10,547 research outputs found
Finite Size Effects in Quark-Gluon Plasma Formation
Using lattice simulations of quenched QCD we estimate the finite size effects
present when a gluon plasma equilibrates in a slab geometry, i.e., finite width
but large transverse dimensions. Significant differences are observed in the
free energy density for the slab when compared with bulk behavior. A small
shift in the critical temperature is also seen. The free energy required to
liberate heavy quarks relative to bulk is measured using Polyakov loops; the
additional free energy required is on the order of at .Comment: LATTICE98(hightemp), talk at Lattice 98, 3 pages, 3 encapsulated
postscript figures, uses espcrc2.st
Kinematics and control algorithm development and simulation for a redundant two-arm robotic manipulator system
An efficient approach to cartesian motion and force control of a 7 degree of freedom (DOF) manipulator is presented. It is based on extending the active stiffness controller to the 7 DOF case in general and use of an efficient version of the gradient projection technique for solving the inverse kinematics problem. Cooperative control is achieved through appropriate configuration of individual manipulator controllers. In addition, other aspects of trajectory generation using standard techniques are integrated into the controller. The method is then applied to a specific manipulator of interest (Robotics Research T-710). Simulation of the kinematics, dynamics, and control are provided in the context of several scenarios: one pertaining to a noncontact pick and place operation; one relating to contour following where contact is made between the manipulator and environment; and one pertaining to cooperative control
Aqueous-Based Extrusion Fabrication of Ceramics on Demand
Aqueous-Based Extrusion Fabrication is an additive manufacturing technique that
extrudes ceramic slurries of high solids loading layer by layer for part fabrication. The
material reservoir in a previously developed system has been modified to allow for
starting and stopping of the extrusion process on demand. Design pros and cons are
examined and a comparison between two material reservoir designs is made. Tests are
conducted to determine the optimal deposition parameters for starting and stopping the
extrudate on demand. The collected test data is used for the development of a deposition
strategy that improves material deposition consistency, including reduced material
buildup at sharp corners. Example parts are fabricated using the deposition strategy and
hardware design.Mechanical Engineerin
Binding-incompetent adenovirus facilitates molecular conjugate-mediated gene transfer by the receptor-mediated endocytosis pathway
Molecular conjugate vectors may be constructed that accomplish high efficiency gene transfer by the receptor-mediated endocytosis pathway. In order to mediate escape from lysosomal degradation, we have incorporated adenoviruses into the functional design of the conjugate. In doing so, however, we have introduced an additional ligand, which can bind to receptors on the cell surface, undermining the potential for cell specific targeting. To overcome this, we have treated the adenovirus with a monoclonal anti-fiber antibody, which renders the virus incapable of binding to its receptor. The result is a multi-functional molecular conjugate vector, which has preserved its binding specificity while at the same time being capable of preventing lysosomal degradation of endosome-internalized conjugate-DNA complexes. This finding indicates that adenoviral binding is not a prerequisite for adenoviral-mediated endosome disruption
Freeform Extrusion of High Solids Loading Ceramic Slurries, Part I: Extrusion Process Modeling
A novel solid freeform fabrication method has been developed for the manufacture of
ceramic-based components in an environmentally friendly fashion. The method is based on the
extrusion of ceramic slurries using water as the binding media. Aluminum oxide (Al2O3) is
currently being used as the part material and solids loading as high as 60 vol. % has been
achieved. This paper describes a manufacturing machine that has been developed for the
extrusion of high solids loading ceramic slurries. A critical component of the machine is the
deposition system, which consists of a syringe, a plunger, a ram actuated by a motor that forces
the plunger down to extrude material, and a load cell to measure the extrusion force. An
empirical, dynamic model of the ceramic extrusion process, where the input is the commanded
ram velocity and the output is the extrusion force, is developed. Several experiments are
conducted and empirical modeling techniques are utilized to construct the dynamic model. The
results demonstrate that the ceramic extrusion process has a very slow dynamic response, as
compared to other non-compressible fluids such as water. A substantial amount of variation
exists in the ceramic extrusion process, most notably in the transient dynamics, and a constant
ram velocity may either produce a relatively constant steady-state extrusion force or it may cause
the extrusion force to steadily increase until the ram motor skips. The ceramic extrusion process
is also subjected to significant disturbances such as air bubble release, which causes a dramatic
decrease in the extrusion force, and nozzle clogging, which causes the extrusion force to slowly
increase until the clog is released or the ram motor skips.Mechanical Engineerin
Freeform Extrusion of High Solids Loading Ceramic Slurries, Part II: Extrusion Process Control
Part I of this paper provided a detailed description of a novel fabrication machine for high solids
loading ceramic slurry extrusion and presented an empirical model of the ceramic extrusion
process, with ram velocity as the input and extrusion force as the output. A constant force is
desirable in freeform extrusion processes as it correlates with a constant material deposition rate
and, thus, good part quality. The experimental results in Part I demonstrated that a constant ram
velocity will produce a transient extrusion force. In some instances the extrusion force increased
until ram motor skipping occurred. Further, process disturbances, such as air bubble release and
nozzle clogging that cause sudden changes in extrusion force, were often present. In this paper a
feedback controller for the ceramic extrusion process is designed and experimentally
implemented. The controller intelligently adjusts the ram motor velocity to maintain a constant
extrusion force. Since there is tremendous variability in the extrusion process characteristics, an
on-off controller is utilized in this paper. Comparisons are made between parts fabricated with
and without the feedback control. It is demonstrated that the use of the feedback control reduces
the effect of process disturbances (i.e., air bubble release and nozzle clogging) and dramatically
improves part quality.Mechanical Engineerin
Combining Cubic Dynamical Solvers with Make/Break Heuristics to Solve SAT
Dynamical solvers for combinatorial optimization are usually based on 2superscript{nd} degree polynomial interactions, such as the Ising model. These exhibit high success for problems that map naturally to their formulation. However, SAT requires higher degree of interactions. As such, these quadratic dynamical solvers (QDS) have shown poor solution quality due to excessive auxiliary variables and the resulting increase in search-space complexity. Thus recently, a series of cubic dynamical solver (CDS) models have been proposed for SAT and other problems. We show that such problem-agnostic CDS models still perform poorly on moderate to large problems, thus motivating the need to utilize SAT-specific heuristics. With this insight, our contributions can be summarized into three points. First, we demonstrate that existing make-only heuristics perform poorly on scale-free, industrial-like problems when integrated into CDS. This motivates us to utilize break counts as well. Second, we derive a relationship between make/break and the CDS formulation to efficiently recover break counts. Finally, we utilize this relationship to propose a new make/break heuristic and combine it with a state-of-the-art CDS which is projected to solve SAT problems several orders of magnitude faster than existing software solvers
On hitting time, mixing time and geometric interpretations of Metropolis-Hastings reversiblizations
Given a target distribution and a proposal chain with generator on
a finite state space, in this paper we study two types of Metropolis-Hastings
(MH) generator and in a continuous-time setting.
While is the classical MH generator, we define a new generator that
captures the opposite movement of and provide a comprehensive suite of
comparison results ranging from hitting time and mixing time to asymptotic
variance, large deviations and capacity, which demonstrate that enjoys
superior mixing properties than . To see that and are natural
transformations, we offer an interesting geometric interpretation of ,
and their convex combinations as minimizers between and the
set of -reversible generators, extending the results by Billera and
Diaconis (2001). We provide two examples as illustrations. In the first one we
give explicit spectral analysis of and for Metropolised independent
sampling, while in the second example we prove a Laplace transform order of the
fastest strong stationary time between birth-death and .Comment: 17 pages. To appear in J. Theoret. Proba
- …