Browsing by Issue Date, starting with "1992"
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- Applications of neural networks to control systemsPublication . Ruano, AntonioThis work investigates the applicability of artificial neural networks to control systems. The following properties of neural networks are identified as of major interest to this field: their ability to implement nonlinear mappings, their massively parallel structure and their capacity to adapt. Exploiting the first feature, a new method is proposed for PID autotuning. Based on integral measures of the open or closed loop step response, multilayer perceptrons (MLPs) are used to supply PID parameter values to a standard PID controller. Before being used on-line, the MLPs are trained offline, to provide PID parameter values based on integral performance criteria. Off-line simulations, where a plant with time-varying parameters and time varying transfer function is considered, show that well damped responses are obtained. The neural PID autotuner is subsequently implemented in real-time. Extensive experimentation confirms the good results obtained in the off-line simulations. To reduce the training time incurred when using the error back-propagation algorithm, three possibilities are investigated. A comparative study of higherorder methods of optimization identifies the Levenberg-Marquardt (LM)algorithm as the best method. When used for function approximation purposes, the neurons in the output layer of the MLPs have a linear activation function. Exploiting this linearity, the standard training criterion can be replaced by a new, yet equivalent, criterion. Using the LM algorithm to minimize this new criterion, together with an alternative form of Jacobian matrix, a new learning algorithm is obtained. This algorithm is subsequently parallelized. Its main blocks of computation are identified, separately parallelized, and finally connected together. The training time of MLPs is reduced by a factor greater than 70 executing the new learning algorithm on 7 Inmos transputers.
- Editing problems of the Romancero: the romantic traditionPublication . Ferré, PereSince the romancero is one of the genres belonging to the so-called oral literature, it may seem, at first, inappropriate as a subject in a volume entitled "The Politics of Editing". One should remember, however, that the romancero, born as many other medieval genres, to be preserved by memory, found another way of life after the fifteenth century, when it started to become available in writing as well.
- Abstract processes in texture discriminationPublication . du Buf, J. M. H.In this study some experiments on texture segmentation are reported using the local Gabor power spectrum. The techniques applied are: (1) supervised pixel classification; (2) boundary detection by spectral dissimilarity estimation; (3) region-based segmentation based on Gaussian spectral estimation; and (4) the same as (3) but based on central moments of the local spectrum. It is shown that very-acceptable-to-excellent results can be obtained. It is argued, however, that the shortcomings of region-based and boundary-based approaches require that both processes should act in parallel, not only in digital image processing but also in the modelling of visual perception.
- Modelling spatial vision at the threshold level.Publication . du Buf, J. M. H.Some available single- and multiple-channel models are reviewed. Multichannel models are generalized and tested against threshold data on various stimulus sets. Without using the explicit assumption of spatial probability summation, simple multichannel models are shown to provide good simultaneous predictions of threshold curves of sinewave gratings and other gratings. They fail in predicting threshold curves of disk-shaped stimuli. If global or local spatial probability summation within channels is incorporated into the models, correctly shaped threshold curves of disks can be predicted. However, the predicted curves appear still too low if compared to measured curves. The same holds for noise gratings. Possible extensions of the models, based on local summation between channel responses and/or models consisting of initial isotropic channels (retina) followed by anisotropic channels (cortex), are discussed.
- Purification and characterization of a pore-forming protein from the marine sponge Tethya lyncuriumPublication . Mangel, A.; Leitão, J. M.; Batel, R.; Zimmermann, H.; Muller, W. E. G.; Schroder, H. C.A pore-forming protein was detected and purified for the first time from a marine sponge (Tethya lyncurium). The purified protein has a polypeptide molecular mass of 21 kDa and a pI of 6.4. Tethya pore-forming protein (also called Tethya hemolysin) rapidly lysed erythrocytes from a variety of organisms. After binding to target membranes, the hemolysin resisted elution with EDTA, salt or solutions of low ionic strength and hence resembled an integral membrane protein. Erythrocytes could be protected from hemolysis induced by Tethya hemolysin by addition of 30 mM dextran 4 (4-6 kDa; equivalent hydrodynamic diffusion radius, 1.75-2.3 nm) to the extracellular medium, but not by addition of uncharged molecules of smaller size [sucrose, raffinose and poly(ethylene glycol) 1550; equivalent hydrodynamic diffusion radii, 0.46, 0.57 and 1.2 nm, respectively]. This result indicates that hemolysin is able to form stable transmembrane pores with an effective diameter of about 2-3 nm. Treatment of osmotically protected erythrocytes with Tethya hemolysin caused a rapid efflux of intracellular K+ and ATP, and a rapid influx of extracellularly added Ca2+ and sucrose. In negative-staining electron microscopy, target erythrocyte membranes exposed to purified Tethya hemolysin displayed ultrastructural lesions but without visible pores.
- Parallel implementation of a learning algorithm for multilayer perceptrons using transputersPublication . Ruano, Antonio; Jones, D. I.; Fleming, P. J.In this paper the parallelization of a new learning algorithm for multilayer perceptrons, specifically targeted for nonlinear function approximation purposes, is discussed. Each major step of the algorithm is parallelized, a special emphasis being put in the most computationally intensive task, a least-squares solution of linear systems of equations.
- Broadband matched-field processing of transient signals in shallow waterPublication . Jesus, S. M.This paper presents a full-wavefield narrowband high-resolution technique that uses the spectral decomposition of the sample covariance matrix to resolve the vertical arrival structure of the harmonic acoustic field. The broadband processor is obtained by weighted averaging of the narrowband range-depth ambiguity estimates within the source signal frequency band. Results obtained with this processor on short transient pulses collected in the North ElbaI. area in 1989 with a 62m aperture vertical array, showed stable and accurate localizations over long time intervals. These results also demonstrate that the sound field, received over a given frequency band, is relatively stable over time and is in agreement with the predictions given by a standard normal-mode propagation model.
- Brightness versus apparent contrast. 2: Large-field asymmetryPublication . du Buf, J. M. H.Experiments were performed on the quasi-static perception of brightness and of apparent contrast of a foveal 1-deg disk, presented either as a luminance increment or decrement against a 300 cd.m-2 background. Results suggest that the perceptual attributes of brightness and apparent (or subjective) contrast should be distinguished. For an equal brightness difference with respect to the background, luminance increments are more effective than decrements. For an equal apparent contrast it is found that increments and decrements, up to 100 cd.m-2, are about equally effective; for higher values luminance decrements are more effective. Brightness increments and decrements can both be described by a Stevens power function of the respective luminance increments and decrements. Apparent contrast can, apart from applying a usual luminance contrast formula, also be described as a power function of the luminance difference with the background.
- Influence of growth-regulators on shoot proliferation in Quercus-Suber LPublication . Romano, Anabela; Noronha, C.; Martins-Loução, M. A.Procedures have been developed to standardize the multiplication stage during mature cork-oak (Quercus suber L.) micropropagation. Axillary and terminal buds were established on Gresshoff and Doy basal medium containing 1 mg I−1 of 6-benzylaminopurine (BAP). Initiation of cultures was possible all over the year. The effects of BAP, Z, IBA, 1AA and NAA and various nutrient formulae on shoot growth and proliferation was investigated. BAP was more suitable than zeatin. Shoot proliferation and elongation were strongly improved by the combination BAP/IAA in the presence of low salt media, like Gresshoff and Doy or Woody Plant medium. Both rates were significantly increased when a double-phase culture system was used. Shoots have been multiplied for 1 year at the rate of three to four-fold every 4 weeks without any decline of vigour. Rooting was achieved by briefly dipping the basal ends of in vitro regenerated shoots in an IBA concentrated solution. The results here reported constitute a promising step towards large scale in vitro propagation of a species in which conventional vegetative propagation by cuttings is very difficult.
- Connectionist approach to PID autotuningPublication . Ruano, Antonio; Fleming, P. J.; Jones, D. I.Proportional, Integral and Derivative (PID) regulators are standard building blocks for industrial automation. The popularity of these regulatores comes from their rebust performance in a wide range of operationg conditions, and also from their functional simplicity, which makes them suitable for manual tuning.
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