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NASA NTRS1988

Evaluation of a methodology for model identification in the time domain

A model identification methodology for structural dynamics has been applied to simulated vibrational data as a first step in evaluating its accuracy. The evaluation has taken into account a wide variety of factors which affect the accuracy of the procedure. The effects of each of these factors were observed in both the response time histories and the estimates of the parameters of the model by comparing them with the exact values of the system. Each factor was varied independently but combinations of these have also been considered in an effort to simulate real situations. The results of the tests have shown that for the chain model, the procedure yields robust estimates of the stiffness parameters under the conditions studied whenever uniqueness is ensured. When inaccuracies occur in the results, they are intimately related to non-uniqueness conditions inherent in the inverse problem and not to shortcomings in the methodology.

Beck, R. T.↗

NASA NTRS1988

Identification and control integration strategies

This paper describes an autonomous control concept for pointing and articulation of science instruments on the Eos (Earth observing system) NASA/NOAA platforms intended to be operational by the late 1990s. Key features of this concept include advanced control adaptation and tuning strategies which provide performance robustness over a wide range of system uncertainties and mission time criticality. System identification-control modification paradigms are synthesized to form an adaptation continuum over this extended regime of autonomous operations.

Milman, Mark↗

NASA NTRS1988

Formulation and verification of frequency response system identification techniques for large space structures

For the past several years much effort has been given to the development of techniques for designing control systems for large space structures (LSS's). The main objective of these efforts has been to develop a LSS control methodology that produces designs that meet strenuous performance requirements and are robust to model inaccuracies. Unfortunately, performance and robustness are conflicting requirements. Because LSS's can not be fully tested on ground, it has become an accepted fact that the design of LSS control systems to meet performance requirements can not be completed until the LSS is placed on-orbit and tested and an accurate model is extracted from on-orbit test results. Modern MIMO sampled-data frequency response design techniques are viable candidates for designing LSS control systems. First, this paper presents techniques for performing MIMO system identification (ID) from test data. Then, techniques for improving the performance of the system ID process in the presence of noise are presented. Finally, practical utility of the system ID approaches are validated by the presentation of results obtained from application on the LSS Ground Test Facility at Marshall Space Flight Center.

Mitchell, Jerrel R.↗

NASA NTRS1988

Modal parameter identification in space structures

The objective of this project is to identify modal properties such as the eigenvalues and eigenfunctions of structures. The formal means for accomplishing this task, Structural Identification, is viewed as a two step procedure: (1) identify the eigensolution; and (2) using the identified eigensolution, identify the mass and stiffness. The eigensolution is identified as a correction on a postulated model based on erroneous parameters.

Baruh, H.↗

NASA NTRS1988

Time domain identification of element matrices

Accurate mathematical models are clearly an important part of the design, modification, control, and damage assessment of large space structures. A critical part of model determination of any type of structure is the use of system identification (SI), a process for using measured excitation and response data to improve the form of a mathematical model or the values of the parameters in it. The reasons for using SI are manifold, but most of them involve uncertainties in either the form of or the parameters in the mathematical model. Some of the more common uncertainties are the following: the nature of the damping, the characteristics of sloppy or sticky joints, inelastic material properties, and parameters in a reduced Degree-of-Freedom (DOF) model. This paper contains a description of the SI algorithm followed by two illustrations: one in which all parameters in a shear building model are determined using pull back and quick release data, and another in which an equivalent reduced DOF model is obtained for a two story frame.

Matzen, Vernon C.↗

NASA NTRS1932

Rapid chemical test for the identification of chromium-molybdenum steel

This note describes a simple, rapid, qualitative test which can be applied to solutions of drilling or chips for the identification of chromium-molybdenum steel. The test is based on the orange-red compound which is formed when thiocyanate and inequivalent molybdenum react. This test is much more reliable than the potassium ethylxanthate test which has been recommended for a like purpose. A list of the apparatus and reagents which are required, and a description of the procedure follows.

Redmond, John C↗

NASA NTRS1930

Identification of Aircraft by Rockwell Test

A large number of tests were made on tubes of 1025 and 4130X steel, in various diameters and wall thicknesses, and after diverse heat treatments. The Rockwell B scale was employed, as being the best suited to the ranges of hardness encountered. Only satisfactory chrome molybdenum tubes were found to show a hardness in excess of 90-B after normalizing. The method therefore provided the desired means of identification of chrome molybdenum steel. It is a qualitative test only. The conditions of a satisfactory test are as follows. 1) The tubing must be normalized. 2) It must be clean inside and out at the point where the test is to be made. 3) The tube must be held in correct alignment with the penetrator and must not move during the test. 4) For thin-walled tubes, the anvil must extend within the tube so as to support the wall.

Knerr, Horace↗

NASA NTRS1994

Frequency-Domain Identification With Composite Curve Fitting

Improved method of parameter identification based on decomposing single wide-band model into two or more component systems in parallel. Each component model predominates in specific frequency range. Wide-band mathematical model of system identified as two narrow-band models: one containing most of information on high-frequency components of dynamics, and one containing most of information on low-frequency components. Applicable to diverse systems, including vibrating structures, electronic circuits, and control systems.

Bayard, David S.↗

NASA NTRS1990

Identification of the connections in biologically inspired neural networks

We developed an identification method to find the strength of the connections between neurons from their behavior in small biologically-inspired artificial neural networks. That is, given the network external inputs and the temporal firing pattern of the neurons, we can calculate a solution for the strengths of the connections between neurons and the initial neuron activations if a solution exists. The method determines directly if there is a solution to a particular neural network problem. No training of the network is required. It should be noted that this is a first pass at the solution of a difficult problem. The neuron and network models chosen are related to biology but do not contain all of its complexities, some of which we hope to add to the model in future work. A variety of new results have been obtained. First, the method has been tailored to produce connection weight matrix solutions for networks with important features of biological neural (bioneural) networks. Second, a computationally efficient method of finding a robust central solution has been developed. This later method also enables us to find the most consistent solution in the presence of noisy data. Prospects of applying our method to identify bioneural network connections are exciting because such connections are almost impossible to measure in the laboratory. Knowledge of such connections would facilitate an understanding of bioneural networks and would allow the construction of the electronic counterparts of bioneural networks on very large scale integrated (VLSI) circuits.

Demuth, H.↗

NASA NTRS1993

Identification and interpretation of patterns in rocket engine data

A prototype software system was constructed to detect anomalous Space Shuttle Main Engine (SSME) behavior in the early stages of fault development significantly earlier than the indication provided by either redline detection mechanism or human expert analysis. The major task of the research project is to analyze ground test data, to identify patterns associated with the anomalous engine behavior, and to develop a pattern identification and detection system on the basis of this analysis. A prototype expert system which was developed on both PC and Symbolics 3670 lisp machine for detecting anomalies in turbopump vibration data was checked with data from ground tests 902-473, 902-501, 902-519, and 904-097 of the Space Shuttle Main Engine. The neural networks method was also applied to supplement the statistical method utilized in the prototype system to investigate the feasibility in detecting anomalies in turbopump vibration of SSME. In most cases the anomalies detected by the expert system agree with those reported by NASA. On the neural networks approach, the results are given the successful detection rate higher than 95 percent to identify either normal or abnormal running condition based on the experimental data as well as numerical simulation.

Lo, C. F.↗

NASA NTRS1993

The star identification, pointing and tracking system of UVSTAR, an attached payload instrument system for the Shuttle Hitchhiker-M platform

We describe an algorithm for star identification and pointing/tracking of a spaceborne electro-optical system and simulation analyses to test the algorithm. The algorithm will be implemented in the guiding system of UVSTAR, a spectrographic telescope for observations of astronomical and planetary sources operating in the 500-1250 A waveband at approximately 1 A resolution. The experiment is an attached payload and will fly as a Hitchhiker-M payload on the Shuttle. UVSTAR includes capabilities for independent target acquisition and tracking. The spectrograph package has internal gimbals that allow angular movement of plus or minus 3 deg from the central position. Rotation about the azimuth axis (parallel to the Shuttle z axis) and elevation axis (parallel to the Shuttle x axis) will actively position the field of view to center the target of interest in the fields of the spectrographs. The algorithm is based on an on-board catalog of stars. To identify star fields, the algorithm compares the positions of stars recorded by the guiding imager to positions computed from the on-board catalog. When the field has been identified, its position within the guiding imager field of view can be used to compute the pointing corrections necessary to point to a target of interest. In tracking mode, the software uses the past history to predict the quasi-periodic attitude control motions of the shuttle and sends pointing commands to cancel the motion and stabilize UVSTAR on the target. The guiding imager (guider) will have an 80-mm focal length and f/1.4 optics giving a field of view of 6 deg x 4.5 deg using a 385 x 288 pixel intensified CCD. It will be capable of providing high accuracy (better than 2 arc-sec) attitude determination from coarse (6 deg x 4.5 deg) initial knowledge of the pointing direction; and of pointing toward the target. It will also be capable of tracking at the same high accuracy with a processing time of less than a few hundredths of a second.

Decarlo, Francesco↗

NASA NTRS1992

In situ identification of the Martian surface material and its interaction with the Martian atmosphere using DTA/GC

Little is known about the mineralogy of the martian surface material. Several techniques have been suggested as candidates for the in situ identification of the martian surface material. The most promising of these techniques include differential thermal analysis (DTA) coupled with gas chromatography (GC) and differential scanning calorimetry (DSC) coupled with either mass spectrometry (MS) or GC. Our studies showed that differential thermal analysis coupled with gas chromatography (DTA/GC) is a more appropriate analytical technique than DSC/MS or DSC/GC to identify the mineralogy of the martian surface material in situ. DTA/GC can be regarded as an advancement from pyrolytic GC analyses that were successfully flown on previous missions, but have supplied only limited mineralogical information.

Mancinelli, R. L.↗

NASA NTRS1993

Experimental modification and identification of the DSS-13 antenna control system model

This work is the first step toward increasing the bandwidth of the DSS-13 antenna position loop controller. A wider bandwidth of the controller allows for faster tracking rates and better pointing performance under windy conditions. To achieve this goal, the antenna control system model has to be improved, such that it will accurately reflect the dynamic properties of the antenna. The existing analytical model, due to its many uncertainties, could not be used in the design of the controller. However, by using experimental data, the analytical model is modified and improved, and a new model is obtained through system identification techniques.

Racho, C. S.↗

NASA NTRS1993

Identification of integrated airframe: Propulsion effects on an F-15 aircraft for application to drag minimization

The application of an adaptive real-time measurement-based performance optimization technique is being explored for a future flight research program. The key technical challenge of the approach is parameter identification, which uses a perturbation-search technique to identify changes in performance caused by forced oscillations of the controls. The controls on the NASA F-15 highly integrated digital electronic control (HIDEC) aircraft were perturbed using inlet cowl rotation steps at various subsonic and supersonic flight conditions to determine the effect on aircraft performance. The feasibility of the perturbation-search technique for identifying integrated airframe-propulsion system performance effects was successfully shown through flight experiments and postflight data analysis. Aircraft response and control data were analyzed postflight to identify gradients and to determine the minimum drag point. Changes in longitudinal acceleration as small as 0.004 g were measured, and absolute resolution was estimated to be 0.002 g or approximately 50 lbf of drag. Two techniques for identifying performance gradients were compared: a least-squares estimation algorithm and a modified maximum likelihood estimator algorithm. A complementary filter algorithm was used with the least squares estimator.

Schkolnik, Gerard S.↗

NASA NTRS1993

Risk identification and reduction in integrated product teams

This brief report summarizes research and planning conducted during Summer 1993 for MSFC on the subjects of risk identification, assessment, and management. Research findings are presented, citing useful references. The major output of this work, the AXAF-S Project Risk Management Plan is outlined.

Batson, Robert G.↗

NASA NTRS1994

Application of star identification using pattern matching to space ground systems at GSFC

This paper reports the application of pattern recognition techniques for star identification based on those proposed by Van Bezooijen to space ground systems for near-real-time attitude determination. A prototype was developed using these algorithms, which was used to assess the suitability of these techniques for support of the X-Ray Timing Explorer (XTE), Submillimeter Wave Astronomy Satellite (SWAS), and the Solar and Heliospheric Observatory (SOHO) missions. Experience with the prototype was used to refine specifications for the operational system. Different geometry tests appropriate to the mission requirements of XTE, SWAS, and SOHO were adopted. The applications of these techniques to upcoming mission support of XTE, SWAS, and SOHO are discussed.

Fink, D.↗

NASA NTRS1994

MME-based attitude dynamics identification and estimation for SAMPEX

A method is described for obtaining optimal attitude estimation algorithms for spacecraft lacking attitude rate measurement devices (rate gyros), and then demonstrated using actual flight data from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) spacecraft. SAMPEX does not have on-board rate sensing, and relies on sun sensors and a three-axis magnetometer for attitude determination. Problems arise since typical attitude estimation is accomplished by filtering measurements of both attitude and attitude rates. Rates are nearly always sampled much more densely than are attitudes. Thus, the absence/loss of rate data normally reduces both the total amount of data available and the sampling density (in time) by a substantial fraction. As a result, the sensitivity of the estimates to model uncertainty and to measurement noise increases. In order to maintain accuracy in the attitude estimates, there is increased need for accurate models of the rotational dynamics. The proposed approach is based on the minimum model error (MME) optimal estimation strategy, which has been successfully applied to estimation of poorly modeled dynamic systems which are relatively sparsely and/or noisily measured. The MME estimates may be used to construct accurate models of the system dynamics (i.e. perform system model identification). Thus, an MME-based approach directly addresses the problems created by absence of attitude rate measurements.

Depena, Juan↗

NASA NTRS1994

Modal Identification Experiment accommodations review

The Modal Identification Experiment (MIE) will monitor the structure of the Space Station Freedom (SSF), and measure its response to a sequence of induced disturbances. The MIE will determine the frequency, damping, and shape of the important modes during the SSF assembly sequence including the Permanently Manned Configuration. This paper describes the accommodations for the proposed instrumentation, the data processing hardware, and the communications data rates. An overview of the MIE operational modes for measuring SSF acceleration forces with accelerometers is presented. The SSF instrumentation channel allocations and the Data Management System (DMS) services required for MIE are also discussed.

Klich, Phillip J.↗