The Experts below are selected from a list of 294 Experts worldwide ranked by ideXlab platform
Basilio Ruiz Cobo - One of the best experts on this subject based on the ideXlab platform.
-
chemical abundances from inversions of stellar spectra analysis of solar type stars with homogeneous and Static Model atmospheres
The Astrophysical Journal, 2001Co-Authors: Carlos Allende Prieto, Paul Barklem, Martin Asplund, Basilio Ruiz CoboAbstract:Spectra of late-type stars are usually analyzed with Static Model atmospheres in local thermodynamic equilibrium (LTE) and a homogeneous plane-parallel or spherically symmetric geometry. The energy balance requires particular attention, as two elements that are particularly difficult to Model play an important role: line blanketing and convection. Inversion techniques are able to bypass the difficulties of a detailed description of the energy balance. Assuming that the atmosphere is in hydroStatic equilibrium and LTE, it is possible to constrain its structure from spectroscopic observations. Among the most serious approximations still implicit in the method is a Static and homogeneous geometry. In this paper, we take advantage of a realistic three-dimensional radiative hydrodynamical simulation of the solar surface to check the systematic errors incurred by an inversion assuming a plane-parallel horizontally-homogeneous atmosphere. The thermal structure recovered resembles the spatial and time average of the three-dimensional atmosphere. Furthermore, the abundances retrieved are typically within 10% (0.04 dex) of the abundances used to construct the simulation. The application to a fairly complete data set from the solar spectrum provides further confidence in previous analyses of the solar composition. There is only a narrow range of one-dimensional thermal structures able to fit the absorption lines in the spectrum of the Sun. With our carefully selected data set, random errors are about a factor of 2 smaller than systematic errors. A small number of strong metal lines can provide very reliable results. We foresee no major difficulties in applying the technique to other similar stars, and obtaining similar accuracies, using spectra with λ/δλ ~ 5 × 104 and a signal-to-noise ratio as low as 30.
Carlos Allende Prieto - One of the best experts on this subject based on the ideXlab platform.
-
chemical abundances from inversions of stellar spectra analysis of solar type stars with homogeneous and Static Model atmospheres
The Astrophysical Journal, 2001Co-Authors: Carlos Allende Prieto, Paul Barklem, Martin Asplund, Basilio Ruiz CoboAbstract:Spectra of late-type stars are usually analyzed with Static Model atmospheres in local thermodynamic equilibrium (LTE) and a homogeneous plane-parallel or spherically symmetric geometry. The energy balance requires particular attention, as two elements that are particularly difficult to Model play an important role: line blanketing and convection. Inversion techniques are able to bypass the difficulties of a detailed description of the energy balance. Assuming that the atmosphere is in hydroStatic equilibrium and LTE, it is possible to constrain its structure from spectroscopic observations. Among the most serious approximations still implicit in the method is a Static and homogeneous geometry. In this paper, we take advantage of a realistic three-dimensional radiative hydrodynamical simulation of the solar surface to check the systematic errors incurred by an inversion assuming a plane-parallel horizontally-homogeneous atmosphere. The thermal structure recovered resembles the spatial and time average of the three-dimensional atmosphere. Furthermore, the abundances retrieved are typically within 10% (0.04 dex) of the abundances used to construct the simulation. The application to a fairly complete data set from the solar spectrum provides further confidence in previous analyses of the solar composition. There is only a narrow range of one-dimensional thermal structures able to fit the absorption lines in the spectrum of the Sun. With our carefully selected data set, random errors are about a factor of 2 smaller than systematic errors. A small number of strong metal lines can provide very reliable results. We foresee no major difficulties in applying the technique to other similar stars, and obtaining similar accuracies, using spectra with λ/δλ ~ 5 × 104 and a signal-to-noise ratio as low as 30.
Michael Posa - One of the best experts on this subject based on the ideXlab platform.
-
WAFR - A Quasi-Static Model and Simulation Approach for Pushing, Grasping, and Jamming
Springer Proceedings in Advanced Robotics, 2020Co-Authors: Mathew Halm, Michael PosaAbstract:Quasi-Static Models of robotic motion with frictional contact provide a computationally efficient framework for analysis and have been widely used for planning and control of non-prehensile manipulation. In this work, we present a novel quasi-Static Model of planar manipulation that directly maps commanded manipulator velocities to object motion. While quasi-Static Models have traditionally been unable to capture grasping and jamming behaviors, our approach solves this issue by explicitly Modeling the limiting behavior of a velocity-controlled manipulator. We retain the precise Modeling of surface contact pressure distributions and efficient computation of contact-rich behaviors of previous methods and additionally prove existence of solutions for any desired manipulator motion. We derive continuous and time-stepping formulations, both posed as tractable Linear Complementarity Problems (LCPs).
-
A Quasi-Static Model and Simulation Approach for Pushing, Grasping, and Jamming
arXiv: Robotics, 2019Co-Authors: Mathew Halm, Michael PosaAbstract:Quasi-Static Models of robotic motion with frictional contact provide a computationally efficient framework for analysis and have been widely used for planning and control of non-prehensile manipulation. In this work, we present a novel quasi-Static Model of planar manipulation that directly maps commanded manipulator velocities to object motion. While quasi-Static Models have traditionally been unable to capture grasping and jamming behaviors, our approach solves this issue by explicitly Modeling the limiting behavior of a velocity-controlled manipulator. We retain the precise Modeling of surface contact pressure distributions and efficient computation of contact-rich behaviors of previous methods and additionally prove existence of solutions for any desired manipulator motion. We derive continuous and time-stepping formulations, both posed as tractable Linear Complementarity Problems (LCPs).
Jaydev P. Desai - One of the best experts on this subject based on the ideXlab platform.
-
Modeling and Control of a 2-DoF Meso-Scale Continuum Robotic Tool for Pediatric Neurosurgery
IEEE Transactions on Robotics, 2020Co-Authors: Yash Chitalia, Joshua J. Chern, Seokhwan Jeong, Kent K. Yamamoto, Jaydev P. DesaiAbstract:This article introduces the analysis and control of a meso-scale two degree-of-freedom robotic endoscopic tool body for minimally invasive surgeries. The design of the robotic tool uses two types of a tendon-driven joint known as a bending flexure joint that allows us to control each degree-of-freedom by minimizing interjoint coupling by design. Pure kinematic Modeling and control for these robots may not provide precise control performance due to kinematic uncertainties arising from tendon elongation, tendon slacking, gear backlash, etc. We propose a Static Model for each of the joints of the robotic tool that avoids several of these problems. Depending on the direction of tendon tension application, the proximal joint displays considerable hysteresis due to the superelastic material characteristics and this is included in our Static Model. The Statics of a highly compliant distal joint is also Modeled and validated using finite element analysis and experimental data. Using these Models, we develop a control system that comprises of a disturbance observer and the proposed Static Model to provide precise force control and compensate for joint hysteresis.
Ulla K. Griffiths - One of the best experts on this subject based on the ideXlab platform.
-
Inclusion of the value of herd immunity in economic evaluations of vaccines. A systematic review of methods used.
Vaccine, 2017Co-Authors: Liv S. Nymark, Tarang Sharma, Alexander Miller, Ulrika Enemark, Ulla K. GriffithsAbstract:Abstract Objective The objectives of this review were to identify vaccine economic evaluations that include herd immunity and describe the methodological approaches used. Methods We used Kim and Goldie’s search strategy from a systematic review (1976–2007) of Modelling approaches used in vaccine economic evaluations and additionally searched PubMed/MEDLINE and Embase for 2007–2015. Studies were classified according to Modelling approach used. Methods for estimating herd immunity effects were described, in particular for the Static Models. Results We identified 625 economic evaluations of vaccines against human-transmissible diseases from 1976 to 2015. Of these, 172 (28%) included herd immunity. While 4% of studies included herd immunity in 2001, 53% of those published in 2015 did this. Pneumococcal, human papilloma and rotavirus vaccines represented the majority of studies (63%) considering herd immunity. Ninety-five of the 172 studies utilised a Static Model, 59 applied a dynamic Model, eight a hybrid Model and ten did not clearly state which method was used. Relatively crude methods and assumptions were used in the majority of the Static Model studies. Conclusion The proportion of economic evaluations using a dynamic Model has increased in recent years. However, 55% of the included studies used a Static Model for estimating herd immunity. Values from a Static Model can only be considered reliable if high quality surveillance data are incorporated into the analysis. Without this, the results are questionable and they should only be included in sensitivity analysis.
