Shaping Inputs to Reduce Vibration in Flexible Space Structures

Kenneth Wonuk Chang

Abstract

Future NASA plans to launch large space structures solicit the need for effective vibration control schemes which can solve the unique problems associated with unwanted residual vibration in flexible spacecraft. In this work, a type of input command shaping called impulse shaping is examined. These shapers require only a simple model of the system and are fairly insensitive to parameter variations.

A theoretical background of impulse shaping is presented along with some insight into the methods of calculating multiple space sequences. The Middeck Active Control Experiment (MACE), an MIT/NASA space shuttle experiment, is then described as the testbed for the experiments. A nonlinear simulation is used to verify the effectiveness of these shapers on a model of MACE. Finally, hardware experimental results are presented and analyzed.

Some conclusions were derived from the results of testing. First, it was found that the shaper was effective in canceling the slowest frequency of the structure, but was less successful in the higher modes. Second, large angle movements decreased the effective vibration cancellation. The nonlinearity involoved with large angle movements causes the shaper to lose some of its effectiveness. Third, although methods exist which can increase the robustness of an impulse shaper, none were found to aid in canceling the effects of nonlinearities caused by large angle moves. Fourth, the bandwidth of the controller was found to be linked to the effectiveness of the shaper. A higher bandwidth produced more favorable results.

Although many inadequacies of impulse shapers when applied to complex nonlinear systems are discussed, it remains a very effective tool in controlling vibration in flexible space structures.

SERC Report #8-92, June 1992

To obtain a copy of this report, contact slbrown@mit.edu.