一种用于异步电机效率优化的混合搜索方法

针对异步电机效率优化问题,提出了一种混合搜索方法。该方法起始于模糊自适应搜索,然后切换至黄金分割法以获取确定收敛速度。这样的搜索步骤能够降低转矩波动,避免在最优点附近发生振荡。利用一个包含铁损和机械损耗的异步电机模型,对该方法进行了矢量控制下的性能验证。仿真结果验证了该方法的有效性。

基于符号时间序列分析技术的电机故障诊断方法研究

随着经济建设的快速发展和电气化程度的不断提高，电机已被广泛应用于工业、农业、国防及人们日常生活的各个领域。从全球范围看，电机的用电量平均占世界用电总量的50％以上、占工业用电量的70％左右，然而在电机消耗的电能中有相当一部分被浪费掉了，其中电机带故障运行是造成电机运行效率偏低，能源浪费严重的主要原因之一。 电机在线监测及故障诊断系统对于减少由于电机故障引发的人员、财产的损失，减少由于故障引发的异常状态而导致的能源浪费有着重要的现实意义。在电机故障危害产生前发现故障并进行维护是电机故障诊断的核心思想，在保证电机故障诊断系统准确性的同时，系统的快速性与鲁棒性显得尤为重要。基于此，本论文从寻求系统的快速、稳定的性能入手，提出了基于符号时间序列分析的感应电机故障诊断框架，重点研究了计算代价小、噪声干扰不敏感的诊断方法，以期提高感应电机故障诊断系统的快速性与鲁棒性。论文的主要工作有： 1． 论文首先构建了一个基于符号时间序列分析的电机故障诊断框架，将电 机故障诊断分解为信号预处理、符号区间划分、符号统计分析三部分，有机地融合了统计分析、信号处理、信息论、模式识别等理论和方法，利用符号时间序列分析技术在强噪声中准确识别系统状态模式的良好性能，可以有效地解决电机故障诊断问题，并实现电机故障诊断量化分析，是对探索电机在线监测与诊断新方法的一次有益的尝试。 2．引入提升小波对信号进行前期处理，并针对常规提升小波固定预测滤波器的局限性，提出了基于梯度信息的自适应提升小波预测方法。该方法中预测滤波器并不是固定的，而是利用梯度的信息来确定预测算子。根据信号的陡峭程度选择预测算子可以更准确地预测信号，从而使原始信号中的平滑特征和陡峭特征可以在小波变换中完好地保留下来。仿真实验及实验室实验结果表明该方法可以有效地保留信号中蕴含的重要的特征信息，对于以提取、识别信号中特征信息为主的故障诊断技术来说具有非常重要的意义。 3．针对所采集现场信号的非均匀分布特点，论文提出了一种自适应符号化划分方法，既可以确保符号在数据密集区间和数据稀疏区间的合理分配，提高符号的利用率，又可以灵活地适应信号的特征，增强诊断系统对微弱故障信号的敏感度。故障诊断实验表明该方法简单有效，实现了故障初期的快速诊断，并且较平均区间划分方法有着更高的计算效率、更明显的诊断效果。 4．将相对熵的概念引入基于符号时间序列分析的电机故障诊断框架中，针对电机故障严重程度量化分析问题，提出了基于模糊相对熵及加权模糊相对熵的符号统计分析方法，并将该方法应用于感应电机的故障诊断与识别，建立了电机故障诊断模型。该方法可以更合理、充分地利用信息丰富的符号区间所蕴含的故障信息，实现了电机故障诊断与故障严重程度的识别。实验结果验证了该方法的合理性、有效性和可靠性。 5．将隐马尔可夫模型（HMM）引入到基于符号时间序列分析的电机故障诊断框架中，构造了基于HMM的电机故障诊断模型，并对HMM阶数选取问题给出了一个基于符号出现不确定信息熵的HMM阶数选取原则，使得模型在满足精度要求的同时，又尽可能地减少模型的计算代价，有效地提高了故障诊断的效率及可靠性。实验结果表明基于HMM的电机故障诊断方法有效地实现了电机转子断条故障、匝间短路故障的诊断与量化分析。

The induction plasma chemical reactor: Part I. Equilibrium model

A mathematical model is presented for the numerical simulation of the flow, temperature, and concentration fields in an rf plasma chemical reactor. The simulation is performed assuming chemical equilibrium. The extent of validity of this assumption is discussed. The system considered is the reaction of SiCl4 and NH3 for the production of Si3N4.

The induction plasma chemical reactor: Part II. Kinetic model

A kinetic model has been developed for the prediction of the concentration gelds in an rf plasma reactor. A sample calculation for a SiCl4/H2 system is then performed. The model considers the mixing processes along with the kinetics of seven reactions involving the decomposition of these reactants. The results obtained are compared to those assuming chemical equilibrium. The predictions indicate that an equilibrium assumption will result in lower predicted temperature fields in the reactor. Furthermore, for the chemical system considered here, while differences exist between the concentration fields obtained by the two models, the differences are not substantial.

Positive selection drives population differentiation in the skeletal genes in modern humans

During the course of evolution, the human skeletal system has evolved rapidly leading to an incredible array of phenotypic diversity, including variations in height and bone mineral density. However, the genetic basis of this phenotypic diversity and the relatively rapid tempo of evolution have remained largely undocumented. Here, we discover that skeletal genes exhibit a significantly greater level of population differentiation among humans compared with other genes in the genome. The pattern is exceptionally evident at amino acid-altering sites within these genes. Divergence is greater between Africans and both Europeans and East Asians. In contrast, relatively weak differentiation is observed between Europeans and East Asians. SNPs with higher levels of differentiation have correspondingly higher derived allele frequencies in Europeans and East Asians. Thus, it appears that positive selection has operated on skeletal genes in the non-African populations and this may have been initiated with the human colonization of Eurasia. In conclusion, we provide genetic evidence supporting the rapid evolution of the human skeletal system and the associated diversity of phenotypes.

The effect of acute stress on LTP and LTD induction in the hippocampal CA1 region of anesthetized rats at three different ages

Not all experiences are memorized equally well. Especially, some types of stress are unavoidable in daily life and the stress experience can be memorized for life. Previous evidence has showed that synaptic plasticity, such as long-term potentiation (LTP) that may be the major cellular model of the mechanism underlying learning and memory, is influenced by behavioral stress. However, the effect of behavioral stress on age-related synaptic plasticity in-vivo was primarily known. Here we found that the LTP induction in the hippocampal CA1 region of anesthetized rats obviously showed inverted-U shape related to ages (4, 10 and 74 weeks old rats), but low-frequency stimulation was unable to induce reliable long-term depression (LTD) in these animals. Furthermore, acute elevated platform (EP) stress enabled reliable LTD significantly and completely blocked LTP induction at these ages. Importantly, LTD after exposure to acute EP stress showed similar magnitude over these ages. The present results that stress enables LTD but impairs LTP induction at these three ages strengthen a view that stress experience-dependent LTD (SLTD) may underlie stress form of aberrant memories. (C) 2004 Elsevier B.V. All rights reserved.