采用单个压电传感器的单模式兰姆波激发频率的选择

考虑了压电传感器(PZT)与板之间的耦合作用,从理论上研究了单个压电传感器激发时产生单模式兰姆波的频率调节方法,实验给出了模式选择在兰姆波结构健康监测中的应用. 在板材、板厚以及PZT尺寸一定的情况下,从理论上能够预测到作为频率函数的各兰姆波模式的幅值变化. 根据某特定兰姆波模式的幅值最大而其他模式幅值相对最小时所对应的频率,即可识别出该兰姆波模式优化的激发频率. 数值仿真验证了理论的有效性以及单模式兰姆波选择的可能性. 在不同的激发频率下,分别激发了优化的A0 模式,优化的S0模式以及共存的A0和S0模 关键词： 兰姆波 压电传感器 激发频率 结构健康监测     

随钻声波测井圆弧状压电阵子的有限元分析*

圆弧状压电阵子是随钻声波测井仪发射换能器的核心部件。利用有限元法对圆弧状压电阵子进行了设计分析,结果表明20 kHz以下频率范围内主要存在弯曲振动模态及径向振动模态,且径向振动模态能够满足随钻声波单极测量模式的工作频率要求。针对径向振动模态,数值模拟了几何尺寸对压电阵子声学性能参数的影响。圆弧状压电阵子的谐振频率随着陶瓷片厚度的增大而升高;随着陶瓷片高度、半径及张开角的增大而降低。圆弧状压电阵子的最大发射电压级幅值随着陶瓷片厚度、张开角的增大而降低;随着半径的增大而升高;随着陶瓷片高度的增大,最大发射电压级幅值先升高后逐渐降低。以外径为6.75 in的钻铤为例,最优化设计了圆弧状压电阵子的尺寸,使其满足钻铤模式波隔声阻带中心频率的要求,同时保证了压电阵子具有最大发射电压级。数值模拟结果可以对随钻声波测井仪发射换能器设计起到良好的指导作用。     

传感器驱动电压对声学温度计中声速测量的影响

声学温度计通过声速的精密测量得到热力学温度,是目前测量热力学温度不确定度最小的方法之一。压电陶瓷传感器结合端盖薄膜结构的声学传感器和传统麦克风传感器相比能够显著提高圆柱共鸣腔的共振频率测量信噪比。在圆柱共鸣腔中,纯轴向声学模式共振峰的幅值和信噪比随着压电陶瓷传感器驱动电压的增加而增加。压电陶瓷传感器驱动电压的变化对共振峰的频率和半宽影响较小,不会引起额外频率扰动。共振频率的随机偏差在气体压力低于150 kPa时随压电陶瓷传感器驱动电压升高逐渐减小;在气体压力高于150 kPa时驱动电压的影响不明显。该研究对于提高采用圆柱定程干涉法声学温度计测量热力学温度的精度具有重要的意义。     

基于动态全息光栅转换技术的压电陶瓷特性测量

观察了光折变LiNbO3晶体中二波耦合动态全息光栅的衍射现象,并利用光栅转换技术实现压电陶瓷的振动频率及快变相位的非接触测量. 实验结果表明,输出光强的变化频率与压电陶瓷的振动频率相等,驱动电压与快变相位幅值呈线性关系.     

随钻声波测井四极子发射换能器的设计*

提出一种基于金属基片结构的随钻四极子发射换能器,其一阶弯曲模式可用于随钻四极子声源。利用有限元法对该换能器进行了设计分析,结果表明在5kHz附近存在一阶弯曲振动模态,其径向位移分布及发射电压级响应均优于单纯的瓦片状压电阵子,非常适合随钻四极子工作频率及声场的测量需求。针对一阶弯曲振动模态,数值模拟了几何尺寸对换能器性能参数的影响。换能器谐振频率随着陶瓷片厚度、陶瓷片高度、陶瓷片张开角度、金属基片厚度的增大而升高。最大发射电压级幅值随着陶瓷片高度、陶瓷片张开角、金属片厚度的增大而升高;随着陶瓷片厚度的增大,最大发射电压级幅值先升高后逐渐降低。金属基片采用铝片可以使换能器有较大的发射功率。数值模拟结果可以对随钻声波测井仪四极子发射换能器的设计起到良好的指导作用。     

非线性工作状态压电换能器匹配技术

针对压电换能器在大功率下存在复杂非线性而导致匹配参数难以优化的问题,以大功率超声金属焊接为例,通过采集测试焊接过程换能器两端电压的幅值与频率,建立换能器反谐振电阻与驱动电压有效值、频率之间的数学模型;提出基于反谐振电阻模型的最优功率匹配方法,推导了匹配电感电容的计算公式。最后实验验证了数学模型的准确性,且当换能器输入功率在最优功率附近时,匹配网络电能传输效率最高。     

水中脉冲放电产生过氧化氢及其影响因素

采用介质涂覆的球-筒电极结构,用以脉冲电压条件下在水中产生放电,通过比色法检测放电产生的过氧化氢,研究了不同电压脉冲幅值、脉冲宽度(储能电容的大小)、水的电导率以及脉冲频率对过氧化氢产生速率的影响,实验结果表明过氧化氢的产率随电压脉冲幅值的增大而增大。当电压脉冲幅值足够高时,水中放电由流注放电形式转换为电弧放电形式,此时过氧化氢产率也大幅提高,而在相同电压条件下,随水的电导率的增大,过氧化氢的产率减小。在相同电压下,脉冲频率的增大,导致放电平均功率增大,水中放电产生过氧化氢的浓度提高。在3.3 W功率时,120 min后水中过氧化氢浓度达到0.2 mmol/L,从而证明了所用电极结构的优越性。     

大气压空气中重复频率纳秒脉冲气体放电模式研究

利用上升沿100ns、脉宽150ns的单级磁压缩纳秒脉冲电源,通过电压电流测量和放电图像拍摄实验,研究了大气压空气中极不均匀电场结构重复频率纳秒脉冲气体放电的放电模式。结果表明纳秒脉冲气体放电存在三种典型的放电模式:电晕放电、弥散放电和火花放电。施加的脉冲电压幅值对放电模式影响显著,随着电压幅值的增加,放电依次经历电晕、弥散和火花放电。固定电压幅值时,放电可能同时存在两种模式。重复频率加强了放电强度,弥散放电的激发电压随重复频率的增加变化不大,但火花放电的激发电压随着重复频率的增加而降低。因此降低重复频率有利于在较大电压范围获得大气压空气弥散放电。     

基于压电陶瓷的激光频率调谐技术

在充分研究各种激光调谐方法优缺点的基础上,针对其调谐范围、调谐速率、调谐线性等方面存在的不足,提出一种基于压电陶瓷的激光频率调谐技术。该方法将压电陶瓷与光纤光栅激光器的布拉格光栅进行粘结,通过调整压电陶瓷的驱动电压来带动布拉格光栅的伸缩,实现波长（即频率）的调谐。同时,利用虚拟仪器中的计算机软件拟合技术,校正压电陶瓷输入电压与输出位移之间的非线性,使系统呈线性频率调谐,以提高测量精度。实验结果表明,当压电陶瓷的驱动电压变化126 V时,可实现0.8 nm（即100 GHz）的调谐范围。     

PVDF梳状换能器接收非线性兰姆波的实验研究*

该文基于PVDF压电薄膜对梳状换能器进行设计制作,并将该换能器应用于接收非线性兰姆波信号实验研究。首先通过输出矢量和方法分析PVDF梳状换能器的工作特性,然后将设计制作的PVDF梳状换能器应用于铝板中非线性兰姆波信号的接收,并与传统压电陶瓷换能器经斜劈接收的非线性兰姆波信号进行比较分析。实验结果表明,两种换能器所接收到的信号幅值随传播距离的变化趋势相近,线性增长的积累效应表征结果相似,且PVDF梳状换能器可以对基波和二次谐波信号同时响应。除此之外,PVDF梳状换能器用于接收非线性兰姆波信号更加稳定。因此,PVDF梳状换能器有望应用于复杂构件的在线检测与监测研究。     

Piezoelectric transducer parameter selection for exciting a single mode from multiple modes of Lamb waves

Excitation and propagation of Lamb waves by using rectangular and circular piezoelectric transducers surface-bonded to an isotropic plate are investigated in this work. Analytical stain wave solutions are derived for the two transducer shapes, giving the responses of these transducers in Lamb wave fields. The analytical study is supported by a numerical simulation using the finite element method. Symmetric and antisymmetric components in the wave propagation responses are inspected in detail with respect to test parameters such as the transducer geometry, the length and the excitation frequency. By placing only one piezoelectric transducer on the top or the bottom surface of the plate and weakening the strength of one mode while enhancing the strength of the other modes to find the centre frequency, with which the peak wave amplitude ratio between the S0 and A0 modes is maximum, a single mode excitation from the multiple modes of the Lamb waves can be achieved approximately. Experimental data are presented to show the validity of the analyses. The results are used to optimize the Lamb wave detection system.     

用分数阶微分实现时频重叠多模式兰姆波的模式分离

为了分离时频重叠多模式超声兰姆波,提出了一种用分数阶微分理论实现多模式兰姆波模式分离的方法。以时频重叠的S1和A1模式混合信号为例,用赛利斯分布作为幅值谱的模型,首先对信号做频散补偿,由补偿后信号幅值谱分数阶微分推导了幅值谱特征参数的计算式并根据信号模型重建幅值谱,然后结合兰姆波的频散特性推导出对应的相位谱,并通过幅值谱和相位谱重构各模式的信号,实现模式分离。对1 mm钢板中仿真时频重叠的S₁和A₁模式信号分离结果显示分离出信号的幅值和带宽相对误差小于6%,中心频率相对误差小于0.25%。实验结果也证实了方法的可行性。因此当混合信号幅值谱不完全交叠时,本方法可以实现多模式兰姆波信号的分离,有助于多模式兰姆波频散信号的分析和识别。      

基于频散补偿和分数阶微分的多模式兰姆波分离

由于兰姆波的多模和频散特性,实际检测时在同一激发频率下存在多种模式的混合信号,而各模式信号有不同的频散特性,使得在时频混叠的情况下兰姆波的检测变得十分复杂.本文在频散补偿的基础上,通过时延函数建模,依靠不同模式频散趋势的差异性,将时频混叠信号的分离问题转化为部分模式混叠信号的分离问题.基于分数阶微分的理论,用信号幅值谱分数阶微分极大值和对应频率分别与微分阶次拟合多项式实现特征参数的提取并依靠特征参数重建幅值谱.结合相位谱重构时域信号以实现部分混叠信号中频散补偿后的模式的分离.最后恢复频散获得分离后的兰姆波信号.仿真和实验结果表明,本文方法不仅可以实现时频混叠多模式兰姆波信号的分离,更能保证分离精度,有助于复杂多模式频散信号的分离与处理的进一步研究.     

用低通滤波方法终止心脏组织中的螺旋波和时空混沌

通过让心肌细胞钠离子通道的触发门变量延迟打开, 使介质具有激发延迟能力, 介质延迟激发时间随控制电压和刺激频率增加而增加, 当控制电压超过一个阈值时, 延迟激发介质具有低通滤波作用:低频波可以连续通过, 而高频波不能连续通过. 本文用Luo-Rudy相I模型研究了介质延迟激发对螺旋波和时空混沌的影响, 数值模拟结果表明: 当控制电压超过阈值时, 介质的延迟激发可有效消除螺旋波和时空混沌; 从小逐渐增大控制电压, 在钙最大电导率较小情况下, 延迟激发会导致介质激发性降低, 使螺旋波漫游幅度增大, 直至传导障碍导致螺旋波消失; 当钙最大电导率较大时, 延迟激发会导致螺旋波失稳变弱, 这样当控制电压增加到一定值时, 时空混沌可以演化成漫游螺旋波, 当控制参数被适当选取时, 观察到漫游幅度大的螺旋波漫游出系统边界消失现象, 继续增大控制电压将导致时空混沌直接消失.     

Optimization of un-tethered, low voltage, 20-100kHz flexural transducers for biomedical ultrasonics applications

This paper describes optimization of un-tethered, low voltage, 20-100 kHz flexural transducers for biomedical ultrasonics applications. The goal of this work was to design a fully wearable, low weight (<100 g), battery operated, piezoelectric ultrasound applicator providing maximum output pressure amplitude at the minimum excitation voltage.Such implementation of ultrasound applicators that can operate at the excitation voltages on the order of only 10-25 V is needed in view of the emerging evidence that spatial-peak temporal-peak ultrasound intensity (I_SPTP) on the order of 100 mW/cm² delivered at frequencies below 100 kHz can have beneficial therapeutic effects. The beneficial therapeutic applications include wound management of chronic ulcers and non-invasive transdermal delivery of insulin and liposome encapsulated drugs.The early prototypes of the 20 and 100 kHz applicators were optimized using the maximum electrical power transfer theorem, which required a punctilious analysis of the complex impedance of the piezoelectric disks mounted in appropriately shaped metal housings.In the implementation tested, the optimized ultrasound transducer applicators were driven by portable, customized electronics, which controlled the excitation voltage amplitude and facilitated operation in continuous wave (CW) or pulsed mode with adjustable (10-90%) duty cycle. The driver unit was powered by remotely located rechargeable lithium (Li) polymer batteries. This was done to further minimize the weight of the applicator unit making it wearable. With DC voltage of approximately 15 V the prototypes were capable of delivering pressure amplitudes of about 55 kPa or 100 mW/cm² (I_SPTP). This level of acoustic output was chosen as it is considered safe and side effects free, even at prolonged exposure.     

Separation of multimode Lamb waves overlapped in time and frequency domains by using fractional differential

To separate multimode Lamb wave overlapped both in time and frequency domains,a multimode Lamb wave signal separation method by using fractional differential is proposed.The Tsallis function is used as the model of the amplitude spectrum.The amplitude spectra of Lamb wave signals are divided into fractional order differential.The characteristic parameters of each mode are extracted by using the polynomial between the maximum amplitude and the differential order as well as the polynomial between the peak frequency and the differential order.The corresponding phase spectrum of each mode is derived from the dispersion characteristics.The signal of each mode is constructed through the amplitude spectrum and phase spectrum.The simulated results of S1 and A1 modes in a 1 mm-thick steel plate show that the relative errors of amplitude and bandwidth of both modes are less than 6%,and that the relative errors of central frequency of both modes are less than 0.25%.Therefore,this method could be used to separate the time-frequency overlapped multimode Lamb wave signals.It is helpful for the analysis and recognition of multimode Lamb wave signals.     

Selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers

In this paper, we describe a modal expansion approach for the analysis of the selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers(EMATs). With the modal expansion approach for waveguide excitation, an analytical expression of the Lamb wave's mode expansion coefficient is deduced, which is related to the driving frequency and the geometrical parameters of the EMAT's meander coil, and lays a theoretical foundation for exactly analyzing the selective generation of Lamb waves with EMATs. The influences of the driving frequency on the mode expansion coefficient of ultrasonic Lamb waves are analyzed when the EMAT's geometrical parameters are given. The numerical simulations and experimental examinations show that the ultrasonic Lamb wave modes can be effectively regulated(strengthened or restrained) by choosing an appropriate driving frequency of EMAT, with the geometrical parameters given. This result provides a theoretical and experimental basis for selectively generating a single and pure Lamb wave mode with EMATs.     

金属板疲劳损伤非线性兰姆波混频检测

针对金属板结构安全运行需要,开展了金属板结构疲劳损伤非线性兰姆波混频检测方法研究.通过数值仿真,研究了两列A0兰姆波与材料损伤间的非线性相互作用.结果 表明,两列共线A0兰姆波在结构材料损伤处产生单向传播的和频S0波,且和频波幅值随传播距离具有积累增长效应.对不同疲劳程度金属板试件进行了共线混频兰姆波检测实验,结果表明...     

General solution of cumulative second harmonic by Lamb wave propagation in a solid plate

A straightforward approach has been developed for the general solution of cumulative second harmonic by Lamb wave propagation in a solid plate. The present analyses of second-harmonic generation by Lamb waves focus on the cases where the phase velocity of the fundamental Lamb wave is exactly or approximately equal to that of the double frequency Lamb wave （DFLW）. Based on the general solution obtained, the numerical analyses show that the cumulative second-harmonic fields are associated with the position of excitation source and the difference between the phase velocity of the fundamental Lamb wave and that of the dominant DFLW component.