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904.
针对多臂空间机器人自主目标抓捕任务,首先建立多臂空间机器人的运动模型和其与目标的相对运动模型,采用Kane方法建立多臂空间机器人的动力学模型;其次,研究基于视觉伺服的机械臂在线轨迹规划算法,并引入零反作用机动,消除机械臂运动对平台姿态的扰动;再次,在不使用零反作用机动功能时,分别使用基于角动量前馈补偿的协调控制算法和逆动力学方法设计了协调控制器,在机械臂运动时保持平台姿态和相对目标的位置。最后,开发了基于Matlab的仿真软件MASS(多臂空间机器人仿真),仿真结果校验了上述方法的有效性。 相似文献
905.
906.
叶盘通道电解加工(ECM)是决定整体叶盘电解加工成形精度的关键工序,其加工间隙不规则、建模困难。基于Realizable k-ε模型建立叶盘通道电解加工的气液两相流模型,根据参数传递关系建立多物理场耦合模型,分析加工间隙内电解液温度、氢气体积分数、电导率等重要参数的分布规律。仿真结果显示电解液在侧面间隙出现涡流现象,使该区域氢气堆积、温度上升,影响工件的成形表面质量,并导致该处的材料蚀除量减小,该结果在试验中得到证实。试验与仿真得出的工件轮廓形状变化趋势一致,试验与仿真获得的端面平衡间隙尺寸分别为0.26、0.33 mm,仿真的相对误差(27%)明显小于经典公式的相对误差(73%)。因此,模型可较为准确地模拟叶盘通道电解加工成形过程,反映各参数对工件成形的影响过程。 相似文献
907.
Yingtai Chen Yumin Li Hong Zhang Yi Xie Xuezhong Chen Jinyu Ren Xiaowei Zhang Zijiang Zhu Hongliang Liu Yawei Zhang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
The aim of this study was to estimate the acute effects of low dose 12C6+ ions or X-ray radiation on human immune function. The human peripheral blood lymphocytes (HPBL) of seven healthy donors were exposed to 0.05 Gy 12C6+ ions or X-ray radiation and cell responses were measured at 24 h after exposure. The cytotoxic activities of HPBL were determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT); the percentages of T and NK cells subsets were detected by flow cytometry; mRNA expression of interleukin (IL)-2, tumor necrosis factor (TNF)-α and interferon (IFN)-γ were examined by real time quantitative RT-PCR (qRT-PCR); and these cytokines protein levels in supernatant of cultured cells were assayed by enzyme-linked immunosorbent assays (ELISA). The results showed that the cytotoxic activity of HPBL, mRNA expression of IL-2, IFN-γ and TNF-α in HPBL and their protein levels in supernatant were significantly increased at 24 h after exposure to 0.05 Gy 12C6+ ions radiation and the effects were stronger than observed for X-ray exposure. However, there was no significant change in the percentage of T and NK cells subsets of HPBL. These results suggested that 0.05 Gy high linear energy transfer (LET) 12C6+ radiation was a more effective approach to host immune enhancement than that of low LET X-ray. We conclude that cytokines production might be used as sensitive indicators of acute response to LDI. 相似文献
908.
Dan-Dan Liu Tao Yu Jing-Song Wang Cong Huang Wei-Xing Wan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
Neural networks (NNs) have been applied to ionospheric predictions recently. This paper uses radial basis function neural network (RBF-NN) to forecast hourly values of the ionospheric F2 layer critical frequency(foF2), over Wuhan (30.5N, 114.3E), China. The false nearest neighbor method is used to determine the embedding dimension, and the principal component analysis (PCA) is used to reduce noise and dimension. The whole study is based on a sample of about 26,000 observations of foF2 with 1-h time resolution, derived during the period from January 1981 to December 1983. The performance of RBF-NN is estimated by calculating the normalized root-mean-squared (NRMSE) error, and its results show that short-term predictions of foF2 are improved. 相似文献
909.
Yuming Fu Hui Liu Lingzhi Shao Minjuan Wang Yu A. Berkovich A.N. Erokhin Hong Liu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
Vegetable cultivation plays a crucial role in dietary supplements and psychosocial benefits of the crew during manned space flight. Here we developed a ground-based prototype of horn-type sequential vegetable production facility, named Horn-type Producer (HTP), which was capable of simulating the microgravity effect and the continuous cultivation of leaf–vegetables on root modules. The growth chamber of the facility had a volume of 0.12 m3, characterized by a three-stage space expansion with plant growth. The planting surface of 0.154 m2 was comprised of six ring-shaped root modules with a fibrous ion-exchange resin substrate. Root modules were fastened to a central porous tube supplying water, and moved forward with plant growth. The total illuminated crop area of 0.567 m2 was provided by a combination of red and white light emitting diodes on the internal surfaces. In tests with a 24-h photoperiod, the productivity of the HTP at 0.3 kW for lettuce achieved 254.3 g eatable biomass per week. Long-term operation of the HTP did not alter vegetable nutrition composition to any great extent. Furthermore, the efficiency of the HTP, based on the Q-criterion, was 7 × 10−4 g2 m−3 J−1. These results show that the HTP exhibited high productivity, stable quality, and good efficiency in the process of planting lettuce, indicative of an interesting design for space vegetable production. 相似文献
910.
Hongping Zhang Peiliang Xu Wenhui Han Maorong Ge Chuang Shi 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
Currently, ground-based Global Navigation Satellite System (GNSS) stations of the International GNSS Service (IGS) are distributed unevenly around the world. Most of them are located on the mainland, while only a small part of them are scattered on some islands in the oceans. As a consequence, many unreasonable zero values (in fact negative values) appear in Vertical Total Electron Content (VTEC) of European Space Agency (ESA) and Center for Orbit Determination in Europe (CODE) IONEX products, especially in 2008 and 2009 when the solar activities were rather quiet. To improve this situation, we directly implement non-negative physical constraints of ionosphere for global ionosphere maps (GIM) with spherical harmonic functions. Mathematically, we propose an inequality-constrained least squares method by imposing non-negative inequality constraints in the areas where negative VTEC values may occur to reconstruct GIM models. We then apply the new method to process the IGS data in 2008. The results have shown that the new algorithm efficiently eliminates the unwanted behavior of negative VTEC values, which could otherwise often be seen in the current CODE and ESA GIM products in both middle and high latitude areas of the Southern Hemisphere (45°S∼90°S) and the Northern Hemisphere (50°N∼90°N). About 64% of GPS receivers’ DCBs have been significantly improved. Finally, we compare the GIM results between with and without the inequality constraints, which has clearly shown that the GIM result with inequality constraints is significantly better than that without the inequality constraints. The inequality-constrained GIM result is also highly consistent with the final IGS products in terms of root mean squared (RMS) and mean VTEC. 相似文献