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Standard Problems (standard + problem)

Distribution by Scientific Domains
Engineering40%

Selected Abstracts

Minimum effort dead-beat control of linear servomechanisms with ripple-free response

OPTIMAL CONTROL APPLICATIONS AND METHODS, Issue 3 2001

Abstract A new and systematic approach to the problem of minimum effort ripple-free dead-beat (EFRFDB) control of the step response of a linear servomechanism is presented. There is specified a set of admissible discrete error feedback controllers, complying with general conditions for the design of ripple-free dead-beat (RFDB) controllers, regardless of the introduced degree of freedom, defined as the number of steps exceeding their minimum number. The solution is unique for the minimum number of steps, while their increase enables one to make an optimal choice from a competitive set of controllers via their parametrization in a finite-dimensional space. As an objective function, Chebyshev's norm of an arbitrarily chosen linear projection of the control variable was chosen. There has been elaborated a new, efficient algorithm for all stable systems of the given class with an arbitrary degree of freedom. A parametrized solution in a finite space of polynomials is obtained through the solution of a standard problem of mathematical programming which simultaneously yields the solution of a total position change maximization of servomechanism provided that a required number of steps and control effort limitation are given. A problem formulated in this way is consecutively used in solving the time-optimal (minimum-step) control of a servomechanism to a given steady-state position with a specified limitation on control effort. The effect of EFRFDB control on the example of a linear servomechanism with torsion spring shaft, with the criterions of control effort and control difference effort, is illustrated and analysed. Copyright © 2001 John Wiley & Sons, Ltd. [source]

An investigation of pulsatile flow in a model cavo-pulmonary vascular system

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 11 2009
K. Chitra
Abstract The complexities in the flow pattern in a cavo-pulmonary vascular system,after application of the Fontan procedure in the vicinity of the superior vena cava, inferior vena cava, and the confluence at the T-junction,are analysed. A characteristic-based split (CBS) finite element scheme involving the artificial compressibility approach is employed to compute the resulting flow. Benchmarking of the CBS scheme is carried out using standard problems and with the flow features observed in an experimental model with the help of a dye visualization technique in model scale. The transient flow variations in a total cavo-pulmonary connection (TCPC) under pulsatile conditions are investigated and compared with flow visualization studies. In addition to such qualitative flow investigations, quantitative analysis of energy loss and haemodynamic stresses have also been performed. The comparisons show good agreement between the numerical and experimental flow patterns. The numerically predicted shear stress values indicate that the pulsatile flow condition is likely to be more severe than steady flow, with regard to the long-term health of the surgically corrected TCPC. Copyright © 2008 John Wiley & Sons, Ltd. [source]

A personal perspective on problem solving by general purpose solvers

INTERNATIONAL TRANSACTIONS IN OPERATIONAL RESEARCH, Issue 3 2010
Toshihide Ibaraki
Abstract To solve the problems that abound in real-world applications, we are proposing an approach of using general-purpose solvers, as we cannot afford to develop special-purpose algorithms for all individual problems. The existing general-purpose solvers such as linear programming and integer programming are very useful but not sufficient. To improve the situation, we have developed solvers for other standard problems such as the constraint satisfaction problem and the resource-constrained project scheduling problem among others. In this article, we describe why general-purpose solvers are needed, what kinds of solvers we considered, how they were developed and where they have been applied. [source]

A hierarchical modelling framework for identifying unusual performance in health care providers

JOURNAL OF THE ROYAL STATISTICAL SOCIETY: SERIES A (STATISTICS IN SOCIETY), Issue 4 2007
David I. Ohlssen
Summary. A wide variety of statistical methods have been proposed for detecting unusual performance in cross-sectional data on health care providers. We attempt to create a unified framework for comparing these methods, focusing on a clear distinction between estimation and hypothesis testing approaches, with the corresponding distinction between detecting ,extreme' and ,divergent' performance. When assuming a random-effects model the random-effects distribution forms the null hypothesis, and there appears little point in testing whether individual effects are greater or less than average. The hypothesis testing approach uses p -values as summaries and brings with it the standard problems of multiple testing, whether Bayesian or classical inference is adopted. A null random-effects formulation allows us to answer appropriate questions of the type: ,is a particular provider worse than we would expect the true worst provider (but still part of the null distribution) to be'? We outline a broad three-stage strategy of exploratory detection of unusual providers, detailed modelling robust to potential outliers and confirmation of unusual performance, illustrated by using two detailed examples. The concepts are most easily handled within a Bayesian analytic framework using Markov chain Monte Carlo methods, but the basic ideas should be generally applicable. [source]

SLICE-S: A Semi-Lagrangian Inherently Conserving and Efficient scheme for transport problems on the Sphere

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 602 2004
Mohamed Zerroukat
Abstract The Semi-Lagrangian Inherently Conserving and Efficient (SLICE) scheme developed for Cartesian geometry is generalized to spherical geometry. The spherical version, SLICE-S, is similarly based on a Control Volume approach and multiple sweeps of a one-dimensional O(,s4) (where s is the spherical distance) conservative remapping algorithm along Eulerian latitudes, then along Lagrangian longitudes. The resulting conservative scheme requires no restriction on either the polar meridional or zonal Courant numbers. SLICE-S is applied to the standard problems of solid-body rotation and deformational flow, and results are compared with those of a standard non-conservative and other published conservative semi-Lagrangian schemes. In addition to mass conservation, and consistent with the performance of SLICE, the present scheme is competitive in terms of accuracy and efficiency. © Crown copyright, 2004. Royal Meteorological Society [source]