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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Analog current mode analog/digital converter

An improved subranging or comparator circuit is provided for an analog-to-digital converter. As a subranging circuit, the circuit produces a residual signal representing the difference between an analog input signal and an analog of a digital representation. This is achieved by subdividing the digital representation into two or more parts and subtracting from the analog input signal analogs of each of the individual digital portions. In another aspect of the present invention, the subranging circuit comprises two sets of differential input pairs in which the transconductance of one differential input pair is scaled relative to the transconductance of the other differential input pair. As a consequence, the same resistor string may be used for two different digital-to-analog converters of the subranging circuit.

Hadidi, Khayrollah↗
DOE OSTI2022

Model predictive control for matrix converter operating in current control mode with load current estimation

A matrix converter system operating in current control mode is provided. The matrix converter includes a switching matrix coupled between a low voltage side and a high voltage side, wherein the matrix converter is coupled at its low voltage side to a generator for receiving an input power including an input current and transforming the input power into an output power at its high voltage side, wherein a load is coupled to the high voltage side. The control system includes a load observer and a model predictive controller (MPC). The load observer is configured to estimate a load current that flows to the load from the high voltage side of the matrix converter as a function of a switching state of the switching matrix, an output voltage output at the high voltage side of the matrix converter, and the input current. The MPC is configured to select the switching state of the switching matrix to meet one or more control objectives defined by minimization of a multi-objective function that tracks the output voltage and the input current using the estimated load current.

24 POWER TRANSMISSION AND DISTRIBUTION↗
NASA NTRS1989

Part B: Pattern control of horn antennas

During this period, the computations of the impedance elements were completed. These include interactions between the two electric current modes, the elecric current mode and the magnetic current mode, and the two magnetic current modes. An accurate and efficient formulation of computing interactions between electric current mode and magnetic current mode was accomplished. This, together with other subroutines allows for the fill-in of all the elements in the matrix. After the fill-in of the impedance elements in the matrix, the forward problem is accomplished. That is, given the specification of the horn and the excitating waveguide mode, the radiation pattern of the antenna based on the integral equation can be obtained. An example case was run for a standard X-band gain-horn (DBG-520). The H- and E-plane patterns of this horn antenna with perfectly conducting walls are compared with the gain pattern available from the manufacturer for up to the first side lobe. Good agreements are obtained although the cross polarization has not yet been accounted for. The effect of the lossy coating on the radiation pattern was also investigated. The resulting E-plane pattern shows about 3-dB improvement in the first sidelobe and 4-dB improvement in the second sidelobe.

Balanis, Constantine A.↗
DOE OSTI2023

Method of transient control for robust enrichment operation in low temperature combustion engine

A method of transient control for enrichment operation in a low-temperature combustion engine. The method includes determining if a current mode of the low-temperature combustion (LTC) engine is a positive valve overlap (PVO) mode. Determining if a previous mode of the LTC engine was also the PVO mode when the current mode is the PVO mode, wherein the previous mode is immediately prior to the current mode. Determining if the previous mode of the LTC engine was a negative valve overlap (NVO) mode when the previous mode was not the PVO mode. Initiating a predetermined enrichment PVO mode for the LTC engine based on the previous mode of the LTC engine. The predetermined enrichment PVO mode includes initiating a deep enrichment PVO mode, when the previous mode of the LTC engine was the NVO mode, and initiating a shallow enrichment PVO mode, when the previous mode of the LTC engine was not the NVO mode.

Yun, Hanho↗
DOE OSTI2020

Matrix converter operating in current control mode using feed forward signals

A matrix converter system having a current control mode operation is provided. The system includes a matrix converter having a switching matrix. The matrix converter is coupled at its low-voltage side to a generator and at its output load side to a load. A controller having a pulse width modulation (PWM) control circuit is configured to control the matrix converter via its switching matrix to increase energy within the internal inductances of the generator when the switching matrix causes a short circuit. A feed forward calculator is configured to calculate a feed forward output phase angle. The feed forward output phase angle is an estimation of an angle between an output current vector and an output voltage vector that represent feedback signals of current and voltage output by the matrix converter. The angular position of the voltage output vector is adjusted as a function of the feed forward output phase angle to align angular position or phase angle of the voltage output vector that represents the voltage output with a selected angular position or phase angle.

42 ENGINEERING↗
NASA NTRS2010

Qualification and Selection of Flight Diode Lasers for Space Applications

The reliability and lifetime of laser diodes is critical to space missions. The Nuclear Spectroscopic Telescope Array (NuSTAR) mission includes a metrology system that is based upon laser diodes. An operational test facility has been developed to qualify and select, by mission standards, laser diodes that will survive the intended space environment and mission lifetime. The facility is situated in an electrostatic discharge (ESD) certified clean-room and consist of an enclosed temperature-controlled stage that can accommodate up to 20 laser diodes. The facility is designed to characterize a single laser diode, in addition to conducting laser lifetime testing on up to 20 laser diodes simultaneously. A standard laser current driver is used to drive a single laser diode. Laser diode current, voltage, power, and wavelength are measured for each laser diode, and a method of selecting the most adequate laser diodes for space deployment is implemented. The method consists of creating histograms of laser threshold currents, powers at a designated current, and wavelengths at designated power. From these histograms, the laser diodes that illustrate a performance that is outside the normal are rejected and the remaining lasers are considered spaceborne candidates. To perform laser lifetime testing, the facility is equipped with 20 custom laser drivers that were designed and built by California Institute of Technology specifically to drive NuSTAR metrology lasers. The laser drivers can be operated in constant-current mode or alternating-current mode. Situated inside the enclosure, in front of the laser diodes, are 20 power-meter heads to record laser power throughout the duration of lifetime testing. Prior to connecting a laser diode to the current source for characterization and lifetime testing, a background program is initiated to collect current, voltage, and resistance. This backstage data collection enables the operational test facility to have full laser diode traceablity.

Liebe, Carl C.↗
DOE OSTI2020

Matrix converter operation in current control mode with regulation of output voltage vector

A matrix converter system having a current control mode operation is provided. The matrix converter system includes a matrix converter that includes a plurality of switches. A unity current reference vector having an amplitude of one is determined that defines an angle and frequency of an output current vector as a function of a reference phase angle, which is a function of the output current vector and an output voltage vector representing feedback signals of a multiphase signal output to a load. The unity current reference vector is used to control the matrix converter, to cause the output current vector to be aligned with the unity current reference vector. A unity voltage reference vector having an amplitude of one is provided that defines a reference angle and frequency as a function of a zero phase angle. The output voltage vector and the unity voltage reference vector are aligned to determine the reference phase angle as an angle between the aligned output voltage vector and the output current vector.

42 ENGINEERING↗
DOE OSTI2023

In-operando synchrotron experiments of flash sintering carried out in current rate mode

Here, we report first-time results for in-operando flash sintering synchrotron experiments carried out in current rate mode where the specimen, held at a constant temperature, is fed current that is increased at a constant rate. These experiments are unique because the time dependence of the sintering behavior can be stretched out over a longer period (by changing the current rate) than in voltage-to-current experiments in which sintering occurs in a burst at the onset of the flash. Two results are presented: (i) A comparison of temperatures measured with the platinum standard to those predicted by the black body radiation model leading to estimates of the emissivity as a function of porosity whereby emissivity increases from 0.65 to 0.9 as the sample sinters from its green state to full density, and (ii) measurements of the excess lattice expansion as a function of density as the sample sinters continuously while the current is increased. The present work highlights the promise of current rate experiments to obtain results while the sample sinters gradually from its green density to full density (somewhat akin to conventional sintering) for gaining further insights into the mechanisms of flash sintering.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Criteria for Grid-Forming Inverters Transitioning Between Current Limiting Mode and Normal Operation

Here, this letter focuses on the fault recovery analysis of inverters using multi-loop droop based grid-forming control, taking into account different current limiting strategies. A criterion is proposed to determine when the inverter can exit the current limiting mode after fault clearance. The theoretical findings are verified through electromagnetic transient simulations using the direct (d)-axis priority-based current limiting method, circular current limiting method, and virtual impedance method.

24 POWER TRANSMISSION AND DISTRIBUTION↗
NASA NTRS1987

Bow-tie antennas on a dielectric half-space - Theory and experiment

A new formulation is discussed for the rigous calculation of the radiation pattern of a bow-tie antenna of finite length and infinitesimal thickness, placed on a lossless dielectric substrate. The analysis is based on a representation of the current density on the metal surface of the antenna as a sum of an imposed (quasistatic) term and a set of current modes with unknown amplitudes. Free-space fields that are expressed in terms of continuous spectra of symmetrized plane waves are matched to the current modes using the method of moments. The resulting set of equations are solved for the unknown current amplitudes. The calculations show that for increasing bow length the antenna impedance spirals rapidly to a value predicted by transmission line theory. The theory also shows that the E-plane pattern of a two-wavelength, 60-deg bow-tie antenna is dominated by low-loss current modes propagating at the dielectric wavenumber. As the bow tie narrows, the loss of the modes increases, and the dominant wavenumber tends to the quasi-static value. Pattern measurements made at 94 GHz are shown to agree well with theoretical predictions. Measurements for a long-wire antenna, a linear array of bow-tie elements, and a log-periodic antenna are also presented.

Compton, Richard C.↗
DOE OSTI2020

Experimental Insights into the Coupling of Methane Combustion and Steam Reforming in a Catalytic Plate Reactor in Transient Mode

The microstructured reactor concept is very promising technology to develop a compact reformer for distributed hydrogen generation. In this work, a catalytic plate reactor (CPR) is developed and investigated for the coupling of methane combustion (MC) and methane steam reforming (MSR) over Pt/Al 2 O 3 -coated microchannels in cocurrent and counter-current modes in transient experiments during start-up. A three-dimensional (3D) computational fluid dynamics (CFD) simulation shows uniform velocity and pressure distribution profiles in microchannels. For a channel velocity from 5.1 to 57.3 m/s in the combustor, the oxidation of methane is complete and self-sustainable without explosion, blow-off, or extinction; nevertheless, flashbacks are observed in counter-current mode. In the reformer, the maximum methane conversion is 84.9% in cocurrent mode, slightly higher than that of 80.2% in counter-current mode at a residence time of 33 ms, but at the cost of three times higher energy input in the combustor operating at ~1000 °C. Nitric oxide (NO) is not identified in combustion products, but nitrous oxide (N 2 O) is a function of coupling mode and forms significantly in cocurrent mode. Furthermore, this research would be helpful to establish the start-up strategy and environmental impact of compact reformers on a small scale.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2002

Focal plane infrared readout circuit with automatic background suppression

A circuit for reading out a signal from an infrared detector includes a current-mode background-signal subtracting circuit having a current memory which can be enabled to sample and store a dark level signal from the infrared detector during a calibration phase. The signal stored by the current memory is subtracted from a signal received from the infrared detector during an imaging phase. The circuit also includes a buffered direct injection input circuit and a differential voltage readout section. By performing most of the background signal estimation and subtraction in a current mode, a low gain can be provided by the buffered direct injection input circuit to keep the gain of the background signal relatively small, while a higher gain is provided by the differential voltage readout circuit. An array of such readout circuits can be used in an imager having an array of infrared detectors. The readout circuits can provide a high effective handling capacity.

Pain, Bedabrata↗
NASA NTRS1995

Modes in Supervisory Control Systems: Structure and Transitions

Mode confusion is becoming a major drawback in operator interaction with systems that allow for multiple levels of automation. This drawback has manifested itself in several mode related accidents and incidents in commercial aviation, military control systems, as well as high technology medical systems. In the domain of commercial aviation, there have been four recent airline accidents, all involving highly automated aircraft, in which mode related problems were present. Mode problems, we argue, stem from three principal factors: (1) mis-identification of the current mode, (2) difficulty in apprehending current mode behavior; and (3) difficulty in predicting the consequences of the next mode transition. This combination of factors may lead to mode confusion and possibly unwanted results. We define in this paper a mode as the system's manner of behavior. Any given system or machine, may have several ways of behaving. A controller, either human or machine, provides the input that triggers the transition from one mode of behavior to another. At any point in time, a machine may be in one mode or another modes are mutually exclusive. Complex systems are typically comprised of several subsystems, or components, each one with its own set of modes. Therefore, the status of the system at any point in time can be described as a vector of all the active modes.

Degani, Asaf↗