320 research outputs found
Simulation, modelling and development of the metris RCA
In partnership with Metris UK we discuss the utilisation of modelling and simulation methods in the development of a revolutionary 7-axis Robot CMM Arm (RCA). An offline virtual model is described, facilitating pre-emptive collision avoidance and assessment of optimal placement of the RCA relative to scan specimens. Workspace accessibility of the RCA is examined under a range of geometrical assumptions and we discuss the effects of arbitrary offsets resulting from manufacturing tolerances. Degeneracy is identified in the number of ways a given pose may be attained and it is demonstrated how a simplified model may be exploited to solve the inverse kinematics problem of finding the âcorrectâ set of joint angles. We demonstrate how the seventh axis may be utilised to avoid obstacles or otherwise awkward poses, giving the unit greater dexterity than traditional CMMs. The results of finite element analysis and static force modelling on the RCA are presented which provide an estimate of the forces exerted on the internal measurement arm in a range of poses
Characteristics, accuracy and reverification of robotised articulated arm CMMs
VDI article 2617 specifies characteristics to describe the accuracy of articulated arm coordinate measuring machines (AACMMs) and outlines procedures for checking them. However the VDI prescription was written with a former generation of machines in mind: manual arms exploiting traditional touch probe technologies. Recent advances in metrology have given rise to noncontact laser scanning tools and robotic automation of articulated arms â technologies which are not adequately characterised using the VDI specification. In this paper we examine the âguidelinesâ presented in VDI 2617, finding many of them to be ambiguous and open to interpretation, with some tests appearing even to be optional. The engineer is left significant flexibility in the execution of the test procedures and the manufacturer is free to specify many of the test parameters. Such flexibility renders the VDI tests of limited value and the results can be misleading. We illustrate, with examples using the Nikon RCA, how a liberal interpretation of the VDI guidelines can significantly improve accuracy characterisation and suggest ways in which to mitigate this problem.
We propose a series of stringent tests and revised definitions, in the same vein as VDI 2617 and similar US standards, to clarify the accuracy characterisation process. The revised methodology includes modified acceptance and reverification tests which aim to accommodate emerging technologies, laser scanning devices in particular, while maintaining the spirit of the existing and established standards. We seek to supply robust re-definitions for the accepted terms âzero pointâ and âuseful arm lengthâ, pre-supposing nothing about the geometry of the measuring device.
We also identify a source of error unique to robotised AACMMs employing laser scanners â the forward-reverse pass error. We show how eliminating this error significantly improves the repeatability of a device and propose a novel approach to the testing of probing error based on statistical uncertainty
Adequacy of Maternal Iron Status Protects against Behavioral, Neuroanatomical, and Growth Deficits in Fetal Alcohol Spectrum Disorders
Fetal alcohol spectrum disorders (FASD) are the leading non-genetic cause of neurodevelopmental disability in children. Although alcohol is clearly teratogenic, environmental factors such as gravidity and socioeconomic status significantly modify individual FASD risk despite equivalent alcohol intake. An explanation for this variability could inform FASD prevention. Here we show that the most common nutritional deficiency of pregnancy, iron deficiency without anemia (ID), is a potent and synergistic modifier of FASD risk. Using an established rat model of third trimester-equivalent binge drinking, we show that ID significantly interacts with alcohol to impair postnatal somatic growth, associative learning, and white matter formation, as compared with either insult separately. For the associative learning and myelination deficits, the ID-alcohol interaction was synergistic and the deficits persisted even after the offspringsĂąâŹâą iron status had normalized. Importantly, the observed deficits in the ID-alcohol animals comprise key diagnostic criteria of FASD. Other neurobehaviors were normal, showing the ID-alcohol interaction was selective and did not reflect a generalized malnutrition. Importantly ID worsened FASD outcome even though the mothers lacked overt anemia; thus diagnostics that emphasize hematological markers will not identify pregnancies at-risk. This is the first direct demonstration that, as suggested by clinical studies, maternal iron status has a unique influence upon FASD outcome. While alcohol is unquestionably teratogenic, this ID-alcohol interaction likely represents a significant portion of FASD diagnoses because ID is more common in alcohol-abusing pregnancies than generally appreciated. Iron status may also underlie the associations between FASD and parity or socioeconomic status. We propose that increased attention to normalizing maternal iron status will substantially improve FASD outcome, even if maternal alcohol abuse continues. These findings offer novel insights into how alcohol damages the developing brain
The First IRAM/PdBI Polarimetric Millimeter Survey of Active Galactic Nuclei. I. Global Properties of the Sample
We have studied the linear polarization of 86 active galactic nuclei (AGN) in
the observed frequency range 80-267 GHz (3.7-1.1mm in wavelength),
corresponding to rest-frame frequencies 82-738 GHz, with the IRAM Plateau de
Bure Interferometer (PdBI). The large number of measurements, 441, makes our
analysis the largest polarimetric AGN survey in this frequency range to date.
We extracted polarization parameters via earth rotation polarimetry with
unprecedented median precisions of ~0.1% in polarization fractions and ~1.2
degrees in polarization angles. For 73 of 86 sources we detect polarization at
least once. The degrees of polarization are as high as ~19%, with the median
over all sources being ~4%. Source fluxes and polarizations are typically
highly variable, with fractional variabilities up to ~60%. We find that BLLac
sources have on average the highest level of polarization. There appears to be
no correlation between degree of polarization and redshift, indicating that
there has been no substantial change of polarization properties since z~2.4.
Our polarization and spectral index distributions are in good agreement with
results found from various samples observed at cm/radio wavelengths; thus our
frequency range is likely tracing the signature of synchrotron radiation
without noticeable contributions from other emission mechanisms. The
"millimeter-break" located at frequencies >1 THz appears to be not detectable
in the frequency range covered by our survey.Comment: 19 pages, 9 figures, 2 long tables (p. 12-19). Accepted by A&A
Resonant Kelvin-Helmholtz modes in sheared relativistic flows
Qualitatively new aspects of the (linear and non-linear) stability of sheared
relativistic (slab) jets are analyzed. The linear problem has been solved for a
wide range of jet models well inside the ultrarelativistic domain (flow Lorentz
factors up to 20; specific internal energies ). As a distinct
feature of our work, we have combined the analytical linear approach with
high-resolution relativistic hydrodynamical simulations, which has allowed us
i) to identify, in the linear regime, resonant modes specific to the
relativistic shear layer ii) to confirm the result of the linear analysis with
numerical simulations and, iii) more interestingly, to follow the instability
development through the non-linear regime. We find that very high-order
reflection modes with dominant growth rates can modify the global, long-term
stability of the relativistic flow. We discuss the dependence of these resonant
modes on the jet flow Lorentz factor and specific internal energy, and on the
shear layer thickness. The results could have potential applications in the
field of extragalactic relativistic jets.Comment: Accepted for publication in Physical Review E. For better quality
images, please check
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Review Article: Rethinking Holder
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68947/2/10.1177_096394709800700104.pd
Introduction: the place of theory today
Zweitveröffentlichun
Non-gray rotating stellar models and the evolutionary history of the Orion Nebular Cluster
Rotational evolution in the pre-main sequence (PMS) is described with new
sets of PMS evolutionary tracks including rotation, non-gray boundary
conditions (BCs) and either low (LCE) or high convection efficiency (HCE).
Using observational data and our theoretical predictions, we aim at
constraining 1) the differences obtained for the rotational evolution of stars
within the ONC by means of these different sets of models; 2) the initial
angular momentum of low mass stars, by means of their templates in the ONC. We
discuss the reliability of current stellar models for the PMS. While the 2D
radiation hydrodynamic simulations predict HCE in PMS, semi-empirical
calibrations either seem to require that convection is less efficient in PMS
than in the following MS phase or are still contradictory. We derive stellar
masses and ages for the ONC by using both LCE and HCE. The resulting mass
distribution for the bulk of the ONC population is in the range 0.20.3
{\msun} for our non-gray models and in the range 0.10.3{\msun} for models
having gray BCs. In agreement with Herbst et al. (2002) we find that a large
percentage (70%) of low-mass stars (M\simlt 0.5{\msun} for LCE;
M\simlt0.35{\msun} for HCE) in the ONC appears to be fast rotators (P4days).
Three possibilities are open: 1) 70% of the ONC low mass stars lose their
disk at early evolutionary phases; 2)their locking period is shorter; 3) the
period evolution is linked to a different morphology of the magnetic fields of
the two groups of stars. We also estimate the range of initial angular momentum
consistent with the observed periods. The comparisons made indicate that a
second parameter is needed to describe convection in the PMS, possibly related
to the structural effect of a dynamo magnetic field.Comment: 17 pages, 11 figure
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