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27 records · Page 2

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NASA NTRS2000

Modulated Fourier Transform Raman Fiber-Optic Spectroscopy

A modification to a commercial Fourier Transform (FT) Raman spectrometer is presented for the elimination of thermal backgrounds in the FT Raman spectra. The modification involves the use of a mechanical optical chopper to modulate the continuous wave laser, remote collection of the signal via fiber optics, and connection of a dual-phase digital-signal-processor (DSP) lock-in amplifier between the detector and the spectrometer's collection electronics to demodulate and filter the optical signals. The resulting Modulated Fourier Transform Raman Fiber-Optic Spectrometer is capable of completely eliminating thermal backgrounds at temperatures exceeding 300 C.

Jensen, Brian J.↗

NASA NTRS1981

The seasonal variations of ozone and temperature in the middle and the upper stratosphere

The seasonal variations in ozone and temperature inferred from the Nimbus-4 BUV (Backscatter Ultraviolet Spectrometer) and the SCR (Selective Chopper Radiometer) experiments are compared with predictions based on a simplified radiative photochemical model. It is shown that the observations, in agreement with the model calculations, show a systematic phase shift from a summer maximum at 10 mb to a winter maximum at 1 mb with equinoctial maxima at intermediate heights. In contrast, the temperature between these pressure levels shows no appreciable phase shift and the summer maximum prevails at all heights. The observed phase differences in ozone and temperature are shown to be a manifestation of the radiative feedback on the photochemistry of the upper stratosphere.

Chandra, S.↗

NASA NTRS1979

Near-term hybrid vehicle program, phase 1. Appendix C: Preliminary design data package

The design methodology, the design decision rationale, the vehicle preliminary design summary, and the advanced technology developments are presented. The detailed vehicle design, the vehicle ride and handling and front structural crashworthiness analysis, the microcomputer control of the propulsion system, the design study of the battery switching circuit, the field chopper, and the battery charger, and the recent program refinements and computer results are presented.

Source record↗

NASA NTRS1977

Electric vehicle power train instrumentation: Some constraints and considerations

The application of pulse modulation control (choppers) to dc motors creates unique instrumentation problems. In particular, the high harmonic components contained in the current waveforms require frequency response accommodations not normally considered in dc instrumentation. In addition to current sensing, accurate power measurement requires not only adequate frequency response but must also address phase errors caused by the finite bandwidths and component characteristics involved. The implications of these problems are assessed.

Triner, J. E.↗

NASA NTRS1977

Electric vehicle power train instrumentation - Some constraints and considerations

The application of pulse modulation control (choppers) to dc motors creates unique instrumentation problems. In particular, the high-harmonic components contained in the current waveforms require frequency-response accommodations not normally considered in dc instrumentation. In addition to current sensing, accurate power measurement not only requires adequate frequency response but also must address phase errors caused by the finite bandwidths and component characteristics involved. This paper discusses the implications of these problems and reports on the degree to which they have been solved at Lewis Research Center.

Triner, J. E.↗

NASA NTRS1991

System and component design and test of a 10 hp, 18,000 rpm AC dynamometer utilizing a high frequency AC voltage link, part 1

Hard and soft switching test results conducted with one of the samples of first generation MOS-controlled thyristor (MCTs) and similar test results with several different samples of second generation MCT's are reported. A simple chopper circuit is used to investigate the basic switching characteristics of MCT under hard switching and various types of resonant circuits are used to determine soft switching characteristics of MCT under both zero voltage and zero current switching. Next, operation principles of a pulse density modulated converter (PDMC) for three phase (3F) to 3F two-step power conversion via parallel resonant high frequency (HF) AC link are reviewed. The details for the selection of power switches and other power components required for the construction of the power circuit for the second generation 3F to 3F converter system are discussed. The problems encountered in the first generation system are considered. Design and performance of the first generation 3F to 3F power converter system and field oriented induction moter drive based upon a 3 kVA, 20 kHz parallel resonant HF AC link are described. Low harmonic current at the input and output, unity power factor operation of input, and bidirectional flow capability of the system are shown via both computer and experimental results. The work completed on the construction and testing of the second generation converter and field oriented induction motor drive based upon specifications for a 10 hp squirrel cage dynamometer and a 20 kHz parallel resonant HF AC link is discussed. The induction machine is designed to deliver 10 hp or 7.46 kW when operated as an AC-dynamo with power fed back to the source through the converter. Results presented reveal that the proposed power level requires additional energy storage elements to overcome difficulties with a peak link voltage variation problem that limits reaching to the desired power level. The power level test of the second generation converter after the addition of extra energy storage elements to the HF link are described. The importance of the source voltage level to achieve a better current regulation for the source side PDMC is also briefly discussed. The power levels achieved in the motoring mode of operation show that the proposed power levels achieved in the generating mode of operation can also be easily achieved provided that no mechanical speed limitation were present to drive the induction machine at the proposed power level.

Lipo, Thomas A.↗

NASA NTRS1976

Stratospheric long waves - Comparison of thermal structure in the Northern and Southern Hemispheres

Stratospheric radiance and temperature data obtained in the Northern and Southern Hemispheres during 1971 and 1972 are analyzed in order to compare planetary-wave behavior in both hemispheres. The data sources include the reduced radiance data of the Nimbus 4 selective chopper radiometer, National Meteorological Center gridded data, and Eole constant-level balloon data. Large-amplitude wave oscillations with energy in the period range from 20 to 30 days are observed along with the eastward propagation of planetary wave 2 in the Southern Hemisphere and a strong vertical coherence in wave phase at all levels between 200 mb and the upper stratosphere. The results show that: (1) despite stationary forcing, eastward phase progression occurs for wave 2 in the south as well as during the amplification phase of wave 1 in both hemispheres; (2) the upper stratosphere in midwinter contains regions of apparent barotropic instability, especially in the south; (3) wave number 2 amplitudes grow from very low values in early winter to high values in late winter; (4) the meridional amplitude structure is similar in the upper stratosphere in both hemispheres; and (5) the equatorward extent of waves is directly related to zonal wind speed in the upper stratosphere.

Leovy, C. B.↗

NASA NTRS1975

Atmospheric variations - Vacillations and index cycles

Zonal indices, momentum flux, and mean and perturbation kinetic energies of the Southern Hemisphere upper troposphere, derived from the EOLE constant density balloon data set, are found to possess strong 18-23 day variations which are statistically significant to a posteriori levels. Cross-spectra and phase relationships suggest that the variation appears as a barotropic interaction between the middle latitude mean westerlies and ultra large-scale perturbations. Long-period and high-amplitude variations in energy conversion between eddy and mean kinetic energies are found which average out in the long term mean to be very small and positive, consistent with previously measured average hemispheric energetics. Furthermore, the variation is shown to possess an ultra-long wave preference. Examination of two independent data sources, time series of total ozone and stratospheric thickness data from the Selective Chopper Radiometer, suggests that the periodicity is a property of general hemispheric motion rather than just a property of the EOLE data set.

Webster, P. J.↗

NASA NTRS1992

Separation of cirrus cloud from clear surface from AVIRIS data using the 1.38 micron water vapor band

Cirrus clouds play an important role in climate systems because of their large area coverage, persistence, and radiative effects. Thin cirrus clouds are difficult to detect in visible images and infrared images in the 10-12 micron atmospheric window region, particularly over land, because these clouds are partially transparent. Ackerman recently developed a method for detecting cirrus clouds using three narrow channels centered near 8, 11, and 12 microns, respectively, based on the analysis of IR emission spectra measured with a high spectral resolution interferometer. Barton also described a method for estimating cirrus cloud height and amount from measurements with two narrow channel radiometers of the Selective Chopper Radiometer on Nimbus 5. Both channels are located within the strong 2.7 micron water vapor band absorption region. One of the channels includes additional carbon dioxide absorption. A differential absorption technique with sets of empirical coefficients was used in the estimation of cirrus cloud heights and amounts. A technique using narrow channels in the strong 1.38 micron water vapor band absorption region for detecting cirrus clouds from spectral imaging data acquired by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) on 5 Dec. 1991 during the FIRE (The First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment) Phase 2 Field Experiment is described.

Gao, Bo-Cai↗