Optimal control of nonlinear system for generator bidding in deregulated power markets

In this paper, considering generator's long-term optimization behavior, the generator bidding problem is studied using optimal control theory. In particular, the system demand is treated as a periodic function, and the competition process is then modeled as a dynamic, nonlinear and feedback system with periodic parameters, where the publicly known market clearing price (MCP) is the system output and the feedback signal, and supplier's outputs are the state variables. A software package MSIER3 for numerically solving the general optimal control problem is used for simulation. The performance of the optimal control is investigated, and a sensitivity analysis of system parameters is done through simulation. ©2005 IEEE.published_or_final_versio

State observation problem for a class of semi-linear hyperbolic systems

2004-2005 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Inertial-Based Filtration Method for Removal of Microcarriers from Mesenchymal Stem Cell Suspensions

© 2018, The Author(s). Rapidly evolving cell-based therapies towards clinical trials demand alternative approaches for efficient expansion of adherent cell types such as human mesenchymal stem cells (hMSCs). Using microcarriers (100–300 µm) in a stirred tank bioreactor offers considerably enhanced surface to volume ratio of culture environment. However, downstream purification of the harvested cell product needs to be addressed carefully due to distinctive features and fragility of these cell products. This work demonstrates a novel alternative approach which utilizes inertial focusing to separate microcarriers (MCs) from the final cell suspension. First, we systematically investigated MC focusing dynamics inside scaled-up curved channels with trapezoidal and rectangular cross-sections. A trapezoidal spiral channel with ultra-low-slope (Tan(α) = 0.0375) was found to contribute to strong MC focusing (~300 < Re < ~400) while managing high MC volume fractions up to ~1.68%. Accordingly, the high-throughput trapezoidal spiral channel successfully separated MCs from hMSC suspension with total cell yield~94% (after two passes) at a high volumetric flow rate of ~30 mL/min (Re~326.5)

A truncated projected Newton-type algorithm for large-scale semi-infinite programming

2005-2006 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Design of broadband beamformers with low complexity

2011-2012 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

A Hybrid Time-Scaling Transformation for Time-Delay Optimal Control Problems

In this paper, we consider a class of nonlinear time-delay optimal control problems with canonical equality and inequality constraints. We propose a new computational approach, which combines the control parameterization technique with a hybrid time-scaling strategy, for solving this class of optimal control problems. The proposed approach involves approximating the control variables by piecewise constant functions, whose heights and switching times are decision variables to be optimized. Then, the resulting problem with varying switching times is transformed, via a new hybrid time-scaling strategy, into an equivalent problem with fixed switching times, which is much preferred for numerical computation. Our new time-scaling strategy is hybrid in the sense that it is related to two coupled time-delay systems—one defined on the original time scale, in which the switching times are variable, the other defined on the new time scale, in which the switching times are fixed. This is different from the conventional time-scaling transformation widely used in the literature, which is not applicable to systems with time-delays. To demonstrate the effectiveness of the proposed approach, we solve four numerical examples. The results show that the costs obtained by our new approach are lower, when compared with those obtained by existing optimal control methods

Optimal design of orders of DFrFTs for sparse representations

This paper proposes to use a set of discrete fractional Fourier transform (DFrFT) matrices with different rotational angles to construct an overcomplete kernel for sparse representations of signals. The design of the rotational angles is formulated as an optimization problem as follows. The sum of the L1 norms of both the real part and the imaginary part of transformed vectors is minimized subject to different values of the optimal rotational angles. In order to avoid all the optimal rotational angles within a small neighbourhood, constraints on the sum of the L1 norms of both the real part and the imaginary part of the product of the individual optimal DFrFT matrices and training vectors being either stationary or nondifferentiable are imposed. Solving this optimization problem is very challenging not only because of the nonsmooth and the nonconvex nature of the problem, but also due to expressing the optimization problem in a nonstandard form. To solve the problem, first it is shown in this paper that this design problem is equivalent to an optimal sampling problem as follows. The absolute sum of the L1 norms of both the real part and the imaginary part of the frequency responses of a set of filters at the optimal sampling frequencies is minimized subject to similar constraints. Second, it is further shown that the optimal sampling frequencies are the roots of a set of harmonic functions. As the frequency responses of the filters are required to be computed only at frequencies in a discrete set, the globally optimal rotational angles can be found very efficiently and effectively

Long-term prognostic implications of visit-to-visit blood pressure variability in patients with ischaemic stroke

Background: Both blood pressure (BP) and its variability (BPV) are established risk factors for the development of atherosclerotic diseases and are associated with an increased risk of cardiovascular and all-cause mortality. The long-term prognostic implications of out-patient clinic visit-to-visit BPV among patients with ischaemic stroke are nevertheless unknown. Methods: We prospectively followed up the clinical outcome of 632 consecutive ischaemic stroke patients without atrial fibrillation. The mean BP and BPV, as determined by the coefficient of variation of the systolic and diastolic BP, were recorded during a mean of 12 ± 6 outpatient clinic visits. Results: The mean age of the patients was 71 ± 11 years. After a mean of 76 ± 18 month’s follow-up, 161 (26%) patients died, 35% (56/161) were due to cardiovascular causes. 16% and 5% developed recurrent stroke and acute coronary syndrome (ACS), respectively. After adjusting for mean systolic BP and confounding variables, patients with a high systolic BPV were at significantly greater risk of cardiovascular mortality (hazard ratio [HR] = 2.36; 95% confidence interval [CI], 1.02-5.49; P < 0.05). A high systolic BPV also predicted all-cause mortality after adjusting for mean systolic BP (HR = 1.79; 95% CI, 1.16-2.75; P < 0.05). There was no association between systolic BPV with non-fatal recurrent stroke nor non-fatal ACS. A raised diastolic BPV did not predict recurrent non-fatal stroke, non-fatal ACS nor mortality. Conclusions: Visit-to-visit systolic BPV predicts long-term all-cause and cardiovascular mortality in patients with ischaemic stroke without atrial fibrillation, independent of other conventional risk factors including average BP control.published_or_final_versio

Violation of the transit-time limit toward generation of ultrashort electron bunches with controlled velocity chirp

Various methods to generate ultrashort electron bunches for the ultrafast science evolved from the simple configuration of two-plate vacuum diodes to advanced technologies such as nanotips or photocathodes excited by femtosecond lasers. In a diode either in vacuum or of solid-state, the transit-time limit originating from finite electron mobility has caused spatiotemporal bunch-collapse in ultrafast regime. Here, we show for the first time that abrupt exclusion of transit-phase is a more fundamental origin of the bunch-collapse than the transit-time limit. We found that by significantly extending the cathode-anode gap distance, thereby violating the transit-time limit, the conventional transit-time-related upper frequency barrier in diodes can be removed. Furthermore, we reveal how to control the velocity chirp of bunches leading to ballistic bunch-compression. Demonstration of 0.707 THz-, 46.4 femtosecond-bunches from a 50 mu m-wide diode in three-dimensional particle-in-cell simulations shows a way toward simple and compact sources of ultrafast electron bunches for diverse ultrafast sciences.ope