323 research outputs found
State-Space Interpretation of Model Predictive Control
A model predictive control technique based on a step response model is developed using state estimation techniques. The standard step response model is extended so that integrating systems can be treated within the same framework. Based on the modified step response model, it is shown how the state estimation techniques from stochastic optimal control can be used to construct the optimal prediction vector without introducing significant additional numerical complexity. In the case of integrated or double integrated white noise disturbances filtered through general first-order dynamics and white measurement noise, the optimal filter gain is parametrized explicitly in terms of a single parameter between 0 and 1, thus removing the requirement for solving a Riccati equation and equipping the control system with useful on-line tuning parameters. Parallels are drawn to the existing MPC techniques such as Dynamic Matrix Control (DMC), Internal Model Control (IMC) and Generalized Predictive Control (GPC)
Robust Control Structure Selection
Screening tools for control structure selection in the presence of model/plant mismatch are developed in the context of the Structured Singular Value (μ) theory. The developed screening tools are designed to aid engineers in the elimination of undesirable control structure candidates for which a robustly performing controller does not exist. Through application on a multicomponent distillation column, it is demonstrated that the developed screening tools can be effective in choosing an appropriate control structure while previously existing methods such as the Condition Number Criterion can lead to erroneous results
Electronic structure and magnetic properties of metallocene multiple-decker sandwich nanowires
We present a study of the electronic and magnetic properties of the
multiple-decker sandwich nanowires () composed of cyclopentadienyl (CP)
rings and 3d transition metal atoms (M=Ti to Ni) using first-principles
techniques. We demonstrate using Density Functional Theory that structural
relaxation play an important role in determining the magnetic ground-state of
the system. Notably, the computed magnetic moment is zero in , while in
a significant turn-up in magnetic moment is evidenced. Two compounds
show a half-metallic ferromagnetic ground state with a gap within
minority/majority spin channel. In order to study the effect of electronic
correlations upon the half-metallic ground states in , we introduce a
simplified three-bands Hubbard model which is solved within the Variational
Cluster Approach. We discuss the results as a function of size of the reference
cluster and the strength of average Coulomb and exchange parameters.
Our results demonstrate that for the range of studied parameters and
the half-metallic character is not maintained in the presence of
local Coulomb interactions.Comment: 9 pages, 9 figures, submited to PR
PLS-Based Robust Inferential Control for a Packed-Bed Reactor
This paper compares the performance of two different inferential schemes when applied to an experimental packed-bed reactor. The first scheme, proposed initially by Brosilow, is designed based on Kalman filter estimation. The second less traditional design uses an estimator computed from the Partial Least Squares regression method (PLS). The second approach was found to give superior performance when the nonlinear system under study is operated is a wide range of operating points. Due to the nonlinearity of the system it is essential to address the issue of robustness of the proposed schemes. This is formally done in this work using Structured Singular Value Theory. For the robustness analysis it is crucial to develop a realistic but not overly conservative uncertainty description. Since the PLS estimator uses a large number of measurements, a robust design based on the uncertainty associated with each one of the measurements would be very conservative. To overcome this problem a lumped uncertainty description is proposed which is identified directly from experiments
Transmission through correlated CuCoCu heterostructures
The effects of local electronic interactions and finite temperatures upon the
transmission across the CuCoCu metallic heterostructure are studied in
a combined density functional and dynamical mean field theory. It is shown
that, as the electronic correlations are taken into account via a local but
dynamic self-energy, the total transmission at the Fermi level gets reduced
(predominantly in the minority spin channel), whereby the spin polarization of
the transmission increases. The latter is due to a more significant
-electrons contribution, as compared to the non-correlated case in which the
transport is dominated by and electrons.Comment: 29 pages, 7 figures, submited to PR
Memory Effect and Triplet Pairing Generation in the Superconducting Exchange Biased Co/CoOx/Cu41Ni59/Nb/Cu41Ni59 Layered Heterostructure
We fabricated a nanolayered hybrid superconductor-ferromagnet spin-valve
structure, the resistive state of which depends on the preceding magnetic field
polarity. The effect is based on a strong exchange bias (about -2 kOe) on a
diluted ferromagnetic copper-nickel alloy and generation of a long range odd in
frequency triplet pairing component. The difference of high and low resistance
states at zero magnetic field is 90% of the normal state resistance for a
transport current of 250 {\mu}A and still around 42% for 10 {\mu}A. Both logic
states of the structure do not require biasing fields or currents in the idle
mode.Comment: 9 pages, 4 figures, Accepted to Applied Physics Letter
Reentrant Superconductivity and Superconducting Critical Temperature Oscillations in F/S/F trilayers of Cu41Ni59/Nb/Cu41Ni59 Grown on Cobalt Oxide
Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers constitute the core
of a superconducting spin valve. The switching effect of the spin valve is
based on interference phenomena occurring due to the proximity effect at the
S/F interfaces. A remarkable effect is only expected if the core structure
exhibits strong critical temperature oscillations, or most favorable, reentrant
superconductivity, when the thickness of the ferromagnetic layer is increased.
The core structure has to be grown on an antiferromagnetic oxide layer (or such
layer to be placed on top) to pin by exchange bias the
magnetization-orientation of one of the ferromagnetic layers. In the present
paper we demonstrate that this is possible, keeping the superconducting
behavior of the core structure undisturbed.Comment: 22 pages, 12 figures, 1 tabl
Screening plant designs and control structures for uncertain systems
Abstract--Screening tools are developed which provide nonconservative estimates of the achievable performance in the presence of general structured model uncertainty. These tools allow the rational selection among plant designs, or can be applied to provide recommedations on how to modify a plant design to improve the closed loop performance. Many of these tools are applicable to the pairing or partitioning of inputs and outputs of decentralized controllers. It is shown through examples that ignoring or improperly characterizing plant/model mismatch while selecting among plant designs can lead to erroneous results
Reentrant superconductivity in superconductor/ferromagnetic-alloy bilayers
We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state
establishing due to the proximity effect in superconducting Nb/Cu41Ni59
bilayers. Using a special wedge-type deposition technique, series of 20-35
samples could be fabricated by magnetron sputtering during one run. The layer
thickness of only a few nanometers, the composition of the alloy, and the
quality of interfaces were controlled by Rutherford backscattering
spectrometry, high resolution transmission electron microscopy, and Auger
spectroscopy. The magnetic properties of the ferromagnetic alloy layer were
characterized with superconducting quantum interference device (SQUID)
magnetometry. These studies yield precise information about the thickness, and
demonstrate the homogeneity of the alloy composition and magnetic properties
along the sample series. The dependencies of the critical temperature on the Nb
and Cu41Ni59 layer thickness, Tc(dS) and Tc(dF), were investigated for constant
thickness dF of the magnetic alloy layer and dS of the superconducting layer,
respectively. All types of non-monotonic behaviors of Tc versus dF predicted by
the theory could be realized experimentally: from reentrant superconducting
behavior with a broad extinction region to a slight suppression of
superconductivity with a shallow minimum. Even a double extinction of
superconductivity was observed, giving evidence for the multiple reentrant
behavior predicted by theory. All critical temperature curves were fitted with
suitable sets of parameters. Then, Tc(dF) diagrams of a hypothetical F/S/F
spin-switch core structure were calculated using these parameters. Finally,
superconducting spin-switch fabrication issues are discussed in detail in view
of the achieved results.Comment: 34 pages, 9 figure
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