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 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 U.S. Pat. Nos. 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 damping vibrations in a spacecraft appendage utilizing a control mechanism responsive to an electric or magnetic field.