48,084 research outputs found
Periodically Controlled Hybrid Systems: Verifying A Controller for An Autonomous Vehicle
This paper introduces Periodically Controlled Hybrid Automata (PCHA) for describing a class of hybrid control systems. In a PCHA, control actions occur roughly periodically while internal and input actions, may occur in the interim changing the discrete-state or the setpoint. Based on periodicity and subtangential conditions, a new sufficient condition for verifying invariance of PCHAs is presented. This technique is used in verifying safety of the planner-controller subsystem of an autonomous ground vehicle, and in deriving geometric properties of planner generated paths that can be followed safely by the controller under environmental uncertainties
The effect of a planet on the dust distribution in a 3D protoplanetary disk
Aims: We investigate the behaviour of dust in protoplanetary disks under the
action of gas drag in the presence of a planet. Our goal is twofold: to
determine the spatial distribution of dust depending on grain size and planet
mass, and therefore to provide a framework for interpretation of coming
observations and future studies of planetesimal growth. Method: We numerically
model the evolution of dust in a protoplanetary disk using a two-fluid (gas +
dust) Smoothed Particle Hydrodynamics (SPH) code, which is non-self-gravitating
and locally isothermal. The code follows the three dimensional distribution of
dust in a protoplanetary disk as it interacts with the gas via aerodynamic
drag. In this work, we present the evolution of a minimum mass solar nebula
(MMSN) disk comprising 1% dust by mass in the presence of an embedded planet.
We run a series of simulations which vary the grain size and planetary mass to
see how they affect the resulting disk structure. Results: We find that gap
formation is much more rapid and striking in the dust layer than in the gaseous
disk and that a system with a given stellar, disk and planetary mass will have
a completely different appearance depending on the grain size. For low mass
planets in our MMSN disk, a gap can open in the dust disk while not in the gas
disk. We also note that dust accumulates at the external edge of the planetary
gap and speculate that the presence of a planet in the disk may enhance the
formation of a second planet by facilitating the growth of planetesimals in
this high density region.Comment: 13 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysic
The Illawarra at Work: A Summary of the Major Findings of the Illawarra Regional Workplace Industrial Relations Survey
This paper summarises the main results of the Illawarra Regional Workplace Industrial Relations Survey (IRWIRS). The data is unique in that it provides the only comprehensive and statistically reliable source of information about workplace employee relations at the regional level in Australia, and compares regional patterns with national trends. The data collected relates to industrial relations indicators, workplace ownership, market conditions, management organisation and decision- making in the workplace, among other things. The results reveal a positive pattern of employment relations in the Illawarra, distinctive in many respects from national trends.Illawarra Regional Workplace Industrial Relations Survey, workplace employee relations, Australia
The architecture of a video image processor for the space station
The architecture of a video image processor for space station applications is described. The architecture was derived from a study of the requirements of algorithms that are necessary to produce the desired functionality of many of these applications. Architectural options were selected based on a simulation of the execution of these algorithms on various architectural organizations. A great deal of emphasis was placed on the ability of the system to evolve and grow over the lifetime of the space station. The result is a hierarchical parallel architecture that is characterized by high level language programmability, modularity, extensibility and can meet the required performance goals
BIOMECHANICAL CHANGES IN A PROFESSIONAL BASEBALL PITCHER: EARLY VS. LATE INNINGS
The purpose of the study was to investigate the effects of extended play on the kinematic and kinetic parameters of the baseball pitch in order to gain a better understanding of injury mechanisms and preventive strategies. Four professional pitchers were videotaped with high-speed (120 Hz) cameras during the second, fourth and sixth innings of the same game. Video data were digitised for one fastball for each inning, and kinematic and kinetic parameters were calculated from the 3D coordinate data. Comparisons between the pitches were based on standard deviations found for the same parameters in a similar study of 40 professional pitchers. Shoulder and elbow ranges of motion, peak varus torque, shoulder and elbow distraction and the rates of angular velocities were among the variables to show significant change
Ejection of Supernova-Enriched Gas From Dwarf Disk Galaxies
We examine the efficiency with which supernova-enriched gas may be ejected
from dwarf disk galaxies, using a methodology previously employed to study the
self-enrichment efficiency of dwarf spheroidal systems. Unlike previous studies
that focused on highly concentrated starbursts, in the current work we consider
discrete supernova events spread throughout various fractions of the disk. We
model disk systems having gas masses of 10^8 and 10^9 solar masses with
supernova rates of 30, 300, and 3000 per Myr. The supernova events are confined
to the midplane of the disk, but distributed over radii of 0, 30, and 80% of
the disk radius, consistent with expectations for Type II supernovae. In
agreement with earlier studies, we find that the enriched material from
supernovae is largely lost when the supernovae are concentrated near the
nucleus, as expected for a starburst event. In contrast, however, we find the
loss of enriched material to be much less efficient when the supernovae occur
over even a relatively small fraction of the disk. The difference is due to the
ability of the system to relax following supernova events that occur over more
extended regions. Larger physical separations also reduce the likelihood of
supernovae going off within low-density "chimneys" swept out by previous
supernovae. We also find that, for the most distributed systems, significant
metal loss is more likely to be accompanied by significant mass loss. A
comparison with theoretical predications indicates that, when undergoing
self-regulated star formation, galaxies in the mass range considered shall
efficiently retain the products of Type II supernovae.Comment: 16 pages, 14 figures, to appear in Astrophysical Journal; higher
resolution figures available through Ap
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