Analysis and experimentation of nonlinear adaptive controllers forthe series resonant converter

It is the purpose of this paper to explore the problem of regulating the output voltage of a DC-to-DC series resonant power converter (SRC). These converters have highly nonlinear dynamics fed by a bipolar square signal generator whose commuting frequency is the only accessible control variable in the control architecture that the authors study. Therefore, they are confronted with the problem of controlling a nonlinear switched system by means of a modulating frequency signal. Two more complications that make this problem more challenging are that the full state is typically not available for measurement, and that the output load, usually represented by a resistance, is unknown. They show here that-for constant control input-SRCs have a unique globally attractive periodic orbit, which motivates them to consider a first harmonic approximation of the system. They then prove that this reduced model consists of a known static nonlinearity in cascade with a first order system with unknown parameters, for which adaptive output feedback solutions can be derived. They propose two different schemes, first a passivity-based controller which, as usual in these schemes, achieves asymptotically the inversion of the nonlinearity. They prove that, under some practically reasonable considerations, this control law reduces to the dissipative controller recently proposed by Stankovic et al. The second scheme directly inverts the static nonlinearity and applies standard adaptive techniques to the resulting linear system. The three controllers are implemented in an experimental setup and the results are presented as a comparative study     

Water electrolysis and pressure drop behaviour in a three-dimensional electrode

Water electrolysis from acidified solution was used as a model system to investigate the net contribution of hydrogen bubbles to the pressure drop increase in a three-dimensional electrode. A bed of silvered glass beads in both fixed and fluidized state was used, assuming an unchanging particle surface during the experiments. Pressure drop behaviour with time was measured for different experimental conditions and presented relative to the pressure drop determined for a bubble free bed. Parameters, such as current density, electrolyte velocity and particle size, greatly influence the relative pressure drop behaviour in the three-dimensional electrode. A sudden increase in the pressure drop occurs with the appearance of a gas phase in the bed, reaching a constant value (plateau) after a certain time; this plateau corresponds to steady state conditions. The pressure drop increases with increasing current density. This increase is in the range 40-150% relative to the bubble free electrolyte flow through the bed. Electrolyte flow-rate also strongly influences the pressure drop in the hydrogen evolving fixed bed electrode. It was observed that the relative pressure drop decreases with increasing electrolyte velocity. At higher flow rates, peaks occur on the pressure drop-time curves, indicating the existence of channeling inside the bed in which spouting occurs. The time to reach the pressure drop plateau decreases with increasing electrolyte velocity as do the time intervals corresponding to maximum pressure drop values. At the minimum fluidization velocity the peaks disappear and the relative pressure drop decreases with time, tending to approach a constant value. For hydrogen evolution in the fluidized bed, the pressure drop is lower than that measured in the absence of gas, and reason for this decrease being the gas hold-up in the bed.     

Optimized error protection of scalable image bit streams [advances in joint source-channel coding for images]

This article focuses on FEC for scalable image coders. For various channel models, we survey recent progress made in system design and discuss efficient source-channel bit allocation techniques, with emphasis on unequal error protection. This article considered JSCC (joint source-channel coding) at the application layer only. Recent research has studied cross-layer optimization where JSCC is applied to both the application layer and the physical layer. The basic task here is to minimize the average distortion by allocating available power, subcarriers, and bandwidth among users at the physical layer and source-channel symbols at the application layer subject to a total resource constraint. Most of the JSCC systems covered in this article can be readily extended to transmit scalable compressed bit streams of video sequences and 3-D meshes. Due to the stringent delay constraints in video communications and the fact that MPEG is currently exploring a scalable video coding standard, fast JSCC algorithms are expected to play a bigger role and bring more performance gains. This article is also expected to stimulate further research efforts into JSCC and more importantly, prompt the industry to adopt some of these JSCC algorithms in their system designs, thus closing the cycle from algorithm development to implementation.     

Evaluation of the antioxidant activity of a series of 4-methylcoumarins using different testing methods

Continuing our research on oxidation reactions of 4-methylcoumarins (4-methyl-2H-1-benzopyran-2-ones), this study presents evaluation of the antioxidant activity of 23 4-methylcoumarin derivatives, using different spectrophotometric and spectrofluorimetric radical testing methods. All studied compounds showed activity comparable to Trolox, an already known antioxidant, which is used as a standard in most testing methods. Observations from the study were made with regard to structural features that regulated the behaviour of the compounds. This paper extends our knowledge of the range of valuable biological activities and possible roles in medicinal therapy associated with this group of compounds.     

A numerical investigation of the refrigerant maldistribution from a header towards parallel channels in an evaporator of automotive air conditioning system

Evaporators in automotive air conditioning are designed as compact heat exchangers with the aim to cool outside air for the widest possible range of weather conditions. Under low operating loads a non-uniformity of the refrigerant liquid and gas phase distribution from the evaporator headers towards the evaporating channels leads to a decrease of the evaporator efficiency and a temperature stratification in the exit stream of cooled air. This malfunction is investigated by numerical simulations and analyses with the Computational Multi-Fluid Dynamic code. The applied numerical approach is validated for available experimental conditions. In order to eliminate the air exit temperature stratification, numerical experiments are performed with design changes of the evaporator header and the effectiveness of applied measures is demonstrated.     

Generalized Representation of Phase Derivatives for Regular Signals

This paper introduces a new generalized complex-lag moment which produces joint time-"phase derivatives" distributions. For the choice of the time-"first-order phase derivative," which stands for time-frequency representation, this distribution can be seen as a form of the Wigner-Ville distribution. Moreover, this generalization leads to distributions with highly reduced inner interferences caused by the nonlinearity of the signal's phase. It can also be seen as a polynomial distribution since the Nth-order distribution produces no inner interferences for polynomial phase law of order N. Implementation of these distributions is addressed. The results are illustrated by examples.     

Analytic phase-factor equations for Talbot array illuminations

Under specific circumstances the fractional Talbot effect can be described by simplified equations. We have obtained simplified analytic phase-factor equations to describe the relation between the pure-phase factors and their fractional Talbot distances behind a binary amplitude grating with an opening ratio of (1/M). We explain how these simple equations are obtained from the regularly rearranged neighboring phase differences. We point out that any intensity distribution with an irreducible opening ratio (M(N)/M) (M(N) < M, where M(N) and M are positive integers) generated by such an amplitude grating can be described by similar phase-factor equations. It is interesting to note that an amplitude grating with additional arbitrary phase modulation can also generate pure-phase distributions at the fractional Talbot distance. We have applied these analytic phase-factor equations to neighboring (0, pi) phase-modulated amplitude gratings and have analytically derived a new set of simple phase-factor equations for Talbot array illumination in this case. Experimental verification of our theoretical results is given.     

Single development project

An often-cited problem in undergraduate software engineering courses states that some topics are difficult to teach in a university setting and, although laboratory work is a useful supplement to the lectures, it is difficult to make projects realistic and relevant. In recognition of this problem, and based on our past experience, we started preparing a new course by examining the pedagogies and curricular aspects of software engineering that are important for the Net Generation of software engineers.The course project described in this paper concentrates on those aspects that can be dealt with effectively within the environment, i.e., the software lifecycle, system interdependences, teamwork, and realistic yet manageable project dynamics, all supported by various means of communication. The workload per students must be balanced with their lack of knowledge and skills, so that their unpreparedness to deal with complex issues does not abate their motivation.The approach was tested on six large projects over the period of one semester. We believe that the results reflect the students’ strong interest and commitment, and demonstrate their ability to stay focused and work at a level that is well above the obvious.     

METAHEURISTICS FOR HANDLING TIME INTERVAL COVERAGE CONSTRAINTS IN NURSE SCHEDULING

The problem of finding a high quality timetable for personnel in a hospital ward has been addressed by many researchers, personnel managers, and schedulers over a number of years. Nevertheless, automated nurse rostering practice is not common yet in hospitals. Many head nurses are currently spending several days per month on constructing their rosters by hand. In recent years, the emergence of larger and more constrained problems has presented a real challenge because finding good quality solutions can lead to a higher level of personnel satisfaction and to flexible organizational procedures. Compared to many industrial situations (where personnel schedules normally consist of stable periodic morning-day-night cycles) health care institutions often require more flexibility in terms of hours and shift types. The motivation for the research presented in this paper has been provided by real-world hospital administrators/schedulers and the approach that we describe has been implemented in over 40 hospitals in Belgium. This paper consists of two main contributions: modeling the real-world situation more accurately than has previously been done in the literature; and presenting and evaluating an efficient and effective tabu search procedure to solve these problems (as represented in the real-world model).

The approach described in this paper concentrates on an advanced representation of the daily personnel requirements of healthcare institutions. We introduce time interval personnel requirements. Instead of formulating the requirements as a number of personnel needed per shift type for each day of the planning period, time interval requirements allow for the representation of the personnel requirements per day in terms of the start and end times of personnel attendance. This formulation enables the provision of a greater choice of shift work and part-time work and reduces the amount of unproductive time because it enables the shifts to be split and combined. We present an algorithmic approach to handle this new formulation. We also set up a series of experiments which indicate that, not only does this approach take into account the requests and requirements of hospital schedulers, but it also generates higher quality schedules when compared with earlier approaches. The obtained results are better in the sense that various specific real-world soft constraints can be satisfied by scheduling appropriate shift type combinations, whereas in the shift type approach, fixed shift types restricted the solution space.     

Determinants of conflict detection: a model of risk judgments in air traffic control

OBJECTIVE: A model of conflict judgments in air traffic control (ATC) is proposed. BACKGROUND: Three horizontal distances determine risk judgments about conflict between two aircraft: (a) Dt(o) is the distance between the crossing of the aircraft trajectories and the first aircraft to reach that point; (b) Dt(h) is the distance between the two aircraft when they are horizontally closest; and (c) Dt(v) is the horizontal distance between the two aircraft when their growing vertical distance reaches 1000 feet. METHODS: Two experiments tested whether the variables in the model reflect what controllers do. In Experiment 1, 125 certified controllers provided risk judgments about situations in which the model variables were manipulated. Experiment 2 investigated the relationship between the model and expertise by comparing a population of certified controllers with a population of ATC students. RESULTS: Across both experiments, the model accounted for 44% to 50% of the variance in risk judgments by certified controllers (N=161) but only 20% in judgments by ATC students (N=88). There were major individual differences in the predictive power of the model as well as in the contributions of the three variables. In Experiment 2, the model described experts better than novices. CONCLUSION: The model provided a satisfying account of the data, albeit with substantial individual differences. It is argued that an individual-differences approach is required when investigating the strategies involved in conflict judgment in ATC. APPLICATION: These findings should have implications for developing user-friendly interfaces with conflict detection devices and for devising ATC training programs.