Showing papers by "University of Twente published in 1994"
TL;DR: In this paper, an instructional design model for the training of complex cognitive tasks should provide instructional strategies that control cognitive load and recommend research in which the cognitive load of instructional manipulations is systematically investigated and determined with mental-effort based measures.
Abstract: Limited processing capacity constrains learning and performance in complex cognitive tasks. In traditional instruction, novices' failure to adequately learn cognitive tasks can often be attributed to the inappropriate direction of attention and the related high or excessive load that is imposed on a learner's cognitive system. An instructional design model for the training of complex cognitive tasks should provide instructional strategies that control cognitive load. We propose such a model and recommend research in which the cognitive load of instructional manipulations is systematically investigated and determined with mental-effort based measures.
753 citations
TL;DR: In this article, it is argued that the proportional reduction in (estimated) variance components is not an attractive parameter to represent the joint importance of the explanatory (independent) variables for modeling the dependent variable.
Abstract: The concept of explained proportion of variance or modeled proportion of variance is reviewed in the situation of the random effects hierarchical two-level model. It is argued that the proportional reduction in (estimated) variance components is not an attractive parameter to represent the joint importance of the explanatory (independent) variables for modeling the dependent variable. It is preferable instead to work with the proportional reduction in mean squared prediction error for predicting individual values (for the modeled variance at level 1) and the proportional reduction in mean squared prediction error for predicting group averages (for the modeled variance at level 2). It is shown that when predictors are added, the proportion of modeled variance defined in this way cannot go down in the population if the model is correctly specified, but can go down in a sample; the latter situation then points to the possibility of misspecification. This provides a diagnostic means for identifying misspecifi...
687 citations
TL;DR: It was concluded that the usefulness of both measurement techniques in instructional research needs to be investigated further.
Abstract: The results of two of our recent empirical studies were considered to assess the usefulness of subjective ratings and cardiovascular measures of mental effort in instructional research. Based on its reliability and sensitivity, the subjective rating-scale technique met the requirements to be useful in instructional research whereas the cardiovascular technique did not. It was concluded that the usefulness of both measurement techniques in instructional research needs to be investigated further.
630 citations
TL;DR: In this paper, the surface exchange kinetics in limiting the rate of oxygen permeation through mixed-conducting oxide ceramics are investigated. But the authors focus on the perovskite type oxides.
448 citations
TL;DR: In this article, the design, modelling and performance characteristics of electrostatically driven vacuum-encapsulated polysilicon resonators are addressed, and an expression for the pull-in voltage of a beam is given.
Abstract: In this paper, the design, modelling and performance characteristics of electrostatically driven vacuum-encapsulated polysilicon resonators are addressed. A one-port configuration is preferably employed for excitation and detection of the vibration. Mechanical instability (pull-in) is discussed on the basis of the energy minimum principle. An expression for the pull-in voltage of a beam is given. The electromechanical behaviour in a limited frequency regime around the fundamental resonance is accurately modelled by an electric circuit consisting of a (static) capacitor shunted by a series (dynamic) RLC branch. The d.c. bias dependence of the circuit components and of the series resonance frequency has been experimentally investigated and is compared with the theory. The large-amplitude behaviour is discussed as well. The plate modulus and residual strain of boron-doped polysilicon are estimated from the resonance frequencies of microbridges of varying lengths. The feasibility of their application as resonant strain gauges is investigated. The 210 m long beams typically have an unloaded fundamental frequency of 324 kHz, a gauge factor of 2400 and an uncompensated temperature coefficient of -135 ppm 0C-1.
417 citations
TL;DR: A quasi-evolutionary model is proposed which underpins the analysis of suggested strategies that could turn electric vehicles from an elusive legend, a plaything, into a marketable product.
414 citations
TL;DR: In this paper, it was shown that integration of a capacitive microphone and a preamplifier will result in a further reduction of the noise level of the FET microphone.
Abstract: Silicon micromachining has successfully been applied to fabricate piezoelectric, piezoresistive and capactive microphones. The use of silicon has allowed the fabrication of microphones with integrated electronic circuitry and the development of the new FET microphone. The introduction of lithographic techniques has resulted in microphones with very small (1 mm2) diaphragms and with specially shaped backplates. The application of corrugated diaphragms seems a promising future development for silicon microphones. It is concluded from a noise consideration that the FET microphone shows a high noise level, which is mainly due to the small sensor capacitance. From this noise consideration, it can be shown that integration of a capacitive microphone and a preamplifier will result in a further reduction of the noise.
374 citations
TL;DR: In this article, a theoretical analysis of the turbulent stress tensor in large-eddy simulation is presented from a theoretical point of view, where realizability conditions for the components of this tensor are derived, which hold if and only if the filter function is positive.
Abstract: The turbulent stress tensor in large-eddy simulation is examined from a theoretical point of view. Realizability conditions for the components of this tensor are derived, which hold if and only if the filter function is positive. The spectral cut-off, one of the filters frequently used in large-eddy simulation, is not positive. Consequently, the turbulent stress tensor based on spectrally filtered fields does not satisfy the realizability conditions, which leads to negative values of the generalized turbulent kinetic energy k. Positive filters, e. g. Gaussian or top-hat, always give rise to a positive k. For this reason, subgrid models which require positive values for k should be used in conjunction with e. g. the Gaussian or top-hat filter rather than with the spectral cutoff filter. If the turbulent stress tensor satisfies the realizability conditions, it is natural to require that the subgrid model for this tensor also satisfies these conditions. With respect to this point of view several subgrid models are discussed. For eddy-viscosity models a lower bound for the generalized turbulent kinetic energy follows as a necessary condition. This result provides an inequality for the model constants appearing in a ‘Smagorinsky-type’ subgrid model for compressible flows.
306 citations
TL;DR: In this article, a simple procedure is provided to write the equations of motion of mechanical systems with constraints as Hamiltonian equations with respect to a?Poisson? bracket on the constrained state space, which does not necessarily satisfy the Jacobi identity.
298 citations
TL;DR: In this article, a number of elements of a general model of quality assessment in higher education are presented, which are put in a historical context and deduced from the recent experiences with quality assessment from both North American and Western European countries.
Abstract: In this article a number of elements of a general model of quality assessment in higher education are presented. On the one hand these elements are put in a historical context of quality assessment in Medieval universities and, on the other hand, deduced from the recent experiences with quality assessment in both North-American and Western European countries. With respect to the historical context a distinction is made between the intrinsic and the extrinsic values of higher education. Two types of quality assessment related to these values are also distinguished. Concerning the recent experiences with quality assessment systems, the practices in the U.S.A., Canada, France, the Netherlands and the United Kingdom are explored. In the final section the general mode of quality assessment is discussed in the context of the distinction between the intrinsic and the extrinsic values of higher education.
236 citations
TL;DR: A new mode of operation is described in which the standard V-shaped silicon nitride cantilever is oscillated under liquid and damped by the interaction between AFM tip and sample surface, making it possible to study physiological processes, such as cell growth, with a minimal level of perturbation and high spatial resolution.
TL;DR: In this article, the authors investigated the mechanisms causing stiction of polysilicon structures fabricated by surface micromachining techniques and found that during drying from rinse liquids attractive dynamic capillary forces are responsible for bringing micromechanical structures into contact with the underlying substrate.
Abstract: The mechanisms causing stiction of polysilicon structures fabricated by surface micromachining techniques have been investigated. It is found that during drying from rinse liquids attractive dynamic capillary forces are responsible for bringing micromechanical structures into contact with the underlying substrate. Measured adhesion energies of sticking microbridges indicate that van der Waals forces are responsible for the stiction of hydrophobic surfaces and that hydrogen bridging is an additional adhesion mechanism for hydrophilic surfaces. Methods to reduce the stiction problem are indicated.
TL;DR: Of the crosslinked DSC samples, ENDSC seems most promising for tissue regeneration, which may explain the minor ingrowth of giant cells and fibroblasts, and the poor formation of new rat collagen.
Abstract: The biocompatibility and tissue regenerating capacity of four crosslinked dermal sheep collagens (DSC) was studied. In vitro, the four DSC versions were found to be noncytotoxic or very low in cytoxicity. After subcutaneous implantation in rats, hexamethylenediisocyanatecrcrosslinked DSC (HDSC) seldom induced an increased infiltration of neutrophils or macrophages, as compared with normal wound healing; whereas new formation of collagen was observed. DSC crosslinked with glutaraldehyde (GDSC) followed by reaction with NaBH4 shortly after implantation showed an increased infiltration of neutrophils with a deviant morphology. Furthermore, a high incidence of calcification was observed, which may explain the minor ingrowth of giant cells and fibroblasts, and the poor formation of new rat collagen. Acyl azide-crosslinked DSC (AaDSC) first induced an increased infiltration of macrophages, and then of giant cells, both with high lipid formation. AaDSC degraded at least twice as slowly as HDSC and GDSC, finally leaving a matrix of newly formed rat collagen. Samples crosslinked with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide (ENDSC) induced the same mild cellular reaction as HDSC; whereas, similar to AaDSC, the degradation rate was slow and an optimal rat collagen matrix was formed. Of the crosslinked DSC samples, ENDSC seems most promising for tissue regeneration.
TL;DR: In this paper, a fabrication process based on surface-micromachining techniques for electrostatically driven vacuum-encapsulated polysilicon resonators is presented, and a novel freeze-drying method that does not require vacuum equipment is presented.
Abstract: Basic design issues and a fabrication process based on surface-micromachining techniques for electrostatically driven vacuum-encapsulated polysilicon resonators are presented. A novel freeze-drying method that does not require vacuum equipment is presented. Reactive sealing with LPCVD silicon nitride is used to create the evacuated cavity, resulting in cavity pressures close to the deposition pressure. Design issues regarding choice of materials, technology and layout are discussed. First experimental results, including an admittance plot of the one-port resonator and a plot indicating the dependence of the Q-factor on the resonator geometry and ambient pressure, are presented.
TL;DR: In this paper, the incorporation of zeolite particles in the micrometer range into polymeric matrices was investigated as a way to improve the gas separation properties of the polymer materials used in the form of membranes.
Abstract: The incorporation of zeolite particles in the micrometer range into polymeric matrices was investigated as a way to improve the gas separation properties of the polymer materials used in the form of membranes. The adhesion between the polymer phase and the external surface of the particles appeared to be a major problem in the preparation of such membranes when the polymer is in the glassy state at room temperature. Various methods were investigated to improve the internal membrane structure, that is, surface modification of the zeolite external surface, preparation above the glass-transition temperature, and heat treatment. Improved structures were obtained as observed by scanning electron microscopy, but the influence on the gas separation properties was not in agreement with the observed structural improvements.
TL;DR: In this paper, a multilevel solver for the circular contact was extended to elliptical contact problems, and it was demonstrated how and when, for contacts with the entrainment directed perpendicular to the major principal axis of the contact ellipse, the pressure and film thickness on the centre-line of a contact can be predicted accurately from an equivalent line contact analysis.
TL;DR: Two kinds of graph polynomials are introduced, clique polynomial and independent setPolynomial, and expansions of these polynmials are focused on.
TL;DR: In this paper, on Intervals, Transitivity = Chaos and Transitivity + Chaos = Chaos, the American Mathematical Monthly: Vol. 101, No. 4, pp. 353-355.
Abstract: (1994). On Intervals, Transitivity = Chaos. The American Mathematical Monthly: Vol. 101, No. 4, pp. 353-355.
TL;DR: In this article, a simple analytical model has been proposed that takes the influence of initial tensile stress into account, and the model predicts that the presence of corrugations increases the sensitivity of the diaphragms, because the initial diaphrasm stress is reduced.
Abstract: Silicon nitride corrugated diaphragms of 2 mm/spl times/2 mm/spl times/1 /spl mu/m have been fabricated with 8 circular corrugations, having depths of 4, 10, or 14 /spl mu/m. The diaphragms with 4-/spl mu/m-deep corrugations show a measured mechanical sensitivity (increase in the deflection over the increase in the applied pressure) which is 25 times larger than the mechanical sensitivity of flat diaphragms of equal size and thickness. Since this gain in sensitivity is due to reduction of the initial stress, the sensitivity can only increase in the case of diaphragms with initial stress. A simple analytical model has been proposed that takes the influence of initial tensile stress into account. The model predicts that the presence of corrugations increases the sensitivity of the diaphragms, because the initial diaphragm stress is reduced. The model also predicts that for corrugations with a larger depth the sensitivity decreases, because the bending stiffness of the corrugations then becomes dominant. These predictions have been confirmed by experiments. The application of corrugated diaphragms offers the possibility to control the sensitivity of thin diaphragms by geometrical parameters, thus eliminating the effect of variations in the initial stress, due to variations in the diaphragm deposition process and/or the influence of temperature changes and packaging stress. >
Journal Article•
TL;DR: There are variations of the dorsal cerebrospinal fluid layer among subjects by more than a factor of 2, and significant variations ofThe mediolateral position of the spinal cord, information on these parameters in patients will be essential for the optimal application of epidural spinal cord stimulation.
Abstract: PURPOSE To investigate intradural geometry, which strongly influences the effects of epidural spinal cord stimulation. METHODS Axial MR images with turbo spin-echo were made of 26 healthy subjects at C-4 through C-6, T-5 and T-6, and T-11 and T-12, at T-11 and T-12 both in the supine and the prone position. Measurements were made of the dorsomedial and the ventromedial cerebrospinal fluid layer and the anteroposterior and transverse sizes of both the spinal cord and the dural sac. The samples of all variables were analyzed statistically. The distance between spinal and vertebral midline was also determined. RESULTS The dorsal cerebrospinal fluid layer was 1.5 to 4.0 mm at C-4 through C-6 and 4.0 to 8.5 mm at T-5 and T-6. At T-11 it was 2.0 to 6.0 mm in the supine position and was increased by approximately 2.2 mm in the prone position. At T-12 these values were 1.5 to 4.5 mm and approximately 3.4 mm, respectively. Differences between the spinal and vertebral midline up to 1.5 to 2.0 mm occurred in approximately 40% of the images. CONCLUSIONS Because there are variations of the dorsal cerebrospinal fluid layer among subjects by more than a factor of 2, and significant variations of the mediolateral position of the spinal cord, information on these parameters in patients will be essential for the optimal application of epidural spinal cord stimulation.
TL;DR: In this article, the host-guest compound and its dynamic behavior was studied in detail by NMR methods, and the structure of the host and guest compound was shown on the cover of this issue.
Abstract: Different orientations of the guest in the cavity of a carcerand lead to a new type of stereoisomerism, “carceroisomerism”. The structure of the host–guest compound (which is shown on the cover of this issue) and its dynamic behavior was studied in detail by NMR methods. Such compounds could perhaps find application as molecular switches for electronic devices.
TL;DR: In this article, the influence of a transverse ridge on the film thickness in a circular EHL contact under rolling/sliding conditions was investigated, and it was shown that the agreement between experiment and Newtonian isothermal theory is good.
Abstract: This paper investigates the influence of a transverse ridge on the film thickness in a circular EHL contact under rolling/sliding conditions. It is a numerical simulation of the optical EHL work of Kaneta et al. (1992). One of the purposes of this investigation is to check the validity of the algorithm and the Newtonian, isothermal lubricant assumption for film thickness predictions under these conditions (ph = 0.54 GPa). It will be shown that, both quantitatively, the film thickness on the central axis Y = 0, and qualitatively, the film thickness profile through “pseudo interference graphs”, the agreement between experiment and Newtonian isothermal theory is good. This supports the argument that the rheological and the thermal behavior of the fluid only slightly influence the film thickness and pressure distribution of the lightly loaded non-smooth contact case.
TL;DR: In this article, the authors investigate the computational complexity of scheduling multiprocessor tasks with prespecified processor allocations, and investigate the complexity of problems when precedence constraints or release dates are involved.
TL;DR: In this paper, two cases of oblique-incidence deposition are discussed: one obliquely-impinging flux and two normalizing fluxes from opposite directions, with respect to the microstructure, both similarities and differences between these deposition will be described.
Abstract: In this paper features are discussed that are related to the nucleation and growth of thin films deposited by physical vapour deposition. Although the paper is mainly focused on oblique-incidence deposition, a normally-impinging vapour flux will also be described. Two cases of oblique-incidence deposition will be discussed: one obliquely-impinging flux and two obliquely-impinging fluxes from opposite directions. With respect to the microstructure, both similarities and differences between these deposition will be described. Further, a special feature of two obliquely-impinging vapour fluxes is dealt with, which is the local composition in the lateral direction (parallel to the substrate). The oblique and opposing directions can induce an inhomogeneous chemical composition. This process is referred to as process-induced compositional separation. In addition to reviewing the literature, the paper summarizes experimental data on obliquely co-evaporated Co-Cr, Co-Ag and Co-Ta films. It will be shown from these experimental data that the two vapour directions can be exploited to tailor compositional inhomogeneities.
TL;DR: In this paper, the L2-gain almost disturbance decoupling problem for SISO nonlinear systems is formulated and sufficient conditions are identified for the existence of a parametrized state feedback controller such that the L 2-gain from disturbances to output can be made arbitrarily small by increasing its gain.
TL;DR: The computer predictions showed that partly coating can only significantly reduce bone atrophy relative to fully coated stems, when the coating is applied at a small region at the utmost proximal part of the stem.
Abstract: Bone atrophy caused by stress-shielding may cause serious complications for the long-term fixation of hip stems. In particular, uncemented total hip arthroplasty is threatened by this problem, because the stems are usually larger and, as a consequence, stiffer than those of cemented implants. In the present study, the effects of fit and bonding characteristics of femoral hip stems were investigated, using the (nonlinear) finite element method in combination with adaptive bone remodeling theory to predict the bone density distribution in a bone or bone/implant configuration. Unknown parameters used in the theory, such as a reference equilibrium loading stimulus and a threshold (dead) zone of this stimulus, were established (triggered) by using the method to predict the density distributions in the natural femur and around fully coated uncemented implants. The computer simulation method can provide long term predictions of remodeling patterns around various implant configurations. Several cases were analyzed, whereby the coating conditions (fully, partly, or noncoated) and the fit characteristics (press fitted or overreamed) were varied. The computer predictions showed that partly coating can only significantly reduce bone atrophy relative to fully coated stems, when the coating is applied at a small region at the utmost proximal part of the stem. For smooth press-fit stems the predicted amount of bone loss (35 percent in the proximal medial region) was less than for a one-third proximally coated or a fully coated stem (50 to 54 percent predicted bone loss in the proximal medial region).(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: In this article, the performance of nanostructured titania ceramics prepared from both peptized and unpeptized sols was compared using XRD, DSC, and nitrogen-gas physisorption techniques.
Abstract: Porosity reduction, packing, pore-size distribution, and anatase to rutile phase transformation behavior of nanostructured titania ceramics prepared from both peptized and unpeptized sols were studied and compared using XRD, DSC, and nitrogen-gas physisorption techniques. Precursor gels prepared from the peptized sol had a green density of about 70% after drying at 40°C, whereas the samples prepared from the unpeptized sol had a green density of only 50%. Samples prepared from the peptized sol showed higher sintering and phase transformation rates compared to the unpeptized sols.
TL;DR: In this article, an extended version of the mass transfer model for a four-component system is evaluated, which is shown to be generally valid for short times, and the thermodynamics under these circumstances are evaluated, together with the kinetics.
TL;DR: In this article, a sociological approach to research councils allows analysis of changes in the reward system of science (where lsquorelevancersquo is becoming an accepted criterion worldwide) and of the complex environment of research councils, where many actors now compete for the intermediary role.
Abstract: Research councils began as channels for state patronage of science (a widespread phenomenon after World War II) and were captured by the scientists: peer review of proposals, panels, board membership. In this way, they became an important organ of the lsquoRepublic of Sciencersquo (Michael Polanyi's concept). Being awarded a grant is now as important for the reputation or status of a scientist as the money value per se: research councils have become part of the reward system of science. Credibility-cycle analysis (Latour and Woolgar) is used to show this; and then applied to the research council itself, between the State and the national scientific community. Current concerns about proposal success rates and conservatism are analysed in terms of dynamics of this research world. This sociological approach to research councils allows analysis of changes in the reward system of science (where lsquorelevancersquo is becoming an accepted criterion world-wide) and of the complex environment of research councils, where many actors now compete for the intermediary role. Research councils must also become entrepreneurial-or become obsolete.
26 Sep 1994
TL;DR: A structured language for the specification of knowledge models according to the CommonKADS methodology, called CML (Conceptual Modelling Language), which provides both a structured textual notation and a diagrammatic notation for expertise models.
Abstract: We present a structured language for the specification of knowledge models according to the CommonKADS methodology. This language is called CML (Conceptual Modelling Language) and provides both a structured textual notation and a diagrammatic notation for expertise models. The use of our CML is illustrated by a variety of examples taken from the VT elevator design system.