222 research outputs found
Creep stability of the proposed AIDA mission target 65803 Didymos: I. Discrete cohesionless granular physics model
As the target of the proposed Asteroid Impact & Deflection Assessment (AIDA)
mission, the near-Earth binary asteroid 65803 Didymos represents a special
class of binary asteroids, those whose primaries are at risk of rotational
disruption. To gain a better understanding of these binary systems and to
support the AIDA mission, this paper investigates the creep stability of the
Didymos primary by representing it as a cohesionless self-gravitating granular
aggregate subject to rotational acceleration. To achieve this goal, a
soft-sphere discrete element model (SSDEM) capable of simulating granular
systems in quasi-static states is implemented and a quasi-static spin-up
procedure is carried out. We devise three critical spin limits for the
simulated aggregates to indicate their critical states triggered by reshaping
and surface shedding, internal structural deformation, and shear failure,
respectively. The failure condition and mode, and shear strength of an
aggregate can all be inferred from the three critical spin limits. The effects
of arrangement and size distribution of constituent particles, bulk density,
spin-up path, and interparticle friction are numerically explored. The results
show that the shear strength of a spinning self-gravitating aggregate depends
strongly on both its internal configuration and material parameters, while its
failure mode and mechanism are mainly affected by its internal configuration.
Additionally, this study provides some constraints on the possible physical
properties of the Didymos primary based on observational data and proposes a
plausible formation mechanism for this binary system. With a bulk density
consistent with observational uncertainty and close to the maximum density
allowed for the asteroid, the Didymos primary in certain configurations can
remain geo-statically stable without including cohesion.Comment: 66 pages, 24 figures, submitted to Icarus on 25/Aug/201
Achieving reliability using behavioural modules in a robotic assembly system
The research in this thesis looks at improving the reliability of robotic as¬
sembly while still retaining the flexibility to change the system to cope with dif¬
ferent assemblies. The lack of a truly flexible robotic assembly system presents
a problem which current systems have yet to overcome. An experimental sys¬
tem has been designed and implemented to demonstrate the ideas presented in
this work. Runs of this system have also been performed to test and assess the
scheme which has been developed.The Behaviour-based SOMASS system looks at decomposing the task into
modular units, called Behavioural Modules, which reliably perform the as¬
sembly task by using variation reducing strategies. The thesis work looks at
expanding this framework to produce a system which relaxes the constraints of
complete reliability within a Behavioural Module by embedding these in a re¬
liable system architecture. This means that Behavioural Modules do not have
to guarantee to successfully perform their given task but instead can perform it
adequately, with occasional failures dealt with by the appropriate introduction
of alternative actionsTo do this, the concepts of Exit States, the Ideal Execution Path, and Alter¬
native Execution Paths have been described. The Exit State of a Behavioural
Module gives an indication of the control path which has actually been taken
during its execution. This information, along with appropriate information
available to the execution system (such as sensor and planner data), allows the
Ideal Execution Path and Alternative Execution Paths to be defined. These
show, respectively, the best control path through the system (as determined by
the system designer) and alternative control routes which can be taken when
necessary
Design and implementation of a compliant robot with force feedback and strategy planning software
Force-feedback robotics techniques are being developed for automated precision assembly and servicing of NASA space flight equipment. Design and implementation of a prototype robot which provides compliance and monitors forces is in progress. Computer software to specify assembly steps and makes force feedback adjustments during assembly are coded and tested for three generically different precision mating problems. A model program demonstrates that a suitably autonomous robot can plan its own strategy
Exploring the NRO Opportunity for a Hubble-sized Wide-field Near-IR Space Telescope -- NEW WFIRST
We discuss scientific, technical and programmatic issues related to the use
of an NRO 2.4m telescope for the WFIRST initiative of the 2010 Decadal Survey.
We show that this implementation of WFIRST, which we call "NEW WFIRST," would
achieve the goals of the NWNH Decadal Survey for the WFIRST core programs of
Dark Energy and Microlensing Planet Finding, with the crucial benefit of deeper
and/or wider near-IR surveys for GO science and a potentially Hubble-like Guest
Observer program. NEW WFIRST could also include a coronagraphic imager for
direct detection of dust disks and planets around neighboring stars, a
high-priority science and technology precursor for future ambitious programs to
image Earth-like planets around neighboring stars.Comment: 76 pages, 26 figures -- associated with the Princeton "New Telescope
Meeting
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LDRD final report: Automated planning and programming of assembly of fully 3D mechanisms
This report describes the results of assembly planning research under the LDRD. The assembly planning problem is that of finding a sequence of assembly operations, starting from individual parts, that will result in complete assembly of a device specified as a CAD model. The automated assembly programming problem is that of automatically producing a robot program that will carry out a given assembly sequence. Given solutions to both of these problems, it is possible to automatically program a robot to assemble a mechanical device given as a CAD data file. This report describes the current state of our solutions to both of these problems, and a software system called Archimedes 2 we have constructed to automate these solutions. Because Archimedes 2 can input CAD data in several standard formats, we have been able to test it on a number of industrial assembly models more complex than any before attempted by automated assembly planning systems, some having over 100 parts. A complete path from a CAD model to an automatically generated robot program for assembling the device represented by the CAD model has also been demonstrated
Introduction to topological defects: from liquid crystals to particle physics
Liquid crystals are assemblies of rod-like molecules which self-organize to
form mesophases, in-between ordinary liquids and anisotropic crystals. At each
point, the molecules collectively orient themselves along a privileged
direction, which locally defines an orientational order. Sometimes, this order
is broken and singularities appear in the form of topological defects. This
tutorial article is dedicated to the geometry, topology and physics of these
defects. We introduce the main models used to describe the nematic phase and
discuss the isotropic-nematic phase transition. Then, we present the different
families of defects in nematics and examine some of their physical outcomes.
Finally, we show that topological defects are universal patterns of nature,
appearing not only in soft matter, but also in biology, cosmology, geology and
even particle physics.Comment: 40 pages, 9 figures, review pape
The design and implementation of vision-based behavioural modules for a robotic assembly system
The work drsrrihrd in this thesis ia about, how to program robots to work re
liably in the presence of uncertainty. Some architectural principle!: are proposed
which address the problem of decomposing robotic assembly tasks into modular
units such that a robot program can be implemented efficiently, tested easily, and
can be maintained or modified without undue complexity. This architecture also
provides a framework to integrate sensors into a robotic, assembly system.These modular units arc called behavioural modules. They perforin their tasks
reliably. The problem of uncertainty is dealt with by encapsulating sensing and
variation reducing strategies inside these modules. Experiments are performed with
a working robotic assembly system using vision based behavioural modules. Analy
sis of this system validates the principles presented in this thesis
Solar High-energy Astrophysical Plasmas Explorer (SHAPE). Volume 1: Proposed concept, statement of work and cost plan
The concept of the Solar High-Energy Astrophysical Plasmas Explorer (SHAPE) is studied. The primary goal is to understand the impulsive release of energy, efficient acceleration of particles to high energies, and rapid transport of energy. Solar flare studies are the centerpieces of the investigation because in flares these high energy processes can be studied in unmatched detail at most wavelenth regions of the electromagnetic spectrum as well as in energetic charged particles and neutrons
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