3,005 research outputs found
Identification and dual adaptive control of a turbojet engine
The objective of this paper is to utilize the design methods of modern control theory to realize a dual-adaptive feedback control unit for a highly nonlinear single spool airbreathing turbojet engine. Using a very detailed and accurate simulation of the nonlinear engine as the data source, linear operating point models of unspecified dimension are identified. Feedback control laws are designed at each operating point for a prespecified set of sampling rates using sampled-data output regulator theory. The control system sampling rate is determined by an adaptive sampling algorithm in correspondence with turbojet engine performance. The result is a dual-adaptive control law that is functionally dependent upon the sampling rate selected and environmental operating conditions. Simulation transients demonstrate the utility of the dual-adaptive design to improve on-board computer utilization while maintaining acceptable levels of engine performance
An inverter/controller subsystem optimized for photovoltaic applications
Conversion of solar array dc power to ac power stimulated the specification, design, and simulation testing of an inverter/controller subsystem tailored to the photovoltaic power source characteristics. Optimization of the inverter/controller design is discussed as part of an overall photovoltaic power system designed for maximum energy extraction from the solar array. The special design requirements for the inverter/ controller include: a power system controller (PSC) to control continuously the solar array operating point at the maximum power level based on variable solar insolation and cell temperatures; and an inverter designed for high efficiency at rated load and low losses at light loadings to conserve energy
A Sensor Failure Simulator for Control System Reliability Studies
A real-time Sensor Failure Simulator (SFS) was designed and assembled for the Advanced Detection, Isolation, and Accommodation (ADIA) program. Various designs were considered. The design chosen features an IBM-PC/XT. The PC is used to drive analog circuitry for simulating sensor failures in real-time. A user defined scenario describes the failure simulation for each of the five incoming sensor signals. Capabilities exist for editing, saving, and retrieving the failure scenarios. The SFS has been tested closed-loop with the Controls Interface and Monitoring (CIM) unit, the ADIA control, and a real-time F100 hybrid simulation. From a productivity viewpoint, the menu driven user interface has proven to be efficient and easy to use. From a real-time viewpoint, the software controlling the simulation loop executes at greater than 100 cycles/sec
CHARACTERIZATION OF CHBrCl2 PHOTOLYSIS BY VELOCITY MAP IMAGING
Halomethanes have attracted extensive research efforts of considerable variety, owing to their relative simplicity and ubiquitous presence in synthetic and environmental settings as well as their amenability to benchmark problems in physical chemistry. Their role in atmospheric processes is well known, most famously as the source of atomic halogens which catalyze the depletion of stratospheric ozone. Indeed, the photolytic cleavage of the carbon-halogen bond is the primary fate of halomethanes in the atmosphere. We utilize laser-induced photolysis to study the C-Br bond cleavage in CHBrCl in a molecular beam. Atomic bromine fragments are probed with resonance enhanced multiphoton ionization (REMPI), which allows ground state and spin-excited products to be independently detected. Action spectroscopy in conjunction with velocity map imaging is used to determine the internal energy of the CHCl partner fragment. Product state distributions as a function of photolysis energy may be discerned with these techniques. Current results will be presented
Studying the Pulsation of Mira Variables in the Ultraviolet
We present results from an empirical study of the Mg II h & k emission lines
of selected Mira variable stars, using spectra from the International
Ultraviolet Explorer (IUE). The stars all exhibit similar Mg II behavior during
the course of their pulsation cycles. The Mg II flux always peaks after optical
maximum near pulsation phase 0.2-0.5, although the Mg II flux can vary greatly
from one cycle to the next. The lines are highly blueshifted, with the
magnitude of the blueshift decreasing with phase. The widths of the Mg II lines
are also phase-dependent, decreasing from about 70 km/s to 40 km/s between
phase 0.2 and 0.6. We also study other UV emission lines apparent in the IUE
spectra, most of them Fe II lines. These lines are much narrower and not nearly
as blueshifted as the Mg II lines. They exhibit the same phase-dependent flux
behavior as Mg II, but they do not show similar velocity or width variations.Comment: 26 pages, 12 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty;
to appear in Ap
Asymmetric polarity reversals, bimodal field distribution, and coherence resonance in a spherically symmetric mean-field dynamo model
Using a mean-field dynamo model with a spherically symmetric helical
turbulence parameter alpha which is dynamically quenched and disturbed by
additional noise, the basic features of geomagnetic polarity reversals are
shown to be generic consequences of the dynamo action in the vicinity of
exceptional points of the spectrum. This simple paradigmatic model yields long
periods of constant polarity which are interrupted by self-accelerating field
decays leading to asymmetric polarity reversals. It shows the recently
discovered bimodal field distribution, and it gives a natural explanation of
the correlation between polarity persistence time and field strength. In
addition, we find typical features of coherence resonance in the dependence of
the persistence time on the noise.Comment: 5 pages, 7 figure
<i>‘What retention’ means to me</i>: the position of the adult learner in student retention
Studies of student retention and progression overwhelmingly appear adopt definitions that place the institution, rather than the student, at the centre. Retention is most often conceived in terms of linear and continuous progress between institutionally identified start and end points.
This paper reports on research that considered data from 38 in-depth interviews conducted with individuals who had characteristics often associated with non-traditional engagement in higher education who between 2006 and 2010 had studied an ‘Introduction to HE’ module at one distance higher education institution, some of whom had progressed to further study at that institution, some of whom had not. The research deployed a life histories approach to seek a finer grained understanding of how individuals conceptualise their own learning journey and experience, in order to reflect on institutional conceptions of student retention.
The findings highlight potential anomalies hidden within institutional retention rates – large proportions of the interview participants who were not ‘retained’ by the institution reported successful progression to and in other learning institutions and environments, both formal and informal. Nearly all described positive perspectives on lifelong learning which were either engendered or improved by the learning undertaken. This attests to the complexity of individuals’ lives and provides clear evidence that institution-centric definitions of retention and progression are insufficient to create truly meaningful understanding of successful individual learning journeys and experiences. It is argued that only through careful consideration of the lived experience of students and a re-conception of measures of retention, will we be able to offer real insight into improving student retention
Mid-infrared spectra of late-type stars: Long-term evolution
Recent ground-based mid-infrared spectra of 29 late-type stars, most with
substantial dust shells, are compared to ground-based spectra of these stars
from the 1960s and 1970s and to IRAS-LRS spectra obtained in 1983. The spectra
of about half the stars show no detectable changes, implying that their
distributions of circumstellar material and associated dust grain properties
have changed little over this time interval. However, many of the stars with
strong silicate features showed marked changes. In nearly all cases the
silicate peak has strengthened with respect to the underlying continuum,
although there is one case (VY~CMa) in which the silicate feature has almost
completely disappeared. This suggests that, in general, an oxygen-rich star
experiences long periods of gradual silicate feature strengthening, punctuated
by relatively rare periods when the feature weakens. We discuss various
mechanisms for producing the changes, favoring the slow evolution of the
intrinsic dust properties (i.e., the chemical composition or grain structure).
Although most IRAS spectra agree well with ground-based spectra, there are a
number of cases where they fall well outside the expected range of uncertainty.
In almost all such cases the slopes of the red and blue LRS spectra do not
match in their region of overlap.Comment: Accepted in ApJ, 20 pages, 5 figures, 1 tabl
Probing the Neutron-Capture Nucleosynthesis History of Galactic Matter
The heavy elements formed by neutron capture processes have an interesting
history from which we can extract useful clues to and constraints upon both the
characteristics of the processes themselves and the star formation and
nucleosynthesis history of Galactic matter. Of particular interest in this
regard are the heavy element compositions of extremely metal-deficient stars.
At metallicities [Fe/H] <= -2.5, the elements in the mass region past barium (A
>= 130-140 have been found (in non carbon-rich stars) to be pure r-process
products. The identification of an environment provided by massive stars and
associated Type II supernovae as an r-process site seems compelling. Increasing
levels of heavy s-process (e.g., barium) enrichment with increasing
metallicity, evident in the abundances of more metal-rich halo stars and disk
stars, reflect the delayed contributions from the low- and intermediate-mass (M
\~ 1-3 Msol) stars that provide the site for the main s-process nucleosynthesis
component during the AGB phase of their evolution. New abundance data in the
mass region 60 <~ A <~ 130 is providing insight into the identity of possible
alternative r-process sites. We review recent observational studies of heavy
element abundances both in low metallicity halo stars and in disk stars,
discuss the observed trends in light of nucleosynthesis theory, and explore
some implications of these results for Galactic chemical evolution,
nucleosynthesis, and nucleocosmochronology.Comment: 47 pages, 2 tables, 11 figures; To appear in PAS
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