Brett P. Masters

Measurement of the modal parameters of a space structure in zero gravity

An analytic and experimental study of the changes in the modal parameters of space structural test articles from 1 to 0 g is presented. Deployable, erectable, and rotary modules were assembled to form three one- and two-dimensional structures in which variations in bracing wire and rotary joint preload could be introduced. The structures were modeled as if hanging from a suspension system in 1 g, and unconstrained, as if free floating in 0 g. The analysis is compared with ground experimental measurements made on a spring/wire suspension system with a nominal plunge frequency of 1 Hz and with measurements made on the Shuttle middeck. The degree of change in linear modal parameters, as well as the change in nonlinear nature of the response, is examined. Trends in modal parameters are presented as a function of force amplitude, joint preload, and ambient gravity level.

Multiple degree-of-freedom force-state component identification

Force-state mapping is extended to allow the quasistatic characterization of realistic multiple degree-of-freedom systems whose constitutive relations are nonlinear, dissipative, coupled, and depend on memory of past states. A general constitutive force-state model is formulated, which includes dynamic hysteresis phenomena typical of that found in deployable structures. A two step parameter identification approach is developed, which employs a linear extended least squares and a nonlinear least squares fit. A six degree-of-freedom force-state testing device was constructed and used to obtain data on one bay of a simple truss and on one bay of the mid-deck 0-gravity dynamics experiment deployable truss.

Evolutionary Design of Controlled Structures

An evolutionary controls/structures design method is developed. The basis of the method is an accurate model formulation for dynamic compensator optimization coupled with a genetic-algori thm-based up- dating of sensor/actuator placement and structural properties. One- and three-dimensional examples from the literature are used to validate the method. Frequency domain interpretation of the controlled structure systems provide physical insight as to how the objective is optimized. Several designs for a stellar inter- ferometer precision structure are found for two different spectra of disturbance under assumed closed- loop optical conditions. Physical limitations in achieving performance are given in terms of average system transfer function gain and system phase loss. PACE-BASED structures that are characterized and com- pensated to submicron levels have found utility in the stel- lar observation sciences. In such structures, control, other than that used in maintaining attitude, is typically introduced in the preliminary and detailed design stages, after the system is found to fail specifications passively. At this stage the structure is fixed in topology and member geometry, leaving the control designer to accept the given plant dynamics. In some cases the actuator/sensor design is also fixed, further limiting the achiev- able performance. Consideration of structural control technology early in the design process of precision structures leads to possible bene- fits, but it also leads to numerous design variables and, sub- sequently, many potential cost functions, making the combined optimization problem very difficult. The payoff is that the ac- tuators/sensors are simultaneously designed with the system, and can subsequently render the controls with greater influence over improving the performance. The drawback is that the combined optimization problem is plagued by large dimen- sions in both the controls and structural problems, and fur- thermore, it becomes combinatorial with the addition of dis- crete choices such as sensor/actuator location (distribution) and type, e.g., inertial or relative. The objective of this paper is to provide a consistent method of designing a precision structure that leverages the use of control to meet performance requirements. In using the method, the necessity of considering both the sensor/actuator design and dynamic compensation in the system design pro- cess is shown. In the past 20 years various investigators have tackled the dynamic controls—structures optimization problem, yielding a myriad of results and sparse implementation. Rao et al., 1 Maghami,2 Canfield et al., 3 and Miller and Shim4 provide ap- proaches where controls and structures topologies are fixed and the control gains, control bandwidth, and member cross- sectional variables optimized with respect to either mass, mo- tion error, or control effort costs. For the most part, the pre- ceding items use nonlinear programming techniques and cover

Experimental investigation of optimized precision optical controlled structures

A spacecraft-like optical interferometry system is investigated experimentally over several different optimized controlled structures configurations. Configurations represent common and not-so-common approaches to mitigating pathlength errors induced by disturbances of two different spectra. Results show that an optimized controlled structure for low frequency broadband disturbances achieves modest performance gains over a mass equivalent regular structure, while an optimized structure for high frequency narrow band disturbances is four times better in terms of rms pathlength. These results are predictable given the nature of the physical system and the optimization design variables. Fundamental limits on controlled performance are discussed based on average system transfer function gains and system phase loss.

System-wide design issues for the stellar interferometer technology experiment (SITE)

The Stellar Interferometer Technology Experiment (SITE) is a near-term precursor mission for spaceborne optical interferometry. Proposed by the MIT Space Engineering Research Center and NASAs Jet Propulsion Laboratory, SITE is a two-aperture stellar interferometer located in the payload bay of the Space Shuttle. It has a baseline of four meters, operates with a detection bandwidth of 300 nanometers in the visible spectrum, and consists of three optical benches kinematically mounted inside a precision truss structure. The objective of SITE is to demonstrate system-level functionality of a space-based stellar interferometer through the use of enabling and enhancing Controlled Structures Technologies such as vibration isolation and suppression. Moreover, SITE will validate, in the space environment, technologies such as optical delay lines, laser metrology systems, fringe detectors, active fringe trackers, and high- bandwidth pointing control systems which are critical for realizing future space-based astrometric and imaging interferometers.