Paris law can reflect the failure mechanism of materials and is usually used to be a method to predict fatigue life or residual fatigue life.But the variable which can represent the health of machine is hardly measure...Paris law can reflect the failure mechanism of materials and is usually used to be a method to predict fatigue life or residual fatigue life.But the variable which can represent the health of machine is hardly measured on line.To a degree,the difficulty of on-line application restricts the scope of application of Paris law.The relationship between characteristic values of vibration signals and the variable in the Paris equation which can describe the health of machine is investigated by taking ball bearings as investigative objects.Based on 6205 deep groove ball bearings as a living example,historical lives and vibration signals are analyzed.The feasibility of describing that variable in the Paris equation by the characteristic value of vibration signals is inspected.After that vibration signals decomposed by empirical mode decomposition(EMD),root mean square(RMS) of intrinsic mode function(IMF) involving fault characteristic frequency has a consistent trend with the diameter of flaws.Based on the trend,two improved Paris models are proposed and the scope of application of them is inspected.These two Paris Models are validated by fatigue residual life data from tests of rolling element bearings and vibration signals monitored in the process of operation of rolling element bearings.It shows that the first improved Paris Model is simple and plain and it can be easily applied in actual conditions.The trend of the fatigue residual life predicted by the second improved Paris model is close to the actual conditions and the result of the prediction is slightly greater than the truth.In conclusion,after the appearance of detectable faults,these improved models based on RMS can predict residual fatigue life on line and a new approach to predict residual fatigue life of ball bearings on line without disturbing the machine running is provided.展开更多
对双主动全桥DAB(dual active bridge)双向DC/DC变换器的调制方案进行了研究,DAB变换器的主要优势在于具有对称结构、双向潮流能力、宽软开关区域和灵活的控制能力等特点。控制这种拓扑结构最简单的方法是通过控制变换器原副边全桥之间...对双主动全桥DAB(dual active bridge)双向DC/DC变换器的调制方案进行了研究,DAB变换器的主要优势在于具有对称结构、双向潮流能力、宽软开关区域和灵活的控制能力等特点。控制这种拓扑结构最简单的方法是通过控制变换器原副边全桥之间的移相角来控制功率传输的方向和大小。然而在轻载条件下,当变换器的输入或输出电压变化较大时,会产生大量的无功功率,同时部分开关管的零电压开关ZVS(zero voltage switching)操作会丢失而直接导致转换效率变低。因此,为了提高DAB变换器的效率,提出了一种混合单移相调制PSM(phase shift modulation)方案,在保持控制简单的基础上,通过减小电感电流有效值,扩大软开关的范围,提高了变换器的效率。首先,通过让拓展移相EPS(extended phase shift)、双移相DPS(dual phase shift)以及三移相TPS(triple phase shift)调制方案中的可控变量相等,从而形成了4种不同的PSM方案。接着,对这些调制方案进行了稳态特征的比较分析,包括传输功率容量、电感电流水平以及软开关性能等。在此基础上,提出了一种混合PSM方案。最后,通过搭建实验平台验证了所提出调制方案的有效性和正确性。展开更多
Considering the relationship between the nuclear radius and the average bindingenergy of nucleons,a simple semi-empirical formula for the nuclear rms charge radii is suggest-ed together with the formula for light neut...Considering the relationship between the nuclear radius and the average bindingenergy of nucleons,a simple semi-empirical formula for the nuclear rms charge radii is suggest-ed together with the formula for light neutron-rich nuclear rms neutron radii.These formulascan well fit the new experimental values.展开更多
以某室外大跨轻柔钢结构人行桥为案例,介绍了该结构的设计概况及调谐质量阻尼器（tuned mass damper,简称TMD）减振设计,分别在结构施工完成后以及TMD装置安装后对其进行了实地动力测试,测得了该结构减振前后的模态特性以及在多种人行...以某室外大跨轻柔钢结构人行桥为案例,介绍了该结构的设计概况及调谐质量阻尼器（tuned mass damper,简称TMD）减振设计,分别在结构施工完成后以及TMD装置安装后对其进行了实地动力测试,测得了该结构减振前后的模态特性以及在多种人行荷载工况下的振动响应,采用加速度峰值和均方根值为评价指标,分析了钢连桥人致振动情况及特性。结果表明：一端设计为滑动连接的钢连桥人致振动主要由第1阶竖向振型控制,扭动及水平振动响应相对较小;安装经优化设计的TMD装置后,整体连桥的频率特性没有明显变化,分布式TMD对多种频率激励工况均有良好的调谐减振作用,减振率达到35%-70%;共振激励下测得的原结构阻尼比较小,安装TMD后结构的阻尼比提高4倍,并且呈现出在自由振动衰减的前期较大,随着振动幅度变弱阻尼比降低;测试中发现TMD装置的减振效率对其阻尼比的变化不敏感。展开更多
为了给下一代无线通信系统(B3G:Beyond 3G)参数设计提供参考以及为算法仿真提供信道建模,针对国内城市室外环境不同的传播场景,在5.8 GHz频段和20 MHz信道带宽的测量条件下,进行了室外信道测量和研究。根据测量数据统计分析,5.8 GHz频...为了给下一代无线通信系统(B3G:Beyond 3G)参数设计提供参考以及为算法仿真提供信道建模,针对国内城市室外环境不同的传播场景,在5.8 GHz频段和20 MHz信道带宽的测量条件下,进行了室外信道测量和研究。根据测量数据统计分析,5.8 GHz频段路径损耗指数在视距通信(LOS:Line of Sight)时为2.53,在非视距通信(NLOS:No Line of Sight)时为3.3～3.8;同时,对COST231-WI路径损耗模型进行了修正,考虑到阴影衰落的影响,修正后的模型能较准确地预测接收功率,为系统覆盖范围预测提供参考。均方根时延扩展(Root Mean Square Delay Spread)的累积概率为0.9时,在0.5～0.8μs之间变化。均方根角度扩展(RMS Azimuth Spread)主要由发射天线位置和传播环境决定。展开更多
电波传播特性预测是无线电系统设计的基础,其中路径损耗特性关系到系统覆盖范围,决定系统布局,时延特性决定数字通信系统的最大数据传输速率.提出一种预测矩形隧道中电波传播特性的方法,该方法可以通过几何光学原理精确地计算出由发射...电波传播特性预测是无线电系统设计的基础,其中路径损耗特性关系到系统覆盖范围,决定系统布局,时延特性决定数字通信系统的最大数据传输速率.提出一种预测矩形隧道中电波传播特性的方法,该方法可以通过几何光学原理精确地计算出由发射天线到达接收天线的电波主要路径,避免了复杂度很高的射线跟踪过程,使传统预测方法的计算复杂度大大降低.仿真结果表明:该模型对电波传播的路径损耗预测精度不低于传统的射线跟踪方法;隧道环境中收发天线相距越近,其接收的多径信号的平均时延扩散与均方根(root mean square,RMS)时延扩散越大;隧道截面积越大,其接收的多径信号的平均时延扩散与RMS时延扩散越大.展开更多
基金supported by National Natural Science Foundation of China (Grant No. 50705096)National Science and Technology Major Project of China(Grant No. 2009zx04014-014)
文摘Paris law can reflect the failure mechanism of materials and is usually used to be a method to predict fatigue life or residual fatigue life.But the variable which can represent the health of machine is hardly measured on line.To a degree,the difficulty of on-line application restricts the scope of application of Paris law.The relationship between characteristic values of vibration signals and the variable in the Paris equation which can describe the health of machine is investigated by taking ball bearings as investigative objects.Based on 6205 deep groove ball bearings as a living example,historical lives and vibration signals are analyzed.The feasibility of describing that variable in the Paris equation by the characteristic value of vibration signals is inspected.After that vibration signals decomposed by empirical mode decomposition(EMD),root mean square(RMS) of intrinsic mode function(IMF) involving fault characteristic frequency has a consistent trend with the diameter of flaws.Based on the trend,two improved Paris models are proposed and the scope of application of them is inspected.These two Paris Models are validated by fatigue residual life data from tests of rolling element bearings and vibration signals monitored in the process of operation of rolling element bearings.It shows that the first improved Paris Model is simple and plain and it can be easily applied in actual conditions.The trend of the fatigue residual life predicted by the second improved Paris model is close to the actual conditions and the result of the prediction is slightly greater than the truth.In conclusion,after the appearance of detectable faults,these improved models based on RMS can predict residual fatigue life on line and a new approach to predict residual fatigue life of ball bearings on line without disturbing the machine running is provided.
基金The project supported by the National Natural Science Foundation of ChinaShanghai Research Funds for Young Scientists
文摘Considering the relationship between the nuclear radius and the average bindingenergy of nucleons,a simple semi-empirical formula for the nuclear rms charge radii is suggest-ed together with the formula for light neutron-rich nuclear rms neutron radii.These formulascan well fit the new experimental values.
文摘以某室外大跨轻柔钢结构人行桥为案例,介绍了该结构的设计概况及调谐质量阻尼器（tuned mass damper,简称TMD）减振设计,分别在结构施工完成后以及TMD装置安装后对其进行了实地动力测试,测得了该结构减振前后的模态特性以及在多种人行荷载工况下的振动响应,采用加速度峰值和均方根值为评价指标,分析了钢连桥人致振动情况及特性。结果表明：一端设计为滑动连接的钢连桥人致振动主要由第1阶竖向振型控制,扭动及水平振动响应相对较小;安装经优化设计的TMD装置后,整体连桥的频率特性没有明显变化,分布式TMD对多种频率激励工况均有良好的调谐减振作用,减振率达到35%-70%;共振激励下测得的原结构阻尼比较小,安装TMD后结构的阻尼比提高4倍,并且呈现出在自由振动衰减的前期较大,随着振动幅度变弱阻尼比降低;测试中发现TMD装置的减振效率对其阻尼比的变化不敏感。
文摘为了给下一代无线通信系统(B3G:Beyond 3G)参数设计提供参考以及为算法仿真提供信道建模,针对国内城市室外环境不同的传播场景,在5.8 GHz频段和20 MHz信道带宽的测量条件下,进行了室外信道测量和研究。根据测量数据统计分析,5.8 GHz频段路径损耗指数在视距通信(LOS:Line of Sight)时为2.53,在非视距通信(NLOS:No Line of Sight)时为3.3～3.8;同时,对COST231-WI路径损耗模型进行了修正,考虑到阴影衰落的影响,修正后的模型能较准确地预测接收功率,为系统覆盖范围预测提供参考。均方根时延扩展(Root Mean Square Delay Spread)的累积概率为0.9时,在0.5～0.8μs之间变化。均方根角度扩展(RMS Azimuth Spread)主要由发射天线位置和传播环境决定。
文摘电波传播特性预测是无线电系统设计的基础,其中路径损耗特性关系到系统覆盖范围,决定系统布局,时延特性决定数字通信系统的最大数据传输速率.提出一种预测矩形隧道中电波传播特性的方法,该方法可以通过几何光学原理精确地计算出由发射天线到达接收天线的电波主要路径,避免了复杂度很高的射线跟踪过程,使传统预测方法的计算复杂度大大降低.仿真结果表明:该模型对电波传播的路径损耗预测精度不低于传统的射线跟踪方法;隧道环境中收发天线相距越近,其接收的多径信号的平均时延扩散与均方根(root mean square,RMS)时延扩散越大;隧道截面积越大,其接收的多径信号的平均时延扩散与RMS时延扩散越大.
