Spring devices – Resilient shock or vibration absorber – Including energy absorbing means or feature
Reexamination Certificate
2000-03-15
2001-03-06
Schwartz, Christopher P. (Department: 3613)
Spring devices
Resilient shock or vibration absorber
Including energy absorbing means or feature
C267S035000, C267S118000, C188S312000
Reexamination Certificate
active
06196528
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention generally relates to motion control mechanisms and, more particularly, to an apparatus for controlling the vibration of an appendage which results from the slewing of a spacecraft or other disturbances.
2. Discussion
Spacecraft are commonly provided with deployable antennas, test equipment, and other appendages. For example, spacecraft antennas typically comprise a dish-like shaped structure formed about a central feed tower. In one such reflector design, the feed tower is secured to the spacecraft at its base while a reflective mesh structure is supported about the axis of the feed tower by a rib structure. A plurality of cables interconnect the remote portions of the dish-like structure with the base of the feed tower. These cables stabilize the ribs by holding the reflective mesh surface in place.
Spacecraft maneuvers such as slewing (i.e., rotating the spacecraft about its yaw or pitch axis) and/or spacecraft disturbances cause the appendages extending therefrom to undergo undesirable oscillations. Typical large diameter reflective structures possess low natural frequencies (1.0 Hz or less) and inherent structural damping of such systems accounts for approximately 0.1% to 0.4% critical damping. Because of the low damping, the antenna requires a significant amount of time to settle down. Prior to settling, the antenna's mission operation is degraded.
Prior art attempts to reduce oscillations have been directed towards stiffening the overall antenna structure. By stiffening the structure, the natural frequency of the antenna can be increased. As such, the oscillations occur faster and the time required for the vibration displacements to drop to an acceptable level is reduced. One method that has been used to increase natural frequency was to increase the tension in the guidelines extending between the rib structure and the feed tower base. Also, cables have been added from the top edge of the rib structure to the top of the feed tower and the diameter of the ribs has been increased. However, these attempts have been only modestly effective as the natural frequency cannot be increased to a sufficient level without adding significant weight to the structure.
Motion control mechanisms for damping oscillations have been used to various degrees in the automotive industry. For instance, U.S. Pat. No. 4,869,476 entitled “Electrically Controlled Viscous Elastic Spring” to Shtarkman and assigned to the Assignee of the present invention and incorporated by reference herein describes a spring for resisting relative movement of parts and for urging the parts to an initial, equilibrium position when the parts are moved therefrom. The spring uses a fluid, such as an electro-rheological fluid or a magnetic fluid, to provide a variable spring rate and a variable load-carrying capacity. The resistance to flow of these fluids varies as a function of the magnitude of either an electric field or a magnetic field applied to the fluid.
Another known device using electro-rheological fluid for damping is disclosed in U.S. Pat. No. 4,896,752 entitled “Vehicle Strut” to Shtarkman and assigned to the Assignee of the present invention which is also incorporated by reference herein. This reference discloses a strut for use in a suspension system wherein the spring and damping characteristics of the strut are controlled, in part, by an electro-rheological fluid disposed within fluid chambers in the strut. When the viscosity of the fluid is varied by changing the magnitude of the electric field applied to the fluid, the spring rate of the strut is varied.
Further, U.S. Pat. Nos. 4,942,947, 4,992,190, 5,167,850, and 5,176,368, all to Shtarkman, and 5,257,681, 5,354,488, 5,367,459, 5,517,096, and 5,655,757 all to Shtarkman, et al., each of which is assigned to the Assignee of the present invention and is incorporated by reference herein, disclose devices utilizing fluid compositions which are responsive to an electric or magnetic field. By controlling the magnitude of the field, the spring and damping characteristics of the devices can be manipulated.
In view of the foregoing, it would be desirable to provide an apparatus for controlling the vibrations of a spacecraft appendage by utilizing a control mechanism responsive to an electric or magnetic field.
SUMMARY OF THE INVENTION
The above and other objects are provided by a motion control apparatus for controlling oscillations of an appendage such as an antenna coupled to a platform such as a spacecraft. The antenna is secured to the spacecraft at its base so as to extend laterally from the spacecraft. A plurality of guidelines extend between remote portions of the antenna's reflector structure and the antenna base. When the reflector is subjected to lateral forces due to a slew maneuver of the spacecraft or other disturbances, the guidelines react to the lateral loads. When the reflector oscillates in its first lateral mode, the guidelines experience a significant portion of the strain energy for that mode, as does the feed tower. Therefore, in order to reduce the settling time of the antenna relative to the spacecraft, a plurality of motion control elements are provided between the antenna base and the spacecraft, along the guidelines between the antenna base and the remote portions of the antenna or both.
The motion control elements preferably comprise either passive or active control elements and include a material having a damping response changed according to the magnitude of an electric or magnetic power source coupled therewith. For instance, the motion control elements may include a magneto- or electro-rheological fluid having its viscosity changed according to the magnitude of an electric or magnetic field applied thereto. Also, the motion control elements may include a piezo-electric stack which exert mechanical force proportional to the magnitude of an electric field supplied thereto. As such, the piezo-electric stack induces a force counteractive to the sensed force which results in damping in a structure.
In a first embodiment, the vibration control mechanism comprises a combination of a piezo-electric stack and electro- or magneto-rheological fluid interposed between an outer housing secured to an anchor and an inner housing secured to an object to be vibrationally controlled.
In a second embodiment the vibration control mechanism of the present invention comprises a piezo-electric stack interposed between an outer housing secured to an anchor and an inner housing secured to an object to be vibrationally controlled.
In a third embodiment, the vibration control mechanism of the present invention comprises an electro- or magneto-rheological fluid interposed between a housing and an inner housing coupled at a first end to an anchor and at a second end to an object to be vibrationally controlled.
In a fourth embodiment the vibration control mechanism of the present invention comprises an electro- or magneto-rheological fluid disposed within an inner housing having flexible bellows type side walls and being fixed at one end to an anchor and at another end to an object to be vibrationally controlled.
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patent: 5
Doherty Kathleen M.
Parker A. Dale
Shtarkman Emil M.
Schwartz Christopher P.
TRW Inc.
Yatsko Michael S.
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