Browsing by Subject "Periodic switching"

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    Analysis and design of switching and fuzzy systems
    (2002-09) Akgül, Murat
    In this thesis we consider the controller design problems for switching and fuzzy systems. In switching systems, the system dynamics and/or control input take dierent forms in different parts of the underlying state space. In fuzzy systems, the system dynamics and/or control input consist of certain logical expressions. From this point of view, it is reasonable to expect certain similarities between these systems. We show that under certain conditions, a switching system may be converted into an equivalent fuzzy system. While the changes in the system variables in a switching system may be abrupt, such changes are typically smooth in a fuzzy system. Therefore obtaining such an equivalent fuzzy system may inherit the stability properties of the original switching system while smoothing the system dynamics. Motivated from this idea we propose various switching strategies for certain classes of nonlinear systems and provide some stability results. Due to the dificulties in designing such switching rules for nonlinear systems, most of the results are developed for certain specific type of systems. Due to the logical structure, obtaining rigorous stability results are very difficult for fuzzy systems. We propose a fuzzy controller design method and prove a stability result under certain conditions. The proposed method may also be applied to function approximation. We also consider a different stabilization method, namely phase portrait matching, in which the main aim is to choose the control input appropriately so that the dynamics of the closed-loop system is close to a given desired dynamics. If this is achieved, then the phase portrait of the closed-loop system will also be close to a desired phase portrait. We propose various schemes to achieve this task.
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    Quantum turnstile operation of single-molecule magnets
    (Institute of Physics Publishing, 2015) Moldoveanu, V.; Dinu, I. V.; Tanatar, Bilal; Moca, C. P.
    The time-dependent transport through single-molecule magnets coupled to magnetic or non-magnetic electrodes is studied in the framework of the generalized master equation method. We investigate the transient regime induced by the periodic switching of the source and drain contacts. If the electrodes have opposite magnetizations the quantum turnstile operation allows the stepwise writing of intermediate excited states. In turn, the transient currents provide a way to read these states. Within our approach we take into account both the uniaxial and transverse anisotropy. The latter may induce additional quantum tunneling processes which affect the efficiency of the proposed read-and-write scheme. An equally weighted mixture of molecular spin states can be prepared if one of the electrodes is ferromagnetic. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.