Robust control strategies for multi-inventory systems with average flow constraints

In this paper, we consider multi-inventory systems in the presence of uncertain demand. We assume that (i) demand is unknown but bounded in an assigned compact set and (ii) the control inputs (controlled flows) are subject to assigned constraints. Given a long-term average demand, we select a nominal flow that feeds such a demand. In this context, we are interested in a control strategy that meets at each time all possible current demands and achieves the nominal flow in the average. We provide necessary and sufficient conditions for such a strategy to exist and we characterize the set of achievable flows. Such conditions are based on linear programming and thus they are constructive. In the special case of a static flow (i.e. a system with 0-capacity buffers) we show that the strategy must be affine. The dynamic problem can be solved by a linear-saturated control strategy (inspired by the previous one). We provide numerical analysis and illustrative examples.     

Translational Parallel Manipulators: New Proposals

Translational parallel manipulators are parallel manipulators wherein the end‐effector performs only spatial translations. This paper presents a new family of translational parallel manipulators. The manipulators of this family are independent constraint manipulators. They have three limbs that are topologically identical and have no rotation singularity. The limbs of these manipulators feature five one‐degree‐of‐freedom kinematic pairs in series. Four joints are revolute pairs and the remaining one, called T‐pair, is a kinematic pair that can be manufactured in different ways. In each limb, three adjacent revolute pairs have parallel axes and the remaining revolute pair has an axis that is not parallel to the axes of the other revolute pairs. The mobility analysis of the manipulators of this new family is addressed by taking into account two different choices for the actuated pairs. One of the results of this analysis is that the geometry of a translational parallel manipulator free from singularities can be defined for a particular choice of the actuated pairs. © 2002 Wiley Periodicals, Inc.     

Evaluation of a material parameter extraction algorithm using MRI-based displacement measurements

The performance of an inversion algorithm is investigated when applied to measured displacement data for a determination of the material parameters /sup /spl lambda/+2/spl mu////sub /spl rho// (longitudinal wave velocity squared) and /sup /spl mu////sub /spl rho// (shear wave velocity squared) throughout an inhomogeneous test phantom. The vector displacement components throughout a test phantom subject to monochromatic shear excitation measured in time using magnetic resonance imaging (MRI) were temporally Fourier transformed to extract the component of monochromatic excitation, and the data was delivered to the inversion algorithm. A series of inversions is presented demonstrating the effects of subsequent wavenumber filtering, polarization selection, and variation in the size of the incremental volume elements. The resulting performance is assessed, and recommendations for future efforts are discussed.     

Cloud Detection Over Sea Surface by use of Autocorrelation Functions of Upwelling Infrared Spectra in the 800-900-cm(-1) Window Region

A new algorithm for cloud detection over sea surface has been developed that makes use of autocorrelation and cross correlation between a real spectrum and either a synthetic or a laboratory spectrum. The scheme is intended for high-spectral-resolution satelliteborne infrared sensors that will measure the Earth's entire emission spectrum rather than the upwelling radiance in a few channels. A new index is defined with which one can determine quantitatively the degree of homogeneity of two spectra in the 800-900-cm(-1) (11.11-12.5 mum) window region. The index makes use of only the observed spectrum along with a reference synthetic spectrum and, therefore, may form the basis for an operational stand-alone cloud-detection algorithm for next-generation high-spectral-resolution infrared sensors. Application both to synthetic spectra obtained in simulation and to sea-surface real spectra recorded through the interferometric monitor for greenhouse gases is considered.     

Effects of plasma total ammonia content and pH on urea excretion in Nile tilapia

Nile tilapia (Oreochromis niloticus) were infused with ammonium salts, acid, and base to investigate the effects of changes in arterial plasma total ammonia content (Tamm) and pH (pHa) on plasma urea-nitrogen (urea-N) levels and urea-N excretory fluxes (Jurea-N). The tilapia did not possess a functional hepatic ornithine urea-cycle (no significant carbamyl phosphate synthetase III activity). Infused substances were dissolved in a saline vehicle and injected twice (5 mL kg-1), the first infusion to "prime" the animal and promote a more marked response to the second infusion, given 2.5 h later. The results reported are those of the second infusion. Infusion of 200 mM NH4Cl increased Tamm, reduced pHa, and increased plasma urea-N and Jurea-N. Two hundred mM NH4HCO3 increased Tamm and arterial plasma total CO2 content (TaCO2), reduced pHa, and increased Jurea-N. Fifty mM HCl reduced pHa but had no effects on urea dynamics. Fifty mM NaOH increased pHa, plasma urea-N levels, and Jurea-N. Two hundred mM NaHCO3 increased pHa, TaCO2, plasma urea-N levels, and Jurea-N. Infusion of the saline vehicle was without effect. The results indicate that ammonia loading and plasma alkalosis both stimulate urea excretion in uricolytic fish. The responses to hyperammonemia or alkalosis were not modified when combined with elevated plasma bicarbonate levels.     

Excitotoxic neurodegeneration induced by deprivation of oxygen and glucose in isolated retina

PURPOSE: Ischemic neurodegeneration contributes to many retinal diseases. An isolated retina model has been used to examine the neuronal cell death induced by deprivation of oxygen and glucose (simulated ischemia) as a model for ischemic disease. METHODS: Neurodegeneration in the isolated chick embryo retina was induced by simulated ischemia and assessed using biochemical (lactate dehydrogenase release) and morphologic (light microscopy) techniques. RESULTS: Simulated ischemia led to lactate dehydrogenase release gradually in a period of 6 to 24 hours. Light microscopic observations demonstrated morphologic cell degeneration well before lactate dehydrogenase release occurred. N-Methyl-D-aspartate (NMDA) and non-NMDA receptor blockers individually provided partial protection, and the combination was fully protective. No protection was provided if the antagonists were added after simulated ischemia. When NMDA receptors were blocked by MK-801, cyclothiazide, an inhibitor of desensitization at non-NMDA receptors, enhanced lactate dehydrogenase released after 1 or 2 hours of simulated ischemia. Low concentrations of glucose effectively prevented lactate dehydrogenase release, despite anoxic conditions. CONCLUSIONS: The isolated retina provided a convenient system to characterize quantitatively ischemic cell death. Retinal ischemic neurodegeneration is an excitotoxic process that involves overactivation of NMDA and non-NMDA glutamate receptors. Blockade of both of these receptor subtypes was necessary for complete neuroprotection. Receptor desensitization played a protective role. If even low concentrations of glucose were delivered to an ischemic retina in vitro, substantial neuroprotection could be achieved. This may have implications for the management of acute retinal ischemic episodes.     

Oligomeric 1,4-phenyleneterephthalamide. Synthesis by interfacial polycondensation and structural properties

The synthesis of a fully aromatic oligoamide by low temperature interfacial polycondensation between terephthaloyl chloride and 1,4-phenylenediamine is reported. The reaction product was characterized with respect to its physical properties and chemical structure by infrared, X-rays, electron spectroscopy for chemical analysis (ESCA), and mass spectroscopic techniques. Elemental and thermal analysis, as well as viscosimetric and titrimetric methods were applied to evaluate the average molecular weight of the oligomer, whose chain length consists mainly of 3 to 4 monomeric units.     

Valyl-transfer RNA formation stimulated by monovalent cations and spermine in rat liver

The effects of hypothalamic disconnection on body temperature and hypothalamo-pituitary-adrenal activity following acute and repeated exposures to heat were studied. Intact male rats, or animals with complete posterior or anterior hypothalamic disconnection, were exposed to a temperature of 36 degrees C and a relative humidity of 35-45%. In the complete posterior and anterior hypothalamic disconnected rats the basal Tre was higher than that of the intact rats; the rise in Tre following heat exposure was lower in the operated rats than in the intact animals. All the experimental animals, except for those with anterior hypothalamic disconnection, showed a significant inhibition of corticosterone release on exposure to heat for 30 min, but no inhibition was observed in any of the disconnected rats when they were exposed to heat for 120 min. These results suggest that the main stimulus for ACTH release, during the first 30 min of heat exposure, is mediated by a neural input through the posteroir hypothalamus and this is followed by a nerural and/or humoral mechanism which enables the animals to increase their corticosterone secretion.     

A case of late pseudomonas ocular infection following scleral buckling

Experiments were carried out on the skeletal muscle fibres of the cricket Acheta domesticus using conventional microelectrode methods. Both potassium and ammonium ions depolarized the fibres but with a considerable divergance from the slope of 58 mV. Calcium ions hyperpolarized the fibres. High concentrations of magnesium ions or dramatic increase in the osmotic pressure of the bathing medium exerted no influence on the resting potential. It seems likely that skeletal muscle fibres of Acheta domesticus at rest are multionic electrodes, but the role played by various ions needs further studies.