高速数字直接电流控制器——一类新的满足未来需要的智能控制器

由于电力电子驱动应用要求不断增加,并越来越重要。为满足未来需求,电力电子控制算法也应该在减少纹波电流和增强鲁棒性的同时,能够快速处理非线性和未知负载。逆变控制有两种基本原理-直接电流控制和间接电流控制。迄今为止,间接电流控制器(例如通过PWM或空间矢量调制)与直接电流调制比较,应用更广泛。然而,直接电流控制器的快速性、鲁棒性能够很好的适应于处理未知甚至是非线性的负载,因而,似乎更适应于未来的需要。但是其中的一个明显不足在于它是用模拟技术来实现的。漂移、温度的影响等非常明显。本文提出了一种新的高速全数字直接电流控制器。该控制器在10M Hz下工作。能够满足未来的需要,并能够处理非线性负载,克服模拟直流控制器的不足。     

Monitoring a reaction at submillisecond resolution in picoliter volumes

Well-established rapid mixing techniques such as stopped-flow have been used to push the dead time for kinetic experiments down to a few milliseconds. However, very fast reactions are difficult to resolve below this limit. We now outline an approach that provides access to ultrafast kinetics but does not rely on active mixing at all. Here, the reagents are compartmentalized into water-in-oil emulsion microdroplets (diameter ～50 μm) that are statically arrayed in pairs, resting side-by-side in a well feature of a poly(dimethylsiloxane) (PDMS) device. A reaction between the contents of two droplets arrayed in such a holding trap is initiated by droplet fusion that is brought about by electrocoalescence and known to occur on a time scale of about 100 μs. A reaction between the reactants (Fe(3+) and SCN(-)) is monitored by image analysis measuring the product formation in the newly merged drop in both space and time, by use of a fast camera. A comparison of the concentration field of the reaction product with the output of a reaction-diffusion system of equations yields a rate constant k ～ 3 × 10(4) M(-3)·s(-1). Since reaction and diffusion are formally included in the mathematical model, measurements can proceed immediately after the drop fusion, removing the need to allow time for mixing. This approach is different from existing methodologies, for example, experiments in a conventional stopped-flow apparatus but also electrofusion of moving droplets where contents are mixed by chaotic advection before a reaction is monitored. Our analysis makes kinetics accessible that are several times faster than techniques that have to allow time for mixing.     

Vorgänge im Walzspalt beim Walzen in Streckkalibern

Untersuchung des Formänderungsverlaufes in der Umformzone gerasterter Schichtproben aus Stahl St 37, die im Walzspalt abgebremst wurden. Umformung in zwei vergleichbaren Kalibern mit Raute- und Ovalform. Betrachtung der örtlichen Formänderungen in kennzeichnenden Schnittebenen.     

Acceptance sampling plans for photovoltaic modules with two‐sided specification limits

The quality control of photovoltaic modules in terms of the output power to satisfy the technical specification is of great importance for producers as well as consumers and also represents a major issue of certification procedures. Previous work focused on one‐sided specification limits to reject underperforming samples (lots) of photovoltaic modules or solar cells. In the present paper, we generalize the classic acceptance sampling methodology and derive sampling plans on the basis of two‐sided specification limits. Those sampling plans can be constructed for arbitrary output power distributions by making use of flash data tables. For the out‐of‐spec setting, the sampling plans are solutions of rather involved nonlinear equations. Explicit formulas, which resemble known sampling plans, can only be obtained under symmetry assumptions. Further, the solution depends on the ratio of overperforming modules to underperforming modules. We investigate by numerical studies to which extent the required sample size depends on that ratio and the shape of the underlying output power distribution. The application to real examples indicates that in practice, the new approach often results in substantially smaller control samples than classic approaches. Copyright © 2012 John Wiley & Sons, Ltd.     

Reduction of energy requirements in distillation

Distillation has been, up to now, the standard method for separating fluid mixtures in the process industry and the consensus is that it will remain so for years to come. The only disadvantage of distillation is its high energy requirement. In the following, it will be demonstrated on the example of an ideal ternary mixture, how the energy requirement of distillation process can be reduced. Very effective are some modifications of single columns, optimization of column sequences and direct or indirect column coupling. It will be shown that, just by column coupling, the energy requirement can be reduced to a third.     

Development of quantitative cell-based enzyme assays in microdroplets

We describe the development of an enzyme assay inside picoliter microdroplets. The enzyme alkaline phosphatase is expressed in Escherichia coli cells and presented in the periplasm. Droplets act as discrete reactors which retain and localize any reaction product. The catalytic turnover of the substrate is measured in individual droplets by monitoring the fluorescence at several time points within the device and exhibits kinetic behavior similar to that observed in bulk solution. Studies on wild type and a mutant enzyme successfully demonstrated the feasibility of using microfluidic droplets to provide time-resolved kinetic measurements.     

Qualifications in the German construction industry: stocks,flows and comparisons with the British construction sector

The paper provides an overview of qualifications in the German construction workforce and draws comparisons with the situation in the British construction industry. The German system of general education and vocational training is outlined. Data from a variety of sources are used to describe both the stocks and the flows of qualifications in the German construction workforce. Particular attention is paid to vocational qualifications, such as apprenticeships and the Meister qualification. Key findings are that the majority of German construction workers are qualified up to apprenticeship level, and that most of these employees have achieved a general schooling certificate from at least a lower secondary school prior to their vocational training. These factors are taken as signs of strength of the German construction workforce. Comparisons between the stocks of qualifications in the construction industry in the two countries suggest that intermediate qualifications of the apprenticeship type are far less prevalent in the British construction workforce.     

A model for perceptual averaging and stochastic bistable behavior and the role of voluntary control

We combine population coding, winner-take-all competition, and differentiated inhibitory feedback to model the process by which information from different, continuously variable signals is integrated for perceptual awareness. We focus on "slant rivalry," where binocular disparity is in conflict with monocular perspective in specifying surface slant. Using a robust single parameter set, our model successfully replicates three key experimental results: (1) transition from signal averaging to bistability with increasing signal conflict, (2) change in perceptual reversal rates as a function of signal conflict, and (3) a shift in the distribution of percept durations through voluntary control exertion. Voluntary control is implemented through the use of a single top-down bias input. The transition from signal averaging to bistability arises as a natural consequence of combining population coding and wide receptive fields, common to higher cortical areas. The model architecture does not contain any assumption that would limit it to this particular example of stimulus rivalry. An emergent physiological interpretation is that differentiated inhibitory feedback may play an important role for increasing percept stability without reducing sensitivity to large stimulus changes, which for bistable conditions leads to increased alternation rate as a function of signal conflict.     

Knowledge construction in an outsider community: Extending the communities of practice concept

We present an empirical analysis of a web forum in which followers of a health-related community exchange information and opinions in order to pass on and develop relevant knowledge. We analyzed how knowledge construction takes place in such a community that represents an outsider position which is not accepted by majority society. For this purpose we applied the Community of Practice (CoP) concept as a guideline for our analysis and found that many well-known activities of CoPs were true of the Urkost community. The social network analysis findings also supported interpreting this community as a CoP. But we found as well that this community had a variety of structural characteristics that the CoP literature deals with insufficiently. We identified the structure of goals, roles, and communication as relevant features that are typical of this outsider CoP. For example, the attitude of the core members towards people of a ‘different faith’ was characterized by strong hostility and rejection. These features provided an effective basis for the development and maintenance of a shared identity in the community. Our findings are discussed against the background of the necessity for further development of the CoP concept.     

Control strategies on stable manifolds for energy-efficient swing-ups of double pendula

Optimal control problems for mechanical systems often arise in technical applications. To find solutions with minimal control effort, the system’s natural, uncontrolled dynamics can be used. Promising candidates to be considered for energy-efficient trajectories are highly dynamic, but uncontrolled motions on (un)stable manifolds of equilibria. In this contribution, we propose a control strategy for mechanical systems which sequences uncontrolled trajectories on (un)stable manifolds with short control manoeuvres to design a feedforward control. In particular, we present optimal swing-up solutions for a double pendulum which are based on trajectories on the stable manifold of the pendulum’s up–up equilibrium. To demonstrate the advantages of our approach compared to a black-box optimisation, we perform a post-optimisation with the optimal control sequence as an initial guess. The numerical results are evaluated in a simulation environment for the double pendulum on a cart and applied to a real test rig.