Communications: radio wave antennas – Antennas – With support for antenna – reflector or director
Reexamination Certificate
2000-04-07
2001-09-04
Le, Hoanganh (Department: 2821)
Communications: radio wave antennas
Antennas
With support for antenna, reflector or director
C343S880000, C248S278100, C248S661000
Reexamination Certificate
active
06285339
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to multi-axis positioning equipment and, more particularly, to a zero-backlash, elevation-over-azimuth, two-axis positioner that is capable of continuous 360°, rotation in azimuth and limited rotation (130°, for example) in elevation.
2. Prior Art
A pair of patents will be mentioned that are exemplary, but not exhaustive, of the prior art in the field with which the present invention is concerned. U.S. Pat. No. 4,580,461 to Sears et al. discloses a gimbal mounting arrangement which provides at least two rotational axes about which a load such as an antenna or optical transducer may be gimbaled. The gimballing arrangement permits both axes of rotation to intersect at a point within the load being gimbaled thereby minimizing the rotational inertia of the load and of the gimbal mechanism itself. Because the rotational inertia is minimized, relatively rapid scan rates can be achieved using only relatively low powered drive inputs. U.S. Pat. No. 5,875,685 to Storaasli discloses a positioner including an output platform and a two-axis bearing suspension which permits the output platform to rotate about two orthogonal axes. The output platform is positioned by a pair of actuators which are mounted, as is the bearing suspension, to a positioner base. The actuators include bevel gears which engage ratchets to couple the actuators and the platform.
Most existing elevation-over-azimuth, two-axis positioners have two separate drives for the azimuth and elevation axes. The elevation axis drive is usually located on the far side of the azimuth axis from the stationary base, requiring an electrical slipring to allow for continuous azimuth rotation. The slipring is typically a very expensive component. Many known constructions of our elevation-over-azimuth, two-axis positioners use small, high-speed electric motors with geartrains to drive the low-speed output axes. Geartrains have one major disadvantage—backlash. Excessive backlash can cause problems with stability in a closed-loop control system, and backlash adds to position error.
Existing elevation-over-azimuth, two-axis positioners in which the position of the elevation axis is determined by the position of the elevation drive in the base relative to the position of the azimuth drive in the base have a major disadvantage. This is that any backlash in the azimuth drive would add to the backlash in the elevation drive, increasing the total backlash at the elevation output axis.
Direct drive electric motors have been used to eliminate backlash. They generally require more size, mass, and input power than a small, high-speed motor with a geartrain for a given output power with a slow-moving output axis.
Many methods have been used for control of backlash in geartrains. George W. Michalec's book Precision Gearing: Theory and Practice, published in 1966, has a good description of many different methods. Most have disadvantages such as increased size, weight, and cost. Examples are split, spring-loaded scissor gears and the use of auxiliary geartrains.
It was with knowledge of the foregoing that the present invention has been conceived and is now reduced to practice.
SUMMARY OF THE INVENTION
The present invention is directed toward a zero-backlash, elevation-over-azimuth, two-axis positioner that is capable of continuous 360° rotation in azimuth and limited rotation (approximately 130°, for example) in elevation. This positioning apparatus utilizes a yoke including a hub mounted by means of bearings on a base for rotation about a first axis, an integral bight member extending radially away from the hub in opposite directions and a pair of upstanding legs distant from the hub extending away from the bight member at spaced apart locations. A load, such as an antenna, extends between and is mounted by means of bearings on the upstanding legs of the yoke member for rotation about a second axis distant from the bight member. A first drive system rotatably positions the yoke about the first axis and a second drive system intermediate the yoke and the load includes a first drive component rotatable about the first axis for rotatably positioning the load about the second axis as a function of the difference in angular movement of the support member about the first axis and the angular movement of the first drive component about the first axis. A spring mechanism biases the load relative to the yoke thereby biasing the second drive system against the first drive system to minimize lost motion in both drive systems.
The motors, gears, position feedback devices, and a controller circuit card for closed-loop positioning of the two orthogonal axes are located in the base. Note that this allows the continuous 360° rotation in azimuth without the use of sliprings. The elevation axis is driven from the base by a mechanism, which in the initial sketch includes a cable and a plurality of pulleys. This mechanism could comprise, for example, a cam, a linkage, a gear train, or other suitable devices instead of the cable and pulleys. The important feature of this mechanism is that the position of the elevation axis is determined by the position of the elevation drive in the base relative to the position of the azimuth drive in the base. The novel part of the invention is the addition of a spring between the load and the yoke.
The spring could take any one of a wide variety of constructions. It could be a helical torsion spring (the form which will be described below as an example), a hairspring, a Neg'ator® type constant force spring, or even a helical extension spring with a cable and pulley. This spring serves to provide a continuous torque preload on the two drivetrains. If the applied torque on the load does not exceed the preload of the spring, this preload will remove from both output axes lost motion due to geartrain backlash and clearances between parts without the additional cost, size, and weight of other known backlash control techniques and constructions.
The purpose of the invention, then, is to position a load in a desired angular orientation with adequate stability and sufficiently small position error. One possible application, and the one primarily described in the ensuing disclosure, is for pointing a directional antenna. It could also be used in a tracking antenna system. Other potential applications include positioning a laser and/or optics for an optical data link. It might also be used in machine tool and robotics applications or for aiming a weapon.
The new and different part of the invention is the addition of a spring between the load and the yoke. This spring would provide a continuous load on the two drivetrains, removing lost motion due to geartrain backlash and clearances between parts.
The concept of eliminating a slipring and driving the elevation axis from the base through a mechanism is not new and commercially available apparatus exists which is so constructed.
Assuming that a two-axis positioner requires two motors and two geartrains, the major advantage over past practice is that this invention uses a single spring to apply a continuous load to these two required geartrains. This eliminates from both axes the lost motion due to gear backlash and part clearances without the additional cost, size, and weight of other backlash control methods.
A primary feature, then, of the present invention is the provision of novel multi-axis positioning equipment.
Another feature of the present invention is the provision of such multi-axis positioning equipment in the form of a zero-backlash, elevation-over-azimuth, two-axis positioner that is capable of continuous 360° rotation in azimuth and limited rotation in elevation.
Still another feature of the present invention is the provision of such multi-axis positioning equipment which includes a yoke mounted on a base for rotation about a first axis, a load mounted on the support member for rotation about a second axis extending transverse of the first axis, a first drive system for rotatably positioning
L-3 Communications Corporation
Le Hoang-anh
Perman & Green LLP
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