Abstract:
Reflective movable mirrors are used to reflect an image from a desired direction into the lens of a camera. This apparatus is preferably used with infrared (IR) cameras. To capture images from different directions, the orientations of the mirrors are modified. The mirrors are light, requiring only miniature motors and actuators for moving them. The mirrors are also much smaller than the focal plane electronics and the IR lens, requiring much less space for moving them than would be required for moving the camera. This provides a pointing capability for an airframe-fixed IR camera and this capability is provided with minimum additional payload, space and power requirements. The apparatus and method can be used for IR and electro-optical (EO) cameras.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on U.S. Provisional Application Ser. No. 60/716,208 entitled “Apparatus and Method for Providing Pointing Capability for Fixed Camera”, filed on Sep. 12, 2005, the teachings of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention (Technical Field) 
         [0003]    The present invention relates to cameras and more particularly to controlling the viewing direction such as pan and tilt angles, of a camera with extreme weight, size, and power restrictions. 
         [0004]    2. Background Art 
         [0005]    The present invention solves the problem where a pan-tilt mechanism must be extremely light and small, because it is intended for a very small unmanned air vehicle (UAV). It must also consume little electrical power to maximize battery life. Presently, pan-tilt units for cameras are known standard devices that one can purchase. However, these units are large, heavy and consume significant electrical power. Conventional pan-tilt units for cameras rotate the camera and its lens using motors and gears and aim it in the desired direction as shown in  FIG. 1 . Camera  100  is rotated in pan direction  102  by gears or other well known method and in tilt direction  104  in a similar fashion to point camera lens  106  towards the desired direction. An example of this type of device, intended for UAV applications, can be found in an advertisement for devices sold by L3 Communications BAI Aerosystems on their netsite: http://www.bai.aero/payloads.html#series66. A system is needed to accomplish the same functionality with a system that is small, light and consumes only a fraction of the power required for the prior art systems. Further, a similar system is needed for use of infrared (IR) cameras and their lenses which are larger and heavier than conventional visual light cameras, making the prior art systems prohibitive for IR camera use in small UAVs. 
         [0006]    Various optical systems, such as scanners and laser range finders, use mirrors to deflect laser beams. In addition, single lens reflex (SLR) cameras use mirrors to deflect images. 
         [0007]    SLR cameras have been around for decades as shown in U.S. Pat. Nos. 6,390,692 and 5,715,003. These cameras use a mirror that deflects the image (that already passed the lens) into the viewfinder. When a picture is taken, the mirror flips up and the image can reach the film that is in the back of the camera. This assures that the photographer sees in the viewfinder exactly what the film will see, even if he changes the lenses between shots. These two prior art patents deal with improving the mirror mechanisms for SLR cameras. 
         [0008]    U.S. Pat. No. 6,678,395 teaches a method of scanning an area with a mirror. The patent deals primarily with algorithms and methods that process data from a multi-spectral camera so as to detect targets with unique spectral and spatial characteristics. They use a gimbaled mirror to scan areas that are not directly under the rescue aircraft. They describe this feature as ‘the target area is scanned by a mirror oscillating about a scan axis across the flight path of the aircraft (cross-track)’. This prior art patent also discloses using the gimbaled mirror to deflect IR illumination at the area that is being captured by the camera. Yet another use of their gimbaled mirror is to stabilize the image by moving the mirror so as to offset aircraft vibrations. This device, in using a single mirror, cannot provide the coverage of the viewing area as the present invention. The present invention provides three hundred and sixty degree (360°) pan motion, preferably a plus or minus twenty-degree (±20°) tilt motion and capability to look down. The arrangement of mirrors and their rotations in the present invention meet all these specifications. 
         [0009]    Another prior art patent is U.S. Pat. No. 6,396,233, which deals with a mirror-based gimbal for a target-tracking seeker for a missile. This is a device that is mounted in the nose of a missile and can point at targets that are not exactly along the longitudinal axis of symmetry of the missile. The scanning range of a seeker can be up to a ninety-degree (90°) cone, meaning that the seeker must be able to deflect plus or minus forty five-degrees (±45°) in all directions. Traditionally, in non-mirror-based designs, these motions are provided by two motors, one scanning left and right forty five-degrees (45°), and the other scanning up and down forty five-degrees (45°). This prior art patent replaces the two-motor design with a mirror suspended on a ball joint, which allows any angular rotation about the joint. This rotational freedom is controlled by four Kevlar lines pulled by computer-controlled capstans. For any angular position of the mirror, there is a set of lengths of the four lines that bring the mirror to that angular position. This prior art device suffers from the same deficiencies of the &#39;395 patent in its limited viewing coverage. 
         [0010]    The present invention is a method and apparatus that uses three light and small mirrors that can be rotated in such a way that they provide the desired viewing coverage and meet the weight and size constraints of a small UAV. 
       SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION) 
       [0011]    The present invention is a method and apparatus for providing a viewing coverage area for a camera that is lightweight, small and that consumes a minimum amount of energy. This apparatus can be preferably used in UAVs such as a micro air vehicle (MAV) as shown in  FIG. 9 . This invention has a complete viewing coverage area of three hundred and sixty degree (360°) pan motion, and a tilt motion preferably of plus or minus twenty-degree (±20°), (i.e., looking above and below the horizon), and capability to look straight down. The tilt motion can be increased or decreased; however, this description describes only the preferred embodiment. This wide viewing coverage is an integral part of the invention. This is accomplished by moving mirrors to achieve the application-specific goal of providing camera pointing capability with minimum weight and size. 
         [0012]    Surveillance tasks for MAVs can include target detection and imaging with electro-optical (EO) and IR cameras. There is a need to pan and tilt these cameras when the vehicle is flying so that it can look away from its direction of flight and capture images of stationary and moving ground targets and of side views. Additionally, forward motion and winds can tilt the MAV from a desired vertical orientation and disable the ability to examine a scene from the needed direction and angle if the cameras are fixed to the airframe. 
         [0013]    Therefore, there is a need for mechanized pan and tilt motions for these cameras. However, an IR camera is relatively heavy and the physical dimension of its focal plane electronics and lens are quite large compared to the size of a vehicle as small as a MAV. Additionally, UAVs, such as the MAV, have limited payload capability and their compact design allows very limited space for mounting of surveillance cameras on them. 
         [0014]    A primary object of the present invention is to provide pointing capability of a camera with minimum weight, size and power consumption. 
         [0015]    Another object of the invention is to provide a camera pointing mechanism for use on a UAV. 
         [0016]    A primary advantage of the present invention is its low weight, enabling its installation on a small UAV with limited payload capability. 
         [0017]    Another advantage of the present invention is its small size, so that it will not significantly increase the size of the UAV and not interfere with its aerodynamics. 
         [0018]    Yet another advantage is the low power consumption because high power consumption requires a large battery that adds to the UAV payload. 
         [0019]    Another advantage of the present invention is that it can also be used in ground-based surveillance applications, and due to the small size of the invention, it improves the covertness of surveillance cameras, and low-power consumption extends battery life. 
         [0020]    Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0021]    The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. In the drawings: 
           [0022]      FIG. 1  shows the prior art method of pointing a camera with a conventional pan-tilt camera gimbal. 
           [0023]      FIG. 2  shows a first embodiment for a forward-looking camera. 
           [0024]      FIG. 3  shows the first embodiment for a pan tilt camera. 
           [0025]      FIG. 4  shows the first embodiment for a down-looking camera. 
           [0026]      FIG. 5  shows a second embodiment for a pan tilt camera. 
           [0027]      FIG. 6  shows the second embodiment for a down-looking camera. 
           [0028]      FIG. 7  shows a third embodiment for a forward-looking camera. 
           [0029]      FIG. 8  shows a fourth embodiment for bore-sighted EO and IR cameras. 
           [0030]      FIG. 9  shows the preferred embodiment of a MAV with a camera mounted in a pod. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Best Modes For Carrying Out the Invention 
       [0031]    The present invention is a method and apparatus for providing a camera viewing area for a camera mounted in a UAV such as a vertical take off and landing MAV. Four embodiments that illustrate the use of the present invention are described. Although the following descriptions are for IR cameras, the present invention can be used with all other types of cameras, thus this disclosure is not limited to IR cameras. The first embodiment is shown in  FIGS. 2 ,  3 , and  4 .  FIG. 2  shows an IR camera  10  mounted horizontally in a payload canister  20  that is attached to the MAV. Target image  12  enters camera lens  14  through a left upper IR-transparent window  16  on the left side  18  of canister  20 . This is a mode of operation where camera  10  is in fixed forward-looking position and the image quality is optimal because image  12  is not reflected off mirrors.  FIG. 3  shows a pan-tilt mode of operation of the first embodiment. IR reflective mirror M 2   22  is tilted forty-five degrees (45°) down, as shown. Target image  12  now enters canister  20  through left lower window  24  on the left side  18  of canister  20  and is reflected off mirrors M 3   26  and M 2   22  into the camera lens  14 . Mirror M 3   26  is mounted on a turntable  28  that can be positioned by lower motor  30  to provide camera rotation about a vertical axis  32 , i.e., camera pan. Note that the left lower IR-transparent window  24  on canister  20  is preferably along the entire circumference of canister  20  to allow three hundred and sixty degree (360°) ‘camera pan’ action. Mirror M 3   26  can also be rotated about a horizontal axis  34  by means of second motor  36  that is mounted on turntable  28 , and linkages  54 . Although each of the embodiments show a motor driving a linkage assembly  54 , other well known methods can be employed such as gears or belts, thus, this disclosure is not meant to limit the embodiments to any particular apparatus for accomplishing this purpose. This motion provides ‘camera tilt’ between about minus twenty degrees (−20°) and plus twenty degrees (+20°), allowing compensation for deviation of the MAV from vertical orientation.  FIG. 4  shows mirror M 1   38  tilted down forty-five degrees (45°), and mirrors M 2   22  and M 3   26  inactive. Image  12  enters canister  20  through bottom window  40  and is reflected off of mirror M 1   38  into lens  14 . This is a down-looking mode of operation. The pointing direction of the camera is determined by the angular positions of mirrors M 1   38 , M 2   22  and M 3   26 . These positions are set by a microcontroller that translates a request to point the camera in a desired direction into the required angular positions of the mirrors. The microcontroller then moves the mirrors into these angular positions by controlling the mirror motors and actuators. To summarize, the first embodiment provides fixed forward-looking and fixed down-looking capability, as well as pan-tilt capability in planes that can preferably be up to about twenty degrees (20°) off horizontal. 
         [0032]    The second embodiment of the invention is shown in  FIGS. 5 and 6 .  FIG. 5  shows camera  10  mounted vertically in canister  20 , and pointing down. Rotating mirror M 3   26  provides three hundred and sixty degree (360°) ‘camera pan’ and ‘camera tilt’ of between about minus twenty degrees (−20°) degrees and plus twenty degrees (+20°), just like in the first embodiment. Image  12  enters through left lower window  24  and is reflected off of mirror M 3   26  into camera lens  14 . Mirrors M 1   38  and M 2   22  are inactive.  FIG. 6  shows a mode of operation where mirror M 2   22  is tilted up forty-five degrees (45°) and mirror M 3   26  is inactive. Target image  12  travels through bottom window  40  and is reflected off fixed mirror M 1   38  and mirror M 2   22  into camera lens  14 . This mode provides a down-looking camera. 
         [0033]    The difference between the first embodiment and the second embodiment is that the first embodiment may provide better forward-looking image quality because it does not use mirrors in its forward-looking mode shown in  FIG. 2 . The second embodiment requires smaller horizontal dimensions and, therefore, may be easier to integrate into a MAV-sized vehicle. 
         [0034]    The third embodiment of the present invention is shown in  FIG. 7 . This embodiment is basically the first embodiment; however it is rotated ninety degrees (90°) counterclockwise, with mirror M 1   38  mounted so that it can tilt forty-five degrees (45°) clockwise or forty-five degrees (45°) counterclockwise.  FIG. 7  shows M 1   38  tilted forty-five degrees (45°) counterclockwise, providing view to the left through left lower window  24 . When mirror M 1   38  is tilted forty-five degrees (45°) clockwise, it provides a view to the right through right lower window  42 . When mirror M 1   38  is inactive and mirrors M 2   22  and M 3   26  are positioned as in  FIG. 3 , the system provides a three hundred sixty degree (360°) tilt and pan between about minus twenty degrees (−20°) and plus twenty degrees (+20°). This would allow continuous pointing capability anywhere ahead of the MAV, under it, or behind it, and could be used to offset large MAV tilt angles required in fast forward flight. 
         [0035]    In another alternative embodiment (not shown), a two (2) mirrored version can be utilized. However, this embodiment is not preferred because of its limitations and extrinsic parts. Mirror M 1  is used to provide pan and tilt, like in the above described embodiments. However, one of the mirrors is eliminated and the looking-down function is achieved by sacrificing the ability to pan backwards (i.e., pan of one hundred eighty degrees (180°)). Instead, the panning range is limited to approximately minus one hundred sixty degrees (−160°) to plus one hundred sixty degrees (+160°). When a user wants to look down, the pan angle is set to one hundred eighty degrees (180°) (i.e., M 1  pointing left), the tilt angle is set to zero degrees (0°), and mirror M 2 , mounted outside of the canister, is reflecting the image from under the canister into mirror M 1 , which is pointed to the left. 
         [0036]    The forth embodiment of the present invention is shown in  FIG. 8 . In this embodiment, two boresighted cameras, one EO camera  44  and the other an IR camera  46 , are recording image  12  simultaneously. The two boresighted camera embodiment can be incorporated into all the camera configurations shown in  FIGS. 2-7 .  FIG. 8  shows the forth embodiment applied to the camera configuration shown in  FIG. 6 . 
         [0037]    In  FIG. 8 , the single camera  10  shown in  FIG. 6  has been replaced with an EO camera  44 , an IR camera  46 , and a commercially available optical device called a cold mirror  48 . Cold mirror  48  is transparent at IR frequencies and it reflects light at visual (EO) frequencies. Therefore, when image  12  light reflected off mirror M 2   22  reaches cold mirror  48 , the image  12  light at IR frequencies passes through cold mirror  48  and reaches IR lens  50 . Image  12  light at EO frequencies (i.e., visual light) is reflected by cold mirror  48  to the left, into EO lens  52  of EO camera  44 . Thus, the two cameras record images of the same scene, but the IR camera records its IR image and the EO camera records its EO image. The two cameras are geometrically aligned (i.e., boresighted) so that image pixels of IR camera  46  correspond to image pixels of EO camera  44 . In this embodiment, all the windows on the payload canister are made of material that is transparent to both IR and EO light. One such material is ALON, marketed by Surmet Corporation. 
         [0038]    In using any of the embodiments as described above, the rotation of the mirror causes rotation of the image. When the pan mirror M 3   26  is at its nominal forward-looking angular position, as shown in  FIG. 5 , the vertical dimension of the scene is projected on the vertical dimension of the camera image. If the camera has an aspect ratio (width/height) of 4:3, for example, then the smaller dimension of the camera image will correspond to the vertical dimension of the scene. This is the normal situation that camera users expect. 
         [0039]    When the pan mirror rotates by 90 degrees from its nominal (forward-looking) angular position, the vertical dimension of the scene is projected on the horizontal dimension of the image, and vice versa. If the rotation is by an angle that is less than 90 degrees, the image is rotated by that angle. A brute-force fix to this optical effect is to rotate the display screen by the camera pan angle, if the display screen is movable. Once rotated, vertical scene direction will correspond to vertical image direction, but the sides of the image rectangle shown on the display will not be along the horizontal and vertical directions of the scene. 
         [0040]    A more reasonable way to handle the image rotation due to pan rotation is to rotate the image in software. Image-rotation algorithms are readily available and are built into many PC applications that handle images. Once the image is rotated in software to offset the mirror-caused rotation, vertical scene direction does correspond to vertical image direction, but the sides of the image rectangle are not along the horizontal and vertical directions of the scene on the display. 
         [0041]    To avoid the problem of image rectangle orientation when software image rotation is used, one could limit the displayed image to a circle with a diameter equal to the vertical dimension of the image frame. Then the circle will always be oriented correctly and there will not be the effect of a rotating rectangle. However, some information will be lost because the circle is only 59% of the rectangular image. A user-option can be included for selecting between the circular display and the rotating rectangle display modes. The circular image is more aesthetically pleasing. The rectangular image shows a larger part of the scene. 
         [0042]    Additionally, since mirrors are used in all of the embodiments, the camera will be recording mirror images of the scene. These mirror images will have to be corrected in software. Since an image processor will always be required to handle the cancellation of the mirror-image effect, it can also be used for the image rotation algorithm execution. 
         [0043]      FIG. 9  shows the preferred embodiment of a MAV with the camera configuration of  FIG. 5  mounted in a pod on the left. The other configurations as set forth above are similarly disposed within a pod. 
         [0044]    The described embodiments provide the capability to control the viewing direction of a camera with minimum additional weight, size, and electrical power. This is especially important for very small UAV&#39;s. 
         [0045]    Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above, are hereby incorporated by reference.