Robert N. Jacques

Frequency domain structural system identification by observability range space extraction

This paper presents the frequency domain observability range space extraction (FORSE) identification algorithm. FORSE is a singular value decomposition based identification algorithm which constructs a state space model directly from frequency domain data. It is numerically well behaved when applied to multivariable and high dimensional structural systems. It can achieve high modeling accuracy by properly overparametering the model. Its effectiveness for structural systems is demonstrated using the MIT Middeck Active Control Experiment (MACE). MACE is an active structural control experiment to be conducted in the Space Shuttle middeck.

Frequency Domain Structural System Identification by Observability Range Space Extraction

This paper presents the Frequency Domain Observability Range Space Extraction (FORSE) identification algorithm. FORSE is a singular value decomposition based identification algorithm which constructs a state space model directly from frequency domain data. The concept of system identification by observability range space extraction was developed by generalizing the Q-Markov Covariance Equivalent Realization and Eigensystem Realization Algorithm. The numerical properties of FORSE are well behaved when applied to multi-variable and high dimensional structural systems. It can achieve high modeling accuracy by properly overparameterizing the system. The effectiveness of this algorithm for structural system identification is demonstrated using the MIT Middeck Active Control Experiment (MACE). MACE is an active structural control experiment to be conducted in the Space Shuttle middeck. Results of ground experiments using this algorithm will be discussed.

MIT multipoint alignment testbed: technology development for optical interferometry

A class of proposed space-based astronomical missions requiring large baselines and precision alignment can benefit from the application of Controlled Structures Technology. One candidate mission, that of a 35 meter baseline orbiting optical interferometer, is studied as a focus mission for a testbed for controlled structures research. Interferometry science requirements are investigated and used to design a laboratory testbed which captures the essential architecture, physics and performance requirements of a full scale instrument. Testbed hardware used for identification and control is presented, including an on-board six-axis laser metrology system using state of the art cats eye retroreflectors. The testbed and research program are discussed in terms of controlled structures design and in terms of the expected benefits to the optical engineering and science communities.

Multivariable model identification from frequency response data

A strategy for synthesising high-fidelity, MIMO models from transfer function data has been developed. It takes advantage of the strengths of three separate model ID methods. The eigensystem realisation algorithm is used to generate a model, and then a curve fit based on the complex log of the transfer functions is used to reduce the model. Finally, the model is fine-tuned using a curve fit to the transfer function data directly. This method has been shown to work well on the SERC Interferometric Testbed, a system with many lightly-damped, closely spaced modes. One facet of identifying a system model from the interferometer data was the absence of noise in the measured transfer functions. Future work will examine performance of this method on noisier data and suggest modifications.<<ETX>>

Identification of highly accurate low order state space models in the frequency domain

Abstract This paper discusses an integrated approach for high accuracy and low order model identification. The approach integrates subspace-based identification algorithms with model reduction and parameter estimation algorithms to generate highly accmate low order models. The specific identification algorithm presented in the paper is the Integrated Frequency domain Observability Range Space Extraction and Least Square parameter estimation algorithm (IFORSELS). IFORSELS is an iterative algorithm which integrates the Frequency domain Observability Range Space Extraction (FORSE) algorithm, the Balanced Realization model reduction algorithm, and the Logarithmic and Additive Least Square parameter estimation algorithms in an iterativp fashion. It is capable of achieving much higher modeling accmacy using a lower order model than that achieved using a higher order model by subspace-based algorithms, such as FORSE.

Preliminary structural control results from the Middeck Active Control Experiment (MACE)

Results are presented of on-going closed-loop ground experiments on the MACE test article, the objective of which is to investigate the extent to which closed-loop behavior of flexible spacecraft in zero gravity can be predicted, as well as to examine orbit system identification and control reconfiguration. The MACE hardware consists of three torque wheels, a two-axis gimballing payload, inertial sensors, and a flexible support structure. With the acquisition of a second payload, this is to represent a multiple payload platform with significant structural flexibility. When linear quadratic Gaussian control is used, payload pointing accuracy is improved by an order of magnitude when disturbed by a broadband torque disturbance. The successes and failures of the design and implementation process are discussed.

TESTING OF AN ACTIVE SMART MATERIAL SYSTEM FOR BUFFET LOAD ALLEVIATION

Modem high-performance aircraft can experience severe buffeting vibrations of the tail empennage during high angle of attack maneuvers, which negatively impacts fatigue life and controllability. ACX and the US Air Force have developed a system to control these vibrations, using piezoelectric strain actuators and active feedback control. This paper details the design, fabrication, and ground-testing of a full-scale prototype Buffet Load Alleviation system for the F/A-18, with an emphasis on ground-test results. In these tests, attenuation of greater than 50% of the RMS buffet response was demonstrated using two channels of control.

Wavelength stabilization in an excimer laser source using piezoelectric active vibration control

Excimer laser light sources for photolithography are subject to a cycle of ever-tightening precision requirements, dictated by the design-rule shrinks planned into the industry roadmap. But pulse-to-pulse stability of the center wavelength of the emitted light is limited by the presence of vibration in key components and structures. This paper covers the application of Active Vibration Control (AVC) technology to an excimer laser to mitigate the effects unwanted vibration, and enable compliance with anticipated future stability specifications. The laser system is described, from a structural-dynamics point of view. A systematic approach to vibration diagnostics is presented, with experimental results to support key conclusions regarding the types and sources of vibrations. Next, analytical assessment of active control performance is discussed, followed by breadboard-type implementation results showing reductions of > 30% in a key stability performance metric.

Typical Section Problems for Structural Control Applications

Two low order problems are studied which capture some of the important fundamental physics associated with the control of structures. The order of the problems is kept low to allow the derivation of a closed-form solution. This identifies the dependency of the solution on the basic parameters of the problem. These two problems derive the optimal H2 and H ∞control for a spring/mass system described by a second order, ordinary differential equation. The H2 solution is compared with the closed-form H2 solution to the optimal regulator for an infinite rod and beam whose behaviors are described by second order, partial differential equations. This comparison identifies the analogies between the typical section problem and a simple structural control problem. The optimal control solutions for the H2 and H∞ problems are expanded upon by optimizing passive parameters. This reduces total closed-loop cost and is analogous to a control/structure optimization problem. It is found that certain levels of finite passive damping and stiffness are desirable even if they are available at no cost in the H2 problem, and that additional stiffness (any amount) and damping (up to ζ = 1/√2) enhances performance in the H∞problem. It is shown that while the structure and control must be designed simultaneously to optimize an H2 performance metric, sequential design will work in some cases which use an H∞ performance metric. These simple problems reveal important properties of more complex structural control problems.