Unconstrained H∞ predictive control with H∞ prediction: single‐input–single‐output case

Does the replacement of the quadratic (H₂) predictor by the worst‐case (H_∞, or cumulative minimax) predictor robustify the predictive control laws? The present work provides a partial answer to this question, positive for the examples considered, representative of three broad classes of systems. The H_∞ prediction is demonstrated to be a powerful and convenient tool for frequency shaping of the gain of the closed‐loop complementary sensitivity function, capable of robustifying the closed loop for systems with different stability properties. The H_∞‐optimal k‐step ahead predictor is derived for an unstable single‐input–single‐ output CARMA model. A BIBO unstable filter for the disturbance rejection is obtained using the internal model principle and included into the closed loop, and the H_∞ predictor is applied to the combination of this filter with the plant. The sum over a finite horizon of the current and the predicted tracking error and control signal power spectral densities (PSDs) is decomposed into two parts, one induced by the worst‐case predicted disturbance and the other—by the known future reference input. A two degrees of freedom algorithm, referred to as the multi‐step closed‐loop polynomial H_∞ predictive control law, is obtained that minimizes the peaks of the PSD of the first part and the integral on the unit circle of the PSD of the second. It is demonstrated on several systems that H_∞ prediction introduces a very intuitive tuning knob in the form of the prediction horizon capable of setting a trade‐off between the steady‐state disturbance rejection perfor mance in terms of the output error variance and the closed‐loop robustness, however the efficacy of the knob strongly depends on the stability properties of the system and its inverse. The trade‐off becomes less pronounced or completely disappears when the H_∞ predictor is replaced by the quadratic one. Copyright © 2000 John Wiley & Sons, Ltd.     

Surface contraction and expansion waves correlated with differentiation in axolotl embryos--I. Prolegomenon and differentiation during invagination through the blastopore, as shown by the fate map

We have discovered a series of expansion and contraction, solitary waves that correlate with discrete steps of differentiation in the urodele amphibian axolotl embryo (Ambystoma mexicanum). Here we examine in detail the proposition that the blastopore is a set of differentiation waves. We superimposed the image of the axolotl fate map onto our digitized video images of normal gastrulation and matched the fate map to pigmentation irregularities on the embryo. We were then able to track the invagination of the fate map by tracking the variegated pigmentation on several embryos as gastrulation proceeded. We show a particular expansion and contraction wave sequence for every tissue in the blastula stage fate map and can now explain precisely why the fate map has the shape it does and its relationship to the embryo at subsequent stages. Each tissue can be assigned a differentiation code and placed on a hierarchical, binary differentiation tree.     

Structure and dynamics of staphylococcal nuclease mutants as studied by fluorescence quenching techniques

Fluorescence techniques have been used to investigate the effectof mutations on the structure and dynamics of staphylococcalnuclease. An estimate of the accessibility to acrylamide ofthe enzyme's single tryptophan residue (Trp140) was obtainedfrom the Stern–Volmer constant for fluorescence quenching.This was Indicative of a partially buried tryptophan in thewild-type nuclease. Five single-site mutant nucleases (H124L,V66L, G88V, G79S and F76V) and one double mutant (V66L + G88V),with widely differing stabilities to denaturants, gave Stern–Volmerconstants which were very similar to that of their parent enzyme.Studies of the temperature- and viscosity-dependence of quenchingsuggest that access by acrylamide to Trp140 is limited by diffusionrather than by protein structural fluctuations. Lifetime-resolvedfluorescence anisotropy studies using steady-state instrumentationsuggest that there is very little segmental motion of the Trp140;most of the anisotropy therefore decays due to protein rotationin the solution. Rotational correlation times for several nucleasemutants have been determined and these are very similar to thatof the native nuclease. Thus it appears that these substitutionsin the primary amino acid sequence, which have significant effectson the stability of the folded proteins, do not cause a significantchange in the protein structure or dynamics around Trp140.     

Chemotherapy-Induced Cognitive Impairment and Hippocampal Neurogenesis: A Review of Physiological Mechanisms and Interventions

A wide range of cognitive deficits, including memory loss associated with hippocampal dysfunction, have been widely reported in cancer survivors who received chemotherapy. Changes in both white matter and gray matter volume have been observed following chemotherapy treatment, with reduced volume in the medial temporal lobe thought to be due in part to reductions in hippocampal neurogenesis. Pre-clinical rodent models confirm that common chemotherapeutic agents used to treat various forms of non-CNS cancers reduce rates of hippocampal neurogenesis and impair performance on hippocampally-mediated learning and memory tasks. We review the pre-clinical rodent literature to identify how various chemotherapeutic drugs affect hippocampal neurogenesis and induce cognitive impairment. We also review factors such as physical exercise and environmental stimulation that may protect against chemotherapy-induced neurogenic suppression and hippocampal neurotoxicity. Finally, we review pharmacological interventions that target the hippocampus and are designed to prevent or reduce the cognitive and neurotoxic side effects of chemotherapy.     

A Study of Scrap Heating by Burners: Part II—Numerical Modeling

A computational fluid dynamics code was developed to model the heating of a bed of porous steel scrap by combustion gases from a burner. The code accounted for nonuniform void fraction in the bed; turbulent, non-Darcian flow, heat transfer from the gas to the scrap; and radiation. The measured bed porosity values were used in the code. Because steel scrap pieces are very irregular in shape and size, the effective particle diameter was fitted to measurements made in a 1-m³ capacity furnace, as reported in part I. It was found that the lower porosity of the scrap was the most beneficial in increasing the efficiency of heat transfer to the scrap bed because the interfacial area is larger. The effect of particle size was much smaller. It was found that the configurations that increased the residence time or path length of the gases increased the efficiency. The measured porosity of the bed approached unity at the walls, so this provided an easy path for the gas to short-circuit the bed, which limited the effectiveness of decreasing the porosity to increase heat-transfer efficiency. Similarly, simulations of nonuniform scrap distributions reduced efficiency of heat transfer due to short circuiting. The implications of the findings for industrial operations are discussed.     

An Alternative Structure for C576

Abstract

Carbon nanotubes with rollup vector indices equal, i. e., (n, n) nanotubes, are calculated to be metallic electrical conductors. Several years ago, a toroidal fullerene structure, C₅₇₆, was proposed and suggested to possess a small bandgap on the basis of its having approximately half of its atoms arranged as in a (4, 4) nanotube. a more recent calculation determined that this structure has a very large anisotropic ring-current diamagnetic susceptibility and is therefore likely to be a good electrical conductor. the present work proposes an alternative structure for C₅₇₆ which is constructed entirely from six copies of a C₉₆ fragment of a (4, 4) nanotube.     

Acid rain: the data and monitoring needs

The need for environmental monitoring has been recognized in the development of air and water quality management programs for many years. The problems of inadequate monitoring have also been recognized for many years. Yet, the lack of monitoring data is acutely evident in the area of acid rain and has seriously hampered efforts to develop responsible policies. This paper focuses on the types of monitoring data needed to address the question of acid rain, viz. terms of reference for acid rain monitoring, acid rain monitoring programs, recognition of needs and problems in air and water quality monitoring, and concluding comments.

Because acid rain involves multiple pollutants with international transboundary effects in air, water, and land, a comprehensive, coordinated, and cooperative system of international monitoring is needed. Coordination must involve not only the international level, but interagency programs in the domestic governments as well. Recent experience with the development of monitoring systems provides hope for the future improvement of U.S. efforts to monitor environmental quality.