The Jitter Beam: An Experiment Demonstrating Pointing Control on a Flexible Structure

The problem of control/structural interaction arises in ground and space systems where flexibility limits performance. In this paper, an experiment called the "jitter beam" simulates the interaction of a pointing control system and a flexible structure. Noncolocation of a sensor and an actuator makes control difficult. A Linear Quadratic Gaussian (LQG) design overcomes the noncolocation problem by use of a bending model. The capability to deal with noncolocation enables the jitter-beam control to coordinate, with a single torquer, the motions of several points on the structure to achieve the pointing goal. The significant achievement of the experiment is the practical demonstration of a control bandwidth two times higher than a critical bending frequency, a factor of ten beyond what a rigid-body design can achieve. The jitter beam experiment resembles small stiff structures like active mirrors. Although the experiment is small and simple, the control methodology can be extended in principle to large structures and complex systems like segmented mirrors. The sparse model frequency spectrum of stiff structures simplifies identification, but because bending frequencies are high, sensor noise, actuator saturation, and computational speed constrain performance. In contrast, the low frequencies of large structures make hardware constraints a small concern, but the dense frequency distribution complicates system identification.