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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Feasibility of using high temperature superconducting magnets and conventional magnetic loop antennas to attract or repel objects at the space station

A study was undertaken to see if magnetic forces can be used at the Space Station to attract or repel spacecrafts such as the Orbital Manuevering Vehicle (OMV) or the Orbiter. A large magnet, in the form of a current loop, is assumed to be placed at the Space Station and another one on the spacecraft. The expression for the force between the two dipoles (loops) is obtained. Using a force of 15 Newtons (3.4 pounds) in order to move the spacecraft, the number of ampere-turn needed in the current loops was calculated at various distances between them. The expression for the force of attraction between a current loop and a soft magnetic material was also examined and the number of amp-turn needed to provide a force of one-tenth of a pound at various distances is also calculated. This one tenth of a pound force would be used in a life line system for the retrieval of an adrift crewman or tool at the Space Station. The feasibility of using conventional antenna on the Station and the incoming vehicle for attraction or repulsion was also examined.

Randhawa, Manjit S.↗
NASA NTRS1989

Fragmentation of cosmic-string loops

The fragmentation of cosmic string loops is discussed, and the results of a simulation of this process are presented. The simulation can evolve any of a large class of loops essentially exactly, including allowing fragments that collide to join together. Such reconnection enhances the production of small fragments, but not drastically. With or without reconnections, the fragmentation process produces a collection of nonself-intersecting loops whose typical length is on the order of the persistence length of the initial loop.

York, Thomas↗
NASA NTRS1989

Fast half-loop maneuvers for a high alpha fighter aircraft using a singular perturbation feedback control law

Singular perturbation analysis is used to derive an outer layer feedback control law for a high alpha fighter aircraft to perform the half-loop maneuver. Pitch rate and angle of attack are treated as fast variables in the derivation. Bang-bang controls are derived to transfer the aircraft state from trim to the outer layer and from the outer layer to specified final half-loop values. The pitch rate is treated as a varibale faster than the angle of attack in the transfer of the state to and from the outer layer. A simulation of the derived control law is conducted at Mach 0.6 and 15,000 feet altitude. The half-loop was performed in 13.12 seconds. It is compared with a NASA pilot simulated half-loop maneuver which took 22.42 seconds for the same initial conditions.

Garrett, Frederick E., Jr.↗
NASA NTRS1989

Closed-form solution for loop transfer recovery via reduced-order observers

A well-known property of the reduced-order observer is exploited to obtain the controller solution of the loop transfer recovery problem. In that problem, the controller is sought that generates some desired loop shape at the plant's input or output channels. Past approaches to this problem have typically yielded controllers generating loop shapes that only converge pointwise to the desired loop shape. In the proposed approach, however, the solution (at the input) is obtained directly when the plant's first Markov parameter is full rank. In the more general case when the plant's first Markov parameter is not full rank, the solution is obtained in an analogous manner by appending a special set of input and output signals to the original set. A dual form of the reduced-order observer is shown to yield the LTR solution at the output channel.

Bacon, Barton J.↗
NASA NTRS1988

An adaptive human response mechanism controlling the V/STOL aircraft. Appendix 3: The adaptive control model of a pilot in V/STOL aircraft control loops

Importance of the role of human operator in control systems has led to the particular area of manual control theory. Human describing functions were developed to model human behavior for manual control studies to take advantage of the successful and safe human operations. A single variable approach is presented that can be extended for multi-variable tasks where a low order human response model is used together with its rules, to adapt the model on-line, being capable of responding to the changes in the controlled element dynamics. Basic control theory concepts are used to combine the model, constrained with the physical observations, particularly, for the case of aircraft control. Pilot experience is represented as the initial model parameters. An adaptive root-locus method is presented as the adaptation law of the model where the closed loop bandwidth of the system is to be preserved in a stable manner with the adjustments of the pilot handling qualities which relate the latter to the closed loop bandwidth and damping of the closed loop pilot aircraft combination. A Kalman filter parameter estimator is presented as the controlled element identifier of the adaptive model where any discrepancies of the open loop dynamics from the presented one, are sensed to be compensated.

Kucuk, Senol↗
NASA NTRS1989

Man-in-the-control-loop simulation of manipulators

A method to achieve man-in-the-control-loop simulation is presented. Emerging real-time dynamics simulation suggests a potential for creating an interactive design workstation with a human operator in the control loop. The recursive formulation for multibody dynamics simulation is studied to determine requirements for man-in-the-control-loop simulation. High speed computer graphics techniques provides realistic visual cues for the simulator. Backhoe and robot arm simulations are implemented to demonstrate the capability of man-in-the-control-loop simulation.

Chang, J. L.↗
NASA NTRS1989

Models of flaring loops

The somewhat questionable concept of an isolated flare loop and the various physical mechanisms believed to be responsible, to some degree, for energy transport within the loop structure is reviewed. Observational evidence suggests a predominant role for high-energy electrons as an energy transport mechanism, and the consequences of such a scenario is explored in some detail, focusing on radiation signatures in the soft X-ray, hard X-ray, and EUV wavebands, as observed by recent satellite observatories. It is found that the predictions of flare loop models are in fact in excellent agreement with these observations, reinforcing both the notion of the loop as a fundamental component of solar flares and the belief that electron acceleration is an integral part of the flare energy release process.

Emslie, A. Gordon↗
NASA NTRS1990

Digital accumulators in phase and frequency tracking loops

Results on the effects of digital accumulators in phase and frequency tracking loops are presented. Digital accumulators or summers are used extensively in digital signal processing to perform averaging or to reduce processing rates to acceptable levels. For tracking the Doppler of high-dynamic targets at low carrier-to-noise ratios, it is shown through simulation and experiment that digital accumulators can contribute an additional loss in operating threshold. This loss was not considered in any previous study and needs to be accounted for in performance prediction analysis. Simulation and measurement results are used to characterize the loss due to the digital summers for three different tracking loops: a digital phase-locked loop, a cross-product automatic frequency tracking loop, and an extended Kalman filter. The tracking algorithms are compared with respect to their frequency error performance and their ability to maintain lock during severe maneuvers at various carrier-to-noise ratios. It is shown that failure to account for the effect of accumulators can result in an inaccurate performance prediction, the extent of which depends highly on the algorithm used.

Hinedi, Sami↗
NASA NTRS1991

Analysis Of Lock Detection In Costas Loops

Report presents analysis of detection of phase lock in Costas loops, used in coherent binary-phase-shift-keying communication systems to track both subcarrier and suppressed carrier signals. Detection of phase lock important part of operation and monitoring of operation of Costas or other tracking loop, provides insight into behavior of loop in real time. Focuses on effects of phase jitter and correlation between samples of phase error in all-digital Costas loop, in which lock detection implemented via algorithm. Applicable to both sinusoidal and square-law carrier signals, incorporates new mathematical models of square-law and absolute-value detectors.

Mileant, Alexander↗
NASA NTRS1991

Phase-lock-loop application for fiber optic receiver

Phase-locked loop circuits are frequently employed in communication systems. In recent years, digital phase-locked loop circuits were utilized in optical communications systems. In an optical transceiver system, the digital phase-locked loop circuit is connected to the output of the receiver to extract a clock signal from the received coded data (NRZ, Bi-Phase, or Manchester). The clock signal is then used to reconstruct or recover the original data from the coded data. A theoretical approach to the design of a digital phase-locked loop circuit operation at 1 and 50 MHz is described. Hardware implementation of a breadboard design to function at 1 MHz and a printed-circuit board designed to function at 50 MHz were assembled using emitter coupled logic (ECL) to verify experimentally the theoretical design.

Ruggles, Stephen L.↗
NASA NTRS1990

Automatic blocking of nested loops

Blocked algorithms have much better properties of data locality and therefore can be much more efficient than ordinary algorithms when a memory hierarchy is involved. On the other hand, they are very difficult to write and to tune for particular machines. The reorganization is considered of nested loops through the use of known program transformations in order to create blocked algorithms automatically. The program transformations used are strip mining, loop interchange, and a variant of loop skewing in which invertible linear transformations (with integer coordinates) of the loop indices are allowed. Some problems are solved concerning the optimal application of these transformations. It is shown, in a very general setting, how to choose a nearly optimal set of transformed indices. It is then shown, in one particular but rather frequently occurring situation, how to choose an optimal set of block sizes.

Schreiber, Robert↗
NASA NTRS1990

HEAO 1 A-2 low-energy detector X-ray spectra of the Cygnus Loop

The Cygnus Loop supernova remnant was observed by the A-2 low-energy detector (LED) proportional counters on the HEAO 1 satellite. Recent improvements to the non-X-ray background rejection and detector response simulation have allowed production of the most accurate spectra of the Cygnus Loop to date. Three separate regions of the Cygnus Loop were observed. Single-temperature, Raymond-Smith models are inadequate to describe the spectra, but two component model fits are good. Temperature, column density, and emission measure variations across the Cygnus Loop are found. These results are interpreted and compared with previous work.

Leahy, D. A.↗
NASA NTRS1990

An estimator-predictor approach to PLL loop filter design

The design of digital phase locked loops (DPLL) using estimation theory concepts in the selection of a loop filter is presented. The key concept, that the DPLL closed-loop transfer function is decomposed into an estimator and a predictor, is discussed. The estimator provides recursive estimates of phase, frequency, and higher-order derivatives, and the predictor compensates for the transport lag inherent in the loop.

Statman, Joseph I.↗
NASA NTRS1991

Blue loops during core helium burning as the consequence of moderate convective envelope overshooting in stars of intermediate to high mass

New calculations of stellar evolutionary sequences without semiconvection in any phase have unexpectedly turned up, for stars in the mass range 3-30 solar masses, an irregular pattern of blue loops on the H-R diagram during the core helium-burning phase. Blue loops occur for 3 and 10 solar masses, but not for 5, 7, 15, and 30 solar masses. It is found, however, that the models near the bottom of the red supergiant branch are only marginallly stable against small inward displacements of the former base of the outer convection zone when it was deepest, for the stellar mass range 5-15 solar masses. As a consequence, convective envelope overshooting need only penetrate a distance of about 0.3 of a local pressure scale height to promote a delayed blue loop in these particular sequences. In conformity with previous work, the triggering mechanism for the blue loop appears to be proximity of the hydrogen-burning shell to the hydrogen discontinuity at the former base of the outer convection zone.

Stothers, Richard B.↗
NASA NTRS1991

Dynamics of closed-loop systems containing flexible bodies

An important characteristic of flexible multibody systems containing closed-loop topologies is that the component modes used to describe individual bodies will no longer be independent because of loop closure constraints. Thus, the issue of component modal selection becomes even more complicated. In addition, the foreshortening effect that has been studied extensively in the literature in the context of open-loop topologies will also be present in these constraint equations. Simulation results presented demonstrate the effects of modal selection and foreshortening on the dynamic response of closed-loop flexible systems.

Tadikonda, Sivakumar S. K.↗
NASA NTRS1992

Behavior Of Costas Loop In Reception Of Telemetry

Report presents study of behavior of Costas loop in reception of binary non-return-to-zero telemetry signal phase-shift-modulated onto sinusoidal or square-wave subcarrier signal in turn phase-modulated onto carrier signal. Equations describe degradation of performance of Costas loop in terms of phase jitter, conditions causing loss of lock, and other effects. Costas loop used to track telemetry signal with residual carrier in presence of Doppler shift. Performance optimized by suitable choice of bit rate, waveform and frequency of subcarrier, and noise bandwidth of filters in arms of Costas loop.

Nguyen, Tien Manh↗
NASA NTRS1992

The behavior of a Costas loop in the presence of space telemetry signals

When the Costas loop is operated in the presence of a residual carrier space telemetry signal, the loop reconstructs its carrier reference from an input signal whose carrier component is not completely suppressed. The telemetry signal investigated in this work is generated by phase shift-keying the data onto a subcarrier and then phase modulating onto the sinusoidal carrier. The telemetry modulation index, telemetry bit rate, subcarrier waveform, and subcarrier frequency are shown to be the key system parameters that contribute to the performance degradation of a Costas loop. Furthermore, the effect of Doppler shift on the loop is also investigated in this work.

Nguyen, Tien M.↗
NASA NTRS1992

Removal of surface loop from stitched composites can improve compression and compression-after-impact strengths

Stitching through-the-thickness (TTT) of composite materials produces a surface loop of yarn between successive penetrations. The surface loop is pressed into the surface layers of the composite material during the curing of the laminate, kinking the in-plane fibers near the surface of the material. The compression strength and compression-after-impact (CAI) strengths of carbon-epoxy specimens were measured with and without the surface loop. Removal of the surface loop had no influence on failure mode or failure mechanism, but did significantly increase the compression and CAI strengths.

Farley, Gary L.↗