Traditional azimuth-elevation (“az-el”) stages are used to point a variety of devices ranging from large optical telescopes to tank guns. Such a stage typically has an “elevation” stage having a horizontal axis mounted on an “azimuth” stage with a vertical axis. Both stages are often motorized.
The diagonal-axes stage described here mitigates or eliminates all of these problems. Instead of one vertical axis and one horizontal axis, a diagonal-axes stage has two horizontal axes, both oriented at 45° to the trajectory of the target. For example, a ground station located on the equator tracking a satellite with an equatorial orbit would have one axis parallel to northeast and southwest, and the other axis parallel to northwest and southeast.
The diagonal-axes stage is considerably less vulnerable to backlash. If it is correctly oriented, its axes rotate in only one direction during an overhead pass by a satellite. As a result, the effects of backlash may be inherently eliminated. If the gravity-induced torque on either axis changes during the pass, then backlash may become important during the part of the pass where the gravity torque, instead of opposing the motion of the stage, pushes the stage in the direction of motion. This can result in the loss of gear-to-gear contact in one or more stages of the gear reduction mechanism. In this case, a preload spring used to eliminate backlash need only be sufficiently strong to overcome the gravity torque, i.e. it need not overcome friction in the gear train.
The diagonal-axes stage is not backlash-free for arbitrary target trajectories such as an aircraft might execute. If properly oriented for any particular satellite, however, it is backlash-free for all passes of that satellite, which will trace out parallel paths on the sky, and for all passes of any other satellite that are perpendicular to the first. It will also be backlash-free for some fraction of other satellite trajectories.
This work was done by Martin W. Regehr and Vachik Garkanian of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47427
This Brief includes a Technical Support Package (TSP).
Diagonal-Axes Stage for Pointing an Optical Communications Transceiver
(reference NPO-47427) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing the Diagonal-Axes Stage designed for pointing an optical communications transceiver. This innovative mechanism is part of ongoing research and development aimed at improving optical communication systems, particularly in aerospace applications.
The diagonal-axes stage represents a significant advancement over traditional azimuth-elevation (az-el) stages, which are commonly used to point various devices, including telescopes and military equipment. The traditional az-el stages consist of two motorized axes: one for elevation (horizontal) and one for azimuth (vertical). While effective, these systems are prone to issues such as backlash, which can hinder precision and responsiveness.
Backlash occurs when there is a delay in the movement of the stage due to slack in the gear train, leading to potential overshoot and inaccuracies. The document discusses various methods to mitigate backlash, including backlash-compensating controllers and anti-backlash gears. However, these solutions often introduce additional complexity, weight, and noise, and may not fully eliminate the problem.
In contrast, the diagonal-axes stage is designed to minimize backlash by ensuring that its axes rotate in a single direction during operations, particularly during overhead passes by satellites. This design inherently reduces the effects of backlash, as the gravity-induced torque on the axes can be managed more effectively. The document explains that when properly oriented, the stage can maintain gear-to-gear contact, thus enhancing its operational reliability.
The prototype of the diagonal-axes stage is depicted in the document, showcasing its design and functionality. The stage is equipped with cylindrical geared stepper motors arranged at right angles, allowing for precise control of movement. The document also highlights the importance of this technology in the context of optical communications, which are critical for maintaining robust links between ground stations and aerial platforms.
Overall, the Technical Support Package emphasizes the potential applications of the diagonal-axes stage in various fields, including telecommunications and aerospace, and underscores NASA's commitment to advancing technology for broader scientific and commercial use. The research is conducted under the auspices of the National Aeronautics and Space Administration, reflecting a collaborative effort to push the boundaries of aerospace technology.
