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At least 1,063 records · Page 59
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NASA NTRS1989

A variable-gain output feedback control design approach

A multi-model design technique to find a variable-gain control law defined over the whole operating range is proposed. The design is formulated as an optimal control problem which minimizes a cost function weighing the performance at many operating points. The solution is obtained by embedding into the Multi-Configuration Control (MCC) problem, a multi-model robust control design technique. In contrast to conventional gain scheduling which uses a curve fit of single model designs, the optimal variable-gain control law stabilizes the plant at every operating point included in the design. An iterative algorithm to compute the optimal control gains is presented. The methodology has been successfully applied to reconfigurable aircraft flight control and to nonlinear flight control systems.

Haylo, Nesim↗
NASA NTRS1989

Application of variable-gain output feedback for high-alpha control

This paper describes a variable-gain optimal discrete output-feedback design approach that is applied to a nonlinear flight regime covering a wide angle-of-attack range that includes stall and poststall. The paper includes brief descriptions of the variable-gain formulation, the discrete-control structure and flight equations used to apply the design approach, and the high performance airplane model used in the application. Both linear and nonlinear analyses are shown for a longitudinal four-model design case with angles of attack of 5, 15, 35, and 60 deg.

Ostroff, Aaron J.↗
NASA NTRS1989

Shape control of high degree-of-freedom variable geometry trusses

Common static trusses are constrained to permit no relative motion between truss elements. A Variable Geometry Truss (VGT), however, is a truss which contains some number of variable length links. The extensible links allow the truss to change shape in a precise, controllable manner. These changes can also be used to control the vibrational response of a truss structure or to perform robotic tasks. Many geometric configurations, both planar and spatial, are possible candidates for VGT manipulators. Here, only two geometries are discussed; the three degree-of-freedom (DOF) spatial octahedral/octahedral truss and the three DOF planar tetrahedral truss. These truss geometries are used as the fundamental element in a repeating chain of trusses. This results in a highly dexterous manipulator with perhaps 30 to 60 degrees of freedom that retains the favorable stiffness properties of a conventional truss. From a fixed base, this type of manipulator could perform shape or vibration control while extending and snaking through complex passageways or moving around obstacles to perform robotic tasks. The approach taken here is to first concentrate on fully understanding the forward and inverse kinematics of the fundamental elements and then utilizing the insight thus gained to solve the more complex problem of the kinematic chains.

Salerno, R. J.↗
NASA NTRS1989

Tropical Pacific moisture variability

Research objectives are to: (1) describe the synoptic scale variability of moisture over the tropical Pacific Ocean; (2) describe the systems leading to this variability; and (3) develop and implement satellite analysis procedures to facilitate (1) and (2) over the data sparse Pacific.

Mcguirk, James P.↗
NASA NTRS1989

Physically-based parameterization of spatially variable soil and vegetation using satellite multispectral data

A stochastic-geometric landsurface reflectance model is formulated and tested for the parameterization of spatially variable vegetation and soil at subpixel scales using satellite multispectral images without ground truth. Landscapes are conceptualized as 3-D Lambertian reflecting surfaces consisting of plant canopies, represented by solid geometric figures, superposed on a flat soil background. A computer simulation program is developed to investigate image characteristics at various spatial aggregations representative of satellite observational scales, or pixels. The evolution of the shape and structure of the red-infrared space, or scattergram, of typical semivegetated scenes is investigated by sequentially introducing model variables into the simulation. The analytical moments of the total pixel reflectance, including the mean, variance, spatial covariance, and cross-spectral covariance, are derived in terms of the moments of the individual fractional cover and reflectance components. The moments are applied to the solution of the inverse problem: The estimation of subpixel landscape properties on a pixel-by-pixel basis, given only one multispectral image and limited assumptions on the structure of the landscape. The landsurface reflectance model and inversion technique are tested using actual aerial radiometric data collected over regularly spaced pecan trees, and using both aerial and LANDSAT Thematic Mapper data obtained over discontinuous, randomly spaced conifer canopies in a natural forested watershed. Different amounts of solar backscattered diffuse radiation are assumed and the sensitivity of the estimated landsurface parameters to those amounts is examined.

Jasinski, Michael F.↗
NASA NTRS1989

Rain attenuation measurements: Variability and data quality assessment

Year to year variations in the cumulative distributions of rain rate or rain attenuation are evident in any of the published measurements for a single propagation path that span a period of several years of observation. These variations must be described by models for the prediction of rain attenuation statistics. Now that a large measurement data base has been assembled by the International Radio Consultative Committee, the information needed to assess variability is available. On the basis of 252 sample cumulative distribution functions for the occurrence of attenuation by rain, the expected year to year variation in attenuation at a fixed probability level in the 0.1 to 0.001 percent of a year range is estimated to be 27 percent. The expected deviation from an attenuation model prediction for a single year of observations is estimated to exceed 33 percent when any of the available global rain climate model are employed to estimate the rain rate statistics. The probability distribution for the variation in attenuation or rain rate at a fixed fraction of a year is lognormal. The lognormal behavior of the variate was used to compile the statistics for variability.

Crane, Robert K.↗
NASA NTRS1989

Electronically Variable Pressure Regulator (EVPR)

A new programmable electronically variable pressure regulator (EVPR) concept accurately controls the local outlet or remote system pressure. It uses an integral pulse width modulated rare earth permanent magnet motor operating in response to redundant pressure transducer feedback signals. The EVPR is a simple single stage device that does not use dynamic seals or pilot valving. Conversion of partial revolution motor torque to poppet lifting force is accomplished by pure flexure action to avoid using bearings. The flexure drive (called the ROTAX) has a variable lead to minimize motor weight and power consumption. Breadboard tests were completed successfully on two critical design elements of the EVPR: the ROTAX and the motor. The ROTAX cable system was tested for 250,000 cycles without failure. The breadboard motor met the basic design requirements including the design torque and power consumption. Prototype parts were fabricated, and testing of the prototype EVPR has started. It is PC computer controlled to facilitate programming, data acquisition and analysis. A lightweight dedicated microprocessor is planned for the flightweight EVPR.

Reinicke, R. H.↗
NASA NTRS1988

Variable mixture ratio performance through nitrogen augmentation

High/variable mixture ratio O2/H2 candidate engine cycles are examined for earth-to-orbit vehicle application. Engine performance and power balance information are presented for the candidate cycles relative to chamber pressure, bulk density, and mixture ratio. Included in the cycle screening are concepts where a third fluid (liquid nitrogen) is used to achieve a variable mixture ratio over the trajectory from liftoff to earth orbit. The third fluid cycles offer a very low risk, fully reusable, low operation cost alternative to high/variable mixture ratio bipropellant cycles. Variable mixture ratio engines with extendible nozzle are slightly lower performing than a single mixture ratio engine (MR = 7:1) with extendible nozzle. Dual expander engines (MR = 7:1) have slightly better performance than the single mixture ratio engine. Dual fuel dual expander engines offer a 16 percent improvement over the single mixture ratio engine.

Beichel, R.↗
NASA NTRS1989

Investigation of x ray variability in highly active cool stars

Ginga x ray observations of highly active cool star coronae were obtained and analyzed in an effort to better understand the nature of their time variability. The possible types of variability studied included x ray occultations via eclipses in a binary system, rotational modulation of x ray emission, flares, and a search for microflaring. Observation of both sigma(sup 2) CrB and Algol were performed successfully by Ginga. The sigma(sup 2) CrB observations occurred on 27 to 30 June 1988, and the Algol observations on 12 to 14 January 1989. In the sigma(sup 2) CrB observation, simultaneous IUE and Very Large Array (VLA) observations were obtained during part of the Ginga observation. Flaring activity was detected on sigma(sup 2) CrB in the Ginga 1.7 to 11 KeV band and in the IUE microwave region. A large flare on Algol which lasted well over 12 hours was detected, began with a maximum temperature of 65 MK which gradually decayed to 36 MK, and evidence was shown of highly ionized Fe line emission.

Stern, Robert A.↗
NASA NTRS1989

The detection of an IR excess toward the globular cluster RV Tauri variable M28 V17

The paper presents the results of a search for IR emission excesses toward three globular cluster stars. Two of the stars are RV Tau variables (M28 V17 and M5 V85) and one is believed to be a W Vir variable (M5 V42) which is historically linked to the RV Tau class. Consideration is given to the implications of the observed detections, in particular, the establishment of M28 V17 as a star in a globular cluster that is indistinguishable from the Preston type A subclass.

Nook, Mark A.↗
NASA NTRS1990

WHT observations of cataclysmic variables in globular clusters. II - The CV candidates in M71

Results are presented from photometric and spectroscopic observations of the six UV-bright objects in M71 which Richer and Fahlman (1988) proposed as candidate cataclysmic variables. It is found that the two brightest of these candidates are probably hot subdwarfs. It is suggested that the remaining candidates are more likely to be white dwarfs and subdwarfs than cataclysmic variables.

Machin, G.↗
NASA NTRS1989

A multiple-time-scale turbulence model based on variable partitioning of the turbulent kinetic energy spectrum

A multiple-time-scale turbulence model of a single point closure and a simplified split-spectrum method is presented. In the model, the effect of the ratio of the production rate to the dissipation rate on eddy viscosity is modeled by use of the multiple-time-scales and a variable partitioning of the turbulent kinetic energy spectrum. The concept of a variable partitioning of the turbulent kinetic energy spectrum and the rest of the model details are based on the previously reported algebraic stress turbulence model. Example problems considered include: a fully developed channel flow, a plane jet exhausting into a moving stream, a wall jet flow, and a weakly coupled wake-boundary layer interaction flow. The computational results compared favorably with those obtained by using the algebraic stress turbulence model as well as experimental data. The present turbulence model, as well as the algebraic stress turbulence model, yielded significantly improved computational results for the complex turbulent boundary layer flows, such as the wall jet flow and the wake boundary layer interaction flow, compared with available computational results obtained by using the standard kappa-epsilon turbulence model.

Kim, S.-W.↗
NASA NTRS1990

Temporal behavior of Hercules X-1 - The long-term variability of its pulse profile and the 35 day X-ray intensity modulation

Data collected by the low-energy detectors of the Hard X-Ray and Low Energy Gamma-Ray Experiment on HEAO 1 were analyzed to study the variability in the temporal behavior of Hercules X-1. Data obtained during thirteen pointed observations distributed in 35-day phase provided details on the time and the energy dependence of the pulse shape and the spectra. It was found that a structural change at the inner edge of the accretion disk, but not at the large radii, was likely to cause the 35-day as well as the observed short-term rapid variations of the pulse profile, which had a considerably more pronounced time dependence than energy dependence. It is suggested that the variable mass-channeling scheme and an intervening effect at the inner accretion disk are directly responsible for the 35-day X-ray modulation.

Soong, Y.↗
NASA NTRS1990

An interim reference model for the variability of the middle atmosphere water vapor distribution

A reference model for the middle atmosphere water vapor distribution for some latitudes and seasons was developed using two data sets. One is the seven months of Nimbus LIMS data obtained during November 1978 to May 1979 over the range 64 deg S - 84 deg N latitude and from about 100-mb to 1-mb altitude, and the other is represented by water vapor profiles from 0.2 mb to 0.01 mb in the mid-mesosphere, measured on ground at several fixed mid-latitude sites in the Northern Hemisphere, using microwave-emission techniques. This model provides an interim water vapor profile for the entire vertical range of the middle atmosphere, with accuracies of better than 25 percent. The daily variability of stratospheric water vapor profiles about the monthly mean is demonstrated, and information is provided on the longitudinal variability of LIMS water vapor profiles about the daily, weekly, and monthly zonal means.

Remsberg, E. E.↗
NASA NTRS1989

Three dimensional simulation of spatial and temporal variability of stratospheric hydrogen chloride

Spatial and temporal variability of atmospheric HCl columns are calculated for January 1979 using a three-dimensional chemistry-transport model designed to provide the best possible representation of stratospheric transport. Large spatial and temporal variability of the HCl columns is shown to be correlated with lower stratospheric potential vorticity and thus to be of dynamical origin. Systematic longitudinal structure is correlated with planetary wave structure. These results can help place spatially and temporally isolated column and profile measurements in a regional and/or global perspective.

Kaye, Jack A.↗
NASA NTRS1990

Orbital variability in the wind of the massive X-ray binary HD 153919/4U 1700-37

A set of forty high-dispersion IUE spectra of HD 153919, the Of-type primary in the 4U 1700-37 X-ray binary system, have been examined for evidence of orbital profile variability associated with the Hatchett-McCray effect. Although the effect is negligible in the resonance lines of C IV, Si IV, and N V, some subordinate lines show clear orbital changes. The variability in the absorption profile of N IV 1718, in particular, is shown to be in agreement with model calculations. The interstellar spectrum is studied, showing that highly ionized species can be maintained by the primary's radiation field alone.

Hammerschlag-Hensberge, G.↗
NASA NTRS1990

The variability of atmospheric equivalent temperature for radar altimeter range correction

Two sets of data were used to test the validity of the presently used approximation for radar altimeter range correction due to atmospheric water vapor. The approximation includes an assumption of constant atmospheric equivalent temperature. The first data set includes monthly, three-dimensional, gridded temperature and humidity fields over global oceans for a 10-year period, and the second is comprised of daily or semidaily rawinsonde data at 17 island stations for a 7-year period. It is found that the standard method underestimates the variability of the equivalent temperature, and the approximation could introduce errors of 2 cm for monthly means. The equivalent temperature is found to have a strong meridional gradient, and the highest temporal variabilities are found over western boundary currents. The study affirms that the atmospheric water vapor is a good predictor for both the equivalent temperature and the range correction. A relation is proposed to reduce the error.

Liu, W. Timothy↗
NASA NTRS1990

Optical variability of extragalactic objects used to tie the HIPPARCOS reference frame to an extragalactic system using Hubble space telescope observations

Observations of a set of 89 extragalactic objects (EGOs) will be made with the Hubble Space Telescope Fine Guidance Sensors and Planetary Camera in order to link the HIPPARCOS Instrumental System to an extragalactic coordinate system. Most of the sources chosen for observation contain compact radio sources and stellarlike nuclei; 65 percent are optical variables beyond a 0.2 mag limit. To ensure proper exposure times, accurate mean magnitudes are necessary. In many cases, the average magnitudes listed in the literature were not adequate. The literature was searched for all relevant photometric information for the EGOs, and photometric parameters were derived, including mean magnitude, maximum range, and timescale of variability. This paper presents the results of that search and the parameters derived. The results will allow exposure times to be estimated such that an observed magnitude different from the tabular magnitude by 0.5 mag in either direction will not degrade the astrometric centering ability on a Planetary Camera CCD frame.

Bozyan, Elizabeth P.↗