September 24, 1997
Much larger orbital structures, like the international space station, are now on the drawing boards. As these larger space habitats are constructed, a new problem will become increasingly important: stability.
If two sections of a space structure begin moving to the beats of different drummers, the structure could easily be severely damaged.
One solution to the problem is to design in added rigidity, but that adds weight, which can significantly increase cost.
An alternate, and potentially more cost-effective, approach is to build in a dynamic control system that actively controls errant oscillations. E. Harrison Teague, a Stanford doctoral student in aeronautics and astronautics, has developed such a system, which employs signals from the Global Positioning System, the Department of Defense's satellite navigation system.
Using inexpensive GPS receivers attached to different portions of a space structure, the method can detect wayward motions with centimeter-level precision and then automatically fire thrusters to compensate for them. The system also can be used to change the orientation of a flexible structure with such accuracy that it moves almost as if it were rigid.
Teague developed the GPS system as part of his doctoral thesis, which he completed in June. An article describing the work will appear in the summer issue of the Navigation Journal, which is still in press. His thesis advisers were aeronautics and astronautics professors Jonathan How and Bradford Parkinson.
Previous methods that provided centimeter-level measurements of position and attitude using GPS relied on the object in question being a rigid body. Teague adapted these techniques to provide the same level of precision with a flexible structure.
Next he had to identify the shapes and frequencies of the various modes of oscillation that could develop in such a structure. Although researchers had some general ideas of what such modes should be, they were not known with enough precision for effective control.
Finally, the student came up with procedures that could control such motions while automatically accommodating processes such as docking and undocking of capsules and the addition and consumption of consumables, processes that can cause major changes in the dynamic properties of space structures.
To try his system, Teague built an ungainly-looking test bed that allows him to simulate the movement of a light structure in weightlessness. The test bed consists of three 100-pound blocks of aluminum connected by two 15-foot long rods. Each block has two arms that are about five feet long extending perpendicular to the rods. On the end of each arm is a small GPS receiver and a cluster of four compressed-air thrusters.
The entire assembly is hung by extremely strong thread. The top of each aluminum block is milled out in a cone shape so that the thread can be attached at its center of mass and the block can rock without contacting the thread. Threads from each of the three blocks extend upward where they are attached to a 30-foot length of heavy steel pipe. Straps from each end of the pipe are tied onto a thrust bearing that allows the entire assembly to rotate. The bearing, in turn, is supported by a heavy, overhead crane.
Because the rods connecting the three blocks are extremely flexible, the testbed can simulate a wide variety of motions. Each of the blocks can be set rocking vertically and horizontally. The rods transmit some of this motion to the other blocks. So waves of motion can travel from one end of the assembly to the other. When the blocks are set rocking in different directions, the waves can combine and cancel in unexpected ways.
Teague's test area is indoors, so he had to use pseudo-satellites, antennas that produce imitation GPS satellite signals. The receivers use these signals to keep track of their precise position. All the positions are sent to a desktop computer that contains a model of the assembly. The computer identifies the oscillation modes when they are still very small and calculates the timing and duration of the air blasts necessary to dampen them out.
The most dramatic demonstration of the system's capabilities comes when Teague vigorously sets the assembly rocking and rolling. When he activates the control system, the thrusters begin hissing, the motions get smaller and smaller and the assembly returns to rest within 5 seconds.
A less dramatic, but more realistic test cares when Teague turns the control system on and then manually moves one of the arms. Thrusters begin hissing immediately and the arm rapidly returns to its proper position when he lets go.
Teague also can use the system to rotate the flimsy assembly as if it were rigid. When he enters the proper command, the thrusters begin to hiss and the assembly begins to turn like a rigid body, with very little shuddering or deviation from its base configuration.
The research was funded by the National Aeronautics and Space Administration.
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