic connectors  865   a,    865   b,    865   c  are included for each respective telescope assembly  220 . As explained in the &#39;221 application, the RLU assembly  100  may be separate and remote from a laser source and other components of the LDV. In such a case, the RLU assembly  100  is coupled to the laser source and other components via fiber optic cables connected to the RLU assembly  100  through fiber optic connectors  865   a,    865   b,    865   c.  Alternatively, the RLU assembly  100  may be made to include additional LDV components such as a laser source. 
         [0026]    An electrical and control signal connector  870  is used to provide electricity and control to the AHRS  590  and heaters  599  in the assembly (including the rod heater  425 ). The electrical and control signal connector  870  could be used to control the wiper motor assembly  140  as well. Alternatively, a separate wiper control connector  875  may be used. A wiper fluid connection  880  is provided to the wiper motor assembly  140  and the washer fluid tube  445 . Other connectors  876  may be included on the center body  860  for mounting the RLU assembly  100  to a structure, such as to a wind turbine. 
         [0027]      FIG. 9  illustrates the RLU assembly  100  mounted on the nacelle  910  of a wind turbine  900  in order to measure wind velocity, as described in the &#39;221 application. Alternatively, the RLU assembly  100  may be mounted at other locations on or near the wind turbine  900 . The RLU assembly  100  is communicatively coupled to a laser source and other LDV components  901 , located, for example, at the base of the wind turbine  900 . The advantages of the RLU assembly  100  enable the assembly with all of its included components to be mounted in exposed and difficult-to-access locations, such as on the wind turbine  900 , while other components  901  of the LDV (e.g., laser source, etc.) may be located elsewhere and out of the elements. Other mounting locations include helicopters and other aircraft, towers, buildings and other moving vehicles such as boats, etc. 
         [0028]    The above description and drawings should only be considered illustrative of embodiments that achieve the features and advantages described herein. Modification and substitutions to specific structures can be made. Accordingly, the claimed invention is not to be considered as being limited by the foregoing description and drawings.