Time filter

Source Type

Schwen D.,University of Illinois at Urbana - Champaign | Bringa E.,National University of Cuyo | Krauser J.,Hochschule Harz University of Applied Sciences | Weidinger A.,Helmholtz Center Berlin | And 2 more authors.
Applied Physics Letters | Year: 2012

The formation of surface hillocks in diamond-like carbon is studied experimentally and by means of large-scale molecular dynamics simulations with 5 × 10 6 atoms combined with a thermal spike model. The irradiation experiments with swift heavy ions cover a large electronic stopping range between ∼12 and 72 keV/nm. Both experiments and simulations show that beyond a stopping power threshold, the hillock height increases linearly with the electronic stopping, and agree extremely well assuming an efficiency of approximately 20 in the transfer of electronic energy to the lattice. The simulations also show a transition of sp 3 to sp 2 bonding along the tracks with the hillocks containing almost no sp 3 contribution. © 2012 American Institute of Physics.


Joncic M.,Hochschule Harz University of Applied Sciences | Kruglov R.,Polymer Optical Fiber Application Center | Haupt M.,Hochschule Harz University of Applied Sciences | Caspary R.,TU Braunschweig | And 2 more authors.
IEEE Photonics Technology Letters | Year: 2014

Wavelength division multiplexing (WDM) is a promising solution for upgrading the capacity of polymer optical fiber (POF)-based telecommunication networks. In this letter, we report on the successful realization of a four-channel high-speed WDM transmission system for 1-mm diameter step-index POF (SI-POF). For combining the optical signals coming from 405-, 450-, 515-, and 639-nm laser diodes onto 1-mm SI-POF, a (4 × 1) coupler with low insertion loss (IL) has been developed. To spatially separate different wavelength channels, a four-channel demultiplexer with low IL (<5.7 dB) and high (non)adjacent channel isolation (>30 dB) has been realized in bulk optics. The 14.77-Gb/s data transmission based on the offline-processed discrete multitone modulation technique has been demonstrated over 50-m SI-POF at a bit-error rate of (10-3). © 2014 IEEE.


Stolzenburg F.,Hochschule Harz University of Applied Sciences
Spatial Cognition and Computation | Year: 2010

Three of the major problems in building autonomous mobile robots are localization, exploration, and navigation. This paper investigates how well different qualitative methods based on angle information, most of them originally invented for representation of spatial knowledge, are suited for addressing these problems. It combines results from discrete and computational geometry with methods from qualitative spatial reasoning, gaining some new insights on the complexity of robot navigation. It turns out that essentially only with panoramas (special roundviews) the qualitative localization problem can be solved in a satisfactory manner. The exploration problem (qualitative map building), remains difficult for all considered approaches. © Taylor & Francis Group, LLC.


Holl S.,Hochschule Harz University of Applied Sciences | Haupt M.,Hochschule Harz University of Applied Sciences | Fischer U.H.P.,Hochschule Harz University of Applied Sciences
Applied Optics | Year: 2013

Optical simulation software based on the ray-tracing method offers easy and fast results in imaging optics. This method can also be applied in other fields of light propagation. For short distance communications, polymer optical fibers (POFs) are gradually gaining importance. This kind of fiber offers a larger core diameter, e.g., the step index POF features a core diameter of 980 μm. Consequently, POFs have a large number of modes (>3 million modes) in the visible range, and ray tracing could be used to simulate the propagation of light. This simulation method is applicable not only for the fiber itself but also for the key components of a complete POF network, e.g., couplers or other key elements of the transmission line. In this paper a demultiplexer designed and developed by means of ray tracing is presented. Compared to the classical optical design, requirements for optimal design differ particularly with regard to minimizing the insertion loss (IL). The basis of the presented key element is a WDM device using a Rowland spectrometer setup. In this approach the input fiber carries multiple wavelengths, which will be divided into multiple output fibers that transmit only one wavelength. To adapt the basic setup to POF, the guidance of light in this element has to be changed fundamentally. Here, a monolithic approach is presented with a blazed grating using an aspheric mirror to minimize most of the aberrations. In the simulations the POF is represented by an area light source, while the grating is analyzed for different orders and the highest possible efficiency. In general, the element should be designed in a way that it can be produced with a mass production technology like injection molding in order to offer a reasonable price. However, designing the elements with regard to injection molding leads to some inherent challenges. The microstructure of an optical grating and the thick-walled 3D molded parts both result in high demands on the injection molding process. This also requires complex machining of the molding tool. Therefore, different experiments are done to optimize the process parameter, find the best molding material, and find a suitable machining method for the molding tool. The paper will describe the development of the demultiplexer by means of ray-tracing simulations step by step. Also, the process steps and the realized solutions for the injection molding are described. © 2013 Optical Society of America.


Mecke R.,Hochschule Harz University of Applied Sciences
2013 15th European Conference on Power Electronics and Applications, EPE 2013 | Year: 2013

The paper presents the energy saving potential of induction machines by using variable rotor flux instead of commonly used constant flux. The efficiency optimized rotor flux depends on the actual load and speed of the motor. At low load it has to be decreased with reference to a pre-calculated lookup table. This table can be included in a field oriented speed control structure as reference value of the rotor flux. The calculation of optimal rotor flux curves includes the non-linear main inductance, the frequency-dependent rotor resistance and rotor leakage inductance, the speed-dependent iron, friction and windage losses and the motor frequency. This distinguishes this paper from former scientific works. The calculations and measurements are carried out at an 11 kW high-efficiency (efficiency classification IE2) asynchronous machine with 2 pole pairs. An increase in efficiency can be reached below 2/3 of rated torque. The maximum increase is about 35 % in motor and 45 % in generator operation mode. The average efficiency improvement is 4,5 % in motor and 6,0 % in generator mode. Additionally, the power factor can be improved over the whole load range and the current consumption can be reduced. Energy saving with variable rotor flux is possible in the whole speed range from 10 to 100 Hz. © 2013 IEEE.


Mecke R.,Hochschule Harz University of Applied Sciences
Proceedings of the 2011 14th European Conference on Power Electronics and Applications, EPE 2011 | Year: 2011

Multilevel inverter topologies are an alternative for a wide range of low voltage electrical drives. In the low voltage range (<1000 V) highly optimised switches with lower conduction and switching losses are available. For 550 V DC link voltage the power semiconductor losses of a two-level VSI with 1200 V-IGBT and a neutral-point-clamped three-level VSI in the power range between 7 to 35 kW are compared. The comparison is carried out by calculation with a linear approximation for conduction and switching losses. The calculated results are proved by simulation with the real loss curves from the manufacturers. The simulated losses are higher than the calculated especially for operation points below the rated power. Therefore, the calculation of losses which is commonly used by the power semiconductor manufacturers is not sufficient for variable loads. At 20 kHz switching frequency and an electrical power of 35 kW the semiconductor losses of the three-level inverter are 38 % lower compared to the two-level inverter. At 7 kW electrical power the reduction of losses is even 66 %. The further increase of the inverter levels (four- and five-level) only leads to a small improvement of the motor current quality. Moreover, suitable power semiconductors (IGBT or MOSFET) with 300 V blocking voltage are not available for five-level inverters. Therefore, four- and five level inverters cannot be included in the losses calculation within this paper. Perhaps in the near future the five-level inverter leads to a further improvement in the inverter efficiency. © 2011 EPE Association - European Power Electr.


Mecke R.,Hochschule Harz University of Applied Sciences
2015 17th European Conference on Power Electronics and Applications, EPE-ECCE Europe 2015 | Year: 2015

Multilevel inverter topologies are an alternative for a wide range of low voltage electrical drives. They can reduce power semiconductor losses, voltage transients at the motor windings, harmonic losses in the cables and common mode disturbance currents. In the voltage range below 1000 V highly optimized switches with lower losses are available. The paper compares the component losses (power semiconductor losses in the frequency inverter, losses of the output filter, harmonic cable and motor losses) and the efficiency of a drive system with two-level and different multilevel NPC inverters. Higher inverter levels reduce the power losses of the inverter and the output filter. For a 22 kW induction motor a losses reduction of 800 W is simulated, if a five-level inverter is used. A five-level inverter requires power semiconductor devices with 300 V blocking voltage. New wide-bandgap semiconductors (SiC, GaN) with low blocking voltage have the potential to reduce the conduction and switching losses in multilevel inverters. This would lead to a reduction of cooling effort and to smaller inverter size. Moreover, the sine filter at the inverter output can be chosen smaller which is accompanied by lower power losses inside the filter. The application of multilevel inverters also in the so called low voltage range can make a contribution to efficiency improvement in electrical drives. © 2015 EPE Association and IEEE.


Spangenberg T.,Hochschule Harz University of Applied Sciences
Information Technology and Tourism | Year: 2014

GPS tracking is a common empirical research method to gain information about human mobility behavior. Based on the recorded GPS trajectories it is possible to analyze and explain motion spatially, and temporally. More comprehensive explanations of specific movement patterns require the coupling with other survey techniques such as trip diaries, interviews, and questionnaires, which are widespread data acquisition methods in tourism research. However, GPS-based surveys require a high effort in the preparation and post-processing of the data, and expertise in both, information technologies and tourism research. In order to prepare the data for the analysis in geographic information systems a number of steps with different tools is necessary. In this paper the GimToP Toolkit (GTK) is presented that integrates the methodological approach and the technology in an easy-to-handle manner. The GTK combines trajectory data with digital questionnaires based on a mobile application that is used for the data collection and preprocessing tasks. Connected to a server, which offers a service-oriented architecture, the GTK has the ability to process the survey data and to provide interoperability for the analysis in other applications. This paper presents the components of the GTK and shows its usage during two field studies, which are carried out in the city of Wernigerode and the Harz National Park in Germany. The results offer new insights in the movement behavior of the tourists and can be used to support a more sustainable development of the destinations. © 2014, Springer-Verlag Berlin Heidelberg.


Holl S.,Hochschule Harz University of Applied Sciences | Haupt M.,Hochschule Harz University of Applied Sciences | Fischer U.H.P.,Hochschule Harz University of Applied Sciences
Proceedings of 22nd International Conference on Plastical Optical Fibers, POF 2013 | Year: 2013

Today, data communication over Polymer Optical Fibers (POFs) is used in areas, in which the advantages of lightweight, cost-effective fibers with easy handling is required. The main applications include automotive communication systems and in-house-networks. Currently only one channel is used for data transmission, which limits the bandwidth. The only way to improve the data transmission rate is the use of more than one data channel. This method is called Wavelength Division Multiplexing (WDM) and it uses different wavelengths for data transmission over one POF. In order to establish a WDM communication link, two main components are required. At the front of the POF link a Multiplexer (MUX) bundles the light sources with different wavelengths to a single fiber. At the end of the transmission path the incoming light must be separated by a Demultiplexer (DEMUX) into the different wavelengths. In order to keep the advantage of cost-effective POFs, it is necessary to mass-produce the MUX and DEMUX component at reasonable prices. For polymers, injection molding is the only technology which achieves this goal. An optical grating on an aspheric mirror is used to separate the chromatic parts of the light in its monochromatic components. The manufacturing of such a micro structured grating leads to a complex machining of the molding tool. Because of the high numerical aperture (NA) of the POFs, it is necessary to design the elements in a 3D-approach. This results in thick-walled molded parts and poses challenges for the injection molding process. Therefore, different experiments are done to optimize the process parameter, find the best molding material, and to find a suitable machining method for this molding tool. In this paper, the process steps and also the realized solutions are described.


Strack H.,Hochschule Harz University of Applied Sciences
Lecture Notes in Informatics (LNI), Proceedings - Series of the Gesellschaft fur Informatik (GI) | Year: 2013

A eCampus security shell architecture was developed and deployed to improve the security of existing university management systems (legacy UMS), integrating innovative eGovernment Standards e.g. the German Electronic Identity Card (GeID), the eGovernment Protocol OSCI and qualified Signatures (QES).

Loading Hochschule Harz University of Applied Sciences collaborators
Loading Hochschule Harz University of Applied Sciences collaborators