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K. Ramesh Kumar

Bio: K. Ramesh Kumar is an academic researcher from University of Johannesburg. The author has contributed to research in topics: Magnetization & Magnetic susceptibility. The author has an hindex of 9, co-authored 28 publications receiving 470 citations. Previous affiliations of K. Ramesh Kumar include Tata Institute of Fundamental Research & Indian Institute of Technology Madras.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the optimal placement of collocated piezoelectric actuator?sensor pairs on flexible beams using a model-based linear quadratic regulator (LQR) controller is considered.
Abstract: This paper considers the optimal placement of collocated piezoelectric actuator?sensor pairs on flexible beams using a model-based linear quadratic regulator (LQR) controller. A finite element method based on Euler?Bernoulli beam theory is used. The contributions of piezoelectric sensor and actuator patches to the mass and stiffness of the beam are considered. The LQR performance is taken as the objective for finding the optimal location of sensor?actuator pairs. The problem is formulated as a multi-input multi-output (MIMO) model control. The discrete optimal sensor and actuator location problem is formulated in the framework of a zero?one optimization problem which is solved using genetic algorithms (GAs). Classical control strategies like direct proportional feedback, constant gain negative velocity feedback and the LQR optimal control scheme are applied to study the control effectiveness. The study of the optimal location of actuators and sensors is carried out for different boundary conditions of beams like cantilever, simply supported and clamped boundary conditions.

186 citations

Journal ArticleDOI
TL;DR: In this paper, the optimal placement of collocated piezoelectric actuator-sensor pairs on a thin plate using a model-based linear quadratic regulator (LQR) controller is taken as objective for finding the optimal location of sensor-actuator pairs.
Abstract: This paper considers the optimal placement of collocated piezoelectric actuator–sensor pairs on a thin plate using a model-based linear quadratic regulator (LQR) controller. LQR performance is taken as objective for finding the optimal location of sensor–actuator pairs. The problem is formulated using the finite element method (FEM) as multi-input–multi-output (MIMO) model control. The discrete optimal sensor and actuator location problem is formulated in the framework of a zero–one optimization problem. A genetic algorithm (GA) is used to solve the zero–one optimization problem. Different classical control strategies like direct proportional feedback, constant-gain negative velocity feedback and the LQR optimal control scheme are applied to study the control effectiveness.

104 citations

Journal ArticleDOI
TL;DR: In this article, the temperature variation of electrical resistance was found to follow the relation Rn=R0n+aTα (α=1.616) and (Rn-normalized electrical resistance) in the temperature range of 25-300 K and almost a temperature independent variation of the electrical resistance below 25-K indicating the absence of spin-flip scattering.

46 citations

Journal ArticleDOI
TL;DR: Magnetic and dielectric properties of the double perovskite Ho2NiMnO6 are reported and signs of inherent Griffiths phase pertaining to the Ni/Mn subsystem are visible.
Abstract: Magnetic and dielectric properties of the double perovskite Ho2NiMnO6 are reported. The compound is synthesized by nitrate route and is found to crystallize in monoclinic P2(1)/n space group. Lattice parameters obtained by refining powder x-ray diffraction data are; a = 5.218(2)angstrom, b = 5.543(2)angstrom, c = 7.480(3)angstrom and the monoclinic angle i beta = 90.18 degrees(4). A phase transition is observed at T-C = 86 K in the temperature-dependent magnetization curve, M(T). The inverse magnetic susceptibility, (1/chi(T)) fits reasonably well with modified Curie-Weiss law by incorporating the paramagnetic response of Ho3+. 1/chi(T) manifests as an upward deviation from ideal Curie-Weiss behaviour well above the ferromagnetic transition. Signs of inherent Griffiths phase pertaining to the Ni/Mn subsystem are visible when one subtracts the Ho3+ paramagnetic contribution from total susceptibility and does the power-law analysis. The magnetic hysteresis at 2 K gives the maximum value of magnetization M-max approximate to 15 mu(B)/f. u. at 50 kOe. Field-derivative of magnetization at 2 K shows discontinuities which indicates the existence of metamagnetic transitions in this compound. This needs to be probed further. Out of the two dielectric relaxations observed, the one at low temperature may be attributed to phononic frequencies and that at higher temperature may be due to Maxwell-Wagner relaxation. A correlation between magnetic and lattice degrees of freedom is plausible since the anomaly in dielectric constant coincides with T-C.

36 citations

Journal ArticleDOI
TL;DR: In this paper, first principles for the full Heusler alloy Co2FeGe within the scheme of density functional theory using plane-wave self-consistent field method were carried out.
Abstract: First-principles calculation has been carried out for the full Heusler alloy Co2FeGe within the scheme of density functional theory using plane-wave self-consistent field method. The spin polarized band structure does not show any energy gap at the Fermi level for both up and down spin electrons. Atom resolved magnetic moment on each site was observed to be 1.3 muB (Co), 2.9 muB (Fe), and 0.0 muB (Ge). X-ray diffraction studies reveal a B2-type structure for the bulk sample and cubic L21 structure for the melt-spun ribbons. The lattice parameter value for the ribbon is 5.736 Aring, and its Curie temperature is around 981 K. The magnetic moment per formula unit at 5 K was observed to be 5.74 muB. The low temperature data ( < 100 K) follows the relationrho = rho0 + AT2 + BT4.5 , indicating the presence of one magnon spin-flip scattering.

30 citations


Cited by
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01 Apr 2009
TL;DR: In this article, the performance of recent density functionals for the exchange-correlation energy of a nonmolecular solid, by applying accurate calculations with the GAUSSIAN, BAND, and VASP codes to a test set of 24 solid metals and nonmetals.
Abstract: We assess the performance of recent density functionals for the exchange-correlation energy of a nonmolecular solid, by applying accurate calculations with the GAUSSIAN, BAND, and VASP codes to a test set of 24 solid metals and nonmetals. The functionals tested are the modified Perdew-Burke-Ernzerhof generalized gradient approximation PBEsol GGA, the second-order GGA SOGGA, and the Armiento-Mattsson 2005 AM05 GGA. For completeness, we also test more standard functionals: the local density approximation, the original PBE GGA, and the Tao-Perdew-Staroverov-Scuseria meta-GGA. We find that the recent density functionals for solids reach a high accuracy for bulk properties lattice constant and bulk modulus. For the cohesive energy, PBE is better than PBEsol overall, as expected, but PBEsol is actually better for the alkali metals and alkali halides. For fair comparison of calculated and experimental results, we consider the zeropoint phonon and finite-temperature effects ignored by many workers. We show how GAUSSIAN basis sets and inaccurate experimental reference data may affect the rating of the quality of the functionals. The results show that PBEsol and AM05 perform somewhat differently from each other for alkali metal, alkaline-earth metal, and alkali halide crystals where the maximum value of the reduced density gradient is about 2, but perform very similarly for most of the other solids where it is often about 1. Our explanation for this is consistent with the importance of exchange-correlation nonlocality in regions of core-valence overlap.

413 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present various optimization criteria used by researchers for optimal placement of piezoelectric sensors and actuators on a smart structure, including modal forces/moments applied by actuators, maximizing deflection of the host structure, minimizing control effort/maximizing energy dissipated, maximizing degree of controllability, and minimizing degree of observability.
Abstract: This article presents in a unified way, the various optimization criteria used by researchers for optimal placement of piezoelectric sensors and actuators on a smart structure. The article discusses optimal placement of piezoelectric sensors and actuators based upon six criteria: (i) maximizing modal forces/moments applied by piezoelectric actuators, (ii) maximizing deflection of the host structure, (iii) minimizing control effort/maximizing energy dissipated, (iv) maximizing degree of controllability, (v) maximizing degree of observability, and (vi) minimizing spill-over effects. Optimal piezoelectric sensor and actuator locations on beam and plate structures for each criterion and modes of interest are presented in a tabular form. This technical review has two objectives: (i) practicing engineers can pick the most suitable philosophy for their end application and (ii) researchers can come to know about potential gaps in this area.

246 citations

Journal ArticleDOI
TL;DR: In this article, a quaternary Heusler half-metallic ferromagnets CoFeCrZ was designed and its first-principles calculations showed that, within a generalized gradient approximation for the electronic exchange correlation functional, both CoFeFeCrGa and CoFeGe are nearly halfmetals.

229 citations

Journal ArticleDOI
TL;DR: This work carried out a full stability analysis of intermetallic Heusler alloys made only of transition metals, and produced two new magnets: Co2MnTi, which displays a remarkably high TC in perfect agreement with the predictions, and Mn2PtPd, which is an antiferromagnet.
Abstract: Magnetic materials underpin modern technologies, ranging from data storage to energy conversion to contactless sensing. However, the development of a new high-performance magnet is a long and often unpredictable process, and only about two dozen magnets are featured in mainstream applications. We describe a systematic pathway to the design of novel magnetic materials, which demonstrates a high throughput and discovery speed. On the basis of an extensive electronic structure library of Heusler alloys containing 236,115 prototypical compounds, we filtered those displaying magnetic order and established whether they can be fabricated at thermodynamic equilibrium. Specifically, we carried out a full stability analysis of intermetallic Heusler alloys made only of transition metals. Among the possible 36,540 prototypes, 248 were thermodynamically stable but only 20 were magnetic. The magnetic ordering temperature, TC, was estimated by a regression calibrated on the experimental TC of about 60 known compounds. As a final validation, we attempted the synthesis of a few of the predicted compounds and produced two new magnets: Co2MnTi, which displays a remarkably high TC in perfect agreement with the predictions, and Mn2PtPd, which is an antiferromagnet. Our work paves the way for large-scale design of novel magnetic materials at potentially high speed.

213 citations

Journal ArticleDOI
TL;DR: In this paper, the optimal placement of collocated piezoelectric actuator?sensor pairs on flexible beams using a model-based linear quadratic regulator (LQR) controller is considered.
Abstract: This paper considers the optimal placement of collocated piezoelectric actuator?sensor pairs on flexible beams using a model-based linear quadratic regulator (LQR) controller. A finite element method based on Euler?Bernoulli beam theory is used. The contributions of piezoelectric sensor and actuator patches to the mass and stiffness of the beam are considered. The LQR performance is taken as the objective for finding the optimal location of sensor?actuator pairs. The problem is formulated as a multi-input multi-output (MIMO) model control. The discrete optimal sensor and actuator location problem is formulated in the framework of a zero?one optimization problem which is solved using genetic algorithms (GAs). Classical control strategies like direct proportional feedback, constant gain negative velocity feedback and the LQR optimal control scheme are applied to study the control effectiveness. The study of the optimal location of actuators and sensors is carried out for different boundary conditions of beams like cantilever, simply supported and clamped boundary conditions.

186 citations