12,949 research outputs found
Automorphism Groups of Geometrically Represented Graphs
We describe a technique to determine the automorphism group of a
geometrically represented graph, by understanding the structure of the induced
action on all geometric representations. Using this, we characterize
automorphism groups of interval, permutation and circle graphs. We combine
techniques from group theory (products, homomorphisms, actions) with data
structures from computer science (PQ-trees, split trees, modular trees) that
encode all geometric representations.
We prove that interval graphs have the same automorphism groups as trees, and
for a given interval graph, we construct a tree with the same automorphism
group which answers a question of Hanlon [Trans. Amer. Math. Soc 272(2), 1982].
For permutation and circle graphs, we give an inductive characterization by
semidirect and wreath products. We also prove that every abstract group can be
realized by the automorphism group of a comparability graph/poset of the
dimension at most four
Revisiting Complex Moments For 2D Shape Representation and Image Normalization
When comparing 2D shapes, a key issue is their normalization. Translation and
scale are easily taken care of by removing the mean and normalizing the energy.
However, defining and computing the orientation of a 2D shape is not so simple.
In fact, although for elongated shapes the principal axis can be used to define
one of two possible orientations, there is no such tool for general shapes. As
we show in the paper, previous approaches fail to compute the orientation of
even noiseless observations of simple shapes. We address this problem. In the
paper, we show how to uniquely define the orientation of an arbitrary 2D shape,
in terms of what we call its Principal Moments. We show that a small subset of
these moments suffice to represent the underlying 2D shape and propose a new
method to efficiently compute the shape orientation: Principal Moment Analysis.
Finally, we discuss how this method can further be applied to normalize
grey-level images. Besides the theoretical proof of correctness, we describe
experiments demonstrating robustness to noise and illustrating the method with
real images.Comment: 69 pages, 20 figure
Local Alignment of the BABAR Silicon Vertex Tracking Detector
The BABAR Silicon Vertex Tracker (SVT) is a five-layer double-sided silicon
detector designed to provide precise measurements of the position and direction
of primary tracks, and to fully reconstruct low-momentum tracks produced in
e+e- collisions at the PEP-II asymmetric collider at Stanford Linear
Accelerator Center. This paper describes the design, implementation,
performance, and validation of the local alignment procedure used to determine
the relative positions and orientations of the 340 SVT wafers. This procedure
uses a tuned mix of in-situ experimental data and complementary lab-bench
measurements to control systematic distortions. Wafer positions and
orientations are determined by minimizing a chisquared computed using these
data for each wafer individually, iterating to account for between-wafer
correlations. A correction for aplanar distortions of the silicon wafers is
measured and applied. The net effect of residual mis-alignments on relevant
physical variables is evaluated in special control samples. The BABAR
data-sample collected between November 1999 and April 2008 is used in the study
of the SVT stability.Comment: 21 pages, 20 figures, 3 tables, submitted to Nucl. Instrum. Meth.
Implementing Rapid Prototyping Using CNC Machining (CNC-RP) Through a CAD/CAM Interface
This paper presents the methodology and implementation of a rapid machining system using a
CAD/CAM interface. Rapid Prototyping using CNC Machining (CNC-RP) is a method that has
been developed which enables automatic generation of process plans for a machined component.
The challenge with CNC-RP is not the technical problems of material removal, but with all of
the required setup, fixture and toolpath planning, which has previously required a skilled
machinist. Through the use of advanced geometric algorithms, we have implemented an
interface with a CAD/CAM system that allows true automatic NC code generation directly from
a CAD model with no human interaction; a capability necessary for a practical rapid prototyping
system.Mechanical Engineerin
Stiffness pathologies in discrete granular systems: bifurcation, neutral equilibrium, and instability in the presence of kinematic constraints
The paper develops the stiffness relationship between the movements and
forces among a system of discrete interacting grains. The approach is similar
to that used in structural analysis, but the stiffness matrix of granular
material is inherently non-symmetric because of the geometrics of particle
interactions and of the frictional behavior of the contacts. Internal geometric
constraints are imposed by the particles' shapes, in particular, by the surface
curvatures of the particles at their points of contact. Moreover, the stiffness
relationship is incrementally non-linear, and even small assemblies require the
analysis of multiple stiffness branches, with each branch region being a
pointed convex cone in displacement-space. These aspects of the particle-level
stiffness relationship gives rise to three types of micro-scale failure:
neutral equilibrium, bifurcation and path instability, and instability of
equilibrium. These three pathologies are defined in the context of four types
of displacement constraints, which can be readily analyzed with certain
generalized inverses. That is, instability and non-uniqueness are investigated
in the presence of kinematic constraints. Bifurcation paths can be either
stable or unstable, as determined with the Hill-Bazant-Petryk criterion.
Examples of simple granular systems of three, sixteen, and sixty four disks are
analyzed. With each system, multiple contacts were assumed to be at the
friction limit. Even with these small systems, micro-scale failure is expressed
in many different forms, with some systems having hundreds of micro-scale
failure modes. The examples suggest that micro-scale failure is pervasive
within granular materials, with particle arrangements being in a nearly
continual state of instability
Geometric path planning without maneuvers for nonholonomic parallel orienting robots
Current geometric path planners for nonholonomic parallel orienting robots generate maneuvers consisting of a sequence of moves connected by zero-velocity points. The need for these maneuvers restrains the use of this kind of parallel robots to few applications. Based on a rather old result on linear time-varying systems, this letter shows that there are infinitely differentiable paths connecting two arbitrary points in SO(3) such that the instantaneous axis of rotation along the path rest on a fixed plane. This theoretical result leads to a practical path planner for nonholonomic parallel orienting robots that generates single-move maneuvers. To present this result, we start with a path planner based on three-move maneuvers, and then we proceed by progressively reducing the number of moves to one, thus providing a unified treatment with respect to previous geometric path planners.Peer ReviewedPostprint (author's final draft
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