1,306 research outputs found
TeleOperator/telePresence System (TOPS) Concept Verification Model (CVM) development
The development of an anthropomorphic, undersea manipulator system, the TeleOperator/telePresence System (TOPS) Concept Verification Model (CVM) is described. The TOPS system's design philosophy, which results from NRaD's experience in undersea vehicles and manipulator systems development and operations, is presented. The TOPS design approach, task teams, manipulator, and vision system development and results, conclusions, and recommendations are presented
On the Performance of Imt-2000 Communication Link Based on Stratospheric Platforms
A new means for providing wireless communication has been currently proposed. It is based on aerial vehicle known asHigh Altitude Platform or Stratospheric Platforms (SPF). The SPF will be operated at an altitude of 17-22 km above theground. Therefore, the channel condition may be different compared with those of the conventional terrestrial orsatellite wireless channel. In this paper, the channel propagation characteristic of such a system is firstly investigated bymeans of ray tracing algorithm. We emphasize our investigation in a typical urban environment, in which the mobileusers mostly exist. We developed building block model for simulation based on building height distribution, which isobtained from measurement inside Tokyo. As a result, propagation loss model and Ricean channel parameter for theSPF channel is reported in different scenarios. By using this result we then estimate the required transmitted power ofSPF to serve the mobile users in a several transmission rate that is used in IMT-2000 services. Finally, an evaluation ofBER of IMT-2000 link is performed in order to estimate the system level performance. From this evaluation, the maincontribution of this paper is to clearly show the critical limitations of both power requirement as well as system levelperformance of mobile communication IMT-2000 by using the concept of the SPF
Structure of Metastable States in Phase Transitions with High-Spin Low-Spin Degree of Freedom
Difference of degeneracy of the low-spin (LS) and high-spin (HS) states
causes interesting entropy effects on spin-crossover phase transitions and
charge transfer phase transitions in materials composed of the spin-crossover
atoms. Mechanisms of the spin-crossover (SC) phase transitions have been
studied by using Wajnflasz model, where the degeneracy of the spin states (HS
or LS) is taken into account and cooperative natures of the spin-crossover
phase transitions have been well described. Recently, a charge transfer (CT)
phase transition due to electron hopping between LS and HS sites has been
studied by using a generalized Wajnflasz model. In the both systems of SC and
CT, the systems have a high temperature structure (HT) and a low temperature
structure (LT), and the change between them can be a smooth crossover or a
discontinuous first order phase transition depending on the parameters of the
systems. Although apparently the standard SC system and the CT system are very
different, it is shown that both models are equivalent under a certain
transformation of variables. In both systems, the structure of metastable state
at low temperatures is a matter of interest. We study temperature dependence of
fraction of HT systematically in a unified model, and find several structures
of equilibrium and metastable states of the model as functions of system
parameters. In particular, we find a reentrant type metastable branch of HT in
a low temperature region, which would play an important role to study the
photo-irradiated processes of related materials.Comment: 19 pages, 11 figure
A state capital for Hawaii
Thesis (M.Arch)--Massachusetts Institute of Technology, Dept. of Architecture, 1961.Accompanying drawings held by MIT Museum.Includes bibliographical references (leaf 41).by Paul S. Shimamoto.M.Arc
On the transient behavior of frictional melt during seismic slip
In a recent work on the problem of sliding surfaces under the presence
of frictional melt (applying in particular to earthquake fault dynamics),
we derived from first principles an expression for the steady state
friction compatible with experimental observations. Building on the
expressions of heat and mass balance obtained in the above study for
this particular case of Stefan problem (phase transition with a migrating
boundary) we propose here an extension providing the full time-dependent
solution (including the weakening transient after pervasive melting
has started, the effect of eventual steps in velocity and the final
decelerating phase). A system of coupled equations is derived and
solved numerically. The resulting transient friction and wear evolution
yield a satisfactory fit (1) with experiments performed under variable
sliding velocities (0.9-2 m/s) and different normal stresses (0.5-20
MPa) for various rock types and (2) with estimates of slip weakening
obtained from observations on ancient seismogenic faults that host
pseudotachylite (solidified melt). The model allows to extrapolate
the experimentally observed frictional behavior to large normal stresses
representative of the seismogenic Earth crust (up to 200 MPa), high
slip rates (up to 9 m/s) and cases where melt extrusion is negligible.
Though weakening distance and peak stress vary widely, the net breakdown
energy appears to be essentially independent of either slip velocity
and normal stress. In addition, the response to earthquake-like slip
can be simulated, showing a rapid friction recovery when slip rate
drops. We discuss the properties of energy dissipation, transient
duration, velocity weakening, restrengthening in the decelerating
final slip phase and the implications for earthquake source dynamics
From slow to fast faulting: recent challenges in earthquake fault mechanics
Faults—thin zones of highly localized shear deformation in the Earth—accommodate strain on a momentous range of dimensions (millimetres to hundreds of kilometres for major plate boundaries) and of time intervals (from fractions of seconds during earthquake slip, to years of slow, aseismic slip and millions of years of intermittent activity). Traditionally, brittle faults have been distinguished from shear zones which deform by crystal plasticity (e.g. mylonites). However such brittle/plastic distinction becomes blurred when considering (i) deep earthquakes that happen under conditions of pressure and temperature where minerals are clearly in the plastic deformation regime (a clue for seismologists over several decades) and (ii) the extreme dynamic stress drop occurring during seismic slip acceleration on faults, requiring efficient weakening mechanisms. High strain rates (more than 104 s−1) are accommodated within paper-thin layers (principal slip zone), where co-seismic frictional heating triggers non-brittle weakening mechanisms. In addition, (iii) pervasive off-fault damage is observed, introducing energy sinks which are not accounted for by traditional frictional models. These observations challenge our traditional understanding of friction (rate-and-state laws), anelastic deformation (creep and flow of crystalline materials) and the scientific consensus on fault operation. This article is part of the themed issue ‘Faulting, friction and weakening: from slow to fast motion’
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