Conic sectors for sampled-data feedback systems

A multivariable analog system can be controlled by a sampled-data compensator. A conic sector that can be used to analyze the closed-loop stability and robustness of this feedback system is presented in this letter.     

The influence of urea on lipogenesis in renal papillae of rats

The osmolality has been determined for the papillary interstitial fluids obtained from the isolated papillae of rats in different states of water balance. Hypertonic buffers for in vitro experiments were prepared by addition of urea to regular Krebs-Ringer phosphate buffers (340 mosmol/kg H₂O) since urea is the most changeable solute of the renal papillary tissue in response to external influences. The effect of such hypertonic buffers on papillary lipogenesis was very pronounced. The fatty acid synthesis from acetate decreased by a factor of 7–9 in response to increased osmolality from 340 to 1370 mosmol/kg H₂O. In the physiological range of osmolalities (500–1800 mosmol/kg H₂O), the de novo synthesis of papillary glycerolipids from glucose decreased by a factor of ca. 5. A possible specific inhibitory effect of hypertonic buffers on the pentose phosphate cycle was studied with a negative result. It is concluded that the addition of urea causes a decrease of total energy metabolism in the tissue.     

DYNAMIC SIMULATION OF METHANATION REACTORS, INTERSTAGE HEAT EXCHANGERS, AND CONTROLLERS

The RMProcess for methanation of coal syngas, which consists of six fixed-bed, adiabatic, catalytic reactors connected in series with interstage waste-heat boilers and feedwater preheaters to remove the heat of reaction, was successfully simulated dynamically using a mixed ordinary/partial differential equation model. Computer integration of the-model was accomplished using the numerical method of lines.

Controls were added to the model to regulate the temperature of the gas feed into the reactors by diverting some gas around the interstage heat-exchangers. It was found that the controller selection and tuning, although generally not a goal in large-scale simulations, was quite important. Conventional methods used to tune the controllers gave controller settings that caused a significant detrimental effect on numerical integration stability and efficiency.     

The PumaPaint Project

The PumaPaint Project is an online robot that allows World Wide Web users to create original artwork. This paper describes the PumaPaint Project at two locations: the original site at Wilkes University and the site at Roger Williams University. Each site allows control of a PUMA robot equipped with four paintbrushes, jars of red, green, blue and yellow paint and white paper attached to a vertical easel. A Java interface executing within a web browser allows interactive control of the robot. This interface contains two windows showing live camera views of the easel and various controls for connecting to the robot, viewing the task status and controlling the painting task. The original site operated from June 1998 to March 2000 with approximately 25,000 unique-addressed machines downloading the interface to produce about 500 canvases. The new site has been in operation since August 2002. This paper discusses the author's experiences in operating the original site, and the motivation for and the challenges of reviving the site in its current location.     

DNA molecules and configurations in a solid-state nanopore microscope

A nanometre-scale pore in a solid-state membrane provides a new way of electronically probing the structure of single linear polymers, including those of biological interest in their native environments. Previous work with biological protein pores wide enough to let through and sense single-stranded DNA molecules demonstrates the power of using nanopores, but many future tasks and applications call for a robust solid-state pore whose nanometre-scale dimensions and properties may be selected, as one selects the lenses of a microscope. Here we demonstrate a solid-state nanopore microscope capable of observing individual molecules of double-stranded DNA and their folding behaviour. We discuss extensions of the nanopore microscope concept to alternative probing mechanisms and applications, including the study of molecular structure and sequencing.     

Decision problems for pushdown threads

Threads as contained in a thread algebra emerge from the behavioral abstraction from programs in an appropriate program algebra. Threads may make use of services such as stacks, and a thread using a single stack is called a pushdown thread. Equivalence of pushdown threads is shown decidable whereas pushdown thread inclusion is undecidable. This is again an example of a borderline crossing where the equivalence problem is decidable, whereas the inclusion problem is not.     

Incorporating Uncertainty via Hierarchical Classification Using Fuzzy Decision Trees

Object hierarchy is often ignored when collecting and classifying geographical objects. Object attributes are defined on the basis of uncertain parameters that may change in space and time. In this paper, we consider fuzzy decision trees for classification and a Bayesian hierarchical model for modeling and handling uncertainty. The study is illustrated with dynamic geographical objects from a coastal management application in the northern part of The Netherlands. Hierarchical modeling is applied to obtain posterior distributions for several boundary regions. The posterior distributions yield lower and upper limits of membership functions describing boundaries between object classes. In this way, a proper fuzzy decision tree for the coastal management application is built, which includes the inherent dynamic uncertainty     

CHALLENGING THE COMPUTATIONAL METAPHOR: IMPLICATIONS FOR HOW WE THINK

This paper explores the role of the traditional computational metaphor in our thinking as computer scientists, its influence on epistemological styles, and its implications for our understanding of cognition. It proposes to replace the conventional metaphor a sequence of steps with the notion of a community of interacting entities, and examines the ramifications of such a shift on these various ways in which we think.