On Probabilistic Parallel Programs with Process Creation and Synchronisation

We initiate the study of probabilistic parallel programs with dynamic process creation and synchronisation. To this end, we introduce probabilistic split-join systems (pSJSs), a model for parallel programs, generalising both probabilistic pushdown systems (a model for sequential probabilistic procedural programs which is equivalent to recursive Markov chains) and stochastic branching processes (a classical mathematical model with applications in various areas such as biology, physics, and language processing). Our pSJS model allows for a possibly recursive spawning of parallel processes; the spawned processes can synchronise and return values. We study the basic performance measures of pSJSs, especially the distribution and expectation of space, work and time. Our results extend and improve previously known results on the subsumed models. We also show how to do performance analysis in practice, and present two case studies illustrating the modelling power of pSJSs.Comment: This is a technical report accompanying a TACAS'11 pape

Determination of Temperature-Dependent Elastic Constants of Steel AISI 4140 by Use of In Situ X-ray Dilatometry Experiments

In situ dilatometry experiments using high energy synchrotron X-ray diffraction in transmission mode were carried out at the high energy material science beamline P07@PETRAIII atDESY (Deutsches Elektronen Synchrotron) for the tempering steel AISI 4140 at defined mechanical loading. The focus of this study was on the initial tempering state ( f errite) and the hardened state (martensite). Lattice strains were calculated from the 2D diffraction data for different hkl planes and from those temperature-dependent lattice plane specific diffraction elastic constants (DECs) were determined. The resulting coupling terms allow for precise stress analysis for typical hypoeutectoid steels using diffraction data during heat treatment processes, that is, for in situ diffraction studies during thermal exposure. In addition, by averaging hkl specific Young's moduli and Poisson ratios macroscopic temperature-dependent elastic constants were determined. In conclusion a novel approach for the determination of phase-specific temperature-dependent DECs was suggested using diffraction based dilatometry that provides more reliable data in comparison to conventional experimental procedures. Moreover, the averaging of lattice plane specific results from in situ diffraction analysis supply robust temperature-dependent macroscopic elastic constants for martensite and ferrite as input data for heat treatment process simulations

How to Play in Infinite MDPs (Invited Talk)

International audienceMarkov decision processes (MDPs) are a standard model for dynamic systems that exhibit both stochastic and nondeterministic behavior. For MDPs with finite state space it is known that for a wide range of objectives there exist optimal strategies that are memoryless and deterministic. In contrast, if the state space is infinite, optimal strategies may not exist, and optimal or ε-optimal strategies may require (possibly infinite) memory. In this paper we consider qualitative objectives: reachability, safety, (co-)Büchi, and other parity objectives. We aim at giving an introduction to a collection of techniques that allow for the construction of strategies with little or no memory in countably infinite MDPs

50 Hz X‐Ray Diffraction Stress Analysis and Numerical Process Simulation at Laser Surface Line Hardening of Web Structures

In situ synchrotron X-ray diffraction experiments were carried out during laser surface line hardening of the common tempering steel AISI 4140 at beamline P05@PETRA III operated by Helmholtz-Zentrum Geesthacht at the Deutsches Elektronen Synchrotron, Hamburg, Germany. A unique process chamber was used to investigate the phase and transverse surface stress evolution during a laser line hardening processes. Synchrotron radiation, in combination with microstrip line detectors, allows for a time resolution of 50 Hz. Specimen geometries were hardened using a high-power diode laser under control of the surface temperature and constant laser beam feed. Herein, it is focused on web-structured specimens in contrast to a flat geometry. The experimental results are discussed with regard to the workpiece geometry effect of the web structure dimensions on the temporal and spatial stress evolution. In addition, numerical process simulations based on the finite element method were carried out to support the drawn conclusions. The presented model is able to predict the surface transverse stresses inside the process zone center, while providing further 3D information. A heat build-up in the web leads to a wider and deeper process zone, however, the absolute hardness increase and the transverse residual stresses at the surface center are not affected

Parity Objectives in Countable MDPs

We study countably infinite MDPs with parity objectives, and special cases with a bounded number of colors in the Mostowski hierarchy (including reachability, safety, Büchi and co-Büchi). In finite MDPs there always exist optimal memoryless deterministic (MD) strategies for parity objectives, but this does not generally hold for countably infinite MDPs. In particular, optimal strategies need not exist. For countable infinite MDPs, we provide a complete picture of the memory requirements of optimal (resp., c-optimal) strategies for all objectives in the Mostowski hierarchy. In particular, there is a strong dichotomy between two different types of objectives. For the first type, optimal strategies, if they exist, can be chosen MD, while for the second type optimal strategies require infinite memory. (I.e., for all objectives in the Mostowski hierarchy, if finite-memory randomized strategies suffice then also MD-strategies suffice.) Similarly, some objectives admit c-optimal MD-strategies, while for others c-optimal strategies require infinite memory. Such a dichotomy also holds for the subclass of countably infinite MDPs that are finitely branching, though more objectives admit MD-strategies here

On Strong Determinacy of Countable Stochastic Games

We study 2-player turn-based perfect-information stochastic games with countably infinite state space. The players aim at maximizing/minimizing the probability of a given event (i.e., measurable set of infinite plays), such as reachability, Büchi, ω-regular or more general objectives. These games are known to be weakly determined, i.e., they have value. However, strong determinacy of threshold objectives (given by an event ε and a threshold c ∈ [0,1]) was open in many cases: is it always the case that the maximizer or the minimizer has a winning strategy, i.e., one that enforces, against all strategies of the other player, that ε is satisfied with probability ≥ c (resp. <; c)? We show that almost-sure objectives (where c = 1) are strongly determined. This vastly generalizes a previous result on finite games with almost-sure tail objectives. On the other hand we show that ≥ 1/2 (co-)Biichi objectives are not strongly determined, not even if the game is finitely branching. Moreover, for almost-sure reachability and almost-sure Biichi objectives in finitely branching games, we strengthen strong determinacy by showing that one of the players must have a memory less deterministic (MD) winning strategy

Starting to get bored: An outdoor eye tracking study of tourists exploring a city

Predicting the moment when a visual explorer of a place loses interest and starts to get bored is of considerable importance to the design of touristic information services. This paper investigates factors affecting the duration of the visual exploration of a city panorama. We report on an empirical outdoor eye tracking study in the real world with tourists following a free exploration paradigm without a time limit. As main result, the number of areas of interest revisited during a short period was found to be a good predictor for the total exploration duration

Smart sensors for autonomous robotic panel assembly

Manufacturing of a typical aluminium airframe panel involves the assembly of a couple of stringers and several dozens of clips. The stringers have to be mounted to the skin by riveting, while the clips are glued by using a fast hardening sealant. Though there are stringers and clips of identical geometry, there is too much part variation to make automation an easy task, and since tolerances are introduced by sagging manual work is still state of the art in order to cope with this. The paper is about the implementation of a computer vision sensor system that is capable of identifying individual clips by their CAD-drill hole pattern, determine the clips position and allow a robot to correctly pick and place them regardless of positional tolerances. In addition to that, a high accuracy drill hole detector that enables to correctly pick and place stringers and compensates the sagging effects, has been implemented. To ensure the quality of the detection processes the position of the individual parts have been investigated by a structured-light 3D-Camera. Within the paper the involved measurement technologies and the robot calibration are focused. An overview of the entire process chain is also given as well as an outlook to future applications in related areas

Teaching Tips – Gute Lehre ist kein Glücksfall!

Fragen wir die Teilnehmerinnen und Teilnehmer unserer hochschuldidaktischen Weiterbildungen nach ihren Erwartungen, so nennen sie an erster Stelle immer „konkrete Hilfestellungen für den Lehralltag“. Schauen wir genauer hin, wie diese Hilfestellungen aussehen könnten, so zeigen sich deutliche Unterschiede in den Zielgruppen der Lehrenden. Stehen sie noch am Anfang ihrer Lehrkarriere, wünschen sie sich besonders Hilfestellungen, die den Einstieg in die Lehre erleichtern bzw. ihnen helfen, effektiv zu lehren und effiziente Lehrende zu werden. Für erfahrene Lehrende hingegen, die zudem mit Prüfungen betraut sind, stellt sich darüber hinaus oft die Frage nach der Auswahl von Prüfungsformen und die Bewertung von Prüfungsleistungen, vor allem da ihnen die Modulabschlussprüfungen eine größere Bandbreite an Formen alternativer Leistungsbewertung eröffnen. Beiden Zielgruppen gemein ist die Suche nach praktischer Hilfe bei der Professionalisierung ihres didaktischen Handelns. Mit Patentrezepten ist ihnen dabei nicht gedient. Das Interesse der Lehrenden an Methoden wird gleichwohl als ein legitimes Interesse gesehen, denn Methodenwissen spielt bei der Vermittlung von Wissen eine große Rolle. Besonders im Kontext von Medialisierung werden die Vermittlungsmethoden wichtiger. Die hochschuldidaktische Weiterbildung will den Lehrenden bei der Entwicklung ihres individuellen und professionellen Lehrprofils theoretisch begründet, serviceorientiert zur Seite stehen. Seit Mitte der 1990er Jahre ist die didaktische Ausbildung für die Lehre nicht mehr dem Zufall überlassen, ebenso wie es keine Notwendigkeit mehr ist, sich hochschuldidaktisches Wissen autodidaktisch anzueignen. Die Entwicklung der Hochschuldidaktik und damit einhergehend die Entwicklung von curriculumbasierten, modularisierten und systematisch aufgebauten Weiterbildungsprogrammen hat in den letzten Jahrzehnten nicht zuletzt dank verschiedener Programme und Initiativen (z.B. Länderprogramme „Qualität der Lehre“, „Exzellenzinitiative“, BMBF, Stifterverband) eine enorme Steigerung erfahren. Die Qualität der Lehre an Universitäten ist zu einem Wettbewerbsfaktor unter den Universitäten geworden und kann zu ihrer Profilbildung beitragen. Die Lehrkompetenz kann als Schlüssel für die Qualität der Lehre gesehen werden und ist ein Merkmal akademischer Personalentwicklung. Dieses Journal soll in kleinem Rahmen als praxisorientierter Ratgeber auftreten. Es soll darüber hinaus das Interesse an hochschuldidaktischer Weiterbildung wecken, Lust machen, sich mit der eigenen Lehre auseinanderzusetzen, sich den kollegialen Fragen der anderen Teilnehmer/inne/n in den Workshops zu stellen, gemeinsam mit ihnen und uns, den Hochschuldidaktikern und Hochschuldidaktikerinnen, mögliche Antworten aufzuspüren und so das eigene Lehrrepertoire zu erweitern