Showing papers by "Polytechnic University of Turin published in 1993"

Routing and scheduling in a flexible job shop by tabu search

Abstract: A hierarchical algorithm for the flexible job shop scheduling problem is described, based on the tabu search metaheuristic. Hierarchical strategies have been proposed in the literature for complex scheduling problems, and the tabu search metaheuristic, being able to cope with different memory levels, provides a natural background for the development of a hierarchical algorithm. For the case considered, a two level approach has been devised, based on the decomposition in a routing and a job shop scheduling subproblem, which is obtained by assigning each operation of each job to one among the equivalent machines. Both problems are tackled by tabu search. Coordination issues between the two hierarchical levels are considered. Unlike other hierarchical schemes, which are based on a one-way information flow, the one proposed here is based on a two-way information flow. This characteristic, together with the flexibility of local search strategies like tabu search, allows to adapt the same basic algorithm to different objective functions. Preliminary computational experience is reported.

On the numerical integration of the linear shape functions times the 3-D Green's function or its gradient on a plane triangle

Abstract: Formulas for the static and dynamic potentials due to linearly varying source distributions defined on a planar triangle are presented. The general linear variation of the source is represented in terms of three linear basis functions, each one associated with a different vertex of the triangular domain. The singular kernels considered are given by the 3-D Green's function and its gradient, both for the static and dynamic case. In the static case the evaluation of potentials is performed analytically and compact form results are given. In the dynamic case the results are given in a form suited for numerical integration. >

On stability of cellular neural networks with delay

Abstract: It is known that symmetric cellular neural networks (CNNs) are completely stable. It is shown that CNNs with delay (DCNNs), though symmetric, can become unstable if the delay is suitably chosen: actually such networks can exhibit periodic cycles. Moreover, a sufficient condition is presented to ensure complete stability: such a condition establishes a relation between the delay time and the parameters of the network. >

Generalized stochastic Petri nets: a definition at the net level and its implications

Abstract: The class of Petri nets obtained by eliminating timing from generalized stochastic Petri net (GSPN) models while preserving the qualitative behavior is identified. Structural results for those nets are derived, obtaining the first structural analysis of Petri nets with priority and inhibitor arcs. A revision of the GSPN definition based on the structural properties of the models is presented. It is shown that for a (wide) class of nets, the definition of firing probabilities of conflicting immediate transitions does not require the information on reachable markings. Identification of the class of models for which the net-level specification is possible is also based on the structural analysis results. The procedure for the model specification is illustrated by means of an example. It is also shown that a net-level specification of the model associated with efficient structural analysis techniques can have a substantial impact on model analysis. >

Strange attractors in delayed cellular neural networks

Abstract: A strange attractor has been found in a system described by a differential-difference equation: a delayed cellular neural network composed by only two cells. A detailed analysis of the set of eigenvalues that give rise to the attractor is presented and the bifurcation phenomena obtained by varying the delay are shown. >

A CAD-oriented analytical model for the losses of general asymmetric coplanar lines in hybrid and monolithic MICs

Abstract: New analytical approximations are derived for the conductor losses of asymmetric coplanar waveguides (ACPW) and coplanar striplines (ACPS) on a finite-thickness dielectric substrate. The expressions hold for lines whose metallizations have thickness much smaller than the slot and strip widths, but suitably larger than the skin penetration depth at the operating frequency. The derivation is based on an extension of the conformal mapping approach formerly proposed by Owyang and Wu (1958) for symmetric lines in air. Comparisons with published data from quasistatic or full-wave numerical analyses are presented to validate the expressions derived for both the symmetric and the asymmetric case. The analytical characterization presented in the paper is well suited for inclusion into CAD codes for MMIC design. >

Techniques for process model evolution in EPOS

Abstract: The authors categorize some aspects of software process evolution and customization, and describe how they are handled in the EPOS PM system. Comparisons are made to other PM systems. A process model in EPOS consists of a schema of classes and meta-classes, and its model entities and relationships. There is an underlying software engineering database, EPOSDB, offering uniform versioning of all model parts and a context of nested cooperating transactions. Then, there is a reflective object-oriented process specification language, on top of the EPOSDB. Policies for model creation, composition, change, instantiation, refinement, and enaction are explicitly represented and are used by a set of PM automatic tools. The main tools are a planner to instantiate tasks, an execution manager to enact such tasks, and a PM manager to define, analyze, customize, and evolve the process schema. >

Evaluation of torque ripple in high performance synchronous reluctance machines

Abstract: Synchronous reluctance motors with multiple-segment rotor structures are considered. The presence of stator slots is taken into account with respect to torque ripple. An analytical model is presented, based on magnetic interaction between d and q axes, due to stator toothing. The effect of skewing is also considered. Its partial effect on torque ripple reduction due to the rotor anisotropy is shown. A closed expression for the torque is obtained, giving the torque ripple as a function of design parameters. Experimental verification of the proposed model is given based on measurements on prototype motors. >

An improved calibration technique for on-wafer large-signal transistor characterization

Abstract: The on-wafer measurement of complex quantities and absolute power levels of active devices is truly significant for nonlinear device characterization and modeling. An original procedure, which allows one to perform both the vector and the power calibrations at the RF wafer probe tips used for on-wafer measurement of two-port devices, is presented. The measurement system is based on an automatic vector network analyzer with coaxial directional couplers and RF coplanar wafer probes. A new error model of the dual directional coupler, which samples the power waves traveling at device output, allows one to take advantage of the coaxial section at the output of the measuring system for calibrating the power level up to the on-wafer probe tips. >

Kinetic equation for classical particles obeying an exclusion principle

Abstract: In this paper we analyze the kinetics of classical particles which obey an exclusion principle (EP) in the only-individual-transitions (OIT) approximation, and separately in the more rigorous contemporary-transitions (CT) description. In order to be able to include the EP into the kinetics equations we consider a discrete, one-dimensional, heterogeneous and anisotropic phase space and, after defining the reduced transition probabilities, we write a master equation. As a limit to the continuum of this master equation we obtain a generalized Fokker-Planck (FP) equation. This last is a nonlinear partial differential equation and reduces to the standard FP equation if the nonlinear term, which takes into account the EP, is neglected. The steady states of this equation, both in the OIT approximation and CT description, are considered. In the particularly interesting case of Brownian particles as a steady state in the OIT approximation we obtain the Fermi-Dirac (FD) distribution, while in the CT description we obtain another distribution which differs slightly from that of the FD. Moreover, our approach permits us to treat in an alternative and efficient way the problem of the determination of an effective potential to simulate the exclusion principle in classical many-body equations of motion.

Wavelet transforms associated with finite cyclic groups

Abstract: Multiresolution analysis via decomposition on wavelet bases has emerged as an important tool in the analysis of signals and images when these objects are viewed as sequences of complex or real numbers. An important class of multiresolution decompositions are the Laplacian pyramid schemes, in which the resolution is successively halved by recursively low-pass filtering the signal under analysis and decimating it by a factor of two. In general, the principal framework within which multiresolution techniques have been studied and applied is the same as that used in the discrete-time Fourier analysis of sequences of complex numbers. An analogous framework is developed for the multiresolution analysis of finite-length sequences of elements from arbitrary fields. Attention is restricted to sequences of length 2/sup n/, for n a positive integer, so that the resolution may be recursively halved to completion. As in finite-length Fourier analysis, a cyclic group structure of the index set of such sequences is exploited to characterize the transforms of interest for the particular cases of complex and finite fields. >

Performance evaluation of a direct torque controlled drive in the continuous PWM-square wave transition region

Abstract: This paper investigates the operation of a direct torque controlled drive when operating under transient conditions and when operating in overmodulation conditions or in the "transition region" to six-step operation. The direct torque control is a dead-beat control of the torque and flux magnitude. In the steady-state, the stator voltage vector which drives the torque and flux to the reference value is calculated during each fixed switching period. Under transient or overmodulation conditions, an alternative switching algorithm must be used since dead-beat control is no longer possible. Two alternatives are presented for operation in overmodulation. The first involves a determination of the switching state a priori, and calculating the duty cycle for each phase based on the torque and flux error. A much simpler scheme is presented which utilizes the voltage reference vector from the direct torque control algorithm. This scheme, although not resulting in dead-beat control, is shown to provide very satisfactory performance in overmodulation. The direct torque control method shows great promise for light traction applications where a large quasi-constant power region is required. The scheme operates very satisfactorily in overmodulation, compared with existing current regulated PWM-based schemes, due to the fact that the voltage space vectors are directly controlled. A complete experimental evaluation of the proposed scheme operating in the transition region is also given. >

About The Effects Of Different Modulation Index On The Iron Losses In Soft Magnetic Materials Supplied By Pwm Inverter

Abstract: The increase in iron-loss in soft magnetic materials due to the modulation depth in pulse width modulation (PWM) inverters is analyzed. In particular, the specific loss variations with different modulation indexes are measured, both in wound cores realized with high quality grain oriented magnetic material, and in induction motors. >

Picard-Fuchs Equations and Special Geometry

Abstract: We investigate the system of holomorphic differential identities implied by special Kahlerian geometry of four-dimensional N=2 supergravity. For superstring compactifications on Calabi-Yau threefolds these identities are equivalent to the Picard-Fuchs equations of algebraic geometry that are obeyed by the periods of the holomorphic three-form. For one variable they reduce to linear fourth-order equations which are characterized by classical W generators; we find that the instanton corrections to the Yukawa couplings are directly related to the nonvanishing of w4. We also show that the symplectic structure of special geometry can be related to the fact that the Yukawa couplings can be written as triple derivatives of some holomorphic function F. Moreover, we give the precise relationship of the Yukawa couplings of special geometry with three-point functions in topological field theory.

Topologies for wavelength-routing all-optical networks

Abstract: Three regular meshed topologies are compared in light of their possible use for the implementation of large all-optical wavelength-routing communication networks (or interconnection systems). These systems provide all source-destination pairs with end-to-end transparent channels that are identified through a wavelength and a physical path. The considered topologies are the K-dimensional bidirectional square lattice, the twin shuffle, and the de Bruijn graph. The comparison is based on the maximum and average distance between source and destination (number of traversed nodes), on the degree of connectivity for each node (number of input and output fibers), and on the minimum number of wavelengths in the WDM comb necessary to discriminate all source-destination pairs. >

Joule-heating effects in the amorphous Fe40Ni40B20 alloy

Abstract: The effects of Joule heating on the amorphous ${\mathrm{Fe}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{B}}_{20}$ alloy are investigated by measuring the time behavior of the electrical resistance of ribbon strips during such a treatment. The structural transformations occurring in subsequent stages of the process are studied by means of x-ray-diffraction, differential-scanning-calorimetry, and magnetic-permeability measurements. A continuous evolution from a fully amorphous to a fully crystalline structure may be followed. The crystallization mechanisms observed in Joule-heated samples differ from the ones occurring under conventional heating conditions. The electrical resistance displays a bump in the course of Joule heating. A quantitative model relating such a bump to the extra heat released to the sample by fast crystallization is proposed and discussed.

Series expansion of the Fermi-Dirac integral Fj(x) over the entire domain of real j and x

Abstract: An algorithm for the evaluation with arbitrary accuracy of the Fermi-Dirac integral F j (x) for any real value of j and x is presented. A new rapidly convergent series representation for x ⩾ 0 is derived. It involves confluent hypergeometric functions, for which several efficient implementations are available. Application of Euler transformation is proposed to improve the convergence of the two classical series expansions for x ⩽ 0 and |xvnb F j (x, b) is defined, and series expansions are provided for its evaluation. Finally, the computation of the inverse functions of F j (x) and F j (x, b) is discussed, and a comparison between three iterative methods is performed.

Measurement and prediction of dynamic loop shapes and power losses in soft magnetic materials

Abstract: Measurements of dynamic hysteresis loops on Fe/sub 78/B/sub 13/Si/sub 9/ amorphous ribbons, in the range 0.5 Hz >

High-quartz solid-solution phases from xerogels with composition 2MgO.2Al2O3.5SiO2 (μ-cordierite) and Li2O.Al2O3.nSiO2 (n = 2 to 4) (β-eucryptite): Characterization by XRD, FTIR and surface measurements

Abstract: The title phases are prepared by the sol-gel route employing prehydrolyzed tetraethylorthosilicate (TEOS) and Al, Li or Mg nitrates in aqueous solution. Amorphous xerogels are obtained at 400°C; they are characterized as high to very high surface oxides with peculiar pore-size distributions. High-quartz solid-solution phases originate from these xerogels by heating at 600°C (lithia-containing compositions) or at 900°C (magnesia-containing composition). They are characterized by X-ray powder diffraction and FTIR spectroscopy as phases with strong disorder in the cation distribution, both in the Al/Si sites and Li sites, with the exception of the stoichiometry LiAlSiO 4 , which shows a certain degree of Li and Si/Al ordering.

A computationally efficient unified approach to the numerical analysis of the sensitivity and noise of semiconductor devices

Abstract: The authors present a computationally efficient unified approach to the numerical simulation of sensitivity and noise in majority-carrier semiconductor devices that is based on the extension to device simulation of the adjoint method for sensitivity and noise analysis of electrical networks. Sensitivity and device noise analysis based on physical models are shown to have a common background, since they amount to evaluating the small-signal device response to an impressed, distributed current source. This problem is addressed by means of a Green's function technique akin to Shockley's impedance field method. To allow the efficient numerical evaluation of the Green's function within the framework of a discretized physical model, inter-reciprocity concepts, based on the introduction of an adjoint device, are exploited. Examples of implementation involving GaAs MESFETs are discussed. >

Discrete-layer models for multilayered shells accounting for interlayer continuity

Abstract: Based on adiscrete-layer approach, in a recent series of papers, the first author has developed the equations governing the elastodynamic behaviour of moderately thick multilayered anisotropic plates by making use of a displacement field which allows a non-linear variation of the in-plane displacements through the laminate thickness and fulfilsa priori the static and geometric continuity conditions at the interfaces between two adjacent layers. Based on this approach, in the present paper we derive the equations of motion and variationally consistent boundary conditions of moderately multilayered anisotropic shells. To show the accuracy and reliability of the proposed approach, closed-form solutions are given and compared with results from three-dimensional elasticity and other approximate bi-dimensional models with and without continuous interlayer stresses. Based on this numerical investigation, the proposed approach appears to work very well.

Geometrically uniform partitions of L*MPSK constellations and related binary trellis codes

Abstract: The theory of geometrically uniform trellis codes is applied to the case of multidimensional PSK (phase shift keying) constellations. The symmetry group of an L*MPSK (M-ary PSK) constellation is completely characterized. Conditions for rotational invariance of geometrically uniform partitions of a signal constellation are given. Through suitable algorithms, geometrically uniform partitions of L*MPSK (M=4,8,16 and L=1,2,3,4) constellations are found, which present good characteristics in terms of the set of distances at a given partition level, the maximum obtainable rotational invariance, and the isomorphism of the quotient group associated with the partition. These partitions are used as starting points in a search for good geometrically uniform trellis codes based on binary convolutional codes. >