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At least 559 records · Page 31

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NASA NTRS1989

Passivity In Analysis Of Robustness Of A Control System

Robustness margin (measure of degree of passivity) defined. Paper presents new approach to analysis of stability of multivariable feedforward-and-feedback control system consisting of exponentially stable linear time-invariant (LTI) feedforward subsystem and nonlinear time-varying (NTV) or dynamic feedback subsystem. New method based on notion of passivity, quantified by robustness margin. Margin provides bound on magnitudes of perturbations, below which perturbations will not make system unstable.

Wen, John Ting-Yung↗

NASA NTRS1987

Reducing model uncertainty effects in flexible manipulators through the addition of passive damping

An important issue in the control of practical systems is the effect of model uncertainty on closed loop performance. This is of particular concern when flexible structures are to be controlled, due to the fact that states associated with higher frequency vibration modes are truncated in order to make the control problem tractable. Digital simulations of a single-link manipulator system are employed to demonstrate that passive damping added to the flexible member reduces adverse effects associated with model uncertainty. A controller was designed based on a model including only one flexible mode. This controller was applied to larger order systems to evaluate the effects of modal truncation. Simulations using a Linear Quadratic Regulator (LQR) design assuming full state feedback illustrate the effect of control spillover. Simulations of a system using output feedback illustrate the destabilizing effect of observation spillover. The simulations reveal that the system with passive damping is less susceptible to these effects than the untreated case.

Alberts, T. E.↗

NASA NTRS1988

Passive and active control of jet turbulence

Techniques for controlling the generation, growth, and interactions of large coherent structures in turbulent flows are discussed, reviewing the results of recent experimental investigations. The fundamental principles of active and passive coherent-structure manipulation are outlined; the effects of geometric modification are examined for the case of an elliptic jet; and particular attention is given to passive control via self-excitation, turbulence and noise suppression, control of pairing interaction via two-frequency excitation, and unsteady boundary-layer separation in wall jets. Diagrams and graphs of typical results are provided.

Fazle Hussain, A. K. M.↗

NASA NTRS1988

Controller synthesis for infinite dimensional systems based on a passivity approach

The author discusses a generalization of hyperstability to systems in Hilbert space and its application to finite-dimensional stabilizing compensator design. The basic idea is to characterize tolerable perturbations in terms of the passivity of the nominal closed-loop system. Controllers achieving the required closed-loop passivity property can then be designed using H(infinity)-optimization. In particular, the author presents a design procedure for a stabilizing a finite-dimensional compensator for a given infinite-dimensional system.

Wen, John T.↗

NASA NTRS1989

Turbulent flow separation control through passive techniques

Several passive separation control techniques for controlling moderate two-dimensional turbulent flow separation over a backward-facing ramp are studied. Small transverse and swept grooves, passive porous surfaces, large longitudinal grooves, and vortex generators were among the techniques used. It was found that, unlike the transverse and longitudinal grooves of an equivalent size, the 45-deg swept-groove configurations tested tended to enhance separation.

Lin, J. C.↗

NASA NTRS1988

Long-term performance of the passive thermal control systems of the IRAS spacecraft

Degradation of passive thermal control systems in space is a matter of serious concern and has been observed in many missions. The performance of the passive thermal control systems of the Infrared Astronomical Satellite (IRAS) over a period of three years is reported here. An exterior temperature of 200 K and a sunshade temperature of approximately 100 K were maintained over this period without significant degradation. The temperature of the telescope contained in the IRAS cryostat was also observed for two years after expenditure of the helium cryogen. It remained at 100 K with no degradation.

Mason, P. V.↗

NASA NTRS1989

Passive navigation using image irradiance tracking

Rotorcraft operating at low altitudes require navigational schemes for locating the terrain and obstacles. Due to the covert nature of missions to be accomplished, a passive navigation scheme is desirable. This paper describes the development of a passive navigation scheme combining image sequences from a vehicle mounted camera with vehicle motion variables. Geometric properties of perspective projection together with an image irradiance tracking scheme at each pixel are used to determine the range to various objects within the field-of-view. Derivation of the numerical algorithm and simulation results are given. Other applications of the proposed approach include navigation for autonomous planetary rovers and telerobots.

Menon, P. K. A.↗

NASA NTRS1989

Integration of active and passive sensors for obstacle avoidance

The automatic obstacle-avoidance guidance problem is studied under the operational constraints imposed by the rotorcraft nap-of-the-earth (NOE) environment. The problem is discussed for two different circumstances. The first assumes that a full range map is available, irrespective of the type of sensor being used. Two approaches are proposed to extend a two-dimensional obstacle-avoidance concept presented by Cheng (1988). The situation where only a sparse range map is available from a passive sensor is also treated. An integrated approach that augments the passive sensor with an active one is discussed, along with the problem of data fusion and how it is affected by the characteristics of NOE flight.

Cheng, Victor H. L.↗

NASA NTRS1990

Transport Of Passive Scalars In A Turbulent Channel Flow

Computer simulation of transport of passive scalars in turbulent channel flow described in report. Shows flow structures and statistical properties. As used here, "passive scalars" means scalar quantities like fluctuations in temperature or concentrations of contaminants that do not disturb flow appreciably. Examples include transport of heat in heat exchangers, gas turbines, and nuclear reactors and dispersal of pollution in atmosphere.

Kim, John↗

NASA NTRS1990

Passive Damping Of Vibrations In Truss Structures

Damping enhanced by redistribution of shear strains in damping materials. Report describes theoretical and experimental investigations of passive damping of vibrations in truss structures. Interest in passive damping revived by proposals to construct large trusses in outer space. Focuses on use of viscoelastic materials to damp longitudinal vibrations in tubular members of such structures.

Chen, Gun-Shing↗

NASA NTRS1990

Passive navigation using image irradiance tracking

Rotorcraft operating at low altitudes require navigational schemes for detecting terrain and obstacles. Due to the nature of the missions to be accomplished and available power onboard, a passive navigation scheme is desirable in this situation. The development of a passive navigation scheme using optical image sequences and vehicle motion variables from an onboard inertial navigation scheme is described. This approach combines the geometric properties of perspective projection and a feedback irradiance tracking scheme at each pixel in the image to determine the range to various objects within the field-of-view. Derivation of the numerical algorithm and simulation results are given. Due to the feedback nature of the implementation, the computational scheme is robust. Other applications of the proposed approach include navigation for autonomous planetary rovers and telerobots.

Menon, P. K. A.↗

NASA NTRS1989

Comparison of rainfall profiles retrieved from dual-frequency radar and from combined radar and passive microwave radiometric measurements

Precipitation profiles retrieved from a dual frequency X- and Ka-band radar and from an X-band radiometer are analyzed. Profiles from combined radar and microwave radiometer measurements are compared to those from dual frequency radar measurements to study the performance of the combined radar-radiometer retrievals. A method for the retrieval of rainfall rate profiles from attenuated radar returns and passive radiometric data is presented. The results suggest that the operation of a single frequency radar with multifrequency passive radiometers may provide cost-effective spaceborne measurements of global tropical rainfall distributions. The possible use of this methodology for the Tropical Rainfall Measuring Mission is considered.

Weinman, J. A.↗

NASA NTRS1990

Investigation of the effects of summer melt on the calculation of sea ice concentration using active and passive microwave data

The effects of ice surface melt on microwave signatures and errors in the calculation of sea ice concentration are examined, using active and passive microwave data sets from the Marginal Ice Zone Experiment aircraft flights in the Fram Strait region. Consideration is given to the possibility of using SAR to supplement passive microwave data to unambiguously discriminate between open water areas and ponded floes. Coincident active multichannel microwave radiometer and SAR measurements of individual floes are used to describe the effects of surface melt on sea ice concentration calculations.

Cavalieri, Donald J.↗

NASA NTRS1990

Interface properties of various passivations of HgCdTe

The effects of different surface treatments of the p-type Hg(x-1)Cd(x)Te (x about 0.2) alloy, including anodic-sulfide, anodic-oxide, and ZnS coatings, on the surface charge density and surface mobility were investigated. Measurements of surface recombination velocities in samples passivated by either a ZnS coating on a freshly etched surface or by anodic native sulfide combined with deposited ZnS showed essentially the same temperature dependence down to 50 K. For the same original material, both the surface state density and the activation energy were found to be similar. It is suggested that, for both passivations, the surface is controlled by traps that can be related to lattice defects, such as vacancies.

Schacham, S. E.↗

NASA NTRS1990

Arctic multiyear ice classification and summer ice cover using passive microwave satellite data

Passive microwave data collected by Nimbus 7 were used to classify and monitor the Arctic multilayer sea ice cover. Sea ice concentration maps during several summer minima are analyzed to obtain estimates of ice floes that survived summer, and the results are compared with multiyear-ice concentrations derived from these data by using an algorithm that assumes a certain emissivity for multiyear ice. The multiyear ice cover inferred from the winter data was found to be about 25 to 40 percent less than the summer ice-cover minimum, indicating that the multiyear ice cover in winter is inadequately represented by the passive microwave winter data and that a significant fraction of the Arctic multiyear ice floes exhibits a first-year ice signature.

Comiso, J. C.↗

NASA NTRS1991

Passive damping concepts for tubular beams with partial rotational and translational end restraints

The main objectives of the study are: (1) identification of potential passive damping concepts for slender tubular structural members with rotational and translational end springs under natural and forced-free vibrations; (2) evaluation of damping efficiencies of the various damping concepts; and (3) evaluation of the suitability of a theoretical finite difference analysis by comparison to the experimental results for the case of natural vibrations. Only member flexural an translation motion is considered. The natural vibration study is conducted on the seven damping concepts and for only one specific initial deflection. The most suitable of the seven dampers is further investigated under forced-free vibrations. In addition only one set of end springs is used for all of the experiments. The results show that passive damping provides a possible approach to structural vibration reduction.

Razzaq, Zia↗

NASA NTRS1991

The advanced microwave precipitation radiometer: A new aircraft radiometer for passive precipitation remote sensing

Past studies of passive microwave measurements of precipitating systems have yielded broad empirical relationships between hydrometeors and microwave transmission. In general, these relationships fall into two categories of passive microwave precipitation retrievals rely upon the observed effect of liquid precipitation to increase the brightness temperature of a radiometrically cold background such as an ocean surface. A scattering-based method is based upon the effect that frozen hydrometeors tend to decrease the brightness temperature of a radiometrically warm background such as land. One step toward developing quantitative brightness temperature-rain rate relationships is the recent construction of a new aircraft instrument sponsored by National Aeronautics and Space Administration/Marshall Space Flight Center (NASA/MSFC). This instrument is the Advanced Microwave Precipitation Radiometer (AMPR) designed and built by Georgia Tech Research Institute to fly aboard high altitude research aircraft such as the NASA ER-2. The AMPR and its accompanying data acquisition system are mounted in the Q-bay compartment of the NASA ER-2.

Hood, Robbie E.↗

NASA NTRS1991

Noise characteristics of passive components for phased array applications

The results of a comparative study on noise characteristics of basic power combining/dividing and phase shifting schemes are presented. The theoretical basics of thermal noise in a passive linear multiport are discussed. A new formalism is presented to describe the noise behavior of the passive circuits, and it is shown that the fundamental results are conveniently achieved using this description. The results of analyses concerning the noise behavior of basic power combining/dividing structures (the Wilkinson combiner, 90 deg hybrid coupler, hybrid ring coupler, and the Lange coupler) are presented. Three types of PIN-diode switch phase shifters are analyzed in terms of noise performance.

Sonmez, M. Kemal↗