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871.
海洋重力传感器伺服回路分析与设计 总被引:1,自引:0,他引:1
李宏生 《中国惯性技术学报》2002,10(5):45-49
海洋重力仪是改善舰船惯导系统精度的重要设备,其重力传感器的伺服回路决定着仪器的精度和动态性能。作在介绍零长弹簧海洋重力传感器结构原理的基础上,研究其伺服回路的结构组成和设计方法,分析了其主要环节参数对系统性能的影响,考察了其时域性能,对海洋重力仪的研制具有重要指导作用。 相似文献
872.
873.
In order to study gas hydrate in media, formation of methane hydrate in three different media including loess, fine and coarse sands were studied. Five cooling rates were applied to form the methane hydrate. The nucleation time of the formation of methane hydrate with each cooling rate were measured for comparison. The experimental results show that the cooling rate is a significant factor affecting nucleation of methane hydrate and gas conversion. Under the same initial conditions, the faster the cooling rate, the shorter the nucleation time and the lower the methane gas conversion rate. The media also affect the formation process of methane hydrate within it. In loess, the gas conversion rate is lowest; in coarse sand, the gas conversion rate is the greatest; and in fine sand, it is in between. According to the study, it is found that the smaller the particle size of the media, the harder the methane hydrate forms within it. 相似文献
874.
Chensheng Ma Dr. Wai Ming Kwok Dr. Hui‐Ying An Xiangguo Guan Dr. Michael Yunyi Fu Patrick H. Toy Dr. David Lee Phillips Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(17):5102-5118
3′,5′‐Dimethoxybenzoin (DMB) is a bichromophoric system that has widespread application as a highly efficient photoremovable protecting group (PRPG) for the release of diverse functional groups. The photodeprotection of DMB phototriggers is remarkably clean, and is accompanied by the formation of a biologically benign cyclization product, 3′,5′‐dimethoxybenzofuran (DMBF). The underlying mechanism of the DMB deprotection and cyclization has, however, until now remained unclear. Femtosecond transient absorption (fs‐TA) spectroscopy and nanosecond time‐resolved resonance Raman (ns‐TR3) spectroscopy were employed to detect the transient species directly, and examine the dynamic transformations involved in the primary photoreactions for DMB diethyl phosphate (DMBDP) in acetonitrile (CH3CN). To assess the electronic character and the role played by the individual sub‐chromophore, that is, the benzoyl, and the di‐meta‐methoxybenzylic moieties, for the DMBDP deprotection, comparative fs‐TA measurements were also carried out for the reference compounds diethyl phosphate acetophenone (DPAP), and 3′,5′‐dimethoxybenzylic diethyl phosphate (DMBnDP) in the same solvent. Comparison of the fs‐TA spectra reveals that the photoexcited DMBDP exhibits distinctly different spectral character and dynamic evolution from those of the reference compounds. This fact, combined with the related steady‐state spectral and density functional theoretical results, strongly suggests the presence in DMBDP of a significant interaction between the two sub‐chromophores, and that this interaction plays a governing role in determining the nature of the photoexcitation and the reaction channel of the subsequent photophysical and photochemical transformations. The ns‐TR3 results and their correlation with the fs‐TA spectra and dynamics provide evidence for a novel concerted deprotection–cyclization mechanism for DMBDP in CH3CN. By monitoring the direct generation of the transient DMBF product, the cyclization time constant was determined unequivocally to be ≈1 ns. This indicates that there is little relevance for the long‐lived intermediates (>10 ns) in giving the DMBF product, and excludes the stepwise mechanism proposed in the literature as the major pathway for the DMB cyclization reaction. This work provides important new insights into the origin of the 3′,5′‐dimethoxy substitution effect for the DMB photodeprotection. It also helps to clarify the many different views presented in previous mechanistic studies of the DMB PRPGs. In addition to this, our fs‐TA results on the reference compound DMBnDP in CH3CN provide the first direct observation (to the best of our knowledge) showing the predominance of a prompt (≈2 ps) heterolytic bond cleavage after photoexcitation of meta‐methoxybenzylic compounds. This provides insight into the long‐term controversies about the photoinitiated dissociation mode of related substituted benzylic compounds. 相似文献
875.
The secondary metabolites of different Ephedra plants are various. Therefore, the discrimination of different Ephedra plants is significant. An objective, easy-to-use, rapid and pollution-free approach is proposed for discriminating Ephedra plants of different species, habitats and picking times on the basis of diffuse reflectance Fourier transform near infrared spectroscopy (FT-NIRS) measurements and multivariate analysis. The Fourier transform near infrared diffuse reflectance spectra (NIRDRS) were acquired from 37 pulverized samples of Ephedra plants put in glass vials in the near infrared (NIR) region between 10 000 and 4000 cm−1, averaging 64 scans per spectrum at a resolution of 4 cm−1. After spectra processing and data pre-processing, spectral data were analyzed respectively with three multivariate analysis techniques: discriminant analysis (DA), self-organizing map (SOM) and back-propagation artificial neural network (BP-ANN). The proposed method could distinguish not only the Ephedra plants of three species and two habitats but also the plants picked at different times of day without special sample treatment and the use of chemical reagents. The performance indexes of the DA model were 84.2-91.9% and the prediction accuracies of both the SOM and the BP-ANN models reached 93.3-100.0%. 相似文献
876.
Juan José Berzas Nevado Gregorio Castañeda Peñalvo Virginia Rodríguez Robledo 《Talanta》2010,82(2):548-554
A ThermoFinnigan sheath liquid flow capillary electrophoresis-mass spectrometry system designed for coupling via a co-axial interface was coupled through an adapted via an alternative, commercially available interface for orthogonal sampling. The affordable, reversible structural alterations made in the commercial LC-MS interface resulted in improved analytical performance.The results of a conventional capillary electrophoresis (CE) method using a commercial co-axial source to determine antioxidant phenolic acids present in virgin olive oil, were compared with those obtained by using a modified orthogonal sampling position. In both cases, separations were done using a 10 mM ammonium acetate/ammonium hydroxide buffer solution at pH 10.0 and a constant applied voltage of 25 kV. The operating variables for the mass spectrometry interface were re-optimized for the modified orthogonal orientation. This allowed the sheath liquid, sheath gas flow rates and capillary voltage to be lowered with respect to the co-axial coupling configuration. In addition, the orthogonal sampling position provided a higher selectivity by effect of ion sampling excluding larger droplets—with an increased momentum along the axis—which were drained through the sink at the bottom of the ion source. Also, the new configuration facilitated sample ionization, improved electrospray stability and led to stronger signals as a result.The new system was validated in terms of precision (repeatability), linearity, and limits of detection and quantification. A comparison of the validation data with the results previously obtained by using a commercial co-axial configuration revealed the adapted orthogonal sampling position to provide better repeatability in both migration times and relative peak areas (<1% and 7% respectively with n = 15 replicates), a good linear range (with levels in the microgram-per-litre region) and lower limits of detection—especially for the compounds detected with the lowest sensitivity when co-axial ESI was used, as HFA, GEN, FER and VAN finding LOD among 24-3.0 μg L−1 respectively. 相似文献
877.
879.
基于分数时延滤波器的宽带数字信号时延的实现 总被引:1,自引:0,他引:1
阐述了基于分数时延滤波器的时延方法设计原理,对其中4种代表性的设计方法的时延性能进行了评估,给出宽带数字线性调频信号的时延仿真结果,该方法可用于宽带雷达数字信号处理。 相似文献
880.
一种基于RTO的业务排序算法 总被引:2,自引:2,他引:0
业务影响分析是业务连续性计划中较为关键的一步,其最终目标是对企业中业务的重要性进行排序。文中的目的是提出一种企业在进行灾难恢复时,各个业务过程之间恢复优先顺序的排序算法。该算法在设计时基于RTO进行排序,并考虑了业务之间的相互依赖关系对恢复顺序的影响。通过实验显示了该算法的合理性:使用该算法可以更好地实施业务影响分析,缩短分析时间,从而提高整个业务连续性实施过程的效率。 相似文献