Abstract:
A device for positioning an antenna on a vehicle. The device includes a first motor-driven timing belt connected to opposite sides of the antenna frame for rotating the antenna about its elevational axis. A constant force spring or a cam compensates for changes in belt path geometry as the antenna rotates. A second motor-driven timing belt is wrapped around a perimeter of a base plate for rotating the antenna about its azimuth axis. Electrical signals for the antenna and the drive motors are multiplexed and passed through a single coaxial cable in a rotary joint along the axis of azimuth rotation.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of satellite antennas, and more specifically to the field of mechanisms for positioning an antenna, and in particular, to the field of mechanisms for positioning an antenna for use on a recreational vehicle. 
     BACKGROUND OF THE INVENTION 
     It is known to place an antenna on a vehicle such as a boat or recreational vehicle (RV) for receiving signals from a satellite, for example, a direct television signal. U.S. Pat. No. 5,517,205 issued to van Heyningen, et al, U.S. Pat. No. 5,528,250 issued to Sherwood, et al, and U.S. Pat. No. 5,585,804 issued to Rodeffer teach various apparatus and methods for mounting and positioning such antennas. Each of these patents is incorporated by reference herein. 
     Prior art devices for positioning antenna are typically driven by electric motors connected to the antenna by a gear mechanism. To reduce the size and cost of the drive motors, high ratio gear trains are often employed. However, such gearing systems create excessive slop in the drive train, thereby limiting the precision with which the antenna can be positioned. Furthermore, prior art devices are often limited in the amount of rotation that can be provided in the azimuth direction. For applications on boats and RV&#39;s an unlimited amount of rotation is desirable. 
     Accordingly, it is an object of this invention to provide a device for positioning an antenna that has small size, low cost, high accuracy of position, and an unlimited range of movement in the azimuth direction. 
     SUMMARY 
     In order to achieve these and other objects of the invention, a device for positioning an antenna is provided having: a frame for mounting the antenna, the frame having a first arm and a second arm connected about an axis of elevational rotation; a first timing belt having a first end connected to the first arm and a second end connected to the second arm; and a means for driving the first timing belt to rotate the frame about the axis of elevational rotation. The device may further have a means for maintaining tension in the first timing belt as the antenna is rotated. A device according to this invention may also have a bracket pivotally connected to the frame about the axis of elevational rotation and a base plate pivotally connected to the bracket for rotation of said bracket about an axis of azimuth rotation. A second timing belt may be wrapped in contact with at least a portion of a perimeter edge of the base plate; with a means for driving the second timing belt being connected to the bracket and operable to rotate the bracket about the axis of azimuth rotation. Electrical signals for the drive motors and the antenna may be multiplexed and passed through a single coaxial rotary joint. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a device in accordance with this invention. 
     FIGS. 2A and 2B are side views of a cam for use with the device of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a device  10  in accordance with this invention for positioning an antenna in both the azimuth (horizontal) and elevation (vertical) directions. The antenna  12  of FIG. 1 consists of a reflector  14  and a low noise block feed (LNBF)  16  associated therewith. In the embodiment of FIG. 1, both of these components are mounted on a frame  18  having a first arm  20  and a second arm  22  connected about an axis of elevational rotation  24 . A belt such as timing belt  26  having a first end  28  and a second end  30  is connected to the first arm  20  and second arm  22  of the frame  18  respectively. The term belt as used herein is meant to include any sort of apparatus capable of exerting a mechanical force over a distance, and may include devices such as a timing belt, rope, wire, ribbon, etc. A belt is generally capable of exerting only a pulling force, although in some embodiments the term belt as used herein, may include a device capable of exerting a pulling and/or a pushing force. For example, a belt may include a flexible plastic rod inserting through a plastic tube wherein the tube is affixed to a structure so as to resist the bending of the rod under a pushing force. Other embodiments of this invention may not include a frame, but may have the timing belt  26  connected directly to the antenna  12 . A means for driving the timing belt is provided. The means for driving the timing belt  32  illustrated in FIG. 1 as a motor  34  connected to the timing belt  26  via a drive pulley  36 . An idler pulley  38  may be used to ensure proper engagement between the timing belt  26  and the drive pulley  36 . The timing belt  26  provides a means for exerting a pulling force on the first arm  20  and the second arm  22 , thereby rotating the frame  18  and antenna  12  about the axis of elevational rotation  24 . The timing belt  26  may be a nylon covered fiberglass reinforced neoprene product as is known in the art, and preferably will have teeth for non-slip connection with drive pulley  36  having matching notches. Other means for exerting a pulling force may include a chain, wire, or rope, with or without a non-slip feature. 
     The device of FIG. 1 may be configured to attach the timing belt  26  to the frame  18  at a variety of locations. Advantageously, by making these connections at a distance removed from the axis of elevational rotation  24 , a mechanical advantage is provided that permits a reduction in the size of motor  34  required and/or a reduction in the gearing ratio required for the motor  34 . A smaller motor results in a lower cost and lighter weight, and a reduction in the gearing ratio results in less slop in the drive train, thereby providing a more precise control of the antenna position. 
     When frame  18  is rotated about the axis of elevational rotation  24 , the required length of the timing belt  26  may change. The amount and direction of change in length will depend on the angle between the first and the second arm  20 , 22 , the location of the connections between the arms  20 , 22  and the timing belt  26 , and the location, number and size of pulleys  36 , 38  in contact with the timing belt  26 . It is possible to design a device with fixed pulley locations that will rotate without changing the length of the timing belt  26 . Alternatively, the embodiment of FIG. 1 illustrates a design that utilizes a spring, preferably a constant force spring  40 , to allow the location of one of the pulleys to change in response to rotation of the frame  18 . Pulley assembly  42  pivots around an axis  44  and is held against the timing belt  26  by a constant force spring  40 . Pulley assembly  42  provides a means for maintaining tension in the timing belt  26 , and preferably a constant tension in the timing belt, during the rotation of the frame  18  and antenna  12 . Any change in belt length required by the geometry of the device during the rotation of the frame  18  would normally result in an increase or a decrease in the tension in the timing belt  26 . Such increase or decrease in tension instead results in compression or expansion of the spring  40  and movement of the pulley assembly  42  about axis  44 , thereby effectively counteracting the required change in length of the timing belt  26  and resulting in a constant tension in the timing belt  26 . Without such a means for maintaining tension, the timing belt  26  may loose tension and begin to slip on the drive pulley  36 , resulting in failure of the device to operate properly. 
     FIGS. 2A and 2B illustrate an alternative means for maintaining tension in the timing belt  26 . FIG. 2A illustrates the second arm  22  and timing belt  26  of FIG. 1 in a first position. Attached to the second arm  22  is a cam  46 . Timing belt  26  is wrapped around the cam  46  and may be fixedly attached to the cam  46  at its end  30 . In the first position illustrated in FIG. 2A, the timing belt  26  is in contact with the cam  46  from its end  30  to a point  48  on the perimeter of the cam  46 . When the frame  18  of FIG. 1 is rotated about its axis of elevational rotation  24 , the second arm  22  will move to a second position illustrated in FIG.  2 B. Note that in this second position the timing belt  26  is in contact with the perimeter of the cam  46  from its end  30  to a point  50 . The change in length of contact between the timing belt  26  and the cam  46  from the positions of FIG. 2A to FIG. 2B may be selected to correspond and to compensate for the change in length of the timing belt  26  resulting from the rotation of the frame  18  around its axis of elevational rotation  24 . The advantage of such a design over the design of FIG. 1 is that the constant force spring  40  may be eliminated. The shape of cam  46  may be round, elliptical, parabolic or other shape as required to maintain tension in the timing belt  26  as the antenna  12  is rotated. The cam  46  is illustrated as being attached to the second arm  22 , although other embodiments may have such a cam  46  attached to the first arm  20  or the antenna  12 . 
     The device  10  of FIG. 1 also includes a bracket  52  pivotally connected to the frame  18  about the axis for elevational rotation  24 . Motor  34  may be mounted to the bracket  52 . The bracket  52  may include a first beam  54  and a second beam  56  connected at their respective centers. First and second beams  54 , 56  may each be attached to rollers or wheels  58  for supporting the bracket  52  on a base plate  60 . 
     Bracket  52  may be pivotally connected to the base plate  60  for rotation of the bracket  52  about an axis of azimuth rotation  62 . A second timing belt  64  or other means for providing a pulling force is wrapped around at least a portion of a perimeter edge of the base plate  60  and is in contact with a pulley  66  connected to motor  68 . Motor  68  is mounted on bracket  52  and is operable to rotate the bracket  52  about the axis of azimuth rotation  62  by driving the second timing belt  64  through pulley  66 . Advantageously, by driving a timing belt  64  wrapped around a perimeter edge of a base plate  60 , a mechanical advantage is provided that allows the size of motor  68  to be reduced when compared to prior art devices. A reduced motor size results in a decrease in cost and weight, and it eliminates the need for a high ratio gear train, thereby providing for more precise control of the azimuth location of the antenna. Note that the length of the second timing belt  64  does not change as the bracket  52  is rotated in relation to the base plate  60  since the geometry of the second timing belt  64  layout does not change during rotation. Therefore, no constant force spring or similar device is required for this drive mechanism. 
     The center connection between the base plate  60  and bracket  52  may include a rotary joint  70  at the axis of azimuth rotation  62 . Rotary joints are known in the art for providing mechanical rotation while maintaining an electrical connection. Rotary joint  70  may preferably connect a single coaxial cable  72 , and the electrical connection may include a means for multiplexing  74  electrical signals for both motors  34 , 68  and for antenna  12 . Because the second timing belt  64  is continuous, and because the rotary joint  70  provides for unlimited rotation, the antenna  12  is provided with an unlimited range of movement in the azimuth direction. 
     Other aspects, objects and advantages of this invention may be obtained by studying the Figures, the disclosure, and the appended claims.