Patent Publication Number: US-6987492-B1

Title: Tetrahedral positioner for an antenna

Description:
GOVERNMENT RIGHTS 
   The present invention was made with support under Contract No. F19628-02-C-0048 awarded by the Government. The Government has certain rights in this invention. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an antenna assembly, more particularly, to a positioner for an antenna. 
   2. Background Information 
     FIGS. 1–2  show an example of an airborne and ground based antenna pointing mechanism, respectively. Antenna mounting systems or positioners for antennas typically utilize X-Y type mounts or have azimuth and elevation rotational axes. Positioners employing gimbal mounts have been used to position antennas, lasers and other devices. As a target moves relative to a gimbal-mounted antenna, the antenna moves to maintain the target within the beam of the antenna. The effectiveness of the device is dependent upon the precision and stability of the positioning. 
   Antennas are typically isolated as much as possible from their host, such as a high-altitude airplane, to avoid pointing errors. Position feedback is often provided to a position control system to effect precise positioning. Cables providing electronic signal interchange and power transfer between the antenna platform and aircraft are also typically routed in a manner to minimize the forces exerted upon the platform. 
   Aircraft antenna mounting devices or positioners are often complex and bulky. Considerations, such as the aerodynamic requirements, space limitations and operational requirements of modern aircrafts create difficulties in the design of effective, light weight and accurate positioners for use with antenna structures, particularly airborne antennas. 
   Accordingly, there is a desire to provide a light weight and accurate positioner for an antenna. There is also a desire to provide an antenna assembly for airborne usage, particularly for use during aircraft operation. The present invention satisfies these needs and others. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention, an antenna assembly is disclosed. The assembly comprises an antenna; and a positioner capable of assuming a tetrahedral shape connected to the antenna and mounted to an antenna mount pedestal. The positioner comprises a substantially flat base portion; and a triangular shaped antenna face bracket rotatably coupled to the base portion. The positioner further comprises an extendable, linear stiffening member connected between the base portion and the triangular shaped antenna face bracket; and an actuator to rotate the triangular shaped face bracket of the antenna assembly and form the positioner in combination with the linear stiffening member, triangular shaped antenna face bracket and base portion. 
   In accordance with another aspect of the present invention, a tetrahedral-shaped positioner for an antenna is disclosed. The positioner comprises a substantially flat base portion; and a triangular shaped antenna face bracket rotatably coupled to the base portion. The positioner further comprises an extendable, linear stiffening member connected between the base portion and the triangular shaped antenna face bracket; and an actuator to rotate the triangular shaped face bracket of the antenna assembly and form the positioner in combination with the linear stiffening member, triangular shaped antenna face bracket and base portion. 
   In accordance with a further aspect of the invention, a method of positioning an antenna is disclosed. The method comprises providing the afore-described antenna assembly and securing the antenna mount pedestal on a substrate selected from the group consisting of an airborne based device and a ground based device. The method further comprises controlling the linear member to position the antenna. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of an airborne antenna pointing mechanism (prior art); 
       FIG. 2  is a perspective view of a ground based antenna pointing mechanism (prior art); 
       FIG. 3  is a perspective view of an antenna assembly, including a tetrahedral positioner for a low-profile, three degree of freedom directional antenna, in accordance with an embodiment of the invention; 
       FIG. 4  is a side view of the antenna assembly of  FIG. 3 ; 
       FIG. 5  is a rear view of the antenna assembly of  FIG. 3 ; and 
       FIG. 6  is a perspective view of an aircraft upon which the antenna assembly of  FIGS. 3–5  may be mounted. 
       FIG. 7  is a perspective view of an antenna assembly inside a randome, in accordance with an embodiment of the invention; 
       FIG. 8  is a side view of an antenna assembly showing an anti-lock spring, in accordance with an embodiment of the invention; 
       FIG. 9  is a side view of an antenna assembly showing a cross-elevation axis, in accordance with an embodiment of the invention; 
       FIG. 10  is an overhead view of  FIG. 3 ; 
       FIG. 11  shows the antenna assembly of  FIG. 3  with the reflector in a folded position; 
       FIG. 12  shows the antenna assembly of  FIG. 3  with the reflector in a partially folded position; 
       FIG. 13  shows a simplified view of the tetrahedral positioner of  FIG. 3 ; and 
       FIG. 14  shows the connector of  FIG. 3  in greater detail. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 3 , there is shown a perspective view of an antenna assembly  6 , incorporating features of the present invention. The antenna assembly  6  advantageously includes a positioner  8  capable of assuming a tetrahedral shape, as shown in this figure. Although the present invention will be described with reference to the embodiment shown in the drawings, it should be understood that the present invention can be embodied in many alternate forms of embodiments. In addition, any suitable size, shape or type of elements or materials could be used. 
   An embodiment of the invention is directed to a tetrahedral positioner, which may be used to mount an antenna such as a directional antenna on an aircraft or other suitable vehicle. New features of embodiments of the invention include a tetrahedral shaped positioner, as well as a linear actuator as one leg of a space-frame antenna support structure to minimize height and weight. A tapered anti-detent spring may also be used beyond zenith to mitigate potential mechanical latch-up. 
   Another feature of embodiments of the invention is that servo design is simplified due to the use of a non-overriding linear actuator. This allows a space frame structural member to act as an active element without requiring a power-consuming brake or heavy counterweight. The tetrahedral space-frame design is preferably triangulated in three dimensions for a high stiffness to weight ratio. High specific stiffness is important for passively obtaining high pointing accuracy while being subjected to aircraft vibration and maneuvers. 
   In the embodiment shown in  FIGS. 3–5  and  7 – 12 , the positioner  8  can be mounted to an aircraft  10  or other suitable device, such as a land or ocean based vehicle including automobiles and ships. Examples of aircraft  10  include manned vehicles, as well as an unmanned aerial vehicle (UAV). An aircraft  10  in the form of a UAV is show in  FIG. 6  generally comprising an air frame  12 , a drive  14  and a viewing unit  16 . The antenna assembly  6  of  FIGS. 3–5  may be conventionally mounted to the aircraft  10  by bolting the base portion to a provided flange on the fuselage. As shown in  FIG. 7 , the antenna assembly  6  may advantageously fit within an aircraft radome  11  of any suitable size and shape. Advantageously, the antenna assembly  6  may be employed in any suitable type of device. 
   The air frame of the aircraft  10  may be a fixed wing type of air frame. However, features of the present invention could alternatively be used in a non-fixed wing aircraft or other suitable vehicles or devices. The drive  14 , in the embodiment shown in  FIG. 6 , comprises a motor and a propeller. However, in alternate embodiments, any suitable type of drive could be used, such as a turbine engine. The viewing unit  16  includes an optical video camera, but could alternatively or additionally comprise an infrared video camera or any other suitable type of viewing device. The antenna assembly  6  may be used to allow remote control of the aircraft and transmission of signals from the viewing unit  16  back to a remote viewing area. 
   Referring to  FIGS. 3–5  and  9 – 12 , the antenna assembly  6  generally comprises a positioner  8 , an antenna  18  and an antenna mount pedestal  20  adapted to be mounted on aircraft  10  or other suitable vehicle. The antenna mount pedestal  20  preferably comprises an azimuth drive and an azimuth motor. The pedestal  20  may also be a direct drive or a gear. This device may include a position feedback mechanism that controls the positioning of the antenna  18 . Alternatively, the antenna mount pedestal  20  may comprise a conventional X-Y pitch arrangement suitable for aircraft radar systems. In a preferred embodiment, the antenna mount pedestal  20  is rigid in design with respect to the platform and does not provide external rotation, but includes internal rotating components. Alternatively, the pedestal  20  may include an external rotational feature. 
   Secured to the antenna mount pedestal  20  is the positioner  8 , as shown in  FIGS. 3–5  and  9 – 12 . The positioner  8  preferably includes a rotatable three legged rigid support structure capable of forming the tetrahedral shape.  FIG. 13  illustrates a simplified view of this tetrahedral shape. 
   In the embodiment shown in  FIGS. 3–5  and  9 – 12 , positioner  8  includes a substantially flat base portion  22 , a triangular shaped antenna face bracket  26  rotatably coupled to the base portion  22 ; an extendable, linear stiffening member  24  connected between the base portion  22  and the triangular shaped antenna face bracket  26 ; and an actuator  29  connected to the member  24 . Preferably, each leg is spaced approximately equidistant from each other. 
   Base portion  22  is preferably triangular in shape and advantageously provides a light weight structure. Other suitable shapes, such as circular, square and rectangular shapes, among others, may also be employed for base portion  22 . 
   Base portion  22  also preferably includes an anti-detent or anti-lock spring  28 , as best seen in  FIG. 3 . For example, during operation the antenna  18  may rotate or fold to approach a closed position. To prevent locking, spring  28  can be employed on base portion  22  as a force to offset the inertia of the folding antenna  18  thereby advantageously preventing lock up. As also shown in  FIG. 8  and in accordance with an embodiment of the invention, a full 100° elevation range may be obtained by the anti-lock feature, including spring  28 . 
   Base portion  22  also preferably includes an opening or aperture  30  through which screws may bolt to the pedestal  20  for securing base portion  22  to the pedestal  20 . A waveguide transmit channel  32  may extend from aperture  30  to a conventional rotary joint connection  34  enabling transmission of RF energy through the azimuth drive unit. In other embodiments, the RF energy could be conveyed through a coaxial cable or optical energy could be used. The optical drive could be mounted to reflector  46  of the antenna assembly  6  to minimize losses. Dual RF/optical systems can also benefit from the teachings of this invention. Moreover, the reflector  46  could be used with laser devices and thus the present invention is not limited to use with RF antennas. 
   Linear stiffening member  24  is coupled to the base portion  22 , as shown in  FIGS. 3–5 , and provides the center leg of the positioner  8 . Preferably, member  24  is a linear actuator  27 , as shown in the figures, advantageously providing an anti-back drive capability. For example, in conventional antenna designs, a brake or high gear ratio is required. Such features are not required in embodiments of the invention with use of linear actuator  27  as part of the positioner  8 . For example, linear actuator  27  may includes a sleeve portion  36  with an internal rotating ball screw or drive mechanism attached to a brushless DC actuator. Alternatively, AC or brush type linear actuators may also be employed. The linear actuator  27  also includes a sliding portion  38 , which can feed into and out of the stationary actuator assembly. Advantageously, the linear actuator  27  is of a stiff construction and is accurate in its positioning of antenna  18 . For instance, the linear actuator  27  may extend and retract to properly position antenna  18 . Linear actuator  27  may also turn the power to the antenna  16  to an on/off position as it is controlled by a drive control motor in the afore-described pedestal  20 . As shown in the  FIGS. 3–5 , the linear actuator  27  preferably includes an actuator or motor  29  located at one end of the linear actuator  27 . In this case, the linear actuator  27  may also function as a dampening mechanism. 
   In some embodiments, the linear stiffening member  24  may simply be a linear stiffener without an actuator or motor  29 . Thus, member  24  may provide a supporting center structure for the positioner  8  with or without a linear sensing device located at one end of the member  24  in place of the actuator or motor  29 . In these embodiments, the actuator or motor  29  may be located on other parts of the positioner  8  to provide the controlling action, as described in further detail below. 
   The antenna face bracket  26  connected to linear member  24  is preferably triangular in shape, as shown in  FIGS. 3–5  and advantageously provides a light weight structure, as well as a high specific stiffness (stiffness to weight ratio). The base of the triangular antenna face bracket  26  is preferably mounted to the base portion  22  of the positioner  8  by hinges  31 , shown in  FIG. 5 . The top portion  40  of the triangular antenna face bracket  26  provides an aperture or opening  42  for linear member  24 , as shown in  FIG. 3 . As in the case of the other legs of the positioner  8 , the triangular antenna face bracket  26  may be of any suitable width, height and length. Preferably, as best seen in  FIG. 5 , the height of the triangular antenna face bracket  26  measured from its top portion  40  to its base is less than about ten inches and preferably less than about six inches. The triangular antenna face bracket  26  also preferably includes hinges  31 , as shown in  FIG. 3 , located in an outward position to enhance the stiffening and stability capabilities of the positioner  8 . A connector  37  may also be provided at the top portion  40 . This connector  37  is shown in further detail in  FIG. 14  and comprises a bracket  39  for rotatably connecting the reflector  46  to the linear member  24 . 
   An actuator or motor  29  may be located at one or both ends of the triangular antenna face bracket  26  adjacent the base portion  22 . Similarly, a cross-elevation actuator could be employed at the top portion  40  of the triangular antenna face bracket  26 . 
   Cross-elevational bands  33 , as shown in  FIGS. 5 and 9 , may also be employed to move the reflector  46  about the cross-elevational axis. Advantageously, at high pointing angles, the bands  33  provide extra ability for accurate pointing. However, if the reflector  46  is in a fully folded position, the bands  33  may not need to be employed for control. Similarly, if a cross-elevation actuator is employed at the top portion  40  of the bracket  26 , the bands  33  may not need to be employed. 
   At an intersection of the triangular antenna face bracket  26  and the base portion  22  of the positioner  8  is typically an angular position detector  44 , such as a three or four wire device or optical encoder, providing position feedback, as shown in  FIG. 3 . Other suitable detectors include resolvers, synchros, magnetic encoders and potentiometers, angular sensing and angular rotation sensing devices. Preferably, this device is a cylindrical, elevation multi-turn resolver. 
   The triangular antenna face bracket  26  supports the antenna  18 . More particularly, the bracket  26  is secured to one side of reflector  46  of the antenna  18 . The reflector  46  may be of any suitable shape. For example, the reflector  46  may be of a dish shape, as shown in  FIGS. 3–5 , or may be rectangular or circular in construction, among other suitable shapes. The reflector  46  may also be of any suitable size and is preferably less than about 12 inches in height, as shown in  FIG. 5 . Similarly, the reflector  46  may be made out of any suitable material including, for example, high strength polymeric, composite and metallic materials. 
   The antenna  18  also preferably includes a tripartite antenna structure  48 , as shown in  FIG. 4 . However, other suitable antenna constructions may also be employed. The antenna  18  is also preferably a directional antenna although other types of antennas, such as ommi-directional, are also suitable. The antenna assembly  6  is also preferably constructed such that during level flight the antenna  18  is substantially parallel to the surface of the earth. 
   The antenna assembly  6  may also advantageously include a cross-elevation axis or three-axis pedestal  50 , as shown in  FIG. 9 , about which the reflector  46  rotates. Advantageously, a cross-elevation of about +/−7 degrees may be achieved in accordance with embodiments of the invention. Alternatively, the cross-elevation axis may include a two-axis pedestal. 
   Advantageously, embodiments of the invention also provide a low cost intrinsically stiff cross-elevation over elevation over azimuth gimbal system for low-profile antenna pointing, which overcomes design challenges. For example, each axis presents a unique design challenge, such as high performance servo (azimuth), size and package constraints (elevation) and clearance at extreme positions (cross-elevation). 
   Thus, an advantage of embodiments of the invention includes the unique EL/X-EL/Azimuth design. For example, advantages of the azimuth axis design include the following: direct drive in azimuth, which is very responsive, accurate and eliminates gearing backlash; preloaded crossed-roller bearings, which provide positioning accuracy, stiffness and robustness, although taper-loaded bearing may also be employed; and slip-ring for 360° continuous rotation, including redundant Au contacts and double seals. Advantages of the elevation and cross-elevation design include its low cost potential and simple design construction. 
   Another advantage of embodiments of the present invention includes providing a light weight and accurate positioner for an antenna, particularly an airborne antenna. The total weight of the positioner  8  may vary depending on, for example, the size of the randome employed, aircraft or other considerations. The components of the positioner  8  may be made out of any suitable material, including conventional high strength polymeric, metallic and composite materials. For example, materials such as stainless steel, carbon fiber, etc. may be employed. 
   It should be understood that the foregoing descriptions are only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.