Showing papers by "University of Twente published in 1996"

L2-Gain and Passivity Techniques in Nonlinear Control

Abstract: 1 Input-Output Stability.- 2 Small-gain and Passivity of Input-Output Maps.- 3 Dissipative Systems Theory.- 4 Hamiltonian Systems as Passive Systems.- 5 Passivity by Feedback.- 6 Factorizations of Nonlinear Systems.- 7 Nonlinear H? Control.- 8 Hamilton-Jacobi Inequalities.

Method engineering: engineering of information systems development methods and tools

Abstract: This paper proposes the term method engineering for the research field of the construction of information systems development methods and tools. Some research issues in method engineering are identified. One major research topic in method engineering is discussed in depth: situational methods, i.e. the configuration of a project approach that is tuned to the project at hand. A language and support tool for the engineering of situational methods are discussed.

Combining kohonen maps with arima time series models to forecast traffic flow

Abstract: A hybrid method of short-term traffic forecasting is introduced; the KARIMA method. The technique uses a Kohonen self-organizing map as an initial classifier; each class has an individually tuned ARIMA model associated with it. Using a Kohonen map which is hexagonal in layout eases the problem of defining the classes. The explicit separation of the tasks of classification and functional approximation greatly improves forecasting performance compared to either a single ARIMA model or a backpropagation neural network. The model is demonstrated by producing forecasts of traffic flow, at horizons of half an hour and an hour, for a French motorway. Performance is similar to that exhibited by other layered models, but the number of classes needed is much smaller (typically between two and four). Because the number of classes is small, it is concluded that the algorithm could be easily retrained in order to track long-term changes in traffic flow and should also prove to be readily transferrable.

On the kinetics between co2 and alkanolamines both in aqueous and non-aqueous solutions. an overview

Abstract: Alkanolamine solutions are frequently used as solvent for the removal of acid compounds from industrial gases (Kohl and Riesenfeld, 1979). Depending on the process requirements, e.g., selective removal of H2S, CO2-bulk removal, several options for alkanolamine based treating solvents with varying compositions of the solution have been proposed. In this paper an overview is presented of the mechanisms that have been proposed in literature and the kinetic data for the various reactions are critically evaluated. Conclusions on the applicability and restrictions are discussed along with perspectives. In addition white spots in the present knowledge are indicated. The reaction between CO2 and primary/secondary amines both in aqueous and non-aqueous solutions can be described over a wide range of conditions and amine concentrations with the zwitterion-mechanism as originally proposed by Caplow (1968) and reintroduced by Danckwerts (1979). All published results, both non-aqueous and aqueous solutions, amine-prom...

Test Generation with Inputs, Outputs and Repetitive Quiescence

Abstract: This paper studies testing based on labelled transition systems, using the assumption that implementations communicate with their environment via inputs and outputs. Such implementations are formalized by restricting the class of transition systems to those systems that can always accept input actions, as in Input/Output Automata. Implementation relations, formalizing the notion of correctness of these implementations with respect to labelled transition system specifications, are defined analogous to the theories of testing equivalence and preorder, and refusal testing. A test generation algorithm is given which is proved to produce a sound and exhaustive test suite from a specification, i.e., a test suite that fully characterizes the set of correct implementations.

Stiction in surface micromachining

Abstract: Due to the smoothness of the surfaces in surface micromachining, large adhesion forces between fabricated structures and the substrate are encountered. Four major adhesion mechanisms have been analysed: capillary forces, hydrogen bridging, electrostatic forces and van der Waals forces. Once contact is made adhesion forces can be stronger than the restoring elastic forces and even short, thick beams will continue to stick to the substrate. Contact, resulting from drying liquid after release etching, has been successfully reduced. In order to make a fail-safe device stiction during its operational life-time should be anticipated. Electrostatic forces and acceleration forces caused by shocks encountered by the device can be large enough to bring structures into contact with the substrate. In order to avoid in-use stiction adhesion forces should therefore be minimized. This is possible by coating the device with weakly adhesive materials, by using bumps and side-wall spacers and by increasing the surface roughness at the interface. Capillary condensation should also be taken into account as this can lead to large increases in the contact area of roughened surfaces.

A survey on the reactive ion etching of silicon in microtechnology

Abstract: This article is a brief review of dry etching as applied to pattern transfer, primarily in silicon technology. It focuses on concepts and topics for etching materials of interest in micromechanics. The basis of plasma-assisted etching, the main dry etching technique, is explained and plasma system configurations are described such as reactive ion etching (RIE). An important feature of RIE is its ability to achieve etch directionality. The mechanism behind this directionality and various plasma chemistries to fulfil this task will be explained. Multi-step plasma chemistries are found to be useful to etch, release and passivate micromechanical structures in one run successfully. Plasma etching is extremely sensitive to many variables, making etch results inconsistent and irreproducible. Therefore, important plasma parameters, mask materials and their influences will be treated. Moreover, RIE has its own specific problems, and solutions will be formulated. The result of an RIE process depends in a non-linear way on a great number of parameters. Therefore, a careful data acquisition is necessary. Also, plasma monitoring is needed for the determination of the etch end point for a given process. This review is ended with some promising current trends in plasma etching.

Compressible mixing layer growth rate and turbulence characteristics

Abstract: Direct numerical simulation databases have been used to study the effect of compressibility on mixing layers. The simulations cover convective Mach numbers from 0.2 to 1.2 and all contain a fully resolved turbulent energy cascade to small spatial scales. Statistical information is extracted from the databases to determine reasons for the reduced growth rate that is observed as the convective Mach number is increased. It is found that the dilatational contribution to dissipation is negligible even when eddy shocklets are observed in the flow. Also pressure-dilatation is not found to be significant. Using an accurate relation between the momentum thickness growth rate and the production of turbulence kinetic energy together with integrated equations for the Reynolds stress tensor it is shown that reduced pressure fluctuations are responsible for the changes in growth rate via the pressure–strain term. A deterministic model for the required pressure fluctuations is given based on the structure of variable-density vortices and the assumption that the limiting eddies are sonic. Simple anisotropy considerations are used to close the averaged equations. Good agreement with turbulence statistics obtained from the simulations is found.

Chapter 10 Dense ceramic membranes for oxygen separation

Abstract: Publisher Summary This chapter reviews the recent developments in the area of mixed ionic-electronic conducting membranes for oxygen separation, in which the membrane material is made dense—that is, free of cracks and connected-through porosity, being susceptible only for oxygen ionic and electronic transport. Emphasis is on the defect chemistry, mass transport, and the associated surface exchange kinetics. The basic elements of mixed ionic and electronic transport through dense ceramic membranes are focused. The chapter discusses mixed-conducting acceptor-doped perovskite and perovskite-related oxides and gives examples to illustrate the fundamental factors determining the oxygen fluxes through dense ceramic membranes. A key factor in the possible application of oxygen ion conducting ceramics is that, for use as solid electrolyte in fuel cells, batteries, oxygen pumps or sensors, their electronic transport number should be as low as possible. Stimulated by the search for candidate materials for electrodes in solid oxide fuel cells (SOFC) and oxygen separation membranes, researchers have explored the possibility of introducing electronic conductivity in oxygen-ion conducting fluorite-type matrices by doping with multi-valent dopants.

Stability of supported liquid membranes: State of the art

Abstract: This paper presents a state of art review on the stability of supported liquid membranes (SLM). The backgrounds of SLM instability phenomena are presented, and various mechanisms for explaining these phenomena are treated in detail. Several suggestions for stability improvement are discussed. ∗ Present address: Agrotechnological Research Institute (ATO-DLO), Fatty Acid and Membrane Technology, P.O, Box 17, NL-6700 AA Wageningen, The Netherlands.

Transcoding of MPEG bitstreams

Abstract: This paper discusses the problem of transcoding as it may occur in, for instance, the following situation. Suppose a satellite transmits an MPEG-compressed video signal at say 9 Mbit/s. This signal must be relayed at a cable head end. However, since the cable capacity is only limited, the cable head end will want to relay this incoming signal at a lower bit-rate of, say, 5 Mbit/s. The problem is how to convert a compressed video signal of a given bit-rate into a compressed video signal of a lower bit-rate. The specific transcoding problem discussed in this paper is referred to as bit-rate conversion. Basically, a transcoder used for such a purpose will consist of a cascaded decoder and encoder. It is shown in the paper that the complexity of this combination can be significantly reduced. The paper also investigates the loss of picture quality that may be expected when a transcoder is in the transmission chain. The loss of quality as compared to that resulting in the case of transmission without a transcoder is studied by means of computations using simplified models of the transmission chains and by means of using computer simulations of the complete transmission chain. It will be shown that the presence of two quantizers, i.e. cascaded quantization, in the transmission chain is the main cause of extra losses, and it will be shown that the losses in terms of SNR will be some 0.5 ? 1.0 dB greater than in the case of a transmission chain without a transcoder.

Conformance testing with labelled transition systems: implementation relations and test generation

Abstract: This paper studies testing based on labelled transition systems, presenting two test generation algorithms with their corresponding implementation relations. The first algorithm assumes that implementations communicate with their environment via symmetric, synchronous interactions. It is based on the theory of testing equivalence and preorder, as is most of the testing theory for labelled transition systems, and it is found in the literature in some slightly different variations. The second algorithm is based on the assumption that implementations communicate with their environment via inputs and outputs. Such implementations are formalized by restricting the class of labelled transition systems to those systems that can always accept input actions. For these implementations a testing theory is developed, analogous to the theory of testing equivalence and preorder. It consists of implementation relations formalizing the notion of conformance of these implementations with respect to labelled transition system specifications, test cases and test suites, test execution, the notion of passing a test suite, and the test generation algorithm, which is proved to produce sound test suites for one of the implementation relations.

Solubilization of NaX Salts in Chloroform by Bifunctional Receptors

Abstract: First the cation, then the anion. This describes the complexation of alkali metal halides with bifunctional receptors like that on the right (R = n‐octyl) based on calix[4]arenes. The uncomplexed ligand has a strong intramolecular hydrogen bond between the urea moieties, which is disrupted by the conformational change induced by complexation of a Na+ ion. Only then is an anion like Cl− bound in the upper portion of the calixarene.

Micromechanical Definition of the Strain Tensor for Granular Materials

Abstract: In order to develop constitutive relations for granular materials from the micromechanical viewpoint, general expressions relating macroscopic stress and strain to contact forces and particle displacements are required. Such an expression for the stress tensor under quasi-static conditions is well established in the literature, but a corresponding expression for the strain tensor has been lacking so far. This paper presents such an expression for two-dimensional assemblies. This expression is verified by computer simulations of biaxial and shear tests. As a demonstration of the use of the developed expression, a study is made of the elastic moduli of two-dimensional, isotropic assemblies of bonded, nonrotating disks. Theoretical expressions are given for the elastic moduli in terms of micromechanical parameters, such as coordination number and contact stiffnesses. Comparison with the results from computer simulations show that the agreement is fairly good over a wide range of coordination numbers and contact stiffness ratios.

Comparison of numerical schemes in large eddy simulation of the temporal mixing layer

Abstract: A posteriori tests of large-eddy simulations for the temporal mixing layer are performed using a variety of numerical methods in conjunction with the dynamic mixed subgrid model for the turbulent stress tensor. The results of the large-eddy simulations are compared with filtered direct numerical simulation (DNS) results. Five numerical methods are considered. The cell vertex scheme (A) is a weighted second-order central difference. The transverse weighting is shown to be necessary, since the standard second-order central difference (A) gives rise to instabilities. By analogy, a new weighted fourth-order central difference (B) is constructed in order to overcome the instability in simulations with the standard fourth-order central method (B). Furthermore, a spectral scheme (C) is tested. Simulations using these schemes have been performed for the case where the filter width equals the grid size (I) and the case where the filter width equals twice the grid size (II). The filtered DNS results are best approximated in case II for each of the numerical methods A, B and C. The deviations from the filtered DNS data are decomposed into modelling error effects and discretization error effects. In case I the absolute modelling error effects are smaller than in case II owing to the smaller filter width, whereas the discretization error effects are larger, since the flow field contains more small-scale contributions. In case I scheme A is preferred over scheme B, whereas in case II the situation is the reverse. In both cases the spectral scheme C provides the most accurate results but at the expense of a considerably increased computational cost. For the prediction of some quantities the discretization errors are observed to eliminate the modelling errors to some extent and give rise to reduced total errors.

A versatile and highly efficient catalyst system for the preparation of polyesters based on lanthanide tris(2,6-di-tert-butylphenolate)s and various alcohols

Abstract: The catalyst system based on lanthanide tris(2,6-di-tert-butylphenolate)s ands various alcohols was found to be very effective in the solution polymerization of L-lactide, e-caprolactone and δ-valerolactone. Good control over molecular weight, molecular weight distribution and end groups can be exerted

Test Generation with Inputs, Outputs, and Quiescence

Abstract: This paper studies testing based on labelled transition systems, using the assumption that implementations communicate with their environment via inputs and outputs. Such implementations are formalized by restricting the class of transition systems to those systems that can always accept input actions, as in input/output automata. Implementation relations, formalizing the notion of conformance of these implementations with respect to labelled transition system specifications, are defined analogous to the theory of testing equivalence and preorder. A test generation algorithm is given, which is proved to produce a sound and exhaustive test suite from a specification, i.e., a test suite that fully characterizes the set of correct implementations.

Kinetics and mechanism of L-lactide polymerization using two different yttrium alkoxides as initiators

Abstract: Polymerization of l-lactide in dichloromethane at 25 °C using an initiating system comprising tris(2,6-di-tert-butylphenoxy)yttrium and 2-propanol was first order in monomer and initiator. A propagation rate constant of 1.92 × 103 L mol-1 min-1 was determined using in situ UV spectroscopy. In the first step the large 2,6-di-tert-butylphenoxy ligands are exchanged for the sterically less demanding 2-propoxide ligands. In the second step the alkoxide is able to attack the carbonyl group with formation of the ring-opened product. After the first two initiating steps the polymerization can proceed until the equilibrium conversion of monomer is reached. Evidence for these three steps is provided by 1H and 13C NMR spectroscopy of the reaction products of each step. The kinetic behavior of commercially available yttrium isopropoxide was more complicated.

1,3-Alternate Calix[4]arenecrown-5 Conformers: New Synthetic Ionophores with Better K+/Na+ Selectivity than Valinomycin

Abstract: New 25,27-dialkoxycalix-[4]arenecrown-5 conformers 8, 10, and 11 have been synthesized and studied. The compounds 8a and 8b, fixed in 1,3-alternate structure, have been obtained in 57 and 40% yield, respectively, by reaction of the corresponding 25,27-dialkoxycalix[4]arenes 7a-b with tetraethylene glycol di-p-toluenesulfonate in the presence of Cs2CO3. The cone 10a and 10b and the partial cone 11 conformers were obtained by selective demethylation of the 25,27-dimethoxycalix[4]arenecrown-5 (6a) and subsequent dialkylation with NaH/DMF and KOtBu/THF, respectively. In the solid state (X-ray), compound 6a adopts a flattened cone conformation, which is also found to be most abundant in CD3CN and CD3OD solution. Upon complexation with potassium picrate compound 6a was converted quantitatively into the 1,3-alternate conformation. All new ligands synthesized were used in the extraction of alkali metal cations from H2O into CHCl3, and as active components in supported liquid membranes and in chemically modified field effect transistors. Results were compared to those obtained with with the natural antibiotic valinomycin 1. All ligands showed high selectivity for potassium. Ligand 8a, fixed in the 1,3-alternate conformation, is more selective than valinomycin and shows the highest K+/Na+ selectivity known so far.

The complementary-slackness class of hybrid systems

Abstract: In this paper we understand a “hybrid system” to be one that combines features of continuous dynamical systems with characteristics of finite automata. We study a special class of such systems which we call the complementary-slackness class. We study existence and uniqueness of solutions in the special'cases oflinear andHamiltonian complementary-slackness systems. For the latter class we also prove an energy inequality.