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At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Optimal identification using inconsistent modal data

This work examines techniques under the general approach of optimal-update identification which produce optimally adjusted, or updated, property matrices (i.e., mass, stiffness and damping matrices) to more closely match the structure modal response. For practical applications, the techniques must perform when the modal response is inconsistent with other constraints on the desired model. An alternate view of the optimal-update problem is presented that leads to new techniques for addressing inconsistent data. Viewpoints used for previously published techniques are also examined to explore issues in optimal-update identification.

Smith, Suzanne Weaver↗
NASA NTRS1991

On Markov parameters in system identification

A detailed discussion of Markov parameters in system identification is given. Different forms of input-output representation of linear discrete-time systems are reviewed and discussed. Interpretation of sampled response data as Markov parameters is presented. Relations between the state-space model and particular linear difference models via the Markov parameters are formulated. A generalization of Markov parameters to observer and Kalman filter Markov parameters for system identification is explained. These extended Markov parameters play an important role in providing not only a state-space realization, but also an observer/Kalman filter for the system of interest.

Phan, Minh↗
NASA NTRS1991

Inverse problems and optimal experiment design in unsteady heat transfer processes identification

Experimental-computational methods for estimating characteristics of unsteady heat transfer processes are analyzed. The methods are based on the principles of distributed parameter system identification. The theoretical basis of such methods is the numerical solution of nonlinear ill-posed inverse heat transfer problems and optimal experiment design problems. Numerical techniques for solving problems are briefly reviewed. The results of the practical application of identification methods are demonstrated when estimating effective thermophysical characteristics of composite materials and thermal contact resistance in two-layer systems.

Artyukhin, Eugene A.↗
NASA NTRS1991

Comments on Frequency Swept Rotating Input Perturbation Techniques and Identification of the Fluid Force Models in Rotor/bearing/seal Systems and Fluid Handling Machines

Perturbation techniques used for identification of rotating system dynamic characteristics are described. A comparison between two periodic frequency-swept perturbation methods applied in identification of fluid forces of rotating machines is presented. The description of the fluid force model identified by inputting circular periodic frequency-swept force is given. This model is based on the existence and strength of the circumferential flow, most often generated by the shaft rotation. The application of the fluid force model in rotor dynamic analysis is presented. It is shown that the rotor stability is an entire rotating system property. Some areas for further research are discussed.

Muszynska, Agnes↗
NASA NTRS1992

Steady and transient performance calculation method for prediction, analysis, and identification

The detailed design and development of turbofans Involves the prediction and identification, by means of test analysis, of the performance of the engine and its components. The thermodynamic simulation and analysis codes integrate existing knowledge and Interpretations of the detailed operating procedure of the components of the engine being developed. The relevance of the predicted performance depends on the quality of the representation of the various physical phenomena affecting the characteristics of the components and, consequently, on the incorporation of experimental correlation in the modelling. In this context, the representation of compressor and turbine characteristics is particularly important. Firstly, we will analyze the ability of corrected parameters to represent MACH similitude at the component inlet under various conditions. The various measurements achieved on the engine during development are used to enhance the modelling. The methods of identifying the thermodynamic calculation code with the various measurements, considered here with their uncertainties, are then presented and described. The analysis of the tests performed on the powerplant, designed to Identify the real characteristics of the engine components, can be undertaken, considering one or more engine test points and incorporating knowledge acquired through experimentation or on the component test bench. There are many possible fields of use for these identification methods ranging from the rematching of components to the optimization of the control system.

Jean Pierre Duponchel↗
NASA NTRS1992

Linear system identification via backward-time observer models

Presented here is an algorithm to compute the Markov parameters of a backward-time observer for a backward-time model from experimental input and output data. The backward-time observer Markov parameters are decomposed to obtain the backward-time system Markov parameters (backward-time pulse response samples) for the backward-time system identification. The identified backward-time system Markov parameters are used in the Eigensystem Realization Algorithm to identify a backward-time state-space model, which can be easily converted to the usual forward-time representation. If one reverses time in the model to be identified, what were damped true system modes become modes with negative damping, growing as the reversed time increases. On the other hand, the noise modes in the identification still maintain the property that they are stable. The shift from positive damping to negative damping of the true system modes allows one to distinguish these modes from noise modes. Experimental results are given to illustrate when and to what extent this concept works.

Juang, Jer-Nan↗
NASA NTRS1990

Identification and control for a manipulator with two flexible links

The authors investigate the effectiveness of an online identification scheme for tracking the modal frequencies of a two-link flexible mechanism executing large-angle movements and carrying an unknown payload. A decentralized, gain-scheduled, adaptive control scheme is employed in conjunction with the identification scheme in order to illustrate the feasibility of online controller adjustment for endpoint position control in terms of vibration suppression after large-angle movements. Motivation for adopting the autoregressive-moving-average-model perspective is based on the convenient representation for online controller tuning and on the assumption that flexibility dynamics, for small deflections after a nonlinear large-angle motion, exhibit linear behavior. Experimental results are presented for a two-link planar mechanism in which both links are very flexible.

Yurkovich, Stephen↗
NASA NTRS1990

Recent developments in learning control and system identification for robots and structures

This paper reviews recent results in learning control and learning system identification, with particular emphasis on discrete-time formulation, and their relation to adaptive theory. Related continuous-time results are also discussed. Among the topics presented are proportional, derivative, and integral learning controllers, time-domain formulation of discrete learning algorithms. Newly developed techniques are described including the concept of the repetition domain, and the repetition domain formulation of learning control by linear feedback, model reference learning control, indirect learning control with parameter estimation, as well as related basic concepts, recursive and non-recursive methods for learning identification.

Phan, M.↗
NASA NTRS1992

On identification of structures with internal resonances

Identification of structures which exhibit modal interactions is considered, and the difficulties experienced due to these interactions are examined. Free oscillations of quadratically and cubically coupled pairs of oscillators are analytically studied to illustrate nonlinear interactions between structural modes involved in two-to-one and one-to-one frequency relationships. In light of this study, results obtained from application of the eigensystem realization algorithm toward identification of a beam-mass structure with a two-to-one frequency relationship and quadratic coupling are presented and discussed.

Balachandran, B.↗
NASA NTRS1991

Design of multi-layer neural networks for accurate identification of nonlinear mappings

Guidelines for the design of multilayer neural networks for the identification of nonlinear mappings are considered. Since nonlinear mappings can be approximated by a one-hidden-layer neural network, an approach to determine the sufficient number of hidden layer nodes to achieve a global minima of the identification error function is considered.

Teixeira, Edilberto↗
NASA NTRS1991

Identification and control of NASA's ACES structures

Results are presented of identification and control experiments on NASA's ACES structure at the Marshall Space Flight Center. The models used for controller design were obtained from identification experiments employing the algorithm Q-Markov cover. The OVC algorithm used for control design produces a controller minimizing the control energy of the closed-loop system, subject to inequality constraints on each of the output variances. The identified model matches the experimental data for the ACES structure reasonably well. The line of sight pointing errors of the structure are substantially reduced by the controllers.

Liu, K.↗
NASA NTRS1991

Variance and bias computations for improved modal identification using ERA/DC

Variance and bias confidence criteria were recently developed for the eigensystem realization algorithm (ERA) identification technique. These criteria are extended for the modified version of ERA based on data correlation, ERA/DC, and also for the Q-Markov cover algorithm. The importance and usefulness of the variance and bias information are demonstrated in numerical studies. The criteria are shown to be very effective not only by indicating the accuracy of the identification results, especially in terms of confidence intervals, but also by helping the ERA user to obtain better results by seeing the effect of changing the sample time, adjusting the Hankel matrix dimension, choosing how many singular values to retain, deciding the model order, etc.

Longman, Richard W.↗
NASA NTRS1991

Scene identification probabilities for evaluating radiation flux errors due to scene misidentification

The scene identification probabilities (Pij) are fundamentally important in evaluations of the top-of-the-atmosphere (TOA) radiation-flux errors due to the scene misidentification. In this paper, the scene identification error probabilities were empirically derived from data collected in 1985 by the Earth Radiation Budget Experiment (ERBE) scanning radiometer when the ERBE satellite and the NOAA-9 spacecraft were rotated so as to scan alongside during brief periods in January and August 1985. Radiation-flux error computations utilizing these probabilities were performed, using orbit specifications for the ERBE, the Cloud and Earth's Radiant Energy System (CERES), and the SCARAB missions for a scene that was identified as partly cloudy over ocean. Typical values of the standard deviation of the random shortwave error were in the order of 1.5-5 W/sq m, but could reach values as high as 18.0 W/sq m as computed from NOAA-9.

Manalo, Natividad D.↗
NASA NTRS1992

Comparison of candidate methods to distinguish noise modes from system modes in structural identification

In modal identification, nonphysical noise or computation modes always appear to help match the input-output data. This paper studies the ability of four criteria to distinguish which modes in a model are noise modes: (1) modal amplitude coherency, (2) the relative contribution of each mode to the pulse response indicated by the mode singular value, (3) the variances of the mode frequencies and damping factors produced by a chosen measurement noise level, and (4) identification of the backward-time in order to let the shift from positive to negative damping of the true system modes distinguish these modes from noise modes. Both simulated and experimental data are used to study the four criteria.

Longman, Richard W.↗
NASA NTRS1993

An application of the Observer/Kalman Filter Identification (OKID) technique to Hubble flight data

The objective of the current research is to identify vibration parameters, including frequencies, damping ratio and uncertainty characteristics, of the Hubble Space Telescope from flight data using an advanced system identification technique. The Observer/Kalman Filter Identification (OKID) technique is used to identify the vibration parameters. The OKID was recently developed by the researchers in the Spacecraft Dynamics Branch at NASA Langley Research Center.

Juang, Jer-Nan↗
NASA NTRS1993

An analytic modeling and system identification study of rotor/fuselage dynamics at hover

A combination of analytic modeling and system identification methods have been used to develop an improved dynamic model describing the response of articulated rotor helicopters to control inputs. A high-order linearized model of coupled rotor/body dynamics including flap and lag degrees of freedom and inflow dynamics with literal coefficients is compared to flight test data from single rotor helicopters in the near hover trim condition. The identification problem was formulated using the maximum likelihood function in the time domain. The dynamic model with literal coefficients was used to generate the model states, and the model was parametrized in terms of physical constants of the aircraft rather than the stability derivatives resulting in a significant reduction in the number of quantities to be identified. The likelihood function was optimized using the genetic algorithm approach. This method proved highly effective in producing an estimated model from flight test data which included coupled fuselage/rotor dynamics. Using this approach it has been shown that blade flexibility is a significant contributing factor to the discrepancies between theory and experiment shown in previous studies. Addition of flexible modes, properly incorporating the constraint due to the lag dampers, results in excellent agreement between flight test and theory, especially in the high frequency range.

Hong, Steven W.↗
NASA NTRS1993

System identification and model reduction using modulating function techniques

Weighted least squares (WLS) and adaptive weighted least squares (AWLS) algorithms are initiated for continuous-time system identification using Fourier type modulating function techniques. Two stochastic signal models are examined using the mean square properties of the stochastic calculus: an equation error signal model with white noise residuals, and a more realistic white measurement noise signal model. The covariance matrices in each model are shown to be banded and sparse, and a joint likelihood cost function is developed which links the real and imaginary parts of the modulated quantities. The superior performance of above algorithms is demonstrated by comparing them with the LS/MFT and popular predicting error method (PEM) through 200 Monte Carlo simulations. A model reduction problem is formulated with the AWLS/MFT algorithm, and comparisons are made via six examples with a variety of model reduction techniques, including the well-known balanced realization method. Here the AWLS/MFT algorithm manifests higher accuracy in almost all cases, and exhibits its unique flexibility and versatility. Armed with this model reduction, the AWLS/MFT algorithm is extended into MIMO transfer function system identification problems. The impact due to the discrepancy in bandwidths and gains among subsystem is explored through five examples. Finally, as a comprehensive application, the stability derivatives of the longitudinal and lateral dynamics of an F-18 aircraft are identified using physical flight data provided by NASA. A pole-constrained SIMO and MIMO AWLS/MFT algorithm is devised and analyzed. Monte Carlo simulations illustrate its high-noise rejecting properties. Utilizing the flight data, comparisons among different MFT algorithms are tabulated and the AWLS is found to be strongly favored in almost all facets.

Shen, Yan↗
NASA NTRS1992

Finite element model and identification procedure

Viewgraphs on finite element model and identification procedure are presented. Topics covered include: interferometer finite element model; testbed mode shapes; finite element model update; identification procedure; shaker locations; data analysis; modal frequency and damping comparison; computational procedure; fit comparison; residue analysis; typical residues; identification/FEM residual comparison; and pathlength control using isolation mounts.

How, Jonathan P.↗