Continuous Stereo Self-Calibration by Camera Parameter Tracking

This paper presents a consistent framework for continuous stereo self-calibration. Based on a practical analysis of the sensitivity of stereo reconstruction to camera calibration uncertainties, we identify important parameters for self-calibration. We evaluate different geometric constraints for estimation and tracking of these parameters: bundle adjustment with reduced structure representation relating corresponding points in image sequences, the epipolar constraint between stereo image pairs, and trilinear constraints between image triplets. Continuous, recursive calibration refinement is obtained with a robust, adapted iterated extended Kalman filter. To achieve high accuracy, physically relevant geometric optimization criteria are formulated in a Gauss-Helmert type model. The self-calibration framework is tested on an active stereo system. Experiments with synthetic data as well as on natural indoor and outdoor imagery indicate that the different constraints are complementing each other and thus a method combining two of the above constraints is proposed: While reduced order bundle adjustment gives by far the most accurate results (and might suffice on its own in some environments), the epipolar constraint yields instantaneous calibration that is not affected by independently moving objects in the scene. Hence, it expedites and stabilizes the calibration process.     

Intergranular crack tip oxidation in a Ni-base superalloy

High-temperature intergranular crack tip oxidation under a single 600 s long sustained tensile load at 700 °C was studied for the Ni-base superalloy Allvac 718Plus. High-resolution analytical techniques showed oxidation to take place at and immediately ahead of the tip of an open crack, forming a closed but layered oxide structure about the prior (now oxidized) grain boundary. Near the prior grain boundary the oxide is Ni-rich, with a Co-enriched layer furthest away from the metal and a Fe-enriched region below this. A Cr-rich oxide is present below the outer Ni-rich oxides throughout the crack, also in the direction of crack growth. This is believed to have a hindering effect on oxidation ahead of the crack. Ni₃(Nb,Al) γ′ precipitates close to the grain boundaries were found to oxidize and form regions of near-stoichiometric NiO within the oxide layers. Remaining constituents of γ′ (e.g. Al and Nb) were found to be enriched in the surrounding oxidized matrix and also to produce thin oxide layers near the interface between the unoxidized metal and the Cr-rich oxide. The formation of the crack tip oxides is discussed with regard to thermodynamics, kinetics and the influence of applied mechanical load.     

Application of neutron radiography to study material processes during hypothetical severe accidents in nuclear reactors

Neutron radiography was applied for investigations of nuclear fuel cladding and control rod behaviour during steam oxidation at temperatures between 1123 and 1673?K under severe nuclear accident conditions. This article gives an overview of these investigations. At KIT, loss of coolant and severe nuclear accidents were experimentally simulated. Post-test examinations of damaged control rods were performed. In order to separate the effect of the neutron absorber and control rod structure materials, radiographs taken with different neutron spectra were combined. It could be seen clearly from these data that local melting resulting from the eutectic reaction between the stainless steel control rod cladding and the Zircaloy-4 guide tube. The uptake of hydrogen during steam oxidation and its diffusion in Zircaloy-4 was investigated in ex situ and in situ radiography experiments at temperatures of 1123 to 1673?K. The kinetics of hydrogen uptake and diffusion was determined. The oxide layer morphology strongly influences the hydrogen concentration in steam oxidized zirconium alloys. Differences of nearly one order of magnitude were found in samples withdrawn from large scale QUENCH experiments. The hydrogen diffusion coefficients were determined for various temperatures. Whereas the diffusion coefficients at 1123 and 1173?K agree well with values expected from literature values for pure Zr, at higher temperatures a faster diffusion was found. The determined activation energy of the hydrogen diffusion is about 10?% higher than published values in the literature.     

Computation of a compact state space model for an adaptive spindle head configuration with piezo actuators using balanced truncation

Finite element models of machine tools or their building blocks are usually very large and thus do not allow for fast simulation or application in controller design. Especially when algebraic constraints come into play the models become differential algebraic equations and therefore are even more difficult to handle in the application. In this contribution we propose a method based on modern system theoretic model order reduction algorithms that allows to generate a first order standard state space reduced order model for a structural model of an adaptive spindle support that is of second order index 1 differential algebraic form due to the piezo actuation applied. The accuracy of the method is demonstrated by a numerical frequency domain error analysis.     

Atom probe tomography of Ni-base superalloys Allvac 718Plus and Alloy 718

Atom probe tomography (APT) allows near atomic scale compositional- and morphological studies of, e.g. matrix, precipitates and interfaces in a wide range of materials. In this work two Ni-base superalloys with similar compositions, Alloy 718 and its derivative Allvac 718Plus, are subject for investigation with special emphasis on the latter alloy. The structural and chemical nuances of these alloys are important for their properties. Of special interest are grain boundaries as their structure and chemistry are important for the materials' ability to resist rapid environmentally induced crack propagation. APT has proved to be suitable for analyses of these types of alloys using voltage pulsed APT. However, for investigations of specimens containing grain boundaries and other interfaces the risk for early specimen fracture is high. Analyses using laser pulsing impose lower electrical field on the specimen thereby significantly increasing the success rate of investigations. Here, the effect of laser pulsing was studied and the derived appropriate acquisition parameters were then applied for microstructural studies, from which initial results are shown. Furthermore, the influence of the higher evaporation field experienced by the hardening γ′ Ni₃(Al,Nb) precipitates on the obtained results is discussed.     

Dopamine and Cognitive Control: The Influence of Spontaneous Eyeblink Rate and Dopamine Gene Polymorphisms on Perseveration and Distractibility.

One fundamental problem of intelligent organisms pursuing goal-directed behavior is how to dynamically regulate the balance between maintenance and flexibility. The authors show that central dopaminergic activity, as indicated by spontaneous eyeblink rate and dopamine gene polymorphisms, plays an important role in the modulation of this balance. Seventy-two young adults were examined. Participants with high blink rates showed increased cognitive flexibility but decreased cognitive stability compared with participants with low blink rates. This pattern of results was even more pronounced for carriers of the DRD4 exon III 4/7 genotype, even though no main effects were found for DRD4 and COMT polymorphisms. Results converge with neuropsychological models that suggest a modulatory role of prefrontal dopaminergic activity for processes of cognitive control. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Atom probe field ion microscope measurements of carbon segregation at an α:α grain boundary and service failures by intergranular stress corrosion cracking

This work reports on a critical measurement to understand the intergranular stress corrosion cracking (IGSCC) of pipeline steels: the atom probe field ion microscope (APFIM) measurement of the carbon concentration at a grain boundary (GB). The APFIM measurement was related to the microstructure and to IGSCC observations. The APFIM indicated that the GB carbon concentration of X70 was ∼10 at% or less, which correlated with a high resistance to IGSCC for X70.