Browsing by Author "Ahmed, S."
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Item Open Access Design of a switched robust control scheme for drug delivery in blood pressure regulation(Elsevier B.V., 2016) Ahmed, S.; Özbay, HitayA control algorithm based on switching robust controllers is presented for a Linear Parameter Varying (LPV) time-delay system modeling automatic infusion of vasodilator drug to regulate postsurgical hypertension. The system is scheduled along a measurable signal trajectory. The prospective controllers are robustly designed at various operating points forming a finite set of robust controllers and then a hysteresis switching is performed between neighboring robust controllers for a larger operating range of the LPV system. The stability of the switching LPV system for the entire operating range is ensured by providing a sufficient condition in terms of bound on the scheduling signal variation using the concept of dwell time. Simulation results are provided to verify the performance of the designed control scheme. © 2016Item Open Access Finite time estimation through a continuous-discrete observer(John Wiley and Sons, 2018) Mazenc, F.; Ahmed, S.; Malisoff, M.We study two broad classes of nonlinear time-varying continuous-time systems with outputs. For the first class, we build an observer in the case where a state dependent disturbance affects the linear approximation. When the disturbances are the zero functions, our observer provides exact values of the state at all times larger than a suitable finite time, and it provides an approximate estimate when there are nonzero disturbances, so our observers are called finite time observers. We use this construction, which is of interest for its own sake, to design a globally exponentially stabilizing dynamic output feedback for a family of nonlinear systems whose outputs are only available on some finite time intervals. Our simulations illustrate the efficacy of our methods.Item Open Access Finite time estimation via piecewise constant measurements⁎(Elsevier B.V., 2018) Mazenc, F.; Ahmed, S.We study a broad class of linear continuous-time time-varying systems that contain piecewise continuous disturbances and piecewise constant outputs. Under an observability assumption, we construct a new type of observer to estimate the state of the system in a predetermined finite time in the presence of the disturbances. In contrast to the well-established finite time observer design techniques which estimate the system state using a continuous output, our proposed observer only requires a piecewise constant output. Our simulations illustrate the efficacy of our observer.Item Open Access Ultra-narrow linewidth photo-emitters in polymorphic selenium nanoflakes(Wiley, 2022-07-12) Hussain, N.; Ahmed, S.; Tepe, H. U.; Huang, K.; Avishan, N.; He, S.; Rafique, M.; Farooq, U.; Kasirga, Talip Serkan; Bek, A.; Turan, R.; Shehzad, K.Photoluminescence (PL) in state-of-the-art 2D materials suffers from narrow spectral coverage, relatively broad linewidths, and poor room-temperature (RT) functionality. The authors report ultra-narrow linewidth photo-emitters (ULPs) across the visible to near-infrared wavelength at RT in polymorphic selenium nanoflakes (SeNFs), synthesized via a hot-pressing strategy. Photo-emitters in NIR exhibit full width at half maximum (Γ) of 330 ± 90 µeV, an order of magnitude narrower than the reported ULPs in 2D materials at 300 K, and decrease to 82 ± 70 µeV at 100 K, with coherence time (τc) of 21.3 ps. The capping substrate enforced spatial confinement during thermal expansion at 250 °C is believed to trigger a localized crystal symmetry breaking in SeNFs, causing a polymorphic transition from the semiconducting trigonal (t) to quasi-metallic orthorhombic (orth) phase. Fine structure splitting in orth-Se causes degeneracy in defect-associated bright excitons, resulting in ultra-sharp emission. Combined theoretical and experimental findings, an optimal biaxial compressive strain of −0.45% cm−1 in t-Se is uncovered, induced by the coefficient of thermal expansion mismatch at the selenium/sapphire interface, resulting in bandgap widening from 1.74 to 2.23 ± 0.1 eV. This report underpins the underlying correlation between crystal symmetry breaking induced polymorphism and RT ULPs in SeNFs, and their phase change characteristics. © 2022 Wiley-VCH GmbH.Item Open Access The value of multi-stage stochastic programming in risk-averse unit commitment under uncertainty(IEEE, 2019) Mahmutoğulları, Ali İrfan; Ahmed, S.; Çavuş, Özlem; Aktürk, M. SelimDay-ahead scheduling of electricity generation or unit commitment is an important and challenging optimization problem in power systems. Variability in net load arising from the increasing penetration of renewable technologies has motivated study of various classes of stochastic unit commitment models. In two-stage models, the generation schedule for the entire day is fixed while the dispatch is adapted to the uncertainty, whereas in multi-stage models the generation schedule is also allowed to dynamically adapt to the uncertainty realization. Multi-stage models provide more flexibility in the generation schedule; however, they require significantly higher computational effort than two-stage models. To justify this additional computational effort, we provide theoretical and empirical analyses of the value of multi-stage solution for risk-averse multi-stage stochastic unit commitment models. The value of multi-stage solution measures the relative advantage of multi-stage solutions over their two-stage counterparts. Our results indicate that, for unit commitment models, the value of multi-stage solution increases with the level of uncertainty and number of periods, and decreases with the degree of risk aversion of the decision maker.