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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Characteristics of GaAs with inverted thermal conversion

GaAs crystals exhibiting inverted thermal conversion (ITC) of resistivity were investigated in conjunction with standard semiinsulating (SI) GaAs regarding characteristics important in device processing. It was established that dislocation density and Si implant activation are unaffected by transformation to the ITC state. However, in ITC GaAs the controlled increase of the EL2 (native midgap donor) concentration during annealing makes it possible to attain resistivities one order of magnitude greater (e.g., about 10 to the 9th ohm cm of 300 K) than those attained in standard SI GaAs (e.g., 10 to the 7th-10 to the 8th ohm cm).

Kang, C. H.↗
NASA NTRS1988

A diagonally inverted LU implicit multigrid scheme for the 3-D Navier-Stokes equations and a two equation model of turbulence

An LU implicit multigrid algorithm is developed to calculate 3-D compressible viscous flows. This scheme solves the full 3-D Reynolds-Averaged Navier-Stokes equation with a two-equation kappa-epsilon model of turbulence. The flow equations are integrated by an efficient, diagonally inverted, LU implicit multigrid scheme while the kappa-epsilon equations are solved, uncoupled from the flow equations, by a block LU implicit algorithm. The flow equations are solved within the framework of the multigrid method using a four-grid level W-cycle, while the kappa-epsilon equations are iterated only on the finest grid. This treatment of the Reynolds-Averaged Navier-Stokes equations proves to be an efficient method for calculating 3-D compressible viscous flows.

Yokota, Jeffrey W.↗
NASA NTRS1989

A diagonally inverted LU implicit multigrid scheme for the 3-D Navier-Stokes equations and a two equation model of turbulence

An LU implicit multigrid algorithm is developed to calculate 3-D compressible viscous flows. This scheme solves the full 3-D Reynolds-Averaged Navier-Stokes equation with a two-equation kappa-epsilon model of turbulence. The flow equations are integrated by an efficient, diagonally inverted, LU implicit multigrid scheme while the kappa-epsilon equations are solved, uncoupled from the flow equations, by a block LU implicit algorithm. The flow equations are solved within the framework of the multigrid method using a four-grid level W-cycle, while the kappa-epsilon equations are iterated only on the finest grid. This treatment of the Reynolds-Averaged Navier-Stokes equations proves to be an efficient method for calculating 3-D compressible viscous flows.

Yokota, Jeffrey W.↗
NASA NTRS1990

Electrical characterization of a Mapham inverter using pulse testing techniques

Electric power requirements for aerospace missions have reached megawatt power levels. Within the next few decades, it is anticipated that a manned lunar base, interplanetary travel, and surface exploration of the Martian surface will become reality. Several research and development projects aimed at demonstrating megawatt power level converters for space applications are currently underway at the NASA Lewis Research Center. Innovative testing techniques will be required to evaluate the components and converters, when developed, at their rated power in the absence of costly power sources, loads, and cooling systems. Facilities capable of testing these components and systems at full power are available, but their use may be cost prohibitive. The use of a multiple pulse testing technique is proposed to determine the electrical characteristics of large megawatt level power systems. Characterization of a Mapham inverter is made using the proposed technique and conclusions are drawn concerning its suitability as an experimental tool to evaluate megawatt level power systems.

Baumann, E. D.↗
NASA NTRS1989

Lyapunov optimal feedback control of a nonlinear inverted pendulum

Liapunov optimal feedback control is applied to a nonlinear inverted pendulum in which the control torque was constrained to be less than the nonlinear gravity torque in the model. This necessitates a control algorithm which 'rocks' the pendulum out of its potential wells, in order to stabilize it at a unique vertical position. Simulation results indicate that a preliminary Liapunov feedback controller can successfully overcome the nonlinearity and bring almost all trajectories to the target.

Grantham, W. J.↗
NASA NTRS1989

Radiation dependence of inverter propagation delay from timing sampler measurements

A timing sampler consisting of 14 four-stage inverter-pair chains with different load capacitances was fabricated in 1.6-micron n-well CMOS and irradiated with cobalt-60 at 10 rad(Si)/s. For this CMOS process the measured results indicate that the rising delay increases by about 2.2 ns/Mrad(Si) and the falling delay increase is very small, i.e., less than 300 ps/Mrad(Si). The amount of radiation-induced delay depends on the size of the load capacitance. The maximum value observed for this effect was 5.65 ns/pF-Mrad(Si). Using a sensitivity analysis, the sensitivity of the rising delay to radiation can be explained by a simple timing model and the radiation sensitivity of dc MOSFET parameters. This same approach could not explain the insensitivity of the falling delay to radiation. This may be due to a failure of the timing model and/or trapping effects.

Buehler, M. G.↗
NASA NTRS1989

20 kHz main inverter unit

A proof-of-concept main inverter unit has demonstrated the operation of a pulse-width-modulated parallel resonant power stage topology as a 20-kHz ac power source driver, showing simple output regulation, parallel operation, power sharing and short-circuit operation. The use of a two-stage dc input filter controls the electromagnetic compatibility (EMC) characteristics of the dc power bus, and the use of an ac harmonic trap controls the EMC characteristics of the 20-kHz ac power bus.

Hussey, S.↗
NASA NTRS1991

HOLLOTRON switch for megawatt lightweight space inverters

The feasibility of satisfying the switching requirements for a megawatt ultralight inverter system using HOLLOTRON switch technology was determined. The existing experimental switch hardware was modified to investigate a coaxial HOLLOTRON switch configuration and the results were compared with those obtained for a modified linear HOLLOTRON configuration. It was concluded that scaling the HOLLOTRON switch to the current and voltage specifications required for a megawatt converter system is indeed feasible using a modified linear configuration. The experimental HOLLOTRON switch operated at parameters comparable to the scaled coaxial HOLLOTRON. However, the linear HOLLOTRON data verified the capability for meeting all the design objectives simultaneously including current density (greater than 2 A/sq cm), voltage (5 kV), switching frequency (20 kHz), switching time (300 ns), and forward voltage drop (less than or equal to 20 V). Scaling relations were determined and a preliminary design was completed for an engineering model linear HOLLOTRON switch to meet the megawatt converter system specifications.

Poeschel, R. L.↗
NASA NTRS1990

Electrical characterization of a Mapham inverter using pulse testing techniques

The use of a multiple pulse testing technique to determine the electrical characteristics of large megawatt-level power systems for aerospace missions is proposed. An innovative test method based on the multiple pulse technique is demonstrated on a 2-kW Mapham inverter. The concept of this technique shows that characterization of large power systems under electrical equilibrium at rated power can be accomplished without large costly power supplies. The heat generation that occurs in systems when tested in a continuous mode is eliminated. The results indicate that there is a good agreement between this testing technique and that of steady state testing.

Baumann, E. D.↗
NASA NTRS1992

Regenerative Snubber For GTO-Commutated SCR Inverter

Proposed regenerative snubbing circuit substituted for dissipative snubbing circuit in inverter based on silicon controlled rectifiers (SCR's) commutated by gate-turn-off thyristor (GTO). Intended to reduce loss of power that occurs in dissipative snubber. Principal criteria in design: low cost, simplicity, and reliability.

Rippel, Wally E.↗
NASA NTRS1991

The Eckhaus and the Benjamin-Feir instability near a weakly inverted bifurcation

We investigate how the criteria for two prototype instabilities in one dimensional pattern forming systems, namely for the Eckhaus instability and for the Benjamin-Feir instability, change as one goes from a continuous bifurcation, to a spatially periodic or spatially and/or time periodic state, to the corresponding weakly inverted, i.e., hysteretic, cases. We also give the generalization to two dimensional patterns in systems with anisotropy as they arise from hydrodynamic instabilities in nematic liquid crystals.

Brand, Helmut R.↗
NASA NTRS1993

Inverted Vs spanning the cusp boundary layer

We present several examples of dayside inverted Vs in which ion convection and energy dispersion appear to span the region of both open and closed field lines. All of these examples occur in the region generally called the low-latitude boundary layer or the cusp boundary layer and suggest a connection between closed field lines of the dayside auroral region and open field lines of the dayside cusp. One explanation is that plasma in the cusp boundary layer on closed field lines convects across field lines in a smooth fashion into the cusp. Such cross-field-line convection may be possible in regions of finite conductivity and strong currents, where the field can diffuse through the plasma. Another explanation for the observations is that they represent the mid-altitude manifestation of merging at the dayside magnetopause and the opening of previously closed field lines.

Menietti, J. D.↗
NASA NTRS1994

Two-Stage Series-Resonant Inverter

Two-stage inverter includes variable-frequency, voltage-regulating first stage and fixed-frequency second stage. Lightweight circuit provides regulated power and is invulnerable to output short circuits. Does not require large capacitor across ac bus, like parallel resonant designs. Particularly suitable for use in ac-power-distribution system of aircraft.

Stuart, Thomas A.↗
NASA NTRS1993

Design of a ZVS PWM inverter for a brushless DC motor in an EMA application

The Component Development Division of the Propulsion Laboratory at Marshall Space Flight Center (MSFC) is currently investigating the use of electromechanical actuators for use in space transportation applications such as Thrust Vector Control (TVC). These high power servomechanisms will require rugged, reliable, and compact power electronic modules capable of modulating several hundred amperes of current at up to 270 Vdc. This paper will discuss the design and implementation of a zero-voltage-switched PWM (Pulse Width Modulation) inverter which operates from a 270 Vdc source at currents up to 100 A.

Bell, J. Brett↗
NASA NTRS1997

Inverting the Pendulum Using Fuzzy Control (Center Director's Discretionary Fund (Project 93-02)

A single pendulum was simulated in software and then built on a rotary base. A fuzzy controller was used to show its advantages as a nonlinear controller since bringing the pendulum inverted is extremely nonlinear. The controller was implemented in a Motorola 6811 microcontroller. A double pendulum was simulated and fuzzy control was used to hold it in a vertical position. The double pendulum was not built into hardware for lack of time. This project was for training and to show advantages of fuzzy control.

Kissel, R. R.↗
NASA NTRS1994

Inverter Matrix for the Clementine Mission

An inverter matrix test circuit was designed for the Clementine space mission and is built into the RRELAX (Radiation and Reliability Assurance Experiment). The objective is to develop a circuit that will allow the evaluation of the CMOS FETs using a lean data set in the noisy spacecraft environment.

CMOS FETs spacecraft environment RRELAX orbit moon↗