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前手翻屈体前空翻2周是男子跳马最高难度动作之一. 迄今,我国只有胡俊捷曾完成 该动作. 本文通过三维运动学分析技术揭示了胡俊捷完成前手翻屈体前空翻2周动作的运动 学规律与技术特点,获得了身体重心和主要关节的位移,水平速度和垂直速度及关节角度等 运动学指标的变化. 这些结果可为我国运动员发展和改进此类动作提供理论依据和技术参 考.     

抗差估计在光纤陀螺信号处理中的应用

为了消除静基座寻北时基座扰动对寻北精度的影响,基于抗差估计提出了一种处理光纤陀螺信号的方法.采用最小二乘估计和抗差估计对光纤陀螺的输出值进行处理,比较了信号中残留的噪声项及干扰项的处理效果.计算结果表明:当载体运动比较剧烈时,抗差估计可以有效剔除异常干扰,从而可以获得较高的寻北精(密)度.     

方波激磁的感应同步器驱动及信号处理技术

一般的感应同步器的使用需要由正弦波信号作为激磁信号,由高精度的轴角转换器作为信号处理的关键器件,因而其在航天产品的使用中受到限制.通过对感应同步器激磁信号频率成分的研究,文章提出了双相方波激磁的驱动方案.在此基础上,对感应信号的频率成分作了进一步分析,采用了Chebyshey低通滤波的信号处理方案,取得了较好的效果.试验数据表明,该方案的测角分辨率达到±0.8",能够满足航天设备中角度测量的需求.     

MIMU信号去噪方法的对比研究

采用“中值平均法“、“FIR数字滤波器滤波法“、“小波去噪“三种不同的方法对MIMU的输出信号进行去噪处理.通过对处理结果的比较研究,证实了小波变换算法在信号去噪处理中的适用性和优越性,并且编写了实时信号处理软件,提高了MIMU的输出精度,从而提高了惯导系统的精度.     

兰姆波测试信号的数字滤波处理

为了增加兰姆波测试信号特征提取的精度,降低噪声的影响,信号的预处理是必要的.本文采用一发一收兰姆波测试技术,在含有人工槽型缺陷的3mm厚铝板上进行了实验.选择带通无限长脉冲响应滤波器对兰姆波信号进行降噪.考虑到兰姆波的激发频率为250kHz,ⅡR滤波器的通带设置为160～330kHz.采用Hilbert变换获取滤波后信号的包络,结果显示,与滤波前兰姆波信号的包络相比,滤波后信号的包络更加平滑和清晰,各个波包的峰值可以唯一确定,为后续兰姆波信号的走时提取提供了方便.在此基础上,根据入射波与S0反射波峰值之间的时间间隔计算S0模式的群速度,结果与理论值较为接近,由此显示了滤波处理在兰姆波测试信号分析中的重要作用.     

基于改进HHT的水中爆炸冲击波信号时频特性分析方法

根据水中爆炸冲击波信号的特征,利用希尔伯特-黄变换(HHT)处理非平稳信号的优势,提出了一种结合小波分解及相关系数筛选的改进HHT方法.该方法首先通过小波分解将信号分为高频部分和低频部分,然后利用相关系数对固有模态函数(IMF)分量进行筛选.理论与实践表明,改进的HHT方法能够有效、准确地对水中爆炸冲击波信号的时频信息...     

激励脉冲信号对基于周向导波的圆环损伤检测的影响

针对含有径向裂纹的圆环,本文主要研究了不同激励脉冲信号对基于周向导波的圆环损伤检测结果的影响.对具有径向裂纹以及没有裂纹的圆环,分别用不同宽度的脉冲信号激励,然后用ABAQUS程序数值计算其应变信号.由Gabor连续小波转换(CWT)处理上述应变信号以确定径向裂纹的位置.结果显示,脉冲宽度对确定裂纹位置的精确性有很大影响.根据圆环的群速弥散曲线,讨论了圆环上裂纹的确定方法.最后,为验证检测圆环损伤所选择脉冲宽度的合理性,进行了两次实验.两次实验提供了相同的结论.     

模平方小波阈值在MEMS陀螺信号降噪中的应用

在深入分析现有软阈值和硬阈值小波去噪方法的基础上,综合二者的优点,提出一种模平方小波阈值去噪方法,并将其应用到MEMS陀螺仪输出信号的滤波处理中,从而有效地减小了信号中的高频噪声,提高了信噪比,抑制了MEMS陀螺仪的随机漂移.文中详细介绍了该方法的原理与实现过程,并结合某项目研究中MEMS惯性测量系统的实地跑车实验,对其中MEMS陀螺仪的实测数据进行了分析研究,肯定了使用该方法对MEMS陀螺仪输出信号进行滤波消噪处理的可行性和有效性.     

不同摩擦工况下纯铁试样表面磁记忆现象研究

采用纯铁圆盘试样与标准碳钢钢球(45钢)组成球-盘摩擦副进行旋转摩擦试验,研究在干摩擦条件下,不同的滑动速度及法向载荷下摩擦表面的磁记忆现象.试验结果表明:磨损前后磁记忆信号存在较大差异,磨损后的磁记忆信号在磨痕处出现信号突变,此突变信号可以准确地表征磨痕的位置及磨痕的宽度.对磁记忆信号进行梯度处理,可以发现不同工况下磨痕处的信号峰面积S、交点距n及梯度峰峰值Vpp三者均不相同,且与磨痕的特征存在一定的关系.此外,通过获取不同干摩擦条件下磨痕处的梯度信号特征值,发现信号峰面积S、交点距n及梯度峰峰值Vpp三者均随滑动速度及法向载荷的增大而增大.     

小波滤波在单轴机抖激光陀螺输出信号处理中的应用

单轴机抖激光陀螺仪采用抖动偏频方案消除闭锁,于是陀螺输出信号中不仅包括外界输入的有用角速率信息,也包含了抖动信号和各种高频噪声,应用之前必须有效地去除抖动信号和各种高频噪声.本文采用小波阈值滤波方法对某型单轴激光陀螺输出信号进行了处理,选用Daubechies小波函数作为小波基,以强制消噪的方法,分别用db8,db6,db4小波和不同的小波分解层数对信号进行了滤波,发现采用db4小波对机抖激光陀螺输出信号的滤波效果最优,为工程应用打下了基础.     

Effects of multiple structural nonlinearities on limit cycle oscillation of missile control fin

Aeroelastic analyses are performed for a 2-D typical section model with multiple nonlinearities. The differences between a system with multiple nonlinearities in its pitch and plunge spring and a system with a single nonlinearity in its pitch are thoroughly investigated. The unsteady supersonic aerodynamic forces are calculated by the doublet point method (DPM). The iterative V-g method is used for a multiple-nonlinear aeroelastic analysis in the frequency domain and the freeplay nonlinearity is linearized using a describing function method. In the time domain, the DPM unsteady aerodynamic forces, which are based on a function of the reduced frequency, are approximated by the minimum state approximation method. Consequently, multiple structural nonlinearities in the 2-D typical wing section model are influenced by the pitch to plunge frequency ratio. This result is important in that it demonstrates that the flutter speed is closely connected with the frequency ratio, considering that both pitch and plunge nonlinearities result in a higher flutter speed boundary than a conventional aeroelastic system with only one pitch nonlinearity. Furthermore, the gap size of the freeplay affects the amplitude of the limit cycle oscillation (LCO) to gap size ratio.     

Time and frequency domain nonlinear system characterization for mechanical fault identification

Mechanical systems are often nonlinear with nonlinear components and nonlinear connections, and mechanical damage frequently causes changes in the nonlinear characteristics of mechanical systems, e.g. loosening of bolts increases Coulomb friction nonlinearity. Consequently, methods which characterize the nonlinear behavior of mechanical systems are well-suited to detect such damage. This paper presents passive time and frequency domain methods that exploit the changes in the nonlinear behavior of a mechanical system to identify damage. In the time domain, fundamental mechanics models are used to generate restoring forces, which characterize the nonlinear nature of internal forces in system components under loading. The onset of nonlinear damage results in changes to the restoring forces, which can be used as indicators of damage. Analogously, in the frequency domain, transmissibility (output-only) versions of auto-regressive exogenous input (ARX) models are used to locate and characterize the degree to which faults change the nonlinear correlations present in the response data. First, it is shown that damage causes changes in the restoring force characteristics, which can be used to detect damage. Second, it is shown that damage also alters the nonlinear correlations in the data that can be used to locate and track the progress of damage. Both restoring forces and auto-regressive transmissibility methods utilize operational response data for damage identification. Mechanical faults in ground vehicle suspension systems, e.g. loosening of bolts, are identified using experimental data.     

Identification of Weakly Nonlinear Systems Using Describing Function Inversion

In this paper describing functions inversion is used and the restoring force of a nonlinear element in a MDOF system is characterized. The describing functions can be obtained using linearized frequency response functions (FRFs). The response of the system to harmonic excitation forces at distinct frequencies close to the resonant frequency results in linearized FRFs. The nonlinear system can be approximated at each excitation frequency by an equivalent linear system. This approximation leads to calculation of the first-order describing functions. By having the experimental describing functions calculated and the system’s responses corresponding to the nonlinear element (measured or interpolated), nonlinear parameter identification can be performed. Two numerical and experimental case studies are provided to show the applicability of this method.     

双颗粒沉降的动力学分析

采用格子Boltzmann-虚拟区域方法对雷诺数范围为50≤Re≤200的双颗粒自由沉降进行了直接数值模拟。首先,研究发现双颗粒最终沉降的位置分别在通道的1/4和3/4位置附近,且与颗粒的初始间距、雷诺数以及通道宽度无关。其次,重点研究了颗粒沉降过程中所受的侧向力(与沉降方向垂直),首次揭示了侧向力的振动频率与雷诺数呈二次关系,且单颗粒的结果始终小于双颗粒的结果;研究还发现侧向力的振动频率与通道宽度近似呈幂律函数关系,且幂律指数与雷诺数有关,雷诺数越大,幂律指数的绝对值越小。最后还研究了雷诺数及通道宽度对侧向力振动振幅的影响。     

Frequency response of primary resonance of electrostatically actuated CNT cantilevers

This paper deals with electrostatically actuated carbon nanotube (CNT) cantilever over a parallel ground plate. Three forces act on the CNTs cantilever, namely electrostatic, van der Waals, and damping. The van der Waals force is significant for values of 50 nm or less of the gap between the CNT and the ground plate. As both forces electrostatic and van der Waals are nonlinear, and the CNTs electrostatic actuation is given by AC voltage, the CNT undergoes nonlinear parametric dynamics. The methods of multiple scales and reduced order model (ROM) are used to investigate the system under soft AC near half natural frequency of the CNT and weak nonlinearities. The frequency–amplitude response and damping, voltage, and van der Waals effects on the response are reported. It is showed that only five terms ROM predicts and accurately predicts the pull-in instability and the saddle-node bifurcation, respectively.     

Vibration suppression in multi-tool ultrasonic machining to multi-external and parametric excitations

Ultrasonic machining （USM） is of particular interest for the machining of non-conductive, brittle materials such as engineering ceramics. In this paper, a multi-tool technique is used in USM to reduce the vibration in the tool holder and have reasonable amplitude for the tools. This can be done via dynamic absorbers. The coupling of four nonlinear oscillators of the tool holder and tools representing ultrasonic cutting process are investigated. This leads to a four-degree-of-freedom system subjected to multi-external and multi-parametric excitation forces. The aim of this work is to control the tool holder behavior at simultaneous primary, sub-harmonic and internal resonance condition. Multiple scale perturbation method is used to obtain the solution up to the second order approximations. The different resonance cases are reported and studied numerically. The stability of the system is investigated by using both phase-plane and frequency response techniques. The effects of the different parameters of the tools on the system behavior are studied numerically. Comparison with the available published work is reported.     

Adaptive neural DSC for stochastic nonlinear constrained systems under arbitrary switchings

This paper focuses on the primary resonance analysis of a dual-rotor system having two rotor unbalance excitations of different rotating speeds and being connected by an inter-shaft ball bearing. Due to the complex excitation condition and the complicated nonlinear bearing forces of the inter-shaft bearing, the general analytical methods, e.g., the multiple scales method or the harmonic balance method, are failed to give the theoretical solutions. Thus, the harmonic balance–alternating frequency/time domain (HB–AFT) method is formulated to deal with this problem. The basic idea of the method is using the inverse discrete Fourier transform and the discrete Fourier transform, instead of the direct analytical relationship between the supposed solutions of the system and the nonlinear forces, to construct the harmonic expressions of the nonlinear forces, which is the so-called alternating frequency/time domain technique. By using the HB–AFT method, therefore, a Newton– Raphson iteration procedure can be performed to demonstrate the approximate solutions of the system. Accordingly, the frequency responses of the system affected by some critical parameters, such as rotating speed ratio, unbalances of both the inner and outer rotors, and clearance of the inter-shaft bearing, are analyzed, respectively. A variety of phenomena including double resonance peaks, biperiodic and quasi-periodic behaviors, and resonance hysteresis phenomenon are obtained, which are discussed in detail through diagrams for separated frequency responses with different frequency components and rotors’ orbits for different combinations of system parameters. Most prominently, for a relatively small unbalance of rotor as well as a relatively large clearance of the inter-shaft bearing, the resonance hysteresis phenomena are more obvious. The results obtained are also compared with the direct numerical simulation results, and the comparisons show good agreements. In addition, the methodology formulated in this paper is a general approach, which can be applied to other engineering systems with multi-frequency excitations.     

Dynamic response of tank trains to random track irregularities

This paper presents a dynamic analytical model for tank train vibrations. The train is considered as a system of 27 degrees of freedom consisting of lateral, roll, yaw, vertical, and pitch motions for the vehicle body and its two bogies and lateral, roll and vertical motions for the four wheel-sets. Liquid sloshing in the tank is modeled using an equivalent mechanical mass-spring model. Coupling between the vehicle system and the railway track is realized through the interaction forces between the train and the rail, where the vertical and lateral irregularity profiles of the track are regarded as stationary ergodic Gaussian random processes and simulated by polynomial functions. Random vibration theory is used to obtain the response power spectral densities. Finally, numerical results for a typical test case including natural frequencies of a coupled system, frequency response functions, and output power spectral densities are presented.     

An investigation on the lift force of a wing pitching in dynamic stall for a comfort control vessel

On the basis of a comfort control system for ocean vessels, the control forces and moments in the form of lift forces from active wings are of important interest. In an ocean vessel comfort control system, active wings or fins are commonly used and constantly adjust their angles of attack to produce optimal sea-keeping conditions. The unsteady nature of the flow field around a wing, and the behaviour of the generated lift force must be understood in order to optimize the comfort control system. This paper presents experimental data on the flow past a pitching wing, paying particular attention to the lagging effects between the fluid dynamic lift force and the motion of the wing at large angles of attack as a function of peak angle of attack and reduced frequency of oscillation. The range of motion investigated has been chosen according to the applicability of a comfort control wing surface. Numerical data is also included to aid explanation on some of the witnessed phenomena.     

Bifurcation and pull-in voltages of primary resonance of electrostatically actuated SWCNT cantilevers to include van der Waals effect

In this work the voltage response of primary resonance of electrostatically actuated single wall carbon nano tubes (SWCNT) cantilevers over a parallel ground plate is investigated. Three forces act on the SWCNT cantilever, namely electrostatic, van der Waals and damping. While the damping is linear, the electrostatic and van der Waals forces are nonlinear. Moreover, the electrostatic force is also parametric since it is given by AC voltage. Under these forces the dynamics of the SWCNT is nonlinear parametric. The van der Waals force is significant for values less than 50 nm of the gap between the SWCNT and the ground substrate. Reduced order model method (ROM) is used to investigate the system under soft excitation and weak nonlinearities. The voltage-amplitude response and influences of parameters are reported for primary resonance (AC near half natural frequency).