How to calculate symmetries of Petri nets

Symmetric net structure yields symmetric net behaviour. Thus, knowing the symmetries of a net, redundant calculations can be skipped. We present a framework for the calculation of symmetries for several net classes including place/transition nets, timed nets, stochastic nets, self–modifying nets, nets with inhibitor arcs, and many others. Our approach allows the specification of different symmetry groups. Additionally it provides facilities either to calculate symmetries on demand while running the actual analysis algorithm, or to calculate them in advance. For the latter case we define and calculate a ground set of symmetries. Such a set has polynomial size and is sufficient for an efficient implementation of the for all symmetries loop and the partition of net elements into equivalence classes. These two constructions are the usual way to integrate symmetries into an analysis algorithm. Received 7 July 1997 / 10 August 1999     

Advanced networking services for distributed multimedia streaming applications

Increased network speeds coupled with new services delivered via the Internet have increased the demand for intelligence and flexibility in network systems. This paper argues that both can be provided by new hardware platforms comprised of heterogeneous multi-core systems with specialized communication support. We present and evaluate an experimental network service platform that uses an emergent class of devices—network processors—as its communication support, coupled via a dedicated interconnect to a host processor acting as a computational core. A software infrastructure spanning both enables the dynamic creation of application-specific services on the network processor, mediated by middleware and controlled by kernel-level communication support. Experimental evaluations use a Pentium IV-based computational core coupled with an IXP 2400 network processor. The sample application services run on both include an image manipulation application and application-level multicasting.

Experimental setup for camera-based measurements of electrically and optically stimulated luminescence of silicon solar cells and wafers

We report in detail on the luminescence imaging setup developed within the last years in our laboratory. In this setup, the luminescence emission of silicon solar cells or silicon wafers is analyzed quantitatively. Charge carriers are excited electrically (electroluminescence) using a power supply for carrier injection or optically (photoluminescence) using a laser as illumination source. The luminescence emission arising from the radiative recombination of the stimulated charge carriers is measured spatially resolved using a camera. We give details of the various components including cameras, optical filters for electro- and photo-luminescence, the semiconductor laser and the four-quadrant power supply. We compare a silicon charged-coupled device (CCD) camera with a back-illuminated silicon CCD camera comprising an electron multiplier gain and a complementary metal oxide semiconductor indium gallium arsenide camera. For the detection of the luminescence emission of silicon we analyze the dominant noise sources along with the signal-to-noise ratio of all three cameras at different operation conditions.     

Performance of genetic algorithms in search for water splitting perovskites

We examine the performance of genetic algorithms (GAs) in uncovering solar water light splitters over a space of almost 19,000 perovskite materials. The entire search space was previously calculated using density functional theory to determine solutions that fulfill constraints on stability, band gap, and band edge position. Here, we test over 2500 unique GA implementations in finding these solutions to determine whether GA can avoid the need for brute force search, and thereby enable larger chemical spaces to be screened within a given computational budget. We find that the best GAs tested offer almost a 6 times efficiency gain over random search, and are comparable to the performance of a search based on informed chemical rules. In addition, the GA is almost 10 times as efficient as random search in finding half the solutions within the search space. By employing chemical rules, the performance of the GA can be further improved to approximately 12–17 better than random search. We discuss the effect of population size, selection function, crossover function, mutation rate, fitness function, and elitism on the final result, finding that selection function and elitism are especially important to GA performance. In addition, we determine that parameters that perform well in finding solar water splitters can also be applied to discovering transparent photocorrosion shields. Our results indicate that coupling GAs to high-throughput density functional calculations presents a promising method to rapidly search large chemical spaces for technological materials.     

Age‐Related Physiological Responses to Working in Deep Cold

Professional order‐picking in deep cold‐storage depots (i.e., storing, picking, and dispatching of frozen groceries) requires employees to remain working at temperatures of approximately ?24°C for a whole workday. Actually, a well‐regulated organization of working times and breaks has not been established, and no competent knowledge exists as to whether an additional age‐differentiated organization is necessary. To assess the physiological effects while working in deep cold, 30 male subjects (Ss) were classified into two age groups (20‐ to 35‐year‐olds and 40‐ to 65‐year‐olds). In whole workday tests, possible age‐dependent effects on the strain were measured to guarantee the preservation of the subjects' ability to work in the long run. For the objectification of the physiological strain, heart rate and skin surface temperature were registered continuously, and blood pressure and body core temperature were measured and recorded discretely during cold exposures of 80, 100, and 120 min, separated by 20‐min warming‐up breaks. Systematic differences of blood pressure could not be found. The heart rate values indicated a high physiological strain for both younger and older Ss, with work‐related increases above the resting level of 30 bpm and more. Due to increases over time, endurance level sometimes was exceeded. Age‐related differences in skin temperature could not be recorded, but the ability to generate heat deteriorated with advancing age, which is shown by more substantial decreases of body core temperature in the group of 40‐ to 65‐year‐olds taken at the tympanum. Regarding physiological strain brought about by maximum heart rate decreasing with age and declined heat generation, correspondingly adapted workday break regimes have to be provided for older employees to ensure their ability to work in the long run. © 2011 Wiley Periodicals, Inc.     

Towards carbon-free flame spray synthesis of homogeneous oxide nanoparticles from aqueous solutions

Flame-assisted spray pyrolysis (FASP) is a versatile process for synthesis of nanoparticles from a broad choice of precursors and solvents. Water is an attractive solvent particularly for inexpensive inorganic precursors (e.g. metal nitrates) as it can effectively reduce the process cost. Furthermore when water usage is combined with a carbon-free fuel (e.g. H₂), nanoparticles can be made without forming CO₂. Here such a FASP process is explored for synthesis of Bi₂O₃ and other oxide nanoparticles from aqueous precursor solutions. The flame temperature was measured by FTIR emission–transmission spectroscopy while powders were characterized by X-ray diffraction and N₂ adsorption. At low FASP fuel gas (H₂ or C₂H₂) flow rates or process temperatures, product powders had a bimodal crystal size distribution. Its large and small modes were made by droplet- and gas-to-particle conversion, respectively. Homogeneous Bi₂O₃ and CeO₂ powders were obtained for sufficiently high flow rates of either C₂H₂ or H₂. Prolonged high temperature residence times promoted precursor evaporation from the spray droplets and yielded homogeneous nanostructured powders by gas-to-particle conversion. In contrast, FASP of aqueous solutions of aluminum nitrate yielded rather large particles by droplet-to-particle conversion at all fuel flows investigated.     

Einlaufverhalten nasslaufender Lamellenkupplungen im Schlupfbetrieb

Forschung im Ingenieurwesen - Nasslaufende Lamellenkupplungen weisen zu Betriebsbeginn eine Phase des Einlaufens auf, in der sich das Reibungsverhalten durch Anpassung der Reibflächen...