An Application of Frequency Entrainment in a DC-to-DC Converter

Inherent simplicity and high efficiency combined with an excellent dynamic behavior make the use of self-oscillating switching circuits an attractive alternative in the design of electronic power conditioning equipment. Unfortunately, these free-running circuits often suffer from large variations in operating frequency under varying line and load conditions, a deficiency which seriously limits their usefulness for many applications. A general approach is described by which this drawback can be remedied without impairing the aforementioned advantages.     

A dual-output d.c. power converter

Electrical equipment often requires power supplies which can furnish several regulated d.c. voltages of the same or opposite polarity. This generally implies a multi-winding power transformer with separate rectifiers and regulators for each voltage.This paper describes a novel d.c.-to-d.c. power converter which provides two independently controlled output voltages, of the same or opposite polarity, from a single power handling switching transistor and a single input voltage. The converter can operate with high efficiency. Circuit complexity and cost are significantly reduced as compared to conventional solutions.The dual-output power converter is an interesting example of a relay system with two input and two output variables. A state-space method is developed for the analysis of this system class, and a theoretical and experimental investigation of the converter circuit is undertaken.     

Cosmetic results of breast conserving therapy for breast carcinoma. Treatment results from the Heidelberg Radiation Clinic in the years 1984 to 1992]

We examined the effect of an antioxidant and protein kinase inhibitors on prostaglandin E2 (PGE2) release from Balb/c 3T3 mouse fibroblast cells induced by quinolone phototoxicity. Simultaneous administration of sparfloxacin (SPFX) or lomefloxacin (LFLX) at 12.5 to 100 microM and ultraviolet-A (UVA) irradiation for 10 min markedly elevated PGE2 concentration in the incubation medium, whereas levofloxacin (LVFX) at concentrations up to 100 microM and UVA irradiation did not increase PGE2 concentration. Pretreatment with 100 microM pyrrolidine dithiocarbamate (PDTC), an antioxidant, or 1 microM calphostin C, a selective inhibitor of protein kinase C (PKC), completely inhibited the elevation of PGE2 in the 24-h incubation medium; pretreatment with 10 microM H7, a cyclic nucleotide-dependent protein kinase, and PKC or 1 microM herbimycin A, a tyrosine kinase inhibitor, inhibited the PGE2 elevation by 29 to 39%. Conversely, 25 nM staurosporine significantly augmented the PGE2 elevation by quinolones plus UVA. Interleukin-1beta (IL-1beta) and tumor necrosis factor alpha (TNFalpha) were not detected in the incubation medium of 3T3 cells after quinolone plus UVA, corresponding to the lack of effect of antibodies against IL-1alpha, IL-1beta, and TNFalpha on PGE2 release from 3T3 cells. These results suggest that PGE2 production in 3T3 cells by quinolone phototoxicity is modulated by reactive oxygen species, PKC, and tyrosine kinase, but not by IL-1 or TNFalpha.     

An Algorithm that Learns What's in a Name

In this paper, we present IdentiFinderTM, a hidden Markov model that learns to recognize and classify names, dates, times, and numerical quantities. We have evaluated the model in English (based on data from the Sixth and Seventh Message Understanding Conferences [MUC-6, MUC-7] and broadcast news) and in Spanish (based on data distributed through the First Multilingual Entity Task [MET-1]), and on speech input (based on broadcast news). We report results here on standard materials only to quantify performance on data available to the community, namely, MUC-6 and MET-1. Results have been consistently better than reported by any other learning algorithm. IdentiFinder's performance is competitive with approaches based on handcrafted rules on mixed case text and superior on text where case information is not available. We also present a controlled experiment showing the effect of training set size on performance, demonstrating that as little as 100,000 words of training data is adequate to get performance around 90% on newswire. Although we present our understanding of why this algorithm performs so well on this class of problems, we believe that significant improvement in performance may still be possible.     

A Symmetry Correcting Pulsewidth Modulator for Power Conditioning Applications

A wide variety of power converters is of the transformer-coupled push-pull type and uses pulsewidth modulation for output regulation. In designing and operating such converters, serious complications usually arise as a result of unsymmetrical switching of the two power transistors. Caused by small parameter dissymmetries of these transistors, asymmetrical switching normally drives the transformer into saturation. Resulting excessive saturation currents reduce efficiency and put a severe strain on the power transistors, which can eventually cause circuit failure. This paper presents a new pulsewidth modulator circuit which can be advantageously utilized in the design of such converters, and which can effectively cope with the aforementioned symmetry problem. Symmetry correction is accomplished by monitoring the two primary currents and by providing proper feedback of the integrated difference between these two currents.     

An artificial intelligence approach to language instruction

This paper describes an implemented, prototype system for a sophisticated, intelligent tutor for instruction in a foreign language. The system is an application of artificial intelligence research in natural language, but it implements several ideas that depart from standard approaches to natural language understanding.

For instance, the semantic analyzer diagnoses several kinds of comprehension problems and semantic errors that a student might make. Some fine distinctions in meaning are represented to detect misuse of words. Not only is a model of good syntax included in the tutor, but also a model of incorrect forms, rich enough to pinpoint specific syntactic mistakes. Finding the intended interpretation is complicated by the likelihood of student errors. Therefore, perfect syntactic form is not necessary for semantic analysis of the student's input.

The problems discussed and solutions presented are closely related to the more general problem of how to respond to a natural language input that surpasses the computer's model of language or of context.     

钢丝绳损坏的讨论

<正>这是一篇讨论钢丝绳损坏和无损检测方法的文章。一台设计正确和使用维护正常的起重机,钢丝绳在寿命周期内损坏的机理主要有两个:◆绕过滑轮和卷筒时由于弯曲造成的     

Natural-language understanding at BBN

Natural-language understanding has evolved from its earliest days in which scientists use an early approach to parsing, to more sophisticated techniques that enable systems to extract information from open-domain text sources to fill data bases automatically. Natural language processing has many potential applications, such as translating foreign-language documents on the Web; automatically routing questions to an appropriate expert at a help/service telephone number; fully automatic question answering; delivering answers to a Web query, as opposed to delivering pointers to Web pages; and automatically filling a structured database with desired information from text or speech sources.     

Regulated self-oscillating convertor with auxiliary feedback circuit

A regulated self-oscillating transformer-coupled d.c./d.c. convertor is presented which has an auxiliary feedback loop to sustain self oscillations. While this circuit retains the inherent simplicity of self-oscillating circuits, it avoids certain starting problems which are usually encountered if transformer coupling is inserted into the self-oscillating feedback loop.