Performance assessment and enhancement of precision controlled structures during conceptual design

Abstract

Future optical space systems such as interferometers and filled-aperture telescopes will extend the resolution and sensitivity offered by their on-orbit and ground-based predecessors. These systems face the challenge of achieving nanometer and milli-arcsecond precision control of stellar light passing through the optical train of a lightweight, flexible structure subjected to various disturbances. It is advantageous to assess the performance of initial concepts of these precision systems early in the design stage to aid in the requirements flowdown and resource allocation process. A complete end-to-end performance assessment methodology is developed which incorporates disturbance, sensitivity, and uncertainty analysis tools within a common state-space framework. The disturbance analysis is conducted using either a time-domain, frequency-domain, or Lyapunov approach to obtain nominal predictions of performance metric root-mean-square (RMS) values. Calculating power spectral density and cumulative RMS functions of the performance metrics allows critical system modes and frequencies to be identified, and in some instances, contributions from each of the disturbances can be determined. A Lagrange multiplier method is used to derive a governing equation for the sensitivities of the performance metrics with respect to model parameters. For a system whose structural dynamic equations are represented in modal form, the ensitivities can be calculated exactly and efficiently with respect to modal frequencies, masses, and damping ratios. The most critical modal parameters are carried into a parametric uncertainty analysis that seeks to identify the worst-case performance RMS values. A constrained optimization technique is described which searches for the worst-case performance over all allowable parameter values. When required, a performance enhancement approach is used to apply controlled structures technologies such as input/output isolation to achieve large performance changes. Structural modifications based on insight provided by a physical parameter sensitivity analysis are then employed to "fine tune" the performance to keep the worst-case values within the requirements. Analytical physical parameter sensitivities are experimentally validated on a truss structure and used to implement stiffness and mass perturbations that reduce the tip displacement of a flexible appendage. The overall framework is applied to an integrated model of the Space Interferometry Mission to demonstrate its practical use on a large order system.