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Results for “timing stability”

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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Numerical spatial marching techniques for estimating duct attenuation and source pressure profiles

A numerical method is developed that could predict the pressure distribution of a ducted source from far-field pressure inputs. Using an initial value formulation, the two-dimensional homogeneous Helmholtz wave equation (no steady flow) is solved using explicit marching techniques. The Von Neumann method is used to develop relationships which describe how sound frequency and grid spacing effect numerical stability. At the present time, stability considerations limit the approach to high frequency sound. Sample calculations for both hard and soft wall ducts compare favorably to known boundary value solutions. In addition, assuming that reflections in the duct are small, this initial value approach is successfully used to determine the attenuation of a straight soft wall duct. Compared to conventional finite difference or finite element boundary value approaches, the numerical marching technique is orders of magnitude shorter in computation time and required computer storage and can be easily employed in problems involving high frequency sound.

Baumeister, K. J.↗
NASA NTRS1984

Two time scale stabilization of systems with output feedback

The problem of constant gain output feedback regulator design for linear systems with ill-conditioned dynamics is considered in the context of singular perturbation theory. A design approach is developed in which gains can be separately calculated to stabilize reduced-order slow and fast subsystem models. By employing the notion of combined control and observation spillover suppression, conditions are derived assuring that these gains will stabilize the full-order system, assuming sufficient frequency separation between the slow and fast subsystems. An LQ design procedure is described in which the spillover suppression conditions are satisfied by adjoining penalty functions to the subsystem performance indices. The theory is demonstrated in a controller design for a flexible space structure.

Moerder, D. D.↗
DOE OSTI2024

How Much Reserve Fuel: Quantifying the Maximal Energy Cost of System Disturbances

Motivated by the design question of additional fuel needed to complete a task in an uncertain environment, this paper introduces metrics to quantify the maximal additional energy used by a control system in the presence of bounded disturbances, compared to a nominal, disturbance-free system. In particular, we consider the task of finite-time stabilization for a linear, time-invariant system. We compare the nominal energy required to achieve this task in the disturbance-free system to the worst-case energy over all feasible disturbances. Solving for the worst-case energy over all disturbances first leads to an optimal control problem with a least-squares solution, and then an infinite-dimensional optimization problem where we derive an upper bound on the solution. The comparison of energies is accomplished using additive and multiplicative metrics, for which we derive bounds. Simulation examples on an ADMIRE fighter jet model demonstrate the practicability of these metrics, and their variation with the distance of the initial condition from the origin and the task completion time.

koopman operator, resilience↗
DOE OSTI2023

Real time detection of multiple stable MHD eigenmode growth rates towards kink/tearing modes avoidance in DIII-D tokamak plasmas

Abstract Real time detection of time evolving growth rates of multiple stable magnetohydrodynamic (MHD) eigenmodes has been achieved in DIII-D tokamak experiments via multi-mode three-dimensional (3D) active MHD spectroscopy. The measured evolution of the multi-modes’ growth rates is in good accordance with the variation of the plasma β N . Using experimental equilibria, resistive MARS-F simulations found the two least stable modes to have comparable growth rates to those experimentally measured. Real time and offline calculations of the modes’ growth rates show comparable results and indicate that cleaner system input and output signals will improve the accuracy of the real time stability detection. Moreover, the shortest real time updating time window of multi-mode eigenvalues can be about 2 ms in DIII-D experiments. This real time monitoring of stable, macroscopic kink and tearing modes thus provides an effective tool for avoidance of the most common causes of tokamak disruption.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1989

A review of atomic clock technology, the performance capability of present spaceborne and terrestrial atomic clocks, and a look toward the future

Clocks have played a strong role in the development of general relativity. The concept of the proper clock is presently best realized by atomic clocks, whose development as precision instruments has evolved very rapidly in the last decades. To put a historical prospective on this progress since the year AD 1000, the time stability of various clocks expressed in terms of seconds of time error over one day of operation is shown. This stability of operation must not be confused with accuracy. Stability refers to the constancy of a clock operation as compared to that of some other clocks that serve as time references. Accuracy, on the other hand, is the ability to reproduce a previously defined frequency. The issues are outlined that must be considered when accuracy and stability of clocks and oscillators are studied. In general, the most widely used resonances result from the hyperfine interaction of the nuclear magnetic dipole moment and that of the outermost electron, which is characteristic of hydrogen and the alkali atoms. During the past decade hyperfine resonances of ions have also been used. The principal reason for both the accuracy and the stability of atomic clocks is the ability of obtaining very narrow hyperfine transition resonances by isolating the atom in some way so that only the applied stimulating microwave magnetic field is a significant source of perturbation. It is also important to make resonance transitions among hyperfine magnetic sublevels where separation is independent, at least to first order, of the magnetic field. In the case of ions stored in traps operating at high magnetic fields, one selects the trapping field to be consistent with a field-independent transition of the trapped atoms.

Vessot, Robert F. C.↗
DOE OSTI2022

Carbon fibers derived from liquefied and fractionated poplar lignins: The effect of molecular weight

Lignin recovered from poplar, a key woody biomass resource proposed for lignocellulosic refineries, was investigated for conversion into carbon fibers. Aqueous solutions of ethanol at selected temperatures and compositions, where the requisite solvent and liquefied-lignin phases form, were used to purify and fractionate hybrid poplar (HP) lignin using the Aqueous Lignin Purification with Hot Agents (ALPHA) process. Sugars (<0.1%) and ash (0.01 ± 0.01%) content of the highest molecular weight fraction (Mw = 52 kDa) approached the limits of detection. This 52 kDa fraction could be dry spun with excellent stability and was subsequently converted into carbon fibers having strength (1.1 ± 0.2 GPa) and modulus (78 ± 8 GPa) essentially double those previously achieved with neat poplar and other hardwood lignins. Of equal importance, stabilization times were decreased by a factor of five. Such stabilization and strength/modulus improvements are essential for reducing the overall manufacturing cost of carbon fibers being proposed for cost-sensitive applications.

09 BIOMASS FUELS↗
NASA NTRS1990

GPS/Loran-C interoperability for time and frequency applications: A survey of the times of arrival of Loran-C transmissions via GPS common mode/common view satellite observations

The results from this survey clearly indicate that the Global Positioning System (GPS) time transfer capability is superior to that of the Loran-C system for absolute timing accuracy, and that even with the most careful calibration of the Loran-C receiver delay and propagation path, inexplicable time of arrival (TOA) biases remain which are larger than the variations across all of the transmitters. Much more data covering years would be needed to show that these biases were stable enough to be removed with a one time site calibration. The synchronization of the transmissions is excellent, all showing low parts in 10(exp 13) offsets versus the United States Naval Observatory (USNO) master clock. With the exception of the Searchlight transmitter, all of the transmissions exhibit timing stabilities over the entire period of less than 300 ns RMS which is at the observed levels of GPS under selective availability (SA). The Loran-C phase instabilities take place over a much greater time interval than those being forced onto the GPS signals under SA, providing for better medium to short term frequency stability. Data show that all but the most distant transmitters offer better than three parts in 10(exp 11) stability at this averaging time. It is in the frequency control area where GPS/Loran-C interoperation will offer some synergistic advantages over GPS alone under SA.

Penrod, Bruce↗
NASA NTRS1994

Remote clocks linked by a fully calibrated two-way timing link

Fully calibrated two-way time transfers between the U.S. Naval Observatory (USNO) and the U.S. Naval Observatory Time Service Substation (NOTSS) have become operational. The calibration method employed was the co-located Earth station method. Results concerning timing, stability, comparison of two-way and Global Positioning System (GPS) timing links, and remote clock contribution to local time scale computation will be presented.

Deyoung, James A.↗